Arsenopyrite and pyrite bioleaching: evidence from XPS, XRD and ICP techniques

ORIGINAL PAPER

Arsenopyrite and pyrite bioleaching: evidence from XPS,XRD and ICP techniques

Marzia Fantauzzi & Cristina Licheri & Davide Atzei &Giovanni Loi & Bernhard Elsener & Giovanni Rossi &Antonella Rossi

Received: 5 April 2011 /Revised: 19 June 2011 /Accepted: 28 July 2011 /Published online: 17 August 2011# Springer-Verlag 2011

Abstract In this work, a multi-technical bulk and surfaceanalytical approach was used to investigate the bioleachingof a pyrite and arsenopyrite flotation concentrate with amixed microflora mainly consisting of Acidithiobacillusferrooxidans. X-ray diffraction, X-ray photoelectron spec-troscopy (XPS) and X-ray-induced Auger electron spec-troscopy mineral surfaces investigations, along withinductively coupled plasma-atomic emission spectroscopyand carbon, hydrogen, nitrogen and sulphur determination(CHNS) analyses, were carried out prior and after bioleach-ing. The flotation concentrate was a mixture of pyrite(FeS2) and arsenopyrite (FeAsS); after bioleaching, 95% ofthe initial content of pyrite and 85% of arsenopyrite weredissolved. The chemical state of the main elements (Fe, Asand S) at the surface of the bioreactor feed particles and ofthe residue after bioleaching was investigated by X-rayphotoelectron and X-ray excited Auger electron spectros-

copy. After bioleaching, no signals of iron, arsenic andsulphur originating from pyrite and arsenopyrite weredetected, confirming a strong oxidation and the dissolutionof the particles. On the surfaces of the mineral residueparticles, elemental sulphur as reaction intermediate of theleaching process and precipitated secondary phases (Fe–OOH and jarosite), together with adsorbed arsenates, wasdetected. Evidence of microbial cells adhesion at mineralsurfaces was also produced: carbon and nitrogen wererevealed by CHNS, and nitrogen was also detected on thebioleached surfaces by XPS. This was attributed to thedeposition, on the mineral surfaces, of the remnants of abio-film consisting of an extra-cellular polymer layer thathad favoured the bacterial action.

Keywords X-ray photoelectron spectroscopy (XPS) .

Surface analysis . Pyrite . Arsenopyrite . Acidithiobacillusferrooxidans . Biohydrometallurgy. Bio-oxidation .

Bioleaching . Biomining . Extra-cellular polymericsubstance (EPS)

Introduction

The term biomining is used to describe the processing of(precious) metal containing ores and concentrates bymicrobial mediation [1]. This process is also known asbiohydrometallurgy or bioleaching especially when bio-mining technologies are applied to base metals and to goldrecovery from gold ores or concentrates where gold is inter-grown with refractory sulphide minerals that are dissolvedby bio-oxidation, i.e. oxidation mediated by microorgan-isms [2]. These techniques are particularly suitable for low-grade ores and are more environmentally friendly comparedwith conventional physicochemical mineral beneficiation

M. Fantauzzi : C. Licheri :D. Atzei :B. Elsener :A. RossiDipartimento di Chimica Inorganica e Analitica, INSTMResearch Unit, Centro Grandi Strumenti Università di Cagliari,09042 Monserrato, Cagliari, Italy

G. LoiDepartment of Medical Sciences Mario Aresu,Università di Cagliari,09042 Monserrato, Cagliari, Italy

G. RossiDepartment of Geoengineering and Environmental Technologies,Università di Cagliari,09100 Cagliari, Italy

A. Rossi (*)Dipartimento di Chimica Inorganica ed Analitica,SS 554 Bivio per Sestu,09100 Cagliari, Italye-mail: [email protected]

Anal Bioanal Chem (2011) 401:2237–2248DOI 10.1007/s00216-011-5300-0

processes [1]. Bio-oxidation pre-treatment of refractorysulphide ores prior to cyanidation represents a successfulapplication of biohydrometallurgy which is currentlypractised in several plants in Africa, Australia, SouthAmerica and Asia [3] to increase gold recovery rates fromsulphide ores such as pyrite (FeS2), arsenopyrite (FeAsS)and pyrrhotite (Fe1−xS; x=0 to 0.2). The Goldfields2BIOX® process at Fairview Mine (South Africa) isprobably one of the bio-oxidation pre-treatment processeswith the longest history of operation. It has been inoperation since 1986, and a refractory arsenopyritic gold-bearing concentrate was beneficiated. The process uses amixed microflora consisting of three naturally occurringbacterial strains i.e. Acidithiobacillus ferrooxidans, Acid-ithiobacillus thiooxidans and Leptospirillum ferrooxidans,which dissolve the sulphide matrix thus extracting thefinely inter-grown gold and making it less expensivelyamenable to subsequent cyanidation. Along with economicinterest, oxidative dissolution mediated by microorganismsis of environmental concern since the latter are usuallypresent in acid mine drainage (AMD) and they play adecisive role in the release of toxic elements into theenvironment. The objective of the present investigation,that was carried out in the framework of a wider researchprogram on the environmental risk associated to theoccurrence of oxidation of sulphide minerals, was to studythe oxidation of a Fairview concentrate, containing bothpyrite and arsenopyrite, by means of a mixed microfloraconstituted by A. ferrooxidans, A. thiooxidans and L.ferrooxidans [4] in a laboratory-scale bioreactor. The studyfocused on bioleaching efficiency but especially on thechemical state of iron, arsenic and sulphur on the surface ofthe mineral particles.

