Electrochemical behaviour of the dissolution and passivation of arsenopyrite in 9K culture medium

Journal Pre-proofs Full Length Article Electrochemical behaviour of the dissolution and passivation of arsenopyrite in 9K culture medium Xiaoliang Liu, Qian Li, Yan Zhang, Tao Jiang, Yongbin Yang, Bin Xu, Yinghe He PII: DOI: Reference:

S0169-4332(20)30025-8 https://doi.org/10.1016/j.apsusc.2020.145269 APSUSC 145269

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Applied Surface Science

Received Date: Revised Date: Accepted Date:

12 October 2019 17 December 2019 2 January 2020

Please cite this article as: X. Liu, Q. Li, Y. Zhang, T. Jiang, Y. Yang, B. Xu, Y. He, Electrochemical behaviour of the dissolution and passivation of arsenopyrite in 9K culture medium, Applied Surface Science (2020), doi: https://doi.org/10.1016/j.apsusc.2020.145269

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Electrochemical behaviour of the dissolution and passivation of arsenopyrite in 9K culture medium Xiaoliang Liu a, b, Qian Li a, *, Yan Zhang a, b, Tao Jiang a, Yongbin Yang a, Bin Xu a, Yinghe He b, * a

School of Minerals Processing and Bioengineering, Central South University, Changsha, Hunan 410083, P.R. China

b

College of Science and Engineering, James Cook University, Townsville, Queensland 4811, Australia

Abstract The behaviour of arsenopyrite (FeAsS) in the acidic iron-free 9K culture medium was investigated using a combination of electrochemical and analytical techniques. The results demonstrated that the oxidative dissolution of arsenopyrite was accompanied by its surface passivation from the beginning of the arsenopyrite dissolution at potentials from OCP to 1000 mV, which limited further dissolution of arsenopyrite. The dissolution was complex and different under different applied potentials. The leaching kinetics of Fe, As and S, as well as the physicochemical properties of the formed passive film, varied with the potentials. A dissolution rate was found to be likely Fe > As > S from FeAsS. The passive film consisted mainly of As2S2 and As2S3 (≤800 mV), and S0 (>800 mV). The passive film was also shown to have a two-layer structure, i.e. a porous outer layer and a compact inner barrier layer that appeared to impart its strongest resistance to the dissolution of arsenopyrite at 750 mV. An applied potential higher than 800 mV could observably improve the dissolution of the film and thus reduce its passivating action. Keywords: Bio-oxidation; Arsenopyrite; Electrochemistry; Passive film. 1. Introduction Significant amounts of gold deposit is accompanied with arsenopyrite which requires pretreatment before leaching. Bio-oxidation or bioleaching is an environmentally friendly, cheap and simple pre-treatment technology for the recovery of valuable metals from a range of minerals/ores and waste materials. The increasing stringent requirements for environmental protection render the bioleaching technology promising, leading to the recent research and evaluation for its applications. However, the long leaching period has been discouraging the large-scale industrial application of this green technology, which is the main problem that demands an urgent solution [1, 2]. *Corresponding authors. E-mail addresses: [email protected] (Q. Li); [email protected] (Y. He).

A number of researches [3, 4] have demonstrated that a surface film will be formed on the arsenopyrite surface during its bioleaching, resulting in the impediment of further oxidation of arsenopyrite. Zhu et al. [3] used quantitative X-ray photoelectron spectroscopy (XPS)-based depth profiling to research a pure arsenopyrite slice which was bioleached by A. ferrooxidans (A.f) strain in acid 9K medium for 10 days, and a film of ~3000 nm (~450 nm without A.f) in thickness was found on the arsenopyrite surface. Jones et al. [4] also observed an overlayer which had a uniform thickness on the surface of arsenopyrite bioleached in 7 days. Further, our recent research on the bioleaching of a pure arsenopyrite cube [5] suggested that a compact surface film could be formed in the initial leaching stage (< 36 h). The passivation of arsenopyrite arising from this surface film is believed to be the main reason for its long bioleaching period. A variety of analytical techniques such as XRD, Raman microspectroscopy and XPS have been adopted in an attempt to reveal the composition of this film, which is found to be very complex and may consist of pyrite (FeS2)-like, S0, As2S2, As2S3, As3O5, As2O3, FeO(OH), FeAsO4·2H2O, KFe3(OH)6(SO4)2, FePO4, FeSO4·4H2O, etc. [4-16]. The formation process of the film is closely related to the oxidative dissolution of Fe, As and S from arsenopyrite. Under an abiotic environment, it is considered that S is dissolved most slowly [17-19], but the dissolution kinetics of Fe and As are still controversial [7, 18-20]. The bioleaching of the elements from arsenopyrite is a more complex process fraught with uncertainties. Corkhill et al. [7] found that the sequence of bioleaching kinetics for FeAsS was As > S > Fe while Zhu et al [3] concluded that it was Fe > S > As. Although a great number of researches on the leaching of FeAsS has been conducted, the roles of Fe, As and S, and their relationship with the formation of passive film, and the properties of the passive film as well as its passivating action remain unclear, which are highly necessary to be further studied. In essence, the leaching of arsenopyrite with or without microorganisms can be considered as an oxidation process where arsenopyrite, acting as a reductant, is oxidised by an oxidant (e.g., Fe3+ and O2), which is accompanied by the leaching of Fe, As and S, and the formation of a passive film. Although As is semimetallic and S is non-metallic, FeAsS is semiconducting and like an ‘alloy’ [21]. It is thus not difficult to associate this oxidation process with the “corrosion” process of alloy-like FeAsS and the “passivation” process of FeAsS by forming a passive film on its surface. In electrochemistry, potential is a useful ‘oxidant’ which can be used to in a controlled manner to research and reveal the active dissolution and passivation behaviours of a metal or alloy. Undoubtedly, it is of great significance to study the electrochemical behaviour, in particular the oxidation process, of FeAsS in the required electrolyte, i.e. acidic 9K culture medium using electrochemical techniques. However, such electrochemical studies have seldom been conducted.

