Modelling the environmental degradation of water contaminants.: Kinetics and mechanism of the riboflavin-sensitised-photo oxidation of phenolic compounds

The aerobic visible-light-photosensitised irradiation of methanolic solutions of either of the phenolic-type contaminants model compounds (ArOH) p-phenylphenol (PP), p-nitrophenol (NP) and phenol (Ph), and for two additional phenolic derivatives, namely p-chlorophenol (ClP) and p-methoxyphenol (MeOP), used in some experiments, was carried out. Employing the natural pigment riboflavin (Rf) as a sensitiser, the degradation of both the ArOH and the very sensitiser was observed. A complex mechanism, common for all the ArOH studied, operates. It involves superoxide radical anion (O-2(center dot-)) and singlet molecular oxygen (O-2((1)Delta(g))) reactions. Maintaining Rf in sensitising concentrations levels (approximate to 0.02 mM), the mechanism is highly dependent on the concentration of the ArOH. Kinetic experiments of oxygen and substrate consumption, static fluorescence, laser flash photolysis and time-resolved phosophorescence detection of O-2((1)Delta(g)) demonstrate that at ArOH concentrations in the order of 10 mM, no chemical transformation occurs due to the complete quenching of Rf singlet excited state. When ArOH is present in concentrations in the order of mM or lower, O-2 is generated from the corresponding Rf radical anion, which is produced by electron transfer reaction from the ArOH to triplet excited Rf. The determined reaction rate constants for this step show a fairly good correlation with the electron-donor capabilities for Ph, PP, NP, ClP and MeOP. In this context, the main oxidative species is O-2(center dot-), since O-2((1)Delta(g)) is quenched in an exclusive physical fashion by the ArOH. The production of O-2(center dot-) regenerates Rf impeding the total degradation of the sensitiser. This kinetic scheme could partially model the fate of ArOH in aquatic media containing natural photo sensitisers, under environmental conditions. (c) 2004 Elsevier Ltd. All rights reserved.