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Comparative study of interaction between pyrite and cysteine by thermogravimetric and electrochemical techniques
Hydrometallurgy 101 (2010) 88–92
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Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Comparative study of interaction between pyrite and cysteine by thermogravimetric and electrochemical techniques Zhaohui Wang a, Xuehui Xie a, Shengmu Xiao a, Jianshe Liu a,b,⁎ a b
College of Environmental Science and Engineering, Donghua University, Shanghai, 201620, China School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 5 November 2009 Accepted 23 November 2009 Available online 27 November 2009 Keywords: Pyrite Tafel plot Chemisorption Electrochemical
a b s t r a c t Adsorption mechanism of L-cysteine on pyrite was investigated by thermogravimetric and electrochemical techniques. TG curves provided the direct evidence for chemisorption of cysteine on pyrite surface. Once cysteine adsorbed to pyrite surface, Ecorr (corrosion potential) sharply lowered whereas Icorr (corrosion current) increased rapidly. Pyrite became more susceptible to be oxidized even at lower potential as cysteine was added. However, the mechanism for pyrite oxidation does not fundamentally change, although cysteine can obviously accelerate oxidation rate of pyrite. These findings have important implications for understanding the mechanism of bacterial adhesion to pyrite and even metal sulfide bioleaching. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Bio-hydrometallurgical operations have been successfully applied to the recovery of metals from sulfide ores in many countries (Ehrlich, 2001; Brierley, 2008). However, slow bacterial oxidation rate is still a major problem to be solved in bioleaching. Therefore, the strategies to enhance bacterial activity at sulfide mineral surface have been widely exploited by many authors (Brierley, 2008). Recently, L-cysteine (Cys), an important sulfureous amino acid (Liu et al., 2003), aroused great concern because of its capacity to accelerate bioleaching (RojasChapana and Tributsch, 2000; Hu et al., 2004; Rojas-Chapana and Tributsch, 2001). The leaching rate of pyrite was enhanced three times compared to the normal process without cysteine (Rojas-Chapana and Tributsch, 2000). Although cysteine has proved highly effective to enhance pyrite leaching, the mechanism involved was not well understood yet. Tributsch et al. speculated that the active group sulfydryl (–SH) of cysteine molecule was involved in bio-leaching process (RojasChapana and Tributsch, 2000). They hypothesized that L-cysteine could interact spontaneously with pyrite, with bonds formation similar to those present in ferredoxin complexes of biological membrane, and consequently disrupt chemical bond of ferrous sulfide and release iron sulfur clusters (Rojas-Chapana et al., 1996, 1998; Tributsch and Bennet, 1981; Tributsch and Rojas-Chapana, 2000; Abd EI-Halim et al., 1995). However, there was little evidence available to support their hypothesis. ⁎ Corresponding author. College of Environmental Science and Engineering, Donghua University, Shanghai, 201620, China. fax: + 86 21 67792523. E-mail address: [email protected] (J. Liu). 0304-386X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2009.11.015
In addition, cysteine may play another role in microbial hydrometallurgy. Blake et al. reported that purified recombinant aporusticyanin and intact acidithiobacillus ferrooxidans showed an identical pattern of adhesion to the same minerals (Blake et al., 2001). Binding of this apoprotein to solid pyrite was accomplished in part by coordination of the unoccupied copper ligands with an iron atom at the exposed edge of the pyrite crystal lattice. As one of four copper binding sites of this protein (Cys138, Met148, His143 and His85) (Walter et al., 1996), cysteine may be probably involved in the adsorption between pyrite and aporusticyanin. Therefore, it is fundamentally interesting and significant to investigate how cysteine interacts to mineral. Our previous study revealed that chemisorption dominated the interaction between pyrite and cysteine based on the results of Langmuir-type adsorption isotherm, FTIR and XRD analyses (Liu et al., 2006). In order to provide further insight into their interaction mechanism, TGA-DSC and electrochemical measurements were employed in this study.
