Pyrite−Arsenopyrite Galvanic Interaction and Electrochemical Reactivity

Jun 24, 2008 - Pyrite (FeS2) and arsenopyrite (FeAsS) are common sulfide minerals associated in base metals, precious metals ores, and concentrates...
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J. Phys. Chem. C 2008, 112, 10453–10461

10453

Pyrite-Arsenopyrite Galvanic Interaction and Electrochemical Reactivity Gustavo Urbano,† Victor E. Reyes,*,† M. Aurora Veloz,† Ignacio Gonza´lez,‡ and Julia´n Cruz§ A´rea Acade´mica de Ciencias de la Tierra y Materiales, A´rea Acade´mica de Quı´mica, UniVersidad Auto´noma del Estado de Hidalgo, Carr. Pachuca-Tulancingo KM 4.5, 42184 Pachuca, Hidalgo, Me´xico, and A´rea de Electroquı´mica, Departamento de Quı´mica, UniVersidad Auto´noma Metropolitana - Iztapalapa, AV. San Rafael Atlixco 186, Col. Vicentina, 09340, Iztapalapa, D. F., Me´xico ReceiVed: January 11, 2008; ReVised Manuscript ReceiVed: April 7, 2008

Comparative voltammetric studies were performed between high purity pyrite mineral (98.86%) and arsenopyrite mineral (content of 85.96% arsenopyrite, FeAsS; 11.84% pyrite, FeS2; 0.98% galena, PbS; 0.06% chalcopyrite, CuFeS2) in order to analyze the galvanic effect on the electrochemical reactivity of two different mineralogical phases when these are associated in the same mineral, using carbon paste electrodes (CPE) in 0.1 M NaNO3 (pH 6.5) as electrolyte. Theoretical studies were performed for a more detailed analysis on the total energy and energy relative of a total optimization of the species involved in the pyrite and arsenopyrite oxidation as pure mineral. The results indicate that the electrochemical reactivity of pyrite in the arsenopyrite mineral was delayed and displaced to more positive potentials with respect to the high purity pyrite mineral electrochemical response, due to a galvanic effect. On the other hand, the voltammetric studies showed the oxidation stages of arsenopyrite mineral, indicating that arsenopyrite was oxidized in a first stage to Fe2+, realgar (As2S2), H3ASO3, and S0. In a second stage, the arsenopyrite and the pyrite were oxidized to FeOOH(s) and S0, followed by oxidation of H3AsO3 to H2AsO4- and S0 to SO4-, besides the scorodite (FeSO4 · 2H2O) formation. Meanwhile, to more negative potentials the regeneration of FeAsS and the reduction to elemental arsenic do not occur due to the reduction of arsenate (H2AsO4-) to H3AsO3 and the residual scorodite (FeAsO4 · 2H2O) that was not dissolved by chemical reaction. The analysis by scanning electron microscopy and energy dispersive spectrometer to surfaces of the electrodes with the mineral (CPE) modified electrochemically was carried out to support the reaction mechanisms. 1. Introduction Pyrite (FeS2) and arsenopyrite (FeAsS) are common sulfide minerals associated in base metals, precious metals ores, and concentrates. The treatment of this kind of ores involves frequently an oxidation stage to improve metal recoveries. Besides, the oxidation of these sulfide mineral residues generates acid rock drainage (ARD) during weathering;1–5 here is the importance of electrochemistry and surface chemistry of arsenopyrite and pyrite minerals for understanding the mineral behavior in the technological and environmental processes. On the other hand, in the flotation process, the galvanic interactions between two or more minerals produce surface coatings, which affect the floatability of sulfide minerals.6 Meanwhile, in the leaching process the galvanic interactions could substantially increase the leaching of one or both of the minerals, constituting a galvanic cell. In this manner, it is observed that the reactivity of the minerals is mainly affected by the galvanic effect between the main associated mineralogical phases; however the fundamental details of this effect on their reactivity are not scarcely taken into account. Therefore, galvanic cells are originated when redox reactions occur on sulfide minerals where the mineral with higher residual potential acts as cathode and becomes * Corresponding author. Tel: +52 771 71 720 00, Ext: 6713 and 2280. Fax: 6730. E-mail: [email protected]. †A ´ rea Acade´mica de Ciencias de la Tierra y Materiales, Universidad Auto´noma del Estado de Hidalgo. ‡ Universidad Auto ´ noma Metropolitana - Iztapalapa. §A ´ rea Acade´mica de Quı´mica, Universidad Auto´noma del Estado de Hidalgo.

galvanically protected. Meanwhile, the mineral with the lowest residual potential acts as anode and its dissolution becomes favored.7,8 Generally, the galvanic interaction between sulfide minerals has been studied using short-circuited galvanic cells or with attached mineral electrodes and by comparing individual electrochemical behavior of the minerals.7–11 In this type of cells, the electrical contact between minerals is through an electrolytic solution while, in an industrial concentrate, the electrical contact between the mineralogical phases are in solid state. This difference could explain the limited reliability of the galvanic cells for the prediction of minerals reactivity. On the other hand, the electrochemical characterization of mineral has been performed in solid electrodes, where a percolation effect on the conductivity of the electrode provokes a potential drop due to changes in current resistance (IR) at the interface. Meanwhile, for a carbon paste electrode (CPE) with a nonconducting binder, the sensitivity is high and its residual currents are low, due to the small double layer capacitance at the electrode-electrolyte interface. The hydrophobic binder used in the CPE lowers the double layer capacitance, and consequently the residual current decreases. Therefore, penetration of the electrolyte into the CPE is unlikely, due to its hydrophobic nature; thus, electrochemical reactions take place entirely at the interface.12,13 Then, the concept of percolation is the probability that any given region of the CPE is sufficiently well connected to the rest to be available for conduction,14,15 playing an important role in the electroactive response of the semiconductor minerals.

