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Environmental fate & pathways

Bioaccumulation: terrestrial

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Description of key information

Data available for uptake of micro- and nanosized TiO2 by different plant and earthworm species indicate an absence of a bioaccumulation and biomagnification potential.

Key value for chemical safety assessment

Additional information

Microsized TiO2:

In a reliable study by Caille et al. (2005), BSAF values of titanium range from 0.0002 to 0.0008 kg/kg for leaves of 3 different plant species (rape, cabbage and red fescue) grown for 54 days on dredged sediment in the laboratory containing 3040 +/- 210 mg Ti/kg dw. Hence, Ti has a low potential for bioaccumulation in plants.

Additionally, in a supporting study by Gogos et al. (2016), uptake of E171 TiO2 bulk particles (primary particle size: 92 ± 31 nm) by Trifolium pratense (red clover) and Triticum spp. (spring wheat) after 14 and 12 weeks of exposure, respectively, was not significantly changed with exposure concentrations up to 1000 mg TiO2/kg soil (nominal) compared to the control.

This low uptake of Ti in plants is assumed to be due to the low solubility of microsized TiO2 at environmentally relevant conditions.

Nanosized TiO2:

According to the recent ECHA guidance on aquatic bioaccumulation (Appendix R 7-2 Recommendations for NM applicable to Chapter R7c) it is not possible to predict the bioaccumulation of nanomaterials from the log Kow or solubility. Hence, BCF/BAF values have to be experimentally determined. Further, for nanomaterials that undergo dissolution it is recommended to obtain information on the form of the substance as present in the animal tissue. Since nano-TiO2 does not to dissolve under environmentally relevant conditions, a potential uptake is unlikely to be associated with the uptake of dissolved Ti forms. Thus, nanosized TiO2 (if any) could potentially only be taken up of via adsorption to biological surfaces, direct penetration, or endocytic processes such as via phagocytosis, pinocytosis, and caveolae-dependent or clathrin-mediated endocytosis.

Furthermore, according to ECHA guidance on terrestrial bioaccumulation of NM (Appendix R 7-2 Recommendations for NM applicable to Chapter R7c, 2017), estimates based on “partitioning” may underestimate exposure in soil and sediment environments while overestimating exposure in water in the case of nanoparticles. Since estimation methods are not available, BCF/BAF values have to be determined experimentally.

In several laboratory bioaccumulation studies with TiO2-NP and plants and earthworms exposed to spiked soil, accumulation kinetics were not addressed and it was not always clarified if steady-state conditions were reached.

Three studies on the bioaccumulation of TiO2 nanoparticles in terrestrial plants are available:

In a reliable study by Gogos et al. (2016), uptake of titanium by Trifolium pratense (red clover) and Triticum spp. (spring wheat) during 14 and 12 weeks of exposure, respectively, did not significantly increase with nominal exposure concentrations up to 1000 mg/kg soil TiO2 nanoparticles (P25, primary particle size: 29 ± 9 nm) compared to the control, indicating a low bioaccumulation potential in plants. Additionally, TEM imaging coupled with EDX analysis did not detect a significant number of Ti-containing nano-sized particles inside plant root cells.

Bakshi et al. (2019, supporting study) investigated uptake and translocation of nano-TiO2 in tomato plants (Solanum lycopersicum) grown on soil containing sludge from a pilot wastewater treatment plant. Micro- and nano-TiO2 in the well characterized sludge (XANES, µXRF, ICP-MS) with average Ti concentration of 819±62 mg/kg (approx. 10 % nanoparticles) originated from wastewater. Compared to control uptake, Ti concentrations of leaves, flowers and fruits were not elevated after 120 days of cultivation. Titanium concentrations of aboveground plant tissues were < 2.59 mg/kg dw, indicating a very low potential for Ti root/shoot translocation at realistic nano-TiO2 concentrations of the substrate.

 

In a supporting study by Klingenfuss (2014), wheat was grown in sandy soil spiked with TiO2-NP for 84 d. TiO2 NPs were not found in wheat roots, suggesting that TiO2 nanoparticles were not taken up. Results on the translocation of Ti to the wheat grain at elevated TiO2 soil concentrations are, however, inconclusive.

Earthworms (Eisenia fetida) were exposed to 10 - 1000 mg/kg of different TiO2-NP (rutile/anatase, rutile, anatase) in artificial substrate for 28 days. After a depuration period of 24 h, whole body burdens were determined. For the different nanomaterials, median biota-soil accumulation factors range from 0.08 -0.15 kg soil/kg earthworm and median biota-food accumulation factors range from 0.09 -0.11 kg food/kg earthworm indicating that earthworms do not bioaccumulate TiO2-NP via soil or food. Further, titanium in the earthworms could not be localised and an adsorption of TiO2-NPs to the cuticle of earthworms cannot be excluded (Hund-Rinke & Klawonn, 2013).

 

In sum, data available for the uptake of TiO2-NP or titanium by different plant species and earthworms indicate a low bioaccumulation and biomagnification potential.