Multilayered alginate–hydrogel-functionalized zeolite based on ionic cross-linking composed with a certain Ca2+ and Mg2+ ratio for ammonium adsorption

To enhance ammonium removal in wastewater treatment, a novel adsorbent, multilayered ionic cross-linked alginate-hydrogel-metal-coated zeolite (Al-H-M-Z), was synthesized. Essential divalent cations, 2.60 mmol of Ca²⁺ and Mg²⁺, were incorporated into Al-H-M-Z (10 g) in a Ca²⁺:Mg²⁺ ratio of 0.73:1.85, facilitating strong alginate-polymer interactions and significantly enhancing the adsorption efficiency. Langmuir and Freundlich isotherm analyses revealed that Al-H-M-Z had a maximum adsorption capacity of 56.5 ± 8.78 mg g^-1 and an adsorption intensity of 0.703 ± 0.122, which were 7.24-fold and 1.59-fold higher than those of natural zeolite (7.80 ± 1.09 mg g^-1 and 0.444 ± 0.254), respectively. These enhanced properties are attributed to the modified structural and surface chemistry. The Al-H-M-Z coating exhibited a 4.76-fold increase in surface area (3.91 m² g^-1), an 8.94-fold increase in micropore volume (17.8 × 10^-3 cm³ g^-1), and a 4.22-fold increase in pore diameter (4.14 nm) when compared to the traditional Al-only coating. For applicability, Al-H-M-Z and Z achieved ammonium removal rates of 7.94% and 5.61%, respectively, in actual fish farm wastewater, representing a 1.41-fold improvement in adsorption efficiency after the coating process.