Transport and retention of silver nanoparticles in soil: Effects of input concentration, particle size and surface coating

Soils are considered as a major sink for engineered nanoparticles (ENPs) because of their inevitable release to the subsurface environment during production, transportation, use and disposal processes. In this context, the transport and retention of silver nanoparticles (AgNPs) with different input particle concentration, particle size, and surface coating were investigated in clay loam using water-saturated column experiments. Our results showed that the mobility of AgNPs in the soil was considerably low, and >73.9% of total injected AgNPs (except for no coating condition) was retained in columns. This is primarily due to the high specific surface area and favorable retention sites in soil. Increased transport of AgNPs occurred at higher input concentration and smaller particle size. The presence of surface coatings (i.e., polyvinylpyrrolidone (PVP) and citrate) further promoted the transport and reduced the retention of AgNPs in soil, which is likely due to their effective blocking of the solid-phase sites that are originally available for AgNPs retention. Although the shape of retention profiles (RPs) of AgNPs was either hyperexponential or nonmonotonic that is different from the colloid filtration theory prediction, the 1-species (consider both time- and depth-dependent retention) and/or 2-species (account for the release of reversibly deposited AgNPs) model successfully described the transport behaviors of AgNPs in soil columns under all the investigated conditions. This study proves the applicability of mathematical model in predicting the fate and transport of ENPs in real soils, and our findings presented herein are significant to ultimately develop management strategies for reducing the potential risks of groundwater contamination due to ENPs entering the environment.