Applied Geochemistry 27 (2012) 2289–2299

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Applied Geochemistry

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Arsenic attenuation in tailings at a former Cu–W–As mine, SW Finland

Annika Parviainen a,⇑, Matthew B.J. Lindsay b,1, Rafael Pérez-López c, Blair D. Gibson b, Carol J. Ptacek b,David W. Blowes b, Kirsti Loukola-Ruskeeniemi d

a Aalto University School of Engineering, Department of Civil and Environmental Engineering, FI-00076 Aalto, Finlandb University of Waterloo, Department of Earth and Environmental Sciences, Waterloo, ON, Canada N2L 3G1c University of Huelva, Department of Geology, Campus ‘El Carmen’, 21071 Huelva, Spaind Geological Survey of Finland, FI-02151 Espoo, Finland

a r t i c l e i n f o

Article history:Received 21 December 2011Accepted 27 July 2012Available online 6 August 2012Editorial handling by K. Savage

0883-2927/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.apgeochem.2012.07.022

⇑ Corresponding author. Present address: UniversitMineralogy and Petrology, Avda. Fuente Nueva s/n, 18958243368.

E-mail address: annika.parviainen@aalto.fi (A. Parv1 Present address: University of British Columbia, De

Sciences, Vancouver, BC, Canada V6T 1Z4.

a b s t r a c t

Nearly half a century after mine closure, release of As from the Ylöjärvi Cu–W–As mine tailings in ground-water and surface water run-off was observed. Investigations by scanning electron microscopy (SEM),electron microprobe analysis (EMPA), synchrotron-based micro-X-ray diffraction (l-XRD), micro-X-rayabsorption near edge structure (l-XANES) and micro-extended X-ray absorption fine structure (l-EXAFS)spectroscopy, and a sequential extraction procedure were performed to assess As attenuation mecha-nisms in the vadose zone of this tailings deposit. Results of SEM, EMPA, and sequential extractions indi-cated that the precipitation of As bearing Fe(III) (oxy)hydroxides (up to 18.4 wt.% As2O5) and Fe(III)arsenates were important secondary controls on As mobility. The l-XRD, l-XANES and l-EXAFS analysessuggested that these phases correspond to poorly crystalline and disordered As-bearing precipitates,including arsenical ferrihydrite, scorodite, kankite, and hydrous ferric arsenate (HFA). The pH within200 cm of the tailings surface averaged 5.7, conditions which favor the precipitation of ferrihydrite.Poorly crystalline Fe(III) arsenates are potentially unstable over time, and their transformation to ferrihy-drite, which contributes to As uptake, has potential to increase the As adsorption capacity of the tailings.Arsenic mobility in tailings pore water at the Ylöjärvi mine will depend on continued arsenopyrite oxi-dation, dissolution or transformation of secondary Fe(III) arsenates, and the As adsorption capacity ofFe(III) (oxy)hydroxides within this tailings deposit.

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1. Introduction

Traditional mining practices employed prior to the implemen-tation of rigorous environmental legislation often included thedeposition of uncontained mine wastes on the land surface andin water bodies. Inadequate segregation of mill tailings or wasterock from surrounding ecosystems can result in severe degradationof water and soil quality over large areas (Salzsauler et al., 2005;Moncur et al., 2006; Nieto et al., 2007; Simpson et al., 2011).Fine-grained tailings containing primary sulfide minerals and sulf-arsenides tend to oxidize under atmospheric conditions releasingAs, trace metals and SO4 to the pore water (Smedley and Kinni-burgh, 2002; Blowes et al., 2003). The sulfide oxidation can bemediated by microbial reactions enhancing this process (Gouldand Kapoor, 2003).

ll rights reserved.

y of Granada, Department of002 Granada, Spain. Fax: +34

iainen).partment of Earth and Ocean

Precipitation of Fe(III) arsenates of varying Fe/As molar ratiosoccurs in tailings piles under a wide range of geochemical condi-tions, acting as a natural As attenuation mechanism. However,the stability of these secondary As-bearing phases is debatable(Paktunc et al., 2008). Iron(III) arsenates exhibiting high Fe/As ratio(>4) are generally considered stable at neutral pH, whereas thosewith low Fe/As ratios or low crystallinity are less stable (Krauseand Ettel, 1989). The bioaccessibility of As solids varies dependingon the speciation; As(V) and As(III) (e.g. Ca ferric arsenate, arseno-lite [As2O3], amorphous ferric arsenates, As-bearing ferric(oxy)hydroxides in decreasing order) are more bioaccessible thanAs(0) and As (1-) (e.g. As-rich pyrite [FeS2], arsenopyrite [FeAsS])which makes the characterization of these phases important (Meu-nier et al., 2010; Plumlee and Morman, 2011). However, crystallinescorodite [FeAsO4�2H2O] is less bioaccessible (Meunier et al., 2010).Additionally, the mobility and toxicity of dissolved As variesdepending on the aqueous speciation. Out of the most commoninorganic aqueous As species, arsenite (generally as H3AsO0

3 underreducing conditions) is more toxic, soluble and poorly adsorbedthan arsenate ðH2AsO�4 Þ under acidic conditions. Therefore, inevaluating the potential stability and bioaccessibility of As inmine wastes, it is important to examine mineralogical occurrences

Fig. 1. Map of the Ylöjärvi Cu–W–As mine area.

2290 A. Parviainen et al. / Applied Geochemistry 27 (2012) 2289–2299

and chemical properties, such as speciation and bondingcharacteristics.

Secondary Fe(III) (oxy)hydroxide or arsenate phases are com-monly very fine, lm-scale precipitates that may contain water.Characterization of these precipitates by traditional mineralogicaltechniques including scanning electron microscopy–energy disper-sive spectrometry (SEM–EDS), electron microprobe analysis(EMPA) and bulk powder X-ray diffraction (XRD) can prove chal-lenging. Combining these methods with synchrotron-based mi-cro-analytical techniques can provide additional information onthe potential stability of As in mine wastes. Previous studies havesuccessfully employed synchrotron-based techniques to examinechemical speciation, bonding characteristics and secondary miner-alogy. These studies have employed micro-X-ray absorption spec-troscopy (l-XAS) (Paktunc et al., 2003, 2004; Beauchemin andKwong, 2006; Kwong et al., 2007; Slowey et al., 2007; Mitsunobuet al., 2008; Chen et al., 2009), micro-X-ray diffraction (l-XRD)(Flemming et al., 2005; DeSisto et al., 2011), and combinations ofboth these methods (Walker et al., 2005, 2009; Endo et al., 2008;Corriveau et al., 2011). Additionally, micro-X-ray fluorescence (l-XRF) has been employed to examine elemental distribution (Endoet al., 2008; Walker et al., 2009).

This study focused on examining As attenuation mechanisms atthe decommissioned Ylöjärvi Cu–W–As mine in SW Finland(Fig. 1). The occurrence and distribution of solid-phase As wasexamined by integrating SEM and EMPA analyses with l-XRF map-ping, l-XAS and l-XRD as well as a sequential extraction method.

