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XPS Characterization of Chalcopyrite, Tetrahedrite, and Tennantite Surface Products after Different Conditioning. 1. Aqueous Solution at pH 10 J. A. Mielczarski,*,† J. M. Cases,† M. Alnot,‡ and J. J. Ehrhardt‡ Laboratoire “Environnement et Mine´ ralurgie” UA 235 CNRS, INPL-ENSG, B.P. 40, 54501 Vandoeuvre-le` s-Nancy, France, and Laboratoire de Chimie Physique pour l’EnvironnementsUMR 9992-CNRS, 54600 Villers-le` s-Nancy, France Received July 17, 1995. In Final Form: December 18, 1995X Characterization of the surface products formed by the interaction of aqueous solution at pH 10 with mineral samples of chalcopyrite (CuFeS2), tetrahedrite (Cu12Sb4S13), and tennantite (Cu12As4S13) was carried out by X-ray photoelectron spectroscopy (XPS). The experimental data collected directly after treatment with aqueous solution and after successive sputtering have led us to determine the in-depth distribution of the different types of surface products formed by the diffusion of metal atoms from the bulk structure to the interface and their interaction with aerated water. In general, the following in-depth surface composition was found for the investigated mineral samples: (i) the outermost thin layer consists of hydrophilic species, mainly ferric or cupric (depending on the mineral sample) oxides/hydroxides and adsorbed water, (ii) the next layer is a sulfur-enriched structure with a gradually changing composition with hydrophobic properties which vary significantly among the minerals, and (iii) the innermost layer has a bulk mineral composition and structure. In the case of chalcopyrite the separation between the outermost hydrophilic layer (iron oxides/hydroxides) and the internal hydrophobic sulfur-rich layer is more pronounced than for other mineral samples. This is the reason that the outermost hydrophilic thin layer can be relatively easily removed mechanically and/or by dissolution during a strong agitation. This involves a strong increase in the hydrophobicity of chalcopyrite. In the case of tetrahedrite and tennantite the outermost hydrophilic layer consists mainly of copper oxides/hydroxides and it is not well separated from the rest of the surface structure. This implies that the two minerals remain hydrophilic. Tetrahedrite forms the copper oxides/hydroxides product much more quickly with a major amount of cupric species, whereas tennantite oxidation, under the same conditions, yields a small amount of cuprous surface species. The differences in surface composition of these three minerals, caused by the different mobilities of copper atoms in their crystalline structures, are vitally important for their separation in the presence of surfactants. It should be noted that although tetrahedrite and tennantite contain a significant amount of other elements, they are not concentrated in the outermost layer. On the contrary their surface concentrations are several times lower than those found for the bulk composition.
Introduction Selective modification of the surface properties of solid components can find various applications. One of the important industrial applications is a selective separation of valuable mineral components from ore by froth flotation. This is realized by the creation of a hydrophobic surface of valuable components, while the surface of the other components is maintained hydrophilic. Changes in the surface properties of sulfide minerals can be achieved by adsorption of organic reagents or by the alteration of the surface composition of the outermost layer of the minerals themselves. Selective flocculation of the small mineral particles, which is also governed by surface properties, is very important to realize the separation process efficiently. The knowledge of the type of the formed surface products, their structure, and surface distribution is vitally important for understanding any changes in the surface properties and manipulating them intelligently for the separation purpose. Lack of direct evidence on the surface composition and structure and the need for it was already underlined by several authors.1-4 †
Laboratoire “Environnement et Mine´ralurgie” UA 235 CNRS. ‡ Laboratoire de Chimie Physique pour l’EnvironnementsUMR 9992-CNRS. X Abstract published in Advance ACS Abstracts, April 15, 1996. (1) Shannon, L. K.; Trahar, W. J. The role of collector in sulphide ore flotation. In Advances in Mineral Processing; Somasundaran, P.; Ed; SME/AIME: New York, 1984; pp 408-424. (2) Chander, S. Int. J. Miner. Process. 1991, 33, 121. (3) Ralston, J. Miner. Eng. 1991, 4, 859. (4) Senior, G. D.; Trahar, W. J. Int. J. Miner. Process. 1991, 33, 321341.
Chalcopyrite was investigated extensively electrochemically and spectroscopically whereas there is no published work on tetrahedrite and tennantite. It was reported5,6 that the surface of chalcopyrite can show strong hydrophobic properties after conditioning in alkaline aqueous solution without any reagent at potentials higher than 100 mV (SHE). The exact nature and structure of the outermost layer of chalcopyrite remained unclear. The suggested hydrophobic species which were formed during the oxidation process were at first elemental sulfur7 and then a metal-deficient surface8,9 and polysulfides.10 Ferric hydroxide was also postulated as the second major surface product of chalcopyrite oxidation at basic and neutral pH’s, mainly on the basis of electrochemical data,7,11 whereas some of the XPS studies do not show supporting evidence for this suggestion.9 The presence of the surface iron hydroxide which is a typical hydrophilic product will cause lower hydrophobic properties of chalcopyrite whereas the sulfur-rich species will involve an increase in hydropho(5) Hayes, G. W.; Trahar, W. J. Int. J. Miner. Process. 1977, 4, 317344. (6) Trahar, W. J. Int. J. Miner. Process. 1983, 11, 57-74. (7) Gardner, J. R.; Woods, R. Int. J. Miner. Process. 1979, 6, 1. (8) Buckley, A. N.; Hamilton, I. C.; Woods, R. Investigation of surface oxidation of sulfide minerals by linear sweep voltammetry and X-ray photoelectron spectroscopy. In Flotation of sulfide minerals; Developments Mineral Processing; Forssberg, K. S. E., Ed.; Elsevier: Amsterdam, 1985; Vol. 6, p 41. (9) Zachwieja, J. B.; McCarron, J. J.; Walker, G. W.; Buckley, A. N. J. Colloid Interface Sci. 1990, 132, 462. (10) Luttrell, G. H.; Yoon, R. H. Colloids Surf. 1984, 12, 239-254. (11) Pang, J.; Chander, S. Miner. Metall. Process. 1989, 7, 149-155.
