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

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 (pH pzc ) 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.