Applied Geochemistry 36 (2013) 49–56

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Applied Geochemistry

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Mobilization of arsenic from mine tailings through reductive dissolutionof goethite influenced by organic cover

0883-2927/$ - see front matter Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.apgeochem.2013.05.012

⇑ Tel.: +1 613 947 7061; fax: +1 613 996 9673.E-mail address: Dogan.Paktunc@NRCan.gc.ca

Dogan Paktunc ⇑Natural Resources Canada, CANMET, Mining and Mineral Sciences Laboratories, 555 Booth Street, Ottawa, Ontario K1A 0G1, Canada

a r t i c l e i n f o

Article history:Received 1 June 2012Accepted 24 May 2013Available online 11 June 2013Editorial handling by K. Savage

a b s t r a c t

Mine tailings at the former Delnite gold mine in northern Ontario were characterized to assess the impactof a biosolids cover on the stability of As species and evaluate options for long-term management of thetailings. Arsenic concentrations in the tailings range from 0.15 to 0.36 wt% distributed among goethite,pyrite and arsenopyrite. Pyrite and arsenopyrite occur as small and liberated particles that are envelopedby goethite in the uncovered tailings and the deeper portions of the biosolids-covered tailings. Sulfideparticles in the shallower portions of the biosolids-covered tailings are free of goethite rims. Arsenicoccurs predominantly as As5+ with minor amount of As1� in the uncovered tailings. Coinciding withthe disappearence of goethite rims on sulfide particles, the biosolids-covered tailings have As3+ speciesgradually increasing in proportion towards the cover. Leaching tests indicated that the As concentrationsin the leachate gradually increase from less than 0.085 to 13 mg/L and Fe from 28 to 179 mg/L towardsthe biosolids cover. These are in sheer contrast to the leachate concentrations of less than 0.085 mg/L Asand 24–64 mg/L Fe obtained from the uncovered tailings confirming the role of biosolids-influencedreduction and mobilization of As in the form of As3+ species. The evidence suggests that reductive disso-lution of goethite influenced by the biosolids-cover caused the mobilization of As as As3+ species.

Crown Copyright � 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction

Mine wastes have the potential to adversely impact the envi-ronment through the generation of acidic drainage and release ofdissolved metals and metalloids. Arsenic is an abundant elementin mine wastes resulting from the exploitation of metalliferousdeposits of base metals and Au. Arsenic is essentially containedin gangue minerals such as arsenopyrite (FeAsS) and pyrite (FeS2)which are discarded as waste, mostly in the form of mine tailings.These sulfide minerals are unstable under atmospheric conditionsand readily decompose by releasing the contained As to the envi-ronment. Arsenic is highly toxic, a known carcinogen at very lowconcentrations, requiring measures for its stabilization and immo-bilization from mine wastes to prevent the contamination of sur-face and groundwater resources. Sulfide-bearing mine tailings areoften covered by water, soil or synthetic materials to limit O2

transport and formation of acid mine drainage. In addition, covermaterials act as physical barriers against erosion and dispersionof fine particulates by wind action.

Remediation and reclamation efforts at mine sites aim to re-store disturbed lands to self-sustaining and ecologically productivestates that are blending with their natural surroundings. For exam-ple, an engineered dry cover system with a successful vegetation

cover at an U mine tailings site helped the establishment of reduc-ing conditions within the tailings that caused the precipitation ofsecondary minerals while isolating the deleterious effects of acidgenerating tailings (Dave et al., 2000; Paktunc and Dave, 2002).

In recent years reutilization of former mine sites for other usessuch as biofuel production has become an attractive closure option.As part of the Green Mines and Green Energy initiative, severalmine tailings sites in northern Ontario are being assessed for theirability with a biosolids cover to supporting crops for biofuel pro-duction as one of the remediation and reclamation options consid-ered for closure. One of the sites is the mine tailings at the formerDelnite gold mine near Timmins, Ontario. During a 27-a periodfrom 1937 to 1964, the Delnite mine processed about 3.5 mil-lion tonnes of Au ore with an average grade of 6.2 g/t (Jamboret al., 1991). Mineral processing operations produced about3 � 106 m3 of tailings which were deposited in a topographicdepression near the mill and impounded to form an area of about27 ha with an elevation of 3–10 m above the surrounding terrain.The tailings are underlain by a discontinuous layer of peat and claydeveloped over a fine-grained sand aquifer and basement till (Jam-bor et al., 1991). The tailings surface was revegetated in 1971 withthe addition of limestone and fertilizer. Currently, the impound-ment is covered by grass and small trees with small patchy barrenareas.

