Characterization of the enhancement of zero valent iron on microbial azo reduction

Background: The microbial method for the treatment of azo dye is promising, but the reduction of azo dye is the rate-limiting step. Zero valent iron (Fe-0) can enhance microbial azo reduction, but the interactions between microbes and Fe-0 and the potential mechanisms of enhancement remain unclear. Here, Shewanella decolorationis S12, a typical azo-reducing bacterium, was used to characterize the enhancement of Fe-0 on microbial decolorization. Results: The results indicated that anaerobic iron corrosion was a key inorganic chemical process for the enhancement of Fe-0 on microbial azo reduction, in which OH-, H-2, and Fe2+ were produced. Once Fe-0 was added to the microbial azo reduction system, the proper pH for microbial azo reduction was maintained by OH-, and H-2 served as the favored electron donor for azo respiration. Subsequently, the bacterial biomass yield and viability significantly increased. Following the corrosion of Fe-0, nanometer-scale Fe precipitates were adsorbed onto cell surfaces and even accumulated inside cells as observed by transmission electron microscope energy dispersive spectroscopy (TEM-EDS). Conclusions: A conceptual model for Fe-0-assisted azo dye reduction by strain S12 was established to explain the interactions between microbes and Fe-0 and the potential mechanisms of enhancement. This model indicates that the enhancement of microbial azo reduction in the presence of Fe-0 is mainly due to the stimulation of microbial growth and activity by supplementation with elemental iron and H-2 as an additional electron donor. This study has expanded our knowledge of the enhancement of microbial azo reduction by Fe-0 and laid a foundation for the development of Fe-0-microbial integrated azo dye wastewater treatment technology.