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Adsorption / desorption

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Description of key information

N,N,N',N',N''-Pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride (diquat C16-18) is mainly governed by ionic interaction and to much lower extent to hydrophobic interaction (Koc).

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 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.

Key value for chemical safety assessment

Koc at 20 °C:
1 000 000

Other adsorption coefficients

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

Other adsorption coefficients

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

Other adsorption coefficients

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

Additional information

Due to the cationic surface-active properties will branched diquat 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,N,N',N',N''-Pentamethyl-N-C16-18 (even numbered) C18 unsat.-alkyl-1,3-propanediammonium chloride was studied in a batch equilibrium experiment according to a refined OECD 106 (Farnback, 2010). 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), 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.












Org C















Eurosoil 4


















From the data it can be observed that sorption onto Speyer 6S is much higher than to Speyer 2.2 despite of the higher organic matter content in 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 diquat C16 -18 can be taken as a worst-case for other diquats with equal or shorter alkyl chain lengths. 

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 (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 5 to 17 µg/L.

For the prediction of the partitioning of the diquat C16-18 in soil, sediment and suspended matter 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. Research sponsored by APAG CEFIC performed at UFZ (K.U. Goss, S. Droge) and IRAS (Y Chen, J. Hermens) to improve the knowledge on bioavailability and partitioning to soil and sediment support these conclusions.

The Kd values observed is for both Speyer 6S and Eurosoil 4 fall outside the by EUSES advised maximum range of 5 * 104 L/kg (or Kocof 1 * 106 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 diquat C16-18.

Because there is besides to the organic carbon content 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. In the table below the distribution constants used in this assessment is summarized:

Distribution constants to be used for branched diquat 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%.