Anoxic oxidation of arsenite linked to chemolithotrophic denitrification in continuous bioreactors

In this study, the anoxic oxidation of arsenite (As(III)) linked to chemolithotrophic denitrification was shown to be feasible in continuous bioreactors. Biological oxidation of As(III) was stable over prolonged periods of operation ranging up to 3 years in continuous denitrifying bioreactors with granular biofilms. As(III) was removed with a high conversion efficiency (>92%) to arsenate (As(V)) in periods with high volumetric loadings (e.g., 3.5-5.1 mmol As L_reactor^-1 day^-1). The maximum specific activity of sampled granular sludge from the bioreactors was 0.98±0.04 mmol As(V) formed g^-1 VSS day^-1 when determined at an initial concentration of 0.5mM As(III). The microbial population adapted to high influent concentrations of As(III) up to 5.2 mM. However, the As(III) oxidation process was severely inhibited when 7.6-8.1mMAs(III) was fed. Activity was restored upon lowering the As(III) concentration to 3.8 mM. Several experimental strategies were utilized to demonstrate a dependence of the nitrate removal on As(III) oxidation as well as a dependence of the As(III) removal on nitrate reduction. The molar stoichiometric ratio of As(V) formed to nitrate removed (corrected for endogenous denitrification) in the bioreactors approximated 2.5, indicating complete denitrification was occurring. As(III) oxidation was also shown to be linked to the complete denitrification of NO₃^- to N ₂ gas by demonstrating a significantly enhanced production of N ₂ beyond the background endogenous production in a batch bioassay spiked with 3.5mM As(III). The N₂ production also corresponded closely to the expected stoichiometry of 2.5 mol As(III) mol^-1 N ₂-N for complete denitrification.