The catalytic effect of copper ion in the bioleaching of arsenopyrite by Acidithiobacillus ferrooxidans in 9K culture medium

Journal Pre-proof The catalytic effect of copper ion in the bioleaching of arsenopyrite by Acidithiobacillus ferrooxidans in 9K culture medium Yan Zhang, Qian Li, Xiaoliang Liu, Huaqun Yin, Yongbin Yang, Bin Xu, Tao Jiang, Yinghe He PII:

S0959-6526(20)30438-8

DOI:

https://doi.org/10.1016/j.jclepro.2020.120391

Reference:

JCLP 120391

To appear in:

Journal of Cleaner Production

Received Date: 12 August 2019 Revised Date:

31 January 2020

Accepted Date: 1 February 2020

Please cite this article as: Zhang Y, Li Q, Liu X, Yin H, Yang Y, Xu B, Jiang T, He Y, The catalytic effect of copper ion in the bioleaching of arsenopyrite by Acidithiobacillus ferrooxidans in 9K culture medium, Journal of Cleaner Production (2020), doi: https://doi.org/10.1016/j.jclepro.2020.120391. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier Ltd.

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

Title: The catalytic effect of copper ion in the bioleaching of arsenopyrite by Acidithiobacillus ferrooxidans in 9K culture medium

Author names and affiliations: Yan Zhang a, b, Qian Li a, Xiaoliang Liu a, b, Huaqun Yin a, Yongbin Yang a, Bin Xu a, Tao Jiang 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

Authors’ E-mail addresses: [email protected] (Y. Zhang); [email protected] (Q. Li); [email protected] (X. Liu); [email protected] (H. Yin); [email protected] (Y. Yang); [email protected] (B. Xu); * Corresponding authors’ E-mail addresses: [email protected] (T. Jiang) [email protected] (Y. He)

Word count: 7395

The catalytic effect of copper ion in the bioleaching of arsenopyrite by Acidithiobacillus ferrooxidans in 9K culture medium Yan Zhang a, b, Qian Li a, Xiaoliang Liu a, b, Huaqun Yin a, Yongbin Yang a, Bin Xu a, Tao Jiang 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 Bioleaching pre-treatment of arsenopyrite (FeAsS), a common mineral occurring in deposits of gold ore that makes the extraction of gold difficult, has attracted significant attention recently due to its simple operation and environmental friendliness. A critical impedance of bioleaching to its large scale industrial application is the slow leaching kinetics. Several metal cations have previously been found to accelerate the leaching process. This paper reports results from an in-depth investigation into the bioleaching of arsenopyrite including leaching experiments, thermodynamic analysis of the leaching system and physicochemical analyses of the materials. The results suggest that a passivating film consisting mainly of As2S2, As2S3 and S0 formed on the arsenopyrite surface in the initial bioleaching of 36 h in the 9K culture containing Acidithiobacillus ferrooxidans. This film disappeared when Cu2+ was added to the system. The bioleaching of arsenopyrite was efficiently improved at the Cu2+ level of 0.05 g/L with its leaching period significantly shortened from 19 d to 13 d. Based on the results, the paper also proposes possible leaching mechanisms and the role of Cu2+ in improving the rate of the leaching process. Specifically, Cu2+ is found to facilitate the oxidative dissolution of the passivating species of As2S2, As2S3 and S0 by forming Cu2S/CuS that can be rapidly oxidised by Fe3+ back to Cu2+. A catalytic cycle of Cu2+/(Cu2S/CuS) is thus likely established on the Fe3+/Fe2+ catalytic cycle, resulting in the elimination of the passivating film. The bacteria utilised the two synergistic cycles to catalyse the leaching of arsenopyrite. Keywords: Bioleaching, Green technology, Arsenopyrite, Metal ion catalyst

* Corresponding authors.

E-mail addresses: [email protected] (T. Jiang); [email protected] (Y. He). 1

1 Introduction Arsenopyrite (FeAsS) is a common mineral occurring in deposits of gold ore. It is recognized as one of the main sulphides that makes the gold ‘invisible’ and the gold ores refractory (Márquez et al., 2012; Miller and Brown, 2016). Pre-treatment of these ores prior to leaching is necessary in the extraction of gold. Currently, there are four pre-treatment methods in use including oxidative roasting, pressure oxidation, chemical oxidation and biological oxidation (Liu et al., 2019). While the first three methods can facilitate the extraction of gold from the refractory ores efficiently, they also suffer from having high energy consumptions, requiring a variety and large quantity of chemicals, and, most importantly, also producing high levels of environmental pollutions (Miller and Brown, 2016; Thomas and Cole, 2005). Bio-oxidation pre-treatment, in contrast, is a simple, cheap, and green technology (Liu et al., 2020; Sepehri and Sarrafzadeh, 2019; Zhang et al., 2019a) that has been explored to extract metals from a range of minerals/ores and waste materials with much reduced impacts to the environment (Borja et al., 2019; Hong and Valix, 2014; Xin et al., 2016). With the increasingly stringent requirements on environmental protection, bioleaching technology has attracted significant attention in the past decade (Miller and Brown, 2016) and its application in the extraction of metals, bio-hydrometallurgy, has been on the increase (Rodrigues et al., 2016; Xin et al., 2016). Several mineral-oxidizing bacteria such as Acidithiobacillus thiooxidans (A. thiooxidans) (Ramírez-Aldaba et al., 2016), Leptospirillum ferrooxidans (L. ferrooxidans) (Corkhill et al., 2008) and Acidithiobacillus ferrooxidans (A. ferrooxidans) (Jin et al., 2012) are reported to exist in gold ores. These indigenous bacteria have been found to be beneficial to the extraction of gold from its ores (Kaksonen et al., 2014; Natarajan and Das, 2003). In particular, A. ferrooxidans (Jin et al., 2012; Ji, 2017) and mixed cultures containing A. ferrooxidans (Falco et al., 2003; Ma et al., 2017; Rohwerder et al., 2003) have been widely used in the bioleaching of sulphide gold ores since they can efficiently oxidise either Fe or S from the ores, thus facilitating the gold extraction. However, although bioleaching offers many advantages, a critical factor that still impedes its large-scale application is its slow dissolution kinetics (Miller and Brown, 2016; Zhang et al., 2019b). Significant research efforts in bio-hydrometallurgy have therefore been directed towards enhancing the leaching kinetics through the use of metal ions as catalysts. A range of metal cations such as Hg2+, Co2+, Bi3+, Ag+, and Cu2+ have been utilised successfully as catalysts in improving the bioleaching of various materials to shorten the long

