Chemical and Morphological Changes of the Pyrite Induced by

The chemical and morphological changes induced in the pyrite surface by the leaching and bioleaching processes in the presence of catalytic Ag ions ha...
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Langmuir 1997, 13, 3355-3363

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Chemical and Morphological Changes of the Pyrite Induced by Leaching and Bioleaching Processes in the Presence of Catalytic Ag Ions J. A. Martı´n-Gago,*,† E. Roma´n,† M. Bla´zquez,‡ C. Quintana,§ and L. Va´zquez† Instituto Ciencia de Materiales de Madrid-CSIC. Campus de Cantoblanco, 28049-Madrid, Spain, Departamento de Ciencia de Materiales e Ingenierı´a metalu´ rgica, Facultad de Quimicas-UCM, 28040-Madrid, Spain, and Instituto de Microelectro´ nica de Madrid-CSIC, PTM Isaac Newton 8, Tres Cantos, 28760-Madrid, Spain Received October 30, 1996. In Final Form: February 28, 1997X The chemical and morphological changes induced in the pyrite surface by the leaching and bioleaching processes in the presence of catalytic Ag ions have been comparatively studied by means of X-ray photoemission spectroscopy, atomic force microscopy, and scanning electron microscopy. Core level photoemission spectra of the S(2p) and Fe(2p) peaks have revealed the formation of different oxides in the leaching and bioleaching processes. Bioleaching leads to the formation of higher valence oxides (jarosites) than those formed in the leaching process. These chemical changes are directly reflected in the surface morphology and particularly as an important increase of the surface roughness. Moreover, the inclusion of catalytic Ag ions in the leaching medium does not produce substantial modification of the topography, and the Ag is incorporated in little grains (∼0.1 µm wide) which are spread along the surface. However, the presence of Ag in the bioleaching medium inhibits the formation of high valence oxides and leads to a different oxidation mechanism. The roughening of the surface due to the catalytic bioleaching is also significantly slowed down by the presence of Ag ions in the solution.

I. Introduction Bioleaching is a process in which an ore is attacked by bacteria, leading to the extraction of the constituent metals. Bioleaching processes of sulfide ores have been extensively used in industry because they represent an economic and nonpolluting alternative to the conventional technologies of metal extraction such as pyrometallurgy and hydrometallurgy.1-3 Important amounts of low-grade sulfide ores exist all over the world. They are identified as complex sulfides or complex pyrites containing profitable amounts of valuable metals such as copper and zinc. Thus, there is a growing interest in the study of processes that lead to the increase in the production of metals from minerals. Particularly the bioleaching technology for recovering metals is becoming a focus of attention in metallurgy. Classically, these kinds of processes have been studied by control of the dissolution rate of the metals. Recently it has been observed that the dissolution rate in the bioleaching process is improved by using catalytic ions in the solution which modify the electrochemical behavior of the sulfides.4,5 Thus, the use of catalytic ions in bioleaching could be of great importance for optimizing the efficiency of these processes. Although leaching and bioleaching of sulfides ores are well established and technologically used techniques, their fundamental mechanisms are still not well understood. * To whom correspondence may be addressed. † Instituto Ciencia de Materiales de Madrid-CSIC. ‡ Departamento de Ciencia de Materiales e Ingenierı´a metalu´rgica, Facultad de Quimicas-UCM. § Instituto de Microelectro ´ nica de Madrid-CSIC. X Abstract published in Advance ACS Abstracts, May 15, 1997. (1) Olson., G. J. Kelly, R. M. Biotechnol. Prog. 1986, 2, 1. (2) Holmes, D. S Minerals and Metallurgical Processing 1988, 5, 49. (3) Smith, R. W.; Misra, M. In Mineral Bioprocessing; Smith, R. W., Misrha, M., Eds.; Mineral, Metals and Materials Society: Warrendale, PA, 1991. (4) Gonza´lez, F.; Ballester, A.; Bla´zquez, M. L.; Go´mez, C.; Mier, J. L. Hydrometallurgy 1992, 29, 145. (5) Coto, O.; Ballester, A.; Bla´zquez, M. L.; Gonza´lez, F. Biorecovery 1993, 2, 121.

