Final Activity Report Summary - ARSENIC REDUCTION (Influence of arsenate adsorption onto Fe- and Al hydroxide mineral surfaces on microbial arsenate reduction rates) Microbial reduction of arsenate (As(V)) to arsenite (As(III)) may elevate not only the mobility but also the toxicity of arsenic in the environment. Adsorption of As to soil solid phases was shown to slow down As(V) reduction rates. However, despite its importance, the reduction kinetics of adsorbed As(V) by microorganisms is not well-understood. Thus, we studied the As(V) reduction kinetics by Shewanella putrefaciens CN-32 with suspension incubation experiments under different As(V) adsorption conditions. The As(V) reduction kinetics decreased with increasing extent of As(V) adsorption, depending on the nature and concentrations of the mineral sorbents. Desorption of As(V) from the solid phases was faster than the microbial reduction of dissolved As(V), suggesting that the overall rate of As(V) reduction was not limited by desorption kinetics. Microbial As(V) reduction may thus proceed predominantly via desorption and subsequent reduction of dissolved As(V). Microbe-mineral surface interactions were additionally able to mobilize adsorbed As(V), which subsequently accelerated As(V) reduction. Oxyanions (e.g. phosphate) and microbial polysaccharide production can increase As(V) desorption and reduction rates. Increased As(V) reduction kinetics were likely due to higher dissolved As(V) at the early stage of the incubation by phosphate addition and steadily increasing the dissolved As(V) concentration during the entire incubation period by spiking exopolysaccharides. In conclusion, the adsorption behaviour of As(V) strongly affects the microbial As(V) reduction kinetics. To estimate the mobility and speciation of arsenic in natural water-soil systems, the microbe-mineral-arsenic interactions must be considered.