Studying colloid transport in porous media using a geocentrifuge

Abstract

Movement of colloids in the subsurface is a concern because mobile colloids may enhance the transport of contaminants. The excessive time required to conduct flow and transport experiments in porous media led to the use of centrifuges to evaluate subsurface transport processes. The objective of this study was to determine the suitability of centrifuges to study colloid transport in saturated porous media. We used a geocentrifuge to run colloid transport experiments under different centrifugal accelerations up to 20 g. Colloids of different densities were used: polystyrene (1.05 g/cm3), silica (2 g/cm3), and hematite (5.26 g/cm3). Deposition coefficients were obtained from the colloid breakthrough curves. We used filtration theory and a theory based on sedimentation‐diffusion to derive functional relationships between centrifugal acceleration and colloid and porous media properties, which allow us to predict the effect of acceleration on colloid transport. Comparison of experimental deposition coefficients with predictions based on filtration theory showed that filtration theory accurately predicted the behavior of polystyrene at higher accelerations but underpredicted colloid deposition for silica and hematite at accelerations higher than ∼10 g. The sedimentation‐diffusion theory allows us to determine whether a system is dominated by sedimentation or diffusion, or is in a transitional state. Theoretical predictions of colloid deposition in a porous medium agreed well with experiments, suggesting that the theory can be used to delineate when centrifugal acceleration will alter colloid transport in flow through column studies conducted in a centrifuge. Common subsurface colloids, such as iron oxides and aluminosilicates, can be affected at accelerations that are used in geocentrifuge transport studies (5 to 300 g). Even colloids with low specific densities, such as polystyrene, will be affected by centrifugal accelerations if their size is large.