2. Experimental 2.1. Materials Pyrite used in experiment was well-crystallized mineral from Hunan Geological Museum. Its chemical composition analysis by scanning electron probe was reported elsewhere (Liu et al., 2006). Ultrasonic treatment was applied to remove surface oxide resulting from exposure to air. Then mineral particles were transferred to agate mortar and well polished. The powder was sealed and reserved in wide-mouth bottle. L-cysteine is a biochemical reagent. Potassium
Z. Wang et al. / Hydrometallurgy 101 (2010) 88–92
nitrate and sodium hydroxide are of laboratory reagent grade. Double-distilled water was used throughout the experiments. 2.2. Experimental method 2.2.1. DSC-TGA measurement DSC-TGA measurement was performed by using a Simultaneous DSC-TGA (SDT) Q600 thermal analyzer under air atmosphere with a flowing rate of 100 mL/min. For each experiment, about 12–25 mg of the sample, either in which cysteine adsorbed or not, was weighed into platinum crucible. The heating rate was 4, 8, 12, and 16 K/min respectively to test its possible effect on sample oxidation. The temperature program was initiated at room temperature and completed at 1000 °C. 2.2.2. Electrochemical analysis All electrochemical measurements were carried out using standard three-electrode cell on Model (2)73A potentiostat/galvanostat (EG&G Princeton Applied Research, USA). The working electrode (cross-sectional area: 1 cm2) was cut from natural pyrite (Liu, 2002). A saturated Ag/AgCl electrode (0.222 V, vs. SHE, 25 °C) was employed as reference electrode. The counter electrode was two graphite electrodes. Unless otherwise stated, all potentials quoted in this study were referred to the standard hydrogen electrode (SHE). Before each measurement, the exposed surface of mineral electrode was polished by 600# sand paper and rinsed by double-distilled water. The potassium nitrate electrolyte (0.1 M, pH = 2.5) was prepared for measurement. The pH values of the solution were adjusted with KOH or HNO3. Note that each experiment was performed at 25 °C without dissolved oxygen removal.
89
catalyze SO2 to SO3. Thus, it is necessarily possible to form FeSO4 at temperatures higher than 550 °C (Eq. (3)). 2FeS2 þ 5:5O2 →Fe2 O3 þ 4SO2
ð1Þ
FeS2 þ 3O2 →FeSO4 þ SO2
ð2Þ
Fe2 O3 þ SO2 þ SO3 →2FeSO4
ð3Þ
It is shown that at the heating rate of 8, 12, and 16 K/min under temperature range between 400 and 500 °C, the mineral weight loss was approximately 30% and nearly equal to the theoretical value (33.33%) when pyrite was totally converted to hematite. Direct oxidation to hematite was preferentially enhanced by faster heating rate (Almeida and Giannetti, 2002). However, there was only 17.67% weight loss at the first stage at a heating rate of 4 K/min. The phenomenon is due to the fact that sulfate formation was favored by slower heating rate and herein oxygen was correspondingly sufficient to pyrite oxidation in terms of Eq. (3). It is worthwhile to note that the small weight gain was observed (as arrow directed), which may be attributed to the formation of hematite (Eq. (3)) (Almeida and Giannetti, 2002). Compared to Fig. 1 (1), the TG curves as shown in Fig. 1 (2) become distinctive between 200 and 250 °C at four heating rates after the cysteine treatment. An obvious weight loss occurred probably due to cysteine decomposition and volatilization. The average weight loss of pyrite under four heating rates was about 8.36%. This result provides a strong evidence for chemisorption of cysteine on pyrite surface.
3. Results and discussion 3.2. Electrochemical analysis 3.1. DSC-TGA The TG curves with different heating rates are shown in Fig. 1. Pyrite without any additive (Fig. 1 (1)) was thermostable until nearly 400 °C. The main oxidation reactions occurring under the temperature range between 400 and 900 °C were (Almeida and Giannetti, 2002): conversion of pyrite to hematite (Eq. (1)) and formation of ferrous sulfate (Eq. (2)). In addition, it is reported the platinum crucible can
3.2.1. Cyclic voltammetry Fig. 2 (A1) depicts the cyclic voltammograms of pyrite in KNO3 electrolyte in the absence of cysteine. The scan began at a starting potential Es of −1.25 V and extended to a switching potential Eλ of 1.70 V. With the scanning rates of 20 and 40 mV/s, respectively, the electrode reaction did not take place at potentials lower than 1.0 V. At slightly more positively potentials, an anodic current appeared and
Fig. 1. TG curves for pyrite before and after cysteine treatment. (1) in the absence of cysteine; (2) in the presence of cysteine.
