Hydrophobic Interaction Effects on the Transport of a Model Nanoplastic in 2D and 3D Porous Media

The attraction between a hydrophobic particle and a hydrophobic surface may be strong enough for irreversible attachment to take place, even under conditions of strong electrostatic repulsion (so called “unfavorable” attachment conditions). This fact has fundamental implications for the transport and retention of hydrophobic nano-colloids (i.e., nanoplastics) in subsurface aquatic environments, where hydrophobic surfaces and interfaces are ubiquitous. Inclusion of hydrophobic attraction in extended DLVO calculations of the total interaction potential between hydrophobic negatively charged ethyl cellulose nanoparticles (a model nanoplastic) and (i) glass surfaces rendered hydrophobic via treatment with octadecyltrichlorosilane (OTS) or (ii) naturally hydrophobic air-water interfaces, indicate the absence of a barrier to attachment and support an expectation of irreversible attachment. We present here a series of experiments in saturated and unsaturated 2D (pore networks etched on glass) and 3D (columns packed with glass beads) porous media which confirm this expectation. The ability of a continuum model accounting for advection, dispersion and irreversible attachment to describe the breakthrough curves is also tested. The results advance the ability to describe the fate of hydrophobic nano-colloids in porous media for a variety of applications.