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Because of the complexity of the sulfide minerals, the study of the basic mechanisms which occur upon bacteria action on the surface is difficult. In order to simplify the study and to gather information about the fundamental processes taking place at the surface region, a high-purity iron sulfide (FeS2, pyrite) has been used in this work. It is well-known that in the bioleaching process the action of the bacteria on a pyrite ore consists of a corrosion and oxidation of the surface layer.6 The aim of this work is to perform a complete and comparative study of the oxidation processes which take place at the pure pyrite surface induced by leaching and bioleaching, in the presence and absence of catalytic silver ions. The chemical transformations induced by the different processes and its influence in the surface morphology will also be discussed. In previous works, some Auger electron spectroscopy results of the transformations which occurs at the surface of other mineral phases (blende and galena) when catalytic ions are added to the bioleaching medium were presented.7,8 In this paper it will be shown that the inclusion of catalytic silver ions in the medium strongly modifies the bioleaching process, producing a different kind of surface oxide from that obtained by direct bioleaching of the pyrite. To achieve this goal, a series of samples were prepared and studied for every step of the leaching and bioleaching processes (see Figure 1). Thus, first of all, a pyrite ore as-received is analyzed. In a second step a pyrite sample is introduced in the so-called 9K medium for a chemical cleaning of the oxide layer (leaching). The chemical effect of the medium on the surface oxides and morphology is shown. Later, the effect of the inclusion of silver catalytic ions in the solution is analyzed. At last, the modifications induced on a surface by standard bioleaching are compared with the bioleaching in the presence of Ag ions. The whole process is sketched in Figure 1. (6) Mustin, C.; Bertheinlin, J.; Marion, P.; de Donato, P. Appl. Environ. Microbiol. 1992, 58, 1175. (7) Blazquez, M. L.; Ballester, A.; Gonza´lez, F.; Roma´n, E.; Bustillo, F. J. Vacuum 1989, 39, 663. (8) Gome´z, C.; Roma´n, E.; Bla´quez, M. L.; Ballester, A.; and Gonza´lez., F. Min. Eng. 1995, 8, 1503.

© 1997 American Chemical Society

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Figure 1. Sketch of the different studied processes. The white upper areas represent the oxide layer.

The efficiency of the bioleaching process is usually determined by a kinetic study of the process performed by an analysis of the metals that are present in the solution. Unfortunately, this method does not provide real information about the chemical state of the elements in the surface of the mineral. However, X-ray photoemission spectroscopy (XPS) is a classical technique used to evaluate the chemical nature of the atomic elements present in a surface by measuring its core level shift. Throughout this work, XPS spectra will be employed to obtain information about the chemical processes which are taking place at the surface in every step (schematically represented in Figure 1). Atomic force microscopy (AFM) and scanning electron microscopy (SEM) were used to obtain information about the surface topography of the samples. In addition, X-ray microanalysis (EDX) in the SEM equipment was performed as a link between the chemical information provided by the XPS and the morphology. AFM imaging provides valuable quantitative information about the surface roughness induced by the processes, i.e., the root mean square surface roughness value (rms). However, further information about the surface morphology can be obtained by analyzing the lateral correlations of the surface roughness. An important variable in this study is the interface width for a horizontal length L, ξ(L), which is defined by

ξ(L) ) [1/NΣ(hi - h)2]1/2 where hi is the surface height at the point i and h is the average film height of the sample with N points and length L. The lateral surface correlations are then analyzed by studying the dependence of ξ(L) with L. Usually ξ changes with L as

ξ(L) ∝ LR R is referred to as the “roughness” exponent, and it quantifies how the roughness changes with length scale (i.e., it gives an idea of the surface disorder). This quantitative process is very helpful to distinguish between those surfaces with similar surface roughness but with different morphologies. R is related to D, the local fractal dimension, of the self-affine surface through the relationship D ) d - R where d is the space dimension of the corroding surface (d ) 3 in our case). Thus, for R f 1 the surface tends to be Euclidean (ordered) and hence D f 2 whereas for R f 0 the surface exhibits an increasing disorder and D f 3, i.e., the surface tends to fill the overall volume.9

Martı´n-Gago et al.

