Closed-loop ammonium removal enabled by EPS-based hydrogel derived from cyanobacterial blooms

Frequent eutrophication in lakes has intensified ammonium (NH₄⁺-N) pollution, while the inefficient utilization of cyanobacterial biomass hinders effective remediation. In this study, a novel cyanobacterial extracellular polysaccharide hydrogel (EPSH) was developed via an ion crosslinking/polymerization-induced strategy using extracellular polysaccharides (EPS) from cyanobacterial slurry. This approach stabilized the EPS structure and enabled a self-sustaining, biomass-derived remediation system. EPSH exhibited a high NH₄⁺-N removal efficiency of 96.19 % in eutrophic water, significantly outperforming NO₃^--N (75.05 %) and Urea-N (62.23 %). The adsorption followed a pseudo-second-order kinetic model (R² = 0.981) and Freundlich isotherm (R² = 0.992), indicating multilayer heterogeneous chemisorption. FT-IR and XPS analyses revealed that the hydrogel surface was rich in carboxyl (-COO^-) and hydroxyl (-OH) groups, which contributed to electrostatic interactions and hydrogen bonding with NH₄⁺-N. Molecular mechanism studies showed that surface-enriched -COO^- groups cooperatively captured NH₄⁺ via strong electrostatic attraction and hydrogen bonding, with an adsorption energy (E_ads) of −120.64 kcal/mol. Type A hydrogen bonds (2.928 Å) exhibited higher binding stability than Type B (3.453 Å). This work offers a robust, eco-friendly, and closed-loop strategy for mitigating NH₄⁺ hotspots in eutrophic lakes.