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This work elucidates spatio-temporal aspects of the biogeochemical transformation of copper mobilized from malachite (Cu2 (CO3 )(OH)2 ) and bioaccumulated within Aspergillus niger colonies when grown on different inorganic nitrogen sources. It was shown that the use of either ammonium or nitrate determined how copper was distributed within the colony and its microenvironment and the copper oxidation state and succession of copper coordinating ligands within the biomass. Nitrate-grown colonies yielded ∼1.7× more biomass, bioaccumulated ∼7× less copper, excreted ∼1.9× more oxalate and produced ∼1.75× less water-soluble copper in the medium in contrast to ammonium-grown colonies. Microfocus X-ray absorption spectroscopy revealed that as the mycelium matured, bioaccumulated copper was transformed from less stable and more toxic Cu(I) into less toxic Cu(II) which was coordinated predominantly by phosphate/malate ligands. With time, a shift to oxalate coordination of bioaccumulated copper occurred in the central older region of ammonium-grown colonies.

Crystalline and glassy phases in the ‘leucite’ systems (K,Rb,Cs)Fe 3 + Si 2 O 6 and (K,Rb,Cs) 2 (Mg,Mn,Fe,Co,Ni,Cu,Zn) 2 + – Si 5 O 12 have been studied using K-edge X-ray Absorption Spectroscopy (XAS) for 3d transition elements and Mg. Crystalline samples of known structure are used as model compounds for deducing the local structures of the equivalent glasses. ‘Ideal’ tetrahedral framework X 2 + –O distances are: Mn 2 + –O 2.02, Fe 2 + –O 1.98; Co 2 + –O 1.96, Ni 2 + –O 1.95, Cu 2 + –O 1.92, Zn 2 + –O 1.93 and Mg 2 + –O 1.93 A. The quenched glasses have Extended X-ray Absorption Fine Structure (EXAFS) first-shell distances and coordination numbers consistent with Mg and 3d elements occurring in tetrahedral coordination in most cases. The X-ray Absorption Near Edge Structure (XANES) spectra of the glasses are basically the same as those for the crystalline samples showing that they have similar medium range order, consistent with most of the Mg and 3d elements acting as network formers along with the associated alkalis as X 2 Y 2 + O 2 complexes. Anhydrous glasses in the system CaO–MgO–Al 2 O 3 –SiO 2 together with a basalt glass from Hawaii show examples of Mg in octahedral- (2.07 A, basalt), tetrahedral- (~ 1.87 A, siliceous glass) and 5-coordinated (2.01–2.04 A) sites. Although most Earth scientists assume that Mg and Fe 2 + act as network modifiers in natural magmas, ultra-potassic, peralkaline compositions ((Na + K)/(Al + Fe 3 + ) > 1) could have had some Fe 2 + and Mg complexes with alkalis in the melt network as K 2 (Mg,Fe 2 + )O 2 and the implications of this to melt density and viscosity are considered.

We thank Richard Pattrick and Vicky Coker for help in collecting XMCD on these samples at the Daresbury SRS and subsequently at the Advanced Light Source (ALS), Berkeley. The ALS is supported by the Director, Office of Science, Office of Basic Energy Sciences (OBES) of the U.S. Department of Energy (DOE) under Contract No. DE-AC02-05CH11231 and we thank Elke Arenholz for her assistance. RJH acknowledges funding from the European Research Council under the European Union's Seventh Framework Programme (FP/2007-2013)/ERC Grant Agreement No. 320750. KMR gratefully acknowledges support from the DOE OBES Chemical Sciences, Geosciences, and Biosciences Division, through the Geosciences Program at Pacific Northwest National Laboratory. We also thank Gerrit van der Laan and Nick Telling for help with XMCD data analysis; David Plant carried out the electron microprobe analyses at Manchester and Paul Schofield provided information on the natural magnesian spinel. We also thank two anonymous referees for constructive comments.

Combined microfocus XAS and XRD analysis of α-particle radiation damage haloes around thorium-containing monazite in Fe-rich biotite reveals changes in both short- and long-range order. The total α-particles flux derived from the Th and U in the monazite over 1.8 Ga was 0.022 α particles per atomic component of the monazite and this caused increasing amounts of structural damage as the monazite emitter is approached. Short-range order disruption revealed by Fe K-edge EXAFS is manifest by a high variability in Fe–Fe bond lengths and a marked decrease in coordination number. XANES examination of the Fe K-edge shows a decrease in energy of the main absorption by up to 1 eV, revealing reduction of the Fe3+ components of the biotite by interaction with the 24He2+, the result of low and thermal energy electrons produced by the cascade of electron collisions. Changes in d spacings in the XRD patterns reveal the development of polycrystallinity and new domains of damaged biotite structure with evidence of displaced atoms due to ionization interactions and nuclear collisions. The damage in biotite is considered to have been facilitated by destruction of OH groups by radiolysis and the development of Frenkel pairs causing an increase in the trioctahedral layer distances and contraction within the trioctahedral layers. The large amount of radiation damage close to the monazite can be explained by examining the electronic stopping flux.

Summary Selenium (Se) is an essential trace element for animals and displays a narrow range between dietary deficiency and toxicity. The toxicity of Se depends on its bioavailability, which is directly related to its oxidation states, of which four occur in the environment (SeVI, SeIV, Se0 and SeII−). Microbial communities drive the cycling of Se between these oxidation states. In order to investigate the effect of microbial activity on Se cycling in the environment, a field site in County Meath, Ireland, was identified with anomalously large concentrations of Se as a result of weathering of black shales within the Lucan formation, leading to cases of Se toxicity in farm animals. Soil cores were extracted from the site for Se speciation and microbial community analysis prior to microcosm experiments to assess Se stability and microbial Se transformations. Selenium was present as a recalcitrant, reduced organic phase that was strongly coordinated with carbon, concordant with suggested hypotheses of Se phyto-concentration within a clay-lined, postglacial marshland. Selenium was not mobilized in microcosm experiments, and supplementation with SeVI resulted in rapid reduction and removal from solution as Se0. Additional electron donors did not affect Se stability or removal from solution, although nitrate did hinder SeVI reduction. Terminal restriction fragment length polymorphism analysis indicated a significant shift in microbial community after amendment with SeVI. This work extends the current knowledge of Se cycling in the environment, and provides information on the bioavailability of Se in the soil, which determines Se content of foodstuffs.

Well-constrained laboratory incubations of a ferruginous marine hydrothermal sediment from Santorini, Greece, were used to elucidate the effect of microbially induced redox transformations on arsenic speciation and mobility. Despite naturally high arsenic concentrations (∼400 mg/kg), the sediment has a low As:Fe ratio (1:1000 wt/wt). Acetate-amendment of sediment, extracted from the naturally-occurring suboxic–anoxic (Eh −60 to −138 mV) transition zone, promoted Fe(III) reduction, and increased the concentration of Fe(II) from ∼40% to ∼60% in the bulk sediment. Sulfate, which was present at lower concentrations, was also reduced. Phylogenetic 16S rRNA and dsr gene analysis suggested that Fe(III) and sulfate were reduced by bacteria related to Malonomonas rubra and Desulfosarcina variabilis , respectively. Arsenic remained predominantly as arsenic trioxide (As 2 O 3 ) throughout the amendment experiment. However, the percentage of total arsenic present within poorly-crystalline iron oxides decreased from ∼69% to ∼32%, while the percentage incorporated within crystalline iron-containing minerals or sorbed to surfaces via inner-sphere complexes increased significantly (to 22% and 30%, respectively). Re-oxidation of the system with nitrate resulted in incomplete reduction of the nitrate pool, and partial re-association of arsenic with the poorly-crystalline iron fraction. Exposure to air led to virtually complete reversal of the arsenic partitioning, and oxidation of 71% As(III) to As(V). During aeration, oxidation of sediment-bound sulfur/sulfide occurred, alongside an observed ∼63% decrease in arsenic bound to this minor component. Analogous trends in arsenic-sediment associations were observed in the natural, unamended sediment depth-profile, whereby a greater proportion of arsenic (34% As(III), 66% As(V)) was bound within poorly-crystalline iron oxides at the sediment–water interface. Arsenic (96% As(III)) was increasingly incorporated within well-crystallized forms of iron with depth and decreasing Eh values. At the greatest depth sampled (35 cm) arsenic increased substantially within the sulfide/organic fraction. Results here contribute to existing evidence that arsenic is not necessarily released from iron-rich sediment systems under conditions of anoxia, but that Fe(II)-bearing minerals forming concomitantly can immobilize arsenic in the solid-phase. Such results may have implications for other systems with high Fe:As ratios.

