Synthesis of functionalized porous montmorillonite via solid-state NaOH treatment for efficient removal of cesium and strontium ions

Solid-state NaOH treatment of montmorillonite clay was used to enhance the removal of Cs⁺ and Sr²⁺. Through this facile and low-cost modification, montmorillonite with a large BET surface area (117.1 m² g⁻¹) and many surface functional groups (SiONa), demonstrated enhanced sorption kinetics (89% removal for 40 mg L⁻¹ Cs⁺ and 23 mg L⁻¹ Sr²⁺ in 1 h) with a sorption capacity of 290.7 mg g⁻¹ for Cs⁺ and 184.8 mg g⁻¹ for Sr²⁺, greatly exceeding the low sorption capacity (137.0 mg g⁻¹ for Cs⁺ and 15.6 mg g⁻¹ for Sr²⁺) of pristine montmorillonite. SEM-EDS and XPS analyses revealed that Cs⁺ and Sr²⁺ were ion-exchanged with Na⁺ on the surface functional groups formed following NaOH treatment. The performance of NaOH-treated montmorillonite was stable following gamma-ray irradiation (at 6 Gy h⁻¹ for 30 min) and across a broad range of pHs (3 to 11), exhibiting a high distribution coefficient (Kd) of 1.5 × 103 mL g⁻¹ for Cs⁺ (1.58 mg L⁻¹) and 3.7 × 10³ mL g⁻¹ for Sr²⁺ (1.64 mg L⁻¹) under groundwater conditions where various cations including Na⁺, K⁺,, and Ca²⁺ (V/m = 1 L/g) were present. The proposed method demonstrated great improvement of the sorption capacity of an abundant and inexpensive montmorillonite.