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Environmental fate & pathways

Adsorption / desorption

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Description of key information

A refined sorption/desorption test according to OECD 106 with 2,2’-(Octadec-9-enylimino) -bisethanol (CAS No 25307-17-9) resulted in equilibrium constants (Kd's) of 2025, 4526 and 4639 L/kg for loamy sand, silt and clay soil. As there is no direct relationship with the sorption behaviour of the substance and the organic carbon content of the soil because other soil properties like the Cation Exchange Capacity and the pH are maybe even more important to predict the sorption behaviour, no Koc's are given.
Despite of that mainly for practical reasons a Koc is calculated from this Kd applying the non-hydrophobics QSAR according to the (TGD, 2003). This Koc can be used to predict the sorption in other compartments than soil and sediment.

Key value for chemical safety assessment

Koc at 20 °C:
90 520

Other adsorption coefficients

Type:
log Kp (solids-water in soil)
Value in L/kg:
4 526
at the temperature of:
20 °C

Other adsorption coefficients

Type:
log Kp (solids-water in sediment)
Value in L/kg:
4 526
at the temperature of:
20 °C

Other adsorption coefficients

Type:
log Kp (solids-water in suspended matter)
Value in L/kg:
9 052
at the temperature of:
20 °C

Other adsorption coefficients

Type:
log Kp (solids-water in raw sewage sludge)
Value in L/kg:
5.2
at the temperature of:
20 °C

Other adsorption coefficients

Type:
log Kp (solids-water in settled sewage sludge)
Value in L/kg:
5.2
at the temperature of:
20 °C

Other adsorption coefficients

Type:
log Kp (solids-water in activated sewage sludge)
Value in L/kg:
5.2
at the temperature of:
20 °C

Other adsorption coefficients

Type:
log Kp (solids-water in effluent sewage sludge)
Value in L/kg:
5.2
at the temperature of:
20 °C

Additional information

Due to the cationic surface-active properties will primary fatty amine ethoxylates adsorb strongly onto the solid phase of soil and sediments. These substances can adsorb both onto the organic fraction and, dependent on the chemical composition, onto the surface of the mineral phase, where sodium and potassium ions can be exchanged against the alkyl ammonium ion. The determination of a Koc from log Kow is not opportune, because the common equations for Koc derivation are not valid for both ionic and surface active substances.

The adsorption behaviour of primary fatty amine ethoxylates was assessed using 2,2’-(Octadec-9-enylimino) -bisethanol (CAS No 25307-17-9) as a worst-case representative of this category. The adsorption/desorption was studied in a batch equilibrium experiment according to a refined OECD 106 (Farnback, 2010). Three soils were used, encompassing a range of clay and organic matter. The test substance adsorbed partially onto the container walls which was considered for the determination of the adsorption coefficients. Adsorption kinetics was determined by measurements at different sampling times (up to 24 h), an equilibrium was reached after 3 hours. Desorption occurred to a lesser extent than adsorption. The table below presents a summary of the most important soil properties and observed partitioning constants.

Table1. Overview of soil properties of soils used for the sorption test and distribution coefficients observed for 2,2’-(Octadec-9-enylimino) -bisethanol (CAS No 25307-17-9)

Soil

Clay

(%)

Silt

(%)

Sand

(%)

CEC

(meq/100g)

pH

Org C

(%)

Caq

(µg/l)

Kd

(103cm3/g)

Koc

(104cm3/g)

Speyer2.2

6.4

12.2

81.4

10

5.4

2.16

107

2.0

8.6

Eurosoil 4

20.3

75.7

4.1

17.3

6.8

1.31

43

4.5

34

Speyer6S

42.1

36.0

21.9

22

7.2

1.75

62

4.6

25

From the results it can be observed that the sorption onto Speyer 6S (clay soil) is much higher than to Speyer 2.2 (loamy sand) despite of the higher organic matter content in the Speyer 2.2 soil. This can be explained that ionic interactions play a more important role than hydrophobic partitioning with organic matter. Alkyl ammonium ions can interact with the surface of mineral particles or with negative charges of humic substances. The influence of the chain length on the sorption behaviour is therefore expected to be less important and the experimental results obtained in the test with 2,2’-(Octadec-9-enylimino) -bisethanol can therefore be taken as a worst-case for other ethoxylated amines with equal or shorter alkyl chain lengths.

The number of soils which was used in this test deviates from the recommendation in OECD guideline 106 (2000) in that three soils were used instead of the recommended five soils. In addition is the partitioning to soil is not based on a Freundlich isotherm but evaluated based on only one test concentration. These deviations are based on results of earlier adsorption desorption tests with cationic surfactants. The ammonium ions will interact with the negative surface of mineral particles or with negative charges of humic substances. The ionic interactions play a more important role than hydrophobic partitioning with organic matter. The log Koc (alone) is therefore considered as a poor predictor of the partitioning behaviour of cationic surfactants in the environment. These earlier results showed that using three soils with at least one loamy sand and a clay soil, can give as much information as using the full number of soils. These earlier tests also revealed that only rarely linear adsorption isotherms were obtained for cationic surfactants and that extrapolation to lower concentrations based on these non-linear isotherms leads to unrealistic results (e. g. RAR primary fatty amines Oct. 2008). According to the Danish EPA (http: //www. mst. dk/udgiv/publications/2004/87-7614-251 -5/html/appd_eng. htm) a more reliable method of extrapolation to lower concentrations, is to use the data originating from the lowest measured concentration and to assume that the coefficient remains constant at lower concentrations. The test as described is therefore performed using only one concentration which is as low as reasonably possible in relation to the detection limit.

The initial concentration used for the determination of the soil partitioning constant was 1 mg/L. The observed aquatic equilibrium concentrations in the experiment range from 20 to 138 µg/L.

For the prediction of the partitioning of the 2,2’-(Octadec-9-enylimino) –bisethanol in soil, sediment and suspended matter not the Kdbased on organic matter (Koc) will used but the uncorrected Kdbecause the relation between the organic matter concentration alone and the sorption observed is not sufficient. Research sponsored by APAG CEFIC is currently performed on these difficult substances at UFZ (K. U. Goss, S. Droge) and IRAS (J. Hermens) to improve the knowledge on bioavailability and partitioning to soil and sediment.

Because there is no principal difference between soil and sediments considering the sorption properties and because for cationic surfactants the degree of sorption is not directly related to the organic carbon content, the Kd value for soil will also be used for sediment and suspended particles. For sludge which consists mainly of organic matter the sorption data as observed for soil is not considered to be representative. This is however not a serious problem because the removal by sorption in a waste water treatment plant will be close to what is observed in the waste water treatment simulation test i. e. 0.16% removal.

Despite of that mainly for practical reasons (e. g. in the exposure models) a Koc is calculated from this Kd applying the non-hydrophobics QSAR according to the (TGD, 2003). This Koc of 90520 L/kg can be used to predict the sorption in other compartments than soil and sediment.

In the table below the distribution constants used in this assessment is summarized:

Distribution constants to be used for 2,2'-(octadec-9-enylimino) bisethanol

Kpsoil

4526 L.kg-1

Ksoil-water

6792 m3.m-3

Kpsusp

9052 L.kg-1

Ksusp-water

2264 m3.m-3

Kpsed

4526 L.kg-1

Ksed-water

2264 m3.m-3

 

With a Kpsuspof 9052 L/kg and a concentration of 15 mg/L suspended matter in surface waters, the adsorbed fraction is calculated as 12%.