ABIOTIC AND BIOTIC PROCESSES IN THE RELEASE AND CONTROL OF
SELENIUM IN THE WESTERN PHOSPHATE RESOURCE AREA
A Thesis
Presented in Partial Fulfillment of the Requirements for the
Degree of Master of Science
with a
Major in Environmental Science
in the
College of Graduate Studies
University of Idaho
by
Jon P. Munkers
Date December 18, 2000
Major Professor: Gregory Möller, Ph.D.
ii
AUTHORIZATION TO SUBMIT THESIS
This thesis of Jon P. Munkers, submitted for the degree of Master of Science with a
major in Environmental Science and titled “Abiotic and Biotic Processes in the
Release and Control of Selenium in the Western Phosphate Area,” has been
reviewed in final form, as indicated by the signatures and dates given below.
Permission is now granted to submit final copies to the College of Graduate Studies
for approval.
Major Professor
_____________________________________Date__________
Gregory MÖller, Ph.D.
Committee
Members
_____________________________________Date__________
Daniel G. Strawn, Ph.D.
_____________________________________Date__________
Thomas F. Hess, Ph.D.
_____________________________________Date__________
Earl Philip Druker
Department
Administrator
_____________________________________Date__________
Margrit von Braun, Ph.D.
Dean, College of
Letters and Science
_____________________________________Date__________
Kurt O. Olsson, Ph.D.
Final Approval and Acceptance by the College of Graduate Studies
_____________________________________Date__________
Charles R. Hatch, Ph.D.
iii
ABSTRACT
The mobilization of selenium from phosphoria mining in Southeastern Idaho
has resulted in concern for environmental contamination and created a challenge to
understand the processes of release and approaches for control. The objective of
this research is the characterization of abiotic and biotic processes affecting the
release and accelerated immobilization of selenium in reclaimed waste rock soils
and nearby waters. In our field studies, we evaluated vadose zone leaching of Se
following subsurface amendment with potato waste and iron metal granules. We
also explored the activity of surface applied iron granules and cheese whey in
limiting plant uptake of Se and release of shallow subsurface Se. Fe0 can act as
both a reducing agent and co-precipitation agent for selenium oxyanions. Potato
waste and cheese whey act as chemical reducing agents and nutrient sources for
facultative anaerobic bacteria that can accelerate selenium immobilization.
Laboratory studies have shown that chemical reduction from added scrap iron metal,
potato waste, and cheese whey can limit Se leachability and aid in co-precipitation of
soluble Se (Bond 1999). Our data suggests promise in the field applications of such
amendments. The primary findings of this research were: 1) Se is substituting for
sulfur in the pyritic component of the middle waste shales, 2) there are naturally
occuring selenium-reducing bacteria present in the soils of the phosphoria region;
and 3) combination amendments containing cheese whey, potato waste, and iron
had leachable selenium concentrations up to 90% lower than those of a control.
iv
ACKNOWLEDGEMENTS
I would like to thank the J.R. Simplot Company for its financial support of both
my research and education as well as to the employees who were involved with my
research. Thank you Dr. Gregory Möller for the opportunity to be involved in this
project as well as your support, guidance, and insight through my graduate
experience. Thanks to the other research associates, graduate students, and
undergraduates that provided their support, specifically Tamara Shokes, Melanie
Bond, Justin Stockwell, Kevin Brackney, James George, Charles Baier, Dave
Cummings, Dr. Frank Rosenzweig, and Montessa Young. Thank you to my
committee members Tom Hess, Dan Strawn, and Phil Druker, as well as the
University of Idaho and the Environmental Science Faculty who are dedicated to the
students they help educate.
I would like to thank my parents and family for their support and guidance
throughout my life. They taught me through hard work I could do anything and be
anything I wanted. Most importantly I would like to thank my wife, Gina, for her
patience, understanding and support during those weeks alone while I was in the
field, especially the evening she gave birth to our son Ben. Lastly, Benjamin Riley
you have given me more inspiration than any other event in my life.
v
TABLE OF CONTENTS
ABSTRACT ...............................................................................................................III
ACKNOWLEDGEMENTS ........................................................................................ IV
TABLE OF CONTENTS ............................................................................................ V
LIST OF FIGURES.................................................................................................... X
LIST OF PHOTOGRAPHS..................................................................................... XIV
1.0 PURPOSE AND OBJECTIVES............................................................................1
2.0 SELENIUM...........................................................................................................2
2.1 CHEMISTRY.............................................................................................. 2
2.2 OCCURRENCE .......................................................................................... 4
2.3 SELENIUM IN THE ENVIRONMENT................................................................ 6
2.3.1 Soils ................................................................................................ 6
2.3.2 Water............................................................................................. 13
2.3 GUIDELINES AND STANDARDS.................................................................. 14
2.4 BIOLOGY AND RECOMMENDED DIETARY ALLOWANCE ................................ 15
2.5 SELENIUM DISEASE ................................................................................ 16
2.5.1 Selenium Deficiency...................................................................... 16
2.5.2 Selenium Toxicosis ....................................................................... 18
3.0 HISTORY OF THE WESTERN PHOSPHATE FIELD ........................................20
3.1 BACKGROUND ........................................................................................ 20
3.2 ACID MINE DRAINAGE ............................................................................. 22
3.3 SELENIUM LEACHING MECHANISM ........................................................... 23
3.3.1 French Drains................................................................................ 23
vi
4.0 REMEDIATION AND TREATMENT APPROACHES .........................................24
4.1 W ATER TREATMENT PROCESSES FOR THE REMOVAL OF INORGANIC SE ..... 24
4.2 BIOREMEDIATION .................................................................................... 25
4.2.1 Selenium-Reducing Bacteria (SRB) .............................................. 25
4.2.1.1 Biochemistry ...........................................................................26
4.2.1.2 Nutrient Amendments .............................................................28
4.3 CHEMICAL REDUCTION AND CO-PRECIPITATION ........................................ 29
4.4 PHYTOREMEDIATION ............................................................................... 31
5.0 MATERIALS AND METHODS ...........................................................................33
5.1 MATERIALS ............................................................................................ 33
5.2 LABORATORY AND ANALYTICAL EQUIPMENT............................................... 33
5.3 ANALYTICAL METHODS FOR WATER AND SOILS .......................................... 34
5.3.1 Waters........................................................................................... 34
5.3.2 Soil ................................................................................................ 36
6.0 CHARACTERIZATION ANALYSIS ....................................................................38
6.1 SCANNING ELECTRON MICROSCOPE (SEM) ............................................. 38
6.1.1 Purpose ......................................................................................... 38
6.1.2 Methods......................................................................................... 38
6.1.3 Results and Discussion ................................................................. 38
6.2 DRILL HOLE CORE ANALYSIS ................................................................... 40
6.2.1 Purpose ......................................................................................... 40
6.2.2 Methods......................................................................................... 40
6.2.3 Results .......................................................................................... 42
vii
7.0 SATURATED PASTE AMENDMENT STUDY....................................................43
7.1 PURPOSE .............................................................................................. 43
7.2 METHODS .............................................................................................. 44
7.3 RESULTS ............................................................................................... 45
7.4 DISCUSSION........................................................................................... 46
8.0 SRB – CULTURING EXPERIMENT...................................................................48
8.1 PURPOSE .............................................................................................. 48
8.2 METHODS .............................................................................................. 48
8.2.1 Standards ...................................................................................... 48
8.2.2 Test solutions ................................................................................ 49
8.2.3 Creek Water .................................................................................. 49
8.2.4 Standard Transfers........................................................................ 50
8.2.4 Media Inoculation .......................................................................... 50
8.3 RESULTS AND DISCUSSION...................................................................... 51
9.0 BATCH REACTOR EXPERIMENT ....................................................................53
9.1 PURPOSE .............................................................................................. 53
9.2 METHODS .............................................................................................. 53
9.2.1 Reactor Construction..................................................................... 53
9.2.2 Procedure...................................................................................... 54
9.2.2.1 Initiating the Experiment .........................................................54
9.2.2.2 Sampling.................................................................................55
9.3 RESULTS AND DISCUSSION...................................................................... 55
10.0 GREENHOUSE STUDY: SELENIUM UPTAKE IN BROMEGRASS IN
AMENDED SOILS ...................................................................................................... 63
viii
10.1 PURPOSE ............................................................................................ 63
10.2 METHODS ............................................................................................ 63
10.4 DISCUSSION ......................................................................................... 72
11.0 SURFACE WASTE ROCK FIELD EXPERIMENTS .........................................74
11.1 PURPOSE ............................................................................................ 74
11.2 METHODS ............................................................................................ 74
11.2.1 Construction ................................................................................ 74
11.2.2 Sampling ..................................................................................... 75
11.3 RESULTS AND DISCUSSION.................................................................... 75
12.0 SUBSURFACE POLE CREEK WASTE ROCK AMENDMENT ANALYSIS .....80
12.1 PURPOSE ............................................................................................ 80
12.2 METHODS ............................................................................................ 80
12.2.1 Subsurface Sampling Assembly (Pan Lysimeters)...................... 81
12.2.2 Collection Drain ........................................................................... 83
12.2.3 Horizontal PVC Connections and Suction Lysimeter
Installation......................................................................................................... 84
12.2.4 Backfill ......................................................................................... 86
12.2.5 Pit Experimental Treatments ....................................................... 87
12.2.6 Central Sampling Facility............................................................. 89
12.2.7 Sampling Protocol ....................................................................... 89
12.2.7.1 Pit Suction Lysimeters ..........................................................89
12.2.7.2 Subsurface Sampling Assemblies (SSAs) ............................90
12.3 RESULTS ............................................................................................. 90
ix
12.3.1 SSAs ........................................................................................... 91
12.3.2 Suction Lysimeters ...................................................................... 93
12.4 DISCUSSION ......................................................................................... 95
13.0 CONCLUSION ...............................................................................................100
REFERENCES.......................................................................................................102
APPENDIX A..........................................................................................................111
x
LIST OF FIGURES
Figure 1. Thermodynamic stability of selenium in water at 10º C, ionic
strength 0.017 and [Se]=8.6µM. Solid lines indicate areas of solid phase
stability, dashed lines indicate aqueous phase stability, and dotted lines
indicate water stability limits. ...................................................................................13
Figure 2. A relief map of Idaho, Montana and Wyoming with the
Southeastern Idaho Phosphate Region circled (USGS)..........................................20
Figure 3. Drill Core analysis of Se and P2O5 concentrations (mg/kg) in a
386-ft core of Smoky Canyon soils measured at two-foot intervals.........................41
Figure 4. Saturated Paste Se concentrations (µM) after 14 days for soils
amended with Star Valley Whey, Star Valley Whey + Fe, WSU Whey and
WSU Whey + Fe compared to a non-amended control...........................................45
Figure 5. Comparison of control, 1 % iron, cheese whey and iron + cheese
whey batch reactor pore water pH over time (n=1). ................................................56
Figure 6. Comparison of control, 1.0 % iron, cheese whey and iron + cheese
whey batch reactor dissolved oxygen changes over time (n=1)..............................57
Figure 7. Comparison of control, 1.0 % iron, cheese whey and iron + cheese
whey batch reactor mV changes over time (n=1)....................................................58
Figure 8. Comparison of the control, 1.0 % iron, cheese whey and iron +
cheese whey batch reactor temperature (°C) changes over time (n=1). .................59
Figure 9. Fluctuation of the dissolved oxygen concentrations (mg/l) for
the1% Fe batch-reactor during exposure to air during the re-aeration
experiment. .............................................................................................................60
Figure 10. Fluctuations in pH during 1% Fe batch reactor exposure to air
during the re-aeration experiment. ..........................................................................61
Figure 11. Fluctuation of the oxidation/reduction potential for the 1 % Fe
Batch Reactor during the re-aeration experiment....................................................62
Figure 12. Comparison of the control, 1 % iron, cheese whey and iron +
cheese whey batch reactor pore water Se concentration (µM) over time
(n=1)........................................................................................................................62
Figure 13. Percent plant biomass compared to the percent root biomass of
the soil amendments and control (n=4). ..................................................................64
xi
Figure 14. Total Bromegrass biomass (grams) in amended and control pots
(n=4)........................................................................................................................64
Figure 15. Average concentration of total Se in the plant matter for the
amended and control pots (n=4). ............................................................................65
Figure 16. Comparison of the concentration of total Se in the Bromegrass
biomass between the amendments and control (n=4).............................................65
Figure 17. Comparison of total Se concentrations of the 3 hr soil leachate in
the different amended soils (n=4)............................................................................66
Figure 18. Comparison of soil phosphorus concentrations (µg/g) for the
different amendments .............................................................................................66
Figure 20. Comparison of soil potassium concentrations (µg/g) for the
different amendments. ............................................................................................67
Figure 19. Comparison of soil pH levels for the different amendments. .................67
Figure 21. Comparison of % soil organic matter for the different
amendments ...........................................................................................................68
Figure 22. Comparison of the soil ammonia concentrations (µg/g) for the
different amendments. ............................................................................................68
Figure 23. Comparison of the % sulfur in the soils for the different
amendments. ..........................................................................................................69
Figure 24. Comparison of the % nitrogen in the soil for the different
amendments ...........................................................................................................69
Figure 25. Comparison of % carbon in the soils for the different
amendments ...........................................................................................................70
Figure 26. Comparison of soil NO3 concentrations (µg/g) for the different
amendments ...........................................................................................................70
Figure 27. Comparison of soil sulfate concentrations (µg/g) for the different
amendments ...........................................................................................................71
Figure 28. Comparison of soil Se concentrations (µg/g) of the different
amendments after harvest.......................................................................................71
Figure 29. Comparison of control, iron and cheese whey pore water pH for
the shallow sub-surface over time (n=3). ................................................................75
xii
Figure 30. Comparison of control, iron and cheese whey pore water
oxidation/reduction potential (mV) for the shallow sub-surface over time
(n=3)........................................................................................................................76
Figure 31. Comparison of the control, iron and cheese whey pore water
temperatures (°C) for the shallow sub-surface over time (n=3)...............................77
Figure 32. Comparison of the control, iron and cheese whey pore water Se
concentration (µM) over time (n=3). ........................................................................78
Figure 33. Illustration of the sub-surface sampling assembly (SSA) used to
collect water samples in the subsurface Pole Creek waste rock amendment
experiment. .............................................................................................................88
Figure 34. Comparison of the control, iron, iron + potato waste and potato
waste pore water pH as sampled by the SSAs over time (n=4). .............................91
Figure 35. Comparison of the control, iron, iron + potato waste and potato
waste pore water oxidation/reduction potential (mV) as sampled by the
SSAs over time (n=4). .............................................................................................92
Figure 36. Comparison of the control, iron, iron + potato waste and potato
waste pore water temperatures (°C) as sampled by the SSAs over time
(n=4)........................................................................................................................92
Figure 37. Comparison of the control, iron, iron + potato waste and potato
waste pore water pH as sampled by the suction lysimeters over time (n=2)...........93
Figure 38. Comparison of the control, iron, iron + potato waste and potato
waste pore water oxidation/reduction potential (mV) as sampled by the
suction lysimeters (n=2). .........................................................................................94
Figure 39. Comparison of the control, iron, iron + potato waste and potato
waste pore water temperature (°C) as sampled by the suction lysimeters
over time (n=2). .......................................................................................................95
Figure 40. Comparison of the control, iron, iron + potato waste and potato
waste pore water selenium concentration (µM) as sampled by the suction
lysimeters over time (n=2).......................................................................................97
xiii
LIST OF TABLES
Table 1. Major naturally occurring forms of selenium separated by oxidation
state. .......................................................................................................................2
Table 2. Standard reduction potentials (Eθ,V) for the stable oxidation states
of selenium..............................................................................................................3
Table 3. Literature review of pH influence on sorption/desorption of
selenium (Adapted from MacGregor 1997) .............................................................9
Table 4. Literature review of cation/anion effects on selenium
adsorption/desorption (Adapted from MacGregor 1997) .........................................10
Table 5. Literature review of the effect of calcite on the adsorption of
selenium (Adapted from (MacGregor 1997). ...........................................................12
Table 6. Se toxicity threshold levels for water, sediment, accumulation in the
food-chain, fish and avian eggs...............................................................................15
Table 7. Comparison of treatment methods for selenite and selenate
removal from water .................................................................................................24
Table 8. Water, food source, and Se species combinations for use in media
that selects for selenium-reducing bacteria .............................................................50
xiv
LIST OF PHOTOGRAPHS
Photograph 1. Scanning electron micrograph (16400X) of a framboidal
pyrite fraction containing 1-3% selenium by weight in freshly fractured
Smoky Canyon shale ..............................................................................................39
Photograph 2. Scanning Electron Micrograph of cubic pyrite fraction with 0.1%
selenium by weight of freshly fractured Smoky Canyon shale ................................40
.