While amorphous calcium carbonate (ACC) has been widely exploited as a precursor to crystalline calcium carbonate in solution, the synthetic potential of crystallising ACC in the solid state has received little attention. Building on our recent results which demonstrate that additives occluded within ACC can facilitate its crystallisation in the solid state, we here demonstrate that annealing of ACC nanoparticles precipitated within Na-AOT (sodium bis(2-ethylhexyl)sulfosuccinate) reverse microemulsions not only leads to remarkable control over the size and morphology of the particles, but that polymorph selectivity can also be achieved through selection of the thermal profile. The ACC precipitated within the microemulsions crystallised at just 70 °C on incubation at this temperature for 12 hours and non-aggregated, sub-micron crystals of either calcite or vaterite – which are often difficult to access using standard synthetic protocols – were produced according to the heating rate employed. The observed low-temperature crystallisation was attributed to surfactant molecules entrapped with the ACC particles during their formation, where these impurities act to facilitate nucleation of a crystalline phase within the ACC. These results therefore highlight the potential of using thermal treatment of dry ACC as a route to achieving control over calcium carbonate formation that is sometimes difficult to achieve in solution.

This work elucidates spatio-temporal aspects of the biogeochemical transformation of copper mobilized from malachite (Cu

A NUMBER of terrestrial plants accumulate large quantities of metals such as zinc, manganese, nickel, cobalt and copper in their shoots1. The largest group of these so-called 'metal hyperaccumulators' is found in the genus Alyssum, in which nickel concentrations can reach 3% of leaf dry biomass2,3. Apart from their intrinsic interest, plants exhibiting this trait could be of value in the decontamination of metal-polluted soils4–6. However, the biochemical basis of the capacity for metal accumulation has not been elucidated. Here we report that exposing hyperaccumu-lator species of Alyssum to nickel elicits a large and proportional increase in the levels of free histidine, which is shown to be coordinated with nickel in vivo. Moreover, supplying histidine to a non-accumulating species greatly increases both its nickel tolerance and capacity for nickel transport to the shoot, indicating that enhanced production of histidine is responsible for the nickel hyperaccumulation phenotype in Alyssum.

Potassium-rich mafic dykes and lavas from the Highwood Mountains Igneous Province, USA were studied by electron-microprobe and bulk-rock analysis. For the mafic phonolites, compositional trends for olivine and augite phenocrysts and groundmass biotite, alkali feldspar and titanomagnetites are presented and substitution mechanisms discussed. Phenocrysts of biotite and augite in the minettes are also characterized, together with groundmass alkali feldspar and titanomagnetite. The alkali feldspars and biotites are commonly enriched in Ba. Olivine, clinopyroxene and biotite phenocrysts are generally quite magnesium-rich, which is consistent with the primitive natures of the least evolved rocks.Bulk-rock major-element compositions are combined with modal and microprobe data for the principal phenocrysts to calculate model residual liquid compositions for mafic phonolites, minettes and a syenitic rock. On the basis of phase-equilibria, it is suggested that the main controls of differentiation are polybaric involving crystallization during transport of primary magmas from the mantle for the minettes, and low-pressure differentiation for the mafic phonolites. Whereas magma mixing might have contributed to petrogenesis, many of the disequilibrium features exhibited by clinopyroxene and biotite phenocrysts can also be attributed to pre-existing phenocrysts undergoing decompression melting during magma uprise from its mantle source, followed by rapid crystal growth and episodic volatile loss in sub-volcanic magma chambers.

X-ray absorption spectroscopy, scanning electron microscopy (SEM) and X-ray diffractometry (XRD) have been used to study the solid-phase speciation of Zn in urban road dust sediments (RDS) in Manchester, UK. X-ray absorption near-edge structure (XANES) analysis using linear combination modelling suggest that the soluble species Zn(NO3)2·6H2O and ZnCl2 represent 70—83%, and Zn-sorbed goethite 17—30%, of the Zn species present. The presence of goethite is not corroborated by extended X-ray absorption fine structure (EXAFS) modelled first shell scattering Zn—O distances of 2.01—2.03 Å, but this may be due to distortion of the Zn octahedra on the goethite surface, or the existence of Zn-sorbed species with other metal hydrous oxides, as inferred by the EXAFS-modelled second shell Fe and Al scatterers. Analysis by EXAFS also suggests that metallic Zn-Cu-Sn-Pb and Zn-silicate phases are present in the RDS, and this is corroborated by SEM and XRD. Other phases suggested by EXAFS include ZnO, franklinite, Zn-sorbed birnessite and zinc formate. Differences between the XANES and other results suggest that model compounds such as Zn-bearing phyllosilicates and metallic Zn phases may have been missing from the XANES fitting. Long-term low-level exposure to the RDS Zn phases identified may lead to an increased risk of cardiovascular or pulmonary diseases.

Radium-226 is a naturally-occurring radioisotope with potentially significant radiological impact and whose environmental behaviour is of concern. The reactions of tracer (0.1–1 nM) dissolved Ra and its chemical analogue Ba with the surfaces of a range of carbonate minerals have been studied. All of the minerals react with Ra but, whereas calcite, dolomite, strontianite, rhodocrosite, ankerite and witherite all show increased uptake with increasing Ra concentration, suggesting a coprecipitation reaction (hence with phase formation limiting uptake), siderite, magnesite and ankerite show behaviour suggesting simple sorption (with decreasing uptake as Ra concentration increases, or with no dependence on [Ra]). Magnesite, in particular, has a low sorption capacity. Barium has been used at higher (0.1–1 mM) concentrations to enable the use of surface analytical and imaging techniques in addition to bulk uptake measurements. Although the same eight carbonates were studied, measurable uptake occurs only on dolomite, magnesite and siderite. For siderite and magnesite, there is an approximately linear relationship between the increasing solid and solution phase Ba concentrations, suggesting a simple sorption process. Dolomite shows more complex behaviour suggesting simple sorption at the lowest concentrations and phase formation at higher concentrations (>0.4 mmol L−1). The latter observation is consistent with spectroscopic evidence for the formation of witherite. Surface analysis and imaging of the three carbonate substrates that react with Ba show a diversity of behaviour, partly as a result of using natural minerals in these experiments. Witherite is commonly formed as a surface precipitate although the presence of even trace SO 4 2 - leads to barite formation. The surface phases display a range of characteristic morphologies, and the surface structure has the effect of templating growth. The presence of even minor amounts of Fe (hydr)oxide phases as alteration products or precipitates on the carbonates is also important, since Ba has a strong affinity for these phases.

It is well known that microbially mediated reduction can result in the removal of U(VI)(aq) from solution by forming poorly soluble U(IV) oxides; however, the fate of U(VI) already associated with mineral surfaces is less clear. Here we describe results from both oxic adsorption and anaerobic microcosm experiments to examine the fate of sorbed U(VI) duringmicrobially mediated bioreduction. The microcosm experiments contained sediment representative of the nuclear facility at Dounreay, UK. In oxic adsorption experiments, uptake of U(VI) was rapid and complete from artificial groundwater and where groundwater was amended with 0.2 mmol l-1 ethylenediaminetetraacetic acid (EDTA) a complexing ligand used in nuclear fuel cycle operations. By contrast, uptake of U(VI) was incomplete in groundwaters amended with 10 mmol l-1 bicarbonate. Analysis of sediments using X-ray adsorption spectroscopy showed that in these oxic samples, U was present as U(VI). After anaerobic incubation of U(VI) labelled sediments for 120 days, microbiallymediated Fe(III)- and SO4 2-- reducing conditions had developed and XAS data showed uranium was reduced to U(IV). Further investigation of the unamended groundwater systems, where oxic systems were dominated by U(VI) sorption, showed that reduction of sorbed U(VI) required an active microbial population and occurred after robust iron- and sulfate- reducing conditions had developed. Microbial community analysis of the bioreduced sediment showed a community shift compared to the oxic sediment with close relatives of Geobacter and Clostridium species, which are known to facilitate U(VI) reduction, dominating. Overall, efficient U(VI) removal from solution by adsorption under oxic conditions dominated in unamended and EDTA amended systems. In all systems bioreduction resulted in the formation of U(IV) in solids. © Taylor & Francis Group, LLC.

