Influence of Ionic Strength on Clay Particle Deposition and Hydraulic Conductivity of a Sand Medium

Abstract

Particulate media filters and infiltrometers play a critical role in separation of solids and liquids in infrastructure applications that require drainage and transmission of water or other liquids. Although the primary mechanisms of particle removal within a filter are well developed (straining, attachment, and detachment), the impact of geochemistry on fine-grained particle removal is not well developed, especially in terms of the impact of clustering of clay particles as a function of ionic strength. This experimental investigation quantifies the impact of ionic strength, median filter-grain diameter, shape of filter grain, and inflow concentration of clay suspensions on the reduction of saturated hydraulic conductivity (K-s) in a sand column. Additionally, three first-order reaction coefficients for each condition are optimized by nonlinear least-squares fit to the data to account for straining, attachment, and detachment mechanisms for the clay deposition. Georgia kaolinite was tested as a representative clay material, and the experimental retention profiles were determined as a function of depth. Results demonstrate that the amount of strained kaolinite decreases as the ionic concentration of the clay suspension increases, which results from the formation of small clay clusters at relatively high ionic strength. Furthermore, the observed reduction in K-s of the sand-filter medium is attributable primarily to straining in the first 7 cm of the depth of the filter. Decreasing the ionic strength of the clay suspension results in increased size of the clay clusters, which leads to flocculation of the clay and a relatively large size ratio between the kaolinite clusters and sand particles (d(c)/d(s)). This produces a significant reduction of K-s as a result of straining at the surface of the filter. The work demonstrates that ionic strength, clay particle-cluster size, and filter sand-grain size must be considered as major factors in particle retention and reduction of hydraulic conductivity within a filter or infiltration trench.