Thermally activated paint-sludge as potential sustainable adsorbent for pollutant removal with insights from...

The continuous rise the release of untreated paint industry wastewater containing dyes, organic pollutants, and heavy metals poses serious environmental and public health concerns, most especially in developing countries where treatment infrastructure remains inadequate. This study examines the potential of thermally activated sludge (TAS) derived from paint industry waste as cost-effective and sustainable adsorbent for the uptake of pollutants from industrial effluents. Comprehensive adsorption analyses through isotherm, kinetic, thermodynamic, and response surface methodology (RSM) modelling were performed to elucidate the adsorption mechanism and optimize operational parameters. Thermally activated sludge was synthesized via pyrolyzing paint sludge at 700 °C and compared with unactivated sludge to explain the effect of thermal modification on adsorption performance. Comprehensive characterization through XRD, FTIR, SEM-EDS, BET, and TGA confirmed that thermal activation enhanced the porosity, surface heterogeneity, mineral composition, and functional groups of the sludge adsorbent. Batch adsorption experiments were performed by varying contact time, pH, temperature, adsorbent dosage, and pollutant concentration. TAS displayed superior removal performance compare to US, achieving substantial reductions in BOD, COD, TSS, TDS, oil and grease, nitrate, and heavy metals including Pb, Cd, and Cr. Maximum adsorption efficiency 86.9% was attained under alkaline conditions. Equilibrium studies revealed that the Freundlich isotherm best explained the adsorption process (R² = 0.998 for TAS), signifying multilayer adsorption on heterogeneous surfaces, while the Langmuir maximum adsorption capacity increased from 58.82 mg g⁻¹ for US to 77.21 mg g⁻¹ for TAS. Kinetic assessment revealed that adsorption onto TAS followed the pseudo-second-order model (R² = 0.995), indicating chemisorption involvement, whereas intraparticle diffusion contributed to the overall adsorption mechanism. Examinations of the thermodynamic data confirmed that the adsorption process was spontaneous and endothermic, with TAS displaying stronger adsorbate-adsorbent interactions compare to US. RSM study identified pH and adsorbent dosage as the most influential operational variables and demonstrated significant interaction effects between process parameters. Thermal activation improved adsorption efficiency, adsorption capacity, and process stability, highlighting the potential of converting industrial sludge waste into a value-added material for sustainable wastewater treatment applications.