Metal Attraction: An Ironclad Solution to Arsenic Contamination? Inorganic arsenic--the more acutely toxic form of this metalloid
element--contaminates drinking water supplies around the world. In
the United States, the most serious arsenic contamination occurs in
the West, Midwest, Southwest, and Northeast; as many as 20 million
people--many getting their water from unregulated private wells--may
be exposed to excess arsenic in their drinking water. In Bangladesh,
it’s estimated that as many as 40 million people may be suffering
from arsenic poisoning; contaminated drinking water is also a problem
in many other countries, including Argentina, China, Chile, Ghana,
Hungary, India, and Mexico.
There are several methods for removing inorganic arsenic from water.
Many take advantage of the strong bond that forms between arsenic
and iron. Now Littleton, Colorado-based ADA Technologies, through
funding from the NIEHS Superfund Basic Research Program, the U.S.
Air Force, the U.S. Environmental Protection Agency (EPA), and the
state of Colorado, has gone a step farther in capitalizing on that
characteristic with a new class of amended silicate sorbents that
remove even more arsenic from water, and do it more easily and more
cheaply. ADA is also working with researchers at Virginia Polytechnic
Institute and State University and Old Dominion University to study
the interaction between arsenic species and iron oxide-based media,
and is collaborating with other partners to develop low-cost approaches
for quantifying the concentration of arsenic in drinking water.
A Host of Health Risks
Most environmental arsenic occurs naturally, appearing in deposits
of minerals and ores including arsenopyrite, enargite, and proustite.
A smaller but still significant source of arsenic exposure is anthropogenic.
Inorganic arsenic as well as various arsenical compounds have been
used in agricultural chemicals and wood preservatives, in the glass
industry, and in the production of lead shot. Elsewhere, emissions
from coal-burning power plants are a significant source of arsenic
exposure.
However, the majority of toxic exposure comes from drinking water
contaminated with naturally occuring arsenic. Chronic arsenic ingestion
through drinking water is known to cause skin cancer, and has been
linked to an increased risk for cancers of the bladder, lung, kidney,
liver, colon, stomach, uterus, and prostate. Arsenic has also been
associated with cardiac, pulmonary, and artery diseases, diabetes mellitus,
and neurological, developmental, and reproductive problems.
In the United States, a revised drinking water standard for arsenic
of 10 micrograms per liter (µg/L) is set to take effect in January
2006, but there is substantial concern that this level is still too
high for public safety. In many other countries, allowable levels are
even higher. With millions of people around the world facing potential
adverse health effects from this contaminant, the need for effective,
affordable ways to remove arsenic from drinking water is critical.
Ridding Water of Arsenic
Arsenic is generally found in two inorganic forms in nature--arsenate
and arsenite. Arsenate is present as a negatively charged ion at typical
drinking water pH (roughly 6.5-8.5), whereas arsenite is neutral in
the same pH range. Many treatment methods rely on a negative arsenic
charge, so they tend to be more successful at capturing arsenate.
One such method is ion exchange, which uses polystyrene-based resins
containing positively charged sites to remove negatively charged
species. Besides being effective only with arsenate, sulfate ions are
removed preferentially to arsenic, so if large amounts of sulfate are
present, those ions will tie up the bonding sites, leaving fewer available
for arsenic to bond with. Activated alumina is a filter medium that
will remove a variety of contaminants, including fluoride, arsenic
(both arsenite and arsenate), and selenium, but it requires periodic
cleaning with an appropriate regenerant such as alum or caustic in
order to remain effective. Activated alumina also is effective only
across a very narrow pH range (6 to 7).
Granular ferric oxide is an iron-based adsorbent that can capture
both arsenate and arsenite, but in general, it functions best at or
below a pH of 7, and both phosphates and silicates can interfere with
its action. A fourth method, a coagulation/filtration process,
uses a ferric chloride liquid and an oxidizing agent such as sodium
hypochlorite to create insoluble ferric hydroxide. Arsenic adsorbs
readily onto the solids, but workers must store and handle corrosive
ferric oxide and oxidant solutions.
The ADA formulation takes a slightly different approach. The basic
ingredient is an iron oxide known as akageneite. According to Craig
Turchi, ADA program manager for the arsenic project, the company focused
on an iron oxide because iron tends to form very strong, stable bonds
with arsenic. Additionally, Turchi explains, oxides tend to be among
the most stable substances in nature, and many of them tend to accumulate
the substances with which they come into contact, including many contaminants. “Iron
oxide is a molecule with hydroxyl groups,” he says. “And
we know that, from a chemical perspective, arsenic behaves like a hydroxyl
in some ways.”
One of the key advantages to ADA’s approach is that the akageneite
is coated onto an inert silicate substrate. In comparison, most other
approaches involve pure iron oxide granules. Turchi says, “Our
tests have shown we can get the same capacity as our competitors, but
with much less iron. And that cuts the cost from around four dollars
per pound down to around two dollars per pound.” Additionally,
because ADA’s akageneite particles are on the nanoscale, they
can be dispersed far more efficiently in water undergoing treatment.
