Electrochemical study of silver telluride (Ag2Te): anodic and cathodic potential dependent-reactions in alkaline cyanide solutions

Accepted Manuscript Electrochemical study of silver telluride (Ag2Te): anodic and cathodic potential dependent-reactions in alkaline cyanide solutions

A.A. González-Ibarra, F. Nava-Alonso, A. Uribe-Salas PII: DOI: Reference:

S0304-386X(18)30334-7 https://doi.org/10.1016/j.hydromet.2018.12.019 HYDROM 4967

To appear in:

Hydrometallurgy

Received date: Revised date: Accepted date:

3 May 2018 14 November 2018 20 December 2018

Please cite this article as: A.A. González-Ibarra, F. Nava-Alonso, A. Uribe-Salas , Electrochemical study of silver telluride (Ag2Te): anodic and cathodic potential dependent-reactions in alkaline cyanide solutions. Hydrom (2018), https://doi.org/ 10.1016/j.hydromet.2018.12.019

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ACCEPTED MANUSCRIPT Electrochemical study of silver telluride (Ag2Te): anodic and cathodic potential dependent-reactions in alkaline cyanide solutions A. A. González-Ibarra*, F. Nava-Alonso and A. Uribe-Salas CINVESTAV Unidad Saltillo, Avenida Industria Metalúrgica 1062, Parque Industrial

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Saltillo-Ramos Arizpe, Ramos Arizpe, Coahuila, Mexico. C. P. 25900

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*Corresponding author: [email protected]; Avenida Industria Metalúrgica 1062, Parque Industrial Saltillo-Ramos Arizpe, Ramos Arizpe, Coahuila, Mexico. C. P. 25900.

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ORCID: 0000-0003-0223-7012. Tel. +52 (844) 438 96 00 ext. 8445 [email protected]; [email protected];

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[email protected]

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Abstract

Although electrochemical characterization has been used to elucidate the species formed

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during hydrometallurgical processes, it has not been commonly applied to the cyanidation of precious metal tellurides. In this study, voltammetric characterization of the silver

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telluride (Ag2Te) reaction system in alkaline cyanide solutions (pH 10.9 and 1250 mg/L CN-) was carried out making use of carbon paste electrodes. For this purpose, the

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electrochemical reactions system of elemental silver and elemental tellurium were first studied by cyclic voltammetry in two aqueous media (i.e., aqueous solution at pH 10.9, and

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1250 mg/L CN- at pH 10.9) and, by comparing the cyclic voltammograms obtained with those for Ag2Te, the electrochemical reaction system was elucidated. The results obtained

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showed that the anodic oxidation of Ag2Te in alkaline cyanide solutions occurred by at least two consecutive steps according to: Step 1 (fast reaction): Step 2 (slow reaction): while the cathodic reduction of Ag2Te in alkaline cyanide solutions occurred according to:

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ACCEPTED MANUSCRIPT Keywords: Cyclic voltammetry; Carbon paste electrode; Silver telluride; Cyanide solutions; Electrochemical reaction 1. Introduction Cyanide leaching has dominated precious metals processing since its implementation in 1887 (Fleming, 1998; Habashi, 1999) and will likely continue to dominate because the

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process is simple and efficient, chemically robust and because the remaining cyanide in

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solution is easy to oxidize and neutralize in precious metals plant tailings. At present, most

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of the gold and silver produced each year is finally extracted from the host rock by cyanidation (Fleming, 1998; Adams, 2016; Bas et al., 2017) and it is increasingly common

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to use it in refractory ores of which precious metal tellurides are an example. The most common precious metal tellurides are calaverite (AuTe2), krennerite ((Au0.8Ag0.2)Te2), sylvanite (AgAuTe4), petzite (Ag3AuTe2) and hessite (Ag2Te) (Henley et al., 2001;

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Echmaeva and Osadchii, 2009; Zhang et al., 2010). Although the extraction of silver by cyanidation has been less intensively studied compared to the extraction of gold, the study

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of refractory silver species is fully justified not only for their mineralogical association but

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also for the fundamental differences that exist between gold and silver cyanidation (e.g., silver minerals have slow cyanidation kinetics and low overall extraction efficiency)

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(Deschenes et al., 2011).

Although many studies have been carried out to elucidate the behavior of silver and that of

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tellurium using electrochemical techniques (Awad, 1968; Dirkse, 1989, 1990; Jayasekera et al., 1994; Zaky et al., 2004), there are just a few electrochemical studies on precious metal

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tellurides (Jayasekera et al., 1996; Salinas et al., 1998; Dyer et al., 2017) and there may exist some misunderstandings about the potential dependent-reactions (e.g., from -2000 to 2000 mV vs. SCE) and the reactions occurring in a conventional cyanidation process, like those described by González-Ibarra et al. (2017a, 2017b) and Nava-Alonso et al. (2017), although in both cases the discussed reactions are of electrochemical nature. According to Dirkse (1989), the result of anodically treating metallic silver in an alkaline solution is a three-step oxidation process. The first step is associated with the formation of Ag2O, the second with AgO and the third one with Ag2O3 (the third step could also be associated with oxygen evolution). An anodic polarization study of metallic silver in 2

ACCEPTED MANUSCRIPT sodium hydroxide solutions carried out by Zaky et al. (2004) agrees with the above, and showed that this is characterized by the formation of Ag(OH)2-, Ag2O and AgO. It is worth noting that although Dirkse (1989) and Zaky et al. (2004) agree in the formation of AgO, this can neither be silver(I) peroxide nor silver(II) oxide. In fact, a more accurate formula for AgO is Ag2O2 and corresponds to a mixed silver(I,III) oxide (Tudela, 2008). Thus, we

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may assume I and III as the oxidation states of the silver stable ions.

