Potassium hexadecyl hydrogen phosphate

Adsorption / desorption

Link to relevant study record(s)

Reference

Endpoint:

adsorption / desorption: screening

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2020-09-25 to 2021-08-26

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)

Version / remarks:

January 21, 2000

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Type of method:

batch equilibrium method

Media:

soil

Specific details on test material used for the study:

- Chemical Name: 1-Hexadecanol, 1-(dihydrogen phosphate), potassium salt (1:1)
- CAS No.: 19035-79-1

Radiolabelling:

yes

Test temperature:

20.4 ± 0.06 °C

Details on study design: HPLC method:

not applicable

Analytical monitoring:

yes

Details on sampling:

- Sampling interval: 1, 3, 16 and 24 hours
- Sample storage before analysis: no

Matrix no.:

#1

Matrix type:

loamy sand

% Clay:

4.1

% Silt:

9.3

% Sand:

86.6

% Org. carbon:

0.63

pH:

4.7

CEC:

3.7 meq/100 g soil d.w.

Matrix no.:

#2

Matrix type:

sandy loam

% Clay:

8

% Silt:

13.7

% Sand:

78.3

% Org. carbon:

1.71

pH:

5.6

CEC:

9.2 meq/100 g soil d.w.

Matrix no.:

#3

Matrix type:

loam

% Clay:

25.9

% Silt:

42

% Sand:

32

% Org. carbon:

1.95

pH:

7.4

CEC:

21.2 meq/100 g soil d.w.

Details on matrix:

Matrix #1
Name: Speyer 2.1

COLLECTION AND STORAGE
- Geographic location: Dudenhofen, Germany; 49°18'N, 8°21'E
- Batch: F2.1 1520
- Sampling date: April 06, 2020
- Sampling depth (cm): 0-20
- Storage conditions: in a closed container at room temperature
- Storage length: stored until use
- Soil preparation: air-dried at ambient temperature; passed through a 2-mm sieve, disaggregation performed with minimal force, so that the original texture of the soil changed as little as possible

PROPERTIES
- % sand: 86.6
- % silt: 9.3
- % clay: 4.1
- Soil taxonomic classification: Loamy sand
- Soil classification system: USDA
- pH: 4.7
- Organic carbon (%): 0.63
- CEC (meq/100 g): 3.7
- Nitrogen content (%): 0.05
- Organic matter (OM)(%): 1.09
- C/N-ratio: 12.60


Matrix #2
Name: Speyer 2.2

COLLECTION AND STORAGE
- Geographic location: Hanhofen, Germany; 49°18'N, 8°20'E
- Batch: F2.2 4018
- Sampling date: October 18, 2018
- Sampling depth (cm): 0-20
- Storage conditions: in a closed container at room temperature
- Storage length: stored until use
- Soil preparation: air-dried at ambient temperature; passed through a 2-mm sieve, disaggregation performed with minimal force, so that the original texture of the soil changed as little as possible

PROPERTIES
- % sand: 78.3
- % silt: 13.7
- % clay: 8.0
- Soil taxonomic classification: Sandy loam
- Soil classification system: USDA
- pH: 5.6
- Organic carbon (%): 1.71
- CEC (meq/100 g): 9.2
- Nitrogen content (%): 0.18
- Organic matter (OM)(%): 2.95
- C/N-ratio: 9.50


Matrix #3
Name: Speyer 2.4

COLLECTION AND STORAGE
- Geographic location: Leimersheim, Germany; 49°07'N, 8°21'E
- Batch: F2.4 1520
- Sampling date: April 06, 2020
- Sampling depth (cm): 0-20
- Storage conditions: in a closed container at room temperature
- Storage length: stored until use
- Soil preparation: air-dried at ambient temperature; passed through a 2-mm sieve, disaggregation performed with minimal force, so that the original texture of the soil changed as little as possible

PROPERTIES
- % sand: 32.0
- % silt: 42.0
- % clay: 25.9
- Soil taxonomic classification: Sandy loam
- Soil classification system: USDA
- pH: 7.4
- Organic carbon (%): 1.95
- CEC (meq/100 g): 21.2
- Nitrogen content (%): 0.22
- Organic matter (OM)(%): 3.36
- C/N-ratio: 8.86

General remark: The sampling site had not been treated with pesticides for at least 4 years prior to sampling.

