The role of sulphate reduction on the reductive decolorization of the azo dye reactive orange 14

The aim of this study was to investigate the impact of a broad range of sulphate concentrations (0-10g SO4-2 L-1) on the reduction of an azo dye (reactive orange 14 (RO14)) by an anaerobic sludge. An increase in the sulphate concentration generally stimulated the reduction of RO14 by sludge incubations supplemented with glucose, acetate or propionate as electron donor. Sulphate and azo dye reductions took place simultaneously in all incubations. However, there was a decrease on the rate of decolorization when sulphate was supplied at 10 g SO42- L-1. Abiotic incubations at different sulphide concentrations (0-2.5g sulphide L-1) promoted very poor reduction of RO14. However, addition of riboflavin (20 mu M), as a redox mediator, accelerated the reduction of RO14 up to 44-fold compared to a control lacking the catalyst. Our results indicate that sulphate-reduction may significantly contribute to the reduction of azo dyes both by biological mechanisms and by abiotic reductions implicating sulphide as an electron donor. The contribution of abiotic decolorization by sulphide, however, was only significant when a proper redox mediator was included. Our results also revealed that sulphate-reduction can out-compete with azo reduction at high sulphate concentrations leading to a poor decolorising performance when no sufficient reducing capacity is available.