Why Silver Might Not Be Gold in Water

Have you ever brushed your teeth? Washed your hair? Then you have been in contact with nanosilver! 

Nanosilver – tiny particles of silver – have a detrimental impact on our environment when it is released into waterways. To address this, a safe level for nanosilver in the environment was found using the aquatic organism Daphnia magna.  

By studying the population and heart rates of Daphnia magna in varying concentrations of nanosilver, a level at which nanosilver does not pose a toxicity risk was discovered! This level was 0.26–0.50 milligrams per litre (mg/L), and exceeding this value would cause toxicity in Daphnia magna, as well as other aquatic organisms

Growing up near the ocean highlighted how much rubbish ends up in our waterways. This led me to wonder about the waste that was too small to be seen by the naked eye. What were they and how could they be managed? This is how I discovered nanosilver.

Daphnia magna were appropriate to use in this project as they are sensitive to changes in water chemistry, and thus commonly used as bioindicators as a reference to indicate toxicity to other organisms. They are an excellent model organism as their body is transparent and their internal organs, including their beating heart, are clearly visible under a microscope. Furthermore, they are invertebrates and lack a central nervous system, and as such, cannot feel pain, minimising their suffering. As a result of this, Daphnia magna are regularly used in ecotoxicology studies - and bred as live fish food!

A bioassay was conducted where the population and heart rates of Daphnia magna were studied in different concentrations of nanosilver, revealing that as the concentration of nanosilver increased, the population and heart rates of the Daphnia magna decreased. This indicates nanosilver is toxic to Daphnia magna, and thus other aquatic life, at certain concentrations, with the minimum concentration of nanosilver which causes toxicity 0.26–0.50 milligrams per litre (mg/L).

While NSW Environmental Protection Authority sets a threshold level for bulk silver, it does not set a separate level for nanosilver as they have identical molecular identities. This is despite abundant scientific literature that indicates nanosilver is MORE toxic, as it has a greater surface area to volume ratio. Thus, it is currently unknown which concentration of nanosilver is toxic in ecosystems. In a scientific context, the results provide a minimum concentration of nanosilver which is not harmful to aquatic organisms. This minimum concentration level can be used as an indicator of water quality in terms of nanosilver pollution, thus ensuring that healthy aquatic ecosystems can be maintained. This threshold level can be used both in environmental monitoring by local and national government agencies, as well by industries to ensure that their waste output will not cause nanosilver levels to exceed their safe threshold level of 0.26–0.50 mg/L.