The value of R can be obtained for each surface from the analysis of the dependence of ξ(L) with L, by plotting log ξAFM versus log L, where L is the length of a segment of the AFM scan of size S measured in the fast x-scan direction. For each scan, 500 pairs of data points (L, ξAFM) are obtained (L being varied from S/128 to 0.98S). Finally, for each L, the corresponding ξAFM represents the average value resulting from 512 scans of the same image. This variable, ξAFM, becomes the rms roughness for lengths L equal to the total scanned size S. This procedure is well established in the scientific literature.10,11 In this work this procedure has been applied to the AFM images recorded for every one of the steps described in Figure 1. Thus, a log ξAFM versus a log L plot is shown for each surface, with the slope being the R value obtained. In some cases two zones with different slopes can be distinguished in a plot leading to the coexistence of two regions with different R values. These experimental crossovers and cutoffs are related to the experimental morphological features i.e., the grain size.10 At last, we would like to remark that is not the aim of this work to perform a dynamical scaling analysis of the morphology evolution of the surfaces subjected to the above-mentioned processes. This goal would have required a surface morphology study for different times of the process which was not our objective. We have just limited this work to the study of the lateral correlations of the surface roughness through the R evaluation, as a means of characterizing the surface morphology as it has been done before for other experimental systems.12 The influence of the tip in the scaling results can be discarded because several tips have been used, and tip geometry artifacts should lead to R ) 1. II. Experimental Details Experiments were made on pyrite ore samples from Brazil. The ores, after being mechanically polished with alumina, present a single phase as evidenced by X-ray diffraction. After the polishing they were subjected to different chemical leaching and bioleaching treatments, using as leaching solution the 9K nutrient medium. The mesophilic bacteria culture, obtained from a mine water were grown in a 9K medium13 consisting of 3 g/L of (NH4)‚2SO4, 0.5 g/L of MgSO4‚7H2O, 0.5 g/L of K2HPO4, 0.1 g/L of KCl, and 0.014 g/L of Ca(NO3)2 using pyrite as an energy source. The main bacteria identified in the culture were the Thiobacillus ferrooxidans, Thiobacillus thiooxidans, and Leptospirillum ferrooxidans. Experiments were carried out in an orbital shaker at 150 rpm using a 250 mL Erlenmeyer flask. The temperature and pH were kept at 35 °C and 2, respectively. The concentration of the catalyst Ag ion was 0.05 g/L, and it was added to the 9K medium in the form of Ag2(SO4). In the bioleaching treatment 5 mL of the bacteria culture was added per 100 mL of the 9K medium. Samples were under bioleaching for 10 days. They were rinsed in distilled water and dried with air blast at room temperature. All samples were exposed to atmospheric pressure for the same time before they were introduced in the ultrahigh vacuum (UHV) system for the XPS analysis. Despite the bacteria culture being rather complex, for a given population and the same experimental conditions (mainly pH and temperature), bioleaching results are quite reproducible. XPS experiments were performed in a UHV system with a base pressure of 2 × 10-10 Torr. The electron energy analyzer used was a hemispherical LEYBOLD with an overall resolution around 0.2 eV; thus the XPS spectra widths are limited by the (9) Meakin, P. Phys. Rep. 1993, 235, 189. (10) Va´zquez, L.; Salvarezza, R. C.; Herrasti, P.; Oco´n, P.; Vara, J. M.; Arvia, A. J. Appl. Surf. Sci. 1993, 70, 413, 1993. (11) Krim, J.; Palasantzas, G. Int. J. of Mod. Phys. B, 1995, 9, 599. Tong, W. M.; Willians, R. S. Annu. Rev. Phys. Chem. 1994, 45, 401. (12) Morales, J.; Esparza, P.; Gonza´lez, S.; Va´zquez, L.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1996, 12 500. (13) Silverman, M. P.; Lundgren, D. G. J. Bact. 77, 642, 1959.

Changes of the Pyrite with Catalytic Ag Ions natural width of the Mg KR radiation. Analysis of the samples is focused on the Fe(2p), S(2p) and Ag(3d) core levels peaks. The AFM is a Nanoscope III system operating in contact mode with Silicon nitride cantilevers. All images were recorded at a resolution of 512 × 512 pixels. Some images are presented in top view (that brighter the pixel, the larger the height) and the rest of them in three-dimensional view (3D). All images in 3D have been represented with the same vertical scale in order to compare heights by a visual inspection. The SEM is a highresolution HITACHI-800 apparatus equipped with a field emission gun and an electron disperse X-ray spectrometer (EDX). The analyses were carried out at 10 kV. The spectrometer is an ultrathin window (UTW) Quantum Kever which allows the detection of light elements with energies inferior to 1 kV.

