Peggy A. O'Day

Arsenic speciation in pyrite and secondary weathering phases, Mother Lode Gold District, Tuolumne County, California

Arsenian pyrite, formed during Cretaceous gold mineralization, is the primary source of As along the Melones fault zone in the southern Mother Lode Gold District of California. Mine tailings and associated weathering products from partially submerged inactive gold mines at Don Pedro Reservoir, on the Tuolumne River, contain approx. 20-1300 ppm As. The highest concentrations are in weathering crusts from the Clio mine and nearby outcrops which contain goethite or jarosite. As is concentrated up to 2150 ppm in the fine-grained (<63 mu-m) fraction of these Fe-rich weathering products. Individual pyrite grains in albite-chlorite schists of the Clio mine tailings contain an average of 1.2 wt. percent As. Pyrite grains are coarsely zoned, with local As concentrations ranging from approx. 0 to 5 wt. percent. Electron microprobe, transmission electron microscope, and extended X-ray absorption fine-structure spectroscopy (EXAFS) analyses indicate that As substitutes for S in pyrite and is not present as inclusions of arsenopyrite or other As-bearing phases. Comparison with simulated EXAFS spectra demonstrates that As atoms are locally clustered in the pyrite lattice and that the unit cell of arsenian pyrite is expanded by approx. 2.6 percent relative to pure pyrite. During weathering, clustered substitution of As into pyrite may be responsible for accelerating oxidation, hydrolysis, and dissolution of arsenian pyrite relative to pure pyrite in weathered tailings. Arsenic K-edge EXAFS analysis of the fine-grained Fe-rich weathering products are consistent with corner-sharing between As(V) tetrahedra and Fe(III)-octahedra. Determinations of nearest-neighbor distances and atomic identities, generated from least-squares fitting algorithms to spectral data, indicate that arsenate tetrahedra are sorbed on goethite mineral surfaces but substitute for SO4 in jarosite. Erosional transport of As-bearing goethite and jarosite to Don Pedro Reservoir increases the potential for As mobility and bioavailability by desorption or dissolution. Both the substrate minerals and dissolved As species are expected to respond to seasonal changes in lake chemistry caused by thermal stratification and turnover within the monomictic Don Pedro Reservoir. Arsenic is predicted to be most bioavailable and toxic in the reservoirs summer hypolimnion.

MOLECULAR STRUCTURE AND BINDING SITES OF COBALT(II) SURFACE COMPLEXES ON KAOLINITE FROM X-RAY ABSORPTION SPECTROSCOPY

X-ray absorption spectroscopy (XAS) was used to determine the local molecular environment of Co(II) surface complexes sorbed on three different kaolinites at ambient temperature and pressure in contact with an aqueous solution. Interatomic distances and types and numbers of backscattering atoms have been derived from analysis of the extended X-ray absorption fine structure (EXAFS). These data show that, at the lowest amounts of Co uptake on kaolinite (0.20–0.32 µmol m−2), Co is surrounded by ≈6 O atoms at 2.04–2.08 Å and a small number or Al or Si atoms (N = 0.6–1.5) at two distinct distances, 2.67–2.72 Å and 3.38–3.43 Å. These results indicate that Co bonds to the kaolinite surface as octahedrally coordinated, bidentate inner-sphere mononuclear complexes at low surface coverages, confirming indirect evidence from solution studies that a fraction of sorbed Co forms strongly bound complexes on kaolinite. In addition to inner-sphere complexes identified by EXAFS spectroscopy, solution studies provide evidence for the presence of weakly bound, outer-sphere Co complexes that cannot be detected directly by EXAFS. One orientation for inner-sphere complexes indicated by XAS is bidentate bonding of Co to oxygen atoms at two Al-O-Si edge sites or an Al-O-Si and Al-OH (inner hydroxyl) edge site, i.e., corner-sharing between Co octahedra and Al and Si polyhedra. At slightly higher surface sorption densities (0.51–0.57/ µmol m−2), the presence of a small number of second-neighbor Co atoms (average NCo < 1) at 3.10–3.13 Å indicates the formation of oxy- or hydroxy-bridged, multinuclear surface complexes in addition to mononuclear complexes. At these surface coverages, Co-Co and Co-Al/Si distances derived from EXAFS are consistent with edge-sharing between Co and Al octahedra on either edges or (001) faces of the aluminol sheet in kaolinite. Multinuclear complexes form on kaolinite at low surface sorption densities equivalent to < 5% coverage by a monolayer of oxygen-ligated Co octahedra over the N2-BET surface area. These spectroscopic results have several implications for macroscopic modeling of metal ion uptake on kaolinite: 1) Primary binding sites on the kaolinite surface at low uptake are edge, non-bridging Al-OH inner hydroxyl sites and edge Al-O-Si bridging oxygen sites, not Si-OH sites typically assumed in sorption models; 2) specific adsorption of Co is via bidentate, inner-sphere complexation; and 3) at slightly higher uptake but still a small fraction of monolayer coverage, formation of Co multinuclear complexes, primarily edge-sharing with Al-OH octahedra, begins to dominate sorption.

