Coupled effects of hydrodynamic and solution chemistry on long-term nanoparticle transport and deposition in saturated porous media

This study aims to systematically explore the coupled effects of hydrodynamic and solution chemistry conditions on the long-term transport and deposition kinetics of nanoparticles (NPs) in saturated porous media. Column transport experiments were carried out at various solution ionic strengths (IS), ionic composition, and flow velocities utilizing negatively charged carboxyl-modified latex NPs of two different sizes (50 and 100 nm). These experiments were designed to obtain the long-term breakthrough curves (BTCs) in order to unambiguously determine the full deposition kinetics and the fraction of the solid surface area (Sf) that was available for NP deposition. The BTCs exhibited a bimodal shape with increasing solution IS; i.e., BTCs were initially delayed, next they rapidly increased, and then they slowly approached the influent particle concentration. NP deposition was much more pronounced in the presence of Ca2+ than Na+ at any given solution IS. Deposition kinetic of NPs was successfully simulated using a two-site kinetic model that accounted for irreversible deposition and blocking on each site, i.e., a decreasing deposition rate as the site filled. Results showed that Sf values were controlled by the coupled effects of flow velocity, solution chemistry, and particle size. Data analyses further demonstrated that only a small fraction of sand surface area contributed in NP deposition even at the highest IS (60 mM) and lowest flow velocity (1 m/day) tested. Consistent with previous studies, our results imply that NP deposition is controlled by physicochemical interactions between the NPs and nanoscale physical and/or chemical heterogeneities on the sand surfaces that produce localized nanoscale favorable sites for deposition. Furthermore, our results suggest that the NP interactions with the collector surfaces tended to strengthen with increasing contact time.