Rapid recovery of inhibited denitrification with cascade Cr(VI) exposure by bio-accelerant: Characterization of chromium distributions, EPS compositions and denitrifying communities

Highlights: • Denitrification was inhibited when Cr(VI) concentration was higher than 80 mg L⁻¹. • Accelerant could shorten the recovery time of denitrification after inhibition. • Microbes could secrete more TB-EPS to protect them from toxicity of Cr(VI). • Accelerant contributed to growth of nirS-type denitrifying bacteria. Hexavalent chromium (Cr(VI)) may inhibit denitrification in biological wastewater treatment systems, and the inhibited denitrification process is difficult to recover in a short time. This study explored Cr(VI) cascade impact (20–125 mg L⁻¹) on denitrification and developed one nontoxic biological accelerant (combination of L-cysteine, flavin adenine dinucleotide, biotin and cytokinin) for denitrification recovery. The results showed that NO₃⁻-N removal efficiency decreased from 75.7% to 21.5% when Cr(VI) concentration increased from 80 to 125 mg L⁻¹. Addition of accelerant could effectively promote the removal of NO₃^--N, and observably reduce the recovery time (42 T) compared with natural recovery (63 T). Furthermore, the main site of Cr(VI) reduction and Cr(III) immobilization was located in the intercellular compartment of the biofilm. Microbes produced more tightly bound extracellular polymeric substances (TB-EPS) to protect them from toxicity under the low Cr(VI) concentrations, while low EPS was secreted when Cr(VI) concentration was higher than 60 mg L⁻¹. Compared to natural recovery system, bio-accelerant addition was beneficial to the recovery of denitrifiers activities, especially for the bacteria containing nirS gene. The results facilitated an understanding of Cr(VI) impact on denitrification, and the proposed bio-accelerant can be potentially applied to heavy metal shock-loading emergency situations.