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Reference
Endpoint:
adsorption / desorption: screening
Type of information:
calculation (if not (Q)SAR)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
accepted calculation method
Justification for type of information:
Log Koc of the test substance was calculated using regression model equations for general, ester class and ionisable compounds. See below under 'methods' for applicability domain.
Principles of method if other than guideline:
The soil adsorption coefficient (Koc) value for the test substance was estimated using the log Pow (Partition coefficient) correlation approach of the Log Koc regression models equations for general, ester class and ionisable compounds.
Key result
Type:
log Koc
Value:
ca. 1.99 dimensionless
Remarks on result:
other: Koc: 97.72 L/kg; calculated using log Kow based regression equation for ionisable compounds
Key result
Type:
log Koc
Value:
>= 2.36 - <= 2.37 dimensionless
Remarks on result:
other: Koc: 229.09-234.42 L/kg; calculated using 'ester class' specific log Kow based regression equations
Key result
Type:
log Koc
Value:
>= 2.29 - <= 2.85 dimensionless
Remarks on result:
other: Koc: 194.98-707.95 L/kg; calculated using 'general' log Kow based regression equations

Results

Koc was calculated using range of regression equations, i.e., ester class specific, general wide variety and ionisable coumpound specific. The test substance is a phosphate ester, therefore the equations related to ester class have been selected as one the criteria to generate the Koc value for the test substance. Apart from chemical class specific equations, general equations also have selected due to their well documented development and large data sets of Koc values. As the test substance is ionisable substance, the regression equation for ionisable compound also used to calculate the Koc. Prediction of log Koc can be improved by treating neutral and ionic fractions separately and therefore probably is superior to methods that merge both fractions without considering the differences between neutral compounds and ions. pKa values of the PSEs are expected to be between 1.5 and 3, the mono-esters will have lower pKas (i.e. higher acidity), the di-esters higher ones. The interval with a maximum фn = 0.1 and a minimum фion = 0.9 is therefore likely to comprise all PSEs having an acidic OH-Group (mono- and di-esters).

Table 1: Calculations of Koc based on regression models equations (General and Ionisable Compound)

Regression Models Used to Estimate Log Koc from Log Kow

 

Ǿneutral fraction

0.1

 

Ǿionic fraction

0.9

Equation Number

Log Kow

 

2.7

 

(I)

EPISuite (Doucette, 2000)

Log Koc = 0.8679 Log Kow - 0.0004

 

Log Koc

2.34

(II)

Variety, mostly pesticides (Kenaga and Goring, 1980)

log Koc = = 1.377 + 0.544 log Kow

 

Log Koc

2.85

(III)

Ester Class specific (Sabljic et al 1995)

log Koc = 0.47 log Kow + 1.09

 

Log Koc

2.36

(IV)

Wide variety (Gerstl, 1990)

log Koc = 0.679 log Kow + 0.663

 

Log Koc

2.50

(V)

Hydrophobics (Sabljic et al 1995)

log Koc = 0.81 log Kow + 0.10

 

Log Koc

2.29

(VI)

Wide variety (Baker et al 1997)

log Koc = 0.903 log Kow + 0.094

 

Log Koc

2.53

(VII)

Franco and Trapp (2008)

Log Koc = Log (Ǿn*10^(0.54 log Kow + 1.11) + Ǿion*10^(0.11 log Kow + 1.54))

 

Log Koc

1.99

(VIII)

Esters class specific (EC, 2003)

Log Koc = 0.49 log Kow + 1.05

 

Log Koc

2.37

Franco and Trapp 2008

Equa. (VII)

1.99

Average of all log Koc values

Equa. (I) + (II) + (III) + (IV) + (V) + (VI) + (VIII)

2.46

Selected Log Koc value

 

1.99

 

KOC

97.72

As the test substance is a weak acid with ionisable property, the Koc value of 97.72 L/kg (Log Koc value of 1.99) calculated from Franco and Trapp (2008) equation has been selected as key value for this endpoint.

Validity criteria fulfilled:
not applicable
Conclusions:
Using the Log Koc regression models equations (general and ionisable compounds) based on partition coefficient value, the estimated Koc of the test substance was determined to be 97.72 L/kg (i.e., equivalent to log Koc 1.99).

