INTRODUCTION. Bacterial oxidation of arsenopyrite plays an important role in bacterial leaching of gold-bearing arsenopyrite concentrates (LindstrOm et al., ...
BIOTECHNOLOGY TECHNIQUES Volume 9 No.2 (February 1 9 9 5 ) p p . l ! l - l I 6 Received 14th December
OXIDATION OF ARSENOPYRITE BY THIOBACILL US FERROOXIDANS DETECTED BY A MINERAL ELECTRODE Josef Zeman 1, Martin Mandl2., Pavtina Mrnugtikovfi2 1Department of Mineralogy, Petrography and Geochemistry and 2Department of Biochemistry, Faculty of Science, Masaryk University, Kotlfi~skfi2, 611 37 Bmo, Czech Republic
SUMMARY A new electrochemical study of arsenopyrite biooxidation was based on process detection by arsenopyrite electrode_ The rate of reaction was evaluated as the exchange current density calculated from polarization curves. Obtained data were used for determination of released electrons from mineral and for evaluation of reaction mechanism of its oxidation. INTRODUCTION Bacterial oxidation of arsenopyrite plays an important role in bacterial leaching of gold-bearing arsenopyrite concentrates (LindstrOm et al., 1992). In environment, this process results in production of toxic arsenic ions. These oxidation studies are usually performed by analyses of bioextracted arsenic. The results of recent electrochemical studies of sulfide oxidation using the electrochemical potentials of grounded pyrite were reported (Chia et al., 1989; Mustin et al., 1993). A compact pyrite electrode was used for the kinetic study of Fe(II) oxidation (Pesic et al., 1989). This paper brings a new approach to the investigation of sulfide oxidation based on a mineral (arsenopyrite) electrode_
PRINCIPLES Arsenopyrite oxidation can be described at various degrees of complexity (Karavaiko, 1988; Torma, 1989). According to minimum and maximum oxidation states of elements, arsenopyrite oxidation can be simply expressed as follows: Anodic (oxidation) reactions:
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FeAsS + 3H20 --- Fe 2+ + AsO33 + SO+ 6I-I+ + 5e"
(1)
FeAsS + 8HzO -- Fe 3+ + AsO43" -Jr 8042- + 16W + 14e"
(2)
Cathodic (reduction) reactions: 02 + 4 e + 4W ~ 2H20
(3)
Fe 3+ + e --- Fe 2+
(4)
The real oxidation state of arsenopyrite oxidation products will be within the limits given by equations (1) and (2). The electrons released from the mineral on the phase boundary into the solution are consumed by the oxidant in the solution. The rates of oxidation and reduction are dependent on the potential of the electrode. The potential of the electrode will be established at the value (rest potential) when the rate of both oxidation and reduction are the same. Under these conditions of steady-state, anodic and cathodic currents are compensated directly on the surface of the arsenopyrite electrode: i~,t = io,oa = io, where io is exchange current density (i =j/A, j - current, A area of the electrode), io is directly proportional to the rate of arsenopyrite oxidation according to Faraday's law: N = Q / (nF) = io t / (nF)
(5)
t i N / d t = io / (nF)
(6)
where N is the amount of released metal at current io, Q is the charge passed across the mineral'solution interface, t is time, n is the number of exchanged electrons, and F is Faraday's constant. At steady-state, io cannot be directly measured. When the system is electrically shiRed from the steady-state by polarization with outside loaded potential, net cathodic or anodic current is obtained according to the direction of polarization. The dependence of net current on potential is expressed by the Buttler-Volmer equation'. i = io {exp[(E-Es)/ba] - exp[-(E-Es)/bc] }
(7)
where i is net current exchange density dependent on the loaded potential E, ba and bc are Tafel's constants for anodic and cathodic reaction, respectively, Es is the rest potential of arsenopyrite electrode at steady-state (before polarization), io is the current passing mineral-solution interface at E = Es. Only Es and i = f(E) can be measured directly, io can be obtained numerically from equation (7)•
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One current becomes negligible at high overpotentials (difference E - E s ) . Then equation (7) can be changed to linear E-i dependence from which constants can be easily obtained. However, the course of reaction can be very different from that of real state under this conditions. Due to the high overpotentials, the mechanism of reaction could be changed (including number of exchanged electrons).
