Natural, Incidental, and Engineered Nanomaterials in Surface Waters: Occurrence and Catalytic Reactivity Influences on Micropollutant Degradation Plus Nutrient Turnover.

Nanomaterials (NMs)─whether natural, incidental, or engineered─are now documented to occur in aquatic environments, with concentrations of elements such as titanium, cerium, and palladium exceeding tens of parts per billion. While prior research has emphasized the toxicology of engineered NMs, their broader geochemical roles remain underexplored. Catalytically active NMs can influence key environmental processes, such as nutrient cycling and pollutant degradation, through photocatalytic, hydrolytic, and hydrogenation mechanisms. Reactive oxygen species (ROS) (including hydroxyl radicals, singlet oxygen, and hydrogen peroxide) produced via UVA or visible light photocatalysis on NM surfaces drive important transformations. Steady-state ROS concentrations (0.1-10 fM) are comparable to those from excited-state dissolved organic matter (DOM*). NMs can hydrolyze and facilitate the conversion of organic phosphorus to bioavailable inorganic forms or organic carbon to low-molecular-weight compounds, potentially fueling microbial food webs. However, major gaps remain regarding NM diversity, reactivity, and persistence. Addressing these requires integrating advanced nanoanalytical tools (e.g., ICP-MS, electron microscopy with EELS) and functional reactivity assays applied to environmentally sourced NMs over ecologically relevant time scales of days to months. This Perspective highlights NMs as dynamic, transient components of aquatic systems introduced through deposition, runoff, and biogenic activity, with implications for global biogeochemical cycling.