The Delnite mine tailings contain pyrite and arsenopyrite withthe potential to generate mine drainage with As releases. The

50 D. Paktunc / Applied Geochemistry 36 (2013) 49–56

tailings, characteristically carbonate-rich, were studied in detail byBlowes (1990) and Jambor et al. (1991) to understand the effects ofsulfide oxidation in a carbonate-rich tailings environment and thebehavior of As in a carbonate-buffered geochemical system, andmore recently by Paktunc and Thibault (2012) to determine thecurrent state of oxidation and stability of As species in relation tothe closure aspects of the tailings impoundment. The tailingsexhibited oxidization features to an average depth of about 0.6 min 1985 (Blowes, 1990). At the time of this study, the oxidation ex-tended to a depth of 1.5 m from the surface evidenced from drillingdown to 6-m depths at six locations within the impoundment(Paktunc and Thibault, 2012). At such sites where mine waste oxi-dation has progressed for more than a decade and the oxidationfront has migrated deeper into the tailings, the emphasis of reme-dial actions should shift from preventing sulfide oxidation to man-aging the fate of dissolved oxidation products (Blowes et al., 2013).

Reutilization of barren mine tailings sites while remediating thesites for closure is a novel concept but such undertakings shouldnot compromise the geochemical stability of the tailings. In addi-tion to supporting crops, biosolids covers would slow down O2 in-gress into tailings, consume O2 and establish reducing conditions.In order to test the ability of a biosolids-cover in such roles whilemaintaining the geochemical stability of the underlying tailings,a field test plot utilizing a 1-m thick pulp and paper biosolids coverwas established in 2008 (Tisch et al., 2012). After about 2 a underbiosolids cover, tailings were sampled with the objectives of char-acterizing the form and nature of As and to assess the impact of theorganic cover on the stability of As species. The emphasis of thepresent study was on the mineralogical aspects and their implica-tions on the stability of the As-carriers. Aqueous geochemistry andgeomicrobiology aspects are currently being assessed as part of along-term column leaching study.

2. Materials and methods

Two cores from the tailings plot covered by biosolids were col-lected by inserting acrylic tubes (�5 � 28 cm) into the tailings afterremoving the biosolids cover. One core from the uncovered tailingswas obtained as a control representing the normal exposed condi-tions of the tailings. The core lengths were about 25 cm each rep-resenting the topmost portions (i.e. upper 25 cm) of the oxidizedtailings in both instances. The cores were capped, sealed andpacked in ice in a cooler before shipping to the laboratory. Thecores were extruded from the acrylic tubes, logged based on theircolor and morphological properties, and subdivided into four por-tions along the core lengths. The samples were dried and processedin an anaerobic chamber for the mineralogical and geochemicalcharacterization studies. Following homogenization, one fractionof each sample was pulverized for chemical and mineralogicalstudies and another fraction, as received, was set in epoxy for pol-ished section preparation. Polished sections were prepared using adiamond paste with no water. The remaining unprocessed tailingswere used in leaching tests.

The bulk tailings samples were dissolved by digestion in sealedvessels under microwave heating and analyzed by Inductively-Coupled Plasma Atomic Emission Spectrometry (ICP-AES) for thedetermination of Si, Al, Fe, Mn, Mg, Ca, Na, K, As and P, and High-Performance Liquid Chromatography (HPLC) for the determinationof SO4 concentrations. Carbon and S determinations were made onoriginal solid samples in ceramic crucibles by the LECO InductionFurnace technique.

Leaching tests involved a modified version of the Toxicity Char-acteristics Leaching Procedure (TCLP) tests (USEPA, 1992) where10 g of dry tailings material was contacted by 200 mL deionizedwater for 20 h. Due to the carbonate-rich nature of the tailings,

pH was not buffered during the tests. The materials were filteredthrough 0.1 lm filters and the solutions were analyzed for As, Feand Ca by ICP-AES.

