2019 Volume 60 Issue 2 Pages 277-286
Pyrite is a common gangue mineral in mine wastes, and its oxidation is the primary cause of acid mine drainage (AMD) formation, which is a very serious environmental problem encountered worldwide. To address this problem, we developed a new technique to suppress pyrite oxidation called galvanic microencapsulation (GME). Galvanic interaction occurs when two conductive or semi-conductive materials with different rest potentials interact with one another. The material with a lower rest potential becomes the anode and is oxidized while the other one with the higher rest potential becomes the cathode and is galvanically protected. In this study, the effects on pyrite oxidation of zero-valent aluminum (ZVAl) or zero-valent iron (ZVI) dosages, leaching time, and pH were elucidated. In addition, the suppression mechanisms involved during GME were investigated by electrochemical measurements and surface-sensitive characterization techniques. The results showed that pyrite oxidation was suppressed in the presence of ZVAl or ZVI. With time, galvanic interaction between pyrite and ZVAl in the first 3 days was negligible, which could be attributed to the Al-oxyhydroxide coating on ZVAl. After 7 days, however, ZVAl exhibited substantial suppressive effects on pyrite oxidation. In comparison, the suppressive effects of ZVI on pyrite oxidation were observed after just 1 day. Cyclic voltammetry and chronoamperometry measurements showed that the suppressive effects of ZVAl and ZVI were predominantly due to galvanic interactions.
Although ZVAl and ZVI could limit pyrite oxidation, their suppressive effects were only temporary because the surface of pyrite was not passivated by an unreactive coating. To induce coating formation and prolong the suppression of pyrite oxidation, phosphate was added together with ZVI. Only ZVI was selected for these experiments because of the potential formation of iron phosphate, a very stable material even under acidic conditions. In the presence of phosphate, suppression of pyrite oxidation by ZVI was dramatically improved because of the combined effects of galvanic interactions and coating formation.
A large amount of waste is generated from coal and metal mining activities that contains sulfide minerals such as pyrite (FeS2) and arsenopyrite (FeAsS).1) More recently, pyrite-rich debris are also excavated during underground and tunnel construction projects in exceptionally large amounts.2–11) When sulfide minerals like pyrite are exposed to the environment, acid mine drainage (AMD) is usually generated. AMD is a very acidic leachate or effluent that contains high concentrations of environmentally regulated heavy metals such as copper (Cu) and zinc (Zn) as well as toxic contaminants like arsenic (As), lead (Pb), and selenium (Se).12–22) Equation (1) shows the initial oxidative dissolution of pyrite by dissolved oxygen (DO) and as this process continues, more ferrous ions (Fe2+) are generated while the pH becomes more acidic. Equation (2) shows the oxidation reaction of Fe2+ with DO to form ferric ions (Fe3+), a strong oxidant that could enhance the oxidization of pyrite and promote AMD formation (eq. (3)).
| \begin{align} &\text{FeS$_{2(\text{s})}$} + \text{3.5 O$_{2(\text{aq})}$} + \text{H$_{2}$O} \\ &\quad = \text{Fe$^{2+}{}_{(\text{aq})}$} + \text{2 SO$_{4}{}^{2-}{}_{(\text{aq})}$} + \text{2 H$^{+}{}_{(\text{aq})}$} \end{align} | (1) |
| \begin{equation} \text{Fe$^{2+}{}_{(\text{aq})}$} + \text{0.25 O$_{2(\text{aq})}$} + \text{H$^{+}{}_{(\text{aq})}$} = \text{Fe$^{3+}{}_{(\text{aq})}$} + \text{0.5 H$_{2}$O} \end{equation} | (2) |
| \begin{align} &\text{FeS$_{2(\text{s})}$} + \text{14 Fe$^{3+}{}_{(\text{aq})}$} + \text{8 H$_{2}$O} \\ &\quad = \text{15 Fe$^{2+}{}_{(\text{aq})}$} + \text{2 SO$_{4}{}^{2-}{}_{(\text{aq})}$} + \text{16 H$^{+}{}_{(\text{aq})}$} \end{align} | (3) |
