Sandians who have used Labs supercomputers to custom-design chemicals with þypaper-like arsenic-trapping properties plan to test the new materials in a city water-purification demonstration plant being built on Albuquerque’s West Side.
Successful trials of the materials, called Specific Anion Nanoengineered Sorbents (SANS) by their Sandia developers, could have major national implications as thousands of communities and other water providers tally up the costs of complying with a controversial new Environmental Protection Agency (EPA) mandate that reduces the maximum allowable amount of arsenic in drinking water from 50 to 10 parts per billion.
About 3,200 of the nation’s 74,000 water systems supply drinking water with arsenic levels that exceed the new limit, according to EPA estimates. Almost half of Albuquerque’s wells will fail to meet the new standard. "In essence the ruling says Albuquerque can’t use half its wells after 2005 without additional treatment," says Dave Teter of Geochemistry Dept. 6118.
The national price tag for complying with the EPA ruling might be in the tens of billions of dollars. Albuquerque’s compliance could cost $150 million, the City estimates.
Water sans arsenic
Inorganic arsenic occurs naturally in some groundwater, seeping out of rock and soils that neighbor the aquifer. Ingesting arsenic over long periods of time can cause cancer of the skin, bladder, liver, kidney, prostate, and other organs and has been linked to a variety of other cardiovascular and neurological illnesses, although scientific data linking low-level, chronic arsenic ingestion to health effects is limited.
The Sandia developers — Dave, Pat Brady, and Jim Krumhansl (all 6118) — think the new arsenic-getting SANS could reduce the sticker shock of complying with the new EPA standard for cities served by water treatment plants, rural communities, and homes, schools, and apartment complexes served by single wells.
They also have proposed using the getters to purify arsenic-laden well water in Bangladesh that is poisoning millions of people there (Lab News, April 7, 2000).
Materials selective for arsenic
"We’ve zeroed in on five classes of materials that are affordable and obtainable and peculiarly selective for arsenic," says Dave.
Most mineral getters have negatively charged surfaces, so they repel similarly charged anions. The SANS selectively attract anions such as arsenate (a toxic arsenic-containing compound) dissolved in water to positively charged sites on the SANS’ surfaces and then grab hold.
"It’s like a guest who eats the cashews out of a nut bowl," says Pat.
To create the materials, the research team selected mineral families with known affinities for anions, then used Labs supercomputers to rapidly simulate the arsenic-
trapping aptitudes of thousands of combinations and variations of the minerals.
"We knew which classes of materials should be highly selective for arsenic at the atomic level," says Pat, "so we asked ourselves what is peculiar or common about those materials. Then we tried to find or make other materials with similar properties."
Because there are nearly infinite variations of chemical species, phases, and surface chemistries, the researchers let the computer sort out the very best performers.
"We got some big hits on materials that had never been considered before," says Dave. "We expected good results, but not this good."
They ruled out those minerals that are difficult or expensive to obtain or produce, that would become saturated too quickly, or that would result in a hazardous byproduct.
"A prerequisite for us was that the solution be at least as simple, safe, and efficient as, and more affordable than, the current technology," he says.
They verified the computers’ results in a lab, pumping arsenic-contaminated water through the powdered materials, then measuring the arsenic content of the outþow.
For proprietary reasons, Dave can’t yet divulge what materials the team found, but they generally are nontoxic mixed metal oxides with high molecular surface areas.
At water treatment plants, groundwater could be pumped through columns containing the powdered materials. Arsenic content in the outþow would be reduced to undetectable levels. After perhaps years of use, the nonhazardous arsenic-saturated getters could be disposed of in a standard landfill.
Sandia’s relationship with the City of Albuquerque’s subcontractor on the new treatment plant, CH2M-Hill, has helped the research team understand the practical needs of municipalities, especially the need for a simple, affordable, available treatment technology.
"Technology has no impact if it isn’t used," says Dave.
Needle in a haystack
He estimates that some of the SANS could be supplied for as little as $200 to $300 a ton, compared to the $4,000 a ton for conventional iron hydroxides used in typical water treatment plants. (Iron hydroxides, adopted for water purification around the turn of last century, sweep out many contaminants simultaneously but don’t selectively remove arsenic.)
"Municipalities filter out dirt, silt, and sewage, but pulling out stuff at the parts-per-billion range cheaply is new," says Pat. "This is harder than finding a needle in a haystack."
The SANS could be adapted for use with smaller water systems, even down to the individual well or household scale, says Dave.
Albuquerque’s Arsenic Removal Demonstration Plant should be operational by next summer, according to City Water Resources Manager John Stomp. The plant will process more than 2 million gallons of water a day using a microfiltration/iron coagulation process, but the facility will reserve space to test developmental technologies such as the SANS.
"We’re very interested in working with Sandia to look at emerging technologies that are cheap and easy to dispose of," says Stomp.
In addition, the same research methodology used to identify the SANS for arsenic removal — computer modeling followed by experimental verification — has been used to design getters for removing other contaminants as well, says Dave.
"We are now able to predict the outcome of sorption processes at the atomic scale and chemically modify inexpensive natural materials to selectively sorb anions," he says.
"Arsenic is the focus right now, but EPA is looking at restricting concentrations of many more micropollutants in the future," he says.
Future custom-designed getters could be used to purify drinking water as well as industrial waste water, process streams, and other efþuents, he says.
The SANS team includes Dave, Pat, Jim, Buddy Anderson (6118), May Nyman, Steve Thoma (both 6233), Joe Chwirka (CH2M-Hill), Nadim Khandaker (OCETA), and Bruce Thomson (University of New Mexico).
The SANS work is part of a larger Water Initiative managed by the Energy and Critical Infrastructure SBU and led by a team of Sandians from centers 6100, 6200, 6800, 5300, and 5800. The initiative seeks technical solutions to key water issues — vulnerability of and threats to water-distribution infrastructures, socio-geopolitical stresses relating to water scarcity, and economic concerns associated with supplying drinkable water. Watch for more about the Water Initiative in future issues of the Lab News.