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 (σ), frontier molecular orbital energies (EHOMO), electronic and zero point energies (E, EZERO)), electrochemical descriptor (half wave potential for the oxidation of phenols to phenoxyl radical (E1/2)), and other descriptors (acidity constants (pKa), maximum absorption wavelength (λmax), 1-octanol/water partition coefficient (Kow)) were correlated with the initial Fenton and photo-Fenton degradation rates (r0). 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.