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III. Experimental Results and Discussion III.1. Modification of Pyrite Samples by the 9K Medium in the Leaching Process. Previous to the analysis of the bioleaching process, it is important to know details about the pyrite surface modification induced by the 9K medium. Thus, one should be able to distinguish between the chemical transformations induced from the medium and those produced by the bacteria action. Therefore, this section is devoted to studying the asreceived pyrite sample (step 1 in Figure 1) by comparing it to leached pyrite in an acid 9K medium (step 2 in Figure 1). Figure 2, part a represents a 8 × 8 µm2 AFM image of the as-received pyrite. The surface topography is characterized by flat terraces with deep holes of different sizes. Terraces and holes seem to be limited by straight lines with angles at 90°, which could be related to crystallographic directions in the pyrite (domains). Figure 2, part b, shows a typical surface profile of the surface where the flat terraces and the depressions between them can be observed. The topographic features described previously are homogeneous all over the sample. Figure 3, part a, represents an AFM image of the pyrite surface after the leaching process in the 9K medium (step 2 in Figure 1). Flat terraces and depressions are also observed in this step. SEM images of both surfaces (not shown) exhibit the same type of morphological features than those observed in AFM images. In Figure 4 the rms roughness at every one of the steps of the process using AFM images has been represented. The total value of the rms roughness has been measured by averaging different images with the same scanning area, and it has been represented in Figure 4 for two different scanning scales. It is clear that steps 1 and 2 do not produce a net surface roughness variation. Only a slight reduction is observed which could be related to a modification of the pyrite surface due to the etching of the 9K acid medium. Although at first glance, images of Figure 2, part a, and Figure 3, part a, resemble each other, a closer analysis of the surface profiles (Figure 2, part b, and Figure 3, part b) and the corresponding log ξAFM versus log L plots (Figure 2, part c, and Figure 3, part c) reveals that there are differences between them. It can be seen that the pyrite substrate has a unique slope R ) 0.89 whereas the leached sample has two different zones with slopes R1 ) 0.81 and R2 ) 0.58 and a crossover value of Log Lc ≈ 2.25, which corresponds to Lc ≈180 nm. The R values in the range 0.8-1 can be explained as Euclidean values defined by the flat terraces observed for the pyrite (Figure 2, part b) and the small granular structures imaged in Figure 3, part b. The Lc value for the leached sample would correspond to the average size of this granular structure. For length scales larger than Lc, an R value close to 0.6 is measured corresponding to height fluctuations between these small grains. This is consistent with the fact that the roughness in the pyrite comes mainly

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Figure 2. (a) AFM image 8 × 8 µm2 in a 3D representation of the as-received pyrite. (b) AFM profile 1.4 µm long of the surface where flat terraces and deep holes are appreciated. (c) log ξ versus log L plot of the as-received pyrite.

from the deep holes between flat terraces whereas in the leached sample it comes from the overall surface morphology (Figure 2, part b, and Figure 3, part b). Thus, although the total rms surface roughness of the original and leached sample are similar, it is evident that the leaching process does alter the surface roughness lateral correlations. The difference between both surfaces is clearly shown from the chemical point of view. Figures 5 and 6 represent series of XPS spectra of the Fe(2p) and S(2p) peaks, respectively, for every step in the leaching and bioleaching process. The total intensity of the spectra of Figure 5, part b, and Figure 6, part b, has been multiplied by 10. From now on, in both figures, the signals coming from the bulk pyrite (FeS2) will be labeled as Fe0 and S0. Peaks situated on the left side of the Fe0 and S0 peaks (higher binding energies) correspond to the different Fe and S oxides present at the surface. In Figure 5, peaks situated at the higher binding energy side correspond to the

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Figure 4. rms roughness (in Å) for the different studied processes. Values were obtained by averaging several images with the same scanned area. The process numbers are referred to in Figure 1.

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Figure 5. XPS spectra of the Fe(2p) peak for the different steps. The position of the main oxide peaks are labeled as I and II: (a) for the as-received pyrite and leached pyrite; (b) pyrite after the bioleaching processes.

Figure 3. (a) 8 × 8 µm2 3D AFM image of the standard leached pyrite. (b) AFM profile 1.4 µm long of the surface where the main morphological features are observed. (c) log ξ versus log L plot of the leached pyrite.

