Arsenic Water Technology Partnership
Arsenic Removal With
Iron-Tailored Activated
Carbon Plus Zero-Valent Iron
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
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©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent
Iron
Prepared by:
Weifang Chen, Robert Parette, Will Sheehan, Fred S. Cannon, Brian A. Dempsey
The Pennsylvania State University
Department of Civil and Environmental Engineering
212 Sackett Engineering Building
University Park, PA 16802
Jointly Sponsored by:
Water Research Foundation
6666 West Quincy Avenue, Denver, CO 80235
and
U.S. Department of Energy
Washington, D.C. 20585-1290
Published by:
WERC, a Consortium for
Environmental Education and
Technology Development at
New Mexico State University
N
OS
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NMSU
SA
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IMT
NM
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UN
Water Research Foundation
A L S ALA
O
A CONSORTIUM FOR ENVIRONMENTAL EDUCATION
AND TECHNOLOGY DEVELOPMENT
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
DISCLAIMER
This study was jointly funded by the Water Research Foundation and the U.S. Department of
Energy (DOE) under Grant No. DE-FG02-03ER63619 through the Arsenic Water Technology
Partnership. The comments and views detailed herein may not necessarily reflect the views of
the Water Research Foundation, its officers, directors, affiliates or agents, or the views of the
U.S. Federal Government and the Arsenic Water Technology Partnership. The mention of trade
names for commercial products does not represent or imply the approval or endorsement of the
Foundation or DOE. This report is presented solely for informational purposes.
Copyright © 2010
by Water Research Foundation and Arsenic Water Technology Partnership
ALL RIGHTS RESERVED.
No part of this publication may be copied, reproduced
or otherwise utilized without permission.
Printed in the U.S.A.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
CONTENTS
LIST OF TABLES ........................................................................................................................ vii LIST OF FIGURES ....................................................................................................................... ix FOREWORD ............................................................................................................................... xiii ACKNOWLEDGMENTS ............................................................................................................ xv EXECUTIVE SUMMARY ........................................................................................................ xvii CHAPTER 1: INTRODUCTION ................................................................................................... 1 Background ......................................................................................................................... 1 Arsenic Removal Technology............................................................................................. 1 pH Effect on Arsenic Removal by ZVI and Iron (hydr)oxides .......................................... 2 Iron Corrosion and Iron Release ......................................................................................... 2 Iron Corrosion by Electrolytic Cells ................................................................................... 3 Research Objectives ............................................................................................................ 4 CHAPTER 2: MATERIALS AND METHODS ............................................................................ 5 Materials ............................................................................................................................. 5 Water Sources for Pilot Columns and rapid small-scale column test (RSSCT) ..... 5 Activated Carbons ................................................................................................... 5 Methods............................................................................................................................... 6 Iron Tailoring by Iron-salt Evaporation .................................................................. 6 Rapid Small-Scale Column Tests (RSSCTs) .......................................................... 6 Zero-Valent Iron (ZVI) and Iron-Loaded GAC for As Removal in RSSCTs ........ 7 Iron Pre-Corrosion, Aging, and Idling in RSSCTs ................................................. 7 Pilot Columns.......................................................................................................... 7 Sampling Protocol and Chemical Analysis........................................................... 12 CHAPTER 3: RESULTS AND DISCUSSIONS ......................................................................... 13 Effect of Temperature on Iron Tailoring by Evaporation ................................................. 13 RSSCT vs. Pilot-scale Columns ....................................................................................... 14 Pilot-scale Studies with Zero-valent Iron Rod and GAC.................................................. 16 Pilot-Scale Studies With Electrolytic Solubilization And Iron-Tailored GAC ................ 22 Pilot-Scale Studies with Electrolytic Solubilization Achieved with 0.01 or
0.02A, Plus Iron-Tailored GAC ................................................................ 23 Pilot-Scale Studies with Electrolytic Solubilization Achieved with 0.1 A, Plus
Iron-Tailored GAC.................................................................................... 28 Zero-Valent Iron With Iron-Tailored GAC In RSSCTs ................................................... 31 RSSCTs: Arsenic Removal with Pre-corroded Galvanized Steel Fittings
Coupled with Iron-tailored GAC .............................................................. 32 Performance of Perforated Steel Chamber Preceding Iron-tailored GAC ............ 34 v
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vi | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
CHAPTER 4: CONCLUSIONS ................................................................................................... 47 CHAPTER 5: SIGNIFICANCE TO UTILITIES ......................................................................... 49 REFERENCES ............................................................................................................................. 51 ABBREVIATIONS ...................................................................................................................... 55 ©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
TABLES
2.1
Water quality characteristics of the groundwater used in this study ...................................5 2.2
Pilot-scale columns operational conditions .........................................................................9 2.3
Composition of zero-valent iron sources (percent %) .......................................................12 3.1
Iron content and BVs to breakthrough for carbons that were iron-preloaded by the
evaporation method, with curing at 50−100°C ..................................................................14 3.2
Bed volumes to 10 µg/L As breakthrough in RSSCT versus pilot columns with iron
tailored carbons ..................................................................................................................16 3.3
Bed volumes to 10 µg/L As breakthrough in pilot columns with zero-valent iron rods
and either iron-tailored GAC or virgin GAC. For Column 5, the rods reached all the
way to the column’s bottom. For all others, the rods remained in the top two-thirds of
the media ............................................................................................................................21 3.4
Theoretical and measured iron dose for electrolytic cells* ...............................................32 3.5
Arsenic distribution in GS #1 (iron-tailored GAC coupled with corrosion of
galvanized steel fittings) after 250,000 BVs ......................................................................34 3.6
Column operating parameters and BVs to 10 µg/L breakthrough* ...................................44 vii
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
FIGURES
2.1
Photograph of pilot-scale columns.......................................................................................8 2.2
Electrolytic solubilization cells ..........................................................................................10 2.3
Branched rod configuration used in Columns #11 to 15. ..................................................11 3.1
RSSCT using arsenic-containing groundwater, for iron-loaded GAC, with preloaded
iron curing at temperatures of 50−100°C. Y-axis µg/L As. ..............................................13 3.2
Arsenic breakthrough for iron-tailored (60oC) carbon in RSSCTs and Pilot Column
#3........................................................................................................................................15 3.3
Arsenic breakthrough for iron-tailored (100oC) carbon in RSSCTs and Pilot Column
#4........................................................................................................................................15 3.4
As breakthrough for pilot columns with virgin AquaCarb and plain steel mesh +
virgin AquaCarb.................................................................................................................17 3.5
Back pressure buildup for Columns #1 and 2 ....................................................................18 3.6
Iron concentration in effluent from Column #2 .................................................................18 3.7
As breakthrough for pilot columns with straight and branched rods (rod diameters ¼
inch) ...................................................................................................................................19 3.8
As for pilot columns, while comparing the effect of pH, EBCT, iron-tailored carbon
and smaller branched rod (rod diameters ¼ inch, unless otherwise listed) .......................20 3.9
Iron concentration in effluent for Column #5, with straight rods that extended through
the full media depth ...........................................................................................................22 3.10
Electrolytic solubilization chamber ...................................................................................23 3.11
Effluent arsenic concentration in Column #17: 0.02 A electrolytic solubilization,
virgin GAC.........................................................................................................................24 3.12
Back pressure and voltage vs. BVs in Column #17: 0.02 A electrolytic solubilization,
virgin GAC.........................................................................................................................25 3.13
Effluent arsenic concentration vs. BVs for Column #18, 19 and 20 .................................26 3.14
Backpressure vs. BVs for Column #18, 19 and 20 ............................................................27 3.15
Electrolytic cell voltages vs. BVs for Column #18, 19 and 20 ..........................................27 ix
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
x | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
3.16
Uneven iron loading in Column #6 when 0.1 A was applied across the ZVI
electrolytic cell ...................................................................................................................29 3.17
Effluent Arsenic Concentration from Pilot Columns #7−10. Influent Concentration
50−60 ppb, targeted EBCTs as listed, and these EBCTs were maintained for the first
1,000−1,500 BVs ...............................................................................................................29 3.18
Release of Iron from Electrolytic Pilot Columns Operated at 0.1A, targeted EBCTs as
listed, and these EBCTs were maintained for the first 1,000−1,500 BVs .........................30 3.19
Pressure Build-up during Operation of Electrolytic Pilot Columns Operated at 0.1 A,
targeted EBCTs as listed, and these EBCTs were maintained for the first 1,000−1,500
BVs ....................................................................................................................................30 3.20
Operational Voltage of Electrolytic Pilot Columns Operated at 0.1 A, targeted EBCTs
as listed, and these EBCTs were maintained for the first 1,000−1,500 BVs .....................31 3.21
RSSCT of iron tailored GAC coupled with galvanized steel (GS#1) and without (#1);
both systems operated at pH 6±0.3 using the arsenic-containing groundwater as
influent (As 47−55 µg/L). Dashed line indicated where the GS#1 system was idled
for 6 days............................................................................................................................33 3.22
pH effect on Mini column performance (A) Arsenic effluent from GAC column. (B)
Arsenic removed by steel chamber. (C) Filterable arsenic after steel chamber. Lines
indicate where the columns were idled for 7 days (Dotted line: pH 7.5; dashed: pH 6;
Solid line: pH 6−6.5) .........................................................................................................35 3.23
Arsenic removal with no idle (PS #3), one idle (PS #1) and 3 idles (PS #2). (A) As
effluent from GAC column. (B) As removal in steel chamber. (C) Filterable arsenic
leaving steel chamber. Solid line indicates where PS #2 idled for 7 days, dashed line
indicates where PS #1 idled for 7 days. All columns were operated at pH 6±0.3 .............38 3.24
Idling effect on Fe release. (A) Total Fe released from steel chamber. (B) Filterable
Fe released from steel chamber. (C) Fe effluent from GAC column. Solid line is
where PS#2 idled for 7 days on 3 occasions. D) Ferrous iron in filtered water released
from steel chamber. Dashed line is where PS#1 idled once for 7 days. ............................40 3.25
Scanning electron microscopy of: (A) Fresh precorroded steel sheets, (B) Aged
precorroded steel sheets, and (C) Steel sheets employed in PS#2, after use. (D) Steel
sheets employed in PS#4, after use. ...................................................................................41 3.26
Effect of pH, pre-corrosion, aging, and idling on: (A) Total Fe released from steel
chamber, (B) Filterable Fe released from steel chamber, (C) Fe accumulated in GAC
column (including preloaded iron).....................................................................................43 ©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
FIGURES
Figures | xi
3.27
X-ray diffraction patterns of the powdered rust collected from steel chamber after
runs PS#3 (no idling-top pattern); PS#3 (replicate-middle pattern) PS #2 ( thriceidled-bottom pattern). Peak designations: G = Goethite-α-FeOOH; M = Magnitite
Fe+2 Fe+3 On; W = Wustite FeO; H = Humboltine (hydrous ferrous oxalate); C =
Clinoferrosilite FeSiO3; As = Bis(arsenic acid)Hydrate (H3AsO4)2H2O; ZVI = zerovalent iron. .........................................................................................................................45 ©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
FOREWORD
The Water Research Foundation is a nonprofit corporation that is dedicated to the
implementation of a research effort to help utilities respond to regulatory requirements and
traditional high-priority concerns of the drinking water community.
The Arsenic Water Technology Partnership (AWTP) program is a partnership between
Water Research Foundation, Sandia National Laboratories (SNL) and WERC, a Consortium for
Environmental Education and Technology Development at New Mexico State University that is
funded by DOE and the Water Research Foundation. The goal of the program is to provide
drinking water utilities, particularly those serving small and rural communities, with costeffective solutions for complying with the new 10 ppb arsenic MCL. This goal is being met by
accomplishing three tasks: 1) bench-scale research to minimize operating, energy and waste
disposal costs; 2) demonstration of technologies in a range of water chemistries, geographic
locales, and system sizes; and 3) cost effectiveness evaluations of these technologies and
education, training, and technology transfer.
The AWTP program is designed to bring new and innovative technologies developed at
the laboratory and bench-scale to full-scale implementation and to provide performance and
economic information under actual operating conditions. Technology transfer of research and
demonstration results will provide stakeholders with the information necessary to make sound
decisions on cost-effective arsenic treatment.
The Foundation participates in the overall management of the program, helps to facilitate
the program’s oversight committees, and administer the laboratory/bench-scale studies. SNL
conducts the pilot-scale demonstrations and WERC oversees the education, training, economic
analysis, and outreach activities associated with this program.
Roy L. Wolfe, Ph.D.
Chair, Board of Trustees
Water Research Foundation
Robert C. Renner, P.E.
Executive Director
Water Research Foundation
xiii
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
ACKNOWLEDGMENTS
This report is the product of a collaborative effort between the members of the Arsenic
Water Technology Partnership and was made possible by funds from congress and the drinking
water community. A special thanks to U.S. Senator Pete Domenici for his support and assistance
in helping to bring low-cost, energy efficient solutions for the removal of arsenic from drinking
water.
The authors of this report are indebted to the following water utilities, companies, and
individuals for their cooperation and participation in this project:
Project Manager:
Hsiao-Wen Chen, WaterRF, Denver, CO
PAC Members:
Dr. Ganesh L. Ghurye,
Engineering Associate
ExxonMobil Upstream Research Company
Thomas J. Sorg,
Director, Research and Development
Drinking Water Treatment, Inorganics Control Technology
US Environmental Protection Agency, Cincinnati, OH
Dr. Bruce M. Thomson
Director of Water Resources Program
Regents Professor of Civil Engineering
University of New Mexico, NM
Southern Nevada Water Authority:
Shane Snyder.
Dave Rexing
Siemens Water Technologies:
James Graham
Douglas Gillen
Tim Peschman
Cool Sandy Beach Community Water System, Inc., Rutland, MA
Thomas C. Cook
xv
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
EXECUTIVE SUMMARY
The executive summary is the most influential part of the report. Besides inclusion in the
report, the executive summary will be posted as a separate document on the Foundation’s Web
site. It is crucial that this summary thoroughly covers the purpose, methods, and results of the
project, emphasizing the practical applications of the research. Write the executive summary in a
narrative style—leave the detailed data in the body of the report. Limit this summary’s length to
2−4 pages of text and include the following sections.
OBJECTIVES
The objectives of the study herein have been to:
1. Devise an innovative arsenic removal system that employs both corroding iron plus
iron-tailored GAC.
2. Test the arsenic removal and iron release as a function of practical operating
parameters such as pH, idle times, empty bed contact time (EBCT) and activated
carbon type.
3. Optimize the rate of iron release using electrolytic cells for arsenic removal and test
the effect of backwashing and plate cleaning.
4. Appraise the validity of our RSSCT design for arsenic by comparing the RSSCT
results with that from pilot columns.
5. Appraise the effect of filtering through pilot columns since rapid small scale column
tests are not designed to simulate filtering.
