Concentration-dependent effects of humic acid and protein on the stability of hematite nanoparticles in an aqueous environment

In this study, humic substance (humic acid, HA) and protein (bovine serum albumin, BSA) were chosen as typical natural organic matter (NOM) to reveal the effects of NOM types and concentrations on the stability of hematite NPs (9 +/- 2 nm) in solutions containing monovalent (Na+) and divalent (Mg2+ and Ca2+) cations at pH 6.5. The results indicated that the surface functional groups and conformational structures of NOM were important factors in determining the stability of NPs in an aqueous environment. HA with a large amount of oxygen-containing groups and a high negative zeta potential (-25.7 mV) could increase the stability of hematite NPs due to the strong electrostatic repulsion and steric repulsion formed in the NaCl solution. The formed cation-bridging effect after hematite NPs incubated in HA induced the fast aggregation of NPs in solutions containing divalent cations. Non-linear relationships between hematite NP stability and BSA concentrations were observed in all three electrolytes. The stability of hematite NPs was significantly depressed at low concentration of BSA due to the decreased electrostatic repulsive force. With the increase in BSA concentration, the steric repulsion formed by the adsorption of BSA molecules on the surface of hematite NPs gradually dominated and stabilized hematite NPs. These results highlight the crucial role of NOM types and concentrations as well as solution chemistry conditions in the colloidal stability of hematite NPs in NOM-rich water.