Chemical Engineering Journal 83 (2001) 123–130 Mechanism of bioleaching of coal fly ash by Thiobacillus thiooxidans A. Seidel a , Y. Zimmels b,∗ , R. Armon b a Department of Chemical Engineering, Environmental Engineering Program, Yale University, New Haven, CT 06520-8286, USA b Faculty of Civil Engineering, Technion — Israel Institute of Technology, Haifa 32000, Israel Received 6 March 1999; received in revised form 31 July 2000; accepted 16 August 2000 Abstract Bioleaching of aluminum and iron from coal fly ash (CFA) by Thiobacillus thiooxidans (T. thiooxidans) bacteria is considered. The interactions between bacteria, metabolic products, CFA particles, and leaching products were studied. It is demonstrated that bacterial growth and the amount of metals leached from the CFA are coupled through biological and chemical interactions, which involve the various components in this system. Bioleaching experiments were performed batch wise by suspending up to 10% (w/v) CFA in T. thiooxidans growth medium containing cell inoculum for a typical 3 week period of time. Samples were taken periodically from leached suspensions and relevant parameters including metals’ concentrations, cell counts, pH and extracellular polymeric substances (EPS) were determined. The results show that under the same conditions, similar leaching levels are obtained by sulfuric acid and bioleaching of CFA, and the contribution of other metabolites is insignificant. CFA inhibits the growth rate through two major effects. The first is due to the alkaline components released by the CFA that cause a rise in the pH, and a corresponding delay in growth. The second is attributed to the random attachment of the bacteria to both the sulfur particles (the energy source) and the barren CFA particles, resulting in a so-called “dilution effect” of the sulfur particles, and an inhibition of the initial growth rate. However, after an adaptation period of the bacteria the subsequent growth rate, the maximal cell concentration and minimal pH were similar to those obtained in the control experiment, irrespective of CFA content. Enhanced excretion of EPS was observed in the presence of CFA as well as in calcium and barium enriched growth media. It is presumed that the mechanism of EPS production is related to the presence of the particulate solid phase. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Coal fly ash; Thiobacillus thiooxidans; Bioleaching; Microbial growth; Bioprocess; Solid-waste treatment 1. Introduction The growing amounts of coal fly ash (CFA), which are produced by coal-operated power plants worldwide, call for their increased utilization. In this context CFA can be used as a source for metal extraction within appropriate economical constraints [1–4]. Applying biohydrometallurgical technolo- gies, for the recovery of valuable metals from solid residues including fly ash was recently considered [5–7]. The leached metals can then be further processed while the residues may be safely deposited or utilized for construction purposes. The advantage of bioleaching is the relatively low cost and mild conditions of the process, and the subsequent low demand for energy or landfill space compared with conven- tional technologies [7]. However, slow kinetics and insuffi- cient selectivity with respect to specific metals, particularly aluminum [3,8,9], can offset the advantages of bioleaching. ∗ Corresponding author. Tel.: +972-4-292-580; fax: +972-4-321-816. E-mail address: zimmels@tx.technion.ac.il (Y. Zimmels). Among the variety of microorganisms that are known to facilitate metal bioleaching reactions, the autotrophic Thiobacilli species are perhaps the most common [7,10]. The bacterium Thiobacillus thiooxidans (T. thiooxidans) is active at low pH and can endure harsh conditions that exist in concentrated solutions of metals [11]. These par- ticular characteristics make it a suitable microorganism for bioleaching. Specific examples of T. thiooxidans growth in environments containing high concentration of metals are: Zn and Cd up to 600 and 400 mM, respectively [12], As 3+ and As 4+ : 67 and 534mM, respectively, iron: Fe 2+ 537 mM and Fe 3+ 180 mM [13], and aluminum: 370 mM [14]. In the case of zinc and cadmium, specific proteins were produced, which were capable of binding these metals [12]. The presence of high content of particulate matter can impede bacterial growth. Kandemir [15] showed that for T. ferrooxidans in a sulfide mineral medium, particulate mat- ter content above 16% becomes the main growth-limiting factor. Here, the impeding effects were due to a slow down in the rates of oxygen transfer and decreased removal of metabolic products of the bacteria. 1385-8947/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S1385-8947(00)00256-4