Electrodes Donate Electrons for Nitrate Reduction in a Soil Matrix via DNRA and Denitrification

Shallow groundwater table fluctuations weaken nitrogen accumulation in the thin layer vadose zone of cropland around plateau lakes, Southwest China

Excessive accumulation of nitrogen (N) in the soil profile in the intensive agricultural region will seriously threaten groundwater quality and safety. However, the impact of shallow groundwater table (SGWT) fluctuations driven by seasonal variations on the N accumulation characterizations in the soil profiles has not been well quantified, particularly in the regions with thin layer vadose zone. Through in-situ monitoring and simulation experiments, the changes in the SGWT and N accumulation of soil profile in intensive cropland around 7 plateau lakes in Yunnan were studied during the rainy season (RS) and dry season (DS), and the N loss in soil profile of cropland driven by SGWT fluctuations was estimated. The results showed that the SGWT and N accumulation in soil profile of cropland around the plateau lakes had obvious seasonal variation characteristics. The proportion of N storage in different forms in 60-100 cm soil layer in the RS was greater than that in the DS, particularly the proportion of NH4+-N storage was as high as 55 %, while N accumulation in surface soil was obvious in the DS. Compared with the DS, due to the rising SGWT in the RS, the maximum storages of TN and NO3--N in the 0-100 cm soil layer decreased by17% and 36 %, respectively. The TN loss intensities from the 0-100 cm soil profiles of cropland around Fuxian Lake, Yilong Lake, Qilu Lake, Dianchi Lake, Yangzong Lake, Erhai Lake, and Xingyun Lake were 74, 54, 127, 105, 93, 72 and 207 kg/ha, respectively. Moreover, if the SGWT was <30 cm, the average TN loss intensity and amount could reach 177 kg/ha and 1250 t, respectively. Therefore, the SGWT regulation was one of the key measures to reducing soil N loss from the thin layer vadose zone of cropland around plateau lakes and improving groundwater quality.

NH2-MIL-101(Fe)-mediated photo-Fenton reaction enhanced simultaneous removal of nitrogen and refractory organics in anammox process through interfacial electron transfer

In this study, H2O2 (0.1 ‰) and NH2-MIL-101(Fe)-driven (150 mg/L) photo-Fenton-coupled anammox were proposed to simultaneously improve the removal efficiency of nitrogen and humic acid. Long-term experiments showed that the total nitrogen removal efficiency was increased by the photo-Fenton reaction to 91.9 ± 1.5 % by altering the bioavailability of refractory organics. Correspondingly, the total organic carbon removal efficiency was significantly increased. Microbial community analyses indicated that Candidatus_Brocadia maintained high activity during photo-Fenton reaction and was the most abundant genus in the reactor. Dissimilatory nitrate reduction to ammonium process and denitrification process were enhanced, resulting in reduced NO3--N production. The establishment of electron transfer between microorganisms and NH2-MIL-101 (Fe) improved the charge separation efficiency of the quantum dots and increased the intracellular adenosine triphosphate content of anammox bacteria. These results indicated that photo-Fenton-anammox process promoted the removal of nitrogen and refractory organics in one reactor which had good economic value and application prospects.

Anammox Coupled with Photocatalyst for Enhanced Nitrogen Removal and the Activated Aerobic Respiration of Anammox Bacteria Based on cbb3-Type Cytochrome c Oxidase

