Quaternary Ammonium Compounds (QACs) are man-made surfactants with uses in everyday products such as shampoo, laundry detergents, and very importantly, disinfectants. During the COVID-19 pandemic, QAC use has greatly increased, leading to increased release to environmental waters.  It is well-known that surfactants such as QACs can strongly affect nanoparticle stability (aggregation, flocculation), indicating that QAC release could impact their behavior in the environment.  Here, we report our studies to examine this possibility with iron oxide nanoparticles due to their environmental ubiquity.  Synthesized α-Fe<sub>2</sub>O<sub>3</sub> nanoparticles (~40 nm diameter) were combined with alkyltrimethylammonium chlorides with varying alkyl chain lengths (8 and 16-carbon, denoted as QAC-8 and QAC-16) at environmentally-relevant concentrations (0, 3.6, 36 and 360 nM).  These solutions were subjected to rotary (end-over-end) mixing for 22 hours at room temperature.  Solution vials were either full (no air-water interface present during mixing) or half-full to examine the role of an air-water interface on nanoparticle stability.  After mixing, the concentration of Fe remaining in solution was determined via elemental analysis and absorption spectroscopy.  Without QACs in solution, in half-full mixing vials, the nanoparticles aggregated during the mixing and settled out or adhered to the vial walls. Half-full vials of nanoparticle solutions with 360 nM QAC-16 were more stable than controls (p = 0.007) , without aggregation or settling  and a ~1.75x greater [Fe].  Solutions with 3.6 and 36 nM QAC-16 were not stabilized relative to control (p &gt; 0.05).  Trials performed with full vials (no air-water interface) remained stable even in the absence of QAC-16, indicating that stabilization by QACs occurs due to surface tension changes.  Preliminary data on QAC-8 - nanoparticle mixtures are also reported.

Influence of quaternary ammonium disinfectants on the aggregation or solution stability of mineral nanoparticles

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