Powder X-ray diffraction (XRD) patterns were collected using aRigaku D/MAX 2500 rotating-anode powder diffractometer withCu Ka radiation at 50 kV, 260 mA, a step-scan of 0.02�, and a scanrate at 1�/min in 2h from 5� to 70�. The polished sections wereexamined by an optical microscope and a Hitachi variable-pressurescanning electron microscope (SEM). In addition to optical micros-copy, mineral identifications were made using backscattered elec-tron (BSE) images reflecting chemical compositions (i.e. averageatomic numbers) of the mineral grains and semi-quantitativemicroanalyses using an energy-dispersive spectrometer (EDX).Optical and SEM characterization studies were supplemented bya Rigaku Rapid-II microfocus rotating anode X-ray diffractometerequipped with confocal multilayer X-ray optics and a large aper-ture curved image plate. Incident X-rays, generated by Cr Ka radi-ation at an accelerating voltage of 35 kV and a current of 25 mA,were focused to 30, 50 and 100 lm spots marked on the polishedsection surfaces.

X-ray absorption spectroscopy investigations were performedat the PNC-CAT’s bending magnet beamline of the Advanced Pho-ton Source. Samples were packed in Teflon sample holders andsealed by Kapton tape. X-ray absorption near edge structure(XANES) spectra were obtained by scanning the Si (111) mono-chromator at 0.2 eV steps over the edge region, representing aver-ages of four to eight individual measurements. Individual scanswere monitored for possible beam-induced oxidation. Three sam-ples were reanalyzed after 15 months to assess the stability ofthe As species in their original containers when exposed to ambi-ent conditions (Fig. SI-1a and b; Table SI-1). A beamsize of1 � 4 mm was used and data collection was made at room temper-ature both in transmission and fluorescence modes. A gold foil wasused for monitoring and calibrating energy shifts. Data reductionand analysis were made by ATHENA (Ravel and Newville, 2005)and the least-squares fitting analyses of the XANES spectra wereperformed with LSFitXAFS (Paktunc, 2004).

3. Results and discussion

3.1. Chemical composition

With 39–50 wt% SiO2 and 9–13 wt% Al2O3, the chemical compo-sitions of the tailings reflect the dominance of the rock-forming sil-icate minerals. Total Fe concentrations range from 6.1 to 9.8 wt%.Sulfide S concentrations are in the 0.3 to 1.2 wt% range, whereasSO4 concentrations are less than 0.17 wt%. Arsenic concentrationsrange from 0.15 to 0.36 wt% (Table 1) and show a crude positivecorrelation with the Fe concentrations. There are no apparent dif-ferences in the bulk chemical compositions of the biosolids-cov-ered and control tailings, and no significant variations exist withdepth.

3.2. Leaching tests

TCLP tests indicate that As concentrations in the leachate of thecontrol tailings are below the detection limit of 0.085 mg/L whichare in line with the porewater As concentration of 0.05 mg/L deter-mined by Blowes (1990) for the oxidized tailings. Corresponding Feconcentrations in the TCLP extracts of the exposed tailings are inthe 24–64 mg/L range. Tailings covered by biosolids, on the otherhand, indicate increased As and Fe concentrations in the leachateas the biosolids cover is approached (i.e. from <0.085 to 13 mg/LAs and 28 to 179 mg/L Fe; Table 1). Changes in the bulk chemicalcomposition of the tailings are too small to be the cause of the

Table 1TCLP results and partial composition of the tailings solids from uncovered (DC1) andbiosolids-covered tailings (DS1 and DS2).

Depth TCLP extracts Tailings solid

(cm) As Ca Fe pH As S� Fe Ca

(mg/L) (wt%)

DC1-A 1.5 <0.085 546.7 64.3 4.3 0.21 0.47 7.37 3.29DC1-B 5.0 <0.085 691.1 54.5 4.6 0.29 0.71 8.64 4.22DC1-C 11.5 <0.085 779.1 38.5 4.7 0.24 0.73 8.17 4.33DC1-D 18.5 <0.085 872.0 23.9 4.8 0.22 0.52 9.02 4.65DS1-A 1.5 10.84 438.9 179.2 4.2 0.33 0.59 8.92 4.17DS1-B 5.5 3.60 479.1 152.6 4.2 0.34 1.17 9.84 3.78DS1-C 12.5 0.86 521.0 112.0 4.3 0.15 0.72 6.10 3.78DS1-D 18.5 0.12 872.0 28.0 5.2 0.24 0.30 8.67 3.99DS2-A 1.5 13.02 323.2 123.7 4.0 0.28 0.71 8.15 3.92DS2-B 6.0 13.65 350.8 124.5 4.1 0.18 0.71 7.16 3.76DS2-C 13.0 5.31 486.3 105.0 4.2 0.36 0.37 9.39 3.79DS2-D 19.0 <0.085 868.3 27.7 4.8 0.24 0.28 8.99 3.88