It is critical to distinguish whether solid phase As is present as anadsorbed species, as a co-precipitate (incorporated into a mineralstructure during its formation), or as discrete As-bearing phases,because of inherent differences in biogeochemical behavior. Previ-ous studies at the Ylöjärvi mine reported mobilization of As andtrace metals from the uncontained tailings pile to adjacent surfacewater and groundwater bodies (Carlson et al., 2002; Kumpulainenet al., 2007; Parviainen et al., 2012). Arsenic concentrations at thesite have approached 2.7 mg L�1 in surface water and 14 mg L�1 ingroundwater. The data presented in this study are central forassessing mechanisms controlling As mobility within mine wastedeposits.

2. Site description

The Ylöjärvi deposit is hosted by a tourmaline breccia, and thesurrounding rocks are mainly tuffites with porphyrite intercala-tions (Himmi et al., 1979). The primary ore minerals in the brecciamatrix were pyrrhotite [Fe(1�x)S], chalcopyrite [CuFeS2], arsenopy-rite, and scheelite [CaWO4], the minor sulfides pyrite [FeS2], sphal-erite [ZnS], galena [PbS], cubanite [CuFe2S3], molybdenite [MoS2],the oxides (magnetite [Fe3O4] ilmenite [FeTiO3], cassiterite[SnO2]), and minute concentrations of native Bi, Ag and Au are alsofound (Himmi et al., 1979). The gangue minerals consist of tourma-line (13–16% of the mill feed), quartz, chlorite (clinochlore), biotite,epidote, plagioclase and hornblende. Sulfur content in the mill feedaverages 1.34%, whereas carbonates (siderite [FeCO3] and ankerite

A. Parviainen et al. / Applied Geochemistry 27 (2012) 2289–2299 2291

[Ca(Fe,Mg,Mn)(CO3)2]) in vein structures are a minor constituentof the mineralization (Himmi et al., 1979).

The Ylöjärvi Cu–W–As mine operated from 1943 to 1966; how-ever, due to changing demand, WO3 concentrate (2020 t) wasrecovered only from 1948 to 1961 and As concentrate (563 t As)solely from 1949 to 1953 (Puustinen, 2003). The mine produceda total of 4 Mt of tailings which were deposited in two tailingsareas measuring 4 ha and 17 ha. Tailings were also deposited inunderground galleries during the final years of operation. The openpit, underground galleries and the smaller tailings area (4 ha) aresituated in the southern end of lake Parosjärvi, and at the time ofthis study were partially submerged (Fig. 1). The larger tailingsarea (17 ha) is situated south of the mine and was operated begin-ning from 1953 receiving mine tailings that contained the unrecov-ered arsenopyrite. According to Himmi et al. (1979), the mill feed(2.51 Mt) contained 4600 mg kg�1 of As on average during the last7 a of production. Hence, roughly 11,500 t of As was stored in thearea during this time. The total amount of As in the tailings is likelymuch higher considering that arsenopyrite was discarded over alonger period of time. The tailings overlay glacial till and peatbog, and are elevated having an average thickness of 9 m, the pri-mary zone being located below the water table approximately at3 m from the tailings surface at the time of this study.

3. Materials and methods

3.1. Sample collection

Twelve samples of undisturbed tailings were collected from thewalls of excavated test pits at two locations in the 17 ha tailingsarea (Fig. 1). In each excavation profile, the samples were collectedinto 15 cm long (5 cm Ø) glass fiber tubes at approximately 50 cmintervals from the surface to a total depth of 250 cm. The sampleswere immediately frozen and transported to the laboratory, wherethey were thawed at room temperature and dried at 40 �C in anoven.

3.2. Solid-phase analyses

A total of 27 tailings samples were set in epoxy resin and pre-pared for mineralogical analysis using standard techniques. Waterwas used only for preliminary cutting of the samples and not forgrinding or polishing of the exposed sample surface. The sampleswere studied with an optical microscope under reflected-light,SEM–EDS, EMPA, l-XRF mapping, and l-XAS and l-XRD tech-niques. Twelve tailings samples from all sampling depths werecharacterized using sequential extractions for elemental distribu-tion. Grain-size distributions were determined by laser diffraction(Mastersizer 2000; Malvern Instruments Ltd., UK) and specific sur-face areas were measured using the BET N2(g) absorption method.

3.2.1. Scanning electron microscopy and electron microprobe analysisThe SEM–EDS (JEOL JSM 5900 LV with Oxford INCA EDS) at the

Geological Survey of Finland was used for a preliminary study ofmineral compositions and to obtain back-scattered images on thetextural relationships of the altered sulfide minerals and secondaryminerals. The EMPA were performed by the wavelength dispersivetechnique using a Cameca SX100 electron probe micro analyzer atthe Geological Survey of Finland. Accelerating voltage and thebeam current were set to 20 kV and 10 nA, respectively. Countingtimes were 10 s on LiF, PET and TAP, and beam size was 3 lm.The X-ray fluorescence lines used were Si Ka, Al Ka, Se La, MgKa, As Ka, Co Ka, Fe Ka, Ni Ka, Mn Ka, Ti Ka, O Ka, Ca Ka, K Ka,S Ka, and natural minerals and metals were employed as stan-

dards. Analytical results were corrected using the PAP on-line cor-rection program (Pouchou and Pichoir, 1986).

3.2.2. Synchrotron-based techniquesSynchrotron-based l-XRF, l-XAS and l-XRD were performed

on the GeoSoilEnviroCARS beamline 13-BM-D at Argonne NationalLaboratory (Argonne, IL, USA) in July 2009 and April 2010. The stor-age ring was operated in continuous top-up mode at a current ofapproximately 102 mA during data collection. The incident X-raybeam was tuned using a Si(111) monochrometer and a 13-elementGe detector (Canberra Industries Inc., USA), and positioned orthog-onally to the incident beam for collection of fluorescence data. Ionchambers positioned in a differential, linear configuration wereused for collection of transmission data. All data was collected un-der ambient atmospheric and temperature conditions.

Three samples, collected from depths of 110 cm, 150 cm and230 cm for profile 2 (Fig. 1) were selected for l-XRF mapping,and l-XAS and l-XRD analysis. A low-speed precision sectioningsaw was used to cut 100 lm thick sections from epoxy samplespreviously analyzed by SEM–EDS and EMPA. One side of theseoff-cuts was adhered to polyimide tape (Kapton� tape; 3 M,USA), which was subsequently adhered to a glass slide with dou-ble-sided tape (Scotch� tape; 3 M, USA). The exposed side of thesample was manually polished to a thickness of �50 lm on a glassplate using powdered alumina. All cutting and polishing were per-formed in the absence of water.

