Revealing Decay Mechanisms of H2O2-based Electrochemical Advanced Oxidation Processes after Long-term Operation for Phenol Degradation.
Hydrogen peroxide (H2O2) based electrochemical advanced oxidation processes (EAOPs) have been widely attempted for various wastewater treatments. So far, stability tests of EAOPs are rarely addressed and the decay mechanism is still unclear. Here three H2O2-based EAOP systems (electro-Fenton, photo-electro-Fenton and photo-electro-generated H2O2) were built for phenol degradation. More than 97 % of phenol were removed in all three EAOPs in 1 h at 10 mA·cm-2. As a key component in EAOPs, the cathodic H2O2 productivity is directly related to the performance of the system. We for the first time systematically investigated the decay mechanisms of the active cathode by operating the cathodes under multiple conditions over 200 hours. Compared with fresh cathode, (H2O2 yield 312  22 mg·L-1·h-1 with current efficiency 84  5% at 10 mA·cm-2), the performance of cathode for H2O2 synthesis alone decayed only 17.8%. Whereas the H2O2 yields of cathodes operated in photo-electro-generated H2O2, electro-Fenton and photo-electro-Fenton systems decayed by 60.0%, 90.1% and 89.6%, respectively with the synergic effect of the salt precipitation, ·OH erosion, organic contamination and optional Fe contamination. The lower current decay of 16.1-32.3% in the electrochemical tests manifested the cathodes did not lost activity severely. Therefore, the significant decrease of H2O2 yield was because the active sites were altered to catalyze four-electron oxygen reduction reaction, which was induced by the long-term erosion of ·OH. Our findings provided a new insight of cathode performance decay, offering significant information for the improvement of cathodic longevity in the future.