Investigations on factors that affect the fate and transport of nanoparticles (NPs) remain incomplete to date. In the present study, we conducted column experiments using 8 and 52 nm silica NPs to examine the effects of NPs' concentration and size on their retention and transport in saturated porous media. Results showed that higher particle number concentration led to lower relative retention and greater surface coverage. Smaller NPs resulted in higher relative retention and lower surface coverage. Meanwhile, evaluation of size effect based on mass concentration (mg/L) vs particle number concentration (particles/mL) led to different conclusions. A set of equations for surface coverage calculation was developed and applied to explain the different results related to the size effects when a given mass concentration (mg/L) and a given particle number concentration were used. In addition, we found that the retained 8 nm NPs were released upon lowered solution ionic strength, contrary to the prediction by the Derjaguin−Landau−Verwey−Overbeek (DLVO) theory. The study herein highlights the importance of NPs' concentration and size on their behavior in porous media. To the best of our knowledge, it is the first report of an improved equation for surface coverage calculation using column breakthrough data.

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Colloidal particles of environmental concern often have nonspherical shapes. However, theories and models such as the classical filtration theory have been developed based on the behavior of spherical particles. This study examined the effect of particle shape on colloid retention (e.g., attachment and straining) and release in saturated porous media. Two‐ and three‐step transport experiments were conducted in water‐saturated glass bead columns using colloids dispersed in deionized water and an electrolyte solution. The particles used in the experiments were carboxylate‐modified latex colloids of spherical (500 nm diam.) and rod (aspect ratio, 7.0) shapes. The rod‐like particles were prepared by stretching the spherical particles. Analysis of the colloid breakthrough curves indicates that particle shape affected transport behavior, but retention did not increase with increasing aspect ratio. Retention of the spherical particles occurred mainly in the secondary energy minimum, whereas retention of rod‐like particles occurred in primary and secondary energy minima. There was less straining of rod‐like particles compared with spherical ones, indicating that the minor axis was the critical dimension controlling the process. Release of spherical particles on elution was instantaneous, whereas release of rod‐like particles was rate limited, giving rise to long tails, implying an orientation effect for rod‐like colloids. The results suggest that the differences in electrostatic properties and shape contributed to the observed different retention and release behaviors of the two colloids.

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