Efficient and recyclable removal of imidazolium ionic liquids from water using resorcinol–formaldehyde polymer resin

Abstract

Resorcinol–formaldehyde (RF) resin is used for the first time to remove imidazolium-based ionic liquids (ILs) from water via adsorption. 1-Butyl-3-methylimidazolium chloride (BMIMCl) was selected as a representative imidazolium IL, and the adsorption kinetics and isotherm of BMIMCl to RF resin were determined. The pseudo first order rate law appeared to be a more appropriate model to interpret the adsorption kinetics, whereas the adsorption isotherm data was well described by the Langmuir isotherm model. The activation energy (E_a) was determined to be 10.8 kJ mol⁻¹, and the maximum adsorption capacity was estimated as 96 mg g⁻¹ at 30 °C. The elevated temperature facilitated adsorption of BMIMCl to RF resin and the adsorption was also much enhanced under alkaline conditions. Charged surfactants could compete with BMIMCl during adsorption or lessen the electrostatic attraction between BMIMCl and RF resin. When a non-ionic surfactant was present, BMIMCl might be confined within the micelles of the non-ionic surfactant, thereby limiting the adsorption of BMIMCl onto RF resin. Cations of ILs with longer alkyl chains showed a higher affinity towards RF resin, whereas the adsorption of BMIMBF₄ to RF resin was much improved owing to the higher hydrophobicity of BF₄⁻ than Cl⁻. The regenerated RF resin also exhibited a stable and efficient adsorption capacity for BMIMCl and the desorption of ILs from RF resin can be also achieved within relatively short periods. These results reveal that RF resin could be a promising and recyclable adsorbent for removing imidazolium ILs from water.