Selective oxidative degradation of azo dyes by hydrogen peroxide catalysed by manganese(ii) ions

摘要

Manganese( II ) ions catalyse the oxidative degradation of Calmagite (H _(3) CAL) dye in aqueous solution at 20 ± 1 °C in the pH range 7.5–9.0 using hydrogen peroxide (H _(2) O _(2) ) as oxidant by a mechanism that involves strong complexation to the Mn ~(II) centre. It is proposed that [Mn ~(III) (CAL)(O _(2) H)] ~(?) i.e. a dye coordinated hydroperoxyl (O _(2) H ~(?) ) Mn ~(III) complex is formed and bleaching of the dye is initiated by an electron-transfer to Mn ~(III) , with the binding of H _(2) O _(2) being the rate determining step. At pH 9.0 in (bi)carbonate, HCO _(3) ~(?) , H _(3) CAL is rapidly bleached via the in situ formation of coordinated peroxycarbonate (HCO _(4) ~(?) ); a TOF (TOF = moles of dye bleached per mole of manganese per hour) of ～5000 h ~(?1) can be achieved. The bleaching of the related azo dyes Orange II and Orange G is different because, unlike Calmagite, they lack an o , o -dihydroxy motif so are unable to complex strongly to Mn ~(II) and no oxidation to Mn ~(III) occurs. At pH 8.0 (phosphate buffer) Orange II and Orange G are not bleached but bleaching can be achieved at pH 9.0 (HCO _(3) ~(?) buffer); the rate determining step is dye coordination and it is proposed bleaching is achieved via an outer-sphere oxygen atom transfer. Mechanisms for dye bleaching at pH 8.0 and pH 9.0 are proposed using data from EPR, UV/VIS and ESI-MS. Mn ~(II) /H _(2) O _(2) /HCO _(3) ~(?) form a potent oxidising mixture that is capable of removing stubborn stains such as curcumin.