90
Z. Wang et al. / Hydrometallurgy 101 (2010) 88–92
Fig. 2. Cyclic voltammograms of pyrite electrode in 0.1 M KNO3 electrolyte at pH 2.5. (A1) without cysteine; (A2) zoom in between − 1.0 and + 0.8 V; (B1) 1 mM cysteine; (B2) zoom in between − 1.0 and + 0.8 V.
then sharply increased. During the reverse scan, a corrosion ring emerged, which was in agreement with the observation by Liu (2002). In order to obtain more explicit image, the curves are magnified at potentials between −1.2 V and +0.8 V (Fig. 2 (A2)). When the potential sweeps were initiated in the positive-going direction, two small anodic currents (designated by a and b respectively), were observed at potential above − 0.5 V. The emergence of these anodic peaks was attributed to the oxidation of pyrite surfaces. The detailed mechanisms of this process may be very complicated. Although experimental setups differed in varied electrolytes and pH values, the anodic reactions of pyrite have been fairly well discussed in the literature (Abd EI-Halim et al., 1995; Almeida and Giannetti, 2002). The anodic peak at − 0.5 V was associated with reactions (4) and (5). FeS2 þ 2e→FeS þ S
FeS þ 2e→Fe þ S
2−
2−
0
E ¼ −0:272V
0
E ¼ −0:492V
ð4Þ
ð5Þ
The peak b ranging from 0.1 V to 0.3 V was related to a series of reactions (6), (7), (8) and (9). The wider shape of peak b was caused by the several anodic peaks overlap. þ
FeS2 þ 2H þ 2e→FeS þ 2HS
−
0
E ¼ 0:105V
ð6Þ
þ
0
FeS þ 2H þ 2e→Fe þ 2H2 SðaqÞ
E ¼ 0:101V
2þ
þ 2HSO4 þ 14H þ 14e
3þ
þ 2SO4 þ 16H þ 15e
FeS2 þ 8H2 O→Fe
FeS2 þ 8H2 O→Fe
−
−
þ
þ
ð7Þ
0
E ¼ 0:349V
ð8Þ
0
ð9Þ
E ¼ 0:398V
During the negative scan, a sharp peak c arose, which was expected to associate with the Fe3+ reduction (Eq. (10)). As the scan was taken more negative values, current peak d appeared. This peak has been attributed to the reverses of reactions (6), (7), (8) and (9). 3þ
Fe
2þ
þ e→Fe
0
E ¼ 0:779V
ð10Þ
Then 1 mM cysteine was added into 0.1 M KNO3 of electrolyte to examine its impact on electrochemical oxidation of pyrite. This solution was stirred over 50 min to ensure the adsorption/desorption equilibrium between cysteine and pyrite electrode. The scanning rates were controlled as 30, 40 and 50 mV/s respectively. The cyclic voltammograms of pyrite electrode after addition of cysteine are shown in Fig. 2 (B1). Fig. 2 (B2) is a local amplification of B1. A comparison of Fig. 2 (A2), (B2) reveals that addition of cysteine
Z. Wang et al. / Hydrometallurgy 101 (2010) 88–92
resulted in the negative-going shifts of both anodic and cathodal potentials, which was of the order of 0.2 V. It proves that cysteine enabled pyrite to be oxidized at lower potential as well as greatly accelerated its oxidation process. 3.2.2. Tafel plot Linear polarization measurement proceeded on M352 software. The entire potential scan was programmed to ±20 mV with respect to rest potential with a scanning rate 0.15 mV/s. In order to examine the possible effect of pH on corrosion kinetics of pyrite electrode, Tafel plots (see Fig. 3) were recorded under acidic (pH = 2.5) and neutral (pH = 6.2) conditions. According to the linear polarization equations (Eqs. (11)–(13)) (Liu, 2002), a series of kinetic parameters, such as corrosion potential and Tafel slope, were calculated using the PARCalc software and listed in Table 1. 2:303ðE−Ecorr Þ −2:303ðE−Ecorr Þ I = icorr exp − exp ba bc ba =
2:303 RT βnF
α = 1−β
bc =
2:303 RT αnF
ð11Þ
ð12Þ ð13Þ
91
Table 1 Electrochemical kinetic parameters calculated by PARCalc software. Parameters
Corrosion potential (Ecorr, mV) Corrosion current (Icorr, μA) Anodic Tafel slope (ba, mV) Cathodal Tafel slope (bc, mV) Transfer Coefficient Number of electron transfer (n)
Without cysteine
Cysteine added
pH = 2.5
pH = 6.2
pH = 2.5
pH = 6.2
226.1
159.1
66.42
− 10.29
0.876
α β
0.250
2.363
1.299
38.44
36.47
35.79
41.51
30.45
19.76
34.41
23.22
0.44 0.56 0.29
0.35 0.65 0.22
0.49 0.51 0.29
0.36 0.64 0.25
It can be obviously observed that corrosion process of pyrite was dependant of pH. Under nearly neutral condition, both corrosion current Icorr and potential Ecorr decreased. This phenomenon may be ascribed to the formation of iron oxide or hydroxide, which inhibited the electron transfer at the pyrite/solution interface. However, when cysteine was introduced into electrochemical system, Ecorr sharply
Fig. 3. Cathodic and anodic polarization curves for pyrite electrode in 0.1 M KNO3 solution without (A: pH = 6.2; B: pH = 2.5) and with 1 mM cysteine (C: pH = 6.2; D: pH = 2.5).
