Enhanced nitrate reduction and current generation by Bacillus sp in the presence of iron oxides

Purpose Fe(III) has been reported as a strictly competitive electron acceptor with respect to other substrate reductions by dissimilatory Fe(III)-reducing bacteria (DIRB). However, the effect of Fe(III) oxides on the substrate reduction by other microorganisms remain unknown. The aims of this study were to investigate the effects of iron oxides on the nitrate reduction and current generation by Bacillus sp., in which the nitrate and carbon anodes served as soluble and insoluble electron acceptors, respectively. Materials and methods Microbial nitrate reduction by Bacillus sp. were conducted in batch cultures in the absence or presence of four chemically synthesized iron(III) oxyhydroxides [i.e., alpha-FeOOH, gamma-FeOOH, alpha-Fe2O3, and gamma-Fe2O3]. Anaerobic techniques were used throughout all the experiments. NO3-/NO2- was determined by ion chromatography, and NH4+ was measured by spectrophotometry at 420 nm after a color reaction with Nessler's reagent. For total Fe(II) determination, samples were extracted using 0.5 M HCl and tested by spectrophotometry at 510 nm, and Fe(II) analyses in NO3- containing samples were performed using a sequential extraction technique. Current generation was tested using a bioelectrochemical reactor that consisted of two identical chambers separated by a cation exchange membrane. Results and discussion The results showed that four iron oxides markedly enhanced the nitrate reduction and current generation by Bacillus sp. Nitrate reduction by the Fe(II) on the oxide surface was proven to take place, but with lower reduction rate than the direct microbial nitrate reduction by Bacillus sp. Al2O3 and TiO2, as control without Fe(II) formation, also enhanced the nitrate reduction and current generation. It was proposed that the electron may be transferred from Bacillus sp. to conduction band of iron oxides to the nitrate or anode, according to their redox potential ranking as outer membrane enzyme of microorganisms < conduction band of iron oxides < electron acceptors. Conclusions This study demonstrated that the presence of iron oxides can obviously enhance both the nitrate reduction and current generation by Bacillus sp., which was in contrast to the previous report with respect to the inhibition effect of Fe(III) on substrate reduction by DIRB. With respect to the semiconductive properties of iron oxides, their roles during the nitrate reduction and current generation were speculated as a conduction band of iron oxides mediating the electron transfer from Bacillus sp. to the nitrate and anode.