The ADA formulation also is notable for its ability to remove both
arsenite and arsenate, and its effectiveness at a wider pH range of
6.5-8.5.
A Super Sorbent
The ADA formulation has been shown to reduce arsenic contamination
as high as 1,000 µg/L to 10 µg/L in as little as 30 minutes. “The
capacity of the adsorbent increases with the concentration of arsenic
in the water,” Turchi says. “This behavior is typical of
most adsorbents.” Capacity varies somewhat with the water content,
says Turchi, but generally ADA’s material removes about 2 milligrams
of arsenic per gram of sorbent at a concentration of 50 µg/L,
and about 40 milligrams of arsenic per gram of sorbent at 1,000 µg/L.
There is a catch, though--as the water gets cleaner and more bonding
sites are taken up by arsenic, it becomes harder and harder to remove
the last bits of arsenic. Turchi says it probably is not possible to
remove all the arsenic from water with processes such as this. “It
comes down to the efficiency of an equilibrium process,” he says. “Arsenic
has an affinity to stay in water, as well as an affinity to attach
to materials like iron. You’ll get diminishing returns until
these affinities balance at some point.”
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The stuff of sorbency. ADA’s amended
silicate sorbent comes in two formulations, one more
suitable for a packed-bed approach (top) and a finer
version that can be sprinkled into water, then filtered
off (bottom). image: ADA Technologies |
Turchi says users will eventually be able to choose from either a
solubilized form of the sorbent that can be sprinkled into contaminated
water, circulated, and then filtered off or allowed to settle out,
or a pelletized form for use in a packed-bed approach. A packed bed
consists of layers of adsorbent material. Contaminated water is poured
in the top, and purified water is collected at the bottom after seeping
through the material. “In the ideal case you periodically add
new sorbent to the clean water end and remove the arsenic-saturated
sorbent from the other end, so that you get the maximum use of your
sorbent,” Turchi says.
Once the binding sites on the iron oxide have been used up, the material
could be reused by acidifying it to break the arsenic-iron bonds, then
filtering off the arsenic. But Turchi says economics--and the logistics
of dealing with the arsenic-laden, high-pH waste--will probably dictate
disposal of the used adsorbent. The spent material has passed EPA tests
to determine the likelihood of contaminants leaching out of landfills
into the surrounding water supply, so Turchi says the sorbent can simply
be disposed of in a regular landfill.
Turchi says ADA’s sorbent system has the benefits of being
both robust and simple: no moving parts, and little training required. “I
think that makes our system more appropriate for smaller-scale uses,” he
says. “Typically, in a large municipal facility, you’d
run the water through the filtration system, but a small village [in
a developing nation] probably doesn’t have a water treatment
facility.”
ADA and collaborator Kinetico Incorporated, a water treatment system
engineering company, are planning a field test involving a packed column
of the amended silicate for summer 2005 at a facility in New Mexico,
where the ADA formulation will be tested against two commercially available
competitors. The formulation also underwent earlier field tests at
two Colorado sites.
Questions of Stability
Though not familiar with ADA’s work specifically, Joshua Hamilton,
director of the Center for Environmental Health Sciences at Dartmouth
College and the Dartmouth Toxic Metals Research Program, says, “I
know a lot of people are working with iron oxides, and it appears to
be a very fruitful area. Iron oxides and arsenic exhibit tight bonding
properties, and oxides are relatively cheap materials. All of these
are pluses--high efficiency, low maintenance, low cost, and easily
renewable.”
Still, says Hamilton, there are a few aspects of the ADA approach
that may be cause for concern. For one, he says, “I think that
assuming you can safely put it in a conventional landfill might be
overstating the strength of the bond.” He explains that a lot
of arsenic is found in granitic deposits throughout his home state
of New Hampshire. “We’re seeing, under normal environmental
circumstances, the mobilization of a good deal of arsenic out of materials
that are basically compounds of arsenic and iron,” he says. “And
it should also be taken into account that there’s a lot of interesting
and somewhat unpredictable chemistry that goes on in landfills.” He
points out that the environmental conditions that allow organics to
remain contained are quite different from those that allow arsenic
to be contained. “So if you focus on arsenic,” he says, “you
could end up releasing organics into the environment and vice versa.”
Turchi agrees that the issue of long-term sorbent stability in landfills
needs to be addressed. He explains that the EPA leach tests examine
the stability of landfilled contaminants under acidic conditions, because
most metals leach off of sorbents under acidic conditions. “Research
indicates that arsenic could conceivably leach off the media under
alkaline conditions, so a different test may be required,” he
says.
Michael Harbut, chief of the Center for Occupational and Environmental
Medicine at Wayne State University, points to another concern: “I’d
[like] to see enough studies to show me the substance that resulted
when the arsenic bonded didn’t have toxic properties of its own--inhalational
studies, cardiac trend studies, the whole suite. Then we’d move
on to worry about bond strength in the environment.” Harbut has
studied low-level arsenic poisoning for many years and has lobbied
for stricter water thresholds and broader testing.
