Philippe Ildefonse

XAFS determination of the chemical form of lead in smelter-contaminated soils and mine tailings: Importance of adsorption processes

Abstract We investigated smelter-contaminated soils from Evin-Malmaison, Nord-Pas-de-Calais, France, and mine tailings from Leadville, Colorado, U.S.A. Bulk Pb concentrations range from 460 to 1900 ppm in the topsoils at Evin-Malmaison site and from 6000 to 10000 ppm in the tailings samples from the Leadville site. These concentrations necessarily raise human health and environmental concerns, but bioavailability and chemical lability of Pb in these materials vary dramatically and show little correlation with bulk concentrations. This study provides detailed information on the speciation of Pb in these materials. Emphasis is on the identification and characterization of poorly crystalline and/or fine-grained species, such as sorption complexes and poorly crystalline (co)precipitates, which are likely to control Pb bioavailability and mobility in these natural systems. Because these samples are heterogeneous, multi-phase mixtures, a variety of bulk analytical methods were used including powder X-ray diffraction (XRD) and Rietveld refinement, scanning electron microscopy, electron probe microanalysis, synchrotron-based Xray absorption, and micro-fluorescence spectroscopies. The synchrotron-based techniques enable identification of amorphous or nanocrystalline Pb-containing phases and the spatial distribution of Pb at the 25 μm scale. These techniques, in conjunction with physical and chemical separation techniques, allowed identification and characterization of several species not amenable to detection by conventional microanalytical techniques. In the Evin-Malmaison samples, direct spectroscopic evidence for Pb sorbed to humic acids was found, as well as to both manganese and iron (oxyhydr)oxides. In the Leadville samples, variations in Pb speciation with pH are consistent with predictions based on simplified model system studies of adsorption processes; specifically, the carbonate-buffered tailings with near-neutral pH contain up to 50% of total Pb as adsorption complexes on iron (oxyhydr)oxides, whereas Pb speciation in sulfide-rich low pH samples is dominated by Pb-bearing jarosites with no evidence for adsorbed Pb in these latter samples.

Chemical and structural changes in Cervus elaphus tooth enamels during fossilization (Lazaret cave): a combined IR and XRD Rietveld analysis

Abstract Tooth enamel from modern and fossil (Lazaret cave) Cervus elaphus was characterized in order to study the chemical and structural changes during fossilization. Calcium, P, Na, Mg, F, Cl, CO 3 , contents were measured by chemical analyses, and infrared (IR) spectroscopy was used to determine H 2 0, OH − , P0 4 3− and C0 3 2− . Carbonate increases during fossilization and substitutes for PO 4 3− at the B-site and for OH at the A-site. The C0 3 2− -for-PO 4 3− substitution experiences the highest increase. Water and OH contents decrease during fossilization. These chemical changes may be traced by Rietveld structure refinement (XRD). Like human enamel, red deer enamel consists of apatite. A good positive correlation has been found between the a cell parameter and C0 3 2− contents. Refinement of atomic positions and site multiplicity allow us to describe site distortions in P0 4 3− polyhedra and along the 6 3 axis; these distortions are indirect probes of the substituent ions in the apatite structure. At Lazaret cave, the karstic environment is thought to control the chemical and structural changes of the fossil enamels. Fossilization conditions have been favorable for a good conservation of the Cervus elaphus tooth enamels whatever their stratigraphic position and location were. These fossil enamels have experienced only slight structural and chemical changes considering their geological age. This accounts for a rapid burial in continental sediments of Lazaret prehistoric cave. These Lazaret fossil enamels could be considered as stable material which may be used for dating by the ESR and U-Th methods.

Occurrence of Zn/Al hydrotalcite in smelter-impacted soils from northern France: Evidence from EXAFS spectroscopy and chemical extractions