The most widely used technique to mitigate the negative environmental impacts of AMD is chemical neutralization.23) In this technique, basic materials such as limestone are added to AMD to increase its pH and precipitate most of the heavy metals as metal oxyhydroxides.24,25) Although this technique is effective, AMD generation could continue for several decades or even centuries, so this approach is unsustainable. Microencapsulation is a promising and potentially more sustainable approach because it limits AMD generation by suppressing pyrite oxidation directly through the formation of a passivating coating on the mineral. The original microencapsulation techniques introduced by Evangelou14) used hydrogen peroxide (H2O2) to oxidize Fe2+ to Fe3+ to coat pyrite with insoluble ferric phosphate. Although this technique effectively suppressed pyrite oxidation, H2O2 is non-selective and could not specifically target pyrite in real, complex wastes leading to unnecessarily large consumption of expensive reagents. Moreover, handling and storage of H2O2 are both difficult especially in large scale applications. Carrier-microencapsulation (CME), a more recent microencapsulation technique developed by Satur and coworkers,26) uses redox-sensitive metal(loid)-organic complexes to carry the coating material to the surface of pyrite where the complexes are adsorbed and decomposed, releasing and rapidly precipitating the insoluble metal(loid) ion of the complex to form a protective coating on pyrite. Because pyrite dissolves via an electrochemical mechanism, the redox-sensitive metal(loid)-organic complexes have been shown to selectively target pyrite and arsenopyrite even in a complex system containing silicate minerals like quartz.26–29) One current limitation of this technique is the use of catechol, which when oxidized in the presence of DO and metal ions like Fe3+ and Cu2+ forms semiquinone radicals, superoxides, and H2O2 that are toxic to cells.30)
Another way to selectively target pyrite in a complex system is via galvanic interaction, a phenomenon that occurs when two semi-conductive or conductive materials having different rest potentials come in close contact with each other. In this process, the material having a lower rest potential becomes the anode and is dissolved while the other material with higher rest potential becomes the cathode and is galvanically protected.10,14,31–36) Using this concept, we developed a new technique to suppress pyrite oxidation called galvanic microencapsulation (GME). In this technique, there are two reactions that would potentially suppress pyrite oxidation: (1) sacrificial effect of the anode, and (2) coating formation using the oxidation products from the anode. GME, similar to CME, would be selective because pyrite oxidation is an electrochemical process and GME involves electron transfer between anodic and cathodic sites. Zero-valent aluminum (ZVAl) and zero-valent iron (ZVI) are potentially good candidates as anodes for this technique because both metals have lower redox potentials (ZVAl = −1.67 V and ZVI = −0.44 V) than pyrite (+0.2 to +0.3 V). Moreover, ZVAl and ZVI are non-toxic and these materials often end up as wastes, so they are easily obtainable. In this study, galvanic interactions of pyrite with ZVAl or ZVI under various conditions were investigated. Specifically, this study aims to: (1) elucidate the effects on pyrite oxidation of ZVAl or ZVI dosage, leaching time, and pH, (2) understand the suppression mechanisms during GME, and (3) improve the coating formation and coverage using phosphate. These objectives were achieved by conducting batch leaching experiments, geochemical modeling, and electrochemical studies like cyclic voltammetry (CV) and chronoamperometry. The surface characteristics of ZVAl and ZVI samples as well as the leaching residues were also analyzed by scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDX) and attenuated total reflectance Fourier transform spectroscopy (ATR-FTIR) to understand not only the nature of the coating formed on pyrite but also the changes in pyrite oxidation dynamics in the presence of ZVAl and ZVI.
The pyrite sample used in this study was obtained from Huanzala Mine, Huanuco, Peru. The X-ray powder diffraction (XRD) pattern shows that it is mainly composed of pyrite and contains Fe (44.6% mass%) and S (54.3%) as the main components with only trace impurities like calcium (0.36%), silicon (0.32%), and aluminum (0.15%) (Figs. 1(a) and (b)). Pyrite was crushed by a jaw crusher, ground with a ball mill, and sieved to obtain a size fraction of 500–710 µm. Ultra-pure ZVAl and ZVI (99.9%) used in this study were obtained from Wako Pure Chemical Industries Ltd., Japan and Figs. 1(c) and (d) show their particle size distributions, which were measured by LASER diffraction (Microtrac® MT3300SX, Nikkiso Co. Ltd., Japan). The D50 of ZVAl and ZVI were similar at around 25 µm. The zeta potential distributions of pyrite (in deionized (DI) water) and ZVI/ZVAl (in 0.01 M NaCl) were measured from pH 2–11 using Zetasizer Nano-series with MPT-2 multi-purpose titration system (Malvern Corporation, UK) (Figs. 1(e)–(g)).
