Performance and mechanism of chromium reduction in denitrification biofilm system with different carbon sources

In the process of biological reduction of Cr(VI), the type of carbon sources affects the rate and effect of Cr(VI) reduction, but its specific performance and influencing mechanism have not yet been explored. In this study, four denitrification biofilm reactors were operated under four common carbon sources (C₆H₁₂O₆, CH₃COONa, CH₃OH, CH₃COONa:C₆H₁₂O₆ 1:1) to reveal the impact of carbon sources on Cr(VI) reduction. Through preliminary experimental concentration research, 75 mg/L Cr(VI) was selected as the dosing concentration. In long-term operation, the composite carbon sources of CH₃COONa and C₆H₁₂O₆ demonstrated excellent stability and achieved an impressive Cr(VI) removal efficiency of 99.5 %. The following sequence was C₆H₁₂O₆, CH₃COONa, and CH₃OH. Among them, CH₃OH was less competitive and the system was severely unbalanced with lowest Cr(VI) reduction efficiency. The toxicity reactions, changes in EPS and its functional groups, and electron transfer revealed the reduction and fixation mechanism of chromium on denitrification biofilm. The changes in microbial communities indicated that microbial communities in composite carbon sources can quickly adapt to the high toxic environment. The proportion of Trichococcus reached 43.6 %, which played an important role in denitrification and Cr(VI) reduction. Meanwhile, the prediction of microbial COG function reflected its excellent metabolic ability and defense mechanism.