Titanium dioxide

Adsorption / desorption

Link to relevant study record(s)

Reference

Reference 1

Endpoint:

adsorption / desorption, other

Remarks:

adsorption/desorption

Type of information:

migrated information: read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: No guideline study, but according to current scientific standards and well documented. Value reported is the mean log Kp value for 20 locations analysed.

Reason / purpose for cross-reference:

reference to same study

Qualifier:

no guideline followed

Principles of method if other than guideline:

The article examines the partitioning of trace metals between surface water and sediments and their fate within the sediments of a river in South Africa in which mine water was drained.

GLP compliance:

not specified

Type of method:

other: monitoring

Media:

sediment

Radiolabelling:

not specified

Analytical monitoring:

yes

Details on sampling:

Surface water and sediment samples were collected from 20 randomly chosen sites that were geographically dispersed throughout the marsh.
At each site, two surface water samples were collected and filtered through a 0.45 µm nylon membrane filter using a hand-held vacuum pump. One of the samples was acidified with 3 M HNO3 to a pH of less than 2 to prevent metal precipitation. The water samples were stored in plastic bottles pre-rinsed with HNO3 and deionized water. The bottles were sealed and stored at 4°C before analysis.
At each site, surface sediments were sampled close to the sediment-water interface by completely inserting an inverted 50 ml polypropylene centrifuge vail. After digging the vail out, they were capped without leaving any head space. Care was taken to prevent further exposure of the sample to the atmosphere by storing the vails in an aerobic jar where anoxic conditions were maintained. The jars were stored on ice for transportation to the laboratory for further analyses.
At each sampling site, surface water pH, electrical conductivity (EC), dissolved O2 (DOC), redox potential (Eh) and temperature were measured.
Bulk sediment samples were analyzed for their trace metal content as well as the partitioning of trace metals among various phases.
To asses any external contamination while sampling, a trip blank was analyzed.

Details on matrix:

pH H2O: mean 7.87 (6.9-9.2)
OM: mean 3.9% (0.9-15.5%)
Clay: mean 14.6% (4-43%)

Details on test conditions:

Samples were diluted 1000 times with an internal standard.
To asses any external contamination while sampling, a trip blank was analyzed.

Phase system:

solids-water in sediment

Type:

log Kp

Value:

4.61 L/kg

Remarks on result:

other: mean log Kp value for 20 locations, range: 3.79-5.13

Validity criteria fulfilled:

not applicable

Conclusions:

Total and dissolved Ti concentrations of surface water and sediments from 20 sites of the stream Blesbokspruit (South Africa) were measured, and the mean log Kd for sediment is 4.61 L/kg dw (range: 3.79-5.13).

Reference 2

Endpoint:

adsorption / desorption, other

Remarks:

monitoring data in water and sediment

Type of information:

other: Calculation of Kd values based on environmental field data

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: see 'Remark'

Remarks:

Kd-values are based on reliable field data of concentration levels in water and sediment; no standard guideline test was conducted to generate Kd-values. Kd-values are calculated based on the hypothesis that equilibrium exists between the baseline levels in water and in sediment.

Reason / purpose for cross-reference:

reference to same study

Qualifier:

no guideline followed

Principles of method if other than guideline:

Evaluation of high quality environmental geochemical data for Europe, which is provided by the Forum of European Geological Surveys (FOREGS), with respect to Ti concentrations in stream water and stream sediment. Baseline levels of titanium in water and sediment were determined in 751 paired sample locations. Assuming equilibrium between both environmental compartments, site-specific Kd's were derived.

GLP compliance:

no

Type of method:

other: field data of baseline levels in water and sediment

Media:

sediment

Radiolabelling:

no

Test temperature:

environmental relevant temperatures

Analytical monitoring:

yes

Details on sampling:

FOREGS:
-The FOREGS sampling grid was based on GTN grid cells developed for Global Geochemical Baseline mapping. This grid divides the entire land surface into 160 km x 160 km cells covering an area of 4,500,000 km2.
- Sampling methodology, preparation and analysis are described by Salminen et al. (2005).
- A total of 808 stream water samples and 852 sediment samples were processed in the FOREGS-program, including 757 paired samples, i.e. samples with the same coordinates for the sampling location of stream water and sediment that are summarised below. However, Ti concentrations in six stream water samples were reported with “0.0” (five samples) or “-1.0” (1 sample), and respective samples were excluded from the analysis, resulting in a total of 751 paired samples summarised below. However, including these six samples with estimated concentrations of ½ LOD (=0.005 µg/L) does shift the summary statistics negligibly.
- High quality and consistency of the obtained data were ensured by using standardised sampling methods and by treating and analysing all samples in the same laboratory of each country.