(a) Chemical composition of the pyrite sample, (b) XRD pattern of the pyrite sample, (c) particle size distribution of ZVAl, (d) particle size distribution of ZVI, (e) zeta potential distribution of pyrite with pH under oxic (with O2) condition, and (f) & (g) zeta potential distribution of ZVI and ZVAl with pH under oxic (with O2) condition, respectively.
Before the leaching experiments, the pyrite sample (500–700 µm) was washed using the method described by McKibben and Barnes37) to remove fine particles and any oxidized layer formed during sample preparation and storage. Leaching experiments were conducted by mixing 1 g of washed pyrite, 10 mL of DI water or phosphate solution, and predetermined amounts of ZVAl and ZVI in an Erlenmeyer flask. For the experiments with various metal powder dosages and pH, the shaking time was fixed for 3 days while in the leaching experiments with time, the dosage of ZVAl or ZVI was fixed at 0.1 g without any pH adjustments. The flasks were shaken (shaking amplitude and rate of 40 mm and 120 strokes/min, respectively) in a constant temperature water bath shaker (25°C). After the predetermined leaching time, the flasks were removed from the shaker, the pH and redox potential (Eh) of suspensions were measured, and the leachates were collected by filtration through 0.2 µm syringe-driven membrane filters (Sartorius AG, Germany). The concentrations of total S as SO42− (dissolved S), dissolved Fe, and dissolved Al in the filtrates were analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES, ICPE-9820, Shimadzu Corporation, Japan) (margin of error ±2%). Meanwhile, the residues were washed thoroughly with DI water and dried in a vacuum drying oven at 40°C for 24 h. The residues were analyzed using a high-magnification 3D digital microscope (VHX-1000, Keyence Corporation, Japan), SEM-EDX (Superscan SSX-550, Shimadzu Corporation, Japan), and ATR-FTIR (FT/IR-6200HFV and ATR Pro One attachment equipped with a diamond prism, Jasco Analytical Instruments, Japan). Some of the leaching experiments were done in triplicates to ascertain that the observed trends were statistically significant.
2.3 Electrode preparation and electrochemical measurementsThe pyrite electrode was prepared by cutting a small cuboid from a large pyrite crystal, placing it in the center of PVC rings, connecting the crystal to Cu wires using silver conducting paste, and fixing it in place with a non-conductive resin (Technovit®, Heraeus Kulzer GmbH, Germany). The pyrite electrode was exposed using Si-carbide papers of decreasing grit size (#400 → #600 → #800 → #1500 → #2000) and then polished with 5 and 1 µm alumina paste on a smooth glass plate. The pyrite electrode was then ultrasonically cleaned for 5 min to remove any residual alumina particles and washed thoroughly with DI water.
For the electrochemical measurements, a conventional three-electrode system attached to an electrochemical measurement unit (SI 1280B, Solartron Instruments, UK) was used. For the electrochemical setup, pyrite was the working electrode, platinum was the counter electrode and Ag/AgCl in saturated KCl solution was the reference electrode. In the electrochemical measurements, pyrite electrode with and without ZVAl or ZVI were measured by CV and chronoamperometry. The pyrite electrode with ZVAl or ZVI was prepared as follows: (1) ZVAl or ZVI was ultrasonically suspended in acetone, and (2) the suspension was put on the surface of the polished pyrite electrode. After the evaporation of acetone, pyrite with ZVAl or ZVI was used as the working electrode.
All CV measurements were carried out in 0.1 M Na2SO4 solution as supporting electrolyte under a nitrogen (N2) atmosphere at 25°C. The CV measurements started after equilibration at the open circuit potential (OCP) and the sweep direction was towards more positive potentials first (i.e., anodic direction) at a scan rate of 30 mV/s. After reaching +0.8 V vs. Ag/AgCl, the sweep direction was reversed until −0.8 V vs. Ag/AgCl, which was then followed by another sweep direction reversal towards more positive potential until the OCP. This constitutes one cycle and for each CV measurement, a total of 5 cycles were recorded.
