Removal of Co²⁺, Sr²⁺ and Cs⁺ from aqueous solution by phosphate-modified montmorillonite (PMM)

Sorptive removal of Co²⁺, Sr²⁺ and Cs⁺ from aqueous solution by phosphate-modified montmorillonite (PMM) was studied considering the influencing factors (initial metal ion concentrations, initial solution pH and temperature). The Freundlich, Langmuir and Dubinin-Radushkevich (DR) models were used to fit single-solute sorption data. The Freundlich model was the best, indicating heterogeneous surface property of PMM. The maximum sorption capacity (q_mL) of Langmuir model was in the order of Cs⁺>Co²⁺>Sr²⁺. The mean sorption energy (E) values of DR model (<8kJ/mol) at pH 5 for all metals indicated physical sorption. However, Cs⁺ sorption mainly occurred by chemical sorption. In bi-solute competitive sorption, the sorption of one metal ion was suppressed by the presence of competing metal ion. The Sheindorf-Rebhun-Sheintuch (SRS) model, the extended Freundlich model (EFM), the modified extended Langmuir model (MELM) and the IAST-Freundlich model predicted the competitive sorption adequately. Sorptions of Co²⁺ and Sr²⁺ were strongly dependent on the initial solution pH but that of Cs⁺ was not. The calculated thermodynamic parameters such as {increment}H, {increment}S, and {increment}G showed that sorptions of Co²⁺ and Sr²⁺ onto PMM were endothermic, whereas that of Cs⁺ was exothermic and that all sorption occurred spontaneously.