Reference materials for l-XAS analysis included As2O3(s) andAs2O5(s) (Sigma–Aldrich, Canada) and the mineral phases arseno-pyrite, scorodite and kankite [FeAsO4�3.5H2O] (Excalibur MineralCorp., USA). A reference material for As(III) adsorbed onto Fe(III)(oxy)hydroxide (As3-Fh) was prepared by suspending two-line fer-rihydrite [Fe2O3�0.5H2O] (Schwertmann and Cornell, 2000) in a0.1 M NaNO3 solution containing 0.01 M of As(III) from NaAsO2(s)

(Sigma–Aldrich, Canada). This solution was buffered to an initialpH of 6, and the suspension was stirred at 22 �C. After 24 h, the sus-pension was vacuum filtered (0.2 lm) and retained solids werefreeze dried. All reference materials were ground using an acid-washed agate mortar and pestle. Resulting powders were spreadonto polyethylene terephthalate (PET) tape (Scotch� Magic™ Tape;3 M, USA), which were layered to achieve a total thickness of 300–500 lm. Additionally, arseniosiderite [Ca2Fe3(AsO4)3�3H2O] dilutedin boron nitride and mounted in a Teflon� sample holder betweentwo layers of Kapton� tape was contributed by Dr. D. Paktunc(Paktunc et al., 2004). The crystallography of mineral standardswas confirmed by powder XRD by the suppliers; however, subse-quent l-XRD analysis indicated that the kankite reference waspoorly crystalline and contained scorodite as an impurity.

High-resolution two-dimensional l-XRF maps were collected toexamine As and Fe distribution, and to identify locations for collec-tion of As K-edge l-XAS spectra. Elemental mapping was per-formed at 16 keV using a focused beam, which measuredapproximately 30 lm (horizontal) � 10 lm (vertical). Maps werecollected using a spatial resolution of 10 lm over1000 � 1000 lm regions of the samples. Fluorescence data wascollected at the As Ka and Fe Ka lines for 1 s at each location.

Characterization of As oxidation states and bonding was per-formed by l-XAS for the X-ray absorption near edge structure(XANES) and extended X-ray absorption fine structure (EXAFS) re-gimes of the As K-edge. Replicate spectra (n = 2 or 3) for referencematerials were collected in transmission mode using a defocusedbeam. Spectra for arsenopyrite were collected at regular intervalsto monitor shifts in incident beam energy over time. Spectra fortailings samples were collected in replicate (n = 5) using the fo-cused beam. An additional As K-edge EXAFS spectra for arsenicalferrihydrite (As5-Fh) was provided by Dr. D. Paktunc (Paktuncet al., 2008). Data reduction and analysis was performed using

2292 A. Parviainen et al. / Applied Geochemistry 27 (2012) 2289–2299

ATHENA, which is a component of the XAS data analysis softwarepackage IFFEFIT (Ravel and Newville, 2005). Replicate l-XAS spec-tra were averaged, and resulting spectra were subsequently edge-step normalized and calibrated using the DE between measuredand theoretical As K-edge positions for arsenopyrite(E0 = 11,867 eV). Least-squares linear combination fitting (LCF) ofthe XANES spectra was performed over an energy range of �20to +40 eV relative to 11,867 eV. Arsenic K-edge energies for refer-ence materials were fixed during LCF analysis.

Fitting of the EXAFS spectra was performed using ARTEMIS, an-other component of the IFFEFIT software package. Scattering pathsfrom crystal structure data for scorodite published by Kitahamaet al. (1975) were used to fit experimental data. Fourier-transfor-mation of l-EXAFS spectra used k3 weighting, and fitting was per-formed over a k range of 3–10 Å to eliminate high-k noise. Theamplitude reduction factor ðS2

0Þ, which was determined from fittingthe scorodite reference, was fixed at 0.86 during fitting. Coordina-tion numbers for the nearest neighbor As–O scattering path andthe As–Fe scattering paths were input as variables, and the typicaluncertainty of these values was estimated at ±20%. Multiple scat-tering paths for As–O–O and As–O–As–O within the AsO4 tetrahe-dron also were included (Chen et al., 2009).

Two-dimensional l-XRD patterns were collected in transmis-sion geometry using a mar345 image plate detector (MarresearchGmbH, Germany). The focused beam and an incident energy of18,000 eV (k = 0.6888 Å) were used to collect patterns. Exposuretimes ranged from 60 to 360 s and analysis was performed atambient temperature. Processing of 2-D diffraction patterns wasperformed using the Area Diffraction Machine (Lande, 2008).Detector geometries were calibrated to CeO2(s) diffraction patterns,and point reflections were masked prior to 2-r integration. Inte-gration was performed over a 2-r range of 0–30 and analysis of dif-fraction patterns was performed with the Xpowder code (Martín-Ramos, 2004).

3.3. Sequential extractions

Tailings samples were subjected to an extraction procedurewhich targeted sequentially water-soluble, adsorbed-exchange-able-carbonate (AEC), Fe(III) hydroxide, Fe(III) oxide, and sulfidefractions to study the elemental distribution in different mineralfractions (modified from Hall et al. (1996) and Dold (2003). Themodified sequential extraction procedure is described in detailby Parviainen (2009). The water soluble fraction targets efflores-cent sulfates (e.g. gypsum [CaSO4�2H2O]), whereas the AEC fractioncontains the portion of adsorbed elements and exchangeable ionsheld on the exchange sites of negatively charged surfaces throughelectrostatic attraction, as well as carbonates. In the Fe(III) hydrox-ide fraction, the poorly crystalline secondary Fe(III) minerals (e.g.,schwertmannite [Fe16O16(OH)12(SO4)2], two-line ferrihydrite, jaro-site [KFe3(SO4)2(OH)6]) are leached, whereas in the Fe(III) oxidefraction some residual secondary Fe(III) minerals with higher crys-tallinity (e.g., high-ordered ferrihydrite, goethite [a-FeOOH], jaro-site) are leached together with primary oxides (e.g., magnetite,hematite [Fe2O3]). The Ylöjärvi tailings samples contain magnetiteonly as a minor constituent; therefore, the sum of the third andfourth fractions was considered as the total amount of secondaryFe(III) minerals for this study. The last extraction step, namelythe sulfide fraction, attacked sulfide minerals including arsenopy-rite, and the residual, containing more inert silicate minerals,was discarded. The extraction method is not As specific, but theFe(III) arsenates are expected to be leached in the reducing ammo-nium oxalate leaches together with other secondary Fe(III) miner-als. However, Corriveau et al. (2011) suggest that crystallinescorodite can be more resistant than other Fe(III) arsenates.

4. Results

4.1. Petrography

Tailings deposited in the larger tailings area consisted generallyof very fine grained material. The samples from profile 1 and fromthe upper part of profile 2 consisted of fine tailings, but a layer ofslightly coarser material was found in the profile 2 below 110 cmto the sampling depth 230 cm. The predominant grain-size was silt(60–90% of <63 lm) in the finer tailings, and fine- to medium-sandsize (70% of 63–595 lm) in the coarser tailings of profile 2. Thespecific surface area of the tailings varied from 0.18 to1.03 m2 g�1 where the lowest value corresponded to sandy tailings,and the highest value to fine tailings with 90% silt and clay fraction.

