Transport and retention of stabilized silver nanoparticles in porous media

Due to the widespread application of silver nanoparticles (AgNPs) and the resulting potential exposure in the environment, information about their environmental transport and fate is essential for risk assessment. AgNPs are commonly modified with functional groups, surfactants, or polymers to increase their stability in liquids and the surface modification greatly influences the environmental behavior of AgNPs. The aim of this study is therefore to investigate the transport and retention of surfactant-stabilized AgNPs under environmentally relevant conditions. Experiments were conducted with water-saturated columns packed with quartz sand, around 90% water-saturated columns filled with undisturbed loamy sand soil, and a lysimeter. Inductively coupled plasma-mass spectrometry/optical emission spectrometry (ICP-MS/OES) was used to analyze the concentrations of AgNPs, Ca 2+ , K + , Fe, and Al. The experimental breakthrough curves (BTCs) and retention profiles (RPs) from column experiments were described using a numerical model that considers timeand depth-dependent retention. Column experiments with quartz sand packing were also conducted in the presence of surfactant to deduce the influence of surfactant on AgNP transport, especially on the spatial distribution of retained AgNPs that determines the long-term transport potential. In addition, to better understand the interactions of AgNPs and the matrix in the environment, remobilization of retained AgNPs from undisturbed soil was studied by changing the solution chemistry such as change of cation types and ionic strength reduction. Experimental results showed that the normalized concentration in BTCs for AgNPs obtained from water-saturated columns increased with a decrease in solution ionic strength (IS), and an increase in flow velocity (q), sand grain size, and input concentration (Co). In contrast to the conventional filtration theory, RPs in sand exhibited uniform, nonmonotonic, or hyperexponential shapes that were sensitive to physicochemical conditions. The simulated retention rate coefficient (k1) and maximum retained concentration on the solid phase (Smax) increased with IS and as the grain size and/or Co decreased. The RPs were more hyperexponential in finer textured sand and at lower Co because of their higher values of Smax, which indicated a larger retention capacity of the porous media. Conversely, RPs were nonmonotonic or uniform at higher Co and in coarser sand that had lower values of Smax, and tended to exhibit higher peak concentrations in the RPs at lower flow velocities and at higher solution IS. These observations indicate that uniform and nonmonotonic RPs occurred under conditions when Smax was filled. The sensitivity of the nonmonotonic RPs to IS and flow velocity in coarser textured sand indicates that AgNPs were partially interacting in a secondary energy minimum according to the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory. However, elimination of the secondary minimum only produced recovery of a small portion (<10%) of the retained AgNPs. These results imply that AgNPs were largely irreversibly interacting in a primary minimum associated with microscopic heterogeneity of the porous media. Nonmonotonic RPs had peak concentrations at greater distances when larger amounts of surfactant were added to AgNP dispersion, suggesting that the competition for filling retention sites between AgNPs and surfactant reduced Smax close to the column inlet. The introduction of surfactant to pre-cover porous media resulted in stronger mobility than adding it to AgNP dispersion. These findings demonstrated that the occupation of retention sites by surfactant reduced the retention capacity for AgNPs to different degrees depending on the surfactant concentration and the sequence of AgNP and/or surfactant application to the column. Experimental and simulated results for undisturbed soil columns showed similar trends with regard to the effects of physicochemical factors, e.g., enhanced transport with decreasing solution IS, increasing AgNP input concentration and flow velocity, and increased k1 and Smax with increasing IS and decreasing Co. In contrast, a significant retardation of AgNP breakthrough and hyperexponential RPs were observed in almost all the transport experiments with soil. A major portion of the injected AgNPs was retained in the first 3 cm of the soil column. The high k1 resulted in retarded BTCs until Smax was filled and then high effluent concentrations were obtained. Hyperexponential RPs were likely caused by the hydrodynamics at the column inlet which produced a concentrated AgNP flux to the solid surface. Higher IS and lower Co produced more hyperexponential RPs because of larger values of Smax. Lysimeter experiments indicated that the major removal of AgNPs occurred in the top layers (first 20 cm) after 12 months intermittent irrigation, but the potential contamination of the groundwater by AgNPs may not be fully excluded. Results from remobilization experiments indicated that retention of AgNPs was much more pronounced in the presence of Ca 2+ than of K + at the same IS, and the amount of AgNP released with a reduction in IS was larger for K + than Ca 2+ systems. The stronger AgNP interactions in the presence of Ca 2+ were attributed to cation bridging. Further release of AgNPs and clay from the soil was induced by cation exchange (K + for Ca 2+ ) that reduced the bridging interaction and IS reduction that expanded the electrical double layer. Results with transmission electron microscopy and energy-dispersive X-ray spectroscopy, and the correlations between released soil colloids and AgNPs indicated that some of the released AgNPs were associated with the released clay fraction. Co-transport of the AgNPs by soil colloids may therefore be of importance for their transport behavior in the environment.