A question that continues to be asked and probably needs to asked given the fact that different environmental conditions may result in unexpected outcomes with the respect to the fate of potential contaminants. The University of Queensland recently highlighted just such an outcome in their news item “Soil increasingly at risk from household products.” Their Researchers found that silver nanoparticles which under ‘normal’ soil conditions converted rapidly to inert and non-toxic forms and did not accumulate in plants, posed more risks and became more bioavailable in saline soils and in soils irrigated with poor-quality water.
You might be surpised to know just how many household products contain silver nanoparticles including washing powders, household sprays, disinfectants, toothpaste and more. These type of products allow silver nanoparticles to enter our wastewater system and eventually accumulate in biosolids at wastewater treatment plants. From here, as with most biosolids, these are applied to agricultural soils, in fact in the UK “Use of sewage sludge as a soil enhancer and fertiliser on agricultural land remains the environmentally favoured option, with around 80% being applied to agricultural land.” *
Concerns over the discharge of silver into the environment with the recent trend in using the perceived widely beneficial properties of silver nanoparticles led DEFRA to commission the report “Assessment of the current exposure of the British natural environment to silver (CB0464), Silver in sewage sludge, soil and river bed-sediments” which was published in 2014.
- Despite its status as a precious metal, the UK is disposing of approximately 4 tonnes of silver (Ag) down the drain each year.
- Study found 3.56 mg/kg dw (median) silver (Ag) in sludge cake from 9 separate sewage treatment plants (2.77 mg/kg dw as the 10%ile and 14.3 mg/kg dw as the 90%ile)
- According to the calculations in this study, a single sludge application to soil would introduce anthropogenic Ag at a level 100 times below a possible predicted no effect concentration (PNEC) for ionic Ag. (NanoFATE project a PNEC of 1,240 μg/Kg for ionic Ag was proposed)
Whilst, as can seen from the key findings above, a single application of sewage sludge does not pose concerns this may not be the case with repeated applications. Furthermore recent research by an international team which included the UK’s Rothamsted Research Ltd and Cranfield University found that the treated sewage sludge with nanomaterials (silver, titanium dioxide and zinc oxide) to typical United States of America maximum limits prevented colonisation of plant roots by nitrogen-fixing bacteria. This resulted in plants with stunted growth and took up far more zinc compared to sewage sludge containing typical forms of the metals. The make-up of microbial communities in the soil was also changed. (For complete news article click here)
In conclusion GEMs would echo one of the closing statements in the soil section of the DEFRA report “There is still much research to be done on the fate and behaviour of nanosilver in soils.”