These objectives were addressed in part with rapid small scale column studies, and also
with pilot columns. The practical aim of this work has been to devise an arsenic removal
technology that would be useful for small water systems.
BACKGROUND
A new arsenic limit of 10 ppb became effective in 2006 for United States drinking water
systems. This new regulation would make small public water facilities face heavy financial
burdens, unless less costly methods of arsenic removal are developed. There is an urgent demand
for an economical, effective, and reliable technique that is capable of removing arsenic species to
this lower level. Arsenic in drinking water is of environmental concern because it is a
carcinogen. The arsenic dilemma is especially serious in Bangladesh and Bengal India, where
arsenic levels as high as 1 mg/L contaminate groundwater.
Our research focused on developing an arsenic removal system that couples the high pore
volume, structural cohesiveness, and low costs of granular activated carbon (GAC) with the
arsenic-sorbing propensity and low costs of iron. Our overall approach has been to couple ironimpregnated GAC with zero-valent iron sources to achieve an effective process for arsenic
removal in pilot columns. We compared virgin carbon with iron-tailored carbon; and coupled
these with solubilization of zero-valent iron that was either electrolytically-induced or non-
xvii
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xviii | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
electrolytic (i.e. passive). We anticipated achieving a useful bed life for removing arsenic in an
easy-to-operate adsorption column, while maintaining low costs.
APPROACH
The driving force of this research was to determine in pilot-scale how we could create
more effective adsorption sites for arsenic with a system that combines granular activated carbon
and zero-valent iron. In this system, corrosion products from the zero-valent iron (ZVI) media
offered a continuous source of dissolved or colloidal iron that could sorb arsenic. GAC provided
extra adsorption sites for arsenic when it was iron-tailored; it also worked as scavenger for the
Fe(II) and Fe(III) colloids and species that resulted from ZVI corrosion. Thus, the main focus of
this study is to enhance the iron solubilization and tailoring processes so as to render a robustness
that is adaptable to a wide array of water compositions.
RESULTS/CONCLUSIONS
This work has focused on several key activities in both RSSCT and pilot columns that
pertain to more favorably removing arsenic with activated carbons that are preloaded with iron,
while coupling this with the solubilization of zero-valent iron (ZVI). Specifically, this work has
discerned the following:
1. We have addressed several scale-up issues from RSSCT using our pilot columns.
RSSCTs offered promise that the concept of blending ZVI with iron-tailored GAC
offered considerably favorable results. RSSCTs are designed to simulate in small
scale the diffusion and mass transfer conditions that occur at full scale. However,
there are several phenomena that RSSCTs are not designed to simulate; and these
include filtration effects. Also, RSSCTs inherently do not provide simulation
information regarding what is the most favorable size of ZVI grains or wires to use
within a full-scale GAC bed. The filtering and ZVI surface area issues were appraised
herein via pilot columns.
2. Studies with iron tailored GAC plus passive iron solubilization (plain steel iron rod
installed in the carbon bed) were effective at removing arsenic. Bed volumes as high
as 52,000 were observed when iron-tailored carbon worked together with iron rod.
3. Iron-tailored carbon can play a significant role in arsenic sorption and filtering. This
is especially significant for carbon bed with passive solubilization since it may take
some time for the corrosion to get fully started. Most importantly, GAC provided
scavenger sites for excessive iron from corrosion.
4. Studies with electrolytic cells showed promising results for this system. The main
issue with electrolytic cells was clogging by excessive corrosion products and
accumulation of an impenetration iron hydr(oxide) barrier on the ZVI surface that
prevented yet further iron solubilization and arsenic removal.
5. With iron-tailored GAC, omparing results from RSSCT and pilot-scale indicates that
for arsenic, it is appropriate to assume proportional diffusivity in RSSCT design.
6. In RSSCTs, pre-corrosion in strong acid and aging created a more porous surface that
offered more contact area for corrosion to occur.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
EXECUTIVE SUMMARY | xix
7. Also in RSSCTs, idling created a more accessible surface for subsequent corrosion;
but the idling also reduced some Fe(III) hydr(oxide) solids to Fe(II) dissolved species,
which proceeds to the GAC bed. This likewise released some of the As that had been
sorbed to the Fe(III) flocs.
APPLICATIONS/RECOMMENDATIONS
Our system with branched rod and iron tailored media could achieve 52,000 bed volumes
via technically viable operations. Moreover, with the granular carbon sheath around the fragile
hydrous ferric oxides, this media has a more favorable skeletal strength. Compared to
conventional Fe oxide media that are synthesized by the sol-gel process, the method for
preparing iron tailored GAC is more environmentally acceptable, cost-effective, and simple due
to less preparation steps and small uses of Fe precursor. In addition, the iron source used is
readily available, and no pretreatment was necessary to achieve a long bed life.
FUTURE AND ONGOING RESEARCH
If this research and development were to continue, the Penn State team would aim to
render this iron-tailored GAC–zero-valent iron combination to be yet more cost effective,
resilient, and robust. To achieve this, we would aim to explore the effect of surface area of the
zero-valent iron source (i.e. use thinner steel rods) and the effect of carbon type (lignite vs.
bituminous carbon). We would aim to optimize the surface area of iron mass that is actively
solubilizing, and is not coated with an impenetrable layer of iron-based scale.
Further, we would aim to gain more insight regarding how to maintain arsenic at very
low levels and maintain the appropriate rate of backpressure buildup when preceding an
electrolytic cell column with iron-tailored GAC. Also, in forthcoming tests, we would aim to
achieve higher iron efficiencies.
The experiments described herein have employed only tap water spiked with arsenic, and
forthcoming experiments would discern whether the same favorable effect can be achieved using
water of a variety of qualities. Our ultimate goal would be to have a system that is robust for a
range of water qualities.
RESEARCH PARTNERS
USEPA.
PARTICIPANTS
Southern Nevada Water Authority; Siemens Water Technologies; Cool Sandy Beach
Community Water System, Rutland MA.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
CHAPTER 1
INTRODUCTION
BACKGROUND
In recent years, arsenic contamination of groundwater has emerged as a major concern on
a global scale. Lifetime exposure to arsenic in water can cause cancer to the liver, lung, kidney or
bladder on consumption of 1L/day of water at 50 µg/L arsenic level (USEPA 2001). Because of
this concern, the WHO in 1993 and USEPA in 2001, lowered the arsenic standard from 50 µg/L
to 10 µg/L; and the USEPA has dictated that all United States public water systems must comply
with a new 10 µg/L standard as of January 1st 2006. In initial projections, USEPA and AwwaRF
had estimated the costs to meet this MCL to be $102 to 550 million per year (Frey 2000; USEPA
2001). The work herein addresses a system that employs iron-tailored GAC coupled with
solubilization of zero-valent iron; and this has been appraised particularly for very small systems.
ARSENIC REMOVAL TECHNOLOGY
Prior studies have revealed that iron (III) has a high affinity and selectivity toward
inorganic arsenic species. For example, modified conventional iron coagulation and filtration can
be cost effective for larger municipalities (Chen et al. 1999); but such treatment may not be
practical for small and very small water utilities, which commonly employ simple well head
treatment systems. Thus there has been an urgent need to devise simple arsenic removal systems
that are suitable for small utilities.
A number of authors have applied zero-valent iron (ZVI) for removing arsenic (Bang et
al. 2005; Karschunke and Jekel 2002; Leupin et al. 2005; Nikolaidis, Dobbs, and Lackovic,
2003; Su and Puls 2003). For example, Nikolaidis, Dobbs, and Lackovic (2003) conducted pilot
tests with a ZVI filter that contained ZVI plus sand with a weight ratio of 1:1. At pH 6, this filter
removed arsenic from 294 µg/L down to 20 ug/L for 18,000–21,600 bed volumes (BVs).
However, such an integrated ZVI/sand filter released effluent iron as high as 70 mg/L
(Nikolaidis, Dobbs, and Lackovic (2003). With a separate sand filter that followed ZVI
corrosion, Bang et al. (2005) was able to control iron effluent to less than 0.3 mg/L; but this
added to system complexity. When a ZVI filter is used alone, the unit clogs with iron oxides, and
excessive iron exits from the unit.
Much research had focused on developing iron-based arsenic adsorbents such as granular
ferric hydroxide (GFH) (Driehaus, Jekel, Hildebrandt, 1998; Selvin et al. 2002; Westerhoff et al.
2005). The GFH can effectively remove both As (V) and As (III) from aqueous solutions, and
this media can be used for a wide range of pH (pH < 9). GFH is reported to have a high treatment
capacity of from 50,000−300,000 BVs to a 10 μg/L breakthrough level (Driehaus, Jekel,
Hildebrandt, 1998; Selvin et al. 2002; Westerhoff et al. 2005). However granular ferric oxide, as
a poorly crystallized β-FeOOH, is physically weak; and this media can crumble and disintegrate
when employed for prolonged use. Indeed, it has been recommended that GFH should be
operated with pressures under 10 psi so as to prevent the media from crumbling; and that GFH
columns should receive backwashing approximately every 3−4 weeks (Westerhoff et al. 2005).
Recent research has focused on creating inexpensive and stable iron bearing adsorbents,
like iron oxide coated sand (Gupta, Saini, and Jain, 2005), iron oxide impregnated activated
1
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
2 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
carbon (Chen et al. 2007, 2008; Jang, Chen, and Cannon, 2008; Gu, Fang, and Deng, 2005), and
iron tailored polymer (Cumbal and Sengupta 2005). The inherently simple features about
activated carbon are that GAC columns are easy to operate and very applicable to small water
systems. Moreover, the GAC grains will remain intact under considerable pressure; and GAC
has been used for decades to treat water.
PH EFFECT ON ARSENIC REMOVAL BY ZVI AND IRON (HYDR)OXIDES
The pH of the water, relative to the pHzpc of iron oxide/hydroxide is a critical factor for
arsenic removal by iron systems. For a specific surface, when pH < pHzpc, the surface tends to be
positively charged and will attract anions such as HAsO42- and H2AsO4-. Conversely, if pH >
pHzpc, the surface tends to be negatively charged and will electrostatically repel arsenic anions.
For Fe3O4, α-FeOOH, γ -Fe2O3, and amorphous Fe(OH)3, values of pHzpc = 6.5–8.5 have been
noted (Cornell and Schwertmann, 1996). The pKa values of arsenate species (As-V) are 2.2,
6.97, 11.53, while those values for arsenite species (As-III) are 9.29, 12.10 and 13.4. Thus,
arsenate removal by iron oxide/hydroxide is favored at acidic pH, while arsenite removal by
those media is favored at pH 7−8.
Mass transfer efficiency was found to play an important role in the removal of arsenic by
iron (hydr)oxides. In a ZVI filter, a scale as thick as 100 μm could be developed on the iron
filing surface over the course of 3 years (Kohn et al. 2005); and it was proposed that the As(III)
removal rate is most likely controlled by the rate of iron corrosion and the diffusion of As(III) to
adsorption sites in ZVI/iron oxides. Diffusion constraints have dictated that the iron scales and
colloids should be porous, so that arsenic mass transfer into these scales and colloids would not
be blocked (Kohn et al. 2005; Yu et al. 2006).
IRON CORROSION AND IRON RELEASE
Under anoxic conditions, iron corrosion can be dictated by auto-reduction reactions in
which ZVI is oxidized (partially) to Fe(II), as per the following Equations 1−4 (Kuch 1988;
Ritter et al. 2002; Ritter et al. 2003). Most Fe(II) species are soluble. Also, Fe(II) can convert to
Fe(II-III) in the form of magnetite (Equation 5); and this can be insoluble. The iron scale formed
by such dissolution/precipitation processes are claimed to be porous (Odziemkowski et al. 1998);
and the authors herein propose that this can be favorable for arsenic sorption (see below).
Fe2O3 + Fe + 6H+ = 3Fe2+ +3H2O
(1)
2FeOOH + Fe + 2H+ = 3Fe2+ + 2H2O
(2)
Fe + 2H2O = Fe2+ + 2OH- + H2
(3)
(4)
Fe2+ + 2OH- = Fe(OH)2
(5)
12Fe2O3 + 3Fe = 9Fe3O4
Moreover, when aerobic conditions prevail at neutral pH, iron reactions can be governed
by the oxidation of ZVI or Fe(II) to Fe(III); and this yields insoluble particles and scales, as per
Equations 6 and 7:
4Fe + 3O2 + 2H2O = 4 Fe(OOH)
4Fe (OH)2 + O2 + 2H2O = 4Fe(OH)3
(6)
(7)
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INTRODUCTION | 3
For a fresh iron surface, the amount of iron released is correlated to the rate of iron
corrosion; whereas after the surface has been covered by corrosion products such as FeOOH and
Fe(OH)3, the amount of iron released could depend upon such parameters as dissolved oxygen
concentration, pH, carbonate / hardness presence. It can also depend upon the rate of diffusion of
reactant and product species through the iron scale or colloids. Higher pH favors the formation of
an impervious passive iron (hydr)oxide layer on the ZVI (Baylis 1926); and this layer can retard
iron corrosion by blocking electron transfer and mass transfer through the layer, thus decreasing
iron release (Karalekas, 1983).
Aged carbon steel and gray cast iron pipes in water distribution systems are generally
covered by layers of scales that formed by corrosion of the iron pipes; and they are claimed to
release colored water that can contain iron above 0.3 mg/L (Sarin et al. 2003; Sarin et al. 2001;
Sarin et al. 2004). Such iron was released by (a) the corrosion of iron metal, (b) the dissolution of
ferrous components of the scales, and (c) hydraulic scouring of particles from the scales (Sarin et
al. 2003). Iron release could be greatly reduced as the corrosion deposits accumulated (Smart,
Blackwood, and Werme, 2002).
IRON CORROSION BY ELECTROLYTIC CELLS
Iron or alum coagulation via electrolytic metal solubilization is an emerging water
treatment technology that has been applied successfully to treat various wastewaters. It has been
applied for treatment of potable water, heavy metal laden wastewater, restaurant wastewater, and
pulp and paper mill wastewater (Vik et al. 1984; Holt et al. 2002; Mills 2000). The advantages of
electrolytic cells over conventional technologies include high removal efficiency, a compact
treatment facility, and the possibility of complete automation. Electrolytic cells also offer the
possibility of anodic oxidation, and the in-situ generation of adsorbents (such as hydrous ferric
oxides, hydroxides of aluminum). Operating conditions for electrolytic solubilization are highly
dependent on the chemistry of the aqueous medium, especially its conductivity and pH. Hansen,
Nunez, and Grandon (2006) observed that arsenic can be removed effectively from smelter
industrial wastewater through EC. Parga et al.(2005) demonstrated the removal of Cr(VI)/Cr(III)
and As(III)/As(V) with an efficiency of more than 99% from both wastewater and wells.
