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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Non-linear sorption has been observed in standard sorption desorption tests with cationic surfactants. Extrapolation based on these non-linear Freundlich isotherms to environmental representative concentration gives frequently unrealistic predictions. A Danish EPA report comes to the conclusion that for these substances the most reliable method of extrapolation is to use the data originating from the lowest aqueous measured concentrations letting the intercept equal zero. Using this approach the observed Kd values for tetramine C16-18 for loamy sand, silt and clay soil are resp. 27000, 52000 and 220000 L/kg. These values are clearly outside the advised range for risk assessment purposes. Normalisation of the adsorption coefficients based on the organic carbon content of the sorbent is not justified considering the properties of the test substance and will lead to erroneous predictions.

Key value for chemical safety assessment

Other adsorption coefficients

Type:
log Kp (solids-water in suspended matter)
Value in L/kg:
5
at the temperature of:
21 °C

Other adsorption coefficients

Type:
log Kp (solids-water in sediment)
Value in L/kg:
4.699
at the temperature of:
21 °C

Other adsorption coefficients

Type:
log Kp (solids-water in soil)
Value in L/kg:
4.699
at the temperature of:
21 °C

Additional information

Due to the cationic surface-active properties will Triamine C16-18 and tetramine C16 -18 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. Research (K. U. Goss, S. Droge, Y. Chen & J. Hermens) has shown that cationic surfactants partitioning to soil and sediment is mainly based on cation exchange. The clay, silt and organic matter fraction are the main fractions in soil and sediment contributing to the cation exchange capacity. Correlating partitioning of cationic surfactants to the organic carbon content alone will therefore not allow a reliable prediction of the partitioning in the environment. The non-normalized adsorption coefficients are therefore used for risk assessment.

The adsorption behaviour of N-(3-aminopropyl)-N’-[3-(C16-18 (evennumbered), C18 unsaturated alkyl amino)propyl]propane-1,3-diamine which is an alkyl tripropylene tetramines and which is identical to Triamine C16-18 which is an alkyl dipropylene triamine but contains one propyleneamine more and is expected to adsorb as strong as the triamine, was studied in a batch equilibrium experiment according to a refined OECD 106 (Lundgren, 2009). Three soils were used a loamy sand a silt and a clay soil, 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 48 h), an equilibrium was reached after 6 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.

 

Soil properties and related soil partitioning constants:

Test system

Texture

% OC

pH

% Clay

Kd

(104cm3/g)

Koc

(106cm3/g)

Speyer 2.2

Loamy sand

2.16

5.4

6.4

2.7

1.1

Speyer 6S

Clay

1.75

7.2

42.1

22

12

Euro Soil 4

Silt

1.31

6.8

20.3

5.2

4.0

 

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 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 tetramine C16-18 can be taken as a worst-case for other tetramines and triamines with equal or shorter alkyl chain lengths. This is supported by the table below where the adsorption/desorption equilibrium constants for the four main reaction products in tetramine C16-18 is presented. 

 


Tab. 4.2.2:Results for adsorption/desorption parameters for individual components

Test system

Component

Kd

(104cm3/g)

Koc*

(106cm3/g)

Kdes

(104cm3/g)

Speyer2.2

C16 tetra

2.5

1.1

4.5

C16 tri

4.2

1.8

7.0

C18' tetra

1.9

0.81

3.6

C18' tri

3.7

1.6

5.7

Speyer6S

C16 tetra

17

9.0

24

C16 tri

24

13

-a

C18' tetra

18

9.9

27

C18' tri

23

12

-a

Eurosoil 4

C16 tetra

4.2

3.2

4.9

C16 tri

9.8

7.4

16

C18' tetra

3.9

3.0

4.6

C18' tri

7.3

5.5

11

*Koc reported but should not be used!!!

aNo analyte detected in water phase during analysis

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 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 amines in the test substance will to a large extent be protonated under ambient conditions and will therefore 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 risly 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 (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. 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 10.7 mg/L. The observed aquatic equilibrium concentrations in the experiment range from 18 to 55 µg/L.

For the prediction of the partitioning of the Triamine C16-18 in soil, sediment and suspended matter as indicated before not the Kd based on organic matter will used but the uncorrected Kd because the relation between the organic matter concentration and the sorption observed alone is not sufficient.

The Kd values observed is for both Speyer 6S and Eurosoil 4 outside the by EUSES advised maximum range of 5 * 104 L/kg for sediment and soil and therefore this maximum value will be used as a realistic worst-case to derive the distribution constants for the Triamine C16-18. 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 only related to the organic carbon content, the value for soil will also be used for sediment and suspended particles. Suspended sediment contains more smaller particles and the Kd (=Kp) for suspended sediment is therefore for precautionary reasons doubled. 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. 10% removal.

 

In table 4.2.2, the distribution constants used in this assessment is summarized:

Tab. 4.2.2:Distribution constants for Triamine C16-18

Kpsoil

50000 L/kg

Ksoil-water

75000 m3/m-3

Kpsusp

100000 L/kg

Ksusp-water

25000 m3.m-3

Kpsed

50000 L/kg

Ksed-water

25000 m3.m-3

 

With a Kpsuspof 100000 L/kg and a concentration of 15 mg/L suspended matter in surface waters, the adsorbed fraction is calculated as 43%.