Enhanced Reduction of Chromium(Vi) in Iron-Carbon Micro-Electrolysis Constructed Wetlands : Mechanisms of Iron Cycle and Microbial Interactions

Chromium(VI) poses a major threat to public health and the ecological environment. Fe-C micro-electrolysis can be utilized as a potential sustainable strategy for improving Cr(VI) reduction in constructed wetlands (CWs). However, to date, the role of the microbial-driven Fe cycle for Cr(VI) reduction in Fe-C-based CWs has barely been studied. In this study, the potential and mechanisms of Cr(VI) removal in Fe-C-based CWs were explored. Efficient removal of total Cr (49%) and Cr(VI) (65%) was detected in Fe-C systems, which was 377% and 34% greater than that of the control, respectively. As a vital sink for Cr, the Fe-C substrate presented better capacity, especially for reducing Cr(VI), which showed a 20.5% lower Cr(VI) content than the control. Moreover, the Fe cycle and microbial electron transfer in different substrate depths were highly concurrent with Cr(VI) reduction. The relative abundances of some strains of Betaproteobacteriales and Comamonadaceae increased in the Fe-C system, which was highly related to chrA, sdhA, mtrC, and nemA, which are associated with Cr(VI) reduction and transformation enzyme-encoding genes. Additionally, other microbes that facilitated Cr(VI) reduction or Fe oxidation-reduction (such as Geobacter, Desulfovibrio, Pseudomonas, and Ferrovibrio) were enriched in the Fe-C system. Correspondingly, the microbial function of electron transport was enhanced and contributed to Cr(VI) removal. Our results may be helpful for understanding the processes and mechanisms of Cr(VI) removal in Fe-C-mediated CWs treating wastewater containing heavy metals