Balasubramanian and Madhavan (2001) reported that the efficient removal of arsenic takes about
7 hours; and the rate of arsenic removal by the electrolytic solubilization technique depends on
the initial arsenic concentration.
In work concurrent to this, Lakshmanan, Clifford, and Samanta (2010) conducted batch
experiments with electrolytic cells for arsenic removal. These experiments employed a liter-sized
batch system that included either iron rods, or graphite rods and stainless steel rods; and Fe+2
corroded from these. Arsenic-spiked challenge water was recirculated through the electrolytic
cell chamber for iron generation times of 60 or more seconds. Then, the coagulated water was
filtered through an 0.2 micron filter paper. 65−100% As(V) removal was achieved when the pH
was 6.5 to 8.5, with the more favorable removal occurring at the lower pH. In comparison, our
Penn State team conducted pilot studies that included either passive iron rods or electrolytic iron
plates (see discussion below). In the case of the electrolytic iron plates, water passed-through the
electrolytic chamber directly into the iron-tailored GAC media. Thus, the Penn State set-up,
combination of unit operations, and the nature of monitoring BVs to breakthrough at Penn State
was considerably different than that of Lakshmanan, Clifford, and Samanta.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
4 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
RESEARCH OBJECTIVES
The objectives of the study herein have been to:
1. Devise an innovative arsenic removal system that employs both corroding iron plus
iron-tailored GAC.
2. Test the arsenic removal and iron release as a function of practical operating
parameters such as pH, idle times, empty bed contact time (EBCT) and activated
carbon type.
3. Optimize the rate of iron release using electrolytic cells for arsenic removal and test
the effect of backwashing and plate cleaning.
4. Appraise the validity of our RSSCT design for arsenic by comparing the RSSCT
results with that from pilot columns.
5. Appraise the effect of filtering through pilot columns since rapid small scale column
tests are not designed to simulate filtering.
These objectives were addressed in part with rapid small scale column studies, and also
with pilot columns. The practical aim of this work has been to devise an arsenic removal
technology that would be useful for small water systems.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
CHAPTER 2
MATERIALS AND METHODS
MATERIALS
Water Sources for Pilot Columns and rapid small-scale column test (RSSCT)
In pilot columns, the arsenic feed solution was spiked into each column via an HPLC
pump. The arsenic feed solution was deionized water that was spiked with about 0.21 g
Na2HAsO4.7H2O / L, and this created a feed solution that contained about 50 mg As(V)/L. Then,
1.2 mL/min of this feed solution was combined with tap water (at 1 L/min) to make a targeted
arsenic concentration for the pilot column influent of 50–60 ppb.
In some of the RSSCTs, arsenic-containing groundwater was used. Table 2.1 is the water
quality of the groundwater.
Table 2.1
Water quality characteristics of the groundwater used in this study
Cations
Calcium (Ca)
Magnesium (Mg)
Sodium (Na)
Potassium (K)
Iron (Fe)
Manganese (Mn)
Aluminum (Al)
Zinc (Zn)
Other
parameters
pH
Turbidity
Conductivity
Concentration
59
11.3
27.5
4.2
< 0.003
0.003
<0.006
0.004
Units
Anions
mg/L CaCO3
mg/L CaCO3
mg/L CaCO3
mg/L CaCO3
mg/L
mg/L
mg/L
mg/L
Bicarb (HCO3)
Fluoride (F)
Chloride (Cl)
Nitrate (NO3)
Phosphate (PO4)
Sulfate (SO4)
Silica (SiO2)
Arsenic*
Concentration
64.2
0.670
9.32
0.041
< 0.080
26.4
12.5
47−55
mg/L CaCO3
mg/L
mg/L
mg/L
mg/L
mg/L
mg/L
μg/L
70.30
0.851
3.6
mg/L CaCO3
mg/L
mg/L CaCO3
Units
Other parameters†
7.4−7.6
0.08
165
NTU
µS
Total Hardness
TOC (C)
Free CO2
* 25% of arsenic was As(III) and 75% of arsenic was As(V), when measured from a 55
gallon barrel of freshly collected water as it arrived at Penn State.
† Dissolved oxygen content was 4–6 mg/L for this groundwater, as used in the Penn State
lab.
(Source: Reprinted from Water Science and Technology, 61.2 pp 441-453, with permission from
the copyright holders, IWA Publishing)
Activated Carbons
AquaCarb, a bituminous based granular activated carbon (GAC), was US mesh # 20 × 50
in size (850 × 300 µm). This carbon was used in 18 of the 20 pilot columns operated. UltraCarb,
a more mesoporous bituminous GAC, was US mesh #12 × 40 mesh in size (1700 × 425 µm); and
this was used in pilot Column #10. Both were obtained from Siemens Water Technologies. We
used activated carbon HD4000 (lignite, US mesh #12 × 40 mesh in size) in Column #15 to test
5
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
6 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
the effect of carbon type on arsenic removal. RSSCTs used AquaCarb or UltraCarb that was
ground down to a mesh size of 200×400.
METHODS
Iron Tailoring by Iron-salt Evaporation
The method of iron tailoring used was adopted from our earlier work (Chen et al. 2008).
For previous rapid small scale column tests, we added 2 g of carbon to 200 mL DI water solution
that contained 2 g of Fe(NO)3·9H2O. To achieve an iron oxide impregnation on activated carbon,
we heated the preloaded media at 50, 60, 80 or 100ºC until dry, cooled at room temperature,
washed with distilled water, dried and sieved. The carbon employed in those RSSCT
experiments was UltraCarb (US mesh #200×400, corresponding to 75 × 38 microns).
For the pilot-scale column tests discussed herein, we mostly followed this same protocol
and tailored carbon at 60 or 100oC (as designated below). The one exception is that we used a
smaller Fe(NO)3·9H2O-to-carbon ratio for our pilot-scale media. Specifically, we added 0.5 kg of
Fe(NO)3·9H2O to 2 kg of activated carbon. The reason for a smaller iron:carbon ration is that
each pilot column contains about 2 kg of carbon. At a 1:1 ratio, we need a much larger amount of
Fe(NO)3·9H2O. The results indicate that when the amount of Fe(NO)3·9H2O decreased from 2
kg/column to 0.5 kg/column, we achieved nearly the same amount of iron loading. Specifically,
in the previous small-scale tests, with an iron:carbon ratio of 1:1, we observed iron loadings of
around 12%. In comparison, for pilot-scale media with this iron:carbon of 0.25:1, the iron
loading amounted to 10.9 to 11.2%.
Rapid Small-Scale Column Tests (RSSCTs)
RSSCTs were conducted with both arsenic-containing groundwater and arsenic-spiked
PSU tap water to evaluate the GAC’s arsenic adsorption capacity in the lab. These tests were
designed to simulate the adsorption conditions that would occur in a full-scale bed. The tests
were conducted with GACs of grain size US mesh #200×400 (75–38 µm) and an empty bed
contact time (EBCT) of 0.53 min, which with proportional diffusivity (Crittenden, Berrigan, and
Hand, 1986) would simulate a full-scale operation with EBCT of 10 min when employing a grain
size US mesh #12×40 (1700–425 µm); or this would simulate an EBCT of 5.4 min when
employing full-scale grains of US mesh #20×50 (850−300 µm). The columns used were 13.5 cm
long and 0.5 cm in diameter. Each test held about 1.67 g of tailored carbon.
Duplicates were run for RSSCT with iron-tailored carbons using the groundwater, which
contained about 55–60 µg/L of arsenic. The native pH range of this water ranged from 7.6–8.0,
and the pH was unaltered in these RSSCTs. The arsenic-containg groundwater was the water
source for all our RSSCTs during all our AwwaRF work, unless specifically listed. We also
conducted RSSCTs with As-spiked PSU tap water when we wanted to compare these two
RSSCT conditions with those from pilot-scale columns. Due to the large amount of water
consumed by pilot-scale columns, we opted to use As-spiked tap water for all pilot columns. Tap
water was spiked to an arsenic concentration similar to that of the groundwater (55–60 µg/L).
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
MATERIALS AND METHODS | 7
Zero-Valent Iron (ZVI) and Iron-Loaded GAC for As Removal in RSSCTs
The effect of ZVI in combination with iron-loaded GAC for arsenic removal by RSSCT
was tested. For iron-loading, Ultracarb GAC was mixed with a citric acid plus ferric chloride
((FeCl3·6H2O)) solution and then agitated on a shaker table at 100−120 RPM for 2−3 days. The
iron tailored GAC was filtered, washed with distilled water until the wash water was clear, dried
at 104ºC overnight, and stored in desiccators before use.
Two ZVI materials were used: the first of these was galvanized steel fittings which had
corroded while processing groundwater before this use and perforated steel sheets (from
McMaster). The second was steel sheets, which were low carbon plain steel. These had a
thickness of 0.5 mm and 0.6 mm diameter holes that netted a total opening area of 23%. The
steel sheets were cut into 0.5−0.6 (±0.2) mm × 0.5−1.2 (±0.2) mm pieces before use.
Iron Pre-Corrosion, Aging, and Idling in RSSCTs
Most perforated steel sheets were pre-corroded before use. This was achieved by soaking
the ZVI materials in 1 M nitric acid + 8 % oxalic acid for 1 to 6 days. After the pre-corrosion, the
perforated steel pieces were washed with DI water until the wash water pH exceeded 5.5. This
pre-corroded ZVI was in some cases further treated by soaking in 50 – 80 ml deionized water
that was open to the air for 1−12 days, as specifically identified in Table 3.5. Galvanized steel
fittings were pre-corroded during prior use, as described above.
Idling involved discontinuing the groundwater from flowing through a GAC column for a
prescribed duration, typically 7−8 days. This idling incurred anoxic conditions within the GAC
column and perforated steel chamber, as no new oxygen entered during the idle time.
Pilot Columns
Pilot columns were produced from 5’ long sections of nominal 3” schedule 80 clear PVC
pipes (inside diameter of 2.875”). The columns were packed with three 2” layers of aquarium
gravel (US mesh #6 × 40). The diameter of the gravel in each layer was progressively smaller. A
2” layer of 1 mm sand was added atop the gravel. The gravel and sand layers served two
purposes. These layers prevented the activated carbon from exiting the bottom of the bed, and
the depth of these layers helped to ensure that there would be no preferential flow through the
bottom of the activated carbon as might occur if the activated carbon was located very near the
exit of the column (the 2.875” diameter column narrows to a ¼” exit port). Activated carbon
with a bed depth of 3’4” was then added atop the sand. End caps, attached with PVC cement,
were schedule 80 and the caps were machined to accommodate ½” straight thread connectors
with o-rings. The activated carbon was backwashed by fluidizing the carbon until no fines were
observed exiting the top of the column.
A photograph of these pilot columns is shown as Figure 2.1. Table 2.2 lists the conditions
used in all the pilot columns.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
8 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
Figure 2.1 Photograph of pilot-scale columns
Column #1 was the control column with virgin AquaCarb. In Column #2, 0.445 kg of
steel mesh was placed atop a bed of virgin AquaCarb. Columns #3 and 4 were columns for irontailored AquaCarb. In Column #5, 2 plain steel rods were used as the iron source. These rods ran
the full length of the carbon bed and were placed on a line that bisected the column’s cross
sectional area, with each rod located ¾” from the column wall.
We also set up pilot-scale studies with electrolytic cells that achieve the electrolytic
solubilization of a zero-valent iron source. Columns #6 to #10 were columns with electrolytic
cells at the top of the activated carbon. Column #6 acted as a control column with no electricity.
Electrolytic cells were comprised of gray cast iron plates that were machined into a 2 ¾” solid
PVC rod that fit snugly into the pilot column.
The cell had one central anode (positive) plate and was flanked by two cathode (negative)
plates. The anode measured 5” in length and was 1 ¾” wide. The cathode plates measured 5” in
length and 1 5/8” in width. All plates had a thickness of 7/16”. The spacing between the plates was
1
/8”. One inch tall spacers were attached to the bottom of the cell to provide clearance between
the cell and the top of the carbon bed. This was
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
MATERIALS AND METHODS | 9
Table 2.2
Pilot-scale columns operational conditions
Column
1
2
Carbon tailoring protocol
1.92 kg of virgin AquaCarb
1.99 kg of virgin AquaCarb
3
5
1.92 kg of iron tailored
AquaCarb by evaporation
method at 60°C (11% Fe)
2.14 kg of Iron tailored
AquaCarb by evaporation
method, 100°C (11% Fe)
2.14 kg of virgin AquaCarb
6
2.08 kg of virgin AquaCarb
7
1.98 kg of virgin AquaCarb
8
2.17 kg of iron tailored
AquaCarb by evaporation
method at 60°C
2.17 kg of iron tailored
AquaCarb by evaporation
method at 60°C
2.06 kg of 12 × 40 mesh iron
tailored UltraCarb
2.20 kg of virgin AquaCarb
4
9
10
11
12
2.15 kg of iron tailored (at
60°C) AquaCarb
13
2.10 kg of virgin AquaCarb
14
1.90 kg of virgin HD4000
15
2.01 kg of iron tailored (at
60°C) AquaCarb
16
2.16 kg of iron tailored (at
60°C) AquaCarb
17
2.09 kg of virgin AquaCarb
18
2.18 kg of iron tailored (at
60°C) AquaCarb
2.20 kg of iron tailored (at
60°C) AquaCarb
2.17 kg of iron tailored (at
60°C) AquaCarb
19
20
Iron sources
Nothing solubilizable
0.445 kg plain steel mesh upstream of carbon, mesh surface area 125
cm2/g
no
no
0.420 kg plain steel iron rods configured vertically within the activated
carbon bed, rod surface area 0.86 cm2/g
1.22 kg of gray cast iron plate preceding activated carbon, rod surface
area 0.36 cm2/g
Electrolytic, 1.22 kg of gray cast iron plate preceding activated carbon,
plate surface area 0.36 cm2/g
Electrolytic, 1.23 kg of plain steel plate preceding activated carbon, plate
surface area 0.36 cm2/g
Electrolytic, 1.22 kg of plain steel plate preceding activated carbon, plate
surface area 0.36 cm2/g, 2× flow rate
Electrolytic release, 1.22 kg of plain steel plate preceding activated
carbon, plate surface area 0.36 cm2/g
0.420 kg plain steel iron rods with branches placed vertically within the
top 2/3 of the activated carbon bed, branched rod surface area 0.91cm2/g
0.420 kg plain steel iron rods with branches placed vertically within the
first 2/3 of the activated carbon bed, branched rod surface area 0.91
cm2/g
0.420 kg plain steel iron rods with branches placed vertically within the
first 2/3 of the activated carbon bed, branched rod surface area 0.91
cm2/g
0.420 kg plain steel iron rods with branches placed vertically within the
first 2/3 of the activated carbon bed, branched rod surface area 0.91
cm2/g.