In addition, bacteria can oxidise arsenopyrite directly or indirectly during bioleaching, but the bacterial oxidation mainly occurs in the indirect mode due to the toxic effect of As in FeAsS to bacteria [22]. In other words, bacteria utilise the Fe3+/Fe2+ redox couple to drive the oxidation of arsenopyrite. Electrochemical potential can be utilised to replace the actual oxidant i.e. Fe3+ or redox couple i.e. Fe3+/Fe2+. Thus, the 9K culture medium used in the electrochemical study was free of iron to eliminate the influence of iron and make sure that the dissolution and passivation behaviour of arsenopyrite are investigated under the effect of only the electrochemical potential. This paper uses standard electrochemical techniques that include cyclic voltammetry, linear sweep voltammetry, chronoamperometry and electrochemical impedance spectroscopy (EIS) assisted with analytical techniques of atomic force microscope (AFM), XPS and inductively coupled plasma–atomic emission spectrometer (ICP-AES) to characterise the electrochemical behaviour of a pure arsenopyrite electrode in iron-free 9K culture medium. The relationships between the elements leached from FeAsS and the properties of the passive film such as space structure, resistance and composition are discussed in detail. The results established the structure of the passive film on the surface of arsenopyrite, and how it influences the further dissolution of the arsenopyrite. They are useful in the understanding and improvement of leaching of arsenopyrite particularly in the presence of microorganisms. 2. Experimental 2.1 Electrochemical experiments All the electrochemical experiments were conducted on a Princeton Model 283 Potentiostat/ Galvanostat (EG&G Princeton Applied Research) coupled with a personal computer using a conventional three-electrode system which consists of the working electrode of FeAsS or Pt electrode, the counter electrode of twin high-density and non-permeable graphite rods, and the reference electrode of Ag/AgCl (saturated KCl) electrode. The investigated arsenopyrite was from Yaogangxian in Hunan province of China, with a composition of Fe1.00As0.99S0.97 and containing Si 0.132 wt.%, Co 0.034 wt.% and Ni 0.062 wt.% as impurities (determined using Xray fluorescence analysis). The high-purity arsenopyrite was cut into a cylindrical electrode of ~14 mm in diameter and ~5 mm in height. The arsenopyrite electrode was sequentially polished with silicon carbide papers of 800, 2000 and 3500 grid, and then ultrasonically cleaned in alternate baths of 5 M HCl, methanol and deionised water for 5 min to remove the surface contaminants [23, 24]. The purity of the Pt electrode (Φ14 mm × 2 mm) was higher than 99.999%. The exposed area of the working electrode was 1 cm2. The test electrolyte was iron-

free 9K culture medium containing K2HPO4·3H2O 0.50 g/L, (NH4)2SO4 3.0 g/L, MgSO4·7H2O 0.5 g/L, KCl 0.1 g/L and Ca(NO3)2·4H2O 0.01g/L. The pH value of the medium was 1.8 adjusted by careful addition of ~3 mol·L-1 sulphuric acid. Nitrogen (purity > 99.9%) was bubbled through the test solution to remove oxygen, and all tests were performed under a protective atmosphere of nitrogen. Prior to each electrochemical test, the working electrode was put into electrolyte for 15 min to ensure the stabilisation. After the chronoamperometry test was finished, EIS measurements and AFM and XPS analyses of the polarised arsenopyrite electrode were performed. The EIS measurement was carried out at open circuit potential (OCP) using an AC perturbation of ±5 mV in the frequency range of 10-2 Hz – 105 Hz. ZSimpWin v3.30 software was used to fit the impedance spectra based on an equivalent electrical circuit. Before the analyses of AFM and XPS, the polarised arsenopyrite electrode was dried in nitrogen atmosphere, sealed in an air-tight plastic bag, and stored in a refrigerator to minimise oxidation. During the chronoamperometry test, samples of solution were withdrawn at regular intervals for chemical analysis. All potentials are reported relative to the standard hydrogen electrode (SHE). All experiments were conducted at 25 (±0.5) ºC. The used chemicals were all AR grade. Deionised water was used throughout all experiments. 2.2 Instrumentation The pH values were measured with a pH meter (PHSJ-4A). The concentrations of Fe, As and S were determined by ICP-AES (America Baird Co. PS-6). The morphology of the arsenopyrite surface was determined with AFM (PicoSPM II). The element chemical state and possible species on the arsenopyrite surface were identified by XPS (ESCALAB 250Xi). The XPS peaks were fitted using Avantage 5.52 software, and the contents (at.%) of elements as well as their chemical state distributions (at.%) were determined based on peak areas. During fitting for the spectra, all S 2p and As 3d components were assigned as doublets with an intensity ratio of 2:1 and 3:2 with the same FWHM, and a spin orbit splitting of 1.19 eV and 0.68 eV, respectively. Based on the theoretical core p level multiplet structures (GS multiplets) for free transition metal ions [25, 26], Fe(II) yields three major multiplet peaks and two extremely small multiplet peaks at higher binding energy whilst Fe(III) has four major multiplet peaks, each with equal full width half maximum peak width (FWHM) and separated by approximately 1.00 eV [27]. 3. Results and discussion 3.1 Cyclic voltammetry measurement

