Leaching of marine manganese nodules by acidophilic bacteria growing on elemental sulfur
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INTRODUCTION
THERE have been many investigations into the recovery of valuable metals from marine manganese nodules because of their potential importance for mineral resources, as reviewed in the literature.[1,2] Although chemical processes have been extensively developed for extracting the metals from manganese nodules, another possible process is the leaching of nodules by microbial means. Because manganese nodules are comprised mainly of oxides of manganese and iron, microbial treatment of nodules requires the use of microorganisms which are capable of reducing oxides of metals, especially manganese as a major element in nodules, in the presence of dissolved heavy metals. A fairly large number of studies have addressed microbial reduction of oxidized manganese, as discussed in reviews of the literature on applied microbiology.[3,4] Evidently, a variety of microorganisms have the desirable ability of solubilizing manganese oxides under aerobic and anaerobic conditions. The mechanisms of the microbial manganese solubilization are discussed in terms of a direct microbial attack on oxides (enzymatic action) and an indirect attack via reductive intermediates from microbial metabolism (nonenzymatic action), as in the case of microbial oxidation of metal sulfides. This article describes the indirect (nonenzymatic) solubilization of manganese nodules by sulfur-oxidizing bacteria growing on elemental sulfur. A similar system was treated by Imai[5] and Ghoshi and Imai,[6] who studied the leaching of manganese dioxide during the growth of Thiobacillus thiooxidans or Thiobacillus ferrooxidans using elemental sulfur as its substrate. Thiobacillus species were found to solubilize the oxide by producing sulfurous acid, a reductive intermediate of sulfur oxidation. An investiga-
tion was recently undertaken to examine the leaching of manganese nodules by a mixed culture of T. thiooxidans and T. ferrooxidans growing on elemental sulfur.[7] The bioleaching mechanism was considered to be that sulfurous acid and sulfuric acid are biologically produced from elemental sulfur added to the solution: S0 1 O2 1 H2O → H2SO3 S0 1 (3/2)O2 1 H2O → H2SO4
[1] [2]
and are simultaneously used to leach manganese nodules. The bioleaching with the sulfur-oxidizing bacteria appears to be a technically possible method, because chemical leaching techniques with sulfurous acid or sulfuric acid have been applied to manganese nodules. However, the previous bioleaching experiments were carried out at a temperature optimum of 30 7C, because these Thiobacillus species are mesophilic acidophiles. In view of the temperature dependence of chemical reaction rate, it is preferable to realize the microbial treatment of manganese nodules at higher temperatures. Thus, the thermophilic acidophile Acidianus brierleyi, which grows optimally at around 65 7C, is a candidate microbe for accomplishing more effective extraction of valuable metals from manganese nodules. The purpose of this work is to obtain rate data on the leaching of raw manganese nodules by A. b
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