10.1021/jp800273u CCC: $40.75  2008 American Chemical Society Published on Web 06/24/2008

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Figure 1. SEM image of arsenopyrite mineral containing the associated mineralogical phases of sulfide minerals exemplified for arsenopyrite (FeAsS) and pyrite (FeS2).

TABLE 1: Mineralogical Species Percentage Contained in Pyrite and Arsenopyrite Minerals sample pyrite

arsenopyrite

species (%) FeS2 PbS CuFeS2 ZnS FeAsS FeS2 PbS CuFeS2

98.86 0.26 0.32 0.03 85.96 11.84 0.98 0.06

In this case, the electrochemical characterization using cyclic voltammetry with carbon paste electrodes containing mineral particles (CPE-mineral) has been, at the past decade, an effective tool to study the overall reactivity3,7,8,13,16–30 and galvanic interaction of the minerals.7,8 The electrochemical behavior of pyrite and arsenopyrite minerals has been widely studied in alkaline and acidic media for several authors.3,16,25–32 Some of them29,30 have found that oxidation of the pyrite and arsenopyrite minerals are similar with a large quantity of sulfate being produced, but the reaction mechanisms involved, and even the reaction products, have not been conclusively identified. Other authors16,27,28 found that the anodic oxidation of pyrite and arsenopyrite, both separated, in alkaline and acid media is carried out in two stages. David et al.17 found that pyrite oxidation proceeds via a complex sequence of series and parallel reaction steps which can be proposed from the electrochemical data and supported by crystal chemical arguments. Beattie and Poling28 determined that the oxidation of arsenopyrite at pH greater than 7 results in the formation of ferric hydroxide deposits on the surface of the mineral. Arsenic is oxidized to arsenate and sulfur is oxidized to sulfate. Below pH 7, soluble iron species are formed and the surface becomes increasingly covered with elemental sulfur when decreasing pH. All the studies on minerals mentioned above deal with only a little part of the whole reaction, because they performed tests in short potential ranges. Moreover, the galvanic effect between pyrite and arsenopyrite has not been reported. In this work, a comparative voltammetric study has been carried out in order to understand the galvanic interactions between pyrite and arsenopyrite minerals and the effect on its reactivity. On the other hand, the voltammetric studies of arsenopyrite mineral, carried out beyond that of the literature, allowed revelation of

Figure 2. Typical cyclic voltammograms obtained on (i) CPE-without mineral in positive direction and CPE-pyrite 20:80 wt % in 0.1 M NaNO3 at pH 6.5 (V ) 20 mV s-1) when the potential scan was initiated in (ii) positive and (iii) negative direction.

the oxidation stages of arsenopyrite and pyrite mineral. Besides, in this paper the reaction mechanisms were supported by analysis of scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) to surfaces of the CPE-mineral, modified electrochemically. 2. Experimental Methods Samples of pyrite and arsenopyrite minerals were collected on the mine “El Monte” of the mining district of Zimapa´n (Mexico). The pyrite and arsenopyrite minerals were characterized mineralogical and chemically to determine the stoichiometric composition of both minerals and the contents of mineralogical impurities. The mineralogical phases were determined with SEM (image shown in Figure 1) and the chemical content was obtained by inductively coupled plasma (ICP) analysis. Mineralogical composition resultant is shown in Table 1. The samples were ground in an agate mortar and then they were sieved to Eλ-> -1.4 V. The positive switching potential was fixed at 2.0 V.

Figure 6. SEM images on CPE-pyrite surface after an imposition of an oxidation potential of 2.0 V during 60 s and later a potential of (a) -0.5 V and (b) -0.935 V, during 60 s.

Figure 4. SEM images on CPE-pyrite surface after electrochemical modification (60 s) in a reduction potential at (a) -0.935 V and (b) -1.385 V.

