Sustainable treatment of textile effluents using enzymes from white-rot fungi

Abstract

In this study, the decolorization of Bezaktiv Navy S-BL, a reactive bifunctional azo dye used for textile dyeing, was investigated by using Cerrena unicolor BRFM 3379 laccase. The fungus was grown on locally available birch bark by semi-solid-state fermentation, and the crude enzyme was recovered and concentrated by ultrafiltration. Plate screening confirmed a strong ligninolytic profile dominated by laccase. In copper-supplemented cultures, laccase activity reached about 12,000 U/L by day 17, and ultrafiltration raised it to 15,016 U/L (38.2 U/mg). Laccase was not able to decolorize the dye on its own, therefore the need for a redox mediator was required. In screening, HBT removed about 90 % of the color within 24 h, clearly outperforming vanillin, and was selected for optimization. The decolorization was subsequently optimized by response surface methodology (RSM) with a three-factor central composite design (CCD). The quadratic model was significant (R² = 0.913), with HBT concentration, the laccase-HBT interaction, and the quadratic HBT term as significant terms. A maximum decolorization of 96.1 % was reached at 1000 U/L laccase, 50 mg/L dye, and 1 mM HBT. A more economical condition (500 U/L laccase, 100 mg/L dye, 0.50 mM HBT) was used for scale-up to flask cultures, where decolorization reached 81 % at 24 h and 91.4 % at 72 h. A stability test showed that HBT caused no additional loss of laccase activity, the decline being due to pH and thermal instability. ATR-FTIR indicated the breakage of the azo bond, shown by the loss of the band near 1489 cm⁻¹ and the appearance of a carbonyl band near 1697 cm⁻¹, consistent with quinone-type transformation products rather than complete mineralization. A Lactuca sativa germination assay showed increased phytotoxicity: the germination index fell from 78.7% for the untreated dye to 0% for the treated sample. Overall, the laccase-mediator system decolorized Bezaktiv Navy effectively, but the higher toxicity of the treated solution shows that color removal alone is not a sufficient measure of detoxification.