Andrea L Foster PhD US Geological Survey GMEG Menlo Park CA Arsenic is an element of concern in mined gold deposits around the world Don Pedro HarvardJamestown KellyRand AuAg Ruth Mine Cu ID: 1022927
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1. Identification and Quantification of Arsenic Species in Gold Mine Wastes Using Synchrotron-Based X-ray TechniquesAndrea L. Foster, PhDU.S. Geological Survey GMEGMenlo Park, CA
2. Arsenic is an element of concern in mined gold deposits around the worldDon Pedro Harvard/JamestownKelly/Rand (Au/Ag)Ruth Mine (Cu)Spenceville (Cu-Au-Ag)Lava Cap (Nevada)Empire Mine (Nevada) low-sulfide, qtz AuArgonaut Mine (Au)Ketza River (Au)sulfide and oxide ore bodiesGoldenville, Caribou, and Montague (Au)
3. The common arsenic-rich particles in hard-rock gold mines have long been knownArsenopyrite FeAsSPrimarySecondary Scorodite FeAsO4 2H2OKankite : FeAsO4•3.5H2OJarosite KFe3(SO4)2(OH)6Tooleite [Fe6(AsO3)4(SO4)(OH)4•4H2OPharmacosiderite KFe4(AsO4)3(OH)4•6–7H2OSecondary/TertiaryIron oxyhydroxide (“rust”) containing arsenic up to 20 wt%arseniden = 1-3n-Arseniosiderite Ca2Fe3(AsO4)3O2·3H2OYukonite Ca7Fe12(AsO4)10(OH)20•15H2O“arsenian” pyriteAs-1 pyrite Fe(As,S)2Reich and Becker (2006): maximum of 6% As-1
4. But it is still difficult to predict with an acceptable degree of uncertainty which forms will be present thermodynamic data lacking or unreliable for many important phases kinetic barriers to equilibrium changing geochemical conditions (tailings management)Langmuir et al. (2006) GCA v70
5. Lava Cap Mine Superfund Site, Nevada Cty, CA
6. Typical exposure pathways at arsenic-contaminated sites are linked to particles and their dissolution in aqueous fluidsingestion of arsenic-bearing watersolid-phase arsenicnear-neutral, low dissolved organic carbon, low salinity watersinhalation or ingestion of arsenic-rich particlessolid-phase arseniclung and gastic/intestinal tract fluids (saline, aqueous solutions-high dissolved organic carbon, enzymes, bile, some low pH, most near-neutral) critical to know the form(s) of arsenic in the solid phase only a subset of those forms may be reactivedissolutiondissolution
7. This talk will review the results of synchrotron X-ray studies of arsenic speciation, with focus on gold mine wastes Brilliant, high flux radiation produced at points tangent toa circular orbit of electrons moving at near-relativistic speeds75 synchrotrons around the world10 in US (4 suitable for environmental work)StanfordBerkeley
8. Large ( 1-10 mm) or small (2 -150 mm) X-ray beams are available for most techniquesBulkMicrobeamPrecision stage(micron)CCD detector(diffraction)Solid-state detectorGas ionization detector X-ray Fluorescence X-ray Absorption Spectroscopy X-ray Diffraction X-ray Photoelectron Spectroscopy Vibrational Spectroscopies Magnetic spectroscopy Small Angle ScatteringCCD Detector
9. Synchrotron X-ray Fluorescence (XRF) SpectrometryBulkMicrobeamOne EDS spectrumPer point (> 1000)Elemental IDElement CorrelationSpatial distributionAsCaFe1800 micronsElemental ID One average SpectrumCaFeAsVoltage (energy)
10. Synchrotron-based X-ray Diffraction (SXRD)2-D patterngoethite/lepidocrocitemixtureSynchrotron XRD20 min1-D patternsConventional XRD8 hours Mineral ID Mineral Mapping Amorphous materials (scattering)
11. X-ray Absorption Fine Structure SpectroscopyX-ray Absorption Near Edge Structure (XANES)Oxidation state, species “fingerprinting”AbsorbanceEnergy (eV)> 1 mg/kgEXAFS function c(k)Photoelectron Wave vector k (Å-1)Extended X-ray Absorption Fine Structure (EXAFS)Speciation = identity, #, distance of neighboring atoms> 75 mg/kg
12. Spectral Deconvolution by Least-Squares FitsOrpiment (As3+-S)As5+ on rust (iron oxyhydroxide)As3+ in water (As3+-O)13.5 % As5+86.5% As3+Bangladesh Aquifer Sediment10-10.4 meters depth As3+As5+Unique spectral shape Energy increases with oxidation stateArsenopyrite (As1-) Energy, (electron volts)11860118801190011920XANES spectra