Figure 7. Typical cyclic voltammograms obtained on (i) CPE-without mineral in positive direction and CPE-arsenopyrite 20:80 wt % in 0.1 M NaNO3 at pH 6.5 (V ) 20 mV s-1) when the potential scan was initiated in (ii) positive direction and (iii) negative direction. Figure 5. SEM images on CPE-pyrite surface after electrochemical modification (60 s) with an oxidation potential of 1.615 V seen at 2000×.

arsenopyrite) in order to better observe the images of the products present at the potentials tested. However, the graphite amount used do not change the electrochemical response respect that of the ratio 80:20 (CPE-mineral). Stoichiometry analyses were carried out discounting the oxygen amount from the silicon oil in the CPE paste. This was achieved carrying out SEM and EDS studies on a CPE-mineral without electrochemical modification. A typical three-electrode cell was conditioned with nitrogen to maintain an inert atmosphere and to carry out the electrochemical experiments at room temperature. A graphite solid bar was used as the auxiliary electrode and a standard sulfate electrode (SSE, E ) 615 mV versus SHE) immersed in a Luggin

capillary, as the reference electrode. All potential values were converted to the standard hydrogen electrode (SHE) scale. Working electrodes were prepared mixing carefully the graphite powder and mineral with silicon oil (0.2 mL) and later homogenizing in the agate mortar. The ratio used for graphite powder and pyrite or arsenopyrite mineral were 20:80 % wt, in which a better reproducibility of the electrochemical response was obtained. Resulting pastes were located inside the CPE with an apparent surface area exposed to the solution of 0.0314 cm2. Fresh surface was created for each experiment by extruding the mineral paste and cutting off the spent part. The electrochemical studies were carried out using a PAR 263A Potentiostat with PowerSuite Software of the same company. Computational Procedure. The geometry and electronic structure of all compounds studied in this work were determined by means of calculations carried out in the spin-restricted and

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Figure 8. Partial view of the typical voltammograms obtained on CPEarsenopyrite 20:80 wt % in 0.1 M NaNO3 at pH 6.5 (V ) 20 mV s-1). The potential scan was initiated in positive direction and the positive switching potential (Eλ+) was varied between 1.0 < Eλ+ < 2.0 V, with increments of 0.1 V. The negative switching potential was fixed at -1.4 V.

the spin-unrestricted formalisms for closed-shell and open-shell systems, respectively. We performed geometry optimizations using density functional theory (DFT) calculations by Gaussian 03;33 for exchange, the Becke functional,34 for correlation, Perdew and Wan (BPW91),35 and for the all-atoms, LANL2DZ basis set were employed.36,37 3. Results and Discussion In order to analyze the electrochemical contribution of pyrite in arsenopyrite reactivity and galvanic interaction, first, electrochemical behavior of CPE-pyrite mineral with high purity (98.8% pyrite) was determined and later the CPE-arsenopyrite mineral (content of 86.95% arsenopyrite, 11.84% pyrite) oxidation is described. Finally, the interaction between the both minerals is explained by comparison of both electrochemical responses. 3.1. Electrochemical Characterization of Pyrite. Figure 2 shows voltammetric responses of a CPE-pyrite when positive and negative scans (ii and iii, respectively) were carried out, beginning at open circuit potential (OCP, E ) 0.136 V) with a sweep rate (V) of 20 mV s-1. The electrochemical behavior of the CPE-without mineral (100% graphite) in the positive direction is also shown in Figure 2 scan i, for comparative purposes. This last one does not show oxidation nor reduction representative responses in the whole of potential range where the mineral has its electrochemical response. When the scan potential was initiated in positive direction (Figure 2, scan ii), an important oxidation process (A1) beginning at ∼1.0 V was observed. On the reverse scan several reduction processes take place. The first reduction peak, C1, was scarcely visible at about 0.73-0.39 V (Figure 2, scan ii, inset), while that of the two reduction peaks, C2 and C3, appeared at more negative potentials (between -0.26 and -1.0 V), and one reduction process C4 was observed at -1.05 V. Finally, when the potential cycle was about to be completed, three oxidation peaks (A2, A3, and A4) were observed. On the other hand, when the scan potential was initiated in negative direction (Figure 2, scan iii, inset), three reduction processes C2′, C3′, and C4 were observed. Due to the low current density involved in those processes (C2′ and C3′), it

Urbano et al. is possible to establish that the high current density in reduction processes C2 and C3 depends on the oxidation processes of the mineral. When the sweep direction was inverted, three oxidation peaks (A2, A3, and A4) were observed in the same potential range and with similar behavior in current density than the voltammogram obtained in positive direction (Figure 2, scan ii, inset). Those peaks could correspond to the oxidation of the same products obtained in the reduction processes C2, C3, and C4. Nevertheless, to more positive potentials the formation of an oxidation peak, A5, is observed between potentials of ∼0.64-0.85 V, which was not observed on the forward scan of the positive direction (Figure 2, scan ii). In addition, the main oxidation process (A1′) presents a different behavior (the potential where oxidation begins) from that seen when the sweep is initiated in a positive direction. It is important to point out that the reduction process, C1′, is more clearly observed when the potential sweep is initiated in a negative direction (Figure 2, scan iii, inset), and this could be associated with the reduction of some oxidized species formed during the handling and preparation of samples38 or due to an activation of the mineral surface, besides the contribution of the process A1. In order to clarify the behavior of these peaks, a study of the voltammetric response of pyrite by Eλ- variation was carried out (Figure 3) with the same conditions as the voltammograms on Figure 2, scan iii. Then, cathodic scans were conducted to different negative switching potentials (Eλ-) and reversed back at 2.0 V. The study allowed showing that oxidation peaks A2, A3, A4, and A5 are associated with the reduction products formed during the cathodic scan. Moreover, the study showed that when the scan was conducted to more negative switching potentials (-1.3 and -1.4 V) an increase in the current density of oxidation on processes A2, A3, A4, A5 and mainly the reduction process C1′ was observed (Figure 3). The behavior of C1 could be associated with an activation of the mineral surface due to the accumulation of oxidized products on the electrode. Figure 4 shows SEM and EDS studies carried out on the surfaces of CPE-pyrite electrochemically modified before and after of the process C4′ (-0.935 and -1.385 V, corresponding to processes C3′ and C4′, respectively), allowed observing the products formed on the mineral surface. According to the stoichiometry, it detected Fe0 before process C4′ (Figure 4a, according to reactions 1a and 1b) and Fe(OH)2 after it (Figure 4b, reactions 1a,c). Last reaction (reaction 1c) occurs where an important change in the pH exists associated with the beginning of the water reduction; this change in pH (i.e., an increase of pH by the formation of OH- at the electrode surface) can provoke undesirable parallel reactions on the nearest coat to the electrode surface, such as the precipitation of metal hydroxides.39

