Dual-deposition rates in colloid filtration caused by coupled heterogeneities in a colloidal population.

Colloidal deposition from an aqueous suspension, during its flow through saturated porous media, is of significance in many natural and man-made processes which lead to water purification. Colloid filtration theory (CFT), which was used to predict removal of homogeneous colloids, in the above systems, is based on a distance-invariant deposition rate. However, many authors over the past decade have reported data which suggest that more than one deposition rate is demonstrated by colloids naturally occurring in the environment on apparently homogeneous media. The observation of two or more deposition rates has been attributed to two modes of deposition as well to the possibility of heterogeneity in the colloidal population, in the recent literature. In this paper, we first examine the variation of liquid-phase concentration C(x) with distance, and find that only two distinct deposition rates are demonstrated by the data under multiple conditions. Since heterogeneity in the colloidal population is expected to produce continuous property variations and hence multiple deposition rates, the appearance of the above dual-deposition behavior is probed further. Depth-wise zeta-potential and particle-size-distribution data reveal that there is a coupling of favorable properties (larger size with positive surface charge) in a section of the population along with a coupling of unfavorable properties in the rest. The above coupling is then confirmed by independent separation experiments. This paper experimentally demonstrates how coupling of two types of heterogeneities in a colloidal population can lead to the appearance of dual-deposition rates while recognizing that the above may be one of the many possible causes for the appearance of dual-deposition rates.