Sustainable metal-phenolic hybrid adsorbent: Double-crosslinked alginate/carboxymethyl cellulose for effective ammonium ion capture

This study develops effective ammonium ion capture beads based on a metal–phenolic network using alginate and carboxymethyl cellulose, crosslinked with ferric ions and tannic acid as secondary crosslinkers. Synthesized via the dripping method, the beads exhibit a rough surface with strong interactions between the polymer carboxylate groups and tannic acid hydroxyl groups. Increasing the tannic acid concentration enhances the gel content of the beads. Physisorption analysis indicates that incorporating carboxymethyl cellulose and tannic acid reduces surface area and pore volume due to pore blocking, while maintaining adsorption efficiency and improving bead stability. Adsorption studies confirm that the beads effectively remove ammonium ions, following a pseudo-second-order kinetic model, indicating a chemisorption process. The Freundlich isotherm model, which suggests heterogeneous adsorption sites, with beads containing 0.050 % w/v tannic acid achieving the highest removal efficiency of 80.3 ± 0.7 % at 298 K. Thermodynamic analysis confirms that the process is endothermic and spontaneous, favoring higher temperatures. Reusability studies show that the beads maintain 80.1 ± 2.2 % removal efficiency in the initial (zero cycle), decreasing to 76.9 ± 1.4 % in the first, 72.2 ± 2.2 % in the second, and 70.1 ± 1.2 % in the third cycle. Chemical oxygen demand analysis indicates improved stability after the first cycle, with fewer residual fragments in the solution. The adsorption mechanism involves electrostatic interactions, hydrogen bonding, cationic exchange, and ion–ion interactions, with gallotannins acting as chelators for ammonium ions. These findings highlight metal–phenolic network beads as a promising, sustainable biosorbent for ammonium ions removal, offering high efficiency, stability, and reusability in water treatment applications.