Key result

Phase system:

sediment-water

Type:

log Kp

Value:

6.57 L/kg

Remarks on result:

other: European median log Kd

Adsorption and desorption constants:

- Regarding the partitioning of titanium in the water column, stream water/sediment partition coefficients range from 182,527 L/kg to 119,329,191 L/kg. Country-specific stream water/sediment partition coefficients are reported in Table 1, and median values range from a log Kd of 6.22 to 7.29 (Table 1). Since FOREGS sampled on a grid aiming to equally represent geochemical baseline concentrations across Europe, a European median log Kd value of 6.57 is derived (Table 2).

- Even though Ti concentrations of sediments can be as high as 3 %, dissolved/dispersed Ti concentrations in the water column of European stream waters are well below 20 µg/L. These monitoring stream water and sediment data provide strong evidence that Ti is sparingly soluble in a wide representative range of European stream waters.

Table 1: Country-specific stream water/sediment partition coefficients

Country	# of data	Area (km^2)	Min	Max	Mean	5^thP	50^thP	90^thP	95^thP
AL	3	28,748	7.38	7.74	7.57	7.39	7.52	7.70	7.72
AT	20	83,879	6.18	7.66	7.12	6.28	6.81	7.56	7.56
BE	5	32,545	6.43	7.20	6.87	6.48	6.68	7.12	7.16
CH	9	41,285	5.92	6.94	6.65	6.17	6.65	6.85	6.89
CZ	10	78,866	6.25	7.19	6.72	6.25	6.44	7.09	7.14
DE	74	357,121	5.60	7.58	6.66	6.04	6.52	6.98	7.03
DK	5	43,098	5.92	6.82	6.48	5.95	6.50	6.72	6.78
EE	11	45,227	6.11	6.87	6.50	6.16	6.31	6.76	6.82
ES	82	504,645	5.26	7.86	7.10	5.77	6.75	7.56	7.76
FI	64	338,144	5.36	7.64	6.67	5.48	6.22	7.07	7.31
FR	117	543,965	5.49	7.80	6.83	6.07	6.62	7.13	7.23
GR	27	131,957	5.96	7.74	6.95	6.16	6.80	7.17	7.28
HR	10	56,542	5.96	7.21	6.80	6.20	6.66	7.05	7.14
IE	11	70,273	6.04	6.67	6.52	6.19	6.56	6.63	6.65
IT	45	301,336	5.42	7.80	6.92	5.67	6.56	7.32	7.50
LT	14	64,589	5.97	6.81	6.45	6.00	6.36	6.65	6.71
LV	7	65,301	5.97	6.60	6.36	6.03	6.36	6.55	6.57
NL	9	41,526	5.68	6.79	6.43	5.83	6.41	6.65	6.72
NO	55	323,759	5.64	8.08	7.42	6.11	7.24	7.80	7.89
PL	56	312,685	5.29	6.98	6.33	5.89	6.22	6.61	6.68
PT	19	92,345	5.84	7.79	7.33	6.53	7.29	7.62	7.70
SE	20	449,964	5.79	7.63	6.89	5.80	6.49	7.24	7.58
SK	15	49,034	6.10	7.02	6.66	6.24	6.58	6.88	6.94
SL	4	20,253	6.95	7.27	7.11	6.97	7.07	7.23	7.25
UK	59	219,331	5.99	7.83	7.03	6.19	6.64	7.39	7.67

Table 2: Paired titanium and titanium dioxide concentrations of stream water, sediments and respective partitioning

