Enhanced reduction of Cr(VI) in iron-carbon micro-electrolysis constructed wetlands: Mechanisms of iron cycle and microbial interactions

The increased pollution of Cr(VI) poses a major threat to public health and the ecological environment. Iron-carbon (Fe-C) micro-electrolysis was a potential sustainable strategy for improving Cr(VI) reduction in constructed wetlands (CWs). This study explored the performance and mechanisms of Cr(VI) removal in Fe-C based CWs that established with iron oxide and biochar, especially the microbial-driven Fe cycle. 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 group without Fe-C, respectively. As a vital sink for Cr, the Fe-C substrate presented better capacity for reducing Cr(VI), which showed a 20.5% lower Cr(VI) content than the control. Moreover, the Fe cycle along substrate depths were highly concurrent with Cr(VI) reduction, which was driven by the enhanced microbial function of electron transport (improved electron transport chain) and Fe oxidation or reduction microbes (such as Geobacter, Desulfovibrio, Pseudomonas, and Ferrovibrio). The relative abundances of some strains of Betaproteobacteriales and Comamonadaceae were increased in the Fe-C system, which was highly related to chrA, sdhA, mtrC, and nemA that associated with Cr(VI) reduction and transformation enzyme-encoding genes. Our results may be helpful for understanding the processes and mechanisms of Cr(VI) removal in Fe-C mediated CWs.