Kinetics of ozone-initiated oxidation of textile dye, Amaranth in aqueous systems

Abstract

The ozone facilitated oxidation mechanism of water soluble azo anionic dye, amaranth (Am) was investigated monitoring the depletion kinetics of the dye spectrometrically at 521 nm. The oxidation kinetics of the dye by ozone was studied under semi-batch conditions, by bubbling ozone enriched oxygen through the aqueous reaction mixture of dye, as function of flow rate, ionic strength, [O₃] and pH variations. With excess concentration of ozone and other reagents and low [amaranth], reaction followed pseudo–first-order kinetics with respect to the dye. Added neutral salts had marginal effect on the reaction rate and the variation of pH from 7 to 2 and 7 to 12 exerted only small increases in the reaction rate suggesting molecular ozone possibly is the principle reactive species in oxidation of dye. The reaction order with respect ozone was near unity and it varied slightly with pH and flow rate variations. The overall second-order rate constant for the reaction was (105 ± 4) M⁻¹ min⁻¹. The main oxidation products immediately after amaranth decolorization were identified. The reaction mechanism and overall rate law were proposed. After spiking the seawater, river water and wastewaters with Amaranth dye, the reaction rates and trends in BOD and COD under control and natural conditions were investigated. The rate of depletion of the dye in natural waters was relatively lower, but the ozonation process significantly decreased both the BOD and COD levels.

Keywords:

• Ozone
• amaranth
• azo dye
• oxidation
• kinetics and mechanism
• pH variation

Acknowledgments

The authors thank the National Research Foundation for providing the financial support to the project and the School of Chemistry at UKZN (Westville) for the facilities.