John M. Charnock

Role of metal-reducing bacteria in arsenic release from Bengal delta sediments

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 Bengal. 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 debate. 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.

Grain Unloading of Arsenic Species in Rice

Rice (Oryza sativa) is the staple food for over half the worlds population yet may represent a significant dietary source of inorganic arsenic (As), a nonthreshold, class 1 human carcinogen. Rice grain As is dominated by the inorganic species, and the organic species dimethylarsinic acid (DMA). To investigate how As species are unloaded into grain rice, panicles were excised during grain filling and hydroponically pulsed with arsenite, arsenate, glutathione-complexed As, or DMA. Total As concentrations in flag leaf, grain, and husk, were quantified by inductively coupled plasma mass spectroscopy and As speciation in the fresh grain was determined by x-ray absorption near-edge spectroscopy. The roles of phloem and xylem transport were investigated by applying a ± stem-girdling treatment to a second set of panicles, limiting phloem transport to the grain in panicles pulsed with arsenite or DMA. The results demonstrate that DMA is translocated to the rice grain with over an order magnitude greater efficiency than inorganic species and is more mobile than arsenite in both the phloem and the xylem. Phloem transport accounted for 90% of arsenite, and 55% of DMA, transport to the grain. Synchrotron x-ray fluorescence mapping and fluorescence microtomography revealed marked differences in the pattern of As unloading into the grain between DMA and arsenite-challenged grain. Arsenite was retained in the ovular vascular trace and DMA dispersed throughout the external grain parts and into the endosperm. This study also demonstrates that DMA speciation is altered in planta, potentially through complexation with thiols.

Role of Oxalic Acid Overexcretion in Transformations of Toxic Metal Minerals by Beauveria caledonica

ABSTRACT The fungus Beauveria caledonica was highly tolerant to toxic metals and solubilized cadmium, copper, lead, and zinc minerals, converting them into oxalates. This fungus was found to overexcrete organic acids with strong metal-chelating properties (oxalic and citric acids), suggesting that a ligand-promoted mechanism was the main mechanism of mineral dissolution. Our data also suggested that oxalic acid was the main mineral-transforming agent. Cadmium, copper, and zinc oxalates were precipitated by the fungus in the local environment and also in association with the mycelium. The presence of toxic metal minerals often led to the formation of mycelial cords, and in the presence of copper-containing minerals, these cords exhibited enhanced excretion of oxalic acid, which resulted in considerable encrustation of the cords by copper oxalate hydrate (moolooite). It was found that B. caledonica hyphae and cords were covered by a thick hydrated mucilaginous sheath which provided a microenvironment for chemical reactions, crystal deposition, and growth. Cryo-scanning electron microscopy revealed that mycogenic metal oxalates overgrew parental fungal hyphae, leaving a labyrinth of fungal tunnels within the newly formed mineral matter. X-ray absorption spectroscopy revealed that oxygen ligands played a major role in metal coordination within the fungal biomass during the accumulation of mobilized toxic metals by B. caledonica mycelium; these ligands were carboxylic groups in copper phosphate-containing medium and phosphate groups in pyromorphite-containing medium.

THE STRUCTURE OF AMORPHOUS COPPER SULFIDE PRECIPITATES : AN X-RAY ABSORPTION STUDY

Abstract X-ray absorption (XAS) and X-ray photoelectron spectroscopy (XPS) analysis of copper sulfide precipitated from aqueous solution at ambient temperatures reveals the existence of a metastable primitive structure that ages to a structure with the characteristics of amorphous covellite; XAS provides direct structural information on the structure of these amorphous copper sulfides. Extended X-ray absorption fine structure (EXAFS) analysis of the primitive structure demonstrates the presence of disulfide (S2-groups) and a Cu S interaction of 2.8A: the latter is not found in covellite. Copper K-edge X-ray absorption near-edge structure (XANES) indicates the dominance of 3- over 4-coordinate Cu in the primitive phase, while Cu L3-edge spectra reveal only Cu(I) to be present in all precipitates formed. XPS data confirm the presence of only Cu(I) and reveal that three types of S are present. Upon ageing, the primitive structure transforms to one with the characteristics of covellite, and this transformation involves the reordering of the S2- and Cu3S CuS3-layers. Development of the primitive phase from either a wurtzite-like structure or planar Cu3S CuS3-layers is possible, with the structural evolution driven by the antipathy of Cu(II) for tetrahedral coordination and anomalous electron densities in the metastable structures.