The introduction of earthworms into soils contaminated with metals and metalloids has been suggested\ud to aid restoration practices. Eisenia veneta (epigeic), Lumbricus terrestris (anecic) and Allolobophora\ud chlorotica (endogeic) earthworms were cultivated in columns containing 900 g soil with 1130, 345, 113\ud and 131 mg kg1 of As, Cu, Pb and Zn, respectively, for up to 112 days, in parallel with earthworm-free\ud columns. Leachate was produced by pouring water on the soil surface to saturate the soil and generate\ud downflow. Ryegrass was grown on the top of columns to assess metal uptake into biota. Different\ud ecological groups affected metals in the same way by increasing concentrations and free ion activities in\ud leachate, but anecic L. terrestris had the greatest effect by increasing leachate concentrations of As by\ud 267%, Cu by 393%, Pb by 190%, and Zn by 429% compared to earthworm-free columns. Ryegrass\ud grown in earthworm-bearing soil accumulated more metal and the soil microbial community exhibited\ud greater stress. Results are consistent with earthworm enhanced degradation of organic matter leading\ud to release of organically bound elements. The degradation of organic matter also releases organic acids\ud which decrease the soil pH. The earthworms do not appear to carry out a unique process, but increase\ud the rate of a process that is already occurring. The impact of earthworms on metal mobility and\ud availability should therefore be considered when inoculating earthworms into contaminated soils as\ud new pathways to receptors may be created or the flow of metals and metalloids to receptors may be\ud elevated.

Microbial processes can affect the environmental behavior of redox sensitive radionuclides, and understanding these reactions is essential for the safe management of radioactive wastes. Neptunium, an alpha-emitting transuranic element, is of particular importance because of its long half-life, high radiotoxicity, and relatively high solubility as Np(V)O(2)(+) under oxic conditions. Here, we describe experiments to explore the biogeochemistry of Np where Np(V) was added to oxic sediment microcosms with indigenous microorganisms and anaerobically incubated. Enhanced Np removal to sediments occurred during microbially mediated metal reduction, and X-ray absorption spectroscopy showed this was due to reduction to poorly soluble Np(IV) on solids. In subsequent reoxidation experiments, sediment-associated Np(IV) was somewhat resistant to oxidative remobilization. These results demonstrate the influence of microbial processes on Np solubility and highlight the critical importance of radionuclide biogeochemistry in nuclear legacy management.

The nematode Caenorhabditis elegans expresses two metallothioneins (MTs), CeMT-1 and CeMT-2, that are believed to be key players in the protection against metal toxicity. In this study, both isoforms were expressed in vitro in the presence of either Zn(II) or Cd(II). Metal binding stoichiometries and affinities were determined by ESI-MS and NMR, respectively. Both isoforms had equal zinc binding ability, but differed in their cadmium binding behaviour, with higher affinity found for CeMT-2. In addition, wild-type C. elegans, single MT knockouts and a double MT knockout allele were exposed to zinc (340 mu m) or cadmium (25 mu m) to investigate effects in vivo. Zinc levels were significantly increased in all knockout strains, but were most pronounced in the CeMT-1 knockout, mtl-1 (tm1770), while cadmium accumulation was highest in the CeMT-2 knockout, mtl-2 (gk125) and the double knockout mtl-1;mtl-2 (zs1). In addition, metal speciation was assessed by X-ray absorption fine-structure spectroscopy. This showed that O-donating, probably phosphate-rich, ligands play a dominant role in maintaining the physiological concentration of zinc, independently of metallothionein status. In contrast, cadmium was shown to coordinate with thiol groups, and the cadmium speciation of the wild-type and the CeMT-2 knockout strain was distinctly different to the CeMT-1 and double knockouts. Taken together, and supported by a simple model calculation, these findings show for the first time that the two MT isoforms have differential affinities towards Cd(II) and Zn(II) at a cellular level, and this is reflected at the protein level. This suggests that the two MT isoforms have distinct in vivo roles.

Precious metals supported on ferrimagnetic particles have a diverse range of uses in catalysis. However, fabrication using synthetic methods results in potentially high environmental and economic costs. Here we show a novel biotechnological route for the synthesis of a heterogeneous catalyst consisting of reactive palladium nanoparticles arrayed on a nanoscale biomagnetite support. The magnetic support was synthesized at ambient temperature by the Fe(III)-reducing bacterium, Geobacter sulfurreducens, and facilitated ease of recovery of the catalyst with superior performance due to reduced agglomeration (versus conventional colloidal Pd nanoparticles). Surface arrays of palladium nanoparticles were deposited on the nanomagnetite using a simple one-step method without the need to modify the biomineral surface, most likely due to an organic coating priming the surface for Pd adsorption, which was produced by the bacterial culture during the formation of the nanoparticles. A combination of EXAFS and XPS showed the Pd nanoparticles on the magnetite to be predominantly metallic in nature. The Pd0-biomagnetite was tested for catalytic activity in the Heck reaction coupling iodobenzene to ethyl acrylate or styrene. Rates of reaction were equal to or superior to those obtained with an equimolar amount of a commercial colloidal palladium catalyst, and near complete conversion to ethyl cinnamate or stilbene was achieved within 90 and 180 min, respectively. © 2010 American Chemical Society.

Ordering of Fe{sup 3+} and Fe{sup 2+} between octahedral (Oh) and tetrahedral (Td) sites in synthetic members of the magnetite (Fe{sub 3}O{sub 4}) - ulvoespinel (Fe{sub 2}TiO{sub 4}) solid-solution series was determined using Fe L{sub 2,3}-edge X-ray magnetic circular dichroism (XMCD) coupled with electron microprobe and chemical analysis, Ti L-edge spectroscopy, Fe K-edge EXAFS and XANES, Fe{sub 57} Moessbauer spectroscopy, and unit cell parameters. Microprobe analysis, cell edges and chemical FeO determinations showed that the bulk compositions of the samples were stoichiometric magnetite-ulvoespinel solid-solutions. Surface sensitive XMCD showed that the surfaces of these oxide minerals were more sensitive to redox conditions and some samples required re-equilibration with suitable solid-solid buffers. Detailed site-occupancy analysis of these samples gave XMCD-Fe{sup 2+}/Fe{sup 3+} ratios very close to stoichiometric values. L{sub 2,3}-edge spectroscopy showed that Ti{sup 4+} was restricted to Oh sites. XMCD results showed that significant Fe{sup 2+} only entered Td when the Ti content was > 0.40 apfu while Fe{sup 2+} in Oh increased from 1 a.p.f.u in magnetite to a maximum of {approx}1.4 apfu in USP45. As the Ti content increased from this point, the steady increase in Fe{sup 2+} in Td sites was clearly observable in the XMCD spectra, concurrentmore » with a slow decrease in Fe{sup 2+} in Oh sites. Calculated magnetic moments showed a steady decrease from magnetite (4.06 {mu}{sub B}) to USP45 (1.5 {mu}{sub B}) and then a slower decrease towards the value for ulvoespinel (0 {mu}{sub B}). Two of the synthesized samples were also partially maghemitized by re-equilibrating with an oxidizing Ni-NiO buffer and XMCD showed that Fe{sup 2+} oxidation only occurred at Oh sites, with concomitant vacancy formation restricted to this site. This study shows the advantage of using XMCD as a direct measurement of Fe oxidation state in these complex magnetic spinels. These results can be used to rationalize the magnetic properties of titanomagnetites, and their oxidized titanomaghemitized analogues, in Earth's crustal rocks.« less

Acid waters and sediments of the Tinto Santa Rosa acid stream (Iberian Pyritic Belt; SW, Spain) were analysed to determine the role of sedimentary phases in the behaviour of arsenic. Aqueous arsenic and iron concentrations decreased markedly from the adit mouth to 300 m downstream indicating iron minerals precipitation as well as arsenic sorption onto these newly-formed phases. This was confirmed by the high arsenic concentrations observed in bed-stream precipitates, which play a major role in controlling arsenic mobility. To unravel the complex nature of the AMD sediments a combination of techniques including X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), total solid digestions, X-Ray Fluorescence (XRF) and X-Ray Absorption Spectroscopy (XAS) were employed. Results showed that (1) arsenic was present predominantly in its pentavalent state; (2) upstream arsenic was sorbed onto the main phase, schwertmannite, whereas downstream it was chiefly associated with goethite and jarosite; and (3) changes in arsenic speciation with depth were observed in the consolidated terrace sediments, where arsenic appeared primarily associated with schwertmannite in the upper part of the terraces, but with goethite at depth. Arsenic mobilization was controlled by sorption onto newly formed precipitates (schwertmannite, goethite and jarosite), causing natural arsenic attenuation.

