ADSORPTION OF LEAD AND ZINC FROM USED AND UNUSED LUBRICANT OIL USING SOIL EQUILIBRIUM, KINETIC AND THERMODYNAMIC STUDIES

This study investigated the adsorption behavior of lead (Pb) and zinc (Zn) from used and unused lubricant oil onto soil, focusing on equilibrium, kinetic, and thermodynamic parameters. Soil and oil samples were characterized, and batch adsorption experiments were conducted to evaluate the effects of pH, temperature, initial metal concentration, adsorbent dosage, and contact time. Metal concentrations were determined using Flame Atomic Absorption Spectrophotometry (FAAS). The results indicated that adsorption was highly dependent on pH, with optimal removal occurring at near-neutral conditions (pH 5-8). The presence of used lubricant oil reduced the adsorption efficiency for both metals compared to unused oil, likely due to the coating of soil active sites by oil degradation products. Equilibrium data were best described by the Freundlich isotherm, suggesting multilayer adsorption on a heterogeneous surface, while kinetic data fitted the pseudo-second-order model excellently (R² ≥ 0.9988), indicating that chemisorption was the rate-controlling mechanism. Thermodynamic analysis revealed that the adsorption process was spontaneous and feasible (ΔG° < 0) for all systems. Zn adsorption was exothermic (ΔH° < 0), characteristic of physisorption, whereas Pb adsorption was endothermic (ΔH° > 0) and entropy-driven, suggesting chemisorption. The study concludes that soil has a significant capacity to adsorb Pb and Zn from lubricant oils, but the efficiency is compromised by the complex matrix of used oil. These findings provide valuable insights for developing soil-based remediation strategies for sites contaminated with spent lubricating oils.