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Adsorption / desorption

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Reference
Endpoint:
adsorption / desorption: screening
Type of information:
experimental study
Adequacy of study:
key study
Study period:
2019
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:
no
GLP compliance:
yes (incl. QA statement)
Type of method:
batch equilibrium method
Media:
soil
Radiolabelling:
no
Analytical monitoring:
yes
Details on sampling:
Preliminary Study
Two soil types three soil/solution ratios described as follows were adopted.
- 10 g soil and 50 mL aqueous Solution of the test substance (ratio 1/5);
- 2 g soil and 50 mL aqueous solution of the test substance (ratio 1/25);
- 1 g soil and 50 mL aqueous solution of the test substance (ratio 1/5 0).
One control sample with only the test substance in 0.01 M CaCl2 solution (no soil) was subjected to precisely the same steps as the test systems, in order to check the stability of the test substance in CaCl2 solution and its possible adsorption on the surfaces of the test vessels.
A blank run per soil with the same amount of soil and total volume of 50 mL 0.01 M CaCl2
solutions (without test substance) were subjected to the same test procedure. This served as a background control during the analysis to detect interfering compounds or contaminated soils.
All the experiments, including controls and blanks, were performed in duplicate.
The air-dried soil samples were equilibrated by shaking with a minimum volume of 45 mL of 0.01 mol/L CaCI 2 overnight (12 h) before the day of the experiment. Afterwards, 5 mL of the stock solution (1000 mg/L) of the test substance was added.
The mixture was shaken 48h until adsorption equilibrium was reached (25±1 °C, 200 rmp).
Parallel method: Samples were collected sequentially over a 48 h period of mixing (in the test at 1, 3, 4, 7, 24, 48 h). After centrifugation and filtration, the aqueous phases of the samples were analyzed. Only the blank run samples per soil with the soil/solution ratio of 1/5 (without test substance) were collected and analyzed at 48 h.

Main experiments
Five soils were selected and used. Each experiment (one soil and one solution) was done at least in duplicate to allow estimation of the variance of the results. In every experiment one blank was run, it consisted of the soil and 0.01 mol/L CaCl2 solution, without test substance, and of weight and volume, respectively, identical to those of the experiment.
The equilibration time, the soil/solution ratio, the weight of the soil sample, the volume of the aqueous phase in contact with the soil and the concentration of the test substance in the solution were chosen based on the preliminary study results. Analysis was performed approximately after 1, 3, 4, 7 and 24 h contact time.

All the mixtures of the soil with the solution is agitating until to reach adsorption equilibrium (as determined before in Tier 2). Then, the phases are separated by centrifugation and the aqueous phases are removed as much as possible. The volume of solution removed is replaced by an equal volume of 0.01 M CaCI 2 without test substance and the new mixtures are agitated again. The aqueous phase of the first tube is recovered as completely as possible and is measured at different time until the desorption equilibrium is reached.
Matrix no.:
#1
Matrix type:
loam
% Clay:
33.04
% Silt:
61.6
% Sand:
5.36
% Org. carbon:
6.54
pH:
7.28
CEC:
28.87 other: cmol/kg
Matrix no.:
#2
Matrix type:
silt loam
% Clay:
42.24
% Silt:
42.76
% Sand:
15
% Org. carbon:
1.36
pH:
4.6
CEC:
11.53 other: cmol/kg
Matrix no.:
#3
Matrix type:
silty clay loam
% Clay:
6.16
% Silt:
74.08
% Sand:
19.76
% Org. carbon:
5.36
pH:
6.81
CEC:
22.9 other: cmol/kg
Matrix no.:
#4
Matrix type:
silt loam
% Clay:
22.16
% Silt:
38.8
% Sand:
39.04
% Org. carbon:
1.39
pH:
7.84
CEC:
7.08 other: cmol/kg
Matrix no.:
#5
Matrix type:
silt loam
% Clay:
14.84
% Silt:
73.2
% Sand:
11.96
% Org. carbon:
6.85
pH:
7.96
CEC:
22.13 other: cmol/kg
Details on matrix:
The soils were characterized by four parameters considered to be largely responsible for the adsorptive capacity: organic carbon, clay content, soil texture, and pH. The soils selected for this study comprises Jilin (black soi #1l), Jiangxi (red soil #2), Jiangsu (paddy soil #3), Shandong (yellow soil #4), and Gansu (meadow soi #5l). In the depth of 10-15 cm surface soil samples were collected and transported using containers and under temperature conditions which guarantee that the initial soil properties were not significantly altered. The physical and chemical properties of the experimental soil were analyzed.

