title: Colloid formation at waste plume fronts.
abstract: Highly saline and caustic tank waste solutions containing radionuclides and toxic metals have leaked into sediments at U.S. Department of Energy (DOE) facilities such as the Hanford Site (Washington state). Colloid transport is frequently invoked to explain migration of radionuclides and metals in the subsurface. To understand colloid formation during interactions between highly reactive fluids and sediments and its impact on contaminant transport, we simulated tank waste solution (TWS) leakage processes in laboratory columns at ambient and elevated (70 degrees C) temperatures. We found that maximum formation of mobile colloids occurred at the plume fronts (hundreds to thousands times higher than within the plume bodies or during later leaching). Concentrations of suspended solids were as high as 3 mass %, and their particle sizes ranged from tens of nanometers to a few micrometers. Calcium carbonate is always one of the dominant phases of the plume front colloids, while the other phases varied with solution pH and temperature. During infiltration of the leaked high-Na+ waste solution, rapid and completed Na+ replacement of exchangeable Ca2+ and Mg2+ from the sediment caused accumulation of these divalent cations at the moving plume front. Precipitation of supersaturated Ca2+/Mg2+-bearing minerals caused dramatic pH reduction atthe plume front. In turn, the reduced pH caused precipitation of other minerals. This understanding can help predict the behavior of contaminant trace elements carried by the tank waste solutions and could not have been obtained through conventional batch studies.
authors.affiliation: Earth Sciences and Material Science Divisions, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA. jwan@lbl.gov
authors.forename: Jiamin
authors.initials: J
authors.lastname: Wan
authors.forename: Tetsu K
authors.initials: TK
authors.lastname: Tokunaga
authors.forename: Eduardo
authors.initials: E
authors.lastname: Saiz
authors.forename: Joern T
authors.initials: JT
authors.lastname: Larsen
authors.forename: Zuoping
authors.initials: Z
authors.lastname: Zheng
authors.forename: Rex A
authors.initials: RA
authors.lastname: Couture
mesh_terms: D000327:Adsorption; D003102:Colloids / Q000737:chemistry*; D004784:Environmental Monitoring; D019015:Geologic Sediments / Q000032:analysis / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D010316:Particle Size; D011850:Radioactive Waste / Q000032:analysis*; D012990:Soil Pollutants, Radioactive / Q000032:analysis; D013696:Temperature; D014865:Waste Disposal, Fluid; D014867:Water / Q000737:chemistry; D014872:Water Movements; D014875:Water Pollutants, Radioactive / Q000032:analysis*
chemical_list: D003102:Colloids; D011850:Radioactive Waste; D012990:Soil Pollutants, Radioactive; D014875:Water Pollutants, Radioactive; D014867:Water