Thermodynamic, isothermal and kinetic studies of heavy metals adsorption by chemically modified Tanzanian Malangali kaolin clay

Abstract

The development of new adsorbent materials exhibiting enhanced properties is recognized as a compelling and technologically significant area of materials research. One of such designs involves chemical modification of native clays leading to materials with improved properties. Herein, native Malangali kaolinite was modified by HCl acid to facilitate improvements in its physicochemical properties. The adsorption kinetics, equilibrium and thermodynamics of cadmium and cobalt ions on chemically modified Malangali kaolin were examined during a series of adsorption experiments. Both native and chemically modified kaolin clays were analysed for their compositional, structural, mineralogical and morphological properties. The resulting chemically modified kaolinite recorded a significant increment in pore diameter, pore volume and surface area from 2.99 to 3.60 nm, 0.45 to 0.81 cc/g and 78.69 to 107.27 m/g, respectively. The heavy metals adsorption kinetics was studied using the pseudo-first-order, pseudo-second-order and intraparticle diffusion models. The pseudo-second-order model demonstrated a better fit to the experimental kinetic data compared to other models. The isothermal equation due to Langmuir model showed good agreement with the equilibrium data when compared to Dubinin–Radushkevich and Freundlich isotherms, indicating monolayer adsorption on homogeneous active sites of the adsorbent. The Langmuir monolayer adsorption capacity of 1.067 and 1.043 mg/g was acquired for cadmium and cobalt ions, correspondingly. Thermodynamic studies indicated an exothermic, spontaneous and physisorption process of heavy metals on the adsorbent. Our findings have demonstrated that chemically modified Malangali kaolinite can be exploited as an affordable, eco-friendly and efficient adsorbent for the adsorption of heavy metals form polluted wastewater.