2 of 2:
Removing Arsenic from Water
of pH, Background Contaminants and Oxidation
By Dennis Clifford, Ph.D.,
Summary: Last month, we discussed the problems of potential
arsenic leaching from certain carbons used in filtration devices.
In Part 2, we cover different species of arsenic and weigh various
methods for effective removal from drinking water supplies.
Since its isolation in 1250 A.D. by Albert the Great, arsenic
has been known for its poisonous nature and acute toxicity. In the
1970s, arsenic was identified as a carcinogen associated with skin
and lung cancer, and more recently it has been linked to bladder
and prostate cancers, cardiovascular disease and diabetes. A reevaluation
of the health effects of arsenic-completed in March 2000 by the
National Academy of Sciences (NAS)concluded the current drinking
water Maximum Contaminant Level (MCL) of 50 parts per billion (ppb)
wasn't sufficiently protective of public health. It recommended
the MCL be lowered as soon as possible. In May 2000, the U.S. Environmental
Protection Agency (USEPA)which is under legal mandate to provide
a final MCL by Jan. 1, 2001proposed an MCL of 5 ppb and announced
it would take comments on levels for 3, 10, and 20 ppb. When the
new MCL is set, it's unlikely it will be above the World Health
Organization (WHO) guideline of 10 ppb, adopted as a standard in
many parts of the world.
According to USEPA estimates, lowering
the MCL to 5 ppb will impact 12 percent of the nation's 54,000 community
water supplies (CWS) that provide water for an estimated 22.5 million
people. A 10-ppb MCL would impact 6 percent of the CWSs and an estimated
10 million people. The USEPA's estimated cost of compliance per
year for the 5- and 10-ppb MCLs are $445 million and $195 million,
respectively.1 Compliance cost estimates sponsored by the American
Water Works Association (AWWA) Water Industry Technical Action Fund
are much higher: $2.3 billion and $780 million per year, respectively.2
Although arsenic occurs in surface
waters, it's primarily a groundwater problem and a widespread one
at that, as evidenced by the U.S. Geological Survey (USGS) arsenic-occurrence
map just published on their website (http://co.water.usgs.gov/trace).
This map and other surveys have shown groundwater arsenic concentrations
above 3 ppb are found in virtually every state. Those with the greatest
number of supplies above 10 ppb include all western states plus
Kansas, Nebraska, the Dakotas, Illinois, Wisconsin, Indiana, Ohio,
Michigan, West Virginia and Pennsylvania.
Arsenic speciation in water
Arsenic is a metalloid (an element that exhibits properties of both
a metal and non-metal), not a heavy metal as is often reported.
It has two common oxidation states, pentavalent arsenic (As-V) and
trivalent arsenic (As-III). Dissolved arsenic is an anion or neutral
molecule-not a cation-in water. Although arsenic can exist in both
organic and inorganic forms, only inorganic arsenic has been found
to be significant in groundwater supplies.
Depending on the reducing or oxidizing
condition in the groundwater, either arsenite (As-III) or arsenate
(As-V) will dominate. The pH of the water is also very important
in determining arsenic speciation. The primary arsenate species
found in groundwater in the 6-to-9 pH range are monovalent H2AsO4- and divalent HAsO4 -2. These anions result
from the dissociation of arsenic acid, H3AsO4. Uncharged
arsenious acid, H3AsO3, is the predominant species of trivalent arsenic
found in natural waters. Only at pH values near and above pH 9.2
does the monovalent arsenite anion, H2AsO3-2, become
Finally, the arsenic in contaminated groundwater will generally
be soluble, but some particulate arsenic may be found associated
with insoluble iron and manganese, clay and other particulates.
To summarize, arsenic contamination of drinking water is predominantly
a groundwater problem associated with soluble inorganic arsenic
in the form of As-V anions or neutral As-III molecules.
