The influence of solution Eh on the rate of ferrous iron oxidation by Acidithiobacillus ferrooxidans is characterized. The experimental approach was based on the use of a two-chamber bioelectrochemical cell, which can determine the ferrous iron oxidation rate at controlled potential. Results enabled the formulation of a novel kinetic model, which incorporates the effect of solution Eh in an explicit form but still integrates the effect of ferrous iron concentration and ferric inhibition. The results showed that at Eh values below 650 mV (standard hydrogen electrode, SHE) the bacterial oxidative activity is mainly dependent on ferrous iron concentration. At Eh values between 650 and 820 mV (SHE) the oxidation rate is mainly controlled by ferric inhibition. Over 820 mV (SHE) the bacterial oxidative activity is strongly inhibited by the Eh increase, being completely inhibited at Eh = 840 mV (SHE).

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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, and once submitted to weathering, may produce acid mine drainage. On the other hand, the high price of copper raised the interest for processing by leaching the low grade ore deposited in large dumps. 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 of the rate of the involved chemical reactions and a decrease or inhibition of microbial growth. In this work we present the results of the isolation of microorganisms from an old tailing deposit, exposed at low temperatures (5oC) during most of the winter. The isolated microorganisms initially showed a low capacity to oxidize 3g/L Fe(II) sulfate at pH 1.6, and tetrathionate 0.01 M, with an initial pH 4 both at 5oC. However, after successive cultures, microorganisms showed a slow capacity to oxidize both substrates, as well as the sulphide contained in the tailings 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 generation of acid mine drainage and in the oxidation and leaching of sulphide ores.

The main environmental problems associated with the mining activities are related to the production of large amounts of wastes; Different pathways are responsible for heavy metals dispersion, by air due to wind action, by water mediated by acid mine drainage and erosion, and the metals could be mobilized in the soil by different transport mechanisms. Different remediation alternatives have been studied and reported in literature. In situ stabilization is a cheaper method. The heavy metals stabilization enables the decrease of metal mobility, reactivity and toxicity in the soil, decreasing heavy metals availability and phytoavailability. Sulphate reducing bacteria (SRB) have been successfully utilized in groundwater bioprecipitation of heavy metals. In this study, this biological agent has been used in the immobilization of heavy metal in the subsurface of the soil due to its dissimilative metabolism. SRB produces hydrogen sulfide that reacts with soluble metals present in the media, generating as final product low soluble metal compounds (metal sulfides). The bio-stabilization was studied at pilot scale to determine the stabilization efficiency using biological agent, SRB. The metals studied were Fe, Cu, Pb and Zn in the contaminated smelter soil. Bioaugmentation and biomagnification were applied. After 4 months, the metal stabilization efficiency was determined by leaching with acid solution at different pH to stimulate the metal mobility. The remediation pilot scale system showed that copper, lead and iron were much more stable at pH 3.0, with only 3.7% and 1% of total metal eluted, and compared with the system without biological agent. In the case of zinc, the elution was similar with or without remediation. The metal stabilization using biological agent was successful in the contaminated smelter soil and these results are promising antecedents for full scale in situ remediation strategy.

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