1
1.0 PURPOSE AND OBJECTIVES
Initial study of selenium in the Southeastern Phosphate area was prompted
by concern for livestock grazing down gradient of mining overburden dumps. In the
Fall of 1996, confirmed cases of selenium toxicosis in a small number of horses
resulted in an action under the Comprehensive Environmental Response,
Compensation, and Liability Act of 1980 (CERCLA) by the United States Forest
Service against the mining company responsible for the release (Bond 1999).
In January 1997, scientists at the University of Idaho started to examine
selenium release from mining waste piles in the Western Phosphate Region (Bond
1999). The first part of the examination was the characterization of release and
laboratory examination of potential control approaches. In May 1999 information
regarding characterization and potential control approaches was published (Bond
1999). Additional work by the United States Geological Survey (USGS) and the
regional Interagency Selenium Working Group characterized regional geology, Se
source minerals and environmental Se levels.
This work continues the selenium research in the region, focusing on possible
release mechanisms and potential control approaches. Knowledge gained from
earlier studies is used for the implementation of large-scale field experiments. This
research focuses on how surface and subsurface chemical and microbial dynamics
change through amendment addition, both in laboratory tested selenium control
approaches and field amendment applications.
2
2.0 SELENIUM
2.1 Chemistry
Selenium (Se), named for Selene the Greek Goddess of the Moon, was
discovered by Berzelius in 1818. Selenium possesses both metallic and nonmetallic properties giving it the distinction of a metalloid. It has an atomic mass of
78.96 and is part of group VI on the periodic table. Selenium shares many of the
attributes of sulfur, which is also a group VI element. Selenium is a major
component of 40 minerals and a minor constituent of 37 others (Cooper and Bennett
1970). Selenium exists in four oxidation states: (Se+6) as selenate, (Se+4) as
selenite, (Se0) as elemental Se, (Se-2) as selenides and organo-selenium
Table 1. Major naturally occurring forms of selenium separated by
oxidation state.
Oxidation State
-2
Se (Selenides)
0
Se (Selenium)
Forms
H2Se
Na2Se
(CH3)2Se
Selenomethionine
Se-methyl-Selenocysteine
Selenocysteine
Selenotaurine
Selenocystathionone
Seleno-di-glutathione
Amorphous selenium
Red selenium
Dark red selenium
Gray selenium
+4
(Selenites)
H2SeO3
Na2SeO3
+6
(Selenates)
Na2SeO4
Se
Se
3
compounds. Table 1 shows the major naturally occurring forms of selenium.
The standard reduction potentials (E°/V) for selenium are shown in Table 2.
Table 2. Standard reduction potentials (E0,V) for the stable oxidation states of
selenium (adapted from Winter 2000).
+6
+4
1.15
Acidic
solution
SeO42-
↔
0.74
H2SeO3
0.03
Basic
solution
SeO42-
↔
0
↔
-0.11
Se
-0.36
SeO32-
↔
-2
↔
H2Se
-0.67
Se
↔
Se2-
Selenium is not mined from any ore as primary product (Oldfield 1994). It is
found principally in sulfide minerals of copper, iron, and lead and is most common in
chalcopyrite, bornite and pyrite. It occurs most abundantly in the North American
porphyry copper deposits of the western U.S. and in the copper-nickel and copperzinc ores of central and eastern Canada. Selenium also occurs in copper ores in
Africa, Asia, Europe, South America, Oceania and Australia. Occasionally it is found
in conjunction with native sulfur and in the form of selenites of other metals (Oldfield
1994).
The first commercial application for selenium was found during World War I,
as a substitute for critically short supplies of manganese, a decolorizer in glass
making. A wide variety of industrial applications have since been developed.
Seventy-five percent of the uses for selenium are in electrical devices, pigments,
4
and glass making (Velinsky and Cutter 1990). Metallurgy, agriculture/biological, and
miscellaneous uses complete the remaining 25% of use.
Selenium is an essential trace mineral found in the soil in its inorganic form.
Plants and microorganisms convert selenium to organic forms, the only true
nutritional source for humans. Because plants do not need selenium, and soil
content can vary from high to barely measurable from one farm to another, there are
no predictable food sources that can yield consistent levels of nutritional selenium
(Bodek 1988).
Selenium can be toxic and dermatologic lesions often characterize the toxicity
of selenium. Selenotic animals and humans develop brittle hair and hooves/nails.
Sporadic cases of selenium poisoning have been reported involving industrial or
accidental exposures to selenium compounds. In certain rural Chinese communities
chronic intakes of very high amounts (several milligrams per day) of selenium were
linked to skin, hair and nail abnormalities, which disappeared upon resuming regular
selenium intakes (Xia et al. 1994).
2.2 Occurrence
Selenium levels in plants and animals depend upon the geographic region
where the product was produced, on agricultural practices, and on the Se
requirements and tolerance of the species. In the United States, the Rocky
Mountains and high central plains may contain 0.20 to 0.30 mg/kg of soil Se, while
the Pacific Northwest and Southeast may contain 2.0 to 3.0 mg/kg (Harr 1978).
5
However, there are considerable variations from one location to another depending
on regional soil minerals, soil genesis, and water geochemistry.
Fossil fuels may contain 1 to 10 mg/kg of Se (Davis, et al. 1988). The annual
release of Se fossil fuels in the U.S. is 1500 tons. In addition, industrial losses are
estimated at 2700 tons of Se and municipal waste accounts for 360 tons. Of this
total Se release, about 25% is in atmospheric emission with the balance in the ash.
Biological use of Se has included the insecticide selenocide to control mites in
citrus groves, grapes, and ornamentals. The dandruff shampoo Selsun contains 1%
Se sulfide. Similar shampoos are used to control dermatitis and mange in dogs.
The greatest current and direct use of selenium in biology is in the
supplementation of animal feeds to provide nutritional requirements for Se (Kishchak
1998). An environmental consequence of such supplementation is the potential
migration of selenium into surface and ground water from field and feedlot runoff.
However, this has been observed to be unlikely (Haws 1997).
In the San Joaquin Valley of California, high Se levels were determined to
cause poisoning of wild waterfowl that inhabited the area (Ohlendorf and Santolo
1994). The Kesterson Reservoir Wildlife Refuge was a major constructed wetland
for agricultural drainage of both surface and ground waters (Ohlendorf and Santolo
1994). After it was determined that high levels of selenium was the cause of the
environmental contamination, reclamation began to limit the hazard of selenium
exposure (U.S. EPA 1992). In 1986 agricultural discharge to the system was
stopped, and in 1988 the reservoir was dewatered and all areas were filled above
the expected seasonal rise of groundwater with clean soil (U.S. Dept. of the Interior
6
1989). Monitoring still continues on the Kesterson Reservoir. Events at Kesterson
were some of the first examples known of wide-scale anthropogenic environmental
Se contamination. Research from the San Joaquin Valley investigations has in turn
produced a great deal of understanding on Se toxicity potential and biogeochemical
cycling.
2.3 Selenium in the Environment
2.3.1 Soils
The abundance of selenium is estimated to be between 0.05 and 0.09 mg/kg
throughout the lithosphere (Davis et al. 1988). The worldwide distribution of
selenium is uneven, and its behavior in the environment is complex due to the
various oxidation states, its chemistry as a Group VI sulfur analogue, and its ability
to be incorporated into organic matter. Selenium can take different solid phase
forms in soils and sediments. The bioavailability of selenium depends considerably
on the phase in which it occurs. These phases include mineral oxides and
hydroxides of iron and manganese; carbonate material, organic matter, and others.
2.3.1.1 Hydrous oxides
The three main hydrous oxides that play an important role in the chemical
processes affecting metals in the soil are Fe, Al, and Mn (MacGregor 1997). A large
quantity of work has been focused on selenium’s association with mineral oxides.
Much of the focus of this research has been to determine the specific complexes
formed between hydrous oxides and selenium, and the specific conditional
influences.
7
Two different complexes can be formed: inner-sphere complexes or outersphere complexes (Jackson 2000). An inner-sphere complex (specific adsorption) is
formed when an aqueous ligand exchanges for a surface hydroxyl group. The
bonding of the inner-sphere complex is the more stable of the two due to the fact
that the inner-sphere consists of either ionic bonds, covalent bonds or a combination
of both.
XOH + SeO3-2 + H+
→
XSeO3- + H2O
(X=Surface)
An outer-sphere complex (non-specific adsorption) is formed when a water
molecule is retained between the surface site and the adsorbed ligand. The bonding
is usually electrostatic in nature and is far weaker than the ionic or covalent bonds of
the inner sphere complexes. For this reason the complex is less stable.
XOH + SeO4-2 + H+
→
XOH2+⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅⋅ SeO4-2
(X=Surface)
Often the surface charge of the hydrous oxide is dependent on pH. At lower
pH, the surface charge tends to be positive, while at higher pH it tends to be
negative. Therefore, the adsorption characteristic of the hydrous oxide is very much
dependent on pH. Since both selenite and selenate are usually present as negative
ions in solution, their adsorption would be greatest at lower pH. Research has
supported this hypothesis. Examples of how pH influences sorption and desorption
are illustrated in Table 3.
Other factors that influence selenium adsorption are oxidation/reduction
potential, temperature, the concentration of selenium present and the effects of
competing ions. The effect of temperature on selenite adsorption onto goethite
showed that raising the temperature from 22 ºC to 32 ºC led to a decrease in
8
selenite adsorption on goethite (Balistrieri and Chao 1987). However, adsorption
studies on bimessite showed no change in adsorption of selenite when the
temperature was raised from 5 ºC to 60 ºC (Scott 1991).
9
Table 3. Literature review of pH influence on sorption/desorption of selenium
(Adapted from MacGregor 1997).
Authors
(Hingston,
Posner et al.
1968, 1972)
(Hamdy and
Gissel-Nielsen
1977)
(Balistrieri and
Chao 1987)
(Hayes,
Charalambos
et al. 1987)
Media
Goethite/
Gibbsite
Species
Selenite
Fe2O3
Selenite
Goethite
Selenite
Goethite
Selenite
Selenate
(Zhang and
Sparks 1990)
Goethite
Selenate
Selenite
Adsorption
greatest at low
pH
(Lipton 1991)
Goethite
Selenite
(Van Der
Hoek,
Bonouvrie et
al. 1994)
(Davis and
Leckie 1980)
Hematite
Selenite
Adsorption
greatest at low
pH
Adsorption
greatest at low
pH
Amorphous
Iron
oxyhydroxide
Amorphous
Iron
oxyhydroxide
Amorphous
Iron
oxyhydroxide
Selenite
(Balistrieri and
Chao 1990)
Hydrous
manganese
dioxide
Selenite
Selenate
Adsorption
greatest at low
pH
(Scott 1991)
Bimessite
Selenite
(Saeki,
Matsumoto et
al. 1995)
Bimessite,l
cryptomelane
and
amorphous
Mn oxide
Selenite
Selenate
Adsorption
greatest at low
pH
Adsorption
greatest at pH 24
(Benjamin
1983)
(Balistrieri and
Chao 1990)
Selenite
Selenite
Selenate
Results: Low pH
Adsorption
greatest at low
pH
Adsorption
greatest at pH3
Results: High pH
Increasing pH leads to
desorption
Adsorption
greatest at pH 4
Adsorption
greatest at low
pH
Increasing pH leads to
desorption at pH 10
Increasing pH leads to
desorption. Selenite almost
completely desorbed at pH 12.
Selenate completely desorbed
at pH 9.
Increasing pH leads to
desorption. Selenite completely
desorbed at pH 11. Selenate
completely desorbed at pH 7.
Increasing pH leads to
desorption
Adsorption
greatest at low
pH
Adsorption
greatest at low
pH
Adsorption
greatest at low
pH
Increasing pH leads to
desorption
Increasing pH from 8-12 leads
to complete desorption
Increasing pH leads to
desorption. Complete
desorption at pH 8.
Increasing pH leads to
desorption. Between pH 4.5
and 6.5 50% was desorbed
Selenate only adsorbed
between pH 4-8. Increasing pH
resulted eventually incomplete
desorption of both selenate and
selenite
No selenate adsorption took
place. Raising the pH from 6 to
10 leads to complete desorption
of selenite
Increasing the pH from 4-7
leads to 50% of the selenite
being desorbed
Increasing pH leads to
complete desorption of selenite
in the pH range of 6-11. Total
desorption of selenate from
amorphous Mn-oxide occurred
at pH 5.5
10
Selenium adsorption will be affected through the competition with other
available anions and cations. Table 4 shows effects of different cation/anions on the
sorption/desorption of selenium species.
Table 4. Literature review of cation/anion effects on selenium adsorption/desorption
(Adapted from MacGregor 1997).
Authors
(Hingston
Posner et
al. 1971)
(Benjamin
1983)
Media
Goethite
Species
Selenite
Cation/Anion Added
Phosphate (13 X the
conc. of Se)
Amorphous
Iron
oxyhydroxide
Goethite
Selenate
Cd, Cu, Co and Zn
Selenite
Nitrate
(Balistrieri
and Chao
1987)
Goethite
Selenite
(Ryden,
Syers et
al. 1987)
Hydrous
ferric oxide
gel.
Selenite
Selenate
Phosphate, silicate,
citrate, molybdate,
bicarbonate/caronate,
oxalate, fluoride,
sulphate
Orthophosphate,
arsenate, silicate,
molybdate, sulphate,
chloride, nitirate
(Naftz and
Rice 1989)
Goethite and
similar
minerals
Amorphous
Feoxyhydroxide
and Mndioxide
Selenite
Various organic
compounds
Selenite
Phosphate, Silicate,
Molybdate, Fluoride,
and sulfate
(Lipton
1991)
Goethite
Selenite
Phosphate
(Glasauer
et al.
1995)
Goethite
Selenite
Na, sulfate and
chloride
(Hayes,
Charalambos
et al. 1987)
(Balistrieri
and Chao
1990)
2.3.1.2 Clays
Effect
Amount of adsorption reduced by
50% compared to a phosphate
free system
Selenate adsorption increased
with the addition of each of the
cations
Almost no effect on adsorption
Selenite is desorbed greatest in
order by phosphate > silicate >
citrate > molybdate >
bicarbonate/carbonate >oxalate >
fluoride>sulphate
Selenite and selenate are
desorbed greatest in order by
orthophosphate>arsenate>silicate
>molybdate>sulphate>chloride>ni
trate
Large amounts of organic saturate
and block adsorption sites on
minerals
Competition sequence for
oxyhdroxide was at H7:
phosphate > silicate molydate.
Competition sequence for Mndioxide was at pH7:
molybdate>phosphate> silicate
Fluoride and sulfate did not effect
adsorption on either oxide except
in extreme amounts
Adsorption was reduced.
Increasing the pH from 6-8
increased desorption
Little effect on the rate and
amount of adsorption occurred
using either NaCl or Na-sulfate
11
Selenium has too large an ionic radius (198 pm) to replace a medium sized
cation in the mineral lattice of a clay mineral. Clays are well known for their
adsorptive qualities with regard to cations, but selenium most often occurs as an
anion or oxyanion. Therefore, the adsorption of selenium to clay would not be
expected. The only way selenium would significantly adsorb to clay would be
through amphoteric pH-dependent charges that arise from broken edges of the clay
where H+ and OH- can adsorb (Gast 1977).
2.3.1.3 Carbonates
Experiments on adsorption of selenium to carbonates have focused on
calcite. Selenium cannot replace calcium in the calcite lattice because the Se ion is
too large. Therefore, adsorption is the only way selenium can be associated with
calcite. There is conflict throughout the literature with regards to the effect of calcite
on the adsorption of selenium. Table 5 summarizes eight of those studies.
