The role of (auto)catalysis in the mechanism of an anaerobic azo reduction

Azo dyes are non-specifically reduced under anaerobic conditions, but the slow rates at which many dyes react may present a serious problem for the application of anaerobic technology as a first stage in the complete biodegradation of these compounds. Therefore, it is significant to explore the mechanism of anaerobic azo reduction, especially with respect to its kinetics. With that purpose, decolouration of the monoazo dye C.I. Acid Orange 7 (AO7) was studied in batch experiments. Experiments indicated that chemical reduction by sulphide is partially responsible for the anaerobic conversions of AO7. Mathematical evaluation of the experimental results pointed out that autocatalysis played an important role in the chemical reduction of AO7. Further tests made clear that 1 -amino-2-naphthol was the dye's constituent aromatic amines that accelerated the reduction process, possibly by mediating the transfer of reducing equivalents. The impact of redox mediation by quinones was further evaluated by testing the catalysing effects of anthraquinone-2,6-disulphonic acid (AQDS) and of autoclaved sludge. AQDS appeared to be an extremely powerful catalyst, capable of increasing the first-order chemical reduction rate constants by a factor 10 to 100. Also autoclaved sludge, possibly because of mediation by sludge organic matter, accomplished accelerated azo reduction rates. Azo reduction in living sulphidogenic anaerobic sludge environments is 3 times more rapid than the chemically catalysed reaction with sulphide. The exact role of the biological activity remains to be clarified.