Combined effects of water temperature and chemistry on the environmental fate and behavior of nanosized zinc oxide.

Information on the effects of water temperature, among several environmental factors, on predicting the behavior, fate, and exposure risks of engineered nanoparticles (NPs), is scarce. In the present work, the behavior and fate of commercial zinc oxide (ZnO) NPs with an average diameter of 52 nm were extensively investigated in U.S. Environmental Protection Agency standard, synthetic freshwater media with varying pH and hardness containing 2mg C/L of humic acid as a natural organic matter (NOM) surrogate, in the temperature range from 4 °C to 45 °C, representing very cold to warm waters. While a constant increase of ZnO hydrodynamic diameter was observed with increasing the temperature, results of analysis of variance showed that the temperature effect was insignificant in the samples with enhanced ionic strength, and water chemistry had more pronounced effects than the temperature on the rate of ZnO NP aggregation. With increase of the water temperature, the NP surface charge was partially reduced. ZnO NP dissolution and surface adsorption of NOM and zinc ions were found to be exothermic processes, and the latter was significantly decreased when temperature was increased in all test matrices. This study provides useful information for assessing environmental risks of ZnO NPs in aqueous matrices with various water chemistries and temperatures.