bacteria such as Thiobacillus ferrooxidans are catalysts of pyrite oxidation because they significantly accelerate the slow chemical oxidation of ferrous iron by ...
APPLIED
AND
ENVIRONMENTAL MICROBIOLOGY, June 1992, p. 1853-1856
Vol. 58, No. 6
0099-2240/92/061853-04$02.00/0 Copyright (© 1992, American Society for Microbiology
Leaching of Pyrites of Various Reactivities by Thiobacillus ferrooxidans FRANCO BALDI,' THOMAS CLARK,2 S. S. POLLACK,2 AND GREGORY J. OLSON2* Dipartimento di Biologia Ambientale, Universita di Siena, Italy,' and Pittsburgh Energy Technology Center, U.S. Department of Energy, Pittsburgh, Pennsylvania 152362 Received 23 December 1991/Accepted 20 March 1992
Wide variations were found in the rate of chemical and microbiological leaching of iron from pyritic materials from various sources. Thiobacilus ferrooxidans accelerated leaching of iron from all of the pyritic materials tested in shake flask suspensions at loadings of 0.4% (wt/vol) pulp density. The most chemically reactive pyrites exhibited the fastest bioleaching rates. However, at 2.0% pulp density, a delay in onset of bioleaching occurred with two of the pyrites derived from coal sources. T. ferrooxidans was unable to oxidize the most chemically reactive pyrite at 2.0%Yo pulp density. No inhibition of pyrite oxidation by T. ferrooxidans occurred with mineral pyrite at 2.0% pulp density. Experiments with the most chemically reactive pyrite indicated that the leachates from the material were not inhibitory to iron oxidation by T. ferrooxidans.
Mechanisms of chemical and microbiological oxidation of pyrite and factors affecting the rate of pyrite oxidation in aqueous solution have been studied widely because of the significance of pyrite oxidation in environmental and industrial processes. The oxidation of pyrite associated with coal and metal ore deposits results in acidic mine drainage, a significant water pollution problem. Conversely, microbiological and chemical oxidations of pyrite and other metal sulfides have beneficial applications in the commercial leaching of metals from low-grade ores and wastes. Chemical and microbiological depyritization of coal have also been studied. In aerated, aqueous solutions, the sulfidic sulfur of pyrite is oxidized and the iron passes into solution in the ferrous form:
coal pyrite were two to four times faster than those of a mineral ore pyrite as determined by X-ray photoelectron spectroscopy (7). Variation in the chemical reactivity of pyrite varies with pyrite morphology and, hence, with surface area (2, 5, 18). However, nonreactive pyrite that is finely ground remains nonreactive (2), indicating that surface area alone is not responsible for variations in pyrite reactivity. Pyrite reactivity also varies with the number of crystal lattice imperfections observed as dislocation densities (natural etch pits) on pyrite surfaces (11). Because bacteria accelerate the rate-limiting step in pyrite dissolution (14), it is of interest to determine how the activity of bacteria on different pyrites correlates with the abiotic, chemical reactivity of the pyrites. Temple and Delchamps (17) mentioned that certain forms of pyrite in coal seams were more reactive than museum-grade mineral pyrite. Aside from these brief early observations, little work has been done to examine the bacterial oxidation of pyrites of various chemical reactivities. Specifically, little quantitative information is available on the rates of chemical and microbial oxidation of different pyrites.
2FeSO4 + 2H2SO4 (1) A more rapid oxidation of pyrite occurs when Fe3` is
2FeS2 + 702 + 2H20
available
as
--
the oxidant:
FeS2 + 14Fe3+ + 8H20 -_ 15Fe2+ + 2SO42- + 16H+ (2) A surface chemical reaction appears to control the rate of pyrite oxidation (18). Both ferrous and ferric ions rapidly adsorb to the surface of pyrite, and the rate of pyrite oxidation is proportional to the fraction of the pyrite surface occupied by ferric ions (6). Moses and Herman (8) suggested that the chemical oxidation of pyrite is limited by the rate that Fe(II) adsorbed to the pyrite surface is oxidized by dissolved oxygen in water. They concluded that adsorbed Fe(II) oxidizes to Fe(III) by donating electrons to dissolved oxygen, reducing it to water. The adsorbed Fe(III), in a rapid step, accepts electrons from pyrite, reducing to Fe(II), which again reacts with dissolved oxygen. Iron-oxidizing bacteria such as Thiobacillus ferrooxidans are catalysts of pyrite oxidation because they significantly accelerate the slow chemical oxidation of ferrous iron by oxygen at low pH. Pyrites from different sources vary significantly in their chemical and physical properties (15), including rate of oxidation by ferric ions (5, 6). Surface oxidation rates of a *
MATERIALS AND METHODS
Organism and culture conditions. T. ferrooxidans 13661 (American Type Culture Collection, Rockville, Md.) was maintained on 9K liquid medium (13) at 29°C on a gyratory shaker at 200 rpm. Pyritic material. Mineral pyrite (Matheson, Coleman and Bell, Norwood, Ohio) and three samples of pyritic material associated with coal deposits were studied. One of the coal pyrites was obtained from the Riverside coal mine, Queensland, Australia; the other two pyritic materials were obtained from strata associated with Pittsburgh seam coal at the Cumberland Mine in Pennsylvania. The pyritic material designated Pittsburgh-A was taken from silver-gray colored strata and appeared unoxidized. Pittsburgh-B material was
taken from a different section of the same piece of rock but contained visible precipitated salts and was considered empirically to be reactive. The pyrites were ground and sieved to 75 to 150 ,um (100/200 mesh), washed in 6 M HCl, rinsed with deionized water and then with absolute ethanol, and dried 1 h at 85°C in a vacuum oven. The pyrites were stored
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BALDI ET AL.
TABLE 1. Analysis of pyritic materials Material
Carbon
Mineral pyrite Queensland Pittsburgh-A Pittsburgh-B
0.26 3.72 4.82 24.02
Organic
(%)
C
Carbonate C
(%)
(%)
0.17 3.72 3.83 24.02
0.09
