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Toxicity to aquatic algae and cyanobacteria

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Dispersions of microsized TiO2 are not toxic to aquatic algae and cyanobacteria up to ≥ 100 mg/L (NOEC) and 5,600 mg TiO2/L (NOEC) in fresh- and saltwater, respectively. Considering the low solubility of microsized TiO2, it is further concluded that microsized TiO2 is not toxic to freshwater algae up to its solubility limit. A low toxic potential of algal toxicity is observed for dispersed nanosized TiO2 with EC10 and EC50 values being > 2 mg/L and > 50 mg/L, respectively.

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Additional information

Microsized TiO2:

Dissolved/dispersed pigment grade TiO2 material, prepared by stirring for 48 h and filtering the dispersion through a 0.1 µm filter, did not significantly affect the growth rate of Pseudokirchneriella subcapitata in a test according to OECD 201 (72 h NOEC ≥ 100 mg/L, nominal). Measured TiO2 concentrations of the test media (10-100 mg TiO2/L, nominal) at test start were below the LOQ (1.44 µg TiO2/L) or ranged from 1.52-1.60 µg TiO2/L and did not increase with the TiO2 loading rate (Schlich et al. 2015). Thus, microsized TiO2 is not toxic to algae up to its solubility limit.

Microsized TiO2 was also not toxic to marine algae. Hudson et al. (2007) exposed Skeletonema costatum to the supernatant of TiO2 dispersions, prepared by stirring for 20 h and siphoning of the middle phase after a sedimentation period of 1 h, in a test according to ISO 10253. A 72 h NOEC value of 5600 mg TiO2/L (nominal) was derived. It is further concluded that microsized TiO2 is not toxic to marine algae up to its solubility limit.

In the study by Hudson et al. (2007), TiO2 particles at these unrealistically high concentrations may have adsorbed nutrients that are necessary for algae growth including phosphate and thereby caused algae growth limitation in the closed, artificial test system. Hence, effect values as derived by Nicolas et al (2015) may be due to an experimental artefact and considered overly conservative.

The algae studies were performed at concentrations several magnitudes above the solubility limit of microsized TiO2. Transformation/dissolution data of different microsized TiO2 materials indicate a low solubility in environmental media as dissolved Ti concentrations after 28 d were below the respective LOD/LOQ (< 0.11 / < 0.34 µg Ti/L). In sum, it is concluded that microsized TiO2 is not toxic to algae up to the solubility limit in fresh- and saltwater.

Nanosized TiO2:

Nicolas et al (2015) tested dispersions of the two titanium dioxide nanoparticles, i.e. NM 101 (< 10 nm) and NM 104 (20 nm), in Erlenmeyer flasks in an algae toxicity test according to OECD 201 (2011). The determined EC10 for inhibition of growth rate amount to 2.1 and 4.7 mg/L for NM 101 and NM 104, respectively, whereas the EC50 were > 50 mg/L indicating a low potential for toxicity to algae.

Additionally, in a guideline algae toxicity test according to OECD 201 (2011) but without detailed documentation by Hund-Rinke et al. (2016), dispersions of the TiO2 nanoparticles NM-104 (26 nm, rutile, Al-coated) were tested in Erlenmeyer flasks. Determined EC10 and EC50 values for growth rate were reported as a range of 5-35 mg TiO2 NP/L (72 h EC10) and 415-1028 mg TiO2 NP/L (72 h EC50), indicating a low potential for algal toxicity.

 

Nano-TiO2 particles may adsorb nutrients necessary for algae growth such as phosphate. In several studies sorption of phosphate to nano-TiO2 was observed (e.g. Luo et al. 2015 and Moharami et al. 2014). Hence, nutrients of the test medium such as phosphate may adsorb to nanosized TiO2, resulting in lower bioavailable concentrations and thereby cause algae growth limitation in a closed, artificial test system without the buffering capacity of natural water systems. Due to a bigger and open nutrient pool in natural open environments, it is not expected that nutrient adsorption to nanosized TiO2 at concentrations applied by Nicolas et al (2015) and Hund-Rinke et al. (2016) would cause algal growth limitations. Hence, effect values as derived by Nicolas et al (2015) and Hund-Rinke et al. (2016) may be due to an experimental artefact and considered overly conservative.

One study on the long-term toxicity of nanosized TiO2 to freshwater polycultures further indicates that nano-TiO2 is not toxic to biofilms at 1 mg/L. In this study by Kulacki et al. (2012), algae biofilms consisting of monocultures of either Synedra ulna, Scenedesmus quadricauda, or Stigeoclonium tenue as well as biofilms consisting of polycultures of these three species were exposed to dispersions of the nanosized TiO2 material P25 (27 ± 4 nm) in a recirculating system for 32 days without any significant effects on biomass resulting in 32-d NOEC values of ≥ 1 mg n-TiO2/L (nominal). Dispersed TiO2 concentrations in the water phase of the 0.1 and 1 mg/L treatments decreased to background levels (0.015 mg/L) or to 0.026 mg/L at the end of the test, respectively.

Miller et al. (2010) also found that nanosized TiO2 was not toxic to marine phytoplanktonic algae up to 1 mg/L. Dispersions of a nanosized TiO2 material (15-20 nm) did not significantly reduce the growth rate of four marine phytoplanktonic species, including diatoms, chlorophytes and prymnesiophytes, in a non-standardized toxicity test. Reported 72 h NOEC-values were ≥ 1 mg n-TiO2/L (nominal).

In a supporting study by Morelli et al. (2018), dispersions of n-TiO2 (Aeroxide P25, primary particle size: 24 ± 7 nm) up to the highest test concentration of 10 mg/L did not significantly affect the growth rate of the marine green alga Dunaliella tertiolecta and thus an unbounded 72-h NOEC of ≥ 10 mg n-TiO2/L (nominal) was derived. Significant aggregation and quick sedimentation in the saline test medium was observed, resulting in a decrease of suspended n-TiO2 particles.

In sum, two GLP-conform guideline tests of the toxicity of microsized TiO2 to aquatic algae and cyanobacteria resulted in an unbounded NOEC value of ≥ 100 mg/L TiO2/L for Pseudokirchnerella subcapitata in freshwater and a NOEC of 5,600 mg TiO2/L for Skeletonema costatum, in saltwater. Nicolas et al (2015) observed some toxicity of two n-TiO2 materials, however, the respective EC10s are well above long-term classification criteria. Studies with n-TiO2 and freshwater polycultures (Kulacki et al. 2012) or saltwater phytoplankton species (Miller et al. 2010) point to a low potential for toxicity to algae. In sum, it can be concluded that microsized TiO2 is not toxic to aquatic algae and cyanobacteria up to ≥ 100 mg/L (NOEC) and up to 5,600 mg TiO2/L (NOEC) in fresh- and saltwater, respectively. Also, a low potential for algal toxicity is observed for nanosized TiO2 with EC10 and EC50 values being > 2 mg/L and > 50 mg/L, respectively.

References:

Luo et al. 2015. The Effect of Phosphate and Sulfate on Arsenate Desorption From Nano-TiO2. J. Res. Sci. Tecnol. 12, Suppl. 1, S17-S-23

Moharami et al. 2014. Effect of TiO2, Al2O3, and Fe3O4 Nanoparticles on Phosphorus Removal from Aqueous Solution. Environ. Prog. Sustain. Energy 33, 1209-1219