Today, areas in many regions of the world face serious problems caused by the contamina- tion of the environment with heavy metals, frequently caused by industrial activities and sewage sludge application [10].
One of these metals is chromium. It exists in different oxidation states in the environment, with trivalent and hexavalent chromium being the most stable and abundant forms in soils [10]. Because of its widespread use combined with a high human toxicity, Cr(VI) is considered as a major pollutant.
In some cases, the contamination with Cr(VI) can be of natural origin. Starting from the mineral chromite (FeCr2O4 and MgCr2O4), chromium occurs naturally in the environment and can be found in multiple oxidation states, including Cr(VI) [8].
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Serpentinites were found to be the rock type with the highest potential to pollute the water in the area [9].
However, anthropogenic release of Cr(VI) is much more frequent. Chromium is used in various industrial processes, including stainless steel plants, preparation of alloys, chrome plating, leather tanning, dye industry, industrial water cooling, paper pulp production, petroleum refining, wood preservation and even nuclear power. As a consequence of the broad use, chromium is present in effluents originated by these activities, polluting sediments, soil, water and air
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Many of these technologies can be applied for the removal of hexavalent chromium [1]:
1. Ex situ treatment technologies for water mostly use chemical reduction of Cr(VI) to Cr(III) followed by precipitation.
2. In situ treatment approaches often use chemical reduction and fixation for remedia- tion. Newer technologies include enhanced extraction, electrokinetics, phytoremedi- ation and natural attenuation.
Most often, a combination of these methods is used for a more efficient and cost-effective treatment of a contaminated site [1]. Today, the remediation of Cr(VI)-contaminated sites is usually based on physical and chemical approaches, but bioremediation is becoming increasingly significant [10].
Bioremediation of chromium is based on the microbial reduction of Cr(VI) to the much less toxic Cr(III). Moreover, it precipitates as Cr(OH)3 (in the pH range 6-9) and thus immobilizes the chromium in the soil matrix [5].
1.3 Scope and structure of the