Registration Dossier

Administrative data

Hazard for aquatic organisms

Freshwater

Hazard assessment conclusion:
PNEC aqua (freshwater)
PNEC value:
0.04 µg/L
Assessment factor:
3
Extrapolation method:
sensitivity distribution

Marine water

Hazard assessment conclusion:
PNEC aqua (marine water)
PNEC value:
0.86 µg/L
Assessment factor:
10
Extrapolation method:
assessment factor

STP

Hazard assessment conclusion:
PNEC STP
PNEC value:
0.025 mg/L
Assessment factor:
1
Extrapolation method:
assessment factor

Sediment (freshwater)

Hazard assessment conclusion:
PNEC sediment (freshwater)
PNEC value:
438.13 mg/kg sediment dw
Assessment factor:
10
Extrapolation method:
assessment factor

Sediment (marine water)

Hazard assessment conclusion:
PNEC sediment (marine water)
PNEC value:
438.13 mg/kg sediment dw
Assessment factor:
10
Extrapolation method:
assessment factor

Hazard for air

Air

Hazard assessment conclusion:
no hazard identified

Hazard for terrestrial organisms

Soil

Hazard assessment conclusion:
PNEC soil
PNEC value:
1.41 mg/kg soil dw
Assessment factor:
3
Extrapolation method:
sensitivity distribution

Hazard for predators

Secondary poisoning

Hazard assessment conclusion:
no potential for bioaccumulation

Additional information

See CSR Annex 4 - PNEC Summary Report

Conclusion on classification

Acute and chronic aquatic toxicity data are available for a range of different freshwater and marine species. For silver and silver compounds, the acute ecotoxicity reference value (ERV) is 0.22 µg Ag/L and the chronic ERV is 0.1 µg Ag/L.

In line with the risk assessment/classification approach adopted for other metals and inorganic metal compounds (ECHA 2013), ecotoxicity data are reported in terms of the concentration of dissolved silver ions from soluble inorganic silver species. Predominantly, these are studies that used silver nitrate (AgNO3) as the source of dissolved silver ions. Silver nitrate is considered to be the form of silver with the greatest toxicity as it dissociates rapidly and completely in aqueous solution. Where data for silver nitrate was not available, data derived from other inorganic salts (e.g. silver chloride) were used, but only after the exposure conditions were determined to be acceptable (e.g. testing was conducted within the limits of solubility and the Ag+ ion was likely to be the dominant dissolved species).

A complete base set of acute ecotoxicity studies is available for soluble inorganic silver species, comprising numerous studies for fish, invertebrates and algae. The lowest reliable acute value is an EC50 of 0.22 µg Ag/L for the invertebrate Daphnia magna (Bianchini et al. 2002).

A complete chronic data set is also available for soluble inorganic silver species, with chronic ecotoxicity data available for various species of fish, invertebrates and algae. The lowest reliable chronic value is an EC10 of 0.1 µg Ag/L for the algae Pseudokirchneriella subcapitata (Fraunhofer 2017). Additional chronic toxicity data of similar sensitivity are also available for the the blue-green algae Nostoc muscorum (Rai et al. 1990), Brown Trout Salmo trutta (217 day EC10 of 0.19 µg Ag/L) and Oncorhynchus mykiss (196 day EC10 of 0.17 µg Ag/L) obtained from Davies et al. (1998).

Based on current guidance (ECHA 2013), soluble silver is classified as R50/53 under the DSD, with specific concentration limits of Cn ≥0.025% for R50-53, ≥0.0025% Cn <0.025% for R51-53, and ≥0.00025% Cn <0.0025% for R52-53. Soluble silver is also classified as Acute 1 and Chronic 1 under the CLP regulations and subject to an acute M factor of 1000 and a chronic M factor of 100.

Silver metal- Transformation/Dissolution tests

Massive and powder forms of silver are considered to be poorly/sparingly soluble. The environmental hazard of poorly/sparingly soluble forms of metals is considered to be associated with their potential to release soluble bioavailable metals ions (e.g. Ag+ ions) or other metal-bearing species into aqueous media, from where they can elicit adverse effects (ECHA 2013). The potential for aqueous transformation or dissolution of elemental or sparingly soluble forms of metals can be measured using standardised Transformation/Dissolution (T/D) tests.

Transformation/Dissolution tests have been performed on silver metal in both massive and powder forms. The massive form of silver was tested using a procedure which controls the available surface area by encasing the sample in glass and exposing a specified surface area.

