Chromium (VI) reduction by two-chamber bioelectrochemical system with electrically conductive wall

Abstract

Hexavalent chromium (Cr(VI)) poses a threat to public health and the environment owing to its high toxicity. Various microbial Cr(VI) reduction methods have been proposed to treat contaminated water; however, they are not effective at high concentrations of Cr(VI), which inhibit microbial growth and viability. To overcome the limitations of conventional microbial methods, we set up a novel bioelectrochemical reactor in which microbial cells were separated from Cr(VI) by an electroconductive barrier consisting of a vertically aligned carbon nanotube (VACNT) composite to remove toxic heavy metal ions. This study demonstrated that chromium compounds could be biologically and sustainably treated with the VACNT composite without growth inhibition, using electron transfer from an electron shuttle secreted by Pseudomonas aeruginosa, even at the microbial inhibitory concentration of Cr(VI). The content of Cr(VI) was measured using the diphenylcarbazide method, and Cr(III), the reduction product, was identified using X-ray photoelectron spectroscopy. Pyocyanin, a phenazine secreted by P. aeruginosa, was identified as an electron mediator in Cr(VI) reduction, using aeration and phz mutant assay. The bioelectrochemical reactor used in this study exhibited a Cr(VI) removal rate of 99.6% at 100 mg/L, which is the highest reduction rate achieved using P. aeruginosa. The microbial Cr(VI) reduction system developed in this study allows microbial Cr(VI) reduction to overcome the hurdles of biological treatment of toxic materials and has advanced the understanding of microbial electron transfer.