Cr(VI) inhibitory mechanism on sulfur-based denitrification: bio-toxicity and bio-electron characteristic and microbial evolutions

Sulfur-based denitrification is a promising technology for efficient nitrogen removal in low-carbon wastewater, while it is easily affected by toxic heavy metals. This study revealed the inhibitory mechanism of Cr(VI) on sulfur-based denitrification, including bio-toxicity and bio-electron characteristics response. NIR was more sensitive to Cr(VI) than NAR, which was inhibited by 21.32% and 19.86% under 5 and 10 mg/L Cr(VI), resulting in 10.12 and 15.62 mg/L NO2--N accumulation. The biofilm intercepted 36.57% chromium extracellularly by increasing 25.78% EPS, thereby protecting microbes from bio-toxicity under 5 mg/L Cr(VI). However, it was unable to resist 20-30 mg/L Cr(VI) bio-toxicity as 19.95 and 14.29 mg/g VSS chromium invaded intracellularly, inducing 165.98% and 169.12% ROS accumulation, which triggered microbial oxidative-stress and damaged the cell. In terms of electron transfer, S2O32- oxidation was inhibited, and parts of electrons flowed back intracellularly to maintain microbial activity, resulting in insufficient electron donors. Meanwhile, FAD and cyt.c contents decreased under 5-30 mg/L Cr(VI), reducing the electron acquisition rate by denitrification enzymes. As the dominant genus in R3-R5, Thermomonas possessed both denitrification and Cr(VI) resistance ability and played an important role in antioxidant stress and biofilm formation.