Sedimentation of TiO₂ nanoparticles in aqueous solutions: influence of pH, ionic strength, and adsorption of humic acid

Abstract

With the development and industrial applications of nanotechnology, increasing productions of engineered nanomaterials enter the aqueous environments by several routes (i.e. remediation purpose and unintentional release). Due to the potential risk deriving from their higher surface activity and greater transferability, engineered nanoparticles bring a new challenge for the environmental systems and human beings. Under favorable particle–particle interactions, nanoparticles aggregate, form large clusters, and settle out from the environmental solutions. The settling process can reduce the mobility of nanoparticles in environmental systems. In the current study, sedimentation experiments were conducted under different environmental conditions to investigate the settling process of nanoparticles. The results indicated that the A/A₀ ratios of titanium dioxide (TiO₂) nanoparticles increased with increasing ionic strength. They also increased near the pH_pzc (point zero of charge). However, the increasing concentrations of humic acids in aqueous solutions reduced the A/A₀ ratios even near the pH_pzc. The measured zeta potentials revealed that the TiO₂ nanoparticles became less charged in the range of pH_pzc and at higher ionic strength. It was due to the double electric layers of TiO₂ nanoparticles compressed with increasing in ionic strength and charge screening effect. Therefore, the electrostatic repulsive force between nanoparticles significantly declined, which lead to favorable aggregation. The interaction energies calculated using the DLVO theory under different pH and ionic strength conditions were consistent with the experimental results from the sedimentation experiments. In addition, the steric interactions between humic acid absorbed on the surface of TiO₂ nanoparticles were responsible for the reduction in the A/A₀ ratios.

Keywords:

• Sedimentation
• Titanium dioxide nanoparticles
• Ionic strength
• Humic acid

Acknowledgements

This work was financially supported by the Funds for Creative Research Groups of China (51121062), the National Natural Science Foundation of China (51278147).