Details on test conditions:

A preliminary test was performed using three soils at one test item concentration (0.1 μg/mL) and three different soil-to-solution ratios (1:20, 1:10 and 1:5). Due to the strong adsorption behaviour observed, additional soil-to-solution ratios of 1:50 and 1:100 were subsequently tested. An adsorption kinetics screening test was performed to investigate the time-dependent adsorption of the test item on all three soils at one test item concentration and a soil-to-solution ratio of 1:100. Finally, an advanced test was carried out using all three soils at a soil-to-solution ratio of 1:100. Five test concentrations covering two orders of magnitude (1.7 – 0.018 μg/mL) were used to investigate the adsorption and desorption isotherms of the radiolabelled test item after 3 hours of adsorption and 2 hours of desorption.

TEST CONDITIONS
- pH: 4.47 to 7.30 (screening test)
- Agitation: agitated on an overhead rotator (approximately 40 r.p.m.) or on a horizontal shaker (approximately 150 r.p.m.)
- Temperature: 20 °C ± 2 °C maintained in a temperature-controlled room

TEST SYSTEM
- Type of reaction vessel: sealed Teflon™ or glass tube
- Number of reaction vessels/concentration: 2 for the screening test, 5 for the advanced test
- Measuring equipment: Liquid scintillation counting (LSC), High-performance liquid chromatography (HPLC),
- Test performed in closed vessels: yes
- Method of preparation of test solution for the preliminary test, screening test and advanced test:
Preliminary test: The test item was delivered dissolved in THF/water/acetonitrile (60:23:17; v/v/v). A stock solution (SS1) was prepared by diluting 2.8 mL of the delivered solution with THF/water (7:3; v/v) to a total volume of 5 mL. The radioactivity content was measured after preparing three dilutions each containing 10 μL of SS1 in 5 mL of acetonitrile/water (1:1; v/v) and measuring quintuplicate aliquots of 100 μL by LSC to be 1'028'980'000 dpm/5 ml, corresponding to a concentration of 0.950 mg [14C]Amphisol K/mL based on a test item specific radioactivity of 3.61 MBq/mg. For the ratio test, an application solution (AS1) was prepared by adding 105 μl SS1 to a 1-mL volumetric flask and filling to the mark with THF/water (7:3; v/v). The solution was then mechanically shaken to ensure homogenisation. The radioactivity content was measured after preparing two dilutions each containing 5 μL in 5 mL of THF/water (7:3; v/v) and measuring quintuplicate aliquots of 500 μL by LSC to be 22'132'200 dpm/mL, corresponding to a concentration of 102.2 μg [14C]Praziquantel/mL based on a test item specific radioactivity of 3.61 MBq/mg. The actual test concentration achieved was determined as follows: prior to or in parallel to dosing of the samples the same volume of the application solution as applied to the samples, i.e. 10 μL, 20 μL and 25 μL, was added to 10 mL, 20 mL and 25 mL of 0.01 M CaCl2 in a volumetric flask, respectively. Thereafter, quintuplicate aliquots of each sample were measured by LSC. The results showing the achieved application rates are summarised in the table in section 3.3.4. For the additional ratio test, an application solution (AS2) was prepared by adding 210 μl SS1 to a 2-mL volumetric flask and filling to the mark with THF/water (7:3; v/v). The solution was then mechanically shaken to ensure homogenisation. The radioactivity content was measured after preparing two dilutions each containing 10 μL in 5 mL of THF/water (7:3; v/v) and measuring quintuplicate aliquots of 500 μL by LSC to be 22'132'200 dpm/mL, corresponding to a concentration of 101.2 μg [14C]Praziquantel/mL based on a test item specific radioactivity of 3.61 MBq/mg.

Screening test: For the adsorption kinetics, an application solution (AS3) was prepared by adding 5 mL of the original substance dissolved in THF/water/acetonitrile (60:23:17; v/v/v) to a 10-mL volumetric flask and filling to the mark with THF/water (7:3; v/v). The radioactivity content in AS2 was determined by quintuplicate 500 μL LSC measurements to be 185'250'600 dpm/mL or 855 μg [14C]Amphisol K/mL. For the desorption kinetics, the same application solution was used.

