Mechanism of anaerobic bio-reduction of azo dye assisted with lawsone-immobilized activated carbon

•Covalent binding of lawsone on granular activated carbon was successfully achieved by Fischer esterification.•Decolorization of congo red increased 2.5-fold with solid-phase redox mediator compared to anaerobic sludge.•Benzidine production was slower than decolorization rate on anaerobic tests being the limiting step of bio-reduction.•One azo bond of congo red is selectively broken and followed by a slower breaking of the second azo bond.•Removal of congo red by adsorption onto LQ-GAC was negligible. Lawsone redox (LQ) mediator was covalently bound to granular activated carbon (GAC) by Fischer esterification. A high LQ adsorption capacity on GAC was achieved (∼230 mg/g), and desorption studies showed strong chemical stability. Furthermore, kinetic experiments with solid-phase redox mediator (RM) and their controls (soluble RM, GAC and anaerobic sludge) were tested for decolorization of congo red dye at initial concentration of 175 mg/L. Benzidine, a by-product of complete congo red reduction, was also measured by HPLC analysis along the kinetic experiments. The highest percentage of decolorization after 24 h of incubation was achieved in cultures with soluble (77%) and immobilized (70%) LQ. In contrast, low decolorization efficiency was reached in anaerobic bio-reduction assays with unmodified GAC (47%) and anaerobic sludge (28%) after 24 h. Removal of congo red by adsorption onto LQ-GAC was negligible. The rate of benzidine production was slower than decolorization rate, suggesting that one azo bond of congo red is selectively broke and followed by a slower breaking of the second azo bond, consequently, appearance of benzidine in solution. These issues could be attributed to the steric rearrangement and the inhibitory effects of the produced aromatic amines in the biotransformation process.