ROLE OF BACTERIA IN DETOXICATION OF CHROMIUM AND AZO DYE CONTAMINANTS RELEASED BY LEATHER PROCESSING INDUSTRY

Abstract

Effluents discharged from tannery industry contain significant amount of chromium and synthetic dyes. Both chromium and dyes can be transformed individually into less toxic forms, but very little is known about their simultaneous treatment. The present study was aimed at isolating bacteria capable of removing toxic hexavalent chromium (CrVI) and reactive black-5 azo dye simultaneously in liquid mineral salt medium (MSM). About 150 bacterial isolates were collected from tannery wastewater and sludge through enrichment of the MSM with CrVI (2 mg L −1 ) and reactive black-5 dye (100 mg L −1 ) under static (batch) condition. Bacterial strains KI (Pseudomonas putida ) and SL14 (Serratia proteamaculans) were able to reduce simultaneously 93% CrVI and 100% color of reactive black-5 azo dye in 24 h at pH 7.2 and 35 oC, in the presence of yeast extract as a co- substrate. Individually, 100% reduction of CrVI and reactive black-5 dye was achieved within 12 h by strain KI and SL14. Bacterial strains were also able to reduce both contaminants at high concentration of salt (up to 50 g NaCl L -1 ). Addition of trace elements Fe(II), Ag(I), Mn(II), Hg(II), Ba(II) and Li(I) had inhibitory effect on the reduction of reactive black-5 and CrVI while the reduction efficiency of the strains increased in the presence of Cu(II). The almost complete (~100%) reduction of the dye and CrVI was achieved by strain KI in 18 h at 5 mg L -1 Cu(II) concentration. In the presence of low concentration (1 mmol L -1 ), hydroquinone and uric acid electron complexes strain KI was able to reduce 100% dye and 92-97% CrVI within 12 h and 18 h, respectively. Potential of selected bacterial strains was also tested either individually or in consortium to degrade eight structurally different azo dyes and CrVI. Complete decolorization of six dyes (reactive black-5, golden ovifix, direct black-38, direct red-81, navy blue, acid red- 88) and 85-100% reduction in CrVI was observed with strain KI in 12 and 24 h. The bacterial consortia were less effective than the individual strains (KI, SL14 and BW3). The most efficient strain KI capable of reducing both dyes and CrVI simultaneously was finally tested in batch and continuous packed bed bioreactors. The biochar prepared from corn cob at 400 oC was used as a packing material in the reactor columns due to its high specific surface area (1275 m 2 g -1 ), porosity (2– 5 μm) and support to facultative cells growth (3.86 x 10 19 cfu g -1 ). Complete degradation (100%) of 100 mg dye L -1 occurred in 5 h in continuous packed bed bioreactor while 20 h were required for complete degradation of the dye in batch bioreactor. Moreover, strain KI was able to reduce completely 300 mg dye L -1 and 10 mg CrVI L -1 within 24 h in continuous packed bed bioreactor. Aniline (10 mg L -1 ), a by-product of reactive black-5, also degraded completely without CrVI while its degradation was 84% in case of simultaneous treatment. LC-MS analysis showed that the degradation products of reactive black-5 and aniline had molecular weight ranging from 110–292 kDa and 119-290 kDa, respectively. This study clearly illustrates that selected strains have potential to degrade azo dyes and CrVI. These bacterial strains are one of the most efficient bacteria capable of reducing toxic CrVI and synthetic dyes simultaneously and could be used for developing bioreactors to treat tannery effluent prior to its discharge into the environment.