Reactivity Differences of Hydroxyl Radicals and Hydrated Electrons in Destructing Azo Dyes

The high-energy radiation-induced degradation of an H-acid derivative azo dye, Apollofix-Red SF-28 (AR-28) was studied in aqueous solution by using pulse radiolysis with kinetic spectroscopic detection for transient measurements. Gamma radiolysis with UV-VIS spectroscopy and gradient ionpair HPLC separation with diode array detection were applied for following the destruction of AR-28 and measuring the products. The reactions of hydrated electron (eaq) and hydroxyl (dOH) radical were investigated separately. dOH reacts with the unsaturated bonds of the molecule. In the further reactions of the dOH adduct radicals, the AR-28 molecules partly reform with a slightly modified structure. The products formed in the first reaction of dOH and AR-28 molecules have also high reactivity towards the dOH radicals. For these reasons the efficiency of dOH radicals in discolouration of the solution is relatively low, it is below 0.5. The efficiency is much higher, close to unity for eaq. The reaction of eaq with AR-28 leads to an immediate destruction of the colour-giving centre.