The surface of the mineral particles is the criticalinterface in the bioleaching system that ultimately controlsits interaction with the bioleaching solution and the micro-organisms; thus, surface analytical techniques such as XPSand X-ray-induced Auger electron spectroscopy (XAES)are particularly well suited. In previous works, the chemicalstate of the elements at enargite (Cu3AsS4) both in AMD-simulating abiotic conditions [5–8] and in the presence ofA. ferrooxidans [9] was analyzed by XPS and XAES. Basedon the Auger parameter, the authors of this paper proposeda mechanism for enargite oxidation in acidic conditions viathe formation of a metal-deficient polysulphide layerpresent at the enargite surface.

The identification of the chemical state of sulphurspecies produced by (bio)oxidation of sulphide minerals isthe subject of heated argument among researchers. Elemen-tal sulphur was found as a product of mineral oxidation onarsenopyrite and pyrite ([10] and references cited therein[11]). Using sulphur K-edge XANES (X-ray absorptionnear edge structure) elemental sulphur, thiosulphate species

and jarosite were revealed [12] after pyrite leaching withthermophile Acidianus manzaensis. Other authors [13, 14]instead reported polysulphide species together with sulphuroxy-anions on oxidized arsenopyrite and pyrite. Differentproducts of oxidation may reflect different mechanisms ofdissolution of sulphide minerals in acidic environment [15,16]: oxidation via thiosulphate (FeS2, MoS2 and WS2) andoxidation via polysulphides (ZnS, CuFeS2 and FeAsS).

As a consequence of mineral oxidation, secondary phasescan precipitate onto the surface of minerals. As such phases,the following were identified: jarosites (MFe3(SO4)2(OH)6)[11, 12, 17, 18], iron oxy-hydroxides (FeOOH) [11] andscorodite (FeAsO4*2H2O) [11] or amorphous ferric arsenate[11, 17]. The precipitation of secondary phases is ofenvironmental concern because they are effective As(V)sorbents in highly acidic environments, thus representing anatural way to limit As(V) concentration in AMD [19]. Onthe other hand, it represents a limit in biomining technologies,since secondary phases behave as a passivating layer [20].

In this paper, the surface of a commercial pyrite/arsenopyrite concentrate (Fairview Mine) was characterizedin view of identifying the chemical state of the elements atthe surface and of the secondary phase precipitated on it.Bulk solution analyses were carried out to demonstrate theefficient dissolution of pyrite and arsenopyrite by thebioleaching process.

Experimental

Materials

Bioreactor feed

The arsenopyrite/pyrite flotation concentrate was suppliedby Fairview Mine (South Africa). This concentrate was dryground in a ceramic ball mill to <75 μm. The concentratewas characterized by different bulk and surface analyticaltechniques (see “Results” section). After dissolution insulphonitric mixture, a residue of 0.39 g/g was found.

Bacterial cultures

The inoculum for the bioleaching experiments consisted ofa mixed microflora called FC [4] isolated from the aciddrainage effluents of the complex sulphide ores mine ofFenice Capanne, Tuscany, Italy. A. ferrooxidans and A.thiooxidans were the main strains of the inoculum alongwith some L. ferrooxidans strains. This inoculum wasgrown on suspensions of Silverman and Lundgren 9Kculture medium [21] with sulphide minerals (pH=2.38) andFairview Mine flotation concentrate to which it was thusadapted.

2238 M. Fantauzzi et al.

The A. ferrooxidans strain was found to be adaptable totoxic metals such as arsenic, probably owing to its DNAcharacteristics [22].

The bioleaching process

The bioreactor for bioleaching

Bioleaching runs were performed in a conventional stirredtank reactor (STR) consisting of a Pyrex glass jacketed 2-dm3

total volume cylindrical tank with hemispherical bottom(Fig. 1). The temperature of the suspension in the bioreactorwas kept constant at 28 °C by circulating constanttemperature water in the jacket. The tank was provided witha Pyrex glass lid with openings for the stirring shaft, for thecondenser and for sampling. The reactor was fitted with fourstainless steel baffles at 90° from each other, a Rushton-typesix-blade stainless steel turbine and an air bubbler locatedbelow the turbine. The turbine Pyrex glass shaft was set inmotion by a variable-speed electric motor. The aeration wasprovided by a non-lubricated air compressor: the air passedthrough a glass-wool sterile-filter for preventing contamina-tion followed by an air-cleaning bottle containing sterilewater. The flow rate, measured with a floating elementflowmeter, was kept slightly higher than that of flooding[23–26], in order to provide the aerobic microflora with thehighest air availability consistent with the flow-dynamiccharacteristics of the reactor.

Procedure

Bioleaching runs were carried out in the bioreactor onsuspensions consisting of 25 g of dry ground flotation

concentrate and 1,500 cm3 of 9k medium [21]. The initialpH of the suspension was adjusted to 2.3 with sulphuricacid. The bioleaching runs were considered terminatedwhen the pH remained constant (around pH=0.9) for3 days. The solid residue was filtered through a conven-tional glass filter funnel fitted with a suitable filter paper;then the filter paper, after repeated rinsing with distilledwater, was dried at 50 °C in a laboratory oven for 10 h andsubsequently stored in Scheibler-type vacuum desiccators atroom temperature. The powder finally was transferred totest tubes closed with screw plugs at the moment they hadto be analyzed with the various instruments.

