A Review of Redox Transformation of Arsenic in Aquatic Environments

Abstract

Arsenic is a redox sensitive element which can exist in As(V), As(III), As(0), and As(-III) oxidation states under redox conditions in the natural environment. The mobility and toxicity of arsenic are determined by its oxidation states. Thermodynamic data in the literature are summarized and used to construct a pe-pH diagram for an arsenic-iron-sulfur system. The chemical species of arsenic in aqueous systems usually are not at equilibrium status because of slow rates of redox reactions. Microbially mediated reductions and oxidations of arsenic play an important role in the transformation of arsenic in sediments, soil, geothermal water, surface water, and water treatment sludge. Heterogeneous oxidation of As(III) species takes place in suspensions containing manganese dioxides, titanium dioxides, and clay minerals. The fate and transport of arsenic is closely related to the redox reactions of sulfur and iron. Oxidized arsenic species such as As(V) are typically adsorbed on iron oxides in soil and sediments under oxic conditions. At low redox potentials, reduced arsenic species such as As(III) are associated with sulfide and pyrite minerals. The occurrence of arsenic in groundwater is mainly attributed to reductive dissolution of iron oxides and oxidative dissolutions of arsenic-rich pyrite and sulfide minerals under moderate reducing conditions (i.e., arsenic mobilization zone).