This study introduced photogenerated electrons into the anammox system by coupling them to a g-C3N4 nanoparticle photocatalyst. A high nitrogen removal efficiency (94.25%) was achieved, exceeding the biochemical limit of 89% imposed by anammox stoichiometry. Photogenerated electrons boosted anammox metabolic activity by empowering key enzymes (NIR, HZS, and WLP-related proteins) and triggered rapid algal enrichment by enhancing the algal Calvin cycle, thus developing multiple anammox-algae synergistic nitrogen removal processes. Remarkably, the homologous expression of cbb3-type cytochrome c oxidase (CcO) in anammox bacteria was discovered and reported in this study for the first time. This conferred aerobic respiration capability to anammox bacteria and rendered them the principal oxygen consumer under 7.9-19.8 mg/L dissolved oxygen, originating from algal photosynthesis. Additionally, photogenerated electrons selectively targeted the cb1 complex and cbb3-type CcO as activation sites while mobilizing the RegA/B regulatory system to activate the expression of cbb3-type CcO. Furthermore, cbb3-type CcO blocked oxidative stress in anammox by depleting intracellular oxygen, a substrate for reactive oxygen species synthesis. This optimized the environmental sensitivity of anammox bacteria and maintained their high metabolic activity. This study expands our understanding of the physiological aptitudes of anammox bacteria and provides valuable insights into applying solar energy for enhanced wastewater treatment.

Microbial biofilm-based hydrovoltaic technology

Hydrovoltaic electricity generation (HEG) utilizes the latent environmental heat stored in water, and subsequently harvests the electrical energy. However, sustainable HEG has remained extremely challenging due either to complex fabrication and high cost, or to restricted environmental compatibility and renewability. Electroactive microorganisms are environmentally abundant and viable in performing directional electron transport to produce currents. These distinctive features have inspired microbial HEG systems that can convert environmental energy into hygroelectricity upon water circulation from raindrops, waves, and water moisture, and has recently succeeded as proof of concept for becoming a cutting-edge biotechnology. In this review, recent advances in microbial biofilm-based hydrovoltaic technology are highlighted to better understand a promising method of electricity generation from environmental energy with the aim of practical applications.

Sunlight Significantly Enhances Soil Denitrification via an Interfacial Biophotoelectrochemical Pathway

Denitrification is an essential step of the nitrogen cycle in soil. However, although sunlight is an important environmental factor for soil, the investigation of the influence of sunlight on soil denitrification is limited to plant photosynthesis-mediated processes. Herein, a new pathway, denoted as a biophotoelectrochemical process, which is induced by the direct photoexcitation of soil, was found to greatly enhance soil denitrification. Using red soil as the research object, the soil with irradiation showed nitrate reduction that was 2.6-4.7 times faster than that without irradiation. The irradiation of soil accelerated the reduction of nitrite and enhanced the conversion of nitrous oxide to nitrogen, indicating that more electron sources were generated. This resulted from the photoinduced generation of ferrous substrates and photoelectrons. The contribution of irradiation to soil denitrification was almost half (45.4%), of which 30.9% was from photoinduced ferrous substrates and 14.5% was from photoelectrons. Moreover, a designed biophotoelectrochemical cell provided solid evidence for direct photoelectron transfer from soil photosensitive substrates to microorganisms. Irradiation promoted the enrichment of Alicyclobacillus, which participates in iron oxidation and electroautotrophy. This finding reveals a role of sunlight in soil denitrification that has been thus seriously overlooked and provides solid evidence for the natural occurrence of photoelectrotrophic effects.

Dissimilatory nitrate reduction to ammonium in four Pseudomonas spp. under aerobic conditions

Dissimilatory nitrate reduction to ammonium (DNRA) has an important role in soil nitrogen retention and is considered to be constrained to anaerobic conditions. However, a recent study found that Pseudomonas putida Y-9 is capable of DNRA under aerobic conditions. In this study, four species of Pseudomonas spp. were found to produce ammonium during the nitrite reduction process under aerobic conditions, similar to the Y-9 strain. The detectable ammonium in the culture supernatant during the nitrite reduction process for each of the four strains originated intracellularly. A subsequent ¹⁵N isotope experiment showed that these four strains were able to transform ¹⁵NO₂ ^- to ¹⁵NH₄ ⁺ in 3 h under aerobic conditions. The NirBD sequence in each of the four strains showed high similarity with that in the Y-9 strain (approximately 94.61%). Moreover, the nirBD sequences in the four strains and the Y-9 strain were all similar to those of other Pseudomonas spp., while they were relatively distant in terms of their phylogenetic relationship from those of other genera. Overall, these results suggest that these four strains of Pseudomonas spp. are capable of DNRA under aerobic conditions, which might be attributed to the existence of nirBD.

