Korean J. Chem. Eng., 26(3), 736-741 (2009) SHORT COMMUNICATION 736 † To whom correspondence should be addressed. E-mail: djkim@kigam.re.kr Effect of Ni 2+ , V 4+ and Mo 6+ concentration on iron oxidation by Acidithiobacillus ferrooxidans Debabrata Pradhan* , **, Jong-Gwan Ahn*, Dong-Jin Kim* ,† , and Seoung-Won Lee** *Minerals and Materials Processing Division, Korea Institute of Geoscience and Mineral Resources (KIGAM), Daejeon 305-350, Korea **Nano Engineering Division, School of Engineering, Chungnam National University, Daejeon 305-764, Korea (Received 1 October 2008  accepted 8 December 2008) Abstract −The ferrous oxidation ability of Acidithiobacillus ferrooxidans was studied in the presence of Ni 2+ , V 4+ and Mo 6+ in 9 K media in order to implement the culture in the bioleaching of spent catalyst. The rate of iron oxidation decreased with increasing concentration of metal ions, but the rate of inhibition was metal-ion dependent. The tolerance limit was critical at a concentration of 25 g/L Ni 2+ , 5 g/L V 4+ and 0.03 g/L Mo 6+ . The growth rate of microorganisms was negligible at concentrations of 6 g/L V 4+ and 0.04 g/L Mo 6+ . Levels and degree of toxicity of these ions have been quantified in terms of a toxicity index (TI). The toxicity order of metal ions was found to be Mo 6+ >V 4+ >Ni 2+ . The sig- nificance and relevance of multi-metal ion tolerance in Acidithiobacillus ferrooxidans has been highlighted with respect to bioleaching of spent refinery catalyst. Key words: Iron Oxidation Rate, Acidithiobacillus ferrooxidans, Adaptation, Tolerance, Toxicity Index, Bioleaching INTRODUCTION Microbial bioleaching is based on the natural ability of microor- ganisms to transform solid compounds into a soluble and extract- able form. This involves enzymatic oxidation or reduction of the solid compounds, or an attack on the solid compounds by meta- bolic products [1,2]. Researchers are employing this technique to remove heavy metals from materials such as low-grade ores, in- dustrial wastes, spent batteries, electronic scraps, and spent petro- leum catalysts. In this regard the acidophilic, chemolithotrophic bac- teria Acidithiobacillus ferrooxidans has been exploited world wide for various metallurgical application, the most common being bio- leaching [3-6]. These autotrophs commonly inhabit the earth’s most metal-rich environments, and in these habitats they develop an indigenous metal- tolerance capability. Therefore these bacteria are considered to be ideal media to study microbial metal-tolerance phenomenon [7,8]. Some toxic cations and anions can affect the ferrous iron oxidiz- ing ability of Acidithiobacillus ferrooxidans . This has been studied using redox potential measurement and other spectroscopic analy- sis. The results suggest that the presence of Hg 2+ , Ag + , Pb 2+ and Cd 2+ in solution at a concentration level of more than 10 ppm inhibits the bio-oxidation of ferrous [9]. No inhibitory effect on the oxidation of ferrous ions is observed in the presence of other heavy metal ca- tions such as As 3+ , Mn 2+ , Sn 2+ , Co 2+ , Cu 2+ and Zn 2+ or with anions such as Cl − and NO 3 − up to concentrations of 10 ppm. It is sug- gested that the iron oxidation is inhibited due to formation of an enzyme-Fe 2+ complex and at the higher concentrations; the mecha- nism is thought to be diffusion of Fe 2+ in to the periplasmic space of the Acidithiobacillus ferrooxidans cells. To design a suitable engineering process to tackle the imbalance in ecosystem, it is crucial to understand the inhibitory effect of dis- solved heavy metals [10]. The work here was conducted to study of the tolerance limits of the acidophilic bacterium, Acidithiobacillus ferrooxidans, in the presence of a range of heavy metals at various concentrations [11]. Biofilm formation is a strategy that microorganisms might use to survive in a toxic flux of inorganic solvents containing heavy metals. Evidence in the literature suggests that biofilm populations are protected from toxic metals by the combined action of chemi- cal, physical and physiological phenomena, and in some instances, linked to phenotypic variation among the constituent biofilm cells [12]. This is termed as “biphasic population killing,” where most of the growing population is rapidly killed by a low concentration of the antimicrobial [13,14]. Since the Acidithiobacillus ferrooxidans can grow at higher con- centrations of iron in the nutrient media, it is convenient to study the kinetics of the iron oxidation rate by analyzing Fe 2+ over time by using a volumetric method. In this note, the iron oxidation rate of the said bacteria has been monitored out in presence of heavy metals such as Ni, V and Mo. These heavy metals form part of the major constituents in spent petroleum refinery catalysts. One other hand adaptation of the heavy metals was carried out in order to apply in bioleaching of the spent refinery catalyst. Also, several toxicity in- dices have been calculated to allow for a better understanding. Bio- leaching of spent refinery catalyst was carried out by using both the unadapted and adapted microorganisms by varying the solid concentrations. MATERIALS AND METHODS 1. Microorganism and Medium The bacteria used in this experiment were a mixed culture iso- lated from the effluent pond water of Dalsung Tungsten and Copper Mines, South Korea. This culture was chemolithotrophic Acidithio-