Effectiveness of Commercial Heavy Metal Chelators with New Insights for the
Future in Chelate Design
Matthew M. Matlock, Kevin R. Henke, and David A. Atwood
Department of Chemistry, University of Kentucky, [email protected].
Abstract.
Toxic heavy metals in air, soil, and water are global problems that are a growing threat
to the environment. There are hundreds of sources of heavy metal pollution, including
the coal, natural gas, paper, and chlor-alkali industries (1,2). To meet the federal and
state guidelines for heavy metal discharge, companies often use chemical precipitation or
chelating agents. In order to be competitive economically, many of these chelating
ligands are simple, easy to obtain, and, generally offer weak bonding for heavy metals.
Poor and indiscriminant metal binding often lead to unstable metal ligand complexes.
Laboratory testing of three commercial reagents, TMT (Trimercaptotriazine), Thio-Red
(potassium/sodium thiocarbonate), and HMP-2000 (sodium dimethyldithiocarbamate),
has shown that the investigated compounds were unable to reduce independent solutions
of 50.00 ppm (parts per million) cadmium, copper, ferrous, lead, or mercury to meet EPA
standards (CFR, 1994). Additionally, the tested compounds displayed high leaching
rates and in some cases decomposed to produce toxic substances. For this reason, a
novel multidentate ligand has been developed for the safe and effective removal of heavy
metals.
Introduction
1
Toxic heavy metals in air, soil, and water are global problems that are a growing threat to
humanity. There are hundreds of sources of heavy metal pollution, including the coal,
natural gas, paper, and chlor-alkali industries (1,2). In response to the growing problems,
federal and state governments have instituted environmental regulations to protect the
quality of surface and ground water from heavy metal pollutants, such as Cd, Cu, Pb, Hg,
Cr, and Fe (3). To meet the federal and state guidelines for heavy metal discharge,
companies often use chemical precipitation or chelating agents. For acid mine drainage
(AMD) and wastewater treatment plants, the typical means of removing heavy metals is
usually accomplished through pH neutralization and precipitation with lime, peroxide
addition, reverse osmosis, and ion exchange (4). Extensive research and financial
resources have been spent on each of these treatment processes with the overwhelmingly
preferred method, based upon cost and effectiveness, being precipitation through
neutralization (4). A major disadvantage of the liming process, however, is the need for
large doses of alkaline materials to increase and maintain pH values of 4.0 to above 6.5
for optimal metal removal (2). Additionally, pH neutralization typically requires that the
materials be appreciably fine-grained to provide the necessary reactive surface area (5).
Furthermore, liming produces secondary wastes, such as metal hydroxide sludges, that
necessitate highly regulated and costly disposal (6).
As an alternative to the liming process many companies have developed chelating
ligands to precipitate heavy metals from aqueous systems. In order to be competitive
economically, many of these chelating ligands are simple, easy to obtain, and, in general,
offer minimal bonding for heavy metals. Poor and indiscriminant metal binding criteria
often lead to unstable metal ligand complexes. These tend to decompose over time
2
releasing the heavy metals back into the environment over varying, but usually short,
periods of time (9). Many of the current remediation ligands on the market require high
ligand to metal dosages to lower metal concentrations to EPA (Environmental Protection
Agency) limits. In addition to high dosage ratios, many of the ligands decompose or are
themselves hazardous materials (9).
The purpose of this article is to examine the effectiveness of three major chelating
ligands that are used commercially, and to introduce new research which promises to not
only produce low-cost, highly selective ligands for heavy metal remediation, but also,
products which can be used at low dosages. The remediation and wastewater treatment
reagents discussed in this article are TMT (trimercaptotriazine), STC (potassium/sodium
thiocarbonate), and SDTC (sodium dimethyldithiocarbamate) (Figures 1A-C).
TMT, or 2,4,6-trimercaptotiazine, trisodium salt nonahydrate, Na3C3N3S3•9H2O
(Figure 1A), is a chemical reagent commonly utilized for the precipitation of divalent and
univalent heavy metals from water. TMT is manufactured and distributed by Degussa
Corporation USA of Allendale and Ridgefield Park, New Jersey (7). Despite the
widespread use of TMT, only limited information is available on how the product reacts
with heavy metals in aqueous solutions and the chemistry and stability of the resulting
heavy metal-TMT precipitates.