Details on test conditions:
The soils were air-dried at ambient temperature (20-25 °C). Disaggregating was performed with minimal force, so that the original texture of the soil would be changed as little as possible. The soils were sieved to a particle size 0.3mm. Careful homogenization was recommended. as this enhances the reproducibility of the results. The moisture content of each soil was determined on three aliquots with heating at 105°C until there was no significant change in weight. For all calculations the mass of soil refers to oven dry mass, i.e. the weight of soil corrected for moisture content.
Sample No.:
#1
Duration:
48 h
Initial conc. measured:
86 other: mg/L
pH:
6.1
Sample No.:
#2
Duration:
48 h
Initial conc. measured:
95.1 other: mg/L
pH:
4.57
Sample No.:
#3
Duration:
48 h
Initial conc. measured:
77.4 other: mg/L
pH:
6.32
Sample No.:
#4
Duration:
48 h
Initial conc. measured:
68.8 other: mg/L
pH:
7.46
Sample No.:
#5
Duration:
48 h
Initial conc. measured:
72 other: mg/L
pH:
7.58
Sample no.:
#1
Duration:
48 h
Remarks:
not detected in supernatant
Sample no.:
#2
Duration:
48 h
Remarks:
not detected in supernatant
Sample no.:
#3
Duration:
48 h
Remarks:
not detected in supernatant
Sample no.:
#4
Duration:
48 h
Remarks:
not detected in supernatant
Sample no.:
#5
Duration:
48 h
Remarks:
not detected in supernatant
Computational methods:
According to guideline.
Sample No.:
#1
Type:
Koc
Value:
664 L/kg
pH:
6.1
% Org. carbon:
6.54
Sample No.:
#2
Type:
Koc
Value:
1 988 L/kg
pH:
4.57
% Org. carbon:
1.36
Sample No.:
#3
Type:
Koc
Value:
1 126 L/kg
pH:
6.32
% Org. carbon:
5.36
Sample No.:
#4
Type:
Koc
Value:
1 724 L/kg
pH:
7.46
% Org. carbon:
1.39
Sample No.:
#5
Type:
Koc
Value:
650 L/kg
pH:
7.58
% Org. carbon:
6.85
Key result
Type:
log Koc
Value:
>= 2.82 - <= 3.3 L/kg
% Org. carbon:
>= 1.36 - <= 6.85
Key result
Type:
Koc
Value:
>= 664 - <= 1 988 L/kg
% Org. carbon:
>= 1.36 - <= 6.85
Key result
Type:
Koc
Value:
ca. 1 230 L/kg
Matrix:
Mean value of soils #1 - #5
% Org. carbon:
>= 1.36 - <= 6.85
Remarks on result:
other:
Remarks:
Mean value
Sample No.:
#1
Phase system:
solids-water in soil
Type:
Kp
Value:
43.5 L/kg
pH:
6.1
Sample No.:
#2
Phase system:
solids-water in soil
Type:
Kp
Value:
27 L/kg
pH:
4.57
Sample No.:
#3
Phase system:
solids-water in soil
Type:
Kp
Value:
60.4 L/kg
pH:
6.32
Sample No.:
#4
Phase system:
solids-water in soil
Type:
Kp
Value:
24 L/kg
pH:
7.46
Sample No.:
#5
Phase system:
solids-water in soil
Type:
Kp
Value:
44.5 L/kg
pH:
7.58
Adsorption and desorption constants:
The test substance adsorption isotherms on #1 (Jilin black soil), #2 (Jiangxi red soil), #3 (Jiangsu paddy soil), #4 (Shandong yellow soil), #5 (Gansu meadow soil) are preferably followed Freundlich adsorption equation, correlative coefficients all are more than 0.9. The Freundlich adsorption coefficient (K ds ) values in above five soils are 27.2. 2.98, 7.29, 1.16 and 9.93 µg^(-1/n)(cm3)^(1/n)g-1 respectively. The 1/n value for different soils are 1.14, 0.840, 0.908, 1.25 and
0,941.
Recovery of test material:
Recovery from water:
The mean recovery rate was 94.1% for the concentration of 10.0 mg/L respectively. The relative standard deviation was 2.2 1%
Recovery from soil:
The mean recovery rates were 92.0% -99.7% for five soils. The relative standard deviation was 1.05% -3.56%.
Nominal concentration (mg/kg) Soil Mean recovery rate (%) RSD (%)
250 #1 99.7 3.01
#2 96.9 1.72
#3 95.7 1.05
#4 92.0 3.56
#5 97.5 1.71
Concentration of test substance at end of adsorption equilibration period:
End of equilibration period after 48 hours