Advanced test (Tier 3): Fresh application solutions were prepared for both the adsorption and desorption parts of the advanced test for each test item concentration.

- Application: After equilibration of the soil with over 90% of the targeted volume of the aqueous phase (0.01 M CaCl2, purified, deionised water), aliquots of the corresponding application solutions were added to the surface of the supernatant. The application scheme is summarized in table 1 in section "Any other information on materials and methods incl. tables" below.
- Other: Test tubes were briefly shaken manually, then either agitated on an overhead rotator (approximately 40 r.p.m.) or on a horizontal shaker (approximately 150 r.p.m.) in a temperature-controlled room (20 °C ± 2 °C). Control samples containing the test item in 0.01 M CaCl2 solution (without soil) were subjected to precisely the same steps as the test samples in order to check the stability of the test item in CaCl2 solution. Blank samples of each soil were subjected to the same test procedure as the test samples and controls by using the same amount of soil and the same total volume of aqueous solution (without test item) of the corresponding test. These samples served as background controls during analysis to detect any contamination.

Sample No.:

#1

Duration:

3 h

Initial conc. measured:

>= 0.02 - <= 1.74 other: mg/L

pH:

4.7

Temp.:

20.4 °C

Remarks:

advanced test

Sample No.:

#2

Duration:

3 h

Initial conc. measured:

>= 0.02 - <= 1.75 other: mg/L

pH:

5.6

Temp.:

20.4 °C

Remarks:

advanced test

Sample No.:

#3

Duration:

3 h

Initial conc. measured:

>= 0.02 - <= 1.74 other: mg/L

pH:

7.4

Temp.:

20.4 °C

Remarks:

advanced test

Sample no.:

#1

Duration:

24 h

Conc. of adsorbed test mat.:

28.8 mg/kg soil d.w.

pH:

4.7

Temp.:

20.4 °C

Remarks:

screening test, mean value, n= 2

Sample no.:

#2

Duration:

2 h

Conc. of adsorbed test mat.:

8.74 mg/kg soil d.w.

pH:

5.6

Temp.:

20.4 °C

Remarks:

screening test, mean value, n= 2

Sample no.:

#3

Duration:

2 h

Conc. of adsorbed test mat.:

31.19 mg/kg soil d.w.

pH:

7.4

Temp.:

20.4 °C

Remarks:

screening test, mean value, n= 2

Key result

Sample No.:

#3

Type:

log Koc

Value:

5.48 dimensionless

pH:

7.4

Temp.:

20.4 °C

Matrix:

Soil #3, Speyer 2.4 (loam)

% Org. carbon:

1.95

Remarks on result:

other: advanced test

Key result

Sample No.:

#2

Type:

log Koc

Value:

4.11 dimensionless

pH:

5.6

Temp.:

20.4 °C

Matrix:

Soil #2, Speyer 2.2 (sandy loam)

% Org. carbon:

1.71

Remarks on result:

other: advanced test

Key result

Sample No.:

#1

Type:

log Koc

Value:

4.35 dimensionless

pH:

4.7

Temp.:

20.4 °C

Matrix:

Soil #1, Speyer 2.1 (loamy sand)

% Org. carbon:

0.63

Remarks on result:

other: advanced test

Key result

Type:

Koc

Value:

112 076 L/kg

Temp.:

20.4 °C

Matrix:

arithmetic mean of key results for soil 1, 2 and 3

Remarks on result:

other: advanced test

Key result

Sample No.:

#1

Type:

Koc

Value:

22 379 L/kg

pH:

4.7

Temp.:

20.4 °C

Matrix:

Soil #1, Speyer 2.1 (loamy sand)

% Org. carbon:

0.63

Remarks on result:

other: advanced test

Key result

Sample No.:

#2

Type:

Koc

Value:

12 908 L/kg

pH:

5.6

Temp.:

20.4 °C

Matrix:

Soil #2, Speyer 2.2 (sandy loam)

% Org. carbon:

1.71

Remarks on result:

other: advanced test

Key result

Sample No.:

#3

Type:

Koc

Value:

