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Sediment toxicity

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Reference
Endpoint:
sediment toxicity: long-term
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key 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 ecotoxicity, nanosilver on an equal mass basis has been found to be significantly less toxic than ionic silver for the majority of REACH endpoints and of equivalent toxicity for some others. None of the empirical information available suggests that nanosilver is consistently more hazardous than ionic silver on an equivalent mass basis, or that “nano-specific” effects that would prejudice the validity of read across from ionic silver to nanosilver occur. In addition, with very limited exceptions which are described further in the report ‘Nanosilver: read-across justification for environmental information requirements’, none of the available data suggested consistent relationships between particle morphology, size, particle size distribution (raw or agglomerated) or coating (surface treatment) and effects.
Notter et al. (2014) presents a meta-analysis of published EC50 values for ionic silver and nanosilver. The authors demonstrate that almost 94% of acute toxicity values assessed for freshwater, seawater and terrestrial systems using algae, annelid, arthropoda, bacteria, crustacea, fish, nematoda, plant, protozoa and rotatoria show that the nanoform of silver is less toxic than the dissolved metal (when normalised for total metal concentration).
In addition, a specific ecotoxicity testing programme 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 was undertaken following the silver substance evaluation. This demonstrated that the nanoform of silver is less toxic than ionic silver (based on EC10 and EC50 values for total, ‘conventional’ dissolved (<0.45μm) and ‘truly’ dissolved (<3kDa) silver). Therefore, taking the full body of evidence into account, the read-across use of toxicity values from ionic to nanosilver as a ‘worst case’ approach is justified and scientifically defensible for environmental endpoints. The Substance Evaluation Conclusion document for silver agreed with this conclusion for the nanosilver forms covered in the REACH dossier (RIVM 2018).

Sediment toxicity
There are two studies reporting the effects of nanosilver exposure on sediment dwelling species included in the REACH dossier as Endpoint Study Records. Both of these studies investigated the effects of spiking nanosilver into overlying water, however, neither of these studies undertook a comparative exposure assessment of the relative toxicity of ionic and nanosilver in their test systems. The critical data for the sediment compartment in the REACH silver dossier is a NOEC of 12 mg/kg silver from a 10d exposure with the amphipod Hyalella azteca (Call et al. 1999). However, this value is not directly comparable with the effects thresholds derived from the nanosilver studies as these exposures were via the water column rather than directly via the sediment phase. As an alternative, an EC10 for ionic silver from the REACH silver dossier of 14.43 μg/L, based on a 10d water-only growth study using Chironomus tentans reported by Call et al. (1999), can be used to compare the relative toxicity of nanosilver and ionic silver in sediment dwelling species. Despite the nanosilver studies both using a significantly longer exposure duration than Call et al. (1999), both studies report effects thresholds for nanosilver at least an order of magnitude less sensitive than for ionic silver. A summary of these supporting studies is available under Section 4.2 of the report ‘Nanosilver: read-across justification for environmental information requirements’ (attached in IUCLID Section 13). Together with the theoretical basis for read-across based on the free-ion, overall these data support robust read-across from the properties of ionic silver to nanosilver for these REACH information requirements.



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
Duration:
10 d
Dose descriptor:
LC50
Effect conc.:
0.084 other: g/kg
Nominal / measured:
nominal
Conc. based on:
element
Basis for effect:
mortality
Remarks on result:
other: Bond lake sediment
Duration:
10 d
Dose descriptor:
LC50
Effect conc.:
2.98 other: g/kg
Nominal / measured:
nominal
Conc. based on:
element
Basis for effect:
mortality
Remarks on result:
other: West Bearskin lake sediment
Duration:
10 d
Dose descriptor:
NOEC
Effect conc.:
0.012 other: g/kg
Nominal / measured:
nominal
Conc. based on:
element
Basis for effect:
growth rate
Remarks on result:
other: Bond lake sediment
Duration:
10 d
Dose descriptor:
NOEC
Effect conc.:
2.15 other: g/kg
Nominal / measured:
nominal
Conc. based on:
element
Basis for effect:
growth rate
Remarks on result:
other: West Bearskin lake sediment.
Conclusions:
The 10-day LC50 values were 0.084 g Ag/kg and 2.98 g Ag/kg for the two sediment types. The 10-day NOECs were 0.012 g Ag/kg and 2.15 g Ag/kg for the two sediment types, based on growth.
Executive summary:

The GLP status of this study is not known. It follows an adapted version of a standard guideline. The methods are well-described and it is considered reliable and suitable for use for this endpoint. The most sendistive

Description of key information

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 included in endpoint summaries and in the appended summary/justification document.

Key value for chemical safety assessment

Additional information

Summary of available data for uncoated and coated nanosilver

There are two studies reporting the effects of nanosilver exposure on sediment dwelling species (Nair et al. 2011, Hund-Rinke and Klawonn 2013). Nair et al. (2011) exposed 4thinstar larvae of Chironomus riparius to nanosilver via the water column in a sediment/ water system and monitored pupation over a 25 day period. The study reports a LOEC of 200 µg/L for nanosilver in the water column (40 – 70 nm uncoated particles) from which a NOEC of 100 µg/L can be derived (where a LOEC is associated with effects >10% but <20% ECHA guidance [ECHA 2008] allows a NOEC to be calculated as LOEC/2). No measured concentrations of silver in sediments were reported.

Hund-Rinke and Klawonn (2013) undertook a standard 28 day OECD 219 sediment/water chironomid toxicity test using spiked overlying water. The study used a static exposure regime and undertook a single spike of overlying water with various concentrations of NM-300K nanosilver (uncoated particles with particle size of 15 nm) and measured chironomid survival, growth and development from 1stinstar larvae to emergence. Nanosilver was observed to rapidly partition to sediments. The study reported effects on development rate and on the total number of observed midges. The most sensitive EC10 from the study was 925 µg/L for effects on the development rate of male and female midges.

Neither of the studies undertook a comparative exposure assessment of the relative toxicity of ionic and nanosilver in their test systems. The critical data for the sediment compartment in the CSR is a NOEC of 12 mg/kg silver from a 10 day exposure with the amphipod Hyalella azteca (Call et al. 1999). However, this value is not directly comparable with the effects thresholds derived from either the Nair et al. (2011) and Hund-Rinke and Klawonn (2013) studies as these exposures were via the water column rather than directly via the sediment phase. As an alternative, an EC10 for ionic silver from the REACH CSR of 14.43 µg/L, based on a 10 day water-only growth study using Chironomus tentans reported by Call et al. (1999), can be used to compare the relative toxicity of nanosilver and ionic silver in sediment dwelling species. Despite the Nair et al. (2011) and Hund-Rinke and Klawonn (2013) studies, both using a significantly longer exposure duration than Call et al. (1999), both studies report effects thresholds for nanosilver at least an order of magnitude less sensitive than for ionic silver.

As both Nair et al. (2011) and Hund-Rinke and Klawonn (2013) use a sediment/water test system, whilst Call et al. (1999) used a water only exposure there could be differences in the partitioning behaviour of silver between the two studies that confounds this simple interpretation of the two tests. Notwithstanding the limitations of the available information, the potential for adverse effects on sediment dwelling species from nanosilver exposure appears to be less than for ionic silver.