Arsenic speciation in the field

Arsenic contamination of drinking water is widely recognised as a major public health concern, with global hotspots in Bangladesh, India, Argentina and China. This is not solely a problem that affects developing countries; it is estimated that over 13 million consumers in North America, primarily in western states, rely on groundwater with an arsenic content of greater than 10µg/L (USEPA) and require treatment. Alaska is typical of many western states in that arsenic contamination of groundwater is both natural and anthropogenic in origin. The latter is directly attributable to metal mining operations that expose sulphide minerals, especially arsenopyrite (FeAsS), to air oxidation and decomposition, with subsequent release of toxic metals. Arsenic is particularly mobile in aqueous environments because it exists as various anionic species in trivalent and pentavalent oxidation states. Further, the toxic trivalent species H3AsO3 has neutral charge in acidic conditions and therefore is not readily absorbed by clay minerals. Consequently, the oxidation state of arsenic in water samples is of great importance in predicting toxicity and mobility. Preservation of arsenic speciation is difficult and a number of different procedures have been proposed, but there is always some possibility of species transformation between the sample site and the laboratory. Separation of arsenic species at the sampling point is therefore an attractive alternative.