A second chemical reagent for precipitating divalent heavy metals from
contaminated waters is STC. STC is a sodium (with or without potassium) thiocarbonate
([Na, K]2CS3•nH2O, where n ≥ 0), which has a trade name of Thio-Red and is
distributed by ETUS Inc. (9; Figure 1B). Previous laboratory studies have demonstrated
that Thio-Red ultimately removes copper, mercury, lead, and cadmium from aqueous
3
solutions through the formation of metal sulfides (that is, CuS, HgS, PbS, and ZnS) rather
than metal thiocarbonates (that is, CuCS3, HgCS3, PbCS3, and ZnCS3) as claimed by
ETUS Inc., 1994 (8,9). A byproduct of the metal sulfide precipitation with STC is
carbon disulfide, which is volatile and toxic liquid.
Another type of commercial remediation agent is SDTC. SDTC is a sodium
thiocarbamate, which has the trade name of HMP-2000 (Figure 1C). Recently, an
accidental release of over 1.5 million gallons of toxic wastewater laced with SDTC
occurred when a local auto parts manufacturing plant in Anderson, Indiana, discharged
large volumes of contaminated water to the city's wastewater (10). The SDTC reportably
decomposed into toxic compounds, tetramethylthiuram and thiram, where it crippled the
Anderson, Indiana, wastewater plant and was later discharged into local state waters (10).
The Indiana Department of Environmental Management reported the deaths of 117 tons
of fish over a 50-mile stretch from Anderson to Indianapolis, Indiana (10).
Analytical Methods
Lead, cadmium, iron (II), and copper analyses were performed with a 1999 Jarrell
Ash Duo HR Iris Advanced Inductive Coupled Plasma (ICP-OES) Spectrometer.
Mercury results were obtained using cold vapor atomic fluorescence spectroscopy
(CVAF) on a Varsal Atomic Fluorescence Spectrometer, model number VI2000, using
EPA techniques for mercury analyses (11). For powder XRD analyses, the samples were
mounted on glass slides with ethanol and analyzed with a Rigaku unit at 40kV and 20
mA using Cu Ka1 (λ=1.540598 Å) radiation. Infrared spectroscopy (IR) data on the TMT
and STDC commercial reagents were obtained from the Aldrich IR library (12). For the
metal-ligand complexes, the IR spectra were collected as potassium bromide pellets using
4
spectroscopy grade potassium bromide (KBr) using a Nicolet-Avatar 320 FT-IR series
spectrometer. Elemental Analyses were determined on a Vario Elementar III. All
masses were obtained with a Sartorius BP2100S or a Mettler AE 240 balance.
Analytical Procedures for ICP Analyses
Cadmium, Copper, Lead, and Iron (II) ICP Analyses
A series of 100 ml 50.00 parts per million (ppm) Cd+2, Cu+2, Pb+2, Fe+2, and Hg+2
samples were prepared separately as water solutions. Based on one to one molar ratio,
each of the reagents was added to the metal solutions. Aliquots (10 ml) were collected
and filtered at 0.2 µm at times 1 hour, 6 hours, and 20 hours following reagent addition
(Tables 1-3).
RESULTS AND DISCUSSION
Characterization of Precipitates.
For reactions between the metal-TMT and the metal-STC complexes, extensive
research has been preformed by Henke and coworkers that describes resulting complexes
(9,13,14). For the STC reactions, it has been proven that the resulting complexes are
metal sulfide and carbon disulfide (9). Research on the STDC-metal complexes has also
been conducted which shows a bidentate metal chelation through the terminal thio groups
of the STDC (13,15). Each of the TMT, STC, and STDC metal complexes was
confirmed to the corresponding literature using elemental analyses, IR, NMR, and XRD
spectroscopy (12,15,16).
Results of Pb+2, Cd+2, Cu+2, and Fe+2 Stoichiometry and Removal as Determined
from the ICP.
