Finding needles in haystacks – Detection of TiO2 nanoparticles in soil

In a recent publication from the Environmental and Soil Chemistry group, we present a method enabling us to detect anthropogenic TiO2 nanoparticles in soil despite the presence of a high natural background. The method relies of colloidal extraction and trace elements ratio using ICP-MS. 

Persistent nature of TiO2 nanoparticles

TiO2 nanoparticles are present in various large scale applications such as surface painting, cosmetics, food pigments, etc. A significant amount of this persistent pollutant is released into the environment through runoff or waste water sludge application where it accumulates mostly on sediments and soils. One of the long terms objectives in our team is to understand how TiO2 nanoparticles are transported in the environment and if it could be of concern in specific situations. That’s why we developed a method to detect TiO2 nanoparticles in soils.

Figure 1: Anthropogenic TiO2 nanoparticles in soil are mixed with natural TiO2 particles, which makes their detection difficult (CC: A. Philippe / Uni Landau)

The study

The biggest challenge in this respect is the presence of relatively large amount of natural TiO2 minerals. For differentiating between them from the anthropogenic nanoparticles, our method relies on two properties of the natural TiO2 particles:

  • They are natural particles of all sizes in the soil but the vast majority of it is much larger than the typical human made nanoparticles (< 300 nm). Therefore, we suspended the soil to be analyzed in a water-based solution and separated the nanoparticulate (“colloidal”) fraction from the larger particles. This way we could considerably reduce the amount of natural TiO2 particles in the samples, while keeping most of the nanoparticles we want to detect. 
  • In nature, Titanium is strongly associated with trace elements such as Niobium in contrast to engineered nanoparticles usually synthesized from purified TiO2. We used this fact in our favor by measuring both Titanium and trace elements in the samples and used a reference value (control sample) to correct for the natural background. This was done using an ICP-MS to determine the concentration of each element after proper sample preparation.

The combination of these two ideas and some further important method optimizations enabled us to reach detection limits in the ppm (µg/g) range for natural backgrounds as high as 5 mg/g. We hope to be able to apply this method in the frame of larger monitoring programs in the future.

Further details about the study can be found in the original paper titled ‘Quantification of anthropogenic TiO2 nanoparticles in soils and sediments combining size fractionation and trace element ratio’ authored by Allan Philippe, Ahmad Bazoobandi and Nadine Göppert, and published in the Journal of Analytical Atomic Spectroscopy (Volume 2)