300 942 L/kg

pH:

7.4

Temp.:

20.4 °C

Matrix:

Soil #3, Speyer 2.4 (loam)

% Org. carbon:

1.95

Remarks on result:

other: advanced test

Key result

Sample No.:

#1

Phase system:

soil-water

Type:

other: K_F

Value:

141 L/kg

Temp.:

20.4 °C

pH:

4.7

Matrix:

loamy sand

% Org. carbon:

0.63

Remarks on result:

other: advanced test, organic carbon normalised

Key result

Sample No.:

#2

Phase system:

soil-water

Type:

other: K_F

Value:

221 L/kg

Temp.:

20.4 °C

pH:

5.6

Matrix:

sandy loam

% Org. carbon:

1.71

Remarks on result:

other: advanced test, organic carbon normalised

Key result

Sample No.:

#3

Phase system:

soil-water

Type:

other: K_F

Value:

6 019 L/kg

Temp.:

20.4 °C

pH:

7.4

Matrix:

loam

% Org. carbon:

1.95

Remarks on result:

other: advanced test, organic carbon normalised

Key result

Sample No.:

#3

Phase system:

soil-water

Type:

Kp

Value:

1 114 L/kg

Temp.:

20.4 °C

pH:

7.4

Matrix:

soil # 3, Speyer 2.4

% Org. carbon:

1.95

Remarks on result:

other: screening test, mean value, n = 2

Key result

Sample No.:

#2

Phase system:

soil-water

Type:

Kp

Value:

844 L/kg

Temp.:

20.4 °C

pH:

5.6

Matrix:

soil #2. Speyer 2.2

% Org. carbon:

1.71

Remarks on result:

other: screening test, mean value, n = 2

Key result

Sample No.:

#1

Phase system:

soil-water

Type:

Kp

Value:

208 L/kg

Temp.:

20.4 °C

pH:

4.7

Matrix:

Soil #1, Speyer 2.1

% Org. carbon:

0.63

Remarks on result:

other: screening test, mean value, n = 2

Adsorption and desorption constants:

Please see table 1 in section "Any other information on results incl. tables".

Recovery of test material:

89.3 % to 98.9 % of initial applied radioactivity during the adsorption part (3 hours shaking time) and 94.6 % to 110.9 % of initial applied radioactivity during the desorption part (total 5 hours shaking time).

Concentration of test substance at end of adsorption equilibration period:

Please see table 2 and 3 in section "Any other information on results incl. tables".

Concentration of test substance at end of desorption equilibration period:

Please see table 4 in section "Any other information on results incl. tables".

Sample no.:

#1

Duration:

24 h

% Adsorption:

39.5

Remarks on result:

other: screening test, mean value of two replicates for soil extract

Sample no.:

#2

Duration:

24 h

% Adsorption:

54.8

Remarks on result:

other: screening test, mean value of two replicates for soil extract

Sample no.:

#3

Duration:

24 h

% Adsorption:

51.4

Remarks on result:

other: screening test, mean value of two replicates for soil extract

Transformation products:

not measured

Remarks:

The missing radioactivity was assigned to a transformation product and was not considered in the scope of the adsoption/desorption study.

Details on results (Batch equilibrium method):

PRELIMINARY TEST
- Soil-to-solution ratio: 1:100
- Analytical test substance concentration in final solution: 102.2 µg/mL

MAIN TEST: PERFORMANCE
- Test material stability during adsorption/desorption phase: yes
- Experimental conditions maintained throughout the study: Yes
- Chemical interactions: none

TRANSFORMATION PRODUCTS
- Range of maximum concentrations in % of the applied amount (screening test): approx. 9%

RESIDUES
- Total unidentified radioactivity (range) of applied amount (screening test): 5.5 – 10.2 %
- Extractable residues (screening test): 76.45 – 91.56 % (sum of amounts extracted from aqueous phase, soil, transfer solution and rinsing solution)
- Non-extractable residues (% of applied amount at end of study period) (screening test): 2.4 – 14.75 %