Effects of carbon load on nitrate reduction during riverbank filtration: Field monitoring and batch experiment

Riverbank filtration (RBF) is a well-established technique worldwide, and is critical for the maintenance of groundwater quality and production of clean drinking water. Evaluation of the decay of exogenous nitrate (NO₃^-) in river water and the enrichment of ammonium (NH₄⁺) in groundwater during RBF is important; these two processes are mainly influenced by denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) controlled by the groundwater carbon load. In this study, the effects of carbon load (organic carbon [OC]: NO₃^-) on the competing nitrate reduction (DNRA and DNF) were assessed during RBF using field monitoring and a laboratory batch experiment. Results show the groundwater OC: NO₃^- ratio did not directly affect the reaction rate of DNRA and DNF, however, it could control the competitive partitioning between the two. In the near-shore zone, the groundwater OC: NO₃^- ratio shows significant seasonal variations along the filtration path owing to the changing conditions of redox, OC-rich, and NO₃^--limited. A greater proportion of NO₃^- would be available for DNRA in the wet season with higher OC: NO₃^- ratio (> 10), resulting in a significantly NH₄⁺-N enrichment rate (from 1.43 × 10^-3 to 9.54 × 10^-4 mmol L^-1 d^-1) in the near-shore zone where the zone of Mn (IV) oxide reduction. However, the activity of DNRA was suppressed with lower OC: NO₃^- ratio (< 10) in the dry season, resulting in a stable NH₄⁺-N enrichment rate (from 3.12 × 10^-4 to 1.30 × 10^-4 mmol L^-1 d^-1). Benefiting from seasonal variation of OC-rich and NO₃^--limited conditions, DNRA bacteria outcompeted denitrifiers, which eventually led to seasonal differences in NO₃^- reduction in the near-shore zone. Overall, under the effect of DNRA induced by continuous high carbon load in RBF systems, nitrogen input is not permanently removed but rather retained in groundwater during RBF.

Nitrate removal by combining chemical and biostimulation approaches using micro-zero valent iron and lactic acid

The occurrence of nitrate is the most significant type of pollution affecting groundwater globally, being a major contributor to the poor condition of water bodies. This pollution is related to livestock-agricultural and urban activities, and the nitrate presence in drinking water has a clear impact on human health. For example, it causes the blue child syndrome. Moreover, the high nitrate content in aquifers and surface waters significantly affects aquatic ecosystems since it is responsible for the eutrophication of surface water bodies. A treatability test was performed in the laboratory to study the decrease of nitrate in the capture zone of water supply wells. For this purpose, two boreholes were drilled from which groundwater and sediments were collected to conduct the test. The goal was to demonstrate that nitrate in groundwater can be decreased much more efficiently using combined abiotic and biotic methods with micro-zero valent iron and biostimulation with lactic acid, respectively, than when both strategies are used separately. The broader implications of this goal derive from the fact that the separate use of these reagents decreases the efficiency of nitrate removal. Thus, while nitrate is removed using micro-valent iron, high concentrations of harmful ammonium are also generated. Furthermore, biostimulation alone leads to overgrowth of other microorganisms that do not result in denitrification, therefore complete denitrification requires more time to occur. In contrast, the combined strategy couples abiotic denitrification of nitrate with biostimulation of microorganisms capable of biotically transforming the abiotically generated harmful ammonium. The treatability test shows that the remediation strategy combining in situ chemical reduction using micro-zero valent iron and biostimulation with lactic acid could be a viable strategy for the creation of a reactive zone around supply wells located in regions where groundwater and porewater in low permeability layers are affected by diffuse nitrate contamination.