FeS2 + H2O + 2e- f FeS + HS- + OH-

-

-

FeS + H2O + 2e f Fe + HS + OH FeS + 2OH- f Fe(OH)2 + S2-

0

(1a) (1b)

∆G°298 ) -180.46 kJ mol-1 (1c)

Even though the cathodic response could reveal interesting information concerning the nature of reduced precipitates on the mineral surface, this paper will only focus on the anodic behavior of pyrite (process A1) to correlate this with oxidative behavior of the arsenopyrite when they are associated. On the other hand, the behavior of the anodic processes A1 and A1′ has been widely studied for several authors.7,22–26 These

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Figure 9. SEM images on CPE-arsenopyrite surface after an imposition of an oxidation potential of 1.55 V during 80 s (a) and later a potential of (b) -0.58 V and (c) -0.95 V during 80 s.

processes are associated with the electrochemical oxidation of pyrite to formed Fe(III) hydroxides, S0, and SO42-, according to the following reactions:

FeS2+2H2O f FeOOH(s) + 2S0 + 3H+ + 3e-

(2a)

FeS2 + 10H2O f FeOOH(s) + 2SO42- + 19H+ + 15e(2b) Besides, the peaks C1 and C1′ have been related with the reduction of iron hydroxides formed on the surface of the mineral during the pyrite oxidation,7,22–26,32,40 according to the following reaction +

-

FeOOH(s) + 3H + e f Fe +2H2O 2+

(3)

Finally, the reduction peaks, C2 and C3, observed when the scan is initiated in positive direction are identified by some authors as the reduction of sulfur phases formed during the oxidation of pyrite and metal-deficient sulfide in two independent processes.23,24,38 The studies of SEM and EDS on the CPE-pyrite surface modified electrochemically at an oxidation potential of 1.615 V (process A1) indicated that the species present are, mainly, FeOOH and S0 (Figure 5). Additionally, SEM and EDS on the potential of 0.5 V (process C1) verify the reduction of the iron hydroxides formed on the mineral surface, when the products formed on the mineral surface diminish, as the oxygen amount of the EDS analysis (Table 2), respective to the potential of 1.615 V. Figure 6 shown the studies of SEM and EDS to the CPEpyrite surface electrochemically modified at an oxidation potential of 2.0 V and later applying a reduction potential. Figure 6a (corresponding to the processes C2, -0.5 V) shows the presence of FeS species, poor in sulfur, deposited on the mineral surface. It can be concluded that recombination of the S and Fe occurs to form FeS poor in sulfur. It is also sustained with the stoichiometry, where an increase of the amount of Fe and S is diminishing (Table 2), respective to the reduction potential of 0.5 V (process C1). Diminishing of the S amount observed in this process (C2) is also due to the formation of soluble species such as H2S (voltammetric studies with solution stirring, not shown in this work, confirm this approach). Meanwhile, in Figure 6b corresponding to the reduction potential of -0.935 V (process C3), the reduced species of FeS poor in sulfur appear, besides that of Fe0, on the mineral surface. The results and the stoichiometry confirm this fact, noting an increase in the concentration of Fe and a diminishing in S in peak C3, respect to the peak C2 (Table 2). Besides, differences on iron and S amounts detected on the electrode surface, from peak C3 to C3′, are due to the amount of compounds formed on the electrode surface during the oxidation and reduction scans.

Figure 10. SEM images on CPE-arsenopyrite surface after an imposition of an oxidation potential of (a) 1.85 V, (b) 2.0 V, during 80 s.