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Jayasekera et al. (1994) carried out a cyclic voltammetric study of the dissolution of tellurium in the pH range from 0 to 14. Among the most interesting results in alkaline

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media are the findings concerning to the formation of an insoluble, or slowly soluble film of H2TeO3 when the scan was initiated in the anodic direction. The results obtained by the

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authors are in agreement with those previously obtained by Awad (1968) in his study of the anodic dissolution of tellurium in HClO4, H2SO4 and HNO3 media.

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Concerning precious metal tellurides, Jayasekera et al. (1996) studied the electrochemical oxidation of AuTe2 in HClO4 solutions. They found that when the potential was swept up to

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500 mV (vs SCE), the reaction products were mainly metallic gold and HTeO2+. When the

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potential was swept to 750 mV or more positive, the reaction products were metallic gold and H2TeO3. They concluded that the H2TeO3 partially passivates the reaction; nevertheless, as the H2TeO3 is unstable in the media employed, it tends to dissolve.

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Electrochemical dissolution of AuTe2 has also been studied in thiourea (an alternative

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lixiviant agent) acidic solutions (Salinas et al., 1998), revealing the presence of three anodic peaks: the first was associated with the decomposition of the thiourea adsorbed on the AuTe2 surface to form formamidine disulfide and with the oxidation and complexation of

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gold; the second was related to the oxidation of tellurium to HTeO2+, and to the formation of a passivating film of H2TeO3; and the third corresponding to the formation of gold hydroxides that might be relatively soluble in thiourea solutions. Recently, Dyer et al. (2017) studied the oxidation behavior of AuTe2, Ag2Te and Ag3AuTe2 using linear voltammetry and they suggested that cyanidation of these precious metal tellurides is likely impeded by the difficulty in oxidizing tellurium. Numerous investigations of the electrochemical behavior of sulfide and oxide minerals have been carried out leaving aside the electrochemical study of refractory species as 3

ACCEPTED MANUSCRIPT precious metal tellurides. According to the above, the aim of the present investigation was to characterize the anodic and cathodic potential dependent-reactions of Ag2Te in alkaline cyanide solutions, making use of carbon paste electrodes (CPEs), for which reason the potentials were far beyond the natural dissolution potentials occurring in real situations. For this purpose, the electrochemical reaction system of elemental silver and elemental tellurium were first studied by cyclic voltammetry (CV) in two aqueous media (aqueous

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solution at pH 10.9, and 1250 mg/L CN- at pH 10.9) and by comparing the typical cyclic

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voltammograms obtained with those obtained with Ag2Te, the electrochemical reactions

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system was elucidated. To complement the electrochemical measurements, the surface of CPE-Ag2Te was subjected to a specific potential for 600 s and then analyzed by scanning

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electron microscopy coupled with energy-dispersive spectrometry (SEM-EDS). Anodic polarization has been frequently carried out in an electrolyte containing cyanide in absence

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of oxygen, whereas the cathodic polarization has been considered in presence of oxygen and absence of cyanide. This study was carried out in the presence of cyanide and

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atmospheric oxygen.

2.1. Materials and reagents

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2. Experimental

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The silver, tellurium and silver telluride (Ag2Te) powders used in this study (SigmaAldrich 99.9%) were ground in an agate mortar and/or sieved to – 38 µm; the phases were

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confirmed by X-ray diffraction. Reagent grade chemicals (i.e., sodium cyanide and sodium hydroxide) and deionized water were used to prepare the alkaline and cyanidation media.

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2.2. Instrumentation

The electrochemical measurements were performed with an alectrochemical analyzerpotentiostat (Radiometer Copenhagen model DEA332) coupled to an electrochemical interface (Radiometer Copenhagen model IMT102), using a typical three-electrode cell arrangement. An Alfa-Aesar graphite bar (6.15 mm diameter × 152 mm long, 99.9995% purity) was used as a counter electrode and a saturated calomel electrode (SCE), immersed into a Luggin capillary, as a reference (all potentials are given vs the reference electrode). The working electrodes used were a silver-modified, tellurium-modified and silver 4

ACCEPTED MANUSCRIPT telluride-modified CPE (CPE-silver, CPE-tellurium and CPE-silver telluride, respectively). Additionally, to CV tests, the surface of CPE-Ag2Te was subjected to a specific potential and analyzed by SEM-EDS (Philips XL30-ESEM). 2.3. Preparation of carbon paste electrodes The modified CPEs were prepared by mixing 0.16 g of graphite powder (2-5 µm,

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99.9995% purity), 0.04 g of electroactive species (i.e., Ag, Te or Ag2Te) and 80 µL of

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silicone oil (Alfa Aesar, ρ = 0.963) in an agate mortar until a completely homogeneous

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paste was obtained. Once the paste was ready, it was packed in a polyethylene syringe of 3 mm internal diameter and 7 cm long, whose plunger was used to renew the electrode

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surface, by gently polishing it on a paper sheet before each electrochemical test. The electrical contacts between the potentiostat and the CPEs-modified were made with copper

CPEs, but without electroactive species.

