Uranyl adsorption onto montmorillonite: Evaluation of binding sites and carbonate complexation

The fate and transport of uranium in contaminated soils and sediments may be affected by adsorption onto the surface of minerals such as montmorillonite. Extended X-ray absorption fine structure (EXAFS) spectroscopy has been used to investigate the adsorption of uranyl (UO₂²⁺) onto Wyoming montmorillonite. At low pH (∼4) and low ionic strength (10⁻³ M), uranyl has an EXAFS spectrum indistinguishable from the aqueous uranyl cation, indicating binding via cation exchange. At near-neutral pH (∼7) and high ionic strength (1 M), the equatorial oxygen shell of uranyl is split, indicating inner-sphere binding to edge sites. Linear-combination fitting of the spectra of samples reacted under conditions where both types of binding are possible reveals that cation exchange at low ionic strengths on SWy-2 may be more important than predicted by past surface complexation models of U(VI) adsorption on related montmorillonites. Analysis of the binding site on the edges of montmorillonite suggests that U(VI) sorbs preferentially to [Fe(O,OH)₆] octahedral sites over [Al(O,OH)₆] sites. When bound to edge sites, U(VI) occurs as uranyl-carbonato ternary surface complexes in systems equilibrated with atmospheric CO₂. Polymeric surface complexes were not observed under any of the conditions studied. Current surface complexation models of uranyl sorption on clay minerals may need to be reevaluated to account for the possible increased importance of cation exchange reactions at low ionic strengths, the presence of reactive octahedral iron surface sites, and the formation of uranyl-carbonato ternary surface complexes. Considering the adsorption mechanisms observed in this study, future studies of U(VI) transport in the environment should consider how uranium retardation will be affected by changes in key solution parameters, such as pH, ionic strength, exchangeable cation composition, and the presence or absence of CO₂.     

An estuarine benthic index of biotic integrity for the Mid-Atlantic region of the United States. II. Index development

A benthic index of biotic integrity was developed for use in estuaries of the mid-Atlantic region of the United States (Delaware Bay estuary through Albemarle-Pamlico Sound). The index was developed for the Mid-Atlantic Integrated Assessment Program (MAIA) of the U.S. Environmental Protection Agency using procedures similar to those applied previously in Chesapeake Bay and southeastern estuaries, and was based on sampling in July through early October. Data from seven federal and state sampling programs were used to categorize sites as degraded or non-degraded based on dissolved oxygen, sediment contaminant, and sediment toxicity criteria. Various metrics of benthic community structure and function that distinguished between degraded and reference (non-degraded) sites were selected for each of five major habitat types defined by classification analysis of assemblages. Each metric was scored according to thresholds established on the distribution of values at reference sites, so that sites with low scoring metrics would be expected to show signs of degradation. For each habitat, metrics that correctly classified at least 50% of the degraded sites in the calibration data set were selected whenever possible to derive the index. The final index integrated the average score of the combination of metrics that performed best according to several criteria. Selected metrics included measures of productivity (abundance), diversity (number of taxa, Shannon-Wiener diversity, percent dominance), species composition and life history (percent abundance of pollution-indicative taxa, percent abundance of pollution-sensitive taxa, percent abundance of Bivalvia, Tanypodinae-Chironomidae abundance ratio), and trophic composition (percent abundance of deep-deposit feeders). The index correctly classified 82% of all sites in an independent data set. Classification efficiencies of sites were higher in the mesohaline and polyhaline habitats (81–92%) than in the oligohaline (71%) and the tidal freshwater (61%). Although application of the index to low salinity habitats should be done with caution, the MAIA index appeared to be quite reliable with a high likelihood of correctly identifying both degraded and non-degraded conditions. The index is expected to be of great utility in regional assessments as a tool for evaluating the integrity of benthic assemblages and tracking their condition over time.     

Mercury surface composition: Integrating petrologic modeling and remote sensing data to place constraints on FeO abundance

A significant opaque component in Mercury’s crust is inferred based on albedo and spectral observations. Previous workers have favored iron-titanium bearing oxide minerals as the spectrally neutral opaque. A consequence of this hypothesis is that Mercury’s surface would have a high FeO content. An array of remote sensing techniques have not provided definitive constraints on the FeO content of Mercury’s surface. However, spectral observations have not detected a diagnostic 1 μm absorption band and have thus limited the FeO in coexisting silicates to <2 wt.% FeO. In this paper, we assess equilibrium among oxide and silicate minerals to constrain the distribution of iron between opaque oxides and silicates under a variety of environmental conditions. Equilibrium modeling is favored here because the geologic process that produced Mercury’s low-albedo intermediate terrain must have occurred globally, which favors a common widespread igneous process. Based on our modeling, we find that iron-rich ilmenite cannot occur with silicates that do not display a 1 μm absorption feature unless plagioclase abundances are high. However, such high plagioclase abundances are precluded by Mercury’s low albedo. Incorporating equilibrium crystallization modeling with spectral and albedo constraints we find the iron abundance of Mercury’s intermediate terrain is ?10 wt.% FeO. This intermediate iron composition matches constraints provided by visible albedo and total neutron absorption observed by MESSENGER. In fact, the total neutron absorption of mixtures of oxide, plagioclase, olivine and pyroxene for the oxide abundances estimated for Mercury, favor Mg-rich members of the ilmenite-geikielite solid-solution series. This work offers compositional constraints for Fe, Ti, and Mg that will be testable by various MESSENGER instrument data sets after it begins its orbital mission.     