92
Z. Wang et al. / Hydrometallurgy 101 (2010) 88–92
lowered whereas Icorr increased rapidly. It provides strong evidence for enhancement of pyrite oxidation in the presence of cysteine. However, it should be noted that transfer coefficient and number of electron transfer, which are indicative of electrode reaction mechanism (Liu, 2002), did not obviously change in the absence and presence of cysteine. It suggests that addition of cysteine just facilitated the electron transfer but did not change the pyrite oxidation mechanism. Cysteine may serve as a bridge or conductor to facilitate electronic charge transfer from pyrite and end-product. Previous study showed that bridge complexes formed by adsorbing – O–, –CN– or –CO– groups to interfacial iron atoms in pyrite could greatly accelerate the transfer of photogenerated charge carriers between pyrite and soluble iron (Schubert and Tributsch, 1990). Based on the discussion above, it is of interest to reexamine the issue of the interaction between aporusticyanin and pyrite (Blake et al., 2001). Their adsorption may be accomplished by their subunits: iron or sulfur atom for pyrite crystal, amino acid residuals for apoprotein. Considering the cysteine's good affinity to pyrite, we predict that cysteine located in the framework of aporusticyanin can act as a mineral-specific receptor. It is still an area of interest that deserves further investigation. For example, the affinity of other copper ligands (Met and His) to pyrite should be examined according to the similar methods as Cys. 4. Conclusion The interaction pattern between pyrite and cysteine was investigated to reveal how cysteine accelerates pyrite dissolution in biomining operation. TG curves provided the direct evidence for chemisorption of cysteine on pyrite surface. It suggests that cysteine may play a critical role in bacterial adhesion to pyrite. Once cysteine was adsorbed to sulfide surface, pyrite can be readily oxidized at lower potential. Cysteine may act as a bridge or conductor to facilitate electronic charge transfer from pyrite and end-product. However, although cysteine can obviously accelerate oxidation rate of pyrite, pyrite oxidation mechanism does not fundamentally change. Acknowledgements The authors gratefully acknowledged the financial support from the Special Fund Project for Authors of National Excellent Ph. D
Theses, China (200549), the National Natural Science Foundation of China (No. 50874032), the Shanghai Leading Academic Discipline Project (B604), and Shanhai Tongji Gao TingYao Environmental Science & Technology Development Fund.
References Abd EI-Halim, A.M., Alonso-Vante, N., Tributsch, H., 1995. Iron/sulphur centre mediated photoinduced charge transfer at (100) oriented pyrite surfaces. J. Electroanal. Chem. 399, 29–39. Almeida, C.M.V.B., Giannetti, B.F., 2002. Comparative study of electrochemical and thermal oxidation of pyrite. J. Solid State Electrochem. 6, 111–118. Blake II, R.C., Sasaki, K., Ohmura, N., 2001. Does aporusticyanin mediate the adhesion of Thiobacillus ferrooxidans to pyrite? Hydrometallurgy 59, 357–372. Brierley, J.A., 2008. A perspective on developments in biohydrometallurgy. Hydrometallurgy 94, 2–7. Ehrlich, H.L., 2001. Past, present and future of biohydrometallurgy. Hydrometallurgy 59, 127–134. Hu, Y.H., He, Z.G., Hu, W.X., Peng, H., Zhong, H., 2004. Effect of two kinds of amino-acids on bioleaching metal sulfide. Trans. Nonferr. Met. Soc. China 14, 794–797. Liu, J.S., 2002. Bioextraction and Corrosion Electrochemistry of Sulfide Minerals. Ph.D. Thesis, Central South University. Liu, Z., Yang, W.B., Bai, G., Tian, W., Jin, Y.J., 2003. Microbial enzyme conversion of L-cysteine and L-cystine. Microbiology 30, 16–21. Liu, J.S., Wang, Z.H., Li, B.M., Zhang, Y.H., 2006. Interaction between pyrite and cysteine. Trans. Nonferrous Met. Soc. China 16, 943–946. Rojas-Chapana, J.A., Giersig, M., Tributsch, H., 1996. The path of sulphur during the biooxidation of pyrite by Thiobacillus ferrooxidans. Fuel 75, 923–930. Rojas-Chapana, J.A., Bartels, C.C., Pohlmann, L., Tributsch, H., 1998. Co-operative leaching and chemotaxis of thiobacilli studied with spherical sulphurrsulphide substrates. Process Biochem. 33, 239–248. Rojas-Chapana, J.A., Tributsch, H., 2000. Bio-leaching of pyrite accelerated by cysteine. Process Biochem. 35, 815–824. Rojas-Chapana, J.A., Tributsch, H., 2001. Biochemistry of sulfur extraction in biocorrosion of pyrite by Thiobacillus ferrooxidans. Hydrometallurgy 59, 291–300. Schubert, B., Tributsch, H., 1990. Photoinduced electron transfer by coordination chemical pathways across pyrite/electrolyte interfaces. Inorg. Chem. 29, 5041–5046. Tributsch, H., Bennet, J.C., 1981. Semiconductor-electrochemical aspects of bacterial leaching: I. Oxidation of metal sulphides with large energy gaps. J. Chem. Technol. Biotechnol. 31, 565–577. Tributsch, H., Rojas-Chapana, J.A., 2000. Metal sulfide semiconductor electrochemical mechanisms induced by bacterial activity. Electrochim. Acta 45, 4705–4716. Walter, R.L., Ealick, S.E., Friedman, A.M., Blake II, R.C., Proctor, P., Shoham, M., 1996. Multiple wavelength anomalous diffraction (MAD) crystal structure of rusticyanin: a highly oxidizing cupredoxin with extreme acid stability. J. Mol. Biol. 263, 730–751.
