A BRIEF HISTORY OF COLLOID-FACILITATED TRANPSORT: ROLE OF SOLUTE DESORPTION KINETICS

Colloid-facilitated transport has now been recognized as a significant mechanism of contaminant transport for about three decades, but only in a few cases has complete evidence been presented for colloid-facilitated transport of contaminants. To address this issue, we investigated the transport of the cations Cs⁺ and Sr²⁺ in flow-through and recirculating saturated quartz porous media in the absence and presence of illite colloids to examine the effect of cation desorption kinetics on colloid-facilitated transport. The illite colloids were equilibrated with radioactive isotopes of the cations (¹³⁷Cs, ⁹⁰Sr) at different ratios of colloid to cation concentrations. The colloid-cation suspensions were injected through the column at pH 7.3 and ionic strength 0.1 mM. In the flow-through column, the presence of colloids accelerated the transport of the cations. The time to breakthrough of the cations decreased as the ratio of colloid to cation concentrations increased. For a given colloid concentration, the breakthrough of Cs⁺ was more rapid than that of Sr²⁺. Analysis of the breakthrough data with a colloid-facilitated transport model showed that time to breakthrough decreased as cation association with the colloids increased and the rate of desorption of cations from the illite decreased. In the recirculating column, Sr²⁺ desorbed from the illite colloids more rapidly than Cs⁺. Initially, Cs⁺ desorbed from the colloids, but over time, the colloidal concentration of Cs⁺ increased. We attribute the accumulation of Cs⁺ on the colloids to the slow transfer of Cs⁺ to strong binding (“frayed edge”) sites on the illite colloids.