On the other hand, the oxidation peaks, A2, A3, and A4 are attributable to the oxidation of the species of sulfur and iron that were formed during the cathodic scan (peaks C4, C3, and C2, respectively). The oxidation peak, A5, is associated with the dissolution of pyrite mineral that forms intermediate compounds such as Fe1-xS2, as shown in the literature.23–25 With these results, some of the main processes of the high purity pyrite oxidation were identified and in such way the results of the arsenopyrite mineral oxidation processes can be separated or assigned to establish the contribution of the pyrite. The following study was to obtain the voltammetric response of the arsenopyrite in presence of an important content of pyrite. 3.2. Electrochemical Characterization of Arsenopyrite Mineral. The electrochemical behavior of the CPE-arsenopyrite was studied in a potential range similar to that used for pyrite. Typical voltammograms of the positive and negative scan on CPE-arsenopyrite electrode are shown in Figure 7 (scans ii and iii, respectively). Both scans were initiated from the OCP with a sweep rate (V) of 20 mV s-1. Response on CPE-without mineral is also shown in Figure 7, scan i for comparative proposes. In Figure 7, scan ii, when the potential scan was initiated in the positive direction, the voltammograms showed an anodic process E1 at about 0.8 V. On the reverse scan, this peak represents a semi passivation process, since the current density of the backward direction is always lower than the forward direction, until potentials ∼1.12 V. This behavior has been generally associated to the formation of a partial passive film on the electrode surface, probably metal hydroxides or S0,3,8,21,40 which later were dissolved. Three reduction peaks were observed to more negative potentials; the first reduction peak, G1, appears in a similar potential range, as peak C1 for pyrite voltammetric characterization (at ∼0.72-0.35 V); then reduction peak G2 was observed at -0.5 V, where the current density increases. The reduction peak G3 presents a sharp current increase until -1.23 V (see inset Figure 7, scan ii); finally, three oxidation peaks E2, E3, and E4 were observed. When the potential scan was started in the negative direction (Figure 7, scan iii), the electrochemical behavior was different

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TABLE 3: Element Percentages (% wt) Obtained on the Surfaces of the CPE-Arsenopyrite Mineral, Modified Electrochemically element (% wt) 1.55 V 1.85 V 2.0 V -0.58 Va -0.95 Va 0.5 Vb -1.23 Vb Fe As S Oc

A1

A1

A1

G2′

G3′

G1

G3

23.56 24.69 15.97 35.78

23.07 29.82 15.02 32.09

16.18 23.44 9.92 50.45

17.81 15.52 11.17 55.50

21.81 26.17 10.34 41.67

18.76 23.64 14.36 43.24

14.65 19.21 9.39 56.75

a Previously, an oxidation potential of 1.55 V was imposed during 80 s. b Previously, an oxidation potential of 2.0 V was imposed during 80 s. c Approximately 29% of the oxygen was attributable to silicon oil used as binding agent; this data was obtained with a sample that was not polarized.

Figure 11. SEM image on CPE-arsenopyrite surface after an imposition of an oxidation potential of 2.0 V during 80 s and later a potential of 0.5 V during 80 s.

Figure 12. Partial view of the typical voltammograms obtained on CPE-arsenopyrite 20:80 wt % in 0.1 M NaNO3 at pH 6.5 (V ) 20 mV s-1). The potential scan was initiated in positive direction (i) with and (ii) without stirring of the solution.

from that shown in Figure 7, scan ii. The cathodic current density began to increase with the formation of a reduction process G2′′ at ∼ -1.0 V; while in the reverse scan only a small and scarcely visible oxidation peak, E3′, was observed (∼ -0.15 V; see inset Figure 7, scan iii), which suggests a lower reactivity of arsenopyrite mineral in relation with the reactivity of the high purity pyrite mineral. The comparison of the voltammograms obtained in both scan directions (curves ii and iii in Figure 7) showed that the reduction process G2 and G3 and oxidation peaks E2, E3, and E4 should be associated with the reduction-oxidation processes of the products developed in oxidation process (E1) of arsenopyrite mineral. In addition, this allows observing that the

Figure 13. SEM image on CPE-arsenopyrite surface after an imposition of an oxidation potential of 2.0 V, during 80 s and later a potential of -0.23 V during 80 s.

TABLE 4: Determination of Cathodic, Anodic and Ratio Charges on CPE-Arsenopyrite, Obtained from Different Positive Switching Potential Voltammetries Eλ+ (V vs SHE)

QA (mC) (E1)

QC (mC) (G1 + G2 + G2′ + G3′ + G3)

QA/QC

1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

0.55 1.47 4.29 5.77 12.72 29.92 48.46 50.06 63.58 101.40 111.60

1.24 1.38 2.23 2.51 3.41 6.81 9.69 10.05 10.37 11.94 12.46

0.45 1.06 1.92 2.30 3.73 4.40 5.00 4.98 6.13 8.49 8.96

current density of process G1 increases when the mineral was previously reduced, like in the high purity pyrite mineral. In this case, the voltammetric responses depended greatly on the initial direction of the scan potential. Taking into account that the main oxidation processes of arsenopyrite mineral could be associated with the anodic process, E1, a study of the CPE-arsenopyrite mineral was carried out, where the scan potential was initiated in positive direction. In this study, the inversion potential was varied in the positive direction (Eλ+), keeping the negative inversion potential fixed in -1.34 V (Figure 8). When the switching potential (Eλ+) was < 1.7 V (Figure 8, where only the reversed part of the voltammograms is illustrated), only two cathodic peaks G2′ and G3′ appear and the associated current density increases as Eλ+ becomes more positive. This behavior indicates that some coupled processes

Figure 14. Evolution of ratio charges on CPE-arsenopyrite, obtained from different positive switching potential voltammetries.