Contents lists available at ScienceDirect
Hydrometallurgy j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / h yd r o m e t
Comparative study of interaction between pyrite and cysteine by thermogravimetric and electrochemical techniques Zhaohui Wang a, Xuehui Xie a, Shengmu Xiao a, Jianshe Liu a,b,⁎ a b
College of Environmental Science and Engineering, Donghua University, Shanghai, 201620, China School of Resources Processing and Bioengineering, Central South University, Changsha 410083, China
a r t i c l e
i n f o
Article history: Received 5 November 2009 Accepted 23 November 2009 Available online 27 November 2009 Keywords: Pyrite Tafel plot Chemisorption Electrochemical
a b s t r a c t Adsorption mechanism of L-cysteine on pyrite was investigated by thermogravimetric and electrochemical techniques. TG curves provided the direct evidence for chemisorption of cysteine on pyrite surface. Once cysteine adsorbed to pyrite surface, Ecorr (corrosion potential) sharply lowered whereas Icorr (corrosion current) increased rapidly. Pyrite became more susceptible to be oxidized even at lower potential as cysteine was added. However, the mechanism for pyrite oxidation does not fundamentally change, although cysteine can obviously accelerate oxidation rate of pyrite. These findings have important implications for understanding the mechanism of bacterial adhesion to pyrite and even metal sulfide bioleaching. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Bio-hydrometallurgical operations have been successfully applied to the recovery of metals from sulfide ores in many countries (Ehrlich, 2001; Brierley, 2008). However, slow bacterial oxidation rate is still a major problem to be solved in bioleaching. Therefore, the strategies to enhance bacterial activity at sulfide mineral surface have been widely exploited by many authors (Brierley, 2008). Recently, L-cysteine (Cys), an important sulfureous amino acid (Liu et al., 2003), aroused great concern because of its capacity to accelerate bioleaching (RojasChapana and Tributsch, 2000; Hu et al., 2004; Rojas-Chapana and Tributsch, 2001). The leaching rate of pyrite was enhanced three times compared to the normal process without cysteine (Rojas-Chapana and Tributsch, 2000). Although cysteine has proved highly effective to enhance pyrite leaching, the mechanism involved was not well understood yet. Tributsch et al. speculated that the active group sulfydryl (–SH) of cysteine molecule was involved in bio-leaching process (RojasChapana and Tributsch, 2000). They hypothesized that L-cysteine could interact spontaneously with pyrite, with bonds formation similar to those present in ferredoxin complexes of biological membrane, and consequently disrupt chemical bond of ferrous sulfide and release iron sulfur clusters (Rojas-Chapana et al., 1996, 1998; Tributsch and Bennet, 1981; Tributsch and Rojas-Chapana, 2000; Abd EI-Halim et al., 1995). However, there was little evidence available to support their hypothesis. ⁎ Corresponding author. College of Environmental Science and Engineering, Donghua University, Shanghai, 201620, China. fax: + 86 21 67792523. E-mail address: [email protected] (J. Liu). 0304-386X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2009.11.015
In addition, cysteine may play another role in microbial hydrometallurgy. Blake et al. reported that purified recombinant aporusticyanin and intact acidithiobacillus ferrooxidans showed an identical pattern of adhesion to the same minerals (Blake et al., 2001). Binding of this apoprotein to solid pyrite was accomplished in part by coordination of the unoccupied copper ligands with an iron atom at the exposed edge of the pyrite crystal lattice. As one of four copper binding sites of this protein (Cys138, Met148, His143 and His85) (Walter et al., 1996), cysteine may be probably involved in the adsorption between pyrite and aporusticyanin. Therefore, it is fundamentally interesting and significant to investigate how cysteine interacts to mineral. Our previous study revealed that chemisorption dominated the interaction between pyrite and cysteine based on the results of Langmuir-type adsorption isotherm, FTIR and XRD analyses (Liu et al., 2006). In order to provide further insight into their interaction mechanism, TGA-DSC and electrochemical measurements were employed in this study.