2.3.1.4 Organic matter
Organic matter in soil is composed of living organisms, organic debris, and
humus. Selenium can be present directly incorporated into organic matter
chemically or complexed with it through adsorption. When comparing the
knowledge of organic chemistry of selenium in soils to that of inorganic, there is
relatively less known about the organic soil chemistry of selenium.
12
Table 5. Literature review of the effect of calcite on the adsorption of selenium
(Adapted from (MacGregor 1997).
Authors
(Hamdy and
Gissel-Nielsen
1977)
(Singh, Singh et
al. 1981)
(Cutter 1985)
Material
Clay loam and sandy
soil
Significance of carbonate materials
Addition of CaCO3 increased selenite desorption
Neutral to alkaline soils
(Neal 1987)
(Goldberg and
Glaubig 1988)
Alkaline alluvial soils
Calcareous,
montmorillonitic soil
and pure calcite
(Fritz and Hall
1988)
(Cowan,
Zachara et al.
1990)
(Van Der Hoek,
Bonouvrie et al.
1994)
Clay rich soil
CaCO3 positively correlated with selenite and selenate
adsorption
The CaCO3 fraction retained only 3% of the total
selenium present in the sediments
CaCO3 plays no part in retention of selenite
Removal of CaCO3 led to a significant decrease in
selenite adsorption. CaCO3 in pure mineral system
adsorbs 4x more selenite than kaolinite and 8x more
than montmorillonite
Removal of CaCO3 led to no effect on selenite
retention
Calcite has 4-5x less exchange sites for sorption than
goethite, kaolinite and montmorillonite. Calcite may be
a important adsorbent phase in calcareous systems
Selenite was found to have a low affinity for calcite
Marine sediments
Pure calcite
Alkaline fly ash
It is known that selenium forms similar organic compounds to those of sulfur,
and the metabolism of selenium is thought to take similar pathways (Shrift 1969).
Plants can synthesize organoselenium compounds (Terry and Zayed 1998). Also, it
is known that microbial processes are capable of producing reduced
organoselenium species like methyl selenides (Chau et al. 1976) and elemental
selenium (Tomei et al. 1995). Many of the organoselenium compounds formed
through microbial processes are not identifiable due to the reactions involving humic
and fulvic acids (Neal 1995).
2.3.1.5 Other Phases
Selenium can replace sulfur through isomorphous substitution in the sulfide
crystal lattice in minerals like pyrite. Therefore, soil and sediments developed as a
13
result of sulfide bearing minerals can often contain certain levels of selenium.
Selenium is also stable in some environments as Se(0).
2.3.2 Water
Selenate and selenite are both soluble in water (Figure 1). In an oxygenated
water column, selenate and selenite would be the most thermodynamically stable
forms of selenium. These oxidized forms of selenium can be adsorbed by biological
media by diffusion or active transport and reduced to either elemental selenium or
selenides, or chelated into organic compounds (Magos and Webb 1980). Though
there have been examples of high levels of organic selenium, most often the
inorganic forms are dominant (Davis et al. 1988).
Se - H2O System Pourbaix Diagram
Eh (Volts)
1.6
1.4
HSeO4
2-
1.2
SeO4
1.0
0.8
0
H2SeO3
0.6
-
HSeO3
0.4
0.2
2-
SeO3
Se
0.0
-0.2
0
H2Se
-0.4
-
HSe
-0.6
-0.8
-1.0
0
2
4
6
8
10
12
14
pH
Figure 1. Thermodynamic stability of selenium in water at 10º C, ionic
strength 0.017 and [Se]=8.6µM. Solid lines indicate areas of solid phase
stability, dashed lines indicate aqueous phase stability, and dotted lines
indicate water stability limits.
14
Elemental selenium is rather insoluble in water. As a result, aquatic systems
can act as large selenium sinks (Bodek 1988). Aquatic organisms do not easily
assimilate elemental selenium, and sedimentary processes remove it in oxygen poor
aquatic systems. If exposed to oxic conditions, elemental selenium may oxidize to
selenite. Inorganic selenide is also insoluble and may combine with hydrogen to
form volatile hydrogen selenide. Though hydrogen selenide is toxic, it is oxidized
quickly in the presence of oxygen. Alternately, metal selenides, such as FeSe, can
form in anoxic metal rich environments.
2.3 Guidelines and Standards
The freshwater criterion for protection of organisms is 5 µg/L (U.S. EPA
1992). Selenium in surface waters is regulated under the Clean Water Act and,
therefore, falls under the hazardous substance domain of the Comprehensive
Environmental Response, Compensation and Liability Act of 1980 (CERCLA).
Specific Se levels with regard to background, threshold, concern, and toxicology are
shown in Table 6.
The maximum concentration level in the drinking water standard is 0.05 µg/L
(EPA 1998). The Occupational Safety and Health Administration (OSHA) limit for Se
compounds in workplace air is 0.2 milligram of selenium per cubic meter of air (mg
Se/m3).
15
Table 6. Se toxicity threshold levels for water, sediment, accumulation in the
food-chain, fish and avian eggs.
Indicator
Normal
background
Threshold
ranges
Level of
concern
ranges
<0.05-1.5
2-5
2-5
Toxicological
and
reproductive
effects certain
>5
<2µg/g
2-4
2-4
>4
Food Chain
<2
2-4 in diet
3-7
>7
Fish
<2
4-12
>12
Avian Eggs
<3
3-8
>8
Water
Sediment
4.2-9.7
Units for water are micrograms per liter (µg/L); all other measurements are micrograms per
gram (µg/g), dry weight. Modified from (Henderson, Maurer et al. 1995).
2.4 Biology and Recommended Dietary Allowance
Selenium is an essential trace mineral. Selenium has a primary role in the
human body as a component of glutathione peroxidase, an antioxidant enzyme that
protects blood cells from highly reactive free radicals. Working synergistically with
vitamin E, another antioxidant, Se aids in the production of antibodies and in
protecting the immune system. Selenium is required to maintain tissue elasticity and
supports the healthy functioning of the pancreas and the heart (Hill 1974).
During the past 15 years, our knowledge of human Se requirements has
improved greatly, primarily because of research on nutritional Se deficiency
conducted in the People's Republic of China. Dietary surveys demonstrated that
Keshan disease, a juvenile cardiomyopathy, was prevalent in parts of China where
the food supply did not provide at least 13 µg Se per day, the intake regarded as a
16
minimum daily requirement (Xia et al. 1994). A study in an area where Keshan
disease was prevalent took place to determine the amount of dietary Se needed to
maximize the activity of the Se containing enzyme, glutathione peroxidase, in
plasma. For men weighing 60 kg, enzymatic activity reached a plateau at selenium
intakes of about 40 µg per day. The Institute of Medicine, through its Food and
Nutrition Board (FNB) and the Standing Committee on the Scientific Evaluation of
Dietary Reference Intakes, released its report on the Dietary Reference Intakes for
Dietary Antioxidants and Related Compounds on April 10, 2000. The recommended
intake level for selenium was set at the amount associated with the highest activity
of enzymes that guard against oxidants in the body. Women and men had
recommended intakes of 55 µg per day. The report set the upper intake level for
selenium at 400 µg per day. The level is based on nutrients from all sources. More
than this amount could cause selenosis. Foods cited as good sources of selenium
included seafood, liver, meat, and grains. In selenium-poor areas of the world (e.g.,
China, New Zealand and Scandinavia), diets may not readily furnish such intakes
and require supplementation.
2.5 Selenium Disease
2.5.1 Selenium Deficiency
The first reference to selenium deficiency was in parts of China. Although
selenium deficiency is rare, there have been times where a deficiency may
exacerbate damage from other diseases. For example, researchers have also found
that the lower the concentration of Se in the blood stream, the higher the risk of
17
developing certain cancers. There is also evidence that children with Down's
Syndrome have lower serum levels which is thought to result in increased free
radical damage to the nerves (Anneren 1990).
The most known pathology associated with Se deficiency is Keshan disease.
Keshan disease is a cardiomyopathy named after the region in China where it was
widespread in 1935. A leading hypothesis is that deficiency allows injury of the heart
by impairing oxidant defenses (Xia et al. 1994).
Areas affected by Se deficiency are usually low in soil Se, resulting in crops of
low Se content or those people receiving total parenteral nutrition without selenium
for extended periods of time. Keshan disease is diagnosed on the basis of signs of
acute or chronic insufficiency of cardiac function, cardiac enlargement, gallop
rhythm, cardiac arrhythmias, and electrocardiographic (EKG) and radiographic
abnormalities (Combs 1986). Some symptoms often observed are dizziness,
malaise, loss of appetite, nausea, vomiting, chills, pericardial and substernal
discomfort, and dyspnea.
In regions of the world that have low Se soils, supplements and Se fortified
fertilizers have been used. In 1983 the Ministry of Agriculture and Forestry of
Finland supplemented multi-mineral fertilizers with Se in the form of sodium selenate
(Varo and Huttumen 1994). Other areas adopted similar supplementation practices
with fertilizers. In areas that are experiencing acute Se deficiency, oral supplements
have been used to increase Se levels. Supplementation of salt supplies with
selenite was instituted in 1983 in areas of China. Selenium supplementation had
18
virtually eliminated the incidence of Keshan disease in China with no reported cases
in the year 1991 (Xia et al. 1994).
2.5.2 Selenium Toxicosis
Toxic Se levels are rare for human populations. Most Se toxicity cases deal
with more susceptible species of confined livestock like horses. For instance, Marco
Polo noticed in the 13th century that if his animals ate certain plants in the mountains
of western China that their hoofs would drop off. However, there have been a few
examples of human toxicity. Those rare examples typically have been through
occupational exposure or in parts of the world with geology that has large
seleniferous formations.
With a few exceptions, documented cases of acute Se toxicity in humans
have involved the occupational exposure of workers in Cu smelting or Se rectifier
plants. Most of these cases involved the inhalation of Se fumes from fires or from
heated metals in various plants. Inhaled Se compounds result in immediate irritation
to mucous membranes of the upper respiratory tract with symptoms of tearing and a
burning sensation of the eyes, running nose, hoarseness, coughing, and sneezing
(Combs 1986). Often patients complain of headache, dizziness, dyspnea, fatigue,
nausea, vomiting, and a bitter taste in the mouth. Some may have a garlic-like odor
of the breath. Clinical signs include conjunctivitis, rhinitis, and bronchitis. After
several hours, pulmonary edema may develop.
Chronic exposure to high levels of Se has been observed in several
populations in seleniferous geographic regions (e.g., the northern Great Plains of the
U.S.A., parts of Venezuela and Colombia, and one county in China). Between 1961
19
and 1964 selenium toxicity occurred in the Enshi County of China. When a drought
damaged the rice crop, the residents supplemented their usual rice intake with corn
and vegetables grown in soil high in selenium. The most frequently reported signs
of symptoms of chronic selenosis include lassitude, depression, fatigue, and
dermatological lesions (Harr 1978). Some examples of symptoms are brittleness
and loss of hair, brittleness and breakage of nails, and scaly dermatitis. Several
reports have linked excessive intakes of Se with increased incidences of dental
cavities and birth defects (Combs 1986).
An effective therapy for Se toxicity has not been identified. However, some
acute signs can be treated. Oxygen therapy for patients with inhalatory Se
intoxication is recommended to minimize pulmonary edema (Combs 1986).
Treatment with p-bromobenzene has been shown to increase urinary excretion of Se
in cattle and dogs, but its hepatotoxicity would preclude its use for humans. Studies
have suggested that cadmium inhibits selenium toxicity by forming a compound or
compounds with selenium in the intestinal tract, which are of low availability and
therefore poorly absorbed (Hill 1974). In a study by Hill, a combination of Se and
cadmium was fed to baby chicks. Se at the same dose alone produced more toxic
effects than when in combination with cadmium.
The best way to relieve Se toxicity is to decrease exposure. Studies have
shown that symptoms of loss of hair and brittle nails have been reversed after
exposure levels have been decreased with little long term negative effects (Xia et al.
1994).
20
3.0 HISTORY OF THE WESTERN PHOSPHATE FIELD
Figure 2. A relief map of Idaho, Montana and Wyoming with the
Southeastern Idaho Phosphate Region circled (USGS).
3.1 Background
The regional stratigraphic variations in rocks of the phosphoria formation and
associated strata indicate a complex history of transgressions and regressions of the
Permian seas across Southeastern Idaho (Williams, et al. 1967). These Permian
seas gave rise to not only the phosphoria, but also was the origin for selenium in the
region.
The Idaho phosphate region covers approximately 10,000 square miles in
parts of Bannock, Bear Lake, Bingham, Bonneville, Caribou, Fremont, Teton, and
Madison Counties (Williams, McKelvey et al. 1967). Most of the area is within the
boundaries of the Caribou National Forest. The Caribou Range phosphate deposit
21
extends in an eastward direction from north of Afton, Wyoming to about 15 miles
east of Idaho Falls, Idaho. The phosphoria formations that crop out in the
northwestern U.S. consist of six members. The six members are: the Lower Chert,
Meade Peak Phosphatic Shale, Rex Chert, Cherty Shale, Retort Phosphatic Shale,
and the Tosi Chert Member. The Meade Peak member is the most phosphatic
enriched. Nowhere are all six members present at one locality (Lane 1983).
Phosphoria formations of the Permian age consisting of chert, carbonaceous
mudstone, and phosphorite are the typical formations in southeastern Idaho.
Recorded geologic studies date as far back as 1872 by members of the
Hayden Survey. The first recorded production of phosphate rock in Idaho is
believed to have come from the Waterloo mine in 1906 (Service 1967). By 1921
there were eight active mining companies in the area. Most of the phosphate rock
mined in Idaho has come from open pit mining. Processing facilities in the area
produce phosphoric acid, fertilizer, and elemental phosphorus from the raw ore.
Currently, five companies mine phosphate in Idaho (Agrium, FMC, Monsanto,
Astaris, and Simplot). These companies produce around 15% of the U.S. supply of
phosphate. Most of the land mined in Idaho is leased from public land, primarily
federal lands. Therefore, many different public and private stakeholders are
involved with the area.
In the fall of 1996, horses in the pastures down gradient of Maybe Canyon
overburden dumps were diagnosed with selenium toxicosis. Testing of mine runoff
and water in the Phosphoria area suggested that significant levels of selenium were
22
being leached from the overburden piles. This selenium can pose a threat to the
aquatic and terrestrial species in the impact area.
3.2 Acid Mine Drainage
Acid drainage (AD) is a severe environmental pollution problem and results
from the oxidation of pyrite and/or other metal sulfides in mining waste, ore tailings,
and overburden. Acid drainage can be extremely acidic (pH as low as 2) and
enriched with iron, aluminum, sulfate, selenate, and heavy metals such as lead (Pb),
mercury (Hg), cadmium (Cd), and in some cases thorium (Th), and uranium (U)
nuclides (Evangelou 1995). However, due to the large amount of limestone and
calcite in the Phosphoria region a pathway for buffering of the acidity exists.
Discharge of AD on land and into river and lakes poses an instant threat to the biota
and ecological balance (Besser and Brumbaugh 1997). The need to prevent AD
formation has, thus, triggered numerous investigations into the mechanisms of pyrite
oxidation and its prevention.
In Smoky Canyon, pyrite oxidation takes place when the small amount of
pyritic minerals in the sedimentary shales are exposed to air and water through the
open pit mining process. The process is complex because it involves chemical,
biological and electrochemical reactions, and varies with environmental conditions.
Factors, such as pH, pO2, specific surface and morphology of pyrite, presence or
absence of bacteria and/or clay minerals, as well as hydrological factors, determine
the rate of oxidation (Ribet, et al. 1995).
23
Selenium, a group VI chemical analog of sulfur, can replace sulfur in pyrite
minerals. Open pit mining exposes the waste rock including pyrite to the air and
water allowing reduced Se species such as FeSe and Se0 to oxidize. In these oxic
environments, the highly soluble selenium oxyanions selenite and selenate can be
formed. These oxidized Se ions are readily leachable and can migrate to ground
and surface waters, potentially contaminating aquatic ecosystems.
