Biooxidation of ferrous iron and sulphide at low temperatures: Implications on acid mine drainage and bioleaching of sulphide minerals

Chilean copper production has been growing in the last 20 years reaching an annual production of 5,557,000 tons of Cu in 2007. For each ton of copper produced, about 200 tons of sterile and low grade ore and 100 tons of tailings are discharged in the environment. Most of these wastes contain significant amounts of sulphide minerals, mainly pyrite, that once submitted to weathering, may produce acid mine drainage. On the other hand, copper high prices raised the interest for processing the low grade ore deposited in large dumps by leaching. An important part of these mining wastes and low grade ores is located in the Andes, where the mean temperature is usually ∼ 5 °C or less. The rate at which bioleaching reactions occur is directly related to the temperature at which the microorganisms (bacteria and archaea) develop. A temperature decrease causes both a decrease on the chemical reactions rates and a decrease or inhibition on microbial growth. The results for microorganism isolation from an old tailing deposit, exposed at low temperatures (5 °C) during most of the winter are presented in this work. Initially the isolated microorganisms showed a limited capacity in oxidizing Fe(II) sulfate (3 g L−1) at pH 1.6, and tetrathionate (0.01 M), with an initial pH of 4.0 at 5 °C. However, after successive cultures, microorganisms showed a low capacity to oxidize both substrates, as well as the sulphide contained in the tailing samples. The terminal Restriction Fragment Length Polymorphism (tRFLP) of the isolated cells grown in basal medium containing Fe(II) showed a nearly pure culture of Acidithiobacillus ferrooxidans. The present study indicates that, even at very low temperatures, microorganisms play an important role in the acid mine drainage generation and also during the oxidation and leaching of sulphide ores.