Vanadium K -edge X-ray Absorption Spectra have been recorded for the native and peroxo-forms of vanadium chloroperoxidase from Curvularia inaequalis at pH 6.0. The Extended X-ray Absorption Fine Structure (EXAFS) regions provide a refinement of previously reported crystallographic data; one short V = O bond (1.54 A) is present in both forms. For the native enzyme, the vanadium is coordinated to two other oxygen atoms at 1.69 A, another oxygen atom at 1.93 A and the nitrogen of an imidazole group at 2.02 A. In the peroxo-form, the vanadium is coordinated to two other oxygen atoms at 1.67 A, another oxygen atom at 1.88 A and the nitrogen of an imidazole group at 1.93 A. When combined with the available crystallographic and kinetic data, a likely interpretation of the EXAFS distances is a side-on bound peroxide involving V–O bonds of 1.67 and 1.88 A; thus, the latter oxygen would be ‘activated’ for transfer. The shorter V–N bond observed in the peroxo-form is in line with the previously reported stronger binding of the cofactor in this form of the enzyme. Reduction of the enzyme with dithionite has a clear influence on the spectrum, showing a change from vanadium(V) to vanadium(IV).

Predicting metal bioaccumulation and toxicity in soil organisms is complicated by site-specific biotic and abiotic parameters. In this study we exploited tissue fractionation and digestion techniques, combined with X-ray absorption spectroscopy (XAS), to investigate the whole-body and subcellular distributions, ligand affinities, and coordination chemistry of accumulated Pb and Zn in field populations of the epigeic earthworm Lumbricus rubellus inhabiting three contrasting metalliferous and two unpolluted soils. Our main findings were (i) earthworms were resident in soils with concentrations of Pb and Zn ranging from 1200 to 27,000 mg kg(-1) and 200 to 34,000 mg kg(-1), respectively; (ii) Pb and Zn primarily accumulated in the posterior alimentary canal in nonsoluble subcellular fractions of earthworms; (iii) site-specific differences in the tissue and subcellular partitioning profiles of populations were observed, with earthworms from a calcareous site partitioning proportionally more Pb to their anterior body segments and Zn to the chloragosome-rich subcellular fraction than their acidic-soil inhabiting counterparts; (iv) XAS indicated that the interpopulation differences in metal partitioning between organs were not accompanied by qualitative differences in ligand-binding speciation, because crystalline phosphate-containing pyromorphite was a predominant chemical species in the whole-worm tissues of all mine soil residents. Differences in metal (Pb, Zn) partitioning at both organ and cellular levels displayed by field populations with protracted histories of metal exposures may reflect theirinnate ecophysiological responses to essential edaphic variables, such as Ca2+ status. These observations are highly significant in the challenging exercise of interpreting holistic biomarker data delivered by "omic" technologies.

The chemical composition oft he natural arsenate-apatite mineral johnbaumite [nominally Ca10(AsO4)6(OH)2] and its alteration product hedyphane [Ca4Pb6(AsO4)6Cl2] have been determined by electron microprobe analysis and the structures ofjohnbaumite and synthetic Sr-, Ba- and Pbarsenate apatites have been studied by As K-edge X-ray absorption spectroscopy and synchrotron X-ray powder diffraction. All samples belong to the holosymmetric apatite space group P63/m with As5+substituted for P5+in the tetrahedral structural site. Johnbaumite contains small amounts ofF and Pb (~0.9 and ~4.4 wt.% respectively) and hedyphane has the ideal composition (formula given above); the compositions of these coexisting phases define the two limbs ofa solvus occurring between Ca- and Pb-arsenate apatite end members. The unit-cell parameters and cation–oxygen bond lengths for the arsenate apatites studied are discussed alongside published data for end-member Ca-, Sr-, Ba- and Pbphosphate apatite analogues with (OH), F, Cl or Br as the anions at the centres of the channels in the apatite structure. This discussion rationalizes the relationships between the two structural sites A(1) and A(2) occupied by divalent cations in terms of the size of the A–O polyhedra and the distortion of the A(1)–O polyhedron as measured by the metaprism twist angle [O(1)–A(1)–O(2) projected onto (001)].

Technetium is a radioactive contaminant found in groundwaters at sites where nuclear wastes have been processed or stored. The redox chemistry of technetium is a major control on its environmental mobility. Under oxic conditions, technetium exists as the pertechnetate ion, Tc(VII)O4 −, which is poorly sorbed by minerals across a wide range of environmentally relevant pH values. Under reducing conditions pertechnetate is converted to lower valency species, of which Tc(IV) tends to predominate. Typically, the Tc(IV) oxidation state readily precipitates as insoluble hydrous Tc(IV) oxides or, at low concentrations, is removed from solution by association with mineral surfaces. Here, we present novel X-ray absorption spectroscopy (XAS) data examining Tc associations with reduced Dounreay soils. In reduced unamended microcosms and in microcosms where we added the co-contaminants ethylenediaminetetraacetic acid (EDTA) or bicarbonate to investigate their effect on Tc biogeochemistry, Tc was removed from solution on exposure to the reduced sediments and was present on solids as hydrous Tc(IV)O2-like phases. Furthermore, to investigate the long term stability and remobilization of solid phase associated Tc in reduced soils, we reoxidized reduced, Tc(IV)-labeled soils, in the presence of air and nitrate. The extent of remobilization of Tc to solution was dependent on the oxidant used. After reoxidation with air for 60 d, (42±6)% of the initial soil bound Tc was resolubilized. In the presence of 25 or 100 mmol L−1 nitrate as an oxidant, negligible microcosm reoxidation or remobilization of Tc to solution occurred. XAS analysis of soils treated with the two oxidants showed that in both systems, the remaining soil associated Tc was present as hydrous TcO2-like phases. The recalcitrance of Tc remobilization under reoxidizing conditions has implications for the fate of Tc in contaminated environments.

Ferrimagnetic nanoparticles have many uses in industry including in magnetic recording media and transformers, however these particles are often expensive to synthesize. In this study, the Fe3+-reducing bacteria Geobacter sulfurreducens and Shewanella oneidensis were used to synthesize spinel ferrite nanoparticles of the general chemical formula M x Fe3– x O4, where M is either Co, Ni, Mn, Zn, or a combination of Mn and Zn. This was done at ambient temperatures through the dissimilatory reduction of Fe3+-oxyhydroxides containing the appropriate substitutional cations. A combination of L -edge and K -edge X-ray absorption spectroscopy (XAS) and L -edge X-ray magnetic circular dichroism (XMCD) was used to determine the site occupancies, valence, and local structure of the Fe and substitutional cations within the spinels. The Ni and Co ferrites produced using each bacterium were very similar and therefore this study concludes that, despite the difference in reduction mechanism of the bacteria used, the end-product is remarkably unaltered. Nickel ferrites contained only Ni2+, with at least 80% in Oh coordination. Cobalt ferrites contained only Co2+ but with a significant proportion (up to 45%) in Td coordination, showing a slight preference for Td sites. The Mn-ferrites contained Mn2+ only on the Oh sites but a mixture of Mn2+ and Mn3+ on Td sites when the amount of Mn exceeded 3% (compared to the amount of Fe) or some Zn was also present. This study successfully produced a range of nanoparticulate ferrites that could be produced industrially using relatively environmentally benign methodologies.

