In Situ Formation of Colloidal Phases and Their Role in Radionuclide Transport.
James Harsh1, Markus Flury1, Youjun Deng1, Kholoud Mashal2, and Gang Chen3. (1) Washington State Univ, Pullman, WA 99164, (2) Hashemite Univ, College of Agriculture, PO Box 150459, Zarqa, 13115, Jordan, (3) FAMU-Florida State Univ, College of Engineering, 2525 Pottsdamer Street, Tallahassee, FL 32310
Radionuclide contamination is often accompanied by other chemical changes in the soil arising from waste organic chelates, strong acid or base solutions, or concentrated salt solutions. These changes can result in neoformation of colloidal material or alteration of existing colloids. As sorbents for radionuclides, colloids may enhance their transport to groundwater if conditions are favorable for dispersion and stable colloidal suspensions. We examine both colloid formation and radionuclide transport related to contaminated sites at the Hanford Reservation in central Washington State. Formation: Waste tanks at the Hanford Reservation have leaked millions of gallons of alkaline waste bearing NaOH, NaNO3, NaAl(OH)4, and various radionuclides and heavy metals into the sediments of the Hanford Formation. Our objectives were (1) to identify colloidal materials formed from the reaction of simulated waste solutions with Hanford sediment; (2) to determine the role of specific anions and cations on the nature of the materials; and (3) to determine their Cs-sorption behavior. Solutions with varying OH and Al concentrations were reacted with Hanford sediments and the products were characterized with x-ray diffraction, FTIR, 27Al and 29Si MAS-NMR, SEM, and x-ray dispersive spectroscopy. Cancrinite and sodalite were the major minerals formed following formation of poorly crystalline aluminosilicates and zeolite. The formation of these minerals was then followed in simulated solutions with dissolved Na2H2SiO4 as the Si source. We varied OH, Na, Al, and Si concentrations to find that cancrinite was the preferred phase and was favored at high NO3, and divalent anions. Cesium present at the time of formation was partially fixed in highly crystalline cancrinite and almost unextractable with K, Ca, or Na in sodalite. Adsorbed Cs could be largely exchanged from cancrinite and sodalite. Transport: The association of Cs with newly formed colloids and with native colloidal minerals suggests that the movement of such colloids could enhance Cs transport. Cesium is normally adsorbed strongly by micaceous minerals of the Hanford Formation; namely, biotite and muscovite. However, if Cs interactions with colloidal phases such as feldspathoids and illite are stable over the period of transport, such colloids could carry Cs to groundwater. We determined the potential for colloidal transport under different levels of water saturation and the potential for Cs transport where Cs was pre-associated with colloids and water saturation was varied. Colloids were transported without removal under saturated conditions while colloids were increasingly removed from the mobile phase as saturation decreased. Cesium associated with mobile colloids was stripped off by the sediment matrix as transport occurred. Up to 70% of the Cs was removed from colloids, this amount decreasing with increasing flow rate. Unless Cs is incorporated into colloids as they are formed, the association is unlikely to result in transport over large distances in the Hanford environment.