MICROBIAL FE(III) REDUCTION KINETICS IN NONTRONITE (NAU-2) IN THE PRESENCE OF ELECTRON SHUTTLE

Bioreduction experiments were performed to understand the rate and extent of bioreduction of Fe(III) in NAu-2 in different electron acceptor [Fe(III) in NAu-2] concentration by a metal reducing bacterium Shewanella putrefaciens, CN32. The 0.5-0.2 mm fraction of NAu-2 (Source Clay Repository, IN) contained 23.4% of iron in its structure with almost all iron (99.5%) as Fe(III). The non-growth reduction experiments were performed in bicarbonate buffer, and total and aqueous Fe(II) production with time were measured by Ferrozine assay. The electron shuttle (AQDS) was added in each experiment to understand the effect of electron shuttle for Fe(III) reduction. The adsorption of Fe(II) onto both NAu-2 and bacterial surfaces was performed using strict anaerobic oxygen trap inside the anaerobic chamber. Since all Fe(III) in the NAu-2 was not available for bioreduction, ‘bioreducible' Fe(III) in NAu-2 was estimated from a separate experiment performed in the presence of AQDS, and was used for kinetic analysis. The reduction rate of Fe(III) normalized by the maximum sorption capacity and initial concentration of NAu-2 followed first order, typical of Monod region where substrate concentration was less than the half saturation constant. The Modified Monod kinetic equation was used to model the bioreduction. The best fit Monod parameters for the bioreduction equation were calculated using non-linear least square regression (DataFit 8.0, Oakdale Engineering). Adsorption isotherm modeled using Langmuir sorption kinetics showed that maximum sorption capacity (qmax) of Fe(II) on NAu-2 was 0.903 mM/g at NAu-2 concentration of 5mg/ml, and to cell was 0.146 mM/1E8cells/ml (0.1 mM/g of cell). It suggested strong affinity of Fe(II) onto the NAu-2 surface than that on cell surface. The time course adsorption measurement showed that NAu-2 surface saturated faster than that of cell surface. The results from measured aqueous iron [Fe(II)aq], which was less than 2-3% of the total biogenic ferrous iron [Fe(II)total], was consistent with extensive sorption of Fe(II) onto NAu-2 and cell surfaces. Therefore the sorption phenomena was supposed as a dominant limiting factor for Fe(III) reduction. It is concluded that bioreduction of Fe(III) in NAu-2 can be quantified by modified Monod kinetics.