Electrochemical oxidation by boron-doped diamond (BDD) electrode is an effective method of degrading refractory organics. Compared with TOC detection, the amount of gas escape can more effectively and intuitively reflect the mineralization and the removal extent. In this study, indole is chosen as a typical pollutant and the detection of its removal rate was compared at current densities of 10, 20, and 30 mA·cm-2. Meanwhile, the degradation mechanism was analyzed based on the changes in the carbon and nitrogen forms and conservation status. As a result, BDD electrodes displayed a higher removal efficiency to indole, which can completely be removed after 8 h, 5 h, and 4 h with current densities of 10, 20 and 30 mA·cm-2, respectively. Changes in TOC removal and CO2 generation were both increased with increasing the current densities, suggesting that the mineralization extent was in accordance with current densities. Furthermore, the escaped CO2, combined with TOC and TIC constituted a conservative carbon system. The byproduct isatin was stable and accumulated at 4-5 h, as TOC, TON, and CO2 generation was unchanged at this stage. Finally, the XPS analysis suggested the adsorption by-products such as isatin and benzoquinone on the BDD surface, which can further be removed by increasing the electrolysis time. This study demonstrated the mineralization process of indole based on the escaped gas detection and the changes in the carbon and nitrogen forms, which will increase the understanding of the electrolysis process.
Keywords: boron-doped diamond anode; carbon conservation; gas escape; indole; mineralization; nitrogen form.