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Environmental fate & pathways

Adsorption / desorption

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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:
equivalent or similar to guideline
Guideline:
OECD Guideline 106 (Adsorption - Desorption Using a Batch Equilibrium Method)
Principles of method if other than guideline:
Method: other (measurement): modified OECD-Guideline 106
GLP compliance:
no
Type of method:
batch equilibrium method
Media:
soil
Radiolabelling:
no
Analytical monitoring:
yes
Details on matrix:
PROPERTIES
1.- Soil texture: Prodzol
- % sand: 89.2
- % silt: 8.2
- % clay: 2.6
- pH: 2.8
- Organic carbon (%): 4.85
- CEC (meq/100 g): 15.1
- Carbonate as CaCO3:0
2.- Soil texture: Alfisol
- % sand: 69.7
- % silt: 14.4
- % clay: 15.9
- pH: 6.7
- Organic carbon (%): 1.25
- CEC (meq/100 g): 12.3
- Carbonate as CaCO3:0.4
3.- Soil texture: Sediment
- % sand: 5.5
- % silt: 58.8
- % clay: 35.7
- pH: 7.1
- Organic carbon (%): 1.58
- CEC (meq/100 g): 13.4
- Carbonate as CaCO3:38.9
Type:
Koc
Value:
>= 2 - <= 31
 The sorption equilibrium was reached within 16 hours.
 The mass balance  resulted in a recovery of > 80 %.

Sorption coefficient Koc determined for different soils:

- Podzol : 31
- Alfisol (agricultural soil): 2
- sediment (Lake Constance) : 2

Executive summary:

Von Oepen (1991) investigated the sorption capacity of three different soils by batch equilibrium studies for phthalic acid similar to the OECD TG 106 . The soils used for testing were an acidic forest soil, an agricultural soil and a sublimnic soil. The sorption equilibrium was reached within 16 hours. Sorption coefficients between 2 and 31 were determined.

Endpoint:
adsorption / desorption: screening
Type of information:
(Q)SAR
Adequacy of study:
supporting study
Study period:
2017
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Justification for type of information:
QSAR calculation: detailed information see "any other information on materials and methods" and "any other information on results"
Guideline:
other: REACH guidance QSARs R6, May/July 2008
Principles of method if other than guideline:
Estimation Program Interface EPI-Suite version 4.11: KOCWIN (v2.00) for the estimation of the organic carbon-normalized sorption coefficient for soil and sediment (Koc).
The Estimation Program Interface was developed by the US Environmental Agency's Office of Pollution Prevention and Toxics, and Syracuse Research Corporation (SRC). © 2000 - 2012 U.S. Environmental Protection Agency for EPI SuiteTM (Published online in November 2012).
GLP compliance:
no
Type of method:
other: Estimation
Media:
soil
Radiolabelling:
no
Type:
log Koc
Value:
1.91 dimensionless
Remarks on result:
other: calculation (MCI method)
Type:
log Koc
Value:
0.56 dimensionless
Remarks on result:
other: calculation (logKow method)
Phase system:
other: Koc
Type:
other: Koc
Value:
80.85 L/kg
Remarks on result:
other: calculation (MCI method)
Phase system:
other: Koc
Type:
other: Koc
Value:
3.63 L/kg
Remarks on result:
other: calculation (logKow method)

Validity of the model:

1. Defined Endpoint: Organic carbon partition coefficient, given as logarithmic Koc and Koc

2. Unambigous algorithm: The molecule is first classified as a polar substance. Based on structure of the molecule, the following fragments were applied: 1 -CO-OH (Organic acid). The number of times of the fragments that occurs in the structure of the substance applied by the program is verified. For estimation of logKoc according to the logKow method the estimated logKow of 0.73 as calculated by the program was used.

3. Applicable domain: With a molecular weight of 166.13 g/mol the substance is within the range of the training set (32 - 665 g/mole) as well as in the validation set (27 - 991 g/mole). Regarding the structure of phthalic acid, the fragment descriptors found by the program are complete.

4a. Statistical characteristics (MCI method): N training set without corrections = 69; N training set with correction = 447; N validation set = 158; Correction coefficient of the total training set without corrections r² = 0.967; Correlation coefficient of the total training set with corrections r² = 0.900; Correlation coefficient of the total validation set r² = 0.850.

4b. Statistical characteristics (Kow method): N training set without corrections = 68; N training set with correction = 447; N validation set = 150; Correction coefficient of the total training set without corrections r² = 0.877; Correlation coefficient of the total training set with corrections r² = 0.855; Correlation coefficient of the total validation set r² = 0.778.

5. Mechanistic interpretation: Log Koc is estimated based on the likeliness of a substance for sorption to surfaces of soil/sediment particles. The log Koc is a physical inherent property used extensively to describe a chemical's likeliness to adsob to organic carbon.

Adequacy of prediction:

The result for phthalic acid falls within the applicability domain described above and the estimation rules applied for the substance appears appropriate.

Validity criteria fulfilled:
not applicable
Conclusions:
The QSAR determination of the carbon partition coefficient for phthalic acid using the model KOCWIN included in the Estimation Program Interface (EPI) Suite v4.11 revealed values of 3.63 L/kg (logKow method) and 80.85 L/kg (MCI method) for the unaffected molecule of the substance as any decomposition (e.g. hydrolysis) of the substance is not taken into account by the program.
Executive summary:

The organic carbon partition coefficient (Koc) for phthalic acid was predicted using the QSAR calculation of the Estimation Program Interface (EPI) Suite v 4.11. The Koc was estimated to be 3.63 L/kg (logKow method), and 80.85 L/kg (MCI method). The results relate to the unaffected molecule of the substance as any decomposition (e.g. hydrolysis) of the substance is not taken into account by the program.

Description of key information

The Koc value range between 2 - 31 suggest the substance is mobile in soil.


The measured Koc value of 2 is used to describe the adsorption potential of phthalic acid to present a worst-case for the aquatic environment.


 

Key value for chemical safety assessment

Koc at 20 °C:
2

Additional information

Von Oepen (1991) investigated the sorption capacity of three different soils by batch equilibrium studies for phthalic acid similar to the OECD TG 106 . The soils used for testing were an acidic forest soil, an agricultural soil and a sublimnic soil. The sorption equilibrium was reached within 16 hours. Sorption coefficients between 2 and 31 were determined. The experimental Koc values suggest that phthalic acid has a high mobility in soil.

Under environmental relevant pHs, phthalic acid exists predominantly in the dissociated form, based upon its pKa values. At pH 5 to 9, the substance will be deprotonated resulting in the corresponding anion. As cationic substances in particular generally adsorb strongly, an adsorption potential of the dissociated form of phthalic acid is not expected.