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Langmuir 1999, 15, 4530-4536

Oxidation of Synthetic and Natural Samples of Enargite and Tennantite: 2. X-ray Photoelectron Spectroscopic Study D. Fullston, D. Fornasiero,* and J. Ralston Ian Wark Research Institute, University of South Australia, The Levels Campus, Mawson Lakes, South Australia 5095, Australia Received October 28, 1998. In Final Form: March 9, 1999 The surface oxidation of synthetic and natural samples of enargite and tennantite has been monitored by X-ray photoelectron spectroscopy, XPS. The minerals were conditioned at pH 11.0 in an aqueous solution purged with nitrogen gas for 20 min or with oxygen gas for 60 min. The XPS results show that the oxidation layer on the mineral surface is thin. The surface oxidation products comprise copper and arsenic oxide/ hydroxide, sulfite, and a sulfur-rich layer made of metal-deficient sulfide and/or polysulfide. The proportion of all of these oxidation products at the mineral surface is more important when the minerals are treated in more oxidizing conditions (i.e., with oxygen gas and for a longer time) for tennantite than for enargite and for the natural samples than for the synthetic samples. Different arsenic sulfide species have been found at the surfaces of enargite and tennantite: As4S4 or As2S3 constitutes the major arsenic sulfide species at the surface of enargite, but these are the minor arsenic sulfide species at the surface of tennantite and in the bulk of both minerals. This difference is not related to a surface impurity in the natural enargite sample as it is also observed in the synthetic enargite sample.

Introduction Arsenic causes serious toxicological and environmental problems during the smelting of minerals containing this element.1-3 Enargite (Cu3AsS4) and tennantite (Cu12As4S13) are such minerals. It is therefore economically and environmentally beneficial to remove these minerals during flotation. Their separation is nevertheless difficult as they generally have similar flotation behavior to the valuable minerals with which they are associated. One of the separation methods relies on the selective oxidation of these minerals due to differences in their electrochemical properties.4 The floatability of sulfide particles is controlled by the level of surface oxidation in the absence or presence of collectors. Mineral separation can therefore be predicted and manipulated by a detailed knowledge of the surface species formed during oxidation. In a recent study,5 we investigated the oxidation of enargite and tennantite by measuring the dissolution of these minerals and their resultant zeta potential as a function of oxidizing conditions. Natural and synthetic mineral samples were also used to study the effect of impurities on the oxidation rate of these minerals. The zeta potential results showed that the surface oxidation is more important for tennantite than for enargite, and for the natural samples than for the synthetic samples. The amount of copper and arsenic dissolved from the minerals confirmed these results. The changes in zeta potential with pH and with oxidizing conditions could be interpreted by involving the presence of a layer of oxidation products covering a sulfur-rich mineral surface. We assumed that these oxidation products are mainly composed of copper oxide/hydroxide because of the isoelectric * Corresponding author. Fax: [email protected].

+61-8-8302-3683. E-mail:

(1) Ozberg, E.; Guthrie, R. I. L. Mat. Sci. Technol. 1985, 1, 12. (2) Dutr’e, V.; Vandecasteele, C. J. Hazard. Mater. 1995, 40, 55. (3) Morizot, G.; Ollivier, P. Miner. Eng. 1993, 6, 841. (4) Byrne, M.; Grano, S.; Ralston, J.; Franco, A. Miner. Eng. 1995, 8, 1571. (5) Fullston, D.; Fornasiero, D.; Ralston, J. Langmuir 1995, 15, 4524.

point of the minerals, the characteristic shape of the zeta potential versus pH curves, and the larger dissolution of copper than of arsenic. In the present study, we have used X-ray photoelectron spectroscopy (XPS) to monitor the surface oxidation of natural and synthetic samples of enargite and tennantite as a function of oxidizing conditions. The mineral samples and their conditioning are exactly the same as those used in our previous study of dissolution and zeta potential of these minerals.5 XPS is a surface-sensitive technique that can identify elements and their chemical state on a solid surface. XPS should therefore provide more detailed information than zeta potential measurements about the species present on the surface of tennantite and enargite during oxidation. Experimental Section XPS measurements were obtained with a Perkin-Elmer Physical Electronics Division (PHI) 5100 spectrometer using an Mg KR irradiation X-ray source operated at 300 W. A pass energy of 17.9 eV was used for all elemental spectral regions. The pressure in the analyzer chamber was 10-7 Pa. The energy scale was calibrated using the Fermi edge and the 4f7/2 line (BE ) 84.0 eV) for gold, while the retardation voltage was calibrated with the position of the Cu2p3/2 peak (BE ) 932.67 eV) and the Cu3p3/2 peak (BE ) 75.13 eV). An accelerated Ar+ ion beam at 3 kV was used to etch the mineral surface for 5 min. In this study, etching time rather than etching depth is used since the etching rate is dependent on the type of material to be etched.6 All chemicals were of analytical grade quality. High purity gas (nitrogen or oxygen from CIG Ltd) was scrubbed by bubbling it through a silica dispersion prior to introduction into the reaction vessel. High purity water, produced by reverse osmosis, two stages of ion exchange, and two stages of activated carbon prior to final filtration, was used in all experimental work. This water was “pretreated” at pH 11.0 with KNO3 (0.01 mol dm-3) as electrolyte and by bubbling the required gas into it. The natural samples of enargite and tennantite were purchased from Continental Minerals (Tucson, AZ). Their chemical analysis revealed that (6) Pratt, A. R.; Muir, I. J.; Nesbitt, H. W. Geochim. Cosmochim. Acta 1994, 58, 827.

