Microbial Reduction and Detoxification of Chromium from Tannery Effluent by Natural Inhabitants

Chromium (Cr), a chemical agent, has long been used extensively in leather tanning. Hexavalent chromium (Cr-VI) found in tannery effluent is highly toxic, carcinogenic and mutagenic to humans. Transformation of Cr-VI to its trivalent counterpart, Cr-III, is the basic process in its detoxification, and microbial transformation of Cr-VI to Cr-III has been one of the most widely studied forms of Cr bioremediation. This study aims to explore the ability of naturally occurring bacteria in reducing and detoxifying Cr in vitro and also from tannery effluent. Five efficient Cr reducing and detoxifying bacteria were isolated from tannery effluent, their morphological, cultural, physiological and biochemical characteristics investigated. They were identified as Aeromonas eucrenophila, Bacillus megaterium, B. carboniphilus, B. licheniformis and B. subtilis. Coincubation of the isolates with varying concentrations of potassium dichromate (K2Cr2O7), a Cr salt, in minimum salts medium, pH 7 revealed notable reduction and detoxification of Cr within 24-72 h as determined by 1,5-diphenylcarbazide colorimetric method and atomic absorption spectrophotometry, respectively. The isolates exhibited substantial resistance or tolerance to 125 to 500 ppm K2Cr2O7. Bacterial detoxification or reduction of was increasingly increased as the incubation period increased from 24 to 48 or 72 h and substrate concentration increased from 125 to 250 or 500 ppm. Most of the isolates exhibited increased reduction and detoxification at 37?C compared to that at 30?C or 45?C, and at pH 7 or 8 compared to that at pH 5 or 6. Furthermore, all the isolates exhibited highest detoxification or reduction when peptone was used as carbon source instead of glucose or ammonium acetate. In a chosen or optimized condition of 37?C temperature, pH 7, 125 ppm K2Cr2O7 concentration and 48 h incubation period, most isolates exhibited 85-99% Cr reduction and detoxification from tannery effluent. It was, therefore, inferred that the isolates have potential as biological agent in reducing and detoxifying Cr from industrial effluent.