Sorption of Cu²⁺ and Co²⁺ using zeolite synthesized from coal gangue: isotherm and kinetic studies

Adsorption of Cu²⁺ and Co²⁺ from aqueous solution using zeolite synthesized from coal gangue was examined at room temperature. Batch experiments were carried out to investigate the effects of contact time, initial ion concentration, and pH on the adsorption process. Under comparable conditions, the adsorption greatly depended on initial ion concentration and pH. Different isotherms were applied to describe equilibrium data, and the results turned out that adsorption data can be better fitted with Langmuir, Temkin, and Dubinin–Radushkevich models, indicating monolayer coverage of adsorption sites on the surface of zeolite NaX and a physical process for Co²⁺ and Cu²⁺ adsorption. Adsorption capacity calculated from Langmuir was 45.05 mg/g for Cu²⁺ and 44.53 mg/g for Co²⁺. The adsorption kinetic could be better described with the pseudo-second-order model, and the removal mechanism of Co²⁺ and Cu²⁺ by the synthesized zeolite NaX was governed by surface adsorption, film diffusion, and intra-particle diffusion. The work suggests that coal gangue-derived zeolite NaX can be used as an alternative economical adsorbent for metal ions removal.