	Parameter	#	Unit	Min.	Max.	Mean	STDEV	5^thp	50^thp	90^thp	95^thp
water	pH^1	743^2 731^4	-	9.8	4.5	6.6 6.6	5.85 5.84	8.5 8.5	7.7 7.7	6.5 6.5	6.1 6.1
water	Ca	751 739^4	mg/L	0.23	592.00	56.58 56.43	62.47 62.02	1.72 1.70	43.10 43.10	117.60 117.76	148.25 148.21
water	Cl	751 739^4	mg/L	0.14	4560.00	34.92 35.35	198.40 199.97	0.49 0.51	9.48 9.50	43.79 44.03	69.78 72.31
water	HCO3	749^5 737^4	mg/L	0.69	1804.42	156.11 156.12	145.92 146.41	5.96 5.84	134.00 131.67	335.72 335.91	372.40 374.55
water	K	751 739^4	mg/L	0.01	182.00	3.15 3.18	7.57 7.62	0.15 0.15	1.65 1.66	6.87 6.91	9.83 9.87
water	Mg	751 739^4	mg/L	0.05	230.00	11.74 11.75	19.60 19.71	0.48 0.49	6.25 6.25	26.55 26.52	38.37 38.80
water	Na	751 739^4	mg/L	0.23	4030.00	24.15 24.41	163.76 165.07	1.01 1.03	6.80 6.80	25.69 25.72	48.31 48.39
water	NO3	751 739^4	mg/L	0.02	107.00	9.38 9.47	13.67 13.75	0.02 0.02	3.10 3.19	29.03 29.08	39.80 39.90
water	DOC	746^3 737^4	mg/L	0.25	57.94	7.38 7.45	8.08 8.10	0.60 0.61	4.78 4.82	16.31 16.40	23.01 23.08
water	SO4	751 739^4	mg/L	0.15	2420.00	54.11 53.73	157.74 157.22	1.19 1.19	16.96 17.49	103.64 103.13	172.22 168.04
water	Dissolved/dispersed Ti	751 739^4	µg/L	0.10	16.80	1.46 1.47	1.96 1.97	0.10 0.10	0.90 0.90	2.90 2.90	4.25 4.21
sediment	Ti	751 739^4	%	0.01	2.99	0.42 0.42	0.24 0.24	0.14 0.14	0.39 0.39	0.63 0.63	0.80 0.81
Partitioning (Kd)	Ti (sed / water)	751 739^4	L/kg	182,527	119,329,191	8,922,560 8,862,395	14,215,816 14,182,762	747,891 733,573	3,792,985 3,695,949	21,616,304 21,348,598	40,635,455 40,084,060
Log Kd	Ti (sed / water)	751 739^4	-	5.26	8.08	6.62 6.62	0.51 0.51	5.87 5.87	6.58 6.57	7.33 7.33	7.61 7.60

¹Statistics are based on H⁺concentrations rather than pH.

²Removal of 2 outliers < pH 4.3 and 6 negative values.

³Removal of 1 outlier > 70 mg/L and 4 negative values.

⁴Values from Switzerland and Albania are not included.

⁵Removal of 2 outliers < 0.01.

Conclusions:

Reliable baseline levels of titanium in pristine water/sediment samples were determined in 751 samples. Sampling and analytical procedures are considered adequate and resulted in reliable data. Assuming equilibrium between the typical concentration in water and sediment, relevant KD-values were generated. Data are therefore considered useful for the determination of a relevant KD for the sediment compartment. Since FOREGS sampled on a grid aiming to equally represent geochemical baseline concentrations across Europe, a European median log Kd value of 6.57 is derived.

Reference 3

Endpoint:

adsorption / desorption, other

Remarks:

adsorption/desorption

Type of information:

migrated information: read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

key study

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: No guideline study, but according to current scientific standards and well described. The Log Kp value reported is the median for samples from 54 rivers at 119 localities.

Qualifier:

no guideline followed

Principles of method if other than guideline:

Partitioning between suspended matter and water was studied by measuring Ti concentrations of water and of corresponding suspended matter.

