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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Cationic surfactants adsorb to soil or sediment mainly via ionic interaction with negatively charged surfaces and there is poor correlation between adsorption and organic carbon content. Hence, sorption should not be normalized to organic matter and it is better to rely on Kd rather than Koc. As a worst case approach, the value for sediment was also used for soils and suspended particles. As a result, the following Kd values were used Kd soil = Kd sediment = Kd sup = 13,630 L/kg.

Key value for chemical safety assessment

Other adsorption coefficients

Type:
other: mean Kd
Value in L/kg:
13 630
at the temperature of:
20 °C

Additional information

A guideline study was conducted on the read-across substance C12-16 ADBAC to determine the soil adsorption / desorption properties using the batch equilibrium method. The test substance adsorbs onto soil (soil 1: Kadsoc = 18,251; soil 2: Kadsoc = 16,679; soil 3: Kadsoc = 812,943) and does not desorb easily for the three soil types used, therefore it can be classified as immobile. The high concentrations of the test substance used in the study (0.5 g test substance for 1 g soil) lead to a less accurate extrapolation to lower concentrations. Adsorption is likely to be higher at lower concentrations than suggested by the outcome of this study (Geffke T, 1999).

In another guideline study, the adsorption / desorption coefficients of the read-across substance C12-16 ADBAC were determined by equilibrating with four soil types (i.e., sand, sandy loam, clay loam, silt loam). Based on the very high percent of 14C-test substance adsorption onto the test soil, i.e., 95.2 to 97.3% in a preliminary study conducted with a 1 : 20 soil : water ratio, the definitive study was carried out at 1 : 200 soil : water. Nominal test concentrations of 0, 0.1, 0.5, 1 and 2 µg/mL (i.e., equivalent to a measured concentration of 0.689, 0.441, 0.887 and 1.85 µg/mL) were prepared by pipetting appropriate aliquots of 14C-test substance at 698 µg/mL directly into test bottles containing 0.01 M CaCl2 solution and test soils. Liquid scintillation counting was employed to measure the test substance concentrations in the aqueous phase. The amount of the test substance remaining adsorbed on the soil was determined by combustion/radio analysis. Although the amount of 14C-test substance adsorbed onto the four test soils was well above the 20 to 80% recommended to ensure Freundlich isotherms, excellent correlations were obtained ranging from 0.9682 to 0.9983. In addition, the 14C-mass balance for the four soils types ranged from 95.3 to 103.2%. The results obtained were as follows:

  • The #32 sand at 0.1% organic carbon had an adsorption Kd and Koc of 6,172 and 6,171,657, respectively, and a desorption Kd and Koc of 7,137 and 7,137,310, respectively.
  • The #45 silt loam at 0.5% organic carbon had an adsorption Kd and Koc of 10,797 and 2,159,346, respectively, and a desorption Kd and Koc of 14,083 and 2,816,590, respectively.
  • The #59 sandy loam at 0.8% organic carbon had an adsorption Kd and Koc of 5,123 and 640,389, respectively, and a desorption Kd and Koc of 96,540 and 12,067,457, respectively.
  • The #58 clay loam at 2% organic carbon had an adsorption Kd and Koc of 32,429 and 1,663,039, respectively, and a desorption Kd and Koc of 165,556 and 8,490,062, respectively.

The mean Kd value was 13,630 L/kg. No apparent degradation of the test substance occurred in any of the four soils. Under the conditions of the study, the test substance was considered to have little or no potential for mobility in the soil and therefore should not pose an environmental risk for contamination of ground water (Daly D and Cranor W, 1988).