10.1021/la981525w CCC: $18.00 © 1999 American Chemical Society Published on Web 05/13/1999

Synthetic and Natural Samples of Enargite and Tennantite the tennantite sample contains 45.7% Cu (weight %), 13.4% As, and 23.6% S with impurities of Fe (4.5%), Pb (0.03%), and Zn (0.04%) whereas the enargite sample contains 57.2% Cu, 11.7% As, and 28.3% S with impurities of Fe (3.0%), Pb (0.02%), and Zn (0.2%). Optical microscopy examination of the polished surface of these minerals revealed the presence of a small amount of bornite and chalcocite. The presence of iron on the mineral surface was also confirmed by XPS. The synthetic samples of enargite and tennantite are very pure; their preparation and characterization have been reported elsewhere.5 Just prior to use, 1 g of mineral was ground in a small amount of “pretreated” water with a ceramic mortar and pestle inside a glovebox flushed with nitrogen gas. The mineral dispersion was then transferred to a water-jacketed, closed reaction vessel and conditioned in 0.4 dm3 of a 0.01 mol dm-3 KNO3 solution at pH 11.0 in the presence of the required gas. The reaction vessel was equipped with a tightly fitting lid containing sealable ports for gas entry and exit, electrodes, and a temperature probe. The temperature inside the reaction vessel was maintained at 21.0 ( 0.2 °C. The mineral dispersion was stirred continuously throughout the experiment. The pH was kept at pH 11.0 by adding small quantities of a concentrated solution of nitric acid or potassium hydroxide. Minerals were conditioned for 20 min in the presence of nitrogen gas bubbling into the solution and for 60 min with oxygen gas. A short conditioning time with nitrogen gas and a long conditioning time with oxygen gas should provide conditions of slow and fast oxidation for the minerals, respectively.7 After decantation, the mineral slurry was washed once with “pretreated” water to remove any suspended colloidal particles and was introduced immediately into the fore-vacuum of the XPS spectrometer as a slurry.

Results To analyze the changes occurring in the XPS spectra during oxidation of the minerals in more detail, a deconvolution of the spectra into individual components is conducted. Each XPS spectrum after subtraction of a Shirley8 background, is fitted by means of an iterative nonlinear least-squares procedure with Gaussian bands.7,9 It is assumed for simplicity that, in each spectrum, the bandwidth is the same for all of the Gaussian bands. In the fitting procedure, the number of Gaussian bands is progressively increased to improve the goodness of fit but also to keep the bandwidth small enough so that the overlap between consecutive bands is minimized. The final results correspond to the minimum number of Gaussian bands required to best fit the experimental spectra. The goodness of fit may be judged from Figures 1 to 5, in which some calculated and experimental XPS spectra are compared. The relevant fitting parameters of the XPS spectra of each atomic element for the synthetic and the natural samples of enargite and tennantite conditioned with nitrogen or oxygen are given in Tables 1 to 4. The overall good fit obtained, together with the similar binding energy and bandwidth of the derived bands, as well as the concordance in their number between the various experiments, supports the reliability of the deconvolution procedure used. The binding energies of the elements present in copper mineral samples have been reported previously9 and are used to assign the individual components identified by the fitting exercise. The position of each peak is corrected for charging effects by fixing the position of the C1s carbon peak to 284.6 eV.10 In this study, the shift of this peak due (7) Fornasiero, D.; Li, F.; Ralston, J.; Smart, R. St. J. Colloid Interface Sci. 1994, 164, 333 and references therein. (8) Shirley, D. A Phys. Rev. B 1972, 5, 4709. (9) Fairthorne, G.; Fornasiero, D.; Ralston, J. Int. J. Miner. Process. 1997, 49, 31 and references therein. (10) Buckley, A. N.; Woods, R.; Wouterlood, H. J. Int. J. Miner. Process. 1989, 26, 29.

