Correlation of colloid collision efficiency with hydraulic conductivity of silica sands

Column experiments were conducted in the laboratory to study the relationship between local-scale colloid filtration parameters and physical heterogeneity as represented by soil or rock hydraulic conductivity. Natural (polydisperse) sand was sieved into distinct classes to produce relatively homogeneous porous media for the experiments. Silica particles 0.45 μm in diameter were used as the transported colloids. The column experiments were carried out under carefully controlled chemical conditions. The hydraulic conductivity of each packed column was determined using a constant-head permeameter arrangement. Conservative transport parameters (dispersivity and effective porosity) were obtained using a salt tracer. Comparison of solute and colloid breakthrough curves indicated that particle deposition rates were unsteady in the column. This behavior was analyzed using a blocking function which describes the alteration of the sand surface due to the accumulation of deposited colloids. By this method, clean-bed collision efficiency factors were obtained for colloid filtration in each of the sand sizes. An inverse relationship was observed between the collision efficiency factor and the grain size of the collector medium. This relationship was attributed to a variation of surface heterogeneity with sand particle size. This theory is supported by the direct correlation of collision efficiency, with surface heterogeneity parameters of the blocking function. The colloid filtration data were used to parameterize a linear correlation between the collision efficiency factor and the natural logarithm of the medium hydraulic conductivity. The collector efficiency, normally calculated from theoretical models for colloid transport in the vicinity of collector grains, was also shown to be a linear function of the natural logarithm of hydraulic conductivity. Correlations of this type can be used effectively in stochastic modeling of colloid transport through heterogeneous porous media.