Synergies of surface roughness and hydration on colloid detachment in saturated porous media: Column and atomic force microscopy studies

Saturated column experiments were conducted to systematically examine the influence of hydration on the detachment of nano- and micro-sized latex colloids (35 nm and 1 μm, respectively) from sand. The colloids were attached on the sand in primary minima (PM) using high ionic strength (IS) NaCl solutions. The PM were predicted to be shallower and located farther from sand surfaces with increasing IS due to the hydration force. Consequently, a greater amount of colloid detachment occurred in deionized water when the colloids were initially deposited at a higher IS. Atomic force microscopy (AFM) examinations showed that both nanoscale protruding asperities and large wedge-like valleys existed on the sand surface. The influence of these surface features on the interaction energies/forces was modeled by approximating the roughness as cosinoidal waves and two intersecting half planes, respectively. The PM were deep and attachment was irreversible at concave regions for all ISs, even if the hydration force was included. Conversely, colloids were weakly attached at protruding asperities due to a reduced PM depth, and thus were responsible for the detachment upon IS reduction. The AFM examinations confirmed that the adhesive forces were enhanced and reduced (or even completely eliminated) at concave and convex locations of sand surfaces, respectively. These results have important implications for surface cleaning and prediction of the transport and fate of hazardous colloids and colloid-associated contaminants in subsurface environments.