Stable isotope signatures of hydration water in secondary mineralization on UO2.

Hydrated secondary mineralization readily forms on the surface of UO2 particles exposed to humidity in an oxidizing environment. The oxygen stable isotope composition of the secondary uranium oxide may reflect that of the water vapor, as well as the hydrogen and oxygen stable isotopic composition of the mineral hydration water. The geospatial organization of δ2H and δ18O values of atmospheric humidity and precipitation is increasingly well understood, which suggests that the hydrogen and oxygen stable isotopes in secondary mineral hydration water may yield information on the environment in which the mineralization formed. UO2 powders were exposed to air with constant 30%, 61%, and 91% relative humidity, and constant H and O stable isotope composition. Aliquots were sampled from the UO2 materials at intervals of 1-10 days through the total humidity exposure duration of 180 days. Scanning electron microscopy, transmission electron microscopy, and x-ray diffraction analysis of the humidity-exposed UO2 indicates that schoepite/metaschoepite [(UO3)•2H2O] secondary phases had formed on the underlying UO2. The δ2H and δ18O values of mineral hydration waters were determined by thermogravimetry-enabled isotope ratio infrared spectroscopy (TGA-IRIS). Results indicate that hydrogen in the surface sorbed and mineral hydration waters is exchangeable and thus their δ2H values are difficult to interpret. However, oxygen in these waters is less exchangeable, and thus the oxygen stable isotope composition of the schoepite/metaschoepite hydration water is likely to be related to that of the exposure water vapor. After formation of schoepite/metaschoepite, the δ18O values of the hydration water in schoepite/metaschoepite does not change in response to changes in exposure vapor δ18O values, which suggests that the δ18O values of the hydration water is relatively durable. These findings suggest that information about the origin and storage history of a UO2 sample may be discernable from δ18O values of schoepite/metaschoepite hydration water.

Water vapor exposure chamber for constant humidity and hydrogen and oxygen stable isotope composition.

RATIONALE: Water vapor exposure experiments have applications for studying water physisorption and chemisorption hydration and hydroxylation reactions on a wide variety of material surfaces. The stable isotopes of hydrogen and oxygen in the water molecule are useful tracers of water exchange mechanisms and/or rates in such vapor exposure experiments. METHODS: We designed and built a humidity chamber system that uses membrane-mediated liquid-vapor exchange of water followed by mixing with dry air to control the relative humidity of air and its δ2 H and δ18 O isotopic composition. We tested the stability and precision of the humidity and its isotopic composition on hourly to 90-day timescales. RESULTS: The humidity chamber design reported here is capable of providing relative humidity control to within ±1%, and consistent δ2 H and δ18 O values of the water vapor that are similar to our cavity ringdown spectroscopy (CRDS) measurement precision (δ2 Hvap ± 0.7‰ and δ18 Ovap ± 0.24‰). We quantify the isotopic enrichment effects of Rayleigh distillation in the system and provide information on water reservoir sizes large enough to buffer isotopic enrichment effects to within measurement precision. CONCLUSIONS: The humidity chamber design reported here provides a means to create constant δ2 H and δ18 O values over the course of an exposure experiment. The design has applications to a wide range of studies of water sorption on material surfaces from foods and pharmaceuticals to geological materials.

Correlated SEM, FIB-SEM, TEM, and NanoSIMS imaging of microbes from the hindgut of a lower termite: methods for in situ functional and ecological studies of uncultivable microbes.

The hindguts of lower termites harbor highly diverse, endemic communities of symbiotic protists, bacteria, and archaea essential to the termite's ability to digest wood. Despite over a century of experimental studies, ecological roles of many of these microbes are unknown, partly because almost none can be cultivated. Many of the protists associate with bacterial symbionts, but hypotheses for their respective roles in nutrient exchange are based on genomes of only two such bacteria. To show how the ecological roles of protists and nutrient transfer with symbiotic bacteria can be elucidated by direct imaging, we combined stable isotope labeling (13C-cellulose) of live termites with analysis of fixed hindgut microbes using correlated scanning electron microscopy, focused ion beam-scanning electron microscopy (FIB-SEM), transmission electron microscopy, and high resolution imaging mass spectrometry (NanoSIMS). We developed methods to prepare whole labeled cells on solid substrates, whole labeled cells milled with a FIB-SEM instrument to reveal cell interiors, and ultramicrotome sections of labeled cells for NanoSIMS imaging of 13C enrichment in protists and associated bacteria. Our results show these methods have the potential to provide direct evidence for nutrient flow and suggest the oxymonad protist Oxymonas dimorpha phagocytoses and enzymatically degrades ingested wood fragments, and may transfer carbon derived from this to its surface bacterial symbionts.

Investigating the affinities and persistence of VX nerve agent in environmental matrices.

Laboratory experiments were conducted to determine environmental variables that affect the affinities and persistence of the nerve agent O-ethyl S-(2-diisopropylaminoethyl) methylphosphonothiolate (VX) at dilute concentrations in environmental matrices. Quantitative analyses of VX and its degradation products were performed using LC-MS. Batch hydrolysis experiments demonstrated an increasing hydrolysis rate as pH increased, as shown in previous studies, but also indicated that dissolved aqueous constituents can cause significant differences in the absolute hydrolysis rate. Adsorption isotherms from batch aqueous experiments revealed that VX has a high affinity for hydrophobic organics, a moderate affinity for montmorillonite clay, and a very low affinity for an iron-oxyhydroxide soil mineral, goethite. The adsorption on goethite was increased with the presence of dissolved organic matter in solution. VX degraded rapidly when dried onto goethite, when specific adsorption was forced. No enhanced degradation occurred with goethite in small amounts of water. These results suggest that aqueous conditions have important controls on VX adsorption and degradation in the environment and a more mechanistic understanding of these controls is needed in order to enable accurate predictions of its long-term fate and persistence.

Geochemical imaging of flow near an artificial recharge facility, Orange County, California.

Critical for the management of artificial recharge operations is detailed knowledge of ground water dynamics near spreading areas. Geochemical tracer techniques including stable isotopes of water, tritium/helium-3 (T/3He) dating, and deliberate gas tracer experiments are ideally suited for these investigations. These tracers were used to evaluate flow near an artificial recharge site in northern Orange County, California, where approximately 2.5 x 10(8) m3 (200,000 acre-feet) of water are recharged annually. T/3He ages show that most of the relatively shallow ground water within 3 km of the recharge facilities have apparent ages < 2 years; further downgradient apparent ages increase, reaching > 20 years at approximately 6 km. Gas tracer experiments using sulfur hexafluoride and xenon isotopes were conducted from the Santa Ana River and two spreading basins. These tracers were followed in the ground water for more than two years, allowing subsurface flow patterns and flow times to be quantified. Results demonstrate that mean horizontal ground water velocities range from < 1 to > 4 km/year. The leading edges of the tracer patch moved at velocities about twice as fast as the center of mass. Leading edge velocities are important when considering the potential transport of microbes and other "time sensitive" contaminants and cannot be determined easily with other methods. T/3He apparent ages and tracer travel times agreed within the analytical uncertainty at 16 of 19 narrow screened monitoring wells. By combining these techniques, ground water flow was imaged with time scales on the order of weeks to decades.