0.420 kg plain steel iron rods with branches placed vertically within the
first 2/3 of the activated carbon bed, branched rod surface area 0.91
cm2/g
0.420 kg plain steel iron rods (1/8”) with branches placed vertically
within the first 2/3 of the GAC bed, branched rod surface area 3.25 cm2/g
Electrolytic, 1.19 kg of gray cast iron plate preceding activated carbon in
a separate vessel, plate surface area 0.36 cm2/g, 0.02A constant current
Electrolytic, 1.20 kg of gray cast iron plate preceding activated carbon,
0.02A constant current, plate surface area 0.36 cm2/g
Electrolytic, 1.20 kg of gray cast iron plate preceding GAC, plate surface
area 0.36 cm2/g, 0.02 A constant current, influent water pH 8.0
Electrolytic, 1.20 kg of gray cast iron plate preceding activated carbon,
plate surface area 0.36 cm2/g, 0.01 A constant current
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
10 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
Figure 2.2 Electrolytic solubilization cells
done in an effort to prevent blockage of the narrow passage between the iron plates. A
photograph of an electrolytic cell is as shown Figure 2.2.
The electrolytic cells in Columns #7−10 were operated with a constant current of 0.1 A
(current density of 0.66 mA/cm2 with an anode surface area of 151.6 cm2). Previous tests by
others in our Penn State group had shown that this current density would result in an iron release
of 2.2 mg/L, when a flow rate of 13 gallons per hour (820 mL/min) was employed. The DC
power supplies for the electrolytic solubilization cells were from BK Precision, model 1627A.
Columns #11 to #15 were GAC beds that contained branched plain steel rods in the top
rds
2/3 of the carbon bed. Figure 2.3 offers a picture of the branched rods. The carbon beds were 3
feet deep, while the branched rod systems were located in the top 2 feet of the bed. The branched
rods in Column #11−15 had a diameter of 1/4”. The influent in Column #13 was adjusted to pH
7.9−8.1, so as to test the effect of pH. The pHs of Columns #11, 12, 14, and 15 were not adjusted
and remained at 7.1−7.4. Studies have shown that the rate of Fe2+ oxidation to Fe3+ becomes
accelerated at higher pH (Snoeyink and Jenkins, 1980). We are interested to see whether the
increased Fe2+ conversion significantly affected the arsenic removal.
The branched rod systems were supported by a 1 foot high structure that was made from
PVC and corrosion-resistant stainless steel (which did not occupy much space—i.e. most of the
bottom foot of bed was GAC grains). We expected that this structure would offer an iron source
that would be somewhat-evenly distributed across the carbon cross section. The design also
aimed to use the last 1/3rd of the carbon bed as a scavenger of the iron products that were
generated in the top 2/3rd of the bed; and thus prevent iron leaking through the bottom of the
media. This 2/3rd – 1/3rd configuration proved to be effective in RSSCTs during Phase I testing;
and it maintained effluent iron levels under 0.3 mg/L.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
MATERIALS AND METHODS | 11
Figure 2.3 Branched rod configuration used in Columns #11 to 15.
Column #16 utilized iron-tailored (at 60oC) Aquacarb, coupled with plain steel rods that
were 1/8” in diameter and 2” long. Twelve of these rods were arranged vertically in the top 2/3rds
of the carbon beds. These rods had the same iron mass as the rods used in Columns #11−15. As
in Columns #11−15, a stainless steel support was placed in the bottom 1/3rd of the bed. The 1/8”
rods were evenly spaced throughout the column; and they were held in place by #6 stainless steel
mesh screen that was secured to the rods by epoxy at the top, middle, and bottom of the rods.
Columns #17−20 included electrolytic cells. We made some changes relative to the
configurations that had been used in Columns #7−10. In Columns #7−10, the electrolytic cells
were placed directly on top of the activated carbon (i.e. within the same vessel). However, in
contrast, for Columns #17−20, the electrolytic cells were installed in a separate vessel that
preceded the activated carbon vessel. By using a separate column to house each electrolytic cell,
we hoped to get a more even distribution of iron into the GAC. When the cell sat directly atop
the GAC, the iron could only access a limited area of the GAC, as we had learned from the
Column #7 to 10 operations (see discussion below). In Column #17, 18 and 19, the new
electrolytic cells were operated with a constant current of 0.02 Amperes (A) (current density of
0.132 mA/cm2). Column #20 was operated at 0.01 A (current density of 0.066 mA/cm2) as
compared to a current of 0.1A in Column #7−10. The 0.01 A was the lowest current that could
be applied with the particular DC power supply that we employed. Under full scale operations,
this level of power supply would be the equivalent to one 60 watt light bulb—for water served to
several thousand people (see below). In Column #19, the influent water pH was also adjusted to
around 8.0 with NaOH.
For all columns, the initial pressure through the column was below the initial readings on
the gauge (<10 psi). A 10 psi of back pressure was applied to each column to prevent dissolved
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
12 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
air in the inlet water from forming into air bubbles as the water passed through the columns. The
inlet tap water was approximately 10oC. The flow rates changed somewhat for the Column
#7−10 studies when there was an extreme back pressure buildup. Specifically, the tap water inlet
provided a pressure of 70 psi. Once the back pressure surpassed 70 psi, the flow rate gradually
decreases. The columns were shut down when the flow rate was too low even if no arsenic was
detected.
The composition of the iron sources used in these columns can be found in Table 2.3, as
reported by the suppliers. All of these pilot columns have operated in the down-flow mode.
Table 2.3
Composition of zero-valent iron sources (percent %)
Plain steel rod and
mesh (1018)
Gray cast iron
plate (G2)
Fe
C
Mn
98.51−9
9.10
92.11−9
4.81
0.15−
0.20
2.60−
3.75
0.60−0.
90
0.60−0.
95
Cr
Ni
Mo
Si
P
S
none
none
none
none
none
none
0.15−0.
30
1.80−3.
00
0−0.0
4
0.−0.
07
0−0.0
5
0−0.1
2
Sampling Protocol and Chemical Analysis
Samples were taken every second day from the influent and effluent ports of the pilot
columns. These were stored in a refrigerator at 4oC until analysis. Arsenic analysis was
conducted via a Shimadzu atomic absorption spectrophotometer-hydride vapor generation
system (AAS–HVG) and later via inductively coupled plasma-mass spectrophotometry (ICP–
MS), or AA with a graphite furnace. Quantitatively, iron was analyzed by a Shimadzu Atomic
Absorption Spectrophotometer (AA-6601F) unit, with flame atomization (unless otherwise
specified). Also, qualitative iron levels were monitored with a HACH kit every time that an
effluent sample was also monitored for arsenic. This protocol had a detection limit of about 0.2
mg/L; and the pilot columns’ effluent that did not include electrolytic cells always exhibited
“non detect” iron per the HACH kit, except as specifically identified below. Influent and effluent
pHs were monitored by an Accumet model 10 pH meter. Column pressure, flow rate, and voltage
(for electrolytic columns) readings were taken on a daily basis.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
CHAPTER 3
RESULTS AND DISCUSSIONS
EFFECT OF TEMPERATURE ON IRON TAILORING BY EVAPORATION
At the beginning of this Phase II study, we decided to compare the effect of temperature
on iron-loading by evaporation. In Phase I, the evaporation was carried out with curing at boiling
temperature (i.e. 100oC). In the Phase II study herein, we appraised curing temperatures of 50,
60, 80 and 100°C. Rapid small scale column tests were then conducted employing the arseniccontaining groundwater. Arsenic concentration in this groundwater was stable at about 55 μg/L.
Figure 3.1 depicts the arsenic breakthrough profiles from these tests; and Table 3.1 summarizes
the iron contents and BVs to 10 μg/L arsenic breakthrough.
Figure 3.1 RSSCT using arsenic-containing groundwater, for iron-loaded GAC, with
preloaded iron curing at temperatures of 50−100°C. Y-axis µg/L As.
Overall, there was no significant difference in the amount of iron that could be preloaded
when employing curing temperatures from 60 to 100°C. The media cured at 60 or 80°C achieved
slightly longer BVs before 10 μg/L arsenic break through than those cured at 50 or 100°C. The
curing temperature has been linked to the iron hydr(oxide) character, with hydrous ferric oxide
predominating at 60oC (Jang et al. 2008).
13
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14 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
Table 3.1
Iron content and BVs to breakthrough for carbons that were iron-preloaded by the
evaporation method, with curing at 50−100°C
Temperature (°C)
50
60
80
100
Iron content (%)
12.0±0.4
12.4±0.4
12.0±0.2
11.7±0.3
Bed volumes to
25,000
29,000
31,000
25,000
breakthrough
RSSCT VS. PILOT-SCALE COLUMNS
In designing RSSCT columns, we had to assume a relationship between empty bed
contact times (EBCTs) in the RSSCT column and in the full-scale bed. The relationship is
contingent on the intraparticle diffusion coefficient, which in turn depends on particle size. If
intraparticle diffusivities do not change with particle size, then a constant diffusivity approach
can be used, and the ratio of the EBCTs is directly proportional to the squared ratio of the carbon
grain diameters. Alternatively, the proportional diffusivity method assumes that intraparticle
diffusivity is a linear function of particle size and that the ratio of the EBCTs is directly
proportional to the ratio of the carbon grain diameters. Specifically, proportional diffusivity is
described by EBCTSC/EBCTFC=DSC/DFC (Hand et al. 1989). In this equation, EBCT is the empty
bed contact time (volume of vessel / flow rate), SC is the small-scale column (either RSSCT or
mini-column), FC is the full-scale column being modeled, D is the geometric average size of the
GAC grains (microns). To determine which method is appropriate for the simulation of a specific
adsorbate, the comparison between the simulated breakthrough curve and the full-scale or pilotscale breakthrough curve must be performed. In our case, we had designed the RSSCTs on the
basis that proportional diffusivity similitude would be appropriate when considering ions the size
of arsenic oxyanions (such as AsO43-, HAsO42-, H2AsO4-, H3AsO3). In the pHs under this study,
HAsO42- and H2AsO4- are the dominant species of arsenate, while H3AsO3 dominates arsenite
species.
Figure 3.2 and 3.3 compare the breakthrough curves as discerned from RSSCTs versus
pilot columns. These experiments employed carbons that were iron- tailored with curing at 60oC
or 100°C. Table 3.2 summarizes the BVs to 10 µg/L breakthrough by tailored temperature in
RSSCT and pilot columns.
As shown by the Table 3.2 data, BVs to breakthrough by RSSCTs were slightly longer
than for pilot columns; but this could be attributed in part to distinctions in iron content: the
tailored carbons made for the pilot columns had an iron content around 11%; while those used in
RSSCTs have had an iron content of 11.7−12.4%. We used a smaller Fe:C ratio when tailoring
carbon for pilot columns (see discussion above). When the BV data is normalized to a uniform
iron content of 12.4 % Fe, the pilot columns achieved BVs to 10 µg/L As breakthrough that were
just 10% lower than the counterpart
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 15
30
As (ug/L)
25
20
15
10
5
0
0
10000
20000
30000
40000
50000
Bed Volumes
iron-tailored carbon (60oC)+Rutland water in RSSCT
iron-tailored carbon (60oC)+Rutland water repeat in RSSCT
iron-tailored carbon (60oC)+As-spiked tap water in RSSCT
iron-tailored carbon (60oC)+As-spiked tap water in pilot column
Figure 3.2 Arsenic breakthrough for iron-tailored (60oC) carbon in RSSCTs and Pilot
Column #3
30
As (ug/L)
25
20
15
10
5
0
0
10000
20000
30000
40000
50000
Bed Voluems
iron-tailored carbon (100oc)+Rutland water in RSSCT
iron-tailored carbon (100oc)+Rutland water repeat in RSSCT
iron-tailored carbon (100oc)+As-spiked tap water in RSSCT
iron-tailored carbon (100oc)+As-spiked tap water in pilot column
Figure 3.3 Arsenic breakthrough for iron-tailored (100oC) carbon in RSSCTs and Pilot
Column #4
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
16 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
Table 3.2
Bed volumes to 10 µg/L As breakthrough in RSSCT versus pilot columns with iron tailored
carbons
Typical
BVs to 10
BVs to 10
Column type
Water
Iron-curing
% Fe*
µg/L As
µg/L As BT
source
temp (oC)
Break(normalized
through
to 12.4% Fe)
RSSCT
Groundwater
60
12.4±0.4
26,200
26,200
RSSCT
Groundwater
100
11.7±0.3
25,500
27,000
RSSCT
As-spiked
60
12.4±0.4
28,200
28,200
PSU Tap
RSSCT
As-spiked
100
11.7±0.3
26,500
28,000
PSU Tap
Pilot column
As-spiked
60
11.0
22,600
25,500
#3
PSU Tap
Pilot column
As-spiked
100
11.0
22,800
25,700
#4
PSU Tap
* % Fe as observed previously or analyzed in split-samples when employing this tailoring
protocol.
values with RSSCTs (see last column of Table 3.2). The authors note that the GAC grains
for the pilot columns had sizes of #20×50 (US standard mesh), as compared to 200×400 for the
RSSCTs. During tailoring, the smaller particle sizes may have helped iron diffuse relatively
further into the individual carbon grains and thus increase overall surface availability of the
preloaded iron. In light of this data and perspective, we discerned that these results indicated that
the proportional diffusivity similitude offers quite a good match to operations with full-scale
GAC grains when monitoring the sorption of arsenate and arsenite in iron-tailored GAC (The
groundwater contained both arsenate and arsenite, per Table 2.1 above).
In addition, for both the pilot columns and RSSCTs, whether a thermal curing
temperature of 60oC or 100oC was used, this made little difference in the BVs to 10 µg/L As
breakthrough. Also, one notes that for the RSSCTs, the BVs to 10 µg/L breakthrough with Asspiked PSU tap water were about the same to those with the groundwater. It is noted that the
PSU tap water had a pH of 7.1−7.3 as compared to 7.6−8.0 for the groundwater. Our studies
have shown that arsenate adsorption in iron-tailored carbon is closely related to pH (Chen et al.,
2008; Jang, Chen, Cannon, 2008). In a pH range of 6−8, the lower the pH, the higher the
adsorption capacity for arsenate. Since there is very little difference in BVs to breakthrough in
RSSCTs or in pilot columns by tailoring temperature, we selected 60oC as the tailoring
temperature for the remainder of our pilot-scale columns where iron-tailored carbon was
employed.
PILOT-SCALE STUDIES WITH ZERO-VALENT IRON ROD AND GAC
In pilot columns, we used plain steel mesh or rods as the iron source. These materials
were easy to install and readily available. No pretreatment was carried out before they were
installed.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 17
As listed in Table 2.2, Column #1 was operated with virgin AquaCarb as a control.
