Reduction of azo dyes with zero-valent iron

The reduction of azo dyes by zero-valent iron metal (Fe-0) 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 bal ance was achieved by summing aqueous concentrations of dye and product amin e. All of the 9 azo dyes tested were reduced with first-order kinetics. The kinetics of decolorization at the lambda(max) of each dye were rapid: a ty pical k(obs) was 0.35 +/- 0.01 min(-1) for Orange II at 130 rpm on an orbit al shaker, corresponding to a surface area normalized rate constant (k(SA)) 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 o f surface sites on the Fe-0, and varied with the square-root of mixing rate (rpm) in a manner indicative of mass transfer limited kinetics. Correlatio n analysis using k(obs) for all of the azo dyes, estimates of their diffusi on coefficients, and calculated energies of their lowest unoccupied molecul ar orbitals (E-LUMO), gave no strong trends that could be used to derive st ructure-activity relationships. Using an authentic sample of wastewater fro m a dye manufacturing operation and construction-grade granular Fe-0, rapid decolorization was achieved that was consistent with reduction of azo dyes . (C) 2000 Elsevier Science Ltd. All rights reserved.