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bicity; therefore, the nature and structure of the outermost layer containing both hydrophobic and hydrophilic species have to be carefully considered. This is very important from a separation point of view where selectivity is closely related to the balance between hydrophobic and hydrophilic surface species. It was reported9 that the surface layers of iron oxidation products formed in air and in aqueous solution showed a different stability in alkaline solution. The layer produced in a strong basic solution can be easily dissolved in the solution, leaving a sulfur-rich surface layer. The difference in the structure of surface layers of chalcopyrite after oxidation in air and in aqueous solution was also reported by other authors,12 who also found a signifcant enrichment in iron oxide/hydroxide type species on all chalcopyrite oxidized samples. There are reported results on the alteration of the surface composition of chalcopyrite after contacting with air, hydrogen peroxide, acetate, and ammonia solutions8,13,14 while only limited information is available on the composition and structure of chalcopyrite after a short-time immersion in basic solution.9,12 The aim of this work is to obtain detailed information on the mineral surface composition in the ultrathin outermost layer of chalcopyrite, tetrahedrite, and tennantite in contact with aerated water at basic pH. In froth flotation separation minerals undergo an interaction with aqueous solution containing oxygen during the grounding process and also during contact with a collector solution as an alternative reaction to the adsorption of the collector. Detailed knowledge of this interaction in a “pure” chalcopyrite-water system is also important to better understand the effect of grinding media15 on the surface composition of chalcopyrite and, thus, also to understand the major reason for an increase of its hydrophobic properties without any surfactant addition. Detailed surface characterization of these minerals after their exposition to an aqueous solution of surfactants will be presented elsewhere.16 In this work particular attention was paid to find differences in the surface composition of these three minerals after their contact with basic aqueous solution, which can be utilized for their selective separation. All the samples are copper minerals; hence, copper chelating reagents, which are typically used, as flotation collectors, for hydrophobization of the chalcopyrite surface, will interact with all three minerals. Therefore, other key differences have to be found that may allow the separation of these minerals. One of the possible ways to accomplish this goal may result from a different behavior of these minerals in contact with a basic solution without collector. The second interesting feature, to which this work was devoted, is the role and the distribution of minor elements (“mineral impurities”) in surface layers and their possible influence on surface interactions with surfactant solutions for each of the three minerals. It was already reported8,17,18 that minor elements in sulfide minerals can concentrate at the surface and in consequence change their surface properties in comparison to “pure” mineral samples. The detailed composition of the adsorbed layer, including depth profiling, will be discussed in this paper. The (12) Cattarin, S.; Flechter, S.; Pettenkofer, C.; Tributsch, H. J. Electrochem. Soc. 1990, 137, 3484-3493. (13) Buckley, A. N.; Woods, R. Aust. J. Chem. 1984, 37, 2403. (14) Brion, D. Appl. Surf. Sci. 1980, 5, 133-152. (15) Ans, J. H.; Gebhard, J. E. Int. J. Miner. Process. 1991, 33, 243262. (16) Mielczarski, J. A.; Cases, J. M.; Alnot, M.; Ehrhardt, J. J. Langmuir 1996, 12, 2531. (17) Mielczarski, J. A. Int. J. Miner. Process. 1986, 16, 179. (18) Laajalehto, K.; Suoninen, E.; Heimala, S. Int. J. Miner. Process. 1991, 33, 95-102.
Mielczarski et al. Table 1. Mineralogical Composition sample
composition
chalcopyrite tetrahedrite
chalcopyrite >99%, sphalerite (ZnS) tetrahedrite ∼98%, chalcopyrite, pyrite (FeS2), galena (PbS), carbonates tennantite ∼55%, sphalerlite ∼42%, pyrite, chalcopyrite, galena
tennantite
Table 2. Open Circuit Potentials of the Investigated Mineral Samples in Water at pH 10 chalcopyrite
tetrahedrite
tennantite
75 mV, 5 min 150 mV, 30 min
280 mV, 5 min 300 mV, 30 min
240 mV, 5 min 270 mV, 30 min
XPS method is particularly effective for obtaining information on inorganic surface composition. The XPS spectra of mineral samples were recorded directly after the conditioning in aqueous solution and after four different sputtering conditions (layer-by-layer microsectioning), which were initially very soft and then gradually stronger. Thus, the obtained results provide very detailed information on the composition of the outermost layer and on the distribution of elements in the interface region at different depths. Experimental Section Materials. The mineral samples chalcopyrite (CuFeS2), tetrahedrite (Cu12Sb4S13), and tennantite (Cu12As4S13) were procured from the Iberian Pyrite Belt. Whereas the two first minerals contain a small amount of other components, the latter sample composition is very complex (Table 1). The slab sample of minerals with dimensions of 10 × 9 mm2 was cut from a massive rock, polished using emery paper and increasingly smaller sizes of alumina powder (to 0.03 µm), and washed with water. Distilled water, 18 MΩ, from the Millipore (Milli-Qplus) system, was used throughout the experiments. The other used reagents were all of an analytical grade. Adsorption. The mineral samples were contacted with aqueous solutions at a basic pH of 10.0 ( 0.1, adjusted by adding KOH, immediately after polishing; hence, they are the samples with fresh surfaces exposed to the solution. The changes in the surface composition after 25 min of immersion in water were studied by XPS directly after removal of the samples from the water solution. If necessary the surface of the mineral samples was quickly blotted in order to remove excess water prior to the spectroscopic analysis. The open circuit potential (OCP) of the mineral samples in aerated water at pH 10 as a function of the immersion time is shown in Table 2. All potentials are reported against the standard hydrogen electrode (SHE). XPS Analysis. The X-ray photoelectron spectra were obtained with a multidetection electron energy analyzer (VSW FAT mode CL150). The nonmonochromatized magnesium and aluminum radiation sources (1253.6 and 1486.6 eV, respectively) operated at 15 kV and 10 mA were employed. The XPS resolution was between 0.9 and 1.0 eV, determined by the use of a gold standard. The operating pressure of the spectrometer analyzer chamber was close to 10-8 Pa. Because of the complexity of the studied mineral samples a two-step procedure was used in these studies. At first wide spectra of the mineral samples were recorded with magnesium and aluminum radiations. Careful inspection of the spectra allows us to assign the observed peaks to particular components and choose the measurement conditions (problem of signal overlapping) at which maximum information can be collected from one measurement. In the second step the most characteristic peaks were recorded in narrow ranges of binding energy in order to obtain a better statistic and resolution required for an optimal spectral mainpulation. The magnesium anode was chosen in these studies, since it was found that, in the case of the aluminum anode, the Cu Auger line has the same position as that expected for the Fe 2p line of iron oxide/hydroxide species. The measurement was performed with a takeoff angle close to 90°. The recorded lines (Zn 2p, Cu 2p, Cu 3p, Cu Auger, Fe 2p, O 1s, Sb 3d, C 1s, S 2p, P 2p, and As 3d) were fitted using a curve-fitting program with Gaussian/Lorentzian or asymmetric