Sulfide-S calculated by subtracting S bound to SO4 from total S.pH measurements are from leachates.Errors in the determinations are within 5% for TCLP extracts, and 2% for Fe, Ca andAs, and 5% for S in the tailings solids.

D. Paktunc / Applied Geochemistry 36 (2013) 49–56 51

differences between the extraction test results from the exposedand biosolids-covered tailings. The leachate concentrations fromthe bottom portions of the cores (�19 cm depth) are similar tothose from the control tailings. The gradual increases in the leach-ate concentrations towards the biosolids cover indicate the forma-tion of readily soluble secondary minerals under the influence ofthe biosolids cover.

3.3. Mineralogy

The mineralogical compositions of the tailings samples are rel-atively uniform with quartz, dolomite, chlorite, albite, muscovite,tourmaline, illite and pyrite being the dominant minerals. Sideriteis present in minor quantities. Semi-quantitative X-ray microanal-yses indicate that dolomite is Fe-bearing and siderite is magnesian.Based on the powder XRD patterns, there are no apparent differ-ences in mineralogy with depth and between the biosolids-coveredand uncovered tailings. Pyrite occurs in concentrations rangingfrom 0.5 to 2.2 wt% and arsenopyrite in lesser quantities. Pyrrho-tite is the other sulfide mineral occurring in trace quantities.

Pyrite and arsenopyrite occur as small particles (<25 lm) andmostly in liberated form. The liberated particles are enveloped bysecondary Fe oxyhydroxides in the uncovered tailings and the dee-per portions of the biosolids-covered tailings (Fig. 1; Figs. SI-2–4).Micro-XRD analyses indicate that the Fe oxyhydroxide rims are

Fig. 1. Backscattered electron images showing pyrite (Py) and arsenopyrite (Apy) particleabbreviations are after Kretz (1983).

goethite in accordance with the findings of Jambor et al. (1991).In contrast, pyrite particles in the shallower portions of the bioso-lids-covered tailings are devoid of such oxidation rims. These sul-fide particles are rugged and display corrosion-like features onparticle surfaces indicating that the particles are relicts of sulfideoxidation (Fig. 2).

Semi-quantitative X-ray microanalyses of pyrite and goethiteoxidation rims indicate that they both contain low levels of As.Quantitative results based on electron microprobe analysis of pyr-ite and goethite in the Delnite mine tailings limit the As concentra-tions in pyrite to be less than 0.7 wt% As (Jambor et al., 1991) and0.9 wt% As (Paktunc and Thibault, 2012), and those in goethite tobe 1.5–18.7 wt% As2O5 (Jambor et al., 1991) and 2.0–14.3 wt%As2O5 (Paktunc and Thibault, 2012).

3.4. Quantitative arsenic speciation

Arsenic K-edge XANES spectra of the tailings are shown in Fig. 3.The uncovered tailings are characterized by the main absorptionpeak centered at about 11,875 eV indicating that the samples aredominated by As5+ species. With the exception of a shoulder atthe lower energy side of the main peak in the uppermost sample,the spectra are broadly similar indicating that the changes in Asspeciation with depth are limited. The shoulder present on thelow energy side of the main absorption peak suggests the presenceof As1� species similar to those of arsenopyrite and arsenian pyrite(Paktunc et al., 2004). Derivatives of the normalized XANES spectrasuggest that As1� is also present in the deeper samples but at lowerconcentrations.

Arsenic K-edge XANES spectra of the tailings covered by bioso-lids are different from those of the uncovered tailings (Fig. 3). Thetailings sampled by both cores are characterized by the presence ofAs species with several oxidation states. The tailings samples rep-resenting 17–20 cm depths at both cores are somewhat similar tothe control tailings in the sense that they are dominated by As5+

species. Tailings above the 17 cm depth display a reduced As spe-cies at about 11,870 eV. Derivatives of this absorption peak are rel-atively uniform at 11869.2 and 11869.6 eV indicating the presenceof As3+ in the biosolids-covered tailings. Plots of the derivatives re-veal that this reduced As species is also present in the lowermostsamples of the biosolids-covered tailings although at much lowerconcentrations (Fig. 3).

