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Toxicity to microorganisms

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Description of key information

 Dispersions of nano- and microsized TiO2 do not inhibit microbial respiration in activated sewage sludge up to 1000 mg/L. 

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Additional information

Micro- and nanosized TiO2:

In one GLP-conform study and one study following the principles of GLP, activated sludge respiration inhibition tests according to OECD 209 were performed (i.e., Egeler and Goth, 2009; Wyrwoll et al. 2014). Results indicate that nano- and microsized TiO2 materials (including anatase and rutile) do not inhibit microbial respiration up to 1000 mg/L (unbounded 3 h NOEC: ≥ 1000 mg/L).

One supporting study with STP microorganisms demonstrates an absence of effects on the chemical oxygen demand of activated sludge microorganisms after long-term exposure (Wang et al. 2012). Exposures of microorganisms of activated sludge in lab-scale sequencing batch reactors to dispersion of the nano-sized TiO2 material Hombikat at concentrations between 0.5 - 2 mg/L (nominal) for 27 d did not significantly affect the chemical oxygen demand of the effluent in comparison to that of control batches. Mass balances of TiO2 in the whole system demonstrate that nominal and measured concentrations did not differ by more than 10%. Since the test duration exceeded typical STP sludge retention times in this study, results are considered as supporting data.

Another supporting study on STP microorganisms further shows no effects of the nano-sized, mixed-phase TiO2 material Aeroxide P25 (21 nm) on the respiration (oxygen uptake rate) and the chemical oxygen demand removal efficiency of activated sludge at a test concentration of 50 mg/L (nominal), for laboratory-scale sequencing batch reactors operated for 8 h under dark conditions (Li et al. 2019). Results are considered as supporting data, since no reference substance (i.e. 3,5-DCP) was tested and the validity criteria could thus not be met.

Furthermore, one supporting study on the environmental fate of TiO2 nanoparticles in laboratory sewage treatment plants conducted according to OECD 303 A (Kuhlbusch et al. 2012; 5.6 Additional information on environmental fate and behaviour) showed no effect of TiO2 nanomaterials (P25, 21 nm) at concentrations between 1 - 10 mg/L (nominal) on DOC elimination and nitrification of the LSTP.

Several supporting studies with freshwater microorganisms are available. Tests investigated either i) effects on not relevant test organisms, such as the single species Vibrio fisheri (Kahru et al. 2007) or freshwater bacteria populations (Ozaki et al. 2016, Jomini et al. 2015), ii) not relevant endpoints, including endpoints that did not measure the function of microorganisms, but number of compromised cells, abundance, diversity and mortality (Battin et al. 2009, Jomini et al. 2015, Johnson et al. 2011). All these studies, except those of Battin et al. (2009) and Jomini et al (2015) indicate an absence of toxicity of nanosized TiO2 dispersions at concentration between 1 and 1000 mg/L to microorganisms. Battin et al. (2009) exposed free-living microorganism communities from natural waters to the nano-sized TiO2 materials P25 (~21 nm, rutile-anatase) and Hombikat UV 100 (< 10 nm, anatase), and found that exposure to P25 at 5.3 mg/L (nominal) for 24 h under dark conditions, caused an increase of compromised cells. Intracellular ROS generation was not significantly enhanced in these cells. Jomini et al. (2015) investigated the abundance and diversity of planktonic and sessile bacteria after exposure to dispersions of the nano-sized TiO2 material P25 (21 nm) in natural river water after 14 d of exposure under dark conditions. A 14 d exposure to 100 mg/L TiO2 caused an increase of planktonic bacteria, but a decrease of sessile bacteria compared to control abundance, however, significant differences were not observed at 1 and 10 mg/L. Further, the bacterial diversity was affected at 100 mg TiO2/L. However, Battin et al. (2009) and Jomini et al. (2015) did not assess endpoints relevant for the hazard assessment of nanosized TiO2 materials in STP.

In sum, all relevant and reliable studies indicate that micro- and nanosized TiO2 is not toxic to microorganisms in activated sewage sludge at concentrations up to 1000 mg/L.