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bioleaching periods (Pathak et al., 2017). In particular, Cu2+, being cheap and readily available, has drawn significant research efforts in its catalytic properties in bioleaching. Chen et al. (2008) used Cu2+ to catalyse the bioleaching of marmatite (Zn,Fe)S. For the same leaching duration, the leaching ratio of Zn was found to have increased from 65% without Cu2+ to 73% in the presence of 5 g/kg-ore Cu2+. A similar catalytic effect of Cu2+ on the dissolution of realgar (As2S2 or As4S4) was also reported by Guo et al. (2011). Apart from enhancing the bio-oxidation efficiency of minerals and ores, Cu2+ was also found to effectively assist the bioleaching of industrial waste materials, e.g., spent zinc-manganese and lithium-ion batteries (Niu et al., 2015; Zeng et al., 2012). Both the bioleaching kinetics and yields of Co, Zn and Mn from these spent batteries were noticeably improved with the presence of Cu2+. In a previous research from our group (Yuan, 2013), Cu2+ was found to be effective in improving the kinetics of bioleaching of an auriferous arsenopyrite. The mechanisms by which the Cu2+ enhanced the leaching process, however, remain unclear. The aim of the research is to improve the practicality of bioleaching pre-treatment method for the extraction of gold from its ores as it is much greener and more environmentally friendly than the current and conventional pre-treatment methods. This is achieved through increasing the kinetics of the bioleaching by addition of copper ion to the leaching solution. The paper reports results from an in-depth study on the effect of Cu2+ in the bioleaching of a typical pure arsenopyrite by A. ferrooxidans. A series of leaching experiments were conducted on natural high purity arsenopyrite samples. The leachate was analysed with ICPAES and a counting chamber with phase contrast microscope for the metal/semimetal ion concentrations and bacteria number, and the leached solid samples were analysed with SEMEDS, XRD and XPS for morphology and surface compositions. Thermodynamics analyses were also performed to elucidate the reasons for slow bioleaching kinetics and the possible roles that Cu2+ played in the bioleaching of arsenopyrite. 2 Methodology 2.1 Thermodynamic calculations Eh–pH diagrams for FeAsS‒H2O and FeAsS–Cu2+–H2O systems were constructed using the EpH module of HSC Chemistry 6.0 for Windows software (Roine, 2006). The EpH module is based on STABCAL (Stability Calculations for Aqueous Systems) developed by Haung and Cuentas (Haung, 1989; Haung and Cuentas, 1989). The species of the elements of Fe, As, S and Cu were firstly searched and selected from the HSC database. The available

3

thermodynamic data (i.e., ∆Gf°298) of the relevant species are listed in Appendix A. Then the solution conditions were set in the window of “Eh–pH Diagram Basic Settings”, and the calculations were performed automatically in this window. The temperature and pressure were set at 25 °C and 1 atm, respectively. The Eh and pH were set in the range of -1.0 – 1.0 V (vs. SHE) and 0 – 3. Under the typical extraction conditions of [Fe2+] 0.16 M and [Cu2+] 0.8 mM (0.05 g/L), the thermodynamical predominance regions of Fe, S, As and Cu species were presented in Eh–pH diagrams on relevant pH and Eh (vs. SHE) scales. Different species for each element can coexist in the aqueous solutions under certain Eh and pH conditions, but the Eh–pH diagrams show only the predominant species in each predominance regions where the element, i.e., Fe, S, As or Cu is highest in content/concentration or distribution fraction. In addition, the lines in the diagrams represent the Eh and pH conditions where the distribution fractions of the element in adjacent species are equal in the equilibrium state. Note that, even though all the leaching experiments were conducted at 30 °C and 1 atm, all the thermodynamic analyses from which all diagrams were constructed were performed for the extraction at 25 °C and 1 atm due to data availability. The small difference in the temperature is expected to have limited, if at all, effect on the outcome of the analyses. 2.2 Minerals, strain and media The natural sample of high-purity arsenopyrite investigated was from Yaogangxian in Hunan province of China, with a composition of Fe1.00As0.99S0.97 and containing only Si 0.132%, Co 0.034% and Ni 0.062% as impurities determined by X-ray fluorescence analysis. Either particles (over 90% -74 µm) or cubes (~15 mm × ~15 mm × ~5 mm) of the arsenopyrite samples were used in this study. The arsenopyrite particles were prepared by wet-milling in a ball mill while the cubes were obtained using a cutter. The samples were stored in air-tight plastic bags to minimise oxidation. A. ferrooxidans and 9K culture medium containing FeSO4·7H2O 44.8 g/L, K2HPO4·3H2O 0.50 g/L, (NH4)2SO4 3.0 g/L, KCl 0.1 g/L, Ca(NO3)2·4H2O 0.01 g/L and MgSO4·7H2O 0.5 g/L were used in the leaching of arsenopyrite. 250-mL Erlenmneyer flasks containing 100 mL 9K culture medium and 15 mL inoculum were shaken at 160 rpm in an orbital thermostat shaker at 30 °C to culture and sub-culture A. ferrooxidans. By careful addition of ~3 M sulphuric acid, the initial pH value of the medium was adjusted to 1.8 that was shown to be optimal for the growth and activity of A. ferrooxidans in a previous research from our group (Ji, 2017). The bacteria had been sub-cultured in the presence of 10 g/L arsenopyrite particles for three months before the bioleaching experiment. 4