2. Experimental 2.1. Materials Pyrite used in experiment was well-crystallized mineral from Hunan Geological Museum. Its chemical composition analysis by scanning electron probe was reported elsewhere (Liu et al., 2006). Ultrasonic treatment was applied to remove surface oxide resulting from exposure to air. Then mineral particles were transferred to agate mortar and well polished. The powder was sealed and reserved in wide-mouth bottle. L-cysteine is a biochemical reagent. Potassium
Z. Wang et al. / Hydrometallurgy 101 (2010) 88–92
nitrate and sodium hydroxide are of laboratory reagent grade. Double-distilled water was used throughout the experiments. 2.2. Experimental method 2.2.1. DSC-TGA measurement DSC-TGA measurement was performed by using a Simultaneous DSC-TGA (SDT) Q600 thermal analyzer under air atmosphere with a flowing rate of 100 mL/min. For each experiment, about 12–25 mg of the sample, either in which cysteine adsorbed or not, was weighed into platinum crucible. The heating rate was 4, 8, 12, and 16 K/min respectively to test its possible effect on sample oxidation. The temperature program was initiated at room temperature and completed at 1000 °C. 2.2.2. Electrochemical analysis All electrochemical measurements were carried out using standard three-electrode cell on Model (2)73A potentiostat/galvanostat (EG&G Princeton Applied Research, USA). The working electrode (cross-sectional area: 1 cm2) was cut from natural pyrite (Liu, 2002). A saturated Ag/AgCl electrode (0.222 V, vs. SHE, 25 °C) was employed as reference electrode. The counter electrode was two graphite electrodes. Unless otherwise stated, all potentials quoted in this study were referred to the standard hydrogen electrode (SHE). Before each measurement, the exposed surface of mineral electrode was polished by 600# sand paper and rinsed by double-distilled water. The potassium nitrate electrolyte (0.1 M, pH = 2.5) was prepared for measurement. The pH values of the solution were adjusted with KOH or HNO3. Note that each experiment was performed at 25 °C without dissolved oxygen removal.
89
catalyze SO2 to SO3. Thus, it is necessarily possible to form FeSO4 at temperatures higher than 550 °C (Eq. (3)). 2FeS2 þ 5:5O2 →Fe2 O3 þ 4SO2
ð1Þ
FeS2 þ 3O2 →FeSO4 þ SO2
ð2Þ
Fe2 O3 þ SO2 þ SO3 →2FeSO4
ð3Þ
It is shown that at the heating rate of 8, 12, and 16 K/min under temperature range between 400 and 500 °C, the mineral weight loss was approximately 30% and nearly equal to the theoretical value (33.33%) when pyrite was totally converted to hematite. Direct oxidation to hematite was preferentially enhanced by faster heating rate (Almeida and Giannetti, 2002). However, there was only 17.67% weight loss at the first stage at a heating rate of 4 K/min. The phenomenon is due to the fact that sulfate formation was favored by slower heating rate and herein oxygen was correspondingly sufficient to pyrite oxidation in terms of Eq. (3). It is worthwhile to note that the small weight gain was observed (as arrow directed), which may be attributed to the formation of hematite (Eq. (3)) (Almeida and Giannetti, 2002). Compared to Fig. 1 (1), the TG curves as shown in Fig. 1 (2) become distinctive between 200 and 250 °C at four heating rates after the cysteine treatment. An obvious weight loss occurred probably due to cysteine decomposition and volatilization. The average weight loss of pyrite under four heating rates was about 8.36%. This result provides a strong evidence for chemisorption of cysteine on pyrite surface.
