Characteristics And Mechanisms Of Arsenic Bioremediation By Arsenic-resistant Fe(Ⅱ)-Oxidizing Bacteria In Aqueous Environment

Arsenic(As) is a common environmental toxin, endangering hundreds of millions of people via consumption of As-contaminated groundwater, which makes As removal from aquatic system of ultmost importance. High As groundwater usually contains high concentrations of ferrous ions. Therefore, As removal by iron oxidizing bacteria-induced iron oxide mineral precipitation attracts increasing attention, which could provide a feasible way of in-situ remediation for high As groundwater.Two strains, including aerobic Pseudomonas sp. strain GE-1 and anaerobic Pseudogulbenkiania sp. strain 2002, were used to investigate the characterstics and mechanisms of Fe(II) oxidation, the characterstics and mechanisms of As removal during the formation of bacteria-induced Fe(III) oxide precipitation.Two line ferrihydrite was formed in the presence of aerobic Fe(II)-oxidizing bacterium, Pseudomonas sp. strain GE-1, using the nitrate as the electron acceptor. Arsenic was effectively immobilized through co-precipitation with and adsorption onto biogenic ferrihydrite. Co-precipitation performed better in As(V) removal than As(III) removal, while adsorption showed higher capacity for As(III) removal. Pseudo-second kinetic model successfully described the anaerobic As adsoroption on the biogenic two-line ferrihydrite, which induced neither As(III) oxidation nor As(V) reduction. However, As(III) oxidation occurred in solid phases during co-precipitation and adsorption under aerobic condition with the presence of strain GE-1. In solutions, As(V) was reduced to As(III) during co-precipitation, although no As(V) reduction occurred during adsorption.Anaerobic Fe(II)-oxidizing bacterium, strain 2002 showed a higher resistance for As(V) than As(III). The maximum tolerance of As(V) was approximately 1000 mg/L. Cells of strain 2002 tended to be shorter in the presence of high As(V) or As(III) concentrations. Results showed that Fe(II) was effectively oxidized, mainly forming lepidocrocite, which immobilized more As(III) than As(V) without changing redox state of As. When the initial Fe/As ratio was kept constant, higher initial Fe(II) concentration immobilized more As with higher As surface coverage of biogenic lepidocrocite. EXAFS analysis showed that variations of initial Fe(II) concentrations did not change the As-Fe complexes(bidentate binuclear complexes(2C)) when fixed As(III) or As(V) initial concentration(13.3 μM). Whereas, variations in initial As concentrations but fixing Fe(II) initial concentration induced the co-occurrence of bidentate binuclear and bidentate mononuclear complexes(2E) and bidentate binuclear and monodentate mononuclear complexes(1V) for As(III) and As(V) treated series, respectively. The coexistence of 2C and 2E complexes(or 1V complexes) would contribute to higher As removal in series with higher initial Fe(II) contents at the same Fe/As ratio.Both strain GE-1 and strain 2002 showed high As removal efficiency during the Fe(II) oxidation using the nitrate as electron acceptor. Therefore, simultaneous removal of soluble As and nitrate by nitrate-reducing Fe(II)-oxidizing bacteria provides a feasible approach for in-situ remediation of As-nitrate co-contaminated groundwater.