The dry residue was characterized by means of X-raydiffraction (XRD), elemental analysis for carbon, hydrogen,nitrogen and sulphur determination (CHNS) and X-rayphotoelectron spectroscopy (XPS). A portion of the residuewas dissolved in nitric acid, and solution analysis by meansof inductively coupled plasma-atomic emission spectrosco-py (ICP-AES) was performed.

Methods

Bulk analysis

X-ray diffraction The diffraction data were collected withCuKα radiation using a Seifert X–3000 diffractometer,equipped with a graphite monochromator in the dif-fracted beam. The copper source was run at 50 KeVand 35 mA. Horizontal slit was 0.03 mm, and detectorslit was 0.31 mm. Analyzer detector was a scintillator.The diffractometer was operated in the step scan mode,8 s per step, between 8.0° and 80.0°. The step size was0.040°.

Fig. 1 Bioleaching equipment.1 agitator shaft, 2 condenser,3 variable speed motor regulator,4 bioreactor tank, 5 thermosta-tion jacket, 6 baffles, 7 variablespeed electric motor, 8 Rushton-type turbine, 9 air sparger, 10flowmeter, 11 air-cleaning bottle,12 air pipe, 13 air compressor,14 thermostatic bath, 15water pipes to and fromthermostated bath

Arsenopyrite and pyrite bioleaching by XPS 2239

ICP-AES Cations present after dissolution in sulphonitricmixture were quantified before and after bioleaching withan ICP instrument model ASH ATOM-SCAN 25 manufac-tured by Thermo-Jarrell.

Micro-elemental analysis CHNS A Carlo Erba EA 1108instrument was used for the analyses of the dry groundconcentrate and the residue after bioleaching.

Surface analysis techniques

XPS spectrometer XPS surface analysis was performed byusing an ESCALAB200 spectrometer manufactured byVacuum Generator Ltd., East Grinstead, UK. The vacuumsystem consists of a nitrogen-trapped turbomolecular pumpand a titanium sublimation pump for the analyzer chamber.The residual pressure in the spectrometer during the runswas about 10−7 Pa. The instrument is equipped with an Al/Mg twin anode. The non-monochromatic Al Kα X-raysource (1,486.6 eV, 15 mA and 20 keV) was used for allexperiments. This allowed acquiring of the X-ray excitedAuger signals of sulphur SKLL at kinetic energies around2,100 eV. The spherical sector analyzer was operated in thefixed analyzer transmission (FAT); a pass-energy of 20 eVwas set for the high-resolution spectra. The full-width athalf-maximum (FWHM) height of the Ag3d5/2 signalrecorded in the same experimental conditions was 1.1 eV.Survey spectra were obtained with 50 eV pass energy. Theintensity/energy response function was evaluated to beequal to 1=

ffiffiffiffiffiffiffi

KEp

[27]. More details regarding the instru-ment configuration are reported in [28].

Sample preparation The samples were mounted as powderon a standard nickel sample holder (VG), covered withdouble-sided adhesive tape and cooled with liquid nitrogenduring the measurement to avoid secondary decompositioneffects.

Data acquisition All the spectra were acquired undercomputer control using VG Eclipse software on an IBM486. To compensate for sample charging during analy-sis, all binding energies were referenced to the carbonC1s signal at 285.0 eV. At least three independentmeasurements were carried out on the samples, and theaccuracy of the measured binding energies was estimat-ed as ±0.1 eV. The instrument was calibrated accordingto the ISO 15472:2001, Surface Chemical Analysis–X-ray Photoelectron Spectrometers–Calibration of EnergyScale [29].

Data processing The spectra were processed usingCASA XPS software (v. 2.3.15 Casa Software Ltd.,Wilmslow, Cheshire, UK). A Shirley–Sherwood back-

ground subtraction was applied before curve fitting [30].Surface quantitative analysis of the concentrate wasdetermined according to [31], assuming sample homoge-neity even if environmental samples could be inhomoge-neous. This assumption is justified by the fact that sampleswere ground and thoroughly mixed prior to XPS analyses;therefore, samples are supposed to be representative of thewhole concentrate. Experimental peak areas were cor-rected for the photoionization cross-sections, σ [32], forthe asymmetry parameter describing the anisotropy ofphotoemission [33] and for the intensity/energy responsefunction that was found to be proportional to KE−0.5 [27].The accuracy for the calculated atomic concentrations isestimated to be ±10%.

Results

Bioreactor feed

The following analyses were carried out on 1 g dry groundsamples of the Fairview flotation concentrate.

Bulk analysis

X-ray diffraction The XRD pattern is shown in Fig. 2. Themain constituents detected are quartz (SiO2), pyrite (FeS2),muscovite (KAl2(Si3Al)O10(OH)2) and clinochlore ((Mg,Al, Fe)12[(Si,Al)8 O20](OH)16). Arsenopyrite (FeAsS) andgypsum (Ca(SO4) (H2O)2) were also detected. Afterdissolution of the bioreactor feed in a sulphonitric mixture,a white precipitate remained (390 mg/g) that was found tobe composed entirely by quartz (Fig 2c).