Interactions between the Fe(III)-Reducing Bacterium Geobacter sulfurreducens and Arsenate, and Capture of the Metalloid by Biogenic Fe(II)

ABSTRACT Previous work has shown that microbial communities in As-mobilizing sediments from West Bengal were dominated by Geobacter species. Thus, the potential of Geobacter sulfurreducens to mobilize arsenic via direct enzymatic reduction and indirect mechanisms linked to Fe(III) reduction was analyzed. G. sulfurreducens was unable to conserve energy for growth via the dissimilatory reduction of As(V), although it was able to grow in medium containing fumarate as the terminal electron acceptor in the presence of 500 μM As(V). There was also no evidence of As(III) in culture supernatants, suggesting that resistance to 500 μM As(V) was not mediated by a classical arsenic resistance operon, which would rely on the intracellular reduction of As(V) and the efflux of As(III). When the cells were grown using soluble Fe(III) as an electron acceptor in the presence of As(V), the Fe(II)-bearing mineral vivianite was formed. This was accompanied by the removal of As, predominantly as As(V), from solution. Biogenic siderite (ferrous carbonate) was also able to remove As from solution. When the organism was grown using insoluble ferrihydrite as an electron acceptor, Fe(III) reduction resulted in the formation of magnetite, again accompanied by the nearly quantitative sorption of As(V). These results demonstrate that G. sulfurreducens, a model Fe(III)-reducing bacterium, did not reduce As(V) enzymatically, despite the apparent genetic potential to mediate this transformation. However, the reduction of Fe(III) led to the formation of Fe(II)-bearing phases that are able to capture arsenic species and could act as sinks for arsenic in sediments.

Oligomerization in As (III) sulfide solutions: Theoretical constraints and spectroscopic evidence

Bond distances, vibrational frequencies, gas-phase energetics, and proton affinities for various thioarsenite molecules and ions are predicted from molecular orbital theory and used to interpret EXAFS and Raman spectra of dissolved thioarsenites in undersaturated, alkaline 1 M NaHS solutions. From MO predictions, Raman peaks at 325 and 412 cm(-1) are assigned to AsS(SH)(2)(-) and a peak at 382 cm(-1) to AsS2(SH)(2-). At alkaline pH, As-S distances in dissolved thioarsenites are 2.21-2.23 Angstrom and no statistically significant As-As interactions are recorded, consistent with predominance of the monomers, AsS(SH)(2)(-) and AsS2(SH)(2-). Estimated proton affinities suggest that thioarsenites with a negative charge greater than 2 are unstable in water. In seeming contradiction to this spectroscopic evidence, a new analysis of published solubility studies reinforces previous inferences that the trimer, As3S4(SH)(2)(-), is the predominant thioarsenite in systems saturated with As2S3. Previously proposed dimeric species of the form, HxAs2S4x-2, are rejected based on predicted thermodynamic properties. Dimer plus tetramer combinations also are rejected. Estimated free energies for AsS(OH)(SH)(-) and AsS(SH)(2)(-) are presented. We reconcile the spectroscopic and solubility evidence by showing that in undersaturated solutions monomers can become thermodynamically favored over oligomers. This pattern should be looked for in other sulfide systems as well. Sulfidic natural waters are in many cases undersaturated with respect to As2S3 phases, so monomeric thioarsenites could be more important in nature than the trimers that have been characterized in saturated solutions. EXAFS spectra show that amorphous As2S3 resembles orpiment in the first shell around As, but that higher shells are disordered. Disorder may be caused by occasional realgar-like, As-As bonds, consistent with the observation that amorphous As2S3 is slightly S deficient.

Arsenic in hair and nails of individuals exposed to arsenic-rich groundwaters in Kandal province, Cambodia.

The health implications of the consumption of high arsenic groundwater in Bangladesh and West Bengal are well-documented, however, little is known about the level of arsenic exposure elsewhere in Southeast Asia, where widespread exploitation of groundwater resources is less well established. We measured the arsenic concentrations of nail and hair samples collected from residents of Kandal province, Cambodia, an area recently identified to host arsenic-rich groundwaters, in order to evaluate the extent of arsenic exposure. Nail and hair arsenic concentrations ranged from 0.20 to 6.50 microg g(-1) (n=70) and 0.10 to 7.95 microg g(-1) (n=40), respectively, in many cases exceeding typical baseline levels. The arsenic content of the groundwater used for drinking water purposes (0.21-943 microg L(-1) (n=31)) was positively correlated with both nail (r=0.74, p<0.0001) and hair (r=0.86, p<0.0001) arsenic concentrations. In addition, the nail and hair samples collected from inhabitants using groundwater that exceeded the Cambodian drinking water legal limit of 50 microg L(-1) arsenic contained significantly more arsenic than those of individuals using groundwater containing <50 microg L(-1) arsenic. X-ray absorption near edge structure (XANES) spectroscopy suggested that sulfur-coordinated arsenic was the dominant species in the bulk of the samples analysed, with additional varying degrees of As(III)-O character. Tentative linear least squares fitting of the XANES data pointed towards differences in the pattern of arsenic speciation between the nail and hair samples analysed, however, mismatches in sample and standard absorption peak intensity prevented us from unambiguously determining the arsenic species distribution. The good correlation with the groundwater arsenic concentration, allied with the relative ease of sampling such tissues, indicate that the arsenic content of hair and nail samples may be used as an effective biomarker of arsenic intake in this relatively recently exposed population.

