Hexavalent Chromium Reduction at Different Growth Phases of Acinetobacter haemolyticus

Abstract

The aim of this work was to evaluate the effects of hexavalent chromium [Cr(VI)] addition at different growth phases on the Cr(VI) reducing capacity of the aerobic Gram-negative bacterium, Acinetobacter haemolyticus. The addition of 10–100 mg Cr(VI) L⁻¹ as potassium dichromate at the time of inoculation (T0) resulted in the extension of time needed to reach the stationary phase, with a total growth inhibition at 70 mg Cr(VI) L⁻¹, a decrease in bacterial count from 10⁸ to 10² CFU mL⁻¹, and a Cr(VI) reduction drop to 35% at 100 mg Cr(VI) L⁻¹. However, bacterial growth was uninterrupted when 10–100 mg Cr(VI) L⁻¹ were added at the early stationary phase (after 12 h of bacterial growth) indicated by a high bacterial count of 10⁸ CFU mL⁻¹. More than 95% of Cr(VI) was reduced at an initial Cr(VI) of 50 mg L⁻¹. However, the Cr(VI) reduction capacity dropped to 50% at 100 mg Cr(VI) L⁻¹. This study showed that the ability of Acinetobacter haemolyticus to reduce Cr(VI) depends strongly on the number of surviving cells present in the solution. Higher Cr(VI) reduction can be achieved when a high number of bacterial cells are present. To achieve this condition, it is suggested to introduce the Cr(VI) into cells suspension at the stationary phase rather than introducing the Cr(VI) at the time of inoculation.