5
It was found that at stoichiometric doses STC, STDC, or TMT was unable to
remediate the cadmium, lead, copper, or iron from a 50.00 ppm (part-per-million) to meet
the EPA discharge limits (3) (Tables 1-3). Even with a 10.00% molar excess dose, no
additional significant removal was observed (Tables 1-3). The SDTC also displayed a
higher affinity for cadmium, lead, copper, and iron than the STC but still EPA discharge
limits were not achieved. TMT displays similar results as compared with the SDTC with
the highest removal seen for lead and copper. Once again, it is seen that even at a 10%
molar increase in dosage TMT was unable to reduce lead or cadmium concentrations to
meet EPA standards (3). For the metal concentrations that increased over time for each
ligand, it is believed that this is either contributed to the formation of more soluble metalligand complexes or a possible high leaching rate of the metal out of the ligand
complexes.
Results of Hg+2 Stoichiometry and Removal as Determined from the CVAF.
For the mercury analyses, it was found that at stoichiometric and an 10% molar
dose increase the STC, SDTC, and TMT were all unable to remove the concentration of
mercury from the a 50.00 ppm solution the EPA limits (3) (Tables 1-3). Maximum
results, for mercury removal with STC, were seen at 20 hours at a 10% molar dose
increase with an average value of 3.97 ppm. At one hour the results of the SDTC, at a
10% molar dose increase, indicate a reasonably high removal of mercury with a final
concentration of 0.690 ppm. Within 20 hours at stoichiometric doses, TMT was able to
reduce the 50.00 ppm mercury concentrations to an average final concentration of 9.82
ppm.
Insights for the Development of New Metal Chelators.
6
There is a definite need for new and more effective reagents to meet the growing
environmental problem. Many reagents on the market today either lack the necessary
binding criteria or pose too many environmental risks to be effectively utilized. For this
reason, ligands utilizing multiple binding sites for heavy metals and mimicking biological
systems for metal binding look to be a possible answer to heavy metal remediation and
wastewater treatment. In order to create more effective and economical ligands,
researchers at the University of Kentucky have developed a serious of new synthetic
ligands, which utilize biological heavy metal binding motifs. These synthetic ligands
provide multiple binding sites for heavy metals to secure the metals. Early results with
the ligands have shown that heavy metal concentrations from aqueous solutions can be
reduced well below EPA discharge limits, and the resulting precipitates have shown no
solubility in organic solvents or aqueous systems over a pH range of 0.0 to 14.0 (17,18).
For example, the synthetic ligands have been able to reduce mercury concentrations from
a 50.00 ppm to 0.094 ppm and lead concentrations of a 50.00 ppm to 0.050 ppm (18).
The newly developed ligands have also been effective in immobilizing mercury from
contaminated soils. In recent tests with soils contaminated with 10270 ppm of elemental
mercury, the multidentate ligand was capable of immobilizing 99.6% of the mercury.
The resulting mercury-ligand compounds were shown to maintain stability even when
subjected to the EPA digestion technique for mercury in solid or semisolid waste (19,20).
Heavy metal pollution is a growing environmental problem, which requires
immediate attention. With current commercial remediation reagents failing to provide
the needed requirements as safe and effective metal chelators, the need for new
technology is critical.
7
Literature Cited
1.
Alloway, B.J. ed. (1995) Heavy Metals in Soils 2nd ed. Chapters 6,8,9,and 11.
Chapman and Hall, Glasgow, UK.
2.
McDonald, D.G.; Grandt, A.F. (1981) Limestone- Lime Treatment of Acid
Mine Drainage-Full Scale. EPA Project Summary (1981). EPA-600/S7-81-033.
3.
Code of Federal Regulations (CFR), 40, 141, 261, 268.40, U.S. Government
Printing Office, Superintendent of Documents, Washington, DC.
4.
Wilmoth, R.C.; Kennedy, J.L. (1979) Industrial Environmental Research
Laboratory, "Removal of Trace Elements from Acid mine Drainage."
5.
