Hypercrosslinked mesoporous poly(ionic liquid)s with high density of ion pairs: Efficient adsorbents for Cr(VI) removal via ion-exchange

[Display omitted] •MPILs with high density of ion pairs are prepared for Cr(VI) removal.•Structure & Activity Relationship (SAR) is studied systematically.•PVIm-6-SCD shows good selectivity and high adsorption capacity of 236.8 mg·g−1 to Cr(VI) at 25 °C.•PVIm-6-SCD has good reusability. Ion-exchange is one of the most efficient methods for removal of water pollutants, like hexavalent chromium (Cr(VI)). Herein, we report the preparation and application of a family of hypercrosslinked imidazolium-based poly(ionic liquid)s (PILs) with different crosslinker lengths, ion contents, and textural properties. We discover the hypercrosslinking porous structure and the anion content (corresponding to the theoretical adsorption capacity) of PILs play decisive roles in their capture capacities towards Cr(VI). Among them, PVIm-6-SCD prepared by free-radical polymerization of monomer with higher anion content and subsequent supercritical CO2 drying (SCD) displays excellent textural properties such as abundant mesoporous structure, high surface area and large pore volume. It facilitates the exposure of high density of exchangeable Br− in PVIm-6-SCD, thereby speed up the mass transfer during ion-exchange for Cr(VI). It shows a high utilization of the adsorption sites (84.5%) and a very high Cr(VI) adsorption capacity (236.8 mg·g−1, 25 °C), which is much higher than those of the literature reported adsorbents. Remarkably, PVIm-6-SCD exhibits good selectivity for Cr(VI) in the presence of competing anions including chloride (Cl−), nitrate (NO3−), dihydrogen phosphate (H2PO4−), and sulphate (SO42−). Moreover, it is effective for Cr(VI) removal under a broad range of pH (2–12). Besides, it has good reusability, showing potential industrial application prospects. This work highlights the PILs with high density of ion pairs for application in wastewater treatment.