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2.4. Experimental procedure

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wires. The unmodified CPE (CPE-blank) was prepared in the same way as the modified

CV studies of silver, tellurium and silver telluride (Ag2Te) were carried out in both alkaline

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aqueous solution and alkaline cyanide solution. In the first, the pH was adjusted at 10.9 by adding NaOH. In the second the pH was adjusted first and NaCN was added to set a 1250

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mg/L concentration of CN-. pH value was set at 10.9 since CN- predominates at alkaline pH values (e.g., 10.5) and, in industrial applications, is common to use cyanide concentrations

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from 1000 to 2000 ppm to recover silver from high-grade ores. CV experiments were carried in the potential range of -2000 mV to 2000 mV (unless otherwise stated) at a 20

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mV/s scan rate and steady-state cycle was considered; no electrolyte stirring was set, unless otherwise stated. The tests were carried out at room temperature (approximately 25°C), and each reported value is the average of at least three independent measurements. To approximate the practical cyanidation process, experiments were performed with saturated atmospheric oxygen (except when specified). 3. Results and discussion 3.1. Silver (Ag) 3.1.1. CPE-Ag behavior in a solution of pH 10.9 5

ACCEPTED MANUSCRIPT Figure 1 shows typical cyclic voltammograms of the CPE-Ag swept between -2000 and 2000 mV in a pH 10.9 solution. When the potential sweep was conducted in the positive direction from the open circuit potential (OCP) which was approximately 50 mV (black line), the anodic peaks A1, A2, A3 and A4 developed at about 350, 390, 780 and 1200 mV, respectively. During the subsequent reverse sweep the cathodic peaks C1, C2, C3 and C4 were observed at about 571, 210, -359 and -1400 mV, respectively. It is worth noting that at

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approximately 600 mV the formation of a grey precipitate was observed, probably due to

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the local high concentration of the species formed during reactions A1 and A2. This grey

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precipitate dissolved as it began to settle, or at least it was more difficult to observe.

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Fig. 1. Typical cyclic voltammograms of the CPE-Ag obtained in an unstirred solution at pH 10.9. The potential was initiated in the OCP (50 mV vs. SCE) in: anodic direction (black line) and cathodic direction (grey line) at a scan rate of 20 mV/s. CPE-blank (dashed line) Peak A1 and A2 correspond to the Ag oxidation probably occurring according to Equation (1). However, the existence of two peaks located very close, suggest that Ag oxidizes to form two distinct compounds. In this case, the Ag2O formed via Equation (1) and AgOH that might be formed by the reaction represented by Equation (2). 6

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Taking into account the free energy of formation of AgOH (i.e., ΔGf° = -21.98 kcal) and Ag2O (i.e., ΔGf° = -2.58 kcal) (Garrels and Christ, 1990) it may be suggested that the species AgOH could be formed during the oxidation peak A1, while Ag2O would develop

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the oxidation peak A2. This phenomenon could occur through Equation (3). This would

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explain the proximity of the current peaks A1 and A2, since both species could be

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electrically equivalent.

Peak A3 located at 780 mV has been observed to occur from 680 to 800 mV (Dirkse, 1989) and it has been associated with the process described by Equation (4), where silver(I) oxide

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is oxidized to silver(I,III) oxide. Following the peak A3, there is a region (i.e., A4) of

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also be the result of H2O oxidation.

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sharply increasing current, associated to the formation of Ag2O3 (Equation 5) but it may

The oxidation processes described above have been discussed by other authors (Dirkse,

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1989, 1990; Zaky et al., 2004) and the peak potential values associated with them are in good agreement with heretofore reported (see Table 1).

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Table 1 Comparison of the potentials (vs. SCE) at which oxidation peaks of Ag in KOH 1M solutions occurred (data from Dirkse (1989)) and oxidation peaks of Ag in a solution of pH 10.9 (Fig. 1) observed in this study. The potentials obtained by Dirkse were measured taking as a reference the Hg/HgO electrode and were converted vs. SCE for comparison purposes Peak Dirkse (1989), mV Fig. 1, mV A1

239

350

A2

299

390

Reaction Oxidation of Ag to Ag2O and AgOH

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ACCEPTED MANUSCRIPT A3

649

780

Oxidation of Ag2O to Ag2O2

A4

720

1200

Oxidation of Ag2O2 to Ag2O3

A few tests conducted by varying the inversion potential showed that peak C1 was associated with peak A4. The reduction process C2 was the cathodic counterpart of the

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anodic peak A3, as well as peak C3 was associated with anodic peaks A1 and A2. It is

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worth noting that when the positive switching potential (Eλ+) was ≤ 1000 mV, the cathodic peak C3 was distinguished in the form of two peaks, confirming C3 as the counterpart of

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the anodic peaks A1 and A2 (reverse of the reactions of Equations (1) and (2)). Peak C4 was due to electrochemical reactions involving the electrolyte solution since it coincides

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with the peak observed when using the CPE-blank.

Figure 1 also shows the negative-going scan (grey line). Peak C4’ was due to the electrolyte

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electrochemical reactions. Following the reversal of the potential sweep, the oxidation peaks A1’ and A4’ were developed. When the sweep was reversed again, it began to

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develop the reduction peak C2’ which did not finish developing upon completing the CV.

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3.1.2. CPE-Ag behavior in a 1250 mg/L CN- solution of pH 10.9 Figure 2 shows typical voltammograms of the CPE-Ag and CPE-blank cycled between -

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2000 and 2000 mV in a 1250 mg/L CN- solution of pH 10.9. As can be observed, the positive-going sweep (black line) gave rise to the oxidation processes B1 and B2 with peak

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potentials at 110 and 1130 mV, respectively. No signs of precipitation were observed, as in the previously discussed case. In the reverse sweep, the reduction peaks D1, D2, D3, and

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D4 were observed.

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Fig. 2. Typical cyclic voltammograms of the CPE-Ag obtained in an unstirred solution at

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pH 10.9 and 1250 mg/L CN-. The potential scan was initiated in the OCP (-150 mV vs. SCE) in: anodic direction (black line) and cathodic direction (grey line) at a scan rate of 20

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mV/s. CPE-blank (dashed line)

The oxidation peak B1 was associated to the Ag oxidation-complexation via Equation (6).

