Adsorption Kinetics and Thermodynamics of Cadmium Removal Using Palm Kernel Shell Biochar from Aqueous Solutions

Heavy metal contamination of water sources is a pervasive environmental problem in Nigeria, and the development of effective, low-cost adsorbents derived from locally abundant agricultural waste is a research priority. This study investigated the adsorption kinetics and thermodynamics of cadmium removal from aqueous solution using biochar prepared from palm kernel shells, a major agricultural by-product in South South Nigeria. Biochar was prepared by pyrolysis at temperatures of 400, 600, and 800 degrees Celsius and characterised by BET surface area, scanning electron microscopy, Fourier-transform infrared spectroscopy, and elemental analysis. Batch adsorption experiments investigated the effects of initial cadmium concentration, contact time, adsorbent dose, pH, temperature, and ionic strength on adsorption performance. Equilibrium data were fitted to Langmuir, Freundlich, Temkin, and Sips isotherm models. Kinetic data were modelled using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models. Thermodynamic parameters including Gibbs free energy, enthalpy, and entropy were calculated. Biochar produced at 600 degrees Celsius exhibited the highest surface area of 428 m2/g and the best cadmium removal efficiency of 96.4 percent at optimised conditions. The Langmuir model provided the best equilibrium fit, with a maximum adsorption capacity of 64.7 mg/g. Adsorption kinetics followed the pseudo-second-order model, indicating chemisorption. Negative Gibbs free energy and positive enthalpy values confirmed spontaneous, endothermic adsorption. Functional group analysis by FTIR identified hydroxyl and carboxylate groups as active binding sites. This study fills a significant gap in biochar adsorption science for Nigerian heavy metal remediation and demonstrates the strong potential of palm kernel shell biochar as a cost-effective adsorbent.

Keywords: biochar, cadmium adsorption, palm kernel shell, adsorption isotherms, kinetics