Prior posts have noted a number of issues related to nanoparticles.
Silver nanoparticles are used in hundreds of consumer products, including food storage containers, clothing, computer keyboards, cosmetics, pillows, cell phones, and medical appliances, because of their antimicrobial properties. Because silver is water soluble, it washes out some products, which sends the nanoparticles into wastewater systems.
Researchers sought to examine what transformations, if any, occurred in the sewage treatment system. They determined that sewage sludge samples which were tested were high in silver, but that the silver could not be attributed to an industrial source. Scanning transmission electron microscopy revealed the nanoparticles were 5-20 nm in diameter and formed small, loosely packed aggregates no more than 100 nm in size. Energy-dispersive X-ray spectrometry showed that sulfur (which is produced by microorganisms that digest sewage) combined with the silver in a 2:1 ratio, and the crystal structure confirmed the formation of silver sulfide nanoparticles as part of the sewage treatment process. No pure silver nanoparticles were found.
What does this portend? Currently, there is a dearth of knowledge about silver sulfide nanoparticles. Their solubility, bioavailability, toxicity, and reactivity are unknown. These are not mere academic questions because sewage sludge is, in some cases, applied to land.
The study can be found at http://pubs.acs.org/doi/abs/10.1021/es101565j and at http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&cmd=Search&doptcmdl=Citation&defaultField=Title%20Word&term=Kim%5Bauthor%5D%20AND%20%20AND%20Environ%20Sci%20Technol%5BJournal%5D%20AND%202010%5BPublication%20Date%5D%20AND%207509%5BPagination%5D.