Research Progress on the Aggregation Behavior and Stability Mechanisms of Organic-Coated Magnetite Nanoparticles

Magnetite nanoparticles (MNPs) have been widely used for heavy metal adsorption, organic pollutant degradation and water remediation owing to their large specific surface area, high reactivity and facile magnetic separation. However, pristine MNPs readily aggregate in aquatic environments, which reduces accessible surface area and active sites, lowers mobility, and ultimately limits remediation efficiency. Surface organic coating is an effective strategy to enhance the colloidal stability of MNPs, yet the diversity of coating types and the complexity of environmental factors make the underlying regulation mechanisms insufficiently clarified. This review summarizes the effects of small organic acids, polymeric ligands and surfactants on the stability of MNPs. Small organic acids such as acetic acid (AA), citric acid (CA) and oleic acid (OA) regulate surface charge via –COOH/–OH groups, typically shifting the zeta potential from about −20 mV to −30 – −35 mV and thus strengthening electrostatic repulsion. Polymeric coatings such as poly(acrylic acid) (PAA), polyethylene glycol (PEG) and carboxymethyl cellulose (CMC) form 5–20 nm steric layers, maintain |ζ| at 30–40 mV, and ensure good dispersion under high salinity and over a broad pH range. For example, CMC-coated MNPs exhibit hydrodynamic diameters of 40–120 nm and a Pb²⁺ adsorption capacity of 152 mg/g, indicating excellent environmental robustness. Environmental pH, ionic strength, ion valence, and natural organic matter (NOM) are identified as primary controls on organic coated MNPs stability; NOM adsorption, typically 50–250 times higher than that of the original organic coating, can reconfigure surface chemistry and aggregation pathways. In addition, light, oxidative ageing and microbial processes may destroy or reconstruct coatings, switching aggregation mechanisms between suppression and promotion. Evidence from DLVO/EDLVO analysis combined with dynamic light scattering (DLS), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) analytical methods demonstrates that aggregation of MNPs is governed by the coupled action of electrostatic repulsion, steric repulsion, cation bridging, and patch-charge attraction, with the dominant interaction shifting in response to environmental conditions. A mechanistic understanding of the co-regulation by organic coatings and environmental factors provides a theoretical basis for designing highly stable and environmentally benign MNP-based remediation materials.