# A Comparative Study on Bieleaching Properties of Various
## 1 Introduction  
Sulfide minerals in ores containing useful metals have been a major source of environmental pollution in mineral and smelting processes [1; 2; 3]. Bioleaching is considered a green alternative to chemical leaching processes as it is a method of extracting metals through biologically produced metabolites [4]. If valuable metals in sulfide minerals are recovered using bioleaching, ore transportation costs can be reduced, and environmental pollution can be mitigated by recirculating the leaching solution [5; 6; 7]. The understanding of bioleaching, the application of microbial processes to extract metals from near-insoluble ores, has been rapidly established over the past few decades [8]. The oxidation of sulfide minerals using bioleaching can be classified into direct and indirect routes. Depending on the pH conditions in the leachate, direct and indirect oxidation occur simultaneously, in a phenomenon termed \"cooperative leaching\" [9; 10].  
When ores undergo chemical or biological oxidation, the minerals present on the surface dissolve. During the dissolution process, the diameter of mineral particles is reduced and the relative surface area increases. This diameter reduction through dissolution was numerically analyzed using the shrinkage model and the shrinkage particle model [11; 12]. However, when the mineral surface is dissolved in solution, all crystal phases of the mineral are not uniformly dissolved, i.e., dissolution occurs selectively. For example, dislocations or stacking faults may exist because of the weak S and Fe bonds in the crystal plane of pyrite (FeS2) [13]. Therefore, a crystal plane that is weakly crystallized (i.e., contains-mineral imperfections) dissolves better than a crystal plane that is highly crystallized [14; 15]. Meanwhile, direct oxidation by bacteria serves to increase the leaching concentration due to the selective attachment of bacteria to mineral imperfections. In particular, bacteria preferentially attach to weakly crystallized crystal planes; hence, the crystal plane is more soluble [16; 17]. In indirect oxidation, bacteria do not attach to the surface of sulfide minerals, whereas in direct oxidation, bacteria directly contact and effectively dissolve the surface of sulfide minerals. However, comparative studies on the effect of the crystal/chemical structure of sulfide minerals and the direct attachment of bacteria to the surface of sulfide minerals on bioleaching properties are still insufficient and need to be further studied.  
Therefore, in this study, we tried to evaluate and compare the bioleaching properties of different sulfide minerals. Sphalerite (ZnS), galena (PbS), pyrite (FeS\({}_{2}\)), and chalcopyrite (CuFeS\({}_{2}\)) collected from local mines were used as sulfide minerals for comparative evaluation. A cultivated indigenous acidophilic bacterium, _Acidiplinium cryptum_ (99.56%), was applied to the bioleaching experiments. The microscopical analyses were conducted to characterize the sulfide minerals before and after bacterial application. The change of pH and mineral leaching properties according to the reaction time were quantitatively evaluated, and bioleaching mechanisms for sulfide minerals were also proposed.