In Situ FTIR−Spectroelectrochemical Study of the Anodic Processes

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In Situ FTIR-Spectroelectrochemical Study of the Anodic Processes on a Galena (PbS) Electrode under Open-Air Conditions in the Absence and Presence of n-Butyl Xanthate I. V. Chernyshova* St. Petersburg State Technical University, Polytechnicheskaya 29, 195251 St. Petersburg, Russia Received January 3, 2002. In Final Form: April 18, 2002 Anodic reactions on a galena (PbS) electrode were studied in situ using attenuated total reflection/ Fourier transform infrared spectroscopy within the -0.5 to +0.7 V (standard hydrogen electrode) potential range in air-saturated borate buffer (pH 9.2) in the absence and presence of n-butyl xanthate. Compared to the deaerated conditions, the anodic decomposition of galena is increased by 2-3 times, being accompanied by the enhanced precipitation of Pb(OH)2 both in the absence and presence of xanthate at low concentrations. The increased precipitation rate of Pb(OH)2 is explained by acceleration of the incongruent dissolution of galena (PbS + H2O + 2h+ w S° + H+ + Pb(OH)+ ) which is caused by an increase in the potential drop in the Helmholtz layer due to the oxygen adsorption. As in the case of the deaerated xanthate solutions, the formation of bulk lead xanthate is preceded by chemisorption of xanthate, and dixanthogen is formed with no overpotential. However, at low concentrations of the reagent, lead xanthate is formed by the precipitation mechanism against the anodic decomposition of galena. At more anodic potentials, this species is substituted by lead hydroxide. At high concentrations of xanthate, xanthate chemisorption inhibits the electrochemical dissolution of galena and the decomposition of adsorbed lead xanthate mediating the formation of lead xanthate and, at higher potentials, that of both lead xanthate and dixanthogen.

Introduction Electrochemically generated ultrathin films on semiconductors have been studied intensively in recent years in the context of flotation, electrocatalysis, solar energy storage, microelectronics, photocatalytic reactions, photodegradation of organics, and other technologies in which interfacial layers with tailored characteristics are required.1,2 Oxidation of semiconducting sulfides in aqueous solution and their interaction with chemical reagents are of long-standing interest for geochemistry, environmental chemistry, solar energy conversion, hydrometallurgy, and flotation.3 Against much research of the anodic processes on galena (PbS) (for review, see refs 4-7), very few studies6-13 have been conducted using in situ spectroscopic * E-mail: [email protected]. Fax: +7 (812) 428-5712. (1) Semiconductor Electrodes (studies in physical and theoretical chemistry); Finklea, H. O., Ed.; Elsevier: Amsterdam, 1988. (2) Nozik A. J.; Memming, R. J. Phys. Chem. 1996, 100, 13061. (3) Environmental Geochemistry of Sulfide Oxidation; Alpers, C. N., Blowes, D. W., Eds.; ACS Symposium Series 550; American Chemical Society: Washington, DC, 1994. (4) Proceedings of the International Symposium on Electrochemistry in Mineral and Metal Processing III; Woods, R., Richardson, P. E., Eds.; The Electrochemical Society: Pennington, NJ, 1992; Vol. 92-17. (5) Proceedings of the International Symposium on Electrochemistry in Mineral and Metal Processing IV; Woods, R., Richardson, P. E., Doyle, F. M., Eds.; The Electrochemical Society, Pennington, NJ, 1996; Vol. 96-6. (6) Chernyshova, I. V. J. Phys. Chem. B 2001, 105, 8185. (7) Chernyshova, I. V. J. Phys. Chem. B 2001, 105, 8178. (8) Kim, B. S.; Hayers, R. A.; Prestige, C. A.; Ralson, J.; Smart, R. St. Langmuir 1995, 11, 2554. (9) Higgins, S. R.; Hamers, R. J. Surf. Sci. 1995, 324, 263. (10) Wittstock, G.; Kartio, I.; Hirsch, D.; Kunze, S.; Szargan, R. Langmuir 1996, 12, 5709. (11) Eggleston, C. M. Geochim. Cosmochim. Acta 1997, 61, 657. (12) Leppinen, J. O.; Basilio, C. I.; Yoon, R. H. Int. J. Miner. Process. 1989, 26, 259.