Abstract Zinc speciation was studied by EXAFS spectroscopy, μ-SXRF elemental mapping, XRD, and chemical extraction methods in two smelter-impacted soils sampled near one of the largest Pb and Zn processing plants in Europe, which is located in northern France about 50 km south of Lille. The tilled and wooded soils chosen for study differ in Zn concentration (≈600 and 1400 mg/kg, respectively), soil pH (7.5 and 5.5, respectively), and organic matter content (1.5 and 6.4 wt% TOC, respectively). In both soils, the occurrence of Fe- and Zn-rich (up to 10 wt% Zn) slag particles ranging in size from a few micrometers to a few millimeters, was shown by m-SXRF elemental mapping of soil thin sections as well as by SEM and chemical analysis of different soil size fractions. For both soils, XRD analysis of the dense coarse fraction, which contains up to 10 wt% Zn, revealed the presence of a minor amount (1-1.5 wt%) of crystalline ZnS (sphalerite and wurtzite). In this fraction, EXAFS data show that Zn is mainly incorporated in the tetrahedral sites of a magnetite- franklinite solid solution. The clay fraction (<2 μm) represents the largest pool of Zn in both soils, with 77 and 62% of the total Zn in the tilled and wooded soils, respectively. However, XRD was not able to detect any Znbearing phases in this fraction. Comparison of Zn K-EXAFS data of untreated and chemically treated samples from the bulk (<2 μm) and the clay (<2 μm) soil fractions with Zn K-EXAFS data from more than 30 model compounds suggests that Zn is present in the following chemical forms: (1) Zn outer-sphere complexes, (2) Zn-organic matter inner-sphere complexes, (3) Zn/Al-hydrotalcite (Zn/ Al-HTLC), (4) phyllosilicates in which Zn is present in the dioctahedral layer at dilute levels, and (5) magnetite-franklinite solid solutions inherited from the smelting process. The presence of exchangeable Zn outer-sphere complexes and of Zn inner-sphere complexes on organic matter is indicated by the relative increase of second-neighbor contributions in the EXAFS RDFs after chemical treatments with 0.01 M CaCl2 and 0.1 M Na4P4O7. The occurrence of Zn/Al-HTLC is demonstrated by the persistence of a Zn-Zn pair correlation at 3.10 ± 0.04 Å (i.e., edge sharing ZnO6 octahedra in the trioctahedral layer structure) in EXAFS data of Na4P2O7 treated soil samples and its disappearance after treatment with 0.45 M HNO3. This latter treatment also revealed the occurrence of Znbearing phyllosilicate minerals, as shown by two Zn-Mg/Al/Si pair correlations at 3.05 ± 0.04 Å and 3.26 ± 0.04 Å, and of magnetite-franklinite solid solutions, as indicated by a Zn-Mn/Fe/Zn pair correlation at 3.50 ± 0.04 Å. Significant changes in the relative proportions of the different forms of Zn between the two soils explain their different responses to chemical treatments and emphasizes the relationships between solid state speciation and mobility of Zn in soils.

Structural chemistry of uranium associated with Si, Al, Fe gels in a granitic uranium mine

Abstract The structure of natural uranium-bearing, recently formed products arising from oxidative weathering of a U deposit (Peny, Massif Central, France) was studied to determine mechanisms of U-trapping by natural gels. Sampled waters are close to saturation with respect to amorphous silica and crystalline-hydrated U-hydroxides and silicates. All products are U-bearing Si/Al- and Fe-rich gels with minor Ca, Mg and water. Fourier Transform InfraRed spectrometry (FTIR) and X-ray absorption spectroscopy at Al–K, Fe–K and U–LIII edges show that these gels are composed of intimate mixtures of short-range ordered aluminosilicates and hydrous ferric oxides. In Si/Al-rich gels, Al is 6-fold coordinated to oxygens and Si tetrahedra are mostly isolated as in Al-rich allophane (Al:Si=2). Fe-rich gels exhibit a local structure dominated by edge-sharing octahedra, with d(Fe–Fe)=3.03 A, likely resulting from hydrolysis and oxidation from Fe2+ solution, and poisoning by Si/Al during Fe3+ precipitation. The local Al environment is close to that measured in Al-substituted goethite, and the Si tetrahedra are poorly polymerized as in natural Si-bearing ferrihydrite. The local structure around U was solved up to 3–4 A by Extended X-ray Absorption Fine Structure (EXAFS). Uranium is present in the hexavalent oxidized state, as uranyl complexes (UO22+) characterized by two axial oxygen atoms at d(U–Oax)=1.80 A and four equatorial oxygen atoms, at two distinct distances (d(U–Oeq1=2.33 A, d(U–Oeq2)=2.48 A). The collinear Oax–U–Oax geometry explains the presence of multiple scattering (MS) events in the EXAFS spectra of the U-bearing gels. In Si/Al-rich gels, U–U pairings at 3.82 A are consistent with polymers based on edge-sharing uranyls, in accordance to the speciation calculated in associated solutions. Alternatively to U–Oax MS, the contribution of Si neighbors at 3.3 and 3.7 A accounts for structural data beyond uranyl polyhedra. It is consistent with the local structure of uranophane, suggesting a coprecipitation process of U and Si. Elements such as Al may have poisoned crystal growth. In contrast, no U–Fe, U–Si/Al nor U–U contributions are evidenced in the Fe-rich gels. Only a weak contribution due to U–Oax MS is present. The geometry of Oeq coordination shell suggests that uranyl is complexed or sorbed onto mineral surfaces. It is proposed that U has been complexed by low Z elements like Si, Al in a first step, then trapped within hydrous ferric oxides during iron precipitation in a second step, i.e., during the final oxidation of the solutions inside the mine galleries.