The electron microscope studies show that the near surface tail-ings up to a depth of least 15 cm were depleted in the principal sul-fide minerals (pyrrhotite, chalcopyrite, arsenopyrite, pyrite) due tooxidation processes. Signs of alteration decreased significantly atdepths greater than 60 cm. At this depth pyrrhotite was notencountered, but small amounts of arsenopyrite and pyrite werepresent. Pyrrhotite and pyrite showed slight alteration on the grainboundaries up to a depth of 200–250 cm. However, in the interfaceof the coarser layer of profile 2 at a depth of 110 cm the effects ofsulfide oxidation intensified again leaving only remnant grains ofarsenopyrite (Fig. 2). A solidified layer was found at the interfaceof the coarser layers. The surficial samples and the sample at110 cm were considered oxidized tailings, whereas deeper sampleswere less altered tailings, and in the primary zone the tailings wereunoxidized. Secondary minerals appeared as brownish horizontallayers to the naked eye in the field, and as continuous layers or ran-dom accumulations filling the pore spaces under the optical micro-scope and SEM (Fig. 2).

4.2. Scanning electron microscopy and electron microprobe analysis

SEM studies provided images of the textural features of the al-tered sulfide and sulfoarsenide grains, and the secondary mineralsreplacing and embedding the primary minerals (Fig. 2). Arsenic-bearing Fe(III) (oxy)hydroxides appeared throughout the samplingdepth being more abundant than Fe(III) arsenates, which wereencountered solely in the sample from 110 cm of profile 2. How-ever, the abundance of Fe(III) (oxy)hydroxides tended to decreasedownward in the profiles with only scarce occurrences noted inthe bottom samples of each profile at a depth of 230 cm.

The As2O5 content varied from 0.5 to 18.4 wt.% (average2.9 wt.%) in the Fe(III) (oxy)hydroxides, whereas it was from 37.3to 46.8 wt.% (average 42.4 wt.%) in the Fe(III) arsenates (Tables1–4). Of three samples examined from profile 2, the sample col-lected from 110 cm exhibited greater compositional variation(Fe2O3 vs. As2O5) of the secondary Fe(III) minerals than the 150and 230 cm samples (Fig. 3). The Fe(III) arsenates had Fe/As molarratios between 1 and 1.5 (Fig. 3). Relatively low concentrations ofAl2O3 (average 0.5 and 0.6 wt.%, respectively), CaO (0.2 and0.2 wt.%), K2O (0.2 and 0.1 wt.%), SiO2 (1.9 and 2.8 wt.%), and SO3

(2.0 and 0.8 wt.%) were detected in Fe(III) arsenates and Fe(III)(oxy)hydroxides.

4.3. Synchrotron-based analyses

4.3.1. Micro-XRF mappingMicro-scale Fe distribution generally was characterized by dis-

crete Fe-bearing grains surrounded by a more diffuse Fe mass(Figs. 4 and 5). These discrete Fe-bearing grains ranged in size from50 lm to 150 lm in cross-section. In general, the 110 cm samplecontained a larger proportion of smaller grains, whereas the deeper

Fig. 2. Back-scattered SEM images of Fe(III) arsenates (FeAs), As-bearing Fe(III) (oxy)hydroxides (FeOx), and arsenopyrite (Asp) grains with varying alteration grades in profile2 at depths of 110 cm (A–C) and 150 cm (D) (Mgt = magnetite).

Table 1EMPA data for Fe(III) arsenates (wt.%) and Fe/As mole ratios at 110 cm depth forprofile 2.

Fe2O3 As2O5 CaO K2O Al2O3 SiO2 SO3 MnO Fe/As

36.7 41.5 0.16 0.13 0.17 0.33 3.30 0.00 1.2738.0 37.2 0.10 0.49 2.92 4.87 1.49 0.10 1.4738.2 41.9 0.08 0.17 0.15 0.26 3.72 0.00 1.3138.6 38.4 0.22 0.17 0.19 9.25 1.03 0.00 1.4539.1 46.8 0.19 0.27 0.05 0.03 2.00 0.03 1.2039.1 44.9 0.28 0.20 0.18 0.16 1.65 0.00 1.2539.1 46.0 0.30 0.20 0.19 0.09 1.77 0.00 1.2339.7 41.1 0.37 0.13 0.36 4.03 1.52 0.01 1.3939.8 47.0 0.11 0.25 0.10 0.04 1.83 0.00 1.2240.4 41.7 0.66 0.39 0.52 0.86 1.99 0.08 1.4046.2 39.9 0.09 0.17 0.27 0.82 1.36 0.11 1.66

Table 2EMPA data for Fe(III) (oxy)hydroxides (wt.%) and Fe/As mole ratios at 110 cm depthfor profile 2.

Fe2O3 As2O5 CaO K2O Al2O3 SiO2 SO3 MnO Fe/As

61.3 18.4 0.23 1.20 2.62 0.58 0.41 7.46 4.7963.1 18.2 0.23 0.96 2.06 0.60 0.37 6.74 5.0067.4 2.65 0.29 0.01 1.09 2.72 4.97 0.00 36.569.8 3.83 0.37 0.19 2.20 4.35 4.14 0.01 26.373.4 2.53 0.68 0.21 1.11 3.17 5.00 0.00 41.874.7 1.69 0.29 0.04 1.29 2.60 4.51 0.00 63.674.8 9.92 0.30 0.06 0.31 2.97 0.52 0.44 10.975.9 1.83 0.28 0.06 0.78 6.08 4.46 0.01 59.677.0 2.10 0.25 0.95 0.55 4.07 0.33 0.54 52.878.6 7.51 0.10 0.05 0.27 2.80 0.32 0.45 15.178.9 5.12 0.15 0.04 0.05 1.32 0.51 0.40 22.279.2 8.86 0.31 0.04 0.26 3.38 0.43 0.43 12.979.5 6.54 0.23 0.04 0.02 1.42 0.60 0.44 17.580.0 8.63 0.35 0.01 0.29 3.33 0.44 0.47 13.381.2 3.98 0.16 0.03 0.09 1.40 0.48 0.42 29.481.4 3.26 0.14 0.02 0.14 1.42 0.45 0.42 35.982.3 3.46 0.06 0.02 0.09 2.12 0.56 0.40 34.283.0 1.53 0.04 0.00 0.06 1.48 0.22 0.49 78.283.7 1.73 0.09 0.00 0.01 1.33 0.25 0.50 69.7

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samples (i.e. 150 cm and 230 cm) were dominated by Fe-bearinggrains at the high end of this size range. Sharp decreases in FeKa counts were observed at the margins of these grains, and a gen-eral trend of decreasing mass with distance from Fe-bearing grainswas observed for the bulk sample matrix. This general trend in Fedistribution was most prominent in samples collected from 110and 150 cm. In contrast, these zones of decreasing Fe mass werelimited in the sample collected from 230 cm. Furthermore, thedeepest sample exhibited the sharpest decreases in Fe mass sur-rounding primary grains and the lowest Fe mass in the bulk samplematrix.

The micro-scale distribution of As was also characterized bydiscrete mineral grains surrounded by zones of decreasing As masswith increasing distance from primary grain margins. These grainswere less abundant than discrete Fe-bearing grains and generallywere smaller, ranging in size from 20 lm to 50 lm across. The dis-tribution of grain sizes was similar between the 110 and 150 cmsamples, whereas two discrete grains observed for the 230 cmsample were at the low end of this size range. A more diffuse Asmass surrounded the primary grains in samples collected from

110 and 150 cm. In contrast, two discrete As-bearing grains wereobserved in the 230 cm sample and these grains were not sur-rounded by a more widely distributed and diffuse As mass.