methods. In particular, it was found7 that the in situ Fourier transform infrared (FTIR) data are consistent with the fact that the initial step of the galena electrochemical decomposition in oxygen-free borate (pH 9.2) is its incongruent dissolution:

PbS + H2O + 2h+ w S° + H+ + Pb(OH)+ (Eh ≈ +0.12 V at [PbOH+] ) 10-5 M and pH 9.2) (1) which is followed by precipitation of lead hydroxide when the solubility product for Pb(OH)2 is reached. Since the band gap for galena is narrow (ca. 0.4 eV) and ore galena has mainly n-type conductivity (the flatband potential Efb in oxygen-free borate is in the -0.6 to -0.2 V (standard hydrogen electrode, SHE) range14,15), reaction 1 starts when the electrode is in the inversion region (at ca. -0.05 V) due to the thermal generation of holes. Beyond +0.1 V, the electrode is p-type degenerate, and reaction 1 takes place in bulk, followed by the formation of lead sulfite and lead thiosulfate:

PbS + 3H2O + 6h+ w PbSO3 + 6H+

(2)

PbS + S° + 3H2O + 6h+ w PbS2O3 + 6H+ (Eh ≈ -0.12 V at pH 9.2) (3) The anodic reactions are different in the presence of (13) Chernyshova, I. V. Elektrokhimiya (Russ. J. Electrochem.) 2001, 37, 679. (14) Richardson, P. E.; O’Dell, C. S. J. Electrochem. Soc. 1985, 132, 1350. (15) Fletcher, S.; Horne, M. D. Int. J. Miner. Process. 1991, 33, 145.

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xanthate (X) at a low and high concentration in solution. This reagent is a typical member of the collector class of O-alkyldithiocarbonates

for example, by redistributing the potential in the electrical double layer. Moreover, the chemical dissolution

2PbS + O2 + 2H2O w 2PbOH+ + 2S° + 2OH- (10)

which is industrially used for selective flotation of sulfides. At high concentrations, chemisorption of X

X- + h+ w Xads

(4)

starts at a lower potential than reaction 1 (at ca. -0.2 V6 in a 10-3 M solution of n-butyl xanthate (IUPAC: nbutoxydithiomethanoate)), preventing the self-decomposition of galena. The chemisorbed xanthate is a precursor of lead xanthate and dixanthogen, which are formed by the following reactions:

PbS + 2Xads w PbX2 + S°

(5)

PbS + 4Xads w PbX2 + X2 + S°

(6)

At low concentrations ( +0.3 V in a 8 × 10-5 M solution of n-butyl xanthate), lead xanthate and dixanthogen were found to decompose into lead monothiocarbonate Pb(ROCSO)2 or the dimer of monothiocarbonate (ROCSO)2, respectively, possibly by the reactions

Pb(ROCS2)2 + Pb(OH)2 + 3OH- + 8h+ w Pb(ROCSO)2 + PbS2O3 + 5H+ (9a) and

(ROCS2)2 + Pb(OH)2 + 3OH- + 8h+ w (ROCSO)2 + PbS2O3 + 5H+ (9b) while galena itself is decomposed into lead sulfite and lead thiosulfate by reactions 2 and 3, respectively. In the previous studies, it has been assumed that both of the processes, the galena oxidation and interaction with xanthate, are electrochemical. The electrochemical mechanism implies that electrons transfer from active sulfide surface sites or physisorbed xanthate anions to oxygen through the sulfide volume, that is, the oxidation and reduction semireactions are spatially separated and, hence, additive. This assumption ignores the well-known fact16 that oxygen adsorption can affect the anodic process, (16) Skorchiletti, V. V. Theoretical Electrochemistry, 4th ed.; Khimia: Leningrad, 1974.

