Effects of Water Chemistry on the Dissolution of ZnO Nanoparticles and Their Toxicity to Escherichia Coli

The dissolution of ZnO nanoparticles (nano-ZnO) plays an important role in the toxicity of nano-ZnO to the aquatic organisms. The effects of water chemistry such as pH, ionic components, and dissolved organic matter (DOM) on the dissolution of nano-ZnO and its toxicity to Escherichia coli (E. coli) were investigated in synthetic and natural water samples. The results showed that the toxicity of nano-ZnO to E. coli depended on not only free Zn(2+) but also the coexisting cations which could reduce the toxicity of Zn(2+). Increasing solution pH, HPO(4)(2), and DOM reduced the concentration of free Zn(2+) released from nano-ZnO, and thus lowered the toxicity of nano-ZnO. In addition, both Ca(2+) and Mg(2+) dramatically reduced the toxicity of Zn(2+) to E. coli. These results highlight the importance of water chemistry on the toxicity evaluation of nano-ZnO in natural waters.