X-ray absorption spectroscopy of Co(II) sorption complexes on quartz (α-SiO2) and rutile (TiO2)

Abstract The local molecular structure of Co(II) surface complexes sorbed to two pure mineral substrates, quartz (α-SiO 2 ) and rutile (TiO 2 ), was examined with X-ray absorption spectroscopy (XAS) and extended X-ray absorption fine structure (EXAFS) analysis. Absorption spectra were collected for samples equilibrated with Co solutions under- and over-saturated with respect to solid Co-hydroxide phases (0.15–3.00 mM) at Co surface coverages of 0.63–9.51 μmol m −2 and for equilibration times of 23 h to 21 days. Quantitative analysis of the EXAFS spectra indicates no significant structural differences in the local atomic environment around Co sorbed to quartz, regardless of the surface coverage or the equilibration time. For all Co/quartz samples, the local Co environment is similar (but not identical) to that of Co in solid, crystalline cobalt hydroxide (Co(OH) 2 (S)), Cobalt is octahedrally coordinated by six O atoms at 2.06–2.11 A; second-neighbor Co backscatterers are found at 3.11–3.12 A, slightly contracted relative to the CoCo distance in Co(OH) 2 (s)(3.173 A). These results and the analysis of Co multiple-scattering suggest the formation of, primarily, large, multinuclear Co complexes or disordered hydroxide-like precipitates for Co sorbed to quartz. For Co sorption on rutile, the EXAFS spectra vary with changes in Co surface coverage and are distinctly different from Co/quartz spectra at all surface coverages examined. At low Co coverages, Co apparently sorbs directly to the rutile surface as mononuclear or small multinuclear complexes. This is indicated by two to three second-neighbor Ti atoms at two distinct distances, 2.95–2.99 A and 3.60–3.63 A, around sorbed Co. These CoTi distances are similar to TiTi distances in the bulk rutile structure. The small number of Ti backscatterers indicates that Co has not diffused into the rutile structure but rather that Co occupies Ti-equivalent sites at the rutile surface. For high surface coverage samples equilibrated for eleven days, the average number of Co second-neighbor backscattering atoms increases, indicating larger-sized multinuclear complexes, but the EXAFS spectra do not resemble those of either the Co/quartz sorption samples or Co(OH) 2 (s). Analysis of these high coverage Co/rutile samples indicates Ti backscatterers at distances near those expected for anatase, a TiO 2 polymorph, suggesting a local geometry for Co and Ti octahedra at the rutile/water interface similar to that of the cation octahedra in anatase. We suggest that differences in Co sorption on quartz and rutile may be attributed to the availability of reactive surface sites on quartz and rutile which are structurally favorable for the sorption of octahedrally-coordinated Co 2+ .

