Hyperexponential and nonmonotonic retention of polyvinylpyrrolidone-coated silver nanoparticles in an Ultisol.

The increasing application of engineered nanoparticles (ENPs) has heightened the concern that these ENPs would eventually be released to the environment and may enter into life cycle of living beings. In this regard, it is essential to understand how these ENPs transport and retain in natural soils because they are considered to be a major repository for ENPs. Herein, transport and retention of polyvinylpyrrolidone (PVP)-coated silver nanoparticles (PVP-AgNPs) were investigated over a wide range of physicochemical factors in water-saturated columns packed with an Ultisol rich in clay-size particles. Higher mobility of PVP-AgNPs occurred at larger soil grain size, lower solution ionic strength and divalent cation concentration, higher flow rate, and greater PVP concentrations. Most breakthrough curves (BTCs) for PVP-AgNPs exhibited significant amounts of retardation in the soil due to its large surface area and quantity of retention sites. In contrast to colloid filtration theory, the shapes of retention profiles (RPs) for PVP-AgNPs were either hyperexponential or nonmonotonic (a peak in particle retention down-gradient from the column inlet). The BTCs and hyperexponential RPs were successfully described using a 1-species model that considered time- and depth-dependent retention. Conversely, a 2-species model that included reversibility of retained PVP-AgNPs had to be employed to better simulate the BTCs and nonmonotonic RPs. As the retained concentration of species 1 approached the maximum solid-phase concentration, a second mobile species (species 2, i.e., the same PVP-AgNPs that are reversibly retained) was released that could be retained at a different rate than species 1 and thus yielded the nonmonotonic RPs. Some retained PVP-AgNPs were likely to irreversibly deposit in the primary minimum associated with microscopic chemical heterogeneity (favorable sites). Transmission electron microscopy and energy-dispersive X-ray spectroscopy analysis suggested that these favorable sites were positively charged sites on montmorillonite edges and goethite surfaces in the soil. Overall, our study highlights that the transport and especially retention of PVP-AgNPs are highly sensitive to the physicochemical factors, but mathematical modeling can accurately predict the fate of these ENPs in porous media which is important for better understanding the fate of these ENPs in point of exit and in the environment.