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Chromium

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Chromium, the seventh most abundant element on earth is a transition metal from group VI-B .It is found in nature as the bound form constituting approximately 0.1 – 0.3 mg.g-1 of the earth’s crust. The name of the element is derived from the Greek word "chroma" meaning color, because many of its compounds are intensely colored. Cr is able to exist in several oxidation states, of which the trivalent Cr (III) and hexavalent Cr (VI) species are the most stable and abundant forms (Zayed & Terry et al., 2003).

The most stable form of chromium is its trivalent oxidation state which is primarily found geologically as chromite (FeCr2O4) (Cervantes et al., 2001). Cr (III) is naturally occurring, whereas Cr (VI) is of anthropogenic origin. This is as a result of the dissolution of chromite and the subsequent oxidation of Cr (III) to aqueous Cr (VI) in the presence of the common manganese mineral, birnessite (Oze et al., 2007). Cr (VI) usually associates with oxygen to form the oxyanions chromate (CrO42-) and dichromate (Cr2O7 2- ). Cr (VI) compounds are highly soluble and therefore mobile within aquatic systems, whereas derivatives of Cr(III) in the forms of hydroxides, oxides and sulphates, are water insoluble and exist mostly bound to organic matter in soils and aquatic systems(Cervantes,C et al.,2001; Zayed & Terry, 2003).

Chromium differs widely in physicochemical properties and biological reactivity with respect to its oxidation states. While Cr (VI) species and dichromate’s are extremely water soluble and mobile in the environment, Cr (III) species are much less soluble and comparatively immobile (Viamajala et al., 2004).

Chromium is an essential micronutrient required for the growth of many microorganisms for the mainten...

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... heavymetals (Ha et al., 2010).Majority of the bacteria used in the biosorption systems produce EPS plays a key role as bio-sorbents for metal remediation and recovery (Sheng GP et al.,2010).. There are several negatively charged functional groups such as hydroxyl, carboxyl, phenolic and sulfhydryl present in the EPS which help in the adsorption of chromium (Sethuraman P et al., 2010). Exopolysaccharides (EPS) which help in attachment, tolerance and reduction of hexavalent chromium (Ozturk et al., 2009).

Studies relating to reduction of hexavalent chromium to trivalent form by native microbial species isolated from different sources have been reported in past (Sethuraman P et al., 2010; Wei-hua XU et al., 2009; K. Sundar et al., 2011). The pivotal role of EPS along with the cell in the adsorption and reduction of chromium has been noted (K. Sundar et al., 2011).
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