Langmuir, Vol. 15, No. 13, 1999 4531

Figure 1. O1s XPS spectrum of natural enargite conditioned at pH 11.0 for 60 min with oxygen gas. The dots represent the experimental spectrum, and the full line represents the calculated spectrum obtained by summing all of the Gaussian bands (- - -).

to charging ranges for + 0.3 to 0.6 eV (Tables 1 to 4) and does not appear to be related to the type of mineral used and its purity, nor to be affected by etching the surface with the Ar+ ion beam. In the C1s XPS spectra, the bands at 284.6 eV and at around 286.4 eV are attributed to hydrocarbon contamination10,11 and to carbonyl species, respectively. The high binding energy component (288.5 eV) of small intensity (except for natural tennantite) is attributed to carbonate species.7,9 Surface carbonate species originate from the adsorption of carbon dioxide onto the surface oxide and hydroxide groups,11 even though high purity nitrogen and oxygen and an airtight conditioning vessel were employed.12 After etching the surface with the Ar+ ion beam, the amount of hydrocarbon and carbonate on the mineral surface is greatly reduced. The O1s spectra (Figure 1) may be divided into three components: oxide species; hydroxide, carbonate, or sulfur-oxy species; adsorbed water.13-17 The latter is the component in the O(1s) spectra at around 532.5-532.8 eV. The bands due to oxides occur at 529.6-530.3 eV while the hydroxide, carbonate, or sulfur-oxy species emit at a higher binding energy of around 531.0-531.7 eV. The proportion of surface oxygen increases after conditioning the mineral for a longer period of time with oxygen but decreases after etching. The proportion of oxide is always equal or less than that of hydroxide. In the Cu2p spectra, the very small intensity of the broad band observed around 942 eV (not shown in Figure 2), which is attributed to Cu(II) shake-up satellites, implies that the surface copper is mainly in a cuprous state with the most intense band at 932.0-932.3 eV attributed to Cu(I) in the sulfide lattice.16 The proportion of surface cupric oxide or hydroxide, 933.9-934.5 eV and 935.4936.2 eV11,18 is relatively small; it increases in more oxidizing conditions but decreases after etching (Tables 1 to 4). (11) Smart, R. St. C. Miner. Eng. 1991, 4, 891. (12) Smart, R. St. C.; Slager, T. L.; Little, L. H.; Greenler, R. G. J. Phys. Chem. 1973, 77, 1019. (13) McIntyre, M. S.; Zeturak, D. G. Anal. Chem. 1977, 49, 1521. (14) Mills, P.; Sullivan, J. L. J. Phys. D: Appl. Phys. 1983, 16, 723. (15) Ferris, F. G.; Tazaki, K.; Fyfe, W. S. Chem. Geol. 1989, 74, 321. (16) Wagner, C. D. In Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; John Wiley: Chichester, England, 1990; Vol. 1, p 595. (17) Nesbitt, H. W.; Muir, I. J.; Pratt, A. R. Geochim. Cosmochim. Acta 1995, 59, 1773.

4532 Langmuir, Vol. 15, No. 13, 1999

Fullston et al.

Table 1. XPS Results for Natural Enargite Conditioned at pH 11.0;a Deconvolution with Gaussian Bandsb,c conditioning gas/ band number N2 BE shift +0.3 eV 1 2 3 N2/etched BE shift +0.4 eV 1 2 3 O2 BE shift +0.3 eV 1 2 3 O2/etched BE shift +0.6 eV 1 2 3

C1s

O1s

Cu2p3/2

As3d

S2p

Fe2p3/2

284.6//2.1/26.9/100 286.5/2.1/2.6/10 288.5/2.1/3.6/13

529.6/1.6/5.0/100 531.0/1.6/5.8/116 532.5/1.6/3.8/76

932.1/1.7/15.7/100 934.0/1.7/2.8/18 935.4/1.7/0.7/4

43.2/1.6/4.4/100 45.1/1.6/0.4/9

161.8/1.6/21.6/100 163.7/1.6/3.2/15 166.5/1.6/0.9/4

(2.6)

284.6/2.0/4.9/100 286.4/2.0/0.7/14

(4.0)

932.3/1.7/40.2/100 934.0/1.7/4.6/11 935.8/1.7/0.9/2

42.5/1.6/3.7/100 44.8/1.6/0.7/19

161.6/1.6/30.7/100 163.6/1.6/3.4/11 166.5/1.6/1.1/4

(5.2)

284.6/2.0/21.0/100 286.3/2.0/4.5/21 288.6/2.0/5.6/27

529.6/1.6/6.9/100 531.0/1.6/11.5/167 532.5/1.6/6.4/93

932.1/1.7/12.9/100 934.0/1.7/4.6/36 935.8/1.7/1.9/15

43.1/1.7/2.8/100 45.5/1.7/0.6/21

161.7/1.6/16.2/100 164.0/1.6/2.1/13 166.8/1.6/0.9/6

(1.9)