Drever, J.I., 1997, The Geochemistry of Natural Waters: Surface and
Groundwater Environments, Prentice Hall, Upper Saddle River, NJ.
6.
Wang,W.; Zhenghe, X.; Finck, J. (1996) "Fundamental Study of an Ambient
Temperature Ferrite Process in the Treatment of Acid Mine Drainage." Env. Sci.
and Tech., 30, (8), 2604-2608.
7.
Degussa Corporation (1993) Data Sheets on TMT-15 and TMT, Ridgefield Park,
New Jersey.
8.
ETUS, Inc. (1994) Product Information on Thio-Red, Sanford, Florida
9.
Henke, K.R. (1998) "Chemistry of Heavy Metal Precipitates Resulting from
Reactions with Thio-Red". Wat. Env. Res., 70, (6), 1178-1185.
10.
State of Indiana's Data Fact's Sheet,
http://www.state.in.us/idem/macs/factsheets/whiteriver/185.
11.
US Environmental Protection Agency (EPA), 1986, "Mercury in liquid waste
(manual cold-vapor technique): Volume 1A: Laboratory Manual:
Physical/Chemical Methods," SW-846, 3rd ed., Office of Solid Waste and
Emergency Response, Washington DC.
8
12
Aldrich FT-IR Library, (1997) Ed. II.
13.
Henke, K.R., Robertson, D; Krepps, M.; Atwood, D.A. (2000) “Chemistry and
Stability of Precipitates from Aqueous Solutions of 2,4,6-Trimercaptotriazine,
Trisodium Salt, Nonahydrate (TMT) and Mercury (II) Chloride” Wat. Res., 34,
(11), 3005-3013.
14.
Matlock, M. M.; Henke, K. R.; Atwood, D.A.; Robertson, J.D. "Aqueous
Leaching Properties and Environmental Implications of Cadmium, Lead, and Zinc
Trimercaptotriazine (TMT) Compounds," Wat.Res., (in press).
15.
Cox, M. J.; Tiekink, E. R. T.; Kristallogr, Z. (1997) “Structural variations in
the mercury (II) bis (1,1-dithiolates). The crystal and molecular structure of
[Hg(S2CNMe2)2].” Z. Kristallogr., 212, (7), 542-544.
16.
Mazalov, L. N.; Parygina, G. K.; Fomin, E. S.; Bausk, N. V.; Erenburg, S. B.;
Zemskova, S. M.; Larionov, S. V. (1999) “X-ray spectral study of the nature of
electronic interactions in transition metal dithiocarbamates.” J. Struct. Chem., 39,
(6), 923-927.
17.
Matlock, M. M.; Howerton, B.S.; Henke, K.R.; Atwood, D.A. 2001.
"A Pyridine-Thio Ligand with Multiple Bonding Sites for Heavy Metal
Precipitation," J. Haz. Mat.,82, (1), 55-63.
18.
Matlock, M. M.; Howerton, B.S.; Atwood, D.A "Mercury and Lead
Precipitation with a Newly Designed Multidentate Ligand," J. Haz. Mat., (in
press).
19.
US Environmental Protection Agency (EPA), 1986, "Test Methods for
Evaluating Solid Wastes: Volume 1A: Laboratory Manual: Physical/Chemical
Methods," SW-846, 3rd ed., Office of Solid Waste and Emergency Response,
Washington DC
20.
Matlock, M. M.; Howerton, B.S.; Atwood, D.A. " Irreversible Binding of
Mercury from Contaminated Soil," Adv. Env. Res., (in press).
9
Figure 1.
A. TMT (2,4,6-trimercaptotiazine, trisodium salt nonahydrate).
B. STC (potassium/sodium thiocarbonate).
C. SDTC (sodium dimethyldithiocarbamate).
A.
SN
-
S
N
3Na+
S-
N
B.
S-
+ 2 Na +
-S
S
C.
S-
+ Na +
N
S
10
Table 1. ICP and CVAF results of SDTC at stoichiometric doses and at 10% molar
dosage increases.
