Thermodynamics and kinetics of the removal of nickel (II) ions from aqueous solutions by biochar adsorbent made from agro-waste walnut shells

Biochar adsorbent (WSBA) classified as a microporous material was obtained via thermal degradation of walnut shells in a nitrogen atmosphere. WSBA was used to remove Ni(II) ions from model aqueous solutions. The thermodynamics and kinetics of the adsorption process were studied on the basis of series of experiments which were conducted under constant agitation, temperatures of 288, 293, 298 and 303 K, and initial solution concentrations of 5, 50, 100 and 150 mg L−1. Pseudo-second-order kinetic equation was in competition with diffusion-chemisorption kinetic model as the best-fitting kinetic model. The adsorption process was well described with the Langmuir isotherm but the best-fitting one is Tóth isotherm model. On its basis, the values of ΔGo, ΔHo and ΔSo were calculated. The thermodynamic parameters showed that the adsorption of Ni(II) ions onto WSBA was spontaneous at all temperatures. The detailed analysis of the data for the explored adsorbent-adsorbate system found out that the adsorption mechanism is ion exchange. It was proven that WSBA could be used as an effective biochar adsorbent for removal of Ni(II) ions from water solutions as an alternative to commercial activated carbons. [Display omitted] •Data analysis is carried out using six kinetic & seven thermodynamic models.•The adsorption kinetics is described by the diffusion-chemosorption equation.•The adsorption thermodynamics is interpreted with the Tóth isotherm.•The estimated thermodynamic parameters show spontaneous adsorption at all temperatures studied.•The maximum adsorption capacity of biochar adsorbent made from walnut shells for Ni(II) ions is 13.25 mg g−1 at 303 K.