Silver

Toxicity to microorganisms

Link to relevant study record(s)

Reference

Endpoint:

activated sludge nitrification inhibition testing

Type of information:

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

Adequacy of study:

key study

Justification for type of information:

Substance considered to fall within the scope of the read-across 'Silver metal: Justification of a read-across approach for environmental information requirements' (document 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

Read across from ionic silver

Plus supporting published data from several studies included in the REACH dossier as Endpoint Study Records with various sizes of nanoparticles and coating types, reporting comparative effects of silver and nanosilver on microorganisms relevant to wastewater treatment

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.