Kinetic investigations into the mechanism of peroxosulfate oxidation of Calmagite dye catalysed by manganese(II) ions

Abstract

Manganese(II) ions were found to be capable of catalysing the peroxosulfate (KHSO₅) oxidation of Calmagite, a dye containing the o,o′-dihydroxy azo structural motif at pH 10 but not the more common monohydroxy azo dyes or those that contain no o-hydroxy group. The catalysed decomposition of Calmagite results in decomposition profiles that resemble an autocatalytic process, i.e. the reaction rate increases as dye concentration decreases. Spectroscopic investigations indicated that two complexes are formed between Mn^II and Calmagite (D), having 1∶1 and 1∶2 stoichiometries, i.e. [MnD] and [MnD₂]. Under experimental conditions [KHSO₅] ≫ [D] ≫ [Mn^II] so the catalyst exists in the form [MnD₂] and it is shown that the unusual kinetics are due to a reaction of order -1 in [D]. This negative order is explained by attributing [MnD] as the active catalyst, formed from the inert [MnD₂], with which it is in equilibrium, by the loss of one dye molecule. The experimental data were found to conform to the rate expression –d[D]/dt = k₀[Mn^II][KHSO₅]/[D] and the rate constant k₀ can be determined from plots of [D]² against time. It is concluded that a key factor for promoting catalysis is specific complexation of Mn^II to the dye substrate, and that the o,o′-dihydroxy azo structural unit is specifically required for efficient manganese catalysis. Some insight into the mechanism of catalysis was obtained by investigating Mn^III as catalyst and from studies where H₂O₂ was employed as the oxidant. It is proposed that the mechanism involves nucleophilic attack by oxidant on the metal centre followed by peroxide-bond scission, leading to formation of manganese-(III) or -(IV) species which subsequently initiate dye oxidation via an inner-sphere reaction mechanism.