ABSTRACT
Arsenic occurs in the natural environment in the oxidation states −3, 0, +3 and +5 and is found in about 200 different minerals including elemental arsenic, arsenides, sulphides, oxides, arsenates and arsenites (Anonymous 2000; Riedel 1994; c 1). The most abundant mineral is arsenopyrite, FeAsS, which is often the host for gold. Arsenic is a well-known poison and is not an element essential for the human body (Anonymous 2000; c 2). In water, arsenic is found in the two oxidation states +3 and +5 (c 11). The trivalent form, As(III), is hydrolysed as arsenous acid H3 AsO3 and is present as the free acid or as one of the species resulting from its dissociation. The pentavalent form, As(V), is also hydrolysed as arsenic acid H3 AsO4, and occurs as the non-dissociated acid or as their dissociated species. However, the two acids show completely different dissociation patterns (Figure 3.1). Arsenic acid is almost completely dissociated at pH values >4, while arsenous acid shows substantial dissociation only at pH values >8.
