Bacterial influence on storage and mobilisation of metals in iron-rich mine tailings from the Salobo mine, Brazil.

In this study we investigated the potential effects of promoting bacterial activity on tailings from the Salobo iron-oxide copper­gold (IOCG) mine, Brazil. In particular we focussed on (1) the potential for mobilising additional Cu and (2) the effects of long-term storage on other metals. Unlike typical sulphide-ore tailings, the pH of the Salobo tailings is circumneutral and these tailings are dominated by Fe-bearing silicates and magnetite, with minor micrometre-scale encapsulated Cu-bearing sulphides. While these tailings do not produce acid mine drainage, an endemic strain of Acidithiobacillus ferrooxidans was isolated from the mine site. These bacteria were used in laboratory column leaching experiments of tailings material, which ran for up to 395 days, without the addition of ferrous iron. Bacteria-tailings interactions were typically maintained at a pH > 5, due to silicate-mediated pH buffering. This was eventually overcome after ~200 days by regular addition of acidic (pH 2.2) nutrient solution, as well as growth and acid generation by bacteria. Copper dissolution was not significantly enhanced by bacterial activity compared to abiotic control experiments while pH was >5. However, as the experiments were progressively acidified, additional Cu was mobilised in the biotic systems. The mineral alteration reactions produced abundant ferrihydrite precipitates within the tailings, which was enhanced by bacterial activity as the pH decreased. Adsorption of metal cations to these precipitates ensured that effluent solutions had only low levels (<0.5 mg/l) of dissolved trace metals such as As, Co, Pb, Zn, Se, Ni and Cr. These adsorption processes will strongly inhibit metal leaching from the tailings during long-term storage, as long as the iron oxidising bacteria are producing the requisite excess of ferrihydrite and the pH is >5. This case study shows that bacterially-mediated silicate weathering, in Fe(II)-bearing silicate rich tailings with only minor sulphides and Acidithiobacillus ferrooxidans can enhance the environmental stability of the tailings.

The influence of hydrogeological disturbance and mining on coal seam microbial communities.

The microbial communities present in two underground coal mines in the Bowen Basin, Queensland, Australia, were investigated to deduce the effect of pumping and mining on subsurface methanogens and methanotrophs. The micro-organisms in pumped water from the actively mined areas, as well as, pre- and post-mining formation waters were analyzed using 16S rRNA gene amplicon sequencing. The methane stable isotope composition of Bowen Basin coal seam indicates that methanogenesis has occurred in the geological past. More recently at the mine site, changing groundwater flow dynamics and the introduction of oxygen in the subsurface has increased microbial biomass and diversity. Consistent with microbial communities found in other coal seam environments, pumped coal mine waters from the subsurface were dominated by bacteria belonging to the genera Pseudomonas and the family Rhodocyclaceae. These environments and bacterial communities supported a methanogen population, including Methanobacteriaceae, Methanococcaceae and Methanosaeta. However, one of the most ubiquitous micro-organisms in anoxic coal mine waters belonged to the family 'Candidatus Methanoperedenaceae'. As the Archaeal family 'Candidatus Methanoperedenaceae' has not been extensively defined, the one studied species in the family is capable of anaerobic methane oxidation coupled to nitrate reduction. This introduces the possibility that a methane cycle between archaeal methanogenesis and methanotrophy may exist in the anoxic waters of the coal seam after hydrogeological disturbance.