Fig. 1 shows the cyclic voltammograms of arsenopyrite and platinum electrodes in iron-free 9K culture medium. Using platinum as the working electrode, no obvious current density was found in the stages of A1 (550(OCP) – 750 mV), A2 (750 – 1000 mV) and C1 (1000 – 0 mV), indicating the high stability of the electrolyte, i.e. iron-free 9K culture medium. On the platinum electrode, gas bubbles were observed to form in stage C2 (0 – -600 mV) and then disappear in stage A3 (-100 – 550 mV). It is clear that the typical hydrogen evolution reaction (2H+ + 2e– = H2) and the oxidation reaction of H2 (H2 = 2H+ + 2e–) occurred. However, the cyclic voltammetry behaviour of arsenopyrite is much more complex than that of platinum as clearly shown in Fig. 1. With the occurrence of an increasing anodic current density, arsenopyrite was oxidised easily in stage A1 but more readily in stage A2. An obvious reduction peak at ~550 mV with relatively low current densities appeared during the negative scanning in stage C1, which results from the reduction of oxidation products from the oxidation process of arsenopyrite [28]. A similar cyclic voltammogram for arsenopyrite and experimental phenomenon in the arsenopyrite/electrolyte interface were also found during the scanning in stages C2 and A3. However, the obvious reduction current peak in stage C2 were reported to likely arise from the formation of H2S, instead of H2, from the reduction of S0 (i.e. S0 + 2H+ + 2e– = H2S) which is one of the typical products from arsenopyrite oxidation [19]. The redox behaviour of arsenopyrite occurring in stages C2 and A3 is unclear at this stage but beyond the research scope of this paper. Since the electrochemical behaviours of arsenopyrite in stages A1 and A2 within OCP  1000 mV are intimately associated with the oxidative dissolution and passivation of arsenopyrite, they were studied and discussed in detail in the following sections. 4

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3.2 Oxidation behaviour of arsenopyrite 3.2.1 Linear sweep voltammetry measurement The oxidation process of arsenopyrite was firstly investigated by the voltammograms with four consecutive scanning in different potential ranges as shown in Figs. 2(a) – (e). 0.05

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As the potential range was within 850 mV, Figs. 2(a) – (c) show that the current densities gradually declined with the increasing scanning number of time. This implies that some oxidation products do form and accumulate on the arsenopyrite surface, impeding its further oxidation. As the upper limit of the potential range exceeded 850 mV (Figs. 2(d) and (e)), a significant passivation of the arsenopyrite surface occurred after the first scanning was finished, which is likely due to the build-up of a substantial amount of oxidation products. In contrast with Figs.

2(a) – (c), voltammograms of the second to fourth scanning in Figs. 2(d) and (e) almost overlapped. This likely lies in the oxidation of not only arsenopyrite but also its oxidation products as well as the formation of a balanced surface structure of arsenopyrite at the end of the first scanning. Clearly, the passivation phenomenon occurs once the arsenopyrite oxidation begins, and the further oxidation of arsenopyrite is significantly influenced by the passivating products and applied potential on its surface. 3.2.2 Chronoamperometry measurement Chronoamperometry responses of the arsenopyrite electrode were also recorded under different constant potentials (650 – 950 mV) as shown in Figs. 3(a) – (e). 0.11

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A dramatic decrease in the initial current density was found under all applied potentials,

further suggesting that arsenopyrite experiences a passivation process since the start of its oxidation. Afterwards, potential was found to have a significant effect on the further oxidation of arsenopyrite. At relatively low potentials of 650 mV (Fig. 3(a)) and 750 mV (Fig. 3(b)), the current densities always decreased with time. The oxidation of arsenopyrite was thus constantly hampered due mainly to the gradual accumulation of passivating products on surface. As the potential was increased to 800 mV (Fig. 3(c)), a constant current density could be finally obtained. It is therefore not difficult to believe that the oxidative dissolution of the passivating products can occur at 800 mV, and an equilibrium between the formation of passivating products (from arsenopyrite oxidation) and their further dissolutions was reached with prolonged time. It is also reasonable to predict that the dissolution of the passivating products can be improved at a higher potential. Not surprisingly, at potentials of 850 mV (Fig. 3 (d)) and 950 mV in (Fig. 3 (e)), the current densities, after experiencing the initial decrease, began to increase with time, and then gradually became relatively stable. In contrast with the obvious passivation occurring at potentials of 650 – 850 mV in Figs. 3(a) – (d) with passive current densities being ultimately attained, the oxidation of arsenopyrite at 950 mV appeared to only undergo a transient stage of passivation in the beginning and then be rather active, leading to high current densities with no occurrence of passivation. This is possibly attributed to the fact that the oxidative dissolution of passivating products can proceed fast at an over-high potential, thus noticeably reducing their passivating action. According to the above research, the formation of passivating products appears to be inevitable due to the oxidation of arsenopyrite, and thus a well-known passive film is formed on the surface. Although this passive film can be dissolved or destroyed in some other way at high potentials (e.g., 950 mV), it is proven to always exist in the actual leaching of arsenopyrite [5]. The passivating action of the passive film is largely dependent on its physicochemical properties such as space structure (thickness, porosity, morphology, etc.), resistance, and composition. It is difficult to exactly characterise these properties due mainly to the complex leaching of arsenopyrite under varying pulp potentials. However, the properties of a passive film that is formed under a controlled potential is stable, making it possible to qualitatively and quantitatively determine its properties and thus passivating action. As a result, the polarised arsenopyrite obtained from the above chronoamperometry test was used to deeply investigate the physicochemical properties of the passive film by performing the EIS measurements and analyses of AFM and XPS as will be shown in Section 3.3.

3.3 Physicochemical properties of the film on polarised arsenopyrite surface 3.3.1 EIS measurement The EIS results are presented in the Nyquist plots as shown in Fig. 4(a). The adopted equivalent electrical circuit model and its values of fitted parameters are presented in Fig. 4(b) and Table 1, respectively. All the impedance spectra in Fig. 4(a) consisted of a small incomplete semicircle in highfrequency and a great incomplete semicircle in low-frequency, and they fitted well (χ2 < 10-3 in Table 1) with the representative model of equivalent electrical circuit (Fig. 4(b)). This model is proposed by Hoar and Wood [29], and has been successfully adopted in many researches to analyse the characteristics of passive film on various material surfaces [30-38]. Specifically, Rs is the solution resistance, and the high-frequency parameters R1 and Q1 represent the properties of the reactions at the outer layer at the passive film/electrolyte interface. The parameter R2 coupled with Q2 describes the processes at the inner barrier layer at the passive film/electrolyte interface. 2100

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χ2 3.64×10-4 8.81×10-5 6.98×10-4 7.08×10-4 6.26×10-4

