Waste denim valorization via co-production of bioethanol and recovered indigo: A sustainable strategy for reducing environmental risk of textile solid waste.

Most existing studies focus on the removal of dyes from wastewater, while generally neglecting the potential soil and groundwater pollution risks posed by the fixed indigo in solid waste denim fabrics after landfilling. Waste denim represents a category of textile solid waste. Leveraging its unique composition-rich in cellulose and indigo, this study proposed a novel high-value conversion concept: the simultaneous hydrolysis of cellulose and release of indigo from waste denim. The results showed that both acid and enzymatic hydrolysis could effectively hydrolyze cellulose, yielding glucose of 0.42-0.95 g/g biomass. For ethanol fermentation, the maximum ethanol conversion rate reached 0.39 g/g biomass, which was much higher than those from conventional lignocellulosic biomass. Moreover, cellulose hydrolysis increased the indigo content in the solid residue, which contained 57-83% indigo. The dyeing effect of the recycled indigo exhibited no significant difference from commercial indigo. Cost estimation and life cycle assessment indicated that indigo, as a high-value-added product, effectively improved the economic viability and environmental performance of bioethanol. This study develops a waste denim valorization route to co-produce ethanol and indigo, mitigating environmental risks of dyed textile waste and achieving harmless disposal of solid waste.