MATERIALS AND METHODS Bacteria. A culture of Thiobacillus ferrooxidans (CCM4253) was used after washing and concentrating on membrane filter (Mandl et al., 1994). Oxidants. Four samples of oxidation solutions were tested based on optimum efficiency of leaching solutions for bioextraction of arsenic from a concentrate (Mandl et al., 1992): Thiobacillusferrooxidans in H20, pH 1_7, and in iron-free mineral 9K medium, pH 1.5 (108 cells/ml), and corresponding controls - water, pH 1.7 and ironfree 9K medium, pH 1.5, without bacteria. Arsenopyrite electrode. Electrodes were made from natural arsenopyrite (obtained from Dr.M.Ku~nierov/t, Institute of Geotechnics, Siovak Academy of Science, Kogice). Its composition was as follows (%,w/w): Fe 33.31, As 4427, Co 1.53, Cu 1.24, Sb 0.49, Bi 0.21, and S 18.95. Crystallites of approximately 1 cm in diameter were sealed in epoxy resin, cut to discs 2.5 cm in diameter, and 2 mm thick. Discs were polished on both sides and mounted with silicon sealant to special glass holders. The electrical connection between the inner surface of the electrode and polarization device was created by liquid mercury and platinum wire. The outer surface of sulfide was in direct contact with oxidative solution. Measuring of Es. Rest potentials of arsenopyrite electrodes were measured against saturated calomel electrode by pH-mV meter OH-108 (Radelkis, Hungary). Polarization curves. The volume of the electrochemical cell was 50 ml. The arsenopyrite electrode was used as a working electrode, saturated calomel electrode as a reference electrode, and platinum net electrode as a counter electrode. The polarization of the working electrode was carried out using potentiostatic unit of polarographic analyzer PA3 (Laboratory Devices, Czech Republic). The linear potential sweep rate was 1 mV/s in both cathodic and anodic directions from the rest potential of working electrode, i.e. to -100 and 100 mV overpotentials, respectively. Determination of io. Equation (7) was fitted to the experimental dependence of exchange current density on the applied potential_ Computer assesment of io was based on the least-squares nonlinear curve-fitting using Gauss-Newton iterative method with Marquardt-Levenberg modification. Number of exchanged electrons. The charge Q passed across arsenopyrite-solution interface over 29 days was determined by integration of experimental dependence io = f(t). The calculation of the number exchanged electrons was started with suggested reaction mechanism for arsenopyrite oxidation according to equation (1), i.e. five electrons: two for iron oxidation and three for arsenic oxidation. The determined Q should theoretically release the amount of iron and arsenic (Ntheor) calculated according to equation (5). The released amout of metals was determined by solution analyses. If equation (1) is valid, the amounts of both metals, calculated and determined, will be the same. If one of the arsenopyrite component is oxidized to a higher oxidation state, the
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released amount of this metal will be lower (see equation (5)). The real number of exchanged electrons was determined as a ratio of calculated and determined amounts of both metals multiplied by the number of electrons according to equation (1) (nF, = 2, hA, = 3). If the number of exchanged electrons was higher than maximum for both metals (three for iron and five for arsenic), the remaining electrons must have been taken from sulfur by oxidation. The total number of exchanged electrons corresponds to a sum of electrons related to As, Fe, and S oxidation. The content of the total iron and arsenic in the solution was determined by atomic absorption spectrometry (Perkin-Elmer 4000, USA) in an acetylen-air flame, Fe(III) by ultraviolet absorption spectrophotometry (Mandl and Novakova, 1993). RESULTS AND DISCUSSION All experiments demonstrated a mineral electrode as a suitable detection system for biooxidation of arsenopyrite. The time dependence of the electrode rest potential is in Fig. 1. However, only the time dependence of the exchange current density (Fig. 2.) shows the instantaneous rate of oxidation.There are considerable differences between samples with and without bacteria.
400
35 30
,---, 350
%
20
X
>
25
300
15 t~
10
25O
I
0 0
10
20
30
0
Time (d)
I
10
20
3 30
Time (d)
Fig. 1. Time dependence of the rest potential Es (A) and exchange current density io (B) of the arsenopyrite electrode. 50 ml of the oxidant solution contained: Thiobacillus ferrooxidans (108 cells/ml) in water, pH 1.7 (1), and in iron-free 9K medium, pH 1.5 (2); iron-free 9K medium, pH 1.5 (3), and water, pH 1.7 (4) without bacteria. The qualitative character of results is in agreement with the rates of bioleaching obtained with concentrates (Mandl et al., 1992). The culture in water, pH 1.7, showed a faster starting bioextraction rate of arsenic than the culture in 9K medium, pH 1.5. A detection system with a moderately polarized mineral electrode enables long-term
1i4
measurements of sulfide oxidation even in cases when the rate of oxidation is not measurable by common analytical methods due to very low oxidation rates. The measurements are approaching natural conditions (sulfide is not in the form of a concentrate, and processes on the surface of the mineral are detected). This method can contribute to the optimization of the process and study of the oxidation mechanism. Determining the number of exchanged electrons is one of these possibilities. If we pay further attention to water samples, pH 1.7, and compare the total number of exchanged electrons in the samples with and without bacteria, then we find that the number of exchanged electrons is greater in the sample with bacteria (Tab. 1.). Tab. 1. Number of exchanged electrons. Q - passed charge, N - amount of released metals detemfined by solution analyses, N t h e o r - calculated amount of released metals, rirninimum axia number of consumed electrons for oxidation of each arsenopyrite component based on the minimum oxidation state of Fe and As, n,~,~,, o~d - number of consumed electrons based on the maximum oxidation state of Fe and As, n - number of total exchanged electrons. water
Q (C)
without 28.3 bacteria with 93.2 bacteria
N ~mol) As Fe 36.7 60.0
(umol) nmini .... As Fe As Fe 58.7 58.7 3.0 2 0
Ntheor
101.4 89.5 193.2 193.2 3.0
2.0
~
n ......