Early activated quorum sensing enhanced a nosZ-deficient strain of Pseudomonas aeruginosa for stably recovering nitrous oxide from incineration leachate in microbial electrolysis cell

In this study, a quorum sensing (QS) signal molecule, 3-oxo-C12-HSL, was supplied into ΔnosZ Pseudomonas microbial electrolysis cell system for strengthening the N₂O recovery from incineration leachate. The resistance for high nitrite loading was strongly improved with nitrite removal efficiency of 80.35% compared to 67.07% from the control on day 13 due to the increasing biomass through early activated QS. Higher N₂O proportion in biogas (85.85% on average) was achieved in the QS early activated reactor, which indicated the better potential for N₂O recovery. Bacterial community analysis showed the purity of ΔnosZ strain with the abundance of 100% in the anode chamber at the end of the operation. This was plausibly related to the increased synthesis of phenazine derivatives by the early activated QS system. These results show a more promising way for N₂O recovery by a single engineering bacteria from the high nitrogen contained actual wastewater.

Shallow groundwater fluctuation: An ignored soil N loss pathway from cropland

Nitrogen (N) pollution originating from agricultural land is among the major threats to shallow groundwater (SG). Soil N losses due to the SG table fluctuation are neglected, although a large number of studies have been conducted to evaluate N losses through leaching and runoff. Herein, the characteristics of N losses driven by SG table fluctuation were investigated using the microcosm experiment and surveyed data from the croplands around Erhai Lake. According to the results achieved, the total N (TN) loss mainly occurred during the initial 12 days when the soil was flooded, then presented N immobilized by soil and finally, basically balanced between influent and effluent after 50 days. The results demonstrated that 1.7% of the original soil TN storage (0-100 cm) was lost. The alternation of drying and flooding could greatly increase TN loss up to 1086 kg hm^-2, which was 2.72 times as much as that of continuous flooding flow. The amount of soil N losses to groundwater was closely related to the soil profile biochemical characteristics (water content, soil microbial immobilization, mineralization, nitrification, and denitrification processes). Soil N loss from crop fields driven by SG table fluctuation is 26 and 6 times of the runoff and leaching losses, respectively, while the soil N loss from the vegetable fields is 33 and 4 times of the runoff and leaching losses. The total amount of N losses from the croplands around the Erhai Lake caused by flooding of shallow groundwater (SG) in 2016 was estimated at 3506 Mg. The estimations showed that N losses would decrease by 16% if vegetables are replaced with staple food crops. These results imply that the adjustment of the planting structure was the key measure to reduce soil N storage and mitigate groundwater contamination.

Microbial investigations of new hydrogel-biochar composites as soil amendments for simultaneous nitrogen-use improvement and heavy metal immobilization

Agricultural sustainability is challenging because of increasingly serious and co-existing issues, e.g., poor nitrogen-fertilizer use and heavy metal pollution. Herein, we introduced a new poly(acrylic acid)-grafted chitosan and biochar composite (PAA/CTS/BC) for soil amendment, and provided a first microbial insight into how PAA/CTS/BC amendment simultaneously improved nitrogen cycling and immobilized heavy metals. Our results suggest that the PAA/CTS/BC amendment significantly promoted soil ammonium retention, and reduced nitrate accumulation, nitrous oxide emission and ammonia volatilization during the rice cultivation. The availability of various heavy metals (Fe, Mn, Cu, Zn, Ni, Pb, Cr, and As) markedly decreased in the PAA/CTS/BC amended soil, thereby reducing their accumulation in rice root. The PAA/CTS/BC amendment significantly altered the structure and function of soil microbial communities. Importantly, the co-occurrence networks of microbial communities became more complex and function-specific after PAA/CTS/BC addition. For example, the keystone species related to organic matter degradation, denitrification, and plant resistance to pathogen or stresses were enriched within the network. In addition to direct adsorption, the effects of PAA/CTS/BC on shaping microbial communities played dominant roles in the soil amendment. Our findings provide a promising strategy of simultaneous nitrogen-use improvement and heavy metal immobilization for achieving crop production improvement, pollution control, and climate change mitigation.

