Metal mining produces large volumes of unwanted materials that are dumped
as waste rock or stored as fine-grain tailings. Both of these pose a potential
environmental hazard, mostly associated with the microbiological oxidation of
residual sulfide minerals (principally pyrite; FeS2) in these waste materials which, in
the absence of neutralising minerals or ongoing application of alkaline chemicals,
results in the production of acidic mine drainage (AMD) waters. These are typically
acidic, sometime extremely so, and contain elevated concentrations of iron and other
soluble metals, metalloids (such as arsenic) and sulfate. The United States
Environmental Protection Agency, categorises water contamination from mining as
one of the top three ecological-security threats in the world. However, metal mining
remains a basic industry underpinning economic growth and development, and so
the question of how to minimise its environmental impact is perceived as an urgent
issue.
In the majority of cases mine-impacted waters are conventionally remediated
either using active chemical (aeration and lime addition) or passive biological
(wetlands or compost bioreactors) treatment. Both of these approaches have major
detractions including: (i) metals present in acid mine drainage are not recovered, and
(ii) the mixed-metal sludges or metal-enriched spent composts generated need to be
carefully disposed (usually in specially-designated landfill sites), and managed to
prevent re-mobilisation of the metals and new cycles of environmental pollution. The
concept of harnessing natural biological processes and systems to remediate AMD
underpins passive bioreactors and wetland systems, and there are numerous
examples of "natural attenuation" of mine waters, though in most cases these result
in improved mine water chemistry though not to regulatory discharge levels. The
current project has sought to understand the potential roles of acidophilic prokaryotic
and eukaryotic microorganisms, in terms of their diversities and the interactions that
occur between them, and to apply this knowledge to devise new approaches for
securing mine wastes and remediating mine waters.
Extremely acidic habitats are inhabited by a range of microorganisms with
contrasting metabolic life-styles. These include chemoautotrophic bacteria and
archaea that catalyze the dissimilatory oxidation of iron, sulfur, and hydrogen,
heterotrophic acidophilic bacteria, many of which can catalyse the dissimilatory
reduction of ferric iron while some reduce sulfate (SRB), and phototrophic microalgae.
While chemolithotrophs are known to have a major role in AMD genesis,
heterotrophic prokaryotes and phototrophic eukaryotes can play important roles in
mitigating AMO pollution.
While some heterotrophic acidophiles can generate alkalinity and immobilise
metals, they require organic carbon to do this. Studies of the nature of organic
materials produced by primary producers in extremely acidic environments
(phototrophs and chemolithotrophs) were therefore carried out. Ch/ore/la
protothecoides var. acidico/a and Euglena mutabi/is, both isolated from abandoned
copper mines were found to exude monosaccharides (glucose and fructose by the
Ch/ore/la isolate, and mannitol and glucose by the Euglena isolate). These exudates
were shown to sustain the growth of iron-reducing heterotrophic bacteria commonly
found in AMO. In contrast, three species of chemoautotrophic acidophiles
(Acidithiobacil/us (At.) ferrooxidans, At. ca/dus and Leptospirillum ferriphi/um) were
shown to secrete glycolic acid into their media. The ability to metabolize this
compound was very restricted among acidophilic prokaryotes, with only species of
Firmicutes (many of which catalyse both iron oxidation and iron reduction) being
capable of utilising glycolic acid in pure culture.
Two continuous-flow bioreactors containing consortia of novel acidophilic
SRB were established and tested over ~1 year for the selective precipitation of
copper and zinc (as sulfide minerals) when fed with synthetic acidic mine waters
containing mixtures of several transition metals and aluminium. The results showed
selective precipitation of Zn (at pH 4.0) from a synthetic AMO containing Fe, Al and
Zn and selective precipitation of Cu (at pH 2.2) from another synthetic AMO that
contained Fe, Zn, Mn, Al and Cu. These experiments demonstrated the potential of
pH-controlled microbial sulfidogenesis for the selective recovery (and recycling) of
metals typically present in elevated concentrations in acidic mine drainage waters.
Finally, a mesocosm experiment was set up using pyritic mine tailings and
test material and various combinations of microbial inocula. The results showed that
that iron- and sulfate-reducing bacteria, sustained by organic carbon provided by
acidophilic micro-algae, had significant impact on acid-generation and metal
mobilisation in the tailings. This part of the study showed the importance of
phototrophic algae in supporting the growth of "benign" bacteria that can help to
minimize the oxidation of sulfide minerals and the consequent generation of acidity
and solubilisation of transition metals, providing a long-term self-sustainable system.
Overall, the results of this study highlighted the importance of the diversity
and interactions of acidophilic microbial consortia, and these might be used to
develop new ecological and biotechnological approaches for reducing the generation
of AMO from abandoned mines and to remediate mine waters.