3.3 Selenium Leaching Mechanism
3.3.1 French Drains
In the Western Phosphate Resource Area, disposal of waste rock in a french
drain configuration was an accepted practice in mine management. Large waste
boulders (1-3 m diameter) would be placed in the bottom of a canyon, and
subsequently covered with mixed waste rock fill. The larger boulders would allow
water from permanent or ephemeral streams and creeks to flow through the pile
rather than damming up behind the pile. Smaller sized waste rock would then be
dumped on top of the large boulders, eventually filling the canyon with millions of
tons of waste rock. This practice provides a pathway for leaching and mobilization
of contaminants such as Se, from the waste rock. Selenium mobilization can occur
from leaching from the creek in the bottom of the canyon, and also from vertical
infiltration and runoff. A french drain located in Smoky Canyon was the focus of
selenium leaching into Pole Creek.
24
4.0 REMEDIATION AND TREATMENT APPROACHES
4.1 Water Treatment Processes for the Removal of Inorganic Se
The best available technologies (BAT) for the removal of inorganic selenium
from water are coagulation-filtration, lime-soda softening, activated alumina (AA),
and reverse osmosis (RO) (Faust and Osman 1998). The effectiveness of the
treatments varies depending on which oxidized species of selenium is predominant.
Comparison of the various treatment methods with regard to selenate and selenite
are shown in Table 7.
Table 7. Comparison of treatment methods for selenite and
selenate removal from water
Conventional Coagulation
Alum
Iron
Selenite
Selenate
P
F pH<7.5
L pH>7.5
L-P
P
P
Lime Softening
P
Ion Exchange
Cation
P
P
Anion
E
E
Activated Alumina
E-G
L-G
Activated Carbon
PAC
P
P
GAC
P
P
Reverse Osmosis
E
E
E-excellent 90-100%, G-good 70-90%, F-fair 40-70%, L-low 20-40%, P-poor 020% Adapted from Faust and Osman 1998.
Chemical coagulation has produced varied results. The success of the
removal is often pH dependent. Lime softening was not a very effective treatment
method for either selenite or selenate (Sorg 1988). Alumina columns have shown
some success in removal; however if there are other anions present, especially
25
sulfate, there is competition resulting in lower removal (Trussell 1980; Sorg 1988).
Anion resins work well with selenate. Often oxidizing selenite to selenate before
treatment produces the best results (Trussell 1980). Alumina adsorption works well
with selenite when the pH range is controlled between 5 and 6. However, selenate
removal was not as successful due to the competition with sulfate. Both powdered
activated carbon (PAC) and granular activated carbon (GAC) did not show
significant removal of either selenite or selenate (Sorg and Logsdon 1978). Reverse
osmosis did have some success at removal of both selenium species (Sorg and
Logsdon 1978). The most effective water treatment methods for inorganic selenium
depend on the Se oxidation state in the target water.
4.2 Bioremediation
4.2.1 Selenium-Reducing Bacteria (SRB)
Because selenium is chemically and structurally similar to sulfur, many
bacteria do not distinguish between the two when using them as their final electron
acceptor in their energy producing cycle. Other anaerobic bacteria specifically
prefer selenium to sulfur for use as an electron acceptor.
Dissimilatory selenate reduction is carried out by a diverse group of strictly
anaerobic bacteria that share the ability to use selenate or selenite as a terminal
electron acceptor in the oxidation of organic matter. In these reactions, selenate is
stoichiometrically reduced to selenide according to the equation:
2CH2O + SeO42- → 2HCO3- + H2Se
26
Sulfate-reducing bacteria (SRB) are particularly active in ecosystems that are
high in sulfate, but they can be isolated from most aquatic and terrestrial
environments that are depleted in oxygen, such as sediments, sewage sludge
digesters, waterlogged soils, and the gastrointestinal tracts of man and animals
(Tortora, et al. 1995). In the biosphere, they play a significant role in the
mineralization reactions that enable cycling of nitrogen, carbon, and phosphorus to
occur (Janzen, et al. 1994).
SRB are known for their destructive potential, for example, the corrosion of
pipes and pumps and the spoiling of coal, oil, and gas (Peng, et al. 1994). The
activities of SRB are recognized as being of considerable economic as well as
scientific importance, and have been of great research interest in recent years.
4.2.1.1 Biochemistry
The biochemistry of selenium-reducing bacteria is thought to be very similar
to that of the sulfate-reducing bacteria, and both are not well understood. That is
one of the reasons that they are the subjects of intense study. Their ability to use
elements other than oxygen as their final electron acceptor in the electron transport
chain makes them useful in the reduction of certain contaminants. One of the
compounds they can use as their final electron acceptor is selenate.
The reduction of SO42- to hydrogen sulfide is an eight-electron reduction that
proceeds through a number of intermediate stages. The sulfate ion must first be
activated by adenosine tri-phosphate (ATP) before being used. The enzyme that
catalyzes this attachment of the sulfate ion to a phosphate of ATP is sulfurylase
(Madigan et al. 1997). This leads to the formation of adenosine phosphosulfate
27
(APS). In dissimilate sulfate reduction APS is reduced directly to sulfite (SO3 2-) with
the release of adenosine monophosphate (AMP). In assimilative reduction another
P is added to APS to form phosphoadenosine phosphosulfate (PAPS), then the
sulfate moiety is reduced. Either way, the first product of sulfate reduction is sulfite.
Once sulfite is formed, the subsequent reductions proceed readily. Several
organisms unable to carry out dissimilative sulfate reduction are able to carry out
dissimilative sulfite reduction. This is because they can convert sulfite to H2S, but
they do not have the APS system and, thus, are unable to reduce sulfate to sulfite.
SRB carry out a cytochrome-based electron transport process (Barton and
Tomei 1995). The electrons from the energy source are transferred to the sulfate
ion in APS and to sulfite. Differing from the mammalian complex IV, this cytochrome
c is very electronegative and is termed cytochrome c3. Other electron carriers in the
electron transport chain of SRB include ferredoxin and flavodoin. Other sulfatereducing bacteria may use a different cytochrome b if the species does not degrade
fatty acids.
The H2 in the electron transport chain comes either from the environment or is
generated from certain organic electron donors like lactate. This hydrogen transfers
electrons to the enzyme hydrogenase, which is located in the periplasm next to
cytochrome c3. Because of where the electron transport components are located in
the membrane, when the H atoms of H2 are oxidized, the protons remain outside the
membrane, whereas the electrons are transferred across the membrane (Madigan,
Martinko et al. 1997). This creates the proton gradient that is used for the synthesis
28
of ATP. In the cytoplasm, the electrons are used for the reduction of the APS and
sulfite.
A great deal of information has been gathered on SRB in the last ten years.
Their ability to metabolize contaminants to less toxic or immobile forms has spawned
new research interests. Because selenium is a sulfur analog, many bacteria that are
able to reduce sulfur can reduce selenium the same way. Selenium is soluble in
water in the +6 and +4 oxidation states. Some SRB have the ability to reduce these
soluble oxidized forms to insoluble reduced forms, thus taking selenium out of
solution (Dungan 1998; Lortie 1992; Baldwin 1985; Oremland 1991; Oremland 1994;
Zehr 1987; Maiers 1988).
Bacteria have been used to reduce selenium in situ (Oremland, et al. 1991).
Studies have shown that some bacteria reduce oxidized forms of selenium through
respiration (Oremland, et al. 1989; Oremland, Blum et al. 1994; Cantafio, et al.
1996).
4.2.1.2 Nutrient Amendments
Because of the size and scope of many environmental contamination
problems, inexpensive control approaches are being sought. Many green chemistry
or “garbage” waste material amendment type technologies have been successful in
stimulating naturally occurring microbial populations, thereby accelerating metal
sequestration. Various amendments have been proposed for different types of acid
mine drainage. The approach is to supply the bacterial populations with an
abundance of food in the form of carbon waste products, like composted manures, in
order to stimulate growth of their population. The more bacteria present the greater
29
the removal or sequestration. The carbon sources include agricultural wastes such
as manure, straw, and sawdust. Any type of waste that is high in nutrients and
carbon for these bacteria has the potential for use. The carbon amendments being
focused on at Smoky Canyon are potato waste and cheese whey because of their
regional availability and chemical composition.
Other organic substrates that have been applied to acid drainage include
sawdust, cellulose, manure, wood chips, leaf mulch, and sewage sludge (Waybrant
1998). One suggested method of implementation is in the form of a reactive barrier.
The substrate would be held in a barrier through which the contaminated water
would travel. Conditions of the barrier would be engineered to promote the proper
flow as well as the desired chemical and microbial conditions that are conducive to
the contaminant removal. Formation of a “hardpan” layer to inhibit the diffusion of
pore gases is an example of one desired condition (Blowes 1994). Suitable
hydraulic permeability of the barrier is important. Assuring that a waste stream
travels through the barrier and is not diverted by it is a key factor in the success of
this approach (Waybrant 1998).
4.3 Chemical Reduction and Co-Precipitation
Accelerating a reducing environment is important to the success of Se
species reduction. Iron metal is one reducing agent that has demonstrated success
in environmental applications (Tashirev, A.B. 1989). Various types of iron have
been studied for their ability to provide a chemically reducing environment or coprecipitation ability (Baldwin, et al. 1985; Balistrieri and Chao 1987; Manning and
30
Burau 1995). When iron metal is in the presence of water, it reacts spontaneously to
hydrolyze water. This provides electrons, which are used to chemically reduce
oxidized forms of selenium. In addition to chemically reducing selenium, the
microbial populations can use the electrons in respiration via the hydrogenase
pathway, and can reduce selenium in their energy production cycle. In providing
electrons, H+ is being consumed, thus increasing the pH in highly acidic
environments to levels at which the bacteria can be productive. The half-reactions
for chemical reduction using iron are
Oxidation:
Fe ↔ Fe2+ + 2eFe2+ ↔ Fe3+ + e-
Reduction
O2(g) +8H+ +8e- ↔ 4H2O
2H+ + 2e- ↔ H2(g)
Studies have been done on possible iron reactors for waste stream
remediation. Co-precipitation of selenium from wastewaters with amorphous iron
oxyhydroxide has been examined (Merrill, et al. 1986). The iron oxyhydroxide forms
when ferric salt was added and the selenium then adsorbed onto and trapped within
the precipitate, therefore settling out of solution.
Removal of selenium from contaminated waters through the use of ferrous
hydroxide has also been shown (Murphy 1992). Use of ferrous iron can result in a
ferric oxyhydroxide precipitate, which incorporates oxyanions like selenium into its
lattice (Manning and Burau 1995). However, optimum pH for this reaction is 9 and
decreases as the pH decreases. An interesting part of the study showed that no
sulfate removal occurred during the removal of selenium. This might suggest that
ferrous hydroxide would be useful for the selective removal of selenate in high
sulfate waters.
31
Another control approach widely used in hazardous waste control is
adsorption. Physical adsorption occurs when electrostatic forces between a solute
and adsorbent are greater than the forces between the solute and solvent. Those
electrostatic forces can be in the form of coulombic attraction, ion exchange, induced
polarization, covalent bonding and Van der Waals forces (Shokes 1997). Adsorption
occurs due to a charge differential on the surface of the adsorbent. Hydrogen and
hydroxide ions mediate the charge on the surface; therefore adsorption is dependent
on pH.
An example of adsorption is illustrated when oxidized selenium ions in the
selenite and selenate oxidation states are adsorbed with alumina doped with
lanthanum oxide (Misra and Nayak 1997). Other adsorption and sorption methods
for Se have been illustrated using calcium, copper, zinc, goethite, zeolite, and
cellulose sorbents (Manceau 1997; Balistrieri 1987; Haggerty 1994).
4.4 Phytoremediation
An approach to reduce selenium levels in the soil is through
phytoremediation, which is the use of plants to remove, stabilize, or detoxify
pollutants. Selenium concentrations in soils have been reduced through plant
uptake in several studies (Banuelos, et al. 1993; Banuelos, et al. 1993; Banuelos, et
al. 1997; Pilon-Smits, et al. 1998; Pilon-Smits, et al. 1999; Pilon-Smits, et al. 1999).
A technique used for the removal of selenium from water has been through aquatic
plants grown in constructed wetlands (Hansen, et al. 1998; Pilon-Smits, et al. 1999;
Zhu et al. 1999). In these constructed wetlands, Se volatilization is often a
32
significant pathway for Se removal. Se volatilization is the process by which
inorganic Se is converted to volatile forms, such as dimethylselenide (Lewis 1966;
Frankenberger and Karlson 1994; Terry and Zayed 1994).
Microbe interactions, in the rhizosphere of the roots of plants, might influence
pathways that are used for the reduction of selenium (Wanek et al. 1999). Other
research has shown that specific plant genes might control selenium uptake (PilonSmits et al. 1999). These genes might be able to be over expressed to increase
selenium uptake (Pilon-Smits et al. 1999). Another method of biotransformation of
selenium has been illustrated in algae mats grown in selenium-laden waters (Fan et
al. 1997; Fan et al. 1998). These algae mats demonstrate Se volatilization (Fan et
al. 1997; Fan et al. 1998).
33
5.0 MATERIALS AND METHODS
5.1 Materials
All chemicals were reagent grade or better and used as received. The batch
reactor and saturated paste studies use 18 MΩ-cm reagent water. The greenhouse
study used deionized water. Water for the microbial culturing experiment was
gathered under field sterile technique from the inlet and outlet waters of Pole Creek
as it traveled through the Pole Creek waste rock dump. Nitrogen gas was used in
the degassing of the batch reactors and the saturated paste studies.
Cheese whey used in the batch reactor and saturated paste studies came
from Washington State University Dairy and the Star Valley Cheese Company. The
types of iron used were colloidal iron (Micropowder iron, 1-3 µm, grade S-3700, ISP
Technologies, Inc.), industrial mixed mesh scrap iron (100% passing 8 US Sieve;
Master Builders, Inc.), and iron shavings (Peerless). The potato processing waste
came from the J.R. Simplot Company potato processing plant in Blackfoot, Idaho.
Homogenized soil for the batch reactors, plant studies, and saturated paste
studies was taken randomly from the Pole Creek waste rock dump. Soil was taken
between 0.5 and 3 meters below ground surface. Exposed surface soil was
avoided.
5.2 Laboratory and analytical equipment
The University of Idaho Analytical Science Laboratory (ASL) performed all
analyses. ASL is a U.S. EPA Drinking Water Program certified facility operation in
34
compliance with Good Laboratory Practice standards (GLPS 1995). Periodic
analyses of laboratory performance check solutions at known concentrations were
used for quality control and to verify the calibration of the instruments.
Instruments used by ASL for analysis include: Hewlett Packard 4500 Series
ICP-MS Plus, Leeman 2000 inductively coupled argon plasma spectrophotometer
(ICP), Perkin Elmer P-40 ICP, Orion model 525A pH-Eh multimeter, YSI model 31
Conductivity Bridge, Milton Roy Spectronic 301, Alpkem Rapid Flow Analyzer, HF
Scientific Turbidimeter DRT 100B, Branson Sonifier 450, and a Zymark TurboVap
evaporator, Amray Model 1830 scanning electron microscope, an anaerobic glove
box (Uni-lab), and a Hydrolab Minisonde4a multi-probe.
5.3 Analytical methods for water and soils
Prior to sampling all containers were either acid washed or cleaned with hot
detergent solution and rinsed three times with deionized water. A final rinse was
done with 18 MΩ-cm reagent water.
5.3.1 Waters
Total selenium Hydride generation ICP (HG-ICP) was used to measure
selenium in water. In order to use the hydride technique, selenium needs to be in
the +4 valence state. In converting selenium to the +4 valent state, a series of
digestions are done. The samples are first digested with nitric acid and then boiled
in perchloric and sulfuric acids to convert all species of Se to selenate. Selenate is
then reduced to selenite with hydrochloric acid and then it is reduced further with
sodium borohydride to hydrogen selenide. The hydrogen selenide is finally analyzed
35
by a Perkin Elmer P-40 ICP. This method does not provide differentiation between
the different selenium species. However, selenite can be determined by directly
injecting undigested water samples. Selenite is the only species present that can be
reduced and detected by the HG-ICP as hydrogen selenide (Tracy and Moller 1990).
