A mechanistic study of stable dispersion of titanium oxide nanoparticles by humic acid

Stable dispersion of nanoparticles with environmentally-friendly materials is important for their various applications including environmental remediation. In this study, we systematically examined the mechanisms of stable dispersion of two types of TiO₂ nanoparticles (TNPs) with anatase and rutile crystalline structures by naturally occurring dissolved organic matter (humic acid) at different pHs, including at, below and above the Point of Zero Charge (PZC). The results showed that stable dispersion of TNPs by humic acid (HA) at all pHs tested can only be achieved with the assistance of ultra-sonication. The dispersion of TNPs by HA differed at the three pHs tested. Generally, HA greatly decreased the hydrodynamic diameters of TNPs at a very low concentration. The dispersion of TNPs became relatively stable when the HA concentration exceeded 5 mg/L, indicating that this HA concentration is required for stable dispersion of TNPs. The mechanisms involved in dispersion of TNPs by HA included electrostatic repulsion, steric hindrance and hydrophobic interaction. Electrostatic repulsion was identified to be the dominant mechanism. The dispersion of TNPs was enhanced when HA was added before ultra-sonication to avoid the partly irreversible re-aggregation of TNPs after sonication. The crystalline phases and concentrations of TNPs were also found to influence their stable dispersion. The findings from this work enhance understanding of the combined effects of HA, pH, ultra-sonication and crystalline structures of TNPs on their stable dispersion. The mechanisms identified can improve applications of TNPs in environmental water pollution control.