Transport of engineered nanoparticles in saturated porous media

Engineered nanoparticles (NPs) can be released into soils as emerging groundwater contaminants because many of them show toxic effects to the ecosystems; however, their fate and transport in soils are largely unknown. The present work examined the transport behavior of two NPs, silver nanoparticles (AgNPs) and carbon nanotubes (CNTs), in saturated porous media. Sodium dodecylbenzene sulfonate (SDBS), an anionic surfactant, was used to disperse the engineered NPs to enhance their stabilities in water. The solubilized NPs were then applied to laboratory columns packed with two types of water-saturated quartz sand to obtain their breakthrough curves. The experimental results showed that the surfactant-solubilized NPs were highly mobile in the saturated porous media. The transport of CNTs in the column was similar to that of colloidal montmorillonite and their recovery rates were around 100%. Less than 15% of the AgNPs were retained in the saturated column during the breakthrough experiments. However, most of the retained AgNPs were released when a SDBS-free water pulse was used to flush the sand column. The Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and a colloid transport model were used to simulate the fate and transport of the engineered NPs in the sand columns. The DLVO theory worked well with AgNPs, but failed to represent the interactions between CNTs and the two sand media. Predictions of the transport model matched the experimental breakthrough data of the two engineered NPs well. Our results indicate that theories and models of colloid transport in porous media may be applicable to describe the fate and behavior of engineered NPs under certain circumstances.