Selective colloid mobilization through surface-charge manipulation

The objective of the current study was to evaluate the use of amine hexadec yltrimethylammonium bromide (HDTMA) to enhance the mobilization and subsequ ent transport of colloidal iron oxides by selectively blocking negatively c harged sites within soil or aquifer sediments. Two materials were used in a series of column leaching studies, a surface soil (Orangeburg Series) and an iron oxide-rich subsurface sediment (Tobacco Rd. Formation) both from Ai ken, SC. For comparison, the same materials were leached with sodium hexame taphosphate (Na-P) as a nonselective dispersing agent. As a cationic surfac tant HDTMA is generally considered a strong flocculent for soils because of its ability to shield the electrostatic repulsion on opposing negatively c harged clays, which was observed in leaching experiments for the surface so il material. Leaching HDTMA solutions through the iron oxide-coated aquifer sediments resulted in the selective dispersion and transport of iron oxide s, relative to the more abundant kaolinite. Despite effluent colloid levels in excess of 6 g L-1, no column plugging was observed for the HDTMA treatm ents. The Na-P treatment, however, produced effluent turbidity levels that were less than HDTMA but induced rapid column plugging. Thermal characteriz ation of the Na-P-derived colloids indicated that they were similar to the same as the bulk clay fraction of the aquifer sediment, indicating that dis persion was nonselective. HDTMA appears to block negatively charged filtrat ion sites that limit iron oxide transport, thus enhancing colloid dispersio n without inducing column plugging observed for nonselective dispersants. I ron oxides have been demonstrated to be the resident phase for many inorgan ic and organic contaminants within highly weathered, organic matter-poor sy stems. This suggests that selective mobilization of colloidal iron oxides a nd their associated contaminants can potentially enhance subsurface remedia tion activities via implementation of pump-and-treat technologies.