The removal of the highly toxic heavy metal, chromium (VI), from industrial effluents before their discharge into the environment presents serious challenges to industries. Many industries use chemical and physical methods to remove the metal from their effluents before their discharge into the environment. However, such methods, despite their efficiency and reliability, are not environmentally friendly. On the contrary, biological chromium (VI) removal from industrial wastewaters has the potential to offer a cheap and environmentally friendly solution to this challenge. The primary objective of this study was to isolate bacteria capable of converting the toxic chromium (VI) to the relatively non-toxic chromium (III) form and evaluate the isolates’ potential use in the bioremediation of chromium-rich industrial effluents. Firstly, chromium (VI)-reducing bacteria were isolated from tannery wastewater. The isolates were then tested for their chromium (VI) tolerance and reducing activities. Chromium (VI) reduction by immobilized or suspended bacterial cells was investigated. The effect of variations in temperature and pH on chromium (VI) reduction by the isolates was also investigated. Seven chromium (VI)-reducing bacterial isolates were obtained. Through the use of 16S rRNA gene sequence analysis, six of the seven isolates were identified as different species of Proteus, while the remaining isolate was separately identified as Klebsiella oxytoca. All the isolates showed varying degrees of tolerance to chromium (VI), ranging from 700 to 1 500 mg/L. Chromium reduction by the isolates ranged from 85.0 to 97.6%, with an average of 91.0%. Immobilised cells achieved better chromium (VI) reduction as compared to those growing in suspension. The optimum pH and temperature for chromium (VI) reduction were found to be 7 and 37°C, respectively. This study clearly demonstrates the feasibility of the biological removal of chromium from industrial wastewater and should therefore motivate researchers towards the possible large-scale application of this technology.
Keywords: Chromium, wastewater, heavy metal, Proteus, bioremediation, environment, tannery.