in which electrons transfer from PbS directly to oxygen, can flow simultaneously with the electrochemical dissolution (reaction 1).17 Although Pillai and Bockris18 reported that the rate of the xanthate oxidation on galena is increased by about 2.5 times in the presence of O2 while the O2 reduction is inhibited by about 10 times in the presence of xanthate, these authors have not interpreted these results. Since in practice flotation of sulfides is implemented in open air, the aim of the present work was to study the effect of dissolved oxygen on the anodic oxidation of galena in borate buffer (pH 9.2) in the absence and presence of n-butyl xanthate. For this purpose, the in situ attenuated total reflection (ATR)/FTIR technique19 was applied under open-air conditions and the results were compared with those obtained in refs 6 and 7 in the deoxygenated buffer. Experimental Section In situ ATR/FTIR spectra of the galena/solution interface were measured in the spectroelectrochemical cell described in detail elsewhere.19 Classical electrochemical equipment (potentiostat PI-50-1 and programmer PR-8, Russia) was employed in a threeelectrode configuration. The counter electrode was a Pt wire. The potentials were measured against a saturated potassium chloride electrode connected via a Luggin capillary to the cell, while all potentials reported here were converted to the SHE. The current-potential (I-V) dependences were measured simultaneously with the spectra with an X-Y recorder. The galena electrode was prepared from a single-crystal galena. The minor elements determined by mass spectrometry with an INA-3 Leybold AG instrument were Ag (3000 ppm), Sb (1200 ppm), and Sn (40 ppm). The galena plate, which was cut parallel to a (100) crystallographic plane, was glued by using a low-melting halcogenide glass to a halcogenide glass hemicylindric internal reflection element (IRE). Before each experimental run, the working surface was wet-polished successively with 1.0, 0.3, and 0.05 µm alumina (Buehler) and washed thoroughly by distilled water. After that, the “glued plate IRE” assembly was attached to the cell, the cell was filled with the buffer, and the galena electrode was reduced at a potential of -0.5 V for 1 h to provide the experiment reproducibility and to remove the surface oxidation products. As shown previously,7 the cathodic flatband potential Efb of the galena electrode in a 0.01 M borate buffer after such a treatment is less than -0.30 V, suggesting14 that the galena electrode is highly n-type with the Fermi level just below the conduction band edge. The spectrum measured at -0.5 V was the background spectrum. Afterward, the electrode potential was increased stepwise. The electrode was kept at each selected potential for either 5 or 8 min, to acquire one or two 200 scan spectra with nonpolarized and polarized radiation, respectively. The single-beam ATR/FTIR spectra were recorded at 8 cm-1 resolution at each step, at the angle of incidence of 37°, with a Perkin-Elmer model 1760X FTIR spectrometer equipped with a Micro ART unit TR-5 (RIIC) and MCT detector, and represented in absorbance units (-log(R/R0)), where R and R0 are the ATR spectra measured at the sample and reference potentials, respectively. No smoothing of the spectra was performed. Solutions of commercial n-butyl xanthate (C4H9OS2K) recrystallized from acetone were prepared in 0.01 M borate buffer (pH 9.2). Bidistilled water and commercial Na2B4O7‚10 H2O (pure for analysis) were used. (17) Tsyrlina, G. A.; Kharkats, Y. I.; Nazmutdinov, R. R.; Petrii, O. A. Elektrokhimiya (Russ. J. Electrochem.) 1999, 35, 23. (18) Pillai, K. C.; Bockris, J. O’M. J. Electrochem. Soc. 1984, 131, 568. (19) Chernyshova, I. V.; Tolstoy, V. P. Appl. Spectrosc. 1995, 49, 665.