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Table 3. Binding Energy (eV) of S, C, O, Fe, and Cu Elements and Kinetic Energy (eV) of Cu Auger Line Measured for Chalcopyrite after Conditioning in Aerated Water, pH 10, and Consecutive Sputtering core level C 1s
treatment
a
b
a
b
a
O 1s b
water treatment and sputtering at (mA min) 0.6 4.6 14.6 19.6
163.0
161.2
286.7
285.1
533.3
531.9
530.0
710.9
708.2
932.0
918.2
162.9 162.9 162.9 162.9
161.1 161.2 161.3 161.2
286.6 286.3 286.1 285.9
285.1 284.8 284.6 284.3
533.5
531.9 531.9 531.9 531.9
530.0 530.0 530.0 530.0
710.5 710.0
708.3 708.4 708.5 708.3
932.2 932.2 932.2 932.2
918.1 918.0 917.9 918.1
S 2p3/2
Fe 2p3/2 c
a
b
Cu 2p3/2 a
Cu Auger
Table 4. Relative Intensities of Observed Lines in XPS Spectra of Chalcopyrite after Conditioning in Aerated Water, pH 10, and Consecutive Sputtering core level S 2p
C 1s
treatment
a
b
a
b
a
O 1s b
c
a
b
Cu 2p3/2 a
water treatment and sputtering at (mA min) 0.6 4.6 14.6 19.6
0.116
0.221
0.146
0.243
0.152
0.423
0.275
0.303
0.307
1.000
0.124 0.037 0.050 0.057
0.275 0.201 0.196 0.222
0.076 0.018 0.008 0.006
0.135 0.039 0.021 0.027
0.044
0.104 0.061 0.020 0.014
0.097 0.095 0.052 0.031
0.203 0.062
0.279 0.399 0.524 0.568
1.000 1.000 1.000 1.000
(for the Fe 2p) peak shapes. The subtraction of the asymmetric background was made before the fitting procedure. A very small charging, about 0.1-0.2 eV (only after ion bombarding) was observed. The copper lines Cu 2p, Cu Auger, and Cu 3p were mainly used as references. The position of the S 2p doublets reported in this work is the position of the S 2p3/2 splitting component obtained from the fitting procedure. After recording the spectra of the mineral samples, directly after their treatment, an in-depth distribution analysis of the chemical composition was performed, where the third dimension perpendicular to the surface of the mineral sample is of primary interest. The sputtering method was used as the preparation technique. The mineral sample was bombarded with Ar+ ions accelerated in an ion gun to an energy of 3 keV. Some energy from the argon ions is transferred to surface atoms which are sputtered away. Thus, the sample surface is successively decomposed during sputtering, and the residual surface can be analyzed. This allows us to obtain information on the composition of the mineral samples at different depths. The amount of atoms removed from the mineral sample during sputtering (in an ideal condition layer-by-layer microsectioning) can be controlled by the magnitude of the current density of the ion gun and the sputtering time. Habitually four different sputtering conditions were used in these studies, to monitor the surface composition of minerals after adsorption, in the following order: (i) 0.3 mA for 2 min, (ii) 1 mA for 4 min, (iii) 1 mA for 10 min, (iv) 1 mA for 5 min. After the last sputtering the obtained spectra are very similar to those characteristic for a “fresh” surface of the mineral samples. The ion current density for the sputtering was determined on a platinum sample, and a value of 3 × 10-8 A/(cm2 s) was found at 1 mA current and an Ar pressure of 2 × 10-5 Pa. Uncertainty of the quantitative depth analysis could be caused by several factors.19 In the present work it could be expected that the major factor will be the variation of the sputtering yield for different elements, which could result in an alternation of the original surface composition during sputtering, with enrichment of the elements with lower sputtering yield. The calculated sputtering yield,20,21 i.e. the number of atoms sputtered per incident argon ion, follows Zn > Sb > Ag > As > Cu > S > Fe > O > C. It should be noted that the sputtering yields are similar for the most interesting elements Cu, S, Fe, and O. Nevertheless another effect induced by ion bombardment, namely reduction (19) Powell, C. J.; Seah, M. P. J. Vac. Sci. Technol., A 1990, 8, 735763. (20) Rivie´re, J. C. In Practical surface analysis by Auger and X-ray photoelectron spectroscopy; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons: New York, 1988; Chapter 2. (21) Hofman, S. In Practical surface analysis by Auger and X-ray photoelectron spectroscopy; Briggs, D., Seah, M. P., Eds.; John Wiley & Sons: New York, 1988; Chapter 4.
Fe 2p3/2
of compounds, causes O and S, which are the more volatile elements, to be lost preferentially. Whereas the preferential sputtering was clearly observed, for example, for alloys,20 the results obtained in our studies do not show significant changes which could be conclusively described by this effect. The most probable reason is that our samples are prepared in water, which produces a composition and structure of the outermost layer which are very heterogeneous and not very compact. There is also water captured in the surface structure (see the results obtained). These probably result in more uniform sputtering because of the matrix effect. To evaluate the sputtering rate, the experiments with a sample of 10 nm thick Al/Ni alloy layer deposited on carbon silicate were performed. This thin layer was removed after 60 min of sputtering at a 10 mA current density of the ion gun. This sputtered sample was free of oxygen content. It was found experimentally that if oxygen either is adsorbed on a metallic surface or forms a part of the surface composition, the sputtering rate increases tremendously by a factor between 10 and 50. Therefore for the calculation of the sputtering rate for the mineral samples used in these studies we assumed a factor of 30. On the basis of the above discussion it is assumed, for a very brief estimation, that 2 mA min argon bombarding removes about a 1 nm thick layer.
Results and Discussion Surface Composition of Chalcopyrite in Contact with an Aqueous Solution at pH 10. The XPS results obtained for a chalcopyrite sample conditioned with water at pH 10, in an open to air vessel and after ion bombarding, are presented in Figure 1 and in Tables 3 and 4. The spectra recorded directly after conditioning (Figure 1a) clearly show that the outermost layer of the mineral is very strongly oxidized. The O 1s line, of high intensity and very broad, shows at least three components at 533.3, 531.9, and 530.0 eV which could be assigned to the adsorbed water (component a), hydroxides (component b) and oxides (component c), respectively. Hence, oxidation products (hydroxides and oxides) of iron and copper are expected at the mineral interface. Because the full width at half maximum (fwhm) of the O 1s component (Figure 1a O 1s line, component b) was found to be larger than two other O 1s components, different types of hydroxide and oxy-hydroxide species are expected at the interface. This is also the reason that in the below discussion we used the term “oxide/hydroxide”, which does not refer to one specific structure such as FeOOH. The presence of the Fe 2p high-intensity component at about 711 eV,
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Figure 1. XPS lines of chalcopyrite after conditioning in aerated water, pH 10, with their fitted components (a), and in-depth compositions after consecutive sputtering (b). In part b the absolute areas of the line components are presented.
