Sustainable Valorization of Sewage Sludge Into NaOH‐Activated Biochar for Malachite Green Removal From Wastewater

ABSTRACT This work investigates a circular economy strategy by converting sewage sludge into efficient adsorbents for malachite green (MG) removal. The adsorption performance of raw sludge (RS) was compared with NaOH‐activated biochar (NAB) prepared by pyrolysis and chemical activation. Physicochemical characterization (XRF, XRD, FTIR, BET, and pH pzc ) confirmed that activation substantially improved the textural properties of the biochar, increasing surface area, pore volume, accessibility, and alkalinity while maintaining its mineral structure. The effects of dye concentration, contact time, adsorbent dose, pH, temperature, and ionic strength were investigated. Kinetic modeling showed that NAB follows pseudo‐second‐order and Elovich models, whereas RS follows pseudo‐first‐order and Elovich models, indicating different adsorption mechanisms. Equilibrium data were described by the Sips model for NAB and the Langmuir model for RS, with maximum adsorption capacities of 314.29 and 53.04 mg.g −1 , respectively. Thermodynamic analysis showed that MG adsorption onto RS was spontaneous and endothermic. NAB retained 91% of its initial capacity after three regeneration cycles and showed robust performance in real wastewater. This work demonstrates a sustainable, low‐cost approach for converting sewage sludge into a high‐performance adsorbent for hazardous dye removal.