ICP-AES

Analyses (ICP-AES) were performed on solutions obtainedby dissolving 1 g of dry ground concentrate in sulphonitricmixture. The concentration of Fe, As, K, Ca, Mg and Alwas determined. Mainly iron (195 mg/g), arsenic (41 mg/g)and aluminium (53 mg/g) were detected (Fig. 3) togetherwith potassium, calcium and magnesium.

Micro-elemental analysis The CHNS analysis revealed17.3±0.1% sulphur (Table 1); carbon and nitrogen werenot detected in the dry ground concentrate.

XPS analysis of dry ground bioreactor feed

In the XPS survey spectrum of the flotation concentrate(Fig. 4a), the signals of iron, arsenic, sulphur, carbon,potassium, magnesium, calcium, aluminium, silicon andoxygen were revealed. In addition to the photoelectron lines


usually examined in surface analytical studies, the X-ray-induced Auger electron lines of arsenic (AsLMM) andsulphur (SKLL) were recorded in this work.

Photoelectron lines—binding energies From the high-resolution spectra of the main elements, the bindingenergies were determined by curve fitting after non-linearbackground subtraction. The measured binding energies are

summarized in Table 2. Figure 5 shows high-resolutionspectra of Fe, As and S of samples prior and afterbioleaching. Together with Fe2p, As3d and S2p signals,Al2p and Si2p signals were acquired (not shown).

The carbon C1s signal (not shown) was always multi-component; the most intense peak was used in this work forcharge compensation (C1s=285 eV). The oxygen O1ssignal (not shown) consisted of three components at ca.531.0±0.1 (oxides)eV, at 532.1±0.1 eV (hydroxides) and at533.3±0.1 eV (adsorbed water), respectively.

Iron and sulphur signals showed components attributableto oxidised species (for example, scorodite FeAsO4*2H2Oand sulphates), together with components due to pyrite andarsenopyrite, whereas arsenic only showed a component thatcan be ascribed to arsenic in scorodite (see “Discussion”section). Details of arsenic, iron and sulfur spectra aredescribed hereafter.

Arsenic showed only one signal, As3d, at 45.6±0.1 eV(Fig. 5a). The spectra were fitted with As3d5/2 and As3d3/2

Fig. 3 Concentrations of Fe, As, K, Ca, Mg and Al in the bioreactorfeed and in the bioleaching residue after dissolution of 1.0000 g insulphonitric mixture acid. Data (milligrams per gram) are averagedover three independent experiments. The confidence interval of themean value corresponds to a 95% probability

Fig. 2 XRD patterns of (a) dryground feed, (b) bioleachingresidue and (c) residue of feedsample after dissolution innitrating acid. Q quartz, Ppyrite, A arsenopyrite, Ggypsum, M muscovite,C clinochlore

Table 1 Results of micro-elemental CHNS analyses on dry groundbioreactor feed and on solid bioleaching residue (weight percent)

C N S

Dry ground feed n.d. n.d. 17.3±0.1

Bioleaching residue 2.39±0.03% 1.45±0.01% 1.00±0.05

n.d. not detected


doublet (binding energy difference of 0.7 eV, area ratio of3:2).

The iron, Fe2p, region exhibited a spin-orbit doublet; thedifference between Fe2p3/2 and Fe2p1/2 was found to be13.5 eV, and the area ratio was 2:1. Only the Fe2p3/2component was considered for curve fitting, since it is themost intense peak of the Fe2p doublet, revealing twosignals at 707.2±0.1 eV (assigned to iron in pyrite) and712.1±0.1 eV (assigned to scorodite), respectively(Fig. 5b).

The most intense sulphur line, S2p, is asymmetric due tospin-orbit splitting. The binding energy difference between2p3/2 and 2p1/2 component was 1.2 eV, and the area ratiowas 2:1. Two sulphur S2p signals (162.2±0.2 and 169.4±0.2 eV) were revealed in the detailed sulphur spectra(Fig. 5c). The signal at high-binding energies was assignedto sulphate and the low-binding energy to sulphur in pyriteand arsenopyrite.

X-ray-induced Auger electron lines No attempt was madefor curve fitting of the Auger signals. The kinetic energieswere determined at the peak maximum. The kinetic energyof the AsLMM signal was 1,217.6±0.2 eV; the Augerparameter calculated according to α′=BE(As3d5/2) +KE(AsLMM) [34, 35] was found to be 1,263.2±0.2 eV.

In the X-ray-induced SKLL Auger signal, two compo-nents were found at 2,116.5±0.2 and 2,106.2±0.2 eV.Auger parameters, determined for the two different kinds ofsulphur, were α′S(I)=(162.2+2,116.5)eV=2,278.7 eV andα′S(II)=(169.4+2,106.3)eV=2,275.7 eV.

Bioleaching liquor

In the bioleaching liquor, dissolved arsenic and iron weredetermined with ICP-AES (Table 3). Results show that143 mg iron and 31 mg arsenic per gram of dry feed to thebioreactor were dissolved during bioleaching.

Bioleaching residue

Bulk analysis

X-ray diffraction After bioleaching, only diffraction peaksof quartz, muscovite and clinochlore were detected in theXRD pattern (Fig. 2b); arsenopyrite and pyrite wereapparently below the detection limit (5%) [36].

ICP-AES In order to determine the composition of thebioleaching residue, 1 g of dry sample was dissolvedin sulphonitric mixture acid, and the concentration ofAs, Fe, K, Ca, Mg and Al was determined by ICP-AES. The results (Fig. 3) show that mainly Al (77 mg/g)was found. The amount of iron decreased by a factor offive (42 mg/g) and that of arsenic by a factor of six(7 mg/g). Also magnesium (13 mg/g) and potassium(22 mg/g) were dissolved. The insoluble solids amountedto 570 mg/g.

