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Nitrate-N in wastewaters is hard to be recovered because it is difficult to volatilize with an opposite charge to ammonium. Here, we have proved the feasibility of dissimilatory nitrate reduction to ammonia (DNRA) by the easy-acclimated mixed electroactive bacteria, achieving the highest DNRA efficiency of 44%. It was then coupled with microbial electrolysis to concentrate ammonium by a factor of 4 in the catholyte for recovery. The abundance of electroactive bacteria in the biofilm before nitrate addition, especially Geobacter spp., was found to determine the DNRA efficiency. As the main competitors of DNRA bacteria, the growth of denitrifiers was more sensitive to C/N ratios. The DNRA microbial community contrarily showed a stable and recoverable ammoniation performance over C/N ratios ranging from 0.5 to 8.0. A strong competition of the electrode and nitrate on electron donors was observed at the early stage (15 d) of electroactive biofilm formation, which can be weakened when the biofilm was mature on 40 d. Quantitative PCR showed a significant increase in nirS and nrfA transcripts in the ammoniation process. nirS was inhibited significantly after nitrate depletion while nrfA was still upregulated. These findings provided a novel way to recover nitrate-N using organic wastes as both electron donor and power, which has broader implications on the sustainable wastewater treatment and the ecology of nitrogen cycling.

Electrochemical catalysis of carbon-based material via two-electron pathway oxygen reduction reaction (ORR) offers great potential for in situ hydrogen peroxide (H2O2) production. In this work, we tuned catalyst mesostructure and hydrophilicity/hydrophobicity by adjusting polytetrafluoroethylene (PTFE) content in graphite/carbon black/PTFE hybrid catalyst layer (CL), aimed to improving the two-electron ORR activity for efficient H2O2 generation. As the only superhydrophobic CL with initiating contact angles of 141.11°, PTFE0.57 obtained the highest H2O2 yield of 3005 ± 58 mg L–1 h–1 (at 25 mA cm–2) and highest current efficiency (CE) of 84% (at 20 mA cm–2). Rotating ring disk electrode (RRDE) results demonstrated that less PTFE content in CLs results in less electrons transferred and better selectivity toward two-electron ORR. Though the highest H2 concentration (2 μmol L–1 at 25 mA cm–2) was monitored from PTFE0.57 which contained the lowest PTFE, the CE decreased inversely with increasing content of PTFE, which proved that the H2O2 decomposition reaction was the major side reaction. Higher PTFE content increased the hydrophilicity of CL for excessive H+ and insufficient O2 diffusion, which induced H2O2 decomposition into H2O. Simultaneously, the electroactive surface area of CLs decreased with higher PTFE content, from 0.0041 m2 g–1 of PTFE0.57 to 0.0019 m2 g–1 of PTFE4.56. Besides, higher PTFE content in CL leads to the increase of total impedance (from 14.5 Ω of PTFE0.57 to 18.3 Ω of PTFE4.56), which further hinders the electron transfer and ORR activity.

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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, photoelectro-Fenton, and photo+ electro-generated H2O2) were built for phenol degradation. More than 97% phenol was 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 h. Compared with the fresh cathode (H2O2 yield of 312 ± 22 mg·L–1·h–1 with a current efficiency of 84 ± 5% at 10 mA·cm–2), the performance of the cathode for H2O2 synthesis alone decayed by only 17.8%, whereas the H2O2 yields of cathodes operated in photoelectro-generated H2O2, electro-Fenton, and photoelectro-Fenton systems decayed by 60.0, 90.1, and 89.6%, respectively, with the synergistic effect of salt precipitation, •OH erosion, organic contamination, and optional Fe contamination. The lower current decay of 16.1–32.3% in the electrochemical tests manifested that the cathodes did not lose activity severely. Therefore, the significant decrease of H2O2 yield was because the active sites were altered to catalyze the four-electron oxygen reduction reaction, which was induced by the long-term erosion of •OH. Our findings provided new insights into cathode performance decay, offering significant information for the improvement of cathodic longevity in the future.