Reduction of azo dyes with zero-valent iron

The reduction of azo dyes by zero-valent iron metal (Fe0) at pH 7.0 in 10 mM HEPES buffer was studied in aqueous, anaerobic batch systems. Orange II was reduced by cleavage of the azo linkage, as evidenced by the production of sulfanilic acid (a substituted aniline). Adsorption of the dyes on iron particles was less than 4% of the initial concentration, and >90% mass balance was achieved by summing aqueous concentrations of dye and product amine. All of the 9 azo dyes tested were reduced with first-order kinetics. The kinetics of decolorization at the λmax of each dye were rapid: a typical kobs was 0.35±0.01 min−1 for Orange II at 130 rpm on an orbital shaker, corresponding to a surface area normalized rate constant (kSA) of 0.21±0.01 L m−2 min−1. The rate of reduction of Crocein Orange G varied with initial dye concentration in a way that suggests saturation of surface sites on the Fe0, and varied with the square-root of mixing rate (rpm) in a manner indicative of mass transfer limited kinetics. Correlation analysis using kobs for all of the azo dyes, estimates of their diffusion coefficients, and calculated energies of their lowest unoccupied molecular orbitals (ELUMO), gave no strong trends that could be used to derive structure-activity relationships. Using an authentic sample of wastewater from a dye manufacturing operation and construction-grade granular Fe0, rapid decolorization was achieved that was consistent with reduction of azo dyes.