**3. Soil chemistry of arsenic and arsenic speciation**

Arsenite (As(III)) exists as the hydroxyl species (H3AsO3 - H2AsO3 1−)), whereas arsenate (As(V)) exists as an oxyanion (H2AsO4 1− or HAsO4 2−). Arsenite and arsenate may: (i) form complexes with soil organic matter, (ii) become adsorbed onto Mn- Al- and Fe-oxyhydroxides, (iii) become adsorbed onto phyllosilicates, (iv) leach or percolate to deeper soil horizons, or (v) undergo plant uptake [6–16]. Aide et al. [6] in a soil chemistry review of arsenic in the soil environment discussed (i) arsenic acid–base chemistry, (ii) commonly occurring As-bearing minerals, (iii) thermodynamics of arsenic oxidation – reduction, (iv) arsenic adsorption onto phyllosilicates and Fe-oxyhydroxides, and (v) competitive adsorption. Plant physiology and arsenic have recently been reviewed [7, 8].
