Plastic movement in variably saturated porous media: The role of ionic strength and surface roughness

We investigated colloid deposition mechanisms in saturated and unsaturated porous medias, focusing on roles of ionic strength (IS, 1-100 mM) and collector surface roughness. Through a customized column setup, experiments were conducted with smooth and rough (HF-etched) glass beads. Rough collectors featured fractal, multi-scale surface craters. Following colloid application, we evaluated released colloid via flushing with 0.4 mM NaHCO₃ solutions. Finally, column dissection was performed to quantify moderately and tightly attached colloids within the primary minimum. Increasing IS reduced mobile colloid while enhanced released, primary minimum moderately and tightly attached colloid. These trends align with DLVO theory when considering various sizes of positively charged heterogeneity (CH) on collector. As IS increased, colloid retention profiles (CRPs) shifted from nonlinear to linear, signaling a transition from unfavorable to favorable attachment. Under unsaturated conditions, colloid retention increased and exhibited linear CRPs at IS >1 mM, driven by enhanced interception and air-water-solid (AWS) interfaces trapping. Roughness unexpectedly enhanced colloid mobility by reducing colloid delivery via larger hydrodynamic forces and weakening attachment on nanoscale surface roughness. Under saturated conditions, surface roughness minimally affected colloid tight attachment in primary minimum due to a trade-off between the inhibitory effect of nanoscale craters and the promotional effect of HF etching-induced additional CH. Conversely, under unsaturated conditions, collloid tight attachment in primary minimum was enhanced, resulting from additional CH formation and enhanced colloid trapping at AWS. These findings highlight the complex interplay between physical roughness and chemical factors, advancing our understanding of environmental colloid transport.