Application of Burman–Plackett and Box–Behnken design for Nickel removal by a novel adsorbent derived from Olive Stones for Sustainable Environment

This study used olive stones as a precursor for conversion to oxygen-free acid-containing carbonate (OSAC), via chemical activation by H3PO4. A Plackett–Burman experimental design was used to identify significant factors, while a Box–Behnken design was used to optimise three parameters for nickel removal. The experimental data obtained were treated using analysis of variance (ANOVA) and fitted to a second-degree polynomial equation by multiple regression. Numerical optimisation using desirability functions identified the optimal conditions for Ni2+ removal: an initial concentration (Co) of 30 mg/L, an initial pH of 3.5 and an adsorbent dose of 0.55 g at 25°C (R2 = 93.38%). The results indicate that the Langmuir model shows the best correlation, with a maximum adsorption capacity of 71.42 mg/g at 298 K. Adsorption follows pseudo-second-order kinetics, similar to chemisorption. The removal of pollutants by new-generation adsorbents proved effective thanks to the application of the BBD statistical methodology. The adsorption isotherms at different temperatures allowed the determination of the free energy ΔG° (−6.446 to −12.146kJ/mol); enthalpy (ΔH° = 61.494 kJ/mol) and entropy ΔS° (228 J/K mol) to predict the nature of the Ni adsorption process. Negative values of (ΔG°) and positive entropy ΔH° indicate spontaneous and endothermic Ni adsorption of the chimisorption type.