Effects of monovalent and divalent metal cations on the aggregation and suspension of FeO magnetic nanoparticles in aqueous solution.

There has been limited research investigating how the mechanisms of aggregation of magnetic nanoparticles (MNPs) are affected by inorganic ions. In this study, Na, Mg, Ca, Sr and Ba were selected to systematically study the aggregation mechanisms of FeO MNPs. The results indicated that divalent cations more significantly affected the stabilities of MNPs than Na at low concentrations (e.g., 0.1mM) in a decreasing order of Ba>Sr>Ca>Mg>Na. Extended DLVO theory did not offer a satisfactory explanation for the above difference due because it ignores specific ion effects. It was also found that the initial adsorption ratios of these metals by FeO MNPs were linearly proportional to the hydrodynamic diameter (HDD) of FeO MNPs before aggregation occurred. In addition to the valence states, the hydration forces and ionic radii of the metal cations were proposed to be other factors that significantly affected the interactions of metal cations with FeO MNPs based on the excellent linear relationships of the HDD of FeO MNPs and these three factors. Moreover, a bridging function of divalent cations might develop after aggregation occurred based on the increases in their adsorption amounts and intensities for binding oxygen-containing functional groups. In addition, an increase in the positive ζ potential of MNPs was observed with the addition of divalent cations until 10.0mM at a pH of 5.0, which potentially enhances the resistance of MNPs to aggregation in aquatic systems compared with Na. Consequentially, the effects of metal cations on the aggregation of MNPs are determined by the hydration forces, valance states, ionic radii and bond types formed on the MNPs. Thus, the specific ion effects of these cations should be considered in predicting the environmental behaviors of specific nanomaterials.