Bioremediation of metals and metalloids by precipitation and cellular binding

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. SRB have the apparently unique potential to simultaneously remove metals, sulphate and acidity through the bioprecipitation of metal sulphides - a phenomenon that still represents one of the most successful biological approaches to metal removal from aqueous media. Other abiotic chemical reactions that can effect the removal of metal(loid) ions from solution take place during active sulphate-reduction.