This research investigated the colloidal stability and reactivity of green iron oxide nanoparticles for removing NO3 – ions from polluted water. These nanoparticles were synthesized by Eucalyptus globulus leaf extract (EL-Fe NPs). Transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FT-IR), and dynamic light scattering (DLS) were utilized to characterize EL-Fe NPs. The effect of contact time, different pH, KCl salt, and anions (PO4 3_, SO4 2_, HCO3 – , Cl_), on NO3 – adsorption using EL-Fe NPs were evaluated. Based on the results, Fe3O4 and a-Fe2O3 nanoparticles were encapsulated by polyphenols and have irregular nanoparticulate structures. After 30 min, the maximum adsorption capacity of 1.5 g L_1 EL-Fe NPs dispersed in 50 mg L_1 NO3 – solution with a pH of 3.7 was acquired at about 12.91 mg/g. Adsorption of NO3 – on EL-Fe NPs was strongly pH dependent, and at pH > 6.6 no significant adsorption occurred. KCl salt by agglomeration of EL-Fe NPs, the mean particle size varied between 25.6 nm and 63 nm, and NO3 – adsorption decreased dramatically. It was observed that EL-Fe NPs significant affinity to adsorb PO4 3_ ions. As a result, no significant adsorption of NO3 – ions onto EL-Fe NPs was detected in the presence of phosphate ions. The experimental data were reasonably fit to a pseudo-second-order kinetic model (R2 = 0.992), which can be concluded that the primary mechanism of NO3 – adsorption is electrostatic. This work indicated that the removal of NO3 – using EL-Fe NPs was influenced by the type and concentration of accompanying cations and anions.

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