GLP compliance:

not specified

Type of method:

other:

Media:

other: Suspended solids

Radiolabelling:

not specified

Analytical monitoring:

yes

Details on sampling:

Filtered and unfiltered water samples were collected under stable hydrological conditions. A volume of 200ml of river water was generally filtered and another 100ml were used for analysis.
Filtration through perforated polycarbonate filters was done in the field immediately after collection of river water samples. Samples were cooled and transported to the laboratory.
Unfiltered samples were acidified by adding 1 ml of HNO3 1:1 purified by sub-boiling distillation on the day of collection, to attain a pH of about 1.5.
The vessels were cleaned with 10% HNO3 for at least 2 days.
Filters were first boiled with distilled water acidified by HNO3 for 10 min before use, dried at 105°C, weighed and transported to the field in glass Petri dishes. A volume of 200 ml of river water was generally filtered and another 100 ml were used for analysis. A total of 155 filters were dried at 105°C, weighed in the laboratory, and the SPM deposited on them was analysed by particle induced X- ray emission (PIXE) and neutron activation analysis (NAA). The values of KD were calculated by dividing the total concentration of the element in SPM (obtained by NAA or PIXE) by its concentration in filtered water (Dissolved fraction=filtered water=true solution+colloids).

Phase system:

solids-water in suspended matter

Type:

log Kp

Value:

5.36 L/kg

Remarks on result:

other: median for samples from 54 rivers in 119 localities

Validity criteria fulfilled:

not applicable

Conclusions:

Titanium concentrations of water and particulate matter from 54 rivers at 119 sites (Czech Republic) were measured, and the median log Kd for suspended matter amounts to 5.36 L/kg dw.

Reference 4

Endpoint:

adsorption / desorption

Type of information:

experimental study

Adequacy of study:

supporting study

Reliability:

3 (not reliable)

Rationale for reliability incl. deficiencies:

significant methodological deficiencies

Remarks:

: Guideline not applicable for nanomaterials; Separation of unadsorbed TiO2 and soil adsorbed TiO2 requires a centrifugation step, which causes the centrifugation of both free and a decisive part of unadsorbed nano-TiO2. This does not allow to determine adsorption constants of nano-TiO2 to soil.

Reason / purpose for cross-reference:

reference to same study

Qualifier:

according to guideline

Guideline:

OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)

Version / remarks:

2000

Principles of method if other than guideline:

No Tier 3 tests (adsorption isotherms and desorption kinetics/desorption isotherms) were conducted.

GLP compliance:

no

Type of method:

batch equilibrium method

Media:

soil

Test temperature:

no information

Analytical monitoring:

yes

Details on sampling:

- Soil/suspension mixtures were centrifuged at 2700 g for 10 min, subsequently 15 ml of the supernatant were transferred and analysed with ICP-OES after digestion with HCl, HNO3 and HF.

Matrix no.:

#1

Matrix type:

other: Dystric Cambisol (RefeSol A01)

% Clay:

5

% Silt:

24

% Sand:

71

% Org. carbon:

0.93

pH:

5.67

CEC:

37.9 other: mmolc/kg

Matrix no.:

#2

Matrix type:

other: Cambic Rendzina (RefeSol A06)

% Clay:

36

% Silt:

55

% Sand:

9

% Org. carbon:

2.46

pH:

6.78

CEC:

236 other: mmolc/kg

Matrix no.:

#3

Matrix type:

other: Stagnic Luvisol (RefeSol A02)

% Clay:

15

% Silt:

84

% Sand:

2

% Org. carbon:

1.3

pH:

6.63

CEC:

133.2 other: mmolc/kg

Matrix no.:

#4

Matrix type:

other: Eutric Cambisol (RefeSol G03)

% Clay:

27

% Silt:

52

% Sand:

21

% Org. carbon:

3.85

pH:

5.64

CEC:

135.8 other: mmolc/kg

Matrix no.:

#5

Matrix type:

other: Gleyic Fluvisol (RefeSol G05)

% Clay:

16

% Silt:

62

% Sand:

22

% Org. carbon:

3.08

pH:

4.78

CEC:

116.1 other: mmolc/kg

Details on matrix:

- Test soils were obtained from Fraunhofer IME (Schmallenberg, Germany)
- Air dried (21 °C, 48 h) and sieved soil (2 mm mesh) was used.

