Color removal of thionine and methyl violet dyes in presence of hydrogen peroxide accelerated by Cu(II) ions

In aqueous solutions, the kinetics of the color removal of two cationic dyes, thionine (TH) and methyl violet (MV), both catalyzed by copper (II) ions and uncatalyzed have been studied. The progress of the reaction was seen by tracking absorbance decay at λmax = 600 and 584 nm for TH and MV dyes respectively. The rate of uncatalyzed reaction increased progressively as the starting peroxide concentration [H2O2]o increased, having a maximum at 0.8 M for TH and 2.0 M for MV, respectively. However, for reactions accelerated with copper (II) ions, the reaction rate rose as [H2O2]o increased, peaking at ≈ 0.4 and 2.0 for TH and MV, respectively, before declining. The creation of the extremely active and non-selective radicals resulting from the interaction of hydrogen peroxide with copper (II) ions was thought to be the source of the catalytic action of copper (II) ions. The order of reaction in [H2O2]o was first at low concentrations with both dyes.. However, employing high TH concentrations reduced the rate and order of the reaction reaching almost to zero-order. In the case of MV dye, the addition of copper (II) ions routinely increases the rate of reaction, reaching a maximum, then decreases gradually. Furthermore, no detectable reaction happened at pH ≤ 6.0 for TH and pH ≤ 5.0 for MV. Besides, the reaction rate increased as the pH of the media increased, particularly in the pH range 9.0-11.0. The interaction between the reacting ions suggests that, in the case of an uncatalyzed reaction, the hydroperoxide anion, HOO-, rather than H2O2, is the active oxidant. This also explains why the concentration of the hydroperoxide 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. The addition of the surfactant SDS also reduced the reaction rate. 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 were proposed.