The transformation of zinc oxide nanoparticles (ZnO NPs) has been largely investigated in wastewater treatment plants, recognized as important intermediates before NPs discharge into the environment. However, considering direct releases of the pristine ZnO NPs forms, additional studies on ZnO NPs fate in different environmental compartments are encouraged. In this work, we investigated ZnO NPs transformation in lacustrine sediments under defined redox conditions. Using X-ray absorption spectroscopy and wet chemical analyses, we followed ZnO NPs and Zn2+ fate over a three-month period in sediments incubated under oxic or anoxic sulfide-rich conditions. Under oxic conditions, ZnO NPs dissolved within a few hours. By contrast, ZnO NPs dissolution under anoxic conditions was much slower, with ~19% of ZnO NPs remaining at the end of the incubation, together with ~41% of ZnS, ~15% of Zn adsorbed onto phyllosilicates and ~27% of Zn-phyllosilicate-like species. The transient formation of Zn-organic complexes under oxic conditions supports that ZnO NPs dissolution is driven by organic compounds, followed by Zn adsorption onto phyllosilicates and the subsequent formation of Zn-layered minerals. Under anoxic conditions, ZnO NPs dissolution is inhibited by the precipitation of amorphous ZnS and controlled by the progressive growth of ZnS NPs. These results improve the understanding of ZnO NPs transformation in slightly alkaline freshwater sediments and highlight the need to assess NPs fate under environmentally relevant conditions.

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