Manipulating subsurface colloids to enhance cleanups of DOE waste sites. 1997 annual progress report

'This research is aimed at (a) developing improved understandings of the processes involved in holding colloids immobile in subsurface media, and (b) exploring the prospects for mobilizing such colloids to enhance cleanups. These colloids are important because they are the many contaminants of concern. chief sorbent media for The following briefly describes the efforts manuscript (currently submitted to Environmental also appended. in the last year. A draft Science and Technology) is Mechanisms Controlling Colloid Releases (Swartz) Using aquifer materials from a Southeastern Coastal Plain site, the authors have explored the mechanisms which control the releases of attached colloids into the groundwater flow. First, the authors have completed electron microscopy observations regarding the intimate particle:particle juxtapositions in the solids (Swartz et al. Geochim. Cosmochim. Acta 61, 707, 1997.) Next, by flushing these aquifer sands with various aqueous solutions, the authors have found that the bulk of the attached colloids appear to be (a) bound to one another via intermediary amorphous iron oxyhydroxides, and (b) attracted to the other colloids by juxtapositions of oppositely charged phases (manuscript submitted to Environ. Sci. Technol. and attached.) The authors suspect that this combination of inter-colloid interactions may be especially important in geologically young deposits where diagenetic processes have not produced highly crystalline solids yet. As a result of these observations and laboratory manipulations, the authors believe the authors understand the steps necessary to initiate colloid mobilization at such a groundwater site. First, one must apply solution constituents which are suited to dissolving a limited portion of the amorphous iron oxyhydroxides. The authors have accomplished this using 0.5 mM ascorbic acid (i.e., vitamin C.) Next, one must reverse the surface charges of either the negatively charged alumino-silicate clays or the adjacent positively charged iron oxyhydroxides. The authors have used 0.5 \265m phosphate solutions (pH 5.2) to drive the iron oxyhydroxides to a net negative surface charge. By working at only millimolar levels, the resulting total solution ionic strength is still low enough to permit the corresponding expanded double layers to push the now like-charge colloids apart. In the laboratory, this results in a sustained presence of about 2 mg colloids per liter of flushing solution for at least many tens of pore volumes. In the author''s continuing efforts, he has returned to the field site, where the aquifer solids were initially collected, and tested the possibility of using such ascorbic acid-phosphate solutions to mobilize colloids in the ground. Efforts have proven successful as measured by the presence of turbidity in suitably altered flushing solutions (and the absence of turbidity in control tests). The author is now completing measurements of the ancillary parameters necessary to interpret the field tests.