Immobilized redox mediators on modified biochar and their role on azo dye biotransformation in anaerobic biological systems: Mechanisms, biodegradation pathway and theoretical calculation

[Display omitted] •Immobilized AQS on BC was manufactured as a solid-phase RM in anaerobic bioreactor.•XPS and FTIR analyses proved BC550-AQS contained more redox moieties (quinone/C = O).•Synergy of BC550-AQS + AGS achieved the highest decolorization efficiency and rate.•DFT revealed RR2 biodegradation pathway of electron attract and azo bond cleavage.•Highest absolute value of ETC proved BC550-AQS had a stronger electron affinity. In this study, a novel solid-phase redox mediator (RM) was manufactured via immobilizing anthraquinone-2-sulfonate (AQS) onto modified biochar (BC). The RM was used to enhance the anaerobic granular sludge (AGS) driven anaerobic biodegradation of azo dyes. Prepared at the optimal pyrolysis temperature of 550 ℃, the AQS modified BC550 exhibits a higher adsorption capacity of about 12.81% toward AQS compared to that of commercial granular activated carbon (GAC). X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy analyses showed that BC550-AQS contained more redox moieties (quinone and C = O) than GAC-AQS, indicating that the BC550 is suitable as a carrier of immobilized AQS. During the 36-h reaction, incubation of BC550-AQS as the solid-phase RM obviously enhanced the capacity of AGS (BC550-AQS + AGS), evidenced by the maximum percentage of Reactive Red 2 (RR2) decolorization (92.27%), which increased up by 80.55% and 14.48% compared to the controls (BC550-AQS, 17.95%) and (GAC-AQS + AGS, 78.61%). Besides, BC550-AQS exhibits good reusability and a higher RR2 decolorization rate (per AQS dosage) over 15 consecutive cycles. The mechanism of biodegradation pathway of RR2 initiated by BC550-AQS was studied using liquid chromatography-mass spectrometry and chemical computation. Density-functional theory calculations successfully predict that the RR2 biodegradation pathway mainly includes azo bond cleavage and electron attraction with a stronger electron affinity. This work not only proposes a deep insight into the biodegradation pathway of RR2 by employing BC550-AQS in an anaerobic biological system but also develops effective solid-phase RMs for efficient removal of contaminants from wastewaters by biological treatment.