Silver in powder form (median particle size >100 nm - <1 mm)

Two sets of full T/D test results are available for silver powder (CIMM 2009, ECTX 2010b). CIMM (2009) report T/D results for a silver powder with a median particle size of 1.9 µm and surface area of 3.0 m2/g after exposure for seven (loading rates of 1, 10 and 100 mg/L) and 28 days (loading rate of 1 mg/L) in OECD media at both pH 6 and pH 8. ECTX (2010b) reports T/D results for a silver flake with a median particle size of 2.6 µm and surface area of 1.17 m2/g after exposure for seven (loading rates of 1, 10 and 100 mg/L) and 28 days (loading rate of 1 mg/L) in OECD media at pH 6 only. The results of the tests performed at pH 6 on the two different silver materials are similar, with average dissolved silver concentrations after seven days of 1.3 and 1.8 µg/L (1 mg/L loading rate) and average dissolved silver concentrations after 28 days of 3.6 and 3.7 µg/L (1 mg/L loading rate) for powder and flake, respectively. Where T/D data are available for the same material under both pH 8 and 6 (CIMM 2009), dissolution of silver powder after 7 days at the highest loading rate (100 mg/L) is reported to be greater at pH 6 (37.36 µg/L) than at pH 8 (26.03 µg/L). However, at the two lower loading rates (1 and 10 mg/L), dissolution is greater at pH 8 (2.55 and 12.14 µg/L, respectively) than pH 6 (1.25 and 8.46 µg/L, respectively). Equally, dissolution of silver powder after 28 days at 1 mg/L was greater at pH 8 (5.71 µg/L) than at pH 6 (3.55 µg/L). The implications of this observation on classification are discussed below.

Discussion on observed effects

At all loading rates the dissolution of silver powder declined throughout the exposure period. Further, at the lower loading rates of 1 and 10 mg/L, the rate of dissolution appeared to be greater for tests performed at pH 8 than for those performed at pH 6, which is not consistent with the behaviour observed with many other metals.

The interpretation of the results observed in the T/D tests for silver powder may be complicated by the elevated chloride concentration in the high pH (pH 8) OECD T/D testing media. The high pH media for T/D tests contains 10 times more chloride than the low pH media. As silver chloride complexes are relatively soluble at concentrations below their solubility limit, the increased dissolution of silver observed at high pH relative to low pH may not be due to test media pH but to the concentration of chloride in the test media. The discrepancy between the dissolution behaviour observed at high and low loading rates is likely to be because the solubility limit of silver chloride complexes is reached in the pH 8 media at high loading rates, limiting dissolved silver concentrations to below those found in the pH 6 media at the highest loading rate due to its lower chloride content.

Bioaccessibility testing has also indicated that the dissolution of silver may be dominated by the solution chloride concentration rather than by the pH. This is supported by chemical speciation calculations performed using VisualMINTEQ that indicate that the formation of soluble silver chloride complexes may be important in silver solubility.

Given that the formation of soluble silver chloride complexes may explain the greater dissolution observed at pH 8, the T/D testing of silver powder at pH 6 may still represent environmentally relevant worst case dissolution of silver. However, classification and the selection of test media for “definitive” T/D testing with massive silver is based on the “worst-case” T/D testing results obtained at pH 8.

Silver in massive form (median particle size >1mm)

Initial studies to determine the T/D behaviour of silver in massive form (ECTX 2010a), which were performed using an epoxy resin carrier to control the exposed surface area of silver during the test, indicated unusual dissolution/solubility behaviour. Dissolved silver concentrations increased rapidly but then declined to a steady state concentration. Further experimentation suggested that the epoxy resin could act as an adsorbent phase for silver, which would result in the decline in the dissolved silver concentrations observed. It was further hypothesised that, during preparation of the test item, minute particles of silver may have become embedded in the epoxy vehicle, considerably increasing the exposed surface area during the test. Given these complications further experiments were proposed using a quartz glass vehicle.

