Comparative electrochemical decolorization of methylene blue through direct and mediated oxidation: Effects of electrode configuration, pH, redox mediators, and chloride electrolyte

This study comparatively evaluates direct and mediated electrochemical decolorization of methylene blue (MB), focusing on electrode configuration, solution pH, redox mediators, and chloride concentration. Batch experiments were performed with 30 mL of 5 mg/L MB at an applied cell voltage of 5 V and 25 ± 2 °C. Pt- and Al-based anode configurations were examined, while Fe²⁺/Fe³ ⁺ and Co²⁺/Co³ ⁺ systems (initial mediator concentration 5 × 10⁻⁷ M) and KCl (0–0.20 M) were evaluated as indirect oxidation approaches. Under near-neutral conditions, the Al-anode configuration reached 55.5% decolorization after 330 min, whereas the Pt-anode configuration reached 33.9% after 210 min. At pH 10, the Pt configuration achieved 99.5% decolorization after 330 min, while the Al configuration reached 21.6% after 210 min. In mediator-assisted experiments, the Co-containing system produced 90.56% decolorization after 450 min compared with 76.38% after 570 min for the Fe-containing system. KCl produced the strongest kinetic enhancement: 0.10 M KCl yielded 99.49% decolorization in 30 min and 0.20 M yielded 99.14% in 20 min, concomitant with a marked increase in solution conductivity. Cyclic voltammetry showed disappearance of the characteristic MB/leuco-MB redox response near −0.5 V vs SCE after electrolysis, supporting loss of the electroactive MB chromophore. The results show that electrode configuration, pH, mediator chemistry, and chloride concentration strongly influence MB decolorization. Because reactive intermediates and mineralization products were not directly quantified, the proposed oxidation pathways are presented as mechanistic interpretations consistent with the observed electrochemical and spectroscopic behavior.