Reduction of Cr(VI) by Escherichia coli BL21 in the presence of redox mediators

Anthraquinone-2,7-disulfonate (2,7-AQDS), anthraquinone-1-sulfonate (α-AQS), anthraquinone-2-sulfonate (AQS), anthraquinone-2,6-disulfonate (AQDS) and anthraquinone-1,5-disulfonate (1,5-AQDS) were selected as redox mediators for Cr(VI) reduction by Escherichia coli BL21. In the presence of 0.8 mmol l−1 AQS, the Cr(VI) reduction efficiency was 98.5% in 7.5 h, whereas it was only 21–34% in the absence of a mediator or in the presence of α-AQS, 1,5-AQDS, AQDS, 2,7-AQDS. A linear correlation, k = 0.768CAQS + 1.5531 (R2 = 0.9935), was found for the reaction constant, k (mg Cr(VI) g−1 dry cell weight h−1) and the AQS concentration (CAQS). The Arrhenius equation described the Cr(VI) reduction in the tested temperature range, and the pre-exponential factor A was 13.249 mg Cr(VI) g−1 dry cell weight h−1,and the activation energy Ea was 28.01 kJ mol−1. Glucose was the best carbon sources, and the optimum pH was 6.0. The Cr(VI) reduction rate obtained with AQS is a significant improvement over low conventional anaerobic reduction rates.