Background: Bioleaching is a process that has been used in the past in mineral pretreatment of refractory sulfides, mainly in the gold, copper and uranium benefit. This technology has been proved to be cheaper, more efficient and environmentally friendly than roasting and high pressure moisture heating processes. So far the most studied microorganism in bioleaching is Acidithiobacillus ferrooxidans. There are a few studies about the benefit of metals of low value through bioleaching. From all of these, there are almost no studies dealing with complex minerals containing arsenopyrite (FeAsS). Reduction and/or elimination of arsenic in these ores increase their value and allows the exploitation of a vast variety of minerals that today are being underexploited.

Bioleaching implies the use of bacteria for either the extraction or purification of minerals, and currently, is considered as one of the modern technologies which have one of the most solid application perspectives [1]. An isolated microorganisms for such task is Acidithiobacillus ferrooxidans (Af), which has attracted a great interest for knowing its bioleaching mechanism. It is known that this bacteria obtains its energy from the oxidation of ferrous ions and from inorganic sulfur [2], suggesting that for this microorganism it is required the interaction bacteriamineral [3]. During this interaction, polysaccharides are synthesized as an evidence of sulfur oxidation, with the consequent mineral dissolution. In the present research, the techniques of scanning electron microscopy (SEM;) and energy dispersive analysis by X-rays EDAX) were used. From these it could be suggested that the mentioned interaction have been taken in specific zones of the mineral. MaterialsStrain T18 of Af, adapted to 1,800 ppm of As, was used as culture medium 9K; crystals of sulfide (pyrite) were used as mineral; a standard methodology for bacteria fixation for its observation by scanning electron microscopy (Jeol Model JSM5800LV) was implemented. Two mineral cultures were prepared for present study, both under the same experimental parameters, one of the cultures without the strain of the Af (control).Results. SEM analysis of both samples indicated a significant difference between the surface of the control sample and the one of the sample that contained the Af. In the first, the surface (Fig 1) apparently smooth and uniform did not present the presence of polysaccharides, while the second, besides showing the presence of Af, it generated polysaccharides and a rugged and irregular surface (Fig.2). The chemical composition of some interest zones, were determined through EDAX, obtaining organic compositions in the mineral with Af, and inorganic compositions in the control sample.Conclusions. The techniques of SEM and EDAX showed that Af acts in a selective way in the minerals surface, showing after the first 12 hours of contact between bacteria and mineral the presence of