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Endpoint:
adsorption / desorption: screening
Type of information:
experimental study
Adequacy of study:
key study
Study period:
15 August 2007 - 13 August 2008
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Reason / purpose for cross-reference:
other: read-across target
Qualifier:
according to guideline
Guideline:
OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)
Deviations:
no
GLP compliance:
yes (incl. QA statement)
Type of method:
batch equilibrium method
Media:
soil
Radiolabelling:
no
Test temperature:
20 ± 2 °C
Analytical monitoring:
yes
Details on sampling:
- Concentrations:
Preliminary test: Three soil-to-solution ratios = 1/50, 1/25, 1/5; one test item concentration = 0.64 mg/L
Screening test: One soil-to-solution ratio = 1/50; one test item concentration = 1.29 mg/L
- Sampling interval:
Preliminary test: 48 hours
Screening test: adsorption phase: 2, 5, 24 and 48 hours; desorption phase: 2, 4, 24 and 48 hours
- Sample storage before analysis: before analyses all samples were stabilised by adding 0.5 mL nitric acid to 5 mL supernatant.
Details on matrix:
COLLECTION AND STORAGE
- Geographic location:
Soil I (SI): Hanhofen (Speyer, Rheinland-Pfalz, Germany; latitude 49°19’N, longitude 08°20’E). The sampling side was a meadow in the sampling year and at least 4 former years.
Soil II (SII): Attenschwiller (68220 Attenschwiller, France, latitude 47°34’N, longitude 7°27’E). The field cover at the time of sampling consisted of weeds.
Soil III (SIII): Siebeldingen (Speyer, Rheinland-Pfalz, Germany, latitude 49°12’N, longitude 08°03’E). The sampling site was uncultivated in the sampling year and at least 4 years before.
- Collection procedures: Sampling and handling of the soils were performed under consideration of ISO 10381-6 “(Soil Quality-Sampling-Guidance on the collection, handling and storage of soil for the assessment of microbial processes in the laboratory)”.
- Sampling depth (cm): The three soils were sampled from the top 20 cm layer.
- Storage conditions and length: no data except that SI, SII and SIII were sampled on August 2006, March 2007 and September 2006, respectively.
- Soil preparation: The soils, sieved to 2 mm, were air-dried at room temperature and homogenised. Disaggregation was performed with minimal force, so that the original texture of the soil was changed as little as possible. In order to eliminate microbial degradation of the test material, the soils were sterilised before use by γ-irradiation (Studer Hard, 4658 Däniken / Switzerland). Thereafter, the soil samples were pre-equilibrated with 50 % of the targeted volume of the aqueous phase by shaking for at least 20 hours at 20 °C prior to application of the test material.

PROPERTIES
- Soil texture
% sand: SI = 79.1, SII = 11.8, SIII = 21.9.
% silt: SI = 13.5, SII = 72.8, SIII = 36.0.
% clay: SI = 7.9, SII = 15.4, SIII = 42.
- Horizon: no data.
- Soil taxonomic classification: SI = Loamy sand, SII = Silt loam, SIII = Clay.
- Soil classification system: The soil types above mentioned were determined according to USDA (United States Department of Agriculture).
- Soil series and order: no data.
- pH: SI = 5.6, SII = 7.4, SIII = 7.0.
- Organic carbon (%): SI = 2.36, SII = 0.99, SIII = 1.89.
- CEC (meq/100 g): SI = 11.0, SII = 10.2, SIII = 20.0.
- Nitrogen content (%): SI = 0.21, SII = 0.11, SIII = 0.21.
- Organic matter (%): SI = 4.07, SII = 1.71, SIII = 3.26.
- C/N-ratio: SI = 11.2, SII = 9.0, SIII = 9.0
- Moisture (in g per 100 g dry soil): SI = 0.95, SII = 1.02, SIII = 5.78.
- Other: None of the soils had been fertilised or treated with pesticides for at least 12 months prior to sampling.
No further data.
Details on test conditions:
TEST CONDITIONS
- Buffer: none.
- pH: The pH of the aqueous phase was measured before and after contact with the soil at the soil-to-solution ratio of 1/50. The pH of the CaCl2-solution was 7.05 before contact with the soil and 6.79, 6.90 and 6.87 after contact with soil I to III, respectively.
- Suspended solids concentration: no data.

