Aggregation of titanium dioxide nanoparticles in water

With the advancement of nanotechnology in today’s world, more products are incorporated with nanoscale materials at atomic level. Equipped with self-cleaning quality and being a disinfectant by itself, titanium dioxide (TiO2) has expanded its application into consumer products in the last few decades and this grow concerns about potential issues in environmental and health aspects. Hence, aggregation behavior of nanoparticles is of great significance for safe application. Starting from preparation of TiO2 suspension using P25 (Evonik, Germany), aggregate size and zeta potential were used to analyze the behavior of nanoparticles in water under the influence of pH, ferric ions and humic acid (HA). Aggregates of TiO2 nanoparticles increased with pH values near the point of zero charge (pH 6 to 7) due to Derjaguin and Landau, Verwey and Overbeek (DLVO) theory. Under the condition of constant pH value of 6 and with incremental addition of ferric ions, aggregate size and zeta potential generally increased with ionic strength which were due to adsorption of ferric ions on TiO2. However, with the variation in pH and at a fixed ferric ions dosage, aggregation of TiO2 nanoparticles only started to enhance at pH 6 to 9. The increase in aggregate size proved that decrease of repulsion force between nanoparticles occurred with increasing pH. Aggregation was limited at low pH due to the presence of strong electrostatic repulsion where zeta potential was highly positive on the surface of the particles. In contrast with ferric ions, influence of HA showed that aggregation could occur, presumably due to the increase of repulsion forces between nanoparticles. Zeta potential decreased from 5.5 mV to -35.0 mV which promoted HA as a barrier in aggregation, and further increase of HA dosage resulted in insignificant change. This was based on an assumption that, for P25 nanoparticles with the presence of HA, zeta potential within -20 mV to 20 mV could create unstable condition which is favorable for aggregation. Repulsion forces below -20 mV or above 20 mV would lead to stabilization of TiO2 nanoparticles, and well-stabilized condition would take place with zeta potential below -40 mV or above 40 mV. Sharp decline in zeta potential indicated that the properties of TiO2 were changed greatly by the addition of HA, which increased the stability of the nanoparticles. This allowed nanoparticles to become persistent in the environment and thus, lowered their risks to the water quality. This phenomenon arises as electrostatic repulsion forces increased with change in surface charges of nanoparticles. With addition of ferric ions, HA became a lower energy barrier in aggregation at a lower concentration of HA but subsequently, aggregate size decreased with decreasing zeta potential, corresponding with increase of electrostatic repulsion forces. This indicated that the HA still acted as the dominant role in the process. Under the conditions investigated in this project, aggregation behavior of TiO2 nanoparticles can be greatly influenced by the variation in pH, ferric ions concentration and HA. This suggests that in natural water of real environment, TiO2 dispersion might occur to a larger extent.Bachelor of Engineering (Environmental Engineering