3. Results and discussion 3.2. Electrochemical analysis 3.1. DSC-TGA The TG curves with different heating rates are shown in Fig. 1. Pyrite without any additive (Fig. 1 (1)) was thermostable until nearly 400 °C. The main oxidation reactions occurring under the temperature range between 400 and 900 °C were (Almeida and Giannetti, 2002): conversion of pyrite to hematite (Eq. (1)) and formation of ferrous sulfate (Eq. (2)). In addition, it is reported the platinum crucible can
3.2.1. Cyclic voltammetry Fig. 2 (A1) depicts the cyclic voltammograms of pyrite in KNO3 electrolyte in the absence of cysteine. The scan began at a starting potential Es of −1.25 V and extended to a switching potential Eλ of 1.70 V. With the scanning rates of 20 and 40 mV/s, respectively, the electrode reaction did not take place at potentials lower than 1.0 V. At slightly more positively potentials, an anodic current appeared and
Fig. 1. TG curves for pyrite before and after cysteine treatment. (1) in the absence of cysteine; (2) in the presence of cysteine.
90
Z. Wang et al. / Hydrometallurgy 101 (2010) 88–92
Fig. 2. Cyclic voltammograms of pyrite electrode in 0.1 M KNO3 electrolyte at pH 2.5. (A1) without cysteine; (A2) zoom in between − 1.0 and + 0.8 V; (B1) 1 mM cysteine; (B2) zoom in between − 1.0 and + 0.8 V.
then sharply increased. During the reverse scan, a corrosion ring emerged, which was in agreement with the observation by Liu (2002). In order to obtain more explicit image, the curves are magnified at potentials between −1.2 V and +0.8 V (Fig. 2 (A2)). When the potential sweeps were initiated in the positive-going direction, two small anodic currents (designated by a and b respectively), were observed at potential above − 0.5 V. The emergence of these anodic peaks was attributed to the oxidation of pyrite surfaces. The detailed mechanisms of this process may be very complicated. Although experimental setups differed in varied electrolytes and pH values, the anodic reactions of pyrite have been fairly well discussed in the literature (Abd EI-Halim et al., 1995; Almeida and Giannetti, 2002). The anodic peak at − 0.5 V was associated with reactions (4) and (5). FeS2 þ 2e→FeS þ S
FeS þ 2e→Fe þ S
2−
2−
0
E ¼ −0:272V
0
E ¼ −0:492V
ð4Þ
ð5Þ
The peak b ranging from 0.1 V to 0.3 V was related to a series of reactions (6), (7), (8) and (9). The wider shape of peak b was caused by the several anodic peaks overlap. þ
FeS2 þ 2H þ 2e→FeS þ 2HS
−
0
E ¼ 0:105V
ð6Þ
þ
0
FeS þ 2H þ 2e→Fe þ 2H2 SðaqÞ
E ¼ 0:101V
2þ
þ 2HSO4 þ 14H þ 14e
3þ
þ 2SO4 þ 16H þ 15e
FeS2 þ 8H2 O→Fe
FeS2 þ 8H2 O→Fe
−
−
þ
þ
ð7Þ
0
E ¼ 0:349V
ð8Þ
0
ð9Þ
E ¼ 0:398V
During the negative scan, a sharp peak c arose, which was expected to associate with the Fe3+ reduction (Eq. (10)). As the scan was taken more negative values, current peak d appeared. This peak has been attributed to the reverses of reactions (6), (7), (8) and (9). 3þ
Fe
2þ
þ e→Fe
0
E ¼ 0:779V
ð10Þ
Then 1 mM cysteine was added into 0.1 M KNO3 of electrolyte to examine its impact on electrochemical oxidation of pyrite. This solution was stirred over 50 min to ensure the adsorption/desorption equilibrium between cysteine and pyrite electrode. The scanning rates were controlled as 30, 40 and 50 mV/s respectively. The cyclic voltammograms of pyrite electrode after addition of cysteine are shown in Fig. 2 (B1). Fig. 2 (B2) is a local amplification of B1. A comparison of Fig. 2 (A2), (B2) reveals that addition of cysteine
Z. Wang et al. / Hydrometallurgy 101 (2010) 88–92
resulted in the negative-going shifts of both anodic and cathodal potentials, which was of the order of 0.2 V. It proves that cysteine enabled pyrite to be oxidized at lower potential as well as greatly accelerated its oxidation process. 3.2.2. Tafel plot Linear polarization measurement proceeded on M352 software. The entire potential scan was programmed to ±20 mV with respect to rest potential with a scanning rate 0.15 mV/s. In order to examine the possible effect of pH on corrosion kinetics of pyrite electrode, Tafel plots (see Fig. 3) were recorded under acidic (pH = 2.5) and neutral (pH = 6.2) conditions. According to the linear polarization equations (Eqs. (11)–(13)) (Liu, 2002), a series of kinetic parameters, such as corrosion potential and Tafel slope, were calculated using the PARCalc software and listed in Table 1. 2:303ðE−Ecorr Þ −2:303ðE−Ecorr Þ I = icorr exp − exp ba bc ba =
