Correlation of colloid collision efficiency with hydraulic conductivity ofsilica sands

Column experiments were conducted in the laboratory to study the relationsh ip between local-scale colloid filtration parameters and physical heterogen eity as represented by soil or rock hydraulic conductivity. Natural (polydi sperse) sand was sieved into distinct classes to produce relatively homogen eous porous media for the experiments. Silica particles 0.45 mu m in diamet er were used as the transported colloids. The column experiments were carri ed out under carefully controlled chemical conditions; The hydraulic conduc tivity of each packed column was determined using a constant-head permeamet er arrangement. Conservative transport parameters (dispersivity and effecti ve porosity) were obtained using a salt tracer. Comparison of solute and co lloid breakthrough curves indicated that particle deposition rates were uns teady in the column. This behavior was analyzed using a blocking function w hich describes the alteration of the sand surface due to the accumulation o f deposited colloids. By this method, clean-bed collision efficiency factor s were obtained for colloid filtration in each of the sand sizes. An invers e 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 he terogeneity parameters of the blocking function. The colloid filtration dat a were used to parameterize a linear correlation between the collision effi ciency factor and the natural logarithm of the medium hydraulic conductivit y. The collector efficiency, normally calculated from theoretical models fo r colloid transport in the vicinity of collector grains, was also shown to be a linear function of the natural logarithm of hydraulic conductivity. Co rrelations of this type can be used effectively in stochastic modeling of c olloid transport through heterogeneous porous media.