Technetium is a long lived (2.13 x 10(5) a), beta emitting radionuclide which is a groundwater contaminant at a number of nuclear facilities throughout the world. Its environmental behaviour is primarily governed by its redox state. Under oxic conditions it forms the highly soluble pertechnetate (TcO4-) ion; under reducing conditions it forms the poorly soluble, reduced forms of Tc, particularly the Tc(IV) ion which is expected to precipitate as hydrous TcO2 above its solubility limit (10(-9) mol l(-1) at similar to pH 7) or to be strongly sorbed to mineral surfaces at lower concentrations. Thus the redox cycling behaviour of Tc is predicted to be key to its environmental behaviour in the natural and engineered environment. Here the results of a series of X-ray absorption spectroscopy (XAS) experiments which examine the oxidation state and coordination environment of Tc in a range of estuarine, aquifer and freshwater sediment suspensions, and in an environmentally relevant amorphous Fe(II) phase under both reduced and reoxidised biogeochemical conditions are presented. In reduced sediments and the amorphous Fe(II) phase prior to reoxidation, XAS results show that Tc was retained as hydrous TcO2-like phases across all samples. Under air reoxidation, experiments showed significant (up to 80%) remobilisation of Tc to solution as TcO4-. In pre-reduced freshwater sediments, aquifer sediments and the amorphous Fe(II) phase oxidised with air, XAS indicated that Tc remained associated with the solids as hydrous TcO2-like phases. By contrast, in air reoxidised estuarine sediment XAS analysis suggested that both hydrous TcO2-like phases and TcO4 were retained within the sediment. Finally, when microbial ly-mediated NO3 reoxidation occurred in estuarine and aquifer sediment slurries, experiments showed comparatively low (

A new protocol for the quantitative determination of zeolite-group mineral compositions by electron probe microanalysis (wavelength-dispersive spectrometry) under ambient conditions, is presented. The method overcomes the most serious challenges for this mineral group, including new confidence in the fundamentally important Si-Al ratio. Development tests were undertaken on a set of natural zeolite candidate reference samples, representing the compositional extremes of Na, K, Cs, Mg, Ca, Sr and Ba zeolites, to demonstrate and assess the extent of beam interaction effects on each oxide component for each mineral. These tests highlight the variability and impact of component mobility due to beam interaction, and show that it can be minimized with recommended operating conditions of 15 kV, 2 nA, a defocused, 20 μm spot size, and element prioritizing with the spectrometer configuration. The protocol represents a pragmatic solution that works, but provides scope for additional optimization where required. Vital to the determination of high-quality results is the attention to careful preparations and the employment of strict criteria for data reduction and quality control, including the monitoring and removal of non-zeolitic contaminants from the data (mainly Fe and clay phases). Essential quality criteria include the zeolite-specific parameters of R value (Si/(Si + Al + Fe3+), the 'E%' charge-balance calculation, and the weight percent of non-hydrous total oxides. When these criteria are applied in conjunction with the recommended analytical operating conditions, excellent inter-batch reproducibility is demonstrated. Application of the method to zeolites with complex solid-solution compositions is effective, enabling more precise geochemical discrimination for occurrence-composition studies. Phase validation for the reference set was conducted satisfactorily with the use of X-ray diffraction and laser-ablation inductively-coupled plasma mass spectroscopy.

Exceptionally high-quality X-ray absorption spectroscopy (XAS) spectra of the As K-edge measured on a garnet from the Central Oslo Rift (Jamtveit et al. 1993) are presented and interpreted. The best fit to the extended X-ray absorption fine structure (EXAFS) spectrum indicates that arsenic occurs predominantly as As5+ in the tetrahedral Si4+ site. The first shell As-O bond lengths of 1.69 angstrom are identical to those observed for As5+-O in sodium arsenate and the edge position of 11872 eV is also characteristic of As5+. Determination of oxidized As within the structure places an important constraint on fluid chemistry, but perhaps even more importantly, EXAFS fitting of outer shells (As-Ca 3.07,3.75, 5.71, 6.11 angstrom; As-Fe/Al 3.47, 5.35 angstrom; As-Si 3.88, 5.60, 5.91 angstrom) in this high-quality data set gives unequivocal evidence that AsO4-3 substitutes for SiO4-4 in the garnet tetrahedral site. This is, to the best of our knowledge, the first Such XAS study yielding direct evidence, through outer shell fitting, for both the coordination environment and oxidation state of arsenic within (as opposed to sorbed onto) a silicate mineral.

Many millions of people worldwide are at risk of severe poisoning through exposure to groundwater contaminated with sediment-derived arsenic. An ever-increasing body of work is reinforcing the link between microbially-mediated redox cycling in aquifer sediments and the mobilisation of sorbed As(V) into groundwaters as the potentially more mobile and toxic As(III) anion. However, to date, few studies have examined the biogeochemical cycling of Fe and As species by microbes indigenous to Cambodian sediments. In this study two contrasting sediments, taken from a shallow As-rich reducing aquifer in the Kien Svay district of Cambodia, were used in a laboratory microcosm study. We present evidence to show that microbes present in these sediments are able to reduce Fe(III) and As(V) when provided with an electron donor, and that the two sediments respond differently to stimulation with Fe(III) and As(V). Shifts in the community composition of the two sediments after stimulation with As(V) suggest a potential role for members of the beta-Proteobacteria in As(V) reduction, a phylogenetic grouping known to contain microorganisms capable of As(III) oxidation, but not previously implicated in As(V) reduction. PCR-based analysis of the sediment microbial DNA using primers specific to the arrA gene, (a gene essential for microbial As(V) respiration), indicates the presence of microorganisms capable of dissimilatory As(V) reduction.

The hydrolysis of palladium was investigated in 0.6 mol•kg-1 NaCl at 298.2 K. Potentiometric titrations of solutions at various total concentrations of palladium(II) revealed that dilute (millimolar) conditions can be used to monitor the proton release due to hydrolysis reactions up to 2 protons per palladium(II) as long as the equilibration time is kept small. Spectrophotometric titrations were used to corroborate the homogeneous changes in speciation for the PdCl3OH2- species and to extract its correlative molar absorption coefficients in the 210-320 nm range. The molar absorption coefficients are similar to those of PdCl42- but exhibit a broader distribution of excitation energies resulting from the blue shift of the dominant charge transfer bands due to the presence of OH-. The longer-term potentiometric titrations systematically yielded, on the other hand, precipitates which matured over a period of 6 weeks and resulted in a more extensive release of protons to the solution. Precipitation experiments at six different total palladium(II) concentrations in the 3-11 pH range showed the dominant precipitating phase as Pd(OH)1.72Cl0.28. The coordination environment of Pd in this solid was investigated by extended X-ray absorption fine structure spectroscopy (EXAFS) and yielded an average 1.75 O and 0.25 Cl per Pd atoms withmore » a Pd-O distance of 2.0 A and Pd-Cl of 2.1 A. Finally, the precipitation experiments showed the final products to be of larger solubility than a literature Pd(OH)2 solubility study in which the KCl media induced a solid phase transformation to Pd(OH)1.72Cl0.28. Polynuclear complexes Pdq(OH)r2q-r with q=r=[3,9] explain the combined precipitation and hydrolysis data and may represent subsets of [Pd(OH)2]n and/or [Pd(OH)1.72Cl0.28]n chains coiled into nanometer-sized spheroids previously described in the literature.« less

Products of anodic dissolution of uranium metal and the cathodic reduction of uranium(IV) ions in a molten eutectic mixture of lithium and potassium chlorides are studied at 450°C using x-ray absorption spectroscopy. It is shown that UCl 6 3− — ions form in the melt as a result of the metal’s anodic dissolution, and the metal-ligand distance in the complex ion is 0.284 nm. No complex ions of another configuration are found to form. Cathodic reduction of the UCl 6 2− — ions also yields the UCl 6 3− — ions, with longer U-Cl bonds.