27Al MAS NMR and Aluminum X-ray Absorption Near Edge Structure Study of Imogolite and Allophanes

This paper compares the results of 27Al nuclear magnetic resonance spectroscopy (NMR) and Al-K-edge X-ray Absorption Near Edge Structure (XANES) of natural imogolite and allophanes and some crystalline reference minerals. All soil allophanes studied contain 4-coordinated Al (AlIV). The highest relative proportion of AlIV, 21% of the total Al, was found in Si-rich allophane. This value is close to that found in spring allophanes, which were previously considered to be different from soil allophanes. For a quantitative determination of the AlIV/Altotal ratio, NMR is more reliable than XANES, because of the sensitivity of the chemical shift to low AlIV concentrations, but XANES may be used even if paramagnetic impurities (mostly Fe) are present. Al-K XANES also yields more information than NMR on the local environment of AlVI and especially site multiplicity. AlVI XANES of imogolite and allophanes are similar regardless of the Al/Si ratio. They yield two well-resolved resonances with maxima near 1568 and 1570 eV, which indicates the presence of a unique AlVI site by comparison with crystalline references. The presence of only one AlVI site indicates that imogolite and allophanes have an octahedral sheet with a structure similar to 2/1 dioctahedral phyllosilicates but different from gibbsite or kaolinite, previously considered as structural analogues. The 27AlIV MAS NMR peak maxima of allophanes are between 58.6 and 59.8 ppm, in the range observed for crystalline and amorphous framework alumino-silicates, and less positive than those of sheet silicates, which are typically in the range 65–75 ppm. 27Al-H1 CPMAS NMR spectra suggest that both AlIV and AlVI have Al-O-H linkages.

Chemical weathering of basaltic lava flows undergoing extreme climatic conditions: the water geochemistry record

This study was dedicated to the early stage of the weathering of historic basaltic flows located in Mount Cameroon. The combination of high relief (i.e. 0 to 4071 m) and high rainfall range (i.e. 1.8 to 12 m/year) lead to strong climatic contrast. Spring and rivers were sampled all around the volcano. We report here the basic chemistry of the waters as well as strontium and uranium isotopic ratios. The combination of the molar proportions of solute obtained with the modal amounts of the minerals in the basalts gives a prediction of what should be the relative molar concentrations of major compounds in the weathering waters issuing from Mount Cameroon. The measured Alkalinity/Si and Mg/Si ratios are higher than the calculated ones while the measured Ca/Si ratio is equal to the calculated value. We suggest that the Si-poor waters of Mount Cameroon are due to biological pumping, trapping of Si in Fe-silicate minerals such as Si containing ferrihydrite and Si interaction with bacterial cell wall leading to the formation of allophane type minerals which were observed in Mount Cameroon soil profiles. Calcium uptake by plants explains the lower Ca/alkalinity ratios measured in the water samples. The water–rock ratio (R) calculated from the strontium isotopic compositions of the water samples, ranges from 29,452 to 367,450. The calculated weathering rates (WR) range from 1 to 20 mm/ky and from 1 to 103 mm/ky for high and low elevations, respectively, and agree with both the thickness and the age of paleosoils found in the same area and with previously published estimates from coupled reaction-transport models. This difference emphasizes the role of vegetation and rainfall at lower elevations as compared to what happens at high elevations.

Crystal chemistry of hydrous iron silicate scale deposits at the Salton Sea geothermal field

The crystal chemistry of Fe-Si scales deposited from geothermal brines at Salton Sea, California, was studied by powder X-ray diffraction and spectroscopic techniques including infrared, 57Fe Mössbauer, 27 Al and 29Si nuclear magnetic resonance (NMR), and Fe and Si K-edge extended X-ray absorption fine structure (EXAFS). Scales precipitated at near 250°C from dissolved ferrous iron and silicic acid are composed of hisingerite. This phase is shown to possess the same local structure as nontronite and is a poorly-crystallized precursor of the ferric smectite. A clear distinction can be made at the local scale between hisingerite and 2-line ferrihydrite because, even in their most disordered states, the former possesses a two-dimensional and the latter a three-dimensional anionic framework. At temperature near 100°C Fe-Si scales are a mix of Al-containing opal and hydrous ferrous silicate, whose local structure resembles minnesotaite and greenalite. This hydrous ferrous silicate is very well ordered at the local scale with an average Fe coordination about Fe atoms of 6 ± 1. The difference in crystallinity between the ferrous and ferric silicate scales was related to variations of growth rates of clay particles precipitated from ferrous and ferric salt solutions. The low crystallinity of the ferric smectite suggests that the oxidation of ferrous iron occurs before polymerization with silica.