284.6/2.3/3.0/100 286.6/2.3/0.8/27

(7.1)

932.2/1.7/39.5/100 934.4/1.7/4.4/11 936.0/1.7/0.6/2

42.6/1.7/3.7/100 44.9/1.7/0.8/22

161.7/1.6/29.1/100 163.9/1.6/3.0/10 166.5/1.6/0.9/3

(7.3)

a The position of each peak is corrected for charging effects by fixing the position of the carbon peak to 284.6 eV.10 b The binding energy (eV)/bandwidth (eV)/percentage of the total/percentage relative to the first band are given for each Gaussian band. c The spectral deconvolution was not attempted when the noise level in the XPS spectra was too high; the percentage of the element is then given in brackets.

Table 2. XPS Results for Natural Tennantite Conditioned at pH 11.0;a Deconvolution with Gaussian Bandsb,c conditioning gas/ band number N2 BE shift +0.4 eV 1 2 3 N2/etched BE shift +0.4 eV 1 2 3 O2 BE shift +0.3 eV 1 2 3 O2/etched BE shift +0.4 eV 1 2 3

C1s

O1s

Cu2p3/2

As3d

S2p

Fe2p3/2

284.6/2.1/21.0/100 286.4/2.1/3.0/14 288.5/2.1/14.8/70

530.1/2.0/9.9/100 531.6/2.0/10.6/107

932.3/1.7/15.6/100 933.9/1.7/1.8/12 935.8/1.7/0.3/2

42.3/1.6/3.3/100 44.4/1.6/0.8/24

161.6/1.4/14.4/100 163.9/1.4/1.0/7 166.5/1.4/0.7/5

(2.7)

284.6/2.3/5.4/100 286.5/2.3/2.0/37 288.7/2.3/1.1/20

(1.1)

932.2/1.7/31.9/100 934.4/1.7/4.4/14 936.0/1.7/1.5/5

42.4/1.7/6.5/100 44.7/1.7/1.4/22

161.6/1.4/34.6/100 163.6/1.4/2.8/8 166.3/1.4/1.0/3

(6.4)

284.6/2.0/21.7/100 286.7/2.0/3.2/15 288.5/2.0/13.2/61

530.1/2.5/11.9/100 531.3/2.5/16.3/137

932.2/2.0/10.6/100 934.2/2.0/7.1/67 936.0/2.0/3.9/37

42.1/1.6/1.9/100 44.3/1.6/1.1/58

161.4/1.7/6.6/100 164.0/1.7/0.8/12 166.8/1.7/0.3/5

(1.4)

284.6/2.2/4.1/100 286.4/2.2/0.6/15 288.8/2.2/1.0/24

530.3/3.2/1.8/100 531.7/3.2/8.6/478

932.3/1.7/29.7/100 934.5/1.7/4.0/13 936.1/1.7/1.7/6

42.4/1.7/5.9/100 44.7/1.7/1.7/29

161.5/1.5/29.1/100 163.8/1.5/2.9/10

(10.7)

a The position of each peak is corrected for charging effects by fixing the position of the carbon peak to 284.6 eV.10 b The binding energy (eV)/bandwidth (eV)/percentage of the total/percentage relative to the first band are given for each Gaussian band. c The spectral deconvolution was not attempted when the noise level in the XPS spectra was too high; the percentage of the element is then given in brackets.

The S2p spectra are composed of three components (Figure 3). Each component consists of two peaks (1/2, 3/2) that are separated by 1.19 eV, with the intensity of the lower binding energy peak being double that of the higher binding energy peak. The component at 161.4-161.8 eV is consistent with metal sulfide as in covellite or chalcocite.19,20 The intermediate binding energy component at 163.5-164.2 eV is attributed to polysulfide since elemental sulfur, if formed, is not expected to remain on the mineral surface at room temperature in the ultrahigh vacuum.21,22 The high binding energy component at 166.2166.8 eV is characteristic of sulfite.7,9 By using a smaller (18) McIntyre, M. S.; Cook, M. G. Anal. Chem. 1975, 47, 2208. (19) Buckley, A. N.; Woods, R. Aust. J. Chem. 1984, 37, 2403. (20) Termes, S. C.; Buckley, A. N.; Gillard, R. D. Inorg. Chim. Acta 1987, 126, 79. (21) Wittstock, G.; Kartio, I.; Hirsch, D.; Kunze, S.; Szargan, R. Langmuir 1996, 12, 5709. (22) Buckley, A. N.; Kravets, I. M.; Shchukarev, A. V.; Woods, R. J. Appl. Electrochem. 1994, 24, 513.

