Selective oxidative degradation of azo dyes by hydrogen peroxide catalysed by manganese(II) ions
RSC ADVANCES(2016)
摘要
Manganese(II) ions catalyse the oxidative degradation of Calmagite (H(3)CAL) dye in aqueous solution at 20 +/- 1 degrees C in the pH range 7.5-9.0 using hydrogen peroxide (H2O2) as oxidant by a mechanism that involves strong complexation to the Mn-II centre. It is proposed that [Mn-III(CAL)(O2H)](-) i.e. a dye coordinated hydroperoxyl (O2H-) Mn-III complex is formed and bleaching of the dye is initiated by an electron-transfer to Mn-III, with the binding of H2O2 being the rate determining step. At pH 9.0 in (bi) carbonate, HCO3-, H(3)CAL is rapidly bleached via the in situ formation of coordinated peroxycarbonate (HCO4-); a TOF (TOF = moles of dye bleached per mole of manganese per hour) of similar to 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 MnII 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 (HCO3- 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/H2O2/HCO3- form a potent oxidising mixture that is capable of removing stubborn stains such as curcumin.
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