Tunable Gellan Gum Hydrogels as High Capacity Adsorbents for Rapid Dye Removal

There is a great need for environmentally friendly high-performance materials for environmental remediation. In this context, the synthesis of functional materials starting from natural biomass is a particularly attractive approach. Here, we report on the synthesis of potent adsorbent hydrogels derived from gellan gum via esterification of gellan gum with maleic anhydride and cross-linking via thiol–ene chemistry. The hydrogel materials obtained and the adsorption of methylene blue were characterized in detail to determine the underlying structure–property relationships. In particular, the impacts of the fractions of −COO– groups and the thiol cross-linker as well as of the thiol functionality on the adsorption capacity, kinetics, and selectivity were unveiled. Likewise, the effects of the operation parameters pH, adsorbent dose, initial concentration, contact time, and ionic strength were examined. The hydrogels were found to exhibit rapid adsorption kinetics (≤15 min to equilibration), to possess a high adsorption capacity of qmax = 547 mg/g (1.71 mmol/g) toward methylene blue, and to outperform a typical commercial cation exchanger in selectively capturing the cationic dye methylene blue from a ternary mixture with methyl orange and sunset yellow FCF with a 27 times higher selectivity. The high selectivity and adsorption capacity as well as the fast dye uptake are attributed to the dominating electrostatic forces and high swelling capacity of the polyanionic adsorbent. The captured dye was successfully desorbed from the adsorbent, which can be regenerated by a simple pH shift and reused at least seven times, sustaining an over 95% dye removal efficiency.