The Effects of Manufactured Nanoparticles on Clay and Soil Dispersion: A Tale of Two Sizes.

Zinc Oxide nanoparticles (ZnO-np) interaction with clay minerals may have a dominant role in regulating their mobility as well as their potential impacts on biogeochemical processes. In a previous study by the authors, it was shown that ZnO-np with a reported primary particle size of 2-6 nm (Pinnacle^AF ZnO Applied Nanoworks) exhibited sorption behavior similar to that of solvated divalent cations held on exchange sites. This year’s research studies the influence of particle size on ZnO-np sorption behavior as well as their effect on clay and soil dispersion. To that end, an 80-nm bZnO-np (ZnO NanoGard^® ) was chosen to contrast with the smaller size (sZnO-np) previously used. Batch sorption experiments were conducted to evaluate bZnO-np sorption to Montmorillonite (Swy-2, Wyoming USA). To overcome experimental limitations associated with discriminating between particle sorption and aggregation in batch reactors, dynamic flow through experiments were carried out. All flow-through experiments were conducted in an up-flow manner to reduce the impact of particle aggregation and settling on observed migration behavior. Plexiglas columns were packed with Ottawa sand that had been coated with the clay sorbent (≈0.5 % by mass) and saturated at 0.5 mL min^-1 with a nano-particle free background solution (1 mM CaCl₂) prior to the introduction of the bZnO-np. Ottawa sand columns without the clay sorbent were used to quantify the impact of straining on sorption behavior. Pressure buildup at the column inlet was monitored as an indicator of column plugging resulting from particle nano-particle capture. Tritiated water was included in the nano-particle solution to determine the physical processes (i.e., flow rate, porosity, hydrodynamic dispersion) impacting bZnO-np migration through the columns. Preliminary results show that in control columns containing only Ottawa sand, less than 2% bZnO-np breakthrough was observed. Despite particle retention, no reduction in column permeability was recorded. .