Copper sulphate pentahydrate (CuSO4⋅5H2O) was used as the copper cation (Cu2+) source. The reagents used in the cultivation of bacteria and the leaching experiments of arsenopyrite were all AR grade. De-ionised water was used throughout all experiments. 2.3 Bioleaching experiment The bioleaching of arsenopyrite samples proceeded in parallel with the growth of bacteria using the identical experimental method and conditions for cultivating bacteria. In each bioleaching experiment for arsenopyrite particles, the pulp density was 10 g/L of FeAsS and the Cu2+ concentration ranged from 0.01 to 0.10 g/L. During the bioleaching process, samples from the supernatant were withdrawn at regular intervals for chemical and bacterial analyses. The pulp from the bioleaching experiment performed in parallel under different conditions was filtered, rinsed with copious de-ionised water and dried in a vacuum oven at 35 °C overnight to obtain the residue for the subsequent X-ray photoelectron spectroscopy (XPS) analysis. The pH values and solution potentials were also measured and recorded during the bioleaching process. All solution potentials were reported with respect to the standard hydrogen electrode (SHE). The arsenopyrite cubes were used to conduct the morphology and mineralogical phase studies. The cubes were first polished sequentially using silicon carbide papers of 800, 1500 and 3000 grid and then cleaned ultrasonically in alternate baths of 5 M HCl, methanol and water for 5 min to remove the surface contaminants (Cruz et al., 1997; Lázaro et al., 1997). At the end of the leaching experiments, the leached cubes were immersed in a glutaraldehyde solution of 2.5% for 3 – 4 h, rinsed using 0.1 M phosphate buffer saline (PBS) of pH 7.2 – 7.4, and then dehydrated successively in 50%, 70%, 85%, 95% and 100% ethanol solutions to maintain cell structure and secure attached cells to the surface. 2.4 Analytical methods The concentration of Fe2+ in solution was determined by titration with potassium dichromate whilst the total concentrations of Fe and As were detected by ICP-AES (PS-6, Baird). The Fe3+ concentration was calculated from the difference between the total Fe concentration and Fe2+ concentration. The cell counts in solution were determined using a counting chamber (Neubauer) with phase contrast microscope (LEICA DMI 3000B). The pH value and solution potential were measured using a pH meter (PHSJ-4A, Shanghai Leici) equipped with a Pt electrode and Ag/AgCl (saturated KCl) reference electrode. The morphology of leached cubes was examined using an SEM (Helios NanoLab G3 UC or Quanta FEG 250, FEI) equipped with EDS. The mineralogical phases of the leached cubes

5

were determined using an X-ray diffractometer (D/Max 2500, Rigaku). The surface species on the leached particles were analysed with XPS (ESCALAB250Xi, Thermo Fisher) and the spectra fitted using Avantage 5.52 software. The contents (at.%) of elements and their chemical state distributions (at.%) were quantified based on the areas under the peaks. 3 Results and discussion 3.1 Bioleaching behaviour of arsenopyrite in the presence of Cu2+ Fig. 1 shows the kinetic results from the bioleaching of arsenopyrite under various Cu2+ concentrations. The leaching behaviour of As, acting as a significant indicator for arsenopyrite bioleaching, is shown in Fig. 1(a). Cu2+ was found to significantly improve the bioleaching of As and thus arsenopyrite with an optimal Cu2+ concentration of 0.05 g/L. At this Cu2+ level, the final concentration of leached As was increased from 1.28 g/L without Cu2+ to 1.77 g/L whilst the bioleaching period was significantly shortened from 19 d to 13 d. The leaching deteriorated when the Cu2+ concentration was further increased from 0.05 g/L to 0.10 g/L. This can be explained by that excessive Cu2+ have adversely affected the bacterial growth and activity, and thus the bioleaching of As. The growth and activity of bacteria is critical to the bioleaching process. Fig. 1(b) shows the effect of Cu2+ addition to the solution on bacteria counts, which demonstrates that the level of 0.05 g/L Cu2+ is the optimal for the growth of bacteria. The reproduction of bacteria requires energy derived from the simultaneous bio-oxidation of Fe2+ to Fe3+ and reduction of O2 with a consumption of H+ to H2O (Jones et al., 2003). Thus, it would be expected that the more the bacteria in the solution, the higher the Fe3+ concentration and pH. In addition, the electric potential of the solution is also intimately associated with the bioleaching of arsenopyrite. A higher solution potential can provide a stronger driving force for the leaching of arsenopyrite. Fig. 1(c) and Fig. 1(d) compare the effect of optimal amount of Cu2+ of 0.05 g/L on the pH, the solution potential and the Fe concentrations. Fig. 1(c) shows that, in the initial leaching period of 3 days, the pH values increased in both cases. Consistent with the higher bacteria counts in solution, the rate of pH rise was slightly higher in the presence of 0.05 g/L Cu2+. At the later stage (> 3 d), the pH dropped markedly due mainly to the formation of H+ from the oxidation of arsenopyrite. Fig. 1(c) also shows the change of the solution potential with time, which is strongly influenced by the redox couple of Fe3+/Fe2+. The variation trend of the solution potential is similar to that of the Fe3+ concentration shown in Fig. 1 (d). It can be seen, from Figs. 1(c) and (d), that the Fe3+ concentration and the

6

solution potential both rose, with a simultaneous Fe2+ concentration drop, in the initial period of 13 days. The addition of Cu2+ at 0.05 g/L caused noticeable concentration changes of Fe species in the solution. The concentration of Fe3+ was much lower, and that of Fe2+ much higher, in the presence of Cu2+, which is beneficial for the bacteria growth and consequently bioleaching. The level of solution potential was also much lower (Fig. 1(c)) but with a much higher leaching yield of As (Fig. 1(a)), further confirming that Cu2+ (0.05 g/L) can catalyse the bioleaching of arsenopyrite. 2.7

2.0

(b)

2.4

1.6

Cell number/(×108/mL)

As concentration/(g/L)

(a)

1.2

0.8

0.00 g/L Cu2+ 0.01 g/L Cu2+ 0.05 g/L Cu2+ 0.10 g/L Cu2+

0.4

0.0

0.00 g/L Cu2+ 0.01 g/L Cu2+ 0.05 g/L Cu2+ 0.10 g/L Cu2+

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pH

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pH with Cu 2+ Solution potential no Cu 580 2+ Solution potential with Cu

2.0

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1.9 540

1.8 1.7 0

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(c)

Solution potential vs. SHE/mV

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Time/d

Time/d

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Fe2+ no Cu2+ Fe2+ with Cu2+ Fe3+ no Cu2+ Fe3+ with Cu2+ Fetotal no Cu2+

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Fetotal with Cu2+ 0 0

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Time/d

Fig. 1 Variation of the (a) As concentration and (b) cell number under various concentrations of Cu2+ as well as (c) pH and solution potential and (d) Fe2+, Fe3+ and total Fe concentrations in the absence and presence of 0.05 g/L Cu2+ with time during the bioleaching of arsenopyrite particles.

