Adsorption behavior of nickel(II) onto cashew nut shell: Equilibrium, thermodynamics, kinetics, mechanism and process design

The adsorption behavior of nickel(II) from aqueous solution onto agricultural waste such as cashew nut shell (CNS) was investigated as a function of parameters such as solution pH, CNS dose, contact time, initial nickel(II) concentration and temperature. The Langmuir, Freundlich, Temkin and Dubinin–Radushkevich models were applied to describe the equilibrium isotherms using nonlinear regression analysis. The equilibrium data fits well for the both Langmuir and Freundlich adsorption isotherms. The Langmuir monolayer adsorption capacity of CNS was found to be 18.868 mg/g. Thermodynamic parameters such as ΔG°, ΔH° and ΔS° have also been evaluated and it has been found that the sorption process was feasible, spontaneous and exothermic in nature. Pseudo-first-order, pseudo-second-order and Elovich kinetic models were used to describe the kinetic data and the rate constants were evaluated. The result of the kinetic study shows that the adsorption of nickel(II) could be described by the pseudo-second-order equation, suggesting that the adsorption process is presumably chemisorption. The adsorption process was found to be controlled by both surface and pore diffusion, with surface diffusion at the earlier stages followed by pore diffusion at the later stages. Analysis of adsorption data using a Boyd kinetic plot confirmed that external mass transfer is the rate determining step in the sorption process. A single-stage batch adsorber was designed for different CNS dose/effluent volume ratios using the Freundlich equation.