4.3.2. Micro-XANES spectroscopyArsenic K-edge XANES indicated that As(V) was the dominant

oxidation state within samples collected from 110 and 150 cm(Fig. 4; Supporting information Table S1). First derivative maximaof XANES spectra collected for these samples ranged from11873.0 to 11873.6 eV (n = 17), which is consistent with theAs(V) oxidation state. In contrast, an As K-edge of 11866.6 eVwas observed for the 230 cm sample. The lower energy of this K-

Table 3EMPA data for Fe(III) (oxy)hydroxides (wt.%) and Fe/As mole ratios at 150 cm depthfor profile 2.

Fe2O3 As2O5 CaO K2O Al2O3 SiO2 SO3 MnO Fe/As

73.6 2.61 0.38 0.65 2.65 7.54 0.49 0.67 40.674.3 2.14 0.35 0.13 1.77 5.46 0.31 0.59 50.174.8 2.36 0.22 0.07 2.78 6.55 0.37 0.64 45.575.8 2.76 0.25 0.19 2.40 5.57 0.50 0.57 39.676.9 2.87 0.21 0.12 2.13 5.54 0.48 0.64 38.677.1 2.86 0.23 0.13 2.12 5.87 0.44 0.62 38.877.3 1.29 0.08 0.03 0.03 5.48 0.25 0.97 86.477.6 2.61 0.14 0.05 0.11 3.06 0.46 0.26 42.879.0 2.31 0.13 0.01 0.10 2.64 0.42 0.28 49.179.9 2.28 0.12 0.00 0.13 2.66 0.49 0.27 50.580.7 2.53 0.10 0.02 0.00 1.48 0.29 0.18 45.881.2 2.61 0.13 0.02 0.01 2.65 0.31 0.59 44.881.7 1.30 0.07 0.00 0.07 2.02 0.21 0.95 90.281.8 1.31 0.11 0.00 0.07 1.78 0.25 1.02 89.882.1 2.71 0.09 0.06 0.02 2.80 0.37 0.37 43.682.4 1.78 0.11 0.02 0.00 0.67 0.37 0.10 66.782.6 2.55 0.25 0.05 0.13 2.94 0.35 0.33 46.682.6 1.90 0.11 0.01 0.00 1.04 0.44 0.14 62.682.7 3.11 0.12 0.04 0.01 3.09 0.39 0.33 38.382.8 3.06 0.13 0.04 0.00 3.07 0.33 0.36 39.082.9 2.61 0.13 0.06 0.05 2.78 0.24 0.43 45.783.1 2.60 0.13 0.03 0.03 2.45 0.38 0.23 46.183.2 3.57 0.14 0.04 0.04 3.42 0.37 0.40 33.584.3 2.74 0.14 0.03 0.10 3.15 0.33 0.33 44.3

Table 4EMPA data for Fe(III) (oxy)hydroxides (wt.%) and Fe/As mole ratios at 230 cm depthfor profile 2.

Fe2O3 As2O5 CaO K2O Al2O3 SiO2 SO3 MnO Fe/As

68.7 0.89 0.22 0.21 1.17 3.22 0.55 0.31 11170.5 1.04 0.19 0.26 1.28 3.66 0.63 0.35 97.670.6 0.91 0.23 0.31 1.88 4.52 0.86 0.32 11274.9 1.74 0.25 0.20 1.00 3.91 0.97 0.66 62.175.1 1.84 0.28 0.13 0.92 3.48 0.93 0.53 58.777.3 1.73 0.16 0.16 0.73 3.39 0.71 0.41 64.377.6 1.42 0.17 0.00 0.31 1.94 0.33 0.58 78.777.6 1.72 0.24 0.12 0.73 3.39 0.76 0.46 64.877.6 1.63 0.25 0.15 0.77 3.69 0.70 0.41 68.578.3 1.31 0.16 0.04 0.53 2.23 0.57 0.29 85.878.6 1.71 0.14 0.00 0.70 2.78 0.45 0.15 66.279.4 1.31 0.13 0.00 0.05 1.93 0.62 0.35 87.080.2 0.66 0.09 0.02 0.20 1.44 0.59 0.26 17580.3 1.26 0.13 0.00 0.11 1.86 0.74 0.44 91.480.4 1.09 0.11 0.03 0.09 1.73 0.60 0.33 10680.5 0.54 0.07 0.01 0.05 1.50 0.32 0.34 21581.0 1.15 0.14 0.00 0.06 1.59 0.51 0.27 10181.5 0.88 0.06 0.00 0.15 1.24 0.55 0.37 13381.6 1.00 0.11 0.03 0.16 1.54 0.58 0.24 11881.6 0.99 0.08 0.04 0.06 1.33 0.48 0.38 11981.7 0.61 0.10 0.03 0.23 1.59 0.60 0.27 19382.5 0.52 0.14 0.01 0.24 1.39 0.66 0.23 22982.7 0.55 0.07 0.02 0.16 1.22 0.45 0.19 218

Fig. 3. Scatter plot of Fe2O3 vs. As2O5 determined by EMPA analysis. Dashed linesrepresent the Fe:As molar ratios. Theoretical molar ratio for Fe(III) arsenates isbetween 1 and 1.5. Sample names in the legend represent sample depth in profile 2.Modified from Paktunc et al. (2003, 2004).

2294 A. Parviainen et al. / Applied Geochemistry 27 (2012) 2289–2299

edge is indicative of As present in a reduced oxidation state similarto that of arsenopyrite.

Speciation of As was performed by LCF in the XANES region (i.e.�20 to +30 eV) of spectra collected for the 110 cm sample in April2010 (Fig. 4). A monochrometer glitch at �11,890 eV preventedLCF of spectra collected during July 2009. Reference spectra uti-lized for LCF of April 2010 spectra included arsenopyrite, As3-Fh,As5-Fh, arsenosiderite, scorodite and kankite. Results of this anal-ysis also indicate that As(V)-bearing compounds dominated(n = 5) at a depth of 110 cm. In general, the primary componentsof fitted spectra were As5-Fh and either scorodite or kankite (Ta-ble 5). The remaining reference minerals consistently comprised<1% of the fitted spectra. The fitted weights of the spectra rangedfrom 30% to 61% (average 52%) for As5-Fh, 0–40% (average 22%)

for scorodite and 0–72% (average 25%) for kankite. Slight discrep-ancies between measured and fitted XANES spectra are indicativeof variations in crystallinity or mineralogy; however, the LCF re-sults suggested that As(V) sorption onto Fe(III) (oxy)hydroxidesand the precipitation of secondary Fe(III) arsenates contribute toAs attenuation.