Rs /(Ω·cm2) 41.2 39.93 40.05 40.49 39.85

R1 /(Ω·cm2) 876.3 1860 457.6 489 402.9

Y0,1 /(μΩ·cm-2·s-n) 106.3 45.72 98.78 100.9 66.3

n1 0.8216 0.8341 0.7478 0.7818 0.8292

R2 /(Ω·cm2) 8656 11270 5670 5578 3473

Y0,2 /(μΩ·cm-2·s-n) 581.3 539.1 958 1054 1201

n2 0.5029 0.5477 0.6024 0.5915 0.5403

In addition, the thickness of the passive film can be estimated using Eq. (1) based on the capacitance data, although changes in the film morphology or composition can alter the dielectric properties, ε, of the film [31, 39, 40].

d = (ε ε0 A)/C

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(2)

where R, Y0 and n are listed in Table 1. Although the actual ε value of the film is difficult to estimate, a change of C can be used as an indicator for a change in the film thickness d. It is assumed that the change in thickness results in the same change trend for R and 1/C, whereas, an opposite change trend for R and 1/C is interpreted as being due to a change in porosity [31, 40, 42]. The variations of R (also listed in Table 1) and 1/C of the film on oxidised arsenopyrite surface with potential are shown in Figs. 5(a) and (b). 4.4

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and reciprocal capacitance (1/C2) with potential for the surface film of oxidised arsenopyrite.

As listed in Table 1, the values of electrolyte resistance (Rs) in the passive film/electrolyte interface under different potentials were almost constant and very low (~40 Ω·cm2), which were much lower than those of film resistance (R1 and R2). According to Fig. 4(b), the passive film had a two-layer structure of an outer layer and an inner layer. Figs. 5(a) and (b) clearly show that there were significant differences between the two layers. The thickness of the outer layer was far greater than that of the inner layer because of 1/C1 (1.5×10-2 – 4.0×10-2 μF-1·cm2) >> 1/C2 (2.4×10-4 – 4.4×10-4 μF-1·cm2), but the inner layer had an evidently higher resistance (R2 = 3000 – 12000 Ω·cm2) comparing with the outer layer (R1 = 400 – 2000 Ω·cm2). This suggests that the outer layer is rather porous while the inner is a much compact barrier layer. The formation of the two-layer structure is likely due to kinetic effects. The passivating products form a compact film immediately upon contact between the arsenopyrite and the leaching solution. Some of passivating products can further react with the leaching solution, leading to the formation of

holes/pores in the film, and the start of the second layer, thus the two-layer structure. It is thus reasonable to assume that the inner barrier layer dominates the passivating action of the film. The passivating action was noticeably improved from 650 mV to 750 mV due to the much increased R and 1/C which are indicative of an enhanced accumulation of passivating products on the arsenopyrite surface. With the increase of potential from 750 mV to 800 mV, a significant decrease in R and 1/C occurred, and thus the film’s passivating action was much reduced. As the potential rose to 850 mV, 1/C decreased, but R remained basically unchanged. As will be discussed in Section 3.3.2, this is most likely caused by the distinct changes in morphology and composition of the film. At a higher potential of 950 mV, with the significant decrease of R2 and 1/C2 the passivating action from the inner layer was considerably reduced. In terms of the outer layer, we found a slight decrease of R1 but with an obvious increase of 1/C1. This opposite change possibly indicates that the outer layer becomes porous. Associating with Section 3.2, the decrease of R1 and R2 with an increasing potential (>750 mV) possibly results from the oxidative dissolution of passivating products, leading to either a decrease in thickness (1/C) or an increase in porosity for the two layers. On the contrary, the increase in R and 1/C from 650 mV to 750 mV suggests that the passivating products build up steadily on the arsenopyrite surface, and thus the strongest passivating action of the film was observed at 750 mV as a result of its highest R and 1/C. 3.3.2 AFM and XPS analyses The morphology of the oxidised arsenopyrite surface under various potentials can be clearly perceived from the AFM images presented in Fig. 6 and the surface roughness (Rq) values listed in Table 2. (a) Pristine

(b) 650 mV

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Fig. 6 AFM images of the (a, a') pristine and (b, b') – (f, f') oxidised arsenopyrite surfaces under a potential of (b, b') 650 mV, (c, c') 750 mV, (d, d') 800 mV, (e, e') 850 mV and (f, f') 950 mV for 1800 s. Table 2 Evolution of roughness Rq (root mean square) values for pristine and oxidised arsenopyrite surfaces with a collection area of 1 μm2. Oxidised arsenopyrite Pristine 650 mV 750 mV 800 mV 850 mV 950 mV

Rq(± standard deviation)/nm 5.62 ± 1.54 10.32 ± 3.79 12.55 ± 2.05 14.70 ± 4.10 26.00 ± 5.90 35.30 ± 10.6

In comparison with the pristine smooth surface (Fig. 6(a, a')), the roughness (Rq) of the arsenopyrite surface continuously rose as the potential increased (Table 2). Consistent with the evolution of roughness, a series of distinct corrosion characteristics were observed from the oxidised arsenopyrite surfaces as presented in Figs. 6(b, b') – (f, f'). At a potential greater than 650 mV, various crystalline particles were also found on the surfaces, indicating the formation of different passivating products for the passive film. The composition of the passive film was further investigated by XPS analysis. A broad scan (not shown) and a series of narrow scans (Fig. 7) for the XPS analysis were performed to identify the chemical states of Fe 2p, As 3d and S 2p, and measure their contents (atom fraction at.%) based on the peak areas (Fig. 8) on the outmost surfaces of the pristine and oxidised arsenopyrite. Fig. 8(a) shows the atom fractions of the elements from the broad scan (see Appendix A for details). The fitted spectra of Fe 2p, As 3d, and S 2p are shown in Fig. 7, and their peak parameters are summarized in Figs. 8(b) – (d) (see Appendices B – D for details). The detailed assignments of Fe 2p(3/2), S 2p(3/2) and As 3d(5/3) were determined according to the references [2, 3, 7, 8, 17-19, 43-48]. Note that Fe(II)-AsS, As(-I)-S, and S2–/(AsS)2– respectively referred to the chemical states of Fe, As and S in FeAsS [2, 3, 45]. The As–S bond in (AsS)2– is difficult to distinguish from the S–S bond in S22– due to their high similarity [3, 7, 17]. The chemical states of S22– and Sn2‒ (n > 2) likely arise from As2S2 (realgar) and/or As2S3 (orpiment), and As2O3 and As3O5 denote the formation of soluble As species such as AsO2‒/AsO33‒, AsO32‒ and AsO43‒ [19, 44, 49-51].

