Physico-chemical properties and photo-reactivity relationship for para-substituted phenols in photo-assisted Fenton system

The reactivity of phenolic compounds can be drastically affected by the electronic nature of the substituting groups. In this work, the effect of physico-chemical properties on the reactivity via photo-assisted Fenton catalysis is reported for several para-substituted phenols (p-nitrophenol (p-NO2), p-chlorophenol (p-Cl), p-hydroxybenzaldehyde (p-CHO), phenol (p-H), p-methoxyphenol (p-OCH3), p-hydroxyphenol (p-OH)) in order to cover a wide range of electronics effects. Electronic descriptors (Hammett constants (sigma). frontier molecular orbital energies (E-HOMO), electronic and zero point energies (E, E-ZERO)). electrochemical descriptor (half wave potential for the oxidation of phenols to phenoxyl radical (E-1/2)), and other descriptors (acidity constants (pK(a)), maximum absorption wavelength (lambda(max)), 1-octanol/water partition coefficient (K-ow)) were correlated with the initial Fenton and photo-Fenton degradation rates (r(0)). Linear relationships were obtained between the initial Fenton and photo-Fenton degradation rates and electronic descriptors. However p-Cl and p-CHO showed higher photo-Fenton degradation rates than ones predicted by the model implying the presence of weaker bonds in these molecules. The biodegradability increase due to the photo-Fenton process was strong but poorly selective suggesting that the produced intermediates present a similar biodegradability. (C) 2009 Elsevier B.V. All rights reserved.