The effects of bacterial leaching on metal partitioning in sewage sludge Ana T. Lombardi 1, *, Oswaldo Jr.Garcia 2 and Waldenir A.N. Menezes 2 1 Centro de Estudos do Mar, Universidade Federal do Parana ´, Avenida Beira-Mar s/n, C. P. 50002 83255-000, Pontaldo Parand, Parana ´, Brazil 2 Dept. Bioquı´mica e Tecnologia Quı´mica, UNESP, Campus Araraquara, C.P. 355 SP CEP: 14801-970, Araraquara, Brazil *Author for correspondence: Tel.: +55 41 4551333, Fax: +55 41 4551105, E-mail: lombardi@ufpr.br Received 23 September 2004; accepted 27 February 2005 Keywords: A. ferrooxidans, A. thiooxidans, bioleaching, metal partitioning, sewage sludge, bacterial leaching. Abstract The partitioning of Mn, Al, Zn, Cu and Ti ions in municipal sewage sludge was investigated before and after bioleaching processes effectuated by Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans. Oxidation– reduction potential increase and pH decrease were obtained as a result of bacterial activity. A less pronounced and constant decrease was obtained with A. ferrooxidans, whereas A. thiooxidans presented a lag phase before a steep pH decrease. Metal solubilization was accomplished in experimental systems supplemented with energy source, Fe 2+ for A. ferrooxidans and S 0 for A. thiooxidans. Solubilization efficiency differed for each metal except for Al, and was relatively similar for either organism. Metal partitioning was conducted using a five-step sequential extraction procedure before and after the bioleaching. The results indicated that Zn and Mn ions were mostly associated with the organic fraction, whereas Cu, Al and Ti ions with the sulphide/residue fraction. The bioleaching process caused prompt solubilization of metals mostly associated with the more labile fractions (exchangeable, adsorbed and organically bound metals), whereas those associated to the less labile ones (EDTA and sulphide/ residue fractions) were exchanged towards more labile fractions. Introduction The disposal of sewage sludge is a worldwide problem. Environmental constraints, population and industrial growth have resulted in a great increase of sewage sludge production. Among nutrients and natural organic com- pounds, sludge composition is characterized by a wide spectrum of highly toxic substances such as metals, pathogenic organisms and persistent organic compounds. Although its use as soil fertilizer has been proposed as an economically viable disposal practice (Epstein 1976; Korentajer 1991), the benefits on soil are counterbalanced by the presence of the toxic compounds (Tack et al. 1999). Metals in sewage sludge are present as several chem- ical species (Stover et al. 1976; Lombardi & Garcia, 2002) and concentrations (Korentajer 1991; Hue & Ranjith 1994; Ravishankar et al. 1994). Literature data show that more than 50% of the sludges are inadequate for use in agricultural areas due to their metal content (Adamu et al. 1989; Couillard & Mercier 1991; Vooneburg & Veen 1993). Therefore, to have a sus- tained agricultural use of the material, it is necessary to remove such toxic elements. The removal of metal ions from sewage sludge by biological methods has been investigated using organ- isms of the acidithiobacilli group. Many of these investigations show that the efficiency of bioleaching is dependent on metal speciation as well as on the con- centration of energy source available to the bacteria (Chen & Lin 2004); the process is temperature-dependent (Liu et al. 2003), with its optimum at 37 o C (Tsai et al. 2003). Chan et al. (2003) have compared iron- and sul- phur-oxidizing bacteria on the bioleaching of heavy metals from sewage sludge and obtained a similar pH decreasing trend but at different rates. Considering the organic-rich composition of sewage sludge and the effectiveness of Acidithiobacillus ferrooxidans on metal bioleaching, Gu & Wong (2004) have identified the organic acetic and propionic acids as substances inhibitory to metal bioleaching and solubilization in sewage sludge. From the literature, we observe that in the 80s and 90s, most related texts investigated the funda- mental aspects of the process, whereas more recently, research has been focused on specific aspects of opti- mizing the process. In spite of the advances obtained, understanding of the process is still incomplete. This is confirmed by 20–25 years of research and a lack trans- ference of such biotechnology process from the labora- tory to the field. To enable this, it is necessary to keep focusing research on the detailed aspects of the interac- tions among bioleaching microorganisms and the several parameters which seems to control the process. World Journal of Microbiology & Biotechnology (2006) 22:1013–1019 Ó Springer 2006 DOI 10.1007/s11274-005-2889-1