Persistence of singly dispersed silver nanoparticles in natural freshwaters, synthetic seawater, and simulated estuarine waters

This investigation focuses on predicting the persistence of citrate-capped 20 nm AgNPs by measuring their colloidal stability in natural freshwaters and synthetic aquatic media. Ultraviolet-visible absorbance spectroscopy, dynamic light scattering, and atomic force microscopy were used to evaluate the colloidal stability of AgNPs in locally-obtained pond water, moderately hard reconstituted water alone or with natural organic matter (NOM), synthetic seawater, and also the individual chemicals most prevalent in seawater. Singly dispersed AgNPs in seawater and waters with greater than 20 mmol L-1 sodium chloride were unstable, with the optical absorbance approaching zero within the first ten hours of mixing. Agglomeration rates as a function of water chemistry and NOM are tested as a hypothesis to explain the rates of disappearance of singly dispersed AgNPs. Other samples, mostly those with lower salinity or NOM, maintained varying degrees of colloidal stability during time studies up to 48 h. This indicates likelihood that some AgNPs will be stable long enough in freshwater to successfully enter estuarine or marine systems. These results should enable a more efficient design of nanoEHS risk assessment experiments by predicting the aquatic or soil compartments at greatest potential risk for accumulation of and exposure to citrate capped 20 nm AgNPs. - Research highlights: → Singly dispersed silver nanoparticles (AgNPs) were stable in some natural freshwaters. → AgNPs exhibit rapid instability in hard waters followed by a persistent fraction stable for days. → Natural organic matter provides increased colloidal stability and thus longer AgNP persistence.