Amines, di-C16-18-alkyl

Adsorption / desorption

Description of key information

Refined sorption/desorption tests according to OECD 106 has been performed with dialkylamine C16-18. The test resulted in equilibrium constants (Kd's) of 2100, 14000 and 56000 L/kg for loamy sand, silt and clay soil. As risk assessment purposes sorption the Kd of silt (Eurosoil 4) of 14000 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:

280 000

Other adsorption coefficients

Type:

log Kp (solids-water in soil)

Value in L/kg:

4.146

at the temperature of:

20 °C

Other adsorption coefficients

Type:

log Kp (solids-water in sediment)

Value in L/kg:

4.146

at the temperature of:

20 °C

Other adsorption coefficients

Type:

log Kp (solids-water in suspended matter)

Value in L/kg:

4.447

at the temperature of:

20 °C

Additional information

Due to the cationic surface-active properties of dialkylamines, 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 dialkylamines C16 -18 was studied in batch equilibrium experiments according to a refined OECD 106 (Farnback, 2010). In both studies 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 6 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 dialkylamine C16 -18 (CAS No 68789 -79 -5)

Soil	Clay (%)	Silt (%)	Sand (%)	CEC (meq/100g)	pH	Org C (%)	Caq (µg/l)	Kd (10^4cm^3/g)	Koc (10^6cm^3/g)
Speyer2.2	6.4	12.2	81.4	10	5.4	2.16	4.2	0.21	0.092
Eurosoil 4	20.3	75.7	4.1	17.3	6.8	1.31	10	1.4	1.1
Speyer6S	42.1	36.0	21.9	22	7.2	1.75	1.6	5.6	3.0

 

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 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 is 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 (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 1.2 to 62 µg/L (0.16 - 28.5 µg/L for the individual substances). For the prediction of the partitioning of the dialkylamines

in soil, sediment and suspended matter not the K_d based on organic matter (Koc) will used but the uncorrected K_d 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.

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 280000 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		14000 L^.kg^-1		Ksoil-water		21000 m^3.m^-3
Kpsusp		28000 L^.kg^-1		Ksusp-water		7000 m^3.m^-3
Kpsed		14000 L^.kg^-1		Ksed-water		7000 m^3.m^-3

 

With a Kp_suspof 28000 L/kg and a concentration of 15 mg/L suspended matter in surface waters, the adsorbed fraction is calculated as 29%.