Electrooxidation of Phenol on Boron-doped Diamond and Mixed-metal Oxide Anodes: Process Evaluation, Transformation By-products, and Ecotoxicity

Abstract

Phenolic pollutants raise health and environmental concerns due to their widespread occurrence in industrial wastewaters. Electrooxidation was studied for phenol degradation in different supporting electrolytes: NaCl, Na2SO4, H2SO4. Experiments were performed at constant current density 20 mA cm−2. Two anode materials were tested ‒ boron-doped diamond (BDD) and mixed-metal oxide (MMO). Degradation process and its impact was investigated from technological, analytical, and ecotoxicological viewpoints. Removal efficiency was monitored by phenol index spectrophotometric method and phenol removal quantified with HPLC-DAD. Additionally, transformation by-products were tracked with GC-MS and LC-MS, as well as ion chromatography. Finally, ecotoxicity was investigated using Daphnia magna. Electrooxidation was efficient and had low energy consumption. The use of BDD anode led to higher removal efficiencies and induced more progressive degradation to simple organic acids, compared to MMO. Selection of electrolyte affected degradation pathways and detoxification pattern. Treatment by BDD in NaCl led to complete phenol removal in 30 min, but undesired chlorinated aromatic by-products were formed. Treatment in sulphate medium led to slower processes irrespective of pH, but less problematic by-products with minimal ecotoxicological impact emerged. By using multi-aspect methodology, this study reevaluates phenol electrooxidative degradation and contributes to better understanding of electrooxidation performance in water treatment.