Reduction Of Azo Dyes By Geobacter Metallireducens

Biological method is usually employed during azo dye wastewater treatment, by which azo dyes can be readily reduced under anaerobic conditions. However, anaerobic decolorization is relatively slow and the decolorization rate needs to be raised.The dissimilatory metal reduction bacterial strain Geobacter metallireducens has recently gained more attention on biodegradation and ecological restoration of organic pollutants,heavy metals and radioactive metals,yet there was no report on its activity of azo dye decolorization. As analog or substituent of humic substance, quinone can act as redox mediator to stimulate the biodegradation and transformation of contaminant. However, no information on humic-substances-mediated reduction of azo dyes by G. metallireducens was available. In order to elucidate these problems, this paper focused on assay of direct and mediated reduction of azo dyes by G. metallireducens.G. metallireducens was found to be capable of decolorizing several azo dyes with different structures to various extents. Different organic matters serving as electron donors could result in varying extents of Acid red27(AR27) decolorization. Enhanced AR27decolorization was observed with the increase of acetate concentration (0to5mM), dye concentration (25to400μM) and biomass (7.5to45mg L-1). AR27decolorization by G. metallireducens was hindered by goethite, which competed for electrons with AR27. Qualitative analysis of intermediate products from acid orange52decolorization by G. metallireducens using high-performance liquid chromatography analysis confirmed the reductive pathway.The study of humic-substances-mediated decolorization of AR27by G. metallireducens found that the tested quinones had diverse impacts on AR27decolorization. Among the various effects. AQDS, AQS, LQ could significantly stimulate the decolorization, while AQC showed unconspicuous facilitation and JQ, DHNQ showed inhibiting effects on the decolorization. Enhanced AR27decolorization was observed with the increase of AQDS concentration ranging from0to10mM. A linear relationship between humic acid (HA) concentration ranging from0to50mg DOC L-1and AR27decolorization rate was observed in initial phase. However, there was no obvious increase in decolorization rate when the HA concentration was further increased to100mg DOC L-1.AQDS-mediated decolorization of AR27enhanced with increase of biomass (7.5-45mg L-1) and dye concentration (50-400μM). The addition of AQDS increased the decolorization efficiencies of AR27. AR73and AO52to around2.9-,1.3-and2.0-fold, respectively,whereas no obvious improvement was observed for decolorization of DB71and RR120. The relationship between specific decolorization rate of AR27in the absence or presence of AQDS and AR27concentration followed Michaelis-Menten kinetics.The Michaelis-Menten kinetics constants of two kinds of reaction were as follows:Km=186.9μM, Vmax=0.65μmol mg protein-1h-1; K’m=118.65μM, V’max=1.325μmol mg protein-1h-1. The V’max of reaction with AQDS was increased to be2folds of that without redox mediator, meanwhile K’m showed a decline,further illustrating that the presence of AQDS would facilitate the reduction of AR27.