Electrochemical advanced oxidation of 2-chlorobenzoic acid using BDD or Pt anode and carbon felt cathode

Abstract

Electrochemical advanced oxidation processes, anodic oxidation and electro-Fenton, have been applied to oxidative degradation of 2-chlorobenzoic acid (2-CBA) in acidic aqueous medium by using Pt or boron-doped-diamond (BDD) anode. Electrolyses were conducted in an open and cylindrical cell with a carbon-felt cathode for H2O2 generation. The main oxidizing species is hydroxyl radical (OH) formed at the BDD surface in both processes and in the bulk from the Fenton’s reaction between initially added and then regenerated Fe2+ (catalyst) and electrogenerated H2O2. Decay of 2-CBA concentration followed a pseudo-first order reaction kinetics. The absolute rate constant of the hydroxylation of 2-CBA was determined by using the competition kinetics method and found to be 2.04×109M−1s−1. The comparative study of TOC removal measurements during electro-Fenton treatment showed a higher mineralization rate with BDD anode than Pt anode due to the higher oxidizing power of the former anode. The chlorine present in 2-CBA is completely released in the form of chloride ion at Pt anode, which is slowly oxidized to Cl2 at the BDD anode. Aromatic oxidation intermediates such as 2-chlorophenol, catechol and benzoquinone are identified by reversed-phase liquid chromatography. Oxalic acid is detected by ion-exclusion-chromatography as the main carboxylic acid. This ultimate end product is finally oxidized into CO2 by hydroxyl radicals. Based on the identified intermediates and inorganic end products, and TOC removal results, a plausible mineralization pathway of 2-CBA by electrogenerated hydroxyl radicals was proposed.