In situ reductive dissolution to remove Iodine-129 from aquifer sediments.

The use of an aqueous reductant (Na-dithionite) with pH buffer (K-carbonate, pH 12) was evaluated in this laboratory study as a potential remedial approach for removing Fe oxide associated iodine and enhancing pump-and-treat extraction from iodine-contaminated sediments in the unconfined aquifer in the 200 West Area of the Hanford Site. X-ray fluorescence data of untreated sediment indicated that iodine was largely associated with Fe (i.e., potentially incorporated into Fe oxides), but XANES data was inconclusive as to valence state. During groundwater leaching, aqueous and adsorbed iodine was quickly released, then additional iodine was slowly released potentially from slow dissolution of one or more surface phases. The Na-dithionite treatment removed greater iodine mass (2.9x) at a faster rate (1-4 orders of magnitude) compared to leaching with groundwater alone. Iron extractions for untreated and treated sediments showed a decrease in Fe(III)-oxides, which likely released iodine to aqueous solution. Solid phase inorganic carbon and aqueous Ca and Mg analysis further confirmed that significant calcite dissolution did not occur in these experiments meaning these phases did not release significant iodine. Although it was expected that, after treatment, 127I concentrations would eventually be lower than untreated sediments, continued, elevated iodine concentrations for treated samples over 750 h were observed for leaching experiments. Stop flow events during 1-D column leaching suggested that some iodide precipitated within the first few pore volumes. Further, batch extraction experiments compared iodine-129/127 removal and showed that iodine-129 was more readily removed than iodine-127 suggesting that the two are present in different phases due to their different origins. Although significantly greater iodine is removed with treatment, the long-term leaching needs to be investigated further as it may limit dithionite treatment at the field scale.

Optical and Chemical Characterization of Uranium Dioxide (UO2) and Uraninite Mineral: Calculation of the Fundamental Optical Constants.

Uranium dioxide (UO2) is a material with historical and emerging applications in numerous areas such as photonics, nuclear energy, and aerospace electronics. While often grown synthetically as single-crystal UO2, the mineralogical form of UO2 called uraninite is of interest as a precursor to various chemical processes involving uranium-bearing chemicals. Here, we investigate the optical and chemical properties of a series of three UO2 specimens: synthetic single-crystal UO2, uraninite ore of relatively high purity, and massive uraninite mineral containing numerous impurities. An optical technique called single-angle reflectance spectroscopy was used to derive the optical constants n and k of these uranium specimens by measuring the specular reflectance spectra of a polished surface across the mid- and far-infrared spectral domains (ca. 7000-50 cm-1). X-ray diffractometry, scanning electron microscopy, and energy-dispersive X-ray spectroscopy were further used to analyze the surface composition of the mineralogical forms of UO2. Most notably, the massive uraninite mineral was observed to contain significant deposits of calcite and quartz in addition to UO2 (as well as other metal oxides and radioactive decay products). Knowledge of the infrared optical constants for this series of uranium chemicals facilitates nondestructive, noncontact detection of UO2 under a variety of conditions.

Microbial functional gene diversity with a shift of subsurface redox conditions during In Situ uranium reduction.

To better understand the microbial functional diversity changes with subsurface redox conditions during in situ uranium bioremediation, key functional genes were studied with GeoChip, a comprehensive functional gene microarray, in field experiments at a uranium mill tailings remedial action (UMTRA) site (Rifle, CO). The results indicated that functional microbial communities altered with a shift in the dominant metabolic process, as documented by hierarchical cluster and ordination analyses of all detected functional genes. The abundance of dsrAB genes (dissimilatory sulfite reductase genes) and methane generation-related mcr genes (methyl coenzyme M reductase coding genes) increased when redox conditions shifted from Fe-reducing to sulfate-reducing conditions. The cytochrome genes detected were primarily from Geobacter sp. and decreased with lower subsurface redox conditions. Statistical analysis of environmental parameters and functional genes indicated that acetate, U(VI), and redox potential (E(h)) were the most significant geochemical variables linked to microbial functional gene structures, and changes in microbial functional diversity were strongly related to the dominant terminal electron-accepting process following acetate addition. The study indicates that the microbial functional genes clearly reflect the in situ redox conditions and the dominant microbial processes, which in turn influence uranium bioreduction. Microbial functional genes thus could be very useful for tracking microbial community structure and dynamics during bioremediation.