Radiation synthesis of imidazolium-based ionic liquid modified silica adsorbents for ReO4− adsorption

The selective elimination of radioactive TcO4− from radioactive waste is becoming increasingly critical for environmental security. In this work, a series of novel adsorbents (Si-IL-X, X = NO3, Cl, BF4, NTf2) modified with ionic liquids containing different anions (X) were synthesized via radiation grafting of 1-vinyl-3-ethylimidazolium tetrafluoroborate onto silanized silica and subsequent anion replacement. The surface and structural properties of the prepared adsorbents were characterized by FT-IR spectroscopy, TGA, XPS, and SEM-EDS. Subsequently, their adsorption properties for ReO4− (a nonradioactive surrogate of TcO4−) were studied in detail via batch and column experiments. The batch experiments indicated that the Si-IL-X showed high ReO4− removal efficiency over a broad pH range of 3–10, and their maximum adsorption capacities were in the range of 147.93–289.02 mg g−1 against ReO4−. It was found that the adsorption ability of Si-IL-X was related to the anion structure of the ionic liquid, and the order was Si-IL-NO3 > Si-IL-Cl > Si-IL-BF4 > Si-IL-NTf2, which was mainly determined by the hydrophilicity and ionic radius of the anions. Besides, their adsorption capacities barely decreased after electron beam radiation of 800 kGy. In column experiments, the adsorbents exhibited excellent selectivity and could capture ReO4− efficiently from simulated radioactive wastewater containing a large number of coexisting ions. In addition, the XPS and FT-IR spectroscopic analyses indicated that adsorption was carried out by an ion-exchange process. The results suggested that Si-IL-X showed excellent application prospect for efficient capture of TcO4− from Hanford waste and other common acidic radioactive wastes.