Details on test conditions:

TEST SET UP:
- Air dried soil was equilibrated for 12 h with 0.01 M CaCl2 solution.
- Thereafter, TiO2 suspensions were added to the soil/CaCl2 solution.
- After shaking for a specific time, test vessels were centrifuged for 10 min at 2700 g.
- Replicates: 2
- Control: blank (Soil, no additional TiO2 applied), two replicates
- Test parameters: pH was checked
- max. Volume: 50 ml

TIER2:
Test soils: RefeSol A01 (pH: 5.1-5.5), RefeSol A06 (pH: 6.1-6.6), RefeSol A02, G03 and G05
Soil/suspension ratio: 1:5 (5 g soil: 25 ml suspension)
Test material: UV Titan M262
Concentration: 2.5 mg
Time points (RefeSol A01 and A06): 1, 2, 4 h
Time points (RefeSol A02, G03 and G05): 1, 2, 4 and 8 h
Test medium: 0.01 M CaCl2

SUSPENSION PREPARATION
- Direct addition of the test material into test vessel: no
- Sonication of suspension: yes, TiO2 (1 g/L) was dispersed in deionised water (pH 5.0, variance of 10%) by means of a ultrasonic homogenizer (200 W, pulse: 0.2/0.8) for 10 minutes.
- Solvents: no
- Filtration or other separation of test material from test dispersion: no
- Appearance of dispersion: no information
- Test medium: deionised water (pH 5.0, variance of 10%)

Adsorption and desorption constants:

no valid adsorption constants determined

Recovery of test material:

not determined

Details on results (Batch equilibrium method):

TIER 1:
- Measurement of the Ti-content in the supernatant after 48 h revealed that around 85-95% of the applied TiO2 was removed by centrifugation from the supernatant.

TIER 2:
- UV Titan M262 + all test soils: Ti content in supernatant < 1% of the applied Ti content at all time points

DLS/ELS RESULTS:
- in deionized water (pH 5), zeta potential: UV Titan M262: +26 mV (SD = 1.06)
- Z.average (d. nm, (SD)): UV Titan M262: 188 (3.01)

Conclusions:

The adsorption potential of the nano-sized TiO2 material UV Titan M262 (20 nm, coated) was investigated in a soil adsorption test according to OECD 106 with five test soils, varying in texture, pH and cationic exchange capacity. Centrifugation step (2700 g, 10 min) removed soil particles and unadsorbed TiO2 agglomerates larger than 177 nm so that a differentiation of adsorbed and unadsorbed TiO2 particles was not possible. Less than 5% of the initial Ti was found in the supernatant after centrifugation at all time points (1, 2, 4 h) and soils. Thus, the OECD guideline 106 is not applicable for measuring the adsorption of TiO2 nanomaterials.

Reference 5

Endpoint:

adsorption / desorption: screening

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

weight of evidence

Study period:

as reported in source record(s)

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

other: as reported in source record(s)

Remarks:

This target record summarises individual studies on partitioning in the environment that ranged in reliability from 2-3.

Justification for type of information:

see attachment "Endpoint-specific read-across justification for sets of nano forms of titanium dioxide" in section 13.

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across source

Reason / purpose for cross-reference:

read-across: supporting information

Reason / purpose for cross-reference:

data waiving: supporting information

Reason / purpose for cross-reference:

read-across: supporting information

Reason / purpose for cross-reference:

read-across: supporting information

Reason / purpose for cross-reference:

data waiving: supporting information

Reason / purpose for cross-reference:

read-across: supporting information

Reason / purpose for cross-reference:

read-across: supporting information

Reason / purpose for cross-reference:

read-across: supporting information

Phase system:

other: as reported in source record(s)

Type:

other: as reported in source record(s)

Remarks on result:

other: as reported in source record(s)

Remarks:

This target record summarises individual studies on partitioning in the environment. Nanosized TiO2 has a very low mobility under almost all environmental conditions and is primarily associated with particles or colloids.

Description of key information

Nano- and microsized TiO2 has a very low mobility under almost all environmental conditions and is primarily associated with particles or colloids. Distribution coefficients are derived from monitoring data of elemental Ti concentrations in water and corresponding sediments or suspended matter. The estimated partition coefficient for soil is based on a standardized soil adsorption test with nanosized TiO2

Key value for chemical safety assessment

Other adsorption coefficients

Type:

other: log Kp (mean for primary and secondary sludge)

Value in L/kg:

3.95

Other adsorption coefficients

Type:

log Kp (solids-water in sediment)

Value in L/kg:

6.57

Other adsorption coefficients

Type:

log Kp (solids-water in suspended matter)

Value in L/kg:

