Elucidating the crystal structure-dependent Cd²⁺-uptake property of benzimidazolium ionic liquid immobilized into macroporous polystyrene

In this study, the crystal structures of the 2-amino-1-methyl-1H-benzimidazol-3-ium(O,O-di-isopropylphosphorodithioate) ionic liquid - [C8H10N3]+ [C6H14O2P1S2]- [(MAB+H)DIPDTP] and bis(1-methyl-1 H-benzimidazol-2-amine)-bis(O,O-di-isopropyl phosphorodithioato)-cadmium(II) - C28H46Cd1N6O4P2S4 [Cd(MAB)2(DIPDTP)2]) are characterized by X-ray diffraction analysis. Organic ligands - O,O-di-isopropylphosphorodithioate (DIPDTP), 1-methyl-1 H-benzimidazol-2-amine (MAB), and new acid-base bi-functional protonic ionic liquid ([(MAB+H)DIPDTP]) - are immobilized into commercial polymeric crosslinked polystyrene-divinylbenzene sorbent (PAD600) and their sorption capacity for Cd2+ from aqueous solution is evaluated. The successful immobilization of DIPDTP and MAB into macroporous polystyrene is confirmed by means of scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), and N2-BET analysis. The Cd2+ sorption efficiency of PAD600-MAB-DIPDTP sorbent increases due to the combination of physical adsorption and extraction, surface complexation, and formation of metal complexes, chelates, and poorly soluble compounds. The adsorption equilibrium data are better fitted to the Freundlich isotherm models. The overall sorption is exothermic and spontaneous and follows a pseudo-second-order kinetic model. This indicates a complex sorption mechanism, including both physical adsorption and strong chemical interaction, dissociation constant, acid-base properties, and reaction condition (pH, temperature, concentration, etc.). The novel composite material can be a promising adsorbent for Cd2+ removal from contaminated water.