Due to the use of titanium dioxide (TiO2) nanoparticles in everyday products, increased exposure in bodies of water (i.e. the aquatic environment) is expected. For example, a washing out of particles from paint or color, which are used outside of buildings and are exposed to wind and rain is conceivable. Likewise, particles from sunscreen-treated skin may reach surface water or wastewater during bathing or showering.

 

It is not easy to detect TiO2 nanoparticles in the environment. However, titanium dioxide nanoparticles were found to leach from facades painted with TiO2-based paint [1]. The particles run down with the rain water in form of aggregates, and often embedded in the paint components, and reached surface waters.

On examination of sediment samples from a region in China, engineered TiO2 particles were detected by electron microscopy and it was possible to distinguish them from naturally occurring titanium [2]. At the same time, this study showed that the accumulation of Ti in sediments has been going on for decades, since also coarser particles produced earlier are detectable. As the sources of Ti pollution, the introduction of treated and untreated waste water is assumed.

An investigation of effluent from sewage treatment plants [3] showed that a large proportion of titanium particles is removed from the wastewater, however, the very small (<700 nm) particles remain in the water, and so again may reach rivers and lakes. The titanium concentrations in the effluent of the treatment plant were 5 to 15 µg/l. During the purification process, most of the titanium particles are bound to solids and get into the sludge. The latter, in turn, is either disposed of as landfill or spread on fields as fertilizer, so that the coarser titanium particles are more likely to get into the soil.

Because the TiO2 concentrations in the environment are so low, both the development of measurement methods [4,5] and the simulation of exposure to titanium dioxide nanoparticles in the environment [6] are currently in the focus of research and development.

 

By means of computer programs, it has been attempted to simulate the probable behavior of titanium dioxide nanoparticles in the environment. Therefore, they most likely occur in natural surface waters and their sediments and in sewage sludge and soils on which sewage sludge was disposed [7,8]. Comparing these predicted environmental concentrations (PEC values) with concentrations not just hazardous for environmental organisms, (PNEC value), it is apparent that currently; particularly TiO2 nanoparticles in discharge from sewage treatment plants may pose an environmental risk. However, for surface water, soil, and air no risk is expected at present. The figure explains in more detail how such a risk is calculated.

 

Risk Ratios for TiO2 in different Regions

Environmental compartment Europe USA Switzerland
Surface waters 0,015 0,002 0,02
STP effluent 3,5 1,8 4,3
Air <0,0005 <0,0005 <0,0005

 

The risk ratio is calculated from the predicted environmental concentrations (PEC) divided by the concentrations that have no effects on environmental organisms (PNEC). If the risk ratio is less than 1, there is no immediate risk to the environment, whereas at levels above 1, there is a risk and further investigations must be carried out [7].

Another computer simulation assumes that in the future, the amount of produced TiO2 will continue to rise and that the proportion of nanoscale TiO2 also further increases [9]. From this, it is concluded that the environmental concentrations will increase in the future.

 

Generally, there are still large gaps in knowledge in this area which are mainly due to inadequate methods of measurement and, hence, accurate knowledge of the environmental concentrations is missing. Further, data are lacking on accurate amounts of substance, as well as on the behavior and distribution in the three environmental compartments water, soil, and air.

 

Literature arrow down

  1. Kaegi, R et al. (2008), Environ Pollut, 156(2): 233-239.
  2. Luo, Z et al. (2011), J Environ Monit, 13(4): 1046-1052.
  3. Kiser, MA et al. (2009), Environ Sci Technol, 43(17): 6757-6763.
  4. Tiede, K et al. (2009), Water Res, 43(13): 3335-3343.
  5. Contado, C et al. (2008), Anal Chem, 80(19): 7594-7608.
  6. Gottschalk, F et al. (2010), Environ Modell Softw, 25(3): 320-332.
  7. Gottschalk, F et al. (2009), Environ Sci Technol, 43(24): 9216-9222.
  8. Mueller, NC et al. (2008), Environ Sci Technol, 42(12): 4447-4453.
  9. Robichaud, CO et al. (2009), Environ Sci Technol, 43(12): 4227-4233.

 

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