Chronoamperometry is an electrochemical technique wherein a fixed potential is applied to the working electrode and changes in the current density are recorded with time. By applying a fixed potential to the pyrite electrode, the anodic and cathodic half-cell reactions during pyrite oxidation could be decoupled and elucidated independently for a prolonged period of time. The chronoamperometry measurements were conducted using the same setup as the CV measurements but with magnetic stirring at 250 rpm. For the anodic polarization measurements, the pyrite electrode was first equilibrated at the OCP, and then anodically polarized at +0.6 V vs. Ag/AgCl for 4.5 hours under N2 atmosphere. Similarly, the cathodic polarization experiments were conducted using a similar setup as the anodic polarization but at an applied potential of −0.2 V vs. Ag/AgCl without N2 purging and was open to the atmosphere. The applied potentials for the cathodic and anodic polarization experiment were selected based on the previous studies of Kelsall et al.38) and Tabelin et al.39,40)
Figure 2 shows the final pH and dissolved S, Fe, and Al concentrations of the leachates after 3 days of leaching with various amounts of ZVAl or ZVI in DI water. Figure 2(a) illustrates that dissolved S concentration decreased as the dosage of ZVAl or ZVI increased, which suggest that both ZVAl and ZVI suppressed the oxidation of pyrite. The initial pH of the control (i.e. the pH measured immediately after mixing pyrite and DI water but before the shaking) was around 5.6 but decreased to pH 4 after 3 days of leaching due to pyrite oxidation (eqs. (1)–(3)).27,39) In the presence of ZVI, higher dosage caused the pH to increase coincident with lower dissolved Fe concentration due to the enhanced precipitation of dissolved Fe at high pH values. The increase in pH in the presence of ZVI could be attributed to the reaction of ZVI with water and DO producing OH−, which leads to the increase in pH (eq. (4)).
| \begin{equation} \text{2Fe$^{0}{}_{(\text{s})}$} + \text{O$_{2}$} + \text{2 H$_{2}$O} = \text{2Fe$^{2+}$} + \text{4 OH$^{-}$} \end{equation} | (4) |
Leaching of pyrite with various dosages of ZVAl or ZVI: (a) pH of the solution after leaching, (b) dissolved Fe concentration after leaching (c) dissolved S concentration after leaching, and (d) dissolved Al concentration after leaching.
Previous studies have shown that higher pH values enhanced pyrite oxidation,41–43) but when ZVI was added into the solution, the dissolved S concentration was low even at higher pH values. There are two possible explanation for this observed discrepancy: (1) oxidation of ZVI consumed DO and the low DO concentration limited pyrite oxidation, and (2) galvanic interaction between pyrite and ZVI suppressed pyrite oxidation.
The consumption of DO by ZVI was determined by directly measuring the decrease of DO concentration with a DO meter and the results showed that 1 g of ZVI consumed only around 20% of DO (eq. (4); Fig. 3). This implies that the slightly lower DO concentration was not the main cause of the suppression of pyrite oxidation. The second possibility will be explained in more detail in the next subsection using electrochemical techniques. Higher dosage of ZVAl also resulted in the decrease of dissolved S and Fe concentrations as illustrated in Figs. 2(a) and 2(c) while dissolved Al was detected due to the dissolution of ZVAl (eq. (5)). In contrast to ZVI, ZVAl addition had negligible effects on the pH. There are two possible explanations for the lower dissolved S and Fe concentrations in the presence of ZVAl. First, dissolved S and Fe could be co-precipitated with dissolved Al via the formation of hydrobasaluminite (Al4(SO4)(OH)10·nH2O) and schwertmannite (Fe8O8(SO4)y(OH)x). According to Sánchez-España et al.,44,45) these two phases could be precipitated at around pH 4, sequestering dissolved Al, Fe, and S from solution. Based on our thermodynamic calculations, saturation indices of these two precursor minerals were positive, which suggest that their precipitation during the experiments were thermodynamically favorable (Table 1). Second, the lower dissolved S and Fe concentrations could be attributed to the suppression of pyrite oxidation via galvanic interaction and will be explained in more detail in the next subsection. Although an increase in pH should occur in the presence ZVAl as explained by eq. (5), Fig. 2(b) illustrates that there was insignificant change in pH after the leaching experiments even at very high ZVAl dosages.
Dissolved O2 with various ZVI dosages.
