Previously I wrote about the medicinal applications of arsenic. The therapeutic uses of this metalloid element are, of course, limited by the serious risk of toxicity. As any addict of crime fiction will be aware, arsenic has acquired a reputation as the favourite poison of murderers.
Unfortunately, arsenic also happens to be among the world’s most common environmental pollutants. It finds its way into drinking water from both natural and man-made sources and is unwittingly or unwillingly consumed by millions of people worldwide. Even at low concentrations, it may be responsible for a range of health problems, including skin lesions and cancers.
Some years ago, because of bacterial pollution in drinking water drawn from rivers, humanitarian organisations working in Bangladesh and neighbouring countries began drilling wells to tap relatively microbe-free underground streams and aquifers. Bacterial diseases declined significantly in communities provided with these wells, but after some years people began showing signs of chronic arsenic poisoning. It turned out that the subterranean streams had passed through layers of rock containing arsenic, which had leached into the water. As a result, the humanitarian projects had resulted in “the largest mass poisoning in history”, to quote the World Health Organization.
So what is to be done about arsenic in drinking water? The existing methods for removing it are too expensive for use in impoverished communities, and so cheap and simple approaches are urgently needed.
A research team at the University of Florida, led by Bin Gao, may have come up with an answer, based on a type of charcoal known as biochar. The researchers produced their biochar from hickory wood chips, which were ground and then heated in nitrogen gas but not burned. The resulting product was treated with a ferrous nitrate bath that impregnated it with iron, using a novel method that directly hydrolysed iron salt on to the biochar.
Investigations using various concentrations of arsenic showed that the iron-enhanced biochar had much better sorption of aqueous arsenic than plain biochar, even though the iron impregnation decreased the specific surface area of the biochar.
Although testing was carried out only under laboratory conditions, the researchers believe their iron-impregnated biochar could have a practical value as a low-cost filter material for removing arsenic from aqueous solutions. They say that it could easily be produced on a large scale, and the hickory wood used in their own experiments could be replaced by any carbon-rich biomass, including waste materials from forestry and agriculture.
Gao suggests that water treatment plants could install large biochar filters to extract arsenic, while homeowners could be supplied with small filters through which to pass their drinking water.
Unfortunately, at the time of writing, Gao and his team are still struggling to attract funding to test their iron-rich biochar on a broader scale.