Micro-elemental analysis The CHNS analysis (Table 1)determined carbon (2.39%), nitrogen (1.45%) andsulphur (1.0%) in the bioleaching residue. Note thatcarbon and nitrogen were not detected in the dry feed;the sulphur concentration greatly decreased after biol-eaching.

Surface characterization of bioleached samples

The XPS survey spectrum (Fig. 4b) shows the presence ofthe same elements as detected in the dry ground feed to thebioreactor. In addition, nitrogen was revealed.

Binding energies of the main peaks recorded in high-resolution spectra are summarized in Table 2, and high-

Fig. 4 XPS survey spectra of dry ground bioreactor feed and ofbioleaching residue. Al Kα X-ray source, 300 W

Table 2 Binding energies values (eV) of photoelectron lines andkinetic energy (eV) of X-ray excited Auger lines of the differentcomponents of bioreactor feed and of bioleaching residue

Element line Dry ground feed Bioleaching residue

As3d5/2 45.6±0.1 45.1±0.1

AsLMM 1,217.6±0.2 1,217.5±0.1

Fe2p3/2 (I) 707.1±0.1 Not present

Fe2p3/2 (II) 712.1±0.1 711.5±0.1

S2p3/2 (I) 162.2±0.1 164.4±0.1

S2p3/2 (II) 169.4±0.1 169.0±0.1

S KLL (I) 2,116.5±0.2 Not detected

S KLL (II) 2,106.3±0.2 2,106.6±0.2

Standard deviations were calculated over three measurements for eachsample. Full-width at half-maximum heights (FWHM) were found tobe constant within 0.1 eV, and their values were As3d5/2=1.8 eV,Fe2p3/2 (I)=1.4 eV, Fe2p3/2 (II)=3.3 eVand S2p3/2=1.6 eV. Line shapewas kept constant in curve fitting procedure


Fig. 5 XPS high-resolution spectra after background subtraction andcurve fitting of dry ground feed sample (left) and of bioleaching residue(right). a Arsenic (As3d5/2) in scorodite-like phase (45.6 eV). b Iron(Fe2p3/2) in pyrite and arsenopyrite (707.1 eV) and in scorodite-like

phase (712.1 eV). c Sulphur (S2p) in pyrite and arsenopyrite (162.2 eV)and sulphate (169.4 eV). d Arsenic (As3d5/2) in arsenates (45.1 eV).e Iron (Fe2p3/2) jarosites and FeOOH (711.5 eV). f Sulphur (S2p) inelemental sulphur (164.4 eV) and sulphate (169.0 eV)


resolution spectra of arsenic, iron and sulphur are shown inFig. 5.

Arsenic, As3d5/2, shows a single signal after bioleachingat 45.1±0.1 eV (Table 2 and Fig. 5d). The kinetic energy ofthe AsLMM Auger line was found at 1,217.5±0.1 eV. TheAuger parameter α′ calculated for arsenic in the bioleachedresidue was 1,262.6 eV (Table 4).

The iron Fe2p3/2 signal (Fig. 5e) showed only onecomponent at 711.5±0.1 eV (Table 2). The component atca. 707 eV (associated to iron in pyrite) present in the dryfeed sample was no more detected.

Sulphur S2p signal (Fig. 5f) consists of two doublets thatwere found at 164.4±0.1 and 169.0 eV (Table 2). TheSKLL Auger line showed only one component at 2,106.2±0.2 eV. The lack of evidence of components at higherkinetic energy may be due to poor signal-to-noise ratio ofSKLL spectrum. Calculated Auger parameters are listed inTable 5.

Surface quantitative analysis

Quantitative XPS analysis was performed according to thefirst principle approach [31]. Results are summarized inFig. 6. A marked decrease in arsenic and iron at mineralsurfaces was found, whereas total sulphur content did notshow any significant variation, and silicon showed amarked increase. Atomic percent calculated only taking

into account arsenic, iron and sulphur (constituent elementsof pyrite and arsenopyrite) is listed in Table 6 and plotted inFig. 7: along with depletion in arsenic and iron content, anenrichment in sulphur was found, in particular, the sulphursignal at ca. 169.0 eV became more intense than lowerbinding energy components.

Discussion

In contrast to studies on pure minerals such as pyrite [12,13, 15, 18], arsenopyrite [14] or enargite [8, 9, 14, 17], inthis work, a technical flotation concentrate (mixture ofpyrite and arsenopyrite) of Fairview mine was investigated.Comparing the bulk composition of the dry ground feed andof the residue after bioleaching, the overall (bio)-leachingefficiency can be assessed and compared to literature values.The focus of this work was on the alteration of the surfacecomposition, thus the formation of by-products of theleaching process and the adhesion of microorganism on thesurface of the mineral particles.