Nitrifier denitrification dominates nitrous oxide production in composting and can be inhibited by a bioelectrochemical nitrification inhibitor

Targeted options to reduce nitrous oxide (N₂O) emission from composting is scarce due to challenges in disentangling the complex N₂O production pathways. Here, combined approaches of nitrogen form analysis, isotopocule mapping, quantitative PCR, and Illumina MiSeq sequencing were used to differentiate N₂O production pathways and decipher the underlying biochemical mechanisms. Results suggested that most N₂O was produced at the latter stage through nitrifier denitrification. The bioelectrochemical assistance through applying an electric potential reduced N₂O emissions by 28.5-75.5%, and the underlying mitigation mechanism was ammonia oxidation repression, as evidenced by the observed reduction in the proportion of the amoA containing family Nitrosomonadaceae from 99% to 83% at the lower voltage and to a negligible level at the higher voltage assessed, which was attributed to their depressed competitiveness for oxygen with heterotrophs. The findings provide evidence that the bioelectrochemical assistance could function as a nitrification inhibitor to minimize compost derived N₂O emissions.

Stimulation of phenanthrene and biphenyl degradation by biochar-conducted long distance electron transfer in soil bioelectrochemical systems

The bioelectrochemical degradation of organic pollutants has attracted considerable attention owing to its remarkable sustainability and low cost. However, the application of bioelectrochemical system (BES) for the degradation of pollutants in soils is hindered by limitations in the effective distance in the soil matrix. In this study, a biochar-amended BES was constructed to evaluate the degradation of organic pollutants. This system was expected to extend the electron transfer distance via conductive biochar in soils. The results showed that biochar pyrolyzed at 900 °C facilitated the degradation of phenanthrene (PHE) and biphenyl (BP) in the soil BES (SBES), reaching 86.4%-95.1% and 88.8%-95.3% in 27 days, respectively. The effective distance of SBESs was estimated to be 154-271 cm away from the electrode, which increased 1.9-3 fold after the addition of biochar. Microbial community and functional gene analysis confirmed that biochar enriched functional degrading bacteria. These findings demonstrate that the promotion of long-distance electron transfer and the formation of soil conductive networks can be achieved by biochar amendment. Thus, this study provides a basis for the effective degradation of for persistent organic pollutants in petroleum-contaminated soils using bioelectrochemical strategy.

Mixotrophic bacteria for environmental detoxification of contaminated waste and wastewater

Mixotrophic bacteria provide a desirable alternative to the use of classical heterotrophic or chemolithoautotrophic bacteria in environmental technology, particularly under limiting nutrients conditions. Their bi-modal ability of adapting to inorganic or organic carbon feed and sulfur, nitrogen, or even heavy metal stress conditions are attractive features to achieve efficient bacterial activity and favorable operation conditions for the environmental detoxification or remediation of contaminated waste and wastewater. This review provides an overview on the state of the art and summarizes the metabolic traits of the most promising and emerging non-model mixotrophic bacteria for the environmental detoxification of contaminated wastewater and waste containing excess amounts of limiting nutrients. Although mixotrophic bacteria usually function with low organic carbon sources, the unusual capabilities of mixotrophic electroactive exoelectrogens and electrotrophs in bioelectrochemical systems and in microbial electrosynthesis for accelerating simultaneous metabolism of inorganic or organic C and N, S or heavy metals are reviewed. The identification of the mixotrophic properties of electroactive bacteria and their capability to drive mono- or bidirectional electron transfer processes are highly exciting and promising aspects. These aspects provide an appealing potential for unearthing new mixotrophic exoelectrogens and electrotrophs, and thus inspire the next generation of microbial electrochemical technology and mixotrophic bacterial metabolic engineering. KEY POINTS: • Mixotrophic bacteria efficiently and simultaneously remove C and N, S or heavy metals. • Exoelectrogens and electrotrophs accelerate metabolism of C and N, S or heavy metals. • New mixotrophic exoelectrogens and electrotrophs should be discovered and exploited.