32000

100000

33000

28000

Fe(II)-(AsS)

70000

Fe(III)-(AsS)

60000

As(-I)-S

24000

20000

As2S2

16000

27000

712

710

708

706

21000

8000 45

704

44

43

Binding energy/eV

42

41

40

18000 167

39

166

160

(AsS)2–/S2– 2

Intensity/(a.u.)

48000

Intensity/(a.u.)

Intensity/(a.u.)

Fe(II)-(AsS)

161

(b'') S 2p650 mV As(-I)-S

51000

162

40000

20000

Fe(III)-O

163

45000

(b') As 3d650 mV

57000

164

Binding energy/eV

24000

(b) Fe 2p650 mV

16000

As2S2 12000

35000

S2– n 30000

S2– 25000

8000 20000

Fe(III)-(AsS)

45000

165

Binding energy/eV

60000

54000

S2–

S2– n 24000

12000

50000 714

(AsS)2–/S2– 2

30000

Intensity/(a.u.)

80000

Intensity/(a.u.)

90000

Intensity/(a.u.)

(a'') S 2pPristine

(a') As 3dPristine

(a) Fe 2pPristine

4000 42000 716

714

712

710

708

706

45

704

44

43

41

40

15000 167

39

165

164

163

161

160

30000

(c) Fe 2p750 mV

(c'') S 2p750 mV

(c') As 3d750 mV

(AsS)2–/S22–

27000

Fe(III)-O

162

Binding energy/eV

2000

90000

1600

80000

75000

Fe(III)-(AsS)

70000

As2S3

Intensity/(a.u.)

Intensity/(a.u.)

85000

166

Binding energy/eV

Binding energy/eV

Intensity/(a.u.)

42

1200

As2S2

800

S2n–

24000

21000

S2– 18000

15000

400

Fe(II)-(AsS) 65000 718

716

714

712

710

708

706

704

47

46

45

44

30000

42

41

12000 167

40

Fe(III)-(AsS) Fe(II)-(AsS)

22000

163

162

161

160

(d'') S 2p800 mV

Intensity/(a.u.)

Intensity/(a.u.)

26000

164

20000

As2S2

4000

Fe(III)-O

165

24000

(d') As 3d800 mV

28000

24000

166

Binding energy/eV

5000

(d) Fe 2p800 mV

Intensity/(a.u.)

43

Binding energy/eV

Binding energy/eV

3000

As2S3 As2O5 As2O3

2000

S2n–

16000

S22–/(AsS)2– 12000

SO42–

SO32–

8000

20000 1000 4000

18000 718

716

714

712

710

708

706

704

49

702

48

47

46

Binding energy/eV

45

44

43

42

41

40

39

172

Fe(II)-(AsS)

34500

Fe(III)-(AsS) 34000

As2S3

6000

5250

As2O5

As2O3 As(-I)-S

4500

714

712

710

708

706

3000 48

704

47

46

45

44

43

42

41

40

(f) Fe 2p950 mV

SO24

22500

SO23

–

15000

2000

Fe(II)-(AsS) Fe(III)-SO

As2O3

Intensity/(a.u.)

Intensity/(a.u.)

15000

–

(AsS)2 /S22

170

168

166

–

As2S3

1600

164

162

160

(f'') S 2p950 mV S2n–

12500

As2O5

Fe(III)-O

–

Binding energy/eV

(f') As 3d950 mV

16500

–

S0

7500 172

39

2400

13500

S2n 30000

Binding energy/eV

Binding energy/eV 18000

160

15000

3750

33500 716

Intensity/(a.u.)

Intensity/(a.u.)

Intensity/(a.u.)

Intensity/(a.u.)

Fe(III)-O

162

37500

36000

35000

164

(e'') S 2p850 mV

6750

35500

166

45000

(e') As 3d850 mV

(e) Fe 2p850 mV

168

Binding energy/eV

7500 36500

170

Binding energy/eV

As(-I)-S 1200

10000

7500

SO42–

SO32– S0

5000

800 12000

718

Fe(III)-(AsS) 716

714

2500 712

710

Binding energy/eV

708

706

400 49

48

47

46

45

44

43

42

41

40

Binding energy/eV

172

170

168

166

164

162

160

Binding energy/eV

Fig. 7 XPS spectra of the (a, a', a'') pristine and potentiostatic oxidised arsenopyrite at (b, b', b'') 650 mV, (c, c', c'') 750 mV, (d, d', d'') 800 mV, (e, e', e'') 850 mV and (f, f', f'') 950 mV for 1800 s.

Fig. 8 Variation of the (a) element contents (at.%) and (b) – (d) chemical states and their distributions (at.%) of (b) Fe 2p, (c) As 3d and (d) S 2p for the pristine and oxidised arsenopyrite under 650 mV, 750 mV, 800 mV, 850 mV and 950 mV for 1800 s.