Speciation and fate of trace metals in estuarine sediments under reduced and oxidized conditions, Seaplane Lagoon, Alameda Naval Air Station (USA)

We have identified important chemical reactions that control the fate of metal-contaminated estuarine sediments if they are left undisturbed (in situ) or if they are dredged. We combined information on the molecular bonding of metals in solids from X-ray absorption spectroscopy (XAS) with thermodynamic and kinetic driving forces obtained from dissolved metal concentrations to deduce the dominant reactions under reduced and oxidized conditions. We evaluated the in situ geochemistry of metals (cadmium, chromium, iron, lead, manganese and zinc) as a function of sediment depth (to 100 cm) from a 60 year record of contamination at the Alameda Naval Air Station, California. Results from XAS and thermodynamic modeling of porewaters show that cadmium and most of the zinc form stable sulfide phases, and that lead and chromium are associated with stable carbonate, phosphate, phyllosilicate, or oxide minerals. Therefore, there is minimal risk associated with the release of these trace metals from the deeper sediments contaminated prior to the Clean Water Act (1975) as long as reducing conditions are maintained. Increased concentrations of dissolved metals with depth were indicative of the formation of metal HS- complexes. The sediments also contain zinc, chromium, and manganese associated with detrital iron-rich phyllosilicates and/or oxides. These phases are recalcitrant at near-neutral pH and do not undergo reductive dissolution within the 60 year depositional history of sediments at this site.The fate of these metals during dredging was evaluated by comparing in situ geochemistry with that of sediments oxidized by seawater in laboratory experiments. Cadmium and zinc pose the greatest hazard from dredging because their sulfides were highly reactive in seawater. However, their dissolved concentrations under oxic conditions were limited eventually by sorption to or co-precipitation with an iron (oxy)hydroxide. About 50% of the reacted CdS and 80% of the reacted ZnS were bonded to an oxide-substrate at the end of the 90-day oxidation experiment. Lead and chromium pose a minimal hazard from dredging because they are bonded to relatively insoluble carbonate, phosphate, phyllosilicate, or oxide minerals that are stable in seawater. These results point out the specific chemical behavior of individual metals in estuarine sediments, and the need for direct confirmation of metal speciation in order to constrain predictive models that realistically assess the fate of metals in urban harbors and coastal sediments.

Geochemical weathering increases lead bioaccessibility in semi-arid mine tailings.

Mine tailings can host elevated concentrations of toxic metal(loid)s that represent a significant hazard to surrounding communities and ecosystems. Eolian transport, capable of translocating small (micrometer-sized) particles, can be the dominant mechanism of toxic metal dispersion in arid or semiarid landscapes. Human exposure to metals can then occur via direct inhalation or ingestion of particulates. The fact that measured doses of total lead (Pb) in geomedia correlate poorly with blood Pb levels highlights a need to better resolve the precise distribution of molecularly speciated metal-bearing phases in the complex particle mixtures. Species distribution controls bioaccessibility, thereby directly impacting health risk. This study seeks to correlate Pb-containing particle size and mineral composition with lability and bioaccessibility in mine tailings subjected to weathering in a semiarid environment. We employed X-ray absorption spectroscopy (XAS) and X-ray fluorescence (XRF), coupled with sequential chemical extractions, to study Pb speciation in tailings from the semiarid Arizona Klondyke State Superfund Site. Representative samples ranging in pH from 2.6 to 5.4 were selected for in-depth study of Pb solid-phase speciation. The principle lead-bearing phase was plumbojarosite (PbFe(6)(SO(4))(4)(OH)(12)), but anglesite (PbSO(4)) and iron oxide-sorbed Pb were also observed. Anglesite, the most bioavailable mineral species of lead identified in this study, was enriched in surficial tailings samples, where Pb concentrations in the clay size fraction were 2-3 times higher by mass relative to bulk. A mobile and bioaccessible Pb phase accumulates in surficial tailings, with a corresponding increase in risk of human exposure to atmospheric particles.

Surface Complexation Model for Strontium Sorption to Amorphous Silica and Goethite

