Alginate/Phosphine-Functionalized Chitosan Beads Towards the Removal of Harmful Metal Ions from Aqueous Medium

Among the many strategies utilized to improve the adsorption capacity of adsorbent materials, the modification of their chemical nature has been widely exploited, especially considering the polysaccharide-based adsorbents. Based on this, a phosphine-functionalized chitosan derivative (CSPPh2) was associated with alginate (Alg), resulting in beads that were utilized to adsorb Cd2+ and Al3+ from water. After a complete characterization [Fourier transformed infra-red (FTIR), X-ray diffraction (XRD), thermogravimetric analyses (TGA/DTG), scanning electron microscopy (SEM), swelling analysis, and pH of point of zero charge (pHPZC)], adsorption experiments were performed to determine the optimal operational conditions. Under similar conditions, the Alg/CSPPh2 showed superior adsorption performance compared to the beads prepared using raw chitosan. Moreover, the maximum adsorption capacities for Cd2+ (78.2 mg/g) and Al3+ (81.5 mg/g) were computed to Alg/CSPPh2 beads under mild conditions (pH 6.0, 25 °C, 50 mg of adsorbent). The adsorption process for both metals was well-fitted by the pseudo-first-order kinetics and Freundlich isotherm model. Overall, the adsorption of these metals on Alg/CSPPh2 occurred via physical interaction forces, and the functional groups on the beads, including the –PPh2 groups, have a pivotal role as binding sites. The post-utilized beads could be regenerated and reused up to 5 times consecutively with a slight decrease in their removal capacity. This study demonstrated that these original beads containing phosphine-functionalized chitosan are enhanced adsorbents to remove metal ions of different valences from the water environment.