SREL Reprint #2210

Characterization of colloids mobilized from Southeastern Coastal Plain sediments

John C. Seaman1, Paul M. Bertsch1, and Richard N. Strom2

1Advanced Analytical Center for Environmental Sciences, University of Georgia, Savannah River Ecology Lab,
Drawer E, Aiken, South Carolina 29802
2Westinghouse Savannah River Company, Savannah River Technology Center, Savannah River Site,
Aiken, South Carolina 29802

Abstract: Two subsurface samples representing highly weathered sediments of the Upper Coastal Plain (Aiken, SC) that differed in Fe-oxide content (7.3 vs 35.9 g Fe kg-1) were evaluated in terms of mobile colloid generation in response to changes in solution composition. In repacked columns, the two sediments were leached (~0.72md-1 Darcy velocity) with either 0.001 N NaCl or CaCl2 solutions for 10 pore volumes followed by 10 pore volumes of deionized water. Bulk clay mineralogy from the two samples and suspensions generated in the column studies was characterized by energy dispersive X-ray analysis (EDXA) using a field-emission scanning electron microscope (SEM). SEM results were confirmed by selected-area electron diffraction (SAED) and EDXA in a transmission electron microscope. In column experiments, the sample lower in Fe oxide displayed increased effluent turbidity that coincided with a decrease in effluent pH during injection of the CaCl2 solutions. In contrast, no effluent turbidity was observed during the leaching of NaCl solutions. The sample higher in iron oxide displayed greater effluent turbidity from the onset of leaching that decreased over the duration of injection for both the CaCl2 and NaCl solutions. Colloids generated from both materials displayed positive electrophoretic mobilities, confirming the importance of iron oxides in controlling dispersion and surface charge properties of the mobile colloids. Minor differences in the clay mineralogy (i.e., iron oxide/kaolinite content) between the samples were reflected in both their sensitivity to pore-solution composition and the relative degree of dispersion observed for each sample. Electron microscopy, EDXA, and thermal gravimetric analysis (TGA) confirmed that the mobile colloids consisted mainly of sub-micron sized Al-rich goethite, with lesser amounts of kaolinite, the predominate layer silicate of the bulk sediments, and varying amounts of crandallite.

SREL Reprint #2210

Seaman, J.C., P.M. Bertsch, and R.N. Strom. 1997. Characterization of colloids mobilized from Southeastern Coastal Plain sediments. Environmental Science and Technology 31:2782-2790.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).