Surface reactivity of anatase and rutile samples - relationship with toxicity on aquatic organisms

The release of engineered titanium dioxide nanoparticles in the environment is nowadays continuously increasing due to their wide range of industrial applications. Their potential toxicity effects became of major concern and several assessment studies in natural waters were already undertaken. However, no consensus arised about the environmental factors influencing their hazardous impact, but rather contrary conclusions were drawn. In this study, the acute toxicity of commercial TiO2 nanoparticles suspensions at different concentrations on microcrustacean (Daphnia magna), marine rotifers (Brachionus plicatilis) and marine microalgae (Phaeodactylum tricornutum) under environmental conditions, in synthetic fresh and marine water, was investigated. Factors driving TiO2 adverse effects on aquatic environment, such as allotropic form, primary particle size, surface area, particle concentration and agglomerate size were studied. A thorough characterization of both surface and bulk properties of nano-sized TiO2 particles was therefore performed. Our results showed that Daphnia magna test is the most sensitive test for assessing toxicity of TiO2 samples on aquatic organisms. For anatase samples, toxicity towards aquatic organisms depends (i) on the primary particle size and the extent of agglomeration (mass median diameter d50), and consequently on surface reactivity (total surface site concentration, specific surface area, pHIEP) (ii) on the presence of rutile impurities in the sample.
Toxicity results of rutile and anatase samples of comparable primary particle size (sub micrometric range) are of same order of magnitude and remained less toxic than nanometric particles. Rutile agglomeration was found to be higher than anatase agglomeration, toxicity results obtained for rutile could be attribute to the shape of particles, or it could be due to the presence of BaTiO3 impurities.
This work emphasized the importance of studying the effects of different bulk and surface parameters of engineered TiO2 nanoparticles to understand their reactivity towards micro-organisms under environmental conditions.