To address the limitations of the 2010 study, a further “definitive test” on massive silver was performed using a quartz glass vehicle. As it was technically challenging to test massive silver at a nominal loading of 1 mg/L “surface equivalent” loading rates of 3, 9, and 27 mg/L were tested, which were then used with the intention to extrapolate to a conventional loading rate of 1 mg/L (ECTX 2013). The corresponding surface area under the loading rates was 3.14, 9.42 and 28.3 mm2, respectively. The surface equivalent loading rate is the mass of material that would result in the same exposed surface area (i.e. the same degree of dissolution). This study was conducted at a worst case pH of 8 only (based on the results of silver powder T/D testing) and used fluorinated ethylene propylene vessels to minimise adsorption of silver to the test system during the course of the 28 day test. The test items were also cleaned ultrasonically prior to the start of the exposure to remove residual particles of silver from the surface of the quartz glass tubes remaining from sample preparation. The average blank corrected dissolved silver concentration in the test medium after seven days exposure was the below the analytical limit of detection of 0.02 µg/L at all loading rates. The average blank corrected dissolved silver concentration in the test medium after 28 days exposure was below the analytical limit of detection of 0.02 µg/L at loading rates of 3 and 9 mg/L and was 0.03 (± 0.07) µg/L at a loading rate of 27 mg/L.

Uncoated and coated nanomaterials (as per EU definition – median particle size <100 nm)

A precautionary classification for silver and silver-based (coated) nanomaterials has been read across from data for soluble silver (see below). Similarly, appropriate M factors have been based on read across from soluble silver.

Classification

The classification strategy for poorly soluble silver substances has been based on ECHA guidance (ECHA 2013). This guidance describes how to determine the classification and appropriate M factors for poorly soluble substances, such as elemental silver powder and massive silver, from acute and chronic ERV and the results of T/D tests (Annex IV: Metals and Inorganic Metal Compounds). Acute and chronic classifications have been undertaken individually.

Chronic classifications for silver metal and powder have been derived from the chronic ERV and 28 day data from T/D tests. The available 28 day T/D data for silver powder and massive silver metal is restricted (for technical reasons) to a minimum loading rate of 1 mg/L. However, as chronic classification requires T/D data obtained from further loading rates of 0.01 and 0.1 mg/L, data for these theoretical loading rates have been extrapolated, as detailed in the guidance (footnote 108, page 608), from tests conducted at higher loading rates using either empirically derived relationships (i.e. linear regression) or precautionary assumptions (i.e. based on analytical limits of detection). Where the available data on which to base assumptions was limited, an alternative classification, based on the surrogate approach to chronic classification (section IV.5.2.2.2) using the acute ERV, is also reported for comparative purposes. The most stringent classification has been adopted.

Silver in powder form (median particle size >100 nm - <1 mm)

DSD / DPD classification

Silver in powder form is classified as R50/53 under the DSD, with specific concentration limits of Cn ≥0.025% for R50-53, ≥0.0025% Cn <0.025% for R51-53, and ≥0.00025% Cn <0.0025% for R52-53 derived using the acute ERV, as set out in Annex III of the DPD.

Acute classification under CLP / GHS

The loading rate of 1 mg/L in full T/D testing at both pH 6 (powder and flake) and pH 8 (powder only) resulted in a seven day dissolved silver concentration above the acute ERV of 0.22 µg/L (Bianchini et al. 2002). Therefore, Ag in powder form is classified as Acute 1 under the CLP regulations.

In addition, according to section IV.5.4 of ECHA guidance, the acute ERV of 0.22 µg/L and the maximum seven day T/D test result at a loading rate of 1 mg/L of 2.6 µg/L (silver powder at pH8) determines that an acute M factor of 10 should be applied to the classification.

Chronic classification under CLP / GHS

The concentration of dissolved silver after 28 days at pH8 in theoretical loading rates of 0.01 and 0.1 mg/L was extrapolated from a linear relationship of dissolution as a function of surface area loading (SAL) derived from a combination of 28 day T/D data at pH 8 for both massive silver and silver powder (calculations after Skeaff et al. 2008), as follows:

Log10[Ag µg/L] = 1.13 x log10(SAL m2/L) + 3.5953 (n=2)

This relationship calculated Ag concentrations in 0.01 and 0.1 mg/L silver powder loading after 28 days at pH 8 of 0.03 and 0.41 µg/L, respectively. The concentration at 0.1 mg/L loading rate is greater than the chronic ERV of 0.16 µg/L resulting in a classification of Chronic 1 under the CLP regulation.

As the available data upon which to base the extrapolation to loading rates of 0.01 and 0.1 mg/L was limited to two results, a complimentary chronic classification derived from the results of T/D testing after 7 days exposure at a loading rate of 1 mg/L and acute ERV according to the surrogate approach detailed in ECHA guidance (for application in instances where either chronic EVR or T/D testing data are not available) was also conducted. This approach also results in a classification of Chronic 1 under the CLP regulation.