Synthesis-dependant structural variations in amorphous calcium carbonate

Amorphous calcium carbonate (ACC) was synthesised in the presence of the additives magnesium and poly(aspartic acid) (pAsp) and the structure and crystallisation of these ACC samples was investigated using a range of techniques including X-Ray Absorption Spectroscopy (XAS), X-Ray Diffraction (XRD) and Infra-Red Spectroscopy (IR). The experiments demonstrated that synthetic ACC can be produced with different short-range structures, according to the solution additives used. While the first Mg–ACC precipitates showed short-range structures most similar to aragonite, with monohydrocalcite short-range structures developing with incubation in solution, the initial pAsp–ACC precipitates possessed short-range structures resembling vaterite. The results show that the influence of these additives on the crystallisation of calcium carbonate is apparent even in the precipitation of the first amorphous precursor phase, and that the first stages of recrystallisation involve the expulsion of water from the structure rather than significant changes in the short-range structure around the calcium ions.

An X-ray absorption (EXAFS) spectroscopic study of aquated Ag+ in hydrothermal solutions to 350°C

Abstract The first direct measurements over a range of temperatures from 25–350°C are reported for cation-oxygen (water) distances in aqueous solutions at equilibrium saturated vapour pressures. The silver-oxygen (first shell water) bond lengths in AgNO 3 and AgClO 4 solutions were obtained using extended X-ray absorption fine structure (EXAFS) measurements on the silver K-edge. At 25°C and 1 bar, the Ag + ion is coordinated by four inner sphere water ligands with an average silver-oxygen distance of 2.32 (±0.01) A. With increasing temperature to 350°C, the silver-oxygen (H 2 O) bond length decreases by about 0.10 (±0.01) A and the coordination number decreases from 4 to ∼3. The Debye-Waller factors remain approximately constant with increasing temperature. Silver-nitrate ion pairing (as AgNO 0 3 ) was observed only at 200 and 300°C in an 0.10 AgNO 3 /3.00 m HNO 3 solution for which the silver-nitrogen (nitrate) distances of 3.24 and 3.27 A, respectively, were obtained.

Reactions of copper and cadmium ions in aqueous solution with goethite, lepidocrocite, mackinawite, and pyrite

Abstract The uptake of Cu and Cd in aqueous solution by interaction with the surfaces of carefully synthesized finely sized particles of goethite, lepidocrocite, mackinawite, and pyrite was measured as a function of initial metal concentration in solution. The results show how reactions between metal ions in solution and mineral surfaces depend in a subtle way on the nature of the surfaces and, in certain cases, on the initial concentration of metal in solution. The uptake curves fall into two groups; type I in which the efficiency of uptake decreases with increasing concentration (Cu and Cd on goethite and lepidocrocite, Cd on pyrite), and type II in which it remains constant (Cu on mackinawite and pyrite, Cd on mackinawite). The total uptake is an order of magnitude greater for the latter group. X-ray absorption spectroscopies (XANES and EXAFS) were used to define the local environments of the metals taken up at the mineral surfaces. All examples showing the type I behavior yield information on local environments consistent with their being bound to the surfaces by an inner sphere complex formation mechanism. Thus, Cu on goethite appeared to form a Jahn-Teller distorted octahedral complex (four O atoms at 1.94 Å ; two O atoms at 2.41 Å ) but with evidence for interaction with two further Fe (or Cu) at 2.92 Å . On lepidocrocite, the first coordination sphere was essentially identical to that on goethite but with a second Cu or Fe shell at 3.04 Å and, for the highest Cu loadings, a third shell (2 Cu or Fe) at 3.67 Å . The Cd on goethite showed best fits for sixfold coordination to O (at 2.26 Å ) but with evidence for a second shell of Fe atoms at 3.75 Å . On lepidocrocite, the first shell was essentially the same as for goethite, but a second shell of Fe atoms appeared to occur at the shorter distance of 3.31 Å . For Cd-loaded pyrite, best fits were given by a single shell of six O atoms at 2.27 to 2.28 Å and with no evidence for a second shell of metal atoms. The systems exhibiting the type II behavior yielded spectra consistent with the formation of new phases on the surfaces, either by precipitation or replacement reactions. In the case of Cu interaction with mackinawite, a chalcopyrite phase appeared to form, whereas interaction with pyrite seemed to produce binary Cu sulfides-covellite at lower loadings and chalcocite at higher loadings. Cd interaction with mackinawite seemed to produce a CdS phase.