Spectral features above the edge and EXAFS spectra between 3and 10 �1 for the uncovered tailings and deeper portions of thebiosolids-covered tailings resemble the spectra of As-bearing goe-thite and ferrihydrite (Paktunc et al., 2004, 2008).

XANES spectra of the samples were fitted with a series of modelcompounds to quantify the distribution of the As species. Model

s with secondary goethite (Gt) rims in samples from the uncovered tailings. Mineral

Fig. 2. Backscattered electron images of pyrite particles (light gray-white) showing oxidative dissolution features without secondary goethite rims in tailings immediatelybelow the biosolids cover.

Fig. 3. (Top) Derivative of normalized As K-edge XANES spectra of the uncovered (DC1) and biosolids-covered tailings (DS1 and DS2); (bottom) least-squares fitting of thenormalized As K-edge XANES spectra of the uncovered (DC1) and biosolids-covered tailings (DS1 and DS2) to end-member compounds shown below the fitted spectra (fromleft to right: arsenopyrite, Fe2+–As3+ and goethite) and listed in Table 2; fitting range: 11,855–11,905 eV. Solid blue lines are experimental spectra; Lines with red circles aresimulated spectra; Uniform vertical scales in all; A, B, C and D refer to sample depths of 1.5, 5–6, 12–13 and 18–19 cm (Table 1); spectra shifted in absorbance values fordisplay purposes; however, vertical scale remains uniform for all the samples.

52 D. Paktunc / Applied Geochemistry 36 (2013) 49–56

D. Paktunc / Applied Geochemistry 36 (2013) 49–56 53

compounds included in the analysis are goethite, ferrihydrite, sco-rodite, ferric arsenate, arseniosiderite, As2O5, As5+–solution, Na–arsenate, As2O3, As3+–solution, Na–arsenite, tooeleite, a biogenicFe2+–As3+, arsenopyrite, pyrite and orpiment (Paktunc et al.,2004, 2008; Smith et al., 2005; Paktunc, 2008; Babechuk et al.,2009) and 18 organic compounds (Smith et al., 2005). Uncertain-ties associated with the estimated quantities of the As specieswould be about 10% (Foster et al., 1998) and up to 19% for quanti-ties of 10% and greater (Paktunc et al., 2003). As-adsorbed goethitemakes up between 83% and 100% of the As species in the uncov-ered tailings (Table 2). The remainder is made of As1� as in arseno-pyrite or pyrite. With concentrations of 84% and 98%, As5+–goethiteis the predominant As-carrier in the lowermost samples from thebiosolids-covered tailings. Arsenic5+–goethite is also present inthe shallower portions of the biosolids-covered tailings but inmuch decreased concentrations. Quantitative speciation resultsshow that As3+ species, best represented by a biogenic Fe2+–As3+

(Babechuk et al., 2009), gradually increase from 16% to 76% and2% to 80% towards the biosolids cover (Table 2). Re-scaning ofthe samples with high As3+ after 15 months exposure to ambientatmospheric conditions indicated minor oxidation of the sampleswith increases of the As5+/(As3+ + As5+) fraction by 0.03 and 0.06(Table SI-1; Fig. SI-1) suggesting that the As3+ species were rela-tively stable in their Teflon sample holders sealed by Kapton tape.

3.5. Influence of biosolids-cover on As mobilization

Numerous studies have attributed the mobilization of As fromnatural and anthropogenic sources in subsurface environments toreductive dissolution of Fe(III) oxyhydroxide carriers (Nicksonet al., 2000; Harvey et al., 2002; Toevs et al., 2008; Fendorf et al.,2010; Charlet et al., 2011; Lindsay et al., 2011). The Delnite test-plots case provides further evidence for the mechanism of As re-leases in a tailings environment. Quantitative changes in the Asspeciation with depth and the differences between biosolids-cov-ered and uncovered tailings clearly indicate that As5+ is reducedto As3+ in the upper portions of the tailings below the biosolidscover. Gradual increases in the proportion of As3+ species towardthe biosolids cover and their complete absence in the uncoveredtailings suggest a direct link between the biosolids and the estab-lishment of reducing conditions influencing the dissolution of goe-thite. Similarity of the As K-edge XANES spectra of the uncoveredtailings to those of goethite and ferrihydrite with adsorbed arse-nate is in agreement with more frequent occurrence of sulfide par-ticles with Fe oxyhydroxide rims in the control tailings and thedeeper portions of the biosolids-covered tailings. The occurrenceof goethite rims on the sulfide particles of the oxidized tailingswas also noted by Jambor et al. (1991) and more recently by