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assigned to iron oxide/hydroxide species, and the relatively low intensity of the second Fe 2p compontent at 708.2 eV, due to the presence of iron in the bulk mineral structure, are consistent with the above observation (made on the basis of the O 1s line) of the strong degree of oxidation of the sample. The broad band at about 711 eV can be assigned to several different ferric and ferrous oxides/ hydroxides such as Fe(OH)3, Fe(OH)2, Fe2O3, Fe3O4, etc.12,14,22 The Cu 2p and Cu Auger spectra do not exhibit any peaks in addition to those characteristic for chalcopyrite. This indicates that there is an insignificant amount of copper oxidation species at the interface. Most probably, only a small amount of cuprous oxides and hydroxides is present. There is no clear evidence for the presence of cupric species. The S 2p spectrum shows a very broad line which cannot be fitted by two doublets in a satisfactory manner, even if the additional doublet is assumed to be much broader than the mineral one (Figure 1a). This suggests a wide distribution of binding energies of possible different sulfur components. It should also be noted that the absence of any types of sulfur oxide species is evident. Fitting with an assumption of two doublets gives the positions of the S 2p3/2 component for each doublet at 161.2 and 163.0 eV with the fwhm values of 1.35 and 1.85 eV, respectively. The doublet positioned at lower energy, i.e., 161.1 eV (component b), is characteristic for chalcopyrite sulfur, which is in agreement with a previous observation.23 Component a at 163.0 eV has a position between the typical ones for sulfide minerals (pyrite has one of the highest positions, BE ) 162.5 eV) and elemental sulfur (BE ) 163.7 eV). This indicates formation of surface intermediate species like metal-deficient sulfides8 or polysulfides.10,24 The presence of elemental sulfur was excluded on the basis of the observation that the S2p3/2 peak at 163.0 eV is stable in ultrahigh vacuum (UHV), at room temperature during the measurement time. It was already demonstrated that the elemental sulfur in the outermost layer is not stable under UHV conditions at room temperature.23 Close inspection of the fitting results of the S 2p line using two doublets (Figure 1) shows that the better fitting is expected if an additional doublet is placed with its position about 162.0 eV. It is difficult to assign this third doublet to a particular surface species which could be present at the mineral interface. Therefore, it is probably correct to assume that the observed broadening of the S 2p line results from gradual changes in the environment of sulfur at the interface. This imposes a multicomponent fitting, and various solutions can be proposed. Therefore Figure 1 shows the results of the most simple fitting of the S 2p line with the assumption, at this point, that the one doublet is characteristic for bulk mineral sulfur (component b) and the second very broad doublet represents enrichment in sulfur (MeSx) in the interface region. For detailed assignment of the MeSx components see also the discussion at the end of this section. This structure is consistent with the finding of a significant excess of sulfur at the interface. The calculated atomic ratio is close to CuFe1.1S3 if the Scofield photoionization cross section25 is used in this estimation or CuFe1.3S2.6 if the Wagner empirical atomic sensitivity factor26 is applied. It (22) Harvey, D. T.; Linton, R. W. Anal. Chem. 1981, 53, 1684-1688. (23) McCarron, J. J.; Walker, G. W.; Buckley, A. N. Int. J. Miner. Process. 1990, 30, 1. (24) Yoon, R.-H.; Lagno, M. L.; Luttrell, G. H.; Mielczarski, J. A. In Processing and utilization of high-sulfur coals IV; Dugan, P. R., Quigley, D. R., Attia, Y. A. Eds.; Elsevier: Amsterdam, 1991; p 241. (25) Scofield, J. H. J. Electron Spectrosc. Relat. Phenom. 1976, 8, 129. (26) Wagner, C. D.; Davis, L. E.; Zeller, M. V.; Taylor, J. A.; Raymont, R. H.; Gale, L. H. Surf. Interface Anal. 1981, 3, 211.
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should be pointed out that these values do not illustrate the stoichiometry of metal-sulfur surface species because the copper and iron surface atoms are bound also as hydroxides and oxides, as was explained above. The atomic ratio determined after the sample was polished by alumina is close to CuFe0.55S2.4 (Scofield) or CuFe0.77S2 (Wagner), respectively. Though the two calculated values are very different, they clearly indicate, independent of the calculation method, a significant excess of sulfur and iron at the chalcopyrite interface after water treatment. In fact the uncertainty in the determination of surface composition as the intensity ratios between different elements is rather low, whereas the uncertainty of the calculated stoichiometry is, as can be seen immediately from the above data, strongly dependent on the calculation method. The empirical sensitivity factor is more recommended by some authors19 for a quantitative determination, and as was reported, a series of standards show a standard deviation of typically 14%; but there are also other factors which influence confidence limits. Detailed discussion of these problems can be found elsewhere.19 In consequence, in the below considerations, the absolute values of the calculated atomic ratios are less meaningful than their variations observed between the samples after different treatments. Empirical sensitivity factors were applied for the quantification of surface compositions. Ion bombarding experiments provide very interesting information on the in-depth distribution in the surface layer (Figure 1b). After very soft sputtering (0.6 mA min) a tremendous decrease in the intensity of the O 1s line (all three components) was observed, which is associated with a strong decrease of iron oxides/hydroxides and adsorbed water content. This clearly indicates that the major part of the observed hydrophilic species forms a very thin outermost layer of the sample. The present carbon contamination species, i.e. hydrocarbon (component b) and carbon bonded to oxygen (component a), are also significantly removed after the very soft sputtering. In consequence, the intensities of all the mineral component lines, i.e. those of iron, copper, and sulfur, increase. An exceptional increase in the intensity of the S 2p components is worthy of notice. This phenomenon will be discussed later in detail. The consecutive ion sputtering (4.6, 14.6, and 19.6 mA min) removes almost all of the carbon and oxygen from the mineral surface (Figure 1b). Low-intensity oxygen peaks present after the last sputtering most likely result from oxide species which are present in the chalcopyrite sample. It is noteworthy that the relative intensity ratios between the total Fe 2p3/2 and Cu 2p3/2 lines at first decrease and then somewhat increase after subsequent ion bombarding (Table 4). This observation supports the above conclusion concerning the diffusion of iron atoms to the outermost layer from the close to the interface vicinity structure of chalcopyrite. The position of the Fe 2p3/2 oxide/hydroxide component (a) shifts after subsequent sputtering (Table 3), which indicates heterogeneity in the depth composition of the iron oxidized layer. The relative intensity ratios between the S 2p and Cu 2p3/2 lines after subsequent sputtering (Table 4) reveal that this ratio increases after the first soft sputtering from 0.33 to 0.40 and after a next sputtering decreases sharply to the value of 0.25, which is almost constant for the last three sputterings. It discloses very interesting information on the sulfur in-depth distribution in the surface layer. The calculated atomic ratios for the first three samples, after water treatment and the first two sputterings, are as follows: CuFe1.3S2.6, CuFeS3.1, CuFeS1.9. The highest sulfur concentration is just below the relatively thin outermost layer containing iron oxy-hydroxy species. This
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sulfur enrichment disappears quickly after the second sputtering, and the composition of the surface layer begins to be almost homogeneous in relation to the content of iron, copper and sulfur, giving a stoichiometry close to that expected for CuFeS2. Close inspection of the changes in intensities of the two components of the S 2p lines (Figure 1b and Table 4) reveals that the assignment of the S 2p line components is more complex. After the first soft sputtering the mineral component (b) shows a much higher intensity ratio than that expected from copper and iron content, indicating that this sulfur component (b) is not in fact a pure mineral component. This supports the above explanation about the formation of the outermost sulfur-rich layer with graduated changes in the sulfur environment. In a simplified analysis one can assume that there are two types of surface sulfur responsible for the observed strong sulfur enrichment. The first one can be assigned to the formation of a metal-deficient layer with the S 2p doublet position very close to that characteristic for the bulk mineral, which is a similar assumption to that recently proposed for lead sulfide,27 and the second is one most probably characteristic of a polysulfide-like structure (MeSx) with a position shifted to higher binding energy. The experimental results from this work suggest a rather gradual distribution of the surface species with the intermediate structures between these two proposed extremes rather than those of the two components, as would be suggested from the used fitting procedure. Another striking feature is that the intensity ratio between the S 2p mineral and the MeSx component is almost constant in all samples analyzed after the second sputtering. The MeSx component was assigned recently13,23 to the metal-deficient layer; however, the above observation and other separately performed experiments28 indicate clearly that the observed strong asymmetry of the S 2p line is also an intrinsic feature of chalcopyrite. It seems that the observed long tail for the S 2p line and no well established shape of the Fe 2p3/2 line in the case of chalcopyrite, in comparison, for example, with pyrite (e.g. ref 29), result from the chalcopyrite bulk structure in which strong covalent bonds with an effective ionic state between Cu+ and Cu+2, and Fe+2 and Fe+3 were reported;30 thus, the different sulfur environments are expected. The XPS spectrum of chalcopyrite is rather consistent with the electronic configuration Cu+Fe3+(S2-)2 accepted also in the literature.31 Another possible reason is that chalcopyrite differs significantly in its semiconductor property from those of the two other mineral samples studied in the work. Whatever the reason, the experimental results indicate that both S 2p components obtained by fitting (Figure 1) contain also information from the bulk structure; therefore, the quantitative determination of the absolute surface sulfur enrichment is rather difficult because of difficulties in finding the “true” baseline. The above results imply the following in-depth distribution of surface products at the mineral-aqueous solution interface at pH 10: (i) the outermost thin layer contains hydrophilic species, mainly iron oxides and hydroxides, and adsorbed water (and carbon contamination), (ii) just below this thin layer, sulfur enrichment in a different type of sulfur (metal-deficient sulfides and (27) Buckley, A. N.; Woods, R. Appl. Surf. Sci. 1984, 17, 401-414. (28) Mielczarski, J. A.; Laajalehto, K. Paper in preparation. (29) Szargan, R.; Karthe, S.; Suoninen, E. Appl. Surf. Sci. 1992, 55, 227. (30) Hall, S. R.; Stewart, J. M. Acta Crystallogr. 1973, B29, 579. (31) Hamajima, T.; Kambara, T.; Gondaira, K. I.; Oguchi, T. Phys. Rev. B 1981, 24, 3349.