The slow bioleaching kinetics of arsenopyrite is believed to be due to the formation of a series of passivating products on the surface of the arsenopyrite such as Fe-containing solids (Corkhill et al., 2008, 2009; Márquez et al., 2012) and As- and/or S-containing solids (Deng et al., 2018; Fantauzzi et al., 2011; Zhu et al., 2014). Some reports suggested that the main passivating product was jarosite (Corkhill et al., 2008, 2009). However, this investigation found that jarosite was formed with or without Cu2+, both in considerable amounts, during the bioleaching process on the surface of arsenopyrite with no significant difference as seen from

7

the SEM images in Figs. 2(a) and (b) as well as the XRD spectrograms in Fig. 2(c). They appeared as fluffy or woolly spherical particles loosely packed in a porous structure on the surface, as reported before (Wang et al., 2007). This implies that it is unlikely that jarosite was the passivating film or that Cu2+ catalysed the bioleaching of arsenopyrite through weakening the formation of jarosite.

(c)

j

a-Arsenopyrite

Intensity/a.u.

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0.05 g/L Cu2+ a

j a jj

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Two theta/°° Fig. 2 (a) – (b) SEM images and (c) XRD spectrograms of the arsenopyrite cube after bioleaching for 8 d (a) with and (b) without 0.05 g/L Cu2+.

3.2 Formation of surface deposits on arsenopyrite To determine the chemical compositions of the passivating products, SEM-EDS and XPS were used to analyse the surface of arsenopyrite cubes or particles leached in different solutions. To avoid the interference from jarosite, the surface was investigated only for the initial stage of leaching (< 5 d). Fig. 3 shows the SEM images and EDS data for the leached arsenopyrite cubes in 9K culture medium under various conditions. Without Cu2+ or bacteria (blank), little oxidation of the arsenopyrite surface occurred after leaching 36 h, as shown in Fig. 3(a). After 120 h (Fig. 3(a')), the O content on the surface (spot a'-1) increased, suggesting that the arsenopyrite was oxidised. But only a small amount of precipitates were observed on the surface (e.g., spot a'-

8

2). The addition of Cu2+ (0.05 g/L) caused a larger amount of precipitates with a higher content of O on the surface after leaching 120 h as seen in Fig. 3(b'). A small quantity of Cu was also detected on the smooth surface (spots b-1 and b'-1). This is likely due to the copper catalyst, similar to a previously reported situation of Cu2+ catalysed chemical leaching of realgar (As2S2) (Guo et al., 2011). The precipitates (e.g., spots b'-2 and b'-3) were visible to the naked eyes in amber yellow or dark brown. EDS analyses show that they have a very similar element composition of jarosite. Introduction of bacteria to the leaching culture resulted in a layer of deposit on the surface of the arsenopyrite cube as seen in Figs. 3(c) and (c'). The layer was structurally dense as a film and was intact in the solution. The cracks appeared only after drying. On the surface below the film (i.e., spots c-1 and c'-1), an atom fraction of nearly 1:1:1 for Fe:As:S and low O contents were detected, indicating that little oxidative dissolution occurred beneath the film. This suggests that the layer has passivated the bulk arsenopyrite. The EDS data for spots c-2 and c'-2 in Figs. 3(c) and (c') suggest that, in addition to O, the film contained much higher contents of As and S than Fe. However, as can be seen later, due to the formation of multiple chemical compounds, the EDS results are insufficient and further chemical analyses had been conducted to determine the exact chemical composition of the film. Above the film, crystal particles with chemical composition similar to jarosite were again apparent (spots c-3 and c'-3).

9

10

Fig. 3 SEM images with EDS data (at.%) for the surfaces of arsenopyrite cubes after leaching 36 h and 120 h in 9K culture medium. Conditions: (a)/(a') blank; (b)/(b') 0.05 g/L Cu2+; (c)/(c') 2×108 cells/mL A. ferrooxidans; (d)/(d') 0.05 g/L Cu2+ and 2×108 cells/mL A. ferrooxidans.

In the presence of both the bacteria and Cu2+ (Figs. 3(d) and (d')), the leached arsenopyrite surface was sparsely covered with some solid particles (e.g. spot d-2), very likely to be jarosite, but the film seen in Figs. 3(c) and (c') disappeared. In addition, EDS data show that the chemical composition of the reacted surface was almost the same as that of the fresh surface (spots d-1 and d'-1). This suggests that the combination of bacteria and Cu2+ in the 9K culture has prevented the formation of a passivating film on the surface of the arsenopyrite cube during bioleaching. It should also be noted that little O and no Cu were detected on the surface of the cube, which is different from the enhanced chemical oxidation of arsenopyrite by Cu2+ (Fig. 3(b')). To determine the chemical composition of the surface layer, XPS was employed on the pristine and bioleached arsenopyrite particles, with and without Cu2+ in the solutions. A broad scan was used to identify and measure the element contents (at.%) on the outmost surface of the arsenopyrite particles. The result is shown in Fig. 4. It can be seen that the bioleached arsenopyrite surfaces without Cu2+ had a similar element content to the pristine surfaces, both having a much higher content of As and S than Fe. In the presence of 0.05 g/L Cu2+, however, the contents of As and S were markedly reduced after bioleaching. Further, consistent with the SEM-EDS results in Fig. 3(d), no Cu was detected on the surface of the bioleached arsenopyrite sample in the presence of Cu2+.

11

Fig. 4 Element contents (at.%) in the pristine and bioleached surfaces of arsenopyrite particles after leaching 36 h in 9K culture medium without and with 0.05 g/L Cu2+.

