Electrochemistry of thiobacillus ferrooxidans interactions with pyrite
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I.
INTRODUCTION
U N D E R S T A N D I N G the interactions of Thiobacillus (T.)ferrooxidans with pyrite is important from both metallurgical and environmental points of view. In metallurgical applications, T. ferrooxidans can oxidize pyrite to liberate encapsulated gold t~J or produce ferric ion and acid for copper leaching in dumps/heaps, t21 From the environmental point of view, this bacterium can clean pyrite from coal t3] but may be detrimental because of its role in the oxidation of sulfide minerals and release of heavy metals and acids into mine waters, t41 In either case, successful control depends on understanding the interactions of T. ferrooxidans with pyrite. Although important, the pertinent reaction mechanisms have received only limited study. Most studies have focused on the suitability of ores, coals, and cultures for bioleaching. Mechanistic studies have been performed by Iwasaki and Natarajan, 151 Natarajan and Iwasaki, t61 Mehta and M u t t , [7,8] and Berry et al.,t91 who all used corrosion principles to explain the reactions involved. Most recently, Chia et al. I~~ studied the electrochemical aspects of pyrite oxidation by T. ferrooxidans during the leaching of a Canadian uranium ore. He found that pyrite oxidation was controlled by solid-state diffusion phenomena. Palencia et al. tm suggested that T. ferrooxidans modulated the solution chemistry (the ferric ion activity) and had a limited effect on the electrochemical reactions on the pyrite surface. This work examines the interfacial phenomena of T. ferrooxidans on pyrite by using the cyclic voltammetric technique. Cyclic voltammetry was selected because it is the most versatile electrochemical technique available for the mechanistic study of redox s y s t e m s . [12,13,14} The cyclic voltammetry results were correlated with the surface reaction products which were characterized by BATRIC PESIC, Professor of Metallurgy, is with the College of Mines and Earth Resources, University of Idaho, Moscow, ID 83843. INBEUM KIM, formerly Graduate Student, College of Mines and Earth Resources, University of Idaho, is Senior Researcher, Rocky Mountain Instrument Company, Longmont, CO 80501. Manuscript submitted June 25, 1992. METALLURGICAL TRANSACTIONS B
using scanning electron microscopy (SEM), X-ray diffraction, and chemical analysis techniques.
II.
MATERIALS AND METHODS
A. Electrochemical Setup Electrochemical measurements were performed with a potentiostat/galvanostat (EG&G PARC, Model 273) controlled by an external computer (EG&G PARC, Model 270 software) (Figure 1). The electrochemical cell was actually a 2-L glass kettle which was also used as a bioreactor for growth of bacteria. One of the kettle' s cover ports was used to supply air, and the other three were used to accommodate a working electrode (pyrite), a standard reference electrode (calomel) with a salt bridge, and a Pt counterelectrode. The kettle was placed into a water bath to maintain a temperature of 25 ~ B. Pyrite Electrode Pyrite electrodes (about 0.5 • 2 x 2 cm) were prepared
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