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

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 (qmL) of Langmuir model was in the order of Cs+ > Co2+ > Sr2+. The mean sorption energy (E) values of DR model (< 8 kJ/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 ΔH, ΔS, and ΔG showed that sorptions of Co2+ and Sr2+ onto PMM were endothermic, whereas that of Cs+ was exothermic and that all sorption occurred spontaneously.