In addition, according to section IV.5.4 of ECHA guidance, the chronic ecotoxicity reference value of 0.16 µg/L and the maximum 28 day T/D test result of 5.71 µg/L (silver powder pH 8) determine that a chronic M factor of 10 should be applied to the classification.

The chronic classification of metals and metal compounds can incorporate information on the environmental fate of dissolved ions. Where there is evidence of rapid environmental transformation of dissolved ions to less bioavailable forms in the water column and sediments, a less stringent chronic classification can be applied. This concept is similar to the use of data on the degradation of organic chemicals to inform classification. However, according to current ECHA guidance on the application of CLP, there is a lack of scientific consensus on the interpretation of rapid removal of dissolved metal species from the water column in the context of classification. Certain parts of Annex IV of the CLP guidance have therefore been removed until the agreement on the validity of use of the concept of rapid removal for classification purposes has been reached. The chronic classification of silver powder is therefore currently based on an assumption that dissolved silver ions are not rapidly partitioned from the water column. However, dissolved silver ions are known to undergo rapid environmental transformation through a combination of geochemical (partitioning to suspended particulate matter and settling) and speciation processes, such as the formation of metastable organic and inorganic reduced sulfide complexes (Kramer et al. 2002). This behaviour, which is dominated by reaction with sulfides, results in short residence times for the free silver ion in the dissolved phase of aquatic systems compared to other metals (Kramer et al. 2002).

The “Tableau Input Coupled Kinetics Equilibrium Transport Unit World Model for Metals in Lakes” (hereafter referred to as TICKET-UWM and available from http://blog.unitworldmodel.net), which was developed to address the complexities of metal speciation (using WHAM V speciation software) and its influence on the fate and effects of metals in the environment, was applied to explore the removal of silver in a theoretical lake system. Processes considered by the TICKET-UWM include complexation by aqueous inorganic and organic ligands such as dissolved organic carbon (DOC), adsorption to particulate phases such as particulate organic carbon (POC) and iron/manganese oxides and cycling of organic matter and sulfide production in lakes. The version of TICKET-UWM used to assess the fate of silver also incorporated a preliminary appreciation of the role of binding to sulfide containing ligands (measured as Chromium Reducible Sulfide - CRS). Modelling of water column removal and sediment sequestration were performed. Performance against criteria for rapid removal and long-term sequestration was assessed.

Simple linear modelling (using EUSES defaults) of a lake water column using TICKET-UWM based on empirically derived Kd values determines that silver is rapidly removed (within 2-4 days) from the water column based on simple equilibrium partitioning to solid-phases (i.e. suspended particles) and particle settling behaviour.

When CRS was considered within speciation-based non-linear modelling, simulations at pH 7 resulted in rapid removal of silver (defined as > 70% removal achieved in 28 days) from the water column using two out of the three modelling approaches adopted at each of the initial concentrations assessed, including the chronic cut-off concentrations of 0.1 and 1.0 mg/L. In the third modelling approach, the formation of silver chloride precipitates, which sustained concentrations of dissolved silver, prevented rapid removal. Removal under additional pH conditions (i.e. between 6 and 8) were not explored. Modelling that did not incorporate CRS binding did not achieve criteria for rapid removal at the lower chronic cut-off concentration (0.1 mg/L) when default modelling assumptions were used. Equally, in a sensitivity analysis, lake depth and settling velocity was determined to influence the ability to achieve rapid removal of silver.

Modelling of long-term sequestration in sediments (predominantly as Ag2S) suggests that binding to acid volatile sulfide (AVS) in anoxic sediments will result in low remobilisation potential. However, within oxic sediments not all of the criteria for sequestration were achieved.

Despite this understanding of fate, there is currently limited information on several elements of the fate of silver in the environment that are critical to any decision of classification, such as the reversibility of the formation of silver-sulfide complexes in the water column, fate processes under various environmental conditions (i.e. different pH values or within riverine systems) and the bioavailability (toxicity) of sulfide-bound silver.

Once guidance on the interpretation of information on rapid environmental degradation becomes available, and if relevant information to address the limitations of understanding described above, the current chronic classification of silver powder should be revisited.

Silver in massive form (median particle size >1mm)

DSD / DPD classification

No classification

Acute classification under CLP / GHS

Dissolved silver concentrations in T/D testing after 7 days exposure were below the analytical limit of detection (0.02 µg/L) in all loading rates (3, 9 and 27 mg/L) and replicates. As the limit of detection is below the acute ERV of 0.22 µg/L, silver massive, according to ECHA guidance, should not be classified for acute hazard.