Strontium sorption to amorphous silica and goethite was measured as a function of pH and dissolved strontium and carbonate concentrations at 25°C. Strontium sorption gradually increases from 0 to 100% from pH 6 to 10 for both phases and requires multiple outer-sphere surface complexes to fit the data. All data are modeled using the triple layer model and the site-occupancy standard state; unless stated otherwise all strontium complexes are mononuclear. Strontium sorption to amorphous silica in the presence and absence of dissolved carbonate can be fit with tetradentate Sr2+ and SrOH+ complexes on the β-plane and a monodentate Sr2+complex on the diffuse plane to account for strontium sorption at low ionic strength. Strontium sorption to goethite in the absence of dissolved carbonate can be fit with monodentate and tetradentate SrOH+ complexes and a tetradentate binuclear Sr2+ species on the β-plane. The binuclear complex is needed to account for enhanced sorption at hgh strontium surface loadings. In the presence of dissolved carbonate additional monodentate Sr2+ and SrOH+ carbonate surface complexes on the β-plane are needed to fit strontium sorption to goethite. Modeling strontium sorption as outer-sphere complexes is consistent with quantitative analysis of extended X-ray absorption fine structure (EXAFS) on selected sorption samples that show a single first shell of oxygen atoms around strontium indicating hydrated surface complexes at the amorphous silica and goethite surfaces.Strontium surface complexation equilibrium constants determined in this study combined with other alkaline earth surface complexation constants are used to recalibrate a predictive model based on Born solvation and crystal-chemistry theory. The model is accurate to about 0.7 log K units. More studies are needed to determine the dependence of alkaline earth sorption on ionic strength and dissolved carbonate and sulfate concentrations for the development of a robust surface complexation database to estimate alkaline earth sorption in the environment.

Experimental abiotic synthesis of methanol in seafloor hydrothermal systems during diking events

Abstract The abiotic synthesis of organic compounds in seafloor hydrothermal systems is one mechanism through which the subsurface environment could be supplied with reduced carbon. A flow-through, fixed-bed laboratory reactor vessel, the Catalytic Reactor Vessel (CRV) system, has been developed to investigate mineral–surface promoted organic synthesis at temperatures up to 400°C and pressures up to 30 MPa, conditions relevant to seafloor hydrothermal systems. Here we present evidence that metastable methanol can be directly synthesized from a gas-rich CO 2 –H 2 –H 2 O mixture in the presence of a mineral substrate. Experiments have been performed without a substrate, with quartz, and with a mixture of quartz and magnetite. Temperatures and pressures in the experiments ranged from 200°C to 350°C and from 15 to 18 MPa, respectively. Maximum conversion of 5.8×10 −4 % CO 2 to CH 3 OH per hour was measured using a mixture of magnetite and quartz in the reactor. After passivation of the stainless steel reactor vessel, experiments demonstrate that methanol is formed at temperatures up to 350°C in the presence of magnetite, and that the formation rate decreases over time. The experiments also show a loss of surface reactivity at 310°C and a regeneration of surface reactivity with increased temperature up to 350°C. Concentrations of CO 2 and H 2 used in the experiments simulate periodic, localized and dynamic conditions occurring within the seafloor during and immediately following magmatic diking events. High concentrations of CO 2 and H 2 may be generated by dike injection accompanied by exsolution of CO 2 and reaction of dissolved H 2 O with FeO in the magma to form H 2 . The experiments described here examine how the ephemeral formation of an H 2 –CO 2 -rich vapor phase within seafloor hydrothermal systems may supply reactants for abiotic organic synthesis reactions. These experiments show that the presence of specific minerals can promote the abiotic synthesis of simple organic molecules from common inorganic reactants such H 2 O, CO 2 and H 2 under geologically realistic conditions.

A web-based library of XAFS data on model compounds.

An archive of XAFS data collected on standard model compounds of transition metals has been constructed and made available by the world wide web. The data in this library have all been taken in transmission on powder samples free of thickness effects. Data are stored in individual files in a standardized ASCII column format. Fields describing the contents of files can be searched easily. The data are intended to provide standards for comparison of EXAFS analysis procedures, for empirical analysis of XANES features, and to test theoretical XANES calculations. Emphasis has been placed on inorganic transition metal compounds to assist the analysis of environmentally relevant~samples.