We have examined alkaline sulfidic (0.5–0.003 M Na 2 S), aqueous solutions of Hg(II)-S complexes (4–370 ppm Hg(II)) by Hg L III edge EXAFS spectroscopy at 296, 348 and 423 K. Data were collected using the ID26 High Brilliance X-ray Spectroscopy beamline at the ESRF. Analysis of these EXAFS spectra shows Hg coordinated by two S atoms at 2.30 A; multiple scattering analyses reveal a linear [–S–Hg–S–] arrangement in the solution complex. These results are in agreement with earlier results on more concentrated solutions of these complexes. There is also evidence in the data for polynuclear sulfide complexes at 296 K and 348 K for samples with the lowest sulfide concentrations although this is complicated by multiple scattering effects.

The biogeochemical activities of free-living and symbiotic fungi must be acknowledged in attempts to understand uranium cycling and dispersal in the environment. Although the near-surface geochemistry of uranium is very complex and a wide variety of mineral phases is known, uranium trioxide (UO3) and triuranium octaoxide (U(3)O(8)) can be used as well characterized models in the study of biotransformations. We have used a complex methodological approach involving advanced solid state speciation and scanning electron microscopy to study the ability of saprotrophic, ericoid and ectomycorrhizal fungi to transform these model oxides. This study has revealed that fungi exhibit a high uranium oxide tolerance, and possess the ability to solubilize UO3 and U(3)O(8) and to accumulate uranium within the mycelium to over 80 mg (g dry weight)(-1) biomass. X-ray absorption spectroscopy of uranium speciation within the biomass showed that in most of the fungi the uranyl ion was coordinated to phosphate ligands, but in ectomycorrhizal fungi mixed phosphate/carboxylate coordination was observed. Abundant uranium precipitates associated with phosphorus were found in the mycelium and encrusted the hyphae. Some of the fungi caused the biomineralization of well-crystallized uranyl phosphate minerals of the meta-autunite group. This is the first experimental evidence for fungal transformations of uranium solids and the production of secondary mycogenic uranium minerals.

Poorly crystalline Fe(III) oxyhydroxides, ubiquitously distributed as mineral coatings and discrete particles in aquifer sediments, are well-known hosts of sedimentary As. Microbial reduction of these phases is widely thought to be responsible for the genesis of As-rich reducing groundwaters found in many parts of the world, most notably in Bangladesh and West Bengal, India. As such, it is important to understand the behavior of As associated with ferric oxyhydroxides during the early stages of Fe(III) reduction. We have used X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) to elucidate the changes in the bonding mechanism of As(III) and As(V) as their host Fe(III) oxyhydroxide undergoes bacterially induced reductive transformation to magnetite. Two-line ferrihydrite, with adsorbed As(III) or As(V), was incubated under anaerobic conditions in the presence of acetate as an electron donor, and Geobacter sulfurreducens, a subsurface bacterium capable of respiring on Fe(III), b...

Uranium (and plutonium) can be separated from spent fuel by treatment in molten carbonates and air sparging. UO 2 is converted into insoluble uranate species that can be filtered off. The rare earth and other fission product elements remaining in solution can be later precipitated using phosphates as precipitants. A sequence is described that can (in principle) be used to recover and recycle the carbonate melt used. Molten chloride eutectics are more convenient solvents than carbonates for obtaining the detailed chemistry of fission products in molten salts and their selective precipitation as phosphates. We have investigated the behavior of: Cs, Mg, Sr, Ba, lanthanides (La to Dy), Zr, Cr, Mo, Mn, Re (to simulate Tc), Fe, Ru, Ni, Cd, Bi and Te, but here report results for the rare earths. The distribution coefficients of these elements between chloride melts and precipitates were determined. Lithium-free melts favored formation of double phosphates. Rare earth elements and zirconium can be removed from chloride melts but Sr, Ba and Mg are melt cation specific. EXAFS and XANES measurements have established the speciation of both transition and non-transition elements in molten chlorides. EXAFS is largely model dependent, and to ensure the correct coordination number requires absorption spectroscopy data. EXAFS provides the element-Cl distance in the melt, not previously available. At high concentrations, the presence of bridging chlorine atoms can be established by EXAFS. The effect of diluting such melts and preliminary data on the solubilities of rare earth fission product elements in carbonate melts are reported.

Arsenic-rich scales are widely associated with geothermal fields and constitute a potential hazard to human health. Such arsenic has hitherto been reported to be almost exclusively hosted by sulphide or oxide phases or occurring as surface species. We report here, however, the occurrence of an arsenic-rich (1500 to 4000 mg kg—1 As) smectite from geothermal precipitates from a geothermal field in northwestern Japan and present evidence that the arsenic is predominantly hosted within this silicate mineral.Consistently ∼80% of the total arsenic determined in these geothermal precipitates was found by selective chemical extractions to be associated with an operationally defined clay mineral fraction, with lesser proportions being associated with operationally defined amorphous silica, Fe oxide and sulphide fractions. Analysis by XRD, ATR IR and XRF showed the clay fraction to be dominated by Mg-rich trioctahedral smectite.Arsenic K-edge XAS spectra of the smectite suggested the dominance of As(III)-O coordinated species with significant contributions from As(V)-O coordinated species. Both XPS and a magnesium chloride chemical extraction indicated minimal adsorption of arsenic on smectite surfaces suggesting that the arsenic was predominantly either dissolved within the smectite or occurred within mineral occlusions. No such occlusions greater than 1 μm in size were observed in the As-rich smectites.The potential occurrence of arsenic-bearing clays should be considered when determining remediation strategies for geothermal environments or evaluating risks associated with the industrial usage of geothermal precipitates.

Arsenic mobilization and Fe(III) reduction in acetate-amended sediments collected from a range of depths from an aquifer with elevated groundwater arsenic concentrations in West Bengal were monitored over a 1 month period. Significant arsenic release was noted in sediment collected from 24 m and 45 m depth, with some Fe(III) reduction also observed in the 24 m sample. The structure of the microbial communities present in the sediments prior to incubation showed marked differences down the sediment column. Profiling of the microbial community in the 24 m and 45 m samples revealed a relatively complex make-up, with Acinetobacter species comprising the bulk of the 24 m sedimentary bacterial population, but no previously characterized As(V)-reducers were detected in either sample.

The structures and interatomic distances of various chloro complexes of uranium in LiCl–BeCl2 eutectic melt at 380 °C have been determined from EXAFS measurements. Dissolution of UCl4 and UO2Cl2 (in the absence of a chlorine atmosphere) generated UCl 6 2 - ions. Similarly, dissolution of UCl3 generated UCl 6 3 - complexes but dissolution of UO2Cl2 under a chlorine atmosphere yielded a mixture of UCl 6 2 - and UO2 Cl 4 2 - , deduced from electronic absorption spectra measurements. Interatomic distances and uranium coordination are compared with earlier data in molten LiCl, both in melts and for quenched samples at room temperature.

Four thallium(I) uranates(VI), Tl4UO5 ,T l 2UO4 ,T l 2U2O7 and Tl2U3O10, were prepared and their IR, and, for the first time, their Raman, electronic and X-ray absorption (EXAFS) spectra have been measured. These uranates are thermally stable under nitrogen up to 660 C but above this temperature they decompose and their uranium content increases, due to loss of Tl2O. The thermal stability of these uranates increases with decreasing thallium content. Comparison with the reported values for the standard enthalpies of formation of the corresponding alkali metal uranates, which are essentially cation independent, allows suggested values for the above uranates, respectively, of � 2437, � 1900, � 3182, and � 4437 kJ mol � 1 . Structural information and atom arrangements were obtained from analysis of powder XRD and EXAFS spectroscopy measurements. U–O distances, both primary and secondary, were obtained from EXAFS measurements and these were essentially identical with those calculated from vibrational IR spectra. 2005 Elsevier B.V. All rights reserved.

The interaction between two contrasting examples of lake sediments and small concentrations of mercury added to the sediments in solution has been studied using X-ray absorption spectroscopy. Whereas one lake (Esthwaite Water) is biologically productive, with a seasonal cycle of phytoplankton activity, including stratification and Fe(III) reduction, and a mineralogy involving quartz, muscovite, and clinochlore, the other (Botany Pond) remains oxic throughout the year. In the latter case, the sediment is predominantly quartz and calcite. Chemical analyses of these two lake sediments reflectthe differences in mineralogy and showthat both contain significant organic carbon (approximately 10-12 wt %) and smaller amounts of S (approximately 0.2-1.7 wt %) and Cl (approximately 0.4-1.1 wt %). Despite the substantial amounts of organic matter in both sediments, the spectroscopic data show that the mercury occurs as a sulfide phase with a local structural environment akin to that in cinnabar. Parallel spectroscopic studies conducted on Hg either coprecipitated or sorbed onto FeS (mackinawite), and on oxidized mackinawite, provide supporting information; the possibility of Hg forming a chloride was eliminated by careful mapping of the relevant elements by an electron microprobe. It appears, therefore, that the high affinity of Hg for S predominates even in substantially oxic environments.

