Kinetics of Reductive Degradation of Azo Dye by Zero-Valent Iron

Abstract

The kinetics of reductive degradation of two different azo dyes by zero-valent iron in aqueous solution were investigated using a laboratory-scale slurry reactor system. Both reductive degradations show pseudo first order reaction kinetics. A typical rate constant for the reactions is 0.735 min−1 for Acid Orange II and 0.694 min−1 for Acid Blue 113 at mixing speeds of 2000 revolutions per minute. The apparent rate constant (k) increased with both mixing speed and iron concentration but decreased with increased initial dissolved oxygen content and pH. The increase with iron concentration for both Acid Orange II and Acid Blue 113 is linear with correlation coefficients of 3 × 10−5mgl−1. The rate constant increases linearly with the inverse square root of mixing speed for both dyes—corresponding directly with increasing reaction rate with decreasing boundary layer thickness. Half lives for Acid Orange II correlate well with values reported in the literature. These figures demonstrate the mass transfer limitation and surface dependency of these heterogeneous reactions and show the trends that it is necessary to understand in order to address them for commercial application. The effect of mass transfer on reaction rate is described qualitatively in terms of mass transfer theory and quantitatively in terms of the empirical Sherwood number and thickness of the stagnant boundary layer that adheres to the iron particle. The effect of surface passivation is shown quantitatively in terms of initial dissolved oxygen content of the aqueous phase.