May 3, 2018 by Dean Leverett

During 2017, I was part of a team of scientists supporting a REACH testing programme aimed at comparing the ecotoxicity of the ionic and nano forms of silver using standardised tests.

As part of the REACH Substance Evaluation for silver, a read-across approach from ionic silver to silver nanoforms was proposed based on existing ecotoxicity data. However, further justification of this read-across approach was requested and therefore standardised ecotoxicity and fate/ behaviour tests were undertaken to directly compare the toxicity of ionic silver and the smallest silver nanoform with the highest specific surface area registered under REACH.

A paper detailing the results of the comparative testing programme in soil will shortly be published.

Bollyn, Willaert, Kerré, Moens, Arijs, Mertens, Leverett, Oorts and Smolders. 2018. Transformation-Dissolution Reactions Partially Explain Adverse Effects of Metallic Silver Nanoparticles to Soil Nitrification in Different Soils. Environmental Toxicology and Chemistry. DOI 10.1002/etc.4161

This paper reports the results of the comparative testing of nano and ionic forms of silver on three soils. These were spiked with equivalent doses of either nanosilver or silver nitrate in a soil nitrification assay. ‘Truly dissolved’ (<1 kDa) silver in the silver nitrate amended soils decreased with reaction half-lives of 4 to 22 days depending on the soil, denoting important silver-ageing reactions. In contrast, ‘truly dissolved’ silver in nanosilver amended soils first increased by dissolution, and subsequently decreased by ageing; the concentration never exceeding that in the silver nitrate amended soils.

The toxic thresholds (EC10, mg silver kg-1 soil) of nitrification, evaluated at 21 or 35 days after spiking, were similar between the two silver forms for two of the soils. For the other soil, they were factors of 3 to 8 more toxic for silver nitrate than for nanosilver, largely corroborating with dissolution differences.

This fate and toxicity assay showed that nanosilver is not more toxic than silver nitrate at equivalent total soil silver concentrations and that differences in silver dissolution at least partially explain toxicity differences between the forms and among soils.

In addition, we will present a poster at SETAC Rome on detailing the aquatic toxicity and dissolution rate testing.

MO410: The aquatic toxicity of a marketed nanosilver product – A direct comparison with ionic silver (Arijs, Leverett, Oorts, Mertens, Schlich and Bruggemann)

The poster will present results that show that silver nitrate was more toxic than nanosilver to both algae growth and Daphnia reproduction using standardised tests, regardless of whether silver concentrations are expressed as total, dissolved (< 0.45 um filtered) or ‘truly dissolved’ (< 3kDa filtered) silver.

We hope to publish the full results of the aquatic ecotoxicity and dissolution studies during 2018.