Adaptation of Acidithiobacillus ferrooxidans to high grade sphalerite concentrate D.F. Haghshenas a , E. Keshavarz Alamdari a, * , M. Amouei Torkmahalleh b , B. Bonakdarpour b,c , B. Nasernejad b,c a Department of Mining and Metallurgical Engineering, Amirkabir University of Technology, P.O. Box. 15875-4413 Tehran, Iran b Department of Chemical Engineering, Amirkabir University of Technology, Tehran, Iran c Food Process Engineering and Biotechnology Research Centre, Amirkabir University of Technology, Tehran, Iran article info Article history: Received 4 April 2009 Accepted 28 July 2009 Available online 27 August 2009 Keywords: Adaptation Acidithiobacillus ferrooxidans High grade sphalerite Bioleaching abstract In this study different strategies were employed for adapting Acidithiobacillus ferrooxidans cells to both Zn 2+ ions and high grade sphalerite concentrate. The serial subculturing was found to be a very efficient strategy for adapting A. ferrooxidans cells to higher Zn 2+ concentrations, as well as high grade sphalerite concentrate, provided that a suitable protocol was employed. Adaptation of A. ferrooxidans cells to 30 g/L Zn 2+ significantly enhanced the rate of bioleaching of Zn 2+ from high grade sphalerite concentrate. Pread- aptation to Zn 2+ ion also shortened the time required to adapt the cells to the concentrate. A. ferrooxidans PTCC 1642, exhibited a higher rate of Zn 2+ bioleaching from high grade sphalerite concentrate compared to A. ferrooxidans DSMZ 583; however, due to the adaptation protocol employed the difference in perfor- mance of the two strains was not very great. Based on the results obtained in this study suggestions were given for the mechanisms during the adaptation of A. ferrooxidans cells to high grade sphalerite concentrate. Ó 2009 Elsevier Ltd. All rights reserved. 1. Introduction Sphalerite is one of the most important sources of zinc produc- tion in the world. Conventional methods of zinc extraction from sphalerite, such as roasting and smelting, result in the production of environmental pollution, which is the result of the production of sulfur dioxide and fumes during these operations. An alternative method for zinc extraction from sphalerite, which has received increasing attention recently, is bioleaching. This process has the advantages of low consumption of energy and environment friend- liness (Liao and Deng, 2004; Rodriguez et al., 2003). Exposure of mesophilic cultures to metal sulfide ore or concen- trate results in inhibition or retardation of the activity of these cul- tures, especially at high pulp densities; the reasons for such a phenomenon have been variously given as reduced oxygen and carbon dioxide transfer rates, mechanical damage to the bacterial cells as a result of attrition between the mineral particles and the cells and the toxic effect of the leached metal ions on the bacterial activity (Marhual et al., 2008; Akcil et al., 2007; Mousavi et al., 2007; Gomez et al., 1999). However, mesophilic bacterial cultures can be adapted to tolerate some of these inhibitory factors (Xia et al., 2008; Mason and Rice, 2002). In bioleaching of high grade sphalerite concentrate, one factor which is likely to retard the activity of bacterial cultures is the tox- icity of the high concentration of Zn 2+ ion likely to exist in the bioleaching solutions. Zn 2+ toxicity is based upon complexation with various cellular components (Dopson et al., 2003). Previous studies have indicated that Acidithiobacillus ferrooxidans can be adapted to Zn 2+ ion concentrations up to 70 g/L (Kondratyeva et al., 1995). Das et al. (1997) reported that the use of A. ferrooxi- dans cultures that were adapted to Zn 2+ ion results in more effi- cient bioleaching zinc from sphalerite; however, in that study the concentration of Zn 2+ in bioleaching media was artificially in- creased and the duration of the experiments was short; so the question which still remains unanswered is whether during the necessary adaptation process of A. ferrooxidans cells to sphalerite in a normal bioleaching media, the bacterial cells will gradually adapt themselves to Zn 2+ ion or not? Pani et al. (2003) used A. fer- rooxidans cells which were adapted to 60 g/L Zn 2+ for bioleaching of zinc sulfide concentrate in a four stage continuous bioreactor sys- tem; however, most other bioleaching studies involving sphalerite have been conducted with A. ferrooxidans cells which had been only adapted to sphalerite ore or concentrate. The most common approach employed by previous investiga- tors to adapt A. ferrooxidans cells to inhibitory metal ion concentra- tions is serial subculturing, in which the bacterial cells are subcultured in media containing increasing metal ion concentra- tions. However, there is no specific protocol followed in these works, and for adaptation to high concentrations of Zn 2+ ion usu- ally several subcultures are necessary (Das et al., 1997; Natarajan et al., 1994; Brahmaprakash et al., 1988). The alternative approach, 0892-6875/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.mineng.2009.07.011 * Corresponding author. Tel.: +98 912 218 1405; fax: +98 21 640 5846. E-mail address: alamdari@aut.ac.ir (E.K. Alamdari). Minerals Engineering 22 (2009) 1299–1306 Contents lists available at ScienceDirect Minerals Engineering journal homepage: www.elsevier.com/locate/mineng