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(N2H4),which is an intermediate product of anammox,participates in intracellular metabolism and extracellular Cr(Ⅵ)reduction.However,the roles of N2H4-induced intracellular metabolism and extracellular reduction in nitrogen removal under Cr(Ⅵ)stress remain unclear.The addition of 3.67 mg/L of N2H4 increased the anammox activity by 17％.As an intermediate,N2H4 enhanced anammox metabolism by increasing the heme c content and electron transfer system activity.As a reductant,N2H4 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 N2H4-induced extracellular Cr(Ⅵ)reduction is the dominant mechanism for the survival of anammox consortia.We also found that N2H4 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).