13. Principal Component Analysis of XANES or EXAFS SpectraEnergy (eV)Component Intensity1234567nEnergySet of Unknown XANES SpectraPCAPrincipal Components(= number of species)Subset “best” model compounds For LSAPC 2 (v2*w2,i) 22% of variancePC 1 (v1*w1,i) 35% of varianceVariance PlotN = 19Energy (eV)Set of Model Compound XANES SpectraTarget TransformationUsing principal componentsN = 30
14. Synchrotron studies of As in Gold Mine WastesEarly Days: 1994-2002individual field and lab projects at “targets of opportunity, typically with limited connection to regulators’ needsBulk XANES + EXAFS2003- presentcollaborative projects at high-profile sites; research focused on addressing needsMicrobeam studies: 2005 and laterCoupled XAFS, XRD XRFCoupled bulk and microbeamMulti-metal XAFSComplimentary Lab-based techniquesMicro Raman, XRD
15. As L50 mFe KEnergy (eV)118501195011900As5+As-Fe = 3.25 ÅFeFeAsAsAlAlAs-Al 3.16 ÅRuth Mine: Ballarat District (Trona, CA)Tailings (ca 1000 mg/kg As) used for residential landscapingFoster et al., (1998) American Mineralogist 83, 553-568D. Lawler, BLM Ruth MineTrona, CA
16. Mesa De Oro: Should be “Mesa de Arsenico”http://www.pbs.org/newshour/bb/environment/superfund_4-16.html (transcript of 1996 show called “Paying for the Past”) Gold tailings with 115 – 1320 mg/kg arsenic 40 homes developed on Mesa between 1975-1985 EPA emergency response halted new home construction removed and replaced about 1 foot of soil shored up sides of MesaGeorge Wheeldon, “geologist”: arsenic from the mine is in a form that is not dangerousTime Magazine Sept 25, 2000San Jose Mercury NewsResidents won 2,000,000 for loss of property valuehttp://consumerlawpage.com/article/environmental_pollution_1.shtml
17. XANES spectra of Mesa de Oro soil samples demonstrate that arsenic is not in arsenopyrite formArsenopyrite FeAsSAs5+ on rust (iron oxyhydroxide)Arsenopyrite (As1-) Energy (KeV)11.8611.8811.9011.92Arsenic (V) on RustRange of Mesa de OroSamples (n =4)Mr Wheeldon’s Error: assuming that arsenic stays in original formA. Foster, R. Ashley, and J. Rytuba, USGS: unpublished data
18. Lava Cap Mine Superfund Site, Nevada Cty, CA
19. Arsenic species in mine-impacted sediment from the Lava Cap MineFoster et al., (2010) Geochemical Transactionspyritearsenopyritepronounced oxidationminimal oxidation-0.6-0.4-0.200.20.40.60.8-0.7-0.6-0.5-0.4-0.3-0.2-0.10PC 2 (v2*w2,i) 22% of variancePC 1 (v1*w1,i) 35% of variancesubareal tailings 30% FeAsSPyrite-rich ore69% Fe(As,S)2 65% FeAsSArsenopyrite-rich ore(FeAsS)submergedtailingstypical ore2468101214subareal tailingsphotoelectron wave vector k (Å-1)k3-weighted EXAFSOur first studies at Lava Cap showed that submerged tailings from the private lake contain As in its original forms. Tailings exposed to air after the burst of a log dam in 1998 have oxidized considerably.
20. Kelly/Rand Mines: Ultra-high arsenic in mine tailings gold-silver mines operated until 1947 approximate volume: 100,000 tons breach of tailings levee; migration of tailings into residences in Randsburg 3000->10,000 mg/kg As remote, Federal Land (BLM), popular with OHVersKim, C.S., Wilson, K.M., and Rytuba, J.J. (2011) Particle-size dependence on metal distributions in mine wastes: implications for water contamination and human exposure. Applied Geochemistry 26, 484-495.
21. Secondary arsenates and As5+-rich sulfate phases predominate in Kelly/Rand tailings020406080100120“background” soilFeAsO4 2H2Oam-FeAsO4Ca2Fe3(AsO4)3O2 3H2OKFe3[(S,As)O4)](OH)6As-FeOOHSecondary crustsTailingsLow-gradeOreLinear Combination fits to EXAFS no evidence for primary sulfide phases (below detection? Oxidized?) solubility and kinetics of dissolution of precipitates is expected to be very different than that of arsenic on ferric oxyhydroxideKim, C.S., Wilson, K.M., and Rytuba, J.J. (2011) Particle-size dependence on metal distributions in mine wastes: implications for water contamination and human exposure. Applied Geochemistry 26, 484-495.