This almost constant pH at high ZVAl dosages could be explained by (eqs. (6)–(7)). Although, the oxidation of ZVAl in water releases OH− (eq. (5)), this alkalinity it is neutralized by the H+ released by the precipitation of hydrobasaluminite and schwertmannite (eq. (6)), which balances the pH of the solution. Comparing the two metals, dissolved S concentration with ZVI was lower than that with ZVAl.
| \begin{equation} \text{2Al$^{0}$} + \text{1.5O$_{2}$} + \text{3H$_{2}$O} = \text{2Al$^{3+}$} + \text{6OH$^{-}$} \end{equation} | (5) |
| \begin{align} &\text{4Al$^{3+}$} + \text{SO$_{4}{}^{2-}$} + \text{14H$_{2}$O} \\ &\quad = \text{Al$_{4}$(SO$_{4}$)(OH)$_{10}{\cdot}$4H$_{2}$O} + \text{10H$^{+}$} \end{align} | (6) |
| \begin{align} &\text{8Fe$^{3+}$} + \text{ySO$_{4}{}^{2-}$} + \text{($24-2\mathrm{y}+\mathrm{x}$)/2H$_{2}$O} \\ &\quad = \text{Fe$_{8}$O$_{8}$(SO$_{4}$)$_{\text{y}}$(OH)$_{\text{x}}$} + \text{($24-2\mathrm{y}$)H$^{+}$} \end{align} | (7) |
Figures 4(a)–(c) show the changes of dissolved S, pH, and dissolved Fe with time for up to 21 days in the presence of ZVAl or ZVI. When ZVAl was added, the concentration of dissolved S slightly decreased compared with the control but was statistically insignificant especially in the first 3 days. In comparison, dissolved S concentration in the presence of ZVI was relatively lower than the control during the same period, which could be attributed to the suppression of pyrite oxidation via galvanic interactions. There are two possible explanations for the better suppressive effects of ZVI compared with ZVAl: (1) ZVI is positively charged while pyrite is negatively charged at pH values less than 7.5 (Figs. 1(e) and (f)), which makes it easier for them to come in contact via electrostatic attraction, and (2) at higher pH, dissolved Fe generated from the ZVI reaction with oxygenated water was precipitated as iron-oxyhydroxide/oxide on pyrite, which inhibited pyrite oxidation. After 7 days, however, the suppressive effects of ZVI on pyrite oxidation disappeared as illustrated by the similar dissolved S concentrations of the control and that with ZVI.
Leaching result of pyrite in the presence of ZVAl or ZVI with time and the equilibrium log a-pH diagram of Al3+ and Fe3+: (a) pH change with time, (b) dissolved S concentration change with time, (c) dissolved Fe concentration change with time, (d) dissolved Al concentration change with time, (e) log a-pH predominance diagram of Fe3+ at 25°C, 1.013 bars, and activity of SO42− = 10−3, and (f) log a-pH predominance diagram of Al3+ at 25°C, 1.013 bars, and activity of SO42− = 10−3.
The pH was buffered at around 7 for 7 days in the presence of ZVI but in the control, the pH decreased from 5.8 to 3.5. The higher pH in the case with ZVI likely enhanced the precipitation of dissolved Fe via two mechanisms: (1) direct precipitation of Fe2+ to Fe(OH)2 (eq. (8)), and (2) Fe2+ was oxidized to Fe3+ and then precipitated as Fe(OH)3 (eq. (9)). With time, the Fe2+-precipitates formed via mechanism (1) could also be oxidized by oxygen to Fe(OH)3 based on eq. (10) and by catalysis reaction with ZVI.46,47) In comparison to the experiments with ZVI, dissolved Fe concentrations in the control were higher because at around pH 3.5, direct precipitation of Fe2+ and the oxidation of Fe2+ to Fe3+ by DO become negligible and sluggish, respectively.48–50)
| \begin{equation} \text{Fe$^{2+}$} + \text{2 H$_{2}$O} = \text{Fe(OH)$_{2}$}+ \text{2 H$^{+}$} \end{equation} | (8) |
| \begin{equation} \text{Fe$^{3+}$} + \text{3 H$_{2}$O} = \text{Fe(OH)$_{3}$} + \text{3 H$^{+}$} \end{equation} | (9) |
| \begin{equation} \text{2Fe(OH)$_{2}$} + \text{0.5 O$_{2}$} + \text{2 H$_{2}$O} = \text{2Fe(OH)$_{3}$} + \text{H$_{2}$O} \end{equation} | (10) |
Although dissolved Fe was precipitated in the case with ZVI, SEM-EDX observations of leaching residues after 7 days show that the pyrite surface was not covered by Fe-oxyhydroxide precipitates (Fig. 5). This means that dissolved Fe was likely precipitated only in the solution and did not attach to pyrite because based on the zeta potential measurements, oxidized pyrite at pH 4–6 is positively charged (Fig. 1(e)) similar to Fe-oxyhydroxides/oxides.39) These results suggest that under the conditions of our experiments, attachment of precipitated Fe-oxyhydroxides onto pyrite was unfavorable, so coating formation was negligible.
