Survey of food-grade silica dioxide nanomaterial occurrence, characterization, human gut impacts and fate across its lifecycle.

There is increasing recognition of the importance of transformations in nanomaterials across their lifecycle, yet few quantitative examples exist. We examined food-grade silicon dioxide (SiO2) nanomaterials from its source (bulk material providers), occurrence in food products, impacts on human gastrointestinal tract during consumption, and fate at wastewater treatment plants. Based upon XRD, XPS and TEM analysis, pure SiO2 present in multiple food-grade stock SiO2 exhibited consistent morphologies as agglomerates, ranging in size from below 100nm to >500nm, with all primary particle size in the range of 9-26nm and were most likely amorphous SiO2 based upon high resolution TEM. Ten of 14 targeted foods purchased in the USA contained SiO2 of the same morphology and size as the pristine bulk food-grade SiO2, at levels of 2 to 200mg Si per serving size. A dissolution study of pristine SiO2 showed up to 7% of the dissolution of the silica, but the un-dissolved SiO2 maintained the same morphology as the pristine SiO2. Across a realistic exposure range, pristine SiO2 exhibited adverse dose-response relationships on a cell model (microvilli) of the human gastro-intestinal tract, association onto microvilli and evidence that SiO2 lead to production of reactive oxygen species (ROS). We also observed accumulation of amorphous nano-SiO2 on bioflocs in tests using lab-cultured activated sludge and sewage sludges from a full-scale wastewater treatment plant (WWTP). Nano-scale SiO2 of the same size and morphology as pristine food-grade SiO2 was observed in raw sewage at a WWTP, but we identified non-agglomerated individual SiO2 particles with an average diameter of 21.5±4.7nm in treated effluent from the WWTP. This study demonstrates an approach to track nanomaterials from source-to-sink and establishes a baseline occurrence of nano-scale SiO2 in foods and WWTPs.