Decolorization of azo dye methyl red by suspended and co-immobilized bacterial cells with mediators anthraquinone-2,6-disulfonate and Fe3O4 nanoparticles

• Aeromonas jandaei strain SCS5 can effectively decolorize methyl red.
• Immobilization of bacteria with AQDS enhanced decolorization of methyl red.
• Immobilization of bacteria with Fe3O4 nanoparticles enhanced decolorization.
• Immobilized bacterial beads retained similar activity up to 10 repeating cycles.
• Methyl red degradation products showed less phytotoxicity compared to methyl red.

In this study, the decolorization and degradation of methyl red (MR) by suspended and immobilized cells of Aeromonas jandaei strain SCS5 under anaerobic and aerobic conditions have been investigated. The complete decolorization of MR at a concentration of 100 mg L−1 by A. jandaei strain SCS5 was obtained within 6 h for both anaerobic and aerobic suspended cultures, where the decolorization rate was faster in acidic conditions than basic conditions. The decolorization efficiency under 6 h increased with increasing cell mass of inoculation and decreased with increasing initial dye concentrations. Immobilized cells of A. jandaei strain SCS5 could decolorize MR, and the decolorization rate was significantly enhanced by cells immobilized with mediators such as anthraquinone-2,6-disulphonate and magnetic Fe3O4 nanoparticles compared to immobilized cells only. Moreover, the immobilized bacterial beads with mediators retained high decolorization activity up to more than 10 repeating cycles. UV–visible spectra (200–800 nm) and gas chromatography-mass spectrometry analysis demonstrated that MR was degraded by A. jandaei strain SCS5 through reductive cleavage of azo bond. MR degradation products showed less phytotoxicity against Triticum aestivum and Phaseolus mungo compared to untreated MR. This study has demonstrated that A. jandaei strain SCS5 could be a promising microbiological agent for the removal of azo dyes from the environment.

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