The soft-bottom macrobenthos of North Carolina estuaries

The structure of macro infaunal (>0.5 mm sieve size) assemblages was examined in samples of unconsolidated substrates collected during the summers of 1994–1997 at 208 stations throughout North Carolina estuaries. Numerical classification (cluster analysis) of stations resulted in 14 distinct site groups that reflected discernible habitatrelated patterns in species distributions. Multiple discriminant analysis, performed on synoptic abiotic variables (depth, salinity, dissolved oxygen, pH, percent silt-clay), showed that the separation of site groups was related primarily to salinity. Percent silt-clay had a secondary influence on the separation of sites within similar salinity zones. Species diversity among site groups generally decreased with decreasing salinity and increasing mud content of sediment. Nodal analysis showed a wide range in constancy and fidelity of species assemblages within site groups. Some assemblages dominated by euryhaline species had no particular affinity with any one site group. The strongest affinities, as evidenced by high values of both constancy and fidelity, were displayed by an assemblage of oligochaetes, insect larvae, gammaridean amphipods, and the clamCorbicula fluminea in tidal freshwater muds; and an assemblage of haustoriid amphipods, the bivalveDonax variabilis, the polychaeteParaonis fulgens, and unidentified echinoids at high-salinity sites in outer Pamlico Sound near ocean inlets. A series of stations with impaired benthic assemblages in polluted habitats emerged from the cluster analysis and was distinguishable from other site groups that reflected a greater influence of natural controlling factors (such as salinity and sediment type) on species distributions. These results suggest that the interaction of both anthropogenic and natural environmental controlling factors is important in defining the structure of these infaunal assemblages.     

Development of a benthic index to assess sediment quality in the Tampa Bay Estuary

The identification, remedial treatment, and monitoring of contaminated sediments are among the priorities for managers of the Tampa Bay Estuary. Tampa Bay, as an urbanized estuary, is subject to the input of watershed sources of chemical contaminants, including metals, pesticides, and organic chemicals. Although the use of biological indicators and their incorporation into multi-metric indices is not new, the refinement and applications of such techniques for determining environmental condition still require further development and exploration. We present a single Tampa Bay Benthic Index (TBBI) that was developed specifically for Tampa Bay. Stepwise discriminant analysis was applied to a comprehensive list of potential benthic metrics. Results from the stepwise procedure identified the metrics that best discriminated between "healthy" and "degraded" conditions, as defined by sediment contaminant effect levels and dissolved oxygen. Discriminant analysis was then applied to the resultant three variables to determine the linear combination for the index.     

Groundwater supply and demand from southern Africa’s crystalline basement aquifer: evidence from Malawi

Failure of borehole sources in weathered and fractured crystalline basement aquifers in Malawi in southern Africa has been linked with poor borehole design, mechanical failure and badly sited boreholes. However, recent work in Malawi indicates that demand may now exceed long-term resource potential in some places and that this is also a cause of water point failure. An 11-year climate cycle (including a wet and dry period) necessitates overdraft from groundwater storage during the dry-cycle years before episodic rainfall events in the wetter part of the cycle again recharge the aquifers. Data, particularly groundwater hydrograph data, are sparse, but sufficient to evaluate the long-term renewable groundwater potential for both fractured and weathered basement-aquifer types in each of the 15 management areas in Malawi. The groundwater potential or long-term renewable resource (recharge) is given by the sum of Darcian throughflow and dry-season depletion of storage. Estimated rural demand exceeds the renewable resource in the fractured-rock aquifer in two management units and in the weathered-rock aquifer in two other units. Although there is inherent uncertainty in the water-balance estimates, the likelihood that rural demand is exceeding long-term average recharge in some areas is cause for concern.     

The Meteoritical Bulletin,No. 100, 2014 June

Meteoritical Bulletin 100 contains 1943 meteorites including 8 falls (Boumdeid [2011], Huaxi, Ko?ice, Silistra, So?tmany, Sutter's Mill, Thika, Tissint), with 1575 ordinary chondrites, 139 carbonaceous chondrites, 96 HED achondrites, 25 ureilites, 18 primitive achondrites, 17 iron meteorites, 15 enstatite chondrites, 11 lunar meteorites, 10 mesosiderites, 10 ungrouped achondrites, 8 pallasites, 8 Martian meteorites, 6 Rumuruti chondrites, 3 enstatite achondrites, and 2 angrites, and with 937 from Antarctica, 592 from Africa, 230 from Asia, 95 from South America, 44 from North America, 36 from Oceania, 6 from Europe, and 1 from an unknown location. This will be the last Bulletin published in the current format. Information about approved meteorites can be obtained from the Meteoritical Bulletin Database (MBD) available online at http://www.lpi.usra.edu/meteor/     

Extent of chondrule melting: Evaluation of experimental textures,nominal grain size,and convolution index

Abstract— Dynamic crystallization experiments on the ordinary chondrite Queen Alexandra Range (QUE) 97008 document textural features that occur in partially melted chondrules with changes in the degree of partial melting and cooling rate. We carried out a matrix of experiments, at peak temperatures of 1250, 1350, 1370, and 1450 °C, and cooling rates of 1000, 100, and 10 °C/h, and quenched. All experimentally produced textures closely resemble textures of porphyritic chondrules. Because peak temperatures were well below the liquidi for typical chondrule compositions, the textural similarities support an incomplete melting origin for most porphyritic chondrules. Our experiments can be used to determine the extent of melting of natural chondrules by comparing textural relationships among the experimental results with those of natural chondrules. We used our experiments along with X‐ray computerized tomography scans of a Semarkona chondrule to evaluate two other methods that have been used previously to quantify the degree of melting: nominal grain size and convolution index. Proper applications of these methods can result in valid assessments of a chondrule's degree of melting, but only if accompanied by careful interpretation, as chondrule textures are controlled by more than just the extent of melting. Such measurements of single aspects of chondrule textures might be coupled with qualitative analysis of other textural aspects to accurately determine degree of melting.