4.3.3. Micro-EXAFS spectroscopyExperimental l-EXAFS spectra were characterized by a moder-

ately defined double hump for the first oscillation located at�5 Å�1 (Fig. 5d). This feature generally was less prominent andwider than that of the scorodite reference, but was more distinctcompared to As5-Fh (Supporting information Fig. S1). Similar fea-tures have been observed in k-space plots of As K-edge EXAFS spec-tra for tailings (Paktunc et al., 2003, 2004), poorly crystalline Fearsenates (Paktunc et al., 2008) and neutralized raffinate solids(Chen et al., 2009). Fourier transforms of these spectra revealedthe presence of a primary peak at 1.3 Å and a secondary peak at2.7 Å (uncorrected for phase shift). These peaks correspond to thepositions of the first shell As–O bond and second shell As–Fe bond.In general, the relative magnitude of the second shell As–Fe peakcompared to the first shell As–O peak was much smaller thanwas observed for the scorodite reference. Additional peaks andshoulders in close proximity to the As–Fe shell are indicative ofmultiple scattering within the AsO4 tetrahedron (Chen et al., 2009).

Fitting of the l-EXAFS spectra utilized the nearest neighbor As–O atomic shell and the second neighbor As–Fe atomic shell. Thesefits were improved by including the As–O–O and As–O–As–O mul-tiple scattering paths within the AsO4 tetrahedron. Including thesecond As–O shell generally did not improve fits and this scatteringpath was excluded from all fits. The fitted DE0 values were consis-tently <10 eV and R-factors were consistently <0.0002. The fittedfirst shell As–O interatomic distances ranged from 1.68 to 1.70 Å,while coordination numbers ranged from 3.90 to 4.72 (Table 6).These values deviate from the nominal four O atoms present inthe AsO4 tetrahedron; however, such variations may arise fromuncertainty in EXAFS fitting and the values are in general agree-ment with previously published data (Foster et al., 1998; Paktuncet al., 2003, 2008).

Interatomic distances fitted for the second shell As–Fe scatter-ing path ranged from 3.28 to 3.31 Å, with the exception of a3.37 Å bond distance at location 4 (Table 6). The shorter distances

Fig. 4. Representative l-XRF maps and corresponding first derivative As K-edge XANES spectra for samples collected at depths of 110, 150 and 230 cm from location 2 in the17 ha tailings area.

Fig. 5. Micro-XRF maps of Fe Ka (a) and As Ka (b) fluorescence for a sample collected from a depth of 110 cm at location 2 in the 17 ha tailings area. Corresponding As K-edgeXANES spectra (c), and k3-weighted EXAFS spectra (d) and Fourier-transformed EXAFS spectra (e). Solid lines and circles represent experimental data (April 2010) and fittedspectra, respectively.

A. Parviainen et al. / Applied Geochemistry 27 (2012) 2289–2299 2295

are consistent with bidentate AsO4 complexes with adjacent Feoctahedra on Fe(III) (oxy)hydroxides (Waychunas et al., 1993).The longer atomic distance for location 4 is indicative of scorodite(Waychunas et al., 1993; Foster et al., 1998; Paktunc et al., 2004)and hydrous ferric arsenate (HFA; Paktunc et al., 2003). Coordina-tion numbers for the second shell As–Fe bond ranged from 0.67 to

1.96, which is less than the nominal coordination number of fourcharacteristic of crystalline Fe(III) arsenates (Kitahama et al.,1975). Paktunc et al. (2003, 2004, 2008) reported coordinationnumbers of <2 for poorly crystalline Fe(III) arsenates and Waych-unas et al. (1993) observed an indirect relationship between Asand Fe coordination and As/Fe ratios in ferrihydrite.

Table 5Results of linear combination fitting of l-XAS spectra collected in April 2010 forsample locations identified in Fig. 5. Fitting was performed in the XANES region (i.e.�20 to +40 eV) with a constant E0 for all reference materials.

Location Fitted weight R-factor

Asp As3-Fh As5-Fh Asd Sco Kan Sum

1 0.01 0.03 0.61 0.00 0.38 0.00 1.04 0.000512 0.00 0.00 0.30 0.00 0.00 0.72 1.02 0.001573 0.01 0.01 0.65 0.00 0.34 0.00 1.01 0.000574 0.02 0.01 0.47 0.00 0.00 0.51 1.01 0.000305 0.01 0.01 0.59 0.00 0.40 0.00 1.01 0.00056

Asp: arsenopyrite.As3-Fh: arsenic (III) ferrihydrite.As5-Fh: arsenical ferrihydrite.Asd: arseniosiderite.Sco: scorodite.Kan: kankite.R-factor: residual factor.

Table 6Summarized results of l-EXAFS fitting for local coordination of O and Fe within 3–10 Å�1 of a central As atom. The amplitude reduction factor ðS2

0Þ was fixed at 0.9during fitting. The data were collected in April 2010 for sample locations identified inFig. 5.

Location As–O As–Fe DE0

(eV)R-factor

N R(Å)

r2

(Å2)N R

(Å)r2

(Å2)

1 4.23 1.69 0.0012 1.96 3.31 0.0009 9.1 0.000042 4.24 1.68 0.0006 0.98 3.28 0.0005 7.7 0.000133 3.90 1.69 0.0012 0.67 3.31 0.0006 8.4 0.000044 4.72 1.69 0.0028 1.89 3.37 0.0102 7.5 0.000065 4.33 1.70 0.0010 1.04 3.32 0.0006 8.4 0.00005

N: coordination number.R: interatomic distance.r2: Debye–Waller parameter.E0: energy offset.R-factor: residual factor.

Fig. 6. Kankite was identified in a l-XRD spectrum corresponding to location 2 inFig. 5.

2296 A. Parviainen et al. / Applied Geochemistry 27 (2012) 2289–2299

4.3.4. Micro-XRD analysisMicro-XRD analyses performed on the locations defined in Fig. 5

lead to identification of kankite as a crystalline Fe(III) arsenate sec-ondary phase, chiefly at analysis points 2 and 4 (Fig. 6). In addition,clinochlore was the main gangue mineral detected in the patterns.Other crystalline Fe(III) arsenates such as scorodite and crystallineFe(III) (oxy)hydroxides were not found. Instead, poorly crystallineor amorphous phases such as arsenical ferric (oxy)hydroxidesand/or HFA, which would contribute weakly to the diffraction ef-fects in the XRD patterns, could exist as they often occur in mining

environments (Paktunc et al., 2008; Walker et al., 2009; DeSistoet al., 2011).

4.4. Sequential extractions

The depletion of sulfide minerals in surficial samples and in110 cm sample in profile 2 was apparent from As and S contentsassociated with the sulfide fraction (Fig. 7). However, the resultsfor Fe were not consistent with the observations made by SEM–EDS. Dissolution of some less resistant silicate minerals (e.g., bio-tite and chlorite) with the sulfide fraction was evident from ele-vated concentrations of silicate-derived elements (e.g., Al, Ca, Fe,Mg). Arsenopyrite is the only As-bearing sulfide mineral in theYlöjärvi tailings, and As showed a wide range of concentrationsin the sulfide fraction from 25 mg kg�1 (<3% of the total As content)in the shallow samples, and up to 6900 mg kg�1 (97.6% of total Ascontent) in the less altered samples at 150 and 230 cm depth. The Sconcentrations in the sulfide fraction followed a trend very similarto As.

