Abstract

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 λ_max of each dye were rapid: a typical k_obs 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 (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 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 k_obs for all of the azo dyes, estimates of their diffusion coefficients, and calculated energies of their lowest unoccupied molecular orbitals (E_LUMO), 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.

Author Keywords: iron metal; decolorization; kinetics; mass transport; correlation analysis

Nomenclature

ρ_a

iron surface area concentration (m² L⁻¹)

λ_max

wavelength of maximum absorbance (nm)

ω

rotation rate (rpm)

A

empirical coefficient describing the kinetics of mass transport (rpm^1/2 min⁻¹)

C, C₀

concentration of substrate at time t, and at time 0 (mM)

D

diffusion coefficient (cm² s⁻¹)

E_LUMO

energy of the lowest unoccupied molecular orbital (eV)

k_obs

pseudo first order disappearance rate constant (min⁻¹)

k_obs′

first order appearance rate constant (min⁻¹)

k_SA

surface-area normalized rate constant (L m⁻² min⁻¹)

k_rxn

first order rate constant for chemical reaction (min⁻¹)

K_1/2

concentration of substrate at half of maximum reaction rate (mM)

V_m

maximum reaction rate (mM min⁻¹)

Article Outline

• Nomenclature

• Introduction

• Materials and methods

• Chemicals

• Batch experiments

• Analytical methods

• Results and discussion

• Pathway and kinetics

• Effect of initial concentration

• Effect of mixing

• Effect of structure

• Wastewater decolorization

• Conclusions

• Acknowledgements

• References