bandwith, the metal sulfide band at 161.4-161.8 eV can be further deconvoluted into two components with a minor component at around 161 eV and a major component at around 162 eV which may be attributed to metal-deficient sulfide or disulfide.17,23 However, this increase in the number of bands does not improve the fit of the S2p spectra (no decrease in the sum of the residual squares between the experimental and calculated spectra) and produces much larger errors for the fitting parameters. Moreover, the intensity ratio of the metal sulfide minor to major components does not change with oxidation conditions, as was also the case in XPS studies of arsenopyrite17 and chalcopyrite.9 For these reasons, only the results obtained with the use of three components to fit the S2p spectra are reported in Tables 1 to 4. The absence of peaks at higher binding energies in all of the S2p spectra indicates (23) Buckley, A. N.; Walker, G. W. XVI Int. Min. Proc. Congress, Forssberg, E., Ed.; Elsevier: Amsterdam, The Netherlands, 1988; p 589.

Synthetic and Natural Samples of Enargite and Tennantite

Langmuir, Vol. 15, No. 13, 1999 4533

Table 3. XPS Results for Synthetic Enargite Conditioned at pH 11.0;a Deconvolution with Gaussian Bandsb,c conditioning gas/ band number N2 BE shift +0.3 eV 1 2 3 N2/etched BE shift +0.5 eV 1 2 3 O2 BE shift +0.6 eV 1 2 3 O2/etched BE shift +0.6 eV 1 2 3

C1s

O1s

Cu2p3/2

As3d

S2p

284.6/1.9/19.9/100 286.4/1.9/11.9/60 288.6/1.9/1.7/9

530.1/1.6/1.5/100 531.3/1.6/3.7/247 532.7/1.6/4.7/313

932.0/1.7/15.4/100 934.1/1.7/1.3/8 935.8/1.7/0.1/1

43.1/1.6/7.0/100 45.7/1.6/0.5/7

161.7/1.5/28.5/100 163.9/1.5/2.6/9 166.6/1.5/0.8/3

284.6/2.1/5.9/100 286.5/2.1/2.2/37

(3.3)

932.2/1.7/37.0/100 934.2/1.7/3.4/9 936.0/1.7/0.1/0

42.4/1.5/8.6/100 44.3/1.5/1.1/13

161.7/1.6/35.1/100 164.2/1.6/3.1/9 166.8/1.6/1.2/3

284.6/2.0/18.5/100 286.4/2.0/8.8/48 288.5/2.0/2.7/15

530.3/1.6/2.5/100 531.4/1.6/4.5/180 532.6/1.6/2.3/92

932.1/1.7/16.0/100 934.3/1.7/1.9/12 936.0/1.7/0.3/2

43.0/1.6/7.2/100 45.3/1.6/0.7/10

161.6/1.5/29.1/100 163.9/1.5/4.0/14 166.7/1.5/1.4/5

284.6/2.1/5.0/100 286.5/2.1/1.6/32

(0.3)

932.3/1.7/40.9/100 934.2/1.7/3.8/9 936.1/1.7/0.6/1

42.3/1.6/7.6/100 44.4/1.6/1.0/13

161.5/1.6/35.4/100 164.0/1.6/2.8/8 166.4/1.6/1.0/3

a The position of each peak is corrected for charging effects by fixing the position of the carbon peak to 284.6 eV.10 b The binding energy (eV)/bandwidth (eV)/percentage of the total/percentage relative to the first band are given for each Gaussian band. c The spectral deconvolution was not attempted when the noise level in the XPS spectra was too high; the percentage of the element is then given in brackets.

Table 4. XPS Results for Synthetic Tennantite Conditioned at pH 11.0;a Deconvolution with Gaussian Bandsb,c conditioning gas/ band number N2 BE shift +0.5 eV 1 2 3 N2/etched BE shift +0.4 eV 1 2 3 O2 BE shift +0.6 eV 1 2 3 O2/etched BE shift +0.5 eV 1 2 3

C1s

O1s

Cu2p3/2

As3d

S2p

284.6/2.0/35.0/100 286.3/2.0/8.0/23 288.3/2.0/3.3/9

530.2/1.6/5.6/100 531.6/1.6/5.7/102 532.7/1.6/2.9/52

932.1/1.9/14.6/100 934.1/1.9/1.9/13

42.2/1.6/6.7/100 44.4/1.6/1.5/22

161.6/1.7/21.8/100 163.8/1.7/3.1/14 166.8/1.7/1.1/5

284.6/2.1/8.3/100 286.5/2.1/2.3/28

(1.3)

