Epichlorohydrin-crosslinked chitosan hydrogels as recyclable adsorbents for phosphate removal from aqueous solutions and municipal wastewater

Phosphorus is an essential nutrient; however, when present in the environment, it can trigger eutrophication. Therefore, its removal from aquatic systems is key to ensure high water quality. Adsorption has shown to be an efficient, simple and versatile water treatment for the removal of a variety of contaminants. Chitosan (CS) is a bio-polymeric material, widely used as adsorbent, capable of self-assembly to form hydrogels. Yet, the material is unstable at pH < 3. The stability can be improved with a crosslinking agent, such as epichlorohydrin (ECH), which also improves the adsorption capacity of the chitosan-based adsorbents. Chemically crosslinked chitosan (CS-ECH) hydrogels are positively charged, having the potential to be used as adsorbents for phosphorus (as orthophosphate) and other anionic contaminants. For this research, CS-ECH hydrogels with chitosan:crosslinker ratios of 1:0, 1:0.5, 1:0.8, 1:1 and 1:1.2 were prepared and evaluated for the removal of phosphorus. The best removal performance corresponded to CS-ECH at 1:0.5 chitosan:crosslinker ratio, at an optimal pH of 3, reaching an adsorption capacity of around 101 mg PO 4 /g of hydrogel. Adsorption kinetics were fitted to a pseudo-first order kinetic model and the Langmuir isotherm. Recyclability tests on phosphate-saturated hydrogels (10 mg PO₄/g and 100 mg PO₄/g) using NaOH treatments revealed a maximum desorption efficiency at pH 11. After three adsorption-desorption cycles, the material retained at least 77 % of its initial removal capacity. Additionally, CS-ECH removed approximately 78 % of phosphate from filtered municipal wastewater, demonstrating its potential for environmental applications. • Epichlorohydrin-crosslinked chitosan hydrogels are efficient phosphate adsorbents. • Adsorption was described by pseudo-second order kinetic model and Langmuir isotherm. • The adsorbents were recycled 3 times, with at least 77 % of its adsorption capacity. • Phosphate adsorption was evaluated on municipal wastewater.