5.36

Additional information

Solid-water partititioning

Solid-solution partitioning of TiO2 is a key property that determines its environmental fate in terrestrial and aquatic systems. The partition coefficient can be used to provide information on potential exposure pathways and which biota are likely to be most relevant once TiO2 is released into the environment. Nano- and microsized TiO2 is poorly soluble and sufficiently stable to not transform to a water-soluble form. Thus, TiO2 itself (and not a soluble form) should be assessed taking into account the particle-specific partitioning characteristics. Further, according to ECHA’s guidance on IR and CSAAppendix R.7.13-2 (July 2008), “the distribution of metals over the solid and liquid phase is not only controlled by pure adsorption/desorption mechanisms. Other processes like precipitation or encapsulation in the mineral fraction also play a role.”

 

Sediment – water partitioning

The assessment of the partitioning of TiO2 in environmental media is based on Kp values derived from monitoring data for elemental Ti concentrations in water and corresponding sediments provided by the FOREGS Geochemical Baseline Mapping Programme that aimed to provide high quality, multi-purpose homogeneous environmental geochemical baseline data for Europe. A total of 757 paired samples, i.e. samples with the same coordinates for the sampling location of stream water (filtered to < 0.45 µm) and sediment (wet sieved in the field to <0.15 mm) were processed (Salminen et al. 2005) and results correspond to steady-state conditions of Ti, independent of Ti speciation.

Sampled stream water and sediments cover a wide range of environmental conditions. Water parameters such as pH, hardness and organic carbon concentrations cover several magnitudes. Dissolved/dispersed titanium water levels range from < 0.10 to 16.80 µg Ti/L with 5th and 95th percentiles of 0.10 and 4.21 µg Ti/L, respectively. The FOREGS dataset reports titanium dioxide concentrations of the sediment. Respective TiO2-data were converted into titanium concentrations to enable comparison of sediment with dissolved/dispersed concentrations in the stream water. Sediment concentrations of Ti range from 0.01 to 2.99 % Ti with 5th and 95th percentiles of 0.14 and 0.81 % Ti, respectively. Taking into account the high quality and representativeness of the data set, the 95th percentile of 4.21 µg Ti/L can be regarded as typical background concentration for dissolved/dispersed titanium in European surface waters and the 95th percentile of 0.81 % Ti as typical background concentration of European stream sediments.

Regarding the partitioning of titanium in the water column, stream water/sediment partition coefficients range from 182,527 L/kg to 119,329,191 L/kg. Since FOREGS sampled on a grid aiming to equally represent geochemical baseline concentrations across Europe, a European median log Kd value of 6.57 is derived for sediment-water partitioning.

Roychoudhury and Starke et al. (2006) measured total and dissolved Ti concentrations of surface water and sediments from 20 sites of the stream Blesbokspruit (South Africa), and the mean log Kd for sediment is 4.61 L/kg dw (range: 3.79-5.13).

Conclusion - Sediment – water partitioning

Even though Ti concentrations of sediments can be as high as 3 %, dissolved/dispersed Ti concentrations in the water column of European stream waters are well below 20 µg/L. These monitoring stream water and sediment data provide strong evidence that Ti is sparingly soluble in a wide representative range of European stream waters and is mostlyassociated with particles or colloids of the sediment.

 

Suspended matter – water partitioning

Vesley et al. (2001) analysed titanium concentrations of water and particulate matter from 54 rivers at 119 sites (Czech Republic), and the median log Kd for suspended matter amounts to 5.36 L/kg dw indicating that titanium in the water column is mostlyassociated with suspended matter.

Soil – water partitioning

Furthermore, in a standardized soil adsorption test according to OECD 106 Kuhlbusch et al. (2012) measured the adsorption of nanosized TiO2 (P25, 21 nm and UV Titan M262, 20 nm, coated). Adsorption was measured at one single concentration (2.5 mg TiO2, 25 mL, 5 g soil) in six different soils varying in texture (from loamy sand to silt loam), and pH from 4.78 to 6.78 (strongly acid to very-sub acid), effective CEC from 37.9 to 236 mmolc/kg and OC content from 0.93 to 3.85 % (very light humic to strongly humic). Centrifugation (2700 g, 10 min) removed soil particles unadsorbed TiO2 agglomerates larger than 177 nm so that a differentiation of adsorbed and unadsorbed TiO2 particles is not possible. Titanium concentration were below 1% and 5 % of the added material in the supernatant after centrifugation of soils mixed with UV Titan M262 and P25, respectively. Based on the solid-solution partitioning of n-TiO2, conservative coefficients of Kd < 495 L/Kg (UV Titan M262) and Kd < 95 L/Kg (P25) can be estimated. Due to the limited test design, the partition coefficients should not be applied without further confirmatory experiments.