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Comparing the oxidation peaks A1 and A2 (Fig. 1, black line) with the oxidation peak B1 (Fig. 2, black line), it is observed that B1 occurred at less positive potential, suggesting that

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presence of CN- favored Ag oxidation. Furthermore, the lack of formation of a precipitate, indicates that Ag oxidation neither produced an oxide nor a hydroxide, since it was

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complexed by CN-. This also explains the presence of only one oxidation peak. When the potential sweep was initiated in the cathodic direction (grey line), the appearance of the same number of peaks than when the potential sweep was initiated in the anodic direction was observed.

Figure 3 shows typical cyclic voltammograms obtained when the inversion potential (Eλ+) was varied (i.e., 500, 1000, 1250 and 1500 mV). This analysis reveals the association between the oxidation peak B1 and the reduction peak D2. It also reveals the association 9

ACCEPTED MANUSCRIPT between oxidation peak B2 and reduction peaks D1, D3 and D4. The dashed line (CPEblank) shows that the processes B2 and D4 could be related with the electrolyte redox

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reactions.

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Fig. 3. Typical cyclic voltammograms of the CPE-Ag obtained in an unstirred solution at pH 10.9 and 1250 mg/L CN-. In all the tests, the potential scan was initiated in the anodic

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direction at a scan rate of 20 mV/s. Eλ+ was 500, 1000, 1250 and 1500 mV vs. SCE

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Figure 4 shows the electrochemical behavior of the associated processes B1 and D2 in a stirred (grey line) and unstirred solution (black line). It was observed that in the stirred solution the reduction process D2 continues developing, indicating that the product formed

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in the oxidation peak B1 was not soluble (e.g., remained in the electrode). This behavior suggests the formation of AgCN(s) during the anodic potential sweep. This species was formed after Ag oxidation via Equation (7). The reduction peak D2 would correspond to the reduction of AgCN(s) (Zheng et al., 2016) via Equation (8). It is worth noting that although only B1 was identified as a formal peak, there is signal occurring before 110 mV at much lower potentials. This may be because silver cyanidation is a slow process at reasonable low potentials, before the oxidation process B1. The fact that stirring increased the reaction rate supports the fact that silver cyanidation would be cyanide diffusion limited 10

ACCEPTED MANUSCRIPT and therefore, stirring would thin or remove the diffusion layer, thus increasing silver

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dissolution.

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Fig. 4. Typical cyclic voltammograms of the CPE-Ag obtained in a solution at pH 10.9 and 1250 mg/L CN-. The potential scan was initiated in the anodic direction at a scan rate of 20

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mV/s, unstirred (black line) and stirred (grey line) solution Since the potential at which the oxidation process B2 occurred matches the one obtained

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with the CPE-blank, the process can be linked to redox electrolyte reactions. These reactions may include the CN- oxidation to CNO- via Equation (9). Thus, the reduction peak D4 may be due to the reverse of Equation (9), since inspecting Fig. 3, it can be observed that peak D4 increased as peak B2 began to develop.

Figure 4 also shows that the cathodic processes D1 and D3 correspond to the formation of soluble species, probably due to the local high concentrations, since the peaks did not occur when the CV’s were carried out in a stirred solution. The stirring of the solution would not 11

ACCEPTED MANUSCRIPT allow the building up of local high concentration of the formed soluble species, avoiding the occurrence of D1 and D3 processes. Analyzing Fig. 3, it is observed that the reduction peak D3 started to develop when the Eλ+ was approximately 1000 mV, while the process D1 was developed when the Eλ+ was 1250 mV. This behavior suggests that when cyanide oxidation occurs, the cyanide complexing silver also oxidizes, although this happens at a more positive potential, compared to that necessary to oxidize the cyanide in solution.

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During the CN- oxidation, the silver will be released to the solution, as described by

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Equation (10).

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Due to the CN- oxidation described above, its concentration on the surface of the electrode decreased, so that silver (see Equation (10)) would be complexed as Ag(CN)2- in the

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interface since in the solution there was a high concentration of CN- and the reduction of

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this complex would be the responsible of the development of peak D3, via Equation (11).

Furthermore, when the switching potential increased to 1250 and 1550 mV, the Ag released

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via Equation (10) was oxidized to Ag3+ and when the potential was inverted, this species

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was reduced generating the process D1, via equation (12).

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It is important to mention that the potential at which the Ag3+ was reduced to Ag+, approximates the potential at which Ag3+ was reduced to Ag+ in a solution without cyanide.

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The analysis to identify peaks B1’, B2’, D1’, D2’, D3’ and D4’ (Fig. 2) is similar to that described above, however, the increase or decrease of the current of these peaks with respect to their analogous, corresponds to changes in the actual surface of the electrode during the potential sweep. 3.2. Tellurium (Te) 3.2.1. CPE-Te behavior in a solution of pH 10.9 Figure 5 shows typical cyclic voltammograms for CPE-Te swept between -2000 and 2000 mV in a pH 10.9 solution. The black line represents the potential sweep when this was 12

ACCEPTED MANUSCRIPT initiated in the positive direction from the OCP, which was approximately -185 mV. It is observed the development of the oxidation peaks E1 and E2 at 135 and 1600 mV, respectively. According to Jayasekera et al. (1994), who performed an extensive study on tellurium, the first oxidation process at alkaline pH was due to the formation of the soluble species TeO32-. The formation of this species depends on the pH and the reaction that could describe it is Equation (13). The oxidation peak E2 was due to the oxidation of the

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electrolyte, since it coincides with peak E2’’, obtained with CPE-blank (dashed line). In the

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reverse sweep, the reduction peaks F1 and F2 were observed at -840 and 1450 mV,

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respectively. The peak F1 begins to develop at -350 mV, just like the peak F1’’, indicating that it was due to the reduction of the electrolyte. The peak F2 involves a series of

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reactions. According to the literature, the red-colored substance that was observed emerging from the electrode surface, is characteristic of the Te22- species (Panson, 1964;

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Jayasekera et al., 1994). While the potential continues sweeping to more negative potentials, it is probable that Te could be reduced to Te2-, indicating that the responsible

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reactions may be those of Equations (14), (15), (16) and (17). When the potential was reversed at -2000 mV, the oxidation peak E3 appeared, which was related with peak F2,

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indicating that the products generated in F2 were oxidized during E3 by the reverse of the

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reactions of Equations (14) and (16).