Table 2Distribution of As species based on least-squares fitting of As K-edge XANES spectra.

Depth (cm) Gt Apy Fe2+–As3+ gff dev

DC1-A 1.5 83 17 0.06 0DC1-B 5.0 95 5 0.07 0DC1-C 11.5 95 5 0.03 0DC1-D 18.5 100 0.13 0DS1-A 1.5 24 76 0.03 �1DS1-B 5.5 29 71 0.04 0DS1-C 12.5 44 8 48 0.09 1DS1-D 18.5 84 16 0.11 �2DS2-A 1.5 16 5 80 0.01 0DS2-B 6.0 14 8 78 0.01 0DS2-C 13.0 34 66 0.07 �2DS2-D 19.0 98 2 0.11 �2

Gt: As-adsorbed goethite; Apy: arsenopyrite; Fe2+–As3+: biogenic ferrous arsenite;gff: goodness-of-fit parameter; dev: deviation from optimal total.

Paktunc and Thibault (2012). Thus, the absence of goethite rimson relict pyrite particles in the upper portions of the biosolids-cov-ered tailings accompanied by gradual increase of As3+ at the ex-pense of As5+ can be attributed to reductive dissolution ofgoethite and the concomitant release of adsorbed As5+ as As3+ spe-cies, directly influenced by the overlying biosolids. The experimen-tal study of Tufano et al. (2008) highlights the prominent role ofAs5+ reduction, coupled by the reductive dissolution of Fe (hydr)o-xides on the increased mobility of As3+ over As5+. It appears thatthe progression of the reducing front has reached to a depth ofabout 17 cm in 2 years. Considering the indispensable role thatbacteria play on the reduction of Fe3+ in soil and tailings environ-ments containing Fe(III) oxyhydroxides (Cummings et al., 2000;Kocar et al., 2006; Tufano and Fendorf, 2008; Tufano et al., 2008;Schippers et al., 2010; Charlet et al., 2011), and correlation of highconcentrations of soluble As and Fe2+ with the populations of Fe-reducing bacteria at a Au mine site (Stichbury et al., 2000), it wouldbe prudent to suggest the influence of bacteria on Fe reduction andAs releases. The role of anaerobic Fe(III)-reducing bacteria can onlybe inferred at this point. Reactive organic C needed for microbialreduction of Fe3+ (Nickson et al., 2000; Stichbury et al., 2000; Fen-dorf et al., 2010; Lindsay et al., 2011) and As5+ is likely to be sup-plied by the biosolids cover.

Iron concentrations in the TCLP extracts of the uncovered tail-ings display a gradual increase from 24 to 64 mg/L towards the sur-face (Table 1). The biosolids-covered tailings display a similarbehavior but with much increased Fe concentrations from 28 to179 mg/L. Accompanying these increases are slight decreases inthe leachate pH suggesting that the Fe releases are in part relatedto the dissolution of residual Fe sulfates such as jarosite andmelanterite that may have formed during the oxidation of sulfideminerals. Dissolution of jarosite and melanterite releases acidityalong with Fe. In addition, it is possible that some Fe in the leachateresults from the dissolution of siderite. Siderite solubility data(Bruno et al., 1992) indicate that siderite would be dissolving atthe observed pH values of 4–5. Increased Fe concentrations inthe leachate near the biosolids-cover are probably related to thedissolution of the reduced Fe species formed under the influenceof the biosolids cover.

