Dissolution kinetics of spent petroleum catalyst using two different acidophiles Debabrata Pradhan a , Debaraj Mishra a , Dong J. Kim a, ⁎, G. Roy Chaudhury b , Seoung W. Lee c a Minerals and Material processing Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, South Korea b Department of Environment and Sustainability, Institute of Minerals and Materials Technology, Bhubaneswar 751013, India c Nano Engineering Division, School of Engineering, Chungnam National University, Daejeon 305-764, South Korea abstract article info Article history: Received 16 June 2009 Received in revised form 13 July 2009 Accepted 29 July 2009 Available online 5 August 2009 Keywords: Spent petroleum catalyst Bioleaching kinetics Bacteria Diffusion Thermodynamic parameter Leaching studies using spent petroleum catalyst containing Ni, V and Mo were carried out using two different acidophiles, iron oxidizing (IOB) and sulfur oxidizing (SOB) bacteria. XRD analysis proved the existence of V in oxide form, Ni in sulfide form, Mo both in oxide and sulfide forms, and sulfur in elemental state. Both bacteria showed similar leaching kinetics at the same leaching parameters, such as pH, nutrient concentration, pulp density, particle size and temperature. The dissolution kinetics for Ni and V was much higher than Mo. Bioleaching kinetics was observed to follow dual rates, initially faster followed by a slower rate. So, dissolution mechanism was based on surface- and pore-diffusion rate. The dissolution process was found to follow 1st order kinetics. Unified dissolution rate kinetics equations were developed using 1st order rate kinetics. Various thermodynamic parameters were also calculated. Rate determining step for both the bacteria were evaluated and the average D 1 (surface) and D 2 (pore) values were found to be around 7 × 10 -9 and 1×10 -10 cm 2 respectively. The lower value of D 2 suggested that the pore diffusion is the rate determining step during bioleaching process. © 2009 Elsevier B.V. All rights reserved. 1. Introduction Spent petroleum catalysts constitute a significant amount of the solid wastes generated by chemical and petrochemical industries. Metals like Ni, Mo, V, Co, Pt, Pd, etc., are widely used as catalyst and are typically deposited on porous materials like alumina and silica through a precipitation or impregnation process (Trimm, 1990). Most of the metals in these solids wastes are in the oxide form and in some other cases they are reduced to active metals for use in catalyzing the appropriate reactions. After the catalysts have been used multiple times, they will become inactive due to the poisoning effect of foreign material and impurities that deposit on the surface of the catalyst (Rapaport, 2000). Mostly catalysts are deactivated by thermal degradation, phase separation or phase transformation and cannot be easily reactivated. In such cases, the spent catalyst is replaced with fresh catalyst (Marafi et al., 1994). The volume of spent catalyst discarded as solid wastes has recently increased significantly due to a steady increase in the processing of feedstock. Since spent catalyst is a hazardous industrial waste (USEPA), its proper disposal is problematic and costly (USEPA, 2003). Exposure of such hazardous waste has long term environmental impacts, particularly on soil and ground water. Therefore, the recovery of metals from these waste catalysts has im- portant economic implications. Several processes, such as hydrometallurgical operations and pyro- techniques, have been adopted to recover metals from spent petroleum catalysts (Toyabe et al., 1995; Veal et al., 2001; Sun et al., 2001). Pyro- techniques are based on a calcinations process, which is energy intensive and emits SO 2 pollutants into the atmosphere. In addition, chemical leaching with an acid is not an eco-friendly process. Due to the several draw backs of the conventional techniques described above, biotechnological leaching processes have been developed as potential alternative methods. Over the last several decades, bioleaching processes have been used in mineral industries to leach sulfide ores (Rawlings et al., 2003). In industrial applications, chemolithotrophic autotrophs have been used extensively. Two different bacteria types (mesophilic and thermophilic) are highly important in the bioleaching process. The mesophilic iron and sulfur oxidizing bacteria, notably Aci- dithiobacillus ferrooxidans and Acidithiobacillus thiooxidans, are the most extensively used microorganisms within the mining and metallurgical industries (Torma, 1977). Currently, these bacteria are being used to leach metals from different industrial solid wastes (Bosshard et al., 1996; Brandl et al., 2001; Cerruti et al., 1998; Blaustein et al., 1993). In addition, several studies have reported on the use of Acidithiobacilli type bacteria for the dissolution of metals from spent petroleum/refinery catalysts (Mishra et al., 2007; Mishra et al., 2008; Bredberg et al., 2004). Bacterial produced metabolites are capable of extracting valuable metals from waste materials and products due to their acidic nature. In order to exploit the intrinsic capabilities of some microorganisms to extract and ultimately recycle metals, more efforts are needed to examine the dissolution kinetics of Hydrometallurgy 99 (2009) 157–162 ⁎ Corresponding author. Tel.: +82 42 8683592; fax: +82 42 8683415. E-mail address: djkim@kigam.re.kr (D.J. Kim). 0304-386X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.hydromet.2009.07.014 Contents lists available at ScienceDirect Hydrometallurgy journal homepage: www.elsevier.com/locate/hydromet