Imidazolium ionic liquid-modified chitosan as an effective adsorbent for phosphate and Cr(VI) ions

In this work, two forms of chitosan (powder and beads) were functionalized with glycidyl imidazolium ionic liquid and compared for the removal of phosphate and Cr(VI) from aqueous solutions. The adsorbents (CS-p/IL, CS-b/IL) were characterized by Fourier-transform infrared spectroscopy, nuclear magnetic resonance spectroscopy, scanning electron microscopy and low-temperature N2 adsorption-desorption. The effect of adsorbent dosage, pH, initial concentration, contact time and coexisting anions was investigated in batch experiments. The results revealed that the modification enhanced the adsorption properties of both adsorbents, confirming that the ionic liquid introduces an additional active centers. CS-b/IL showed better removal efficiency of both anions over a wide pH range. The adsorption onto CS-p/IL occurred almost immediately, whereas on CS-b/IL the uptake increased gradually but eventually reached a higher adsorption capacity. The results obtained suggest that CS-b/ IL is a more effective form of chitosan for interaction with contaminants. The experimental data were fitted with various isotherm and kinetic models. The Sips isotherm best described the results obtained with maximum adsorption capacities of CS-p/IL and CS-b/IL for phosphate of 44.43 and 79.59 mg/g, respectively, and for Cr(VI) ions of 110.1 and 119.9 mg/g, respectively, suggesting that the adsorption mechanism is complex and does not follow an ideal monolayer adsorption. The adsorption kinetics followed both the pseudo first-order and pseudo second-order models indicating both physical and chemical adsorption, and involved intraparticle diffusion as a significant role in the process. The possible removal mechanism of phosphate and Cr(VI) ions involved electrostatic attraction, ion exchange and hydrogen bonding.