First Ever Accurate Test for Arsenic in Soils

If you have a cat or dog who likes to hide under the deck or children who play on equipment made with pressure-treated wood, you’ll be glad to hear that analytical chemist Julian Tyson and colleagues at the University of Massachusetts Amherst recently developed the first-ever accurate test for arsenic compounds in soil, promising a significantly improved environmental and health impact assessment. The method holds some promise for detecting naturally occurring high arsenic levels in Asian rice, as well.

In North America, arsenic is found most commonly under decks and near structures such as playground gyms made of pressure-treated wood, which is impregnated with heavy metals. The squeezed-in chromium, copper and arsenic make wood weather-resistant and durable but they also slowly leach out into the environment, mainly soil. The potential health impact, called by some an “environmental time bomb,” has been difficult to assess in an objective, quantitative way until now, according to Tyson and his graduate student co-author Khalid Al-Assaf, because the key arsenic compounds stick so tightly to iron oxides that they couldn’t be isolated and measured separately.

“It’s been very hard to know if this source of contamination was staying put, evaporating into the air or getting into the groundwater,” Tyson explains. Several laboratories have long sought a soil test for arsenic, but his research team is the first to develop a procedure for isolating all the compounds of interest, including the mono- and dimethylated species in soil and accurately measuring them.

Their paper describing the new technique is the cover story for the April 4 issue of the Journal of Analytical Atomic Spectrometry published by the British Royal Society of Chemistry, now available online. With the new procedure, chemists can now help to answer questions about whether arsenic compounds are getting into drinking water supplies, being taken up by plants, and whether soil bacteria are involved in the production of methylated compounds.

It’s already known that arsenic is easily ingested by children who touch pressure-treated wood play equipment and then put their hands in their mouths, and it’s brought into homes on pets that get into dirt under pressure-treated wood decks. Tyson says earlier attempts to trace arsenic movement through the environment by sampling dog toenails were not conclusive.

The chemist adds that because some bacteria in soil are able to convert arsenic to volatile products, and iron oxides can bind it tightly, the residue in the soil may not travel very far, so “we probably shouldn’t be unduly alarmed. However, to be prudent you don’t want to see children eating it, either. Our new method will help to determine how much can be considered bioavailable near these sources.”

The new method calls for scientists to extract arsenic compounds with sodium hydroxide and phosphoric acid, separate them by chromatography, convert them to volatile hydrides and measure them by the light emitted from a high-temperature inductively-coupled plasma (atomic emission spectroscopy).

Arsenic is found in high concentrations in chicken manure from factory farms. Interestingly, another source of surface-soil arsenic in the United States is Civil War-era cemeteries. At that time, so many casualties could occur in a short period that bodies had to wait for burial. In the interim, corpses were preserved in arsenic-laden embalming fluid until individual graves could be dug. Overall, there is little evidence that surface arsenic ends up in our groundwater, but “the jury is still out” on this point, Tyson adds.

Elsewhere in the world, notably Southeast Asia, high arsenic levels occur naturally in soil and groundwater, according to the chemist. Unfortunately, because rice is “an accumulator” plant species, millions of people in Bangladesh, China and India face the risk of toxic contamination in their food. Tyson and colleagues’ method for isolating the arsenic compounds from soils may be adaptable to determine arsenic concentrations in batches of rice, he says, to improve food safety.

Further, as one of the world’s leading research groups with expertise in arsenic but also in the mineral selenium, Tyson’s UMass Amherst analytical chemistry team is collaborating with researchers in Bangladesh to explore preliminary evidence that the mineral might mitigate arsenic poisoning in at-risk populations if taken as a dietary supplement.

At UMass Amherst, Tyson runs the “arsenic project” in which first-year undergraduates get an early, authentic research experience with some aspect of the environmental and analytical chemistry of arsenic. Here, “they get to see how science is really done,” he says. The chemist hopes students find their study so exciting they “get hooked,” seek out further research experience as undergraduates and seriously consider graduate study.

Posted April 1st. 2009

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