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Adsorption / desorption

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Description of key information

Refined sorption/desorption tests according to OECD 106 have been performed with DETA based tall oil imidazoline. The test resulted in equilibrium constants (Kd's) of 47249, 19022 and 145295 L/kg for loamy sand, silt and clay soil for the DETA based imidazoline. As risk assessment purposes sorption the Kd of loamy sand (Speyer 2.2) for DETA of 47249 L/kg will be used as realistic worst-case

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. Because there is no direct relationship of the sorption with the organic matter content in the soil, there is no principal difference between soil and sediments on respect to the sorption properties. Therefore the same sorption Kd is considered to be acceptable for both soil and sediment.

Key value for chemical safety assessment

Koc at 20 °C:
944 980

Other adsorption coefficients

Type:
log Kp (soil-water)
Value in L/kg:
4.674
at the temperature of:
20 °C

Other adsorption coefficients

Type:
log Kp (sediment-water)
Value in L/kg:
4.674
at the temperature of:
20 °C

Other adsorption coefficients

Type:
log Kp (suspended matter-water)
Value in L/kg:
4.975
at the temperature of:
20 °C

Additional information

Due to the cationic surface-active properties of tall oil poly ethyleneamine based imidazolines, these substances will adsorb strongly onto the solid phase of soil and sediments. The substance 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 DETA based tall oil poly ethyleneamine imidazolines was therefore studied in batch equilibrium experiments according to a refined OECD 106 (Farnback, 2010). In this study 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), equilibrium was reached after 24 hours. Only a limited difference was observed between the results of 3 and 24 hours. Desorption occurred to a lesser extent than adsorption. The tables below present a summary of the most important soil properties and observed partitioning constants for both imidazolines.

Overview of sorption test results of Tall oil diethylenetriamine imidazoline (CAS No 68442 -97 -7)

Soil

Clay

(%)

Silt

(%)

Sand

(%)

CEC

(meq/100g)

pH

Org C

(%)

Caq

(µg/l)

Kd

(104cm3/g)

Koc

(106cm3/g)

Speyer2.2

6.4

12.2

81.4

10

5.4

2.16

4.2

4.7249

2.0

Eurosoil 4

20.3

75.7

4.1

17.3

6.8

1.31

10

1.9022

1.4

Speyer6S

42.1

36.0

21.9

22

7.2

1.75

1.6

14.295

7.8

 

From the data it can be observed that the sorption onto Speyer 6S is much higher than to Speyer 2.2 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. Only for the hydrophobic interaction with the organic matter in the soil or sediment some influence of the alkylchain length is anticipated. 

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 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. According to the Danish EPA (2004) 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 (e.g. RAR primary fatty amines Oct. 2008)

. 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 10.2 mg/L (1.9 mg/L for the individual components). The observed aquatic equilibrium concentrations in the experiment range from 1.6 to 10 µg/L. For the prediction of the partitioning of the alkyl polyethylene imidazolines

in soil, sediment and suspended matter not the Kd based on organic matter (Koc) will used but the uncorrected Kd because the relation between the organic matter concentration and the sorption observed alone is not sufficient. Research sponsored by APAG CEFIC is currently performed 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 related to the organic carbon content, the 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. Here the removal via adsorption in the OECD 303A can be used.

Despite of the fact that sorption is poorly predicted when corrected for organic matter content it was decided, mainly for practical reasons (e.g. in the exposure models) to derive a Koc is from the Kd applying the non-hydrophobics QSAR according to the (TGD, 2003). This Koc of 944980 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

Kpsoil

47000 L.kg-1

Ksoil-water

28200 m3.m-3

Kpsusp

94000 L.kg-1

Ksusp-water

23500 m3.m-3

Kpsed

47000 L.kg-1

Ksed-water

23500 m3.m-3

 

With a Kpsuspof 94498 L/kg and a concentration of 15 mg/L suspended matter in surface waters, the adsorbed fraction is calculated as 59%.

The relevant emission factors to water and STP sludge as determined in the CAS test have been used in the assessment:

In STP

(From CAS test)

Fstp.water 0.001

Fstp.sludge 0.0004

Fstp.degr 0.9995

Removal from water >99.999%

USES 2.1.2 as incorporated in Chesar was used for the environmental assessment. Due to the restricted possibilities in Chesar, in some cases the modelling results do not accurately reflect the actual situation and where possible some adjustments were made.

A waste water treatment simulation test (CAS test) is available. This test shows a removal of 99.999% is achieved in a waste water treatment plant. This removal is mainly due to biodegradation (99.95 %) and additionally a small percentage (0.04%) goes to sludge. The Kp values for solids-water in raw, settled, activated and effluent sewage sludge were adjusted in such a way that the results for removal to sludge in the STP as modelled by EUSES (and therefore Chesar) match the test results as found in the CAS test.

[LogKoc: 5.98]