As shown in Figs. 7 and 8, substantial changes in the atom fractions of Fe, As and S and their chemical states as well as possible species in the oxidised surface or passive film occurred comparing with the pristine FeAsS surface (approximately 1:1:1 for Fe:As:S). Under different potentials, the atom fractions of the elements were found in the order of Fe < As < S. With an easy dissolution of Fe in 650 – 950 mV, As seemed to readily leave the mineral surface into solutions since the potentials >750 mV. At potentials >850 mV, mainly S appeared to stay at the film. This is likely due to the corresponding dissolution kinetics of Fe > As > S as will be discussed in Section 3.4. For the passive film formed at potentials ≤750 mV, Fe was found to markedly dissolve into solutions while As and S were both left at the film. Under a potential of 650 mV, in addition to a small amount of Fe(III)-O likely deriving from iron hydroxides/oxides [7, 8], some As2S2 was found in the film. As the potential rose to 750 mV, the passive film appeared to consist mainly of As2S3 and As2S2. Consistent with the electrochemical results, obvious As2O3, As3O5, SO32‒ and SO42‒, in addition to arsenic sulphides (As2S3 and As2S2), occurred with a significant decrease in the As fraction at 800 mV, indicating the oxidative dissolution of the arsenic sulphides. At higher potentials of 850 mV and 950 mV, mainly S occurring as various species were found in the film. In particular, substantial amounts of polysulphides (Sn2‒) and S0 were formed, of which Sn2‒ is liable to decompose to S0 and H2S in

acidic solutions [52]. This would likely allow S0 to be formed in the film as its main composition. In addition, the occurrence of SO32‒ and SO42‒ was possibly attributed to the dissolution of arsenic sulphides and S0 at high potentials (Sections 3.2.2 and 3.3.1), and thus a small quantity of Fe(III)-SO [7, 8] likely originating from jarosite was observed at 950 mV. The above results make it reasonable to believe that the passive film mainly consists of these As- and S-containing species, i.e. As2S2 and As2S3 (≤800 mV), and S0 (>800 mV). 3.4 Element dissolution from arsenopyrite during chronoamperometry measurement Since the leaching of elements from FeAsS to solutions is closely related to the oxidative dissolution and passivation behaviour of arsenopyrite as discussed above, the concentrations of Fe, As and S in electrolyte were also determined during the chronoamperometry tests. It should be noted that the quantity of leached S (the difference of total S content before and after leaching) was difficult to exactly calculate and found to be “negative” (not shown) under all potentials. This is most likely caused by the loss of S from the release of a colourless gas with the characteristic foul odor of rotten eggs (possibly H2S). However, as suggested by results from the above XPS analyses as well as previous researches [17-19], the leaching of S is very likely slower than Fe and As. In terms of Fe and As, their leaching behaviours under different potentials are shown in Fig. 9. Figs. 9(a) – (c) show that the leaching of Fe was observably faster due to its higher concentrations than those of As in the potential range of 650 – 800 mV. In particular, the concentrations of As were much lower compared to Fe at potentials ≤750 mV. Only at high potentials of 850 mV and 950 mV (Figs. 9(d) and (e)), As could be leached at a comparable rate to Fe. An obvious passivation took place according to the occurrence of plateau for Fe concentrations after leaching around 60 min at potentials of 650 – 800 mV. As was difficult to be leached at potentials ≤750 mV, and its leaching appeared to be readily at 800 mV but still not fast. As discussed in the XPS analyses, a passive film consisting mainly of arsenic sulphides (As2S2 and As2S3) was likely formed on the surface that appears to completely impede the leaching of Fe from arsenopyrite at potentials ≤800 mV. Similar to the leaching of Fe, As could also be constantly leached from FeAsS at an appreciable rate at potentials >800 mV. It is thus not surprising that mainly S was left on the surface as a result of its slowest leaching kinetics, leading to the formation of a passive film consisting mainly of S0, in agreement with the XPS analyses. In particular, no significant passivation phenomenon occurred in the leaching of FeAsS under 850 mV and 950 mV. As mentioned in Figs. 3(d) and (e), there was only a shortlived severe

passivation in the initial leaching process (< ~100 s), but afterwards this passivation would be reduced due to the observable dissolution of the passive film, thus resulting in the constant leaching of Fe and As both at an appreciate rate. 3.2

(a) 650 mV

2.8

Ion concentration/(molL-1)

Ion concentration/(molL-1)

3.2

2.4 2.0 1.6 1.2

Fe As

0.8 0.4 0.0

(b) 750 mV

2.8 2.4 2.0 1.6 1.2

Fe As

0.8 0.4 0.0

0

20

40

60

80

100

120

140

160

0

20

40

Time/min

80

100

120

140

160

Time/min

5.6

11.2

(c) 800 mV

Ion concentration/(molL-1)

Ion concentration/(molL-1)

60

4.8 4.0 3.2 2.4 1.6

Fe As

0.8 0.0

(d) 850 mV

9.8 8.4 7.0 5.6 4.2 2.8

Fe As

1.4 0.0

0

20

40

60

80

100

120

140

160

Time/min

0

20

40

60

80

100

120

140

160

Time/min

Ion concentration/(molL-1)

112

(e) 950 mV 96 80 64 48 32

Fe As

16 0 0

20

40

60

80

100

120

140

160

Time/min

Fig. 9 Variation of the Fe and As concentrations in electrolyte with time under a constant potential of (a) 650 mV, (b) 750 mV, (c) 800 mV, (d) 850 mV and (e) 950 mV.

Consistent with the results from XPS analyses, the leaching kinetics of Fe, As and S from arsenopyrite likely follows the order of Fe > As > S. The leaching behaviour of the elements leads to the above oxidative dissolution and passivation behaviours of arsenopyrite. In association with the electrochemical results and the AFM and XPS analyses, we can find that, from 650 mV to 800 mV, most of Fe and part of As were leached from FeAsS, and thus As2S2 and As2S3 were produced and accumulated on the arsenopyrite surface, forming the passive film and reaching its highest thickness and resistance at 750 mV. At potentials >800 mV, most of Fe

