Geochemistry
Extensive geochemical studies have been performed on samples from four depth profiles collected beneath the North and South Process Ponds through the vadose zone to groundwater (Zachara et al. 2005, Zachara et al. 2007). This research has sought to identify the mineral association and molecular speciation of sorbed U(VI), quantify adsorption/precipitation and desorption/dissolution rates for contaminant U(VI), and develop a surface complexation model for U(VI) on 300 Area sediments. The overall goal of this research has been to develop better conceptual and numeric models of microscopic U(VI)-sediment interactions for improved predictions of future plume behavior.
Molecular speciation measurements by X-ray absorption and cryogenic laser induced fluorescence spectroscopy (CLIFS) have shown that the chemical speciation of U(VI) changes from precipitated forms in the upper vadose zone near the historic source term to adsorption complexes in the deeper vadoze zone and aquifer sediments (Bond et al. 2006, Catalano et al. 2006, Wang et al. 2005 a,b). Precipitated U(VI) exists within secondary mineral coatings on millimeter-sized lithic fragments in the form of coprecipitates with calcite, and as discrete uranyl phases including metatorbernite [Cu(UO2PO4)2·8H2O; Arai et al. 2007; McKinley et al. 2007] and autunite. Adsorption occurs within aggregates of fine-grained phyllosilicates that include chlorite, vermiculite, and smectite, with variable ferrihydrite in low concentration that results from chlorite weathering. Adsorption is dominated by surface complexes to phyllosilicates in some locations and depths, and by surface complexes to Fe(III)-oxides (ferrihydrite) in others. Adsorption is significant within the fines fraction (e.g., log Kd≈0.5 to 2.5 L/kg), but its overall effect is diluted strongly by mass-dominant gravel.
In comparing different sediments from the 300 Area, mass transfer rates are observed to vary with U molecular speciation, sediment texture, and aqueous pH/carbonate content. For sediments containing adsorbed U(VI), the mass transfer rate decreases with increasing sorption strength and ferrihydrite content. Desorption is invariably slower than adsorption. These findings indicate that pore scale mass transfer rates will exhibit significant heterogeneity in the field depending on sediment properties and whether desorption or adsorption is occurring. Pore-scale mass transfer is thought to result from diffusive transport within particle coatings (precipitated U) and phyllosilicate aggregates (adsorbed U) of limited porosity.
In the deeper vadose zone and groundwater where adsorption is the primary retardation process, sorption strength is found to vary strongly with groundwater geochemical composition (e.g., pH and carbonate content) and mineral properties of the fines fraction, such as sediment surface area, ferrihydrite content, and phyllosilicate distribution (Bond et al., 2006). Accordingly, the distribution coefficient (Kd) varies by two orders of magnitude over the range in water composition and sediment characteristics present at the site. A surface complexation model has been developed for the 300 Area sediments (Bond et al. 2006) that explicitly includes aqueous speciation effects through surface complexation reactions: SOH + UO22+ + H2O = SOUO2OH + 2H+ SOH + UO22+ + H2CO3 = SOUO2CO3 + 2H+, where SOH is an undifferentiated surface site with concentration of 3.84 mol/m2. This model semi-quantitatively describes carbonate and pH effects on U(VI) adsorption and reduces the magnitude of Kd variation. However, different surface-area normalized model parameters are required for each sediment indicating the presence of unaccounted variability (e.g., difference between NPP and SPP models, a factor of 5 in Kd) from sediment mineralogic heterogeneity. Thus, field experiments performed by the IFRC must consider and characterize sorption site spatial heterogeneity explicitly as it effects U(VI) retardation and mass transfer.
ReferencesBond D. L., Davis J. A., and Zachara J. M. 2007. Uranium(VI) release from contaminated vadose zone sediments: estimation of potential contributions from dissolution and desorption. In Adsorption of Metals by Geomedia II(eds. M.O. Barnett and D.B. Kent), pp. 379-420. PNNL-SA-58541. Academic Press, San Diego.
Catalano JG, JP McKinley, JM Zachara, SM Heald, SC Smith, and GE Brown, Jr. 2006. "Changes in uranium speciation through a depth sequence of contaminated Hanford sediments." Environmental Science & Technology 40(8):2517-24. PNNL-SA-46734, Pacific Northwest National Laboratory, Richland, Washington.
Qafoku NP, JM Zachara, C Liu, OS Qafoku, and SC Smith. 2005. "Kinetic desorption and sorption of U(VI) during reactive transport in a contaminated Hanford sediment." Environmental Science & Technology 39(9)3157-3165. PNNL-SA-42960, Pacific Northwest National Laboratory, Richland, Washington.
Wang Z, JM Zachara, JP McKinely, and SC Smith. 2005a. "Cryogenic laser induced U(VI) fluorescence studies of a U(VI) substituted natural calcite: implications to U(VI) speciation in contaminated Hanford sediments." Environmental Science & Technology 39(8):2651-2659 DOI:10.1021/es048448d. PNNL-SA-42959,Pacific Northwest National Laboratory, Richland, Washington.
Wang Z, JM Zachara, PL Gassman, C Liu, O Qafoku, and JG Catalano. 2005b. "Fluorescence spectroscopy of U(VI)-silicates and U(VI)-contaminated Hanford sediment." Geochimica et Cosmochimica Acta 69(6):1391-1403. PNNL-SA-42812, Pacific Northwest National Laboratory, Richland, Washington.
Zachara JM, JA Davis, C Liu, JP McKinley, N Qafoku, DM Wellman, and SB Yabusaki. 2005. Uranium Geochemistry in Vadose Zone and Aquifer Sediments from the 300 Area Uranium Plume. PNNL-15121, Pacific Northwest National Laboratory, Richland, Washington.
Yuji A, MA Marcus, N Tamura, JA Davis, JM Zachara. 2007. Spectroscopic Evidence for Uranium Bearing Precipates in Vadose Zone Sideiments at the Hanford 300-Area Site.pp. 4633-4639, Environmental Science Technology.
Zachara JM, C Brown, J Christensen, JA Davis, E Dresel, C Liu, S Kelly, JP McKinley, RJ Serne, W Um. 2007. A Site-Wide Perspective on Uranium Geochemistry at the Hanford Site. PNNL-17031, Pacific Northwest National Laboratory, Richland, Washington.
McKinely JP, JM Zachara, J Wan, DE McCready, and SM Heald. 2007. Geochemical Controls on Contaminant Uranium in Vadose Hanford Formation Sediments at the 200 Area and 300 Area, Hanford Site, Washington. vol. 6, No. 4, November 2007. Vadose Zone Journal.