Enhanced microbial nitrate reduction using natural manganese oxide ore as an electron donor

Algae promotes the biogenic oxidation of Mn(II) by accelerated extracellular superoxide production

Microbial manganese (Mn) oxidation, predominantly occurs within the anaerobic-aerobic interfaces, plays an important role in environmental pollution remediation. The anaerobic-aerobic transition zones, notably riparian and lakeside zones, are hotspots for algae-bacteria interactions. Here, we adopted a Mn(II)-oxidizing bacterium Pseudomonas sp. QJX-1 to investigate the impact of algae on microbial Mn(II) oxidation and verify the underlying mechanisms. Interestingly, we achieved a remarkable enhancement in bacterial Mn(II)-oxidizing activity within the algae-bacteria co-culture, despite the inability to oxidize Mn(II) for the algae used in this study. In addition, the bacterial density almost remains constant in the presence of algal cells. Therefore, the increased Mn(II) oxidation by QJX-1 in the presence of algae cannot be due to the increased biomass. Within this co-culture system, the Mn(II) oxidation rate surged to an impressive 0.23 mg/L/h, in stark contrast to 0.02 mg/L/h recorded within pure QJX-1 system. The presence of algae could inhibit the Fe-S cluster activity of QJX-1 by the produced active substance in co-culture, and result in the acceleration of extracellular superoxide production due to the impairment of electron transfer functions located in QJX-1 cell membranes. Moreover, elevated peroxidase gene expression and heightened extracellular catalase activity not only expedited Mn(II) ions oxidation but also facilitated conversion of intermediate Mn(III) ions into microbial Mn oxides, achieved through the degradation of hydrogen peroxide. Therefore, the acceleration of extracellular superoxide production and the decomposition of hydrogen peroxide are identified as the principal mechanisms behind the observed enhancement in Mn(II) oxidation within algae-bacteria co-cultures. Our findings highlight the need to consider the effect of algae on microbial Mn(II) oxidation, which plays an important role in the environmental pollution remediation.

New insights into substrates shaped nutrients removal, species interactions and community assembly mechanisms in tidal flow constructed wetlands treating low carbon-to-nitrogen rural wastewater

A limited understanding of microbial interactions and community assembly mechanisms in constructed wetlands (CWs), particularly with different substrates, has hampered the establishment of ecological connections between micro-level interactions and macro-level wetland performance. In this study, CWs with distinct substrates (zeolite, CW_A; manganese ore, CW_B) were constructed to investigate the nutrient removal efficiency, microbial interactions, metabolic mechanisms, and ecological assembly for treating rural sewage with a low carbon-to-nitrogen ratio. CW_B showed higher removal of ammonia nitrogen and total nitrogen by about 1.75-6.75 % and 3.42-5.18 %, respectively, compared to CW_A. Candidatus_Competibacter (denitrifying glycogen-accumulating bacteria) was the dominant microbial genus in CW_A, whereas unclassified_f_Blastocatellaceae (involved in carbon and nitrogen transformation) dominated in CW_B. The null model revealed that stochastic processes (drift) dominated community assembly in both CWs; however, deterministic selection accounted for a higher proportion in CW_B. Compared to those in CW_A, the interactions between microbes in CW_B were more complex, with more key microbes involved in carbon, nitrogen, and phosphorus conversion; the synergistic cooperation of functional bacteria facilitated simultaneous nitrification-denitrification. Manganese ores favour biofilm formation, increase the activity of the electron transport system, and enhance ammonia oxidation and nitrate reduction. These results elucidated the ecological patterns exhibited by microbes under different substrate conditions thereby contributing to our understanding of how substrates shape distinct microcosms in CW systems. This study provides valuable insights for guiding the future construction and management of CWs.

Spatial confinement: A green pathway to promote the oxidation processes for organic pollutants removal from water

Organic pollutants removal from water is pressing owing to the great demand for clean water. Oxidation processes (OPs) are the commonly used method. However, the efficiency of most OPs is limited owing to the poor mass transfer process. Spatial confinement is a burgeoning way to solve this limitation by use of nanoreactor. Spatial confinement in OPs would (i) alter the transport characteristics of protons and charges; (ii) bring about molecular orientation and rearrangement; (iii) cause the dynamic redistribution of active sites in catalyst and reduce the entropic barrier that is high in unconfined space. So far, spatial confinement has been utilized for various OPs, such as Fenton, persulfate, and photocatalytic oxidation. A comprehensive summary and discussion on the fundamental mechanisms of spatial confinement mediated OPs is needed. Herein, the application, performance and mechanisms of spatial confinement mediated OPs are overviewed firstly. Subsequently, the features of spatial confinement and their effects on OPs are discussed in detail. Furthermore, environmental influences (including environmental pH, organic matter and inorganic ions) are studied with analyzing their intrinsic connection with the features of spatial confinement in OPs. Lastly, challenges and future development direction of spatial confinement mediated OPs are proposed.

