Abstract
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 t
Highlights
Industrial pollution is a major factor to cause significant degradation to the environment around us
Tannery wastewater is found extremely polluted in terms of total dissolved solids (TDS), total suspended solids (TSS), biochemical oxygen demand (BOD), chemical oxygen demand (COD), heavy metals, etc. (Mondal et al 2005)
Cr-III exists at a narrow range of concentration, and is an essential element because it regulates the metabolism of glucose in human body (Srinath et al 2001)
Summary
Industrial pollution is a major factor to cause significant degradation to the environment around us. In a developing country like Bangladesh, solid waste or effluent from a tannery is directly discharged into water bodies or sewers without treatment (Verheijen et al 1996; Favazzi 2002). Bangladesh has 165 leather and footwear factories It has 161 tanning industries that process rawhides into finished leather. Other than Cr, the most commonly occurring heavy metals in tannery wastewater include lead, manganese, cadmium and copper. Occurrence of these toxic substances in surface water and soil leads to serious public health threat and significant risk to the ecological system (Sundar et al 2010). The present study aims to investigate the ability of naturally occurring bacteria in reducing and detoxifying Cr at privileged nutritional and environmental conditions, and evaluate their potentiality as candidate agent of bioremediation of the heavy metal
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