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A simultaneous partial nitrification, anammox and denitrification(SNAD) micro-aerobic system was constructed. The effects of different particle sizes of ZVI on the micro-aerobic system were investigated from the aspects of operation, microbial performance and community structure by adding 120 mg of zero-valent iron(ZVI) into the system. The results showed that the TIN removal rate of the system stabilized at 83.591% and the electron transport system activity(ETSA) was 1.48 times of the inoculated sludge under the domestication of ZVI with a partical size of 50 nm (50 nZVI), which was higher than the influence of other particle sizes of ZVI. The smaller the particle size, the more production of extracellular polymers promoted by ZVI, the more stable of the activated sludge structure. This was due to the fact that ZVI of smaller particle size was more easily adsorbed and utilized by microorganisms. The results of microbial community analysis showed that the smaller the particle size, the higher the diversity of the microbial community mediated by ZVI, and the more complex the community structure. ZVI mediation promoted the enrichment of microorganisms that were not sensitive to oxygen demand. Candidatus_Brocadia and Candidatus_Kuenenia, two kinds of AnAOB were the main bacteria detected in the samples with the highest abundance of 4.650% and 0.692% under the influence of 50 nZVI. Under nZVI, AnAOB acted together with two facultatively anaerobic denitrifying bacteria, Gemmobacter and norank_f_Comamonadaceae, which constituted the denitrification pathway of SNAD. In contrast, facultatively anaerobic denitrification was the main denitrification pathway mediated by mZVI.

Anaerobic digestion of waste-activated sludge (WAS) towards biogas recovery is constrained by the limited hydrolysis and inhibited acetogenesis steps that hinder subsequent energy recovery. This study employed Fe(VI)/S(IV) oxidation to enhance the WAS solubilization and coupled it with the syntrophic interaction of hydrogen-producing acetogen (HPA) and sulfate-reducing bacteria (SRB) to stimulate the successive procedure towards methane production. Results showed that the dosage ratio of HPA-SRB to WAS (H-S-W) with 1:1:50 outperformed with the highest methane production potential (11.63 ± 1.87 mL CH4/(g VSS·d). Meanwhile, the efficient and sequential process from acetogenesis to methanogenesis stimulated by HPA-SRB was evidenced by a significant decrease of 30.2% in the acetate concentration. The microbial community structure further manifested the crucial role of HPA-SRB with increased abundance of Desulfobulbus (2.07%), Syntrophomonas (1.24%) and Smithella (1.63%), which stimulated acetophilic methanogen boost with Methanobacterium dominating with 77.51% in H-S-W100. Furthermore, the positive syntrophic relationships among HPA-SRB and acetophilic methanogens towards methane production were confirmed via molecular ecological network and canonical correspondence analysis. This study highlighted the syntrophic cooperation of the mixed consortia of HPA and SRB on methane production based on Fe(VI)/S(IV) pretreatment and provided the theoretical and technical basis for the potential implementation of novel methanogenesis technology for WAS treatment. [ABSTRACT FROM AUTHOR]

The Volmer step in alkaline hydrogen evolution reactions (HERs), which supplies H* to the following steps by cleaving H–O–H bonds, is considered the rate-determining step of the overall reaction. The Volmer step involves water dissociation and adsorbed hydroxyl (*OH) desorption; Ru-based catalysts display a compelling water dissociation process in an alkaline HER. Unfortunately, the strong affinity of Ru for *OH blocks the active sites, resulting in unsatisfactory performance during HER processes. Hence, this study investigates a series of key descriptors (ΔG*H2O, ΔG*H–OH, ΔG*H, and ΔG*OH) of TM (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, or Pt)–Ru/Mo2Ti2C3O2 to systematically explore the effects of bimetallic site interactions on the kinetics of the Volmer step. The results indicate that bimetallic catalysts effectively reduced the strong adsorption of *OH on Ru sites; especially, the NiRu diatomic state shows the highest electron-donating ability, which promoted the smooth migration of *OH from Ru sites to Ni sites. Therefore, Ru, Ni and MXenes are suitable to serve as water adsorption and dissociation sites, *OH desorption sites, and H2 release sites, respectively. Ultimately, NiRu/Mo2Ti2C3O2 promotes Volmer kinetics and has the potential to improve alkaline HERs. This work provides theoretical support for the construction of synergistic MXene-based diatomic catalysts and their wide application in the field of alkaline HERs. [ABSTRACT FROM AUTHOR]

To break the bottlenecks of slow hydrolysis and low acid production efficiency of waste-activated sludge (WAS) in the traditional anaerobic fermentation process, this study investigated the employment of ferrous-iron (Fe(II))-activated sulfite to produce hydroxyl, sulfate, and other highly oxidizing radicals on WAS floc cracking and short-chain fatty acid (SCFAs) production during anaerobic fermentation. The effect of the dosage ratio of Fe(II)/S(IV) was also studied. Results showed that the combined pretreatment of Fe(II)-activated sulfite significantly promoted the exfoliation of extracellular polymers and the subsequent SCFAs production. The highest concentration of SCFAs reached 7326.5 mg COD/L under the optimal dosage of 1:2 for Fe(II)/S(IV), which was 1.1~2.1 times higher than that of other research groups. Meanwhile, the analysis by 3D fluorescence spectroscopy and EPR (electron paramagnetic resonance) showed that Fe(II)-activated sulfite had a synergistic effect on the rupture of sludge cells and the stripping of extracellular polymers, with SO4− and OH as the key radicals generated and being much stronger in the 1:1 and 1:2 groups. High-throughput sequencing showed that the Fe(II)-activated sulfite system significantly changed the functional microbial diversity. The anaerobic fermentation bacteria and sulfate-reducing bacteria were significantly enriched. The underlying mechanism of Fe(II)-activated sulfite oxidation and molecular ecological network of key microbiomes were unveiled. [ABSTRACT FROM AUTHOR]