5. Sources and basic chemistry of arsenic in water

Arsenic is a metalloid with the atomic number 33, atomic weight 74.9216, symbol As. It is redox-sensitive, which means that it changes its form through the addition or loss of electrons. Its state depends on geochemical factors, including pH conditions (measures of acidity or alkalinity), distribution of other ions, and microbial activity.

Although arsenic can be introduced into groundwater by mining activities, most groundwater contamination is of geological origin, typically by the weathering of arsenic-bearing rocks, minerals and ores. Arsenic occurs as a major constituent in more than 200 minerals. Arsenic is highly soluble and can appear in different compounds. The inorganic forms found in water are considered the most toxic. Soluble arsenic in drinking-water is mostly found as trivalent arsenite, As(III), or pentavalent arsenate, As (V).

Chemical speciation (the form in which the arsenic occurs) is a critical element in arsenic removal. As(III) is generally more toxic to humans and four to ten times more soluble in water than As(V), which can also be more easily removed from water. The conversion of arsenite to arsenate, using an oxidising agent, is therefore critical in many treatment processes.