Immobilized redox mediators on anion exchange resins and their role on the reductive decolorization of azo dyes.

Quinoid redox mediators (RM), including 1,2-naphthoquinone-4-sulfonate (NQS) and anthraquinone-2,6-disulfonate (AQDS), were adsorbed on anion exchange resins (AER) in order to explore their catalytic effects on the reductive decolorization of azo dyes by anaerobic granular sludge. Immobilized quinones preserved their catalytic properties once adsorbed on the surface of AER. Addition of different concentrations of immobilized quinones to sludge incubations increased up to 8.8-fold the rate of decolorization of azo dyes compared to controls lacking quinones. The catalytic effects of immobilized quinones also resulted in a greater extent of decolorization in quinone-amended incubations compared to controls lacking external RM. Spectrophotometric screening did not show any detachment of either AQDS or NQS during decolorization assays confirming that the enhanced decolorization accomplished was exclusively attributed to quinones immobilized on AER. Sterile controls including the maximum concentration of immobilized quinones supplied (4.8 mM) did not show any removal of azo dyes suggesting that physical-chemical processes, such as adsorption or chemical reduction, were not responsible for the enhanced decolorization reached. To our knowledge, this is the first study demonstrating the catalytic contribution of RM immobilized on AER on the reductive (bio)transformation of azo dyes.