Calcite, a filter for water-borne arsenic?

An experiment at the Institut Laue-Langevin raises great hopes.

The health of millions of people is threatened by the presence of arsenic in their drinking water. In countries such as India, Bangladesh, Vietnam, Taiwan and Argentina, tens of millions of people drink untreated water containing levels of arsenic between ten and one hundred times higher than that recommended by the World Health Organisation ! Even in France, many sources of water, including mineral waters, will no longer satisfy new standards introduced on 1st January 2004 (which limit the concentration of arsenic in drinking water to 10 ìg L-1).

In order to decontaminate these water supplies, we urgently need to find an inexpensive and efficient method of trapping the arsenic. Studies have recently been conducted at the ILL into the capacity of calcite to capture arsenic. Unlike other substances such as manganese sand, iron oxides or ettringite, which though more efficient are much rarer and more expensive, calcite is in fact the main constituent of limestone and therefore an extremely common mineral.

The experiments using neutrons have been carried out to study the incorporation of arsenic beyond the surface and into the bulk of the sample. Due to the very high resolution of the diffractometer D20, it has been shown that up to 80 mg of arsenic per gram of calcite can be captured by this material ! AsO33– replaces the carbonate (CO32–) in the calcite lattice, which expands to accommodate it. The higher the degree of substitution, the larger the lattice cell becomes.

This discovery raises great hopes, since calcite can be synthetically produced in water purification systems simply by allowing a milk of lime (used in water purification plants and power stations to neutralise effluents and separate pollutants) to react with the carbon dioxide in the air.

Pure (synthetic) calcite could therefore offer a cheap and simple solution for filtering water contaminated with arsenic.