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

Authors: LI, TIANTIAN - China Agricultural University; SHEN, CHONGYANG - China Agricultural University; WU, SEN - Tianjin University; JIN, CHAO - Sun Yat-Sen University; Bradford, Scott

Submitted to: Water Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/15/2020
Publication Date: 6/20/2020
Citation: Li, T., Shen, C., Wu, S., Jin, C., Bradford, S.A. 2020. Synergies of surface roughness and hydration on colloid detachment in saturated porous media: Column and atomic force microscopy studies. Water Research. 183. https://doi.org/10.1016/j.watres.2020.116068.
DOI: https://doi.org/10.1016/j.watres.2020.116068

Interpretive Summary: The release of colloids (e.g., small particles like microorganisms and clays) from surfaces is important for many industrial (e.g., water treatment) and environmental applications (e.g., fate of pathogens). This paper explores the roles of solution ionic strength and surface roughness on colloid release. The solution chemistry sequence was an important factor contributing to colloid release. More release occurred with a reduction in ionic strength when colloids were initial deposited at higher ionic strength in the presence of hydrated cations. Furthermore, the release of colloids from the surface roughness tops was much more pronounced than for the valleys due to differences in the strength of colloid interactions. This information will be of interest to scientists and engineers concerned with the removal of colloids from surfaces.

Technical Abstract: 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 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 were modeled by approximating the roughness as cosinoidal waves and two intersecting half planes, respectively. The PM were deep and attachments were irreversible at convex 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. These results have important implication to surface cleaning and prediction of the transport and fate of hazardous colloids and colloid-associated contaminants in subsurface environments.