Bioleaching efficiency

The composition of the samples determined by combiningthe results of the XRD (Fig. 2), ICP (Fig. 3) and CHNS(Table 1) analyses is shown in Table 7. Several assumptionswere made for the calculation (numerical values shown arefor dry ground bioreactor feed):

– Calcium is present in gypsum (Ca(SO4) (H2O)2) only;the quantity of gypsum in 1 g sample amounted to0.026±0.001 g;

– Potassium only derived from muscovite (KAl2(Si3Al)O10(OH)2), which thus amounted to 0.10±0.01 g;

– All of the arsenic belonged to arsenopyrite (FeAsS),which thus amounted to 0.09±0.01 g;

– From total sulphur content determined by CHNSanalyses, after subtraction of sulphur from gypsum

Table 3 Concentrations of arsenic and iron measured in the solutionafter bioleaching

Liquor after bioleaching Fe As

mg/L 2,381±150 513±44

mg/g 143±14 31±3

mmol/g 2.6±0.2 0.41±0.04

Data in the second and third row are calculated from the first row andare related to the amount of iron and arsenic dissolved by bioleachingper gram of feed

BE (As3d5/2), eV KE (AsLMM), eV Auger parameter, α′, eV

Bioreactor feed 45.6±0.1 1,217.6±0.2 1,263.1±0.2

Bioleaching residue 45.1±0.1 1,217.5±0.2 1,262.6±0.2

Literature data [6]

Arsenopyrite (FeAsS) 42.1 1,224.8 1,266.9

As2S3 44.2 1,221.2 1,265.4

As2O3 44.9 1,218.8 1,263.7

As2O5 46.8 1,216.8 1,263.6

Scorodite (FeAsO4·2H2O) 45.6 1,217.9 1,263.5

Na2HAsO4·7H2O 44.9 1,217.3 1,262.2

KH2AsO4 44.9 1,217.6 1,262.5

Table 4 As3d binding energies(eV), AsLMM kinetic energies(eV) and Auger parameterα′ (eV) of arsenic in compoundsof the bioreactor feed, of thebioleaching residue and ofstandards

Standard deviations were calcu-lated over three measurementsfor each sample. FWHM ofAs3d5/2 was 1.8±0.1 eV


and arsenopyrite, it is possible to determine content ofpyrite (FeS2) which thus amounted to 0.281±0.006 g;

– Magnesium, the remaining aluminium and iron belongedto clinochlore, which amounted to 0.178±0.001 g;

– Silicon contained in muscovite and clinochlore wasassumed to be present in the form of SiO2, whichamounted to 0.08±0.01 g. By subtracting 0.08 g fromthe weight of the insoluble residue (0.39 g) identifiedas pure SiO2 (Fig. 2), the amount of pure SiO2 in thesample can be calculated to 0.31±0.02 g.

The sum of the above-calculated weights is 0.988 g,which is in very good agreement with the weighted amount.Calculations similar to those made on the bioreactor feedwere carried out for the solid residue found after bioleach-ing (Table 7). The sum of the calculated weights in this caseis 0.984 g.

As qualitatively shown already in the XRD scans (Fig. 2),the results (Table 7) clearly show a marked reduction of theamount of arsenopyrite (from 9.1% to 1.5%) and pyrite

(from 28.1% to 1.3%) as consequence of their bio-oxidativedissolution. The bioleaching efficiency for arsenopyrite isabout 85% and for pyrite about 95% in the presentexperimental conditions. This fact is reflected in the analysisof the bioleaching liquor (Table 3) where high arsenic andiron contents were found. The amount of arsenic and irondissolved during bioleaching (millimoles per gram of feed tothe bioreactor) can be calculated from the difference of solidpyrite (FeS2) and arsenopyrite (FeAsS) prior and afterbioleaching (Table 7). This calculation results in 0.47±0.01 mmol/g for dissolved arsenic and 2.70±0.05 mmol/gfor dissolved iron in very good agreement with resultsobtained from ICP determination of dissolved iron andarsenic in bioleaching liquor (Table 3).

Surface composition of ground concentrate samples

XRD spectra of the ground feed concentrate show thepresence of pyrite and arsenopyrite mineral (Fig. 2a).

BE (S2p3/2), eV KE (SKLL), eV Auger parameter, α′, eV

Bioreactor feed 162.2±0.1 2,116.5±0.2 2,278.7±0.2

169.4±0.1 2,106.3±0.2 2,275.7±0.2

Bioleaching residue 164.4±0.1 – –

169.0±0.1 2,106.6±0.2 2,275.6±0.2

Literature data [6 and literature cited therein]

Pyrite 162.5 2,116.2 2,278.7

Enargite (Cu3AsS4) 162.1 2,115.8 2,277.9

FeS 162.8 2,116.1 2,278.9

As2S3 163.6 2,113.4 2,277.0

CuSO4 169.2 2,106.6 2,275.8

Elemental sulphur 164.4 2,112.8 2,277.2

Table 5 S2p binding energies(eV), SKLL kinetic energies(eV) and Auger parameter α′of sulphur in compounds of thebioreactor feed, of the bioleach-ing residue and of standards

Standard deviations were calcu-lated over three measurementsfor each sample. FWHM forS2p3/2 was 1.6±0.1 eV

Fig. 7 XPS quantitative analyses (at percent) determined by the firstprinciple method for dry ground feed and for the bioleaching residue.Arsenic, iron and sulphur are only considered (uncertainty±10%)

Fig. 6 XPS quantitative analyses (at percent) determined by the firstprinciple method for dry ground feed to the bioreactor and for thebioleaching residue (uncertainty±10%)


