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

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. 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(-), H2, and Fe(2+) 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 H2 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). 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 H2 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.