Retention and Remobilization of Stabilized Silver Nanoparticles in an Undisturbed Loamy Sand Soil

Column experiments were conducted with undisturbed loamy sand soil under unsaturated conditions (around 90% saturation degree) to investigate the retention of surfactant stabilized silver nanoparticles (AgNPs) with various input concentration (Co), flow velocity, and ionic strength (IS), and the remobilization of AgNPs by changing the cation type and IS. The mobility of AgNPs in soil was enhanced with decreasing solution IS, increasing flow rate and input concentration. Significant retardation of AgNP breakthrough and hyperexponential retention profiles (RPs) were observed in almost all the transport experiments. The retention of AgNPs was successfully analyzed using a numerical model that accounted for time- and depth-dependent retention. The simulated retention rate coefficient (k1) and maximum retained concentration on the solid phase (Smax) increased with increasing IS and decreasing Co. The high k1 resulted in retarded breakthrough curves (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. Retention of AgNPs was much more pronounced in the presence of Ca2+ than K+ at the same IS, and the amount of AgNP released with a reduction in IS was larger for K+ than Ca2+ systems. These stronger AgNP interactions in the presence of Ca2+ were attributed to cation bridging. Further release of AgNPs and clay from the soil was induced by cation exchange (K+ for Ca2+) that reduced the bridging interaction and IS reduction that expanded the electrical double layer. Transmission electron microscopy, energy-dispersive X-ray spectroscopy, and correlations between released soil colloids and AgNPs indicated that some of the released AgNPs were associated with the released clay fraction