Boosting Cesium Retention Efficiency with Polyoxometalate-Ionic Liquid Composites: Insights into {WO6} Octahedral Affinity

Abstract

Commonly used polyoxometalate (POM) adsorbents exhibit high cesium (Cs) adsorption capacity. However, its impotent ability to sequester adsorbates poses a risk of Cs migration in radioactive waste decontamination and disposal. By leveraging the cesium affinity of POMs and the regulatory properties of ionic liquids (ILs), POM-IL composites hold promise for mitigating the risk of cesium migration through efficient adsorption and retention. Herein, we designed two POM-ILs composed of the phosphotungstate anion (PW) and tetrabutylammonium cation (TBA), i.e., the {WO₆}-saturated TBA-PW₁₂ and mono-lacunary TBA-PW₁₁, for the capture and retention of aqueous Cs. The composites were then characterized, and the performance and mechanism of Cs uptake were investigated. TBA-PW₁₂ exhibited ∼300 nm aggregate of particles around 20 ∼ 60 nm. TBA-PW₁₂ retained the Keggin structural features of PW₁₂O₄₀³⁻, which was proven essential for the binding of Cs⁺. With a retention efficiency of 97.3% in an NH₄Cl eluent and a maximum adsorption capacity of 400.9 mg/g at 338 K, TBA-PW₁₂ exhibited superior Cs capture and retention compared to the POM-IL derived from common Cs adsorbent phosphomolybdate. The Cs adsorption onto TBA-PWs achieved equilibrium within 180 minutes. Cs uptake by TBA-PWs was monolayer chemisorption of Cs⁺ accompanied by H⁺ release, and the removal efficiencies stabilized around initial pH 5 – 10. Due to intense affinity between {WO₆} and Cs⁺, TBA-PW₁₂ efficiently sequestered Cs⁺ through {WO₆} octahedra so that the {WO₆}-saturated TBA-PW₁₂ presented better Cs uptake performance than the mono-lacunary TBA-PW₁₁. The IL anion [N(C₄H₉)₄]⁺ provided a composite-water biphasic interface for aqueous Cs adsorption, while retaining the ability of POM cation PW₁₂O₄₀³⁻ to form a stable structure with Cs via its affinity to {WO₆} octahedra. These findings support the design of POM-IL composites as efficient adsorbents for capturing and retaining Cs, aiding in the decontamination and safe disposal of radioactive waste, thereby minimizing the risk of nuclide leakage into the environment.