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FeOOH + H2AsO4- + H2O f FeAsO4 · 2H2O + H+ + 1/2O2 + 2e- (6d)

Figure 15. Partial view of typical voltammograms obtained on (a) CPE-pyrite mineral and (b) CPE-arsenopyrite mineral in 0.1 M NaNO3 at pH 6.5 (V ) 20 mV s-1). The potential scan was initiated in positive direction and the positive switching potential (Eλ+) was varied between 1.0 < Eλ+ < 2.0 V. The negative switching potential was fixed at -1.4 V.

were associated with a first oxidation stage. According with EhpH diagram for the Fe-As-S system41,42 and the data reported in the literature3,16,43 it is likely that the following coupled oxidation processes were involved:

FeAsS T Fe2+ + 1/2As2S2 + 2e-

(4a)

As2S2+6H2O f 2H3AsO3+2S0 + 6H+ + 6e-

(4b)

From the above reactions, it is possible to determine that the reduction peak G2′, corresponds to the reduction process of As(III) species (H3AsO3), formed in the forward scan, to realgar (As2S2), according with reaction 5a. Meanwhile, peak G3′ corresponds to the regeneration of the FeAsS through the reduction of realgar (As2S2) and recombination of Fe2+ present in the interface (reaction 4a).

2H3AsO3+2S0+6e- f As2S2 + 6OH-

(5)

SEM and EDS studies on the surface of the CPE-arsenopyrite electrochemically modified at oxidation potential of 1.55 V (corresponding to process E1) show the oxidation products of the arsenopyrite mineral (Figure 9a). According to the stoichiometry, these species correspond to the products of the reactions 4a and 4b (As2S2 and S0, mainly). On the other hand, microscopies of the Figure 9b,c (corresponding to peaks G2′ and G3′; -0.58 and -0.95 V, respectively) show the reduction of products formed in the processes E1. The stoichiometric analysis indicate the presence of As2S2 (reaction 5) at the peak G2′. Meanwhile, at the peak G3′ the presence of FeAsS and residual As2S2 was detected, which is in concordance with reactions 4a and 5. Meanwhile, when Eλ+ was higher than 1.7 V, the shape of peaks G2′ and G3′ changes; then the peaks G1 and G3 appeared and the associated current density increase when Eλ+ was more positive than 1.8 V, indicating that the oxidation processes of arsenopyrite proceeds by a second-stage dissolution mechanism, as shown with the following reactions

FeAsS + 5H2O f FeOOH(s)+S0 + H3AsO3 + 6H+ + 6e(6a) H3AsO3+H2O f H2AsO4-+3H+ + 2e-

(6b)

S0+4H2O f SO42- + 8H+ + 6e-

(6c)

SEM and EDS studies carried out to the surfaces of CPEarsenopyrite mineral altered at a oxidation potential of 1.80 V (Figure 10a, corresponding to process E1) show the products of the arsenopyrite oxidation. Stoichiometry of the species present is consistent with the products of the reactions 4a,b and 6a; it means, species such as FeOOH, S0, and As2S2 are present in the surface of the electrode. To more positive potentials (>1.80 V), the oxidation of As(III) species (H3AsO3) to arsenate (H2AsO4-) and oxidation of S0 to SO4- occurs, according with reactions 6b,c. Subsequently, the scorodite (FeAsO4 · 2H2O) formation occurs (reaction 6d; near to 2.0 V); where an important change in the pH begins and the predominant As(V) aqueous species and Fe(III) species coexist.44,45 It confirms what was said by some authors,46 which suggested that the scorodite formation in the presence of iron could also be occurring after arsenopyrite oxidation. The microscopy on the Figure 10b shows the formation of scorodite (FeAsO4 · 2H2O) species at 2.0 V on the surface of the electrode and the EDS analysis and the stoichiometric results confirm it (see Table 3). During the reverse scan, a reduction peak, G1, like to that in pyrite is identified by some authors23,24,32,40 as the reduction of iron hydroxides (according to reaction 3) present in the interface. The microscopy of the Figure 11 (corresponding to peak G1, 0.5V) shows some of the Fe hydroxides that have not been reduced completely and residual scorodite. An analysis of the cathodic region showed that reduction peak, G2, appeared when the inversion potential became more positive, especially when Eλ+ is greater than 1.8 V. This process was associated with the reduction of SO42- to H2S (reaction 7). A voltammetric study on CPE-mineral arsenopyrite (while the solution was agitated) shows that the current density of peak G2 decreases (Figure 12). This fact confirms the presence of the ionic specie (SO42-) removed from the interface, avoiding its reduction to H2S by reaction 7.

SO42- + 6H+ + 8e- f H2S(aq) + 4OH-

(7)

On the other hand, peak G3 can be associated to the regeneration of the FeAsS through the reduction of As (V) to As(III) species (reaction 8a) formed in the forward scan and by reaction 8b.

