Degradation kinetics of hydroxy and hydroxynitro derivatives of benzoic acid by fenton-like and photo-fenton techniques: A comparative study

The oxidative degradation of a series of hydroxy and hydroxynitro derivatives of benzoic acid by Fenton-like and photo-Fenton processes was compared under identical conditions (initial concentrations, pH and temperature). In spite of closely related chemical structures, (2-hydroxybenzoic (2H-BA), 2,4-dihydroxybenzoic (24DH-BA), 2-hydroxy-5-nitrobenzoic (2H5N-BA), 4-hydroxy-3-nitrobenzoic (4H3N-BA) and 2-hydroxy-4-nitrobenzoic (2H4N-BA) acids), the degradation timescales were remarkably different. A common feature was, however, that autocatalytic decay profiles were displayed by the substrates and H2O2. A simple equation, which may be used as a valuable tool for a semiquantitative analysis of the main kinetic features of the inverted “S” profiles, is presented. In addition, a method for the estimation of the relative contribution of photoinduced pathways in photo-Fenton systems (photoenhancement factors) is proposed.In order to assess the key processes governing the kinetic profiles observed, complementary studies were performed to evaluate the formation of ferric complexes, the reactivity towards HO and Fe(II) production efficiencies. Except for 4H3N-BA, the model substrates form highly stable complexes with Fe(III). Competition experiments showed that the reactivities of both the substrates and the ferric complexes with hydroxyl radicals cannot explain the large timescale differences observed in Fenton-like and photo-Fenton systems. The comparison of Fe(II) production under irradiation in the absence of H2O2 with the decay profiles observed in both Fenton-like and photo-Fenton systems confirms that the main factor controlling the autocatalytic behavior is the formation of organic intermediates that are capable of reducing Fe(III) species. An additional factor in the photo-Fenton process may be the efficiency of photoinduced Fe(II) production, which is affected by complex formation since the studied complexes exhibit a lower efficiency of Fe(III) photoreduction than the Fe(III)–aquo complex.