Microplastic and Silica Colloid Transport in a Saturated Porous Medium Under Various Flow Directions: the Effect of Gravity

Most experiments into colloidal transport and retention in lab-scale columns are performed in the up-flow mode of liquid movement in order to minimize air entrapment, but ignore the effects both of gravity and of the complex actual hydro-geological regime. In contrast, this study evaluated the roles of gravitational force and liquid flow direction on the transport of two types of colloid (2-μm-diameter microplastic and silica microspheres) through sand columns arranged in various orientations. Analysis of breakthrough curves, transport modeling, calculations of the contact efficiency, DLVO interaction energy, and particle settling velocities were combined to quantify the effect of gravity on each flow regime. Both types of microsphere were observed to experience frequent collisions with sand surfaces, and attachment to matrix particles with energies corresponding to the secondary minimum of DLVO energy contributed to their retention. Due to the larger collector efficiency elicited by greater material density and gravity, a higher retention of SiO₂ than PS microspheres was observed under the same flow conditions, and the average attachment coefficient of the former was 4.77 times higher than the latter. Greater deposition was observed under upward mode operation versus that under downward mode. The average attachment coefficient under upward mode was approximately 2.0 times that found under downward mode, for both types of microsphere. The calculation of settling velocity and effective particle velocity verified that the deposition of two microspheres was affected by gravity, and it had a greater influence on the transport of SiO₂ than on the PS microspheres.