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Ecotoxicological information

Toxicity to microorganisms

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Reference
Endpoint:
toxicity to microorganisms
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).

Toxicity to aquatic micro-organisms
Published data from several toxicity to aquatic micro-organisms studies using various sizes of nanoparticles and coating types are included in the REACH dossier as Endpoint Study Records. Radniecki et al. (2011) reported that smaller nanoparticles (20 nm) were more toxic than larger nanoparticles in their nitrification batch assay. However, in terms of weight of evidence there is sufficient available data to support a read across to nanosilver from ionic silver. There is insufficient data available to make any conclusion on the influence of coating on the toxicity of nanosilver to aquatic microorganisms. Together with the theoretical basis for read-across based on the free-ion, 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 4.1.8 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:
reference to other study
Reason / purpose for cross-reference:
read-across source
Duration:
13.3 min
Dose descriptor:
LOEC
Effect conc.:
0.05 other: mg Ag/L
Remarks on result:
other: ~10-15% inhibition
Duration:
13.3 min
Dose descriptor:
NOEC
Effect conc.:
0.025 other: mg Ag/L
Remarks on result:
other: calculated as LOEC/2
Details on results:
Results based on nominal concentrations.
Results with reference substance (positive control):
Not applicable
Reported statistics and error estimates:
No data reported
Validity criteria fulfilled:
not applicable
Conclusions:
NOEC for inhibition of autotrophic nitrification by AgNO3 was determined to be 0.025 mg Ag/L.
Executive summary:

In a non-standard batch extant respirometric assay the NOEC for inhibition of autotrophic nitrification by AgNO3 was determined to be 0.025 mg Ag/L.

Description of key information

Key value for chemical safety assessment

Additional information

Summary of available data for uncoated and coated nanomaterials

Reliable and relevant data on the effects of silver and silver-based (coated) nanomaterials are available from 15 studies. These studies describe the effects of nanosilver on microorganisms relevant to wastewater treatment (i.e. cultures of nitrifying bacteria or activated sludge populations) or directly on wastewater treatment processes (i.e. effects on nitrification rates or phosphorus removal efficiency). Some studies report the results of a comparative assessment between the effects of nano silver and ionic silver. Additional studies reporting the effects on nanosilver on yeast (Saccharomyces cerevisiae), Escherichia coli and naturally occurring assemblages/communities of microorganisms in the environment (i.e. rivers, lakes and wetlands) were identified in the literature search, but after initial assessment were not considered to be relevant to the REACH endpoint and are not considered further.

The most sensitive effects for nanosilver on microorganisms are reported in a series of five associated studies from the University of Missouri by Choi and Hu (2008 and 2009), Choi et al. (2008 and 2009) and Liang et al. (2010). These studies report the results of a series of experiments investigating the potential impacts of nanosilver on nitrification and organic matter removal during simulated wastewater treatment, including a comparison with the effects of ionic silver. The ionic silver data presented in Choi and Hu (2008) was the key data identified for the toxicity to aquatic microorganism endpoint in the REACH CSR for silver. Their studies consistently report that nanosilver is more toxic (by approximately 50%) than ionic silver when assessed on an equivalent mass basis. However, many of the studies used a single exposure concentration (Choi et al. 2009, Choi and Hu 2009, Liang et al. 2010), which makes robust characterisation of relative toxicity difficult. Where multiple concentrations were used to characterise the dose-response relationship (Choi and Hu 2008, Choi et al. 2008), these exposures also indicated that nanosilver was more toxic than ionic silver to nitrifying microorganisms. However, at the lowest portion of the dose-response (i.e. NOEC/LOEC values) it was not possible to distinguish between the relative toxicity of nanosilver and ionic silver. As such, the NOEC value used to derive the STP PNEC for the REACH registration of silver based on ionic silver is also the NOEC for nanosilver. Jeong et al. (2012) also investigated the effects of exposure to nanosilver on nitrification, reporting that the inhibition observed in their 12-hour batch testing system was “much lower” than that observed by Choi and Hu (2008) and Liang et al. (2010) resulting in a NOEC for nitrification of 0.5 mg/L and denitrification of 10 mg/L, which they consider could be the consequence of “different characteristics of activated sludge”. Jeong et al. (2012) do not report any comparative assessment of nanosilver and ionic silver.

Other studies (Radniecki et al. 2011, Sheng et al. 2011, Dams et al. 2011, Wang et al. 2012, Chen et al. 2012) also report the comparative effects of silver and nanosilver on microorganisms relevant to wastewater treatment. However, in contrast with the University of Missouri studies reported above, these all report that ionic silver is more toxic than nanosilver to aquatic microorganisms. For example, Chen et al. 2012 report the relative effects of ionic silver and nanosilver (uncoated, with a particle size of 20-40 nm) on enhanced biological phosphorus removal (EBPR). No effects on phosphorus removal were observed in nano silver exposures up to 5 mg/L. However, a NOEC of 0.5 mg/L was reported for ionic silver.

Radniecki et al. (2011) reported that smaller nanoparticles (20 nm) were more toxic than larger nanoparticles in their nitrification batch assay, which is consistent with the effects observed in Daphnia reported in section 4.2.4.

There is insufficient data available to make any conclusion on the influence of coating on the toxicity of nano silver to aquatic microorganisms.