Kuhlbusch et al. (2012) applied dispersions of the nano-sized TiO2 materials P25 (21 nm), PC 105 (15-25 nm) and UV Titan M262 (20 nm) to columns of three test soils varying in pH, texture and cationic exchange capacity (CEC) according to OECD 312 soil leaching tests. After 48 h of artificial rain onto the soil columns, the Ti concentration in the eluates were below the LOD (< 5 µg/L), indicating that less than 0.04% of the applied TiO2 leached out of the columns. Except for the TiO2 material UV Titan M262, the highest TiO2 concentrations were measured in the first segment (0 - 1 cm) of the soil columns and Ti concentrations did not rise in subsequent segments. Apparently, up to 14 % and 19 % ofUV Titan M262 were transported to the second (3 – 4 cm) and last segment (29 - 30 cm), respectively.

Nickel et al. (2013) applied dispersions of nanosized TiO2 (P25, 21 nm) to columns of three test soils varying in pH, texture and CEC according to OECD 312 soil leaching tests. The Ti concentrations of eluates were below the LOD (< 0.12 µg/L), and Ti concentration did not increase in all subsequent soil segments.

Conclusion - soil – water partitioning

Results of soil leaching studies indicate a low leachability and mobility of nanosized TiO2 materials in soil and provide further evidence for the adsorption/ retention in soil. A similar lack of mobility may be assumed for microsized material based on common physica-chemical properties including the poor solubility. 

 

Sewage sludge – waste water partitioning

Johnson et al. (2011) studied the fate of TiO2 particles in a sewage treatment plant (STP) serving over 200,000 people. Apparently, concentrations of Ti in the fraction < 0.45 µm differ significantly between influent and effluent. Whereas the TiO2 particles do not undergo removal during the primary settlement stage, the greatest decline (ca. 90%) followed the biological stage of activated sludge with mean Ti concentrations in the fraction < 0.45μm in raw sewage influent, following primary settlement and in final effluent of 30.5μg/L; 26.7μg/L; and 3.2μg/L, respectively.

Results from a lab-scale sequencing reactor indicate that biological STPs that operate with suspended biomass such as activated sludge have the potential to remove 97 ± 1% of n-TiO2 from wastewater (Wang et al. 2012). These results are confirmed by Kuhlbusch et al. (2012) who investigated the fate of nano-sized TiO2 (P25, 21 nm) stabilized with sodium metahexaphosphate in a laboratory STP according to OECD TG 303 A. Apparently, 4 % of the initially applied TiO2 were recovered in the overflow after 22 d and consisted mainly of particles < 0.6 µm. However, these results have to be considered carefully since the influence of the stabilizing agent on the fate of P25 in the laboratory STP was not clarified.

Pronounced sedimentation and adsorption to the solid phase of sewage was also observed in a non-standardized study by Barton et al. (2015). Mean distribution coefficients of the solid and liquid phase of primary and secondary wastewater sludge at steady state of nanosized TiO2 (P25, 21 nm) after 1 h of mixing and 30 min of sedimentation amounted to 8800 L/kg for primary and 9100 L/kg for secondary sludge. These results provide further evidence that the majority of the applied nano-TiO2 is removed from the water phase of wastewater by sedimentation within 30 min. Thus, the mean sludge – waste water distribution coefficient of 8950 L/kg was applied in the CSA.

Conclusion - Sewage sludge – waste water partitioning

Studies on the fate of TiO2 particles in wastewater provide evidence that up to 97% are removed during sewage treatment. Thus, the emission of TiO2 materials into the aquatic system via the effluent of STPs is low since, for example, measured Ti concentrations of the effluent of an operating STP amount to 3.2 µg Ti/L despite higher Ti concentrations of the respective influent.