Production of CO2 and H2 by Diking-Eruptive Events at Mid-Ocean Ridges: Implications for Abiotic Organic Synthesis and Global Geochemical Cycling

We have calculated the amounts of CO2 and H2 produced by complete degassing of mid-ocean ridge basalt (MORB) magma, and by degassing during transient diking-eruptive events. Our CO2 calculations are based on a model estimate of an initial CO2 content of 1800 ppm in MORB magma, which is equivalent to 2.2 × 1012 mol CO2 per year for magma production at worldwide ocean ridges. Observations indicate that many MORB magmas are emplaced in numerous small pulses of dikes and associated lava flows with very short emplacement times, which would result in release of relatively large amounts of CO2 over short intervals. For example, a dike injected into the oceanic crust that extends from the top of its magma chamber at 2 km depth to the seafloor would degas 2.3 × 104 mol CO2 per m2 surface area of dike, and produce another 4.0 × 104 mol CO2 per m2 on complete crystallization. Unlike CO2, which is not strictly governed by crystallization-alteration processes, H2 is produced from MORB by the reduction of H2O by ferrous iron in the magma to form magnetite and H2 as the magma cools and crystallizes. From published paired analyses of MORB glass and crystalline rock, we estimate that the amount of H2 produced from one cubic meter of rock averages 301 mol. We suggest that the oxidizing agent is H2O dissolved in the magma, which results in rapid generation of H2. The amount of pre-alteration oxidation may be limited by the amount of H2O dissolved in the magma; thus relatively water-rich magmas will undergo greater oxidation. For the case of the two-kilometer-high dike reaching the seafloor, the amount of H2 released is 6.2 × 105 moles H2 per m2 surface area of the dike. This is 10 times greater than the total CO2 released by degassing and crystallization of the dike. Assuming that the H2 generation rate for the entire basaltic layer of the oceanic crust is the same as for MORB lavas (312 mol/m3), then the annual global H2 production rate is 6.3 × 1012 mol H2 per year. This amount is about three times greater than our calculated annual CO2 production from MORBs. Given that the annual CO2 production rate from MORBs over 3.3 Ga can account for all CO2 found in the Earths crust, hydrosphere, and atmosphere, it is likely that the H2 produced at mid-ocean ridges plays a significant role as a reducing agent in the global redox state of the Earths surface. In contrast to time-averaged global production rates, the rapid release of CO2 and H2 in diking-eruptive events may locally result in formation of a separate gas phase containing H2-CO2-H2O in that order of abundance. The amounts of CO2 and H2 produced could provide a significant energy source for autotrophic microorganisms. It has been demonstrated that such a CO2-H2-H2O gas mixture yields methanol in magnetite-surface catalyzed reactions at seafloor hydrothermal conditions. Such abiotic synthesis reactions could have been important in early Earth processes.

Changes in zinc speciation with mine tailings acidification in a semiarid weathering environment.

High concentrations of residual metal contaminants in mine tailings can be transported easily by wind and water, particularly when tailings remain unvegetated for decades following mining cessation, as is the case in semiarid landscapes. Understanding the speciation and mobility of contaminant metal(loid)s, particularly in surficial tailings, is essential to controlling their phytotoxicities and to revegetating impacted sites. In prior work, we showed that surficial tailings samples from the Klondyke State Superfund Site (AZ, USA), ranging in pH from 5.4 to 2.6, represent a weathering series, with acidification resulting from sulfide mineral oxidation, long-term Fe hydrolysis, and a concurrent decrease in total (6000 to 450 mg kg(-1)) and plant-available (590 to 75 mg kg(-1)) Zn due to leaching losses and changes in Zn speciation. Here, we used bulk and microfocused Zn K-edge X-ray absorption spectroscopy (XAS) data and a six-step sequential extraction procedure to determine tailings solid phase Zn speciation. Bulk sample spectra were fit by linear combination using three references: Zn-rich phyllosilicate (Zn(0.8)talc), Zn sorbed to ferrihydrite (Zn(adsFeOx)), and zinc sulfate (ZnSO(4) · 7H(2)O). Analyses indicate that Zn sorbed in tetrahedral coordination to poorly crystalline Fe and Mn (oxyhydr)oxides decreases with acidification in the weathering sequence, whereas octahedral zinc in sulfate minerals and crystalline Fe oxides undergoes a relative accumulation. Microscale analyses identified hetaerolite (ZnMn(2)O(4)), hemimorphite (Zn(4)Si(2)O(7)(OH)(2) · H(2)O) and sphalerite (ZnS) as minor phases. Bulk and microfocused spectroscopy complement the chemical extraction results and highlight the importance of using a multimethod approach to interrogate complex tailings systems.