Binary adsorption of Cu(II) and Ni(II) on Lai'yang bentonite: Kinetics, equilibrium, competition quantitative and mechanisms investigation

In this study, the competitive adsorption of Cu(II) and Ni(II) adsorption onto bentonite were investigated. The effects of initial adsorbate concentration, pH, contact time, temperature on adsorption and the kinetics, thermodynamic, quantification of adsorption competition and competitive mechanisms have been studied. The results indicated that Cu(II) has a higher affinity than Ni(II) for adsorption on bentonite. But the inhibition effect was not noticeable under alkaline conditions. The Langmuir and pseudo-second-order kinetics models can be applied to simulate the isothermal adsorption and adsorption dynamic, respectively. Under a single system/binary system, the maximum adsorption capacity of bentonite for Cu(II) and Ni(II) is 12.76/10.57 and 11.12/9.11 mg/g, respectively. The equilibrium adsorption capacity reduction rate is suitable for quantitative characterization of adsorption competition under any concentration conditions; adsorption competition coefficient is more suitable for low concentration adsorption; although the distribution coefficient is not suitable for quantitative adsorption competition at high concentrations, it can be used to characterize the variation of adsorption competitiveness of Cu(II) at different concentrations. The higher electronegativity, lower first hydrolysis constant and Misono softness, and a better guest-host consistency between Cu(II)O-6 and the vacancies in the montmorillonite octahedral sheet make the higher affinity of Cu(II) to bentonite and result in the possible ion exchange between Cu(II) in solution and adsorbed Ni(II). In addition, at high concentrations, Cu(II) and Ni(II) precipitates formed on the surface of montmorillonite may cause remained Cu(II) ions in the solution unable to enter the internal space of the montmorillonite octahedron.