932.2/1.7/32.2/100 934.2/1.7/2.6/8 935.9/1.7/0.2/1

42.3/1.5/12.7/100 44.4/1.5/1.2/9

161.6/1.6/34.4/100 163.9/1.6/3.3/10 166.6/1.6/1.4/4

284.6/1.9/28.2/100 286.3/1.9/7.6/27 288.5/1.9/2.4/9

530.1/1.6/13.8/100 531.6/1.6/10.5/76 532.8/1.6/0.8/6

932.2/2.1/9.7/100 934.2/2.1/4.6/47 936.2/2.1/1.4/14

42.1/1.7/4.0/100 44.3/1.7/1.5/38

161.5/2.0/12.6/100 163.9/2.0/2.4/19 166.8/2.0/1.0/8

284.6/2.1/5.2/100 286.7/2.1/1.9/37

(0.2)

932.3/1.7/35.2/100 934.5/1.7/2.8/8 936.2/1.7/0.2/1

42.2/1.5/12.2/100 44.4/1.5/1.6/13

161.5/1.7/34.9/100 163.5/1.7/4.4/13 166.2/1.7/1.3/4

a The position of each peak is corrected for charging effects by fixing the position of the carbon peak to 284.6 eV.10 b The binding energy (eV)/bandwidth (eV)/percentage of the total/percentage relative to the first band are given for each Gaussian band. c The spectral deconvolution was not attempted when the noise level in the XPS spectra was too high; the percentage of the element is then given in brackets.

that thiosulfate and sulfate are not present on the mineral surface under the conditions used in this study. The proportions of polysulfide and sulfite on the surface of all minerals increase when nitrogen is replaced with oxygen but decreases after etching (Tables 1 to 4). The As3d spectra (Figures 4a and 5a) are composed of two components (3/2, 5/2). Each component consists of two peaks that are separated by 0.7 eV, with the intensity of the higher binding energy peak being two-thirds that of the lower binding energy peak.17,24,25 For tennantite (Figure 4a), the higher binding energy component at 44.344.7 eV is consistent with As(III)-O as in As2O3,17,26,27

whereas the lower binding energy component at 42.142.4 eV is associated with arsenic copper sulfide. Peaks for FeAsS17,28 and GaAs24-27 have been found at even lower binding energies, 41.2 eV and 41.1-41.7 eV, respectively, whereas elemental arsenic produces a peak at 41.5 eV.16 The peaks for As4S4, As2S3, and As2S5 are reported at 43.1, 43.4, and 44.4 eV, respectively, at approximately 1 eV lower than their corresponding arsenic oxides.16,29 The binding energy of the As3d peaks reported in this study are similar to those obtained in an earlier XPS study of tennantite.30 Mielczarski et al. 30 reported peaks at 42.7 and 44.5 eV for tennantite; the high binding energy peak

(24) Cho, J.; Pawlowicz, L. M.; Saha, N. C. J. Appl. Phys. 1992, 72, 4172. (25) Song, Z.; Shogen, S.; Kawasaki, M.; Suemune, I. J. Vac. Sci. Technol. B 1995, 13, 77. (26) Campo, A.; Cardinaud, Ch.; Turban, G.; Dubon-Chevallier, C.; Amarger, V.; Etrillard, J. J. Vac. Sci. Technol. A 1993, 11, 2536. (27) Hirota, Y. J. Appl. Phys. 1994, 75, 1798.

(28) Buckley, A. N.; Walker, G. W. J. Appl. Phys. 1988-1989, 35, 227. (29) Handbook of X-ray Photoelectron Spectroscopy; Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., Muilenberg, G. E., Eds.; Perkin-Elmer Corporation, 1979. (30) Mielczarski, J. A.; Cases, J. M.; Alnot, M.; Ehrhardt, J. J. Langmuir 1996, 10, 2519.

4534 Langmuir, Vol. 15, No. 13, 1999

Figure 2. Cu2p3/2 XPS spectrum of natural enargite conditioned at pH 11.0 for (top) 20 min with nitrogen gas and (bottom) 60 min with oxygen gas. The dots represent the experimental spectra, and the full lines represent the calculated spectra obtained by summing all of the Gaussian bands (- - -).

Figure 3. S2p XPS spectrum of natural enargite conditioned at pH 11. 0 for (top) 20 min with nitrogen gas and (bottom) 60 min with oxygen gas. The dots represent the experimental spectra, and the full lines represent the calculated spectra obtained by summing all of the Gaussian bands (- - -).

was attributed to “arsenic oxides”. For enargite, the interpretation of the As3d spectra (Figure 5a) is more complex. After etching, the peaks at 42.3-42.6 eV and at 44.3-44.9 eV are similar to those found for tennantite; however, before etching, these peaks are obtained at

Fullston et al.

