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Key value for chemical safety assessment

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Larson and Vashon (1983) is a sound reference, considered reliable and suitable for use as a key study for the assessment of adsorption for DODMAC (named DSDMAC in the article).

The adsorption of long-chain dialkyl quaternaries (QAC's) to EPA and river sediments has been determined. Equilibrium adsorption coefficients (Kd) for the three QAC's tested were decreasing in the order STAC>CTAB>DSDMAC. Adsorption coefficients for the strongest binding QAC ( STAC) to river sediments were comparable to values determined for raw wastewater solids and activated sludge, and desorption experiments indicated that strong binding of this material occurred to both organicand inorganic particulate matter. Kinetic studies also indicated that adsorption was rapid, reaching equilibrium values within a few hours. In general, the QAC's we tested had a high affinity for particulate matter and were capable of strongly binding to a variety of environmentally relevant sorbents.

DSDMAC has been shown to adsorb onto both the mineral and the organic fraction of soil and sediments. Kappeler (1982) found that on average 27% of the DSDMAC in river water is adsorbed onto suspended matter (mean 22 mg/L suspended solids). For exposure assessment purpose, a value of 10,000 L/kg dw is chosen for both Kpsed and Kpsoil. Indeed, because there is no principal difference between soil and sediments on respect to the sorption properties the same value of Kp can be used.

With an assumed Kpsusp of 10,000 l/kg and a concentration of 15 mg suspended matter per litre river water, about 87% of the DSDMAC would remain in the water phase. Kappeler (1982) found that in river water on average 27% of the DSDMAC is adsorbed onto suspended matter (average concentration 22 mg/L). From these values, the Kpsusp is calculated to 16,800 L/kg and the Log Kpsusp to 4.2. As the latter value has a better empirical basis, it is used in the exposure calculation.

For both DODMAC and DHTDMAC, the determination of a Koc from log Kow is not opportune because the common Koc derivations are not valid for both ionic and surface active substances. Moreover, the log Koc is considered as a poor predictor of the partitioning behaviour of cationic surfactants in the environment as the ionic interactions play a more important role than hydrophobic partitioning with organic matter. For the same reasons, it is not relevant to use calculated values of Koc from QSARs (e.g. EPIWIN). Therefore no Koc values have been assessed but Kp values for the exposure assessment. Since the sorption to soil and sediment of these substances is mainly governed by ionic interaction, similar sorption is expected for DODMAC and DHTDMAC. Therefore this is relevant to use the Kpsusp value determined with DODMAC.

In addition, the adsorption behaviour of DHTDMAC would be difficult to determine related to the complexity of analysis for mixtures that are clearly not encouraged by the respective guidelines, OECD 106 and OECD 121. Finally, the shortest C chain lengths of DHTDMAC are expected to have a less affinity because the adsorption is inversely proportional to alkylchain length. The value of Kpsusp determined for DODMAC (C chain length of C18) can be considered as the worst-case for the other C chains of DHTDMAC. As a conclusion DODMAC and DHTDMAC can be bound very strongly by some minerals, while in others relatively small distribution constants were estimated.

Other adsorption coefficient indicated as dimensionless:
- log Kp (suspended matter-water) ,4.22 at the temperature 20.0°C