Silver

Bioaccumulation: terrestrial

Link to relevant study record(s)

Reference

Endpoint:

bioaccumulation: terrestrial

Type of information:

read-across from supporting substance (structural analogue or surrogate)

Adequacy of study:

supporting study

Justification for type of information:

1. HYPOTHESIS FOR THE READ-ACROSS APPROACH (ENDPOINT LEVEL)
The REACH registration of silver (powder and massive forms of zero-valent, elemental, silver) is underpinned, in common with other metals, by a read-across (or analogue) approach from the properties of the free ion. This principle of read-across from the free ion has been extended to also include nanosilver.
The scientific validity of read-across from the hazard properties of ionic silver (source) to nanosilver (target) under REACH is underpinned by both theoretical and empirical considerations.
The theoretical basis for the use of ionic silver data as the foundation of the risk assessment of nanosilver is based on the premise that the free metal ion (Me+) is the most toxic metal form/species (Starodub et al. 1987). This consideration was implicit in the development of the Free Ion Activity Model [FIAM] (Morel 1983, Paquin et al. 2002, Campbell 1985, Brown and Markich 2000) and, more recently, the Biotic Ligand Model [BLM] (Paquin et al 2002, Niyogi and Wood 2004) that has underpinned the risk assessment of several metals (e.g. Cu, Ni, Zn) under the Existing Substance Regulations and REACH; and most recently the development of the Environmental Quality Standard (EQS) for nickel and nickel substances under the Water Framework Directive (WFD). When considered on an equal mass basis ionic silver would therefore be expected to have greater toxicity than nanosilver simply on the basis that silver ions are released over time from the surface of particles (via oxidative dissolution). As the properties of nanosilver are read-across directly from ionic silver (not just to the fraction of silver ions released from nanosilver), this read-across is also expected to introduce considerable precaution into the hazard component of the risk assessment of nanosilver as all nanosilver, irrespective of coating, morphology or particle size distribution is assumed to behave similarly to the free ion.
This theoretical consideration has been tested by conducting a comprehensive review of the available scientific literature for nanosilver, with particular emphasis on the comparative effects on REACH relevant biotic systems (REACH information requirement) of ionic silver and nanosilver. This review is described for each endpoint in subsequent sections of the report ‘Nanosilver: read-across justification for environmental information requirements’ (attached in IULCID Section 13) and is summarised below. Furthermore, this theoretical consideration was confirmed by a specific ecotoxicity testing programme undertaken by the EPMF following the silver substance evaluation and designed to compare the effects of the smallest nanosilver form registered under REACH (‘Nano 8.1’ or ‘Silberpulver Typ 300-30’) and silver nitrate to algae, Daphnia (long-term) and soil microorganisms.

2. READ-ACROSS APPROACH JUSTIFICATION (ENDPOINT LEVEL)
In terms of the fate and bioaccumulation of nanosilver, based on the available information, the properties of nanosilver are considered to be sufficiently similar to ionic silver such that read across from the properties of ionic silver during exposure modelling and risk characterisation will result in a precautionary risk assessment for nanosilver.

Bioaccumulation
Recent ECHA guidance on aquatic bioaccumulation assessment for REACH registration observes that it is not possible to make log Kow or solubility based estimates of nanomaterial bioaccumulation as nanomaterials within test systems are “dispersed” and not in solution. As such, measured BCF values are required to fulfil data requirements under REACH. The guidance also states that it is also of vital importance to consider the influence of aggregation/agglomeration as well as dissolution on bioaccumulation. If possible, information on bioaccumulation of nanomaterials should be supported with information on the form of the substance present in the animal tissue (i.e. are nanoparticles of silver bioaccumulated or just ionic silver released from nanomaterials).
Handy et al. (2012) outline several problems with the performance of conventional bioaccumulation tests using nanomaterials. Critically, they question the founding assumption of the “steady-state” required for BCF measurements from aqueous exposures as colloidal dispersions (of nanomaterials) are dynamic systems which do not achieve steady equilibrium state (Handy et al. 2008). Equally, uptake by endocytosis (a potential mechanism of accumulation of nanoparticles) may also confound the use of standard kinetic relationships employed in bioaccumulation tests that are based on diffusion (i.e. the Fick equations). Handy et al. (2012) warn against the application of bioaccumulation tests without an appreciation of the underlying mechanism of uptake and kinetics. Handy et al. (2012) also discuss the use of diet borne studies. However, limited potential for the verification of particle size distribution of nanomaterials when incorporated into food (as per studies conducted in soils) are considered to restrict the usefulness of diet-based bioaccumulation tests with nanomaterials.
Read-across from the dissolved silver ion is also applied to fulfil information requirements for silver and silver-based (coated) nanomaterials. Supporting information for this read-across is summarised in endpoint summaries, here below and in further detail in the report ‘Nanosilver: read-across justification for environmental information requirements’ (attached in IUCLID Section 13).
Importantly, where silver is accumulated after exposure to nanosilver, the current studies are insufficient to distinguish if nanosilver particles are accumulated or if ionic silver has dissolved from a nanosilver particle, which is then accumulated.

