Transport and Retention of Unstable Nanoparticle Suspensions in Porous Media: Effects of Salinity and Hydrophobicity Observed in Microfluidic Pore Networks 

Transport of hydrophobic nanoparticles in porous media is of growing interest in relation to the presence of nanoplastics and engineered NPs in the subsurface, where both hydrophobic collector surfaces and high salt concentrations may be simultaneously present by accident or design. This study investigates the effects of NP input concentration, surface wettability, and salt valence on the transport and deposition of model hydrophobic ethyl cellulose (EC) NPs in 2D hydrophilic and hydrophobic microfluidic networks under flow conditions representative of subsurface environments. Uniform porous geometries and low hydrodynamic dispersion enhance NP residence time, promoting aggregation and irreversible retention, particularly in immobile zones. Our results show that higher NP concentrations and the presence of divalent cations (Ca²⁺) result in fractal aggregate formation, permeability loss, and formation of secondary porosity, altering flow paths and elution behavior. Direct porous medium visualization during and after experiments reveals transient flocculation, post-flush release, and pore structure changes such as pore throat occlusion and dead-end zone accumulation. Surface wettability further modulates transport; hydrophobic collectors enable irreversible attachment of mostly single particles via hydrophobic interactions. Fluorescence imaging, extended DLVO theoretical calculations, particle remobilization and permeability measurements corroborate observations of nanoparticle elution, showing that the effect on NP and aggregate deposition of surface hydrophobicity outweighs that of monovalent salt, whereas divalent salt accelerates and promotes irreversible deposition. Traditional interpretation of breakthrough curves does not resolve key microscale retention mechanisms under varying physicochemical conditions. In the presence of hydrophobic attraction between particles and collector surfaces, coagulation induced by high ionic strength results in complex interactions that shape transport, aggregation, and retention of hydrophobic NPs in porous media. These findings offer critical insights into the fate of hydrophobic NPs in subsurface environments for risk assessment and design of nanoremediation interventions in the form of injectable permeable adsorptive barriers.