Executive summary:

The soil adsorption coefficient (Koc) value for the test substance, ' mono- and di- C16 PSE, K+ and C16-OH and isostearyl isostearate', was determined using the well-known log Kow based log Koc regression models equations. To calculate a more reliable value and to reduce the overall uncertainty, multiple equations, which could be categorised as general, class-specific (i.e., ester) (Doucette WJ, 2000) and ionisable compound based (Franco and Trapp, 2008), were used for the calculations. The log Koc was calculated from the equations using the log Kow value of 2.7 determined for the test substance (based on individual solubility ratio) and a maximum фn of 0.1 and a minimum фion of 0.9, for the Franco et al., equation. The log Koc values were calculated to range from 2.29 to 2.85, using general equations, 2.36 to 2.37, using ‘ester class’ specific equations, and was 1.99 using the ionisable compound based equation. This range of Koc indicates low to moderate sorption to soil / sediment and moderate to slow migration potential to ground water (US EPA, 2012). Given that the test substance is ionic, the prediction of log Koc by treating neutral and ionic fractions separately is considered superior to methods that merge both fractions without considering the differences between neutral compounds and ions (Franco and Trap, 2008). Therefore, the log Koc of 1.99 (i.e., equivalent to Koc of 97.72 L/kg) calculated from Franco and Trapp (2008) equation has been selected as key value for this endpoint.

Description of key information

The log Koc of 1.99 (i.e., equivalent to Koc of 97.72 L/kg) calculated from Franco and Trapp (2008) equation has been selected as key value for soil adsorption endpoint.

Key value for chemical safety assessment

Koc at 20 °C:
97.72

Additional information

Given the limitation of the publicly available QSAR models for log Koc estimation of ionic compounds, the endpoint has been assessed using log Kow based log Koc regression equations proposed for ionisable compounds, along with other general and class specific equations, as a comparison.

The soil adsorption coefficient (Koc) value for the test substance, ' mono- and di- C16 PSE, K+ and C16-OH and isostearyl isostearate', was determined using the well-known log Kow based log Koc regression models equations. To calculate a more reliable value and to reduce the overall uncertainty, multiple equations, which could be categorised as general, class-specific (i.e., ester) (Doucette WJ, 2000) and ionisable compound based (Franco and Trapp, 2008), were used for the calculations. The log Koc was calculated from the equations using the log Kow value of 2.7 determined for the test substance (based on individual solubility ratio) and a maximum фn of 0.1 and a minimum фion of 0.9, for the Franco et al., equation. The log Koc values were calculated to range from 2.29 to 2.85, using general equations, 2.36 to 2.37, using ‘ester class’ specific equations, and was 1.99 using the ionisable compound based equation. This range of Koc indicates low to moderate sorption to soil / sediment and moderate to slow migration potential to ground water (US EPA, 2012). Given that the test substance is ionic, the prediction of log Koc by treating neutral and ionic fractions separately is considered superior to methods that merge both fractions without considering the differences between neutral compounds and ions (Franco and Trap, 2008). Therefore, the log Koc of 1.99 (i.e., equivalent to Koc of 97.72 L/kg) calculated from Franco and Trapp (2008) equation has been selected as key value for this endpoint.

Possible processes behind the sorption of organic chemicals to soil and sediment are ion bonding or ligand exchange, chemiosorption (formation of a bond, usually covalent, with the soil molecular structure), ion–dipole and dipole–dipole interactions, charge transfer, hydrogen bonding, and hydrophobic bonding (Van der Waals forces). The most chemically active component of the soil is the colloidal fraction, which consists of organic matter and inorganic clay minerals. Both components display a negative electrical charge at the surface. The effect of this charge can be measured by the cationic exchange capacity, which on average is 50 meq/100 g for clays and 290 meq/100 g for humic acids. Electrical forces involving charge transfer (40 kJ/mol) are stronger than hydrophobic bonding (4 kJ/mol) so that they dominate when present. Thus, a different degree of sorption of anions, cations, and neutral molecules can be expected, with cations showing the highest potential for sorption, due to electrical attraction (Franco and Trapp, 2008).

Therefore, considering that the test substance is an anionic surfactant, its sorption potential can be expected to be much lesser than other known cationic surfactants, which is in line with the calculated log Koc derived based on Franco and Trapp, 2008 proposed equation for ionisable compounds.

[LogKoc: 1.99]