Processes for arsenic removal
Arsenic can be removed from water using a variety of processes including
iron and alum coagulation, lime softening (LS), activated alumina
adsorption (AAl), granular ferric hydroxide adsorption (GFH), ion
exchange (IX), coagulation assisted microfiltration (C-MF), reverse
osmosis (RO) and nanofiltration (NF). Experience with all of these
processes has demonstrated that As-V is better to far better removed
than is As-III. Thus, oxidation of As-III to As-V will generally
be required before any other treatment processes to meet the new
effluent MCL requirement when using these processes to treat waters
This discussion will focus on AAl,
GFH, IX, C-MF, RO and NF, because they're applicable to point-of-use/point-of-entry
(POU/POE) and small community systems. Keep in mind, amendments
to the Safe Drinking Water Act (SDWA) during its 1996 reauthorization
established for the first time that POU/POE technologies could be
used as "best available technology" (BAT) for small systems
to come into compliance with federal drinking water regulations.
Figure 1. Effect
of pH on As-V removal by activated alumina
AAl adsorption is typically carried out using 28×48 mesh alumina
in packed beds with 2-to-6 foot (ft) media depth and an empty bed
contact time (EBCT) of 3-to-5 minutes. The major factors influencing
run length are pH, competing ions, EBCT and arsenic oxidation state.
The longest runs-20,000 bed volumes (BV) and greater-are achieved
in the 5-to-6 pH range with run length decreasing as pH increases
(see Figure 1). Silica, phosphate and fluoride ions compete
strongly with arsenates for adsorption sites. As-III is poorly adsorbed
compared with As-V (see Figure 2). However, AAl can be used
to remove As-III for short run lengths (typically less than 1,000
BV) at neutral to slightly alkaline pHs. AAl is regenerable using
a fairly complicated base and acid regeneration process, which dissolves
some of the media.
Figure 2. Effect of oxidation
state on arsenic removal
by activated alumina@pH 6.0
Granular ferric hydroxide
GFH adsorption is a relatively new process3 developed in Germany
and used in Europe to meet the new 10-ppb guideline. It employs
packed beds of GFH, a poorly crystallized beta-FeOOH, which is available
from only one source, GEH Wasserchemie. It generally gives longer
runs than AAl and is less sensitive to pH and EBCT. As with AAl,
silica, phosphate and fluoride ions compete strongly with arsenates
for adsorption sites, and As-V is much better removed than As-III.
The exhausted GFH media isn't considered regenerable and is simply
thrown away. Spent GFH is considered a non-hazardous waste, because
it reportedly passes the applicable leaching tests. For comparison,
spent AAl can usually be disposed of as a non-hazardous waste, too.
Generally, EBCTs for GFH can be less than those required for AAl.
The IX process removes only As-V. In it, the source water (oxidized
if necessary) is passed through a 2-to-6 ft deep bed of chloride-form,
strong-base anion exchange resin in which the chloride-arsenate
ion-exchange reaction takes place in the 6.5-to-9 pH range. (See
Equation 1, where "R" represents a positively charged
resin exchange site.) Regeneration with excess NaCl according to
Equation 2 is readily accomplished, and returns the resin to the
chloride form, prepared for another exhaustion cycle.
Equation 1: 2 RCl + HAsO42- = R2HAsO4
+ 2 Cl- Exhaustion
Equation 2: R2HAsO4 + 2NaCl = 2 RCl + Na2HAsO4 Regeneration
This process is like IX softening
except that strong-base anion (SBA) resin is used in place of strong-acid
cation (SAC) resin. Generally, IX will give the lowest effluent
arsenic concentrations of any of the potential treatment processes
including RO. Run lengths, which decrease as sulfate increases,
are commonly in the 400-to-4,000 BV range. Recently, it was discovered
the spent IX regenerant could be reused up to 25 times without removing
the arsenic.4 A significant disadvantage
of ion exchange is the potential for arsenic peaking or dumping,
i.e., effluent arsenic concentration exceeds influent concentration.
For dumping to occur, an ion more preferred than arsenate, e.g.
sulfate, must be present and the run must be continued beyond arsenic
breakthrough. Thus, the USEPA isn't considering IX for POU/POE compliance
to the MCL.5 IX has an advantage over AAl and GFH in that
EBCTs of one minute and even less can be used.
In the C-MF process, 1-to-10 ppm of ferric iron (Fe-III) is added
to the raw water, which is mixed for 20-to-60 seconds before direct
0.2-micron microfiltration to remove the arsenic-contaminated Fe(OH)3
precipitate that's formed. Arsenic removals (up to 98 percent for
As-V) depend on Fe-III dosage, pH, competing ions (silicate, phosphate
and fluoride). Typically, only 10-to-30 percent As-III removal is
achieved. Complicated systems are required to recycle the backwash
to increase water yield. Coagulation systems are prone to upsets
if the feed water chemistry changes and aren't generally suitable
for POU/POE devices.