The April 1998 breaching of the Aznalcollar tailings dam at Boliden Apirsa's Spanish Aznalcollar/Los Frailes Ag–Cu–Pb–Zn mine resulted in the flooding of the Rios Agrio and Guadiamar with arsenic (As)-rich tailings and water. Mineralogical and geochemical characterisation of water and sediment from five ephemeral pools on the Agrio–Guadiamar floodplain has been carried out to understand how the As is sequestered and to evaluate the long-term stability of the As-bearing phases. Dissolved As concentrations in pool waters range from an extreme of 85,000 μg l − 1 down to 5.5 μg l − 1 . XANES, EXAFS, XRD, X-ray mapping, electron microprobe analysis and single chemical extractions suggest that As in the sediments is sequestered in sulphide (As 0 in arsenopyrite and pyrite, coordinated by 1 S and 3 Fe) and in Fe oxide (AsV in arsenate, coordinated by 4 O, bound to Fe oxide and Fe sulphate, probably jarosite). The pools evolved as a result of the interaction of rainwater with the sediments, and As concentrations are controlled by evaporation and by the amount of tailings (and specifically, arsenopyrite) in the sediments that remained even after clean-up efforts. The arsenopyrite remaining in the floodplain sediment is potentially highly reactive under the prevailing pH and Eh conditions on the floodplain, and will probably continue to provide As to ephemeral pools and possibly, Agrio–Guadiamar river water.

The contamination of ground waters, abstracted for drinking and irrigation, by sediment-derived arsenic threatens the health of tens of millions of people worldwide, most notably in Bangladesh and West Bengal1,2,3. Despite the calamitous effects on human health arising from the extensive use of arsenic-enriched ground waters in these regions, the mechanisms of arsenic release from sediments remain poorly characterized and are topics of intense international debate4,5,6,7,8. We use a microscosm-based approach to investigate these mechanisms: techniques of microbiology and molecular ecology are used in combination with aqueous and solid phase speciation analysis of arsenic. Here we show that anaerobic metal-reducing bacteria can play a key role in the mobilization of arsenic in sediments collected from a contaminated aquifer in West Bengal. We also show that, for the sediments in this study, arsenic release took place after Fe(iii) reduction, rather than occurring simultaneously. Identification of the critical factors controlling the biogeochemical cycling of arsenic is one important contribution to fully informing the development of effective strategies to manage these and other similar arsenic-rich ground waters worldwide.

Synthetic bornites with compositional variations within 2 at% of the stoichiometric formula of Cu5FeS4 show dramatic changes in their Cu L,,3 absorption spectra, determined using synchrotron radiation. Many of the bornites show L-3 spectra with a strong peak at 932.6 eV and a broader region of intensity in the region 933 eV to 940 eV with specific features at 935.6 eV and 938 eV. However, the intensity of the peak at 932.6 eV varies greatly, and there is a clear correlation between the loss of intensity of this peak and the gain of intensity of the peak at 935.6 eV. EPMA reveals that this change in relative intensities is directly correlated to the excess of valence electrons present in the bornites, calculated the using formal valences of Cu+, Fe3+ and S2- [The Cu L,,2 spectra are typical of Cu(I) sulfides and Fe L3 edge spectra reveal the Fe to be present as Fe3+]. A simple cluster model can explain this transfer in intensity among the three different peaks. The three peaks in the bornite XAS 2p spectrum can be assigned (in order of increasing energy) to the final states cd(10)Ls(2), cd(10)s, and cd(9)s(2); the transition to an empty s state is possible for all three configurations, d(10)Ls-->cd(10)Ls(2), d(10)--> cd(10)s, and d(9)s-->cd(9)s(2). When the amount of excess electrons increases, the holes are filled and the weight of the d(10)Ls and d(9)s configurations will decrease, whereas that of the d(10) configuration will increase, exactly as observed in the spectra. This study demonstrates the value of metal L-edge spectroscopy, and Cu L-2,L-3 spectra in particular, in investigations of minerals with variations in stoichiometry and chemistry; the information derived on electronic structure can be used to explain bonding and other variations in physical and chemical properties of the minerals.

Oxidative dissolution of a primary Mn-silicate phase (rhodonite) was studied via synchrotron X-ray techniques. The study was designed to combine the element-specific chemical technique of Glancing Incidence X-ray Absorption Spectroscopy (GIXAS) with the surface structural technique of X-ray scattering in order to produce the first depth resolved study of Mn-silicate low-temperature reactivity. A chemo-mechanically polished polycrystalline rhodonite sample was characterized and then reacted with pH 3.5 nitric acid. The surface originally had a mosaic structure and 15.5 (±1) Å r.m.s. roughness. Surface composition was not measurably different from bulk rhodonite before reaction, indicating that the surface preparation regimen had not produced an altered surface. After 1 h of reaction, the roughness of the mineral surface decreased and reflectivity oscillations developed, resulting from the formation of a leached layer. This layer was 74.7 (±2) Å thick with an electron density equal to 72% of that of bulk rhodonite (equal to the loss of ~1 in 2 Mn atoms). Both the primary and the buried interfaces had similar roughnesses; 4.9 and 4.5 (±1.0) Å , respectively. Diffuse scatter indicated that the correlation length between surface features also decreased. The GIXAS analysis showed that the Mn remaining in the surface had become oxidized, with the degree of oxidation decreasing as a function of depth. Oxidation penetrated at least 140 Å into the structure. A further 2.5 h of reaction at pH 3.5 caused dissolution of the leached layer and reduced the thickness of this altered region to 16.0 (±2) Å , while surface roughness increased slightly to 6.2 (±1.0) Å . Depletion of Mn in this region increased only slightly relative to the first reaction step; the electron density was 67% that of bulk rhodonite, equivalent to the loss of 2 in 3 Mn atoms. The thickness of the oxidized region however, persisted. Analysis by XPS on the same specimen corroborates the X-ray results.

Knowledge of the solid-phase speciation of As in Bengali sediments associated with hazardous As-rich groundwaters is crucial to understanding the processes controlling As release. The local coordination environment of As in such a sediment has been probed using K-edge As EXAFS. This revealed that As exists predominantly in its oxidized form, As(V), probably adsorbed as bidentate arsenate tetrahedra on metal (Fe and/or Al) oxide/hydroxide surfaces, although incorporation of As into a metal oxide structure cannot be ruled out. Arsenic was found to occur in several different coordination environments and this, together with the low concentration (–1) of As in the sediment prevented the unambiguous assignment of the second coordination sphere. The EXAFS analysis of the sediment after incubation under anaerobic conditions in the presence of added electron donor for metal reduction indicated changes in the relative concentrations of different solid-phase As species, providing circumstantial evidence for differential susceptibility to microbial action.

We have examined alkaline sulfidic (0.5–0.005 M NaHS−) aqueous solutions of Hg(II)–S complexes [2.8–0.28 mM Hg(II)] using extended X-ray absorption fine structure (EXAFS) and an especially designed spectroscopic cell. The structural environment of Hg in the complex, determined by EXAFS analysis of Hg LIII edge fluorescence spectra taken at 298 and 348K, shows Hg coordinated by 2 S atoms at 2.30 A. Multiple-scattering analysis reveals a linear [–S–Hg–S–] arrangement in the solution complex. These results are consistent with a low coordination number for Hg as predicted by relativistic models, ab initio calculations, and as determined from the structural environment of Hg in crystalline mercury compounds.

