The current study investigates the characteristics of arsenic precipitation during the bioleaching of realgar. The bioleaching performance of Acidithiobacillus ferrooxidans BY-3 (A. ferrooxidans) was investigated through scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and Fourier transform infrared (FT-IR) spectrophotometry. SEM and XRD analyses revealed that the arsenic-adapted strain of A. ferrooxidans was more hydrophobic and showed higher attachment efficiency to realgar compared with the wild strain. The arsenic precipitation using A. ferrooxidans resulted in the precipitation of an arsenic-rich compound on the surface of the bacterial cell, as shown in the TEM images. The FT-IR spectra suggested that the ？OH and ？NH groups were closely involved in the biosorption process. The observations above strongly suggest that the cell surface of A. ferrooxidans plays a role in the induction of arsenic tolerance during the bioleaching of realgar. 1. Introduction Realgar, which has a chemical formula of α-As4S4, is a sulfide mineral with high arsenic content (over 90% As4S4) and has been widely used as a traditional Chinese herbal medicine [1]. However, realgar is not soluble in water, and thus, pretreatment is frequently required to enhance its solubility and/or to prepare it in an appropriate form for clinical use. Recent considerable efforts have focused on developing protocols to increase the solubility and bioavailability of realgar. Among the protocols, bioleaching, which is a simple and effective technology in hydrometallurgy, has attracted extensive interests [2]. For instance, our group previously succeeded in developing a bio-arsenic aqueous solution from realgar particles by taking advantage of an arsenic-resistant indigenous strain of A. ferrooxidans BY-3 using the bioleaching method and confirmed the anticancer activities of the developed bioarsenic aqueous solution in vitro and in vivo [3]. On the other hand, arsenic is known to be poisonous for many organisms [4]. A high concentration of arsenic in the leaching solution could inhibit bacterial growth and activity, which have negative effects on realgar bioleaching. Based on our previous observations during the entire bioleaching process [3], the concentration of arsenic interestingly increased to a peak value at first and then decreased, instead of increasing continuously, but there have been no thorough studies on this reason for this phenomenon. The decrease in arsenic concentration may be attributed to the arsenic adsorption and/or
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