Potential impacts of titanium dioxide nanoparticles on trace metal speciation in estuarine sediments

Engineered titanium dioxide (TiO₂) nanoparticles (NPs) are widely used and consequently released into the environment. The subsequent accumulation of TiO₂ NPs in depositional environments may affect the geochemical behavior of trace metals, which needs to be assessed. Here, we performed experiments to investigate the speciation change for molybdenum and tungsten in the presence of TiO₂ NPs. Laboratory results show that the rate constant for MoS₄^2- hydrolysis associated with TiO₂ NPs is ~1.75 × 10^-9 L m^-2 s^-1, whereas it is 5.95 × 10^-10 L m^-2 s^-1 for WS₄^2- hydrolysis. In addition, we estimated the maximum rate for MoS₄^2- hydrolysis to be ~1.24 × 10^-1 μM hr^-1, whereas the maximum rate for WS₄^2- hydrolysis is ~4.91 × 10^-2 μM hr^-1. However, the modeling results suggest that the TiO₂ NPs accumulated in estuarine sediments might play a relatively minor role in affecting the speciation of trace metals prior to the current time. This is because the relatively low accumulation (i.e., < 8 × 10^-3 mol kg^-1) of TiO₂ NPs before 2021 results in the lower rate (>100 times) for speciation changes of both molybdenum and tungsten compared to the rate for natural geochemical processes. On the other hand, our results suggest that TiO₂ NPs will likely impact the oxyanion cycling in the near future owing to the increasing accumulations of TiO₂ NPs in estuarine sediments.