2:303 RT βnF
α = 1−β
bc =
2:303 RT αnF
ð11Þ
ð12Þ ð13Þ
91
Table 1 Electrochemical kinetic parameters calculated by PARCalc software. Parameters
Corrosion potential (Ecorr, mV) Corrosion current (Icorr, μA) Anodic Tafel slope (ba, mV) Cathodal Tafel slope (bc, mV) Transfer Coefficient Number of electron transfer (n)
Without cysteine
Cysteine added
pH = 2.5
pH = 6.2
pH = 2.5
pH = 6.2
226.1
159.1
66.42
− 10.29
0.876
α β
0.250
2.363
1.299
38.44
36.47
35.79
41.51
30.45
19.76
34.41
23.22
0.44 0.56 0.29
0.35 0.65 0.22
0.49 0.51 0.29
0.36 0.64 0.25
It can be obviously observed that corrosion process of pyrite was dependant of pH. Under nearly neutral condition, both corrosion current Icorr and potential Ecorr decreased. This phenomenon may be ascribed to the formation of iron oxide or hydroxide, which inhibited the electron transfer at the pyrite/solution interface. However, when cysteine was introduced into electrochemical system, Ecorr sharply
Fig. 3. Cathodic and anodic polarization curves for pyrite electrode in 0.1 M KNO3 solution without (A: pH = 6.2; B: pH = 2.5) and with 1 mM cysteine (C: pH = 6.2; D: pH = 2.5).
92
Z. Wang et al. / Hydrometallurgy 101 (2010) 88–92
lowered whereas Icorr increased rapidly. It provides strong evidence for enhancement of pyrite oxidation in the presence of cysteine. However, it should be noted that transfer coefficient and number of electron transfer, which are indicative of electrode reaction mechanism (Liu, 2002), did not obviously change in the absence and presence of cysteine. It suggests that addition of cysteine just facilitated the electron transfer but did not change the pyrite oxidation mechanism. Cysteine may serve as a bridge or conductor to facilitate electronic charge transfer from pyrite and end-product. Previous study showed that bridge complexes formed by adsorbing – O–, –CN– or –CO– groups to interfacial iron atoms in pyrite could greatly accelerate the transfer of photogenerated charge carriers between pyrite and soluble iron (Schubert and Tributsch, 1990). Based on the discussion above, it is of interest to reexamine the issue of the interaction between aporusticyanin and pyrite (Blake et al., 2001). Their adsorption may be accomplished by their subunits: iron or sulfur atom for pyrite crystal, amino acid residuals for apoprotein. Considering the cysteine's good affinity to pyrite, we predict that cysteine located in the framework of aporusticyanin can act as a mineral-specific receptor. It is still an area of interest that deserves further investigation. For example, the affinity of other copper ligands (Met and His) to pyrite should be examined according to the similar methods as Cys. 4. Conclusion The interaction pattern between pyrite and cysteine was investigated to reveal how cysteine accelerates pyrite dissolution in biomining operation. TG curves provided the direct evidence for chemisorption of cysteine on pyrite surface. It suggests that cysteine may play a critical role in bacterial adhesion to pyrite. Once cysteine was adsorbed to sulfide surface, pyrite can be readily oxidized at lower potential. Cysteine may act as a bridge or conductor to facilitate electronic charge transfer from pyrite and end-product. However, although cysteine can obviously accelerate oxidation rate of pyrite, pyrite oxidation mechanism does not fundamentally change. Acknowledgements The authors gratefully acknowledged the financial support from the Special Fund Project for Authors of National Excellent Ph. D
Theses, China (200549), the National Natural Science Foundation of China (No. 50874032), the Shanghai Leading Academic Discipline Project (B604), and Shanhai Tongji Gao TingYao Environmental Science & Technology Development Fund.
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