New insights about the bacterial oxidation of arsenopyrite: A mineralogical scope

Polished sections of relatively pure samples of arsenopyrite taken from El Violin gold mine (Anorí, Antioquia, Colombia), were bio-oxidized. An initial mineralogical characterization was performed using X-ray diffraction and plane polarized optical microscopy (reflected light mode). The samples were treated in a 100 ml Erlenmeyer flask, in a T&K medium; using the Acidithiobacillus ferrooxidans strain ATCC 23270, with a cell concentration between 107 and 108 cells/ml, on orbital shaker at 180 rpm and 30 °C. Samples were taken after 0, 2, 4, 8, 20, 32, 120, 360 and 720 h, and analyzed using a confocal Raman spectrometer, plane polarized light optical microscopy, and X-ray diffraction.Mineral coatings were detected on the biotreated arsenopyrite, as follows: (i) in the early stages of the bio-oxidation, orpiment and jarosite films; and (ii) at the end of the bio-oxidation process, films formed by a mixture of jarosite, elemental sulphur and arsenolite. The role of iron in the process is also discussed.In this work, it was found the presence of orpiment (As2S3) rather than realgar. However, the presence of orpiment may be explained due to the lack of arsenopyrite stoichiometry, often depleted in arsenic and enriched in sulphur. It was also found that arsenolite occurs as fine particles embedded in a matrix of jarosite, avoiding it is the dissolution.These results show that the generation of coatings is quite dynamic and changing in time. It is also clear that the generation of films on the arsenopyrite studied is more complex than reported results, indicating that it is important to review either kinetic models, and concepts about the mechanisms involved in its bio-oxidation and the implications for mineral technology and the environment.

► Arsenopyrite was studied under the action of Acidithiobacillus ferrooxidans.
► Surface characterization of the samples was performed by Micro-Raman spectroscopy.
► Arsenolite was the main arsenic mineral product of the bio-oxidation of arsenopyrite.
► Orpiment instead realgar modifies the biooxidation model proposed by other authors.