Kaolinite colloid transport in porous media and its implications on contaminant transport

Abstract

Organic and inorganic colloidal particles are ubiquitous in the geologic subsurface environment and they transport readily through porous media due to their relatively small size when compared to pore spaces. While an extensive body of research exists that quantifies colloid and colloid-associated contaminant transport in porous media, few studies have investigated the physical impact of clay colloid retention and transport on the hydraulic properties of the formation, and its subsequent impact on contaminant transport. Specifically, changes in the geochemistry and groundwater flowrate within the formation can result in aggregation or dispersion of colloids. Depending on the prevailing attachment mechanisms, colloids can clog the porous media, and can either facilitate or retard contaminant transport. The work performed in this study used DLVO theory to quantify colloidal clay attachment and transport mechanisms within sand column experiments, and its subsequent impact on hydraulic conductivity and contaminant transport through the soil media. Column tests were performed to measure the reduction of hydraulic conductivity due to physical clogging by colloids, as a function of flowrate and solution chemistry. In addition, stochastic and pore network modeling were applied to predict clay colloidal transport in order to account for the polydispersed characteristics of both the sand medium and the colloidal clay particles. Finally, column tests and batch adsorption tests were performed to quantify clay colloid associated heavy metal transport (facilitated or retarded) to determine the impact of the presence of clay colloids on single-ion and multi-ion metal transport. All soil-column experiments performed in this work were quantified by evaluating optimized first-order rate coefficients associated with clay-sand interaction, clay-metal interaction, and sand-metal interaction, which provided quantitative representation of the experimentally obtained retention profiles and breakthrough curves. The major findings of this research are as follows: i) solution chemistry played an important role in hydraulic conductivity and contaminant transport, particularly at low flow rates, ii) the reduction of hydraulic conductivity was not only a function of retained clay colloids but also depended on the dominant retention mechanism (attachment and/or straining), iii) polydispersed characteristics of clay colloids and uncertainty in the size of clay clusters should be considered in predicting clay colloid transport, and iv) retention of clay colloids retarded heavy metal transport while the transport of clay colloids facilitated heavy metal transport.