CTR diffraction and grazing-incidence EXAFS study of U(VI) adsorption onto α-Al2O3 and α-Fe2O3 (11?02) surfaces

Evaluation of the long-term health risks of uranium contamination in soils, sediments, and groundwater requires a fundamental understanding of the various processes affecting subsurface transport of uranium, including adsorption processes at mineral/water interfaces. In this study, the sites of binding and surface complexation of U(VI) adsorbed on the (11?02) surfaces of α-Al₂O₃ and α-Fe₂O₃ have been determined using crystal truncation rod (CTR) diffraction and grazing incidence extended X-ray absorption fine structure (GI-EXAFS) spectroscopy. The available binding sites on the (11?02) surfaces were constrained through bond valence and steric analyses. On both surfaces, U(VI) forms uranyl-carbonato ternary complexes to surface oxygens that are singly coordinated to aluminum or iron. On the α-Al₂O₃ (11?02) surface, a monodentate complex results, whereas on the α-Fe₂O₃ (11?02) surface, the binding is bidentate to adjacent singly coordinated oxygen sites (i.e., binuclear). Differences in protonation of the singly coordinated oxygen atoms, surface charging, U(VI) aqueous speciation, substrate structure, or the electronic structure of surface functional groups may be the cause of these differences in adsorption geometry. Both XPS and CTR diffraction reveal higher U(VI) surface coverages on the α-Fe₂O₃ (11?02) surface than on the α-Al₂O₃ (11?02) surface. This difference cannot be the result of differences in defect concentration alone as CTR diffraction is not sensitive to U(VI) sorbed to defect sites, implying that the α-Fe₂O₃ (11?02) surface has an intrinsically higher affinity for U(VI). The surface complexes observed in this study are different from the bidentate, mononuclear complexes typically derived for U(VI) on powdered aluminum- and iron-(oxyhydr)oxides and clay minerals using U L_III-edge EXAFS spectroscopy. However, the presence of monodentate, mononuclear and bidentate, binuclear complexes may have been overlooked in past EXAFS studies on such substrates, as these complexes have U-Al or U-Fe interatomic distances that are too large to be easily detected by EXAFS spectroscopy.     

The Pitman Canyon paleoseismic record: A re-evaluation of southern San Andreas Fault segmentation

Correlation of three well-resolved paleoseismic records, including the Pitman Canyon site with its emerging record, presents a new possibility to understand fault segmentation. To be a useful concept, fault segment boundaries must be relatively stationary over multiple seismic cycles and must appear frequently in limiting the rupture extent of earthquakes; thus, sites on the same segment should share more paleoseismic events than those on different segments. A conclusive event correlation between sites is difficult or impossible due to dating uncertainties. However, often the data are adequate to preclude correlation and thus provide firm limits on rupture extent for those events. Thus clear non-correlations provide more information about segmentation than do unprovable potential correlations.The southern end of the most recent rupture in 1857, between Wrightwood and Pitman Canyon, is often used to define a segment boundary. However, there is an absence of significant non-correlation between the previous five Pitman Canyon events and the Wrightwood events. While both Pallett Creek and Wrightwood ruptured in 1857, only two of the previous five Wrightwood events can correlate with Pallett Creek events, which may or may not indicate that they actually do. These paleoseismic records do not support the existence of a segment boundary between Wrightwood and Pitman Canyon as defined by the 1857 rupture extent, suggesting a reevaluation of southern San Andreas Fault segmentation, and using historic ruptures to define segments in general.     

