Facile fabrication of a phosphonium-based ionic liquid impregnated chitosan adsorbent for the recovery of hexavalent chromium

Chitosan adsorbents impregnated with a phosphonium-based ionic liquid (Chi_IL), trioctyldodecyl phosphonium chloride, were prepared for the adsorption of hexavalent chromium and compared to the performance of native chitosan. The physical and chemical properties of the adsorbents were characterized by Fourier-transform infrared spectroscopy, X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Effects of various parameters, such as pH, adsorbent dosage, contact time, temperature, and multi-component systems, were systematically examined. Chi_IL showed a high adsorption capacity (282.6 mg g⁻¹) compared to native chitosan (238.1 mg g⁻¹). The adsorption kinetics of the metals followed a pseudo-second-order kinetic model, and the experimental data were a good fit for the Freundlich isotherm model. Following the isotherm and activation energy parameter, adsorption of Cr(vi) onto Chi_IL follows a chemisorption process, possibly through an anionic exchange with the anion of the IL. The thermodynamic parameters suggested that the adsorption of Cr(vi) is a spontaneous and exothermic reaction. In the column adsorption, Chi_IL exhibited a longer column exhaustion time than that of native chitosan owing to the enhanced adsorption capacity caused by the introduction of IL. Moreover, the column with the parameters of 6 cm bed depth, 5 mL min⁻¹ flow rate, and 50 mg L⁻¹ was able to achieve the best performance in Cr(vi) adsorption.