This study demonstrates the role of electron shuttles and trace metals in the biotransformation of azo dye
reactive black-5 and hexavalent chromium (CrVI) that are released simultaneously in tannery effluent.
Previously isolated bacterial strain Pseudomonas putida KI was used for the simultaneous reduction of
the dye (100 mg L1
) and CrVI (2 mg L1
) in a mineral salts medium (MSM). Among various trace metals,
only Cu(II) had a stimulating effect on the bacterial-mediated reduction process. Application of electron
shuttles such as hydroquinone and uric acid at a low concentration (1 mM) had a positive effect on the
reduction process and caused simultaneous reduction of 100% dye and 97% CrVI in 12–18 h. Mannitol,
EDTA and sodium benzoate at all concentrations (ranging from 1 to 9 mM) showed an inhibitory effect
on the reduction of reactive black-5 and CrVI. An inverse linear relationship between the velocity of reaction
(V) and the concentration [S] of electron shuttles was observed. The results imply that both types and
concentration of an electron shuttle and trace metals can affect the simultaneous reduction of reactive
black-5 and CrVI.