The stability of Titanium Dioxide Nano-particles in Aquatic Environment

The increasing use of nano-materials in consumer products has led to much concerns about their release to the environment and the subsequent health impacts. To better predict the fate and behavior of nano-particles in aquatic systems, it is essential to understand their interactions with different components of natural waters including natural organic matter (NOM), such as humic acid and fulvic acid.
The objective of this study is to investigate the stability of nano- titanium- dioxide (nano-TiO2) under different pH, ionic strength and concentration of NOM in aqueous phase. The stability was determined by measuring the concentration of Ti in supernatant of the suspension.
In this study, the surface charge of nano-TiO2 particles changed from positive to negative at pH 5.5. When the pH of the solution approaching 5.5, the average particle size increased rapidly to the micrometer level, and residual supernatant concentration was only 7.8% of the original concentration after three days. The effects of the ionic strength on the stability of nano-TiO2 particle were investigated. The result showed that when ionic strength was greater than 30 mM, the supernatant residual concentration of Ti would not influenced by pH value. The residual concentration decreased as ionic strength increased. Theses results were validated by the value of the estimated energy barrier between nano-TiO2 particles, which is calculated by following Derjaguin Landau Verwey Overbeek (DLVO) theory.
The results from the settling experiments revealed that initial concentration of humic substances lower than 1 mg/L would cause nano-scale titanium dioxide particles unstable. Zeta potential for particles had no significant influence when humic substances were added under different concentrations. But the residual concentration of Ti in the supernatant would increase when the equilibrium concentration of humic substances increased. It seemed that the static-electric repulsion become more apparent. Under high equilibrium concentration of humic acid (14.1 mg/L) the surfaces of nano-TiO2 particles were covered with humic acid, the amount of adsorbate caused bridging between particles, and the particles easily aggregated and settled.