A pathway of the generation of acid mine drainage and release of arsenic in the bioleaching of orpiment.

Arsenic in acid mine drainage (AMD) is commonly associated with the bioleaching of arsenic sulfide minerals. Orpiment is iron free and one of the most common arsenic sulfide minerals, but no studies are involved with the relationship between the iron free bioleaching of orpiment and the generation of arsenic-containing AMD. In this study, the iron free bioleaching experiments with Acidithiobacillus thiooxidans (T.t) or Acidithiobacillus caldus (A.c) were carried out. In the experiments with T.t, the pH value decreased with time, and the leached arsenic increased significantly. Meanwhile, the density of planktonic bacteria increased gradually, suggesting that T.t survived in the orpiment pulp. However, in the experiments with initial pH of 1, pH changed little and arsenic was nearly not leached, implying that the bioleaching of orpiment can be inhibited when the initial pH was too low. The XRD patterns and the TFESEM-EDS analyses showed that no elemental sulfur was detected on the orpiment surface. It was supposed that the sulfur was converted to sulfuric acid in the bioleaching process. The CFESEM images showed that no corrosion pits were formed though a few cells adhered to the orpiment surface, and the TEM images showed that no extracellular polymeric substances (EPS) were excreted by the attached cells on the orpiment particles. In the experiments with A.c, similar results were obtained. It is inferred that the bioleaching of orpiment under iron deficient conditions in mining areas generates arsenic-containing AMD, but can be inhibited when the initial pH is too low.

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