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polysulfides) is observed, and (iii) below this is the structure with the composition very similar to that expected for bulk chalcopyrite. For schematic representation see also Figure 4. The multilayer structure of different species formed during oxidation of chalcopyrite in basic aerated solution was suggested recently11 on the basis of the AC impedance results, and the direct spectroscopic observations made in this work in general agree with that proposition. However, these results do not support the suggestion11 that in the presence of oxygen the Cu2+ and S2O42- species are present in the outermost layer of chalcopyrite. The results obtained are consistent with the proposed9 surface reaction:
CuFeS2 + 0.75xO2 + 1.5xH2O ) CuFe1-xS2 + xFe(OH)3 with the following meaning, coming from this study, that CuFe1-xS2 represents all sulfur-rich surface intermediates from metal-deficient sulfides to polysulfides and Fe(OH)3 represents all iron (ferrous and ferric) oxides/hydroxides. The results obtained clearly show that the surface structure does not consist of well defined homogeneous layers with clear interfaces. Moreover, a lateral heterogeneity with patchwise structures is expected, which is in good agreement with the model of electrochemical type interactions between sulfide mineral and aerated water at open circuit potential, when one part of the mineral surface serves as anode and another one as cathode. The determined layer by layer surface structure indicates that chalcopyrite contacted with water at pH 10 in quiescent solution will be virtually hydrophilic. This conclusion seems to be in opposition to the reported observation of the self-induced hydrophobicity of chalcopyrite under basic solution conditions at a potential above 100 mV (SHE).5 The difference between these two experiments is that the hydrophobicity determination was performed at pH 11 under strong agitation conditions. At the same pH preoxidized in air chalcopyrite immersed in nitrogen-saturated solution during 1 h shows only a negligible amount of oxidized iron.9 This indicates the importance of pH for the formation process of the outermost layer. At pH 11 the produced iron oxides/ hydroxides are not stable at the interface and they are readily dissolved in solution whereas at pH 10 the iron oxides/hydroxides show higher stability. In another work6 it was shown that the potential dependence of the hydrophobic properties of chalcopyrite is independent of pH over the pH range 8-11. If we believe that there is no effect of the potential control reagent added to the solution, i.e. NaOCl, on the surface composition of chalcopyrite, in order to explain the controversy between the high hydrophobicity observed at lower basic pH and the hydrophilic outermost layer found in this work, it could be assumed that the hydrophilic outermost layer can be very easily removed from the mineral surface mechanically, by a strong agitation, which is a common hydrodynamic condition applied in the froth-flotation process. When the iron oxy-hydroxy thin layer is removed, the layer strongly enriched in sulfur (metal-deficient sulfide and polysulfide structures) begins to be exposed to solution, providing a high hydrophobicity. The oxidation iron surface species are present as ferrous and ferric compounds. The ferrous hydroxide species are more soluble in water than the ferric one. Obviously the strong agitation significantly helps to remove these partially soluble species, which is another way to uncover the hydrophobic sulfur-rich layer. The hydrophobicity experiments performed on the same chalcopyrite sample
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Table 5. Binding Energy (eV) of S, C, O, Sb, Zn, Ag, and Cu Elements and Kinetic Energy (eV) of Cu Auger Line Measured for Tetrahedrite after Conditioning in Aerated Water, pH 10, and Consecutive Sputtering core level C 1s
S 2p3/2
O 1s + Sb 3d5/2 a b c
treatment
a
b
a
b
water treatment and sputtering at (mA min) 0.6 4.6 14.6 19.6
162.9
161.4
286.5
284.8
162.9 162.8 162.7 162.9
161.5 161.4 161.3 161.4
286.4 286.5 286.5 286.6
284.8 284.8 284.6 284.8
532.5
Zn 2p3/2 a
Ag 3d5/2 a
Cu 2p3/2 a b
Cu Auger
530.8
529.7
1021.4
367.8
934.4
932.4
917.1
531.1 531.1 530.8 530.8
529.8 529.8 529.5 529.5
1021.6 1021.7 1021.5 1021.7
367.8 367.9 367.9 368.0
933.9
932.4 932.4 932.3 932.4
917.3 917.4 917.4 917.4
Table 6. Relative Intensities of Observed Lines in XPS Spectra of Tetrahedrite after Conditioning in Aerated Water, pH 10, and Consecutive Sputtering core level S 2p
C 1s
treatment
a
b
a
b
water treatment and sputtering at (mA min) 0.6 4.6 14.6 19.6
0.127
0.288
0.307
0.626
0.049 0.013 0.013 0.012
0.133 0.077 0.099 0.099
0.112 0.010 0.005
0.199 0.023 0.014 0.016
with and without agitation provide results32 which fully support the above explanation. Hence, these observations clearly indicate that a strong agitation of chalcopyrite in aerated water is the required condition for a so-called self-induced hydrophobicity of chalcopyrite at pH’s lower than 11. At a pH above 11 and an acidic pH the iron oxides/hydroxides layer is not stable at the chalcopyrite interface even in quiescent solution hence, the outermost sulfur-rich hydrophobic layer can be formed without any strong agitation. Surface Composition of Tetrahedrite in Contact with an Aqueous Solution at pH 10. The XPS results obtained for the tetrahedrite sample conditioned in water at pH 10, in an open to air vessel and after ion bombarding, are presented in Figure 2 and in Tables 5 and 6. As was expected on the basis of bulk composition of the mineral sample (Table 1), the XPS spectra show also the presence of zinc and silver in the surface layer of the mineral. It should also be noted that the O 1s component characteristic of the oxide species (component c, Figure 2a) overlaps the Sb 3d5/2 component. This complication can be overcome easily by the use of the Sb 3d3/2 component at 539 eV, for the characterization of the antimony species in the sample and for the subtraction of the overlapped Sb 3d5/2 component from the mixed O 1s and Sb 3d5/2 lines positioned at about 531 eV. There is a well established intensity ratio and splitting distance between both Sb 3d components.33 The spectra of the mineral sample, recorded immediately after removal from water (Figure 2a), clearly show that the outermost layer of the mineral is strongly oxidized. The O 1s line, which is high in intensity, very broad, and asymmetric, shows at least three components at 532.5, 530.8, and 529.7 eV, which could be assigned to the adsorbed water, hydroxides, and oxides and/or antimony Sb 3d5/2 component, respectively. The abovementioned subtraction of the antimony component allows us to determine the O 1s oxide component at about 530 eV, which has a very low intensity for the investigated sample. This implies that mainly hydroxides and adsorbed water are the surface products. This mineral sample contains only traces of iron, which can be deduced (32) Houot, R.; Joussement, R.; Bourgeois, L. Unpublished data. (33) Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., Muilenberg, G. E., Eds.; Handbook of X-ray Photoelectron Spectroscopy; Perkin Elmer Corporation: Eden Prairie, MN, 1979.
