The influence of dissolved organic carbon and surface chemistry on the toxicity of silver nanoparticles in Daphnia magna

Silver nanoparticles (AgNPs) are by far the most extensively used nanomaterial in general consumer products. Concurrent with this use is the major concern regarding the toxicity of AgNPs to aquatic organisms. The most persistent question regarding the toxicity of AgNPs is whether this toxicity is due to the nanoparticles (NPs) themselves or the ions they release. Two primary factors influencing the aquatic toxicity of AgNPs are the surface coating, which presumably controls the release of Ag+, and the dissolved organic carbon (DOC) of the media, which serves as a source for ligands to bind the Ag + and reduce bioavailability to aquatic organisms. This study was conducted to investigate the role of these variables on the toxicity of AgNPs to the aquatic pelagic invertebrate, Daphnia magna and to test the hypothesis that the toxicity of AgNPs to Daphnia magna can be explained by their generation of silver ions. Acute toxicity bioassays (48h) were conducted using silver nitrate (AgNO3) as the positive control and three AgNPs with different surface coatings: gum arabic-coated (AgGA), polyethylene glycol-coated (AgPEG), and polyvinylpyrrolidone-coated (AgPVP) silver nanoparticles. Bioassays were conducted in moderately hard reconstituted water (MHW) augmented with two different concentrations of Suwannee River dissolved organic carbon (SRDOC). Measurements of the total silver and dissolved silver concentration were determined at the 48h median lethal concentration (LC50). The findings indicated that in MHW, AgNO3 was the most toxic to Daphnia magna (LC50 =1.06 &mgr;g Total Ag/L), and AgPVP was the least toxic (LC50 =14.81&mgr;g Total Ag/L). The dissolved silver concentrations at the Total Ag LC50 value were similar for all nanoparticles and AgNO 3. The presence of SRDOC reduced the toxicity (48h Total Ag LC50) of AgNO3, AgGA and AgPEG while that of AgPVP was unchanged. The results also showed that there was a significant decrease in the dissolved silver concentration with all the AgNPs in SRDOC. These results supported our hypothesis that toxicity of AgNPs to Daphnia magna is a function of ionic silver concentration.