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Environmental distribution

Primary alkyl amines are surface-active substances. They consist of a hydrocarbon part which on its own would be poorly soluble in water, and a hydrophilic amino group. The substances are strong organic bases with pKa values for C8 – C18 amines around 10.6. Under environmental conditions the main fraction will exist as alkyl ammonium ions. Salts with anorganic acids like sulfate, phosphate or silicate are poorly or not soluble and are not surface-active in water. Salts with organic acids like acetic acid, benzoic acid, benzoesulfinic acid and others are more soluble (Hoechst AG, 1980b).

Adsorption/desorption

Due to the surface-active properties, long-chained alkyl amines adsorb strongly onto the solid phase of soil and sediments. The 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(Hoechst AG, 1980b).

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.

Slangen (2000) studied the adsorption behaviour of 1-14C-labelled n-octadecylamine in a batch equilibrium experiment according OECD 106. Two soils collected in UK (Cranfield 164 soil, 21.8% clay, 6.6% organic matter, silt loam; Cranfield 266 soil, 50.2% clay, 2.6% organic matter, clay), one sediment collected in The Netherlands (18.7% clay, 4.1% organic matter, silt loam) and a sewage sludge (45.9% clay, 51.9% organic matter, silty clay) were used, encompassing a range of % clay and organic material. 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: for Cranfield 164 soil 24.4% desorption after 47 hours and 24.2% after 166 hours were determined, while desorption for Cranfield 266 soil was 13.7% after 47 hours and 19.1% after 166 hours. The Freundlich adsorption isotherms were determined to:

 

Table:Freundlich adsorption isotherms determined by Slangen (2000):

Compartment

KFAds

(μg1-1/n(cm3)1/ng-1)

1/n

Soil: Cranfield 164 silt loam

3065

1.5384

Soil: Cranfield 266 clay

30053

1.8897

Sediment: Oostvaardersplassen silt loam

6433

1.4478

Sewage sludge: DB1 silty clay

821

1.0322

 

 

Apparently, the sorption onto Cranfield 266 soil is much higher than to Cranfield 164 despite of the higher organic matter content in Cranfield 164 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 low, and the experimental results obtained in the test with octadecyl amine can be taken as representative for the other products. As well, an influence of the double bond (in octadecenylamine) onto sorption is not expected.

The adsorption isotherms determined by Slangen (2000) are non-linear. The distribution constants for soils and sediment decrease dramatically as the concentrations decrease. The lowest aquatic equilibrium concentration in the experiment (5 µg/l) is more than one order of magnitude higher than the calculated PEC values. For example, with the isotherm determined for the sediment and an aquatic concentration of 10 ng/l, a Kp value of 37 l/kg is calculated, which is far below the constants determined in the experiment (707 – 3140 l/kg). Apparently, extrapolation to low concentrations would lead to unrealistic results.

According to the Danish EPA (2004) a more reliable method of extrapolation is to use the data originating from the lowest measured concentrations and to assume that the coefficient remains constant at lower concentrations. At the 2 lowest concentrations, values of 707 and 687 l/kg were experimentally determined, the mean value (697 l/kg) is used for the exposure assessment.

The mean values for the two soils are 252 and 342 l/kg, respectively. Because there is no principal difference between soil and sediments on respect to the sorption properties, as a worst case approach the value for sediment is also used for soils and suspended particles.

For the adsorption onto sludge, values of 687 and 674 l/kg were determined for the 2 lowest concentrations. The mean value (680 l/kg) is used for the exposure calculation.

 

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

 

                                           Table:Distribution constants for primary alkyl amines

Kpsoil

697 l.kg-1

Ksoil-water

1050 m3.m-3

Kpsusp

697 l.kg-1

Ksusp-water

175 m3.m-3

Kpsed

697 l.kg-1

Ksed-water

349 m3.m-3

Kpsludge

680 l.kg-1

 

 

 

With a Kpsuspof 697 l/kg and a concentration of 15 mg/l suspended matter in surface waters, the adsorbed fraction is calculated as 1.0%.

Volatilisation:

Considering the low calculated values for protonated Primary alkyl amines, volatilisation at environmental conditions is expected to be negligible. For the model calculations, a mean value of 0.01Pa.m3/mol is used.

As primary alkyl amines are not released into air in relevant amounts, volatilisation from surface waters is not expected, and degradation half-lives in the atmosphere are relatively short (about 7.5 h), the compounds are not expected to occur in precipitation.