Rusty but New

Innovative Treatment Relies on Rust to Prevent Arsenic Contamination

by Chris Bryant

This beaker contains a solution central to an experimental method of treating arsenic contaminated sites. (Chip Cooper)
This beaker contains a solution central to an experimental method of treating arsenic contaminated sites. (Chip Cooper)

Odorless and tasteless, it occurs naturally in soil, water, air, plants and animals.

It can also be deadly.

Arsenic has been used as a poison for thousands of years, and, in this country, arsenic trioxide was commonly used for decades as a herbicide and pesticide, as well as in treating lumber to prevent rotting. North America is peppered with sites first contaminated 50 or more years ago that still contain significant levels of arsenic.

A University of Alabama geochemist, in conjunction with industrial partners, is developing and testing a method to prevent arsenic at contaminated sites from leaching, or filtering, through the soil and into drinking water supplies. Central to its effectiveness is, oddly enough, rust.

Dr. Rona Donahoe, professor of geological sciences, along with four UA graduate students, is using more than $350,000 in external funding to test the method in both the laboratory and at an industrial site.

“We’ve developed a method to treat arsenic contaminated soil so that the arsenic is chemically bound within the soil and immobilized,” Donahoe says. Immobilization prevents the contaminant from leaching into water supplies.

In January, the Environmental Protection Agency significantly lowered the level of arsenic deemed acceptable in drinking water. “That decrease in the maximum contaminant level (or MCL) had been a long time coming,” Donahoe says. Years ago, the World Health Organization set the maximum arsenic level for drinking water at 10 parts per billion, while the U.S. used 50 parts per billion. Before deciding to lower the levels to match the WHO, the EPA considered lowering the standard to a value between the two figures.

“I remember seeing statistics,” Donahoe says, “that if the MCL should be lowered to 20 parts per billion, more than 25 percent of the water supplies in the United States would fail to meet that level. So, at 10 parts per billion, you can see we’re looking at a more serious situation.”

Dr. Rona J. Donahoe, center, and UA doctoral students and staff display a Geoprobe, a drilling instrument similar to one used in the geochemist's experimental treatments of arsenic contaminated soil. Joining Donahoe are, from left, Ziming Yue, Sidhartha Bhattacharyya, both doctoral students, Elizabeth Graham, manager of UA's Geochemical Analytical Laboratory, and doctoral student Ghanashyam Neupane. (Chip Cooper)
Dr. Rona J. Donahoe, center, and UA doctoral students and staff display a Geoprobe, a drilling instrument similar to one used in the geochemist’s experimental treatments of arsenic contaminated soil. Joining Donahoe are, from left, Ziming Yue, Sidhartha Bhattacharyya, both doctoral students, Elizabeth Graham, manager of UA’s Geochemical Analytical Laboratory, and doctoral student Ghanashyam Neupane. (Chip Cooper)

The new treatment method involves injecting into the soil a solution of ferrous sulfate through 2-inch diameter holes created with an instrument called a Geoprobe. Pushed to a depth near the water table, (some 8 feet at the field test site) the holes are centered 5-6 feet apart. Enough solution is injected through the holes to saturate the soil throughout the treated area.
“We’ve treated one site, and we’re tracking that site with monitoring wells,” Donahoe says. “We’re in the third year after treatment, and the arsenic levels have fallen and are staying low, so that looks very promising.” In laboratory experiments, adding lanthanum chloride to the treatment solution further immobilizes the arsenic, she said.

“The leachability is reduced to virtually zero over long time periods. We cannot detect any leaching of arsenic using our normal analytical methods.”

While using ferrous sulfate to treat contaminated waste is commonplace, it’s never previously been used to treat large contaminated areas on-site.

“It’s possible that the actual application method, itself, might be patented, but we’re a long way from that right now,” Donahoe says. “To satisfy regulators that this method is going to be successful, we must demonstrate, over a long time period, that there will be no remobilization of the arsenic.”

If accepted, this new method could prove more effective, and more economical, than current clean-up, or remediation, efforts.

Decontamination efforts of large sites have traditionally involved excavating the contaminated soil and transporting it to specially designed landfills equipped for hazardous waste.

“That’s very expensive and, in some cases, it’s impractical,” Donahoe says, “if you are dealing with an industrial site where access by heavy equipment is impossible or dangerous. In those cases, chemical fixation (the treatment method UA is studying) is the only reasonable way to deal with those sites. Potentially, it offers a huge cost savings – even at sites where you don’t have the kind of restricted access that would cause excavations to be a problem.”

Plus, moving the soil, as done in excavation, doesn’t really solve the problem, Donahoe points out, rather it just transfers it, albeit to a safer site.

Chemically, Donahoe’s method relies on rust to remedy the problem. “The solution injected into the soil oxidizes, forming ferric hydroxide. “Ferric iron is very insoluble, and it forms, basically, rust,” she says.

Shortly after the ferric sulfate solution is injected into the soil, arsenic actually co-precipitates with the ferric hydroxide and becomes part of that material’s structure, the researchers believe. In addition, the rust material coats particles in the soil and provides a very high adsorption surface which binds arsenic remaining in solution.

Limitations on the new method appear to be few. Because the treatment does result in the release of two less harmful contaminants, iron and sulfate, the method would not be used in the protected area around municipal water wells.

“With enough distance, the iron will precipitate fully and won’t be a problem in the groundwater,” Donahoe says. Nor would the method typically be effective below the water table where oxygen is not typically present. “This technique would not be good at a site where you would expect the water table to fluctuate greatly and get close to the surface.”

Arsenic has been linked to multiple types of cancer as well as partial paralysis and blindness. It is most troubling when it contaminates drinking water supplies in unsafe amounts and in organometallic form.

“Organometallic forms of any toxic element are more dangerous because they are more available for uptake by organisms,” Donahoe said. Arsenic contamination in drinking water is most problematic in the western part of the U.S. where it occurs naturally at high levels, she said.

In addition to the field tests, Donahoe and students conduct column tests in the lab to mimic how sites contaminated with arsenic change over time.

Today, some industrial sites, golf courses and agricultural areas still use herbicides containing arsenic, Donahoe says, despite the potential dangers.

“There are alternative herbicides and pesticides, but anything that kills an insect is potentially harmful to higher animals. There is no good solution.”

However, the technique Donahoe is developing may prove a good treatment solution. And that brings the UA researcher much satisfaction.

“Theoretical research is all well and good, and it has merits in its own right, but, personally, I prefer to apply the research so that I can see there is some good coming from it. It gives me more satisfaction than only publishing theoretical results in a journal.”

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