Chronic classification under CLP / GHS

Similar to the results of 7 day T/D testing, the results of 28 day T/D testing at loading rates of 3 mg/L and 9 mg/L were also below the analytical limit of detection (0.02 µg/L). Only a single replicate at the highest equivalent loading rate of 27 mg/L reported concentrations of dissolved silver above the analytical limit of detection. As the analytical limit of detection used in the T/D testing was significantly lower than the chronic ERV of 0.16 µg/L, it is not considered necessary to classify silver massive for long-term hazard as even a precautionary extrapolation to concentrations of dissolved silver at lower loading rates of 0.01 and 0.1 mg/L (assuming that silver is present at a concentration equivalent to the analytical detection limit of 0.02 µg/L) would also result in values of dissolved silver lower than the chronic ERV.

This reasoning is supported by a linear relationship of dissolution as a function of surface area loading (SAL) derived from a combination of 28 day T/D data at pH 8 for both massive silver and silver powder (calculations after Skeaff et al. 2008), as follows:

Log10[Ag µg/L] = 1.13 x log10(SAL m2/L) + 3.5953 (n=2)

This relationship, whilst based on limited data, results in dissolved silver concentrations in 0.01, 0.1 and 1.0 mg/L silver massive loading after 28 days at pH 8 of 0.000004, 0.00004 and 0.0007 µg/L, respectively. The concentration at 1.0 mg/L loading rate is, as expected, less than the chronic ERV of 0.16 µg/L, which necessitates no chronic classification is required under the CLP regulation.

Uncoated and coated nanomaterials (as per EU definition – median particle size <100 nm)

The classification of uncoated and coated nanomaterials has been directly read across from soluble forms of silver without recourse to a T/D test. Silver and silver-based nanomaterials would therefore be classified as per soluble silver substances: Acute and Chronic Category 1.

Appropriate M factors have also been read across from soluble forms of silver. This read-across results in acute and chronic M factors of 1000 and 100, respectively, for uncoated and coated nanomaterials.

As the weight of evidence available suggests that the ecotoxicity of uncoated and coated nanomaterials is lower than soluble silver on an equivalent mass basis (see read-across matrix for environmental endpoints), read across from soluble silver in this case is expected to result in a precautionary classification and M-factors for nanosilver.

Equally, as the dissolution rates of uncoated and coated nanomaterials are expected to be variable dependent on morphology, particle size, particle size distribution and coating, this approach encompasses the range of likely properties of nano silver and is consistent with the read across rationale applied to uncoated and coated nanomaterials for environmental hazard endpoints.

For the purposes of this read-across, silver-based (coated) nanomaterials are considered to be those that are primarily based on elemental silver and fulfil the criteria of the Commission Recommendation for the definition of nanomaterials, but which also comprise a surface functionalisation/coating/capping. The scope of the read-across from soluble silver to elemental silver and elemental silver-based (coated) nanomaterials for classification is identical to the scope of the environmental assessment of silver and silver-based (coated) nanomaterials in the REACH chemical safety assessment (CSA), i.e. the read-across is only considered to be applicable where applied to elemental silver nanomaterials or elemental silver-based nanomaterials with additional surface functionalisation/coating/capping that is demonstrated to be non-toxic, biodegradable, “passive” and applied primarily to increase the stability of nanomaterial dispersions in aqueous or non-aqueous solutions (i.e. to prevent aggregation or agglomeration) through steric, electrostatic or comparable mechanism. Surface functionalistion/coating/capping that are specifically designed to interact with biological receptors or which have hazardous properties in their own right (e.g. meet the criteria for SVHC) would not be compatible with this read-across approach for classification. Equally, multi-metal nanomaterials incorporating elemental silver are outside of the scope of this read-across.

References cited

ECHA (2013) Guidance on the Application of the CLP Criteria. Guidance to Regulation (EC) No 1272/2008 on classification, labelling and packaging (CLP) of substances and mixtures. Version 4.0, November 2013.

Kramer J, Benoit G, Bowles K, DiToro D, Herrin R, Luther G, Manolopoulos H, Robillard K, Shafer M, Shaw J. 2002. Environmental chemistry of silver. pp 1-25. in Silver in the Environment: Transport, Fate, and Effects. Edited by Andren A, Bober T. Society for Environmental Toxicology and Chemistry. ISBN 1-880611-44-9

Skeaff JM, Hardy DJ, King P. 2008. A new approach to the hazard classification of alloys based on transformation/dissolution. Integrated Environmental Assessment and Management. 4 (1): 75-93