Adsorption mechanisms of Co²⁺ and C²⁺ from aqueous solutions using natural clinoptilolite: Equilibrium and kinetic studies

The study was aimed at determining the adsorption mechanisms involved in the ion-exchange of Co²⁺ and Cu²⁺ in their aqueous solutions using Southern African natural clinoptilolite in its original (non-activated form). The clinoptilolite was characterized with FTIR and XRF and was found to consist of quartz, alumino-silicates calcite and dolomite was shown to be effective at removing Co²⁺ and Cu²⁺ ions from an aqueous solution. Both Co²⁺and Cu²⁺were readily adsorbed from dilute solutions (0.0020 M) followed by a 0.0698 M solution and the least removal was recorded with 0.2000 M solution. After mixing Co²⁺ and Cu²⁺ to generate a mixed Co/Cu synthetic solution the removal rates variable results and depended on the salt concentration ratios of the mixed solution. It was observed that Co²⁺ was generally easily removed from these solutions compared to Cu²⁺. The adsorption data of the metal ions were postulated by Langmuir and Freundlich isotherms over the entire concentration range. Kinetic data for adsorption kinetics accurately fitted the pseudo-first order with external diffusion models indicating that the ion-exchange of Co²⁺ and Cu²⁺ ions could be happening on the exterior surface of the sorbent while the contribution of internal diffusion mechanism was considered to be minimal.