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Biosorption studies in batch mode were carried out to investigate the adsorption of Ni(II) ions from aqueous solution on A. barbadensis Miller leaves (ABL) residue powder. The effects of Ni(II) ion concentration, pH, biosorbent dose and temperature on removal process were investigated. The maximum biosorption of 60.2% for an initial Ni(II) ion concentration of 100 ppm was achieved. The five equilibrium adsorption isotherms, namely Freundlich, Langmuir, Temkin, Dubinin-Radushkevich (D-R) and BET, were analysed to fit the equilibrium data. Freundlich isotherm was found to best fit (R 2 = 0.9975) in this study. Among pseudo-first-order, pseudo-second-order, intraparticle diffusion and Elovich and Bangham kinetics models, pseudo-second-order model was found to be the best fitted model (R 2 = 0.9853) in this adsorption process. Separation factor (R L) value of less than 1.0 indicates that biosorption of Ni(II) on ABL is favourable. Thermodynamics parameters such as Gibbs free energy (∆G°), enthalpy (∆H°) and entropy change (∆S°) of the biosorption process were also estimated, and it was found that biosorption of Ni(II) ion on ABL was spontaneous, feasible and exothermic in nature. The sticking probability (S* ≪ 1) indicates that the biosorption process was physical in nature. The scanning electron microscopy (SEM) analysis was also carried out to study the morphology of the biosorbent used. The results showed that ABL residue has the potential to be used as a low-cost biosorbent for the removal of Ni(II) from wastewater.

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