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Reference
Endpoint:
adsorption / desorption: screening
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)
Deviations:
yes
Remarks:
Only stage 1 and 2 of the OECD 106 study was performed and 3 soils instead of 5 were used. However, this was considered adequate for the information requirements.
Principles of method if other than guideline:
The standard test guideline (OECD 121, Koc estimation by HPLC) is not suitable for inorganic compounds as the method has not been validated for any inorganic compounds and none of the recommended reference substances are inorganic compounds. Therefore, a limited partitioning study with three soils following the OECD 106 Batch equilibrium method was performed to determine the Kd values. The approach was considered adequate for the information requirements.
GLP compliance:
yes (incl. QA statement)
Type of method:
batch equilibrium method
Media:
soil
Radiolabelling:
no
Test temperature:
The test was conducted at ambient temperature.
Details on study design: HPLC method:
Not applicable
Analytical monitoring:
yes
Details on sampling:
- Concentrations: 10 µg test item/L
- Sampling interval: Duplicate samples were taken after 0 h, 2 h, 4 h, 8 h, 16 h, 24 h and 48 h after initiation of the experiments.
Details on matrix:
SOIL PROPERTIES - Lufa 2.3
Organic carbon in % C 0.67 ± 0.04
Nitrogen in % N 0.08 ± 0.01
pH-value (0.01 M CaCl2) 5.8 ± 0.7
Cation exchange capacity (meq / 100g) 7.3 ± 1.0
Particle size distribution (mm) according to German DIN (%)
< 0.002 6.4 ± 2.0
0.002 – 0.006 5.0 ± 0.2
0.006 – 0.02 11.5 ± 0.7
0.02 – 0.063 19.5 ± 1.9
0.063 – 0.2 24.7 ± 1.8
0.2 – 0.63 30.4 ± 0.6
0.63 – 2.0 2.5 ± 0.9
Soil type Silty sand
Particle size distribution (mm) according to USDA DIN (%)
< 0.002 6.3 ± 1.9
0.002 – 0.05 33.8 ± 0.2
0.05 – 2.0 59.9 ± 1.9
Soil type Sandy loam
Max water holding capacity (g / 100 g) 35.6 ± 1.7
Weight per volume (g / 1000 mL) 1334 ± 6

SOIL PROPERTIES - Lufa 2.4
Organic carbon in % C 1.98 ± 0.18
Nitrogen in % N 0.21 ± 0.01
pH-value (0.01 M CaCl2) 7.2 ± 0.2
Cation exchange capacity (meq / 100g) 30.6 ± 5.1
Particle size distribution (mm) according to German DIN (%)
< 0.002 26.0 ± 1.5
0.002 – 0.006 8.1 ± 1.0
0.006 – 0.02 15.0 ± 1.2
0.02 – 0.063 23.3 ± 0.7
0.063 – 0.2 18.9 ± 0.3
0.2 – 0.63 7.0 ± 2.2
0.63 – 2.0 1.7 ± 0.2
Soil type Clayey loam
Particle size distribution (mm) according to USDA DIN (%)
< 0.002 26.0 ± 1.9
0.002 – 0.05 41.0 ± 1.4
0.05 – 2.0 33.0 ± 2.0
Soil type Loam
Max water holding capacity (g / 100 g) 43.4 ± 0.8
Weight per volume (g / 1000 mL) 1284 ± 29

SOIL PROPERTIES - Lufa 6S
Organic carbon in % C 1.73 ± 0.05
Nitrogen in % N 0.18 ± 0.01
pH-value (0.01 M CaCl2) 7.1 ± 0.1
Cation exchange capacity (meq / 100g) 25.9 ± 2.5
Particle size distribution (mm) according to German DIN (%)
< 0.002 41.7 ± 1.1
0.002 – 0.006 8.8 ± 1.2
0.006 – 0.02 13.0 ± 0.6
0.02 – 0.063 14.2 ± 0.9
0.063 – 0.2 9.2 ± 0.6
0.2 – 0.63 10.2 ± 0.9
0.63 – 2.0 2.9 ± 0.5
Soil type Clayey loam
Particle size distribution (mm) according to USDA DIN (%)
< 0.002 40.0 ± 2.1
0.002 – 0.05 34.4 ± 2.6
0.05 – 2.0 25.6 ± 3.2
Soil type clay
Max water holding capacity (g / 100 g) 40.5 ± 0.9
Weight per volume (g / 1000 mL) 1369 ± 40
Details on test conditions:
TEST CONDITIONS
- Buffer: no, experiment performed at natural soil pH, solution was made in 0.01 M CaCl2 solution
- pH: 4.7 - 7.2