Soil #1 #2 #3 #4 #5
1/50 1/50 1/50 1/50 1/50
Concentration 51.8 63 45.4 67.6 52.9
in supernatant in mg/L
Concentration of test substance at end of desorption equilibration period:
During the desorption equilibrium time of 48 h, the test substance was not detected in the
supernatant of all soils.
Sample no.:
#1
Duration:
24 h
% Adsorption:
94
Sample no.:
#2
Duration:
24 h
% Adsorption:
95.9
Sample no.:
#1
Duration:
48 h
% Desorption:
ca. 0
Sample no.:
#2
Duration:
48 h
% Desorption:
ca. 0
Sample no.:
#3
Duration:
48 h
% Desorption:
ca. 0
Sample no.:
#4
Duration:
48 h
% Desorption:
ca. 0
Sample no.:
#5
Duration:
48 h
% Desorption:
ca. 0
Transformation products:
not measured

  Table 1. Recovery of test item Stand Solution in Aqueous Samples

Nominal Concentration (mg/L) Measured Concentraion (mg/L) Recovery Rate (%) Mean Recovery rate (%) RSD (%)
10 9.23 92.3 94.1 2.21
9.37 93.7
9.64 96.4

  Table 2. Recovery of test item Stand Solution in Soil Samples

Nominal Concentration (mg/L) Soil Measured Concentraion (mg/kg) Recovery Rate (%) Mean Recovery rate (%) RSD (%)
250 #1 242 96.8 99.7 3.01
257 103
249 99.6
#2 239 95.6 96.9 1.72
247 98.8
241 96.4
#3 237 94.8 95.7 1.05
239 95.6
242 96.8
#4 228 91.2 92 3.56
239 95.6
223 89.2
#5 245 98 97.5 1.71
247 98.8
239 95.6

Table3. Results of Adsorption Isotherms
Soil Freundlich adsorption equation Freundlich adsorptiion coefficient
 Kf (µg^(1-1/n) * (cm^3)^1/n * g^-1)		 1/n
#1 y = 1.1439 x + 1.4351, r2 = 0.9252 27.233 1.144
#2 y = 0.8403 x + 1.8175, r2 = 0.9184 2.975 0.8403
#3 y = 0.9076 x + 1.9436, r2 = 0.9598 7.286 0.9076
#4 y = 1.2497 x + 0.8311, r2 = 0.9470 1.163 1.2491
#5 y = 0.941 x + 1.8281, r2 = 0.9935 9.933 0.941
Validity criteria fulfilled:
yes
Conclusions:
CAS 13431-57-7 was found to adsorb to the 5 tests soils. The test substance did not desorb significantly from the 5 soils.
Executive summary:

According to the results, the pH of the slurry was measured before and after contact with the test substance which was basically kept at the same level with little fluctuation. From the data, a depletion more than the standard error of the analytical method of control samples was not observed, so it was stated that the test substance was stable in the test period and hadn't adsorbed to the surface of test vessels. The preliminary study results showed that optimal soil/aqueous ratio was 1:50,the appropriate adsorption equilibrium time was 24 h.

The mass balance results showed that the mass balance ratio (MB) of #1 (Jilin black soil) and #2 (Jiangxi red soil) soils at the 24 h adsorption equilibrium were 94.0% and 95.9%,respectively, both greater than 90%,meeting the mass balance requirements. So it was only necessary to analyse the concentration of the test substance in the aqueous phase in the subsequent kinetic test and isotherm test.

The adsorption coefficient (Kd) values of Neopentylglycol diacetate in #1 (Jilin black soil), B #2 (Jiangxi red soil), #3 (Jiangsu paddy soil), #4 (Shandong yellow soil), #5 (Gansu meadow soil) are 43.5, 27.0, 60.4, 24.0 and 44.5 cm3/g, respectively. The determined Koc values of are 664, 1988, 1126, 1724 and 650 cm3/g in above soils respectively. The average Kocvalue was determined as 1230cm3/g.

The test substance adsorption isotherms preferably followed Freundlich adsorption equation, correlative coefficients all are more than 0.9. The Freundlich adsorption coefficient (Kf)values in above five soils are27.2. 2.98, 7.29, 1.16and9.93 µg^-1/n (cm^3)^1/n / g respectively. The 1/n value for different soils are 1.14, 0.840 , 0.908, 1.25 and 0.941.

During the desorption equilibrium time of 48 h, the test substance was not detected in the supernatant of #1 (Jilin black soil), #2 (Jiangxi red soil), #3 (Jiangsu paddy soil), #4 (Shandong yellow soil) and #5 (Gansu meadow soil). Desorption isotherm test shows: the test substance doesn't desorb significantly in the five soils, and the desorption isotherm doesn't conform to the Freundlich desorption equation.

Description of key information

CAS 13431-57-7 was found to adsorb to the 5 tests soils. The test substance did not desorb significantly from the 5 soils.

Key value for chemical safety assessment

Koc at 20 °C:
1 230

Additional information