Functionalization of thiosemicarbazide/gellan gum for the enhancement of cadmium removal from aqueous solutions – Grafting of tributyl phosphate derivative

Two sorbents are successfully synthesized based on the chemical modification of gellan gum (GG). First, thiosemicarbazide is grafted onto GG to produce GEG-C. In a second step, GEG-C is functionalized with a derivative of tributyl phosphate (dTBP, giving GEG-P sorbent). The sorption properties are compared for recovery of Cd(II) from aqueous solutions. GEG-C is expected to bind Cd(II) through carboxylic, amine, and thiocarbonyl groups, while phosphate groups in GEG-P contribute to increase metal sorption. The chemical modification improves not only the uptake kinetics (equilibrium time ≈30 min, vs. 45 min) but also the sorption capacities (≈2.79 vs. 1.60 mmol Cd g−1), at optimum pH (i.e., pH 5). The functionalization also improves (a) the sorbent stability (weaker reduction of the loss in sorption efficiency after five cycles of reuse compared with GEG-C) and (b) the sorption selectivity in the presence of competitor ions (from equimolar multicomponent solutions), especially at pH 4. Different sorption mechanisms may be involved depending on the pH (deprotonation of reactive groups) including chelation (onto deprotonated amine and phosphonate groups) and ion exchange (protonated groups with metal cations). Cadmium sorption is reversed by contact with 0.3 M HCl solution. The functionalization (GEG-P) strongly increases the selectivity coefficient for Cd(II) against major elements in mining effluent, especially at pH higher than 5. These results confirm the interest of multifunctionality in the development of new sorbents based on renewable resources (such as gellan gum).