Kinetic investigations into the copper(II)-catalysed peroxosulfate oxidation of Calmagite dye in alkaline media

Abstract

Addition of Cu^II catalysed the peroxosulfate (KHSO₅) oxidation of Calmagite dye (D) in alkaline media and it was observed that the dye decomposition profiles exhibit extremely complex kinetic behaviour, with typical induction periods associated with rapid autocatalysis. A comprehensive kinetic model has been derived which not only can predict changes in speciation of reactants with time, but can also satisfactorily explain all the kinetic profiles. Much higher catalyst concentrations are required for copper(II) catalysis than for manganese(II) catalysis ([KHSO₅] > [Cu^II] > [D] as opposed to [KHSO₅] > [D] > [Mn^II]), i.e. there needs to be excess Cu^II over D. This arises for two main reasons; first, binding of Cu^II to the dye inhibits its reaction with peroxosulfate, protecting the dye against oxidation, and secondly the active catalyst is a heterogeneous copper hydroxide species which arises when there is an excess of Cu^II in alkaline media. The unusual decomposition profiles are attributed to release of Cu^II into alkaline media via direct oxidation of the [CuD] complex by peroxosulfate. This oxidation is relatively slow, but when sufficient Cu^II is released ‘true’ autocatalysis is exhibited. The best simulated fits were obtained when the catalytic step was assumed to be first order in [SO₅^2–] (i.e. peracid primarily acting as a nucleophile) and zero order in [CuD] as it is adsorbed onto the colloidal copper hydroxide.