Dissolved Oxygen (DO) A Hydrolab probe (Minisonde water quality
multiprobe) was used to analyze for DO, pH, oxidation/reduction potential, and
temperature. A Clark cell probe measures DO using the current resulting from the
electrochemical reduction of oxygen (at a gold cathode) diffusing through a selective
membrane.
Acidity (pH) pH was determined using a probe consisting of an ion selective
glass membrane, whose potential is measured with respect to a Ag/AgCl reference
electrode.
Oxidation/Reduction Potential (mV) The oxidation/reduction potential
probe consisted of a platinum electrode, whose potential was measured with respect
to a reference electrode; this electrode rapidly comes into equilibrium with the
potential of the sample solution. The reference electrode consisted of a silver
chloride wire immersed in a solution of constant potassium chloride (KCl)
concentration. The solution is in ionic contact with the water sample through a
porous junction that restricts KCl leakage.
Temperature (C°) Temperature was measured from the calibrated
thermocouple located in the hydrolab multiprobe.
36
5.3.2 Soil
Selenium Total selenium was measured the same way as described above
in section 5.3.1 using hydride generation. The sample was digested with nitric acid
and by boiling with a mixture of sulfuric and perchloric acids. All of the selenium
species were converted to selenate. Hydrochloric acid and heat was then used to
reduce the selenate to selenite. The selenite was then reduced further with acidic
sodium borohydride to hydrogen selenide, which was then measured by HG-ICP.
Trace micro-element screen One gram of dry weight sample was digested
in nitric acid and hydrogen peroxide at 150° C. The digestate is refluxed at 150° C
with hydrochloric acid, filtered if necessary, and analyzed for target elements by ICP.
Sulfate-sulfur Ten grams of dried soil were analyzed using ion
chromatography.
Phosphorous and potassium Determination of phosphorous and
potassium is a two-step process. Potassium on exchange sites was replaced by the
sodium ion in a 0.5 M sodium bicarbonate solution and AA or ICP reads potassium
in the extract. Phosphorous was determined using a single reagent containing
sulfuric acid, ammonium molybdate, ascorbic acid and antimony potassium tartrate.
An ammonium molybdiphosphate complex is formed that is reduced by ascorbic
acid and color-stabilized by antimony. Concentration of the complex was read on a
spectrophotometer.
Percent C N S A LECO (CNS) 2000 combustion analyzer analyzed total
carbon, nitrogen and sulfur. Infrared adsorption was used for detection of carbon
and hydrogen and thermal conductivity was used for the detection of nitrogen.
37
Nitrogen – nitrate and ammonium Five grams of dried and ground soil was
mixed with 2M KCL, and the extract is analyzed by automated colorimeter and/or ion
selective electrode.
38
6.0 CHARACTERIZATION ANALYSIS
6.1 Scanning Electron Microscope (SEM)
6.1.1 Purpose
Characterization of selenium located in the waste rock shales was explored
using the scanning electron microscope (SEM). Using the SEM X-ray fluorescence,
areas of high Se concentration in a sample could be determined. Analyzing the
mineral morphology of selenium in the waste shales helped focus laboratory study
as well as understanding mechanisms of release and control.
6.1.2 Methods
Samples were obtained as surface and shallow sub-surface grab samples
from the Smoky Canyon waste rock impoundment, and from nearby road cuts. An
Amray 1830 Scanning Electron Microscope with an acceleration potential of 20.0 kV
was used to analyze the middle waste shales from the Smoky Canyon waste rock
dump. The rock was freshly fractured and mounted on carbon stubs with carbon
tape and placed under the SEM. Using the X-ray fluorescence backscatter
technique, inclusions containing high Fe were found. During the analysis, Fe was
found primarily associated with sulfur. Upon further analysis of the inclusions,
selenium was found to be present.
6.1.3 Results and Discussion
Selenium was found in the middle waste shales sampled from the Smoky
Canyon waste rock pile. Selenium was found substituting for sulfur in the form of
39
Photograph 1. Scanning electron micrograph (16400X) of a framboidal
pyrite fraction containing 1-3% selenium by weight in freshly fractured
Smoky Canyon shale.
pyrite (FeS2). Selenium can replace sulfur in the crystal lattice by isomorphous
substitution. When analyzed with the SEM the structure of pyrite found was
framboidal and cubic (photographs 1 and 2). Some inclusions analyzed had
selenium substituting for sulfur as high as 6%. Most of the pyritic inclusion samples
ranged from 0.1% to 1% selenium. Thus, this study supports the hypothesis that
leached selenium is originating from the middle waste shales, specifically from pyrite
where it has isomorphically substituted for sulfur.
40
Photograph 2. Scanning Electron Micrograph of cubic pyrite fraction
with 0.1% selenium by weight of freshly fractured Smoky Canyon shale.
6.2 Drill Hole Core Analysis
6.2.1 Purpose
J.R. Simplot exploration geologists took a representative 356 ft core of the
geologic profile from the Smoky Canyon Mine. The core material was analyzed for
selenium and other elements to determine the location of selenium and P2O5 with
regard to depth and geographic strata. This information can provide understanding
of possible high Se concentration areas in the geologic strata.
6.2.2 Methods
The drill core and sample submission was done by the J.R. Simplot
Corporation. A copy of the results showing selenium and P2O5 concentration with
samples at two-foot intervals were provided to the Environmental Chemistry and
Toxicology Laboratory at the University of Idaho.
41
P2O5
Figure 3. Drill Core analysis of Se and P2O5 concentrations (mg/kg) in a 386ft core of Smoky Canyon soils measured at two-foot intervals.
42
6.2.3 Results
Figure 3 shows P2O5 concentrations as a percentage to the left, and Se
concentrations in mg/kg to the right. The general type of rock is noted on the right
side of the figure. The length of the core is approximately 356 feet.
Selenium analysis of the drill core shows maximum selenium levels in the
middle waste shales at a depth between 200 and 300 ft below the surface. This
suggests that the release of selenium is coming from the middle waste shales as
they are disposed into waste piles in typical mining practice. This middle layer of
“waste rock” separates the two rich phosphoria ore layers and is removed as waste
during the mining process. Past reclamation protocol has been to use these shales
as a top dressing during the revegetation process. This procedure was done
because the high organic component of the shales increased waste rock soil fertility
for reclamation forage production. However, this practice exposed waste rock
fractions that were high in selenium to air and water where it could then oxidize to
selenite and selenate, the soluble forms of selenium. This knowledge is now used to
address best management practices of specific types of waste rock in mine
operations to prevent selenium oxidation and environmental contamination.
43
7.0 SATURATED PASTE AMENDMENT STUDY
7.1 Purpose
The saturated paste study was an extension of a previous study done on
Smoky Canyon waste rock soil (Bond 1999). Bond tested the effectiveness of
various amendments on selenium removal. This work tested two additional
amendments: cheese whey alone and mixed with iron. The impetus for this work
was the potential of using cheese whey from a cheese plant, Star Valley cheese,
located near the Smoky Canyon mine site. The close proximity to the Se impacted
site might make cheese whey a feasible amendment in the field. This study was
done to test the effectiveness of cheese whey on selenium sequestration in the
laboratory before using it as a field amendment.
Whey is a collective term referring to the serum or watery part of milk that
remains after the manufacture of cheese. For every 10 pounds of cheese
manufactured, 90 pounds of whey are produced. Acid whey has a pH usually <5.1
and results from the production of cottage cheese. Sweet whey, with a pH of >5.6,
results from rennet-coagulated cheese manufacture. Although the composition of
each whey type is somewhat different and variable, both sweet and acid whey
contain about 0.7%-0.8% protein on a liquid basis, with whey proteins only
representing about 10%-12% of the total solids of whey.
Whey is abundantly available, and poses a potential disposal problem itself.
Whey has high concentrations of milk sugars like lactose. Acetic acid is used during
the Ricotta cheese precipitation process. This addition results in a residual amount
44
of acetate ion as part of the waste whey. Acetate, like lactate, has the potential to
act as a good energy source for naturally occurring bacteria that have the ability to
reduce oxidized selenium anions. Therefore, the effectiveness of four different
amendment combinations using cheese whey and iron metal on sequestering
selenium from Smoky Canyon waste rock soil was tested.
7.2 Methods
Four treatment trials and one control were performed in triplicate for 14 days.
For each trial, 12.5 g of soil was measured into a 100 ml polyethylene cup and
mixed with 30 ml of 18 MΩ-cm water. Iron metal (colloidal powder) was added to
the soil prior to addition of the water, wet amendments were added to the water prior
to addition to the soil. The soil was amended with:
A.
Star Valley Cheese Whey
B.
Washington State University Cheese Whey
C.
Star Valley Cheese Whey + Elemental Fe
D.
Washington State University Cheese Whey + Elemental Fe
One ml of cheese whey was added with the 30 ml of water (roughly 3%). The
iron was used to amend the test soils at 1% dry weight. Pastes were sealed under
nitrogen and two layers of parafilm and placed into an incubator at 25° C for 14
days. At the conclusion of the experiment the soil pastes were mixed and filtered
using a Buchner funnel and glass-fiber filter paper. The filtrate was analyzed for
total selenium.
45
6
5.1
5
Selenium µM
4
3
2
0.7
1
0.6
0.3
0.0
0
Control
Star Valley
Whey
WSU Whey
WSU Whey +
Fe
Star Valley
Whey + Fe
Figure 4. Saturated Paste Se concentrations (µM) after 14 days for soils
amended with Star Valley Whey, Star Valley Whey + Fe, WSU Whey and WSU
Whey + Fe compared to a non-amended control (n=3).
7.3 Results
All of the treated soils had selenium levels lower than the control (Figure 4).
The two amendments of cheese whey alone had similar removal levels at around
85%. Washington State University whey in combination with colloidal iron had
removal levels around 94%. The highest removal was observed in the combination
of Star Valley cheese whey and colloidal iron. The combination of Star Valley
cheese whey and colloidal iron had 99% removal of soluble selenium.
46
7.4 Discussion
Though all the amendments had removal values over 80%, the greatest level
of removal was the combination of cheese whey and iron. These lower levels might
suggest the possibility of a synergism between the two that assists both microbial as
well as chemical reduction and/or co-precipitation. Another possibility is that the iron
adds another energy source for selenium reducing microbes via the hydrogenase
pathway. As iron oxidizes it releases electrons that are available to the microbes to
use in respiration. With this additional source of energy their populations and activity
may have increased.
It is also likely that the iron provides a more reducing environment where
oxidized selenium can be chemically reduced. It has been shown that reduction of
selenium oxyanions to elemental selenium by zero-valent iron is best observed
under acidic conditions (Marchant 1976). The cheese whey provides acidity and
may allow direct chemical reduction of selenium by zero-valent iron, which has a
-0.44V reduction potential.
The Star Valley cheese whey alone and in combination with iron was more
successful than the whey used from WSU. This difference could be from the type of
cheese and the method from which the cheese is processed. At Star Valley Cheese
the last step in the processing is the precipitation of Ricotta cheese with vinegar from
the whey. The addition of vinegar provides a large amount of acetate. Acetate has
been shown to be a preferred substrate for certain organisms (eg. T. selenatis) that
under anoxic conditions respire selenate as their terminal electron acceptor and
acetate as the electron donor and carbon source (Bledsoe 1999).
47
Classification of local selenium-reducing bacteria could provide further
information on the specific mechanism of respiration, and the specific components
used from the whey as an electron donor and carbon source. Along with bacterial
classification, studies limiting the bacteria to different components found in the whey
might provide information on key components in their success for selenium
stabilization.
Future studies could also differentiate between sulfate-reducing bacteria,
which prefer lactate as an electron-donor/carbon-source, and selenium-reducing
bacteria that might need the lactose to be fermented to acetate for optimal growth.
Stimulation of sulfate-reducing bacteria might lead to high concentrations of sulfide
and the subsequent inhibition of selenium-reducing bacteria. However, respiratory
acids like acetate select for certain selenium-reducing bacteria over sulfate-reducing
bacteria. Further classification of the cheese whey and the microbes present in the
whey could provide some information that may suggest how to manipulate the
indigenous bacteria to provide the electron-donor/carbon-source most effective for
selenium-reducing bacteria stimulation.
48
8.0 SRB – CULTURING EXPERIMENT
8.1 Purpose
The purpose of the culturing experiment was to observe naturally occurring
bacteria in the Smoky Canyon waste rock soil that have the ability to reduce oxidized
forms of selenium. If there are naturally occurring bacteria in the soils,
bioremediation technologies can be evaluated for possible treatments.
8.2 Methods
8.2.1 Standards
Several reagent solutions were needed prior to the culturing of selenium
reducing bacteria. For the solutions, approximately 500 ml of 18 MΩ-cm reagent
water was used. A 1-L Erlenmeyer flask was filled with 600 ml of 18 MΩ-cm
degassed water. The flask was placed over a Bunsen burner and boiled. During the
heating a tube delivering N2 gas was placed into the beaker. After 5-10 min of
boiling the flask was removed from the heat. The water was allowed to cool under
N2 to room temperature.
A 0.2 M stock solution of sodium selenite was prepared in an autoclavable
150 ml culture flask. 3.46 g of sodium selenite was placed into a flask. To the flask
100 ml of cool 18 MΩ-cm water was added under N2. After 100 ml of water was
pipetted into the flask, it was purged with N2 for 30 sec to 1 min. After degassing, a
rubber septa was placed into the top of the flask while the degassing needle
remained. The septa was pushed in while the needle was pulled out, limiting O2
49
introduction. The septa was sealed with an aluminum crimped seal. The sealed
flask was placed in an autoclave for 20 minutes at standard operating temperature
and pressures.
All the other sterile solutions were prepared in the same manner. The
solutions included:
0.2 M soldium selenate solution (Na2SeO4)
0.1 M lactic acid solution
0.1 M cysteine solution
8.2.2 Test solutions
Sample water from above the waste rock impoundment (Upper Pole CreekUPC) and below the waste rock impoundment (Lower Pole Creek-LPC) was used in
the test solution. The use of site water allowed for any minerals or vitamins that are
specific to the area to be included in the culture broth.
8.2.3 Creek Water
50 ml of Upper Pole Creek water and 50 ml of Lower Pole Creek water were
put into separate Erlenmeyer Flasks. The flask was allowed to boil over heat. A
continuous N2 head was then applied to the flask. After the water boiled for 5-10
minutes it was cooled under N2. After cooling, 50 ml of creek water was pipetted
into the various glass culturing flasks under N2, making sure that both flasks were
under nitrogen when doing transfers to limit any oxygen exposure. After all the
flasks were filled, they were sealed using anaerobic technique and autoclaved.
50
8.2.4 Standard Transfers
To the culturing flasks the appropriate solutions were added using anaerobic
technique. The different combinations of food source and selenium species are
shown in Table 8. The combinations were used to differentiate changes in culture
behavior due to energy source or selenium species.
8.2.4 Media Inoculation
Media inoculation was performed in an anaerobic glove box. All the
instruments used in the glove box were sterilized. The culture flasks were opened in
the N2 environment of the glove box. Three grams of waste rock soil were placed
into each flask, which were then resealed. After all the flasks were inoculated and
sealed, they were removed from the glove box and placed into a drawer at room
temperature. As an experimental control, a soil sample was sterilized by autoclaving
and placed into flasks containing the same growth media. The control flasks would
indicate positive selenium precipitation in the event of microbial contamination or
Table 8. Water, food source, and Se species combinations for use in media that
selects for selenium-reducing bacteria.
Type of Water
Food Source
Selenium Species
UPC
UPC
UPC
UPC
UPC – Control
Lactic Acid
Lactic Acid
Cheese Whey
Cheese Whey
Lactic Acid
Selenate
Selenite
Selenate
Selenite
None
LPC
LPC
LPC
LPC
LPC - Control
Lactic Acid
Lactic Acid
Cheese Whey
Cheese Whey
Cheese Whey
Selenate
Selenite
Selenate
Selenite
None
51
abiotic reduction of the introduced Se species.
8.3 Results and Discussion
Within 48 hrs, all the test combinations except the experimental controls
showed positive results for microbial selenium reduction. The positive results were
in the form of a color change as a result of oxidized forms of selenium being reduced
to Se0, which precipitates out of solution as an orange/red solid. The control
cultures had no color change. This suggests that there is an abundant population of
selenate and selenite-reducing bacteria naturally present in the environment at the
Smoky Canyon Pole Creek waste pile. With the natural presence of seleniumreducing bacteria in the soils of Smoky Canyon, selenium control treatments that
focus on microbial stimulation can be further explored.