TEST SYSTEM
- Type, size and further details on reaction vessel: The study was performed in Teflon centrifuge tubes (preliminary test) or glass bottles (screening test).

- Water filtered: No.

- Number of reaction vessels/concentration: All experiments including controls were performed in duplicate.

- Measuring equipment: The concentration of the test material in the samples was determined by analysis of dissolved lanthanum using inductively coupled plasma mass spectroscopy (ICP-MS).

- Test performed in closed vessels (due to significant volatility of test substance) or in open system: No data.

- Method of preparation of test solution, incubation conditions and study conduct:
1) Preparation of the application solution:
Preliminary test: An application solution (AS1) was prepared for the preliminary test by weighing 1.21 mg of the test material and diluting it in 350 mL 0.01 M CaCl2 solution. Its final concentration was calculated to be 0.0034 mg/mL, taking into account the purity of the test material. The application solution was thoroughly mixed before use.
Screening test: An application solution (AS2) was prepared for the screening test in the same way as for the preliminary test only using an amount of 5.05 mg test material in 1500 mL 0.01 M CaCl2 solution. Its final concentration, corrected for purity, was calculated to be 0.0034 mg/mL.

2) Application: After pre-equilibration of the soil with 0.01 M CaCl2 solution, aliquots of the corresponding application solutions were added on the surface of the supernatant using a pipette as described in the table reported in the field “Any other information on materials and methods incl. tables”. Besides the treated samples, controls (duplicates, without soil) and blanks (duplicates, without test material) were subjected to precisely the same steps as the test samples. The control samples checked the stability of the test material in CaCl2 solution whereas the blanks served as a background control during the analysis to detect interfering compounds or contaminated soils. Analysis was only performed for the longest interval (48 hours). All test tubes were shaken briefly by hand, and then mechanically shaken on a rotary shaker in a temperature-controlled room.

3) Test concentrations: for the different parts of the adsorption/desorption test, the following test concentrations were measured by ICP/MS:
Preliminary test: 0.64 mg/L.
Screening test: 1.29 mg/L.

4) Conditions for adsorption/desorption:
Adsorption: All test tubes were shaken horizontally on a rotary shaker at about 150 strokes per minute. The agitation device kept the soil in suspension during shaking. After shaking, the samples were centrifuged (10 minutes at about 2800 or 3000 rpm) and the supernatants decanted into tarred, labelled sample containers. The volumes of the supernatants were determined gravimetrically. At the different sampling intervals, the samples were removed for ICP/MS analysis for determination of the test material concentration. The equilibrium concentration of the test material, and its total amount in the aqueous phase, were calculated based on the results of the analyses. The amount of test material adsorbed onto soil particles was obtained from the difference between the initial and final amount of test material in the aqueous phase.
Desorption: After separation of the soil from the adsorption solution, the same volume of 0.01 M CaCl2 as removed after the adsorption step was added. Thereafter, the mixtures were agitated in the same way as for the adsorption part. The samples were centrifuged and the supernatant analysed by ICP/MS at the different shaking intervals. The desorption equilibrium concentration of the test material, and its total amount in the aqueous phase, were calculated based on the results of the ICP/MS analyses. The remaining soil water in all samples was determined gravimetrically, and the mass of the test material left over from the adsorption equilibrium and the desorption step, respectively, was calculated.