O 1s + Sb 3d5/2 a b c 0.996
Cu 2p3/2
Zn 2p3/2 a
Ag 3d5/2 a
a
b
1.885
1.219
0.371
0.038
3.536
1.000
1.045 0.201 0.123 0.098
0.811 0.441 0.450 0.466
0.264 0.193 0.149 0.156
0.033 0.008 0.009 0.019
2.069
1.000 1.000 1.000 1.000
after successive sputtering. Copper is the major element which forms the hydroxides species in the outermost layer. The presence of a very high intensity broad component at 934.4 eV, with a fwhm of 3.2 eV, indicates the formation of cupric hydroxide species. The presence of a satellite, between the Cu 2p components, at about 943.5 eV supports the conclusion that Cu2+ ions form the outermost layer (Figure 2a). The Cu Auger line at a kinetic energy of 917.1 eV (Table 5) is shifted by 0.3 eV to lower kinetic energy, also indicating the formation of the cupric oxidation species. Antimony is another element which can form surface oxidation species. A small but well visible asymmetry of the Sb 3d3/2 component may indicate the presence of a small amount of antimony hydroxides. The formation of antimony oxide species involves the chemical shift about 1 eV to higher binding energy,18 which is not observed in this work. The relative intensity ratio between the Sb 3d5/2 and Cu 2p3/2 lines indicates about a three times increase in the copper surface concentration in comparison to the bulk composition. This indicates at first that the observed asymmetry of the Sb 3d3/2 line can be caused by the formation of a copper-deficient mineral structure and secondly that antimony hydroxides are not the major species present at the interface. Zinc and silver, which are other metal ions present in the sample, show very low intensity lines in comparison with the “fresh” sample. Since the first very soft sputtering (0.6 mA min) causes a significant increase in the intensity of the zinc and silver lines, it indicates that the concentration of these atoms is several times lower at the interface than in the bulk. The S 2p spectrum shows a very broad line with an almost flat maximum (Figure 2a). On the basis of the above discussion of the chalcopyrite results, it was also assumed for tetrahedrite that the observed broadening of the S 2p line results from a multicomponent distribution of sulfur in the sulfur-rich surface layer. Therefore, the fitting of this line was performed with an assumption of two doublets, with the positions of the S 2p3/2 component for each doublet at 161.4 (doublet b) and 162.9 eV (doublet a) with the fwhm values of 1.35 and 1.55 eV, respectively. The second broad doublet represents a multicomponent distribution of sulfur in the surface layer. Because the mineral sample contains other minor metal elements (“impurities”) and this mineral could have up to 20% of
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Figure 2. XPS lines of tetrahedrite after conditioning in aerated water, pH 10, with their fitted components (a), and in-depth compositions after consecutive sputtering (b).
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Chalcopyrite, Tetrahedrite, and Tennantite
the copper atoms replaced by iron, zinc, silver, and mercury,34 it cannot be excluded that this modification will have an influence on the shape of the two S 2p doublets. Hence, it is probable that at least a part of the intensity of the MeSx doublet is caused by the replacement of copper by “impurity” metals, which could produce a significantly different environment for sulfur atoms in comparison to the one in the pure tetrahedrite sample. Ion bombarding experiments provide additional information on the in-depth distribution in the surface layer (Figure 2b). After very soft sputtering (0.6 mA min) the O 1s component at 532.5 eV disappears, and component a of the Cu 2p3/2 line shifts to the 933.9 eV position. This supports the above conclusion that the outermost layer of the sample consists of cupric hydroxide species with coadsorbed water. The change in the position of the Cu 2p3/2 indicates the heterogeneity of the outermost cupric oxide/hydroxide layer. Some increase in the intensity of component a of the Cu 2p3/2 line together with the O 1s hydroxide component is observed and most probably results from the removal of the carbon contamination layer and the adsorbed water during the first sputtering. The next sputtering (additional 4 mA min) removes almost all the cupric hydroxides species. The Cu 2p3/2 line shows a strong component at 932.4 eV which is due to copper from the mineral. Because the second sputtering does not cause disappearing of the O 1s hydroxide component, it indicates the presence of hydroxide species within the adsorption layer. They are cuprous hydroxides, which do not show a shift in the position of Cu 2p3/2 line. After two sputterings the relative intensities of the antimony and zinc lines reach their highest values (almost constant during the following sputterings), which are characteristic for the bulk mineral composition. The carbon contamination species (hydrocarbons and species containing carbon and oxygen) are also significantly removed after these two sputterings. Another observation is the shift of the Sb 3d5/2 component to a lower position with consecutive sputtering (Table 5), which confirms the above conclusion about the formation of a small amount of antimony oxides/hydroxides and/or a copper-deficient structure in the interface region of tetrahedrite. Comparison of Figures 2b and 1b shows that the changes in the intensity of the two sulfur components after subsequent sputtering are very different. A gradual increase in the intensity of the sulfur mineral component and an almost constant intensity of the MeSx component are observed (Figure 2b). This implies gradual changes in the surface sulfur composition, which is very different from the observation made for the chalcopyrite sample after similar conditioning in water (Figure 1b). Close inspection of the relative intensity ratio between the two components of the S 2p lines and the mineral component of the Cu 2p3/2 lines (Table 6) reveals that this ratio decreases gradually after the first two sputterings and that after the subsequent sputtering it remains almost constant. These together suggest that the MeSx component originates from surface enrichment. There is observed much lower sulfur enrichment at the interface of tetrahedrite than that of chalcopyrite and a gradual decrease in sulfur enrichment which results mainly from diffusion of copper to the outermost layer and the formation of cupric oxide/hydroxide species. These results do not suggest the sulfur-enriched layer-like structure which was found for chalcopyrite. Moreover, the presence of minor elements, as was already mentioned above, could modify the S 2p doublet at about 163 eV. A detailed discussion (34) Shuey, R. T. Semiconducting Ore Minerals; Elsevier: Amsterdam, 1975; p 336.
