Guanylated chitosan derivatives for the adsorption of anionic dyes: Performance and mechanism

Chitosan is an emerging adsorbent in pollution management, but its solubility in acidic aqueous solutions and its weak affinity towards pollutants limit its adsorption performance. The development of novel cationic chitosan containing guanidinium groups, has facilitated the advancement of various applications. This work presents a comprehensive methodology for chitosan guanylation through the reaction of neat chitosan with carbodiimide reagents in ionic liquid media. The resulting two guanidinium chitosan derivatives, guanidinium chitosan containing dicyclohexyl and guanidinium chitosan containing dimethylaminopropyl hydrochloride were thoroughly characterized by FT-IR, XRD, thermogravimetry, solid-state NMR and scan electron microscopy, and then investigated as adsorbents for anionic dyes, i.e. methyl orange. The impact of pH, contact time, dye concentration and temperature on the adsorption of methyl orange dye were explored. Both materials showed high adsorption efficiency of 274 and 320 mg/g, respectively. Due to the lower acidity of the guanidinium groups, the gunaylated materials display efficient anion exchange properties even at basic pH. Furthermore, the guanylation leads to decreased solubility via the construction of intermolecular hydrogen bonds. The adsorption properties of cationic chitosan derivatives are outstanding, displaying a high degree of recyclability. The utilization of guanylated chitosan derivatives presents new opportunities in the field of water purification.