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Fig. 5. Typical cyclic voltammograms of the CPE-Te obtained in an unstirred aqueous

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solution at pH 10.9. The potential scan was initiated in the OCP (-185 mV vs. SCE) in: anodic direction (black line) and cathodic direction (grey line) at a scan rate of 20 mV/s.

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CPE-blank (dashed line)

The peaks E1’, E2’, F1’, F2’ and F3’ (Fig. 5, grey line) were developed due to the reactions

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described by Equations (13), (14), (15), (16) and (17).

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3.2.2. CPE-Te behavior in a 1250 mg/L CN- solution of pH 10.9 Figure 6 shows typical cyclic voltammograms for CPE-Te swept between -2000 and 2000

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mV in a 1250 mg/L CN- solution of pH 10.9, which shows a similarity to the cyclic voltammograms of Fig. 5, in terms of the number of redox reactions and their peak potentials at which they developed, indicating that in both solutions the same reactions took place. The current appeared on voltammograms for all reactions was larger in the solution containing cyanide than in the solution without cyanide, most probably because the ionic strength of this solution (e.g., the electrical conductivity) was greater, thus favoring the development of greater current during the redox reactions. Continuing with the comparison, the peak E1’ (Fig. 5, grey line) becomes two peaks (i.e., G1’ and G1’’, grey line) in Fig. 6;

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ACCEPTED MANUSCRIPT furthermore, in the same Figure, the peaks G2 (black line) and G2’ (grey line) were

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observed but they did not occur in the plain aqueous solution (Fig. 5).

Fig. 6. Typical cyclic voltammograms of CPE-Te obtained in an unstirred solution at pH

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10.9 and 1250 mg/L CN-. The potential scan was initiated in the OCP (-160 mV vs. SCE)

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in: anodic direction (black line) and cathodic direction (grey line) at a scan rate of 20 mV/s This comparative analysis revealed that peak G1 (see Fig. 6) developed according to Equation (13) and that peaks G3 and H1 are most probably due to redox reactions of the

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electrolyte. The process H2 and H2’ occurred according to Equations (14), (15), (16) and

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(17), while peaks G4 and G4’ are due to the reverse of Equations (14) and (16). When the sweep was initiated in the cathodic direction (Fig. 6, grey line) the Te was reduced to produce soluble species (see Equations (14), (15) and (16)). When the potential was reversed at -2000 mV, the species in solution in the vicinity of the electrode surface were oxidized, generating peaks G1’ and G1’’. Both peaks are due to the oxidation reaction of the process G1, however, two peaks were observed since elemental tellurium was found in two different media: (1) in the solid working electrode and (2) in the electrodeelectrolyte interface, precipitated during the process G4’. This nature of the elemental tellurium is explained by the proximity of these two oxidative processes. The potential at 15

ACCEPTED MANUSCRIPT which the peak G1 was observed (i.e., 240 mV) coincides with the potential at which the process G1’’occurred, corresponding to the oxidation of the tellurium in the solid working electrode, while the peak G1’ is due to the oxidation of the Te precipitated in the electrodeelectrolyte interface. Based on the potential-pH diagram presented by Murase et al. (1999), the process G2’ was due to the oxidation of TeO32- formed by G1’ and G1’’ according to

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the reaction of Equation (18).

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This reasoning also applies for the process G2 of Fig. 6. The potential difference between the peaks G2 and G2’ is caused by the different nature of the species that are being oxidized. G2 is due to H2TeO3 oxidation, in the solid working electrode, to form TeO42- and

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G2’ is due to H2TeO3 oxidation, in the electrode-electrolyte interface, to form TeO42-. This behavior suggests that the oxidation of the species that are in the electrode-electrolyte

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interface occurs easier (i.e., the peaks develop at less positive potentials) than the oxidation

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of the species in the working solid electrode. 3.3. Silver telluride (Ag2Te)

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3.3.1. CPE-Ag2Te behavior in a solution of pH 10.9

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Taking as a reference the typical cyclic voltammograms obtained for CPE-Ag and CPE-Te in an unstirred solution of pH 10.9 (Figs. 1 and 5), it was possible to identify the redox

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processes developed when using the CPE-Ag2Te under the same conditions (Fig. 7). The peak potentials at which the redox processes were observed are shown in Table 2. The

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reduction peaks were also identified by varying the inversion potential as in the previously discussed cases.