RESULTS OF SUPPLEMENTARY EXPERIMENT (Advanced Test):
The distribution of [14C]Amphisol K between the aqueous phase and soil after 3 hours of adsorption in the advanced test is presented in Table 16 to Table 18. In all soils, adsorption was generally slightly lower at higher test concentrations, except for soil 3. At the highest concentration of 1.74 mg/L, 14.9%, 9.6% and 50.6% of the applied Amphisol K was adsorbed to soils 1 through 3, respectively. At the lowest concentration of 0.02 mg/L, the corresponding values were 24.5%, 11.9% and 25.2% of applied. Recovered radioactivity in the control samples was between 89.3% and 99.6%. HPLC analysis showed stability of the test item in soilless control solutions and aqueous phases of soil 1 and 2. Different degrees of degradation were observed during the 3-hour of agitation period in the aqueous phase of soil 3 and extracts of each soil. The arithmetic mean Freundlich isotherm coefficient for adsorption KFoc was 112'076 with a standard deviation of 163'631. The regression constants (1/n) for adsorption ranged from 1.033 and 1.613, indicating a slight to strong dependence of adsorption on the test item concentration of [14C]Amphisol K for all soils.

The arithmetic mean Freundlich isotherm coefficient for desorption Kdes,Foc was 190'874 with a standard deviation of 303'018. The 1/n values for the desorption ranged between 0.885 and 1.453, and were similar to the adsorption values for the respective soils. The organic carbon normalised Freundlich desorption coefficient (Kdes, Foc) of soil 1 was lower when compared to adsorption (KFoc), demonstrating in general the reversible sorption of the test item to this soils. For soils 2 and 3, the Kdes, Foc were higher when compared to adsorption (KFoc), demonstrating irreversible binding of the test item to those soils. All relevant quality checks as part of confirming the acceptability of the study and of the reported endpoints were performed according to the EFSA quality criteria evaluator's checklist.

Statistics:

Calculations were performed with a commercially available computer program (Microsoft Office Excel).

Table 1: Adsorption and desorption parameters resulting from the Freundlich isotherms obtained in the advanced test

Parameter	Soil			Mean
	Speyer 2.1	Speyer 2.2	Speyer 2.4	Arithmetic	Standard deviation
KF [mL/g]	141	221	6019	-
Log(KFOC)	4.35	4.11	5.48	-
KFOC [mL/g]	22379	12908	300942	112076	163631
KFOM [mL/g]	20652	4389	87483	37508	44037
1/n	1.056	1.033	1.613	-
r2	0.9798	0.9768	0.9679	-
Kdes,F [mL/g]	108	253	10815
log(Kdes,FOC)	4.23	4.17	5.73
Kdes,FOC  [mL/g]	17072	14782	540767	190874	303018
Kdes,FOM [mL/g]	15755	5026	157200	59327	84930
1/n	0.885	0.981	1.453	-
r2	0.9581	0.926	0.9797	-

K_F = Freundlich adsorption/desorption coefficient

Kdes, F = Freundlich desorption coefficient
Kdes, Foc = Freundlich desorption coefficient related to organic carbon content of soil
Kdes, Fom = Freundlich desorption coefficient related to organic matter content of soil
1/n = Regression constant
r2 = Regression coefficient

 

Table 2: Concentration of [14C]Amphisol K in aqueous phase after 3 hours of adsorption for soil 1, 2 and 3.

Nom. test item conc.	Concentration of [14C]Amphisol K in the aqueous phase [mg/L]
	Soil: Speyer 2.1		Soil: Speyer 2.2		Soil: Speyer 2.4
[mg/L]	c(ads)aq (mean)	relative SD (%)	c(ads)aq (mean)	relative SD (%)	c(ads)aq (mean)	relative SD (%)
1.74	0.191	3.5	0.108	16.0	0.054	11.1
0.87	0.108	8.6	0.063	19.8	0.048	2.2
0.17	0.037	26.4	0.015	2.9	0.015	2.7
0.08	0.02	8.4	0.008	5.7	0.008	6.8
0.02	0.004	26.3	0.001	8.3	0.002	2.0

 

Table 3: Concentration of [14C]Amphisol K in soil after 3 hours of adsorption for soil 1, 2 and 3.

