Microbial reduction of Fe(III) in nontronite: Role of biochar as a redox mediator

Biochar naturally occurs in soils and sediments as a result of wildfire, but its role in Fe redox cycling is poorly known. In this study, bioreduction experiments were conducted with lactate as the electron donor, Fe(III) in nontronite (NAu-2) or ferric citrate as the electron acceptor, and Shewanella putrefaciens CN32 as the Fe(III)-reducing bacterium in the absence and presence of biochar. High biochar concentrations increased but low concentrations decreased the reduction rate and extent. When NAu-2 was replaced with ferric citrate [aqueous Fe(III)-citrate complex], while the rate was enhanced, the extent of bioreduction remained unchanged at low biochar concentration but declined at high biochar concentration. These different redox behaviors of biochar can be explained by the interplay among four factors: (1) the electron shuttling role of biochar; (2) the electron buffering capacity of biochar; (3) the effect of biochar surface on cell distribution; (4) the effect of biochar on cell attachment and growth. In the absence of biochar, the electron transfer pathway is from cells directly to Fe(III) (Pathway 1). However, in the presence of biochar, there is a second pathway (Pathway 2), i.e., from attached cells to biochar and from biochar to Fe(III). At low biochar concentrations, the presence of biochar decreases the efficiency of Pathway 1 because some cells are diverted to biochar surface, but this deficiency is not made up by Pathway 2 because of physical separation between negatively-charged biochar and NAu-2 particles. Therefore, even though biochar particles receive electrons from attached cells, they are not able to transfer them to NAu-2, thus retaining a proportion of electrons. Possible cell clumping on biochar surface may decrease effective cell concentration and thus pose additional inhibition of NAu-2 bioreduction. However, in the treatment of soluble Fe(III)-citrate complex, Pathway 2 is still effective, because there is no physical barrier between biochar and soluble Fe(III). The electron shuttling role of biochar actually increases the rate of Fe(III) bioreduction. As biochar concentration increases, large surface area of biochar spreads out cells and increases the probability of encounter between cells and NAu-2 particles. In this case, both pathways are effective. Increasing biochar concentrations result in fewer electron retention per gram of biochar. Possible cell growth on biochar surface would further enhance Pathway 2. Therefore, the overall result is the enhanced rate and extent of Fe(III) bioreduction in NAu-2. However, for soluble Fe(III)-citrate complex, both pathways may be impaired because high concentration of biochar would accumulate toxic substances and decrease concentration of cells, which would inhibit Fe(III) bioreduction. As biochar accumulates in soils, the role of biochar is expected to greatly impact Fe redox reactions and other environmental processes.