Checking the XPS high-resolution spectra (Fig. 5) of thedry ground concentrate, it can be observed that all thesignals (arsenic, iron and sulphur) show the presence ofoxidized compounds (Table 2). Arsenic binding energy(45.6±0.1 eV) is clearly higher than that expected forarsenic in arsenopyrite (40.7 [37], 41.2 [13, 38] and42.1 eV [6]). According to the modified Auger parameterα′ [6, 34, 35], the chemical state of arsenic is similar toarsenic in scorodite (FeAsO4*2H2O), a hydrated iron (III)arsenate (Table 4). This is in agreement with the fact thatscorodite is one of the most common natural arsenates,being it a weathering product of arsenic-bearing oredeposits [39]. The reaction that may lead from arsenopyriteto scorodite is [40]

FeAsSþ 14Fe3þ þ 10H2O T 14Fe2þ þ SO2�4 þ FeAsO4

� 2H2Oþ 16Hþ

The presence of scorodite on feed sample is alsoconfirmed by the iron Fe2p3/2 signal (Fig. 5b) composedof two components that may be attributed to iron in FeS2and FeAsS (707.1±0.1 eV) and to iron (III) in scorodite-like phase (712.1±0.1 eV) according to [41]. Iron arsenatepresence at the surface of air-oxidized arsenopyrite and inarsenopyrite exposed to acidic environment was alsorevealed by XANES study [42 and references cited therein].The sulphur S2p peak (Fig. 5c) of ground sample showedtwo components: the lower-binding energy doublet(162.2 eV) can be attributed to S2

2− species present atpyrite and arsenopyrite surfaces [13, 14] and a smallerdoublet at 169.4±0.1 eV that can be attributed both tosulphate in gypsum and to sulphate produced by mineral

sulphide oxidation. These results lead to conclude thatpyrite and arsenopyrite minerals present in the dry groundfeed are oxidized at the surface.

Surface modifications induced by bioleaching

The surface composition of the residue after bioleaching(Fig. 6) confirms the dissolution of most of pyrite andarsenopyrite present in the feed as a large decrease in ironand arsenic content was observed accompanied by amarked increase in silicon (present in quartz). A moredetailed analysis, considering only elements associated topyrite and arsenopyrite (As, Fe and S), is shown in Fig. 7where also the chemical state of the elements is considered.

Iron chemical state

After bioleaching, iron Fe2p3/2 signal showed only onecomponent at 711.5±0.1 eV (Fig. 5e) with a higherFWHM. This binding energy value is close to 711.6 eVreported in [41] for jarosite KFe3(SO4)2(OH)6 and 711.6 eVfor Fe(III) oxy-hydroxide [43–45]. Jarosites are ferrichydroxysulphates with the general formula MFe3(SO4)2(OH)6 where M can be K+, Na+, NH4

+, Ag+ orH3O

+. As the 9K culture medium used in this workcontains both K+ and NH4

+ ions, jarosite was mostprobably ammonium jarosite NH4Fe3(SO4)2(OH)6 andpotassium jarosite KFe3(SO4)2(OH)6. Jarosite can be foundin sediment of AMD [46 and reference cited therein], sinceit precipitates at low pH values in the presence of highconcentrations of Fe(III) and SO4

2− [47]. These are typicalconditions in bioleaching environment since the overalloxidation reactions involved are, for pyrite and arsenopy-rite, respectively, produce protons and Fe(III) [48]:

4FeS2 þ 15O2 þ H2OT 2Fe2 SO4ð Þ3 þ 2H2SO4

FeAsSþ 3=2O2 þ H2O T FeAsO4 þ H2SO4

Jarosites were found together with metastable ferricarsenate on enargite after bioleaching [17]. Jarosites andscorodite were found as solid-phase products of an

Table 6 XPS quantitative analyses (at percent) determined by the firstprinciple method for dry ground feed to the bioreactor and for thebioleaching residue

As Fe(I) Fe(II) S(I) S(II)

Bioreactor feed 37 3 43 11 6

Residue 16 – 33 13 38

Only arsenic, iron and sulphur are considered (uncertainty±10%)

Components Bioreactor feed Bioleaching residue

Quartz (SiO2) 31±2 57±2

Muscovite (KAl2(Si3Al)O10(OH)2) 10.2±0.9 21±1

Clinochlore (Mg4Al4Fe4)[(Si4Al4)O20](OH)16 17.8±0.1 17.6±0.1

Gypsum (Ca(SO4)(H2O)2) 2.6±0.1 0.05±0.01

Arsenopyrite (FeAsS) 9.1±0.1 1.50±0.04

Pyrite (FeS2) 28.1±0.6 1.30±0.01

Total calculated 98.8 98.4

Table 7 Amount of differentphases determined according toXRD, ICP-AES and CHNSresults for bioreactor feed andbioleaching residue(weight percent)


arsenical pyrite oxidative dissolution by moderately ther-mophilic bacteria [49], whereas jarosites, amorphous Fe(III)arsenate and elemental sulphur were found as solid-phaseproducts of arsenopyrite bio-oxidation by A. ferrooxidans[10] and pyrite bioleaching by extreme thermophile A.manzaensis [12]. In this work, the XRD pattern of theresidue does not show any contributions due to jarosite,FeOOH, scorodite or amorphous arsenate and elementalsulphur species because they might be below the detectionlimit of the technique.