The chemical states of Fe, S and As were determined using narrow scans. The spectra are presented in Fig. 5. All the spectra of S 2p and As 3d were fitted as doublets with an intensity ratio of 2:1 and 3:2 as well as a spin orbit splitting of 1.19 eV and 0.68 eV, respectively. In terms of the fitting for Fe 2p spectra, Fe(III)-(AsS), Fe(III)-O and Fe(III)-SO were resolved using four GS peaks (Corkhill et al., 2008; Nesbitt et al., 1995) and Fe(II)-(AsS) using three GS peaks (Gupta and Sen, 1974, 1975; Pratt et al., 1994). The detailed assignments of Fe 2p(3/2) (Corkhill et al., 2008, 2009; Mikhlin et al., 2006), S 2p(3/2) (Costa et al., 2002; Hu et al., 2017; Zhu et al., 2014) and As 3d(5/2) (Hacquard et al., 1999; Nesbitt et al., 1998; Schaufuss et al., 2000) were determined according to the references, and summarized in Appendices B − D. The corresponding fitted peaks and chemical states for Fe 2p, S 2p and As 3d as well as their distributions (at.%) are given in Figs. 5 and 6. 5000

32500

90000

(a') S 2p-Pristine

(a) Fe 2p-Pristine

Intensity/(a.u.)

75000

27500

25000

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Sn

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164

162

48

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42

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Binding energy/eV

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S 12000

2– n

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Fe(III)-SO

5500

Fe(III)-(AsS)

Intensity/(a.u.)

6500

Intensity/(a.u.)

Fe(III)-O

7000

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3000

Fe(III)-(AsS)

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SO4

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168

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162

As2O5

3000

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Intensity/(a.u.)

7000

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

As2S3

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Binding energy/eV 1200

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(c'') As 3d-0.05 g/L Cu2+ 1000

2500

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SO42–

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As2S2 4500

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Binding energy/eV

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6000

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Intensity/(a.u.)

Intensity/(a.u.)

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(a'') As 3d-Pristine

S2– /(AsS)2– 2

30000

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Fe(III)-O

85000

As2O5 800

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708

706

704

172

170

168

166

164

Binding energy/eV

12

162

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200 48

47

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45

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42

Binding energy/eV

Fig. 5 XPS spectra for Fe 2p, S 2p and As 3d in the (a, a', a'') pristine and bioleached surfaces of arsenopyrite particles after leaching 36 h in 9K culture medium (b, b', b'') without Cu2+ and (c, c', c'') with 0.05 g/L Cu2+.

As further shown in Figs. 5 and 6, the chemical state distributions for Fe 2p, S 2p and As 3d have noticeable differences in the three cases. Note that Fe(II)-(AsS) from Fe 2p, S2– and (AsS)2– from S 2p, and As(-I)-S from As 3d are all originated from arsenopyrite (FeAsS). The occurrence of the other chemical states is due to the oxidation of FeAsS. It is difficult to distinguish S22– from (AsS)2– due to their high similarity (Lázaro et al., 1997). In particular, S22– and Sn2– (n > 2) are intimately associated with As, thus likely occurring in the form of sulphides such as As2S2 and As2S3 (Fantauzzi et al., 2006, 2011; Schaufuss et al., 2000) while As2O3 and As2O5 are considered soluble As in water to become AsO33– (Fantauzzi et al., 2006, 2011) and AsO43– (Hollinger et al., 1994; Soma et al., 1994) ions. Figs. 5(a, a', a'') and 6 show that the outermost surfaces of the pristine arsenopyrite particles were oxidised, possibly by the dissolved oxygen during wet grinding. 60% of Fe(II) in FeAsS was converted to its oxidation states of Fe(III)-(AsS) and Fe(III)-O. In addition to the formation of some soluble As (19.5%), a small fraction (14.6%) of As and S was oxidised to As2S2. This surface oxidation of the arsenopyrite before bioleaching appears to be unavoidable as it happens during wet grinding process under ambient conditions.

13

Fig. 6 Chemical state distributions (at.%) for (a) Fe 2p, (b) S 2p and (c) As 3d in the pristine and bioleached surfaces of arsenopyrite particles after leaching 36 h in 9K culture medium without and with 0.05 g/L Cu2+.

In addition to this surface oxidation, Figs. 5(b), 5(c) and 6(a) show that Fe in the pristine surfaces of FeAsS were further oxidised during bioleaching with or without Cu2+, and more than 90% of the Fe occurred in the form of Fe(III)-O and Fe(III)-SO, likely originated from jarosite. As suggested from the SEM-EDS and XRD results in Figs. 2 and 3, the passivating film was found to be rich in As and S with little Fe, i.e., jarosite or other forms of iron precipitates. Figs. 5(b', b''), 5(c', c''), 6(b) and 6(c) show that S and As in the pristine surfaces were also oxidised after bioleaching with or without Cu2+. However, the distributions of their chemical states were significantly different in these two cases. Most (~90%) of S and As existed in the water-insoluble form of As2S2, As2S3 and S0 after bioleaching without Cu2+. It is thus reasonable to consider that the passivating film on the arsenopyrite surface seen in Fig. 3(c) consists mainly of these As- and S-containing species. In contrast, these passivating species was not measured on the surface of the arsenopyrite particles after bioleaching with the addition of 0.05 g/L Cu2+ in the solution. Instead, only As2O3/As2O5 and SO42– were detected. This suggests that Cu2+ enhanced the oxidative dissolution of As2S2, As2S3 and S0 to soluble arsenates and sulphate, and thus leading to the elimination of the passivating film. In summary, the leaching test results presented in Section 3.1 demonstrated that addition of 0.05 g/L Cu2+ substantially improved the leaching yield and kinetics. The SEM-EDS results indicate that the main difference caused by the inclusion of 0.05 g/L Cu2+ in the leaching solution is the disappearance of a visible film on the surface of the arsenopyrite during bioleaching without Cu2+, which the XPS results suggest to be the water-insoluble As2S2, As2S3 and S0. To further ascertain the role of Cu2+ in the bioleaching of arsenopyrite, detailed thermodynamic analyses were conducted. 3.3 The role of Cu2+ in the bioleaching of arsenopyrite Fig. 7 shows the thermodynamics of arsenopyrite dissolution under the typical acidic bioleaching condition (pH = 0 – 3). This pH range was chosen based on the research that the optimal pH range for the growth and activity of Acidithiobacillus ferrooxidans was 1.5~2.0 (Ji, 2017). The Eh–pH diagrams of FeAsS–H2O and FeAsS–Cu2+–H2O systems are presented in Fig. 7(a) and (b), respectively. Fig. 7(a) shows that, in the absence of Cu2+, arsenopyrite can be oxidised at Eh > -0.4 V. The predominance species for Fe, S and As in the pH 0 – 3 and Eh -0.4 – 0.4 V region are Fe2+, As2S2/As2S3/S0 and As2S2/As2S3, respectively. In other words, if bioleaching occurred in this pH and electric potential ranges, insoluble compounds 14