H2AsO4- + 3H+ + 2e- f H3AsO3 + H2O

(8a)

Fe2+ + H3AsO3+S0 + 3H+ + 5e- f FeAsS + 3H2O (8b) However, SEM and EDS analysis to the electrode surface of CPE-arsenopyrite mineral, electrochemically modified (corresponding to peak G3, -1.23V), show the presence of an amorphous film of scorodite (Figure 13). The presence of this specie is due to the fact that scorodite formed at 2.0 V (process E1) was not dissolved completely according to reaction 9; the rate of arsenopyrite oxidation declines and pH decreases47 in association with water oxidation. Therefore, the process of peak G3 corresponds to reduction of aqueous arsenate (H2AsO4-) to H3AsO3 at the interface of the electrode, according to the reaction 8a. This fact was also verified through a voltammetric study on CPE-mineral arsenopyrite (stirring the solution) which has shown the decrease of current density of peak G3 (Figure

10460 J. Phys. Chem. C, Vol. 112, No. 28, 2008

Urbano et al.

Figure 16. Typical geometry and electronic structure of all compounds studied; showing distances, angles bond and total energy (∆E) for (a) pyrite and (b) arsenopyrite.

TABLE 5: Calculation of Related and Total Energies of Products and Reagents Conducting an Optimization of the Species Involved to Produce FeOOH from the Oxidation of Pyrite and Arsenopyrite reactants

Etotal (a.u)

FeS2 2(H20) 1/2(O2) FeSAs 5(H20) (O2)

-143.775 -152.813 -75.128 -139.760 -382.032 -150.256

Σreactants (au) -371.716 -672.048

products

Etotal (au)

FeOOH 2(S) H3O+ FeOOH (S) 2(H3O+) H3AsO3

-274.536 -19.967 -76.698 -274.536 -9.984 -153.397 -233.515

12 scan i), indicating the presence of ionic specie of arsenate (H2AsO4-) that is removed of the interface, avoiding its reduction to H3AsO3.

FeAsO4 · 2H2O f FeOOH + H2AsO4- + H+

(9)

At this moment, only some mechanisms of the electrochemical behavior of the arsenopyrite have been identified; nevertheless, it is important remembering that the arsenopyrite mineral is not pure since the content of pyrite is significant in the arsenopyrite mineral (see Table 1). Because the arsenopyrite mineral with high purity is difficult to obtain (since it is always associated with pyrite), a study of the integration of the cathodic and anodic charges (in Table 4) of the Figure 8, was carried out. The chart in Figure 14 shows the charge ratio (QA/QC), which yielded a linear behavior between the potentials range of 1.0 to 1.8 V. When Eλ+ > 1.8 V, the charge ratio was higher and the associated current density of reduction peak G1 increase, due to the beginning of the contribution of pyrite by the reduction of iron hydroxides, indicating that reaction 2a is taking place together with reaction 6a. This fact allows observing that pyrite is oxidized after that arsenopyrite in the arsenopyrite mineral; which is attributed to the galvanic protection offered by the arsenopyrite to the pyrite, avoiding its free oxidation. On the other hand, Figure 15 shows a comparative study of peaks C1 and G1 obtained from the electrochemical responses of pyrite and arsenopyrite (respectively) on the reduction of iron hydroxides, when the potential was varied in the positive direction (Eλ+). Figure 15a shows that current density begins to increase from Eλ+ > 1.1 V for the pyrite; meanwhile, in arsenopyrite mineral (Figure 15b) it was initiated from Eλ+ > 1.6 V, being greater in current density at Eλ+ > 1.8 V. This behavior indicates that the voltammetric response of pyrite in arsenopyrite mineral begins to more positive potentials, than in pyrite mineral with high purity, due to the galvanic protection offered by the arsenopyrite to the pyrite. As mentioned above, the arsenopyrite mineral with high purity is difficult to obtain; therefore, a theoretical study was carried

Σproducts (au)

∆E ) Σproducts Σreactants (au)

∆E (eV)

-371.202

0.514

13.979

-671.432

0.616

16.773

out for a more detailed analysis on the formation of FeOOH from the pyrite and arsenopyrite (reaction 2a and 6a). DFT calculations were performed for reagent and products, i.e., a total optimization of the species involved. Figure 16 shows the geometry and electronic structure of all compounds studied, as distances and angles bond. Table 5 shows the total energy and energy relative. The theoretical results obtained indicate that pyrite oxidation to form FeOOH, occurs first, than the oxidation of arsenopyrite. Therefore, the electrochemical response of the pyrite contained in arsenopyrite mineral is expected first, as seen in the electrochemical study of the pyrite with high purity and the theoretical study. However, in the electrochemical study of arsenopyrite mineral first occurs an oxidation stage of arsenopyrite, followed by a second stage where the arsenopyrite produces FeOOH and then the contribution of pyrite, which produces iron hydroxide. This fact confirms that the pyrite oxidation is displaced to more positive potentials, after of the arsenopyrite oxidation, due to a galvanic protection. 4. Conclusions Voltammetric studies of pyrite and arsenopyrite mineral, carried out beyond that of the literature, allowed elucidate the reaction mechanisms involved in the oxidation stages of arsenopyrite mineral. In the first stage the arsenopyrite is oxidized to Fe2+ and realgar (As2S2), followed by the oxidation of realgar to As(III) specie (H3AsO3) and S0. In the second stage, the arsenopyrite mineral is oxidized to FeOOH, S0, and As(III) specie (H3AsO3) followed by the oxidation of As(III) specie (H3AsO3) to As(V) species (H2AsO4-), sulfur to sulfate (SO42-), and the electrochemical formation of scorodite (FeAsO4 · 2H2O). At this stage, the contribution of the pyrite oxidation was observed by an increase of the amount of iron hydroxides produced. Meanwhile, on the reverse scan (cathodic region) during the second stage the reduction to elemental As0 or the regeneration to arsenopyrite (FeAsS) was not observed due to the reduction of arsenate (H2AsO4-) to H3AsO3 and residual

