Microbial electrochemical reduction of chromate in groundwater driven by Thiobacillus denitrificans

Chromium is a strategic metal widely used in various industrial applications. However, its toxic form—hexavalent chromium [Cr(VI)]—is frequently detected in groundwater, posing serious risks to both the environment and human health. This study explored the microbial electrochemical reduction of Cr(VI) in groundwater using Thiobacillus denitrificans with electrode-derived free electrons as the electron donor. In bioelectrochemical systems (BES), 76.9 ± 1.9 % of Cr(VI) was removed by the end of the batch experiment—significantly higher than the removal observed in abiotic controls and biological systems without an applied voltage. Notably, T. denitrificans attached to the cathode and achieved a much greater Cr(VI) removal efficiency (70.8 ± 1.5 %) compared to the suspended cells (30.6 ± 3.4 %). Analysis of the cathode precipitates confirmed that Cr(VI) was microbially reduced to its less toxic trivalent form [Cr(III)]. Protein analysis showed a substantial increase in cathode-attached biomass, reaching 207.4 ± 22.7 mg/g VSS. Enzyme assays demonstrated that the levels of oxidoreductases (e.g., dehydrogenase, catalase) and electron transfer mediators (e.g., cytochrome c) were significantly elevated under electrical stimulation. Furthermore, RT-qPCR analysis of chromate reductase genes (e.g., chrA, yieF) and electron transfer components (e.g., mtrC, omcB) showed substantial upregulation in gene expression. Collectively, these findings indicate that Cr(VI) reduction by T. denitrificans in BES is facilitated by the combined action of Cr(VI) reductases and electron transfer-related pathways. This study provides strong evidence supporting microbial electrochemical systems as a promising and sustainable approach for remediating Cr(VI)-contaminated groundwater.