of As2S2, As2S3 and S0 would form and precipitate on the surface of the arsenopyrite, in agreement with the SEM-EDS and XPS results presented in Section 3.2. At a higher Eh (> 0.2 – 0.4 V), the predominant Fe and S species become H3OFe3(SO4)2(OH)6 while the As species are converted to HAsO3– and H2AsO4–. Jarosite [KFe3(SO4)2(OH)6] can readily be formed between H3OFe3(SO4)2(OH)6 and K+ in the 9K culture (Liu et al., 2009; Wang et al., 2007), as presented in Fig. 2. If 0.8 mM (0.05 g/L) Cu2+ is added to the solution, Fig. 7(b) shows that the Fe in the arsenopyrite can be leached as Fe2+ to the solution at a lower Eh (> -0.5 – -0.65 V for pH 0 – 3) while the conversion of Fe2+ to H3OFe3(SO4)2(OH)6 occurs at a much higher Eh (> 0.3 – 0.45 V). The stability areas of S and As species are mainly Cu2S, CuS and HAsO2 (a), which can be formed at a lower Eh (> -0.5 – -0.65 V). In addition, the predominant Cu species are Cu2S and CuS that are readily oxidised to Cu2+ at an Eh greater than 0.3 – 0.45 V. Thermodynamically, therefore, the presence of Cu2+ destabilises As2S2, As2S3 and S0 by forming CuS and Cu2S. In the presence of Fe3+, CuS and Cu2S would in turn be dissolved into the solution, which is why no Cu was detected on the surface of the arsenopyrite cubes or particles by either EDS or XPS.

Fig. 7 Eh–pH diagrams of (a) FeAsS–H2O and (b) FeAsS–Cu2+–H2O systems without and with 0.8 mM Cu2+. Conditions: [Fe2+]tot 0.16 M, [FeAsS] 0.4 mM; 25 °C and 1 atm.

3.4 Possible mechanisms for the bioleaching of arsenopyrite Based on the EDS and XPS results, the possible mechanisms and reactions involved in the copper catalysed bioleaching of arsenopyrite can be proposed and are shown in Fig. 8 and Table 1. Fig. 8(a) shows that, during the conventional bioleaching without Cu2+, the Fe2+ is oxidised by the bacteria to become Fe3+, which in turn reacts with the arsenopyrite according to reactions 1 – 3 in Table 1. In the process, solid precipitates of As2S2, As2S3 and S0 are 15

produced, forming a passivating film on the surface of the particles, as evidenced by the EDS and XPS results. Although these solid precipitates could be further oxidised according to Eqs. 10 – 12 in Table 1, their accumulation at the initial bioleaching stage caused the formation of a passivating film that would severely limit the subsequent leaching of arsenopyrite.

Fig. 8 Schematic diagram of the mechanism of arsenopyrite bioleaching by bacteria (a) without and (b) with Cu2+. Table 1 Possible reactions involved in the leaching of arsenopyrite in the presence of Cu2+. Oxidative leaching of arsenopyrite 3+

2–

2+

–

+

3FeAsS + 2H2O + 7Fe = As2S2 + S + 10Fe + AsO2 + 4H 4FeAsS + 4H2O + 12Fe3+ = As2S3 + S2– + 16Fe2+ + 2AsO2– + 8H+ 2FeAsS + 6H2O + 10Fe3+ = S0 + 2HAsO3– + S2– + 12Fe2+ + 10H+ 3FeAsS + 2H2O + Cu2+ + 7Fe3+ = CuS + As2S2 + 10Fe2+ + AsO2– + 4H+ 3FeAsS + 2H2O + 2Cu2+ + 5Fe3+ = Cu2S + As2S2 + 8Fe2+ + AsO2– + 4H+ 4FeAsS + 4H2O + Cu2+ + 12Fe3+ = CuS + As2S3 + 16Fe2+ + 2AsO2– + 8H+ 4FeAsS + 4H2O + 2Cu2+ + 10Fe3+ = Cu2S + As2S3 + 14Fe2+ + 2AsO2– + 8H+ 2FeAsS + 6H2O + Cu2+ + 10Fe3+ = CuS + S0 + 2HAsO3– + 12Fe2+ + 10H+ 2FeAsS + 6H2O + 2Cu2+ + 8Fe3+ = Cu2S + S0 + 2HAsO3– + 10Fe2+ + 10H+

∆Gr0/( kJ/mol) a

No.

-504.203 -816.451 -531.546 -711.916 -657.857 -1024.165 -970.106 -739.260 -685.201

1 2 3 4 5 6 7 8 9

-282.116 -177.281 116.431 -697.544 -589.426 -384.995 -330.936 -91.283 -37.224

10 11 12 13 14 15 16 17 18

-280.531 -172.413

19 20

Oxidation of passivating products from arsenopyrite leaching 2As2S2 + 15H2O + 16Fe3+ = 4HAsO3– + 2S2– + HS2O3– + 16Fe2+ + 25H+ As2S3 + 9H2O + 10Fe3+ = 2HAsO3– + S2– + HS2O3– + 10Fe2+ + 15H+ 2S0 + 3H2O + 2Fe3+ = HS2O3– + S2– + 2Fe2+ + 5H+ 2As2S2 + 15H2O + 2Cu2+ + 16Fe3+ = 2CuS + 4HAsO3– + HS2O3– + 16Fe2+ + 25H+ 2As2S2 + 15H2O + 4Cu2+ + 12Fe3+ = 2Cu2S + 4HAsO3– + HS2O3– + 12Fe2+ + 25H+ As2S3 + 9H2O + Cu2+ + 10Fe3+ = CuS + 2HAsO3– + HS2O3– + 10Fe2+ + 15H+ As2S3 + 9H2O + 2Cu2+ + 8Fe3+ = Cu2S + 2HAsO3– + HS2O3– + 8Fe2+ + 15H+ 3S0 + 3H2O + Cu2+ + 2Fe3+ = CuS + HS2O3– + 2Fe2+ + 5H+ 3S0 + 3H2O + 2Cu2+ = Cu2S + HS2O3– + 5H+ Oxidation of Cu2S and CuS 2Cu2S + 12Fe3+ + 3H2O = 12Fe2+ + 4Cu2+ + HS2O3– + 5H+ 2CuS + 8Fe3+ + 3H2O = 8 Fe2+ + 2Cu2+ + HS2O3– + 5H+ a

