Bioremediation of soils polluted with hexavalent chromium using bacteria: A challenge

Abstract

The contamination of the environment with heavy metals is a serious problem, because industrial activities and sewage sludge applications have largely contributed to a wide spread of these elements in the terrestrial environment. Therefore, over some years, the discarding of solid and/or liquid waste products containing heavy metals due to industrial processes has received a lot of attention, and legislation for the protection of the environment has become more rigid (Benedetti 1998; Chen and Hao 1998). Chromium, considered as one of the main pollutants in the United States by the Environmental Protection Agency (EPA) (Fig. 1), is widely used in many industrial activities (Fig. 2). Its world production is in the order of 10,000,000 tons per year (Cervantes et al. 2001). Chromium is able to exist in several oxidation states, ranging from Cr+2 to Cr+6, but in soils the most stable and common forms are trivalent Cr(III) and hexavalent Cr(VI) species (Fendorf 1995), which display quite different chemical properties and affect organisms in different ways. In fact, in contrast to other metals, the hazard of chromium is dependent on its oxidation state. Hexavalent chromium is water-soluble, highly toxic and mutagenic to most organisms and carcinogenic for humans, while trivalent chromium is essential (in low concentrations) for human and animal nutrition, relatively waterinsoluble and less toxic than Cr(VI) (Francisco et al. 2002). In many countries, chromate, which is the most prevalent form of Cr(VI) present in solid/liquid waste due to human activities, such as electroplating, steel and automobile manufacturing, production of paint pigments and dyes, wood preservation, is a hazardous contaminant (Kamaludeen et al. 2003), because it is a serious threat to human health and it readily spreads beyond the site of initial contamination through aquatic systems and groundwater. © 2007 Springer-Verlag Berlin Heidelberg.