and As could be dissolved and thus significant changes in the properties of the passive film were accompanied. The main composition of the film became S0 at 850 mV and 950 mV, and a higher potential rendered the film either thinner in its thickness (or lower in resistance) or higher in its porosity. 4. Conclusions The oxidative dissolution of arsenopyrite in acidic iron-free 9K culture medium is complex as suggested from its complicated and changeable electrochemical behaviours and relevant AFM, XPS and ICP-AES analyses. Since the dissolution of arsenopyrite begins, the passivation resulting from the formation of so-called passive film inevitably takes place on its surface. In addition, the dissolution and passivation of arsenopyrite changes significantly with the potentials from OCP (~550 mV) to 1000 mV. This is largely attributed to the differences in the dissolution kinetics among Fe, As and S from FeAsS and the physicochemical properties of the formed passive film at different potentials. The dissolution kinetics of the elements was shown to be likely Fe > As > S. With the easy dissolution of Fe, various As- and S-containing species were likely produced on the arsenopyrite surface forming a passive film that mainly consists of As2S2 and As2S3 (≤800 mV), and S0 (>800 mV). The passive film was found to have a two-layer structure of a thick but porous outer layer and a thin but compact inner layer. The inner layer is responsible for the passivating action of the film due mainly to its much higher resistance than the outer layer. The passivating action appears to be strongest at 750 mV due to the constant accumulation of passivating products. At higher potentials (≥800 mV), an appreciable oxidative dissolution of the passivating products also occurs and thus noticeably reduce the film’s passivating action. In the practical bioleaching process of arsenopyrite, the solution potentials commonly vary in the range of 500 – 850 mV. It is therefore not difficult to understand why the As- and Scontaining species, i.e. As2S2, As2S3 and S0 are often found in the passive film. However, consistent with our recent research [5], these passivating species could be generated from the beginning of the bioleaching process (<36 h), and would form a clear passive film on the arsenopyrite surface that severely limits its further bioleaching. In order to reduce or eliminate the passivation from this passive film and thus shorten the long bioleaching period, effective measures seem to be maintaining a higher level of solution potential than 800 mV or adding some additives that can promote the dissolution and break-down of the passive film.

Acknowledgments Financial supports from the National Natural Science Foundation of China (Grant No. 51574284), the Fundamental Research Funds for the Central Universities of Central South University (Grant No. 2017zzts194), the China Scholarship Council (Grant Nos. 201706370222 and 201606370128) and the National Key Research and Development Program of China (Grant No. SQ2018YFE011066) are all gratefully acknowledged.

Appendices Appendix A. Element contents (at.%) of Fe 2p, As 3d and S 2p on the surface of the pristine and potentiostatic oxidised arsenopyrite in 1800 s under 650 mV, 750 mV, 800 mV, 850 mV and 950 mV. Content/at.% As 3d 35.25 40.15 45.45 20.67 4.39 6.40

Fe 2p 28.40 13.17 4.44 5.81 1.28 11.63

Pristine 650 mV 750 mV 800 mV 850 mV 950 mV

S 2p 36.35 46.68 50.11 73.52 94.33 81.97

Appendix B. Peak parameters and chemical states of Fe 2p(3/2) in the pristine and potentiostatic oxidised arsenopyrite in 1800 s under 650 mV, 750 mV, 800 mV, 850 mV and 950 mV.

Pristine

650 mV

750 mV

800 mV

Binding energy/eV 706.6 707.6 708.6

FWHM 1 1.2 1.1

Chemical state Fe(II)-(AsS) Fe(II)-(AsS) Fe(II)-(AsS)

709.6 710.6 711.6 712.6 706.4 707.4 708.4

1.6 1.6 1.6 1.6 1 1 1

Fe(III)-(AsS) Fe(III)-(AsS) Fe(III)-(AsS) Fe(III)-(AsS) Fe(II)-(AsS) Fe(II)-(AsS) Fe(II)-(AsS)

709.5 710.5 711.5 712.5

1.1 1.1 1.1 1.1

Fe(III)-(AsS) Fe(III)-(AsS) Fe(III)-(AsS) Fe(III)-(AsS)

711.2 712.2 713.2 714.2 706.6 707.6 708.6

1.7 1.7 1.7 1.7 1.1 1.0 1.0

Fe(III)-O Fe(III)-O Fe(III)-O Fe(III)-O Fe(II)-(AsS) Fe(II)-(AsS) Fe(II)-(AsS)

709.7 710.7 711.7 712.7

1.3 1.3 1.3 1.3

Fe(III)-(AsS) Fe(III)-(AsS) Fe(III)-(AsS) Fe(III)-(AsS)

711.2 712.2 713.2 714.2 706.5 707.5 708.5

1.8 1.8 1.8 1.8 1 1 1

Fe(III)-O Fe(III)-O Fe(III)-O Fe(III)-O Fe(II)-(AsS) Fe(II)-(AsS) Fe(II)-(AsS)

Content/at.% 85.52

14.48

57.78

11.08

31.14

40.04

14.65

45.31

64.02

850 mV

950 mV

709.5 710.5 711.5 712.5

1.5 1.5 1.5 1.5

Fe(III)-(AsS) Fe(III)-(AsS) Fe(III)-(AsS) Fe(III)-(AsS)

711.7 712.7 713.7 714.7 706.5 707.5 708.5

1.7 1.7 1.7 1.7 1 1 1

Fe(III)-O Fe(III)-O Fe(III)-O Fe(III)-O Fe(II)-(AsS) Fe(II)-(AsS) Fe(II)-(AsS)

709.7 710.7 711.7 712.7

1.1 1.1 1.1 1.1

Fe(III)-(AsS) Fe(III)-(AsS) Fe(III)-(AsS) Fe(III)-(AsS)

711.7 712.7 713.7 714.7 706.8 707.8 708.8

1.5 1.5 1.5 1.5 1.0 1.1 1.0

Fe(III)-O Fe(III)-O Fe(III)-O Fe(III)-O Fe(II)-(AsS) Fe(II)-(AsS) Fe(II)-(AsS)

709.7 710.7 711.7 712.7

1.2 1.2 1.2 1.2

Fe(III)-(AsS) Fe(III)-(AsS) Fe(III)-(AsS) Fe(III)-(AsS)