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ACCEPTED MANUSCRIPT Table 2 Comparison of the peak potentials at which oxidation and reduction for CPE-Ag, CPE-Te and CPE-Ag2Te in a solution of pH 10.9 were detected; data obtained from Figs. 1, 5 and 7 CPE-Ag

CPE-Te

CPE-Ag2Te

Oxidation processes

-

E1

135

-

-

-

-

-

-

J1

A1

350

-

-

A2

390

-

-

A3

780

-

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A4

1200

-

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E2

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Peak Potential, mV Peak Potential, mV Peak Potential, mV

581 -

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J2

770

-

J3

1260

1600

-

-

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K1

340

-

K2

-150

-

-

K3

-270

-

-

-

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F2

-1450

K4

-1000

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-

Reduction processes 571

-

C2

210

-

C3

-359

C4

-1400

-

-

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C1

As observed in Fig. 7, the positive-going sweep causes to the oxidation peaks J1, J2 and J3

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at 581, 770 and 1260 mV, respectively. When the potential was reversed at 2000 mV, the reduction peaks K1, K2, K3 and K4 occurred at 340, -150, -270 and -1000 mV, respectively. The oxidation peak J1 is most probably due to the oxidation of the species Te2- or Ag+ contained in the Ag2Te. Ag+ oxidation could occur before Te2- oxidation, leaving behind a Te-rich layer. However, the oxidation of Ag+ to Ag2+ (i.e., peak A3) occurs almost at the same potential at which the peak J2 was detected (see Table 2) via Equation (4), indicating that peak J1 is due to Te2- oxidation, according to Equation (19).

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Fig. 7. Typical cyclic voltammograms of CPE-Ag2Te obtained in an unstirred solution at

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pH 10.9. The potential scan was initiated in the OCP (-90 mV vs. SCE) in: anodic direction (black line) and cathodic direction (grey line) at a scan rate of 20 mV/s

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When the reaction described by Equation (19) took place, a grey precipitate was observed emerging from the electrode surface, in a similar manner to the one produced by the

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processes A1 and A2 (section 3.1.1). This behavior suggests that when the Te2- of the Ag2Te was oxidized, the accompanying silver was released as Ag+ (see Equation (19)),

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which reacted immediately to produce Ag2O, a solid species. The chemical reaction that

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was probably involved in this process is described by Equation (20).

When comparing the voltammograms presented in Figs. 1 and 7, and by analyzing Table 2, it is noted that peaks J3, K1, K2 and K3 were observed at similar potentials to those of peaks A4, C1 and C2 and C3. The peak K1 could be represented by the inverse of Equation (5), while process K2 is to the inverse of Equation (4) and process K3 is due to the inverse of Equations (3) and (1). The oxidation peak J3 is associated to Ag2O3 formation and electrolyte redox reactions.

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ACCEPTED MANUSCRIPT The reduction process K4 (see Fig. 7) occurred at approximately -1000 mV, associated with the formation of a reddish substance emerging from the electrode surface, as the one observed during the reduction of elemental Te (Fig. 5, peaks F2 and F2’). It is probable that the reduction peak K4 was due to the reactions described by Equations (14), (15), (16) and (17). It is worth to noting that the reddish substance was also observed when the potential sweep was initiated in the cathodic direction (Fig. 7, grey line, K4’), although none of the

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species formed by Equations (14), (15) and (16) were present. This may suggest that the

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reddish substance was caused by a different reaction, probably the one described by

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Equation (21).

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When the silver of the Ag2Te was reduced, the Te2- was released to the solution and this species is unstable in presence of oxygen (Panson, 1964). According to this, the Te2- is

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oxidized by the dissolved oxygen producing the species Te22-. This hypothesis was corroborated by bubbling pure nitrogen into the electrolyte to purge the oxygen, thus

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avoiding the formation of Te22- and consequently the appearance of the reddish substance

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through the process K4’ while sweeping the potential. Thus, additional to the reaction

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described by Equation (21), the reaction described by Equation (22) also occurred.

According to the above, it is probable that through the development of the process K4, the

(16) and (17).

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reaction described by the Equation (21) also occur, additional to the Equations (14), (15),

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3.3.2. CPE-Ag2Te behavior in a 1250 mg/L CN- solution of pH 10.9 Figure 8a shows typical cyclic voltammograms for CPE-Ag2Te swept between -2000 and 2000 mV in a pH 10.9 and 1250 mg/L CN- solution. When the potential sweep was initiated in the anodic direction from the OCP (i.e., -390 mV, black line), the anodic peaks L1, L2 and L3 were observed at 233, 910 and 1850 mV, respectively. During the subsequent reverse sweep, the cathodic peaks M1, M2, M3 and M4 were observed at about -320, -600, -1260 and -1850 mV, respectively. As mentioned in section 3.3.1, the process L1 was due to the oxidation of the Te2- of the crystal lattice of the Ag2Te (see Equation 19

ACCEPTED MANUSCRIPT (19)), except that in the presence of cyanide the Ag+ was immediately complexed to form Ag(CN)2- and AgCN(s); therefore, no precipitate indicating the formation of oxides or hydroxides was observed. Furthermore, the maximum current of peak L1 was obtained at the same potential of the maximum current obtained for peak G1 (i.e., 233 mV). Interestingly, in most voltammograms there is a flat line from the OCP to the first oxidation process, but not in the one of Figure 8a; there was a reaction occurring at negative

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potentials. Therefore, some oxidation reaction occurred between -350 and -150 mV, as can

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be observed in Figure 8b, the first of which has a maximum between -100 and -200 mV,

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these multiple peaks could include the process of Te2- oxidation to form elemental Te releasing the silver contained in Ag2Te, which is complexed by the free cyanide. It is

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important to mention, that the dissolution mechanism described above, would correspond to an oxidative dissolution process, although the cyanidation of Ag2Te under standard

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conditions could behave differently; since there is no evidence showing that cyanide impacts the oxidation potential of tellurium, but it does for silver, meaning that silver

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complexation would occur leaving bound Te unreacted at high concentration on the Ag2Te

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surface, thus forming a layer of a tellurium-enriched silver telluride species.

