Tyson's toxic technique

The first accurate test for arsenic compounds in contaminated soil has been developed by US chemists. Their atomic emission approach to the problem could provide improved environmental and health assessments. The work also has implications for improving safety for children and pets near treated wood in the garden and as well as detecting high arsenic levels in some Asian rice supplies.

Arsenic is an insidious environmental hazard we have discussed in SpectroscopyNOW on several occasions, not least because of problems with contamination of water supplies and aquifers in Asia and arid regions elsewhere.

Julian Tyson and colleagues at the University of Massachusetts Amherst have now developed an accurate test for arsenic compounds in soil that could be useful for people concerned about pet safety in the garden or children playing around equipment made with pressure-treated wood.

A quantitative way to assess the potential health impact of pressure-heated wood, which is loaded with heavy metals and arsenic has not been available until now. According to Tyson, Peter Uden and graduate student co-author Khalid Al-Assaf this apparent "environmental time bomb," could now be defused. They have explained that some important arsenic compounds bind so tightly to iron oxides that they couldn't previously be isolated and measured separately. Moreover, bacteria metabolise arsenic compounds into volatile species.

"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. He and his colleagues have now achieved a first in the long search for an arsenic soil test. They have developed a procedure to isolate all the compounds of interest, including the mono- and di-methylated species in soil and to measuring them accurately.

The new test will allow environmental chemists to answer questions about the concentrations of arsenic compounds that might be leaching into drinking water supplies, being taken up by plants, and whether soil bacteria are involved in the production of the methylated compounds.

Because of the iron oxide binding and conversion to volatiles it arsenic contamination of soil may not be the environmental time bomb some have claimed. "We probably shouldn't be unduly alarmed," says Tyson, "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 analysis involves extracting any arsenic compounds from a soil sample using sodium hydroxide and phosphoric acid. They are then separated by chromatography and converted to volatile hydrides, which can be determined to a high degree of precision by atomic emission spectroscopy.

As an educational endeavour, Tyson runs the "arsenic project" in which first-year undergraduate chemistry students are given an early induction into authentic research with some aspect of the environmental and analytical chemistry of arsenic. Here, "they get to see how science is really done," he says. "In some ways, I am as proud of the success of this effort as I am of the method for extracting and determining the the four arsenic species," he told SpectroscopyNOW.

The new AES approach to testing for arsenic used in this program could have widespread applications beyond the back garden and children's playground.

Arsenic is present in high concentrations in chicken guano from factory farms, it is present in Civil War-era cemeteries where arsenic-containing embalming fluids were used liberally to preserve corpses ahead of the burial bottleneck. Tyson's technique could trace the evidence as to whether such sources are capable of contaminating groundwater, although Tyson says that "the jury is still out" on this point.

As mentioned previously, arsenic contamination of groundwater caused by exposure of safe arsenic rock salts in the aquifers to the air during drought and after excessive drawing of water for intensive irrigation have left countless villages without a safe water supply. This affects the people drinking the water and has led to hundreds of thousands, if not millions, of people being slowly poisoned by their water supply. Moreover, the food crops, including rice, irrigated with this water are also accumulating arsenic in places such as Bangladesh, China, and India, increasing the toxic risk facing those relying on this staple 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.