An integrated approach involving the use of ion chromatography–inductively coupled plasma-mass spectrometry (IC–ICP-MS), X-ray absorption spectroscopy (XAS) and sequential extraction procedures has been employed to elucidate the solution and solid phase speciation and partitioning of As in a polluted urban watercourse. Dissolved As concentrations exceeding 130 μg l −1 and comprising entirely inorganic species were determined in the waters of Tinker Brook, a contaminated stream. Upon mixing with a relatively As-free stream, White Ash Brook, both the total concentration of dissolved As and the proportion of As(V) were observed to decrease dramatically below values expected for conservative mixing. This was ascribed to adsorption onto the Fe (oxyhydr)oxides that characterise White Ash Brook on the basis of sequential extraction and direct analysis of the solids via XAS . The shift in oxidation state is speculated to be due to the faster rate of adsorption of As(V) on Fe (oxyhydr)oxides than As(III) in this fast flowing stream system. During periods of reduced supply of anthropogenic As, a small, secondary input of As(III) to White Ash Brook is detectable, delivered by a small ochreous seepage. The Fe (oxyhydr)oxide As-rich deposits surrounding this discharge may also act as a significant source of As upon dissolution during stormflow conditions.

The gold thiosulfate complex, Au(S2O3)(2)(3-), has been studied using extended X-ray absorption fine structure (EXAFS) spectroscopy and high level quantum mechanical calculations, which include the effect of aqueous solvent via a dielectric continuum model. EXAFS measurements on Au(S2O3)(2)(3-) show gold coordinated by two sulfurs at 2.29 Angstrom. Density functional calculations, incorporating the effect of solvent using the COSMO method, show the 2-fold S coordination of An to be linear in geometry. This computational approach is further employed to examine the conformational potential energy surface of the complex in aqueous solution. As expected from the significant deviation (108degrees) between the calculated conformation in aqueous solution and the observed crystal structure, internal rotation about the S-Au-S axis is facile: calculation predicts a rotational barrier in solution of less than 3 kcal/mol. Calculated Au-S bond dissociation energies indicate a strong metal-S bond in aqueous solution. Lower solution stability of Au(S2O3)(2)(3-) with respect to another thio gold complex, Au(SH)(2)(-), is predicted in accord with measured overall stability constants. These calculations, together with our experimental observation that Au(S2O3)(2)(3-) decomposes at 393 K, support the view that gold transport by the thiosulfate complex of Au(I) may be restricted to ambient and moderate temperatures. This contrasts with the Au(I) thiolate complex which can mobilize gold at elevated temperature and pressure.

The solution chemistry of uranyl ion with iminodiacetate (IDA) and oxydiacetate (ODA) was investigated using NMR and EXAFS spectroscopies, potentiometry, and calorimetry. From the NMR and EXAFS data and depending on stoichiometry and pH, three types of metal:ligand complex were identified in solution in the pH range 3-7: 1:1 and 1:2 monomers; a 2:2 dimer. From NMR and EXAFS data for the IDA system and previous studies, we propose the three complex types are [UO(2)(IDA)(H(2)O)(2)], [UO(2)(IDA)(2)](2)(-), and [(UO(2))(2)(IDA)(2)(mu-OH)(2)](2)(-). From EXAFS spectroscopy, similar 1:1, 2:2, and 1:2 complexes are found for the ODA system, although (13)C NMR spectroscopy was not a useful probe in this system. For the 1:1 and 1:2 complexes in solution, EXAFS spectroscopy is ambiguous because the data can be fitted with either a long U-N/O(ether) value (ca. 2.9 A) suggesting 1,7-coordination of the ligand or a U-C interaction at a similar distance, consistent with terminal bidentate coordination. However, the NMR data of the IDA system suggest that 1,7-coordination is the more likely. The stability constants of the three complexes were determined by potentiometric titrations; the log beta values are 9.90 +/-, 16.42 +/-, and 10.80 +/- for the 1:1, 1:2, and 2:2 uranyl-IDA complexes, respectively, and 5.77 +/-, 7.84 +/-, and 4.29 +/- for the 1:1, 1:2, and 2:2 uranyl-ODA complexes, respectively. The thermodynamic constants for the complexes were calculated from calorimetric titrations; the enthalpy changes (kJ mol(-)(1)) and entropy changes (J K(-)(1) mol(-)(1)) of complexation for the 1:1, 1:2, and 2:2 complexes respectively are the following. IDA: 12 +/- 2, 230 +/- 8; 8 +/- 2, 151 +/- 9; -33 +/- 3, -283 +/- 11. ODA: 26 +/- 2, 198 +/- 12; 20 +/- 2, 106 +/- 8; -24 +/- 2; -219 +/- 8.

Eight uranyl compounds containing the dicarboxylate ligands iminodiacetate (IDA) or oxydiacetate (ODA) have been characterized in the solid state. The published polymeric structures for [UO(2)(C(4)H(6)NO(4))(2)] and [UO(2)(C(4)H(4)O(5))](n) have been confirmed, while Ba[UO(2)(C(4)H(5)NO(4))(2)] x 3H(2)O, [(CH(3))(2)NH(CH(2))(2)NH(CH(3))(2)][UO(2)(C(4)H(4)O(5))(2)] [orthorhombic space group Pnma, a = 10.996(5) A, b = 21.42(1) A, c = 8.700(3) A, Z = 4], and [C(2)H(5)NH(2)(CH(2))(2)NH(2)C(2)H(5)][UO(2)(C(4)H(4)O(5))(2)] [monoclinic space group P2(1)/n, a = 6.857(3) A, b = 9.209(5) A, c = 16.410(7) A, beta = 91.69(3), Z = 2] contain monomeric anions. The distance from the uranium atom to the central heteroatom (O or N) in the ligand varies. Crystallographic study shows that U-heteroatom (O/N) distances fall into two groups, one 2.6-2.7 A in length and one 3.1-3.2 A, the latter implying no bonding interaction. By contrast, EXAFS analysis of bulk samples suggests that either a long U-heteroatom (O/N) distance (2.9 A) or a range of distances may be present. Three possible structural types, two symmetric and one asymmetric, are identified on the basis of these results and on solid-state (13)C NMR spectroscopy. The two ligands in the complex can be 1,4,7-tridentate, giving five-membered rings, or 1,7-bidentate, to form an eight-membered ring. (C(4)H(12)N(2))[(UO(2))(2)(C(4)H(5)NO(4))(2)(OH)(2)] x 8H(2)O [monoclinic space group P2(1)/a, a = 7.955(9) A, b = 24.050(8) A, c = 8.223(6) A, beta = 112.24(6), Z = 2], (C(2)H(10)N(2))[(UO(2))(2)(C(4)H(5)NO(4))(2)(OH)(2)] x 4H(2)O, and (C(6)H(13)N(4))(2)[(UO(2))(2)(C(4)H(4)O(5))(2)(OH)(2)] x 2H(2)O [monoclinic space group C2/m, a = 19.024(9) A, b = 7.462(4) A, c = 2.467(6) A, beta = 107.75(4), Z = 4] have a dimeric structure with two capping tridentate ligands and two mu(2)-hydroxo bridges, giving edge-sharing pentagonal bipyramids.

The mechanisms whereby As(III) and As(V) in aqueous solution (pH 5.5-6.5) interact with the surfaces of goethite, lepidocrocite, mackinawite, and pyrite have been investigated using As K-edge EXAFS and XANES spectroscopy. Arsenic species retain original oxidation states and occupy similar environments on the oxyhydroxide substrates, with first-shell coordination to four oxygens at 1.78 A for As(III) and 1.69 A for As(V). In agreement with other workers, we find that inner sphere complexes form, apparently involving bidentate (bridging) arsenate or arsenite. Interaction of As(III) and As(V) with the sulfide surfaces shows primary coordination to four oxygens (As-O: 1.69-1.76 A) with further sulfur (approximately 3.1 A) and iron (3.4-3.5 A) shells suggesting outer sphere complexation. Arsenic species were also coprecipitated with mackinawite (pH 4.0), and these samples were further studied following oxidation. At high As(III) or As(V) concentrations, arsenate or arsenite species form, probably as sorption complexes, along with poorly crystalline arsenic sulfide (the only product at low As(V) concentrations). All oxidized samples show primary coordination to four oxygens at 1.7 A, indicating As(V); these arsenates may show both outer sphere complexation with residual mackinawite and inner sphere complexation with new oxyhydroxides. These experiments help to clarify our understanding of As mobility in near-surface environments.