A significant portion of dissolved As originating from the oxida-tion of arsenopyrite was retained in secondary Fe(III) minerals,which included both Fe(III) hydroxide and Fe(III) oxide fractions(Fig. 7). The amount of As in these fractions ranged from 82.5% to93.7% (up to 3685 mg kg�1) of the total As in the highly oxidizedshallow samples of both profiles and in sample 110 cm in profile2. The bulk Fe/As ratio of these samples for the secondary Fe(III)minerals fractions was on average 1.9, suggesting the presence ofFe(III) arsenates, whereas the Fe/As ratio value increased withdepth and reached a maximum of 13 in the deepest samples of pro-file 2. The sequential extraction results suggested that As was co-precipitated with the secondary Fe(III) minerals rather than ad-sorbed on the mineral surfaces as indicated by the low concentra-tions of As in the AEC fraction (average 52 mg kg�1, or 2.1% of totalAs content). In contrast, concentrations of S within the secondaryFe(III) minerals were low (average 61 mg kg�1), whereas, for in-stance, sample 110 cm in profile 2 contained up to 409 mg kg�1

of S in the AEC fraction. The elevated S concentrations (up to1200 mg kg�1) in the water-soluble fraction were likely derivedfrom high pore-water concentrations by precipitation of tertiarysulfates upon drying the samples (Jambor, 1994). Arsenic concen-trations in the water-soluble fraction were relatively low (average8 mg kg�1).

5. Discussion

5.1. Occurrence of secondary As phases

Extensive sulfide oxidation was observed for surficial samplesand at 110 cm in profile 2. The presence of arsenate in the shal-lower samples, shown in the As K-edge XANES spectra (Fig. 4), isindicative of a higher degree of sulfide-mineral alteration and Asoxidation, whereas redistribution of Fe and As among oxidationproducts was minimal at greater depth. Results of EMPA examina-tion of the secondary Fe(III) phases and sequential extraction anal-ysis for sample 110 cm (profile 2), where secondary mineralogyexhibited greater compositional variation, indicated that As oc-curred as three primary components: (1) as an adsorbed phase(minor component); (2) as a coprecipitate in Fe(III) (oxy)hydrox-ides; and (3) as Fe(III) arsenates (Tables 1 and 2; Figs. 2 and 7). Re-sults of LCF suggested that scorodite, kankite and arsenicalferrihydrite were the major components of the l-XANES spectra(Table 5). However, l-XRD analysis indicated that the kankite ref-erence likely contained impurities or was poorly crystalline. None-theless, LCF indicated that the l-XANES spectra for kankite was abetter fit than scorodite (i.e., locations 2 and 4, Fig. 5). Micro-

Fig. 7. Sequential extraction results for As, Fe, and S (mg kg�1). (AEC = adsorbed-exchangeable-carbonate, depth of sample surface in cm).

A. Parviainen et al. / Applied Geochemistry 27 (2012) 2289–2299 2297

XRD patterns (Fig. 6) for locations 2 and 4 (Fig. 5) confirmed thepresence of kankite. In contrast, scorodite was not detected byXRD, but other secondary Fe(III) arsenates such as HFA exhibitinglow crystallinity or amorphous character may occur with kankite.A broad diffraction peak at 3 Å has previously been attributed topoorly crystalline HFA in weathered As-rich tailings (Walkeret al., 2009) and in synthesis experiments (Paktunc et al., 2008).

Results of l-EXAFS analysis were in accordance with the EMPAand sequential extraction data, and provided additional insight onmechanisms of As attenuation. Interatomic distances for the sec-ond shell As–Fe scattering path suggested the presence of Fe(III)oxyhydroxide and Fe(III) arsenate (i.e., scorodite, kankite), whilethe coordination numbers for As–Fe (<4) are indicative of poorlycrystalline or disordered precipitates (Waychunas et al., 1993; Fos-ter et al., 1998; Paktunc et al., 2003, 2004, 2008). Fitting of the l-EXAFS spectra indicated that outer-sphere complexation of AsO4

tetrahedra with Fe(III) phases likely contributed to As attenuation;however, co-precipitation with poorly crystalline Fe(III)(oxy)hydroxides cannot be dismissed. These results suggested thatmultiple mechanisms of As attenuation occurred within oxidizedtailings at the study site.

5.2. Potential stability of As phases

Arsenic was most commonly associated with secondary Fe(III)minerals and sulfide phases (i.e., arsenopyrite) rather than withreadily-mobilized fractions (i.e., water-soluble and AEC fractions;Fig. 7). Therefore, the biogeochemical stability of As within the tail-ings is largely dependent on the potential for dissolution of thesephases. Arsenopyrite is relatively stable under limited Fe(III) andO2 availability, which was characteristic of conditions in the pri-mary zone below the water table. Yet, abundant arsenopyriteremaining in the vadose zone is potentially prone to oxidation.

Moreover, arsenical Fe(III) (oxy)hydroxides and Fe(III) arsenates,carrying 82.5–93.7% of the total As in the highly altered samples,exhibit varied stability that is dependent on the composition andcrystallinity of these phases, as well as biogeochemical conditionswithin the tailings.

The precipitation of Fe(III) arsenates with a Fe/As ratio <1.5 haspotential for greater As mass attenuation than As-bearing Fe(III)(oxy)hydroxides. However, differences in the stability of Fe(III) ars-enates with varied compositions complicate assessments of the po-tential long-term effectiveness of this As attenuation mechanism.Krause and Ettel (1989) note that the pH stability range for Fe(III)arsenates broadens with increasing Fe/As ratios and precipitateswith Fe/As ratios P 8 are stable over a wide pH range (3–8). Theseauthors also report that crystalline scorodite solubility is approxi-mately 100 times lower than for amorphous FeAsO4�xH2O.Although sorption onto Fe(III) (oxy)hydroxides and co-precipita-tion with poorly-crystalline phases can limit As mobility in porewater, attenuation resulting from co-precipitation with more crys-talline phases likely will facilitate greater long-term stability (Fos-ter et al., 1998). Recrystallization of poorly-crystalline Fe(III)arsenates may, therefore, increase As stability over time. Le Berreet al. (2008) studied the transformation of poorly crystalline ferricarsenate into crystalline scorodite and stated that upon recrystalli-zation the Fe/As molar ratio of the precipitate decreased from 1.22to 1. These authors deduced the decrease as the stepwise conver-sion of ferrihydrite, initially precipitated together with poorly crys-talline Fe(III) arsenate, to crystalline scorodite as well. Thetransformation of ferrihydrite or lepidocrocite [c-FeOOH] to morecrystalline phases over time, for example goethite or hematite, alsohas potential to enhance As stability at trace concentrations(Pedersen et al., 2006). Das et al. (2011) confirmed the importanceof As in the long-term stability of ferrihydrite in alkaline condi-tions. However, rates of transformation may decrease for Fe(III)(oxy)hydroxides with lower Fe/As ratios (Das et al., 2011).