Long-Term Fertilization Shapes the Putative Electrotrophic Microbial Community in Paddy Soils Revealed by Microbial Electrosynthesis Systems

Electrotrophs play an important role in biogeochemical cycles, but the effects of long-term fertilization on electrotrophic communities in paddy soils remain unclear. Here, we explored the responses of electrotrophic communities in paddy soil-based microcosms to different long-term fertilization practices using microbial electrosynthesis systems (MESs), high-throughput quantitative PCR, and 16s rRNA gene-based Illumina sequencing techniques. Compared to the case in the unfertilized soil (CK), applications of only manure (M); only chemical nitrogen, phosphorous, and potassium fertilizers (NPK); and M plus NPK (MNPK) clearly changed the electrotrophic bacterial community structure. The Streptomyces genus of the Actinobacteria phylum was the dominant electrotroph in the CK, M, and MNPK soils. The latter two soils also favored Truepera of Deinococcus-Thermus or Arenimonas and Thioalkalispira of Proteobacteria. Furthermore, Pseudomonas of Proteobacteria and Bacillus of Firmicutes were major electrotrophs in the NPK soil. These electrotrophs consumed biocathodic currents coupled with nitrate reduction and recovered 18-38% of electrons via dissimilatory nitrate reduction to ammonium (DNRA). The increased abundances of the nrfA gene for DNRA induced by electrical potential further supported that the electrotrophs enhanced DNRA for all soils. These expand our knowledge about the diversity of electrotrophs and their roles in N cycle in paddy soils and highlight the importance of fertilization in shaping electrotrophic communities.

Microorganisms in sediment microbial fuel cells: Ecological niche, microbial response, and environmental function

A sediment microbial fuel cell (SMFC) is a device that harvests electrical energy from sediments rich in organic matter. SMFCs have been attracting increasing amounts of interest in environmental remediation, since they are capable of providing a clean and inexhaustible source of electron donors or acceptors and can be easily controlled by adjusting the electrochemical parameters. The microorganisms inhabiting sediments and the overlying water play a pivotal role in SMFCs. Since the SMFC is applied in an open environment rather than in an enclosed chamber, the effects of the environment on the microbes should be intense and the microbial community succession should be extremely complex. Thus, this review aims to provide an overview of the microorganisms in SMFCs, which few previous review papers have reported. In this study, the anodic and cathodic niches for the microorganisms in SMFCs are summarized, how the microbial population and community interact with the SMFC environment is discussed, a new microbial succession strategy called the electrode stimulation succession is proposed, and recent developments in the environmental functions of SMFCs are discussed from the perspective of microorganisms. Future studies are needed to investigate the electrode stimulation succession, the environmental function and the electron transfer mechanism in order to boost the application of SMFCs for power generation and environmental remediation.

Elevated CO2 concentration modifies the effects of organic fertilizer substitution on rice yield and soil ARGs

Antibiotic resistance and rising CO2 levels are considered among the most significant challenges we will face in terms of global development over the following decades. However, the impact of elevated CO2 on soil antibiotic resistance has rarely been investigated. We used a free-air CO2 enrichment system to investigate the potential risks posed by applying mineral and organic fertilizers to paddy soil at current CO2 concentration (370 ppm) and future elevated CO2 (eCO2, 570 ppm predicted for 2100). Organic fertilizer substitution (substituting the mineral fertilizer by 50% N) alone increased the plant uptake and soil residue of sulfamethazine, and enriched sulfonamide resistance genes (sul1, sul2), tetracycline resistance genes (tetG, tetM) and class 1 integron (intl1). But it decreased the rice grain yield (by 7.6%). Comparatively, eCO2 decreased the sul2, tetG and intl1 gene abundances by organic fertilizer substitution, and meanwhile increased grain yield (by 8.4%). Proteobacteria and Nitrospirae were potential hosts of antibiotic resistance genes (ARGs). Horizontal gene transfer via intl1 may play an important role in ARGs spread under eCO2. Results indicated that future elevated CO2 concentration could modify the effects of organic fertilizer substitution on rice yield and soil ARGs, with unknown implications for future medicine and human health.

