Suppressing Methane Production to Boost High-Purity Hydrogen Production in Microbial Electrolysis Cells.

Hydrogen gas (H2) is an attractive fuel carrier due to its high specific enthalpy; moreover, it is a clean source of energy because in the combustion reaction with oxygen (O2) it produces water as the only byproduct. The microbial electrolysis cell (MEC) is a promising technology for producing H2 from simple or complex organics present in wastewater and solid wastes. Methanogens and non-archaeal methane (CH4)-producing microorganisms (NAMPMs) often grow in the MECs and lead to rapid conversion of produced H2 to CH4. Moreover, non-archaeal methane production (NAMP) catalyzed by nitrogenase of photosynthetic bacteria was always overlooked. Thus, suppression of CH4 production is required to enhance H2 yield and production rate. This review comprehensively addresses the principles and current state-of-the-art technologies for suppressing methanogenesis and NAMP in MECs. Noteworthy, specific strategies aimed at the inhibition of methanogenic enzymes and nitrogenase could be a more direct approach than physical and chemical strategies for repressing the growth of methanogenic archaea. In-depth studies on the multiomics of CH4 metabolism can possibly provide insights into sustainable and efficient approaches for suppressing metabolic pathways of methanogenesis and NAMP. The main objective of this review is to highlight key concepts, directions, and challenges related to boosting H2 generation by suppressing CH4 production in MECs. Finally, perspectives are briefly outlined to guide and advance the future direction of MECs for production of high-purity H2 based on genetic and metabolic engineering and on the interspecific interactions.

Electro-activation of peroxymonosulfate by a graphene oxide/iron oxide nanoparticle-doped Ti4O7 ceramic membrane: mechanism of singlet oxygen generation in the removal of 1,4-dioxane.

Electro-activation of peroxymonosulfate (PMS) has been widely investigated for the degradation of organic pollutants. Herein, we employ graphene oxide (GO)/Fe3O4 nanoparticles (NPs) doped into a Ti4O7 reactive electrochemical membrane through strong chemical bonding as the cathode to activate PMS for the degradation of 1,4-dioxane (1,4-D). The strong chemical interaction between GO, Fe3O4-NPs, and Ti4O7 via Fe-O---GO---O-Ti bonds enhances the electron-transfer efficiency and provides catalytically active sites that boost the electro-activation of PMS. As a result, the 1,4-D oxidation rate of the GO/Fe3O4-NPs@Ti4O7 REM cathode is ~3 times higher (7.21 × 10-3 min-1) than those of other Ti4O7 ceramic membranes, and 1O2 plays a key role (59.9%) in the degradation of 1,4-D. The 1O2 generation mechanism in the electro-activation process of PMS was systematically investigated, and we claimed that 1O2 is mainly generated from the precursors H2O2 and O2•-/HO2• rather than by O2 or •OH, as has been reported in previous studies. A flow-through mode test in the PMS electro-activation system is firstly reported, and the 1,4-D decay efficiency is 7.1 times higher than that obtained by a flow-by mode, showing that an improved PMS mass transfer efficiency enhances the conversion to reactive oxygen species.

Interface engineering strategy of a Ti4O7 ceramic membrane via graphene oxide nanoparticles toward efficient electrooxidation of 1,4-dioxane.

Although Ti4O7 ceramic membrane has been recognized as one of the most promising anode materials for electrochemical advanced oxidation process (EAOP), it suffers from relatively low hydroxyl radical (•OH) production rate and high charge-transfer resistance that restricted its oxidation performance of organic pollutants. Herein, we reported an effective interface engineering strategy to develop a Ti4O7 reactive electrochemical membrane (REM) doped by graphene oxide nanoparticles (GONs), GONs@Ti4O7 REM, via strong GONs-O-Ti bonds. Results showed that 1% (wt%) GON doping on Ti4O7 REM significantly reduced the charge-transfer resistance from 73.87 to 8.42 Ω compared with the pristine Ti4O7 REM, and yielded •OH at 2.5-2.8 times higher rate. The 1,4-dioxane (1,4-D) oxidation rate in batch experiments by 1%GONs@Ti4O7 REM was 1.49×10-2 min-1, 2 times higher than that of the pristine Ti4O7 REM (7.51×10-3 min-1) and similar to that of BDD (1.79×10-2 min-1). The 1%GONs@Ti4O7 REM exhibited high stability after a polarization test of 90 h at 80 mA/cm2, and within 15 consecutive cycles, its oxidation performance was stable (95.1-99.2%) with about 1% of GONs lost on the REM. In addition, REM process can efficiently degrade refractory organic matters in the groundwater and landfill leachate, the total organic carbon was removed by 54.5% with a single-pass REM. A normalized electric energy consumption per log removal of 1,4-D (EE/O) was observed at only 0.2-0.6 kWh/m3. Our results suggested that chemical-bonded interface engineering strategy using GONs can facilitate the EAOP performance of Ti4O7 ceramic membrane with outstanding reactivity and stability.

Peroxymonosulfate activation by oxygen vacancies-enriched MXene nano-Co3O4 co-catalyst for efficient degradation of refractory organic matter: Efficiency, mechanism, and stability.

Cobalt-based catalysts have been widely explored in the degradation of organic pollutants based on peroxymonosulfate (PMS) activation. Herein, we report an MXene nano-Co3O4 co-catalyst enriched with oxygen vacancies (Ov) and steadily fixed in nickel foam (NF) plates, which is used as an efficient and stable PMS activator for the removal of 1,4-dioxane (1,4-D). Ti originating from MXene was doped into the Co3O4 crystal, generating large amounts of Ov, which could provide more active sites to enhance PMS activation and facilitate the transformation of Co2+ and Co3+, causing a high stability. As a result, the 1,4-D removal efficiency of the NF/MXene-Co3O4/PMS system (kapp: 2.41 min-1) was about four times higher than that of the NF/Co3O4/PMS system (kapp: 0.62 min-1). In addition, singlet oxygen was the predominant reactive oxygen species. Notably, the 1,4-D removal of the NF/MXene-Co3O4/PMS system was over 95% after 20 h operation in the single-pass filtration mode with only 3.72% accumulative Co leaching, showing excellent stability and reusability of NF/MXene-Co3O4. This work provides a defect engineering strategy to design a robust and stable catalytic system for water treatment, which expands the application of MXene in the field of environmental remediation.