Applicability of colloid filtration theory in size-distributed, reduced porosity, granular media in the absence of energy barriers

The vast majority of colloid transport experiments have been conducted in granular porous media with narrow size distribution, which allows a single collector size and narrow values of porosity to be used in colloid filtration theory to predict deposition rates under favorable conditions (absence of energy barriers). In this work, deposition of colloids (ranging from 0.21 mm to 9.1 mm) in packed columns was examined with three different borosilicate glass bead porous media: uniform, monomodal nonuniform and bimodal nonuniform. The corresponding porosities to these media were 0.38, 0.34 and 0.28. The effect of reduced porosity on the flow field was studied using high resolution computerized X-ray micro tomography (HRXMT), Lattice-Boltzmann (LBM) flow simulation methods, and direct observations in packed flow cells. This allowed discerning the influence of preferential flow paths on fluid velocities, and it demonstrated that straining in pore throats too small to pass was not a significant contributor to colloid retention even up to particle to grain size ratios of 0.05. Mechanistic simulations successfully predicted the experimentally-observed sensitivity to porosity and absolute values of the deposition rate coefficients when a number-based mean grain size was used to represent the size-distributed porous media.