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Figure 1. Current-potential dependences for the galena electrode in air-saturated and deaerated 0.01 M borate buffer (pH 9.2), measured simultaneously with the ATR/FTIR spectra shown in Figures 2 and 3.

Results and Discussion Anodic Oxidation in the Absence of Xanthate. The voltammetric curves for a galena electrode in air- and N2-saturated 0.01 M borate buffers are shown in Figure 1. One can see that at Eh > +0.35 V the rate of anodic oxidation is substantially higher in the presence of oxygen. The ATR spectra, which were measured simultaneously, are shown in Figures 2 and 3 for the air-saturated and deaerated buffers, respectively. To distinguish the spectral changes at each step of the potential increase, the spectrum at the potential marked is referenced to the spectrum at the preceding potential. Broad absorption bands at around 1400, 1150, and 800-900 cm-1 (marked by solid arrows in Figure 2a) are observed in open air at potentials up to +0.25 V, while they are hardly distinguished in the case of the oxygen-free solution (Figure 3a). The bands mentioned are attributable to physisorbed hydroxyl and borate ions.7 According to the electrochemical data,14,20,21 oxygen reduction

O2 + 2H2O + 4e w 4OHtakes place in this potential range, the potential being partitioned nearly equally between the solid and solution phases. Hence, the appearance of these bands can be explained by a shift in the proton equilibrium of the borate buffer system. Since no typical band22,23 of adsorbed O2 and O2- is observed, adsorbed oxygen, if any, is present in another state, possibly as O22-, OOH•, or O-. Lead hydroxide, which is characterized7 by a band at 13901396 cm-1, is the only surface product seen in the ATR spectra measured from +0.35 to +0.4 V (Figure 2b). Comparing these spectra with those shown in Figure 3b, one can conclude that the increase in the anodic current in air-saturated borate is accompanied by a higher rate of precipitation of Pb(OH)2. The increase in the rate of precipitation can be explained by the contribution of chemical oxidation by reaction 10 with direct participation of dissolved oxygen in the oxidation of the sulfide sulfur, which however, leaves unexplained the increase in the anodic current. An alternative interpretation consists of acceleration of reaction 1 by an increase in the potential drop in the Helmholtz layer (HL) due to adsorbed oxygen. (20) Woods, R. In Principles of Mineral Flotation. The Wark Symposium; Jones, M. H., Woodcock, J. T., Eds.; AIMM: Parkville, Victoria, Australia, 1984; pp 91-116. (21) Buckley, A. N.; Woods, R. Int. J. Mineral Process. 1990, 28, 301. (22) Che, M.; Tench, A. J. In Advances in Catalysis; Eley, D. D., Pines, H., Weiz, P. B., Eds.; Academic Press: New York, 1983. (23) Li, C.; Domen, K.; Maruya, K.-i.; Onishi. J. Am. Chem. Soc. 1989, 111, 7683.