Initial metal
Solution concentration
(ppm)
pH
Final metal
concentration
(ppm)
EPA Discharge
Limit (ppm) (3)
50.00
50.00
50.00
21.9
21.9
23.8
5.0
5.0
5.0
4.0
4.0
4.0
50.00
50.00
50.00
15.5
16.2
16.3
5.0
5.0
5.0
1
6
20
3.5
3.5
3.5
50.00
50.00
50.00
12.6
12.6
12.5
10% dose increase
10% dose increase
10% dose increase
1
6
20
4.0
4.0
4.0
50.00
50.00
50.00
7.08
7.10
7.19
Cd
Cd
Cd
stoichiometric
stoichiometric
stoichiometric
1
6
20
3.0
3.0
3.0
50.00
50.00
50.00
11.5
11.5
12.1
1.0
1.0
1.0
SDTC
SDTC
SDTC
Cd
Cd
Cd
10% dose increase
10% dose increase
10% dose increase
1
6
20
4.0
4.0
4.0
50.00
50.00
50.00
10.5
10.5
11.0
1.0
1.0
1.0
SDTC
SDTC
SDTC
Fe (II)
Fe (II)
Fe (II)
stoichiometric
stoichiometric
stoichiometric
1
6
20
4.0
4.0
4.0
50.00
50.00
50.00
25.2
23.1
23.9
2.0
2.0
2.0
SDTC
SDTC
SDTC
Fe(II)
Fe(II)
Fe(II)
10% dose increase
10% dose increase
10% dose increase
1
6
20
4.5
4.5
4.5
50.00
50.00
50.00
24.3
23.2
23.9
2.0
2.0
2.0
SDTC
SDTC
SDTC
Hg
Hg
Hg
stoichiometric
stoichiometric
stoichiometric
1
6
20
4.0
4.0
4.0
50.00
50.00
50.00
1.01
1.50
2.78
0.2
0.2
0.2
SDTC
SDTC
SDTC
Hg
Hg
Hg
10% dose increase
10% dose increase
10% dose increase
1
6
20
4.0
4.0
4.0
50.00
50.00
50.00
0.69
1.24
1.63
0.2
0.2
0.2
Chelating
Agent
Metal
Dose
Time
(hours)
SDTC
SDTC
SDTC
Pb
Pb
Pb
stoichiometric
stoichiometric
stoichiometric
1
6
20
3.5
3.5
3.5
SDTC
SDTC
SDTC
Pb
Pb
Pb
10% dose increase
10% dose increase
10% dose increase
1
6
20
SDTC
SDTC
SDTC
Cu
Cu
Cu
stoichiometric
stoichiometric
stoichiometric
SDTC
SDTC
SDTC
Cu
Cu
Cu
SDTC
SDTC
SDTC
Table 2. ICP and CVAF results of STC at stoichiometric doses and at 10% molar
dosage increases.
Dose
Time
(hours)
Solution
pH
Initial metal
concentration (ppm)
Final metal
concentration
(ppm)
EPA Discharge
Limit (ppm) (3)
Pb
Pb
Pb
stoichiometric
stoichiometric
stoichiometric
1
6
20
6.0
6.0
6.0
50.00
50.00
50.00
38.2
44.8
48.2
5.0
5.0
5.0
STC
STC
STC
Pb
Pb
Pb
10% dose increase
10% dose increase
10% dose increase
1
6
20
5.5
5.5
5.5
50.00
50.00
50.00
33.7
41.6
48.0
5.0
5.0
5.0
STC
STC
STC
Cu
Cu
Cu
stoichiometric
stoichiometric
stoichiometric
1
6
20
5.0
5.0
5.0
50.00
50.00
50.00
27.8
29.0
28.9
STC
STC
STC
Cu
Cu
Cu
10% dose increase
10% dose increase
10% dose increase
1
6
20
4.5
4.5
4.5
50.00
50.00
50.00
27.1
25.8
26.8
STC
STC
STC
Cd
Cd
Cd
stoichiometric
stoichiometric
stoichiometric
1
6
20
5.5
5.5
5.5
50.00
50.00
50.00
