Insight into the generation of hydroxyl radicals by photo-electrocoagulation process via active chlorine

Photo-electrocoagulation with aluminum electrodes is becoming an increasingly attractive process for the degradation of organic pollutants in low concentration levels, such as those found in secondary effluents (biological processes). The technological potential of this approach relies on the synergistic effect of the electrochemical generation of coagulants, active chlorine species, and photochemical reactions promoted by UV irradiation (λ = 254 nm) which results in the formation of highly reactive oxygen species such as the ·OH radical. In this context, the goal of this work was to demonstrate and evaluate the influence of the operating parameters of a laboratory-scale photo-electrocoagulation process on the generation of ·OH radicals. Using fluorescence spectrophotometry, it was possible to detect and quantify ·OH radicals by means of coumarin and terephthalic acid assessment. The production of ·OH radicals was highly influenced by the pH of the solution, observing the highest concentration at pH 3. Chloride concentration also showed a positive effect on the generation of ·OH radicals; however, this influence was only observed up to a chloride ion concentration of 30 mM. In addition, phenol mineralization of 20.1% was achieved by applying a current density of 2.7 mA cm−2 under 2 h of irradiation. To the best of our knowledge, this is the first study that demonstrates the production of ·OH radicals in a photo-electrocoagulation process with aluminum electrodes.