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This mechanism is illustrated in Figure 4. Since the anodic current was found to be independent of the stirring conditions, the latter hypothesis seems to be more realistic. An argument supporting the “HL” interpretation is that the rate of reaction 1 is controlled by the buffer concentration and the surface layer composition, as found in our previous study.7 Therefore, the rate of reaction 1 depends not only on the potential bias but also on the potential of the diffuse part of the double layer, ψ1 (Figure 4a). Oxygen adsorption can change the structure of the double layer and, hence, ψ1 in the following way. Because of the high electronegativity of oxygen, an electron from the PbS lattice is completely transferred onto the adsorbed oxygen molecule, charging it negatively and creating a hole at the PbS surface. The species “O2-...localized h+” has the dipole moment with the negative pole directing toward the solution, and its electric field increases a drop in the potential in the HL (Figure 4a, dashed line). In addition, one can expect that oxygen adsorption also increases the gradient of the electric potential (the electric field) in the HL since the radius of the O2- species is less than that of physisorbed borate anions which constitute the solution side of the HL in the absence of dissolved oxygen. At high coverages, oxygen adsorption can even change the sign of ψ1 (Figure 4b). In this case, the lead cations which have transferred through the electrode/electrolyte boundary accumulate in the double layer. Thus, oxygen adsorption can increase the rate of reaction 1 and promote precipitation of lead hydroxide. At +0.50 V (Figure 2b), bands at 1220, 1110, 1005, and 983 cm-1 of lead thiosulfate and a broad band at ca. 950 cm-1 of lead sulfite appear in the spectra (for a more detailed assignment of the bands, see ref 7). The band intensities are higher than those in the deaerated buffer (compare with the corresponding band intensities in Figure 3b), implying that diffusion of lead cations is still the rate-determining step in the galena oxidation. Absorption of free carriers (electron or holes),7,24 whose concentration in the space-charge region layer is modulated by the potential of the semiconducting electrode, presents a smooth baseline at low wavenumbers (dotted curves in Figure 2). The value of this absorption can be estimated as the absorbance at the baseline measured at a fixed wavenumber, for example, at 800 cm-1 (dashed arrows in Figure 2a). The free-carrier absorption decreases at -0.4 V due to a decrease in concentration of conductionband electrons in the space-charge layer of the electrode. It is practically constant in the -0.3 to 0 V region, indicating that here the potential mainly changes in the HL. From +0.1 V, generation and accumulation of holes at the surface produce an abrupt growth in the absorption at low wavenumbers. This process is slowed at +0.5 V when lead-sulfoxy compounds start to form. This behavior is qualitatively similar to that observed under deaeration of the buffer (Figure 3). The difference is that in addition to elementary sulfur, the galena electrode oxidized anodically from 0 to +0.25 V in the presence of dissolved oxygen is covered by lead hydroxide (the 1390 cm-1 band) due to an increase in the rates of reactions 1-3. This means that, as opposed to galena oxidized in deaerated borate, such a galena is hydrophilic throughout all of the potential range under study, which is consistent with the surface hydrophobicity and flotation data.25 Anodic Oxidation in the Presence of Xanthate. Figures 5 and 6 show the s- and p-polarized difference (24) Erne, B. H.; Ozanam, F.; Chazalviel, J.-N. J. Phys. Chem. B 2000, 104, 11591. (25) Chanturia, V. A.; Vigdergauz, B. E. Electrochemistry of Sulfides; Nauka: Moscow, 1993; p 203 (in Russian).

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Figure 2. ATR nonpolarized difference spectra of the galena/open-air 0.01 M borate (pH 9.2) interface during a positive scan. The smooth background at low frequencies (dotted lines) arises from free-carrier absorption. The reference for the spectrum marked by “-0.40 V” is the spectrum measured at -0.5 V after reduction. Each of the other curves is the spectrum measured at the indicated potential after subtracting the spectrum measured at the preceding potential. Thus, the spectra present the differences taking place at the potential increase from the preceding to the indicated value. The horizontal lines indicate zero absorption. The solid arrows indicate absorption bands of the double-layer species. The dashed arrows show the technique of evaluating the free-carrier absorption (see the text).

spectra obtained for the galena electrode in 0.01 M borate buffer (pH 9.2) at an n-butyl xanthate concentration of 8 × 10-5 M. Xanthate adsorption is seen from -0.1 V. Up to + 0.05 V, the adsorbed species show different s- and p-polarized spectra. The relative intensity of the νasCOC band at ca. 1195 cm-1 in the p-polarized spectrum is significantly higher than that in the s-polarized one, suggesting that xanthate is adsorbed with the COC groups

oriented preferentially perpendicular to the surface. In contrast, the s- and p-polarized spectra of the species adsorbed at +0.1 V are similar to each other and to the spectrum of bulk PbX2.6 This observation can be explained by the formation of bulk lead xanthate at +0.1 V. The bands at 1265 and 1023 cm-1 of dixanthogen are distinct at +0.2 V, which is close to the reversible reaction potential for the X-/X2 pair. At potentials higher than +0.2 V, the

