This study focuses on the colloidal stability of green-synthesized iron nanoparticles utilizing Eucalyptus extract (EL-FeNPs), for water treatment, particularly in the removal of phosphate. The influence of phosphate on the surface zeta potential (ζ) and average particle size (APS) of EL-FeNPs in various electrolyte solutions (NaCl, KCl, CaCl2) was assessed using dynamic light scattering (DLS). Furthermore, the impacts of different conditions on phosphate adsorption were explored. The point of zero charge (pHpzc) for EL-FeNPs was determined to be at pH 3.5. At acidic pH levels, EL-FeNPs exhibited an amorphous structure with APS of 25.6 nm; however, an increase in pH facilitated their aggregation and crystallization when the APS increased to 84.5 nm. Optimal phosphate adsorption was achieved in 100 mM CaCl2 (31.88 mg g-1), followed by KCl (21.13 mg g 1), and NaCl (15.70 mg g 1), at pH 7. Nonetheless, EL-FeNPs tended to aggregate in these electrolyte solutions. The introduction of phosphate to the suspension significantly reduced the APS and enhanced the negative ζ. For example, in the ELFeNPs- Ca system, the APS was approximately 6887 nm (ζ ≈ -18.77), which decreased to about 4505 nm (ζ ≈ -24.04) with the addition of phosphate. Phosphate adsorption onto EL-FeNPs predominantly involved innersphere complexation with monolayer adsorption, aligning closely with the pseudo-second-order kinetic model (R2 = 0.999) and Langmuir-Freundlich isotherm (R2 = 0.999). Thermodynamic assessments indicated that the adsorption process was both endothermic and spontaneous. The findings suggest that phosphate can notably improve the stability of EL-FeNPs in environmental settings.

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