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Bioremediation of metals and metalloids by precipitation and cellular binding
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INTRODUCTION
Interactions between dissimilatory sulphate-reducing bacteria (SRB)
and metal(loid) ions have been studied since the first half of the twentieth
century and the ability of SRB to bring about changes in the speciation of
metal(loid)s has been recognized for much of this time. Early work focused
on the role of SRB as nuisance organisms and metal(loid) interactions with
SRB were often studied in the context of their use as metabolic poisons
to control SRB activity (Postgate, 1952; Newport and Nedwell, 1988; Nemati
et al., 2001). With growing awareness of the importance of microorganisms
in biogeochemical cycling, the emphasis of research has shifted to the
environmental roles of SRB. Their capacity to control the mobility of metals
in aqueous sediments by the formation of poorly-soluble metal sulphides is
now apparent and SRB-generated metal sulphides constitute an important
environmental sink for many metals (Morse et al., 1987; see also Chapter 13,
this volume). Bioremediation of dissolved metal(loid)s is an application
for which SRB may be particularly suitable, given that sulphate frequently
co-occurs with toxic metal ions in, e.g. metal-processing wastes
and acid mine-drainage waters.
METAL REMOVAL BY SULPHIDE GENERATION
While dissimilatory sulphate-reducing bacteria are a metabolically and
taxanomically diverse group, they are united by the common ability to utilize
sulphate as a terminal electron acceptor to conserve energy for growth:
Sulphur in the S(VI) oxidation state is stoichiometrically reduced to S(-II)
and, under the circumneutral conditions in which SRB are generally
encountered, the main product is bisulphide (HS), with a small proportion
of volatile H2S (Stumm and Morgan, 1996). Bisulphide is a highly reactive
species, with the propensity to bond with metal cations in solution forming
metal sulphide solids. Metal sulphides are chemically characterized as
sparingly soluble, in equilibrium between the solid and aqueous phases.
The degree of solubilization is represented by the solubility product constant.
OTHER INDIRECT CHEMICAL EFFECTS
The use of chemical treatments to stimulate the alkaline precipitation
of heavy metals from acidic wastewaters is well established, but while such
processes may be highly efficient in terms of metal removal abilities, they
require the continuous addition of alkaline reactants and are thus expensive.
The use of biologically generated alkalinity, based on cheap and readily
available substrates, is therefore a potentially attractive alternative.
SRB are able to remove acidity from local and bulk environments and
some SRB strains are reported to maintain sulphate-reducing activity in
waters with an initial bulk acidity of pH 24. Fortin and Beveridge (1997)
found that SRB were active and important in the localized cycling of iron.