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 min. 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.
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