5) Study conduct:
Preliminary test: The preliminary test was conducted using the three soils and three soil-to-solution ratios (1/50, 1/25 and 1/5) in order to select the optimal soil/solution ratio (for the subsequent screening test), to determine the adsorption of the test material onto the surfaces of the test vessels and the stability of the test material during the test period. A test material concentration of 0.64 mg/L and a sampling time of 48 hours were used. After 48 h, duplicate tubes were sampled and the supernatants analysed by ICP/MS for the test material concentration. Based on the results of the preliminary test, the soil-to-solution ratio of 1/50 was selected for the subsequent screening test.
Mass balance: The mass balance was carried out on all soils directly after the 48-hour adsorption step in the screening test at the soil-to-solution ratio of 1/50 (two tubes per soil). After sampling, the supernatant was removed and the soil was extracted three times, each using 5 mL nitric acid (10 %). The extracts resulting from the same tube were combined and analysed by ICP/MS.
Screening test: The adsorption kinetics were studied using the same soils as used for the preliminary test at the soil-to-solution ratio of 1/50 and a test material concentration of 1.29 mg/L. Additionally, the distribution coefficients Kd and Koc were determined after 48 hours of adsorption. After 2, 5, 24 and 48 hours of shaking during the adsorption phase, duplicate tubes were sampled and the supernatants analysed by ICP/MS. In order to determine whether the test material is readily desorbed from soil or not, a desorption experiment was performed directly after the adsorption step using sampling times at 2, 4, 24 and 48 hours.
Computational methods:
The following parameters were calculated using a commercially available Excel computer program:
- A: Percentage of substance adsorbed
- Kd: Distribution coefficient
- D: Percentage of substance desorbed
- Kdes: Desorption coefficient
- Koc: Organic carbon normalized adsorption/desorption coefficient
Type:
Koc
Value:
5 480 283 dimensionless
Type:
log Koc
Value:
6.36 dimensionless
Adsorption and desorption constants:
Adsorption: The Kd values amounted to 5451, 46861 and 216905 mL/g for soils I to III, respectively, with corresponding Koc values of 230978, 4733400 and 11476472 mL/g, respectively.
Desorption: The Kd des values were 15203, 2641, 30858 mL/g for soils I to III, respectively, with corresponding Koc des of 644212, 266724 and 163702 mL/g, respectively.
Recovery of test material:
The mass balance showed overall recoveries of 62.0, 79.1 and 75.9 % of the initial amount applied for soils I to III, respectively. Most of the applied test material was found in soil (62 to 79 %). Insignificant amounts (0.1 % of applied or less) were detected in the supernatant.
Concentration of test substance at end of adsorption equilibration period:
After addition of 64.5 μg of test material, 63.9 to 64.5 μg were adsorbed to the soils, with only 0.01 to 0.59 μg detected in the supernatant.
Concentration of test substance at end of desorption equilibration period:
The amount of test material desorbed remained low and constant with time, with <0.1 to 3.3 % of the adsorbed amount desorbed for all three soils within the 48-hour desorption experiment.
After 48 hours of shaking in the desorption experiment 0.28, 1.23 and 0.54 μg was recovered in the desorption solution of soils I to III. In the soils, between 63.20 and 63.95 μg were recovered after the desorption step.
Sample no.:
#1
Duration:
48 h
% Adsorption:
62
Sample no.:
#2
Duration:
48 h
% Adsorption:
79.1
Sample no.:
#3
Duration:
48 h
% Adsorption:
75.9
Sample no.:
#1
Duration:
48 h
% Desorption:
0.4
Sample no.:
#2
Duration:
48 h
% Desorption:
1.9
Sample no.:
#3
Duration:
48 h
% Desorption:
0.8
Transformation products:
no
Details on results (Batch equilibrium method):
See Tables 1 and 2 below
Statistics:
All calculations were done using the program VALIDATA 3.0 according to DIN 38402-part51.

Table 1: Concentration of the Test Material in the Solid and Liquid Phases at the end of Adsorption Equilibration Period (Mean ± S.D.)

Concentration

(mg/L)

Soil 1 (loamy sand)

Soil 2 (silt loam)

Soil 3 (clay)

On soil (µg)

In solution (µg a.i./mL)

% Adsorbed

On soil (µg)

In solution (µg a.i./mL)

% Adsorbed

On soil (µg)

In solution (µg a.i./mL)

% Adsorbed

Control*

1.238 µg a.i./mL in solution; 96.0 % adsorbed

1.29

63.92

0.0117

99.1

64.43

0.001

99.9

64.49

0.0003

100.0

*Control = without soil

 

Table 2: Concentration of the Test Material in the Solid and Liquid Phases at the end of Desorption.

Concentration

(mg/L)

Soil 1 (loamy sand)

Soil 2 (silt loam)

Soil 3 (clay)

On soil (µg)

In solution (µg)

% Desorbed as % of the Adsorbed

On soil (µg)

In solution (µg)

% Desorbed as % of the Adsorbed

On soil (µg)

In solution (µg)

% Desorbed as % of the Adsorbed

1.29

63.67

0.28

0.4

63.20

1.23

1.9

63.95

0.54

0.8

Validity criteria fulfilled:
yes
Conclusions:
The test material was strongly adsorbed to soil.
Executive summary:

The adsorption/desorption behaviour of the test material on soil was determined using three soils and the batch equilibrium method in accordance with the standardised guideline OECD 106.

