Fate of TiO2 nanoparticles entering sewage treatment plants and bioaccumulation in fish in the receiving streams

- Ti in colloids of WWTP effluents and receiving streams were mainly from nano-TiO2.
- Most of nano-TiO2 (74-85%) in the influent was removed by the activated sludge.
- WWTP effluents and other sources, such as runoff, contribute to Ti in receiving water.
- Ti was accumulated in edible fish tissue due to the long term exposure.

Wastewater treatment plants (WWTPs) can be important point sources for the discharge of engineered nanomaterials to natural surface waters. In this study, we investigated the fate of nano-TiO2 in two full scale WWTPs and their receiving streams in northern China (i.e., the Binhe and Minxin rivers, which flow to the Xiaohe River). High resolution transmission electron microscopy, energy dispersion X-ray spectroscopy and X-ray photoelectron spectroscopy analyses indicated that Ti in colloids present in the WWTP effluents and receiving streams originated mainly from engineered nano-TiO2. Most of the nano-TiO2 (74-85%) in the influent sewage was removed by the activated sludge, but significant concentrations (27-43 μg/L Ti) remained associated with colloids in the effluents. Surprisingly, the total concentration of Ti in the receiving river streams was higher (52-86 μg/L), indicating the importance of other sources such as urban runoff. These relatively high Ti concentrations are unlikely to originate from natural sources since they are significantly higher than those in the nearby Yuqiao Reservoir (< 5 μg/L), representing regional background levels. Consistently, Ti levels in fish collected from the Xiaohe River (including muscle and other tissues) were significantly higher than those of fish from the reservoir. Overall, TiO2 releases through WWTP effluents and other sources (including runoff) are contributing to the relatively high observed concentrations of suspended Ti in the Xiaohe River, which results in accumulation in edible fish tissue.