Biotransformation of Hexavalent Chromium from Wastewater Using Bacillus firmus

Abstract

Chromium is an element present in the Earth’s crust that acts as an impurity in food, water, soil and air and causes benign effects on the human body. Among the various oxidation states, the hexavalent form has higher toxicity, higher solubility in water, and higher mobility in soil. The maximum recommended permissible limit of Cr in drinking water is 0.01 mg/L by the United States Environmental Protection Agency (USEPA), whereas 0.05 mg/L by the Bureau of Indian Standards (BIS) and World Health Organization (WHO). Several physical and chemical methods have been developed for the removal of chromium from contaminated water. In most techniques, there is a higher chance of re-contamination after the disposal of generated sludge. In this study, Bacillus firmus has been exploited for in-situ bioremediation of hexavalent chromium (Cr⁶⁺) from wastewater under batch mode. The effects of operating conditions such as pH, initial concentration, centrifugation rate (rpm), external carbon, and nitrogen sources were investigated. The maximum Cr⁶⁺ conversion of 99.9% was obtained at 0.5 and 1 mg/L initial concentration, pH 7, 30°C, and 120 rpm. Dextrose was found to be the potential C source for enhancing microbial growth and efficient conversion at higher Cr⁶⁺ concentrations. Furthermore, living and dead biomass revealed a comparatively high biosorption of 18.3% at 100 mg/L Cr⁶⁺ concentration.