Hyperbranched polymeric ionic liquid with imidazolium backbones for highly efficient removal of anionic dyes

• A novel hyperbranched polymeric ionic liquid (hb-PIm+PF6−) was synthesized.
• The hb-PIm+PF6− exhibited high adsorption capacity toward anionic dyes.
• It selectively adsorbed the anionic dye from a mixture of cationic and anionic dyes.

A hydrophobic hyperbranched polymeric ionic liquid (hb-PIL) with an imidazolium (Im+)-salt backbone (hb-PIm+PF6−) was proposed for efficient adsorption of anionic dyes. For this purpose, a hydrophilic hb-PIL was first synthesized via the thiol–ene addition polymerization by the “A2 + B3” method. Then, the anion exchange reaction of hb-PIm+Cl− with KPF6 afforded the target hb-PIm+PF6− adsorbent. hb-PIm+PF6− has high adsorption capacity toward anionic dyes. In contrast, it exhibits less adsorption amount toward cationic dyes. By using congo red (CR) as the model adsorbate, the adsorption mechanism of hb-PIm+PF6− was investigated in detail by Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive X-ray, and transmission electron microscopy analyses. The adsorption behavior of hb-PIm+PF6− towards CR was analyzed by two kinetic and four isotherm models. These results indicate the characteristic structure of hb-PIm+PF6− was responsible for its high adsorption capacity for CR. The main driving force for the CR adsorption was electrostatic attraction, carried out by the anion-exchange between CR and counter-ion PF6− of hb-PIm+PF6−. The presence of the cavities formed by the branched chains provided storage sites for CR binding. In addition, the selective adsorption of hb-PIm+PF6− towards anionic dye could be used to purify cationic solution containing anionic dye.