Simulating the fate and transport of ZnO nanoparticles in a Tidal River: Coupling a form-specific material flow analysis model to a hydrodynamic fate model

Predicting the environmental fate of engineered nanomaterials (ENM) with high spatial resolution under realistic environmental conditions is key for a high-tier assessment of ENM exposure in the environment. A crucial step in this process is to link release assessments based on material flow analysis (MFA) with the fate and transport models. This paper presents a novel model that couples a form-specific probabilistic material flow analysis (PMFA) release model with a highly spatiotemporally resolved fate and transport river model. The effects of tides and the experimentally derived dissolution rate are incorporated into the modeling to accurately reflect the realistic environmental conditions of the study area, a coastal river in southern Taiwan. The PMFA results show that the pristine form of ZnO nanoparticles released into surface waters accounts for 89 % of the total ZnO nanoparticles released to surface waters, due to the limited coverage of the wastewater treatment system. Dissolution was the predominant fate process for ZnO nanoparticles in the Yanshuei River, while heteroaggregation was less important. Free ZnO nanoparticles only occurred sporadically and were noticeable at the discharge points, with the highest mean steady-state concentration of 0.9 μg/L. Free Zn ion was the major ZnO nanoparticles-derived product species, with an average steady-state concentration that can accumulate downstream to 7 μg/L. A sensitivity analysis indicated the importance of dissolution at dissolution rates k_diss > 3 d⁻¹, while heteroaggregation became important when k_diss ≤ 0.1 d⁻¹. The tides significantly affected the distributions of Zn species along the river. Within 2 months of simulation time, the high tide resulted in the accumulation of Zn species as much as 3 times higher at the river sections receiving large loads, while the low tide drained the plumes of Zn species. The study highlights the important considerations of the realistic local ENM release, including the ENM forms, in combination with the highly spatiotemporal fate and transport modeling, which is essential for the exposure assessment as a part of ecological risk assessment.