In future experiments, specific attention should be placed on understanding
microbial mechanisms associated with selenium reduction. One such study would
be to track how microbial populations change over time in different amended soils.
Along with population numbers, the characterization of specific species would be of
interest. The possibility exists that numerous species are able to reduce selenium.
Alternately, there may be a single species capable of selenium reduction naturally
present. The dominant microbial populations may also change over time as the
chemical environment changes. One species might prefer one amendment to
another, or certain compounds in the amendment. In addition, one specific species
of bacteria may demonstrate better removal of one specific selenium anion.
52
Many questions remain regarding the microbiology associated with selenium
reduction, and understanding of specific mechanisms and pathways. This
experiment was an initial step in showing that there is microbes present capable of
Se reduction in culture. These bacteria might have the potential of being stimulated
to aid in the control of selenium in the impact areas.
53
9.0 BATCH REACTOR EXPERIMENT
9.1 Purpose
The batch reactor experiments were designed as a pilot study to test the
dynamics of release and control of oxidized selenium from Smoky Canyon
homogenized waste rock, as well as the usefulness of cheese whey and iron as
possible amendments in the reduction of soluble selenium levels. A batch reactor
and sampling system was designed to collect water from within the reactor without
introducing oxygen. As the saturated soil went through chemical changes a multianalyzer-probe located within the reactor measured ph, mV, DO, and temperature.
Water samples were taken from the saturated soil from a suction type filter sampler
located in the slurry. The sample traveled through a tube to the outside of the
reactor where it was prepped for selenium analysis.
9.2 Methods
9.2.1 Reactor Construction
With a hole saw, a 2-in hole was drilled in the middle of the lid of a 19 Liter
plastic bucket. Three #2-rubber stopper size holes were drilled in a triangle around
the 2-in hole in the center.
Numerous holes were drilled in a 24-in long piece of 2-in PVC from the
bottom to ½ way up the piece of PVC. A 2-ft long piece of filter sock was then tied in
a knot at one end. The open end of the filter sock was placed over the PVC pipe,
and the sock was fastened to the pipe with a zip-tie. The open end of the PVC was
54
then fastened to a 2-in x 1-in coupler. The coupler was placed into the 2-in hole in
the lid of the bucket.
A piece of slotted 1-in PVC pipe was cut to a length of 7-3/4 -in long. A 1-in
PVC cap was placed on the bottom of the slotted PVC. No glue was used to fasten
the cap. A piece of filter sock twice as long as the slotted PVC was tied in a knot at
one end. The sock was slipped over the slotted PVC and attached with a zip tie.
Two lengths of 1/8-in plastic tubing were cut to a rough length of 6-in. The
plastic tubing was placed through two rubber stoppers with the appropriate diameter
hole. The first stopper was placed into the open end of the slotted PVC allowing 5-in
of the plastic hose to penetrate inside the slotted PVC. The other stopper was
pushed down so that it fit into one of the holes in the lid of the bucket. The slotted
PVC was on the bottom side of the lid. Two 6-in pieces of plastic tubing were each
placed through a stopper. The two stoppers were put in the remaining holes of the
lid.
9.2.2 Procedure
9.2.2.1 Initiating the Experiment
Soil taken from Smoky Canyon was screened to a diameter less than ¾-in.
The 2-in PVC pipe was held in the center of the 19-L bucket, and the bottom of the
bucket was filled with soil. Soil and 18 mΩ-cm water were then alternated into the
bucket. When applying an amendment, the amendment was homogenized in the
soil prior to placing it into the bucket. When the bucket was 1/3 full, the water
sampling apparatus was placed into the bucket and soil water mix was placed
around it until a total of 5-L of water were added and the soil slurry was 2-in from the
55
top of the bucket. The lid was then placed over the two inch PVC and fastened.
The headspace was then purged using N2 gas, which enters through one of the
plastic lines and vents through the other. After purging for 5-min., the hose
delivering the gas is pulled off, folded and the vent lines were sealed. The coupler
was then placed on the 2-in piece of PVC and tightened. A Minisonde4A Hydrolabprobe, capable of logging pH, DO, oxidation/reduction potential, and temperature,
was programmed and placed into the 2-in PVC through the coupler and secured at
the coupler. The Hydrolab oxidation/reduction potential probe has a range of –999
to 999 mV, and accuracy of +/-20 mV, and a resolution of 1 mV. All the holes in the
lid were sealed with silicon sealant.
9.2.2.2 Sampling
Water samples were taken on a regular basis while the probe measured pH,
mV, temperature, and dissolved oxygen. Water samples, 10 ml, were taken from
the water sampling apparatus and filtered with a 0.45 µm syringe filter. The samples
where then submitted to the Analytical Science Laboratory (ASL) for total selenium
analysis.
9.3 Results and Discussion
As shown in Figure 6, the dissolved oxygen in all reactors was below 1 mg/L
after 15 hours. As the reactors achieved this anaerobic state, the
oxidation/reduction potential decreased (Figure 7). However, the oxidation/reduction
potential change varied somewhat between the amendments. The control reactor
initially started to become more reducing as the dissolved oxygen decreased, but
56
12
Control
11
Cheese Whey
1 % Fe
10
Cheese Whey + Fe
9
pH
8
7
6
5
4
3
0
50
100
150
200
250
300
350
Hours
Figure 5. Comparison of control, 1 % iron, cheese whey and iron + cheese
whey batch reactor pore water pH over time (n=1).
fluctuated more than the other amendments. The control seemed to trend upward
after 150 hours. The cheese whey amendment dropped to a negative
oxidation/reduction potential around 120 hours, and then stayed between zero and
70 mV to the end of the experiment. The 1% Fe amendment had the quickest
decrease in oxidation/reduction potential. By 50 hours, the Fe amendment had an
oxidation/reduction potential below zero. The combination of cheese whey and iron
had a gradual decrease to around 50 mV. All of the amendments, with the
exception of the cheese whey-iron combination, had a slight upward trend towards
the end of the experiment from 150 to 200 hours.
The pH values in the cheese whey amended soils were slightly more acidic
than that of the control and the iron amendment alone (Figure 5). This slight
reduction in pH may have occurred as a result of the low pH of the cheese whey
57
8
Control
7
Cheese Whey
1 % Fe
6
Dissolved Oxygen mg/L
Cheese Whey + Fe
5
4
3
2
1
0
0
50
100
150
200
250
300
350
-1
Hours
Figure 6. Comparison of control, 1.0 % iron, cheese whey and iron +
cheese whey batch reactor dissolved oxygen changes over time (n=1).
amendment (pH 5.0-6.5). The control pH was around 7.5 for the entire experiment.
The 1% Fe reactor dropped initially from around 8.0 and remained constant
throughout the experiment. The cheese whey reactor dropped from a pH of 7.0 to
around a pH of 6.0. The combination reactor had an initial drop from around 6.5 to
5.5, and then steadily rose to around 7.0. Many of the iron co-precipitation and
chemical reduction mechanisms are dependent on pH; therefore, the lower pH
provided by the cheese whey in the combination reaction might have contributed to
the facilitation of removal by iron that did not take place in the iron reactor alone.
The temperature for all the reactors was between 20° C and 25° C (Figure 8).
There were small variations between the night and day temperature due to the
58
600
Control
500
Cheese Whey
1 % Fe
400
Cheese Whey + Fe
mV
300
200
100
0
0
50
100
150
200
-100
Hours
Figure 7. Comparison of control, 1.0 % iron, cheese whey and iron + cheese whey
batch reactor mV changes over time (n=1).
cooling system of the building. There was no significantly higher or lower
temperature readings for any one of the reactors compared to the others.
In field applications, the soil profile will be exposed to periodic oxygen influx.
To observe if the system would recover from this exposure to oxygen the 1% Fe
batch reactor was temporarily exposed to air using compressed air and the reactor
vent tubes. The maxima of the graphs near 300 hours show when the air was
introduced and the recovery back to an anaerobic state at about 600 hours (Figure
11). The oxidation/reduction potential also recovered from exposure to oxygen
dosing to slightly reducing conditions (Figure 9).
The oxygen levels rose in the reactor as air was bubbled through the vent
lines (Figure 9). As the system was resealed at just past 350 hours, it started to
recover back to an anaerobic state. The oxidation/reduction potential was slower to
59
40
Control
Cheese Whey
1 % Fe
Cheese Whey + Fe
35
Degree Celsius
30
25
20
15
10
0
50
100
150
200
250
300
350
Hours
Figure 8. Comparison of the control, 1.0 % iron, cheese whey and iron +
cheese whey batch reactor temperature (°C) changes over time (n=1).
recover (Figure 11). The oxidation/reduction potential stayed constant at 400 mV
until around 600 hours when it dramatically dropped down to a more reducing level.
The reason for this hysteresis is unclear; however, a redox couple involving ferric
iron is probable. The pH of the system rose slightly with the addition of oxygen to
the system from a pH of 7.9 to a pH of 8.1 (Figure 10). As the system went back to
an anaerobic state, the pH dropped back to 7.9 and then fluctuated back and forth to
the end of the experiment. The pH change was not a large fluctuation, but there was
a small response to the oxygen exposure, possibly due to accelerated corrosion of
the iron metal amendment.
The selenium concentrations all dropped as the systems went anaerobic
(Figure 12). This suggests one of the main mechanisms in selenium reduction is an
anoxic environment and a lower oxidation/reduction potential. All of the amended
60
3
Dissolved Oxygen mg/L
2.5
2
1.5
1
0.5
0
0
100
200
300
400
500
600
700
800
-0.5
Hours
Figure 9. Fluctuation of the dissolved oxygen concentrations (mg/l) for
the1% Fe batch-reactor during exposure to air during the re-aeration
experiment.
reactors maintained an mV and pH consistent with the Se (0) stability field shown in
Figure 1. The systems seem to be resilient to air exposures in the design of these
experiments. In future experiments, all the reactor-amended systems should be
exposed to oxygen. This will allow a conclusive determination of the response of the
amended system to oxic challenge.
The initial selenium concentrations were different for each system. The initial
concentrations differ as a result of the mixing time required during preparation, as
well as the spontaneous reactions between the oxidized Se species and the
amendments. The highest Se concentration was that of the control at 850 µM,
whereas the lowest concentrations were the cheese whey and iron combination at
150 µM. It is apparent that the soil amendments cause some Se immobilization
61
upon initial mixing. Follow-up experiments to this pilot study will have three batch
reactors for each treatment and have three sampling apparatuses per batch reactor
8.8
8.7
8.6
8.5
pH
8.4
8.3
8.2
8.1
8
7.9
7.8
0
100
200
300
400
500
600
700
800
Hours
Figure 10. Fluctuations in pH during 1% Fe batch reactor exposure to air
during the re-aeration experiment.
at different depths. The results could then provide more statistical confidence in
comparisons of the various amendments to reduce soluble selenium levels.
62
600
500
400
mV
300
200
100
0
0
100
200
300
400
500
600
700
800
-100
Hours
Figure 11. Fluctuation of the oxidation/reduction potential for the 1 % Fe
Batch Reactor during the re-aeration experiment.
12
11
10
Control
Cheese Whey
1 % Iron
Cheese Whey + Iron
9
8
Se µM
7
6
5
4
3
2
1
0
0
50
100
150
200
250
300
Hours
Figure 12. Comparison of the control, 1 % iron, cheese whey and iron +
cheese whey batch reactor pore water Se concentration (µM) over time
(n=1).
350
63
10.0 Greenhouse Study: Selenium Uptake in Bromegrass in amended soils
10.1 Purpose
Plant uptake of selenium has proven to be a primary pathway for selenium
bioavailability. However, little is known about plant uptake of selenium in amended
soils. Soil amendments used in field experiments, including elemental iron, cheese
whey and potato waste creates selenium-reducing environments. Study objectives
included quantifying selenium uptake in Smooth Bromegrass grown in different
selenium reducing soil amendments.
10.2 Methods
The study involved a randomized cellblock design with six (6) block
replications. Each block includes six pots with the following six (6) soil types:
Smoky Canyon waste rock soil (Control)
0.1% Fe3+ by weight added as ferric chloride solution
1% by weight elemental granular iron
0.5% by weight elemental granular iron
0.6% by weight cheese whey solids
1.3% by weight potato waste solids
The soil used in the study was collected by sieving middle waste rock shale
collected from the Smoky Canyon field site. Soils were added to standard 1 gal
pots. Each pot represents one experimental unit. After a 14 day incubation period
the pots were seeded following a re-mixing of the soil in the pots. The seed pots
were watered and maintained in a temperature and humidity controlled greenhouse.
64
The experimental units were harvested and analyzed for total selenium.
Harvest occurred at the “pre-boot” stage before the plants begin the reproductive
stage.
100%
90%
80%
Percentage By Weight
70%
60%
Plant Biomass
50%
Root Biomass
40%
30%
20%
10%
0%
Control
0.1% Fe3+
1% Fe
0.5% Fe
Cheese Whey
Potato Waste
Figure 13. Percent plant biomass compared to the percent root biomass of
the soil amendments and control (n=4).
12.00
10.00
Mass (grams)
8.00
6.00
4.00
2.00
0.00
Control
0.1% Fe3+
1% Fe
0.5% Fe
Cheese Whey
Potato Waste
Figure 14. Total Bromegrass biomass (grams) in amended and control pots
(n=4).
65
25
20
Se mg/kg
15
10
5
0
Control
Fe (III)
1% Fe
0.5% Fe
Whey
Potato Waste
Figure 15. Average concentration of total Se in the plant matter for the
amended and control pots (n=4).
120
100
Total Se (mg)
80
60
40
20
0
Control
Fe (III)
1% Fe
0.5% Fe
Whey
Potato Waste
Figure 16. Comparison of the concentration of total Se in the Bromegrass
biomass between the amendments and control (n=4).
66
0.3
30
0.25
25
µg/g
Se (µ M)
0.2
20
0.15
15
0.1
10
0.05
5
0
0
Control
Control
Fe (III)
0.1% Fe
1% Fe
0.5% Fe
0.5% Fe
1% Fe
Whey
Potato Waste
Cheese Whey Potato Waste
Figure 17. Comparison of total SeAmendments
concentrations of the 3 hr soil leachate
in the different amended soils.
Figure 18. Comparison of soil phosphorus concentrations (µg/g) for the
different amendments.
67
7.5
7.4
7.3
7.2
pH
7.1
7.0
6.9
6.8
6.7
6.6
Control
0.1% Fe
0.5% Fe
1% Fe
Cheese Whey Potato Waste
Amendments
Figure 20. Comparison of soil pH levels for the different amendments.
400
350
300
µg/g
250
200
150
100
50
0
Control
0.1% Fe
0.5% Fe
1% Fe
Cheese Whey Potato Waste
Amendments
Figure 19. Comparison of soil potassium concentrations (µg/g) for the different
amendments.
68
6.00
Organic Matter %
5.00
4.00
3.00
2.00
1.00
0.00
Control
0.1% Fe
0.5% Fe
1% Fe
Cheese
Whey
Potato Waste
Amendments
Figure 21. Comparison of % soil organic matter for the different amendments.
4.0
3.5
3.0
µg/g
2.5
2.0
1.5
1.0
0.5
0.0
Control
0.1% Fe
0.5% Fe
1% Fe
Cheese Whey
Potato Waste
Amendments
Figure 22. Comparison of the soil ammonia concentrations (µg/g) for the different
amendments.
69
0.06
Sulfur %
0.05
0.04
0.03
0.02
0.01
0.00
Control
0.1% Fe
0.5% Fe
1% Fe
Cheese Whey
Potato Waste
Amendments
Figure 23. Comparison of the % sulfur in the soils for the different amendments.
0.35
0.30
Nitrogen %
0.25
0.20
0.15
0.10
0.05
0.00
Control
0.1% Fe
0.5% Fe
1% Fe
Cheese Whey Potato Waste
Amendments
Figure 24. Comparison of the % nitrogen in the soil for the different
amendments.
