Additive and competitive effects of bacteria and Mn oxides on arsenite oxidation kinetics.

Arsenic (As) is a redox-active metalloid whose toxicity and mobility in soil depend on oxidation state. Arsenite [As(III)] can be oxidized to arsenate [As(V)] by both minerals and microbes in soil however, the interaction between these abiotic and biotic processes is not well understood. In this study, the time dependency of As(III) oxidation by two heterotrophic soil bacteria (Agrobacterium tumefaciens and Pseudomonas fluorescens) and a poorly crystalline manganese (Mn) oxide mineral (δ-MnO(2)) was determined using batch experiments. The apparent rate of As(V) appearance in solution was greater for the combined batch experiments in which bacteria and δ-MnO(2) were oxidizing As(III) at the same time than for either component alone. The additive effect of the mixed cell- δ-MnO(2) system was consistent for short (<1 h) and long (24 h) term coincubation indicating that mineral surface inhibition by cells has little effect the As(III) oxidation rate. Surface interactions between cells and the mineral surface were indicated by sorption and pH-induced desorption results. Total sorption of As on the mineral was lower with bacteria present (16.1 ± 0.8% As sorbed) and higher with δ-MnO(2) alone (23.4 ± 1%) and As was more easily desorbed from the cell-δ-MnO(2) system than from δ-MnO(2) alone. Therefore, the presence of bacteria inhibited As sorption and decreased the stability of sorbed As on δ-MnO(2) even though As(III) was oxidized fastest in a mixed cell-δ-MnO(2) system. The additive effect of biotic (As-oxidizing bacteria) and abiotic (δ-MnO(2) mineral) oxidation processes in a system containing both oxidants suggests that mineral-only results may underestimate the oxidative capacity of natural systems with biotic and abiotic As(III) oxidation pathways.