Values were calculated using equation of ∆Gr° = ∑[νi∆Gf°(i)], where ∆Gf° values are listed in Appendix A. 16

In the presence of Cu2+, the oxidative leaching of arsenopyrite would proceed according to Eqs. 4 – 9. The formation of the passivating products was largely prevented based on Eqs. 13 – 18. No Cu2S or CuS was detected on the bioleached arsenopyrite surface, as suggested in the SEM-EDS and XPS results, because, thermodynamically, they can easily be oxidised by Fe3+ back to Cu2+ as shown in Eqs. 19 and 20. Experimentally, it was demonstrated in previous researches (Gu et al., 2013; Zhao et al., 2015) that the oxidative leaching of chalcocite (Cu2S) by the Fe3+/Fe2+ couple was rapid, particularly in the presence of bacteria, which oxidise Fe2+ back to Fe3+. Thus, as presented in Fig. 8(b), a catalytic cycle between Cu2+ and Cu2S/CuS was also formed from the Fe3+/Fe2+ redox cycle, which leads to the removal of the passivating film during the bioleaching process. 4. Conclusions Experimental results from bioleaching of arsenopyrite by A. ferrooxidans suggested that addition of 0.05 g/L Cu2+ to the solution could not only noticeably enhance the leaching yield of As, but also significantly shorten the leaching period from 19 d to 13 d. Morphological studies showed that a porous layer of jarosite would form on the surface of arsenopyrite, regardless if Cu2+ was present in the solution. The main difference the Cu2+ caused was preventing the formation of a compact passivating film on arsenopyrite. EDS and XPS analyses indicated that this passivating film was comprised mainly of As2S2, As2S3 and S0. Thermodynamic analyses implied that Cu2+ prevented the formation of the passivating film through the formation of Cu2S/CuS, which would readily be dissolved to become Cu2+ again in the presence of Fe3+. Based on the combined results from bioleaching, surface characterisation and thermodynamic analyses, a possible mechanism for the catalytic effects of Cu2+ in the bioleaching of arsenopyrite is proposed that the bacteria utilised the two synergistic cycles of Cu2+/(Cu2S/CuS) and Fe3+/Fe2+ to enhance the leaching of arsenopyrite. The finding that Cu2+ can considerably increase the yield as well as shorten the time of bioleaching of arsenopyrite makes this cleaner and more environmentally friendly process much more commercially viable thus a step closer to its large-scale application. It also provides useful information for the extraction of valuable metals from other As- and Scontaining materials. 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 17

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. 2018YFE0110200) are all gratefully acknowledged. References Borja, D., Nguyen, K.A., Silva, R.A., Ngoma, E., Petersen, J., Harrison, S.T.L., Park, J.H., Kim, H., 2019. Continuous bioleaching of arsenopyrite from mine tailings using an adapted mesophilic microbial culture. Hydrometallurgy 187, 187–194. Chen, S., Qin, W., Qiu, G., 2008. Effect of Cu2+ ions on bioleaching of marmatite. Trans. Nonferrous Met. Soc. China 18, 1518–1522. Cruz, R., Lázaro, I., Rodríguez, J.M., Monroy, M., González, I., 1997. Surface characterization of arsenopyrite in acidic medium by triangular scan voltammetry on carbon paste electrodes. Hydrometallurgy 46, 303–319. Corkhill, C.L., Wincott, P.L., Lloyd, J.R., Vaughan, D.J., 2008. The oxidative dissolution of arsenopyrite (FeAsS) and enargite (Cu3AsS4) by Leptospirillum ferrooxidans. Geochim. Cosmochim. Acta 72, 5616–5633. Corkhill, C.L., Vaughan, D.J., 2009. Arsenopyrite oxidation – A review. Appl. Geochem. 24, 2342– 2361. Costa, M.C., Botelho Do Rego, A.M., Abrantes, L.M., 2002. Characterization of a natural and an electro-oxidized arsenopyrite: A study on electrochemical and X-ray photoelectron spectroscopy. Int. J. Miner. Process. 65, 83–108. Deng, S., Gu, G., Xu, B., Li, L., Wu, B., 2018. Surface characterization of arsenopyrite during chemical and biological oxidation. Sci. Total Environ. 626, 349–356. Falco, L., Pogliani, C., Curutchet, G., Donati, E., 2003. A comparison of bioleaching of covellite using pure cultures of Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans or a mixed

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Appendices Appendix A. ∆G°298 (kJ/mol) values of relevant species a FeAsS FeAsO4 Fe3(AsO4)2 Fe(OH)2 Fe(OH)3 FeO·OH Fe3+ Fe2+ FeOH2+ FeOH+ Fe(OH)2+ Fe2(OH)24+ H3OFe3(SO4)2(OH)6 b

∆Gf°298 /(kJ/mol) -49.7616 -772.727 -1766.73 -492.158 -705.885 -489.439 -17.1907 -91.5644 -242.064 -275.615 -452.391 -467.733 -3230.36

H2O

-237.177

Species

Species As2S2 As2S3 As2O3 As2O4 As2O5 As4O6 AsO2– AsO43– As(OH)4– HAsO3– HAsO42– H2AsO3– H2AsO4– H3AsO3 (a) H3AsO4 (a) HAsO2 (a)

∆Gf°298 /(kJ/mol) -68.5508 -91.4907 -576.899 -701.161 -782.437 -1152.42 -349.991 -648.477 -824.457 -606.638 -714.732 -587.149 -753.399 -638.142 -764.001 -402.951

