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EC number: 236-675-5
CAS number: 13463-67-7
Dispersions of nano- and microsized TiO2 do not inhibit microbial respiration in activated sewage sludge up to 1000 mg/L.
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
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.
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