Figure 4. As3d XPS spectra of natural tennantite conditioned at pH 11.0 for 60 min with oxygen gas. The dots represent the experimental spectra, and the full lines represent the calculated spectra obtained by summing (a) two or (b) four Gaussian bands (- - -).

slightly higher binding energies, 43.0-43.2 eV and 45.145.5 eV (Tables 1 to 4). Since the separation between these two peaks is approximately the same before and after etching, one might conclude that these peaks are the same in both cases and only differential charging is at the origin of the shift in binding energies of these peaks.31 The presence of a surface impurity or an insulating oxidation layer, subsequently removed after etching, may cause this differential charging. If this were the case, one would have expected similar binding energy shifts for other elements present on the enargite surface and to a broader C1s spectra for enargite before etching. As shown in Tables 1 to 4, this explanation of differential charging is not validated. The problem of surface impurity has also to be discarded since this shift in the position of the As3d peaks only occurs for enargite and for both the synthetic and natural samples. Furthermore, the observation that the surface of the natural samples oxidizes more than that of the synthetic samples and that tennantite is more oxidized than enargite eliminates the explanation of a surfaceinsulating oxidation layer on the enargite samples. To solve this problem of bands with different binding energies, the deconvolution of the all of the As3d spectra is further refined with inclusion of additional bands covering the appropriate spectral range from 42 eV up to 46 eV, at approximately 42 eV for non-oxidized arsenic copper sulfide, at 43.4 eV for As4S4 and As2S3,16,29 at 44.5 eV for As2O3,17,26,27 and at 45.3 eV for As2O5.17 As these four bands invariably merge into two bands in the fitting procedure, several restrictions are incorporated to keep these bands apart: the same bandwidth is used for the four bands, (31) Barr, T. L. In Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; John Wiley: Chichester, England, 1990; Vol. 1, p 357.

Synthetic and Natural Samples of Enargite and Tennantite

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Figure 6. Intensity of the four Gaussian bands used to calculate the As3d XPS spectra of the natural and synthetic samples of enargite and tennantite conditioned at pH 11.0 with nitrogen gas for 20 min (O) before and (4) after surface etching, and with oxygen gas for 60 min (b) before and (2) after surface etching (the first, second, third, and fourth bands are attributed to non-oxidized arsenic copper sulfide, As4S4 or As2S3, As2O3, and As2O5, respectively). Figure 5. As3d XPS spectra of natural enargite conditioned at pH 11.0 for 60 min with oxygen gas. The dots represent the experimental spectra, and the full lines represent the calculated spectra obtained by summing (a) two or (b) four Gaussian bands (- - -).

and its upper value is limited to 1.2 eV; the binding energy of the second, third, and fourth bands was not allowed to change by more than 0.1 eV from their initial values (no restriction is imposed on the band with the lowest binding energy, the first band). These restrictions result in slightly worse spectral fits (Figures 4b and 5b) and much larger errors for the fitting parameters, especially for the high binding energy bands of low intensity. The final results show that a bandwidth of 1.2 eV is obtained for all of the bands and their binding energies vary from 42.00 to 42.35 eV for the first band, from 43.35 to 43.50 eV for the second band, from 44.50 to 44.60 eV for the third band, and from 45.35 to 45.40 eV for the fourth band. As the binding energies of the four bands are almost identical in all of the As3d spectra, interpretation of these spectra is only restricted to a comparison of band intensity (or percentage) with etching and with oxidation treatment. The As3d band intensities for the natural and synthetic samples of enargite and tennantite are shown in Figure 6. These figures reveal that similar trends in band intensity are observed for the natural and synthetic samples. The intensity of the first (non-oxidized arsenic copper sulfide) and second (As4S4 and As2S3) bands is always much higher than that of the last two bands (As2O3 and As2O5). The first band (lowest binding energy) is always more intense than the second band except for enargite before etching. Etching has a larger effect on the intensity of the first two bands; they increase after etching except for the second band of enargite which decreases in intensity. No attempt is made to deconvolute the Fe2p spectra into individual components because of the small amount of iron present on the mineral surface and the lack of features of the Fe2p spectral envelope. However, the broadness of the spectra up to a binding energy of 714 eV

