Sustainable approaches in reviving reactive green dyes: biochar-based dye recovery from textile dyeing sludge

• Textile dyeing sludge was converted into biochar for reactive green dye removal. • Optimization of biochar through RSM–BBD for better char yield. • Structural features enable an adsorption capacity of 12.8 mg/g. • The adsorption - desorption cycle yields 6.2% of recoverable RG dye. • Recovered dye achieves 78–85% uptake in fabric redyeing. In recent years, the usage of char from biomass-derived sludge as an adsorbent has been rising in popularity for wastewater treatment, impacting both economic and environmental aspects. In this study, the biochar obtained from textile dyeing sludge was employed for the adsorption and recovery of Reactive Green (RG) dye from aqueous solutions. Characterization of the textile dyeing sludge with thermogravimetric analysis and the biochar with scanning electron microscopy, Fourier transform infrared spectroscopy and Brunauer–Emmett–Teller analysis confirmed the porous structure, with a pore size of 6.49nm and a surface area of 48.5 m 2 /g. Response surface methodology using Box–Behnken design was used to optimize operating parameters, indicating a temperature of 400°C, residence time of 20-25°C /min and heating rate of 20-25 minutes for increased char production. Adsorption behaviors followed a pseudo-second-order kinetic model and the Freundlich isotherm, showing a q e value of 12.82 mg/g that reflects chemisorption and multilayer adsorption on heterogeneous sites. Thermodynamic parameters confirmed the endothermic and spontaneous nature of the process. Desorption studies using 0.1 M HNO₃ recovered up to 70% of dye at 6.2% of the initial concentration recovered, and the dyeing process showed 78-85% dye uptake, confirming partial resource recovery. Overall, this performance is slightly reduced compared to virgin RG, but the recovered dye offers clear economic and environmental advantages. Unlike most studies restricted to adsorption and desorption assessment, this work demonstrates the feasibility of dye recovery and reuse, aligning with the UN SDGs 6 and 12 on water management and responsible production.