Tracing titanium dioxide nanoparticles in the environment
Titanium dioxide nanoparticles are one of the most commonly produced nanomaterials worldwide. They are present in many consumer products, such as sunscreen and confectionery. After use, a large proportion of TiO2 ends up in agricultural soils, through the application of sewage sludge. A prerequisite for risk assessment is to be able to distinguish this released TiO2 from the elevated TiO2 natural background, and therefore detect anthropogenic inputs. Furthermore, the current models of nanomaterial fluxes in environmental compartments have large uncertainties, therefore field monitoring is needed to be able to quantify actual anthropogenic inputs. Therefore, an evaluation of the differences in terms of composition and structure between natural and anthropogenic nanomaterials may provide clues to help interpret data on toxicity.
Within the framework of LabEx SERENADE and with the support of thenanoID platform (EquipEx), scientists from the Institut des Sciences de la Terre (ISTerre/OSUG, CNRS / UGA / IRD / IFSTTAR / USMB), ESRF, Particle Laboratory - Eawag and ECOLAB evaluated the potential of physical techniques (micro and nano X-ray fluorescence (XRF), X-ray absorption spectroscopy (XANES), X-ray diffraction (XRD) and (DRX) and transmission electron microscopy coupled with X-ray microanalysis (TEM-EDX)) to distinguish natural versus anthropogenic particles has been investigated. Three matrices were compared : sewage sludge, agricultural soil that had never received sewage sludge, and sludge-amended soil.
Finally, the morphology of the crystals observed by electron microscopy and the status of TiO2 particles within the organo-mineral aggregates proved to be relevant criteria to discriminate natural versus anthropogenic TiO2. In the sludge, smooth particles typical of TiO2 pigments were evidenced (Figure 1c, 1f), whereas the soil contained rough and irregular TiO2 particles. Moreover, TiO2 particles in the sludge were present as weakly evolved aggregates, dominated by organic matter (Figure 1a-b and Figure 2), whereas in the soil they were intimately associated with organo-mineral assemblages forming the soil micro- and macroaggregates (Figure 1d-f and Figure 2), the building blocks of soils.
Although TiO2 phases are generally considered as very weakly soluble, they undergo weathering and transformations in acidic soils or in the rhizosphere . Thus, the observed differences in particle morphology may attenuate over time due to the weathering of TiO2 minerals. Likewise, even if it is slow (in the range of decades, although this has been debated), the formation of soil aggregates is a dynamic and continuous process, so the progressive incorporation of anthropogenic TiO2 within soil organo-mineral assemblages is expected. It is likely that with time, engineered TiO2 becomes indistinguishable from the natural background in soils.
To conclude, the X-ray studies complemented microscopy methods providing chemical identification of the TiO2 species and location of the nanoparticles within soil and sludge matrixes.
- Figure 1. TEM analyses of the TiO2 particles present in sludge (a–c) and soil (d–e). (f) Energy-dispersive X-ray microanalysis of the zones highlighted in (c) and (e).
- Figure 2. Tricolour μXRF maps for sewage sludge (a,b) and for sludge-amended soil (c,d), showing the distribution of Ti, S, Ca, Al and Si.
Searching for relevant criteria to distinguish natural vs. anthropogenic TiO2 nanoparticles in soils, A.E. Pradas del Real (a,b), H. Castillo-Michel (b), R. Kaegi (c), C. Larue (d), W. de Nolf (b), J. Reyes-Herrera (b), R. Tucoulou (b), N. Findling (a), E. Salas-Colera (b) and G. Sarret (a), Environ. Sci. : Nano 5, 2853- 2863 (2018) ; doi : 10.1039/c8en00386f.
(a) ISTerre (Institut des Sciences de la Terre), Univ. Grenoble Alpes, CNRS, Grenoble (France)
(c) Eawag, Particle Laboratory, Dübendorf (Switzerland)
(d) ECOLAB, Universite de Toulouse, CNRS, INPT, UPS, Toulouse (France)
Local scientific contact
Géraldine Sarret, ISTerre/OSUG | geraldine.sarret univ-grenoble-alpes.fr
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The european synchrotron (ESRF)
 M. Schindler and M.F. Hochella, Geology, 44, 515–518 (2016).