Transport of Engineered Nanoparticles in Saturated Porous Media: Comparisons with Natural Colloids.

Engineered nanoparticles (NPs) can be released into soils as emerging groundwater contaminants because many of them showed toxic effects to the ecosystem; however, their fate and transport in soils are largely unknown. The present work compared the transport behavior of two ENPs, silver nanoparticles (AgNPs) and carbon nanotubes (CNTs), in saturated porous media with that of colloidal montmorillonite, a natural clay mineral. Sodium dodecylbenzene sulfonate (SDBS), a surfactant, was used to disperse the natural colloids and engineered NPs to enhance their stabilities in water. The solubilized NPs and colloids were then applied to laboratory columns packed with water-saturated quartz sand to obtain their breakthrough curves. The experimental results showed that transport of CNTs in the column was similar to that of natural clay colloids and their recovery rates were close to 100%. The transport AgNPs were little different and about 10% of the AgNPs was retained in the saturated column at the end of the breakthrough experiments. However, the retained AgNPs were released when a SDBS-free water pulse was used to flush the sand column. An advection-dispersion model was used to simulate the transport behavior in the sand column. The model results matched the experimental breakthrough data very well for both engineered NPs and natural colloids. Our results indicate that theories and models of colloid transport in porous media are applicable to describe the fate and behavior of engineered NPs.