SEM-EDX of pyrite after leaching with ZVAl or ZVI: (a) leaching with ZVAl for 3 days, and (b) leaching with ZVI for 3 days.
In contrast, the suppressive effects of ZVAl on pyrite oxidation in the first 3 days were negligible. After 7 days, however, the concentration of dissolved S became lower than the control. When ZVAl is present with pyrite, galvanic interactions should occur as shown in eq. (5) that would suppress pyrite oxidation. The ATR-FTIR spectrum of ZVAl shows the stretching vibration of Al = O (475 and 565 cm−1), bending vibration of Al = O (769 cm−1), angle bending of Al = O (976 cm−1), and bending vibration of O–H (1145, 1340, and 3100 cm−1) (Fig. 6), which are characteristic IR bands of AlO(OH)·xH2O.51) These results suggest that ZVAl is covered by a thin oxyhydroxide film, so galvanic interactions were limited. After 3 days, the pH of suspension became slightly acidic (pH < 4) that enhanced the dissolution of the oxyhydroxide film on ZVAl (Fig. 4(e)), which promoted galvanic interactions and suppressed pyrite oxidation. Moreover, the less positive surface charge of ZVAl at this pH range likely improved its electrostatic interaction with pyrite (Fig. 1(e) and (f)). The results showed that ZVAl could continue to suppress pyrite oxidation (up to 21 days) if it is not passivated by Al-oxyhydroxides (e.g., under acidic conditions).
ATR-FTIR spectrum of ZVAl used in the experiment.
As described previously, pH is an important parameter during pyrite oxidation, but because the presence of ZVI strongly affected this parameter, the effects of ZVI on pyrite oxidation remain unclear. In this subsection, the effects of initial pH were investigated using DI (pH 5.8), 0.01 M HCl (pH 2), and 0.001 M NaOH (pH 11). Figure 7 shows the changes in pH, dissolved S, and dissolved Fe with time. In the presence of ZVI, the pH in the three solutions increased as follows: from 5.8 to 7 in DI water, from 2 to 6.2 in 0.01 M HCl, and from 9 to 10 in 0.001 M NaOH. This initial increase in pH could be attributed to the pH buffering effects of ZVI as discussed previously. It is interesting to note that the initial pH buffering effects of ZVI became smaller as the pH becomes more alkaline; that is, the buffering effects of ZVI in the 0.001 M NaOH were negligible compared with that in 0.01 M HCl. At higher pH, ZVI is coated with an Fe-oxyhydroxide film that limited its suppressive effects on pyrite oxidation via galvanic interactions, so the pH decreased with time due to pyrite oxidation. In the case of 0.01 M HCl, the pH rapidly decreased with time most likely due to the proton generated by the precipitation of Fe3+ (eq. (9)) formed from the gradual oxidation of Fe2+ from ZVI by DO. Dissolved S concentrations were remarkably lower in the 0.01 M HCl solution until 21 days, indicating that the suppression of pyrite oxidation by ZVI via galvanic interactions was very effective under acidic conditions. The strong suppressive effects of ZVI on pyrite oxidation under acidic conditions could be attributed to two reasons: (1) ZVI is positively charged while pyrite is negatively charged at pH values between 3 and 4 (Figs. 1(e) and (f)), which makes it easier for them to come in contact via electrostatic attraction, and (2) Fe-oxyhydroxide/oxide layer on ZVI, which could inhibit the transfer of electrons, was most likely not formed.52)
Effects of initial pH on the (a) final pH with time, (b) dissolved S concentraction with time, and (c) dissolved Fe concentration with time.