22. Lungs of tortoises collected near mines contain particles similar to those found in mine tailings687 lung tissue11860118801190011920scoroditejarositeSpot 2Spot 14.6 mm3.5 mmAs12A. Foster, unpublished data
23. Ultra-high As gold mines in Nova Scotia, CanadaWalker et al (2009) Canadian Mineralogist v 47
24. Current and Future directions in synchrotron-based arsenic researchValidated method for As bioaccessibility testCoupled to geochemistry, As speciationBioreactors (anaerobic and aerobic)Aerobic: naturally-occurring microbial consortia?Anaerobic: relative bioaccessibility of As in neo-sulfides vs. organic compounds (biomass)PlantsFinding more accumulators, maximizing uptakeCoupling genetics, protein expression, and location of metal (As) sequestration
25. Arsenic Relative Bioavailability Project (Empire Mine State Historic Park)N =2525ChemistryX-ray diffractionElectron ProbeQEMSCANParticle size analysis“Bulk” Synchrotron studies“Microfocused” Synchrotron studiesIn vitroIn vivoModel of mineralogical control on As bioaccessibilityCorrelation with easily-measured parameterHelping to find a cost-effective means of evaluating the potential for re-development of mined lands contaminated with arsenic% Bioaccessible% BioavailableThis study is conducted through a proposal by CA Department of Toxic Substances Control and USGS that was funded by US-EPA (Brownfields Program)
26. Principal Component Analysis predicts 4-5 unique arsenic species Component 1Component 2Component 1Component 2N =19N =18Outlier removedVariance can be visualized on additional Component axes (3, 4, 5)
27. Bioaccessibility and Bioavailability have similar trends with key arsenic species% relative bioavailabilityCa-Fe-ArsenatePyrite/ArsenopyriteSEE MORE AT ALPERS TALK TOMORROW SESSION 10
28. Arsenic in roasted ore treated by an anerobic biochemical reactorMaghemite-richMore As3+Hematite-richLess As3+Paktunc et al. (2008) Proceedings of the 9th Intl Conference for Appl. MineralogyAnerobic Reactor SamplesMostly As3+, but organic NOT SULFIDE (??)As3+SCH3CH3
29. New and improved pyrite: now with As3+Deditius et al (2008)Yanacocha , PeruShould be present in other low-sulfidation epithermal deposits: Nevada Au?(Fe,Au)(As,S)2
30. Arsenic attenuation by naturally-occurring microbial consortia: Lava Cap Mine (NPL), CA050001000015000200002500030000Arsenic (mg/kg) LL1 LL10 LL1adj LL2sed LL2 Fe floc algaeLL2LL1LL1adjLL10Foster et al, USGS Open File Report 2009-1268LL1LL10LL2 algaeLL2 fe flocLL1 adjLL2 sedimentBiogenic Fe-(hydroxide) accumulates arsenic to levels several times to orders of magnitude greater than the original mine tailngs (yellow horizontal line)
31. Monitoring the performance of a natural passive bioreactorLL1 LL100102030405060708090100Jun 06Oct 06Nov 06Feb 07Mar 07Aug 07Oct 07Mar 08AsFeMnAs: 12-30 mg/l 10 mg/lMn: 235-2000 mg/l 300 mg/lEPA cleanup goal:% removed between LL1 and LL10Concentration range at LL10
32. Arsenic is associated with Fe Oxyhydroxides rather than with biological materials (contrast the anerobic treatment of Paktunc)-0.100.160.110.4699AF-wLL500AF-LCD1a post-rinse00AF-LCD1av2*w2,iv1*w1,iAs(V)-FeOOHdominant1000xEPA region 9 Superfund has a pilot aerobic /anerobic treatment in place at the adit, but is not supplanting it with the native microorganisms
33. As Speciation in Pteris vittata Hyperaccumulating FernWebb et al (2003) ES&TPickering et al. Environ. Sci. Technol., 40 (2006) 5010-5014.
34. Synchrotron techniques have had great utility in the study of arsenic speciation in gold mines..there is more to come!Arsenopyrite Primary (ore, concentrates)Pyriten = -1, +3n3+5+OSFeAs-pooracidicNear-neutralCa minerals+ CaFeOOHCa-Fe arsenates As-richFe arsenates Fe sulfates
35. The EndLeptothrix ochracea from the Lava Cap Mine