Effects of bacterial activity on the release of trace metals from sphalerite oxidation Nurgul Çelik Balcı Istanbul Techniqual University, Faculty of Mines, Department of Geology, Maslak, stanbul 34469, Turkey e-mail: ncelik@itu.edu.tr tel/fax: +902122856118 Abstract The aim of the present paper was to determine the role of Fe-oxidizing bacteria in trace metal solubilization during sphalerite oxidation. For this purpose, a trace element bearing sphalerite mineral was used in the oxidation/leaching experiments (biotic & abiotic). The experiments were conducted biologically and abiotically under aerobic conditions at pH 3. Acidithiobacillus ferrooxidans was used in the biological sphalerite oxidation experiments. The results showed that bacteria play the major role in the release of trace metals from sphalerite by enhancing the oxidation kinetics. These results indicate that in order to develop proper rehabilitation strategies for Acid mine drainage systems the role of bacteria should be determined. Key words: Acidithiobacillus ferrooxidans, trace elements, sphalerite, acid mine drainage. Introduction The behavior of hazardous metal elements in acid drainage for environmental monitoring and pollution forecasting in mining districts has become a focused research area in recent years. Previous research on the transport and fate of metals in acid mine drainage showed that the metal contents in AMD are probably not controlled by a single geochemical factor but by several factors (Dai et al. 2006a; Liu et al. 2004). Microorganisms play a predominant role in the solubilization, transport, and deposition of metals and minerals in the environment, processes that are still poorly understood ((Pronk et al. 1990; Suzuki et al. 1994; Balci et al. 2006). One of the most important bacteria in the metal sulfide oxidation is the acidophilic bacteria A. ferrooxidans. These bacteria are responsible for the oxidation of insoluble metal sulfides with a high acidification of sediments, controlling the transport of the metals in the environment. The trace metal contents in most natural waters are controlled by adsorption or co- precipitation processes. Co-precipitation may involve adsorption, cluster formation, homogeneous solid solution, heterogeneous solid solution, or a combination of these processes (Karthikeyan et al. 1997; Martinez and Mcbride 1998). Waters contaminated by AMD have a wide range of chemical composition and contain some elements such as Fe, Al, and Mn, which rapidly form precipitates when AMD is neutralized (Lee et al. 2002). The solubility varies for each element, causing a different efficiency of precipitation depending on pH values (Nordstrom and Alpers 1999). The precipitates formed by metal hydrolysis may include oxyhydroxides and hydroxysulfates, depending on local geochemical conditions (Bigham et al. 1996; Nordstrom and Alpers 1999). Therefore, knowledge about the release of hazardous metals from tailings, especially the key controlling factors affecting the evolution of specific metals in AMD, is important. In order to understand and determine the cycles of trace elements in the environment, it is necessary to elucidate the role of bacteria. The aim of the present paper is to evaluate the influence of A. ferroxidans on the solubilization of trace metals during sphalerite oxidation under conditions which mimic the AMD environment. Methods Sphalerite preparation Trace element bearing sphalerite was obtained from the United States Geological Survey (USGS), in Denver, Colorado (USGS). The trace element content of the sphalerite sample was checked by analyzing the elemental composition by ICP-MS at the United States Geological Survey (USGS), in Denver, Colorado. Prior to use, the samples were ground and sieved. A grain size of < 63 m was used for the experiments. The surface of the sphalerite was cleaned according to the methods described by Moses et al. (1987), and subsequently sterilized (Balci et al. 2007). Batch experiments Leaching experiments were conducted as biological and abiotic in order to determine the part of abiotic processes in metal mobilization during sphalerite oxidation. For the biological experiments, the