EXAFS evidence of sorbed arsenic (V) and pharmacosiderite in a soil overlying the Echassières geochemical anomaly, Allier, France

At Echassieres (Allier, France), arsenic speciation was determined in a soil developed over a micaschist where Hercynian hydrothermal mineralization, including arsenopyrite FeAsS and lollingite FeAs2, has lead to a regional As anomaly. The overlying soils which have developed from long term weathering exhibit As levels as high as 900 ppm in the richest area, where the saprolite contains up to 5200 ppm As. Analysis by powder XRD, [nu]-X-ray diffraction on the 20[nu]m scale, SEM-EDS and electron micro-probe analyses revealed that As, released from arsenopyrite and/or lollingite alteration, is concentrated in a secondary iron arsenate, pharmacosiderite, (Bax,K2-2x) (Fe,Al)4 (AsO4)3 (OH)5 · 6H2O. Quantitative mineralogical analysis by Rietveld refinement indicates that the proportion of As hosted by this mineral decreases systematically from the saprolite to the topsoil (from 70 % to 30 % of the total bulk As content, respectively). EXAFS spectroscopy indicates that the main form of the As occurring with pharmacosiderite, consists of As(V) ions sorbed on iron oxides. Sorption processes, which dominate As speciation in the topsoil horizons, appear as a key mechanism able to delay As dissemination from soils to plant and surface waters, provided that pH and Eh conditions remain sufficiently acidic and oxidizing, respectively.

Mineralogy of lead in a soil developed on a Pb-mineralized sandstone (Largentière, France)

Abstract Lead speciation was determined in a soil developed on a geochemical anomaly arising from a Pb-Zn stratabound deposit in Largentière (Ardèche, France). This geological setting offers the opportunity to determine the preferred form(s) of Pb following soil formation on this unique anomaly. In the soil profile studied, Pb concentrates in the B-horizon (2055 mg/kg Pb) relative to both the A- (1330 mg/kg Pb) and C- (1874 mg/kg Pb) horizons. Plumbogummite (PbAl3(PO4)2(OH)5·H2O) is the main host of Pb in the soil profile. Pb also appears to be associated with Mn-(hydr)oxides, as shown by micro-analyses (EMPA, SEM-EDS, and μ-SXRF), in the form of inner-sphere Pb2+ complexes, as suggested by Pb LIII-edge EXAFS spectroscopy. Linear least-squares fitting of background-subtracted, k3-weighted Pb LIII-edge EXAFS functions derived from bulk soil samples was carried out using Pb LIII-EXAFS spectra of 22 Pb-containing model compounds. Quantitative assessment of Pb speciation revealed that, whereas plumbogummite is the most abundant Pb phase in the soil profile, Pb2+-Mn-(hydr)oxide surface complexes are gradually replaced by Pb2+-surface complexes with other phases, possibly Pb2+-organic complexes, upward in the soil profile. The presence of large amounts of Pb-phosphate in the Largentière soil suggests that low solubility phosphates may be important long-term hosts of Pb in Pb-contaminated soils that have sufficiently high phosphorous activities to cause formation of these phases.

Shock recovery experiments on dolomite and thermodynamical calculations of impact induced decarbonation

We have studied experimentally shocked dolomites and calcites by scanning electron microscopy (SEM), analytical transmission electron microscopy (ATEM), X ray diffractometry (XRD) using Rietveld refinements, and mass spectrometry analysis of the abundance ratios of the stable isotopes of carbon and oxygen. The shock-recovery experiments have been perfomed by the multiple reverberation technique on natural dolomite rocks at 60 GPa, using steel devices and highexplosive driver-flyer plates as plane shock wave generators. Modified assemblies with grooves and holes were built in order to facilitate the escape of CO 2 , in case the conditions of breakdown of the carbonates into oxides and CO 2 would be reached during the shock or postshock history of the samples. In contrast with the results from previous studies, almost no evidence for outgassing, expressed, for example, by the identification of CaO or MgO, could be observed. Consequently, no isotope fractionation occured in the shocked samples. This result is consistent with the calculations of peak-shock and postshock temperatures, as well as with the examination of outgassing conditions of carbonates, calculated in this study up to 80 GPa. We have shown that in a direct shock, outgassing in air of nonporous dolomites and calcites should occur in impacts at 55-65 GPa and 35-45 GPa, respectively. The effect of porosity, which strongly lowers these values, has been estimated. The experimental setup for shock-recovery experiments is shown to be an important parameter : reverberated shocks lead to lower peak-shock and postshock temperatures than direct shocks at the same pressure ; differences among experimental setups might explain part of the discrepancies between previous studies. In this study, the only surviving shock-induced phenomenon is pulverization leading to grain sizes smaller than in the starting material. The decrease in grain size has been quantified via a structure refinement by Rietveld analysis of X ray powder patterns, which also allows the estimation of the lattice strains. A future pressure scale of shock effects in carbonates could probably be based on such parameters.