TEST SYSTEM
- Type, size and further details on reaction vessel: centrifugation tubes, agitated on horizontal shaker
- Amount of soil and water per treatment (if simulation test): 2.0 - 12.5 g soil and 22.5 - 45 mL 0.01 M CaCl2 solution per tube in preliminary test, 2.0 g of soil and 45 - 50 mL 0.01 M CaCl2 solution per tube in the kinetic determination test.
- Soil/water ratio: 1/2, 1/5 and 1/25 (preliminary test), 1/25 (kinetic determination test)
- Number of reaction vessels/concentration: 1 blank per soil, one control sample (for all three soils), 13 tubes per soil (duplicate tubes for each sampling point and 1 blank tube)
- Measuring equipment: ICP-MS and ICP-OES for the analysis of gold in aqueous samples were used. Digested samples for the mass balance were measured by ICP-OES.
- Method of preparation of test solution: After equilibration a specified volume of the diluted stock solution was applied to the soil solution mixture resulting in a loading of 10 µg test item/L.
Sample No.:
#1
Type:
Koc
Value:
677 717.7
Temp.:
20 °C
Matrix:
Soil 2.3
% Org. carbon:
0.67
Remarks on result:
other: room temperature, sampling time 8 h
Sample No.:
#2
Type:
Kd
Value:
4 540.71
Temp.:
20 °C
Matrix:
Soil 2.3
% Org. carbon:
0.67
Remarks on result:
other: room temperature, sampling time 8 h
Sample No.:
#3
Type:
Koc
Value:
55 855.1
Temp.:
20 °C
Matrix:
Soil 2.4
% Org. carbon:
1.98
Remarks on result:
other: room temperature, sampling time 24 h
Sample No.:
#4
Type:
Kd
Value:
1 105.93
Temp.:
20 °C
Matrix:
Soil 2.4
% Org. carbon:
1.98
Remarks on result:
other: room temperature, sampling time 24 h
Sample No.:
#5
Type:
Koc
Value:
67 870.3
Temp.:
20 °C
Matrix:
Soil 6S
% Org. carbon:
1.73
Remarks on result:
other: room temperature, sampling time 24 h
Sample No.:
#6
Type:
Kd
Value:
1 174.16
Temp.:
20 °C
Matrix:
Soil 6S
% Org. carbon:
1.73
Remarks on result:
other: room temperature, sampling time 24 h
Details on results (HPLC method):
Not applicable.
Adsorption and desorption constants:
The equilibrium times for tetrachloroauric acid are visually reached for soil 2.3 after 8 h, for soil 2.4 and 6S after 24h.
Soil 2.3 (8 h)
Adsorption Ati= 99.3 %
Adsorption coefficient Kd = 4540.71 cm3/g
Adsorption coefficient normalized to organic carbon Koc = 677717.7 cm3/g

Soil 2.4 (24 h)
Adsorption Ati = 97.6 %
Adsorption coefficient Kd = 1105.93 cm3/g
Adsorption coefficient normalized to organic carbon Koc = 55855.1 cm3/g

Soil 6S (24 h)
Adsorption Ati = 97.6 %
Adsorption coefficient Kd = 1174.16 cm3/g
Adsorption coefficient normalized to organic carbon Koc = 67870.3 cm3/g

After 48 h (test end)
Soil 2.3
Kd: 16185.05 cm3/g
Koc: 2415679.1 cm3/g

Soil 2.4
Kd: 1827.01 cm3/g
Koc: 92273.4 cm3/g

Soil 6S
Kd: 595.32 cm3/g
Koc: 34411.7 cm3/g
Recovery of test material:
ICP-MS: The recoveries of fortified samples were in the range of 88.4 – 106.9 % (n = 8).
ICP-IOS: Recoveries from fortified samples: 101.3 – 106.2 % (n = 3)
Concentration of test substance at end of adsorption equilibration period:
Concentration of test item in aqueous phase at 48 h:
Soil 2.3: sample A: 0.0076 ug/L, sample B: 0.0068 ug/L
Soil 2.4: sample A: 0.0818 ug/L, sample B: 0.0704 ug/L
Soil 6S: sample A: 0.085 ug/L, sample B: 0.078 ug/L

Concentration of test item adsorbed on soil at 48 h:
Soil 2.3: sample A: 0.1162 ug/g, sample B: 0.1161 ug/g
Soil 2.4: sample A: 0.1382 ug/g, sample B: 0.1383 ug/g
Soil 6S: sample A: 0.1207 ug/g, sample B: 0.1209 ug/g
Concentration of test substance at end of desorption equilibration period:
Not applicable
Sample no.:
#1
Duration:
8 h
% Adsorption:
99.3
Remarks on result:
other: Soil 2.3
Sample no.:
#2
Duration:
24 h
% Adsorption:
97.6
Remarks on result:
other: Soil 2.4
Sample no.:
#3
Duration:
24 h
% Adsorption:
97.6
Remarks on result:
other: Soil 6S
Details on results (Batch equilibrium method):
PRELIMINARY TEST
- Weighed soil: 2.0 - 12.5 g
- Volume of CaCl2 solution: 22.5 - 45 mL
- Initial test substance concentration: 10 µg test item /L

Validity criteria fulfilled:
yes
Conclusions:
At equilibrium, the Koc values of 677717.7, 55855.1 and 67870.3 were detemined for Soil 2.3, 2.4 and 6S, respectively. The Kd values of 4540.71, 1105.93 and 1174.16 were determined for Soil 2.3, 2.4 and 6S, respectively.
Executive summary:

A study was conducted to determine the adsorption – desorption of tetrachloroauric acid according to OECD 106. A limited partitioning study with three soils was performed to determine the Kd values. The approach was considered adequate for the information requirements.

