Effect of magnetite on the performance of anaerobic reduction of methyl orange by Shewanella oneidensis MR-1

Abstract:

[Objective] Studies have discovered that magnetite could be used as an extracellular electron transfer mediator to increase or decrease the microbial reduction rate of dyes. However, the mechanisms underlying these two distinct results remain to be elucidated. [Methods] In this study, magnetite was synthesized by the hydrothermal method and used for the anaerobic reduction of methyl orange (MO), a typical azo dye, by Shewanella oneidensis MR-1. [Results] Magnetite exerted concentration-dependent effects on the degradation of MO. Specifically, low concentrations (20-50 mg/L) of magnetite increased the decolorization efficiency of MO by 4.07%-10.64%, while high concentrations (100-200 mg/L) of magnetite decreased the efficiency by 3.92%-18.35%, compared with the group with only bacteria for degradation. Furthermore, the changes in magnetite concentration affected cell surface morphology, metabolic activity, and electron transfer efficiency rather than the distribution of dyes on the microbial surface in the microbial reduction of MO. Low concentrations of magnetite increased ATP production by 1.08%-7.65% and led to the production of 0.033-0.051 mg/L Fe²⁺, while the high concentrations of magnetite decreased ATP production by 38.74%-60.14% and increased Fe²⁺ concentration to 0.091 mg/L. In addition, exogenous Fe²⁺ showed similar concentration-dependent effects on the anaerobic reduction of MO, i.e., promoting the MO reduction at low concentrations (0.01-0.02 mg/L) and inhibiting the reduction at high concentrations (＞0.05 mg/L). [Conclusion] Low concentrations of magnetite did not affect the bacterial cell morphology and improved the cell metabolic activity. A small amount of dissolved Fe²⁺ in the system favored the reduction of MO by bacteria, whereas high concentrations of magnetite showed an opposite influencing trend. This work enriches our understanding about the effect of magnetite on extracellular electron transfer and its application in the reductive transformation of organic pollutants.