34.4
39.5
47.1
1.0
1.0
1.0
STC
STC
STC
Cd
Cd
Cd
10% dose increase
10% dose increase
10% dose increase
1
6
20
5.0
5.0
5.0
50.00
50.00
50.00
27.1
34.9
41.5
1.0
1.0
1.0
STC
STC
STC
Fe (II)
Fe (II)
Fe (II)
stoichiometric
stoichiometric
stoichiometric
1
6
20
6.0
6.0
6.0
50.00
50.00
50.00
35.2
34.4
33.0
2.0
2.0
2.0
STC
STC
STC
Fe(II)
Fe(II)
Fe(II)
10% dose increase
10% dose increase
10% dose increase
1
6
20
5.0
5.0
5.0
50.00
50.00
50.00
34.8
34.6
33.6
2.0
2.0
2.0
STC
STC
STC
Hg
Hg
Hg
stoichiometric
stoichiometric
stoichiometric
1
6
20
6.0
6.0
6.0
50.00
50.00
50.00
8.59
8.07
6.85
0.2
0.2
0.2
STC
STC
STC
Hg
Hg
Hg
10% dose increase
10% dose increase
10% dose increase
1
6
20
6.0
6.0
6.0
50.00
50.00
50.00
6.72
5.20
3.97
0.2
0.2
0.2
Chelating
Agent
Metal
STC
STC
STC
Table 3. ICP and CVAF results of TMT at stoichiometric doses and at 10% molar
dosage increases.
Chelating
Agent
Metal
Dose
Time
(hours)
Solution
Initial metal
Final metal concentration
pH
concentration (ppm)
(ppm)
EPA Discharge
Limit (ppm) (3)
TMT
TMT
TMT
Pb
Pb
Pb
stoichiometric
stoichiometric
stoichiometric
1
6
20
5.0
5.0
5.0
50.00
50.00
50.00
18.2
18.5
21.1
5.0
5.0
5.0
TMT
TMT
TMT
Pb
Pb
Pb
10% dose increase
10% dose increase
10% dose increase
1
6
20
5.5
5.5
5.5
50.00
50.00
50.00
16.1
16.6
17.3
5.0
5.0
5.0
TMT
TMT
TMT
Cu
Cu
Cu
stoichiometric
stoichiometric
stoichiometric
1
6
20
5.0
5.0
5.0
50.00
50.00
50.00
16.2
13.3
10.1
TMT
TMT
TMT
Cu
Cu
Cu
10% dose increase
10% dose increase
10% dose increase
1
6
20
5.5
5.5
5.5
50.00
50.00
50.00
16.2
14.2
12.6
TMT
TMT
TMT
Cd
Cd
Cd
stoichiometric
stoichiometric
stoichiometric
1
6
20
5.0
5.0
5.0
50.00
50.00
50.00
37.1
36.1
38.2
1.0
1.0
1.0
TMT
TMT
TMT
Cd
Cd
Cd
10% dose increase
10% dose increase
10% dose increase
1
6
20
5.5
5.5
5.5
50.00
50.00
50.00
21.0
21.0
21.6
1.0
1.0
1.0
TMT
TMT
TMT
Fe (II)
Fe (II)
Fe (II)
stoichiometric
stoichiometric
stoichiometric
1
6
20
5.0
5.0
5.0
50.00
50.00
50.00
25.0
25.5
25.3
2.0
2.0
2.0
TMT
TMT
TMT
Fe(II)
Fe(II)
Fe(II)
10% dose increase
10% dose increase
10% dose increase
1
6
20
5.5
5.5
5.5
50.00
50.00
50.00
23.6
22.4
21.8
2.0
2.0
2.0
TMT
TMT
TMT
Hg
Hg
Hg
stoichiometric
stoichiometric
stoichiometric
1
6
20
5.5
5.5
5.5
50.00
50.00
50.00
18.1
13.4
9.82
0.2
0.2
0.2
TMT
TMT
TMT
Hg
Hg
Hg
10% dose increase
10% dose increase
10% dose increase
1
6
20
5.5
5.5
5.5
50.00
50.00
50.00
15.2
16.9
10.5
0.2
0.2
0.2