Column #2 had a zero-valent iron source in the mesh shape. Figure 3.4 shows the breakthrough
curves for Column #1 and #2. As shown, the virgin AquaCarb had very limited adsorption
capacity for arsenic. Arsenic breakthrough occurred at about 900 BVs.
70
As in influent
60
As (ppb)
50
40
pilot column 1: virgin Ultracarb
30
20
pilot column 2: plain steel mesh+virgin Ultracarb
10
0
0
5000
10000
Bed Volumes
15000
20000
Figure 3.4 As breakthrough for pilot columns with virgin AquaCarb and plain steel mesh +
virgin AquaCarb
Although no arsenic was detected for the duration of Column #2 (steel mesh upstream of
virgin GAC), the back pressure in this column reached such a level that we had to terminate the
operation prematurely since little water was able to pass through at this pressure. Specifically,
with the development of iron corrosion, there was a gradual increase in back pressure from 10 to
55 psi after 12,000 BVs (Figure 3.5). The high pressure buildup is attributed to the large amount
of iron corrosion products that deposited at the top of the carbon bed; and these corrosion
products were very fine particles that essentially clogged up the pathway between carbon
particles. Besides the problem with back pressure, Column #2 also shows iron in effluent
constantly higher than 0.3 mg/L as shown in Figure 3.6. This is problematic since the secondary
drinking water for iron is 0.3 mg/L. In comparison, for the Column #1 run that included no
solubilizable iron source, there was no pressure buildup (Figure 3.5).
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
18 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
60
pilot column 1: virgin AquaCarb
Back pressure (psi)
50
pilot column 2: virgin AquaCarb+plain steel
mesh
40
30
20
10
0
0
5000
10000
15000
20000
Bed Volumes
Figure 3.5 Back pressure buildup for Columns #1 and 2
1.2
Iron (mg/L)
1
pilot column 2: virgin AquaCarb+plain steel
mesh
0.8
0.6
0.4
0.2
0
0
1000
2000
3000
4000
5000
6000
7000
Bed Volumes
Figure 3.6 Iron concentration in effluent from Column #2
Column #5 and #11−15 were set up as potential solutions for both the back pressure and
iron leaching problems. In Column #5, two plain steel rods were installed vertically throughout
the length of the carbon bed. As comparison, in Column #11 to 15, a branched rod with the same
mass was installed, but only within the first 2/3 of the bed.
In these runs, we compare the effects of several variables: (a) operation at native pH
(7.1−7.3) versus operation at pH 7.9−8.1; (b) bituminous-based AquaCarb versus lignite-based
HD4000, (c) empty bed contact time of 5 minutes versus 2.5 minutes, (d) ¼” diameter rods
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 19
versus 1/8” diameter rods. In order to achieve these comparisons, we operated the pilot units as
follows: Column #11 contained iron-tailored GAC that was cured at 60oC with an iron content
around 11%. The influent pH of Column #13 was adjusted to 7.9−8.1. Column #14 employed
HD4000 carbon. Column #15 used iron-tailored AquaCarb and an empty bed contact time 2.5
min together with a branched rod. Column #16 used 1/8” rods, so as to create a higher exposed
iron source surface area. The total surface area of the 1/8” rods was 3.25 m2/g vs. 0.91 m2/g of
that for ¼” diameter.
Figures 3.7 and 3.8 are the As breakthrough for these columns and Table 3.3 summarizes
the results.
35
Effluent As (ug/L)
30
25
20
15
10
5
0
0
10000
20000
30000
40000
50000
60000
Bed Volumes
pilot column 5: virgin AquaCarb+plain steel rod
pilot column 11: iron-tailored carbon (60oC)+branched plain steel rod
pilot column 12: virgin AquaCarb+ branched plain steel rod
Figure 3.7 As breakthrough for pilot columns with straight and branched rods (rod
diameters ¼ inch)
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
20 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
Effluent As (ug/L)
20
15
10
5
0
0
10000
20000
30000
40000
50000
60000
70000
Bed Volumes
pilot column 13: virgin AquaCarb+ branched steel rod, pH 7.9-8.1
pilot column 14: HD4000+branched plain steel rod
pilot column 15: iron-tailored AquaCarb+branched plain steel rod, EBCT=2.5
pilot column 16: iron-tailored AquaCarb+1/8" branched steel rod
Figure 3.8 As for pilot columns, while comparing the effect of pH, EBCT, iron-tailored
carbon and smaller branched rod (rod diameters ¼ inch, unless otherwise listed)
The most favorable result was for Column #16, where iron-tailored activated carbon was
coupled with 1/8” diameter steel rods that were placed in the top 2/3 of the GAC bed. In this
case, the pilot column processed 52,000 BVs before 10 µg/L As was consistently reached. This
compared to 43,000 BVs when coupling ¼” steel rods with iron-tailored GAC (Column 11), and
to 23,000 BVs when using just iron-tailored GAC (columns 3 and 4). In further comparison,
when mere virgin Aquacarb was coupled with ¼” steel rods (Column 12), breakthrough occurred
at 36,500 BVs (i.e. 6,500 BVs less). This indicated that iron-tailored AquaCarb did play a
significant role in adsorbing arsenic. Moreover, the combination of the iron-tailored AquaCarb
plus branched rods offered a double barrier against arsenic breakthrough.
Column #12 contained virgin AquaCarb and operated at native pH, while the Column
#13 run operated at a pH of 0.5−1 units higher than native. As a result, Column #13 had arsenic
breakthrough at 31,100 BVs, which was 5,400 BVs lower than that of Column #12. This is in
accordance with our previous studies, which showed that in a pH range of 6−8, the lower the pH,
the higher the adsorption capacity for arsenate (Chen et al., 2008; Jang, Chen, and Cannon,
2008).
When comparing the combination of virgin AquaCarb plus ¼” rods versus virgin
Hydrodarco 4000 plus ¼” rods, it is observed that the HD4000 source was more favorable, as 10
µg/L As breakthrough occurred at 37,000 BVs when the HD4000 was included.
Columns #11 and #15 were compared to evaluate the effect of EBCT, while using irontailored GAC and ¼” steel rods. When EBCT changed from 5 to 2.5 min, BVs to breakthrough
dropped from 43,000 to 35,000. Longer EBCT provides more time for arsenic to diffuse into the
carbon’s pore structure thus offering a higher removal efficiency.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 21
Table 3.3
Bed volumes to 10 µg/L As breakthrough in pilot columns with zero-valent iron rods and
either iron-tailored GAC or virgin GAC. For Column 5, the rods reached all the way to the
column’s bottom. For all others, the rods remained in the top two-thirds of the media
ColpH
Carbon type
EBCT
Iron rods
BVs to 10
Ave As before
umn
µg/L As
10 µg/L
Break-through breakthrough
start (µg/L)
3
7.1−7.
Iron tailored
5
none
22,600
3.1
4
AquaCarb
(60oC)
5
none
22,800
2.5
4
7.1−7.
Iron tailored
4
Aquacarb
(100oC)
5
7.1−7.
virgin
5
1/4” straight to
See footnote* See footnote*
4
AquaCarb
bed bottom
5
1/4” branched;
43,000
3.0
11 7.1−7.
iron-tailored
top 2/3 bed
4
AquaCarb
(60oC)
12 7.1−7.
virgin
5
1/4” branched;
36,500
2.7
4
AquaCarb
top 2/3 bed
13 7.9−8.
virgin
5
1/4” branched;
31,100
3.6
1
AquaCarb
top 2/3 bed
14 7.1−7. virgin HD4000
5
1/4” branched;
37,000
4.0
4
top 2/3 bed
15 7.1−7.
Iron-tailored
2.5
1/4” branched;
35,000
6.5
4
AquaCarb
top 2/3 bed
(60oC)
16 7.1−7.
Iron-tailored
5
1/8” rod; top 2/3
52,000 BVs
3.7
4
AquaCarb
bed
(60oC)
* Consistent continuous breakthrough at 29,500 BVss. This followed release of 12−22 µg/L As
at 1000−4000 BVs.
Column #5 exhibited breakthrough at 1,000−4,000 BVs, and then 29,500 BVs. The initial
breakthrough may have been because of the uneven distribution of iron corrosion products in the
beginning stages of the test with these vertical rods. Arsenic may have leaked through the bed
from locations that did not possess much iron (especially under plug flow conditions). A
configuration with the branched rod had resolved this issue, and resulted in considerably less
arsenic leakage during the first 2,000 BVs.
Effluent iron concentrations were monitored via a Hach kit for all these columns. When
the straight rods reached all the way to the bottom of the media (Column #5), effluent iron
remained above 0.3 mg/L for most of the run (Figure 3.9). However, in contrast, when the rods
were placed in the top two thirds of the GAC media (Column #11−15), effluent iron
concentrations constantly remained under 0.2 mg/L for the duration of the operation, as
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
22 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
determined from these weekly semi-quantitative Hach kit measurements. Thus, results were
favorable when the bottom 1/3 of the carbon served as a filter/sorber of the corroded iron
products.
1.2
pilot column 5: virgin AquaCarb+ plain steel rod
Effluent iron (mg/L)
1
0.8
0.6
0.4
0.2
0
0
10000
20000
30000
40000
Bed Volumes
Figure 3.9 Iron concentration in effluent for Column #5, with straight rods that extended
through the full media depth
PILOT-SCALE STUDIES WITH ELECTROLYTIC SOLUBILIZATION AND IRONTAILORED GAC
The authors also appraised electrolytic solubilization with current amperage ranging from
0.01−0.1 A. Overall, we ran eight pilot columns with electrolytic cells. Columns #7−10 were
operated first, with a constant current of 0.1 A, and this amount of current was soon found to be
too much, as it incurred so much release of iron that the beds experienced extreme clogging and
pressure buildup within 2,500 BVs. For these runs, the electrolytic solubilization occurred in the
region that was immediately above the GAC beds and in the same chamber as the GAC beds (see
discussion of Columns #7−10 below in Section 3.4.2). On the basis of what was learned from
these, we operated Columns #17−20 with a smaller current of 0.01 or 0.02 A; and our aim was to
solubilize less iron and incur less pressure loss. For the sequence of the report herein, we will
first discuss the results when using 0.01 or 0.02 A.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 23
Pilot-Scale Studies with Electrolytic Solubilization Achieved with 0.01 or 0.02A, Plus IronTailored GAC
Electrolytic Solubilization pilot studies were conducted while using a current across the
electrolytic cells of 0.01 or 0.02 A and these have been identified as Columns #17−20 herein.
The voltage drop across these cells ranged from 1 volt (V) initially up to 5 V eventually, when an
iron (hydr)oxide scale had built up between the ZVI plates. For these runs, we placed the
electrolytic solubilization cell in a separate chamber that was above the GAC (see photos, Figure
3.10). This separate chamber configuration allowed for even distribution of iron colloid dispersal
over the full surface of the GAC media.
Figure 3.10 Electrolytic solubilization chamber
This was favorable, when compared to the uneven iron distribution that had occurred
when the electrolytic solubilization had occurred immediately above the GAC media (see
discussion in Section 3.4.2 for Columns #7−10 below).
On the basis of the 0.1 A trials (see discussion in Section 3.4.2 below), we anticipated
that 0.01 A would release about 200 µg/L iron, while 0.02 A would release about 400 µg/L iron.
Relative to the 50 µg/L arsenic, this represented an Fe:As mass ratio of 4:1 (in the first case) or
8:1 (in the latter case). In comparison, from earlier RSSCT tests, we had observed that an Fe:As
ratio as low as 6:1 was effective for removing arsenic) (see discussion in Section 3.5 below).
Also, the 0.01 A was the smallest current that our particular DC power unit could supply.
We also wanted to compare iron-tailored AquaCarb versus virgin AquaCarb, while also
appraising several pH’s. So Column #17 operated with virgin AquaCarb while using a 0.02 A
current at native pH. Column #18 operated with iron-tailored AquaCarb while using a 0.02 A at
native pH. Column #19 operated with iron-tailored AquaCarb, while using a 0.02 A current, at
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
24 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
an adjusted influent pH of 7.9−8. Column #20 employed 0.01 A of current and iron-tailored
AquaCarb at native pH (see Table 2.2 above).
When using virgin GAC and 0.02 A of current, 10 µg/L arsenic breakthrough did not
occur until 20,000 BVs, as shown for the Column #17 data in Figure 3.11. The pressure and
voltage buildup that accompanied this appears in Figure 3.12. By 12,000 BVs, the pressure had
risen from an initial 5−10 psi to 45−50 psi while the voltage had risen from an initial 1.1 V to 3.4
V. At this time, the GAC media was backwashed and this diminished the pressure to 5−10 psi
again. Then after another 15,000 BVs, (total cumulative 30,000−35,000 BVs) the pressure had
again reverted to 45 psi.
The backwashing only temporarily diminished the voltage drop, which then continued to
climb until it reached 7 V at 35,000 BVs of total cumulative water processed (Figure 3.12). The
backwashing also did not improve (or only briefly improved) arsenic removal, as by 20,000 BV,
effluent arsenic had reached 10 µg/L, and then it continued to climb to 20−25 µg/L at
25,000−35,000 BVs (Figure 3.11). For this Column #17 run, backwashing involved an upflow of
5 gph, which achieved a 30% expansion. This proceeded through 20 L of water processed, or 4.3
BVs of water.
30
25
backwashing
As (ug/L)
20
15
10
idle
5
0
0
5000
10000
15000
20000
25000
30000
35000
40000
Bed Volumes
Figure 3.11 Effluent arsenic concentration in Column #17: 0.02 A electrolytic
solubilization, virgin GAC
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 25
50
8
backwashing
45
7
idle
40
5
30
25
4
20
3
Voltage (V)
Back pressure (psi)
6
35
15
2
back pressure
voltage
10
5
0
0
5000
10000
15000
20000
25000
30000
35000
1
0
40000
Bed Volumes
Figure 3.12 Back pressure and voltage vs. BVs in Column #17: 0.02 A electrolytic
solubilization, virgin GAC
From RSSCTs, we had learned that when a ZVI source was idled, subsequent As
effluents could be diminished (see Section 3.5 below). However, with this electrolytic
solubilization chamber, the idling did not enhance performance. Rather, effluent arsenic levels
continued to climb after a 7-day idling event at 19,900 BVs (Figure 3.11). An important
operations principles learned from this pilot column study was that an impermeable iron
hydr(oxide) scale developed on the electrolytic plates, and neither backwashing nor idling altered
this conditions.
