Uranium(VI) Release from Contaminated Vadose Zone Sediments: Estimation of Potential Contributions from Dissolution and Desorption

A key difficulty in developing accurate, science-based conceptual models for remediation of contaminated field sites is the proper accounting of multiple coupled geochemical and hydrologic processes. An example of such a difficulty is the separation of desorption and dissolution processes in releasing contaminants from sediments to groundwaters; very few studies are found in the literature that attempt to quantify contaminant release by these two processes. In this study, the results from several extraction techniques, isotopic exchange experiments, and published spectroscopic studies were combined to estimate the contributions of desorption and dissolution to U(VI) release from contaminated sediments collected from the vadose zone beneath former waste disposal ponds in the Hanford 300-Area (Washington state). Vertical profiles of sediments were collected at four locations from secondary pond surfaces down to, and slightly below, the water table. In three of the four profiles, uranium concentration gradients were observed in the sediments, with the highest U concentrations at the top of the profile. One of the vertical profiles contained sediments with U concentrations up to 4.2x10-7 mol/g (100 ppm). U(VI) release to artificial groundwater solutions and extracts from these high-U concentrations sediments occurred primarily from dissolution of precipitated U(VI) minerals, including the mineral metatorbernite, [Cu(UO2PO4)2⋅8H2O]. At the bottom of this profile, beneath the water table, and in all three of the other profiles, U concentrations were \u3c5.88x10-8 mol/g (14 ppm), and U(VI) release to artificial groundwater solutions occurred primarily due to desorption of U(VI). When reacted in batch experiments with artificial groundwater solutions with compositions representative of the range of chemical conditions in the underlying aquifer, all samples released U(VI) at concentrations greater than regulatory limits within a few hours. A semi-mechanistic surface complexation model was developed to describe U(VI) adsorption on sediments collected from near the water table, as a function of pH, alkalinity, and Ca and U(VI) concentrations, using ranges in these variables relevant to groundwater conditions in the aquifer. Dilute (bi)carbonate solution extractions and uranium isotopic exchange methods were capable of estimating adsorbed U(VI) in samples where U(VI) release was predominantly due to U(VI) desorption; these techniques were not effective at estimating adsorbed U(VI) where U(VI) release was affected by dissolution of U(VI) minerals. The combination of extraction and isotopic exchange results, spectroscopic studies, and surface complexation modeling allow an adequate understanding for the development of a geochemical conceptual model for U(VI) release to the aquifer. The overall approach has generic value for evaluating the potential for release of metals and radionuclides from sediments that contain both precipitated and adsorbed contaminant speciation

The Crystal Structure of the Extracellular 11-heme Cytochrome UndA Reveals a Conserved 10-heme Motif and Defined Binding Site for Soluble Iron Chelates

Members of the genus Shewanella translocate deca- or undeca-heme cytochromes to the external cell surface thus enabling respiration using extracellular minerals and polynuclear Fe(III) chelates. The high resolution structure of the first undeca-heme outer membrane cytochrome, UndA, reveals a crossed heme chain with four potential electron ingress/egress sites arranged within four domains. Sequence and structural alignment of UndA and the deca-heme MtrF reveals the extra heme of UndA is inserted between MtrF hemes 6 and 7. The remaining UndA hemes can be superposed over the heme chain of the decaheme MtrF, suggesting that a ten heme core is conserved between outer membrane cytochromes. The UndA structure has also been crystallographically resolved in complex with substrates, an Fe(III)-nitrilotriacetate dimer or an Fe(III)-citrate trimer. The structural resolution of these UndA-Fe(III)-chelate complexes provides a rationale for previous kinetic measurements on UndA and other outer membrane cytochromes

R08. Characterization of Key Metabolites in Serum of Fuzheng Huayu Phase II Clinical Trial

Corresponding author (NCNPR): Yan-Hong Wang, [email protected]://egrove.olemiss.edu/pharm_annual_posters/1007/thumbnail.jp

Kemod: A Mixed Chemical Kinetic And Equilibrium Model of Aqueous and Solid Phase Geochemical Reactions

This report presents the development of a mixed chemical Kinetic and Equilibrium MODel (KEMOD), in which every chemical species can be treated either as a equilibrium-controlled or as a kinetically controlled reaction. The reaction processes include aqueous complexation, adsorption/ desorption, ion exchange, precipitation/dissolution, oxidation/reduction, and acid/base reactions. Further development and modification of KEMOD can be made in: (1) inclusion of species switching solution algorithms, (2) incorporation of the effect of temperature and pressure on equilibrium and rate constants, and (3) extension to high ionic strength

Characterization of an electron conduit between bacteria and the extracellular environment

A number of species of Gram-negative bacteria can use insoluble minerals of Fe(III) and Mn(IV) as extracellular respiratory electron acceptors. In some species of Shewanella, deca-heme electron transfer proteins lie at the extracellular face of the outer membrane (OM), where they can interact with insoluble substrates. To reduce extracellular substrates, these redox proteins must be charged by the inner membrane/periplasmic electron transfer system. Here, we present a spectro-potentiometric characterization of a trans-OM icosa-heme complex, MtrCAB, and demonstrate its capacity to move electrons across a lipid bilayer after incorporation into proteoliposomes. We also show that a stable MtrAB subcomplex can assemble in the absence of MtrC; an MtrBC subcomplex is not assembled in the absence of MtrA; and MtrA is only associated to the membrane in cells when MtrB is present. We propose a model for the modular organization of the MtrCAB complex in which MtrC is an extracellular element that mediates electron transfer to extracellular substrates and MtrB is a trans-OM spanning ß-barrel protein that serves as a sheath, within which MtrA and MtrC exchange electrons. We have identified the MtrAB module in a range of bacterial phyla, suggesting that it is widely used in electron exchange with the extracellular environment

Structure of a bacterial cell surface decaheme electron conduit

Some bacterial species are able to utilize extracellular mineral forms of iron and manganese as respiratory electron acceptors. In Shewanella oneidensis this involves decaheme cytochromes that are located on the bacterial cell surface at the termini of trans-outer-membrane electron transfer conduits. The cell surface cytochromes can potentially play multiple roles in mediating electron transfer directly to insoluble electron sinks, catalyzing electron exchange with flavin electron shuttles or participating in extracellular intercytochrome electron exchange along “nanowire” appendages. We present a 3.2-Å crystal structure of one of these decaheme cytochromes, MtrF, that allows the spatial organization of the 10 hemes to be visualized for the first time. The hemes are organized across four domains in a unique crossed conformation, in which a staggered 65-Å octaheme chain transects the length of the protein and is bisected by a planar 45-Å tetraheme chain that connects two extended Greek key split ß-barrel domains. The structure provides molecular insight into how reduction of insoluble substrate (e.g., minerals), soluble substrates (e.g., flavins), and cytochrome redox partners might be possible in tandem at different termini of a trifurcated electron transport chain on the cell surface