Evidence for the critical role of nanoscale surface roughness on the retention and release of silver nanoparticles in porous media

Authors: LIANG, YAN - Guangxi University; ZHOU, JINI - Guangxi University; DONG, YAWEN - Guangxi University; KLUMPP, ERWIN - Agrosphere Institute; SIMUNEK, JIRI - University Of California; Bradford, Scott

Submitted to: Environmental Pollution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2019
Publication Date: 12/13/2019
Citation: Liang, Y., Zhou, J., Dong, Y., Klumpp, E., Simunek, J., Bradford, S.A. 2020. Evidence for the critical role of nanoscale surface roughness on the retention and release of silver nanoparticles in porous media. Environmental Pollution. 258. https://doi.org/10.1016/j.envpol.2019.113803.
DOI: https://doi.org/10.1016/j.envpol.2019.113803

Interpretive Summary: Soil grain roughness has been theoretically demonstrated to influence the fate of colloids (e.g., microorganisms, clays, and nanoparticles) in soils and aquifers, but little experimental evidence has quantitatively examined this issue. Experiments were conducted to systematically study the mobility of silver nanoparticles (AgNPs) in sands with different roughness properties under various solution chemistry conditions. Results demonstrated that AgNP transport and release was greatly enhanced on smooth sand in comparison to the rough sand, at higher pH and lower ionic strength (IS), and for larger input concentrations. This behavior was explained by the pronounced influence of roughness on AgNPs interactions with grain surfaces. This information will be of interests to scientists and engineers concerned with the fate colloids in the environment that pose a risk to human and/or ecosystem health (e.g., pathogenic microorganisms and toxic nanoparticles).

Technical Abstract: Although nanoscale surface roughness has been theoretically demonstrated to be a crucial factor in the interaction of colloids and surfaces, little experimental research has investigated the influence of roughness on colloid or silver nanoparticle (AgNP) retention and release in porous media. This study experimentally examined AgNP retention and release using two sands with very different surface roughness properties over a range of solution pH and/or ionic strength (IS). AgNP transport was greatly enhanced on the relatively smooth sand in comparison to the rougher sand, at higher pH, and lower IS and fitted model parameters showed systematic changes with these physicochemical factors. Complete release of the retained AgNPs was observed from the relatively smooth sand when the solution IS was decreased from 40 mM NaCl to deionized (DI) water and then the solution pH was increased from 6.5 to 10. Conversely, less than 40% of the retained AgNPs was released in similar processes from the rougher sand. These observations were explained by differences in the surface roughness of the two sands which altered the energy barrier height and the depth of the primary minimum with solution chemistry. Limited numbers of AgNPs apparently interacted in reversible, shallow primary minima on the smoother sand, which is consistent with the predicted influence of a small roughness fraction (e.g., pillar) on interaction energies. Conversely, larger numbers of AgNPs interacted in deeper primary minima on the rougher sand, which is consistent with the predicted influence at concave locations. These findings highlight the importance of surface roughness and indicate that variations in sand surface roughness can greatly change the sensitivity of nanoparticle transport to physicochemical factors such as IS and pH due to the alteration of interaction energy and thus can strongly influence nanoparticle mobility in the environment.