Boosting cesium retention efficiency with polyoxometalate-ionic liquid composites: Insights into {WO6} octahedral affinity

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 {WO 6 }-saturated TBA-PW 12 and mono-lacunary TBA-PW 11 , for the capture and retention of aqueous Cs. The composites were then characterized, and the performance and mechanism of Cs uptake were investigated. TBA-PW 12 exhibited ∼300 nm aggregate of particles around 20–60 nm. TBA-PW 12 retained the Keggin structural features of PW 12 O 40 3− , which was proven essential for the binding of Cs + . With a retention efficiency of 97.3% in an NH 4 Cl eluent and a maximum adsorption capacity of 400.9 mg/g at 338 K, TBA-PW 12 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 {WO 6 } and Cs + , TBA-PW 12 efficiently sequestered Cs  +  through {WO 6 } octahedra so that the {WO 6 }-saturated TBA-PW 12 presented better Cs uptake performance than the mono-lacunary TBA-PW 11 . The IL anion [N(C 4 H 9 ) 4 ] + provided a composite-water biphasic interface for aqueous Cs adsorption, while retaining the ability of POM cation PW 12 O 40 3− to form a stable structure with Cs via its affinity to {WO 6 } 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.