Synthesis of Sulfur-Grafted Chitosan Biopolymers and Improvement to Their Sorption of Silver Ions from Different Aqueous Solutions

Abstract

A functionalized chitosan thiourea composite (CH-TU) was successfully synthesized using formaldehyde as a crosslinking agent for enhancing silver recovery from different aqueous solutions. Comparison sorption studies with a non-functionalized composite (CH-F) as a reference material were conducted. Grafting led to an improvement in the sorption performances, i.e., 0.763 mmol Ag g−1 for CH-F vs. 2.125 mmol Ag g−1 for CH-TU. The pseudo-first-order rate equation (PFORE) was fitted to the sorption kinetics at saturation times of 40 and 30 min for CH-F and CH-TU, respectively, while the sorption isotherms were fitted with Langmuir and Sips equations for both sorbents. Fourier transform infrared (FTIR), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), nitrogen sorption–desorption isotherms (BET-surface area), elemental analysis, thermogravimetric analysis (TGA) and pH of the zero charge (pHpzc) were used to characterize and investigate the sorption mechanism. Sorption was performed three times to check the reproducibility, while the sorption performances were stable over 20 cycles, with a limited decrease in performance (5 and 3% for CH-F and CH-TU, respectively). Nitric acid solution (0.3 M) was efficient for desorbing the adsorbed metal ions. The grafted sorbent with thiourea is considered as a promising tool for recovering Ag(I) from acidic waste leachate derived from waste spent films.