Increased Fe and As concentrations in the extracts as the bioso-lids cover is approached indicate that reduced As and Fe species(i.e. As3+ and Fe2+) are temporarily retained in the tailings as solu-ble species, similar to that hypothesized by Masue-Slowey et al.(2010). Detailed mineralogical examinations failed to detect anysecondary precipitates of Fe2+ and As3+ such as magnetite (Tufanoand Fendorf, 2008; Masue-Slowey et al., 2010). Possible occur-rences of fougerite and siderite were suspected based on micro-XRD analyses; however, their positive confirmation was not possi-ble due to the complex and heterogeneous nature of the tailingsmatrices. Releases of As were much higher than those of Fe dis-playing exponential trends (Fig. 4) similar to those resulted froma bacterial reduction experiment involving As-bearing ferrihydrite(Burnol et al., 2007). Such co-variation patterns could not have re-sulted from competitive sorption between As3+ and Fe2+ evidencedfrom the experimental work of Dixit and Hering (2006). Other pos-sibilities for the observed co-variation of As3+ and Fe2+ would bethe sorption of As3+ on newly formed reduced Fe minerals.Recently, Charlet et al. (2011) emphasized the importance ofnano-sized Fe2+ and Fe3+ minerals on the effectiveness of As3+

sorption and its immobilization. Burnol et al. (2007) interpretedthe decoupling behavior of As and Fe releases during bacterialreduction as resulting from reduction of As to weakly sorbed As3+

and competition of the aqueous carbonate species for the sorptionsites on ferrihydrite. The displacing effect of dissolved carbonateon As adsorption has been considered to be an important causefor the elevated concentrations of As in groundwater (Appelo

Fig. 4. Variation of As with Fe in TCLP leachates; solid black squares: uncoveredtailings; solid red squares: biosolids-covered tailings (DS1); solid red triangles:biosolids-covered tailings (DS2). Blue circles: incubation experiments involvingadsorbed and coprecipitated As-bearing ferrihydrite digitized from Fig. 8 of Burnolet al. (2007); Curves are exponential fits to data with reduced Chi-square values of0.011 and 0.0068 for TCLP leachates.

Fig. 5. Variation with depth of As concentrations in the TCLP extracts and fractionof As3+ in the tailings solids from the biosolids covered tailings. Uncovered tailingsplot along the y-axis.

54 D. Paktunc / Applied Geochemistry 36 (2013) 49–56

et al., 2002). Modeling results (Burnol et al., 2007) suggest that thecompetition becomes worse for As3+ at lower pH. For instance, atpH 5 the fraction of sorbed As3+ on ferrihydrite drops to about35% from 92% in the absence of carbonate species. In this case,availability of carbonate species in solution due to siderite dissolu-tion and degradation of the organic matter from the biosolids coverwould promote As release in the biosolids-covered tailings. Inaddition, the leveling off of Fe releases at around 125 and175 mg/L can be explained by the precipitation of a reduced Femineral.

Based on electron microprobe analyses, the average molar Fe/Asratio of goethite rims on sulfide minerals in the Delnite mine tail-ings is 18 (Jambor et al., 1991) and 20 (Paktunc and Thibault,2012). These are broadly comparable to the molar Fe/As ratios ofthe TCLP extracts from the tailings near the biosolids cover (i.e.22 for DS1-A and 13 for DS2-A) suggesting that Fe and As releasedfrom the reductive dissolution of goethite are precipitated as solu-ble compounds or retained as weakly sorbed species. Changes withdepth in the As3+ fractions of the tailings are similar to those of theAs concentrations in the TCLP extracts (Fig. 5) suggesting that thefraction of As released during the leaching tests is essentially thereduced fraction of As. The mechanism of such releases can beattributed to chemical or microbial reductive dissolution of goe-thite driven by organic C supplied by the biosolids cover.

3.6. Implications for remediation and closure

It appears that As is effectively stabilized by goethite in the oxi-dized tailings which is about 1.5 m thick covering the entire tail-ings impoundment. Goethite occurring as rims on sulfide mineralparticles not only protects the sulfide mineral surfaces from furtheroxidation by limiting the transport of oxidants and products, butalso immobilizes As through sorption. Arsenic releases in the oxi-dized portions of the uncovered tailings are effectively controlledat <0.085 mg/L by goethite, which is also evidenced from theporewater concentrations of As at 0.05 mg/L (Blowes, 1990). Theselevels are much lower than the Canadian metal-mine effluentslimit of 0.5 mg/L (Government of Canada, 2002) and the Québecprovincial mine effluent guideline of 0.2 mg/L (Gouvernement duQuébec, 2005). Provided that reducing conditions are not imposedon the oxidized tailings, and arsenopyrite and pyrite are protected

from further oxidation, As releases from the Delnite mine tailingswill be limited.