9.85

712.0 713.0 714.0 715.0

2.0 2.0 2.0 2.0

Fe(III)-O Fe(III)-O Fe(III)-O Fe(III)-O

71.05

714.2 715.2 716.2 717.2

1.3 1.3 1.3 1.3

Fe(III)-SO Fe(III)-SO Fe(III)-SO Fe(III)-SO

5.96

13.22

22.76

25.06

39.29

35.65

13.14

Appendix C. Peak parameters and chemical states of As 3d(5/2) in the pristine and potentiostatic oxidised arsenopyrite in 1800 s under 650 mV, 750 mV, 800 mV, 850 mV and 950 mV. Pristine 650 mV 750 mV 800 mV

Binding energy/eV 41.5 42.8 41.6 42.7 42.3 43.6 42.3 43.3 44.7

FWHM 1.3 0.7 1.1 1.1 1.0 1.3 0.8 0.9 1.5

Chemical state As(-I)-S As2S2 As(-I)-S As2S2 As2S2 As2S3 As2S2 As2S3 As2O3

Content/at.% 91.47 8.53 78.22 21.78 11.95 88.05 65.98 14.57 13.4

850 mV

950 mV

46.2 41.7 43.2 44.5 45.8 41.8 43.3 45.4 46.1

1.0 1.2 1.1 1.0 1.1 0.8 1.1 1.4 1.4

As2O5 As(-I)-S As2S3 As2O3 As2O5 As(-I)-S As2S3 As2O3 As2O5

6.05 15.23 63.17 9.79 11.81 12.43 24.79 29.69 33.09

Appendix D. Peak parameters and chemical states of S 2p(3/2) in the pristine and potentiostatic oxidised arsenopyrite in 1800 s under 650 mV, 750 mV, 800 mV, 850 mV and 950 mV. Pristine 650 mV 750 mV

800 mV

850 mV

950 mV

Binding energy/eV 161.5 162.5 163.7 161.4 162.6 163.7 161.4 162.4 163.4 162.9 163.7 168.1 169.0 162.5 163.6 164.1 168.3 169.1 163.7 164.9 167.1 168.6

FWHM 1.4 1.3 1.1 0.8 1.3 1.2 0.7 1.1 1.2 1.5 0.88 1.2 1.2 1.0 1.1 0.9 1.6 1.2 1.1 0.9 1.4 1.4

Chemical state S2‒ 2‒ S2 /(AsS)2‒ Sn2‒ (n > 2) S2‒ 2‒ S2 /(AsS)2‒ Sn2‒ (n > 2) S2‒ S22‒/(AsS)2‒ Sn2‒ (n > 2) S22‒/(AsS)2‒ Sn2‒ (n > 2) SO32‒ SO42‒ 2‒ S2 /(AsS)2‒ Sn2‒ (n > 2) S0 SO32‒ SO42‒ 2‒ Sn (n > 2) S0 SO32‒ SO42‒

Content/at.% 11.28 74.2 14.52 5.64 60.93 33.42 5.74 42.45 51.81 35.91 46.73 8.39 8.97 9.01 53.16 11.39 15.63 10.81 76.9 11.73 7.17 4.2

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Figure captions Fig. 1 Cyclic voltammograms (the first cycle) for the arsenopyrite and platinum electrodes at 20 mV∙s-1 starting from the OCP for arsenopyrite in the positive direction. Fig. 2 Linear sweep voltammograms for the arsenopyrite electrode with four consecutive scanning at 10 mV∙s-1 in the potential range of (a) 550–650 mV, (b) 550–750 mV, (c) 550–850 mV, (d) 550–950 mV and (e) 550–1000 mV. Fig. 3 Variation of current density on the arsenopyrite electrode with electrolysis time at a constant potential of (a) 650 mV, (b) 750 mV, (c) 800 mV, (d) 850 mV and (e) 950 mV for 1800 s. Fig. 4 (a) Nyquist plots for the oxidised arsenopyrite electrode at a potential of 650 mV, 750 mV, 800 mV, 850 mV and 950 mV for 1800 s, and (b) the equivalent electrical circuit model for EIS. Fig. 5. Variation of (a) outer layer resistance (R1) and reciprocal capacitance (1/C1) and (b) inner layer resistance (R2) and reciprocal capacitance (1/C2) with potential for the surface film of oxidised arsenopyrite. Fig. 6 AFM images of the (a, a') pristine and (b, b') – (f, f') oxidised arsenopyrite surfaces under a potential of (b, b') 650 mV, (c, c') 750 mV, (d, d') 800 mV, (e, e') 850 mV and (f, f') 950 mV for 1800 s. Fig. 7 XPS spectra of the (a, a', a'') pristine and potentiostatic oxidised arsenopyrite at (b, b', b'') 650 mV, (c, c', c'') 750 mV, (d, d', d'') 800 mV, (e, e', e'') 850 mV and (f, f', f'') 950 mV for 1800 s. Fig. 8 Variation of the (a) element contents (at.%) and (b) – (d) chemical states and their distributions (at.%) of (b) Fe 2p, (c) As 3d and (d) S 2p for the pristine and oxidised arsenopyrite under 650 mV, 750 mV, 800 mV, 850 mV and 950 mV for 1800 s. Fig. 9 Variation of the Fe and As concentrations in electrolyte with time under a constant potential of (a) 650 mV, (b) 750 mV, (c) 800 mV, (d) 850 mV and (e) 950 mV.

Author Contribution Statement: Yan Zhang and Qian Li: Conceptualization, Methodology. Tao Jiang, Yongbin Yang and Bin Xu: Resources. Xiaoliang Liu and Yan Zhang: Investigation, Validation, Writing- Original draft preparation. Yinghe He: Writing- Reviewing and Editing. Qian Li: Funding acquisition.

Declaration of interests ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Highlights:     

Dissolution and passivation of FeAsS in 9K culture medium vary with potential. Surface passivation of the FeAsS occurs as soon as oxidative dissolution begins. The passivating products are mostly As2S2 and As2S3 (≤800 mV), and S0 (>800 mV). The passive film on the surface of FeAsS has a two-layer structure. A potential higher than 800 mV helps reducing the passivating action of the film.