Transport of citrate-coated silver nanoparticles in saturated porous media

In this study, the influences of physical and chemical factors [e.g., ionic strength (IS), pH, and flow rate] on the fate and transport of citrate-coated silver nanoparticles (AgNPs) were investigated through experiments using saturated columns. For the transport behavior of AgNPs under various conditions, retardation was confirmed with an increase in ionic strength (IS) while early elution developed with an increase in pH and flow rate. These transport experiment outcomes were simulated through Hydrus-1D, and the observed breakthrough curves were confirmed to have a significant correlation with the fitted results. Interestingly, the AgNPs and quartz sand used in this study showed a negative charge in the investigated experimental conditions. Although the reaction between AgNPs and quartz sand was expected to be unfavorable, AgNPs were observed to have been deposited onto the sand surface during the column test. To clarify the mechanism of the deposition of AgNPs even in unfavorable conditions, the interaction energy profiles were calculated based on the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. From the results, unfavorable interactions were expected in the NP–NP and NP–sand interactions in every condition. It was concluded that the deposition of AgNPs onto the sand surface under the unfavorable conditions in this study was mainly because of the physical roughness of the sand surface. Moreover, this hypothesis was supported by the zone of influence calculation in accordance with IS, the interpretation results of the fractional sand surface coverage in accordance with concentration changes of AgNPs, and series column tests.