Nanoparticle transport in partially saturated porous media: Attachment at fluid interfaces

• The interface between water and a trapped non-wetting phase in porous media acts as a nanoparticle collector in a manner vastly different from the solid-water interface. • Interplay between kinetically-limited irreversible attachment and nanoparticle exchange via diffusion between advective and non-advective parts of the pore space determines macroscopic behavior in the presence of a trapped nonwetting phase. • Pore network modeling is an effective tool for upscaling pore-level physics of nanoparticle attachment at fluid interfaces. Like the solid-water interface (SWI), air-water and oil-water interfaces (AWI and OWI) also act as collectors for nano-sized particles in porous media. The attachment of hydrophobic nanoparticles, which is often favorable and irreversible, is of particular interest because the transport and retention of such particles is closely linked to the fate of nanoplastics in unsaturated subsurface environments and the success of nanoremediation practices. Here, we show how a pore-network model (PNM) can be used to upscale the kinetics and extent of irreversible nanoparticle attachment at a single fluid-fluid interface under conditions of advection and dispersion in a sphere packing. By focusing on a trapped (immobile) non-wetting phase, we highlight a fundamental difference between the single-collector contact efficiency of AWI/OWI and SWI. Namely, AWI/OWI collectors, which are largely by-passed by the flowing aqueous phase, are exposed to a hydrodynamic environment dominated by diffusion. This difference has profound implications for the modelling of nanoparticle transport in porous media at the continuum (Darcy) scale. This study reveals the potential of pore network modelling as an essential complement to continuum models for upscaling the behavior of nanocolloids in porous media.