Degradation of 2,4-Dichlorophenoxyacetic Acid by Electro-oxidation and Electro-Fenton/BDD Processes Using a Pre-pilot Plant

The degradation of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) has been studied by electro-oxidation (EOx) and electro-Fenton/BDD (EF/BDD) using a 3-L pre-pilot plant with a BDD/BDD cell of 64 cm2 of electrode area. Electrolyses were performed with 60 mg L−1 2,4-D in 0.05 M Na2SO4 at pH 3.0, current densities between 7.8 and 31 mA cm−2, and liquid flow rates between 4 and 10 L min−1. In these electrochemical advanced oxidation processes, the main oxidants are hydroxyl radicals (•OH) formed from water oxidation at the BDD anode and/or from Fenton’s reaction between added Fe2+ and H2O2 generated at the BDD cathode. The EOx treatment at 31 mA cm−2 yielded 70 % mineralization as maximal at 160 min, which decreased at lower liquid flow rate because it was limited by the mass transport of organics toward the anode. Faster degradation was found for EF/BDD with 0.7 mM Fe2+, leading to 81–83 % of mineralization in 120 min. High current efficiency was determined at the beginning of this process, and the lowest energy cost of 95 kWh kg−1 TOC (2.0 kWh m−3) was obtained at 7.8 mA cm−2. 2,4-D was rapidly removed by EF/BDD. Four chloroaromatic products (2,4-dichlorophenol, 4,6-dichlororesorcinol, chlorohydroquinone, and chloro-p-benzoquinone) and three hydroxylated derivatives were detected by liquid chromatography–mass spectrometry, whereas hydroquinone and p-benzoquinone were identified by reversed-phase HPLC. A reaction sequence for the degradation of 2,4-D by EOx and EF/BDD involving all detected aromatics is proposed.