Mineralization of phthalic acid by solar photoelectro-Fenton with a stirred boron-doped diamond/air-diffusion tank reactor: Influence of Fe³⁺ and Cu²⁺ catalysts and identification of oxidation products

Here, the substrate decay and mineralization rate for 100 cm³ of a 2.0 mM phthalic acid solution in 0.10 M Na₂SO₄ of pH 3.0 have been studied by electro-Fenton (EF) and solar photoelectro-Fenton (SPEF). The electrochemical cell was a stirred tank reactor containing a 3 cm² boron-doped diamond (BDD) anode and a 3 cm² air-diffusion cathode that generates H₂O₂. Cu²⁺ and/or Fe³⁺ were added as catalysts with total concentration of 0.50 mM and a constant current density of 33.3 mA cm^-2 was applied. In EF with Cu²⁺ or Fe³⁺ alone and SPEF with only Cu ²⁺, phthalic acid decayed slowly and poor mineralization was reached because the main oxidant was OH produced at the BDD surface from water oxidation. In contrast, the substrate destruction was largely enhanced using SPEF with 0.50 mM Fe³⁺ since a high quantity of oxidant OH was produced in the bulk induced by photo-Fenton reaction. This treatment led to an almost total mineralization by the photolysis of generated Fe(III)-carboxylate complexes. In all cases, the decay of phthalic acid obeyed a pseudo-first-order reaction. The combination of Cu²⁺ and Fe³⁺ as catalysts accelerated the mineralization process in SPEF because Cu(II)-carboxylate complexes were also removed with OH formed from photo-Fenton reaction. The best SPEF process was found for 0.125 mM Cu²⁺ + 0.375 mM Fe³⁺, giving rise to 99% mineralization with 40% current efficiency and 0.294 kWh g^-1 TOC energy consumption. Eleven aromatics and six short-linear carboxylic acids were identified as oxidation products. A plausible reaction sequence for phthalic acid mineralization involving all the detected products was finally proposed.