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Langmuir, Vol. 12, No. 10, 1996 2527
of the uncertainty in the assignment of the broadest sulfur components (MeSx) for different tetrahedrite samples can be found elsewhere.16 After the last sputtering the relative intensity ratio of Cu:As:S is about 1:0.41:0.111, giving the stoichiometry Cu12Sb4.9S10.3, which is very similar to that found for the bulk sample. The above results imply the following in-depth distribution of surface products at the interface of tetrahedritebasic aqueous solution: (i) the outermost thin layer contains hydrophilic species, mainly cupric hydroxides and adsorbed water, (ii) the intermediate layer is characteristic of low sulfur enrichment with a gradual decreasing in sulfur excess, and (iii) the interior is a structure with a composition very similar to that of the bulk tetrahedrite sample. The tetrahedrite sample contains a significant amount of silver and zinc, whose concentrations are much higher in bulk than at the interface. It is also interesting to note that carbon was found as one of the components of the bulk mineral sample. For details see also the schematic diagram presented in Figure 4. The results obtained allow us to propose the following major surface reaction:
Cu12Sb4S13 + 0.5xO2 + xH2O ) Cu12-xSb4S13 + xCu(OH)2 where Cu12-xSb4S13 represents all the changes in the interface region including sulfur-rich surface intermediates and Cu(OH)2 represents all copper (cuprous and cupric) oxides/hydroxides. These surface species do not form well defined homogeneous layers with clear interfaces; moreover, a lateral heterogeneity with patchwise structures is expected. This is in good agreement with the model of electrochemical type interactions between sulfide mineral and aerated water. The formation of antimony oxide/hydroxide species was not included in the reaction scheme. The obtained results indicate that tetrahedrite in contact with water at pH 10 will remain hydrophilic. Because there is not a good separation between the outermost hydrophilic species and the internal hydrophobic species, it seems that even a strong agitation of tetrahedrite in basic water will not produce a strong hydrophobicity of the mineral. Surface Composition of Tennantite in Contact with an Aqueous Solution at pH 10. The XPS results obtained for a tennantite sample conditioned in water at pH 10, in an open to air vessel and after sputtering, are presented in Figure 3 and in Tables 7 and 8. The XPS spectra also show that this mineral is not a pure tennantite and that a significant amount of zinc, a small amount of iron, and traces of antimony were also observed at the interface of the sample directly after polishing. X-ray diffraction analysis shows a significant amount of sphalerite and a small amount of pyrite and chalcopyrite with the sample (Table 1). The spectra recorded immediately after removal from water clearly show that the outermost layer of the mineral is oxidized, but the amount of the oxidation products formed on this mineral is much lower than those observed for chalcopyrite and tetrahedrite. The O 1s line is very broad and asymmetric. It shows three components at 532.6, 531.0, and 529.4 eV, which could be assigned to the adsorbed water, hydroxides, and oxides, respectively (Figure 3a). The adsorbed water and metal hydroxides are the major surface products. This mineral sample contains only a small amount of iron and antimony, which can be concluded from the XPS spectra only after successive sputtering; hence, iron and antimony do not form the outermost layer. Copper, arsenic, and zinc are
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Figure 3. XPS lines of tennantite after conditioning in aerated water, pH 10, with their fitted components (a), and in-depth compositions after consecutive sputtering (b).
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Langmuir, Vol. 12, No. 10, 1996 2529
Table 7. Binding Energy (eV) of S, C, O, Zn, As, and Cu Elements and Kinetic Energy (eV) of Cu Auger Line Measured for Tennantite after Conditioning in Aerated Water, pH 10, and Consecutive Sputtering core level C 1s
treatment
a
b
a
b
a
O 1s b
water treatment and sputtering at (mA min) 0.6 4.6 14.6 19.6
162.9
161.5
286.4
284.8
532.6
531.0
529.4
1021.7
162.8 162.9 163.0 162.9
161.5 161.5 161.5 161.5
286.3 286.1
284.8 284.6 284.7 284.7
532.7
531.2 531.9 531.9 531.9
529.7 529.7 529.7 529.7
1021.7 1021.7 1021.7 1021.7
S 2p3/2
As 3d
c
Zn 2p3/2 a
a
b
Cu 2p3/2 a
Cu Auger
44.5
42.7
932.3
917.3
42.7 42.7 42.7 42.7
932.3 932.4 932.3 932.4
917.3 917.2 917.3 917.3
Table 8. Relative Intensities of the Observed Lines in XPS Spectra of Tennantite after Conditioning in Aerated Water, pH 10, and Consecutive Sputtering core level S 2p
C 1s
treatment
a
b
a
b
a
O 1s b
water treatment and sputtering at (mA min) 0.6 4.6 14.6 19.6
0.053
0.179
0.049
0.080
0.113
0.060
0.017
0.698
0.041 0.025 0.030 0.033
0.164 0.169 0.312 0.186
0.032 0.004
0.061 0.022 0.011 0.007
0.060
0.041 0.027 0.022 0.013
0.024 0.027 0.018 0.018
0.766 1.172 1.440 1.555
the elements which can form the hydroxides species in the adsorption layer. The intensities of the Cu 2p3/2 and As 3d lines show a slow increase after successive sputtering while the intensity of the Zn 2p3/2 line increases rather sharply (Figure 3b). It signifies that copper and arsenic are the elements present in the outermost layer. There is no evidence for the formation of cupric hydroxide species (no satellite); hence, only cuprous species are expected at the interface. The Cu Auger line does not show an observable shift or additional component (Table 7) which could indicate the presence of cuprous oxides and hydroxides in a detectable amount. These observations are in good correlation with the finding that the amount of cuprous hydroxides is very low. The small intensity component of the As 3d line at 44.5 eV (Figure 3a) indicates the formation of arsenic oxides33 at the outermost layer. These species disappeared after the first sputtering. The S 2p line shows some broadening which cannot be fitted assuming only one doublet (Figure 3a). As in the case of other mineral samples, it was assumed that the observed broadening of the S 2p line results from the multicomponent distribution of sulfur in the surface-rich structure. In consequence the fitting of this line was performed assuming two doublets characteristic for the bulk mineral sulfur, one doublet at 161.5 eV (component b) and the second doublet at about 162.9 eV, which represents the multicomponent distribution of sulfur in the surface structure. The highest intensity ratio between both sulfur components is observed in the spectra recorded directly after conditioning in water, which indicates a sulfur enrichment in the outermost layer also in the case of tennantite. Nevertheless, the S 2p broad component at about 163 eV does not necessarily have to be fully attributed to the sulfur-rich layer. The tennantite sample contains a significant amount of other minerals (Table 1); moreover, some copper atom replacement by zinc, or other minor elements, in the structure of tennantite is very possible. Diversity in sulfur environment of the complex mineral structure will also participate in the broadening of the S 2p line. The problem is to determine how strong is this influence. At present it is difficult to find a very pure sample of tennantite which could be used as reference. Ion bombarding experiments provide additional information on the in-depth surface distribution (Figure 3b). After two soft sputterings (0.6 and 4.6 mA min) the O 1s broad line attributed to hydroxides and oxides species
