Role of Immobile Kaolinite Colloids in the Transport of Heavy Metals

Abstract

Predicting the transport of contaminants in porous media is crucial to protecting public health and remediating contaminated soil and groundwater. However, the prediction of contaminant transport is challenging due to the presence of mobile and immobile colloids. The work performed in this experimental investigation quantified the role of immobile clay colloids on metal transport through sets of column breakthrough experiments under varying solution chemistry, clay content, and flow rate. Georgia kaolinite was chosen as the colloidal material, and Pb(II) was chosen as the dissolved contaminant. The silica sand used as the bed material was sized to ensure that the kaolinite colloids remained stationary during the column experiments. Results indicated that retardation of the Pb(II) breakthrough curve was observed as ionic strength decreased and kaolinite content and pH increased, while no significant variation of Pb(II) breakthrough was observed at any kaolinite content as flow rate decreased. This work demonstrated that, in the presence of immobile kaolinite colloids, Pb(II) breakthrough curves strongly depended on the pH and ionic strength, which controlled the charge on the surface functional groups and the surface availability of metal adsorption sites on immobile kaolinite colloids. In addition, the evaluation of unknown first-order coefficients in the continuum governing equation, bed efficiency, and Pb(II) saturation provided a quantitative description of Pb(II) breakthrough curves.