Evaluation of industrial palm oil sludge as an effective green adsorbing substrate for toxic aqueous cadmium removal

Palm oil sludge (POS), an industrial waste from the palm oil industry, was utilised as a new adsorbent for the removal of toxic cadmium (Cd²⁺). Response surface methodology was carried out to investigate the effects of initial concentration (C₀), contact time (t), dosage (W) and pH and optimise the process conditions. The maximum adsorption capacity was evaluated to be 18.49 mg g⁻¹ at these optimised conditions: C₀ = 200 mg L^-1, t = 60 min, W = 0.3 g and pH 5.8. The adsorption equilibrium was achieved after 60 min, whereby the adsorption equilibrium and kinetic were best described by the Freundlich isotherm and pseudo-second-order kinetic models, respectively. The adsorption of Cd²⁺ onto POS was thermodynamically spontaneous (ΔG = -35.63 to -31.97 kJ mol⁻¹), exothermic (ΔH = -35.92 kJ mol⁻¹) with increased randomness at the solid–liquid interface (ΔS = 0.283 kJ mol⁻¹ K⁻¹). POS exhibited a high regeneration potential using 0.4 mol L^-1 hydrochloric acid, with a desorption efficiency over 90% after 5 adsorption–desorption cycles. Conclusively, POS is an effective adsorbent for Cd²⁺ removal.