Pyrite-Arsenopyrite Galvanic Interaction and Reactivity scorodite (FeAsO4.2H2O) that was not dissolved by a chemical reaction at the interface of electrode and pH changing interfacial. The SEM and EDS studies on the CPE-mineral surfaces electrochemically modified, and a study of the charges integration (cathodic and anodic) obtained of the voltammetric response of the CPE-arsenopyrite mineral in presence of an important content of pyrite support the reaction mechanisms proposed. Then, results obtained of electrochemical and theoretical studies showed the galvanic interaction between pyrite and arsenopyrite when they are present in the same mineral. In this case, the pyrite oxidation is delayed in the arsenopyrite mineral, due to the galvanic protection by the arsenopyrite. Acknowledgment. The authors want to thank to CONACYT and Hidalgo’s government for the financial of the Fomix project 2002-01-9166. Gustavo Urbano wishes to thank the doctorate scholarship provided by CONACYT and the PIFOP program. References and Notes (1) Lowson, R. T. Chemical ReViews 1982, 82, 5. (2) Lawrence, W. Biotreatment of Gold Ores. In Microbial Mineral RecoVery; Ehrlich, H. L., Brierley, C. L., Eds.; McGraw-Hill, Inc.: New York, 1994. (3) Cruz, R.; La´zaro, I.; Rodrı´guez, M. J.; Monroy, M.; Gonzalez, I. Hydrometallurgy 1997, 46, 303–319. (4) Pratt, A. R.; Nesbitt, H. W.; Muir, I. Geochim. Cosmochim. Acta 1994, 58, 23–5147. (5) Price, W. A.; Montrin, K.; Hutt, N. Guideliness for the prediction of acid rock drainage and metal leaching from mines in British Columbia: Part I. General procedures and information requirements. In Fourth International Conference on Acid Rock Drainage; Price, W.A., Morin, K., Hutt, N., Eds.; Energy and Minerals Division: Vancouver, BC, 1997; I, 15-30. (6) Pozzo, R. L.; Malicsi, A. S.; Iwasaki, I. Int. Miner. Metall. Process. 1990, 7 (1), 16–20. (7) Cruz, R.; Luna-Sa`nchez, R. M.; Lapidus, G. T.; Gonza`lez, I.; Monroy, M. Hydrometallurgy. 2005, 78, 198. (8) Urbano, G.; Mele´ndez, A. M.; Reyes, V. E.; Veloz, M. A.; Gonza´lez, I. Int. J. Miner. Process. 2007, 82, 148–155. (9) Metha, A. P.; Murr, L. E. Hydrometalurgy. 1983, 9, 235. (10) Nowak, P.; Krauss, E.; Pomianowski, A. Hydrometallurgy. 1984, 12, 95. (11) Paramguru, R. K.; Nayak, B. B. J. Electrochem. Soc. 1996, 143 (12), 3987. (12) Wang, J.; Freiha, B. A. Talanta. 1983, 30, 317. ` sbjo¨rnsson, J. Hydrometallurgy 1993, 34, 171. (13) Ahlberg, E.; A (14) Broadbent, S. R.; Hammersley, J. M. Proc. Cambridge Philos. Soc. 1957, 53, 629. (15) Hammersley, J. M. Proc. Cambridge Philos. Soc. 1957, 53, 642. (16) La´zaro, I.; Cruz, R.; Gonza´lez, I.; Monroy, M. J. Int. J. Miner. Process. 1997, 50, 63. (17) David, J. V.; England, K.E. R.; Kelsall, G. H.; Yin, Q. In Journal of Conference Abstracts, Vol. 2, No. 1; University of Cambridge: Cambridge, UK, 1997. ` sbjo¨rnsson, J. Hydrometallurgy 1994, 36, 19. (18) Ahlberg, E.; A (19) Nava, J. L.; Oropeza, J. L.; Gonza´lez, I. Electrochim. Acta 2002, 47, 1513. (20) Cisneros, I.; Oropeza, M. T.; Gonza´lez, I. Hydrometallurgy 1999, 53, 133. (21) Cisneros, I.; Oropeza, M. T.; Gonza´lez, I. Electrochim. Acta 2000, 45, 2729.

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