The mechanism of bacterial action in the leaching of pyrite by Thiobacillus ferrooxidans

In many of the experiments reported in the literature on the leaching of py rite by Thiobacillus ferrooxidans, the concentrations of ferric and ferrous ions in the presence of bacteria differ significantly from experiments con ducted in their absence. In addition, these concentrations change throughou t the course of the experiment. This makes it difficult to determine whethe r the presence of bacteria increases the rate of leaching above that for ch emical leaching at the same solution conditions. We have designed an experi mental apparatus to overcome this problem. This apparatus controls the redo x potential in one compartment of an electrolytic cell by manipulating the current to the cell. In this manner, the concentrations of ferrous and ferr ic ions are maintained at their initial values for the duration of the expe riment. Two types of experiments are reported in this paper In the first, p yrite electrodes were exposed to solutions of the same bulk conditions in t he presence and absence of bacteria, and their mixed potentials were determ ined. In the second, particulate pyrite was leached with and without bacter ia to determine the effect that bacteria have on the rate of leaching. The mixed potential of bacterially dissolved pyrite decreases as microcolonies and biofilms form on the surface of pyrite electrode over a 14 day period. On the other hand, the mixed potential of chemically dissolved pyrite is co nstant over the same period. The results of the leaching experiments show t hat Thiobacillus ferrooxidans enhances the rate of leaching above that foun d in the absence of bacteria at the same conditions in solution. An electro chemical model of pyrite dissolution is derived that describes the mixed po tential and the kinetics of pyrite leaching. This analysis indicates that t he decrease in mixed potential and the increase in the leaching rate in the presence of bacteria are due to an increase in the pH at the surface. (C) 1999 The Electrochemical Society. S0013-4651(98)12-007-4. All rights reserv ed.