Nanoscale zero-valent iron coupling with Shewanella oneidensis MR-1 for enhanced reduction/removal of aqueous Cr(VI)

Abstract

The surface passivation of nZVI is considered one of the most challenging in successfully applying nZVI to environmental remediation. Shewanella oneidensis MR-1 (MR-1) are widely distributed in the various aquatic environments, which can use Fe(III) (hydr)oxides as electron acceptor, converting Fe(III) to Fe(II). In this study, the nZVI coupling with MR-1 process is proposed for efficient reduction/removal of high concentration of hexavalent chromium [Cr(VI)]. By studying the experimental conditions such as initial Cr(VI) concentrations, nZVI dosages, and cell concentrations, the optimal experimental conditions for Cr(VI) removal were obtained. Under the conditions of nZVI dosage of 1.32 mM, cell concentration of 3 × 107 cells/mL and Cr(VI) concentration of 80 mg/L, the removal efficiency of Cr(VI) and total chromium [Cr(total)] can reach 98.65% and 98.23% within 6 days, respectively. The key point of this study is to demonstrate that the addition of MR-1 can promote dissolution of the passivation layer on the nZVI surface and prolong its reactivity, significantly improving Cr(VI) reduction/removal efficiency. In this process, most Cr(VI) is reduced to Cr(III), which then is removed by forming Cr2O3 and Cr(CH3C(O)CHC(O)CH3)3 complexes, and a small amount of the unreduced Cr(VI) is removed through adsorption and complexation with oxygen-containing functional groups. The extracellular polymeric substances (EPS) derived from MR-1 play a crucial role in removing the passivation layer on the nZVI surface and Cr(VI) removal. This method may have great potential application for reduction and detoxification of Cr(VI) in the aqueous environment.