Sulphate-reducing bacteria from mining environments for metals bioremediation

Toxic heavy metals and metalloids constitute an international pollution problem that not only impacts public health but also is of environmental and economic importance. Several conventional treatment technologies for removing metals are available. These techniques, based on chemical methods of neutralisation and precipitation, even though quick and effective, present several disadvantages, such as the need for building additional treatment plants, the high cost of the chemical reagents used and the generation of an important volume of sludge which need further treatment. Prokaryotes with physiological activity of sulphate reduction are found in several environmental sites containing metals and these microorganisms have developed several different strategies for detoxification and resistance to toxic elements that are potentially useful for bioremediation. Since sulphate-reducing bacteria (SRB) are found in a large number of contaminated sites containing toxic metals, it is apparent that these organisms have a functional defence system that enables them to persist and even grow under metal stress. The enzymatic metal reduction by SRB offers an alternative to chemical processes to remediate environments containing metals. Acid mine drainage (AMD) is one of the most important source of heavy metal environmental pollution. AMD is characterised by its high acidity (≤3), high concentration of metals (e.g. Cu, Fe, Zn, A1, Pb, As and Cd) and high concentration of dissolved sulphates (≥3000 ppm). Taking advantage of the fact that SRB are present in these mining environments, several samples were collected from S. Domingos abandoned mine (Portugal) and screened for their ability to sulphate reduction, metal resistance and bioremediation. In this context, the most promising consortium of SRB (SRB no. 6) was grown in matrices column reactors, using lactate and ethanol as an electron donor source for the production of H2S from sulphates. Metal resistance was tested in batch using different metal concentration for Cu, Zn and Fe. The results obtained for this SRB inoculum, showed 97% of sulphate reduction (3750—350 mg/l) in the reactors, and a metal tolerance to Fe, Cu and Zn, at concentrations until 260 mg/l, 40 mg/l and 80 mg/l, respectively, in the batch assays, being observed a sulphate reduction (2000—0 mg/l) and a significant metals concentration decrease in the growth medium. Further studies are being carried out to apply this mining inoculum for the design of reactors for AMD bioremediatio