Amino acid–functionalized magnetic iron oxides for selective removal of metals and nutrient ions: A critical review

This review critically evaluates magnetic micro- and nanoparticles for water treatment, with a specific focus on amino acid functionalization as a mechanism-driven route to selectivity and reusability. Iron oxide cores (Fe₃O₄/γ-Fe₂O₃) provide rapid magnetic recovery, while amino acid ligands bearing –NH₂, –COOH, –SH, and guanidinium functionalities enable tunable binding toward metal(loid) ions (Cd²⁺, Pb²⁺, Cu²⁺) and nutrient ions (nitrate, phosphate) through coordinated complexation, electrostatic interactions, ion exchange, and hydrogen bonding. Beyond summarizing synthesis and composite design options, the review foreground’s structure–function relationships linking surface chemistry, solution speciation, and matrix complexity to observed selectivity and regeneration performance. Key practical limitations are highlighted, including suppression of uptake in real waters by competitive ions and natural organic matter, fouling, oxidative aging of iron oxides, ligand loss, capacity/magnetization decay during reuse, secondary waste from regeneration, and batch-to-batch variability. Finally, we outline research priorities for standardized selectivity reporting under mixed-ion conditions, durability-focused regeneration protocols, scalable production routes, and pilot-scale validation coupled with inline magnetic recovery.