Chromate detoxification using combination of ChromeBac™ system and immobilized chromate reductase beads

• Negligible role of NADH during Cr(VI) reduction by bacteria.
• Role of Co(II) ions to enhance Cr(VI) reduction.
• Demonstration on the application of immobilized chromate reductase beads in an existing chromate treatment system.

Incomplete removal of chromium from discharge effluent of chromium-based industries is a serious problem due to its deleterious effect. ChromeBac™ is a locally developed Cr(VI) reduction system carried out by aerobic Cr(VI) resistant bacteria followed by chemical precipitation. The locally isolated Cr(VI) resistant-reducing Acinetobacter haemolyticusEF369508 was immobilized onto carrier materials inside a 50 L bioreactor. Mixture of 10% (v/v) liquid pineapple waste and neutralized Cr(VI) solutions (30–60 mgL−1) was fed into the bioreactor at 0.11 m3 h−1. Around 90% of the initial Cr(VI) was reduced after 24 h of contact inside the bioreactor. Residual Cr(VI) was then further reduced to between 1.0 and 1.5 mgL−1 by immobilized chromate reductase alginate-beads packed in a 10 L flow-through column, after 15 h of contact. Some important characteristics for the chromate reductase activities for A. haemolyticus are as follows; not NADH-dependent, associated with CFE with notable contribution from the membrane fraction, enhanced in the presence of glucose, optimal at pH 7.0, 30 °C, in the presence of 1 mM Co2+ (highest) with Michaelis–Menten constant, Km and maximum reaction rate, Vmax of 184.47 μM and 33.3 nmol/min/mg protein respectively. Ag+ and Hg2+ inhibited the enzyme activity. This study demonstrated the potential of using immobilized chromate reductase beads to further reduce residual Cr(VI) present in the effluent of a ChromeBac™ process, hence reducing the time for overall Cr(VI) treatment process.