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Figure 4. Scheme of the potential distribution in the double layer at the positive-charged galena electrode: (a) without specific adsorption (solid line) and with adsorption of oxygen at low coverages (dashed line) and (b) with oxygen adsorption at high coverages. δ is the thickness of the HL, ψ is the potential drop in the HL, φa is the potential difference between the electrode and the bulk solution, and ψ1 is the potential of the diffuse part of the double layer. Figure 3. ATR nonpolarized difference spectra of the galena/ deaerated 0.01 M borate (pH 9.2) interface during a positive scan. Each of the other curves is the spectrum measured at the potential indicated after subtracting the spectrum measured at the preceding potential. Thus, the spectra present the differences taking place at the bias from the preceding to the indicated potential. The horizontal lines indicate zero absorption. The dotted lines indicate free-carrier absorption. The arrow indicates the νs(SO32-) band of PbS2O3.

negative band from lead xanthate appears at 1200 cm-1 and the positive broad bands arise at 1370-1400 cm-1 from lead hydroxide, implying that lead xanthate is substituted by lead hydroxide. The reversible potential of reaction 8 is +0.276 V at pH 9.2, which is somewhat higher than the potential at which this reaction is observed. However, if the reversible potential of this reaction is calculated with the surface value of pH, instead of the bulk one, then this process can be described by reaction 8. This points out the necessity to take into account the potential distribution between a semiconductor and the HL when comparing experimental and thermodynamic data. Contrary to the case of deaerated borate,6 a band at 1740 cm-1 due to monothiocarbonate species (reaction 9) is not observed in the spectra shown in Figures 5 and 6, probably because of the low limit of the upper potential in this set of measurements. A band at ca. 1720 cm-1 which arises at +0.25 V can be attributed to the bending vibration of either the OH3+ ion26 or lattice water,27,28 since it (26) Pliuskina, I. I. IR spectra of minerals; Moscow University Press: Moscow, 1977 (in Russian).

accompanies the growth of the 1396 cm-1 band of Pb(OH)2 (Figure 2b). A closer analysis of the spectra shown in Figures 5 and 6 reveals free-carrier absorption. The background absorption in the long-wavelength region in both the p- and s-polarized spectra increases as the galena electrode is biased to potentials higher than +0.05 V (zero absorption is indicated for each spectrum), suggesting that the galena surface in this region is degenerate by holes before the formation of bulk lead xanthate at ca. +0.1 V. This is consistent with the sequence of reactions 4, 1, and 7. The experiments were repeated at a xanthate concentration of 1 × 10-3 M (Figure 7). At potentials from -0.2 to ca. 0.0 V, the resulting spectra show typical bands of a xanthate ion coordinated to Pb2+. The band at 11771190 cm-1 is due to νasCOC, the band at ca. 1130 cm-1 is due to νsCOC, and the band at 1020-1030 cm-1 is due to νasSCS. Compared to the spectra of bulk PbX2 (see ref 6), the relative intensity of the νasCOC band in the potential region mentioned is higher, which can be ascribed to a different coordination of the xanthate radical to the surface than in bulk PbX2. The rate of xanthate adsorption increases at 0.0 V. The νasCOC band at 1265 cm-1 from dixanthogen appears in the spectra at +0.05 V, in agreement with thermodynamic data, and afterward the surface film composition remain unchanged. The electrode (27) Soptrajanov, B.; Petrusevski, V. J. Mol. Struct. 1997, 408/409, 283. (28) Cvetkovic, J.; Petrusevski, V.; Soptrajanov, B. J. Mol. Struct. 1997, 408/409, 463.