Chemical state of sulphur

Significant changes occur in the chemical state of sulphur.In effect, sulphur S2p peaks acquired before (Fig. 5c) andafter bioleaching (Fig. 5f) showed considerable differences.After bioleaching, the component at high-binding energyassociated to sulphate is much more intense and slightlyshifted in binding energy (Table 5). The Auger parameter α′of this signal remains constant (2,275.6±0.2 eV), confirm-ing the presence of sulphate (Table 5). Thus, furtherevidence of jarosite precipitation on the surfaces of feedgrains (Fig. 7) is obtained. The lower-binding energydoublet after bioleaching was less intense, and it shiftsfrom 162.2 eV (associated to sulphur in pyrite andarsenopyrite) to 164.4±0.1 eV (Fig. 5f and Table 5), whichis the expected value for elemental sulphur [6 and literaturecited therein]. Unfortunately, the sulphur SKLL signalcould not be recorded due to poor S/N ratio. Based on thebinding energy values, elemental sulphur is formed on thebioleached grains of pyrite and arsenopyrite.

Implications for the dissolution mechanism

Though pyrite and arsenopyrite bio-oxidation occur accord-ing to different mechanisms [15, 16], respectively, viathiosulphate and via polysulphides, elemental sulphur canbe produced by both mechanisms as an intermediateproduct. In thiosulphate mechanism, the first step of pyriteoxidation in the presence of Fe(III) ions leads to thiosulphateproduction that can be oxidized to tetrathionate S4O6

2−.Tetrathionate thus formed undergoes a series of complexreactions [15] that lead to elemental sulphur formation.According to polysulphide mechanism, elemental sulphurformation is due to a series of radicalic reactions that lead toelongation of polysulphide chains. The last term of polime-rization reactions is H2S9 that in acidic solutions decomposesto rings of elemental sulphur S8. Evidence of polysulphidemechanism was reported by these authors in previous paperson enargite chemical leaching [7, 8] and bioleaching [9],since they had evidence of the formation of a copper-deficient enargite surface layer. Also, the arsenic Augerparameter did not change with bioleaching in spite of the

differences in binding energy values of As3d prior to andafter bioleaching. They hypothesized that mechanicallydeformed surfaces produced by grinding may have beeneasily dissolved owing to their higher energy. In thementioned paper, no occurrence of secondary phases formedat enargite surface both prior to and after bioleaching wasreported. As opposed, in this work, the arsenic Augerparameter changed (Table 4) from a value typical ofscorodite-like phases in ground samples to values close toarsenic, As(V), bound to oxygen in arsenates (1,262.2±0.2 eV) after bioleaching. Arsenates can be adsorbed inFeOOH phases; in fact, it is known that FeOOH as well asjarosites plays an important role in arsenate removal fromAMD by adsorption and co-precipitation [19], but if sulphateions concentration is too high, like in our bioleachingsolution, jarosites capability of arsenate removal is stronglyreduced, and arsenate adsorption takes place more likely onFeOOH.

Role of EPS in dissolution mechanism

In a previous work on bioleaching of enargite [9], thepresent authors found evidence of adhesion of microbialcells to the enargite surface by means of extra-cellularpolymeric substances (EPS) [50] produced by the livingmicroorganisms [51, 52]. The EPS layer is able to complexFe(III) ions into glucuronic acid–iron complexes [51], thusincreasing the actual concentration of Fe(III) ions on themineral surface and the electrochemical potential of thelatter [8]. As a result, the dissolution rate of minerals ishigher in the presence of microorganisms. In this work, theevidence of the presence of microbial cell remnants isfound in carbon and nitrogen content determined by CHNSanalyses (Table 1) and in XPS survey spectrum (Fig. 4) ofthe bioleaching residue that showed a peak at ca. 400 eV,due to nitrogen of bacterial cells.

Conclusions

Samples of a pyritic and arsenopyritic flotation concentratewere characterized by bulk techniques (XRD, CHNS andICP-AES analyses) and by XPS and XAES surfaceanalyses prior to and after bioleaching. The results obtainedallowed the following conclusions:

& Bioleaching of both pyrite and arsenopyrite leads toenhanced dissolution due to adhesion of microbial cellsto the mineral particles by means of EPS—for theoccurrence of remnants of which evidence was provid-ed—which creates an environment rich in Fe(III) ions.As a consequence, ca. 95% of pyrite and 85% ofarsenopyrite were dissolved during bioleaching.


& Elemental sulphur detected at the surface of pyrite andarsenopyrite on the bioleaching residue can be consid-ered as intermediate species in the bio-oxidation process.

& Using the Auger parameter concept, it was possible toshow that oxidized arsenic species at the mineralsurface change from a scorodite-like phase prior tobioleaching to arsenates (probably adsorbed on FeOOHand jarosites phases) after bioleaching.

& Secondary phases mainly constituted of iron oxy-hydroxides and jarosites precipitate on mineral par-ticles. They do not seem to have an adverse effect onmineral dissolution.

Acknowledgments The authors wish to express their appreciation toProf. Cristina Trois, University of Natal, Durban, South Africa, for herkind assistance with the purchase and shipment of the commercialpyrite/arsenopyrite concentrate sample of the Fairview Plant. Theyalso wish to thank the Management of the Fairview Mine for grantingpermission to purchase the concentrate. This research was carried outwith the financial support of the University of Cagliari and of theItalian Ministry for Education, Universities and Research in theframework of the Research Project of National relevance. Finally, thepatience and skilful cooperation of the Technicians of the Inorganicand Analytical Chemistry Department of the University of Cagliari areworthy of being acknowledged.

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Arsenopyrite and pyrite bioleaching: evidence from XPS, XRD and ICP techniques

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