Enhanced azo dye Reactive Red 2 degradation in anaerobic reactors by dosing conductive material of ferroferric oxide

•Dosing Fe3O4 on anaerobic treatment of azo dye was investigated.•Fe3O4 significantly improved anaerobic treatment of RR2.•Complete cleavage of the NN bond in RR2 were obtained dosing Fe3O4.•Paludibacter, Trichococcus and Methanosarcina abundances increased with Fe3O4. Effect of dosing ferroferric oxide (Fe3O4) on the anaerobic treatment of azo dye Reactive Red 2 (RR2) was investigated in two anaerobic sequencing batch reactors (ASBRs). System performance, dye degradation pathways, and microbial activities and structure were examined. The addition of Fe3O4 significantly improved treatment efficiency, with the removal efficiency of RR2 increased by 116%, the maximum methane (CH4) yield potential and the peak CH4 production rate improved by 7.7% and 22.3%, and the lag phase shortened by 39.6%, respectively. The activity of the electron transport system was significantly enhanced by dosing Fe3O4, with the maximum value increased by 77% and conductivity of the anaerobic sludge increased by 178%. According to the proposed pathway for the degradation of RR2, the degradation products from complete cleavage of the NN bond in RR2 were obtained at the presence of Fe3O4, while were absent without Fe3O4. At high initial dye concentrations, the dosage of Fe3O4 alleviated the inhibition to microbes by RR2, and high degradation rate and removal efficiency were maintained. The microbial community structure changed during the long-term acclimation with the dosage of Fe3O4. Paludibacter, Trichococcus and Methanosarcina were predominant and their relative abundances increased with the addition of Fe3O4.