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Environmental hazard of selected TiO 2 nanomaterials under consideration of relevant exposure scenarios

Year: 2015
Type: dissertation
Abstract: In the last decades the production and use of nanomaterials, such as titanium dioxide nanomaterials (nano-TiO2), increased extensively. To support a sustainable nanotechnology, it is essential to investigate the environmental risks of nanomaterials. However, it remains unclear whether in the context of the European Chemical Regulation Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) the nano and bulk form of a substance have to be registered separately or not, and if parameters considered with approved and standardized test guidelines (OECD) are sufficient to describe the potential environmental implications of manufactured nanomaterials. These guidelines do not consider relevant exposure scenarios such as solar radiation and mixture toxicity which might be of special importance for manufactured nanomaterials, including nano-TiO2 which is known to be photoactive and to adsorb organic co-contaminants. Therefore, we conducted water flea tests with nano-TiO2 (Daphnia magna, OECD 202) under laboratory light (LL) and simulated solar radiation (SSR) and examined the potential mixture toxicity of nano-TiO2 materials and the organic antimicrobial substance triclocarban to earthworms (Eisenia fetida, OECD 207 and 222), daphnids, fish embryos (Danio rerio OECD 236) as well as to activated sludge (OECD 209). Different sized uncoated, anatase TiO2 materials were tested to account for particle size specific effects (nanomaterials: NM 101, 7-10 nm and NM 102, 15-25 nm; bulk: NM 100, 200-220 nm). Furthermore, we investigated: a) whether the ionic strength of the test medium has an influence on the outcome of Daphnia sp. acute immobilisation tests of nano-TiO2 materials; b) whether solar radiation influences the aquatic toxicity of TCC and the extent of potential mixture toxicity of nano-TiO2 and organic co-contaminants; c) whether TCC has an effect on the reproduction of earthworms; d) whether oxidative stress was induced in earthworms by nano-TiO2, TCC and mixtures of both. We showed that in contrast to Daphnia sp. acute immobilisation tests in which solar radiation was considered (median effect concentration (EC50), 48 h, SSR, NM 101, NM 102, NM 100 and TCC: 1.28, 0.53, 3.88 and 0.006 mg/L), standard tests were not able to adequately detect toxicity associated with nano and non-nano scale TiO2 materials, as well as TCC (EC50, 48 h, LL, NM 101, NM 102, NM 100 and TCC: 79.52, > 50, >50 and 0.027 mg/L), resulting in an underestimation of the hazard associated with these materials to daphnids in the aquatic environment. Thus, we propose to develop guidance for including solar radiation in standardized OECD guidelines used for testing photoactive chemicals and nanomaterials. Phototoxicity of the TiO2 materials was linked to the photoactivity measured by electron paramagnetic resonance, i.e., the total hydroxyl radical (•OH) formation rate of the TiO2 particles comprising free and surface adsorbed •OH. The latter as well as phototoxicity were maximal for the intermediate sized particle NM 102. Thus, phototoxicity of TiO2 materials to D. magna, although not inversely correlated, was shown to depend on particle size supporting the claim to the importance of testing nano and bulk forms of TiO2 separately in the context of REACH. Our results confirm that 10-fold diluted ISO medium (EC50, 48 h, SSR, NM 102/NM 101: 0.53/1.28 mg/L) may be regarded as a more appropriate medium than ISO medium (EC50, 48 h, SSR, NM 102/NM 101: 1.1/2.9 mg/L) for testing nanomaterials in the Daphnia sp. acute immobilisation test, in order to represent natural waters which have a lower ionic strength than ISO medium. Mixture toxicity tests revealed that TCC toxicity was either not influenced or significantly lowered by up to 64%, 53% and 74% to daphnids, fish and earthworms at an TiO2 application level of 1 mg/L (aquatic) and 1000 mg/kg dw (terrestrial). We suggest that toxicity was reduced because TCC adsorbed to nano-TiO2, and consequently the bioavailability and toxicity of TCC was lowered. Mixture toxicity experiments with fish embryos revealed that TCC toxicity (EC50, 96 h, dark/SSR: 74/80 µg/L) under solar radiation was lowered to a lesser extent (53%) than under laboratory light (78%) in the presence of nano-TiO2 (NM 102), demonstrating that the interaction of nano-TiO2 and organic co-contaminants can be influenced by environmental factors such as solar radiation. We showed that TCC negatively affected the survival as well as reproduction of earthworm at environmentally irrelevant concentrations (lethal concentration causing 10% effect, LC10, 14 d: 262 mg/kg dw; EC50, 56 d: 243 mg/kg dw). However, we provided evidence that TCC uptake via food ingestion (bioaccumulation factor, BAF: 10.3 ± 1.1) results in a more pronounced uptake of TCC than via soil exposure (BAF: 2.8 ± 0.7). For an adequate risk assessment it has to be clarified whether TCC exposure via food (e.g. sludge) leads to a more pronounced toxicity of TCC to E. fetida than exposure via soil. Measurements of oxidative stress revealed that chronic TCC earthworm toxicity was not related to oxidative stress and that co-exposure to nano-TiO2, as well as nano-TiO2 itself also did not induce oxidative stress within earthworms. TiO2 materials alone induced no effects to all test organisms, resulting in no observed effect concentrations (NOEC) of ≥ 50, 100 and 1000 ppm for daphnids, fish embryos, earthworms (acute and chronic) and activated sludge. In conclusion, this study shows that it is necessary to establish relevant exposure scenarios to assess the environmental risks of nano scale and non-nano scale TiO2 materials and TCC. Furthermore, primary particle size of TiO2 materials has to be considered in future regulations concerning nanomaterials to ensure their safe use.
Language: English
Open Access status: green
OpenAlex ID: W2777841590
Cites: 74
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Related to: 10