Fe(2p3/2) component of the spin-orbit splitting and peaks at the lower binding energies to Fe(2p1/2). In Figure 6 spin-orbit splitting of S is seen as a shoulder close to S0. In Figure 5, part a, and previously to the leaching process (step 1), the signal from the FeS2 substrate, Fe0, is hardly seen, whereas in Figure 6, part a, the S0 peak is clearly resolved. This fact is related to the different free mean path of the emitted photoelectrons for Fe and S. Photoelectrons emitted from Fe atoms have a kinetic energy of 542 eV and those from S atoms have 1087 eV. The electron mean free path in the high-energy region varies as xE, (E being the electron kinetic energy), and thus, it can be affirmed that the S signal is coming from a region around 1.4 times deeper than that of the Fe peak. The Fe0 signal is very low because it is attenuated by the superficial oxide layer. If a value of around 10 Å is assumed for the mean free path at 500 eV,14 it can be estimated that the thickness of the oxide layer formed by exposure of the sample to air should be approximately of the same order. (14) Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2.

Figure 6. XPS spectra of the S(2p) peak for the different steps. The position of the main oxide peaks are labeled as I and II: (a) for the as-received pyrite and leached pyrite; (b) pyrite after the bioleaching processes.

The maximum of the main oxide peak (labeled as I) in Figure 5, part a, appears at 3.8 eV with respect to Fe0 and at 6.2 eV from S0 in Figure 6, part a. These values correspond to Fe and S in chemical states close to Fe+2 and S+4,15 indicating that the surface of pyrite is formed by different oxides of the type of FeO and sulfite and other

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(a)

(b)

(c) Figure 7. Ratio between the oxides type I (FeI and SI) and II (FeII and SII) and the signal from the pyrite (Fe0, S0) for the different steps. Values obtained from Figures 5 and 6. (a) Fe oxides. (b) S oxides.

different combinations where these groups are included. These values are in good agreement with previously published results.15,16 The origin of the oxides is the atmospheric action, and from now on, they will be referred to in the text as native oxides. It is important to remark that the broad features referred to as I or II in Figure 5, part a, and Figure 6, part a, include emission from different oxide states; and thus, Fe+2 states (labeled as I) form mainly FeO and Fe(SO4), and Fe+3 states (labeled as II) can lead to the formation of several products, such as Fe(SO4), Fe2(SO4)3, FeOOH, or Fe2O3. The total intensity of the S0 and Fe0 peaks is not a relevant parameter to estimate the amount of oxide at the surface because it can change from one sample to another due to either a different surface termination or slight variations in the experimental setup. Thus, to estimate the total amount of oxide for every step in the process, we showed in Figure 7 the ratio between the oxides at the I and II positions (Feox and Sox) with respect to the Fe0 peak (Figure 7, part a) and with respect to the S0 peak (Figure 7, part b) after background subtraction. This figure shows that the ratio of both oxides decreases greatly from step 1 to 2, indicating that either the surface is passivated by the 9K medium or the native layer is thinner because the surface has been exposed to the air for a shorter time. The binding energy of the oxide lines for step 2 are at the same positions as those of the as-received sample (Figure 5, part a, and Figure 6, part a), indicating (15) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. L.; Muilenberg, G. E. Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1978. (16) de Donato, Ph.; Mustin, C.; Benoit R.; Erre, R. Appl. Surf. Sci. 1993, 68, 81.

Figure 8. (a) 8 × 8 µm2 3D AFM image of the leached pyrite in the presence of Ag ions. (b) AFM profile 1.4 µm long of the surface where the main morphological features are appreciated. (c) log ξAFM versus log L plot of the leached pyrite in the presence of Ag ions.

that the oxides have the same chemical nature. Thus, they could be formed by the action of the atmospheric gases after the exposure to the air. As already described above, a slight topographic difference can be appreciated when image 2, part a, and image 3, part a, are compared: The surface of the sample treated in a 9K medium shows a slightly lower number of holes and a typical grain, as evidenced by Figure 3, part c, of around 180 nm which is not present in the as-received sample. Thus, the first layers of the surface could become altered by the action of the acid medium, resulting in a more difficult oxygen penetration. III.2. Leaching in the Presence of Catalytic Silver Ions. Before studying the effect of catalytic Ag ions in the bioleaching process it is important to know the effect of the ions in a standard leaching process (step 3 in Figure 1). Figure 8, part a represents an 8 × 8 µm2 AFM image of this kind of sample. At the bottom part of the image a clean pyrite terrace is seen. AFM images show a typical grain with an average size of around 0.2 µm. The total rms roughness of the sample in this stage of the process

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Figure 9. XPS spectra of the Ag(3d) peak for the (a) leached and (b) bioleached pyrite in the presence of catalytic Ag ions. Arrows indicate the position of minor components which suggest the presence of Ag in a different chemical state.

