Insights into the Electrolyte Roles of Electrochemically Activated Peroxymonosulfate in a CeO2 Electrode for RhB Degradation

In the electrochemically activated peroxymonosulfate (EC-PMS) process, the electrolyte plays a pivotal role affecting the degradation efficiency, reusability, and cost. However, the function of the electrolyte on active species and degradation pathways remains uncertain to date. To address this, this study explores the degradation of Rhodamine B (RhB) using EC-PMS with a CeO 2 electrode (EC/CeO 2 + PMS) under three electrolytes (H 2 SO 4, Na 2 SO 4, and NaOH). While Na 2 SO 4 was used as the electrolyte, the EC/CeO 2 + PMS system achieved 46.6% RhB removal within 60 min (100 mg/L RhB, 0.5 g/L PMS, and −10 mA/cm 2 ), outperforming standalone PMS (30.8%) or EC (17.7%) systems. Notably, Ce ion leaching was electrolyte-dependent. The value reached 6.2 mg/L in H 2 SO 4, while it remained much lower in Na 2 SO 4 (0.06 mg/L) and NaOH (0.08 mg/L), demonstrating exceptional stability in neutral/alkaline conditions. Further analysis indicated that electrochemical activation could efficiently accelerate the Ce 4+ /Ce 3+ circulation, thereby significantly enhancing the degradation process. Quenching experiments and electron spin resonance studies exhibited that the primary radicals varied depending on the electrolyte. Surface-bound radicals were the dominant reactive oxidation species in the H 2 SO 4 electrolyte, while surface-bound radicals and singlet oxygen ( 1 O 2 ) codominated in the Na 2 SO 4 electrolyte. In contrast, 1 O 2 was the main species in the NaOH electrolyte. Finally, the degradation pathways in different electrolytes were proposed based on the analysis of intermediates. The findings underscore a previously underestimated significance of electrolyte choice in the EC-PMS process and provide valuable understanding for efficient removal of organic pollutants using electrochemical PMS activation.