TiO2 nanoparticles enhance bioaccumulation and toxicity of heavy metals in Caenorhabditis elegans via modification of local concentrations during the sedimentation process

Unintentionally released titanium dioxide nanoparticles (TiO2 NPs) may co-occur with pre-existing heavy metal pollutants in aquatic environments. However, the interactions between NPs and heavy metals (HMs) and their co-effects in living organisms are largely unknown. The aim of this investigation was to illustrate the influence of TiO2 NPs (5 and 15 nm) on the bioaccumulation and toxicity of cadmium (Cd), arsenate (As(III)), and nickel (Ni) in Caenorhabditis elegans (C. elegans) during the process of sedimentation in aquatic environment. Our data showed that HMs accelerated the aggregation of TiO2 NPs. The rapid aggregation and sedimentation of TiO2 NPs changed the vertical distribution of HMs through adsorption and induced increased and prolonged exposure of benthic species. Aggregate particle size along with ionic strength were the major factors affecting the rate of sedimentation. TiO2 NPs at non-toxic concentrations efficiently enhanced the bioaccumulation and reproductive toxicity of HMs to C. elegans in a dose- and size-dependent manner; however, the effect of TiO2 NPs on As(III) was obviously lower than that on two valence metals. These data provided clear evidence that TiO2 NPs could serve as environmental regulators to significantly facilitate the toxicity and the accumulation of HMs in C. elegans, indicating that the interaction and fate of TiO2 NPs and HMs on their co-toxic responses during the sedimentation should be considered as a necessary and integral part of risk assessment in the ecological system.