As 3d
c
Zn 2p3/2 a
a
b
Cu 2p3/2 a
0.003
0.025
1.000
0.023 0.023 0.033 0.024
1.000 1.000 1.000 1.000
with a low intensity indicates the presence of a small amount of cuprous and arsenic oxides/hydroxides at the interface. Also carbon contamination species (hydrocarbons and species containing carbon and oxygen) are removed from the surface after these sputterings. At the same time the relative intensities of the zinc and copper lines reach their highest values (almost constant during the next sputterings), which are characteristic for the bulk mineral composition. Close inspection of the relative intensity ratio between the S 2p and Cu 2p3/2 lines reveals that this ratio is almost constant after all the sputterings at a value of 0.18 with the exception of the result obtained after a sputtering of 14.6 mA min when a sharp increase to 0.34 was observed. The same observation was made for the As 3d line. The simultaneous and exclusive changes of the S 2p and As 3d lines suggest that they are caused by an arsenic sulfide impurity which was removed from the surface by the following sputtering (19.6 mA min). This experiment was repeated with the same sample after a new surface polishing, and the above phenomenon was not observed again. After the last sputtering the relative intensity ratio of the elements Cu:As:S:Zn is about 1:0.024:0.22:1.55, which gives the stoichiometry Cu12As2.3S20.5Zn16.3. This quantitative estimation suggests that the surface is composed of a mixture of two major mineral components, tennantite with copper atoms replaced partly by zinc and zinc sulfide, analogous to the bulk tennantite sample. The above results suggest the following in-depth distribution of the surface products at the mineralaqueous solution interface: (i) the outermost thin layer contains a hydrophilic species, mainly cuprous hydroxides, adsorbed water, and arsenic hydroxides, (ii) the intermediate layer is only slightly enriched in sulfur, and (iii) the inner layer is a structure which has a composition very similar to that expected for the bulk structure. For details see also the schematic diagram presented in Figure 4. The tennantite sample contains a significant amount of zinc sulfide in the bulk structure, but the zinc surface concentration is much lower than that in the bulk. The results obtained imply that tennantite in contact with water at pH 10 will be hydrophilic. The results obtained allow us to propose the following major surface reaction:
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Figure 4. Schematic diagram of surface layer composition in the interface region for three minerals. Areas filled by patterns represent approximate distributions of major surface species in the interface region whose thickness is roughly estimated.
Cu12As4S13 + (0.25x + 0.75y)O2 + (0.5x + 1.5y)H2O ) Cu12-xAs4-yS13 + xCu(OH) + yAs(OH)3 where Cu12-xSb4-yS13 represents all the changes in the interface region, including sulfur-rich surface intermediates, and Cu(OH) and As(OH)3 represent all copper and antimony oxides/hydroxides. The experimental results clearly show that these surface species do not form well defined homogeneous layers with clear interfaces, and a lateral heterogeneity with patchwise structure is expected, similarly as for other mineral samples. Zinc sulfide, which is another major component of the investigated mineral sample, is not expected to be oxidized strongly. Comparison of XPS Results of Interaction of Chalcopyrite, Tetrahedrite, and Tennantite with an Aqueous Solution. The compositions of the mineral sample surfaces are strongly modified after conditioning with water at pH 10. In general, the following in-depth distribution of the surface composition was found for the investigated mineral samples: (i) the outermost thin layer contains hydrophilic species, mainly ferric or cupric (depending on the mineral sample) oxides/hydroxides and adsorbed water, (ii) the intermediate layer is a sulfurenriched structure with a gradually changing composition with hydrophobic properties which vary significantly among minerals, and (iii) the innermost layer has a bulk mineral composition and structure. A summarized sche-
Mielczarski et al.
matic diagram of the surface layer compositions for the three minerals is presented in Figure 4. The thicknesses of the surface oxidation layers marked on this scheme are very rough estimates. In the case of chalcopyrite the separation between the outermost hydrophilic layer and the internal hydrophobic layer is more pronounced than for the other mineral samples. Therefore, the outermost hydrophilic thin layer can be relatively easily removed mechanically and/or by dissolution during a strong agitation. This treatment involves a strong increase in the hydrophobicity of chalcopyrite without any collector, resulting in a so-called self-induced hydrophobicity. The surface layer formed on the tetrahedrite sample does not show such an established separation between the outermost hydrophilic species and the internal sulfurrich hydrophobic structure as in the case of chalcopyrite. Moreover, the produced hydrophilic layer is relatively thicker and the hydrophobic layer much thinner and not so well established as those observed for chalcopyrite. Therefore, the mechanical stripping of the hydrophilic outermost layer seems to be more difficult and in fact cannot be fully accomplished because of the mixed hydrophilic and hydrophobic surface structure. Hence, a strong agitation will not provide a high hydrophobicity, and this mineral will remain hydrophilic. The surface composition of tennantite is similar to that observed for tetrahedrite, where the gradual changes between the outermost hydrophilic metal hydroxide layer and the metal-deficient hydrophobic layer are observed. The difference between these minerals is that in the case of tennantite a much lower amount of oxidation products is formed in slow oxidation processes which yield the lower oxidation state (only cuprous) species. This obviously involves only limited changes in the composition of the interface region of the mineral. The surface structure of tennantite does not fulfill the conditions for the selfinduced hydrophobicity of the mineral. The quantitative determination of sulfur enrichment is difficult for tetrahedrite and tennantite because of the problem of separating the spectroscopic signals coming from the surface alternation and those from the presence of minor elements. A very strong mobility of copper atoms34 in the crystalline structure of the tetrahedrite and tennantite is probably the main reason that at their interface region the well established and separated outermost hydrophilic and internal hydrophobic layers are not formed. Therefore, the surface sulfur enrichment does not reach the level observed for chalcopyrite, for which iron atoms show much a higher tendency to diffuse to the interface than copper. This indicates that the differences in the diffusion of metal atoms in crystalline structures of the minerals, during contact with basic aqueous solution, have a significant influence on their surface composition. It is also interesting to note that although the tetrahedrite and tennantite contain a significant amount of other elements, they are not concentrated in the outermost layer; on the contrary, their surface concentrations are several times lower than those found for the bulk compositions. Acknowledgment. This work was supported by the European Community (Project MA2M-CT92-0062). The mineralogical analysis by Dr. P. Marion is gratefully acknowledged. LA9505881
