Disilver oxide

Adsorption / desorption

Link to relevant study record(s)

Reference

Endpoint:

adsorption / desorption

Remarks:

adsorption

Type of information:

experimental study

Adequacy of study:

key study

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

other: Non-guideline study, well reported, very large range and number of soils, fully acceptable.

Qualifier:

no guideline followed

Principles of method if other than guideline:

Nearly 500 soil samples from across Europe were equilibrated with Ca(NO3)2 (1:10 m/v). After 48 hours shaking on an end-over-end shaker soil suspensions were spiked with 100 µL of a solution containing silver isotope (110Ag 12KBq as AgNO3) mixed for a further 72 hours. Samples were centrifuged at 1200 g for 15 min and supernatants filtered to <0.22 µm. The 110Ag activity in the filtrates was determined by gamma spectrometry.

GLP compliance:

no

Type of method:

other: Isotope dilution

Media:

soil

Specific details on test material used for the study:

Details on properties of test surrogate or analogue material (migrated information):
Not applicable

Radiolabelling:

yes

Test temperature:

No data reported

Details on study design: HPLC method:

No data reported

Analytical monitoring:

yes

Details on sampling:

The large number of soil samples lead to every 10th sample being analysed, using duplicate soil samples, and assessed for silver and variability in soil Kd values. Spiked soils were allowed to equilibrate for 72 hours before analysis. Every batch of samples (150-200 centrifuge tubes) included 5 blanks, containing 0.01 M Ca(NO3)2), and 5 reference standards (0.01 M Ca(NO3)2) with 100 µL of 110Ag spike.

Details on matrix:

Each 50mL centrifuge tube contained 2.0 +/-0.05 g of <2mm seived soils and 20 mL of 0.01 M calcium nitrate before being spiked. The experimental soil Kd values were found to vary less than 7.1% for Ag.

Details on test conditions:

After spiking and equilibration for 72, samples were centrifuged to 15 minutes at 1200g and filtered through 0.22 µm syringe filters.

Computational methods:

Percentiles calculated

Type:

Kd

Remarks:

L/Kg

Value:

4 023

Temp.:

20 °C

Remarks on result:

other: Median value of 497 soils. Range 159 -> 4700. Standard temperature assumed.

Type:

Kd

Remarks:

L/kg

Value:

159

Temp.:

20 °C

Remarks on result:

other: Minimum partitioning. Standard temperature assumed.

Type:

Kd

Remarks:

L/kg

Value:

> 4 700

Temp.:

20 °C

Remarks on result:

other: Maximum partioning. Standard temperature assumed.

Details on results (HPLC method):

No data reported

Adsorption and desorption constants:

No data reported

Recovery of test material:

No data reported

Concentration of test substance at end of adsorption equilibration period:

No data reported

Concentration of test substance at end of desorption equilibration period:

No data reported

Transformation products:

not specified

Details on results (Batch equilibrium method):

The range of Kd values was 159 - >4700 L/kg, with a median of 4023. The 10th percentile of values was 1200 L/Kg and the 90th percentile >4700 L/Kg.

Statistics:

Median values and percentiles were calculated.

Validity criteria fulfilled:

not specified

Conclusions:

Nearly 500 soils collected from across Europe were tested to determine the Kd of silver, assilver nitrate using isotope dilution. The range of Kd values was 159 - >4700 L/kg, with a median of 4023. The 10th percentile of values was 1200 L/Kg and the 90th percentile >4700 L/Kg.

Executive summary:

Janik et al. (2010) determined the Kd values of silver in nearly 500 soils collected from across Europe. Isotope dilution was used with 110 Silver Nitrate (12 KBq as Ag NO3). After 72 hours equilibration, post spike, the solid solution partitioning coefficient for each soil was determined on the soil solution filtrates using gamma spectrometry. The measured Kd values ranged from 159 -> 4700 L/Kg, with a median value of 4023 L/kg. It should be noted that 42% of the 497 soils tested gave Kd values that were greater than 4700 L/Kg. The median Kd value of 4023 L/Kg will be used through this assessment.

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

Other adsorption coefficients

Type:

log Kp (solids-water in suspended matter)

Value in L/kg:

5.28

at the temperature of:

20 °C

Other adsorption coefficients

Type:

log Kp (solids-water in sediment)

Value in L/kg:

4.05

at the temperature of:

20 °C

Other adsorption coefficients

Type:

log Kp (solids-water in soil)

Value in L/kg:

3.6

at the temperature of:

20 °C

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.