(step 3) with respect to previous steps has been slightly reduced (see Figure 4), indicating a smoother surface termination. From the log ξAFM versus log L plot a result similar to that obtained for the leached sample is obtained (Figure 8, part c). The main difference with that sample is that the small grain structure is better defined in the Ag-leached sample than when the silver ions are not present as it can be observed by comparing the cross section of Figure 8, part b. A crossover value of Lc ≈ 180 nm is also observed, coinciding with the grain size observed in Figure 8, part a. Thus, the silver presence does not affect appreciably the surface roughness correlation originated by the leaching process. The presence of Ag on the surface was corroborated by the detection of an XPS Ag(3d) peak which is shown in Figure 9, curve a. The spectrum shows two peaks corresponding to the spin-orbit splitting of the Ag(3d) electrons. In this spectrum a single component (no shoulders) is appreciated, with a total full width at half maximum (FWHM) of 1.83 eV. From the thermodynamic point of view, the most favorable reaction is

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FeS2 + 4Ag+ f Fe+2 + 2Ag2S Nevertheless, the formation of other Ag compounds as metallic Ag or AgO can not be directly ruled out. XPS spectra of the Fe and S for this stage are shown in Figure 5, part a, and Figure 6, part a. These spectra show an oxidized surface with the same type of oxides as those obtained after the standard leaching (i.e., native oxide). The total amount of oxides has not significantly changed (see Figure 7). The pyrite surface morphology induced by the leaching process in the presence and absence of catalytic silver ions does not present any topographic difference (Figure 3, part c, and Figure 8, part c). On the other hand, the Ag signal is detected by XPS on the surface. The question that arises is where on the surface is the Ag located. SEM images from this sample show a similar topography (data not shown for conciseness). Particularly, in some areas of the sample, grains of 0.1-0.2 µm are detected clustered together on top of the clean pyrite surface. This kind of

Figure 10. (a) 8 × 8 µm2 3D AFM image of the bioleached pyrite. (b) 1.5 × 1.5 µm2 top view AFM image of the same sample showing a detail of the surface. (c) log ξ versus log L plot of the bioleached pyrite.

feature was not detected on the surface after a leaching process without catalytic silver ions (step 2). It is reasonable to think that the Ag should be incorporated into the grains detected by the SEM and AFM images. Thus, it can be established that when catalytic Ag ions

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are present in the leaching process, the only difference is the formation of little Ag conglomerates (likely Ag2S) without significant chemical modification of the pyrite surface morphology. III.3. Bioleaching Process. It is well-known and -established17 that in an aqueous system the oxidation of pyrite is a result of both chemical and biological activities. The chemical reaction is

(a)

FeS2 + 3/2 O2 + H2O f Fe+2 + 2SO4- + 2H+ and the biological reaction is

Fe+2 + 1/4O2 + H+ f Fe+3 + 1/2H2O The Fe+3 produced by the biological oxidation of Fe+2 is available to oxidize additional pyrite according to the following global reaction:

FeS2 + 8H2O + 14Fe+3 f 15Fe+2 + 2(SO4)-2 + 16H+ Thus, the presence of the iron and sulfur oxidizing bacteria catalyzes the pyrite oxidation. The morphological changes induced by the bioleaching process on the pyrite surface are evident in Figures 10 (AFM) and 11 (SEM). A very rough topography (four times rougher than that in the previous AFM images) is produced by the process (Figure 4). The morphology consists of deep valleys (0.5-1 µm deep) with the dimensions in the order of those of the bacteria and large grains (∼1 µm wide). On top of the large grain structure, we can resolve a smaller granular morphology (30-60 nm wide) as shown in Figure 10, part b. From Figure 10, part c, a unique R value of 0.76 can be obtained, quite different from those obtained for the leached samples. In Figure 11, part a, it is clear that the surface looks porous and not so dense (spongelike) as in steps 1-3 (Figure 8). Moreover, many remaining or broken material pieces are detected scattered on the surface. All these findings support the idea that the surface has been drastically altered by the bioleaching process. The corresponding XPS spectra for this sample (see Figure 5 and Figure 6, part b) show a very oxidized surface, where Fe0 and S0 peaks are seen albeit with difficulty. The energy position of the maximum of these oxide peaks (labeled as II) is different from the position of the native oxide (peak I). They have shifted toward the higher binding energy region of the spectra, indicating that bacteria have acted by transformation of the native oxide into a higher valence oxide as already known from previous studies.18 The energy position of the Fe oxide corresponds to an oxidation state close to Fe+3 which could be associated to Fe in a complex jarosite material. For the S the situation is similar; the final oxide corresponds to (SO4)-2 oxides, a group which is also present in the jarosite material. These mixed high-valence oxides seem to be all over the surface, and this could be the reason for the morphological changes appreciated in the AFM and SEM images. The total photoemission intensity after the bioleaching process has decreased by a factor of 10. The low emission is related to the significant increase of the surface roughness evidenced in Figure 4. Also, from the XPS spectra, it can be inferred that the kind of oxides created at this stage of the process is different from the native oxide formed during the leaching process. The total amount of high(17) Rossi, G. Biohydrometallurgy; McGrawn-Hill Book Co.: Hamburg, 1990; Chapter 5. (18) Karthe, S.; Szargan, R.; Suonimen, E. Appl. Surf. Sci. 1993, 72, 157.

