Generation of hydroxyl radicals in the peroxi-coagulation process with an air-diffusion cathode: Fluorescence analysis and kinetic modeling

Abstract

Conventional peroxi-coagulation process is based on the combined occurrence of electrocoagulation and oxidation in a single unit, since H2O2 and Fe2+ are electrogenerated on site from cathodic and anodic reactions, respectively. The present work aims to evaluate the effect of pH and applied current on the generation of hydroxyl radicals (•OH) from Fenton’s reaction during peroxi-coagulation treatment. The electrochemical cell consisted of a gas-diffusion cathode with graphite cloth, and an iron plate anode. By using a fluorescence probe (coumarin) and kinetic modeling, it has been possible to estimate the •OH concentration at different current values and to determine the effect of pH on their production. A system with six ordinary differential equations was established and solved to predict the concentrations of the main species of Fenton’s reaction. In addition, the interference of possible by-products and side coumarin hydroxylation reactions in the determination of •OH was considered. The simulation results reveal that current increase from 10 to 50 mA positively influences the •OH generation, further decreasing when operating at 70 mA. This is attributed to: (i) the negative effect of an excessive H2O2 generation, and (ii) the increase in pH during the electrolysis. This latter phenomenon is detrimental because of the partial precipitation of Fe2+ catalyst. A sensitivity analysis was performed to determine the most influential kinetic constants of the model on •OH and 7-hydroxycoumarin concentrations. This work demonstrates the importance of considering possible side reactions, which may occur when coumarin is used as a probe compound to quantify the •OH.