Nom. test item conc.	Concentration of [14C]Amphisol K in the aqueous phase [mg/L]
	Soil: Speyer 2.1		Soil: Speyer 2.2		Soil: Speyer 2.4
[mg/L]	c(ads)s (mean)	relative SD (%)	c(ads)s (mean)	relative SD (%)	c(ads)s (mean)	relative SD (%)
1.74	25.907	7.9	16.742	0.0	86.853	3.3
0.87	15.148	8.8	14.513	3.5	35.929	3.2
0.17	3.99	6.0	3.55	6.3	4.539	2.6
0.08	1.759	10.1	1.748	8.6	2.356	4.2
0.02	0.443	25.1	0.214	3.1	0.456	1.8

 

Table 4: Advanced test desorption: Percentage of [14C]Amphisol K in the aqueous phase and the soil extract after 3 hours of adsorption and 2 hours of desorption (determined by HPLC) at the highest test concentration.

% Amphisol K of HPLC chromatogram
Replicate	Phase	Soil			Soilless control
		Speyer 2.1	Speyer 2.2	Speyer 2.4
A	Aqueous	78.20	86.18	82.71*	100
B		77.07	94.62	74.14*	100
A	Soil extract	77.40	35.00	75.33	n.a.
B		80.84	39.41	74.94	n.a.

n.a not applicable
* Samples of soil 3 could only be analysed after a prolonged storage time. LSC measurements of those samples after storage showed insufficient recovery of radioactivity when compared to the original measurement. For Amphisol K storage stability was repeatedly confirmed in aqueous phases of control samples, with no adherence of the test item to the vessel wall during storage, hence missing radioactivity can be attributed to a transformation product. Freshly prepared samples of soil 3 after 3 hours adsorption have shown that radioactivity settled from solution if ultracentrifuged, but not if acidified, indicating adherence to soil particles, which can be prevented through acidification of the sample. Stored samples were acidified, which should bring precipitated material into solution, recovering the total amount of radioactivity originally present in the sample before storage. However, this was not the case, indicating that it was a transformation product that could not be recovered. Therefore, the recovered amount of radioactivity measured by LSC after storage corresponded to a maximum amount of Amphisol K in those samples. The missing radioactivity was assigned to a transformation product and is not of interest in the scope of an adsoption/desorption study.

Validity criteria fulfilled:

yes

Conclusions:

In an Adsorption/Desorption Test using a Batch Equilibrium Method according to OECD TG 106 the KFoc of the substance at 20 °C was determined to be 112076 L/kg (arithmetic mean, n = 3; SD: 163631), corresponding to a log(KFoc) ranging between 4.11 and 5.48, which indicates a strong to very strong adsorption to soil matrices. 

Executive summary:

The adsorption and desorption characteristics of the 14C-labelled test item in soil were investigated in a GLP-compliant Adsorption/Desorption Test (Batch Equilibrium Method) according to OECD TG 106 using three different European soil types, covering a range of soil properties: soil 1 (Speyer 2.1, loamy sand), soil 2 (Speyer 2.2, sandy loam) and soil 3 (Speyer 2.4, loam). Preliminary and screening tests were performed in order to find the optimal parameters for the advanced test. Due to the amphiphilic structure of the test item, several preliminary tests were conducted in order to investigate adsorption to test vessel and the optimal soil-to-solution ratio.

 

The test item showed strong adsorption to the test vessel (Teflon and glass tubes), also in the presence of soil. The optimal conditions to minimize/recover adsorption to any vessel surface were the addition of tetrahydrofurane to the aqueous phase prior to LSC measurement and rinsing of the original test vessel with tetrahydrofurane/water 7:3 (v/v) + 1 % formic acid.

 

The preliminary test was performed using all soils at one test item concentration (0.1 μg/mL) and three different soil-to-solution ratios (1:20, 1:10 and 1:5). High adsorption was observed, reaching maximum mean amounts of 94.4 %, 96.6 % and 77.8 % AR in soils 1 to 3, respectively, at the soil-to-solution ratio of 1:5, thereby leaving low amounts of applied radioactivity in the aqueous phase. The entire amount of radioactivity could be recovered in soilless control samples by rinsing of the test vessel.

