Polymeric ionic liquid gels composed of hydrophilic and hydrophobic units for high adsorption selectivity of perrhenate

Abstract

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 ΔH^Θ and ΔS^Θ 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.