Degradation of biorefractory compounds by anodic oxidation on Magnéli phase and electro-Fenton process

Abstract The degradation of Rhodamine B (RhB) was studied using advanced electrochemical oxidation processes (EAOP): electro-Fenton (EF), anodic oxidation (AO) and electro-Fenton coupled with anodic oxidation (EF/AO) on a carbon felt (CF) cathode paired with either a sub-stoichiometric titanium dioxide Magnéli phase (Ti₄O₇) anode or a platinized titanium (Ti/Pt) anode. The study revealed that operating conditions significantly impacted the kinetics of electrochemical reactions. Increasing the current density from 10 to 50 mA cm⁻ ² significantly accelerated the degradation of RhB, with 30 mA cm⁻ ² identified as the optimal current density, balancing energy consumption and degradation efficiency. The RhB concentration also influenced the degradation kinetics: higher concentrations required longer treatment times. The EF/AO coupling process proved particularly effective in degrading higher concentrations of RhB. Toxicity evaluation using the Microtox® bioluminescence inhibition test showed reduced toxicity during the electrochemical degradation of the pollutant, with EF/AO presenting the most significant reduction. The EF/AO coupling also demonstrated superior total organic carbon (TOC) removal, reaching 90% at pH 3 compared to EF and AO alone, suggesting efficient mineralization of RhB and its by-products. Energy consumption (EC) remained relatively stable across all processes during a 480 min electrolysis period. High-resolution mass spectrometry confirmed the degradation pathways of RhB, involving chain oxidation reactions leading to the formation of intermediate products and mineralization to CO₂ and H₂O. This study demonstrates the potential of EF, AO and EF/AO coupling as effective methods for mineralizing Rhodamine B (RhB) from wastewater, contributing to sustainable and environmentally friendly wastewater treatment strategies.