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Environmental fate & pathways

Adsorption / desorption

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Description of key information

Soil
The results of the GEMAS survey (Janik et al. 2010) are used to derive the Kd value for soil. 497 Kd values for silver were measured in agricultural soils sampled from throughout Europe. The median value is log Kd 3.60 (Kd 4023), 10th and 90th values percentile are log Kd 3.08 and >3.67 (Kd 1200 and >4,700). The value of 4,700 is the upper limit of detection and so is both the maximum value and 90th percentile.
Suspended sediment
The selected value, the mean of four log Kd values reported from separate studies, is log Kd 5.28 (Kd 190546).
Sediment
The selected value, the mean of two log Kd values reported from separate studies, is log Kd 4.05 (Kd 11092).
STP
The partitioning of silver in an STP is expressed in terms of the fraction of influent silver in the final effluent (0.199) and the sludge (0.801). The fraction partitioning to sludge is derived as the geometric mean of 14 measurements reported from three separate studies.

Key value for chemical safety assessment

Additional information

Transport of metals between aqueous phase and soil/sediment/suspended matter should be described on the basis of measured soil/water, sediment/water and suspended matter/water equilibrium partition coefficients (Kd), instead of using common mathematical relationships based on, for example, octanol-water partition coefficients, as is usually done for organic chemicals (TGD, 1996).

In order to reduce the influence of a particular study or sampling site where a large number of Kd determinations have been made, often within a limited geographic area, a single Kd value was identified for each individual study. In some cases where Kd values (or the data from which they could be calculated) were not stated explicitly in the studies these values were estimated from graphs: in these cases, the selected values are used as supporting information only. In other cases, the values were calculated as the mean log Kd value across sampling sites in the study. Mean values are calculated where log transformed Kd values are reported: where Kd values (not log transformed) are summarised the median value is used. In cases where multiple determinations of Kd were made at each sampling site, the mean log10 Kd value for each individual site was used as the input for the overall log Kd calculation. A distribution of Kd values is therefore derived for each partitioning phase (suspended sediment, settled sediment), with equal weighting attributed to each study. For soils, the results of a single large European study are used.

The Kd values for soil were derived from partition coefficient information collected within the GEMAS project (Janik et al. 2010). This project used agricultural soils collected from throughout Europe and is a consistent and comprehensive study of silver partitioning in European soils. A weight of evidence approach has been taken to determine the log Kd values for soil, suspended matter and sediment. Available data on the partitioning of silver in the environment show that a wide range of partition coefficients are observed, depending upon the local conditions. The values for the partition coefficients used in the assessment are median values based on measured data.

The log Kd values that will be used in the assessment are: log Kd soil 3.60 (Kd 4023), log Kd suspended matter 5.28 (Kd 190546) and log Kd sediment 4.05 (Kd 11092).

The partitioning of silver during sewage treatment has also been addressed empirically based on measured data in STP influent and effluent from several studies. Due to the extremely low volatility of silver, and the fact that it cannot be degraded, all silver which is removed from wastewater is assumed to be partitioned to the sewage sludge. The partitioning of silver in STP is based on the geometric mean of measurements reported for 14 separate STPs in three separate studies and is expressed as the fraction of influent silver in the final effluent (0.199) and the sludge (0.801) – removal efficiency of 80.1 %. Mean and median removal efficiencies were 82.9 and 87.5, respectively.