Heavy metal bioprecipitation

MARÍA S. URBIETA; GRACIANA WILLIS PORATTI; MARISA R VIERA; EDGARDO R DONATI

Bioprocesses: Clean Technologies for the Protection and Sustainability of the Environment

Ediciones Universitarias de Valparaíso

Lugar: Valparaiso; Año: 2010;

Heavy metal pollution is one of the most important environmental problems today. Several industries produce and discharge wastes containing different heavy metals into the environment, such as mining and smelting of metalliferous, surface finishing industry, energy and fuel production, fertilizer and pesticide industry and application, metallurgy, iron and steel, electroplating, electrolysis, electro-osmosis, leatherworking, photography, electric appliance manufacturing, metal surface treating, aerospace and atomic energy installation etc. Most heavy metals are toxic to organisms including humans. This has led to an increased interest in developing systems that can remove or neutralise their toxic effects in soil, sediments and wastewater. Unlike organic contaminants, which can be degraded to harmless chemical species, heavy metals cannot be destroyed. To remediate the pollution cause by heavy metals, the available possibilities are either their immobilisation in a non-bioavailable form, or their re-speciation into less toxic forms. The simplest and cheapest method of removing most heavy metals from solution is to increase the pH of the effluent, thus converting the soluble metal into an insoluble form (i.e. its hydroxide). Precipitation by adjusting the pH is, however, not selective. Consequently, precipitation by alkali addition (usually lime) produces large quantities of solid sludge and in addition the solubility is not low enough to avoid concentrations of sulfates and metals into the effluents that do not meet environmental standards. Other conventional methods for removing metal ions from aqueous solution have been suggested, such as ion exchange, electrochemical treatment, membrane technologies, adsorption on activated carbon, evaporation etc. However, some of them are ineffective, especially at low (but still high for the environment) metal ion concentration in aqueous solution (lower than 100 mg.L−1). Ion exchange, membrane technologies and activated carbon adsorption process are very efficient but extremely expensive when treating large amount of water and wastewater containing heavy metal in low concentration avoiding their use at large scale. Microorganisms play an active role in certain geological processes, as they may serve as agent of concentration, dispersion, or fractionation of matter including metals usually as a consequence of heavy metal stress. Some microorganisms can solubilize metals by acid leaching or chelation while others can immobilize metals by accumulation/sorption or promote an external precipitation of insoluble compounds. These biological processes may be exploited for the treatment of contaminated solids and liquid wastes due to the flexibility of microorganisms to handle different physico-chemical conditions as those found in effluents, their selectivity to remove specific compounds and the cost-effectiveness (Lovley and Coates, 1997; White et al, 1997; Gadd, 2000; Valls and de Lorenzo, 2002; Viera and Donati, 2004). In this chapter we focus on bioprecipitation which is one of the most interesting biotechnology methods to remove heavy metal pollution from liquid effluents.