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Read across from structurally similar Quats (DHTDMAC, DODMAC) can be applied.

ADSORPTION

In the EU Risk Assessment DODMAC the following assessment for Distribution is given:

Both DODMAC and DHTDMAC have to be considered as nearly insoluble in water (cf. section 1). However, the compounds form stable dispersions in water containing unilamellar or multi­lamellar particles such as vesicles. The size of the dispersed particles depends on temperature and the sheer forces applied when making the dispersion (e.g. by stirring or ultrasonication). Both substances can also form mixed aggregates with other substances, e.g. anionic tensides or humic substances (ECETOC, 1993).

All relevant concentrations in environment, waste water or toxicity test solutions are far above the water solubility. It is evident that in the hydrosphere DODMAC resp. DHTDMAC is not really dissolved but always adsorbed onto suspended matter or included in vesicles together with other lipophilic organics (e.g. humic acids, tensides). The water solubility is not a limiting factor for emissions into waste water or pollution of the hydrosphere.

The determination of a Koc from log Pow is not opportune, because the common Koc deriva­tions are not valid for surface active substances like DODMAC. As revealed by the following investigations, DODMAC adsorbs onto both the mineral and the organic fraction of soil and sediments.

In a test with14C-DODMAC and 3 different sediments, sediment-water partitioning coeffi­cients from 3,833 to 12,489 l/kg dw were analytically determined. The results indicate that the coefficient is more dependent on the nature of the mineral phase than on the organic carbon content. Kinetic studies indicated that adsorption was rapid, reaching equilibrium values within a few hours (Larson & Vashon, 1983).

During a test on toxic effects on sediment organisms, the14C-DODMAC concentrations were measured in sediment (organic carbon 4.2%) and in interstitial water. From these results, a distribution coefficient can be calculated. The Ksed-water values are in the range of 2,150 to 15,000 (related to dry weight and dimensionless). The Ksed-water values are increasing with rising DODMAC concentrations (Pittinger et al., 1989).

The partitioning of 14C-DODMAC between porewater and the whole sediment (collected from a gravel pit) was studied by Conrad et al. (1999). The equilibrium was reached before the first sampling time (2 days). Sediment and porewater concentrations were measured applying different DODMAC amounts, and from the regression slope the partitioning coefficient was determined to 3018 l/kg.

ECETOC (1993) cites a Kpsuspvalue of 85,000 l/kg, however it is not clear how it was derived.

Kappeler (1982; cited in 3.1.2.1) found that on average 27% of the DHTDMAC in river water is adsorbed onto suspended matter (Æ22 mg/l suspended solids). Assuming that the DODMAC distribution can be set equal to DHTDMAC, the Kpsuspis calculated to 16,800 l/kg from these values.

Hellmann (1984) examined the remobilisation of DODMAC adsorbed onto bentonite. Activa­ted bentonite (loaded with 34% DODMAC) was treated with water. The substance could not be detected in the water phase. With the detection limit, a distribution coefficient above 105 l/kg was calculated.

The distribution of DODMAC in a clay-mineral/water-methanol system was determined by Hellmann (1987). Different mixtures between 100% methanol and methanol : water 60 : 40 were used for the adsorption experiment. After equilibrium was reached, DODMAC was measured in the solutions and the distribution coefficients were calculated. The higher the water content, the higher the K value was. Extrapolated to pure water, the value was estimated to 30.106 l/kg.

McAvoy et al. (1994) examined the mobility of DHTDMAC in an aquifer containing 78% sand, 5.8% clay, 16.1% silt, and 0.26% organic carbon. The estimated sorption coefficients were relatively low, varying from 25 to 62 l/kg.

These investigations demonstrate that DODMAC can be bound very strongly by some mine­rals, while in others relatively small distribution constants were estimated. Under environmental conditions, the sorption properties of DODMAC resp. DHTDMAC probably vary in a wide range depending on the nature of the adsorbant. We assume that the sorption properties of DODMAC and DHTDMAC are nearly identical.

 

In the following exposure assessment, a value of 10,000 l/kg dw is chosen for both Kpsedand Kpsoil.

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 DHTDMAC would remain in the water phase. Kappeler (1982; cited in 3.1.2.1) found that in river water on average 27% of the DHTDMAC is adsorbed onto suspended matter (average concentration 22 mg/l). From these values, the Kpsusp is calculated to 16,800 l/kg. As the latter value has a better empirical basis, it is used in the exposure calculation.

AIR WATER PARTITIONING (HENRY 'S LAW CONSTANT, HLC)

With the Property estimation program US HENRYWIN a HLC for DODMAC can be calculated with a fragment method to be 6.5E-03 Pa*m3/mol (low volatilty).

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