Unveiling a novel mechanism for anaerobic hexavalent chromium reduction mediated by extracellular reductases

Anaerobic biological treatment technology are considered as promising way for toxic Cr(VI)-containing wastewater removal. However, the inherent mechanisms of bioremediation of Cr(VI) by microbial enzymatic reduction is unclear, which limited the regulation for enhancing Cr(VI) removal. In this work, the effects of Cr(VI) on anaerobic process and the bio-reduction of Cr(VI) by intracellular and extracellular reductase were investigated. Results showed that the Cr(VI) exposure with concentration >5 mg/L significantly inhibited the anaerobic digestion (e.g., glucose degradation and CH₄ production) and reduced the reduction efficiency of Cr(VI). Moreover, the non-enzymatic and enzymatic test on Cr(VI) reduction indicates that enzymatic reduction dominated the transformation of Cr(VI) to Cr(III), occupying 81.6 %-89.3 % of total Cr(VI) reduction. Furthermore, the extracellular enzymatic effect showed a comparable contribution to the intracellular enzymatic effect for Cr(VI) reduction, indicating that the reduction of extracellular enzymes is an important aspect of Cr(VI) reduction. The findings revealed the vital role of extracellular enzymatic reduction in the transformation of Cr(VI), which contributes to clarifying the mechanisms of Cr(VI) reduction through anaerobic biological treatment.