Hamilton raises a third concern: the fact that even $2 per pound
could prove to be an insurmountable barrier in many countries. With
comparable technologies costing $4-8 per pound, ADA’s price does
seem a bargain. But with the annual per capita income in Bangladesh,
for example, hovering around US$360, the outlay of even $2 per pound
of sorbent (enough to remediate about 800,000 liters of water) could
well prove prohibitively high.
“One factor that could mitigate that cost would be to make
the sorbent material in the country where it will be used,” says
Turchi. “Using less expensive labor and avoiding the costs of
transportation could lower the cost significantly.”
In the meantime, even affluent countries such as the United States
are still searching for an effective, affordable response to arsenic. “You
can put a reverse osmosis filter on your sink at a cost of six to eight
hundred dollars,” Harbut says. “That’s not much to
some, but for too many in this country, that’s just more than
they can afford. We need, as a society, to fund these systems for those
who can’t afford them.” Municipal water systems appear
to have the technology to address the arsenic issue, but private well
owners, as well as developing nations whose populations are scattered
far beyond the reach of any centralized system, need a fast, safe,
reliable system for removing arsenic from water. Further testing will
tell if ADA’s amended silicate technology can provide one answer.
Lance Frazer
Suggested Reading
Hussain MD, Haque MA, Islam MM, Hossen MA. 2001.
Approaches for removal of arsenic from tubewell water
for drinking purpose. In: Ahmed MF, Ali MA, Adeel
Z, eds. Technologies for Arsenic Removal from Drinking
Water. Tokyo; Dhaka: The United Nations University;
Bangladesh University of Engineering and Technology;
69-75. Available: http://www.unu.edu/env/Arsenic/Hussain.pdf.
Mushak P. 2000. Arsenic and Old Laws: A Scientific
and Public Health Analysis of Arsenic Occurrence
in Drinking Water, Its Health Effects, and EPA’s
Outdated Arsenic Tap Water Standard. New York: Natural
Resources Defense Council. Available: http://www.nrdc.org/water/drinking/arsenic/aolinx.asp.
Robins RG, Nishimura T, Singh P. 2001. Removal
of arsenic from drinking water by precipitation,
adsorption or cementation. In: Ahmed MF, Ali MA,
Adeel Z, eds. Technologies for Arsenic Removal from
Drinking Water. Tokyo; Dhaka: The United Nations
University; Bangladesh University of Engineering
and Technology; 31-42. Available: http://www.unu.edu/env/Arsenic/Robins.pdf.
Other
Arrows in the Arsenic Arsenal
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Juggling strategies. Simple
arsenic mitigation methods that can be used at
home are one target of ongoing research.
image: Shehzad Noorani/World Bank |
Many other researchers are seeking the magic combination
of a cheap and effective arsenic remediation process.
All too often, if a treatment process is effective,
it’s not cheap, and if it’s cheap, it’s
not effective. A couple of other new ideas being tested
might, like the ADA strategy, meet both goals.
Ashok Gadgil, a researcher at Lawrence Berkeley
National Laboratory, is working with a by-product
of coal burning called bottom ash. Bottom ash
(which differs from fly ash in that the former
contains no heavy metals) is an ultrafine substance,
with particles one-tenth to one-hundredth the
width of a human hair. Gadgil and his team
coat the ash particles with ferric hydroxide,
which in turn bonds powerfully with available
arsenic. Initial laboratory tests indicate
the substance can reduce arsenic concentrations
from 2,400 µg/L to only 10 µg/L
within an hour.
Gadgil envisions loading this material
into a teabag-sized filter to go in a water
jug, providing a Bangladeshi family of six
with a day’s safe drinking water. Costs,
he estimates, might run around 30¢ per
person per year. Gadgil is also testing the
material for possible use in a water treatment
system for small U.S. municipal water treatment
facilities.
On the opposite coast, an engineering team
under the direction of Massachusetts Institute
of Technology engineering professor Susan
Murcott has hit on
the idea of a filtration system utilizing layers of sand, brick chips,
gravel, and iron nails. Once again, the strong attraction between arsenic
and iron comes into play, as tests indicate arsenic contamination can
be reduced to 10 µg/L within an hour. Cost of the initial system
is about US$16 per year.
Only time--and much more testing--will
tell whether these approaches or any of the
others being developed around the world will
meet all of the criteria of simplicity, reliability,
and ease of use. One additional incentive
to find such an approach is the new Grainger
Challenge Prize for Sustainability. The National
Academy of Engineering is offering this $1
million prize to help solve the massive public
health problem of arsenic contamination.
The prize will be awarded to an individual
or group for the design and creation of a
workable, sustainable, economical point-of-use
water treatment system for arsenic-contaminated
groundwater in Bangladesh, India, Nepal,
and other developing countries. The first
Grainger Challenge prize will be awarded
in February 2007.
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