Water requirements for cultivatingSalicornia bigeloviiTorr. with seawater on sand in a coastal desert environment

The forage and oilseed halophyte,Salicornia bigeloviiTorr., was grown in gravity-drained lysimeters set in open plots of the same crop over two seasons in a coastal desert environment in Sonora, Mexico. The lysimeters were irrigated daily with seawater (40 g l⁻¹salts) at rates ranging from 46–225% of potential evaporation. Biomass and seed yields increased with increasing irrigation depth over the range of treatments. Biomass yields ranged from 13·6–23·1 t DM ha⁻¹, equivalent to conventional forage crops, on seasonal water application depths of 2·3–3·8 m, but were markedly lower at lower irrigation depths. Increasing the irrigation depth lowered the soil solution salinity, resulting in greater growth and water use, and hence leaching fractions that were nearly even over irrigation treatments, averaging 0·5. Evapo-transpiration rose in direct proportion to the irrigation depth. Potential evaporation was estimated by site pan evaporation and by the Blaney-Criddle and Penman models using climatological data; the methods agreed within 15%. The ratio of evapo-transpiration to potential evaporation increased over the growing season and approached 1·5 by pan on the highest irrigation treatment due to the combined effects of high transpiration and high evaporation from the permanently moist soil surface. The best field predictor of biomass yield was the salinity of the soil moisture in the top 15 cm of soil profile, which constitutes the root zone for this crop. Root zone salinity must be kept at 70–75 g l⁻¹for high yields. Although irrigation and drainage requirements were high compared to conventional crops, seawater irrigation appears to be feasible in medium sand and could augment crop production along coastal deserts. The possibility of using this crop for animal production is discussed.     

Using Q-mode factor analysis in biostratigraphic ordination

Although traditionally restricted to the analysis of closed compositional data, the Imbrie Q-mode method appears also to be of utility when applied to binary data sets common in biostratigraphy. Its advantages include its utilization of the tally concept, and its ability to analyze very large numbers of species simultaneously. When the structure of the data set is simple, the method appears to be capable of ranking samples along a temporal gradient, or coenocline, defined by the second unroated factor. Varimax and oblique rotations do not appear to increase the efficacy of this ranking. Even when facies effects are important, a temporal ranking of samples can be achieved, providing that the variation due to time is greater than that due to facies.     

ALH85085: a unique volatile-poor carbonaceous chondrite with possible implications for nebular fractionation processes

Allan Hills 85085 is a unique chondrite with affinities to the Al Rais-Renazzo clan of carbonaceous chondrites. Its constituents are less than 50 μm in mean size. Chondrules and microchondrules of all textures are present; nonporphyritic chondrules are unusually abundant. The mean compositions of porphyritic, nonporphyritic and barred olivine chondrules resemble those in ordinary chondrites except that they are depleted in volatile elements. Ca-, Al-rich inclusions are abundant and largely free of nebular alteration; they comprise types similar to those in CM and CO chondrites, as well as unique types. Calcium dialuminate occurs in several inclusions. Metal, silicate and sulfide compositions are close to those in CM-CO chondrites and Al Rais and Renazzo. C1-chondrite clasts and metal-rich “reduced” clasts are present, but opaque matrix is absent. Siderophile abundances in ALH85085 are extremely high (e.g., Fe/Si= 1.7 × solar), and volatiles are depleted (e.g., Na/Si= 0.25 × solar, S/Si= 0.03 × solar). Nonvolatile lithophile abundances are similar to those in Al Rais, Renazzo, and CM and CO chondrites.ALH85085 agglomerated when temperatures in the nebula were near 1000 K, in the same region where Renazzo, Al Rais and the CI chondrites formed. Agglomeration of high-temperature material may thus be a mechanism by which the fractionation of refractory lithophiles occurred in the nebula. Chondrule formation must have occurred at high temperatures when clumps of precursors were small. After agglomeration, ALH85085 was annealed and lightly shocked. C1 and other clasts were subsequently incorporated during late-stage brecciation.     

TL dating studies of tephra, loess and lacustrine sediments

A brief review is presented of my efforts to improve the accuracy of dating unheated sediments by TL and of the development of a new tephrochronometer. Specific applications to known-age deposits are outlined, as are studies of the depositional environment of subaqueous sediments. Removal of anomalous fading in loess by storage at 75°C for 8 days is demostrated.Throughout, for unheated sediments the preferential use of the partial bleach (R-gamma or R-beta) technique is emphasized, especially in situations where the growth curves are sublinear. Contrary to a widespread misconception, sublinearity does not invalidate the partial bleach method. Furthermore, examples are given of TL ‘sensitivity’ (or ‘efficiency’) changes occurring with the regeneration technique — changes that are variable and seemingly sample dependent.     