Species S S2– S22– SO32– S2O32– S2O42– S2O52– S2O62– S2O72– S2O82– HS2– HSO3– HS– HS2O3– H2S (a)

∆Gf°298 /(kJ/mol) 0 86.00982 79.76167 -486.755 -518.87 -600.825 -791.217 -969.453 -795.432 -1115.35 11.51053 -527.84 12.44438 -532.363 -27.656

Species Cu(AsO2)2 Cu3AsO4 Cu3(AsO4)2 CuO Cu2O Cu(OH)2 CuS Cu2S Cu2+ Cu+ CuO22– CuOH+ Cu(OH)3– Cu(OH)42– Cu2OH3+ Cu2(OH)22+ Cu3(OH)42+ Cu(OH)O–

∆Gf°298 /(kJ/mol) -748.217 -624.325 -1316.56 -128.132 -147.907 -372.869 -56.6355 -86.2559 65.06826 50.00357 -172.539 -126.453 -501.731 -657.045 -68.7476 -283.902 -633.814 -251.55

a

Data from HSC database 6.0 (Roine, 2006). ∆Gf°298 value was calculated using the equation of ∆Gr° = -RT ln K = ∑[νi∆Gf°(i)], where relevant ∆Gf° values are also listed in Appendix A and the ln K value was from the Hydrochemical log K Database of HYDRA/MEDUSA software (Puigdomenech, 2004).

b

Appendix B. Parameters for the fitted XPS peaks of S 2p(3/2) on the arsenopyrite surfaces

Pristine

0 g/L Cu2+ 0.05 g/L Cu2+

Binding energy/eV 161.5 162.5 163.7 163.7 164.9 167.1 168.6 168.9

FWHM 1.4 1.3 1.1 1.1 0.9 1.4 1.4 1.25

Chemical state S2– S22–/(AsS)2– Sn2– Sn2– S0 S2O32– SO42– SO42–

Distribution/at.% 11.28 74.2 14.52 76.9 11.73 7.17 4.2 100

Appendix C. Parameters for the fitted XPS peaks of As 3d(5/2) on the arsenopyrite surfaces

Pristine

0 g/L Cu2+ 0.05 g/L Cu2+

Binding energy/eV 41.7 42.7 44.1 45.6 42.6 43.7 45.4 44.2 45.5

FWHM 0.8 0.9 1.5 1.0 0.8 1.0 1.6 0.9 1.4

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Chemical state As(-I)-S As2S2 As2O3 As2O5 As2S2 As2S3 As2O5 As2O3 As2O5

Distribution/at.% 65.98 14.57 13.4 6.05 74.82 13.07 12.11 10.7 89.29

Appendix D. Parameters for the fitted XPS peaks of Fe 2p(3/2) on the arsenopyrite surfaces

Pristine

0 g/L Cu

2+

Binding energy/eV 706.6 707.6 708.6

FWHM 1.1 1.0 1.0

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

Distribution/at.%

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)

711.2 712.2 713.2 714.2 709.7 710.7 711.7 712.7

1.8 1.8 1.8 1.8 1.2 1.2 1.2 1.2

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

711.4 712.4 713.4 714.4

1.8 1.8 1.8 1.8

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

713.8 714.8 715.8 716.8 709.7 710.7 711.7 712.7

1.6 1.6 1.6 1.6 1.2 1.2 1.2 1.2

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

711.7 712.7 713.7 714.7

1.7 1.7 1.7 1.7

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

78.00

713.5 714.5 715.5 716.5

1.3 1.3 1.3 1.3

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

9.90

715.6 716.6 717.6 718.6

1.3 1.3 1.3 1.3

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

4.95

40.04

14.65

45.31

9.08

83.42

7.50

7.15

0.05 g/L Cu2+

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List of figure captions Fig. 1 Variation of the (a) As concentration and (b) cell number under various concentrations of Cu2+ as well as (c) pH and (d) Fe2+, Fe3+ and total Fe concentrations in the absence and presence of 0.05 g/L Cu2+ with time during the bioleaching of arsenopyrite particles.

Fig. 2 (a) – (b) SEM images and (c) XRD spectrograms of the arsenopyrite cube after bioleaching 8 d (a) with and (b) without 0.05 g/L Cu2+.

Fig. 3 SEM images with EDS data (at.%) for the surfaces of arsenopyrite cubes after leaching 36 h and 120 h in 9K culture medium. Conditions: (a)/(a') blank; (b)/(b') 0.05 g/L Cu2+; (c)/(c') 2×108 cells/mL A. ferrooxidans; (d)/(d') 0.05 g/L Cu2+ and 2×108 cells/mL A. ferrooxidans.

Fig. 4 Element contents (at.%) in the pristine and bioleached surfaces of arsenopyrite particles after leaching 36 h in 9K culture medium without and with 0.05 g/L Cu2+.

Fig. 5 XPS spectra for Fe 2p, S 2p and As 3d in the (a, a', a'') pristine and bioleached surfaces of arsenopyrite particles after leaching 36 h in 9K culture medium (b, b', b'') without Cu2+ and (c, c', c'') with 0.05 g/L Cu2+.

Fig. 6 Chemical state distributions (at.%) for (a) Fe 2p, (b) S 2p and (c) As 3d in the pristine and bioleached surfaces of arsenopyrite particles after leaching 36 h in 9K culture medium without and with 0.05 g/L Cu2+. Fig. 7 Eh–pH diagrams of (a) FeAsS–H2O and (b) FeAsS–Cu2+–H2O systems without and with 0.8 mM Cu2+. Conditions: [Fe2+]tot 0.16 M, [FeAsS] 0.4 mM; 25 °C and 1 atm.

Fig. 8 Schematic diagram of the mechanism of arsenopyrite bioleaching by bacteria (a) without and (b) with Cu2+.

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Highlights: Bioleaching of FeAsS is slow due to a passive film of mainly As2S2, As2S3 and S0. Addition of Cu2+ to leaching solution prevented the formation of the film. Cu2+ enables synergies among bacteria, Cu2+/(Cu2S, CuS) and Fe3+/Fe2+ in solution. Optimal Cu2+ amount is 0.05 g/L, shortening leaching period from 19 d to 13 d. Lab experiments, SEM-EDS, XPS and thermodynamic analyses used to develop mechanisms

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: