New understanding of the reduction mechanism of pyrolusite in the Acidithiobacillus ferrooxidans bio-leaching system

Electrochimica Acta 297 (2019) 443e451

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New understanding of the reduction mechanism of pyrolusite in the Acidithiobacillus ferrooxidans bio-leaching system Jin-xing Kang a, Ya-li Feng a, *, Hao-ran Li b, Zhu-wei Du b, Xiang-yi Deng a, Hong-jun Wang a a b

School of Civil and Resource Engineering, University of Science and Technology, Beijing 100083, China State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 19 August 2018 Received in revised form 2 December 2018 Accepted 4 December 2018 Available online 4 December 2018

The reductive dissolution of pyrolusite in simulated Acidithiobacillus ferrooxidans bio-leaching medium was investigated. This study was performed in three stages. First, the advantageous electrochemical test conditions, parallel to the optimal bio-leaching conditions and adopting the Mn reduction rate, were determined by imitated electrolysis. The facilitation of A. ferrooxidans on MnO2 reduction is sensitive to pH and Fe(III) concentration. Second, electrochemical tests revealed that the reductive dissolution of manganese dioxide incorporated two single electron and proton steps-the first exchange of MnO2 to MnO$OH, and then conversion to Mn(OH)2 for diffusion. The results of transient and steady electrochemical measurements indicated that the first electron-transfer significantly affects the rate controlling step of Mn-leaching in control(9K) medium, while using A. ferrooxidans and Fe(III) in the solution tends to enable the leaching rate to be controlled by the latter electron transfer step. Third, the analysis of semiconductor and carrier properties of passive films of pyrolusite formed in the different solutions, illustrated that the reductive dissolution of manganese dioxide tends to depend on the movement of the holes. The first electron preferentially reacts with the shallow energy level of the O-vacancy to form þ MnO$2 , which then absorbs H to become MnO$OH. The second electron participates in the transformation of MnO$OH to (MnOH)(OH) and then to Mn(OH)2. A. ferrooxidans increases the carrier densities of the passivating film accelerating electron and proton transfer and Fe(III) primarily influences the shallow donor density of oxygen during the first electron-exchange. Additionally, the synergistic effect of A. ferrooxidans and Fe(III) on manganese dioxide ore reductive leaching is confirmed. © 2018 Elsevier Ltd. All rights reserved.

Keywords: Acidithiobacillus ferrooxidans Manganese dioxide Electrochemistry Electron transfer Carrier density

1. Introduction Manganese oxide ores, as important mineral carriers of manganese, are increasingly appreciated in value due to the rising demand of manganese products and the aggravation of the intractable manganese resource shortage [1]. China is a country in which the reserves of manganese oxide ore exist mainly in the form of lowgrade pyrolusite resources, accounting for 93% of the supply, with poor mining conditions and actionable benefits [2]. Many studies show that the manganese presents as manganese dioxide in pyrolusite exhibits the octahedral structure of [MnO6] as a basic unit [3]. Several reasons have emerged for the wet process preference, including growing external pressure and energy costs [4]. The

* Corresponding author. E-mail address: [email protected] (Y.-l. Feng). https://doi.org/10.1016/j.electacta.2018.12.031 0013-4686/© 2018 Elsevier Ltd. All rights reserved.

critical aspect of the process of manganese extraction from manganese dioxide mineral using a wet method is restoring highvalence manganese oxide minerals to soluble low-valence manganese ions [5]. However, a critical review of the literature indicated that traditional wet reductive routes are related to high costs if the grade of raw Mn ore is low [4]. Hence, an alternative and economical process must be developed, particularly for low-grade manganese-oxide ores. Microbial leaching based on the direct or indirect biochemical action of bacteria can economically and environmentally extract valuable metals from ores and tends to be more popular, especially for the application of low-grade ores [6]. The bio-leaching of manganese dioxide is a redox process converting insoluble Mn(IV)bearing minerals to dissolvable Mn(II) or Mn(III)-containing substances using microbiological agents or their metabolites [7]. Many heterotrophic and autotrophic microorganisms have been declared

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useful for manganese bio-leaching [8]. Acidithiobacillus ferrooxidans (A. ferrooxidans) is by far the most widely used bioleaching microorganism in ore leaching processes, including Au, Cu, Ni, U and Mn ores [9e13]. Early in the 1960s, Japanese scholars conducted research on the extraction of manganese from low-grade pyrolusite ore with sulfur as reductant by using A. ferrooxidans sulfuric leaching systems. Later, many researchers adopted sulfide mineral ores as the energy substance and electron donor for A. ferrooxidans leaching of manganese-dioxide ore [14]. Of these systems, the electrochemical behaviour of sulfide donor and manganese-dioxide acceptor may have the foremost topics of investigation. Since the electrode process of sulfide or manganese oxide dissolution is based on the interfacial reaction of A. ferrooxidans/solution/ore, it is particularly important to determine the relationship between its dissolution system and electrode electrochemical behaviour [15]. At present, the auxo-action of A. ferrooxidans on the redox reactions of the iron cycle and the sulfur cycle have been proved repeatedly to facilitate sulfide ore leaching [16e18]. However, the mechanism of the A. ferrooxidans effect on Mn-oxides reductive dissolution still represents a difficult challenge due to the use of reduction materials and the influence of bacteria on these substances in manganese bio-leaching. Moreover, iron is a commonly associated element in Mn- and S-bearing ores and often affects bio-leaching. In a weakly acidic A. ferrooxidans-leaching system, the dissolution of pyrolusite involves acidic and reductive corrosion, as well as dissolution under the influence of Fe(III) and/or A. ferrooxidans. Understanding the roles of Fe(III) and A. ferrooxidans is necessary to optimally employ acidic culture, Fe(III)-containing culture, A. ferrooxidans-containing culture, and bacteria mixed Fe(III) culture. While most bio-leaching studies were performed in culture medium with the presence of A. ferrooxidans and Fe(III), research regarding the functional analysis of the single action of bacteria or Fe(III) on minerals dissolution is relatively limited. This work attempted to recover a function of A. ferrooxidans on the reductive dissolution of manganese-dioxide ores in bioleaching system by using highly purified pyrolusite as raw material. The optimal conditions corresponding to the bioleaching of manganese dioxide ores were obtained by simulated electrolytic tests in bacteria and abiotic sulfuric acid systems. The electrochemical behaviour and electrode corrosion of pyrolusite in A. ferrooxidans-coupled and A. ferrooxidans-free electrolysis solutions were analysed using multiple electrochemical methods. The dissolution mechanism of the manganese-dioxide ore was discussed in the context of an A. ferrooxidans sulfuric acid system. 2. Materials and methods 2.1. Materials and bacteria culture The pyrolusite samples were obtained from Chaoyang, Liaoning, China. The main minerals in the ore include pyrolusite and quartz. The sample was air-dried, crushed and ground to 74 mm in the laboratory for the experiments, with a specific surface of 7.4 m2/g. The chemical compositions analysis illustrated that the manganese dioxide sample contained Mn 59.73%, and Fe 0.11% (94.49% total MnO2). Fig. 1 shows the XRD pattern of the manganese ore which is characterized by 28.7, 37.5 , and 56.8 peaks respectively corresponding to (110), (101), (211) lattice face, indicating b-MnO2 in terms of PDF#72-1984. All other reagents were of analytical grade. A. ferrooxidans was obtained from the Environmental Biological Science and Technology Research Center, Institute of Geochemistry, Chinese Academy of Science. The abundance of A. ferrooxidans species was determined by sampling bacteria cultivated in 9K basic medium (g$L1) [19]: (NH4)2SO4 0.15, MgSO4$7H2O 0.05, K2HPO4

Fig. 1. XRD patterns of manganese dioxide sample.

0.05, Ca(NO3)2 0.01. The growth culture was 9K culture supplemented with 44.5 g L1 FeSO4$7H2O. The bath solutions included a control(9K), Fe(III), A. ferrooxidans and an A. ferrooxidans þ Fe(III) solution. The control solution was a sterile and iron ion-free 9K nutrient solution, which was autoclaved at 120  C for 30 min before use. A selected mass of Fe2(SO4)3 dissolved in a sterile 9K basic medium constituted the Fe(III) solution. The A. ferrooxidans solution consisted of bacteria suspended in control 9K solution without iron ion from which A. ferrooxidans was separated by a centrifuge at 12000 r$min1 for 20 min from the post-log growth culture. The A. ferrooxidans þ Fe(III) solution was a combination of 9K basal nutrient, ferric ion and A. ferrooxidans solution. A solution of 20% H2SO4 was used to adjust the pH of the solution. 2.2. Dissolution performance tests Dissolution tests of MnO2 were performed in a three-electrode system using an electrochemical workstation (CHI660D, CH Instruments, USA). The schematic of the bioleaching was presented in Fig. 2. The preparation of electrolysis electrodes deposited a thin layer of pyrolusite ink over a graphite felt (1.0 cm2). The mineral grain ink was prepared by mixing the manganese dioxide ore (90 wt.%) and, polytetrafluroethylene (10 wt.%) in N-methyl-2pyrrolidone (the mass of pyrolusite was fixed at 0.1 g) [20]. A Pt plate (1 cm2) electrode was used as an auxiliary electrode and a saturated calomel electrode(SCE) was used as a reference (0.24 V vs standard hydrogen electrode (SHE)). The electrodes dried out at 60  C. The simulated bath solutions, in line with Section 2.1, were prepared as the A. ferrooxidans þ Fe(III), Fe(III), A. ferrooxidans, and control(9K) solution, and 200 mL was used for each test. Leaching characteristic of input voltage, pH value, Fe(III) concentration, and A. ferrooxidans number density were measured by Mn extraction efficiencies calculated from the manganese content in lixiviums at an appointed time. Simulated agitation bio-leaching were performed at 25  C by connecting a circulating thermostatically controlled water loop with magnetic stirring. Evaporative loss was compensated with distilled water. The pH of the bath solution was kept constant by adjustment with dilute sulfuric acid and potassium hydroxide solution. Mn(II) concentration was determined by the potassium periodate spectrophotometric method [39] (UV1750, SHIMADZU, Japan). The leached electrodes were air dried to send for SEM observations performed with a scanning electron microscope (SEM, ULTRA55, Carl-ZeissSMTrecLMD, Germany).

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Fig. 2. Schematic diagram of the bio-leaching measurement of manganese-dioxide ores.

2.3. Electrode and electrochemical measurements Electrochemical evaluations were performed in a typical electrochemical cell with three electrodes on a CHI660D microcomputer-based electrochemical system. The working electrodes were platinum paste-pyrolusite electrode (Pt disk of 0.1 cm2). The preparations of the clad layer and electrolyte, and the uses of the reference electrode and the counter electrode, were consistent with Section 2.2. The three electrodes were immersed in the biotic or abiotic sulfuric acid leaching medium for 30 min and the scan was activated. The simulated bath solutions were made independent of the influence of dissolved oxygen by purging with nitrogen. The cyclic voltammetry measurements were performed by sweeping from 0.6 V (positive-going potential scan) to 1.5 V (negative-going potential scan) and back to the beginning potential with a sweep rate of 25 mV s1. The impedance spectra were obtained at the open circuit potential (OPC) by applying a sine wave voltage amplitude of 5 mV in the frequency range of 0.01~105 Hz. The EIS data were displayed as Nyquist plots and analysed using Zview 2 software. Potentiodynamic polarization curves were generated from 0.4 to 0.8 V with a scanning rate of 5 mV s1. The polarization of pyrolusite electrodes at 0.2 V for 20 min was completed, before the Mott-Schottky measurement started. The Mott-Schottky curves were generated from 0.6 V to 1.5 V with a step size of 20 mV under a frequency of 1000 Hz. All electrochemical measurements were performed at 25  C. Refer to SCE for all potentials shown in the paper.

A. ferrooxidans within limits. Fe(III) does have a vital catalytic role in the reductive dissolution of pyrolusite and it could be enhanced by A. ferrooxidans. A high-level of input voltage leads to a driving force enhancement of Mn dissolution (Fig. 3 (a)). However, the application of excessive voltage has created an adverse environment for the activities of live cells, as a result, when the input voltage is higher than 800 mV (vs SCE) the dissolution of Mn in the Fe(III) bath solution is better than that of the bacterial system. A. ferrooxidans performs a facilitating role on manganese leaching under the input of 600 mV with a maximum leaching rate of 21.94 mg L1 h1 Mn

3. Results and discussion 3.1. Determination of dominant factors on extraction of Mn The effects of the bath voltage, pH, initial Fe(III) concentration and initial bacteria amount on the pyrolusite leaching in A. ferrooxidans and non A. ferrooxidans solutions, as determined by the extraction rate of Mn, are summarized in Fig. 3. The manganese dissolution positively corresponds with the presence of

Fig. 3. Effects of (a) input voltage, (b) pH, (c) Fe(III) concentration, and (d) bacterial concentration on Mn leaching rate at temperature of 25  C ((a) pH 2.0, Fe(III) concentration 1.0 g L1, A. ferrooxidans 1.0  108/mL; (b) 600 mV(vs SCE), Fe(III) concentration 1.0 g L1, A. ferrooxidans 1.0  108/mL; (c) 600 mV(vs SCE), pH 2.0, A. ferrooxidans 1.0  108/mL; (d) 600 mV(vs SCE), pH 2.0, Fe(III) concentration 1.0 g L1).

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in the A. ferrooxidans þ Fe(III) solution. High or low pH conditions are without benefit to the biological corrosion of manganese dioxide (Fig. 3 (b)). The reductive dissolution of pyrolusite is a reaction of hydrogen consumption [21]. Thus, the Mn leaching rate is nearly tripled at pH 1.0 as compared to pH 2.0 in the control(9K) solution. A. ferrooxidans is harmful to Mn leaching under high acidity (pH 1.0), suggesting that the adsorption of hydrogen ions on the surface of the electrode evokes a decrement in Mn dissolution rate attributed to the adversity resistance of bacteria [22]. With pH increase, A. ferrooxidans performs a beneficial role in the extraction of Mn in Fe(III)-containing or Fe(III)-free solutions. However, admixing Fe(III) into the control(9K) solution at pH above 2.0 is detrimental to the Mn leaching rate because of the coverage of iron-bearing insoluble minerals on the mineral surface [23]. As seen in Fig. 3(c&d), the synergistic effect of Fe(III) in the A. ferrooxidans leaching system is significant. A higher initial Fe(III) concentration, comparable to the A. ferrooxidans number density, correlates with a faster leaching rate of Mn. Fe(III) ions could be reduced to Fe(II) ions on the surface of the pyrolusite electrode, then Fe(II) ion preferred to reduce MnO2 to Mn2þ [24]. With the presence of 0.2 g L1 Fe(III), Mn leaching rate increased 30%, it may exhibit a 12% additional increase through the introduction of A. ferrooxidans to the bath solution. Increasing A. ferrooxidans number density increased Mn leaching rate in Fe(III)-free solution. Moreover, in Fe(III)-containing solution, Mn leaching rate weakly corresponds to A. ferrooxidans concentration. It is clear that the facilitation of A. ferrooxidans on Mn leaching rate is based on the balance of Fe(III)/Fe(II), which is faintly affected by bacterial number [25]. According to the results in Fig. 3(c&d), 1.0 g L1 Fe(III) and 1.0  108/mL A. ferrooxidans should be appropriate for the increase of Mn extraction efficiency. Hence, it could be concluded that pH 2.0, Fe(III) concentration of 1.0 g L1 and A. ferrooxidans concentration of 1.0  108/mL can serve as the optimal conditions for bioleaching of pyrolusite and electrochemical measurements.

3.2. Characterization of leaching residues Fig. 4 shows the SEM images of pyrolusite leached by sterile acidic 9K medium in the presence and absence of Fe(III) or A. ferrooxidans. It can be seen that the raw pyrolusite ore is generally characterized by blocky, granular shape with smooth surface (Fig. 4(a&b)). The mineral particles in each solution shrank with the reductive corrosion of the surface of manganese dioxide ore. The

Fig. 4. SEM micrographs of pyrolusite after leaching for 6 h in various solutions. (a) Raw pyrolusite ore, (b) unreduced acid-treated ore, (c) reduction in the control(9K) solution, (d) reduction in the A. ferrooxidans solution, (e) reduction in the Fe(III) solution, (f) reduction in the A. ferrooxidans þ Fe(III) solution ((c)e(f) 25  C, pH 2.0, reductive voltage 600 mV(vs SCE), Fe(III) concentration 1.0 g L1, A. ferrooxidans 1.0  108/mL).

surface of the large granule adheres many tiny mineral grains during corrosion. The corrosion character of pyrolusite in the Fe(III) solution is similar to the control(9K) solution. When admixing A. ferrooxidans into the control(9K) solution, the eroding holes of the corrosion layer of pyrolusite are finely and densely formed, suggesting that A. ferrooxidans increases corrosive sites. It is considered that A. ferrooxidans contains many surface groups which have a beneficial effect on the diffusion of protons and electrons [26]. With the use of A. ferrooxidans in the Fe(III)-containing solution, a featured corrosion is detected the corrosion products adhere to the surface of the electrode, leading to a thin layer displaying a strong hydrophilic nature similar to hydroxide, indicating that A. ferrooxidans with the presence of Fe(III) facilitates the dissolution of MnO2 to Mn-containing hydroxide, which evokes changes in the dynamic conditions of pyrolusite reduction. 3.3. Electrochemical studies 3.3.1. Cyclic voltammetry analysis Fig. 5 presents cyclic voltammograms of pyrolusite in various solutions. A. ferrooxidans catalyses the oxidation-reduction of the MnO2/Mn2þ couple. The pair of redox currents corresponding to MnO2/Mn2þ appears between 0.8 and 1.2 V (vs SCE) as shown in Fig. 5(a), indicating that the dissolution of manganese is mainly obtained from the reduction of MnO2 to Mn2þ in bio-leaching systems. With the introduction of A. ferrooxidans in iron-free solution, the redox peak current of pyrolusite electrode increased, the ratio of peak current was closer to 1, and the potential difference narrowed suggesting that A. ferrooxidans accelerates the exchange rate of MnO2/Mn2þ to promote the decomposition of pyrolusite. A significant acceleration of the corrosion and electron-transfer rate of pyrolusite was obtained from employing Fe(III), indicating that the circulation of Fe(III)/Fe(II) facilitates MnO2 reduction. The reduction peak observed at nearly 0.3 V(vs SCE) suggests that the reduction of Fe(III) to Fe(II) occurred on the surface of electrode and the reduction was increased by 90% with the admixture of bacteria to the Fe(III)-containing solution, suggesting that A. ferrooxidans promotes the redox process of Fe3þ/Fe2þ. Fig. 5(c), (d), (e) and (f) present the effect of scanning rate(v) on the redox of MnO2/Mn2þ in the different systems, and Fig. 5(b) shows the relationships between scan rate (v) and peak current (ipc). It can be seen that, with increasing sweep rate v, a) two reduction peaks appeared at nearly 0.9 V(vs SCE) in simulated solutions; b) the redox potential (Ep, (Epc þ Epa)/2) [27] positively

Fig. 5. Plots of cyclic voltammograms analysis in various solutions (25  C, pH 2.0, (a) Cyclic voltammograms (25 mV s1, 0.61.5 V (vs SCE), (c)e(f) solutions; (b) relationship between peak current (ipc) and scanning rate(v) of pyrolusite obtained from (c)e(f) cyclic voltammograms (0.6e1.4 V(vs SCE), (c) A. ferrooxidans 1.0  108/mL, Fe(III) concentration 1.0 g L1; (d) Fe(III) concentration 1.0 g L1; (e) A. ferrooxidans 1.0  108/ mL; (f) control(9K)).

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shifted and, with increasing scanning rate from 10 mV s1 to 100 mV s1, the redox potential positively moved from 1.034 to 1.068 V(vs SCE) in the control(9K) solution; c) the ipa/ipc is smaller than 1.0 when v is low, and it increased with increasing v, and then the higher the v, the further away it is from 1.0; d) in iron-free solutions, the ipc$v1/2 first decreases, and then tends to be stable, while in iron-containing solutions, the ipc$v1/2 is independent of scanning rate; e) the reduction peak potential Epc becomes more negative (when scanning rate increased tenfold the Epc negatively shifted by nearly 30 mV), and the peak currents ipc increase. Those calculated results suggest that the reduction of MnO2 is characterized by two electron transfer steps and that the first step impacts the leaching rate dramatically. The ipc vs. v curve shows a downward incline with increasing scan rate v in the control(9K), A. ferrooxidans or Fe(III) solutions but an upward incline in the A. ferrooxidans þ Fe(III) solution. The plots of ipc against v1/2 for the control, A. ferrooxidans, and Fe(III) solutions are straight lines with slopes of 0.99, 0.99 and 0.99, respectively, indicating that the interfacial chemical reaction is the ratelimiting step in those systems. However, after introducing A. ferrooxidans into the Fe(III) solution, the results show a monotonously increasing curve, suggesting that the influence of the intermediate product increases. In the admixture of A. ferrooxidans and Fe(III) bio-leaching solution, the rate-determining step of MnO2 reduction was influenced by product diffusion. The course of pyrolusite dissolution could be expressed as [28]: A:ferrooxidans

MnO2 þ Hþ þ e






! MnO$OH þ Hþ þ e⇔Mn2þ þ H2 O (1)

3.3.2. Polarization curves analysis When the potential was located from 0.4 V to 0.8 V(vs SCE), pyrolusite could be treated as a scarcely polarized electrode in Fe(III)-free solutions and as a quasi-reversible electrode in Fe(III)containing solutions; this is because it exhibited no redox current peak in the former medium, as measured by cyclic voltammograms, and a strong response to the presence of Fe(III) in the latter solution. Therefore, the potential of 0.4e0.8 V was chosen for the tests of reduction slope of pyrolusite in this work. Potentiodynamic polarization curves are shown in Fig. 6. With the presence of A. ferrooxidans, the reduction of MnO2 has a higher current density under the low potential related to the absence of bacteria, indicating that A. ferrooxidans promotes the reductive dissolution of MnO2. The smaller the slope of the Tafel curve for the MnO2 reduction, the lower the value of overpotential for increasing the same current density. The cathode slope of the reductive dissolution of manganese dioxide decreased in the presence of A. ferrooxidans and/or Fe(III), suggesting that Fe(III), as well as A. ferrooxidans, facilitates the reduction of MnO2. According to the

447

relationship between overpotential and net current density listed as Eq. (2), the decrease in the slope of polarization curve corresponding to MnO2 reductive corrosion correlates with two possibilities-the change in transfer coefficient and electron number of the rate-determining step [29,30].

I ¼ i0

  nF anF bnF þ hc ¼ i0 hc RT RT RT

(2)

where I is the net current density of the cathode reaction, i0 is the self-corrosion current density, hc is overpotential of the cathodic reaction, a as well as b represent transfer coefficients and a þ b ¼ 1, R is molar gas constant (8.314 J mol1 K1), F is Faraday constant (96585 C mol1), and T is temperature in K. Setting a reduction of two electrons from MnO2 to Mn2þ as two separated one electron transfer procedures connected in series, i,e, the transformations to the intermediate product and from the intermediate product to the final product are independent of each other, the dissolution of MnO2 could be listed as Eq. (3). 0

* i1 MnðIIIÞ

MnðIVÞ þ e ) 0

* i2 MnðIIÞ

þe)

(3)

where Mn(III) is intermediate product, and i01 and i02 represent the current density of each step. If the corrosion of MnO2/Mn2þ occurs in stable conditions, leading to a steady-state response to the concentration of intermediate, each single electron course supplies half of the total current density. Thus, the kinetic equations could be supposed:

"  #   cMnðIIIÞ I a1 F b1 F 0 ¼ i1 exp h  0 exp  h 2 RT c RT c cMnðIIIÞ

(4)

"   #  cMnðIIIÞ I a F b F ¼ i02 0 exp 2 hc  exp  2 hc 2 RT RT cMnðIIIÞ

(5)

where a1, b1 and a2, b2 are the transfer coefficients of the first electronic step and the second electronic step, and a1þb1 ¼ a2þ b2 ¼ 1; cMn(III) and c0Mn(III) are the concentrations of intermediate at the beginning and balanced by polarization. With respect to the concentration of immobilized intermediate, the polarization curve could be expressed as:

i h i h a2 ÞF exp ða1 þ hc  exp  ðb1 þRTb2 ÞF hc I RT     ¼ 1 exp a2 F h 2 þ 1 exp  b2 F h 0

i1

RT

c

0

i2

RT

When i01 is much larger than i02, it can simplify Eqs. (6) and (7).

   

ð1 þ a2 ÞF b F I ¼ 2i02 exp hc  exp  2 hc RT RT

Fig. 6. Plots of polarization analysis in various solutions ((a) polarization curves; (b) Tafel slope of cathodic reaction (25  C, pH 2.0, 2 mV s1, 0.4e0.8 V (vs SCE), A. ferrooxidans 1.0  108/mL, Fe(III) concentration 1.0 g L1).

(6)

c

(7)

Comparing Eq. (7) with Eq. (2), it is clarified that i0, a, and b are equal to 2i02, (1þa2)/2 and b2/2, respectively. The apparent transfer coefficient of a is often significantly larger than that of b, illustrating that the dissolution of Mn(IV) dioxide is controlled by the second electron transfer process corresponding to the transfer from Mn(III) to Mn(II). Similarly, if i01 is much smaller than i02, Eq. (8) could descript Eq. (6). It is found that i0, a, and b are equal to 2i01, a1/2 and (1þb1)/2, respectively, indicating that the first electron transfer controls the reaction rate of MnO2 dissolution.

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a F ð1 þ b1 ÞF I ¼ 2i01 exp 1 hc  exp  hc RT RT

(8)

The cathodic reaction of two electrons transfer is the main cause of the reductive leaching of pyrolusite. When the cathodic slope is 118 mV$des1, a, equivalent to a1/2, is trending towards zero with a1 trending towards zero, suggesting that the rate of the first electron transfer is very slow. If the cathodic slope is 59 mV$des1, a corresponding to (1þa2)/2 is close to 0.5 with a small enough a2 value, indicating that the redox process of Mn(III) to Mn(II) of the second electron transfer determines the reaction rate. Therefore, the dissolution of manganese dioxide is controlled by the diffusion of intermediates in the A. ferrooxidans þ Fe(III) dissolution as the Tafel slope of 59 mV$des1, and the formation of intermediate determines the leaching rate in the control(9K) solution with a cathodic reaction slope of 119.4 mV$des1. Using A. ferrooxidans or Fe(III) alone, the control step is still the first electron transfer. Moreover, the use of A. ferrooxidans leads to a lower slope of cathodic polarization than that of Fe(III), suggesting that A. ferrooxidans decreases the activation energy barrier of MnO2 dissolution more than Fe(III). 3.3.3. Alternating-current impedance analysis Fig. 7 shows Nyquist(a) and Bode(b) plots for the pyrolusite electrodes in the different solutions. Two incomplete semicircles were detected in all Nyquist curves and two characteristic parameters were observed for all Bode curves, indicating that pyrolusite underwent two electrochemical courses. The high-frequency time constant corresponded to the electrode reaction, and the lowfrequency time constant associated with the interfacial diffusion. With the use of A. ferrooxidans, the maximum peak of phase angle moves to high frequency, illustrating that bacteria decrease the time for electron travel through the interfacial electric double layer. Under the low frequency range (f  103 Hz), the rate change of phase degree of ores in the A. ferrooxidans solution is faster than in the Fe(III) solution, and under the high frequency region (f  104 Hz) using A. ferrooxidans leads to a lower phase degree than using Fe(III) alone, suggesting that A. ferrooxidans facilitates the permeation of electrons and protons to the electrode surface better than does Fe(III). The Nyquist curves present a full semi-circle and a small part of the circle arc, suggesting that the pyrolusite reduction in bio-leaching solution is mainly controlled by the interfacial chemical reaction. A simple equivalent circuit of Rs(RactCPE1)(RpCPE2)(Fig. 6(a)) was employed to fit the impedance data [31]. The fitting results obtained using the equivalent circuit were summarized in Table 1. Rs, Ract, and Rp represent the solution resistance, the charge transfer resistance and the redox resistance of the passive film during pyrolusite reduction. CPE1 and CPE2 reflected the double-layer capacitances between electrode/electrode interfaces and surface layer/electrolyte interfaces, respectively. Y0

Fig. 7. Nyquist(a) and Bode(b) plots of the pyrolusite in various solutions (25  C, pH 2.0, frequency 0.01e105 Hz, potential perturbation OCP, A. ferrooxidans 1.0  108/mL, Fe(III) concentration 1.0 g L1).

and h correspond to the parameters of CPE. Combining the previous statements, the corrosion of pyrolusite in A. ferrooxidans bio-leaching solution can be expressed as follows: the single electron discharge of MnO2 preferentially generates MnO$OH on the surface of pyrolusite, then this intermediate product absorbs onto the surface and reacts with protons and electrons, leading to production of Mn2þ, and then the diffusion of Mn2þ. The decrease in electron transfer resistance is pronounced in response to the presence of A. ferrooxidans and/or Fe(III), and the mobility of CPE1 is much smaller than that of CPE2. Therefore, it is considerable that the electron transfer resistance is mainly depended on the conversion of MnO2 to MnO$OH. As observed, both A. ferrooxidans and Fe(III) promote electron transfer reactions, and the decrement in electron transfer resistance with the presence of A. ferrooxidans is larger than that of Fe(III) alone by 11%, suggesting that A. ferrooxidans evokes a significant drop in impedance because of the acceleration by bacteria on the transference of electrons and protons. Adding A. ferrooxidans and Fe(III) to the 9K basic solution, there is a lower resistance and a higher transmittance than in the cases of using A. ferrooxidans or Fe(III) alone. In addition, the dispersion of corrosion film increased with the presence of A. ferrooxidans, illustrating that A. ferrooxidans is beneficial to the diffusion and transformation of the intermediate product. In Fe(III)-containing solutions, the dispersion coefficient decreased compared to the control solution, indicating that the polarized response of the intermediate film increased with the presence of Fe(III). These results support the assertion that A. ferrooxidans influences two electrons transfer of MnO2 reduction, while Fe(III) affects the first electron transfer course. 3.3.4. Mott-Schottky curves analysis Pyrolusite is a semiconductor regarding electrical conductivity and, comparable to its acid reduction product, the Mott-Schottky equation could depict the relationship between differential capacitance of space charge layers of manganese dioxide and the interfacial potential of semiconductor relative to the bulk solution in A. ferrooxidans bio-leaching systems [32]. There is no redox current peak observed over the potential range from 0.2 to 0.4 V(vs SCE), as shown in Fig. 5, and pyrolusite electrodes would form a depletion layer under a polarization potential of 0.2 V(vs SCE). Thus, it is reasonable to conduct a 20 min prepassivation before the MottSchottky test. Mott-Schottky relations for two types of semiconductors could be listed as:

P  type :

N  type :

1 C 2sc 1 C 2sc



kT

¼

2 εε0 eNa

¼

  2 kT E þ Efb  εε0 eNd e

 E þ Efb 

e

 (9)

(10)

where Csc is the differential capacitance of space charge layer, ε is the relative dielectric constant of passive film(a value of 5 used for ε of manganese oxide [33,34]), ε0 is the permittivity of vacuum (8.854  1012 F m1), e is the electron charge (1.602  1019 C), Nd and Na are carrier concentrations corresponding to donor density of n-type and acceptor density of p-type semiconductor respectively, E is impressed voltage (0.2 V(vs SCE) used in this work), Efb is the flat-band potential of electrode, k is Boltzmann constant (1.38066  1023 J K1), and T is temperature. Fig. 8 shows plots of Mott-Schottky tests in the different bath solutions. The passivation film of pyrolusite in simulated bath solutions performs as p-n-p-n type semiconductor structures when the test potential begins at 0.6 and ends at 1.5 V(vs SCE). The Mott-Schottky curves are divided into two p-type regions and two

J.-x. Kang et al. / Electrochimica Acta 297 (2019) 443e451

449

Table 1 Fitting results using the equivalent circuit(Fig. 6) for pyrolusite electrodes. Simulated solution

Rs/(U$cm2)

Y0 of CPE1 (106 U1 cm2$sh1)

h1

Ract/(U$cm2)

Y0 of CPE2 (106 U1 cm2$sh2)

h2

Rp/(U$cm2)

A.ferrooxidans þ Fe(III) Fe(III) A. ferrooxidans Control(9 K)

31.6 23.7 32.0 27.5

1.94 1.78 2.14 1.39

0.998 0.990 0.981 0.974

2472 3116 2748 4274

793.3 678.7 368.6 360.1

0.895 0.898 0.972 0.952

7180 8256 12260 8679

located at region I. When the potential is higher than 0.7 V, only a donor density is detected in Fe(III)-free solutions; in other words, the dissolution of manganese dioxide performs as merely a potential donor, illustrating that it is difficult to enable direct electron absorption for manganese dioxide dissolution. It can be stated that the reductive dissolution of manganese dioxide tends to be realized by the electron-hole movement. Fe(III) facilitates the corrosion of pyrolusite due to Fe(III) effects on the electron-hole of oxygen in the surface of the electrode. Comparing the Fe(III) solution with the control(9K) solution, there is an increase in the shallow acceptor concentration at a low potential, indicating that the transformation of Fe2þ to Fe3þ is attenuated, which leads to increased production of MnO$OH by Fe2þ reducing MnO2 on the surface of the electrode. On the other hand, the declines in donor densities are related to the acceleration of the transformation of Fe2þ to Fe3þ with the increase in potential and Fe3þ would compete electron with MnO2. The density of oxygen vacancies increased when potential is lower than 0.7 V (vs SCE), while the concentration of unbound electrons decreased with the improvement of the solution potential caused by admixing Fe(III) into the control(9K) solution, leading to lower rates of formation and growth of the inactive layer which depended on Mn3þ and electron holes of oxygen for a reduced anti-corrosive quality. Adding A. ferrooxidans to the Fe(III)-containing solution, the carrier densities significantly increased, suggesting that A. ferrooxidans decreases the decay resistance of the corrosion scale. The increment in acceptor density, corresponding to holes of oxygen and Mn3þ, suggests that A. ferrooxidans promotes the mobility of electrons and protons to cavitation of oxygen and facilitates the transfer of Mn4þ to Mn3þ when A. ferrooxidans is introduced to Fe(III)containing solution. Moreover, A. ferrooxidans leads to a thinner inner layer consisting of Mn4þ and electron holes of oxygen; as a result, the reduction of pyrolusite accelerates.

Fig. 8. Mott-Schottky curves of the manganese dioxide ores in various solutions (25  C, pH 2.0, voltage 0.2 V (vs SCE), frequency 1 kHz, increment 20 mV, A. ferrooxidans 1.0  108/mL, Fe(III) concentration 1.0 g L1).

n-type regions, corresponding to potential region I (0.4 to 0.3 V), region III (0.8e1.2 V) and region II (0.3e0.6 V), region IV (1.2e1.4 V), respectively. The slope transformation potentials appeared nearly at 0.3, 0.75 and 1.2 V(vs SCE), respectively. This suggests that the passivation film of manganese dioxide reduction contains two donor density and two acceptor density. The passivation film is concerned with the shallow level acceptor concentration (Na1) of Mn4þ, Mn3þ and oxygen vacancy and the shallow level donor concentration of Mn4þ, Mn3þ and oxygen vacancy(Nd2) in low potential areas (below 0.6 V). In a high potential region (higher than 0.8 V), it is corresponding to the deep level acceptor density (Na2) of Mn4þ, Mn3þ, and oxygen vacancy and the deep level donor density (Nd2) of Mn4þ and oxygen vacancy. The fitting results of carrier density and flat-band potential of the passivating films in the simulated bio-leaching solutions were listed in Table 2. The results demonstrated that using A. ferrooxidans increased carrier densities related to the equivalent sterile group, suggesting that A. ferrooxians weakens the corrosion resistance of the passivating membrane. The influence of A. ferrooxidans on acceptor density is more remarkable than on donor density. In the A. ferrooxidans solution the acceptor density of the passivation film increased by 43% related to the control(9K) solution when potential

3.4. Mechanism of MnO2 reduction in A. ferrooxidans bioleaching The reductive leaching of MnO2 in bio-leaching solutions is a process of multiple electron and proton participation. Based on the earlier findings, the reduction of pyrolusite in A. ferrooxidansleaching solutions could be expressed as:

MnO2 þ 4Hþ þ 2e ¼ Mn2þ þ 2H2 O

(11)

In accordance with the principle of microscopic reversibility, the above reaction could be interpreted as a course of electron and proton participation in reducing Mn(IV) oxide to Mn(II) hydroxide

Table 2 Carrier densities and flat-band potentials of the passive films on pyrolusite in various solutions (fitting results of Fig. 8). Region

I II III IV

A.ferrooxidans þ Fe(III)

Fe(III)

A. ferrooxidans

Control(9K)

Nd1/1017 cm3

Efb1/V

Na2/1017 cm3

Efb2/V

Nd3/1017 cm3

Efb3/V

Na4/1017 cm3

Efb4/V

3.90 1.97 3.74 4.48

0.50 0.01 1.93 0.76

2.91 1.49 1.53 2.45

0.55 0.02 1.66 1.03

3.52 2.71 e 7.16

0.47 0.01 e 0.47

2.48 2.08 e 5.54

0.48 0.01 e 0.26

450

J.-x. Kang et al. / Electrochimica Acta 297 (2019) 443e451

Fig. 9. Bio-electrolytic reaction of MnO2 in A. ferrooxidans bio-leaching.

and a dissolution of the reductive products. Therefore, Eq. (11) could consist of Eqs. (12) and (13).

MnO2 þ 2H þ þ 2e ¼ MnðOHÞ2

(12)

MnðOHÞ2 þ 2Hþ ¼ Mn2þ þ 2H2 O

(13)

A. ferrooxidans impacts the electron and proton transfer significantly, so Eq. (12) is the major concern in this work. The application of square scheme and the consideration of reductive conditions determined that the possible route of MnO2 to Mn(OH)2 is as follows [35e38]: First, the movement of the hole of the valence shell of O2 of pyrolusite electrodes turns MnO2 into MnO$2 , then the absorption of hydrogen ion produced by the static electric force on the surface of the electrode combines Hþ with MnO$2 to form MnO$OH; Next, hydrogen ions of the solution continuously spread and adhere to the surface of electrode, and the following adhesion of Hþ evokes mobility of the proton from the surface of MnO$OH to the inner core with the deep-seated MnO$OH, creating (MnOH)(OH); In the meantime the superficial MnO$OH generates MnO$2 for the adsorption cycle of Hþ, and then the hole-transfer of Mn3þ of (MnOH)(OH) leads to a more stabilized substance of Mn(OH)2, and finally, Mn(OH)2 dissolves to Mn2þ. Fig. 9 shows the schematic diagram of the reductive leaching of pyrolusite in the A. ferrooxidans bio-leaching system. As discussed above, A. ferrooxidans accelerates the electron and proton transfer resulting from the increment in the hole concentration of the oxygen of manganese dioxide followed by the diffusion of hydrogen ions to the surface of the electrode leading to the generation of intermediate of MnO$OH. The facilitation of Fe(III) on pyrolusite reduction mainly depends on the contribution of the hole concentration of oxygen at shallow energy levels. In Fe(III)containing solution, A. ferrooxidans catalyses the redox of the Fe3þ/Fe2þ pair generating an acceleration of the dissolution of pyrolusite. 4. Conclusions The reductive leaching of manganese dioxide undergoes two processes of single electron transfer in bio-leaching systems including the prior formation of MnO$OH and its re-conversion. The first e-interchange imposed restriction on the Mn extraction

rate during leaching in iron-free and sterile 9K medium. With the employment of A. ferrooxidans or Fe(III) the Mn extraction rate increased within limited conditions but did not alter the ratedetermining step. While utilizing A. ferrooxidans and Fe(III) in the medium, the dynamic conditions of Mn(IV)-reduction were apt to be dependent on the diffusion-rate-limited nature of the second electron. The reductive dissolution of manganese dioxide is inclined to be realized by the movement of the holes. The corrosion films in bioleaching systems performed as p-n-p-n semiconductor structure. They preferentially act on the shallow energy level of O-vacancy during the first electron interchange. The acceleration of Fe(III)-ion is mainly related to the increase in the density of shallow acceptors. A. ferrooxidans can enhance the dissolution of pyrolusite by the increment in carrier densities and the acceleration of proton and electron transfer. The possible route of manganese dioxide reductive leaching is obtained. Acknowledgement This study was supported by the “China Ocean Biological Resources Development Program (DY135-B2-15), Major Science and Technology Program for Water Pollution Control and Treatment (2015ZX07205-003), National Natural Science Foundation of China (21176026, 21176242)”. References [1] W. Zhang, C. Cheng, Manganese metallurgy review. Part I: leaching of ores/ secondary materials and recovery of electrolytic/chemical manganese dioxide, Hydrometallurgy 89 (2007) 137e159. [2] Z. Hao, H. Fei, Q. Hao, L. Liu, China's manganese geological research and prospecting have achieved great breakthrough, ACTA Geol. Sin. Engl. Ed. 90 (2016) 749e750. [3] C.M. Julien, M. Massot, C. Poinsignon, Lattice vibrations of manganese oxides. Part I. Periodic structures, Spectrochim. Acta Mol. Biomol. Spectrosc. 60 (2004) 689e700. [4] D. Dwivedi, N.S. Randhawa, S. Saroj, R. K Jana, An overview of manganese recovery by hydro and pyro-metallurgical routes, J. Inst. Eng. 98 (2017) 1e8. [5] Z. You, G. Li, J. Dang, W. Yu, X. Liu, The mechanism on reducing manganese oxide ore with elemental sulfur, Powder Technol. 330 (2018) 310e316. [6] P. Martinez, M. Vera, R.A. Bobadilla-Fazzini, Omics on bioleaching: current and future impacts, Appl. Microbiol. Biotechnol. 99 (2015) 8337e8350. [7] Z. Dan, Y. Zhang, J. Cai, X. Li, N. Duan, B. Xin, Reductive leaching of manganese from manganese dioxide ores by bacterial-catalyzed two-ores method, Int. J. Miner. Process. 150 (2016) 24e31.

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