Surfactant-Induced Reductions in Soil Hydraulic Conductivity

Surfactant solutions are being proposed for in situ flushing of organic contaminants from soils and aquifers. The feasibility of surfactant additives in remediation may depend in large part on how these chemicals affect the hydraulic conductivity of the porous media. While there is evidence in the literature of conductivity loss during surfactant flushing (Miller et al. 1975; Nash et al. 1987), there has been little research on quantifying the process for unconsolidated sediments. Surfactant-affected hydraulic conductivity reductions were measured in two soils (Teller loam and Daugherty sand). Testing was done with eight surfactants at a variety of concentrations (10^-5 to 10^-l mole/kg), surfactant mixtures, and added solution electrolytes. The Teller was also tested with its organic matter removed. Maximum hydraulic conductivity decreases were 47 percent for the sand and more than two orders of magnitude for the loam. Surfactant concentrations, surfactant mixtures, soil organic content, and added solution electrolytes all affected the degree of conductivity reduction. Results indicate that surfactant-affected hydraulic conductivity losses should be considered prior to in situ remediation and may preclude surfactant use in some fine grain soils.     

The anthracite smokers of eastern pennsylvania:P_{S_2 (g)} -T stability diagram by TL analysis

Coal seams and culm banks associated with mine fires in the anthracite region of eastern Pennsylvania have been burning for decades. Many of the fires may have ignited by spontaneous combustion or by the burning of trash. Minerals associated with the combustion of anthracite form by the condensation of gas exhaled through surficial gas vents or “anthracite smokers.” A Pressure-Temperature (P-T) stability diagram is constructed for the condensation of orthorhombic sulfur from anthracite gas using “Thermodynamic Loop Analysis” (TL analysis). This method of analyzing chemical systems incorporates Kirchhoff's Law into a four step procedure structured around a closed thermodynamic cycle or “thermodynamic loop.” The four steps, referred to us “The Four S ′S of Thermodynamic Loop Analysis,” include: (1) “Set Up”—graphical characterization of the problem. (2) “Sum”—the application of thermodynamic principles. (3) “Substitute”—the use of materials data available from the literature, and (4) “Solve”—computation of one or more variables. The example presented demonstrates that thermodynamic loops can incorporate any number of polymorphic phase transformations. In addition, thermodynamic loop analysis is applicable to any geologic process involving the condensation of minerals from a gas. The stability diagram derived by TL analysis may have applicability in monitoring the release of sulfur gas into the atmosphere.     

Chronostratigraphic studies of sediments in the Negev desert, using amino acid epimerization analysis of land snail shells

The chronostratigraphy of mostly Holocene sediments in the Northern Negev desert of Israel was studied through amino acid epimerization analysis (alloisoleucine/isoleucine ratio, A/I) of individual land snail shells and ¹⁴C analysis of bulk samples of land snail shells contained within the sediments. Analysis of 31 deposits shows a very strong correlation between A/I and ¹⁴C age. A/I analyses indicate that age mixtures occur within all colluvial and in some fluvial deposits. Consequently, radiocarbon dates of bulk samples, which give an average age, overestimate the time of deposition. Sedimentation rates were generally rapid in both fluvial and colluvial deposits. Colluviation shows a maximum in the early Holocene and decreases progressively thereafter, in accordance with the change in average rainfall. Accumulation of fluvial sediments shows a distinct mid-Holocene maximum and apparently relates to the interaction between variation in rainfall, extent of bedrock exposure, and vegetation density. Late Holocene fluvial deposition may relate to agricultural activity. The understanding of the chronology of sedimentation is dependent on an integrated approach employing amino acid epimerization analysis to study age variation within a layer and ¹⁴C analyses for establishment of an absolute chronology.