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Figure 11. (a) SEM image of the bioleached pyrite. (b) Corresponding EDX spectra recorded at 8 kV in the numbered zones of (a).

valence oxides is higher than that of the native oxide; however both of them coexist as a result of the chemical and biological reactions proposed at the beginning of the section. Image 11, part b, represents EDX spectra of the most relevant topographic features of Figure 11, part a. The numbers written on Figure 11, part a, correspond to the locations of the surface where the spectra of Figure 11, part b, have been recorded. Curve 1 has been recorded on the main surface, and it shows two main peaks at the energies corresponding to Fe and S that are assigned to the pyrite. A low O intensity is detected as a shoulder of the Fe peak. Because of the low-absorption rate of X-rays in matter, the EDX signal is coming from the inner pyrite bulk and thus, the oxide layer should not be very thick, in agreement with the photoemission results of Figure 7

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(XPS signal from the bulk pyrite is also detected). Also, it seems clear that the jarosite layer is quite uniform. EDX curves recorded in points 2 and 3 correspond to spectra taken on the broken pieces. In all these spectra the O signal is important, indicating that these grains consist of different oxides, which originate the wide XPS peaks in Figure 5, part b, and Figure 6, part b, labeled as I and II. III.4. Bioleaching in the Presence of Catalytic Ag+ Ions. The presence of Ag+ in the solution induces important changes in the bioleaching process (step 5). Figure 12 shows the morphological transformation experienced by the surface during step 5. In Figure 4 it can be observed that step 5 induces a smaller surface roughness than the standard step 4. At this stage, the surface consists of larger grains (width and height in the 1-7 and 0.7-2.5 µm ranges, respectively). On top of this structure a fine granular (50 nm wide) structure can be resolved (Figure 12, part b). The analysis of the roughness correlations gives the graph shown in Figure 12, part c. Two zones can be distinguished. The first one, for length scales less than the average small grain size, corresponds to the lateral roughness correlations inside the grain. Thus, a Euclidean value of R ) 0.95 is obtained. A crossover value of 1.5 is found in Figure 12, part c, which corresponds to a Lc ≈ 50 nm as can be seen in Figure 12, part b. For L > 50 nm the slope ranges between 0.6 and 0.7. It is important to note that the first zone extends over a very small length range due to the large scanned areas. In the standard bioleached sample the first zone is not clearly seen. One possible contribution to this effect can be that, even at this small scale, the large surface roughness predominates. The S and Fe spectra for bioleached samples with and without inclusion of Ag ions (step 5 and 4, respectively) are represented in Figure 5, part b, and Figure 6, part b. As it has been described above, oxide-related features in XPS spectra appear in the left part of the Fe0 and S0 peaks (higher binding energies). The oxide intensity is higher than those obtained in the leaching steps (steps 1-3 in Figure 1), but it has decreased with respect to the standard bioleaching process (see Figure 7). Oxides induced by the bacteria in the presence of Ag are mostly of the same type as those in the native oxide that were created in the leaching steps, whereas the oxide formed in the normal bioleaching process (step 4) has mainly produced a higher valence oxide. This fact could be understood as a slow down of the biological reaction proposed above to favor other reactions. One of those alternative reactions could be the association with the formation of the Ag jarosite:

Martı´n-Gago et al.

