Bioreduction of Hexavalent Chromium Using Moderate Thermophilic and Thermophilic Microorganisms

Hexavalent chromium is one of the toxic heavy metals considered to be potentially harmful for life of a wide range of organisms, including human life, due to its high mobility in soil and water media. Wastewaters containing chromium(VI) are produced from different sources including many industries such as electroplating, leather tanning procedures, and chromite ore processing. When these practices are not conveniently regulated, contamination problems associated with irresponsible waste disposal of effluents appear in the surrounding environments, such as water bodies and sediments. Conventional treatment of chromium(VI) implies its reduction to chromium(III)-a less toxic species-and its later precipitation. This process can be carried out through widely known physicochemical techniques although it is expensive mainly when low concentrations of the pollutant need to be treated. Microorganism-based techniques are considered very cost-effective alternatives, especially for the first step of the treatment, i.e., chromium(VI) reduction. Isolation and identification of chromium(VI)-resistant and chromium(VI)-reducing strains are fundamentally significant for this purpose. Microorganisms capable of living when exposed to high temperatures seem to be great candidates for bioremediation applications due to the increase of the speed of the process. The objective of the present chapter is to study thermophilic chromate-resistant and chromate-reducing bacteria toward Cr(VI) bioreduction. Cr (VI) removal by immobilized bacterial cells and their advantages are also summarized for a better understanding on how to transfer this technology from laboratory to a large-scale application for wastewater treatments. An example of bioreduction of chromium(VI) by immobilized bacteria isolated from the geothermal area of Domuyo (Neuquen-Argentina) is presented as case study.