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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Key value for chemical safety assessment

Koc at 20 °C:

Additional information


For metals, adsorption/desorption translates in the distribution of the metals between the different fractions of the environmental compartment, e.g. the water (dissolved fraction, fraction bound to suspended matter), soil (fraction bound or complexed to the soil particles, fraction in the soil pore water,...).This distribution between the different compartments is translated in the partition coefficients between these different fractions. Study records on partition coefficients are given under 5.6.

Partition coefficients for zinc in freshwater has been reviewed in the RAR (ECB 2008). Based on the extensive experimental evidence, a partition coefficient for the distribution between solid particulate matter and water (Kpsusp) of 5.04 (log value) has been defined for EU waters and used throughout the RAR.

The Kp for the distribution between sediment and water (Kpsed) was estimated in the RAR from that for particulate matter, as follows: Kpsed= Kpsusp/ 1.5, based on the average difference in concentrations of zinc and other metals in both media. For zinc this results in a Kpsedof 73,000 L/kg. (ECB 2008). The marine Kd was derived based on data from several marine waters. the geometric mean value for zinc in seawater is 6010 L/kg.

For soil, a solids-water partitioning coefficient of 158.5 L/kg (log value 2.2) was determined experimentally on 11 American soils. This value was used in the RA Zinc.

Naphthenic acids:

Modelled values indicate a minor to strong adsorption for naphthenic acids to soil, whereas experimental values seem to indicate a low degree of adsorption.

Based on the analysis results (IUCLID section 1.4) of a number of representative naphthenic acids samples, a list of potential chemical components was compiled. The criteria for choosing these specific structures were the occurrence of C-numbers, the number of rings and their relative share in the analysed samples. The log Koc of these structures was estimated with the equation of Di Toro et al. (1985). The log Koc range is 2.02-6.59.

Janfada et al. (2006) reported values for the Kd of naphthenic acids from oil sands, between 1.3 and 17.8 L/kg at 4°C, which corresponds to calculated values of Log Koc of 1.85 to 2.82, corresponding in turn to the lower end of the estimated values (2.02-6.59).