70
4.9
4.8
Carbon %
4.7
4.6
4.5
4.4
4.3
Control
0.1% Fe
0.5% Fe
1% Fe
Cheese Whey Potato Waste
Amendments
Figure 25. Comparison of % carbon in the soils for the different amendments.
6
5
µg/g
4
3
2
1
0
Control
0.1% Fe
0.5% Fe
1% Fe
Cheese Whey
Potato Waste
Amendments
Figure 26. Comparison of soil NO3 concentrations (µg/g) for the different
amendments.
71
200
180
160
140
µg/g
120
100
80
60
40
20
0
Control
0.1% Fe
0.5% Fe
1% Fe
Cheese Whey
Potato Waste
Amendments
Figure 27. Comparison of soil sulfate concentrations (µg/g) for the different
amendments.
45
40
35
Se (µg/g)
30
25
20
15
10
5
0
Control
.1% Fe
0.5% Fe
1% Fe
Cheese Whey
Potato Waste
Figure 28. Comparison of soil Se concentrations (µg/g) of the different
amendments after harvest.
72
10.4 Discussion
The relative percentage of plant biomass distributed between the roots and
the shoot of the plants was not significantly affected by any of the treatments (Figure
13). However, total biomass was increased by the carbon amendments of cheese
whey and potato waste (Figure 14). This increase in biomass may be due to an
increase in nutrients, such as K and % Carbon, from cheese whey and potato waste.
A current disposal practice for cheese whey is application on agricultural fields.
All plants in the amended soils were lower in selenium than that of the control
(Figure 15). The total forage selenium biomass levels were higher in the cheese
whey and potato waste amended plants because there was more growth (Figure
16). A soil leach test illustrated that all of the soils with the exception of Fe (III) had
less leachable selenium than that of the control. However, the soil total selenium
levels after harvest was not significantly different in the amendments (Figure 28).
Based on the soil selenium levels after harvest in the soil, the concentration of
selenium in the soil does not significantly change as a result of plant uptake.
Soil analysis did not show any significant variation other than that of
potassium and nitrate (Figures 18-27). Potassium was elevated in the potato waste
amendment and nitrate was somewhat elevated in the 0.1% Fe amendment (Figure
21). The potassium levels in the potato waste (87 µg/g) most likely cause the higher
level of soil potassium.
The data shows a lower level of selenium in the uptake of the plants grown in
amended soils. This suggests that amending the soils may limit uptake and thus
limit exposure to animals that feed on the forage. Others have found a reduction in
73
plant Se with increased organic C in soils (Bisbjerg and Gissel-Nielsen 1969;
Levesque 1974; Blaylock and James 1994; Ajwa, Banuelos et al. 1998). Animals
consuming plants that contain Se levels ranging from 5-20 mg/kg dry mass for a
prolonged period of time are likely to suffer from chronic or possibly acute Se
poisoning (Girling 1984). The Bromegrass Se uptake in this study is above 5 mg/kg
for all treatments. The lowest was in the 1% Fe amendments at a little over 6 mg/kg.
74
11.0 SURFACE WASTE ROCK FIELD EXPERIMENTS
11.1 Purpose
The surface/shallow sub-surface waste rock field experiments were designed
to test reactivity of a surface application of cheese whey and granular iron on soil
chemistry, and the subsequent selenium chemistries. A better understanding of how
plants uptake selenium can be gained through observing the chemistry changes and
observing how the available selenium changes within the iron and cheese whey
treatment zones.
11.2 Methods
11.2.1 Construction
Three test fields measuring 30 meters long and 8 meters wide were surveyed
on a level waste rock field and cleared of plant growth. The test fields were
separated from each other by a 6-m buffer zone. Three suction lysimeters were
placed in each field. The Soil Moisture Equipment Model #1920F1 pressure-vacuum
soil water sampler (ceramic) lysimeters were placed 3.8-m from the width side of the
field, and where separated equidistant from each other by 6-m. Each lysimeter was
placed at a depth of 1-m.
Row 1 contained no treatment amendment (control). Row 2 contained
approximately 450 kg of granulated iron. The iron metal granules were spread
uniformly by shovel and rake. Row 3 was treated with approximately 15,000 L of
75
cheese whey. The cheese whey was spread directly from the truck from a 4-in
hose.
A network of tubing connected all of the suction lysimeters and runs to the
central sampling shed constructed on site. Lysimeter tubing for individual rows were
contained in a separate 2-in PVC conduit. During the winter, the mine reclamation
personnel spread a seed mixture on top of the snow pack.
11.2.2 Sampling
Water samples were drawn from the suction lysimeters during all seven
research-sampling trips. On-site analysis included pH, oxidation/reduction potential,
and temperature. The samples were then submitted for total selenium analysis.
11.3 Results and Discussion
9
8
pH
7
Cheese Whey
Iron
Control
6
5
4
3
3/16/99
4/5/99
4/25/99
5/15/99
6/4/99
6/24/99
7/14/99
8/3/99
Sample Date
Figure 29. Comparison of control, iron and cheese whey pore water pH for
the shallow sub-surface over time (n=3).
76
As the sampling season progressed, the pH values rose slightly (Figure 29).
The control and amendment samples maintained a slightly increasing pH, fluctuating
between pH 6 and pH 8. The pH of the cheese whey amendment did not reduce the
pH of the pore waters as much as expected. The soils contain limestone and calcite
and this may act to counter acidification.
The oxidation/reduction potential of the amended zones drop from the first
sampling visit in March 1999 by approximately 150 mV in April 1999 (Figure 30).
The data shows the control and treatments following similar trends until the last
sampling date in August 1999. Small volumes of sample might have caused the
oxidation/reduction fluctuation of the last sampling trip. The initial drop in the
700
650
Cheese Whey
Iron
Control
600
550
mV
500
450
400
350
300
250
200
3/16/99
4/5/99
4/25/99
5/15/99
6/4/99
6/24/99
7/14/99
8/3/99
Sample Date
Figure 30. Comparison of control, iron and cheese whey pore water
oxidation/reduction potential (mV) for the shallow sub-surface over time (n=3).
77
25
20
°
C
15
Cheese Whey
Iron
10
Control
5
0
3/16/99
4/5/99
4/25/99
5/15/99
6/4/99
6/24/99
7/14/99
8/3/99
Sample Date
Figure 31. Comparison of the control, iron and cheese whey pore water
temperatures (°C) for the shallow sub-surface over time (n=3).
oxidation/reduction potential might have resulted from the amendments or it might
have been normal sample variance. The control did not vary significantly from the
amended samples.
The temperature of the samples followed a trend upward as spring turned to
summer (Figure 31). The control as well as the amended zones rose from around 5°
C in March to around 20° C in August. The increase in temperature could be one
key factor in the stimulation of selenium-reducing bacteria. Often, seleniumreducing bacteria prefer temperatures warmer than 5° C (Omil 1997).
The surface amended soils were close to 10 times lower in soluble selenium
in the first sampling (Figure 32). As the sampling season progressed, both the
control and the amended soil levels decreased. We hypothesize that as the snow
pack and spring precipitation moved through the shallow subsurface, selenium
oxidized in the past year was leached out of the test zone into the deeper
78
6
Control
Granular Fe
5
Cheese Whey
Se µ M
4
3
2
1
0
10-Apr
30-Apr
20-May
9-Jun
29-Jun
19-Jul
8-Aug
28-Aug
Sample Date
Figure 32. Comparison of the control, iron and cheese whey pore water Se
concentration (µM) over time (n=3).
subsurface. There were no increases of selenium in the amended soil water as the
spring melts migrated down the soil profile. This suggests that the selenium was
prevented from migrating when compared to the control. The data support a
conclusion that the amendments provided an environment that sequestered the
selenium, preventing migration. It is interesting to note that the Se levels in the
control area samples rose in the latter part of the sampling season, whereas the
treated zones appeared to remain stabilized.
Keeping the selenium in the soil less soluble with the cheese whey and iron
may decrease availability to plants in the shallow sub-surface. Amendments
preventing the oxidation of selenium and possible uptake by plants may reduce the
possible exposure pathway through reclamation forage. Along with reducing uptake
by plants, amendments may decrease surface mobilization of Se in the form of
runoff and infiltration.
79
To further test this hypothesis, it would be of interest to perform a randomized
block experimental design for different concentrations of cheese whey and iron. By
varying the concentrations, a preferred application rate could be determined. Along
with additional studies, continued analysis of the current field site should provide
longer-term success of the treatments. The seasonal cycling of precipitation
compared to selenium leaching through the shallow sub-surface could provide
insight about times of high release. This knowledge may assist in the development
of robust Se control strategies that may include the use of surface amendments.
80
12.0 SUBSURFACE POLE CREEK WASTE ROCK AMENDMENT ANALYSIS
12.1 Purpose
The purpose of the subsurface field research was to test how amendments
that were successful in the laboratory, such as iron and potato waste, might change
soil chemistry and prevent leaching of selenium on a large scale under field
conditions. Through the selenium and chemistry data, a better understanding of the
release/control mechanistic dynamics was gathered, and then used for future study
towards the development of possible control approaches for environmental selenium
release.
12.2 Methods
A large trench was excavated approximately 60-m long, 15-m wide, and 5.5m deep. The side slopes were subsequently excavated to 1:1 slope, and the bottom
of the trench was leveled. The trench was divided into four treatment cells:
•
•
•
•
Cell 1
Cell 2
Cell 3
Cell 4
experimental control
iron treatment
mixed iron and potato waste
potato waste.
Each cell was roughly square with 7.5-m sides and an approximate area of
56-m2. Approximately 3-m separated each of the cells. Roughly 1200-m3 of soil
excavated from the pit was homogenized for use in the cells. The four soil piles
were cone piled and mixed with a front-end loader. The piles were further
81
homogenized by repeatedly spreading with a road grader before being applied to the
test cells. Two types of soil water samplers were installed in each cell: pan
lysimeters to passively collect and totalize saturated flow and suction lysimeters to
actively collect either saturated or unsaturated flow.
12.2.1 Subsurface Sampling Assembly (Pan Lysimeters)
Sixteen subsurface sampling assemblies (SSA), 4 per treatment cell (n=4),
were constructed on site (Figure 33). The SSA consisted of a 55-gal collection
barrel, collection pan, PVC piping, and connective polyethylene (poly) tubing.
Holes 2-½ inches in diameter were drilled in the lid of the barrel, and 7-in
separated the center of the holes. A 2-½ in diameter hole was drilled in the side of
the barrel 7-3/4 in down from the top of the lid for the drain. Threaded couplers 2
inches in size were placed in the holes in the lid and the side of the barrel so that the
threads are on the inside of the barrel. The couplers were then secured by
tightening a nut on the threaded end of each coupler.
A 2-in 90° elbow was attached to the coupler in the side of the barrel. A 2-in
PVC pipe 18-in long was slotted ½ way through the length of the pipe. This 18-in
long pipe was attached to the 90° elbow. A piece of filter sock was placed over the
slotted pipe and secured with a zip-tie. The end of the sock was knotted.
A 2-in schedule 80 PVC cross was attached at one opening with a 2-in x 1-in
PVC reducing bushing. A 1-in piece of PVC 5 in long had a hole ¼ inches in
diameter drilled through it ¼ in away from one end. This 1-in piece of PVC pipe
approximately 5 in long was attached through the 1-in end of the 2-in x 1-in bushing
so that the ¼-in diameter hole was visible when viewed through the 2-in PVC cross.
82
This apparatus is needed to hold the poly tubing in place. A 1-in cap was placed on
the open end of the 1-in piece of PVC resulting in the closing of the pipe. This
procedure was done for all 16 2-in crosses.
The same procedure for the 16 crosses was applied to the 16 2-in PVC T’s.
The bushing was attached to the T so that when the 1-in piece of PVC was placed
through you could see the ¼-in hole in the 1-in piece of PVC when looking through
the T. The 1-in cap was then placed on the 1-in piece of PVC to close the system.
The following lengths were cut from the 2-in schedule 80 PVC:
•
•
•
32 pieces at 22-in
16 pieces at 23-3/4-in
16 pieces at 37-in
•
•
•
16 pieces at 3-in
16 pieces at 2.5-in
16 pieces at 39-1/3-in.
Six of the 22-in pieces were attached to the bottom of the 2-in PVC crosses.
At the other end of the 22-in piece of PVC pipe was attached a 2-in 90º elbow. A 2 x
1 reducing bushing was placed on the open end of the elbow. A 1-in hose barb was
attached to the 1-in end of the bushing. The 2-in crosses were then connected,
directly across from the pipe that was put in to secure the 1-in poly tubing, to a 2-in
90º elbow with the 16 3-in pieces. The 23-3/4 in pieces of PVC were then attached
to the other end of the 90º elbows.
The remaining sixteen 22-in pieces were attached to the bottom of the 2-in
T’s. The other end of the 22-in piece of PVC was placed into one of the 2-in
couplers on the lid of the 55-gal barrel. The 23 3/4-in piece of PVC pipe that was
attached to the cross was placed into the other 2-in couple on the lid of the barrel.
83
The ¼-in poly tubing was inserted through the top of the 2-in T. The tubing is
then run through the ¼-in hole in the 1-in PVC sticking through the opening. From
the 1-in piece of PVC the poly tubing was run down the 22-in long PVC through the
barrel lid and into the barrel. After the poly-tube is through the lid, it was placed
through a circular scrap piece of plastic large enough to prevent it from traveling
back through the lid.
The ¼-in poly tubing was inserted through the top of the 2-in PVC cross. The
poly tubing was inserted through the ¼-in hole in the 1in PVC sticking through the
opening. The tubing was then run to the bottom of the 90º elbow.
To the top of the cross the 39 1/3-in piece of PVC was connected. Before
connecting the poly tubing was run through the bottom of the 39 1/3-in piece of PVC
and out the top. The 37-in piece of PVC was attached to the top of the 2-in T after
having the poly tubing run through it. The two PVC pieces were attached with an
uni-strut to help stabilize them. As the PVC was attached, gravel was placed around
the barrels and the PVC to protect them from being crushed during backfilling.
12.2.2 Collection Drain
A pan measuring 10-in square and 5-in deep was used as a collection drain.
To increase the surface area, from which the pan would collect water, fabric was
attached to the drain. Two 35-in by 35-in pieces of fabric felt and one 35-in by 35-in
piece of plastic were use per pan. The plastic and the bottom piece of felt had a 2-in
hole cut in the middle of them. The plastic was sandwiched between the two pieces
of filter fabric, with the piece of filter fabric that had a hole in it at the bottom. The
84
sandwiched fabric was placed over the top of the drain and attached to the cap of
the drain with a zip tie.
The bottom of the drain was attached to a 1-in hose barb via a 3-in x 1-in
PVC bushing that was placed into the drain hole. To the 1-in hose barb was
attached a 15-ft long 1-in hose. The hose was secured with a 1-in hose clamp. The
other end of the hose was attached to the 1-in hose barb on the PVC-cross
apparatus on the lid of the 200-L barrel. Each barrel was connected to one
collection pan. The pans were labeled North West (NW), North East (NE), South
West (SW) and South East (SE) according to their orientation in the pit. The pans
were placed in the corners of the pit at a height above that of the 2-in cross. This
allowed water collected to flow into the 22-in piece of PVC and over to the 23 3/4-in
piece of PVC and down into the 55-gal barrel. The water could be sampled using
the ¼-in poly tubing located in the 22-in piece of PVC.
12.2.3 Horizontal PVC Connections and Suction Lysimeter Installation
During all of the PVC connections the poly tubing was being continuously run
through the PVC pipe, making one continuous connection. If the poly tubing ran
short, it was coupled to another piece of poly tubing with a "Fast and Tite" full flow
thermoplastic polypropylene fitting with nitrile o-rings and continued all the way
through the PVC. This poly tubing provided soil water samples to a centrally located
sampling shed that was used during the winter months.
A 2-in PVC T coupler was attached to the 37-in and 39 1/3-in piece of PVC
that was attached to the barrel apparatus (Figure 33). To the top of the T a Piece of
2-in PVC was attached, as the pit was backfilled, until it was roughly 5-ft above the
85
ground surface. To the horizontal part of the T, a full length of PVC was attached.
