Colloid release and transport processes in natural and model porous media

Abstract

Abstract Colloid release was observed from packed columns for two natural porous media (sands) and one model system (glass beads with deposited latex colloids). Colloid release was found to occur in all cases when the ionic strength was reduced in columns that were in equilibrium with Na + ions. Most of the released colloids from the natural porous media were smaller than 1 μm in size, and comprised pure and impure forms of silica (60–70% by mass) and clay minerals (20–30% by mass). For greater reductions in ionic strength, the total mass of released colloids increased, although the shape of the effluent mass concentration profile did not change. Release rate coefficients were obtained by fitting a colloid transport model (an advection-dispersion transport model with source/sink terms for colloid release and deposition) to the column effluent data. To fit the data for different ionic strengths, the total available mass of releasable colloids had to be adjusted, and fitted release rate coefficients were not sensitive to the ionic strength. In contrast, calculations based on Derjaguin-Landau-Verwey-Overbeek (DLVO) theory indicate a strong dependence of release rate constants on ionic strength for homogeneous colloids. This discrepancy can be attributed to charge and size heterogeneity in the colloids, and to our inability to determine accurately interparticle forces at small separations. The trend of greater mass release for greater reductions in ionic strength could be explained qualitatively by computing interparticle interactions with a constant-charge boundary condition (albeit with a charge density much lower than that experimentally determined) which showed a decreasing energy barrier for particle detachment with decreasing ionic strength.