(a)

(b)

(c)

3Fe2+ + 2HSO4- + Ag+ + 6H2O f AgFe3(SO4)2(OH)6 + 8H+ Thus, the surface oxide layer is mainly formed of lowvalence oxides (native oxides), and it can be stated that the presence of the silver on the surface inhibits bacteria action. Ag was detected on the surface by XPS (Figure 9, curve b). This fact means that when the Ag is present in the solution the bacteria react, including the Ag in the chemical reactions induced by them. The chemical state of the Ag in the surface is different from that resulting from the leaching process (step 3) as evidenced by the shift in the spectra of Figure 9. In this figure, it is clearly observed that upon bacteria action the Ag peak shifts around 0.6 eV, indicating a new chemical state for the Ag. It has

Figure 12. (a) 4.5 × 4.5 µm2 3D AFM image of the bioleached pyrite in the presence of Ag ions. (b) 0.91 × 0.91 µm2 top view AFM image of the same sample showing a detail of the surface. (c) log ξ versus log L plot of the bioleached pyrite in the presence of catalytic Ag ions.

been reported that clustering of Ag compounds19 could instigate a core level shift; nevertheless the expected shift (19) Donato, Ph.; Mustin, C.; Benoit, R.; Erre, R. In Biohydrometallurgical Technologies; Torma, A. E., Wey, J. E., Lakshmanan, L. V., Eds.; The Minerals, Metals and Materials Society: Warrendale, PA, 1993; Vol. 1, p 163.

Changes of the Pyrite with Catalytic Ag Ions

due to the different grain size is inferior to 0.2 eV. In curve b of the Figure 9 one also observes a shoulder at the low-energy side, which could mean that the Ag is not in a single state but at least in two of them. Unfortunately reference samples have not been measured, and then is not possible to assign unambiguously these peaks to a given compound. When the surface topography described above is compared with those of step 3 where Ag was added to the medium without bacteria, a difference is appreciated. The grain size has changed from around 0.35 µm for the leached sample to around 0.05 µm for the bioleached one. This reduction of the grain size can be associated with a change in the chemical state of the Ag compounds formed at the surface. In order to get some further information about the chemical state of the Ag, SEM images and EDX spectra have been recorded at the most relevant points of the surface (data not shown for conciseness). On the eroded flat regions, EDX spectra look similar to that obtained on the flat areas of the standard bioleaching process (Figure 11, part b, curve 1), indicating a smooth oxide layer. Both AFM and SEM found grains of around 0.05 µm. EDX performed on one of those grains have revealed the presence of Fe, O, S, and Ag. This composition matches with that of the Ag jarosite proposed at the beginning of the section. Due to the small size of these grains, most of the recorded intensity for S and Fe originated in the bulk pyrite. Also, in some parts of the sample, big grains 1.5 µm long were detected. They are made of S, Cl, and Ag. Cl is present in the 9K medium, and it could form AgCl. Therefore, EDX spectra, in good agreement with XPS, suggest that Ag is present on the surface in two different chemical states, one as a foreign element to the catalytic process combined with Cl and secondly as little crystallites of the Ag jarosite. To summarize the results obtained, one can conclude that the leaching process, either standard or Ag-activated, does not change appreciably the surface roughness. However the study of the surface roughness correlations

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show that from a morphology defined by the pyrite polishing with an R coefficient close to 1 (Euclidean value) the leaching process leads to a morphology characterized by two zones. The first zone corresponds to length scales less than the average grain size and has also a Euclidean R value. The second one, for length scales larger than the grain size, shows an R value in the 0.5-0.6 range which corresponds to the height fluctuations between grains. This is consistent with the different morphologies observed for both samples: flat terraces separated by deep holes for the initial substrate and a more compact granular structure for the leached sample. However, from the morphological point of view the Ag-activated leaching process does not induce significant changes on the surface structure (except for the grain structure which is better defined). The grain size observed after the leaching processes and the slight smoothing of the surface can be related to the modification of the pyrite surface due to the 9K acid medium. When bioleaching is performed on the pyrite, the changes in the surface morphology are marked. The surface roughness clearly increases, and the surface presents different roughness correlations. The standard bioleaching process leads to the formation of higher valence oxides than those formed in the leaching process. The inclusion of Ag in the process significantly modifies the morphology of the final surface as it reduces the surface roughness and better defines a small granular morphology which is also evident on the log ξAFM versus log L plot. It also alters the rate of formation of high-valence oxides, suggesting a different oxidation mechanism where Ag ions would lead to the formation of Ag jarosites, blocking then the biological oxidation of the pyrite. Acknowledgment. We acknowledge J. Alberca for sample preparation and I. Jimenez for his assistance with the XPS measurements. LA961054I