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Environmental fate & pathways

Additional information on environmental fate and behaviour

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Endpoint:
additional information on environmental fate and behaviour
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
3 (not reliable)
Rationale for reliability incl. deficiencies:
significant methodological deficiencies
Remarks:
Use of stabilizing agent (sodium hexametaphosphate) might have influenced the agglomeration/adsorption behaviour and thus the fate of TiO2 with in the laboratory sewage treatment plant; mass balance in trial 2 below 80%, consequently this result of trial 2 are not considered reliable
Cross-reference
Reason / purpose for cross-reference:
reference to same study

Data source

Reference
Reference Type:
publication
Title:
Unnamed
Year:
2012
Report date:
2011

Materials and methods

Test guideline
Qualifier:
according to guideline
Guideline:
other: OECD 303 A: Simulation Test - Aerobic Sewage Treatment -- A: Activated Sludge Units; B: Biofilms
Version / remarks:
2001
Deviations:
yes
Principles of method if other than guideline:
- The dosage of TiO2 was increased from 1 to 5 and then to 10 mg/L after each week, instead of a constant dosage of one concentration.
- Tests were run with 1 g/L sodium metahexaphosphate (SHP) stabilizing agent.
- Synthethic drinking water (DIN EN ISO 6341, Annex III - A3-3 synethetic drinking water) instead of tap water was used. It contained the 3.5 fold amount of sodium hydrogen carbonate as that recommended in the guideline.

- The study had to be divided into two trials due to a leakage in the treatment system when the system was changed from 5 mg/L to 10 mg/L.
- Trial 1 (22 d): adaption phase of seven days followed by seven days dossage of 1 mg/L and seven days dosage of 5 mg/L.
- Trial 2: seven days dosage of 10 mg/L
GLP compliance:
no

Test material

Reference
Name:
Unnamed
Test material form:
solid: nanoform
Details on test material:
P25

- Purity: no information
- Crystalline phase: anatase 86% and rutile 14%
- Primary particle size: 21 nm
- Coating: none
- BET specific surface area: 50 ± 15 m²/g
- Particle size distribution in used suspension (pH 5; DLS): 220 nm
- Surface charge: see results section
- Shape: spherical
- Density: 4.13 g/cm³
- Refractive index: 2.7

Results and discussion

Any other information on results incl. tables

STABILITY OF THE TIO2 SUSPENSION

- Addition of 1 wt% SHP and sonication resulted in the formation of stable suspensions for 24 h (hydrodynamic diameter in dispersion, DLS: 230 nm; zeta potential, ELS: -35 mV)

FUNCTIONALITY OF THE TREATMENT PROCESS:

- DOC elimination (above 97% at test end) and nitrification within the test unit was comparable to that of the reference unit.

- Increase of suspended solids was observed in the outflow of the test unit (4 -17 mg/L) compared to the reference unit (1 -2 mg/L). This is explained with the use of the dispersant SHP. However, the concentrations of filterable solids were still within an acceptable range. In technical STP filterable solids above 30 mg/L are considered as a malfunction.

FATE OF TIO2 IN LABORATORY SEWAGE TREATMENT PLANT:

- High correlation between the TiO2 concentration in the surplus sludge and in the outflow of the laboratory sewage treatment plant (LSTP): R² = 0.84; Concentration in the activated sludge was 23 times higher than in the outflow.

- SEM and EDX scans of the sewage sludge and the effluent show agglomerates of P25 in sludge but not in the effluent.

- Total TiO2 amount added to the LSTP in trial 1 and 2 were 504 mg and 840 mg, respectively.

- Mass balance trial 1 (after 22 d): 78% activated sludge, 4% outflow and 18% balance gap

- Mass balance trial 2 (after 32 d): 60% activated sludge, 3% outflow and 37% balance gap

-The measured TiO2 concentration in the unfiltered outflow highly correlates with the calculated TiO2 concentration, which is calculated by multiplying the TiO2 concentration in the activated sludge by the concentration of filterable solids and divided by the dry solids of the activated sludge in the aeration vessel (R² = 0.86). However, the measured TiO2 concentration was four times higher than the calculated TiO2 concentration based on filterable solids. It is assumed that the TiO2 concentration in the outflow is mainly determined by the presence of smaller particles below 0.6 µm and not by the sludge overflow as mesured by the filterable solids (> 0.6 mm).

Applicant's summary and conclusion

Conclusions:
The fate of the nano-sized TiO2 material P25 (21 nm), which was stabilized with sodium metahexaphosphate, was investigated in laboratory sewage treatment plant according to OECD 303 A. 4% of the initial mass of the applied TiO2 was found in the overflow after 22 d, and was mainly determined by particles < 0.6 µm. The test was split into trial 1 (0-22 d, 1 and 5 mg/L) and trial 2 (23-32 d, 10 mg/L), due to a leakage at day 22, which necessitated a restart of the system. Only the results of trial 1 are considered as valid, showing a mass balance of 79%, compared to a mass balance of 63% for trial 2. These results have to be interpreted carefully, as it remains unclear whether the stabilizing agent had an influence on the fate of P25 in the laboratory sewage treatment plant.