 

Due to the strong adsorption behaviour observed, additional soil-to-solution ratios of 1:50 and 1:100 were subsequently tested. Maximum mean amounts adsorbed of 90.7 %, 90.9 % and 97.1 % AR were determined in soils 1 to 3, respectively, at the soil-to-solution ratio of 1:100, thereby slightly increasing the amounts of applied radioactivity in the aqueous phase.

 

Based on the preliminary tests, the soil-to-solution ratio of 1:100 (1 g soil/100 mL CaCl2) was selected for all soils as the optimal ratio to obtain as high as possible amounts in the aqueous phase for accurate concentration determination.

 

The adsorption kinetics screening test was performed to investigate the time-dependent adsorption of the test item on all three soils at one test item concentration and a soil-to-solution ratio of 1:100. The adsorption was determined by measuring the radioactivity in the aqueous phase after 1, 3, 16 and 24 hours. Adsorption of radioactivity decreased with time and equilibrium was not fully reached, indicating degradation of the test item during the adsorption process. HPLC analyses of selected supernatant solutions and soil extracts revealed different degrees of degradation. In soilless control samples no degradation was observed after the longest adsorption time. A short interval, i.e. three hours for the adsorption, was considered to be adequate for the subsequent tests.

 

A complete mass balance of the radioactivity applied was also obtained after 24 hours of adsorption, resulting in total mean recoveries ranging from 90.7% to 93.6% of applied radioactivity. The HPLC chromatograms of the mass balance samples showed variable stability of the test item in the aqueous phase and soil extracts. These results indicated that the direct measurement had to be used for the determination of Kd and KOC values. The mean KOC values amounted to 33041, 49'346 and 571242 mL/g for soils 1 to 3, respectively.  The desorption kinetics were performed to investigate the behaviour of the test item during the desorption process. Desorption was performed using all three soils after a previous 3-hour period of adsorption. The soil-to-solution ratio was1:100 and the test was performed with intervals of 2, 4, 6 and 24 hours. The desorption kinetics experiment showed relatively constant concentrations of radioactivity 6 hours of desorption for all soils. However, degradation of the test item was confirmed in supernatants and soil extracts. It was decided to continue using adsorption time of 3 hours and a desorption time of 2 hours for the advanced test to reduce the total sorption time and to consequently minimize degradation of the radiolabelled test item. The Koc,des values amounted to 34317, 48742 and 59048 mL/g for soils 1 to 3, respectively.

 

The advanced test was performed using all three soils at a soil-to-solution ratio of 1:100. Five test concentrations covering two orders of magnitude (1.7 – 0.018 μg/mL) were used to investigate the adsorption and desorption isotherms of the radiolabelled test item after 3 hours of adsorption and 2 hours of desorption. The direct method was employed, i.e. soil samples were extracted after the adsorption step as well as after the desorption step and both phases (extract and aqueous phase) were analysed by HPLC. The degradation of the test item was considered for the evaluation of the advanced test. The results obtained were evaluated by applying the linear Freundlich equation and soil adsorption coefficients, including the Freundlich adsorption constants K_F and K_Foc and the regression constant 1/n, were determined. As a result, the K_Foc of the test item was determined to be 112076 L/kg (arithmetic mean; SD: 163631).

Description of key information

In an Adsorption/Desorption Test using a Batch Equilibrium Method according to OECD TG 106 the K_Foc of the substance at 20 °C was determined to be 112076 L/kg (arithmetic mean, n = 3; SD: 163631), corresponding to a log(K_Foc) ranging between 4.11 and 5.48, which indicates a strong to very strong adsorption to soil matrices. 

Key value for chemical safety assessment

Koc at 20 °C:

112 076

Additional information

A new GLP-compliant guideline study on the adsorption/desorption behaviour of the substance is available and summarized below.

 

Key information

The adsorption and desorption characteristics of the 14C-labelled test item in soil were investigated in a GLP-compliant Adsorption/Desorption Test (Batch Equilibrium Method) according to OECD TG 106 using three different European soil types, covering a range of soil properties: soil 1 (Speyer 2.1, loamy sand), soil 2 (Speyer 2.2, sandy loam) and soil 3 (Speyer 2.4, loam). Preliminary and screening tests were performed in order to find the optimal parameters for the advanced test. Due to the amphiphilic structure of the test item, several preliminary tests were conducted in order to investigate adsorption to test vessel and the optimal soil-to-solution ratio.