At the other end of the horizontal PVC a 45º T was attached. A 2-in cap was placed
on the 45º outlet on the T. A ¼-in hole was drilled in the cap from which the tubing
can enter from the suction lysimeter. The open end of the T was attached to a full
length of PVC continuously until the PVC reached the location for the shed. When
at the location of the shed a 90º elbow was attached to the end, and an appropriate
length of PVC was attached to the elbow so that the pipe was accessible in the
shed.
Two Soil Moisture Equipment Model #1920F1 pressure-vacuum soil water
sampler (ceramic) lysimeters were installed per treatment cell. The lysimeters
contained two 0.25-in OD polyethylene tubes. A white tube on the apparatus was
the pressure-vacuum tube while the tube marked with black tape is the sample tube.
The sample tube was connected to another tube inside the sampler that drew water
from the bottom of the sampling cup, while the pressure-vacuum tube terminated at
the top of the sampler.
Lysimeter installation required sand sized and fine screened soil to bed
around the lysimeter. A 1:1 mixture of silica flour (200 mesh) and screened soil was
mixed with water to form a slurry. The slurry was used to provide hydraulic contact
between the lysimeter and the pit run rock. The balance of the screened soil was
then used to surround the lysimeter and protect it from breakage and crushing by
coarse rocks during backfilling. The lysimeters were installed at a 30° angle from the
horizontal with the porous ceramic cup located approximately at 6-in above the
elevation of the collection pan.
86
The lysimeters were placed approximately ¼-in below the treated zone.
Sampling tubing was run into the horizontally located PVC at the 45° T’s. The poly
tubing from the suction lysimeters was continuously run through the PVC with the
SSA tubing. All the tubing was continuously labeled to keep track of its location.
12.2.4 Backfill
During assembly of the SSAs, gravel back fill was placed directly around the
barrel and PVC pipes for protection during burial. The barrels sat approximately 12ft below ground surface. Waste rock soil was placed into the pit to a level of 10-ft
below ground surface (BGS). At 10 ft BGS the amended soil was placed into the
appropriate cell. The depth of the Fe amendment was roughly 2-in. The depth of
the potato waste and potato waste plus Fe amendment was roughly 1-2 ft. The rest
of the pit was filled with homogenized waste rock from the pit extraction.
The assemblies were constructed so that infiltrated water would pass through
waste rock soil. As the water moved down the soil profile it would encounter the
treatment zone. In its downward migration, the water would enter the collection pan
sampling assembly. After sampling, the port would be purged and measured to
ensure a fresh sample. If the sampling port should become full, the water would
overflow into the collection barrel. The sampling port can be accessed from directly
above the treatment zone and in the centralized shed. Heavy snow levels during the
winter make vertical access difficult; therefore, the poly tubing allows for sampling
from within a sampling shed located onsite.
Four SSAs were placed into each cell (n=4). Gravel was used to protect the
PVC pipes and conduit from breakage by large rocks during pit backfilling. The
87
SSAs allow for water samples to be obtained after the seasonal water infiltration
processes. Comparison of the different samples from the different treatment cells
provided comparative data on selenium reduction and subsurface hydrology.
12.2.5 Pit Experimental Treatments
Each of the four cells contained different treatments:
•
•
•
•
Cell 1 - (control) - contained pit rock
Cell 2 - granular iron
Cell 3 - granular iron and potato waste
Cell 4 - potato waste.
The granular iron was applied uniformly by hand with shovels and rakes
making an approximate 5-cm layer. A tanker with 14,000 kg of waste potato sludge
was delivered to the site. It was estimated that 35-45% of the potato sludge was
water. At an average 40% moisture content the estimated dry mass would be 8400
kg. The potato waste was blended with soil prior to refilling the treatment cell by
mixing it with a loader and then spreading it in a long windrow and turning with a
grader until it visually appeared that the moisture content was uniform. One third of
the potato waste soil blend was applied to cell 3, and the remaining 2/3 of the potato
waste soil blend was applied to cell 4.
Cell 1, the control, provides subsurface water samples with no treatment at
all. This served as a control in which the other amendments can be compared to, as
well as allowing for analysis of water dynamics at the site. The granular iron in cell 2
and 3 is thought to provide a chemical environment where the selenium will be coprecipitated or reduced so that it cannot migrate through the ground water. The
88
Figure 33. Illustration of the sub-surface sampling assembly (SSA) used to
collect water samples in the subsurface Pole Creek waste rock
amendment experiment.
89
potato waste amendment can act as a carbon source for a group of specialized
bacteria called Sulfate Reducing Bacteria (SRB). These bacteria, which occur
naturally almost everywhere, have the unique ability to use oxidized selenium as
their final electron acceptor in their energy producing cycle. This left the selenium in
a non-soluble form; therefore, it could not migrate in water.
12.2.6 Central Sampling Facility
All of the pan lysimeters had two riser pipes, one for the sample collection
chamber and the other to measure the water in the barrel. However, during heavy
snowfall collection of samples from the vertical riser pipes is difficult. In order to
allow for sampling during winter conditions, a series of horizontal risers were
plumbed into the pan lysimeters. They all ran to a central location were they were
plumbed to the surface. A shed was built over the well pipe stub ups. Sampling
from all of the cells could then be performed during winter freeze from inside the
shed.
12.2.7 Sampling Protocol
Six sample trips were conducted in the spring and early summer of 1999.
The dates of the trips were:
March 16, 1999
June 23, 1999
April 17, 1999
July 13, 1999
May 26, 1999
August 17, 1999
12.2.7.1 Pit Suction Lysimeters
Upon arriving at the field site, the suction lysimeters were drained and a
vacuum was placed on the sample line with a hand vacuum pump. After 24 hours
90
the lysimeter sample line was pumped into sample bottles. The sample bottles were
placed chilled and returned for analysis.
12.2.7.2 Subsurface Sampling Assemblies (SSAs)
A vacuum was placed on the sample line of the SSAs. Water samples were
taken in bottles and chilled. Any excess water in the collection line was returned to
the 55 gal barrel. This allowed for the water in the collection tube to always be the
most recently collected by the pans. Temperature, pH, and mV measurements were
taken at the time of sampling.
Water volumes were measured in the sample collection tube and the 200-L
barrel using an electrical conductivity meter. The meter was lowered down the
sampling pipe. By measuring the distances between the bottom of the chamber and
the height of the water, the volume of water in the chamber or barrel could be
calculated.
12.3 Results
Data was gathered from six research trips to the Smoky Canyon field
research site. The water sampled from the SSAs was measured for pH, mV,
temperature and total soluble selenium (Figures 34-40). The pH of the treatment
zones varied little, staying around pH =7 (Figure 34). The Eh of all amendments
followed similar trends (Figure 35). The temperature gradually increased in all SSAs
corresponding to the increase in temperature as winter turned to spring (Figure 36).
All of the treatment zones were at or below the level of the control for the
duration of the sampling period (Figure 37). There is no indication of any increased
91
mobilization from any of the treatments. The most successful treatment was the
combination of potato waste and granular iron. Selenium levels were over 70%
lower than that of the control for the duration of the sampling period. The potato
waste amendment alone illustrated a decrease of selenium for the first three months,
but had an increase in the last sample taken. The iron treatment’s selenium
concentrations fluctuated between 3 and 4 µM, and always below that of the control.
12.3.1 SSAs
8.5
8
7.5
pH
7
6.5
Control
Fe
6
Fe+ potato waste
Potato Waste
5.5
5
4/17/99
5/7/99
5/27/99
6/16/99
7/6/99
7/26/99
8/15/99
Sample dates
Figure 34. Comparison of the control, iron, iron + potato waste and potato
waste pore water pH as sampled by the SSAs over time (n=4).
92
600
500
mV
400
300
200
Control
Fe
Fe + Potato Waste
Potato Waste
100
0
4/17/99
5/7/99
5/27/99
6/16/99
7/6/99
7/26/99
8/15/99
Sample Dates
Figure 35. Comparison of the control, iron, iron + potato waste and potato
waste pore water oxidation/reduction potential (mV) as sampled by the
SSAs over time (n=4).
25
Control
Fe
20
Fe + Potato Waste
Potato Waste
°
C
15
10
5
0
4/17/99
5/7/99
5/27/99
6/16/99
7/6/99
7/26/99
8/15/99
Sample dates
Figure 36. Comparison of the control, iron, iron + potato waste and potato
waste pore water temperatures (°C) as sampled by the SSAs over time
(n=4).
93
12.3.2 Suction Lysimeters
The suction lysimeter data was similar to that from the SSAs. The pH was
somewhat higher for the iron amendment during the first sampling trip pH=7.6, but
through the remaining season the pH was around 6.8 to 7 for all amendments
(Figure 37). The mV readings rose slightly for the amended soils during the first
three sampling points, and were 40 to 100 mV higher for the remaining sampling
dates (Figure 38). Temperature rose as the season changed to summer. The range
of temperature was at the coldest 5º C, and at the warmest was around 20º C
(Figure 39).
The most successful amendment from both monitoring methods was the
combination of potato waste and iron (Figure 40). Both the SSAs and the suction
Control
Fe
Fe + Potato Waste
Potato Waste
7.8
7.6
7.4
7.2
pH
7
6.8
6.6
6.4
6.2
6
4/17/99
5/7/99
5/27/99
6/16/99
7/6/99
7/26/99
8/15/99
Sample Dates
Figure 37. Comparison of the control, iron, iron + potato waste and potato
waste pore water pH as sampled by the suction lysimeters over time (n=2).
94
lysimeter selenium levels for the potato waste and iron mixture were around one
mg/L and below. The pattern of selenium concentration fluctuation is similar for the
treatments of the two different sampling types. The correlation between the two
methods strengthens confidence in the precision of selenium concentration.
800
Control
Fe
Fe + Potato Waste
Potato Waste
mV
600
400
200
0
4/17/99
5/7/99
5/27/99
6/16/99
7/6/99
7/26/99
8/15/99
Sample Date
Figure 38. Comparison of the control, iron, iron + potato waste and potato
waste pore water oxidation/reduction potential (mV) as sampled by the
suction lysimeters (n=2).
95
25
20
°
C
15
10
Control
Fe
5
Fe + Potato Waste
Potato Waste
0
4/17/99
5/7/99
5/27/99
6/16/99
7/6/99
7/26/99
8/15/99
Sample Date
Figure 39. Comparison of the control, iron, iron + potato waste and
potato waste pore water temperature (°C) as sampled by the
suction lysimeters over time (n=2).
12.4 Discussion
The reducing environment of the subsurface was one variable sought in the
analysis. However, we believe the results of the oxidation/reduction potential
readings may have been somewhat skewed due to temperature instability of the
samples. The sample chambers where designed to trap water in a collection
chamber. Once in the collection chamber the waters were no longer in contact with
the soil profile, and the chemistry possibly could change. Also, as the samples were
pumped from the sample chamber and collected, they were exposed to higher levels
of oxygen. For this reason we believe that actual oxidation/reduction conditions in
the subsurface soil would be more reducing, and possibly negative under some of
the treatment zones. Though the oxidation/reduction values may not be the actual
96
values of the saturated soil, they may contain the same trends as the saturated soil.
Possibly in future studies a probe can be buried in the collection pan, and the data
can be taken in-situ.
Temperature data followed the ambient air temperatures as the seasons
started to turn from early spring to summer. Temperature might have a large
influence on the activity of selenium reducing microbes. During the warmer times of
the year, the activity of such microbes would most likely increase. In the shallow
subsurface experiment, we noticed drops in Se levels as the temperatures
increased. Isolation of the selenium reducing bacteria of the area would make a
temperature study possible. The activity could be monitored for selenium reduction
at different temperatures with different amendments to understand how these
bacteria react to temperature variations, and how some amendments might help
mitigate those influences by providing an abundant source of food.
The pH of the system did not fluctuate much with the addition of the
amendments. However, a small reduction in pH could provide a preferential
environment for selenium sequestration. One possible reason for the success of the
combination of iron and potato waste could be that the potato waste provides a pH
low enough for iron to chemically reduce selenium. It has been shown that selenium
reduction with zero-valent Fe is most successful in slightly acidic environments
(Marchant 1976).
97
The lowest total selenium levels were observed in the treatment that
combined iron and potato waste. This amendment caused Se removal to be over
80% in both methods of sampling the subsurface waters. These data suggest the
possibility of the amendments stimulating multiple selenium sequestration
mechanisms in the sub-surface. There could be a synergism between the two
chemistries that make for an environment conducive to selenium stabilization
thereby limiting leaching. The potato waste and iron amendment could possibly be
Control
Fe
Fe + Potato Waste
Potato Waste
5.00
4.50
4.00
3.50
Se µM
3.00
2.50
2.00
1.50
1.00
0.50
0.00
10-Apr
20-Apr
30-Apr
10-May
20-May
30-May
Sampling Date
Figure 40. Comparison of the control, iron, iron + potato waste and potato
waste pore water selenium concentration (µM) as sampled by the suction
lysimeters over time (n=2).
stimulating a microbial reduction of selenium. Another possibility is that they might
be sequestering selenium via completely different mechanisms resulting in an
additive effect. However, the data does not suggest that they are additive but rather
98
that one amendment might help provide an environment from which the other is
more successful.
The combination of similar carbon amendments and iron were equally
successful in the laboratory batch reactors and saturated paste experiments. This
strengthens the possibility that combining a carbon source and iron provides an
atmosphere that is conducive to limiting selenium leaching through a combined
chemical-microbial pathway.
Another possible microbial mechanism may be that two different species of
bacteria are reducing selenium by contributing to the selenium sequestration at
different times. Each species could have a preferential source of energy. One could
prefer potato waste while the other could prefer iron, or the microenvironment
provided by each. Or, the preferential species might change as the chemistry of the
potato waste changes. As certain components of the potato waste are consumed by
bacteria-A, the chemistry of the amendment changes. Due to those changes,
bacteria-B may then out-compete or prefer the new environment, therefore, having
two different species contributing to the selenium sequestration at different times.
This could possibly explain why the potato waste amendment had better staying
power. Doing a batch reactor time trial that counts and identifies bacteria species
over time for different amendments could test this hypothesis. There is a possibility
that a genus or species of microbe reduces selenium better with a certain
combination of amendments, like what was observed with potato waste and iron.
A piece of data that should provide more information on the staying power of
these amendments will be the selenium concentrations in the second field sampling
99
season. The main goal of sequestering selenium is to do it in a way that makes reoxidation difficult. Data from the second sampling season coupled with further
laboratory work will help provide insight into amendments with the best long-term
sequestration properties.
100
13.0 CONCLUSION
The mobilization of selenium from waste rock soil at the Pole Creek Canyon
impoundment poses an environmental concern. High selenium levels in the waters
and forage down gradient of the waste rock provide pathways of Se exposure to
wildlife and livestock. Selenium has been observed in the reduced forms of selenopyrite and zero-valent selenium, specifically in the middle waste shale fraction of the
waste rock. The conformation observed in the seleno-pyrite forms were framboidal
pyrite and cubic pyrite.
Partial sequestration of Se from waste rock soils is observed in zones
amended with potato waste, potato waste-iron granules, iron granules and cheese
whey. Studies have shown that bacteria present in the soil have the ability to reduce
selenium. The success of cheese whey and potato waste in the field and in the
laboratory experiments suggests that a microbial mechanism for Se sequestration is
present and is stimulated with the addition of these amendments. Though the iron
amendments had somewhat mixed results, when used in combination with cheese
whey or potato waste, the effectiveness was always greater than any amendment
alone. This may suggest that there is a chemical pathway associated with the iron,
and that its activity is somehow increased when in combination with potato waste or
cheese whey. The specific reactive pathway is not clear at this time; however, the
laboratory experiments in collaboration with the field data strongly suggests this
chemical-microbial pathway.
Further focus should be directed towards understanding the reactive
pathways and specific mechanisms associated with the various amendments and
101
how they react to the various species of selenium. Classification and tracking of
microbial numbers may provide information on the specifics of the different carbon
amendments. Tracking both microbial numbers and water/soil chemistry in future
studies would provide insight on the combined pathways associated with the
combination amendments.
The results presented in this thesis encourage the possibility of “green
chemistry” approaches for controlling environmental selenium contamination through
bioremediation and chemical reduction/sequestration. However, work remains on
specific mechanisms of release and control.
102
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APPENDIX A
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