In Situ/Operando Methods for Understanding Electrocatalytic Nitrate Reduction Reaction

With the development of industrial and agricultural, a large amount of nitrate is produced, which not only disrupts the natural nitrogen cycle, but also endangers public health. Among the commonly used nitrate treatment techniques, the electrochemical nitrate reduction reaction (eNRR) has attracted extensive attention due to its mild conditions, pollution-free nature, and other advantages. An in-depth understanding of the eNRR mechanism is the prerequisite for designing highly efficient electrocatalysts. However, some traditional characterization tools cannot comprehensively and deeply study the reaction process. It is necessary to develop in situ and operando techniques to reveal the reaction mechanism at the time-resolved and atomic level. This review discusses the eNRR mechanism and summarizes the possible in situ techniques used in eNRR. A detailed introduction of various in situ techniques and their help in understanding the reaction mechanism is provided. Finally, the current challenges and future opportunities in this research area are discussed and highlighted.

Advances in iron-based electrocatalysts for nitrate reduction

Excessive nitrate has been a critical issue in the water environment, originating from the burning of fossil fuels, inefficient use of nitrogen fertilizers, and discharge of domestic and industrial wastewater. Among the effective treatments for nitrate reduction, electrocatalysis has become an advanced technique because it uses electrons as green reducing agents and can achieve high selectivity through cathode potential control. The effectiveness of electrocatalytic nitrate reduction (NO₃RR) mainly lies in the electrocatalyst. Iron-based catalysts have the advantages of high activity and low cost, which are well-used in the field of electrocatalytic nitrates. A comprehensive overview of the electrocatalytic mechanism and the iron-based materials for NO₃RR are given in terms of monometallic iron-based materials as well as bimetallic and oxide iron-based materials. A detailed introduction to NO₃RR on zero valent iron, single-atom iron catalysts, and Cu/Fe-based bimetallic electrocatalysts are provided, as they are essential for the improvement of NO₃RR performance. Finally, the advantages of iron-based materials for NO₃RR and the problems in current applications are summarized, and the development prospects of efficient iron-based catalysts are proposed.

Effects of magnetite on microbially driven nitrate reduction processes in groundwater

Nitrate is a common pollutant in the aquatic environment. Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are the main reduction processes of nitrate. In the relatively closed sediment environment, the competitive interaction of these two nitrate reduction determines whether the ecosystem removes or retains nitrogen. In the process of NO₃^--N bioreduction, Magnetite, which is a common mineral present in soil and other sediments can play a crucial role. However, it is still not clear whether magnetite promotes or inhibits NO₃^--N bioreduction. In this paper, the effect of magnetite on NO₃^--N bioreduction was studied by batch experiments. The results show that magnetite can increase the NO₃^--N reduction rate by 1.48 %, and can inhibit the DNRA process at the beginning of the reaction and then promote the DNRA process. Magnetite changed the microbial community structure in our experiment systems. The relative abundance of Sphingomonas, which mainly exists in a high carbon and low nitrogen environment, increased under sufficient carbon source conditions. The relative abundance of Fe-oxidizing and NO₃^--N reducing bacteria, such as Flavobacterium, increased in the absence of carbon sources but in the presence of magnetite. In addition, magnetite can significantly increase activity of the microbial electron transport system (ETS). the added microbial electronic activity of magnetite increased nearly two-fold under the same experiment conditions. The acid produced by the metabolisms of Pseudomonas and Acinetobacter further promotes the dissolution of magnetite, thus increasing the concentration of Fe (II) in the system, which is beneficial to autotrophic denitrifying bacteria and promote the reduction of NO₃^--N. These findings can enhance our understanding of the interaction mechanism between iron minerals and nitrate reducing bacteria during nitrate reduction under natural conditions.

Intensifying anoxic ammonium removal by manganese ores and granular active carbon fillings in constructed wetland-microbial fuel cells: Metagenomics reveals functional genes and microbial mechanisms

The conventional biological ammonium removal process is challenged for lack of electron acceptors. A lab-scale integrated constructed wetland coupled with microbial fuel cells (CW-MFC) filling manganese ores (MO) and granular active charcoal (GAC) has been developed, named CW-CM. It enhanced the nitrogen removal two times over the control. A metagenomic-based study illustrated the functional genes and taxonomic groups related to N transformations, explored metabolic mechanisms of nitrogen and carbon sources, and then revealed some characteristics of the extracellular electron transfer (EET). Many nitrifying bacteria and autotrophic and heterotrophic denitrifiers were enriched in CW-CM. Furthermore, most nitrification and denitrification reactions except for the conversion of ammonium to hydroxylamine were significantly enhanced in CW-CM. Glycolysis and the TCA cycle were also improved. Overall, a novel anoxic ammonia removal process was achieved in the experimental group with no need of anammox functional bacteria and anammox key genes.