Impact of natural organic matter and divalent cations on the stability of aqueous nanoparticles

The stability of nanoparticles in aquatic environment plays an important role in determining their environmental implication and potential risk to human health. This research studied the impact of natural organic matter (NOM) and divalent cations (Ca²⁺) on the stability of engineered metal oxide nanoparticles (e.g. ZnO, NiO, TiO₂, Fe₂O₃ and SiO₂). When nanoparticles were present in neutral water, a relatively weak electrolyte concentration (0.01 M KCl) could result in their aggregation; however, with the addition of 1 mg/L NOM, the negative surface charge of nanoparticles increased significantly and therefore their propensity to aggregate is reduced. 4 mg/L NOM stabilized most nanoparticles by producing -30 mV or higher zeta potentials. On the other hand, the negative charge that NOM imparted to nanoparticles could be neutralized by divalent cations (calcium ions). 0.04 M-0.06 M Ca²⁺ induced the aggregation of NOM-coated nanoparticles. It should be noted that among all the studied nanoparticles, SiO₂ exhibited the unique stability due to its low NOM adsorption capacity and small Hamaker constant. SiO₂ remained stable no matter whether the solution contained NOM or Ca²⁺.