,~
n
S 1.8
Fe 4.8
As 2.0
S 0.0
6.8
5.0
5.0
3.0
2.0
10.0
As can be seen from Tab. 1, the most probable case corresponds to "n,,,,, . . . . ,a.". In the presence of bacteria, sulfur (n = 5) is oxidized to a higher oxidation state, possibly to S(VI) (only minor amounts to S(0)), and probably accompanied by oxidation of arsenic to As(III), and iron to Fe(II). Without bacteria, sulfur (n = 18) is oxidized to S(0), with a minor amount to higher oxidation state, possibly to S(VI). The higher number of exchanged electrons influenced by bacteria may correspond to the expected fact that oxidation of sulfur on the mineral surface proceeds primarily to the sulfate anion, while this process is negligible without bacteria. Expected course of "indirect"
io
was found in solution of Fe(IlI), pH 1.7 (chemical oxidant in
bacterial attack) without bacteria (18
decreasing of the time dependence of
io
mM, not shown). Hyperbolic
to very low values corresponded to a
decreasing concentration of Fe(III) during its reduction by sulfide and a lack of Fe(Ill) regeneration by bacteria. All considerations based on
io
refer only to the reactions connected with oxidation of
the mineral and not to the consecutive reactions in solution. It is clear that mere solution
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analyses do not necessarily lead to correct ideas about the oxidation mechanisms of minerals. It is particularly evident with arsenopyrite, where As(m) in solution can be further oxidized to As(V) by Fe(III) under pyrite catalysis supported by bacteria (Barrett et al., 1993). According to Mandl and Vygkovslo) (1994), the process probably corresponds to the reaction H3AsO3 + Fe2(S04)3 + H20 = H3AsO4 + 2FeS04 + H2S04 Similarly, Fe(II) is oxidized by bacteria to Fe(III) in solution. This is also not connected to the electron exchange between mineral and solution.
ACKNOWLEDGEMENTS
This work was supported by grant No. 511/94/0487 from the Grant Agency of the Czech Republic. We also thank the Czechoslovak Commercial Bank (~SOB) for partial support.
REFERENCES
Barrett, J., Ewart, D.K., Hughes, MN., and Poole, R.K. (1993). FEMSMicrobioL Rev. 11, 57-62, Chia, L.M, Choi, W.K., Guay, R., and Torma, A.E (1989). In:Biohydrometallurgy 1989, J. Salley, R.G.L. McCready and P.L. Wichlacz, eds_, pp.35-47, Canada Centre for Mineral and Energy Technology, Ottawa. Karavaiko, G.I. (1988). In: Biogeotechnoiogy of Metals, Manual, G.I. Karavaiko, G. Rossi, A.G. Agate, S.N. Groudev, and Z.A. Avakyan, eds., pp. 10-45, Centre for International Projects UNEP, Moscow. Lindstr6m, EB., Gunneriusson, E, and Tuovinen, O.H. (1992). Crit. Rev. BiotechnoL 12, 133-155. Mandl, M_, Matulovfi, P., and Dotekalova, H. (1992). AppL MicrobioL BiotechnoL 38, 429-431. Mandl, M., and Novhkov/t, O. (1993). Biotechnol. Technique 7, 573-574. Mandl M, and Vygkovslo) M (1994). Biotechnol. Lett. 16, 1199-1204. Mustin, C, de Donato, Ph., Berthelin, J., and Marion, Ph. (1993). FEMS MicrobioL Rev. 11, 71-78. Pesic, B., Oliver, D.J., and Wichlacz, P. (1989). BiotechnoL Bioeng. 33, 428-439. Torma, AE. (1989). JOM June, 32-35.
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