Three pilot columns appraised performance when iron-tailored GAC was coupled with
0.01−0.02 A electrolytic solubilization, and these columns reached 10 µg/L arsenic breakthrough
at 22,000−25,000 BVs (Figure 3.13). Longer bed life occurred when a 0.02 A cell was used
while operating at pH 7.1−7.4 (Column #18) than when a 0.02 A cell was used while operating at
pH 7.9−8.1 (Column #19) or when a 0.01 A cell was operated with a pH 7.1−7.4 water (Figure
3.13). These bed lives were only slightly longer than when iron-tailored GAC was used alone
without any electrolytic solubilization (compare to Figure 3.2 above). When comparing Figure
3.12 to 3.13, one observes that when iron-tailored GAC was coupled to electrolytic solubilization
(Column #18), the bed life to 10 µg/L As breakthrough was 6,000 BVs longer than for the
counterpart that used virgin GAC (Column #17). Also, when using the iron-tailored GAC
(Column #18), the average effluent As before breakthrough started was 2 µg/L; and this was
lower than the 4 µg/L for the counterpart run that employed virgin GAC (Column #17).
The Figure 3.11 data indicates that by about 20,000 BVs, the electrolytic plates surface
had accumulated an impervious scale of iron oxide. After that, electrolytic solubilization was not
preceding in a manner than enhanced arsenic removal.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
26 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
30
25
As (ug/L)
20
plate acid rinsing
15
10
backwashing
5
backwashing
0
0
5000
10000
15000
20000
25000
30000
35000
40000
Bed Volumes
pilot column 18: iron-tailored AquaCarb +electrolytic cell (0.02A), influent pH 7.1-7.4
pilot column 19: iron-tailored AquaCarb+electrolytic cell (0.02A), influent pH 7.9-8.1
pilot column 20: iron-tailored AquaCarb+electrolytic cell (0.01A), influent pH 7.1-7.4
Figure 3.13 Effluent arsenic concentration vs. BVs for Column #18, 19 and 20
The pilot columns that operated with 0.02 A at pH 7.9−8.1 (Column #19) built up a back
pressure of 45 psi within 12,000 BVs (Figure 3.14). This was faster than for the pilot column
with 0.02 A at pH 7.1−7.4 (Column #18), which built up 45 psi in 13,000−15,000 BVs. In
contrast, with operations at pH 7.1−7.4 and 0.01 A, the 45 psi pressure drop did not accumulate
until 27,000 (Column #20). Backwashing diminished the pressure drop to 10 psi, and then the
pressure climbed back up to 45 psi at about the same rate as it had during the first cycle of
operation for Columns #18 and #19.
Backwashing also only temporarily diminished the voltage drop, as discerned from the
Figure 3.15 data. Indeed, before the backwashing of Columns #18 and 19, voltage had climbed
from 0.8−1.1 V initially up to 3.0−3.3 V by 10,000−12,000 BVs. Then backwashing diminished
voltage to 1.3−2.0 V for about 2000 BVs, before it reverted to the voltages that had occurred
before backwashing. Thus, although backwashing could dislodge some of the iron (hydr)oxide
floc that accumulated between the electrolytic plates, this diminished voltage only briefly.
For the pilot column that were operated at 0.01 A and pH 7.1−7.4 (Column #20), we yet
more aggressively rejuvenated the electrolytic plates. Specifically, when the effluent arsenic had
climbed to 10 µg/L, we took the electrolytic plates out of service and submerged it in a solution
of 100 mL of concentrated sulfuric acid. This acid rinsing of the electrolytic plates diminished
the voltage to 0.8 V, and the slop of increasing voltage after acid washing was about the same as
it had been when this pilot column first started as per Figure 3.15 for Column #20. However, this
acid rinsing did not diminish arsenic breakthrough, which instead continue to climb.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 27
60
backashing
Back pressure (psi)
50
plate acid rinsing
40
30
20
10
0
0
5000
10000
15000
20000
25000
30000
35000
40000
Bed Volumes
pilot column 18: iron-tailored AquaCarb (60oC)+electrolytic cell (0.02A), influent pH 7.1-7.4
pilot column 19: iron-tailored AquaCarb+electrolytic cell (0.02A), influent pH 7.9-8.1
pilot column 20: iron-tailored AquaCarb+electrolytic cell (0.01A), influent pH 7.1-7.4
Figure 3.14 Backpressure vs. BVs for Column #18, 19 and 20
6
5
backwashing
Voltage (V)
4
3
plate acid rinsing
2
1
0
0
5000
10000
15000
20000
25000
30000
35000
40000
Bed Volumes
pilot column 18: iron-tailored AquaCarb (60oC)+electrolytic cell (0.02A), influent pH 7.1-7.4
pilot column 19: iron-tailored AquaCarb+electrolytic cell (0.02A), influent pH 7.9-8.1
pilot column 20: iron-tailored AquaCarb+electrolytic cell (0.01A), influent pH 7.1-7.4
Figure 3.15 Electrolytic cell voltages vs. BVs for Column #18, 19 and 20
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
28 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
Monitoring of iron (data not shown) for pilot columns #17−20 indicates that iron
concentration in effluent was stable at less than 0.2 mg/L, which is lower than the secondary
drinking water standard for iron of 0.3 mg/L. The iron here was monitored thus far by means of a
HACH portable iron kit with a detection limit is 0.2 mg/L. None of the samples had iron higher
than the detection limit. We have yet to analyze iron with our newly installed Atomic Absorption
Spectrophotometer-Graphite Furnace Atomizer system (AAS−GFA). Comparing this to effluent
iron from Column #7−10 runs (Figure 3.17 below), These results indicate that the lower current
of 0.01 to 0.02 A prevented excessive iron leaching while also considerably diminishing
colloidal iron clogging.
Pilot-Scale Studies with Electrolytic Solubilization Achieved with 0.1 A, Plus Iron-Tailored
GAC
Prior to conducting the 0.01 or 0.02 A pilot columns, we conducted several pilot columns
with cells that employed 0.1 A current between the electrolytic plates. These higher-amperage
conditions caused too much iron to solubilize, and this caused rapid pressure loss through the
filters and iron leakage through the filter media. Nonetheless, the arsenic removal was very good,
as the following results exhibits.
In these pilot trials (Columns #7−10), the electrolytic plates were placed immediately
above the GAC media. This proximate arrangement incurred an uneven distribution of iron
colloids into the GAC media, as the Figure 3.16 photograph depicts for the Column #6 media
after its operation was completed. The non-uniform media clogging caused water to
preferentially short-circuit through the media regions that were loss clogged. Even with this
excessive iron release and media clogging, we operated these columns for about 1,800−2,300
BVs. During this time, effluent arsenic remained below 0.6 µg/L, as shown in Figure 3.17.
The excessive iron solubilized from the electrolytic cells in turn caused excessive leakage
of iron through the GAC media, as indicated by the Figure 3.18 data. Specifically, effluent iron
ranged from 0.6 to 1.8 mg/L, as determined by Atomic Absorption with flame atomization.
These values were considerably higher than the 0.3 mg/L secondary iron standard.
Pressure buildup quickly in these runs, up to the available system pressure of 45−55 psi
within 1,000−2,000 BVs (Figure 3.19). This pressure buildup also meant that the design flow
rate (and contact time) could not be maintained. For example, with Columns #7−9, the targeted
EBCT was 5 minutes, but this could only be maintained out to 1,000−1,500 BVs. By the end of
these 1,700−2,100 BVs, the actual EBCT amounted to 50 minutes. Likewise, for Column #10,
where the design EBCT was 2.5 minutes, this flow could only be maintained until 2,000 BVs,
and then by the end of 2,300 BVs, the actual EBCT amounted to 60 minutes.
The voltage also climbed in these runs, from 1 V initially to 4−6.5 V at the end of runs
(Figure 3.20).
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 29
Figure 3.16 Uneven iron loading in Column #6 when 0.1 A was applied across the ZVI
electrolytic cell
As (ug/L)
0.6
0.4
0.2
0
0
500
1000
1500
2000
2500
Bed Volumes
pilot
pilot
pilot
pilot
column 7: electrolytic cell+virign AquaCarb, EBCT=5 min
column 8: electrolytic cell+iron-tailored (60oC) AquaCarb, EBCT=5 min
column 9: electrolytic cell+virign AquaCarb, EBCT=2.5 min
column 10: electrolytic cell+virign UltraCarb, EBCT=5 min
Figure 3.17 Effluent Arsenic Concentration from Pilot Columns #7−10. Influent
Concentration 50−60 ppb, targeted EBCTs as listed, and these EBCTs were maintained for
the first 1,000−1,500 BVs
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
30 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
Iron (mg/L)
2
1.5
1
0.5
0
0
500
1000
1500
2000
2500
Bed Volumes
pilot
pilot
pilot
pilot
column 7: electrolytic cell+virign AquaCarb, EBCT=5 min
column 8: electrolytic cell+iron-tailored (60oC) AquaCarb, EBCT=5 min
column 9: electrolytic cell+virign AquaCarb, EBCT=2.5 min
column 10: electrolytic cell+virign UltraCarb, EBCT=5 min
Figure 3.18 Release of Iron from Electrolytic Pilot Columns Operated at 0.1A, targeted
EBCTs as listed, and these EBCTs were maintained for the first 1,000−1,500 BVs
Pressure (psi)
80
60
40
20
0
0
500
1000
1500
2000
2500
Bed Volumes
pilot
pilot
pilot
pilot
column 7: electrolytic cell+virign AquaCarb, EBCT=5 min
column 8: electrolytic cell+iron-tailored (60oC) AquaCarb, EBCT=5 min
column 9: electrolytic cell+virign AquaCarb, EBCT=2.5 min
column 10: electrolytic cell+virign UltraCarb, EBCT=5 min
Figure 3.19 Pressure Build-up during Operation of Electrolytic Pilot Columns Operated at
0.1 A, targeted EBCTs as listed, and these EBCTs were maintained for the first
1,000−1,500 BVs
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 31
7
Voltage (V)
6
5
4
3
2
1
0
0
500
1000
1500
2000
2500
Bed Volumes
pilot
pilot
pilot
pilot
column 7: electrolytic cell+virign AquaCarb, EBCT=5 min
column 8: electrolytic cell+iron-tailored (60oC) AquaCarb, EBCT=5 min
column 9: electrolytic cell+virign AquaCarb, EBCT=2.5 min
column 10: electrolytic cell+virign UltraCarb, EBCT=5 min
Figure 3.20 Operational Voltage of Electrolytic Pilot Columns Operated at 0.1 A, targeted
EBCTs as listed, and these EBCTs were maintained for the first 1,000−1,500 BVs
After these runs of 1,800−2,500 BVs, Columns #7−10 electrolytic plates had released
17−25 g of iron as determined by mass balance for iron plates before and after. Table 3.4 lists the
iron that theoretically should have been solubilized from these anodes (per parallel ongoing
research), as compared to that which was actually measured. There was no change in mass
measured for the cathode plates in Columns #7−10. Theoretically, 3 × 10-5 g Fe/s would have
been expected to be solubilized from these anode plate when the applied current was 0.1 A. As
shown in Table 3.4, the actual iron dose supplied in Columns #7−10 matched reasonably well
with the theoretical dose (76−109%). Also shown in Table 3.4 is the number of hours of
electrolytic cell operation for each column.
ZERO-VALENT IRON WITH IRON-TAILORED GAC IN RSSCTS
The effect of zero-valent iron on arsenic removal was appraised first in RSSCTs. The
pilot column studies described in Sections 3.1−3.4 built on these RSSCT results. A portion of
the data that appears in Section 3.5 has been presented in an earlier AwwaRF Report for Project
3080. Specifically, the data that appears in an earlier AwwaRF report (although in different
format) includes data in Figures 3.21, 3.22A, 3.23A, 3.24A; and Tables 3.5 and 3.6. However,
this RSSCT work continued on beyond that earlier study; and the following section also includes
this significantly additional work and interpretation. The prior and continued work are merged
into one contiguous section herein for cohesiveness.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
32 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
Table 3.4
Theoretical and measured iron dose for electrolytic cells*
Column
#
7
8
9
10
Hours of Cell
Operation
275
220
216
220
Total Theoretical Iron
Dose (g)
28.6
22.9
22.5
22.9
Measured Mass Loss
(g)
25
25
17
20
% of Theoretical
87
109
76
87
*In 2,500 BVs, 0.54 g As was removed while the influent As concentration was 55 μg/L.
Using an iron release of 20 grams, this would translate to about 37 g Fe/g As.
RSSCTs: Arsenic Removal with Pre-corroded Galvanized Steel Fittings Coupled with Irontailored GAC
The first set of these RSSCT runs employed iron-tailored GAC that was coupled with
passively corroded galvanized steel fittings. The results showed that the corroded galvanized
steel fittings substantially extended bed life (Figure 3.21). Ultracarb GAC in these columns was
pre-treated using citrate-Fe(III) to achieve an iron loading of 1.2 − 1.3%. To explore the iron
corrosion effect, one column employed 316 stainless steel fittings as the non-corrosion control
and the other column employed corroded galvanized steel fittings. Without corroded iron, the
iron – tailored GAC column (#1) exhibited 10 µg/L breakthrough at 7,000 BVs. When
precorroded galvanized steel fittings were coupled with iron – tailored GAC (GS #1), effluent
arsenic remained below 10 µg/L for 150,000 BVs, and showed no sign of progressing to full
exhaustion even at 250,000 BVs. GS #1 had remained idle at 26,000 BVs for 6 days. A similar
column (GS #2) with citrate-Fe(III)-Mg(II) preloaded GAC and iron content of 0.97% exhibited
breakthrough of 10 µg/L As at a slightly earlier time around 130,000 BVs; otherwise, results
were very similar to GS #1.
Arsenic and Fe accumulated in the GAC-Galvanized Steel system
The authors suspected that the prolonged arsenic removal in GS #1 could be attributed to:
(1) arsenic adsorption and coprecipitation by the iron (hydr)oxides on the corroded galvanized
steel fittings surface; and (2) arsenic coprecipitation and adsorption by iron (hydr)oxides
particles that were released from the galvanized steel fittings. Such As−Fe particles could have
been filtered by the glass wool plugs or GAC column; and they may also have deposited on the
inside wall of the effluent tubing.
To appraise these possibilities, GS #1 was stopped after 250,000 BVs and the Fe and As
distribution were determined for the GAC media, the glass wool plugs, the galvanized steel
fittings and the effluent tubings (see Table 3.5). Over the course of the 250,000 BVs, the glass
wool plugs at the inlet and outlet had been replaced twice; and the Fe and As analysis were
appraised for the third of these plugs. Comparing influent and effluent As as a function of
processed BVs, the total arsenic removed by the entire system was 29.1 mg. The 1.67 g carbon
bed captured 40%, the third plug of inlet glass wool accumulated 29%, the galvanized steel
fittings retained 5%, and outlet tubing 3% of the removed arsenic. By difference, the first and
second glass wool plugs accumulated about 22% of the removed As. The third glass wool plug
and the carbon bed also accumulated 80 to 110 mg of iron.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
As ef f l uent ( ug/ L)
RESULTS AND DISCUSSIONS | 33
30
Wi t hout i r on
20
Wi t h
gal v ani z ed
s t eel f i t t i ng
10
0
0
50000
100000
150000
Bed Vol umes
200000
250000
Figure 3.21 RSSCT of iron tailored GAC coupled with galvanized steel (GS#1) and without
(#1); both systems operated at pH 6±0.3 using the arsenic-containing groundwater as
influent (As 47−55 µg/L). Dashed line indicated where the GS#1 system was idled for 6 days
(Source: Reprinted from Water Science and Technology, 61.2 pp 441-453, with permission
from the copyright holders, IWA Publishing)
Arsenic removal by glass wool (alone) when placed after galvanized steel
The glass wool in the GS #1 accumulated a considerable amount of arsenic. This
instigated an evaluation of performance in the absence of GAC, with just galvanized steel and
glass wool (GS #3). Arsenic in the effluent reached 20 µg/L within a mere 600 BVs. Effluent
iron increased from to over 0.3 mg/L within 15,000 BVs. Thus, glass wool without GAC offered
only limited capacity for capturing iron and arsenic.
