A review on the generation, discharge, distribution, environmental behavior, and toxicity (especially to microbial aggregates) of nano-TiO2 in sewage and surface-water and related research prospects

This article reviews the nano-effects and applications of different crystalline nano‑titanium dioxide (nano-TiO2), identifies their discharge, distribution, behavior, and toxicity to aquatic organisms (focusing on microbial aggregates) in sewage and surface-water, summarizes related toxicity mechanisms, and critically proposes future perspectives. The results show that: 1) based on crystal type, application boundaries of nano-TiO2 have become clear, extending from traditional manufacturing to high-tech fields; 2) concentration of nano-TiO2 in water is as high as hundreds of thousands of μg/L (sewage) or several to dozens of μg/L (surface-water) due to direct application or indirect release; 3) water environmental behaviors of nano-TiO2 are mainly controlled by hydration conditions and particle characteristics; 4) aquatic toxicities of nano-TiO2 are closely related to their water environmental behavior, in which crystal type and tested species (such as single species and microbial aggregates) also play the key role. Going forward, the exploration of the toxicity mechanism will surely become a hot topic in the aquatic-toxicology of nano-TiO2, because most of the research so far has focused on the responses of biological indicators (such as metabolism and damage), while little is known about the stress imprint caused by the crystal structures of nano-TiO2 in water environments. Additionally, the aging of nano-TiO2 in a water environment should be heeded to because the continuously changing surface structure is bound to have a significant impact on its behavior and toxicity. Moreover, for microbial aggregates, comprehensive response analysis should be conducted in terms of the functional activity, surface features, composition structure, internal microenvironment, cellular and molecular level changes, etc., to find the key point of the interaction between nano-TiO2 and microbial aggregates, and to take mitigation or beneficial measures to deal with the aquatic-toxicity of nano-TiO2. In short, this article contributes by 1) reviewing the research status of nano-TiO2 in all aspects: application and discharge, distribution and behavior, and its aquatic toxicity; 2) suggesting the response mechanism of microbial aggregates and putting forward the toxigenic mechanism of nanomaterial structure; 3) pointing out the future research direction of nano-TiO2 in water environment.