Credit: © 2007 ACS

The potential toxicity of metal oxide nanoparticles in aquatic environments is influenced by a wide range of factors, including the particles' size, charge and the degree to which they stick together to form larger aggregates. Another important consideration, however, is that of chemical composition, and whether any toxic effects arise from the nanoparticles themselves or simply from their metal ions that dissolve into solution.

Now, a team of researchers in Australia led by Natasha Franklin at CSIRO Land and Water have studied the behaviour of ZnO nanoparticles in freshwater systems to better understand their environmental fate and potential toxicity1. Using a dialysis membrane with very small pores (approximately 1 nm) — through which only zinc ions or small zinc complexes can pass — the rates at which both bulk and nanoparticulate ZnO dissolve in water were measured. At a pH value of 7.6, the different material forms were found to have similar rates of dissolution, producing Zn ion solutions with similar saturated concentrations.

The toxicity of various zinc compounds — including both nanoparticulate and bulk ZnO, and ZnCl2 as a control — to freshwater algae were found to be roughly the same when based on the total amount of dissolved zinc. This observation suggests that care should be taken when assessing the toxicity of nanoparticles because such effects could be derived simply from dissolved metal ions as well as properties intrinsic to the particles themselves.