The authors also conducted a shortened run (GS #4), using similar operating conditions as
for GS#1 but operated for just 24,000 BVs before it was stopped (prior to breakthrough) so as to
monitor iron and arsenic accumulation. At this interim point in this run, over 90% of the arsenic
removal had occurred within the GAC column; with 10% in the glass wool. Comparing the
results from GS #1, #2, and #4 with those from the glass wool column (GS #3), one observes that
the arsenic was at first preferably removed by the GAC media and subsequently the inlet glass
wool accumulated more iron particles.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
34 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
Table 3.5
Arsenic distribution in GS #1 (iron-tailored GAC coupled with corrosion of galvanized
steel fittings) after 250,000 BVs
Adsorbents
Fe (μg)
As (μg)
% Asremoved
/ Astotal
Outlet tubing
Inlet Glass wool (third plug)
Outlet Glass wool (third plug)
HCl washed rust from Galvanized
steel fittings
GAC
Monitored Total (with 1.67 g
carbon)
1st and 2nd plug of glass wool
(compute by difference)
26,200
82,400
1,480
760
8,380
230
3
29
<1
0.022
0.076
0.12
--
1,600
5
--
113,557
11,588
40
0.075
223,637
22,558
78
0.072
As/Fe
(mole/mole)
22
Total As removed (compute from
29,100
100
Fig 3.21)
(Source: Reprinted from Water Science and Technology, 61.2 pp 441-453, with permission from the
copyright holders, IWA Publishing)
Performance of Perforated Steel Chamber Preceding Iron-tailored GAC
Another round of mini-column trials (PS#1−#7) appraised arsenic removal two sequential
chambers. The first chamber contained perforated steel (identified herein as the steel chamber)
that was pre-corroded and pre-aged. The second column in series contained GAC that was pretailored with iron-citrate.
Effect of pH on Arsenic Removal—Through GAC Column
Using this two-chambered dual system, the authors appraised how water pH influenced
arsenic removal. The first run operated at pH 6.0 (PS #1), the second run was mostly at pH 6.0
but with an imposed brief interval at pH 6.5 (PS #4), and the third run was at pH 7.5 (PS #5). All
three runs were idled for 7 days at BV times around 20,000−30,000 BVs, as depicted in Figure
3.22.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 35
A
As ef f l uent ( ug/ L)
60
pH 7. 5 ( PS #5)
pH 6 or 6. 5 ( PS #4)
pH 6 ( PS #1)
50
40
30
20
10
0
0
80000
160000
Bed Volumes
240000
320000
B
As r emoved i n st eel
chamber ( ug/ L)
25
pH 6 ( PS #1)
pH 6 or 6. 5 ( PS #4)
pH 7. 5 ( PS #5)
20
15
10
5
0
0
20000
40000
60000
80000
100000
120000
140000
Fi l t er abl e As f r om
s t eel c hamber ( ug/ L)
Bed Vol umes
C
50
pH 6 ( PS #1)
40
pH 6 or 6. 5 ( PS # 4)
30
20
10
0
0
50000
100000
Bed Vol umes
150000
200000
Figure 3.22 pH effect on Mini column performance (A) Arsenic effluent from GAC column.
(B) Arsenic removed by steel chamber. (C) Filterable arsenic after steel chamber. Lines
indicate where the columns were idled for 7 days (Dotted line: pH 7.5; dashed: pH 6; Solid
line: pH 6−6.5). (Source: Reprinted from Water Science and Technology, 61.2 pp 441-453,
with permission from the copyright holders, IWA Publishing)
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
36 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
The water pH clearly influenced system performance. The pH 6.0 run (PS #1) did not
exhibit As breakthrough until 248,000 BVs, while breakthrough occurred at only 20,000 BVs for
the pH 7.5 run (PS #5) (Figure 3.22A). Indeed, even a short period of increased pH diminished
both the concurrent As removal and the long term BVs to continuous As breakthrough. These
observations come from evaluating the PS #4 data (Figure 3.22A). This run was operated at pH
6.0±0.3 for 39,000 BVs, then the water pH was intentionally increased to 6.5−6.7 from 39,000 to
58,000 BVs, after which the water pH was returned to 6.0±0.3. While the pH was raised to
6.5−6.7, the As effluent from PS #4 gradually increased from 8 to 33. When the pH was returned
to 6.0, the As effluent dropped back below 10 µg/L until consistent breakthrough of >10 µg/L
occurred at about 70,000 BVs.
The PS #1 run also experienced one relatively short period of arsenic breakthrough from
45,000 to 70,000 BVs, which occurred when pH rose to 6.4 during an 8-day holiday. A strict pH
adjustment protocol was maintained in all other runs. In summary, the system resulted in long
runs with low effluent As when the pH was 6.0, but performance was compromised with an
increase of pH to 6.4 or higher.
Effect of pH on Arsenic Removal through the Steel Chamber
The authors also monitored the As removal within the perforated steel chambers (Figure
3.22B). The cumulative mass of arsenic that was removed in the steel chambers after 55,000 BVs
of operation was 0.54 mg for constant pH 6 (PS#1), 0.46 mg for pH 6.0 except for a brief
deviation in pH (PS#4), and 0.1 mg for pH 7.5 (PS#5). This result highlights the higher arsenic
removal that can be achieved in a corroded ZVI bed at lower pH. However, the GAC columns
removed significantly more As than did the steel chambers (Figure 3.22A).
These observations of better arsenic removal performance at the lower pH are in
agreement with previous studies of arsenic removal via iron (hydr)oxides and zero-valent iron
(Bang et al. 2005; Jain, Raven, and Loeppert, 1999; Lenoble et al. 2002). The pH effect on
arsenic removal by iron (hydr)oxides could be attributed both to the amount of surface charge on
the perforated steel, and to the arsenic speciation, as per the literature discussed above.
Specifically, the pHzpc value of iron (hydr)oxides typically range from 6.5 to 8.5 (Cornell and
Schwertmann, 1996). When the water pH is less than the pHzpc, the surfaces tend to be
positively charged, and these surfaces will attract the As(V) anions such as the HAsO42- and
H2AsO4- that prevail at low pH. In contrast, when pH > pHzpc, the surface tends to be negatively
charged and will repel these oxyanions. As(V) accounts for 75% of total As in the groundwater,
while 25% of total As was As(III) (as H3AsO3 at pH below pH 9.29). All of these factors favored
more arsenic removal at pH 6.0 than at pH 7.5.
Effect of Idling on Arsenic Removal
It was reported above that idling resulted in increased As removal, in terms of the effluent
As concentration and also in the number of BVs before breakthrough. We further investigated
this effect by comparing no idle interval (PS #3), one idle interval (PS #1), and three idle
intervals (PS #2) (Figures 3.23 and 3.24). All of these runs were at pH 6 ± 0.3 except as
otherwise noted; and each idle period lasted 7 days.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 37
A smooth breakthrough curve was observed for no idle periods (PS#3), with103,500 BVs
before consistent As break through at 10 µg/L. Comparatively, from 215,000 − 248,000 BVss
were successfully treated before consistent breakthrough when the column was idled for either
one time (PS #1) or three times (PS #2) (Figure 3.23A). However, the idling intervals caused
relatively brief periods of arsenic breakthrough from the GAC column, (Figure 3.23A), total and
filterable iron release from the PSS chamber (Figure 3.24A and B), and iron release from the
GAC columns (Figure 3.24C). In particular, ferrous iron was released from the PSS chamber
immediately after idling; Fe(II) concentration averaged about 0.25 mg/L (Figure 3.24D),
accounting a quarter of the total iron discharged from the PSS chamber.
Mechanism Regarding Idling Effects
In interpreting these iron and arsenic results, we consider that the groundwater contained
4−6 mg/L dissolved oxygen when used in the Penn State lab. Thus, when the mini columns were
operating in continuous mode of operation, sufficient O2 was available to produce all of the
corrosion products that were observed. As discussed previously, it is likely that the surface
became passivated against further rapid corrosion. However, during the idling time, when no
new water flowed through the columns, O2 was probably depleted, resulting in anoxic conditions
and the formation of Fe(II) (see discussion of x-ray patterns below). Many of these Fe(II) species
are dissolved species (Kuch 1988; Ritter et al. 2002, 2003), but these conditions also result in
formation of wustite (FeO) and magnetite. These precipitates tend to form porous scales with
excess dissolved Fe(II), as proposed by Odziemkowski et al. (1998). When feed to the column
was resumed, O2 was once again provided, resulting in oxidation to ferric oxides and release of
these particulates from the PSS chamber as filterable iron.
This proposed mechanism for the idling reactions is consistent with the observed release
of “filterable” arsenic from the PSS chamber, as demonstrated by the Figure 3.22C and 3.22C
data for PS#1 and PS#4. Much of the “filterable” arsenic (i.e. filterable through 0.2 µ
membranes) subsequently was captured in the GAC filter, some arsenic bled through the GAC
bed immediately after an idling event, likely carried by iron particulates, as per Figure 3.23A.
Overall, these results revealed that week-long idling posed both favorable and
unfavorable impacts, when operations relied on passive ZVI solubilization as the arsenic-sorbing
source of iron (hydr)oxide.
Influencing Iron Release Rate from the Perforated Steel Chamber
Several factors influenced release of iron from the PSS chamber and capture in the GAC.
In addition to idling, these factors included the operating pH, pre-corrosion condition, and aging
of the PSS prior to use. These effects are qualitatively described by Figures 3.25A-C. It is
proposed that both surface roughness and porosity of the corrosion precipitates were increased
by pre-corrosion (Figure 3.25A) and by aging (Figure 3.25B). Moreover, after the three idling
events for PS#2, the perforated steel material was characterized by considerable surface floc
formation (Figure 3.25C).
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
38 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
A
As ef f l uent ( ug/ L)
80
no i dl e ( PS #3)
i dl e once ( PS #1)
i dl e 3 t i mes ( PS #2)
60
40
20
0
0
50000
100000
150000
200000
250000
300000
350000
Bed Vol umes
50
As i n Fe ( ug/ L)
B
no i dl e ( PS #3)
i dl e once ( PS #1)
i dl e 3 t i mes ( PS #2)
40
30
20
10
0
0
50000
100000
150000
200000
C
Fi l t er abl e As f r om i r on
c ol umn ( ug/ L)
Bed Vol umes
50
45
40
35
30
25
20
15
10
5
0
I dl e once ( PS #1)
no i dl e ( PS #3)
0
50000
100000
150000
200000
Bed Vol umes
Figure 3.23 Arsenic removal with no idle (PS #3), one idle (PS #1) and 3 idles (PS #2). (A) As effluent from
GAC column. (B) As removal in steel chamber. (C) Filterable arsenic leaving steel chamber. Solid line
indicates where PS #2 idled for 7 days, dashed line indicates where PS #1 idled for 7 days. All columns were
operated at pH 6±0.3. (Source: Reprinted from Water Science and Technology, 61.2 pp 441-453, with
permission from the copyright holders, IWA Publishing)
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 39
A
Fe conc. ( mg/ L)
4. 5
4
i dl e 3 t i mes ( PS #2)
i dl e once ( PS #1)
3. 5
no i dl e ( PS #3)
3
2. 5
2
1. 5
1
0. 5
0
0
50000
100000 150000 200000 250000 300000 350000 400000
Bed Vol umes
3
Fe c onc . ( mg/ L)
B
i dl e 3 t i mes ( PS #2)
i dl e onc e ( PS #1)
no i dl e ( PS #3)
2. 5
2
1. 5
1
0. 5
0
0
50000
100000
150000
200000
250000
300000
350000
Bed Vol umes
Fe c onc . ( mg/ L)
C 1. 4
i dl e 3 t i mes ( PS #2)
i dl e onc e ( PS #1)
no i dl e ( PS #3)
1. 2
1
0. 8
0. 6
0. 4
0. 2
0
0
50000 100000 150000 200000 250000 300000 350000 400000
Bed Vol umes
(continued)
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
40 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
Fer r ous i on( mg/ L)
D
0.
0.
0.
0.
0.
0.
0.
0.
8
7
6
5
4
3
2
1
0
PS #1
PS #2
0
50000
100000
150000
200000
250000
Bed Vol umes
Figure 3.24 Idling effect on Fe release. (A) Total Fe released from steel chamber. (B)
Filterable Fe released from steel chamber. (C) Fe effluent from GAC column. Solid line is
where PS#2 idled for 7 days on 3 occasions. D) Ferrous iron in filtered water released from
steel chamber. Dashed line is where PS#1 idled once for 7 days. (Source: Reprinted from
Water Science and Technology, 61.2 pp 441-453, with permission from the copyright
holders, IWA Publishing)
These operational parameter effects on iron release are further depicted in Figure 3.26. In
particular, Figure 3.26A presents the cumulative iron released from the steel chamber for runs
PS#1−5. As shown, PS#2 released the most iron; and this release was maintained throughout the
column operating duration. The PS#2 steel was pre-corroded for six days, then aged for no days;
and then experienced idling on three occasions, while operating at pH 6.0. The longer
precorrosion, lower pH, and idling enhanced the iron release. PS#1 released the second most
amount of iron; and this steel was pre-corroded for 3 days, aged for 5 days, then operated at pH 6
while idling once. Apparently, the additional aging somewhat made up for the less extensive precorrosion. The steel chamber that released the least amount of iron was PS#4, which was precorroded for 3 days, aged for no days, operated at pH 6 with an incurred spike to pH 6.5; and
idled once. This lower extent of pre-corrosion and aging diminished the iron release. In between
were PS#3 (3 days pre-corrosion, one day aging, pH 6.0 operation); and PS#5 (1 day precorrosion, 2 days aging and pH 7.3−7.5 operation). Thus, under these circumstances, the extent
of precorrosion and aging posed a greater influence on iron release rate than did even the
operating pH.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 41
A
B
100
50 μm
D
C
20 μm
100
20 μm
Figure 3.25 Scanning electron microscopy of: (A) Fresh precorroded steel sheets, (B) Aged
precorroded steel sheets, and (C) Steel sheets employed in PS#2, after use. (D) Steel sheets
employed in PS#4, after use. (Source: Reprinted from Water Science and Technology, 61.2
pp 441-453, with permission from the copyright holders, IWA Publishing)
Also, one can deduce from this data that small differences in chemical and operating
parameters made big differences in iron release rates; and this in turn influenced the arsenic
removal. This is a factor that must be addressed when a unit operation relies on passive
solubilization of ZVI as the source of colloidal iron (hydr)oxide that is used to capture arsenic insitu in a subsequent GAC bed.
