Polymeric ionic liquid gels composed of hydrophilic and hydrophobic units for high adsorption selectivity of perrhenate†
The removal of TcO4− from aqueous solutions has attracted more and more attention recently, and ReO4− has been widely used as its natural analog. In this work, polymeric ionic liquid gel adsorbents, PC2-C12vimBr, with high adsorption capacity and selectivity towards ReO4− were synthesized by radiation-induced polymerization and crosslinking. PC2-C12vimBr was composed of two monomers: a hydrophobic unit, 1-vinyl-3-dodecylimidazolium bromide for high selectivity, and a hydrophilic unit, 1-vinyl-3-ethylimidazolium bromide for improved kinetics. A gel fraction up to 90% could be achieved under 40 kGy with varied monomer ratios. The adsorption of PC2-C12vimBr gels for ReO4− was evaluated by batch adsorption. The PC2-C12vimBr gel containing 20 mol% hydrophilic unit (named PC2-C12vimBr-A) could significantly improve the adsorption kinetics, which had an equilibrium time of ca. 24 h. The adsorption capacity obtained from the Langmuir model was 559 mg g−1 (Re/gel). The selective factor against NO3− was 33.4 ± 1.9, which was more than 10 times higher than that of PC2vimBr, and it could maintain ReO4− uptake as high as 100 mg g−1 in 0.5 mol kg−1 HNO3. The ΔHΘ and ΔSΘ of the NO3−/ReO4− ion-exchange reaction of PC2-C12vimNO3-A were −16.9 kJ mol−1 and 29 J mol−1 K−1, respectively, indicating physical adsorption. The adsorption mechanism of ReO4− onto PC2-C12vimBr-A gel was ion-exchange, and it could be recovered using 5.4 mol kg−1 HNO3.