Influence of Dissolved Sodium and Cesium on Uranyl Oxide Hydrate Solubility DANIEL E. GIAMMAR* AND JANET G. HERING California Institute of Technology, 1200 E. California Boulevard, Environmental Engineering Science, Pasadena, California 91125 The solubility of uranium-containing minerals can control the mobility of uranium in contaminated soil and groundwater. The identity and solubility of these minerals are strongly influenced by solution composition. The influence of dissolved sodium and cesium on the solubility of uranyl oxide hydrates has been investigated in a series of batch experiments conducted with synthetic metaschoepite ((UO 2 ) 8 O 2 (OH) 12 ‚10H 2 O). During reaction of metaschoepite in NaNO 3 , CsNO 3 , and NaF solutions, an initial increase in the dissolved uranium concentration was followed by a decrease as uranium was incorporated into a secondary solid phase. Given sufficient reaction time, metaschoepite was completely transformed to a clarkeite-like sodium uranyl oxide hydrate or a cesium uranyl oxide hydrate that has not previously been described. These secondary solid phases exhibited X-ray diffraction patterns and Raman spectra that were distinct from those of the original metaschoepite. Dissolved uranium concentrations in equilibrium with the sodium and cesium uranyl oxide hydrates can be more than 2 orders of magnitude lower than those in equilibrium with metaschoepite. Initial changes in metaschoepite solubility may also result from particle growth induced by sodium and cesium incorporation into the solid phase. Introduction Uranium (U) contamination of soil and groundwater has resulted from mining, refining, and waste disposal activities associated with nuclear weapons and energy programs. U is a principal contaminant in soils at Department of Energy weapons processing plants (1), and the proposed high-level radioactive waste repository at Yucca Mountain would receive large amounts of U in the form of spent nuclear fuel and vitrified high-level waste (2). The mobility of U in oxic soil and groundwater can be controlled by U(VI) minerals, and U(VI) oxide hydrates are particularly important because of their occurrence at contaminated sites and crucial role in the corrosion of spent nuclear fuel. Schoepite ((UO2)8O2- (OH)12‚12H2O) or schoepite-like phases have been identified in soils at the Fernald Environmental Management Site (3, 4), Oak Ridge National Laboratory (5), and in catch-box soils used in the ballistics testing of depleted uranium metal projectiles (6). Schoepite is the first phase in a sequence of secondary U(VI) phases formed during the corrosion of spent nuclear fuel (7) and the weathering of natural ore deposits (8). The formation of schoepite may control the solubility of U in a corrosive environment, and schoepite is the starting material for the formation of more stable secondary phases. While U sorption on Fe(III) oxide (9-11) and clay minerals (12-14) has been widely observed, surface precipitation of schoepite and schoepite-like phases has also been observed on Fe(III) oxide (11, 15) and phyllosilicate minerals (16). Despite their simple compositions, U(VI) oxide hydrates are a diverse family of minerals, whose structures are composed of sheets of uranyl bipyramidal polyhedra con- nected through edge and corner sharing of equatorial oxygen atoms (17). Metaschoepite ((UO2)8O2(OH)12‚10H2O) and schoepite are closely related phases with nearly identical interlayer spacings and minor differences in sheet arrange- ments (18, 19). Water molecules occupy the interlayers of schoepite and metaschoepite (20), but these solids have a strong tendency to incorporate cations into their interlayer spaces (8, 21). New mineral phases are formed by incorpo- ration of Ca 2+ (19, 22-25), Na + (8), K + (23), Ba 2+ , and Pb 2+ (22). While not observed in nature, synthetic layered cation U(VI) oxide hydrates have also been formed with Sr 2+ (26, 27), Cs + (28), Na + (29, 30), K + (31), Mg 2+ , Mn 2+ , and Ni 2+ (32). Schoepite can also undergo transformation without the incorporation of interlayer cations, including transformations to U(VI) phosphates (24) and carbonates (33). In contaminated environments, cations available for incorporation may be present in the contacting groundwater or may be co-contaminants with U. In unsaturated corrosion tests of UO2 (the primary component of spent nuclear fuel) with groundwater from the Nevada Test Site adjacent to the Yucca Mountain repository, initial schoepite formation was followed by the formation of becquerelite (Ca(UO2)6O4(OH)6‚ 8H2O) and compreignacite (K2(UO2)6O4(OH)6‚7H2O) (7). Formation of U(VI) oxide hydrates with interlayer cations is also likely in high ionic strength aqueous waste solutions containing U. The formation of Na-containing U(VI) oxide hydrates may be expected in systems that store or have received U(VI)-containing aqueous solutions with high NaNO3 and/or NaOH concentrations such as the tank wastes at the Hanford Site and Savannah River Site (34, 35). In Na- rich geological settings, the mineral clarkeite (Na[(UO2)O- (OH)]‚H2O) can form through the alteration of preexisting uranium minerals (8). Cations produced by fission such as Cs + and Sr 2+ that are present in waste solutions and spent nuclear fuel (7) may also be incorporated into U(VI) oxide hydrates that form in these systems. While cation incorpora- tion influences U(VI) oxide hydrate solubility, incorporation may also serve to retard the mobility of cations (36), a process which is particularly important for fission products such as 137 Cs and 90 Sr. The objectives of this work were to examine the influence of dissolved Na + and Cs + on the solubility of U(VI) oxide hydrates and to relate changes in solubility to transformations of the solid phase. The influence of Na + and Cs + was probed by performing a series of experiments with synthetic met- aschoepite exposed to solutions of varying chemical com- position. By combining bulk solution chemistry measure- ments with solid-phase characterization, observed changes in U solubility were related to metaschoepite transformation and the formation of U(VI) oxide hydrates containing Na + and Cs + . Experimental Section Materials. Two synthetic metaschoepite materials were prepared following an adaptation of published methods (24, * Corresponding author phone: (314)935-6849; fax: (314)935-5464; e-mail: degiammar@seas.wustl.edu. Present address: Environmental Engineering Science, Washington University in St. Louis, Campus Box 1180, One Brookings Drive, St. Louis, MO 63130. Environ. Sci. Technol. 2004, 38, 171-179 10.1021/es0345672 CCC: $27.50  2004 American Chemical Society VOL. 38, NO. 1, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 171 Published on Web 11/26/2003