Nanofiltration and reverse osmosis
Assuming one can afford to use them, NF and RO processes will effectively
remove arsenic. NF membranes can do a good job (50-to-95 percent)
of As-V removal but achieve much less removal of As-III. Typically,
NF is not as good as RO, which removes >95 percent of As-V and
>75 percent of As-III. Both processes are relatively expensive
and produce a brine stream, which can be greater than 20 percent
of the feed water flow rate and must be disposed of properly.
Oxidation of As-III to As-V
About 15 years ago, research revealed one ppm of chlorine was an
effective oxidant for converting As-III to As-V in less than five
seconds.6 In that same research, oxygen was shown to be
ineffective, and in-situ-formed monochloramine was shown to be 50
percent effective. Subsequent studies confirmed the partial effectiveness
of in-situ-formed monochloramine, but demonstrated that pre-formed
monochloramine was completely ineffective as an oxidant for As-III.7 This study also showed permanganate, ozone and
manganese oxide media (Filox) were effective oxidants for As-III,
while chlorine dioxide and UV light were ineffective. In the latter
work it was shown as well that interfering reductants, particularly
sulfide, slowed the oxidation rate.
A new 5-ppb arsenic MCL has been proposed, which will require thousands
of water suppliers to treat for arsenic removal. Activated alumina
adsorption, granular ferric hydroxide adsorption, ion exchange,
Fe-III coagulation-microfiltration, nanofiltration, and reverse
osmosis are proven arsenic removal processes. When choosing among
the processes, one must consider arsenic oxidation state, pH, competing
ions (especially silicate, phosphate, fluoride and sulfate) and
the point of treatment (POU, POE or small community systems). In
all these processes, As-V is more easily removed than is As-III,
both of which are naturally present in many groundwaters. Chlorine,
permanganate, ozone and solid oxidizing media (Filox) are effective
oxidants, while oxygen, monochloramine, chlorine dioxide and UV
light are ineffective.
1. U.S. Environmental Protection
Agency, "Technical Fact Sheet: Proposed Rule for Arsenic in
Drinking Water and Clarifications to Compliance and New Source Contaminants
Monitoring," EPA 815-F-00-11, Office of Water, May 2000.
2. Frey, M., M. Edwards, G. Amy, D.
Owen, and Z. Chowdhury, "National Compliance Assessment and
Costs for the Regulation of Arsenic in Drinking Water," American
Water Works Association, Denver, January 1997.
3. Driehaus, W., M. Jekel and U. Hildebrandt,
"Granular Ferric Hydroxide-a New Adsorbent for the Removal
of Arsenic from Natural Water," J. Water SRT-Aqua, V. 47, No.
1, pp. 30-35, 1998
4. Clifford, D., G. Ghurye, A.R. Tripp
and J. Tong, "Field Studies on Arsenic Removal in Albuquerque,
New Mexico," University of Houston Report, August 1998.
5. Kempic, J.B., "Centrally-managed
POU/POE Option for Compliance with the Arsenic Regulation,"
Office of Ground Water and Drinking Water, USEPA, Washington, D.C.,
6. Frank, P., and D.A. Clifford, "Arsenic
(III) Oxidation and Removal from Drinking Water," EPA/600/52-86/021,
8 pp., USEPA, Cincinnati, April 1986.
7. Ghuyre, G., and D.A. Clifford,
"Laboratory Study on the Oxidation of Arsenic (III),"
75 pp., submitted to USEPA, June 2000.
About the author
Dr. Dennis Clifford is a professor of environmental engineering
and chairman of the Department of Civil and Environmental Engineering
at the University of Houston. Also a professional engineer, he has
more than 30 years experience in teaching, research and consulting
related to water and soil treatment. During the last 20 years, he
and his students-notably, Ganesh Ghurye, Jean Tong, C.C. Lin, A.R.
Tripp, Eric Rosenblum, Phyllis Frank and L.L. Horng-have extensively
researched the subject of arsenic speciation, oxidation and treatment.