Colloid Stability in Vadose Zone Hanford Sediments

We experimentally determined colloid stability of natural colloids extracted from vadose zone sediments from the U.S. Department of Energy's Hanford Reservation. We also used reference minerals, kaolinite, montmorillonite, and silica, for comparative purposes. Colloid stability was assessed with two different methods:  the batch turbidity method and dynamic light scattering. Critical coagulation concentra tions (CCCs) were determined for pure Na and pure Ca electrolyte solutions, as well for mimicked Hanford vadose zone pore waters with varying sodium adsorption ratios (SARs). Critical coagulation concentrations obtained from the batch turbidity method were sensitive to initial colloid mass concentrations, settling time, and CCC criteria. The lower the initial colloid concentration and the shorter the settling times were, the larger was the CCC. The CCCs determined from the dynamic light scattering, where diluted colloidal suspensions are used, were not dependent on settling time and arbitrary CCC criteria, so dynamic light scattering is therefore the preferred method to determine colloid stability. The CCC values determined from dynamic light scattering ranged from 90 to 200 mmol/L for Na systems and 1.7 to 3.8 mmol/L for Ca systems. The stability of natural colloids was intermediate between that of pure kaolinite and montmorillonite. The results indicate that colloids in the Hanford vadose zone form stable suspensions, i.e., are in the slow aggregation regime. Nonetheless, due to the long travel times in the vadose zone, nearly all colloids will aggregate and be removed from the water column before reaching groundwater levels.