Bioaccumulation in terrestrial invertebrates
Published data from two bioaccumulation studies in the earthworm Eisenia fetida using nanoparticles are included in the REACH dossier as Endpoint Study Records. Both studies report BAF bioavailable values for nanosilver below the BAF bioavailable value for ionic silver of 0.62. This supports the use of ionic silver as the ‘worst case’ basis to read across properties to nanosilver. A summary of these supporting studies is available under Section 3.2.3 of the report ‘Nanosilver: read-across justification for environmental information requirements’ (attached in IUCLID Section 13).

For further information and data matrix see 'CSR Annex 9 - Read Across Justification Nanosilver ENV_SUMMARY_200706' attached in IUCLID section 13.

Reason / purpose for cross-reference:

read-across source

Type:

BCF

Value:

0.62 dimensionless

Basis:

whole body w.w.

Time of plateau:

30 d

Calculation basis:

kinetic

Remarks on result:

other: 30 day BAF bioavailable value for ionic silver

Kinetic parameters:

The time course of accumulation during the exposure period had high initial accumulation which leveled off and approached a plateau after 28 days.

Metabolites:

Not applicable

Details on results:

- Control: During the five months duration of the experiment, no worms died in either the treated or the control terraria. Their average mass increased from 394 ± 76 mg ww to 566±144 mg ww and cocoons and juveniles were found in all treatments during the whole duration of the experiment.
- Results: At the end of the exposure period (day 28), but prior to emptying their gut content, worms had body concentrations (mean±SEM, n=30) corresponding to 5.1±0.5% and 11.0±0.3% of the concentration in the food for Ag NPs and Ag ions. After emptying their gut content for 48 hours, the corresponding value was 2.3±0.1%, for Ag ions. 80% of Ag ions were excreted within 48 hours and 97% during the two-month depuration period. At this time the remaining 110m Ag concentration in individual worms was close to the detection limit and the depuration study was terminated. Further localization of Ag in worms’ organs was not possible due to extremely low amounts of Ag in worms.
- Conclusion: 30 day BAF bioavailable value of 0.62 ± 0.11 was reported for ionic silver.

Reported statistics:

The counting uncertainties were calculated from the counting uncertainty of individual activity measurements using the square root of sum of squares. Evaluation of statistical differences between ionic and particulate silver was performed by a Student’st-test.

Validity criteria fulfilled:

not applicable

Conclusions:

30 day BAF bioavailable value of 0.62 ± 0.11 was reported for ionic silver.

Executive summary:

Coutris et al. 2012 used neutron activated soluble silver (silver nitrate) to asses the uptake, excretion and bio-distribution of silver in the earthworm Eisenia fetida exposed in standard OECD artificial soil (pH 5.95) amended with spiked, air dried, horse manure (0.55 ± 0.15 µg/g dissolved silver). A 30 day BAF_bioavailablevalue of 0.62 ± 0.11 was reported.

Additional information

Summary of available data for uncoated and coated nanosilver

Bioaccumulation in terrestrial invertebrates

Coutris et al. (2012) used neutron activated nanosilver particles (20.2 ± 2.5 nm by TEM) and soluble silver (silver nitrate) to assess the uptake, excretion and bio-distribution of silver in the earthworm Eisenia fetida exposed in standard OECD artificial soil (pH 5.95) amended with spiked, air dried, horse manure (0.55 ± 0.15 µg/g dissolved silver; 0.77 ± 0.15 µg/g nanosilver). 30 day BAF_bioavailable values of 0.31 ± 0.12 and 0.62 ± 0.11 were reported for nanosilver particles and ionic silver, respectively. Ionic silver was approximately twice as bioavailable as nanosilver. Silver accumulated from soluble salts and nanoparticles was excreted rapidly.

Hunde-Rinke and Klawonn (2013) also measured bioaccumulation of nanosilver (NM-300K, 15 nm particle size) with Eisena fetida. Worms were exposed to silver via either soil or feed in a long-term (28 day) chronic toxicity study. No depuration phase was included and measurements were made at 28 days only. Despite these limitations, a bioaccumulation factor (BAF) of 0.52 can be calculated from silver accumulation observed in the lowest test concentration (13.43 mg/kg dry weight) of the soil exposure after 28 days (no adverse effects were observed on survival or reproduction at this nanosilver concentration). It is not clear if steady-state concentration was achieved after this exposure. Exposures to higher concentrations of silver in soil (by factors of up to 13), whilst resulting in toxicity, did not result in broadly greater accumulation of silver. It is unclear if the silver was located in the tissues or the gut of the worm. These results compare favourably to those reported by Coutris et al. (2012). No comparative exposure to ionic silver was conducted.