Enhanced mineralization of phenol and other hydroxylated compounds in a photocatalytic process assisted with ferric ions

Abstract

This study shows that the presence of low ferric ion concentrations accelerates the rate of photocatalytic oxidation and mineralization of different phenolic components. Ferric ions not only considerably affect the rates of disappearance and total mineralization of phenol, but also the concentration-time profiles of its major intermediates. At reaction conditions, three major aromatic intermediates were identified: ortho-dihydroxybenzene (o-DHB), para-dihydroxybenzene (p-DHB), and 1,4-benzoquinone (1,4-BQ). Additionally, four carboxylic acids were also detected as intermediates: fumaric acid (FuAc), maleic acid (MeAc), oxalic acid (OxAc) and formic acid (FoAc). Fe ions also increase the rate of disappearance and mineralization of the aromatic intermediates when tested separately. A new refined series-parallel kinetic network for the photocatalytic mineralization of phenol is proposed describing the reaction in higher detail than previous studies. It is concluded that the proposed mechanism adequately describes both unpromoted photocatalytic oxidation (unpromoted PC) and Fe-assisted photocatalytic oxidation (Fe-assisted PC) of phenol with the various mechanistic steps involving detectable species. Experimental data also reveal that while unpromoted PC and Fe-assisted PC reactions share a close mechanism, the latter leads to changes in the step-related kinetic constants. It is found that the overall quantum yield (OQY) increase in the Fe-assisted PC reactions for phenol ranges between 30% and 50%. For the aromatic intermediates the increment is between 25% and 38%. Two kinetic models are developed to represent the oxidation of phenol in both unpromoted PC and Fe-assisted PC systems. The first model predicts the rate of disappearance of the major aromatic components while the second one includes the carboxylic acids and allows the prediction of the total mineralization rate of phenol. Estimates of the parameters along with their statistical indicators are also reported for both systems.