Adsorption properties of manganese oxides prepared in aqueous-ethanol medium toward Sr(II) ions

In this study, nanostructured manganese oxides were synthesized using sol–gel method by reduction of KMnO₄ with ethyl alcohol. Firstly, the effect of K⁺, Na⁺, Ca²⁺, and Mg²⁺ ions as templates on their structure and adsorption properties was studied. According to X-ray diffraction analysis, all samples were identified as amorphous phases. Fourier-transform infrared spectroscopy confirmed that prepared sorption materials had a band around 760 cm⁻¹, characteristic for the tunnel-type manganese oxides. The BET surface area of the obtained samples was 188–293 m² g⁻¹, the sorption volume of 0.431–1.136 m³ g⁻¹, and the average pore size of 6–20 nm. The maximum Sr²⁺ sorption capacity of 120 mg g⁻¹ was observed for the non-templated manganese oxide sorbents synthesized at 25 °C during 5 h. The calculated isotherm parameters and corresponding correlation coefficients revealed that the Sr²⁺ uptake process followed the Freundlich and Redlich-Peterson models for metal ion-templated manganese oxides. The increase in temperature and time of synthesis, as well as the application of K⁺, Na⁺, Ca²⁺, and Mg²⁺ ions as templates led to the decrease of sorption capacity down to 40–60 mg g⁻¹ and 60–80 mg g⁻¹, respectively. The highest sorption efficiency of metal ion-templated manganese oxides in Na⁺, K⁺, and Ca²⁺-form was shown for ⁹⁰Sr, its K_d being (3.8–5.1) × 10³ cm³ g⁻¹. For Na⁺ ion-templated sample, as typical, the K_d values were 2.0 × 10⁻¹–8.6 × 10⁴ cm³ g⁻¹ in the pH range of 0.5–12.0 and 3.5 × 10²–6.2 × 10¹ cm³ g⁻¹ for Ca²⁺ ion concentration varied from 10 to 80 mM, respectively.