Initially, a preliminary test was performed in order to investigate the adsorption behaviour of the test material. For this purpose, three soils, a test concentration of 0.64 mg/L and three soil (dry weight) to aqueous phase ratios: 1/50 (1 g/50 mL), 1/25 (1 g/25 mL) and 1/5 (5 g/25 mL) were used. Very high adsorption was observed for all three ratios. After 48 hours of agitation, virtually all of the initial amount of test material was adsorbed (≥99 %).

Therefore, for the subsequent screening test, the maximum soil-to-solution ratio of 1/50 was selected together with a test material concentration of 1.29 mg/L and the same three soils. The adsorption/desorption kinetics of the test material were determined after 2, 4 or 5, 24 and 48 hours of agitation. After 2 hours of agitation, adsorption represented 96, 100 and 100 % for soils I to III, respectively. Only low desorption was observed during the 48-hour desorption period (maximum of 3 % of test material desorbed for all three soils).

The mass balance was performed during the screening test at the soil to solution ratio of 1/50. The amount of applied test material recovered from the samples represented 62, 79 and 76 % for soils I to III, respectively. Most of the applied amount was extracted from the soils, with only ≤0.5 % of applied detected in the supernatants. The mass balance results confirmed the strong adsorption of the test material to soil.

The following adsorption/desorption distribution coefficients were obtained for the soils at the selected ratio of 1/50:

- Mean Koc value: 5480283

- Mean Koc, des value: 358213

Endpoint:
adsorption / desorption: screening
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Study conducted on read-across material
Justification for type of information:
Read-across to structurally similar substance, lanthanum oxide is justified since the adsorption/ desorption potential of the registered substance is due to the metal cation which is analogous in the case of the registered substance and the read-across substance (La3+).
Reason / purpose for cross-reference:
read-across source
Type:
Koc
Value:
5 480 283 dimensionless
Type:
log Koc
Value:
6.36 dimensionless
Sample no.:
#1
Duration:
48 h
% Adsorption:
62
Sample no.:
#2
Duration:
48 h
% Adsorption:
79.1
Sample no.:
#3
Duration:
48 h
% Adsorption:
75.9
Sample no.:
#1
Duration:
48 h
% Desorption:
0.4
Sample no.:
#2
Duration:
48 h
% Desorption:
1.9
Sample no.:
#3
Duration:
48 h
% Desorption:
0.8

Description of key information

The following adsorption/desorption distribution coefficients were obtained for the analogous test material Lanthanum oxide at the soil-to-solution ratio of 1/50:

- Mean Koc value: 5480283

- Mean Koc, des value: 358213

Key value for chemical safety assessment

Koc at 20 °C:
5 480 283

Additional information

The adsorption/desorption behaviour of the test material on soil was determined using three soils and the batch equilibrium method in accordance with the standardised guideline OECD 106.

Initially, a preliminary test was performed in order to investigate the adsorption behaviour of the test material. For this purpose, three soils, a test concentration of 0.64 mg/L and three soil (dry weight) to aqueous phase ratios: 1/50 (1 g/50 mL), 1/25 (1 g/25 mL) and 1/5 (5 g/25 mL) were used. Very high adsorption was observed for all three ratios. After 48 hours of agitation, virtually all of the initial amount of test material was adsorbed (≥99 %).

Therefore, for the subsequent screening test, the maximum soil-to-solution ratio of 1/50 was selected together with a test material concentration of 1.29 mg/L and the same three soils. The adsorption/desorption kinetics of the test material were determined after 2, 4 or 5, 24 and 48 hours of agitation. After 2 hours of agitation, adsorption represented 96, 100 and 100 % for soils I to III, respectively. Only low desorption was observed during the 48-hour desorption period (maximum of 3 % of test material desorbed for all three soils).

The mass balance was performed during the screening test at the soil to solution ratio of 1/50. The amount of applied test material recovered from the samples represented 62, 79 and 76 % for soils I to III, respectively. Most of the applied amount was extracted from the soils, with only ≤0.5 % of applied detected in the supernatants. The mass balance results confirmed the strong adsorption of the test material to soil.

The following adsorption/desorption distribution coefficients were obtained for the soils at the selected ratio of 1/50:

- Mean Koc value: 5480283

- Mean Koc, des value: 358213

[LogKoc: 6.36]