Kinetics of adsorption and redox processes on iron and manganese oxides: Reactions of arsenic(III) and selenium(IV) at goethite and birnessite surfaces

Selenium and arsenic are naturally-occurring, non-metallic elements with complex chemical and biological behavior in aquatic environments. In this study, rates and mechanisms of adsorption, desorption, and electron transfer reactions involving selenium and arsenic oxyanions and two naturally occurring metal oxides, goethite (;Goethite does not oxidize Se(IV) or As(III) in solution at pH 4 and above. However, redox products (Mn(II), Se(VI), As(V)) are observed when Se(IV) or As(III) is added to aqueous suspensions of birnessite. In the arsenite-birnessite system, the rate of As(V) appearance in solution is equal to the rate of As(III) disappearance from solution while the appearance of Mn(II) in solution is slightly slower. In the selenite-birnessite system, uptake of Se(IV) occurs in minutes. Extent of adsorption decreases with increasing pH. The appearance of measurable Se(VI) occurs slowly (time scale of days to weeks) and is a function of adsorbed selenite. This indicates that the rate of selenite oxidation by birnessite is limited by the rate of electron transfer. Rate data from both arsenic an selenium redox systems are successfully described by a reversible four-step kinetic model that accounts for adsorption of the reduced species, electron-transfer, release of the oxidized species, and release of reduced Mn(II).;The data suggest that iron oxides provide an adsorptive sink for mobile Se and As oxyanions, while manganese oxides play a major role in accelerating the oxidation of Se(IV) and As(III). Results on the rates of key chemical processes affecting selenium and arsenic should be useful in understanding complex geochemical cycles and in finding solutions to problems in pollutant transport and accumulation in water-sediment systems.