Enhancement of extracellular Cr(VI) reduction for anammox recovery using hydrazine: performance, pathways, and mechanism

The hexavalent chromium (Cr(VI)) would frequently impose inhibition to anaerobic ammonium oxidation (anammox) process, hindering the efficiency of nitrogen removal in wastewater treatment. Hydrazine (N 2 H 4 ), which is an intermediate product of anammox, participates in intracellular metabolism and extracellular Cr(VI) reduction. However, the roles of N 2 H 4 -induced intracellular metabolism and extracellular reduction in nitrogen removal under Cr(VI) stress remain unclear. The addition of 3.67 mg/L of N 2 H 4 increased the anammox activity by 17%. As an intermediate, N 2 H 4 enhanced anammox metabolism by increasing the heme c content and electron transfer system activity. As a reductant, N 2 H 4 accelerated the reduction of c -Cyts-mediated extracellular Cr(VI) to the less toxic Cr(III). Extracellular Cr(III) accounts for 74% of the total Cr in a Cr(VI)-stressed anammox consortia. These findings highlight that N 2 H 4 -induced extracellular Cr(VI) reduction is the dominant mechanism for the survival of anammox consortia. We also found that N 2 H 4 increased the production of extracellular polymeric substances to sequester excessive Cr(VI) and produced Cr(III). Taken together, the study findings suggest a potential strategy for enhancing nitrogen removal from ammonium-rich wastewater contaminated with Cr(VI).