Valorisation of solid fraction from pretreated sewage sludge for cationic dye removal: mono-adsorbate and multi-adsorbate approach

This study presents a novel and sustainable approach to waste valorisation, transforming solid fractions from sewage sludge treated by thermal hydrolysis (TH) and wet oxidation (WO) into highly effective biosorbents. This innovative methodology offers an eco-friendly solution for both sludge management and water pollution control, significantly advancing sustainable environmental remediation. Among the materials developed, the biosorbent derived from WO-treated sludge (RSWO) showed exceptional adsorption performance for cationic dyes (methylene blue (MeB) and safranin (SF)) in both mono- and multi-adsorbate systems. RSWO achieved maximum adsorption capacities of 196 mg/g for MeB and 144 mg/g for SF in mono-adsorbate systems under alkaline conditions at 25 °C. In multi-adsorbate scenarios, SF exhibited 44 -67 % higher capacities than MeB due to competitive adsorption. Kinetic studies for both systems consistently fitted to the pseudo-second-order model, indicating chemical adsorption as the rate-limiting step. The Langmuir model accurately described the mono-adsorbate adsorption equilibrium, while the JAMM model proved particularly suitable for the multi-adsorbate system, effectively accounting for both heterogeneity and inter-component interactions. Thermodynamic analysis indicated predominantly physical adsorption, being spontaneous for both dyes in both mono- and multi-adsorbate systems. Notably, RSWO exhibited excellent reusability, maintaining efficiency over four adsorption-desorption cycles for MeB with 0.1 M HCl regeneration, and showing superior performance for SF in combined systems. FTIR and XPS analyses confirmed interactions involving hydroxyl, carboxyl, aldehyde, and amide groups, alongside π-π and n-π interactions. This comprehensive research establishes RSWO as a promising, renewable, and eco-friendly adsorbent for efficient dye-contaminated wastewaters.