Most living cell systems exploited to date have been used for decontamination of effluents containing metals at concentrations below toxic concentrations (Kapoor and Viraraghavan 1995; White et al. 1995). Some of those technologies employ a consortium of undefined microorganisms as well as higher plants. The "Meander System" used at the Homestake lead mine (MO, USA) passes effluents containing Pb, Cu, Zn, Ni, Fe, and Cd ions through various ditches or channels containing autotrophic cyanobacteria, algae, and higher plants. Metal ion removal is achieved with an efficiency greater than 99% (Erlich and Brierley 1990; Jennet and Wixson 1983). These complex systems likely utilize precipitation and entrapment of particulates, in addition to biosorption and uptake into cells and plant tissues, in the removal process and concentrate the metals in the sediment in forms which greatly reduce environmental mobility and bioavailability (Brierley and Brierely 1993). In order to render sparse minerals in low grade ores soluble, those involved in biohydrometallurgy have exploited other micro-organisms to facilitate metal recovery (Brombacher et al. 1997) with the aim of mobilizing precious metals, Au, Ag, and sometimes Pt, but Ga, Ge. Patents (described in the above references) outlining various processes employ reactor technology and the most simple heap-leach or in situ operations to strip metals from their ores. Bioleaching of metals from ores has successfully employed bacteria such as Thiobacilli, Sulfobacilli, and Sulfolobus, as well as consortia of organisms and some unidentified indigent species. The same practices, which are in use in the mining industry to liberate metals from ores perhaps have applications in the remediation of soils contaminated with those same metals by mobilization, followed by biosorption. Commercial processes using biosorbents for metal ion removal from aqueous waste streams are still in their infancy.
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