Oxidation of AsIII by Several Manganese Oxide Minerals in Absence and Presence of Goethite

Abstract

Oxidation of AsIII by three types of manganese oxide minerals affected by goethite was investigated by chemical analysis, equilibrium redox, X-ray diffraction (XRD) and transmission electron microscopy (TEM). Three synthesized Mn oxide minerals of different types, birnessite, todorokite, and hausmannite, could actively oxidize AsIII to AsV, and greatly varied in their oxidation ability. Layer structured birnessite exhibited the highest capacity of AsIII oxidation, followed by the tunnel structured todorokite. Lower oxide hausmannite possessed much low capacity of AsIII oxidation, and released more Mn2+ than birnessite and todorokite during the oxidation. The maximum amount of AsV produced during the oxidation of AsIII by Mn oxide minerals was in the order: birnessite (480.4 mmol/kg) > todorokite (279.6 mmol/kg) > hausmannite (117.9 mmol/kg). The oxidation capacity of the Mn oxide minerals was found to be relative to the composition, crystallinity, and surface properties. In the presence of goethite oxidation of AsIII by Mn oxide minerals increased, with maximum amounts of AsV being 651.0 mmol/kg for birnessite, 332.3 mmol/kg for todorokite and 159.4 mmol/kg for hausmannite. Goethite promoted AsIII oxidation on the surface of Mn oxide minerals through adsorption of the AsV produced, incurring the decrease of AsV concentration in solutions. Thus, the combined effects of the oxidation (by Mn oxide minerals)—adsorption (by goethite) lead to rapid oxidation and immobilization of As in soils and sediments and alleviation of the AsIII toxicity in the environments.