2298 A. Parviainen et al. / Applied Geochemistry 27 (2012) 2289–2299

Changes in pH and redox conditions also influence the stabilityof Fe(III) arsenates of varying Fe/As ratios. Paktunc et al. (2008)demonstrated that scorodite and Fe(III) arsenates exhibit greaterstability at lower pH, whereas arsenical ferrihydrite stability in-creases at higher pH. In a more recent study, Paktunc and Brugg-eman (2010) found that synthetic nanocrystalline scoroditeexhibits higher solubility at both low and high pH (i.e., <2 and>6), with the minimum solubility of about 0.25 mg L�1 As occur-ring at pH 3–4. These results for nanocrystalline scorodite are com-parable to those reported for crystalline scorodite by Krause andEttel (1989). In a recent review of secondary As minerals, Drahotaand Filippi (2009) stated that solubility and thermodynamic datahave not been reported for kankite. Cech et al. (1976) stated thatkankite in the type locality is less stable than scorodite due to atendency to transform into scorodite. Although less commonly ob-served than scorodite, kankite has been detected in several As-richmine tailings facilities, generally in close association with scorodite(Kato et al., 1984; Walker et al., 2009; Kocourková et al., 2011;DeSisto et al., 2011).

A development of reductive conditions may promote the liber-ation of As through reduction of As(V) or reductive dissolution ofsecondary Fe(III) phases (McCreadie et al., 2000; Kocar and Fen-dorf, 2009; Lindsay et al., 2011). For instance, remedial actionsafter implementation of a soil cover or addition of organic C can in-duce the transition to anaerobic conditions. According to Salzsauleret al. (2005) and Simpson et al. (2011), As-bearing alteration prod-ucts (e.g., scorodite, As-bearing jarosite) were remobilized underreducing near-neutral (pH 6.7–8.6, average 7.9) conditions aftercapping arsenopyrite-rich mine wastes. These authors suggestedthat under these conditions, Fe(III) (oxy)hydroxides are the princi-pal scavengers of As (mostly present as arsenite) acting as animportant attenuation mechanism.

The pH of tailings pore water at Ylöjärvi was 5.7 (ranging from4.9 to 7.7) within 200 cm of the surface, and increased to 8.9 (Ehdecreased to 212 mV, corrected to SHE) with depth in 2008 (Par-viainen et al., 2012). Lower pH values and higher redox potentialwere detected in surficial tailings (median pH 5.4 and Eh320 mV) and in the coarser grained material in the profile 2 (med-ian pH 5.4 and Eh 348 mV), whereas just above the interface ofcoarse tailings (approx. 100 cm depth) the pH and Eh were 7.7and 223 mV, respectively (Parviainen et al., 2012). Although, sul-fide oxidation generally decreases downward, the pH–Eh condi-tions and mineralogical observations imply that oxidationprocesses are reactivated at 110 cm in profile 2 probably due toan insertion of more oxygenated pore water in the coarser layerof tailings, or due to oxidation during a hiatus in tailings deposi-tion. Sulfide oxidation is expected to continue leading to moderatedecreases in the tailings pH in the vadose zone, but decreases in pHare expected to cease with depletion of sulfide minerals in the lessaltered tailings due to ongoing oxidation over time. Iron(III) arse-nates similar to kankite and scorodite were observed primarily inthe sample from 110 cm, and presumably occurred in the top sam-ples (suggested by the low bulk Fe/As ratios) where oxidation pro-cesses are most intensive and the lowest pore water pH valuesoccur. In the remainder of the samples, Fe(III) (oxy)hydroxidessimilar to arsenical ferrihydrite were the dominant secondaryFe(III) phases. Poorly crystalline Fe(III) arsenates occurring in theYlöjärvi tailings likely are not effective long-term sinks for As, aschanging biogeochemical conditions may promote As remobiliza-tion over time. The pH of around 5.7 favors the precipitation of fer-rihydrite and can promote dissolution of Fe(III) arsenate, therebyincreasing the role of ferrihydrite in the uptake of As (Paktuncet al., 2008). Arsenic mass transport in tailings pore water will de-pend on multiple processes under changing pH–Eh conditions,including (1) oxidation and dissolution of primary arsenopyrite;(2) precipitation and re-dissolution of poorly crystalline Fe(III) ars-

enates; (3) As adsorption on the precipitating ferrihydrite; and (4)recrystallization of ferrihydrite into more stable Fe(III) phases.

6. Conclusions

A combination of mineralogical and geochemical investigationswas performed to assess mechanisms of As attenuation in the va-dose zone of tailings at the Ylöjärvi (Cu–W–As) mine site. Arseno-pyrite, which was a primary As source in these tailings, wasgenerally depleted in samples near the tailings surface and in azone of extensive weathering in coarse grained tailings located110 cm below the tailings surface. In general, sulfide-mineral alter-ation decreased downwards with the deepest samples collectedfrom 230 cm depth exhibiting minimal signs of oxidation. In thehighly altered samples, redistribution of As and Fe was detectedand secondary phases found were poorly crystalline As-bearingFe(III) (oxy)hydroxides (arsenical ferrihydrite type) and Fe(III) ars-enates (HFA, scorodite and kankite type). These phases containedconsiderable amounts of As and were important controls on Asmobility within the oxidized tailings. However, pH conditions(median 5.7) observed by Parviainen et al. (2012) are not favorableto the stability of poorly crystalline Fe(III) arsenates (especiallyscorodite), and transformation of these precipitates to arsenicalferrihydrite can be expected (Langmuir et al., 2006; Bluteau andDemopoulos, 2007). This process has been shown to enhance theadsorption capacity of ferrihydrite (Salzsauler et al., 2005), whichcould facilitate long-term As attenuation if circumneutral pH con-ditions persist and organic C inputs are limited.

Acknowledgments

This study was funded by the Finnish Graduate School in Geol-ogy, K.H. Renlund Foundation, and Finnish Cultural Foundation.Additionally, Dr. R. Pérez-López acknowledges the Spanish Minis-ter of Science and Innovation and the ‘Ramón y Cajal’ Subpro-gramme (MICINN-RYC 2011). Funding for synchrotron-basedresearch was provided by the Natural Science and EngineeringCouncil of Canada (NSERC). Synchrotron-based techniques wereperformed at GeoSoilEnviroCARS (Sector 13), Advanced PhotonSource (APS), Argonne National Laboratory. GeoSoilEnviroCARS issupported by the National Science Foundation – Earth Sciences(EAR-0622171) and Department of Energy – Geosciences (DE-FG02-94ER14466). Use of the Advanced Photon Source was sup-ported by the US Department of Energy, Office of Science, Officeof Basic Energy Sciences, under Contract No. DE-AC02-06CH11357. Lassi Pakkanen and Bo Johanson provided assistancewith the micro analytical work at the Geological Survey of Finland.We thank Matt Newville and Yeong Choi for technical assistancewith synchrotron-based techniques. We also thank Dogan Paktuncfor contributing an As K-edge EXAFS spectrum for arsenical fer-rihydrite and the arseniosiderite reference sample.

Appendix A. Supplementary material

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.apgeochem.2012.07.022.

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