 

The test item showed strong adsorption to the test vessel (Teflon and glass tubes), also in the presence of soil. The optimal conditions to minimize/recover adsorption to any vessel surface were the addition of tetrahydrofurane to the aqueous phase prior to LSC measurement and rinsing of the original test vessel with tetrahydrofurane/water 7:3 (v/v) + 1 % formic acid.

 

The preliminary test was performed using all soils at one test item concentration (0.1 μg/mL) and three different soil-to-solution ratios (1:20, 1:10 and 1:5). High adsorption was observed, reaching maximum mean amounts of 94.4 %, 96.6 % and 77.8 % AR in soils 1 to 3, respectively, at the soil-to-solution ratio of 1:5, thereby leaving low amounts of applied radioactivity in the aqueous phase. The entire amount of radioactivity could be recovered in soilless control samples by rinsing of the test vessel.

 

Due to the strong adsorption behaviour observed, additional soil-to-solution ratios of 1:50 and 1:100 were subsequently tested. Maximum mean amounts adsorbed of 90.7 %, 90.9 % and 97.1 % AR were determined in soils 1 to 3, respectively, at the soil-to-solution ratio of 1:100, thereby slightly increasing the amounts of applied radioactivity in the aqueous phase.

 

Based on the preliminary tests, the soil-to-solution ratio of 1:100 (1 g soil/100 mL CaCl2) was selected for all soils as the optimal ratio to obtain as high as possible amounts in the aqueous phase for accurate concentration determination.

 

The adsorption kinetics screening test was performed to investigate the time-dependent adsorption of the test item on all three soils at one test item concentration and a soil-to-solution ratio of 1:100. The adsorption was determined by measuring the radioactivity in the aqueous phase after 1, 3, 16 and 24 hours. Adsorption of radioactivity decreased with time and equilibrium was not fully reached, indicating degradation of the test item during the adsorption process. HPLC analyses of selected supernatant solutions and soil extracts revealed different degrees of degradation. In soilless control samples no degradation was observed after the longest adsorption time. A short interval, i.e. three hours for the adsorption, was considered to be adequate for the subsequent tests.

 

A complete mass balance of the radioactivity applied was also obtained after 24 hours of adsorption, resulting in total mean recoveries ranging from 90.7% to 93.6% of applied radioactivity. The HPLC chromatograms of the mass balance samples showed variable stability of the test item in the aqueous phase and soil extracts. These results indicated that the direct measurement had to be used for the determination of Kd and KOC values. The mean KOC values amounted to 33041, 49'346 and 571242 mL/g for soils 1 to 3, respectively.  The desorption kinetics were performed to investigate the behaviour of the test item during the desorption process. Desorption was performed using all three soils after a previous 3-hour period of adsorption. The soil-to-solution ratio was1:100 and the test was performed with intervals of 2, 4, 6 and 24 hours. The desorption kinetics experiment showed relatively constant concentrations of radioactivity 6 hours of desorption for all soils. However, degradation of the test item was confirmed in supernatants and soil extracts. It was decided to continue using adsorption time of 3 hours and a desorption time of 2 hours for the advanced test to reduce the total sorption time and to consequently minimize degradation of the radiolabelled test item. The Koc,des values amounted to 34317, 48742 and 59048 mL/g for soils 1 to 3, respectively.

 

The advanced test was performed using all three soils at a soil-to-solution ratio of 1:100. Five test concentrations covering two orders of magnitude (1.7 – 0.018 μg/mL) were used to investigate the adsorption and desorption isotherms of the radiolabelled test item after 3 hours of adsorption and 2 hours of desorption. The direct method was employed, i.e. soil samples were extracted after the adsorption step as well as after the desorption step and both phases (extract and aqueous phase) were analysed by HPLC. The degradation of the test item was considered for the evaluation of the advanced test. The results obtained were evaluated by applying the linear Freundlich equation and soil adsorption coefficients, including the Freundlich adsorption constants K_F and K_Foc and the regression constant 1/n, were determined. As a result, the K_Foc of the test item was determined to be 112076 L/kg (arithmetic mean; SD: 163631).