It is generally assumed that titanium dioxide (TiO2) particles regardless of their size do not dissolve. Thus it is likely that TiO2 retain their particulate form in the environment. Studies on the environmental behaviour of TiO2 nanoparticles were carried out under three main aspects: (1) Influence of environmental conditions on the presence and mobility of the particles, (2) binding of environmental contaminants by the particles, and (3) the influence of particles on processes in the environment.

 

(1) Studies on the influence of environmental conditions on the mobility are concerned in particular with naturally occurring substances in soil or water and their interaction with the nanoparticles. This is particularly the influence of agglomeration , as well as stability and deposition of particles. The natural materials include organic materials (degradation products of plants or animals), such as humic or fulvic acids, which are included in all waters and soils in different proportions.

In most cases, binding of such materials to TiO2 leads to stabilisation of the suspension particles and prevents their agglomeration [1,2,3,4,5,6,7]. As a result, particles rather "float" in the water, thus remain mobile and do not sediment. Stabilisation by organic materials is largely independent of the pH and salinity of the environment, i.e. takes place under various environmental conditions. The agglomeration-preventing effect of proteins is similar to that of organic materials [8].

However, certain substances like organic acids, e.g. oxalic acid, can also have opposite effects or may not exert any stabilising effect [9]. Minerals or salts also increase agglomeration of the particles [10] and reduce their mobility [1].

The behaviour of the particles is also influenced by their chemical and physical properties. Hence, contaminants (e.g. residues from production) may affect the surface charge and, thus, the behaviour of the particles [11,5]. However, according to the present studies, the different crystalline structures of TiO2 and different shapes and sizes have no effect on sedimentation and agglomeration of the particles.

 

(2) In addition to the natural organic substances, also inorganic substancescan bind to titanium dioxide particles and thereby influence their behaviour in the environment and their effects on environmental organisms. Titanium dioxide particles can bind toxic heavy metals such as cadmium and arsenic [12,13,14].

Carp exposed to cadmium and arsenic-containing water and titanium dioxide particles took up more cadmium and arsenic than carp in particle-free water [13,14]. In algae, however, this effect was not observed, because algae do not absorb cadmium-laden particles [12]. Due to the photocatalytic effect of TiO2, arsenic is transformed into a less toxic form [15]. Similarly, an increased binding of phosphorus to TiO2 has been described [16].

Thus, TiO2 nanoparticles in principle bind many substances and increase the bioavailability for organisms. Whether TiO2 particles have a significant impact on the availability of other pollutants under field conditions has not been investigated yet.

 

(3) TiO2 nanoparticles can also affect processes in the environment, e.g. by changing the properties of sediments with respect to surface area, pore size, and ability to bind to other substances [16]. This could for example increase the binding of nutrients in TiO2-contaminated soils. The precise effects of TiO2 enrichment, however, are still unexplored.

Also, water-purification processes in water treatment plants can be affected by high TiO2 concentrations. Effects on the removal of nitrogen compounds have been described [17].

 

Literature

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  8. Allouni, ZE et al. (2009), Colloids Surf B Biointerfaces, 68(1): 83-87.
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  10. French, RA et al. (2009), Environ Sci Technol, 43(5): 1354-1359.
  11. Liu, X et al. (2011), J Colloid Interface Sci, 363(1): 84-91.
  12. Hartmann, NB et al. (2010), Toxicology, 269(2-3): 190-197.
  13. Sun, H et al. (2009), Environ Pollut, 157(4): 1165-1170.
  14. Zhang, X et al. (2007), Chemosphere, 67(1): 160-166.
  15. Pena, ME et al. (2005), Water Res, 39(11): 2327-2337.
  16. Luo, Z et al. (2011), J Hazard Mater, 192(3): 1364-1369.
  17. Zheng, X et al. (2011), Environ Sci Technol, 45(17): 7284-7290.

 

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