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Figure 5. The p-polarized difference ATR spectra of the galena electrode/electrolyte interface at potentials starting at -0.5 V. The electrolyte is 8 × 10-5 M potassium n-butyl xanthate solution in borate buffer (pH 9.2) at ambient atmosphere. Figure 7. ATR nonpolarized spectra of surface compounds on a galena electrode in 10-3 M potassium n-butyl xanthate solution in borate buffer (pH 9.2) at ambient atmosphere at potentials starting at -0.3 V. Reference has been taken at -0.5 V.

Figure 6. The s-polarized difference ATR spectra of the galena electrode/electrolyte interface at potentials starting at -0.5 V. The electrolyte is 8 × 10-5 M potassium n-butyl xanthate solution in borate buffer (pH 9.2) at ambient atmosphere.

processes are not accompanied by the increase in freecarrier absorption that was observed in galena oxidation both in the absence of xanthate (Figures 2 and 3) and at low xanthate concentrations (Figures 5 and 6). Therefore, lead xanthate must be produced by reactions 5 or 6 rather than by the precipitation, since in the latter case holes would accumulate at the interface. Finally, there are also some trends observed in the bending band δH2O of water near 1620-1650 cm-1 in the in situ ATR spectra (Figures 5, 6, and 7). For galena in 10-3 M xanthate (Figure 7), the overall interval within which the potential was changed can be divided into three parts. From -0.5 to -0.1 V, where xanthate is chemisorbed, no pronounced change in the δH2O region is observed. The precipitation of lead xanthate at 0 V results in the appearance of the negative band at 1630 cm-1, which increases steeply and shifts to 1636 cm-1 at 0 to +0.05 V and to 1648 cm-1 at +0.2 V when dixanthogen appears. One might ascribe the negative intensity of the water band to the optical effect (screening of the electrode/ solution interface by the growing organic film). However, this interpretation fails to explain the sudden change of the band intensity and position at 0 to +0.1 V when the amount of adsorbed substance is relatively small, the positive intensity of the δH2O band observed at the formation of lead hydroxide (Figures 5 and 6), and the difference in the δH2O band in the s- and p-polarized

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spectra (Figures 5 and 6). Since the presence of dixanthogen makes the galena surface hydrophobic while due to bulk oxidation the galena surface becomes hydrophilic, one can conclude that some of the spectral changes described are due to reorganization of interfacial water caused by changing wetting properties of the electrode surface. Namely, one can suggest that a blue shift of the δH2O band against the intensity increase correlates with an increase in the surface hydrophobicity. Conclusions The FTIR spectra measured in situ indicated that the anodic oxidation of galena in borate buffer in the presence of dissolved oxygen is substantially intensified both in the absence of xanthate and in its presence at low concentrations, in agreement with electrochemical data.18,29 However, contrary to the latter, the FTIR spectra allowed interpretation of this effect. Namely, in contrast to the deaerated conditions,6,7 the main surface oxidation product both in the absence and presence of xanthate in the air(29) Lekki, J.; Chmielewski, T. Fizykochem. Probl. Mineralurg. 1989, 21, 127 (in Polish).

Chernyshova

saturated borate (pH 9.2) was found to be Pb(OH)2. The increase in the precipitation rate of this species is explained by an increase in the potential drop in the HL and, possibly, by redistribution of the potential in the double layer (Figure 4). As in the case of the deaerated buffer,6 independently of the xanthate concentration in solution, the formation of bulk lead xanthate is preceded by chemisorption of xanthate. Dixanthogen is formed with no overpotential. However, at low concentrations, PbX2 is mainly formed by the precipitation mechanism, since the onset of the formation of bulk PbX2 coincides with the onset of increasing hole absorption. At high concentrations of xanthate, the electrochemical dissolution of galena (eq 1) and the decomposition of adsorbed lead xanthate (eq 8) and dixanthogen (eq 9b) are inhibited by the formation of lead xanthate (eq 5) and, at higher potentials, by the formation of both lead xanthate and dixanthogen (eq 6). Acknowledgment. I thank the Russian Foundation for Basic Research (RFBR) for the financial support under Grant No. 99-03-32614. LA020014D