Effect of Pre-corroded Steel Mass in Steel Chamber
The steel chamber-GAC systems discussed above (PS#1−5) each employed 3.3−4.0 g of
pre-corroded steel. In comparison, when less steel was employed, bed life was shorter.
Specifically, runs PS#6 and #7 employed 0.65−0.75 g of pre-corroded steel; and these exhibited
As breakthrough above 10 µg/L at 1,000−1,300 BVs, as per Table 3.6. These results can be
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
42 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
construed in relative terms, as the iron corrosion rates and extents would be specific to the
configurations of the steel’s surface-to-volume ratios.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 43
Tot al Fe r el eased ( mg)
A
300
250
200
150
pH
pH
pH
pH
pH
100
50
0
000000
050000
100000
150000
6
6
7.
6
6
( PS #1)
or 6. 5 ( PS #4)
5 ( PS #5)
( PS #2)
( PS #3)
200000
250000
300000
Bed Vol umes
Fi l t r abl e Fe r el eased
( mg)
B
100
80
60
pH
pH
pH
pH
pH
40
20
0
0
50000
100000
6
6
7.
6
6
( PS #1)
or 6. 5 ( PS #4)
5 ( PS #5)
( PS #2)
( PS #3)
150000
200000
Bed Vol umes
250000
300000
Fe ac c umul at ed i n GAC
( mg/ g)
C
350
300
250
200
pH 6 ( PS #1)
pH 6 or 6. 5 ( PS #4)
pH 7. 5 ( PS #5)
pH 6 ( PS #2)
pH6 ( PS#3)
150
100
50
0
0
50000
100000
150000
200000
250000
300000
Bed Vol umes
Figure 3.26 Effect of pH, pre-corrosion, aging, and idling on: (A) Total Fe released from
steel chamber, (B) Filterable Fe released from steel chamber, (C) Fe accumulated in GAC
column (including preloaded iron). (Source: Reprinted from Water Science and Technology,
61.2 pp 441-453, with permission from the copyright holders, IWA Publishing)
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
44 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
Table 3.6
Column operating parameters and BVs to 10 µg/L breakthrough*
Run Name
Iron
source
#1†
None
GS #1 & #2
GSF
GS #3‡
GSF
GS #4§
GSF
PS #1
PS
PS #2
PreCorrosion
time
(days)
PreAged
(day)
-
-
-
ZVI
Mass
after
preCorrosion
(gram)
-
Operation
pH
-
6
-
-
6
no
22,000 −
26,000
no
-
6
no
3
5
3.3
6
PS
6
0
4
6
PS #3
PS
PS
1
0
3.5
PS #4
3
3
PS #5
PS
1
2
PS #6 & #7
PS
1
10
6
6−
6.5§
7.3−
7.6
7.3−
7.6
27100
16,220,
124,400,
180,800
no
4
3.9
0.65−0.
75
6
Idle at
what BVs
BVs to 10 µg/L
As consistent
breakthrough
6000
135,000 −
150,000
600
(stopped at
24,000)
248,000
215,000
103,500
27,300
70,000
18,700
20,000
20,000
1,000−1,300
*
Run #1 and Runs GS #1−4 were RSSCTs, while PS #1−7 were mini -column tests. GSF =
galvanized steel fittings; PS = perforated steel sheets.
† All columns employed GAC that was preloaded with Fe (1.0−1.35%) plus citric acid, except
GS #3, which had no GAC. RSSCT of iron tailored GAC without corroded iron.
‡ RSSCT of glass wool with galvanized steel fittings, no GAC.
§ GS #4 had only been operated for 25,000 BVs.
** PS #4 was running at pH 6 mostly, but at 35, 500 – 54,500 BVs, pH was increased to
6.5−6.7.
(Source: Reprinted from Water Science and Technology, 61.2 pp 441-453, with permission from the
copyright holders, IWA Publishing)
X-Ray Diffraction
X-Ray Diffraction patterns appraised the nature of the iron that was sampled from the
perforated steel chambers after the completion of runs PS#2 as compared to PS#3. The PS#2
steel had experienced three idling episodes, whereas the PS#3 had not experienced any idling.
As shown in Figure 3.27, when no idling had occurred (PS#3-duplicates), the predominating
peaks included those for goehthite – α- FeOOH with its Fe(III) valence; and magnetite, with its
Fe(II / III) valence. In contrast, following three idling events, the PS#2 sample exhibited
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
RESULTS AND DISCUSSIONS | 45
predominance of wustite FeO, with its Fe(II) valence, along with magnetite and a slight peak for
zero-valent iron (ZVI). The non-idled sample exhibited scant wustite or ZVI; while the thriceidled sample exhibited considerably less goehthite. These X-ray patterns highlight the proposed
mechanism (above), that idling caused the corroded iron to become more reduced. The Figure
3.27 X-ray patterns also exhibited the presence of As(V) in the form of (H3As)4)2H2O-Bis
(arsenic acid) Hydrate, at a 2θ of 12 degrees.
M
160
M
M
140
As
120
H
G
G
G
G GG
M
M
M
H C
100
80
60
W
40
W ZVI
20
0
10
20
PS#2
30
PS #3
40
50
PS #3 ( dupi cat e)
60
70
2 t het a
Figure 3.27 X-ray diffraction patterns of the powdered rust collected from steel chamber
after runs PS#3 (no idling-top pattern); PS#3 (replicate-middle pattern) PS #2 ( thriceidled-bottom pattern). Peak designations: G = Goethite-α-FeOOH; M = Magnitite Fe+2 Fe+3
On; W = Wustite FeO; H = Humboltine (hydrous ferrous oxalate); C = Clinoferrosilite
FeSiO3; As = Bis(arsenic acid)Hydrate (H3AsO4)2H2O; ZVI = zero-valent iron. (Source:
Reprinted from Water Science and Technology, 61.2 pp 441-453, with permission from the
copyright holders, IWA Publishing)
Other iron peaks on PS#3 included Humboltine (hydrous ferrous oxalate and
Clinoferrosilite (FeSiO3).
Results from 0 – 55,000 BVs could be appraised for runs PS #1 to PS #5. Compared with
data collected from 0 – 24,500 BVs, the differences indicated that after the system had been
operating for some time, the effects of pre-corrosion and aging became less of a factor, while the
operating pH affected performance more.
We compared the BVs to 10 µg/L As breakthrough versus the amount of iron that
accumulated in the GAC at the time of consistent As breakthrough.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
CHAPTER 4
CONCLUSIONS
The research in Phase II focused on pilot-scale studies on arsenic removal by
technologies developed in earlier rapid small-scale column tests (RSSCTs). Arsenic removal by
a system of solubilized zero-valent iron plus iron-tailored GAC showed valuable technical
promise for practical applications. The best performance thus far was achieved when carbon
tailored by an iron-salt evaporation method was coupled with passive zero-valent iron
solubilization. This pilot column was able to operate for about 52,000 BVs before 10 µg/L
breakthrough and this was accomplished by only slight pressure buildup. Iron-tailored carbon
played a significant role in sorbing and filtering arsenic and extended the overall system bed life.
In addition, iron concentration was kept under 0.2 mg/L (lower than its secondary drinking water
standard of 0.3 mg/L) when part of the carbon was used as a scavenger bed for the solubilized
iron. In addition, investigation on the effect of pH proved that arsenic removal was highly related
to water pH. An increased from pH 7.1−7.4 to 7.9−8.1 caused a decrease in bed life of about
15%.
The feasibility of electrolytic cells as an iron corrosion sources were also investigated
extensively, and this appraisal will continue. Thus far, we have learned that pressure and voltage
buildup were two of the main issues with this electrolytic cells system. Systems with electrolytic
cells were able to operate more than 20,000 BVs before breakthrough. Under the current column
configuration, acid washing of the plates seems to have little effect on restoring the arsenic
adsorption capacity after breakthrough.
For the iron-tailored GAC, pilot scales studies were further compared with results from
RSSCTs. In this comparison, filtering effects were excluded since ZVI solubilization was not
involved. The similarity in arsenic breakthrough curves between these two systems serves to
prove that it is appropriate to assume proportional diffusivity for arsenic sorption in RSSCT
design.
We have also included results from studies in RSSCTs on arsenic removal by a Fepreloaded GAC plus precorroded iron system. In RSSCTs, the system offered some promise for
practical applications, but several limitations remain to be worked out. Best performance was
achieved when the pH was continuously adjusted down to 6.0. In this case, consistent 10 µg/L
arsenic breakthrough occurred at 100,000 to 250,000 BVs as measured by the GAC bed.
Moreover, effluent iron could usually be controlled under 0.2 mg/L when no idling occurred.
This longevity was somewhat comparable to GFH media, although different metrics applied
(Driehaus et al. 1998; Selvin et al. 2002; Westerhoff et al. 2005).
In these RSSCTs, the study of idling effects was important relative to practical
applications of the system, because many groundwater systems are operated in on-off mode. By
stopping the column and leaving it idle for several days, operations extended the system’s bed
life by two-fold. But as an unfavorable effect, this idling also caused a brief spike right after the
system was restarted; and this represented a limitation of a system such as this, where a bed of
passively corroding iron precedes a bed of GAC or other media that can participate in redox
reactions.
Arsenic removal through the GAC media was generally proportional to the amount of
iron that accumulated in the GAC column, and this extent of iron accumulation was generally
47
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48 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
controlled by the operating pH, the pre-corrosion and pre-aging conditions of the ZVI source,
and the idling of the whole system.
Overall, main finding from both the pilot and RSSCT studies include
1. Studies with passive iron solubilization +GAC (1/8” diameter plain steel iron rod
installed in the carbon bed) were effective at removing arsenic. Bed volumes as high as
52,000 were observed when iron-tailored carbon worked together with iron rods.
2. Iron-tailored carbon can play a significant role in arsenic sorption and filtering. This is
especially significant for carbon bed with passive solubilization since it may take some
time for the corrosion to get fully started. Most importantly, GAC provided scavenger
sites for excessive iron from corrosion.
3. Studies with electrolytic cells showed somewhat favorable results for this system. The
main issue with electrolytic cells was clogging by excessive corrosion products and
accumulation of an impenetrable iron hydr(oxide) barrier on the ZVI surface that
prevented yet further iron solubilization and arsenic removal.
4. With iron-tailored GAC, comparing results from RSSCT and pilot-scale indicates that for
arsenic, it is appropriate to assume proportional diffusivity in RSSCT design.
5. In RSSCTs, pre-corrosion in strong acid and aging created a more porous surface that
offered more contact area for corrosion to occur.
6. Also in RSSCTs, idling created a more accessible surface for subsequent corrosion; but
the idling also reduced some Fe(III) hydr(oxide) solids to Fe(II) dissolved species, which
proceeds to the GAC bed. This likewise released some of the As that had been sorbed to
the Fe(III) flocs.
©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
CHAPTER 5
SIGNIFICANCE TO UTILITIES
As significance to utilities, a new arsenic limit of 10 µg/L became effective in 2006 for
the United States drinking water systems. This new regulation would make small public water
facilities face heavy financial burdens, unless less costly methods of arsenic removal are
developed. There is an urgent demand for an economical, effective, and reliable technique that is
capable of removing arsenic species to this new level.
Adsorption onto iron-tailored GAC is considered to be one of the more promising
technologies because it is economical and easy to set up, and because the skeletal structure of the
GAC is strong, whereas granular iron media are fragile. For the research herein, we focused on
test on pilot scales that would utilized both virgin and iron-tailored GAC so that this active
sorptive material could complex both arsenite and arsenate with high sorption capacities. Our
aim was to develop a media preparation method that is environmentally acceptable, costeffective, and simple.
Yet further, our research has focused on developing an arsenic removal system that
couples the high pore volume, structural cohesiveness, and low costs of granular activated
carbon (GAC) with the arsenic-sorbing propensity and low costs of iron. Our overall approach to
combining iron-loaded GAC with zero-valent iron had shown promising results in RSSCT. In
pilot-scales, systems of iron sources with iron rods or electrolytic cells were further investigated.
We anticipate that we would achieve the longest bed life for removing arsenic in an easy-tooperate adsorption column, while maintaining low costs. Thus far, through the coupled research
activity, we have developed an iron-preloaded media that can remove arsenic to below 10 µg/L
for 26,000−33,000 BVs, and when this iron tailoring was coupled with solubilization of zerovalent iron in the same vessel, arsenic was removed to below 10 µg/L for 52,000 BVs. We have
also observed that when iron solubilization preceded the GAC vessel, we could achieve
150,000−250,000 BVs.
49
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©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
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©2010 Water Research Foundation and Arsenic Water Technology Partnership. ALL RIGHTS RESERVED.
ABBREVIATIONS
A
AAS
As(III)
As(V)
Ampere
atomic absorption spectrometry
arsenite
arsenate
BV
bed volume
C
C0
CA-Fe-SD
CPC
CS-Fe-SD
CTAC
the concentration of arsenic in the solution
the initial arsenic concentration
citrate ions added iron incorporated super darco
cetylpyridinium chloride monohydrate
cationic surfactant and iron incorporated super darco
cetyltrimethylammonium chloride

Dapp
the liquid film thickness
the apparent diffusivity of arsenic
EBCT
empty bed contact time
Fe(II)-Fe-SD
Fe-GAC
Fe-SD
Fe-UC
ferrous ions sequential impregnated iron incorporated surper darco
iron incorporated granular activated carbon
iron incorporated super darco
iron incorporated ultra carb
GAC
granular activated carbon
HFO
HFO-GAC
HVG
hydrous ferric oxide
hydrous ferric oxide incorporated granular activated carbon
hydride vapor generator
IWI
incipient wetness impregnation
µg/L
mg/g
mg/L
mm
microgram per liter
miligram per gram
miligram per liter
milimeter
55
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56 | Arsenic Removal With Iron-Tailored Activated Carbon Plus Zero-Valent Iron
POE/POU
ppb
point of use/point of entry
part per billion
R
RSSCTs
the average radius of adsorbent particles
rapid small-scale column tests
SD
super darco
t
TDS
time
total dissolved solid
UC
ultra carb
V
Volt
WHO
world health organization
X
XRD
the fraction of the arsenic adsorbed to adsorbent
X-ray diffraction
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