Kinetics and mechanism of the uncatalyzed and copper (II) catalyzed color removal of thionine and methyl violet dyes in presence of hydrogen peroxide

Abstract

In aqueous solutions, the kinetics of the color removal of two cationic dyes, thionine and methyl violet, both catalyzed by copper (II) ions and uncatalyzed have been studied. The progress of the reaction was seen by tracking the decay in absorbance at λ_max = 600 and 584 nm for methyl violet and thionine, respectively. The rate of an uncatalyzed reaction increased progressively as the starting peroxide concentration [H₂O₂]_o increased, peaking at 0.8 M for thionine and 2.0 M for methyl violet, respectively. However, for the catalyzed reactions, the rate of reaction rose as [H₂O₂]_o increased, peaking at ≈ 0.4 and 2.0 for methyl violet and thionine, respectively, before declining. The creation of the extremely active and non-selective hydroxyl radicals produced by the reaction of hydrogen peroxide with copper (II) ions was thought to be the source of the catalytic action of copper (II) ions. However, a first order dependence with respect to the original dye concentration was noted. Nevertheless, employing high methyl violet concentrations reduced the rate and order of the reaction, with the latter becoming zero-order at such concentrations. In the case of thionine dye, the addition of copper (II) ions routinely increases the rate of reaction, and the concentration of copper (II) ions gradually decreases the order of enhancement. In contrast, the rate increased, peaked, and then declined when methyl violet was used. Furthermore, no detectable reaction happened at pH ≤ 6.0 for thionine and pH ≤ 5.0 for methyl violet; rather, the rate of reaction increased with increasing pH of the reaction media, particularly in the pH range 9.0–11.0. The interaction between two single, oppositely charged ions is indicated by the influence of ionic strength on the reaction rate. This suggests that, in the case of an uncatalyzed reaction, the hydropeorxide anion, HOO⁻, rather than H₂O₂, is the active oxidant. This also explains why the concentration of the hydropeorxide anion increases with increasing pH, leading to the rate enhancement seen. When a radical scavenger was added to the catalyzed reaction, the rate of reaction was slowed down, indicating that active free radicals were involved in the reaction process. Even at low concentrations below the threshold micelles concentration, the addition of the surfactant SDS also reduced the rate. Anionic micelles electrostatic interaction with the cationic dye may be the cause of this rate drop. The work could be used for the reuse of dye bath water to lower costs in textile and dyeing industries as well as for environmental reasons. Reaction mechanisms for both catalyzed and uncatalyzed reactions were proposed.