Efficient Decolorization of the Poly-Azo Dye Sirius Grey by Laccase-Mediator System: Process Optimization and Toxicity Assessment

The textile industry produces high volumes of colored effluents that require multiple treatments to remove non-adsorbed dyes which could be recalcitrant due to their complex chemical structure. Most of the studies have dealt with the biodegradation of mono or diazo dyes but rarely with poly-azo dyes. Therefore, the aim of this paper was to study the biodegradation of a four azo-bond dye (Sirius grey) and to optimize its decolorization conditions. Laccase-containing cell free supernatant from the culture of a newly isolated fungal strain, Coriolopsis gallica strain BS9, was used in the presence of 1-hydroxybenzotriazol (HBT) to optimize the dye decolorization conditions. A Box-Benken design with 4 factors namely: pH, enzyme concentration, HBT concentration and dye concentration, was performed to determine optimal conditions for the decolorization of Sirius grey. The optimal conditions were pH 5, 1 U/mL of laccase, 1 mM of HBT, and 50 mg/L of initial dye concentration ensuring a decolorization yield and rate of 87.56 % and 2.95 %/min respectively. Decolorized dye solution showed a decrease of its phytotoxicity (Germination index GI = 80 %) compared to the non-treated solution (GI = 29 %). This study suggests that laccase-mediator system could be a promising alternative for dyes removal from textile wastewaters.