Impact of solar UV radiation on toxicity of ZnO nanoparticles through photocatalytic reactive oxygen species (ROS) generation and photo-induced dissolution.

The present study investigated the impact of solar UV radiation on ZnO nanoparticle toxicity through photocatalytic ROS generation and photo-induced dissolution. Toxicity of ZnO nanoparticles to Daphnia magna was examined under laboratory light versus simulated solar UV radiation (SSR). Photocatalytic ROS generation and particle dissolution were measured on a time-course basis. Two toxicity mitigation assays using CaCl2 and N-acetylcysteine were performed to differentiate the relative importance of these two modes of action. Enhanced ZnO nanoparticle toxicity under SSR was in parallel with photocatalytic ROS generation and enhanced particle dissolution. Toxicity mitigation by CaCl2 to a less extent under SSR than under lab light demonstrates the role of ROS generation in ZnO toxicity. Toxicity mitigation by N-acetylcysteine under both irradiation conditions confirms the role of particle dissolution and ROS generation. These findings demonstrate the importance of considering environmental solar UV radiation when assessing ZnO nanoparticle toxicity and risk in aquatic systems.

Comparison of nanosilver and ionic silver toxicity in Daphnia magna and Pimephales promelas.

The increasing use of nanosilver in consumer products and the likelihood of environmental exposure warrant investigation into the toxicity of nanosilver to aquatic organisms. A series of studies were conducted comparing the potency of nanosilver to ionic silver (Ag(+)) at acute and sublethal levels using two test organisms (Daphnia magna and Pimephales promelas). The 48-h D. magna median lethal concentration (LC50) of multiple sizes (10, 20, 30, and 50 nm) of commercially prepared nanosilver (nanoComposix) ranged from 4.31 to 30.36 µg total Ag L(-1) with increasing toxicity associated with decreasing particle size. A strong relationship between estimated specific particle surface area and acute toxicity was observed. Nanosilver suspensions (10 nm) treated with cation exchange resin to reduce the concentration of Ag(+) associated with it were approximately equally toxic to D. magna compared to untreated nanosilver (48-h LC50s were 2.15 and 2.79 µg total Ag L(-1), respectively). The 96-h LC50 and 7-d sublethal 20% effective concentrations (EC20s) for P. promelas were 89.4 and 46.1 µg total Ag L(-1), respectively, for 10 nm nanosilver and 4.70 and 1.37 µg total Ag L(-1), respectively, for Ag(+); the resulting ratios of 96-h LC50 to 7-d EC20 were not significantly different for nanosilver and ionic silver. Overall, these studies did not provide strong evidence that nanosilver either acts by a different mechanism of toxicity than ionic silver, or is likely to cause acute or lethal toxicity beyond that which would be predicted by mass concentration of total silver. This in turn suggests that regulatory approaches based on the toxicity of ionic silver to aquatic life would not be underprotective for environmental releases of nanosilver.

Temporal and spatial variation in solar radiation and photoenhanced toxicity risks of spilled oil in Prince William Sound, Alaska, USA.

Solar irradiance (W/m2) and downwelling diffuse attenuation coefficients (Kd; 1/m) were determined in several locations in Prince William Sound (AK, USA) between April 2003 and December 2005 to assess temporal and spatial variation in solar radiation and the risks of photo-enhanced toxicity from spilled oil. Weekly irradiance measurements of surface visible light, ultraviolet B (UVB), and ultraviolet A (UVA) radiation in Valdez (AK, USA) followed expected trends of maximum solar irradiance at each summer solstice and minimum values at each winter solstice. Variation from weekly maximum expected surface irradiances was attributed to large variations in environmental conditions over the 142-week monitoring period. Season and proximity to glacial meltwater were significant determinants of Kd, with 1% attenuation depths ranging from 0.4 to 15 m (UVB and UVA) and from 0.5 to 28 m (visible light). The probability of photo-enhanced toxicity risks estimated from UVA dosimetry decreased with increasing water depth, with higher risks during spring and summer and lower risks during fall and winter. These results demonstrate substantial temporal and spatial variation in solar radiation in Prince William Sound and the potential for significant season- and location-specific photo-enhanced toxicity risks from spilled oil.