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Fig. 8. Typical voltammograms of CPE-Ag2Te obtained in an unstirred solution at pH 10.9 and 1250 mg/L CN-. The potential scan was initiated in the OCP (-390 mV vs. SCE) in: (a) anodic direction (black line) and cathodic direction (grey line) at a scan rate of 20 mV/s; and (b) as a function of Log I for the process L1, in anodic direction. The oxidation of Te2- to TeO32- is probably occurring via the formation of elemental tellurium, which was confirmed by analyzing the products obtained on the surface of the 21

ACCEPTED MANUSCRIPT electrode subjected to a chronoamperometry at a specific potential during 600 s, in order to generate enough reaction product to be observed by SEM-EDS. A sample of the unreacted CPE-Ag2Te surface was also analyzed for comparison purposes. By comparing the micrographs of Fig. 9, it is noted that when a potential of 300 mV was applied, the CPE-Ag2Te surface shows reaction zones caused by anodic dissolution of the

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Ag2Te. EDS analysis of the unreacted CPE-Ag2Te revealed an Ag and Te composition of

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62.4 and 37.2 wt-%, respectively; while the EDS analysis of a CPE-Ag2Te subjected to a 300 mV potential for 600 s revealed an Ag and Te content of 56.9 and 42.4 wt-%,

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respectively. Furthermore, a EDS point analysis on the reaction zones revealed a composition with much less Ag (i.e., 16.2 wt-% Ag and 80.2 wt-% Te). This sharp decrease

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in Ag content reveals that the Ag was dissolved, while Te remained on the surface (corroborating that the dissolution of silver occurred faster than that of tellurium). This

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behavior may be explained by the formation of elemental Te, which accumulates on the Ag2Te surface, while the silver is dissolved. The peak L1 and the presence of elemental

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tellurium suggest that the oxidation of Te2- was carried out in at least two consecutive steps represented by Equations (23) and (24). Silver dissolution occurred as Te2- was oxidized,

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suggesting that the diffusion of cyanide to complex Ag2+ occurred fast, while the further oxidation of Te was slow. It is likely that the silver has been leached leaving bound

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unreacted tellurium, surely increasing its concentration in the Ag2Te surface.

Step 2 (slow reaction):

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Step 1 (fast reaction):

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Fig. 9. SEM micrographs of the CPE-Ag2Te surfaces (a) unreacted surface and (b) after applying a potential of 300 mV vs. SCE during 600 s in a solution at pH 10.9 and 1250 mg/L CN-

The sum of both reactions (Equations (23) and (24)) results in a reaction similar to that of Equation (19). It is well known that the presence of cyanide enhances the dissolution of Ag2Te (see Equation (23)), which is observed by comparing the peak potentials at which the processes L1 (see Equations (23) and (24)) and J1 (see Equation (19)) occurred (233 and 581 mV, respectively). The reactions represented by Equations (23) and (24) suggest 23

ACCEPTED MANUSCRIPT the formation and accumulation of elemental Te on the surface of the Ag2Te, which may passivate the anodic dissolution of the Ag2Te. SEM-EDS analysis of the CPE-Ag2Te surface subjected to 1000 mV during 600 s (Fig. 10) shows that the reaction zones due to the anodic dissolution increased both in number and size, with respect to those shown in Fig. 9b, and revealed an Ag and Te content of 6.68 and

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88.40 wt-%, respectively. It is worth noting that oxygen content was of 0.45 wt-%. This

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suggests that besides the reaction described by Equation (18), the reactions described by Equations (22) and (23) are still occurring at 1000 mV. The anodic peak L3 is assigned to

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the water oxidation. On the reverse sweep, the reduction peaks M1, M2, M3 and M4 were observed at -320, -600, -1260 and -1850 mV, respectively. The reduction peaks M1 and M2

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were due to the reduction of the silver cyanide complexes (e.g., Ag(CN)2- and AgCN(s)) via

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the reactions described by Equations (8) and (11), respectively.

Fig. 10. SEM micrograph of the Ag2Te surface after applying a potential of 1000 mV vs. SCE during 600 s in a solution at pH 10.9 and 1250 mg/L CNIn order to identify the products of the reduction peak M3, a SEM-EDS analysis of the CPE-Ag2Te subjected to -1400 mV for 600 s was carried out (Fig. 11). The analysis revealed a Ag and Te content of 88.22 and 5.83 wt-%, respectively. These results suggest 24

ACCEPTED MANUSCRIPT that the peak is due to the reduction of Ag2Te via Equation (21). When the potential scan was inverted at -2000 mV, two oxidation peaks occurred (i.e., L4 and L5). Peak L4 was associated with the reduction peak M3, most probably according to the inverse of Equations

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(14), (15) and (16), while peak L5 occurred according to Equation (6).

Fig. 11. SEM micrograph of the Ag2Te surface after applying a potential of -1400 mV vs.

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SCE during 600 s in a solution at pH 10.9 and 1250 mg/L CNIn the negative-going transit of the potential sweep, the peaks L1’, L2’, M1’, M3’, M4’,

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L4’ and L5’ were observed, which occurred according to the same reactions for L1, L2, M1, M3, M4, L4 and L5. It is worth mentioning that the differences of the current of the

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peaks during the potential scan are due to changes in the actual surface of the CPE’s-Ag2Te caused by the redox reactions. It is also worth noting that the oxidation peak L3 (i.e., water oxidation) was not detected when the potential sweep was initiated in the cathodic direction, which may be due to the formation, development and accumulation of elemental Te on the surface of the Ag2Te. The discussion of the CPE-Ag2Te behavior in a 1250 mg/L CN- solution of pH 10.9 assumes that Ag2Te is an intermetallic compound, but Dyer et al. (2017) suggested that precious metal tellurides behave electrochemically like if its components were in metallic 25

ACCEPTED MANUSCRIPT state. This behavior could be to the fact that intermetallic compounds appear as an intermediate phase that exist over a very narrow range of compositions. The free energy of formation values (i.e., ΔG°f) of the species formed by the oxidative processes of CPE-Ag, CPE-Ag and CPE-Ag2Te are shown in Table 3. This thermodynamic data complements the comparison of the peak potential at which oxidation processes occur

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(see Table 2) and simplify the interpretation of the reactions concerned.