To confirm that the suppression of pyrite oxidation was primarily due to galvanic interactions between pyrite and ZVAl, electrochemical studies were conducted. Figure 8 shows the first CV cycles of pyrite with and without attached ZVAl. The OCP measured in the case without ZVAl on the pyrite electrode (i.e., freshly polished surface) was +0.2 V but with ZVAl, the OCP shifted to a more negative value (−0.1 V). This OCP shift could be attributed to the effects of ZVAl on the surface of pyrite because partially oxidized ZVAl has an OCP of around −0.2 V, which was measured during our preliminary experiments. Although the OCP shifted with attached ZVAl, the current density profiles with and without ZVAl were similar. The duration of CV measurements was very short (one cycle = 107 s), so even though the CV results showed that ZVAl only had little effect for a short time, it might strongly affect the pyrite oxidation dynamics for longer periods of time as illustrated by the leaching results discussed previously (Fig. 4).
Cyclic voltammetry and chronoamperometry of pyrite with and without attached ZVAl: (a) cyclic voltammetry of pyrite with and without attached ZVAl, (b) anodic polarization curves with time of pyrite with and without attached ZVAl, and (c) cathodic polarization curves with time of pyrite with and without attached ZVAl.
To evaluate the effects of ZVAl on pyrite oxidation for a longer period of time, chronoamperometry was conducted. Figure 8(b) shows the changes in current density with time during the anodic polarization of pyrite with and without ZVAl at +0.6 V (eq. (11)).
| \begin{equation} \text{FeS$_{2}$} + \text{8 H$_{2}$O} = \text{Fe$^{2+}$} + \text{2 SO$_{4}{}^{2-}$} + \text{16 H$^{+}$} + \text{14e$^{-}$} \end{equation} | (11) |
Initially, the current density of pyrite with ZVAl was similar to that without ZVAl and is consistent with the CV results. As discussed previously, ZVAl is very reactive, so an Al-oxyhydroxide film is easily formed on its surface. Al-oxyhydroxide is an insulating material that likely prevented the transfer of electrons, which could explain why the suppressive effects of ZVAl on pyrite oxidation in the first 3 days of the leaching experiments were negligible (Fig. 4). However, after about 500 seconds the current density of pyrite with ZVAl started to become higher than that without ZVAl, which continued until the end of the experiment. The higher current density could be explained by the galvanic interaction between ZVAl and pyrite after the dissolution of Al-oxyhydroxide layer on ZVAl with time at slightly acidic pH (electrolyte pH = 5.72) (Fig. 4(e)). This means that galvanic interactions between pyrite and ZVAl could occur when the Al-oxyhydroxide film on ZVAl is removed. Figure 8(c) shows the changes in current density with time during the cathodic polarization of pyrite with and without ZVAl at −0.2 V. The current density of pyrite with ZVAl was smaller than that without ZVAl, which means that ZVAl suppressed the cathodic half-cell reaction of pyrite oxidation. When ZVAl particles cover the surface of pyrite, the cathodic sites are physically protected from oxidants and eqs. (11) and (12) are suppressed.40)
| \begin{equation} \text{O$_{2}$} + \text{4 H$^{+}$} + \text{4e$^{-}$} = \text{2 H$_{2}$O} \end{equation} | (12) |
| \begin{equation} \text{Fe$^{3+}$} + \text{e$^{-}$} = \text{Fe$^{2+}$} \end{equation} | (13) |
Figure 9 shows the first CV cycles of the pyrite electrode with and without attached ZVI. With attached ZVI, the current density was higher compared with that without ZVI (Fig. 9(a-1)), which could be attributed to the oxidation of ZVI on the pyrite surface (eq. (4)). In the cathodic sweep, the current density was smaller with attached ZVI than that without ZVI likely because ZVI limited the electron transfer to oxidants by physical protection (Fig. 9(a-2)).40)
Cyclic voltammetry of pyrite with and without attached ZVI: (a-1) anodic sweep of the first cycle, and (a-2) cathodic sweep of the first cycle.