implies that a large proportion of the surface iron is oxidized to ferric oxide/hydroxide.9,17 We also note that the proportion of iron on the mineral surface is greater when the minerals are conditioned in nitrogen than in oxygen and after etching the surfaces. Discussion The oxidation of tennantite and enargite has been investigated in three recent studies. From their electrochemical study of enargite, Cordova et al.32 proposed that only copper is involved in the first oxidation step with the formation of a copper-depleted surface and copper oxide. Mielczarski et al.30 in their XPS study of tennantite found a small amount of cuprous and arsenic hydroxides covering a sulfur-rich surface. The formation of copper oxide/ hydroxide on a sulfur-rich surface was confirmed in a zeta potential study of tennantite and enargite.5 Furthermore, it was shown in that study5 that the amount of copper dissolution and surface copper oxidation products increases with oxidizing conditions and is more important for tennantite than for enargite and for the natural samples than for the synthetic samples. It was also found that the amount of arsenic dissolved from the minerals was much lower that that of copper. The present XPS study confirms our earlier findings and gives more detailed information on the species present on the surface of tennantite and enargite during oxidation at pH 11.0. The mineral surface is not heavily oxidized. The surface oxidation layer is thin as XPS can probe the non-oxidized mineral. Cupric oxide/hydroxide is the major oxidation product. It is found at the top surface together with a sulfur-rich phase made of metal-deficient sulfide and/or polysulfide, as the proportion of both these species decreases after etching. Conditioning the minerals with oxygen for 60 min produces more metal-deficient sulfide (32) Cordova, R.; Gomes, H.; Real, S. G.; Schrebler, R.; Vilche, J. R. J. Electrochem. Soc. 1997, 144, 2628.

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and polysulfide, more sulfite, and more copper and arsenic oxide/hydroxide that when the minerals are conditioned with nitrogen for 20 min. The O1s, Cu2p, As3d, and S2p XPS spectra indicate that the natural and the synthetic samples of tennantite are more oxidized than the corresponding samples of enargite (Tables 1 to 4). The As3d spectra are particularly interesting since they reveal differences in oxidation between the two arsenic minerals at the top surface layer. According to the XPS analysis, As4S4 or As2S3 constitutes the major arsenic sulfide species at the surface of enargite but are the minor arsenic sulfide species at the surface of tennantite and in the bulk of both minerals. The presence of As2S2 and As2S3 at the enargite surface has been postulated by Cordova et al.;32 As2S2 was formed with Cu2S by the electroreduction of a copperdepleted arsenic sulfide phase in the presence of cupric oxide. Furthermore, the natural arsenic samples contain a small amount of iron as impurity. Iron is mainly oxidized, but its proportion at the mineral surface does not increase with oxygen conditioning and is less than that in the bulk of the mineral. The presence of iron impurity in the natural samples may increase galvanic interactions in these samples and explain the larger amount of oxidation products observed on the surface of the natural samples. From the XPS results, the following reaction in alkaline solutions is proposed for the initial oxidation of enargite (or tennantite)

Cu3AsS4 + (x/2 + 3/4y)O2(aq) f Cu3-xAs1-yS4 + xCuO + 1/2yAs2O3 Copper and, to a lesser extent, arsenic (x > y) migrate to the surface to form an outermost layer of copper and arsenic oxide/hydroxide covering a metal-deficient, sulfurrich surface. A high level of metal-deficiency results in surface restructuring to form polysulfide.23 With further oxidation, polysulfide oxidizes to sulfite, and As2O3 oxidizes to As2O5 at the surface of enargite and tennantite. The mechanism by which As4S4 and As2S3 are formed at the mineral surface is uncertain. These species may result from a surface restructuring following the diffusion of cuprous sulfide, leaving a copper and sulfur-depleted

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arsenic-rich phase33 or after the formation of polysulfide. The fact that they are formed preferentially at the enargite surface seems to indicate that the crystal structure of the minerals, orthorhombic for enargite and cubic for tennantite, is important for the formation of As4S4 and As2S3 and for the rate of oxidation of the arsenic minerals. Conclusions The surface oxidation of synthetic and natural samples of enargite and tennantite has been monitored by XPS. The minerals were conditioned at pH 11.0 in an aqueous solution purged with nitrogen gas for 20 min or with oxygen gas for 60 min. The XPS results show that the oxidation layer on the mineral surface is thin. The surface oxidation products comprise copper and arsenic oxide/hydroxide, sulfite, and a sulfur-rich layer made of metal-deficient sulfide and/or polysulfide. The proportion of all of these oxidation products at the mineral surface is more important when the minerals are treated in more oxidizing conditions (i.e., with oxygen gas purging and for a longer time) for tennantite than for enargite and for the natural samples than for the synthetic samples. Different arsenic sulfide species have been found at the surface of enargite and tennantite: As4S4 or As2S3 constitutes the major arsenic sulfide species at the surface of enargite but are the minor arsenic sulfide species at the surface of tennantite and in the bulk of both minerals. This difference is not related to a surface impurity in the natural enargite sample since it is also observed in the synthetic enargite sample. Mineral impurities and different crystal structures may be at the origin of the difference in oxidation rate observed between the natural and synthetic mineral samples, and between enargite and tennantite, respectively. Acknowledgment. The financial support for this work from the Australian Research Council and Rio Tinto Ltd is gratefully acknowledged. LA981525W (33) Chen, T. T.; Dutrizac, J. E.; Owens, D. R.; Laflamme, J. H. G. Can. Mineral. 1980, 18, 173.