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Table 3 Free energy of formation values (i.e., ΔG°f) (Garrels and Christ, 1990) of the solution at pH 10.9, and 1250 mg/L CN- at pH 10.9

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species formed by the oxidation processes of CPE-Ag, CPE-Ag and CPE-Ag2Te in aqueous

Formula ΔG°f, kcal

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Process

AgOH

-21.98

A2

Ag2O

-2.58

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A1

20.8

Ag(CN)2-

72.05

E1, G1, J1, L1

TeO32-

-93.79

G2, L2

TeO42-

-109.1

A3, J2

Ag2O3

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B1

2.6

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A4, J3

AgO

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4. Conclusions

The voltammetric characterization of Ag2Te in solutions of pH 10.9 in presence and

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absence of 1250 mg/L CN- has shown the following: (1) When the potential was cycled between -2000 and 2000 mV in a solution of pH 10.9, the oxidation reaction products were TeO32- via the oxidation of Te2- (at 581 mV), AgO via the oxidation of Ag+ (at 770 mV) and Ag2O3 via the oxidation of AgO (at 1260 mV). The reduction reaction products are AgO via the reduction of Ag2O3 (at 340 mV), Ag2O via the reduction of AgO (at -150 mV), Ag via the reduction of Ag2O and AgOH (at -270 mV), Te22- via the reduction of Te (-1000 mV) and Te2- via the subsequent reduction of Te22-. When the potential sweep was initiated in the cathodic direction the Te2- was produced via the reduction of Ag2Te. 26

ACCEPTED MANUSCRIPT (2) When the potential was cycled between -2000 and 2000 mV in a solution of pH 10.9 containing 1250 mg/L CN-, the oxidation products were Te via a fast oxidation of Ag2Te and TeO32- via a slow oxidation of Te. These reactions still occurred up to 1000 mV. The reduction products were Ag via the reduction of AgCN(s), Ag(CN)2- and Ag2Te. Due to the reduction of the Ag2Te to form Ag and Te2-, and when the potential was inversed at -2000 mV to complete the cycle, the products Te22- via the oxidation of Te2-, Te via the

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subsequent oxidation of T22- and Ag+ via the oxidation of Ag were generated. (3) The initial oxidation of the Ag2Te occurs at approximately 233 mV. This potential is far

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beyond the natural dissolution potentials occurring in standard cyanidation systems and therefore, the Ag2Te dissolution will not occur or will proceed at a very low rate.

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In summary, the oxidation of Ag2Te in a cyanide alkaline solution (i.e., 1250 mg/L CN- and pH 10.9) was carried out in at least two consecutive steps represented by Equations (23)

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and (24), the first being a fast reaction and the second a slow reaction. Furthermore, when Ag+ was released due to the oxidation of Ag2Te, no silver oxides or hydroxides were

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generated (as in the plain aqueous solution of pH 10.9) since Ag+ was immediately complexed by the cyanide. The reduction of Ag2Te produced elemental silver and released

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Te2-, according Equation (21).

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Acknowledgements

The authors are grateful to Consejo Nacional de Ciencia y Tecnología (CONACYT)

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(Mexico) for the postgraduate scholarship awarded to González-Ibarra, and for the research

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project CB2015/257115. References

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ACCEPTED MANUSCRIPT Bas, A.D., Ghali, E., Choi, Y., 2017. A review on electrochemical dissolution and passivation of gold during cyanidation in presence of sulphides and oxides. Hydrometallurgy 172, 30-44. https://doi.org/10.1016/j.hydromet.2017.06.021 Deschênes, G., Rajala, J., Pratt, A.R., Guo, H., Fulton, M., Mortazavi, S., 2011. Advances in the cyanidation of silver. Miner. Metall. Proc. 28 (1), 37-43.

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Dirkse, T.P., 1990. A potentiostatic study of the electrolytic formation of AgO. Electrochim. Acta 35 (9), 1445-1449. https://doi.org/10.1016/0013-4686(90)85019-J

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Nava-Alonso, F., González-Ibarra, A.A., Pérez-García, E., Castillo-Ventureño, E.N., UribeSalas, A., Fuentes-Aceituno, J.C., 2017. Leaching alternatives to recover gold and silver from tellurides. In: Annual Conference of Metalurgist, COM 2107, hosting World Gold (7th

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ACCEPTED MANUSCRIPT Zhang, J., Zhang, Y., Richmond, W., Wang, H., 2010. Processing technologies for goldtelluride ores. Int. J. Min. Met. Mater. 17 (1), 1-10. https://doi.org/10.1007/s12613-0100101-6 Zheng, B., Wong, L.P., Wu, L.Y.L., Chen, Z., 2016. Cyclic voltammetric study of high speed silver electrodeposition and dissolution in low cyanide solutions. Int. J. Electrochem.

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2016, 1-11. http://dx.doi.org/10.1155/2016/4318178

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ACCEPTED MANUSCRIPT Highlights Anodic and cathodic potential dependent-reactions of Ag2Te were investigated.

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Carbon paste electrodes in alkaline cyanide solutions were used.

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The anodic oxidation of Ag2Te occurred in at least two consecutive steps.

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The cathodic reduction of Ag2Te produced elemental silver and released Te2-.

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SEM-EDS results support electrochemical results.

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