Competitive adsorption of metal ions onto goethite–humic acid-modified kaolinite clay

A binary mixture of humic acid and geothite was prepared and used to modify kaolinite to produce geothite–humic acid (GHA)-modified kaolinite adsorbent useful for the adsorption of Pb²⁺, Cd²⁺, Zn²⁺, Ni²⁺ and Cu²⁺ from Single and Quinary (5) metal ion systems. The cation exchange capacity (CEC) and specific surface area of GHA-modified kaolinite clay adsorbent were found to be 40 meq/100 g and 13 m²/g, respectively, with the CEC being five times that of raw kaolinite clay (7.81 meq/ 100 g). The Langmuir–Freundlich equilibrium isotherm model gave better fit to experimental data as compared with other isotherm models. In Quinary metal ion system, the presence of Zn²⁺ and Cu²⁺ appears to have an antagonistic effect on the adsorption of Pb²⁺, Cd²⁺ and Ni²⁺, while the presence of Pb²⁺, Cd²⁺ and Ni²⁺ shows a synergistic effect on the adsorption of Zn²⁺ and Cu²⁺. The GHA-modified kaolinite showed strong preference for the adsorption of Pb²⁺ in both metal ion systems. Brouers–Weron–Sotolongo (BWS) kinetic model gave better fit to kinetic data compared with other kinetic models used. Data from BWS kinetic model indicate that adsorption of metal ions onto GHA-modified adsorbent in both metal ion systems followed strictly, diffusion-controlled mechanism with adsorption reaction proceeding to 50 % equilibrium in <2 min in the Single metal ion system and <1 min in the Quinary metal ion system. Adsorption of metal ions onto GHA-modified kaolinite is fairly spontaneous and endothermic in nature in both metal ion systems although the rate of metal ion uptake and spontaneity of reaction are reduced in the Quinary metal ion system.

Keywords
Competitive adsorption; Kaolinite; Goethite; Humic acid; Diffusion-controlled transport; Kinetics