CHARACTERIZATION OF COLLOIDS MOBILIZED FROM SOUTHEASTERN COASTAL-PLAIN SEDIMENTS

Two subsurface samples representing highly weathered sediments of the Up per Coa sta I 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 gen eration in response to changes in solution composition. In repacked co lumns, the two sediments were leached (similar to 0.72 m d(-1) Darcy v elocity) with either 0.001 N NaCl or CaCl2 solutions for 10 pore Volum es followed by 10 pore Volumes of deionized water. Bulk clay mineralog y 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 co nfirmed by selected-area electron diffraction (SAED) and EDXA in a tra nsmission electron microscope. In column experiments, the sample lower in Fe oxide displayed increased effluent turbidity that coincided wit h a decrease in effluent pH during injection of the CaCl2 solutions. i n contrast, no effluent turbidity was observed during the leaching of NaCl solutions. The sample higher in iron oxide displayed greater effl uent turbidity from the onset of leaching that decreased over the dura tion of injection for both the CaCl2 and NaCl solutions. Colloids gene rated from both materials displayed positive electrophoretic mobilitie s, confirming the importance of iron oxides in controlling dispersion and surface charge properties of the mobile colloids. Minor difference s 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 sa mple. Electron microscopy, EDXA, and thermal gravimetric analysis (TGA ) confirmed that the mobile colloids consisted mainly of submicron siz ed Al-rich goethite, with lesser amounts of kaolinite, the predominate layer silicate of the bulk sediments, and varying amounts of crandall ite.