Cr(VI) generated due to natural oxidation of chromite mineral present in chromite mine overburden (COB) dumps of Sukinda, India, has been characterized by different physico-chemical methods. The Cr(VI) was found to be associated with goethite matrix at a contamination level of 500 mg Cr(VI) kg(-1) of COB. Bacillus sp. isolated from the overburden sample exhibiting high tolerance to the hexavalent chromium, was used for the remediation of Cr(VI) in the overburden. The process was optimized while varying the parameters such as pH (2-9), pulp density (10-60%) and temperature (25-40 degrees C). Optimal reduction of more than 98% of Cr(VI) in the COB sample was achieved in 16 h at pH similar to 7.0 and 60% pulp density with the Bacillus sp. (4.05 x 10(7) cells mL(-1)) in absence of media. The exponential rate equation yielded rate constant value of 2.14 x 10(-1) h(-1) at 60% pulp density. The mode of bio-reduction of Cr(VI) in the overburden sample was established by FT-IR, XRD, EPMA and SEM-EDS studies. (C) 2013 Elsevier B.V. All rights reserved.