Mechanistic insights into phosphate sequestration by protein-functionalized Fe3O4 magnetic nanocomposite derived from Morinda citrifolia.

Phosphate pollution is a significant factor in eutrophication and declines in water quality. This study presents a protein-functionalized magnetic nanocomposite (MCSP-Fe3O4) synthesized through a green method using crude biomolecules from Morinda citrifolia seeds as natural modifiers for phosphate adsorption. This approach allowed the formation of magnetic nanoparticles while introducing bioactive surface functionalities from the seed proteins. Structural and physicochemical characterization confirmed the formation of crystalline magnetite with mesoporous characteristics and the presence of biomolecule-derived surface functionalities. The MCSP-Fe3O4 nanocomposite showed a maximum phosphate adsorption capacity of 118.45 mg g-1. Adsorption kinetics were best described by the pseudo-second-order and Elovich models, indicating that surface-controlled interactions on heterogeneous adsorption sites governed phosphate uptake. Equilibrium data aligned well with the Langmuir and the Freundlich isotherms, suggesting both monolayer adsorption behavior and heterogeneous surface interactions. Based on adsorption behavior, surface chemistry characteristics, and previous studies of Fe3O4-based phosphate adsorbents, phosphate uptake is proposed to involve a combination of ligand exchange at Fe-OH sites, electrostatic attraction, and hydrogen-bonding interactions. The mesoporous structure and magnetic recoverability allowed efficient adsorption and easy post-treatment separation using an external magnetic field. Although the adsorbent maintained magnetic recoverability during repeated use, further studies on desorption efficiency, competing ions, and advanced surface characterization are required to elucidate the adsorption mechanism and its practical applicability fully. These findings demonstrate the potential of underutilized Morinda citrifolia seed-derived biomolecules as sustainable functionalization agents for magnetic phosphate adsorbents.