Impact of Ionic Strength on Colloid Mobility in Saturated and Unsaturated Porous Media
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of Chemistry, Washington State University, Tri-Cities, Richland, WA 99352 Westinghouse Savannah River Company, Aiken, SC 29808 3 EPRI, Inc., Palo Alto, CA 94304. ABSTRACT A model system consisting of well-characterized quartz sand, negatively charged carboxyl-modified latex microspheres, and synthetic J- 13 groundwater was used as a first-step in defining limiting conditions for colloid transport at the proposed Yucca Mountain Repository for radioactive waste. Colloid (280-nm diameter) and tracer (bromide) transport was determined using column methods under steady-state flow conditions for both saturated and unsaturated moisture conditions. Experiments were conducted at three water contents (13%, 70%, and 100% saturated) and four ionic strengths (deionized water, and 0.1 x, Ix, and 1Ox the ionic strength of J13 groundwater, which has an ionic strength of 0.0116). Colloid mobility in the 13% saturated columns was appreciably less than in the 70% and 100% saturated columns, whereas colloid mobility in the 70% and 100% saturated columns were quite similar. Ionic strength had a significant impact on colloid retention, becoming more pronounced in the drier systems. Colloids were mobile in deionized water and the 0.1 x J-13 groundwater. Retention was approximately 20% in lx J-13 groundwater and 100% in the lOx J-13 groundwater. Compared with the 70% and 100% saturated conditions, colloid mobility decreased for the 10% saturated condition, with greater than 50% retention for the Ix J-13 groundwater system. As observed for the 70% and 100% saturated conditions, colloids in the lOx J-13 solution were essentially immobile. Colloid mobility is greatly affected by the ionic strength of the soil solution and this effect is moisture saturation dependent. INTRODUTION Subsurface contaminants are distributed among the stationary solid, mobile aqueous, and potentially mobile colloid phases. Mobile colloids may enhance contaminant transport in subsurface sediments [1]. This research considers the effect of ionic strength and moisture content on colloid transport through porous media. The investigations were initiated as a firststep in defining limiting conditions for colloid transport at the proposed Yucca Mountain Repository for radioactive waste. EXPERIMENT A new technique for studying colloid transport through unsaturated systems was recently evaluated at Pacific Northwest National Laboratory [2]. The technique uses the Unsaturated Flow Apparatus (UFATM, Beckman Scientific), which is a temperature-controlled centrifuge with a continuous flow rotor head. A rotating seal assembly allows simultaneous fluid delivery by two separate flow paths, to two soil samples or columns. Centripetal acceleration is used in combination with a fluid delivery system, or pump, to establish steady-state flow and moisture conditions. 737 Mat. Res. Soc. Symp. Proc. Vol. 556 © 1999 Materials Research Society
This method was adapted from previous efforts to determine unsaturated hydraulic conductivity, flow and transport [3,4,5,6]. A model system consist
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