Figures 10(a) and (b) show the anodic and cathodic polarization curves with and without ZVI on the pyrite electrode, respectively. In the presence of ZVI, the current density was larger than that without ZVI, and this trend continued until the end of the experiment. This higher current density with ZVI, which was also noted in the CV results, could be explained by the galvanic interactions between ZVI and pyrite; that is, when ZVI is in contact with pyrite under oxidizing conditions (i.e., positive Eh), it protects pyrite by acting as a sacrificial anode and is preferentially dissolved. It is also interesting to note that the oxidation of ZVI on the pyrite surface occurred quickly and continued for quite a long period of time (ca. 4.5 h), which suggests that so long as conditions are slightly acidic, ZVI could protect pyrite under oxidizing conditions. These results further support our earlier deduction that the suppressive effects of ZVI on pyrite oxidation during the leaching experiments was largely due to galvanic interactions. Figure 10(b) shows the changes in current density with time of pyrite with and without ZVI polarized at −0.2 V. The current density of pyrite with ZVI was smaller than that without ZVI, which means that ZVI suppressed the cathodic half-cell reaction of pyrite oxidation by physically protecting cathodic sites against oxidants similar to that observed in the cathodic sweep of the CV measurements and the cathodic polarization results of ZVAl.
Chronoamperometry measurements of pyrite with and without ZVI: (a) anodic polarization curves with time of pyrite with and without attached with ZVI, and (b) cathodic polarization curves with time of pyrite with and without attached with ZVI.
In the leaching experiments and electrochemical measurements discussed above, it was observed that pyrite oxidation was suppressed via galvanic interactions. Although this effect was relatively rapid, it was only temporary because a passivating coating on pyrite was not formed. To induce coating formation and prolong the suppression of pyrite oxidation, a combination of ZVI and phosphate was used. Only ZVI was selected because it generates dissolved Fe that could react with phosphate to form Fe-phosphates that are very stable under a wide range of pH.14) Figure 11 shows the changes in pH and dissolved S concentration after 1 day of leaching with and without ZVI in pyrite suspensions containing phosphate as well as the SEM-EDX results of the leaching residues. The presence of phosphate only could already slightly suppress the oxidation of pyrite as illustrated by the lower dissolved S concentration in phosphate solution compared with that in DI water. Suppression was even stronger when both ZVI and phosphate were present. SEM-EDX observations of the leaching residues showed that a coating composed of Fe, P, and O was present on the surface of pyrite when ZVI and phosphate were added into the system (Fig. 11(c)). This means that combining ZVI and phosphate improved the suppression of pyrite oxidation because both galvanic interactions and coating formation occurred.
Evolution of leachate chemistry of pyrite with ZVI and phosphate and SEM-EDX of leaching residue: (a) pH (b) dissolved S concentration (c) SEM photomicrograph of pyrite after 1 day of leaching with ZVI and phosphate, and the corresponding elemental maps of Fe (c-1), O (c-2), P (c-3), and S (c-4).
The results of this study have two important implications. Firstly, phosphate-enhanced GME could be used to treat mine tailings and pyrite-rich waste rocks to limit the formation of AMD. One possible way for the application of this technique is by treating pyrite-rich wastes in reactors together with scrap iron and phosphate rocks like apatite or industrial wastes containing phosphate. Unreacted iron particles could be regenerated by magnetic separation while the treated wastes are disposed of in impoundments. Secondly, phosphate-enhanced GME could be applied in the separation of coal and pyrite during coal cleaning. This process could be done during the wet grinding operation to coat pyrite with Fe-phosphate prior to flotation. Dissolved Fe for the phosphate coating may directly come from the dissolution of grinding balls that are typically made of steel via galvanic interactions with pyrite. Coal and pyrite are both inherently hydrophobic, which means that they are difficult to separate by flotation. After phosphate-enhanced GME treatment, the Fe-phosphate coating formed on pyrite would make the mineral more hydrophilic53) and facilitate better separation by flotation. Because most of pyrite in the tailings after flotation is already coated with Fe-phosphate, they could be directly disposed of in typical impoundments and AMD formation would most likely be suppressed. Preliminary experiments of the authors showed that Fe-phosphate coating formation during ball milling using only steel balls as the source of dissolved Fe was possible. Moreover, leaching tests of pyrite after ball milling suggest that the ferric phosphate coating effectively suppressed pyrite oxidation (Data not shown).
In this study, galvanic interactions of pyrite with ZVAl or ZVI under various conditions were elucidated using leaching experiments, thermodynamic calculations, electrochemical methods, and surface characterization techniques. The results of this study are summarized as follows:
This study was financially supported by the Japan Society for the Promotion of Science (JSPS) grant-in-aid for scientific research (Grant numbers: JP26820390, JP17H03503, and JPK12831).
