Dye-laden sludge-derived biochar for wastewater remediation: A review on pyrolytic engineering, adsorptive interactions, and environmental prospects

The persistent release of synthetic dyes from industrial effluents constitutes a substantial ecological and toxicological hazard owing to their persistent molecular structure and biological recalcitrance. This review presents a focused evaluation of biochar derived from dye-laden sewage sludge (DLSS-B), highlighting its potential as a low-cost, sustainable adsorbent for dye removal from wastewater. DLSS, a byproduct rich in organic matter, inorganic minerals, and residual dyes, is thermochemically converted into biochar through pyrolysis. A critical assessment is conducted on how variations in pyrolysis temperature, thermal ramping rate, and inert gas environment influence the resultant biochar’s specific surface area (reaching up to 405 m²/g), pore architecture, and surface functional chemistry. DLSS-B exhibits high adsorption capacities ranging from 200 to 405 mg/g for dyes such as malachite green and crystal violet, primarily through π–π stacking, electrostatic interactions, and hydrogen bonding. Chemical modifications, including acid/alkali activation and metal doping, further enhance its adsorptive performance. Regeneration studies show that DLSS-B can retain up to 92 % efficiency over five cycles, indicating strong reusability. Beyond dye removal, secondary applications such as soil amendment (non-food use) and energy recovery (∼20 MJ/kg) are discussed. This review emphasizes the integrated role of pyrolysis engineering, surface chemistry, and reuse strategies in developing DLSS-B as a viable material for industrial dye remediation.