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Adsorption / desorption

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Description of key information

The observed Kd values 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 and therefore the maximum of the advised range of 50000 L/kg is used.

Branched triamine C16-18 is a strong base with a pKa for the first amine of >9. This means that it will be protonated under environmental conditions.

Sorpion of the branched triamine C16-18 is therefore mainly governed by ionic interaction and to a much lower extent to hydrophobic interaction.

This means that there will be 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. Despite of that mainly for practical reasons a Koc is calculated from this Kd. This Koc can be used to predict the sorption in other compartments than soil and sediment. A Kd of 50000 L/kg corresponds with a Koc of 1000000 L/kg.

Key value for chemical safety assessment

Koc at 20 °C:
1 000 000

Additional information

Two sorption tests are available one with a branched triamine C12 and one with a linear tetramine C16-18. For the branched triamine C12 a Kd is used of 4083 L/kg and for the linear tetramine C16 -18 a Kd of 50000 L/kg. It is expected that the Kd of the branched triamine C16 -18 will be somewhere in between these two observed values but as a worst-case the sorption as observed for the linear tetramine C16 -18 will be used for read across. The results of this test are therefore described in more detail.

Due to the cationic surface-active properties will branched Triamine 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, 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 is not valid for both ionic and surface active substances.

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 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.


Tabel 12 a: Soil properties and related soil partitioning constants.


Test system


% OC


% Clay

Kd (104cm3/g)

Koc (106cm3/g)


Loamy sand






Speyer 6S







Euro Soil 4








From the data it can be observed that the sorption onto6S is much higher than to2.2 despite of the higher organic matter content in2.2 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. 

Table 12b: Results for adsorption/desorption parameters for individual components


Test system









C16 tetra




C16 tri




C18' tetra




C18' tri





C16 tetra




C16 tri




C18' tetra




C18' tri




Eurosoil 4

C16 tetra




C16 tri




C18' tetra




C18' tri




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 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 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 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( 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 Tri- and tetramine C16-18 in soil, sediment and suspended matter not the Kdbased on organic matter will used but the uncorrected Kdbecause 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,) and IRAS (J. Hermens) to improve the knowledge on bioavailability and partitioning of cationic surfactants to soil and sediment.

The Kdvalues observed is for both Speyer 6S and Eurosoil 4 outside the advised maximum range of 5 * 104L/kg (or Kocof 1 * 106L/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 tri- and tetramine 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 related to the organic carbon content, the value for soil will also be used for sediment and suspended particles. For sludge which consists for a large extent 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 12C, the distribution constants used in this assessment is summarized:

Table 12C: Distribution constants for Triamine C16-18





75000 m3.m-3




25000 m3.m-3




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%.