In contrast, As concentrations in the TCLP extracts indicate thateffluents originating from the biosolids-covered tailings withinabout 17 cm would exceed both mine effluents criteria. UnlessAs3+ is further reduced and stabilized as secondary sulfides withprolonged reductive conditions, accumulation of mobile As3+ spe-cies in the tailings through reductive dissolution of goethite influ-enced by the biosolids cover would pose concerns since As may bereleased to the receiving water bodies. This forms another pathwayby which aquifers can be contaminated through the accumulationof dissolved As3+ in addition to the settings where redox conditionsinfluence the concentrations of reactive Fe and S through SO2�

4

reduction and precipitation of sulfide minerals (Toevs et al.,2008) and dissolved concentrations of O2 and Fe2+ (O’Day et al.,2004).

Increased amounts of As3+ species that are more mobile andtoxic than As5+ would require reconsideration of the suitability ofthe biosolids cover at the Delnite mine tailings and reutilizationof the tailings area for the production of biofuel crops. This wouldalso be applicable to other tailings sites with oxidized sulfide tail-ings that are rich in As. In such cases, no-cover or dry cover optionswithout organic materials appear to be more viable for long-termAs control from a geochemical stability viewpoint. This does notnecessarily imply that other types of tailings covers would imposesimilar conditions.

4. Summary and conclusions

Pyrite and arsenopyrite particles are rimmed by secondaryoxidation products in the exposed tailings and the lower portionsof the biosolids-covered tailings. Such secondary oxidationproducts are not present around the relict sulfide particles in the

D. Paktunc / Applied Geochemistry 36 (2013) 49–56 55

tailings immediately below the biosolids cover suggesting the dis-solution of goethite.

With leachate As concentrations of less than 0.085 mg/L, As spe-cies appear to be stable in the exposed tailings. Tailings covered bybiosolids on the other hand indicate increased As concentrations inthe leachate as the biosolids cover is approached. Iron concentra-tions in the leachate of the biosolids-covered tailings also displayincreases towards the top.

The biosolids-covered tailings display changes in the solid Asspeciation with depth. The proportion of As3+ species gradually in-crease towards the biosolids cover from about 20 cm depthwhereas there are no changes in the As speciation with depth forthe uncovered tailings where they are dominated by the As5+ spe-cies. These changes mimic those of the leaching tests suggestingthat As3+ species represent the mobilized portion of As. Thesechanges also coincide with the disappearance of goethite rims onpyrite and arsenopyrite and suggest reductive dissolution of goe-thite and accompanying As5+ reduction as the main cause of Asmobilization from the tailings influenced by the biosolids cover.In this case, closure options imposing anaerobic conditions onthe oxidized top portions of the tailings such as the biosolids coverdo not appear to be suitable for the Delnite tailings impoundment.

The study results provide new information for improved under-standing of mineral behavior, and changes in As speciation and po-tential mobility under reducing conditions imposed by organiccover materials. These, in turn, can be used to better assess thelong-term stabilities of the arsenical minerals in tailings impound-ment and develop long-term storage or disposal options along withimproved environmental impact assessments for environmentalprotection.

Acknowledgements

The contributions of the following are acknowledged: BryanTisch for discussions and collection of the core samples, AnalyticalServices Group of CANMET-MMSL for bulk chemical analyses andextraction tests, Derek Smith for powder XRD analyses, Dr. DougGould for reviewing the manuscript. The study was funded in partby Goldcorp. Organic arsenic compounds were kindly provided byDrs. Paula Smith and Ken Reimer of Royal Military College. X-rayabsorption spectroscopy experiments were performed at thePNC/XOR beamline, Advanced Photon Source (APS), Argonne Na-tional Laboratory which is supported by the US Department of En-ergy under Contracts W-31-109-Eng-38 (APS) and DE-FG03-97ER45628 (PNC-CAT) through a General User Proposal to theauthor and a Partnership Proposal funded by the Natural Sciencesand Engineering Research Council of Canada through a MRS grant.Constructive comments from two anonymous journal referees andeditorial handling of the manuscript by Ron Fuge and Kaye Savageare acknowledged.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.apgeochem.2013.05.012.

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