Uptake of Ni(II) ions from aqueous solution by kaolinite and montmorillonite: Influence of acid activation of the clays

Kaolinite and montmorillonite were treated with 0.25 M H2SO4 and the acid activated clays along with the parent clays were tested for their uptake capacity for Ni(II) ions from aqueous solution. The batch adsorption experiments were conducted under a set of variables (concentration of Ni(II) ion, amount of clay, pH, time and temperature of interaction). Increasing pH favored Ni(II) uptake till the ions were precipitated as the insoluble hydroxides at pH 8.0. The uptake was rapid up to 40 min and equilibrium was obtained within 180 min. The kinetics of the process was evaluated by subjecting the results to a number of models like the pseudo-first order, second order, Elovich equation, liquid film diffusion, and intra-particle diffusion and it was found that the data more closely resembled a second order process. The experimental data conformed to both Langmuir and Freundlich isotherms showing that the interactions were mostly chemical in nature. The clays had reasonable monolayer adsorption capacity of 10.4, 11.9, 28.4, and 29.5 mg g(-1) for kaolinite, acid activated kaolinite, montmorillonite, and acid-activated montmorillonite respectively. Montmorillonite had much better adsorption capacity than kaolinite and the acid activation boosted the adsorption capacity of both kaolinite and montmorillonite. The interactions were exothermic in nature, accompanied by decrease in both entropy and Gibbs energy. The results have established good potentiality for kaolinite, montmorillonite and their acid-activated forms to take up and separate Ni(II) from aqueous medium through adsorption-mediated immobilization.