Optimization of eco-friendly laccase decolorization for monochlorotriazine reactive dyed waste cotton fabrics using response surface methodology

The high color fastness of reactive dyes in waste cotton textiles impedes effective decolorization and recycling. This study employs an eco-friendly enzymatic approach utilizing a fungal laccase (extracted from Aspergillus oryzae) to decolorize monochlorotriazine-type reactive dyes (C.I. Reactive Blue 5, C.I. Reactive Yellow 3, and C.I. Reactive Red 24) on cotton fabrics. Key process parameters were optimized via standard orthogonal experimental and Box–Behnken design response surface modeling. Optimal decolorization conditions were identified as 0.5% HBT (1-hydroxybenzotriazole), pH 7, 50°C, 0.5 U/ml laccase, 1:20 bath ratio, and 1 h treatment. Under these conditions, decolorization efficiencies reached 62%, 30%, and 27% for blue, yellow, and red dyed cotton fabrics, respectively, with fiber strength retention exceeding 95%. Fourier-transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD) analyses confirmed no significant changes in the chemical structures of Blue 5, Yellow 3, and Red 24 dyed cotton fabrics, whereas their crystallinity values increased by 10.24%, 8.87%, and 21.97%, respectively. X-ray photoelectron spectroscopy revealed partial cleavage of N=N bonds in Red 24 and Yellow 3, and C=O bonds in Blue 5, indicating that laccase disrupts the conjugated chromophore structures under optimized conditions. Unlike conventional chemical treatments, which often degrade fiber integrity, this study achieves effective decolorization of cotton fabrics with high color fastness under neutral, mild conditions, while maintaining the physical properties of fibers. It provides an environmentally friendly solution for textile recycling, significantly enhances the economic and environmental benefits associated with their recycling, and thus exhibits promising industrial application potential.