Simple salts could prevent arsenic contamination

Encouraging the growth of special microbes could help to clean up sites where groundwater has been contaminated with arsenic, US scientists have claimed.

"Microbial processes ultimately determine whether arsenic builds to dangerous levels in groundwater," researchers at the University of Illinois said. "So remediation could be as simple as stimulating certain microbes to grow."

Arsenic contamination is a serious threat to human health and has been linked with various medical conditions, including hypertension, cardiovascular disease and a number of cancers.

Some parts of Illinois are exposed to high levels of arsenic contamination in the local water, which led to the study being conducted, according to Craig Bethke, a professor of geology at the university.

"We also discovered important links between the amount of organic material dissolved in the groundwater and the concentrations of sulphate and arsenic," he explained.

The researchers analysed water samples from 21 wells at various depths at the Mahomet aquifer, a regional water supply for Central Illinois. Mr Bethke said they were surprised to see that the arsenic concentrations varied heavily from well to well. Moreover, they found that the concentration of arsenic varied inversely with the concentration of sulphate.

"Concentrations may reach hundreds of micrograms per litre in one well, which is enough to make people very sick, but then fall below detection limits in a nearby well," he said. "Our analyses suggest the aquifer is divided into zones of mixed microbial activity, some dominated by sulphate-reducing bacteria, other by methanogens."

Graduate student Matthew Kirk explained that sulphate-reducing bacteria consumed sulphate and reduced it into sulphide, which then reacts to precipitate arsenic, leaving little in solution.

However, Mr Kirk said that where the bacteria ran out of sulphate, methanogenic bacteria took over as the dominant force. As this does not produce sulphide there is no precipitation path for the arsenic, which therefore accumulates to high levels in the groundwater in these areas.

"In the aquifer, the balance between the amount of organic material and the amount of sulphate that leaches into the groundwater appears to control whether the water becomes contaminated," Mr Kirk said. "Where the supply of sulphate is high relative to organic matter, sulphate remains low. But where the organic matter is high relative to sulphate, the sulphate has depleted and arsenic accumulates."

Adding sulphate to naturally contaminated groundwater might, therefore, be a simple but highly effective way to sequester the arsenic, he added.

"The bacteria are already present, so all you have to do is stimulate them," Mr Kirk pointed out. "Sulphate salts are inexpensive, readily soluble and easily obtained."

By Jane Kettle




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