Green walnut husk and pomegranate peel for nickel removals from industrial wastewater by absorption process: batch and column experiments

Biosorption studies were conducted using batch and column experiments to analyze the adsorption of nickel ions onto green walnut husk and pomegranate peel, two low-cost, eco-friendly biosorbents. While previous research has explored the biosorption of various heavy metals, this study uniquely compares the efficiency of these agricultural waste materials for nickel removal under different operational conditions, including particle size, pH, biosorbent dose, and initial nickel concentration. The results showed that the optimum pH for both adsorbents ranged from 4 to 6, and the adsorption process reached equilibrium within approximately 1 h. Based on the Langmuir isotherm, the maximum adsorption capacities for green walnut husk and pomegranate peels were 99 mg/g and 11.4 mg/g, respectively, indicating that both adsorbents effectively removed nickel from wastewater. In the study, the adsorption data were fitted to three linear adsorption isotherm models: Langmuir, Freundlich, and Temkin. Among linear adsorption isotherms, the observed data for both adsorbents were better fitted by the Freundlich isotherm (R²_{(pomegranate peel)} = 0.97 and R²_{(green walnut husk)} = 0.99). Among the five nonlinear adsorption kinetic models tested, the pseudo-second-order model with R² = 1 was the most suitable for modeling. The separation factor for both adsorbents was below one across all concentrations confirming favorable adsorption. The negative value of the Gibbs free energy change indicates that the adsorption process was spontaneous. In addition, the Gibbs free energy values absorbed for both biosorbents revealed that the absorption process in green walnut husks was physisorption, whereas for pomegranate peels, both physisorption and chemisorption occurred spontaneously. Based on column studies, the dose–response curve best matches the breakthrough curve data.