A calcium chloride solution containing the test item was added to the soils 2.3, 2.4 and 6S to obtain approx. 10 µg test item/L and agitated over up to 48 hours.

The equilibrium times for tetrachloroauric acid are visually reached after 8 h for soil 2.3, and after 24h for soils 2.4 and 6S. At equilibrium, the Koc values of 677717.7, 55855.1 and 67870.3 were detemined for Soil 2.3, 2.4 and 6S, respectively. The Kd values of 4540.71, 1105.93 and 1174.16 were determined for Soil 2.3, 2.4 and 6S, respectively. The Kd values of 16185.05, 1827.01 and 595.32 were determined for Soil 2.3, 2.4 and 6S, respectively, after 48 h. The Koc values of 2415679.1, 92273.4 and 34411.7 were detemined for Soil 2.3, 2.4 and 6S, respectively, after 48 h.

Description of key information

The Kd values of 16185.05, 1827.01 and 595.32 were determined for Soil 2.3, 2.4 and 6S, respectively, after 48 h. The Koc values of 2415679.1, 92273.4 and 34411.7 were determined for Soil 2.3, 2.4 and 6S, respectively, after 48 h. An equivalent Koc value of 197215 was calculated from the three soils, and was then used to calculate partition coefficients for the substance to other environmental matrices in addition to soil (e.g. suspended sediment). The equivalent Koc value results in a Kd value of 3944.3 for standard soil with 2 % organic carbon. 

Key value for chemical safety assessment

Koc at 20 °C:
197 215

Other adsorption coefficients

Type:
log Kp (solids-water in suspended matter)
Value in L/kg:
4.299
at the temperature of:
20 °C

Other adsorption coefficients

Type:
log Kp (solids-water in sediment)
Value in L/kg:
4.294
at the temperature of:
20 °C

Other adsorption coefficients

Type:
log Kp (solids-water in soil)
Value in L/kg:
3.596
at the temperature of:
20 °C

Additional information

A study was conducted to determine the adsorption – desorption of tetrachloroauric acid according to OECD 106. A limited partitioning study with three soils was performed to determine the Kd values (Klawonn 2016). The approach was considered adequate for the information requirements.

The equilibrium times for tetrachloroauric acid are visually reached for soil 2.3 after 8 h, for soil 2.4 and 6S after 24h.

At equilibrium, the Koc values of 677717.7, 55855.1 and 67870.3 were determined for Soil 2.3, 2.4 and 6S, respectively. The Kd values of 4540.71, 1105.93 and 1174.16 were determined for Soil 2.3, 2.4 and 6S, respectively.

The Kd values of 16185.05, 1827.01 and 595.32 were determined for Soil 2.3, 2.4 and 6S, respectively, after 48 h. The Koc values of 2415679.1, 92273.4 and 34411.7 were determined for Soil 2.3, 2.4 and 6S, respectively, after 48 h.

Organic carbon normalised partition coefficients (Koc) for the three tested soils showed a similar degree of variability to the calculated Kd values. In order to extrapolate the results of the soil partitioning study to other environmental media the average organic carbon normalised partition coefficient (Koc) was calculated for the three soils, and this was used to calculate the partitioning in the various standard environmental matrices. The average value was calculated as the mean of the log transformed values, which was then back transformed to give the average Koc value. An equivalent Koc value of 197215 was calculated from the three soils, and was then used to calculate partition coefficients for the substance to other environmental matrices in addition to soil (e.g. suspended sediment). The equivalent Koc value results in a Kd value of 3944.3 for standard soil with 2 % organic carbon. Unitless partition coefficients (Kcomp-water) can then be calculated from the equivalent Koc value to define the partitioning of this substance in standard suspended sediment (10 % organic carbon). A value of 5917 is calculated for Ksoil-water, and a value of 4931 is calculated for Ksusp-water. These partition coefficients are used to calculate soil and sediment PNEC values by equilibrium partitioning.

The equivalent Koc value is calculated as:

Koc= Kd/ Foc

Koc values were calculated for each individual soil using the known organic carbon content of each sample, and the mean value calculated from log Koc values.

Kd values for standard environmental media are then calculated from the equivalent Koc value as:

Kd = Foc x Koc

using fixed Foc values for the relevant environmental compartments (2 % for soil and 10 % for suspended sediment). This results in Kd values of 3944 for soil and 19721 for suspended sediment. The calculated Kd value for standard soil is slightly higher than the values determined in any of the experiments due to the higher content of organic carbon in the standard soil compared to the test soils.