Effects of biochar on nitrification and denitrification-mediated N2O emissions and the associated microbial community in an agricultural soil

Nitrous oxide (N₂O) is a strong greenhouse gas, and it is of great significance for N₂O reduction to study the effects of biochar on its production pathway. In this research, the contributions and mechanisms of biochar on autotrophic nitrification (ANF), heterotrophic nitrification (HNF), and denitrification (DF) to N₂O emissions were studied by using ¹⁵N stable isotopes and high-throughput sequencing after laboratory incubation. The results showed that biochar addition at 2% (B2) significantly reduced the N₂O emissions from the ANF by an average of 20.6%, while adding 5% biochar (B5) had no significant effect on the ANF. Both B2 and B5 significantly reduced the N₂O emissions from the HNF by 15.7% and 13.2%, respectively, and reduced the N₂O emissions from the DF by 40.9% and 11.7%, respectively. B2 enhanced the relative contribution rate of the ANF to N₂O emissions by 6.3%, while B5 had little effect on it. Biochar addition significantly changed the copy numbers of the AOA and AOB, as well as the nirK, nirS, and nosZ genes, but it had no significant effect on the community composition of the AOA and had minimal effect on the AOB community. B2 significantly increased the abundance of the genus Rhodococcus of nirK type denitrifiers and had a significant effect on the relative abundance of Cupriavidus and Pseudomonas of the nosZ type denitrifiers. These results revealed that the inhibitory effects of biochar on N₂O emissions from nitrification might be attributed to the direct immobilization and adsorption of inorganic N by biochar and to its promotion of the genus Rhodococcus of nirK-type denitrifiers and the genera Cupriavidus and Pseudomonas of the nosZ-type denitrifiers. The soil exchangeable NH₄⁺-N and NO₃^--N concentrations were the primary factors affecting the N₂O emission rates. These results help to elucidate the effects and mechanisms of biochar on N₂O production pathways in agricultural soil.

Remediation of nitrate contamination by membrane hydrogenotrophic denitrifying biofilm integrated in microbial electrolysis cell

Complete biological denitrification is usually restricted in electron donor lacking waters. Hydrogenotrophic denitrification attracts attention for its clean and cost-efficiency advantages. Therein, the hydrogen could be effectively generated by microbial electrolysis cells (MECs) from organic wastes. In this study, a gas diffusion membrane (GDM) integrated MEC (MMEC) was constructed and provided a novel non-polluting approach for nitrate contaminated water remediation, in which the hydrogen was recovered from substrate degradation in anode and diffused across GDM as electron donor for denitrification. The high overall nitrogen removal of 91 ± 0.1%-95 ± 1.9% and 90 ± 1.6%-94 ± 2.2% were respectively achieved in Ti-MMEC and SS-MMEC with titanium and stainless-steel mesh as cathode at all applied voltages (0.4-0.8 V). Decreasing applied voltage from 0.8 to 0.4 V significantly improved the electron utilization efficiency for denitrification from 26 ± 3.6% to 73 ± 0.1% in Ti-MMEC. Integrating MEC with GDM greatly improved TN removal by 40% under applied voltage of 0.8 V. The hydrogenotrophic denitrifiers of Rhodocyclaceae, Paracoccus, and Dethiobacter, dominated in MMECs facilitating TN removal. Functional denitrification related genes including napAB, nirKS, norBC and nosZ predicted by PICRUSt2 based on 16S rRNA gene data demonstrated higher abundance in MMECs.

Efficient recovery of Cu(II) by LTA-zeolites with hierarchical pores and their resource utilization in electrochemical denitrification: Environmentally friendly design and reutilization of waste in water

Water pollution seriously endangers human health and the environment. Here we prepared and tested mesoporous LTA zeolites for the adsorption of Cu(II) from aqueous media and the captured copper was further used for electrochemical nitrate reduction. The prepared hierarchically porous LTA exhibited a high capacity (341.5 mg g^-1) for Cu(II) adsorption, following the pseudo-second-order kinetic and Freundlich adsorption isotherm models well. The Cu-LTA sample was characterised by various analytical methods, and Cu(I) species were identified as the active sites for nitrate electrochemical reduction. Based on the spectral characterization and reducibility, strong metal-support interaction was found between copper and LTA, which is beneficial to the dispersion of active sites and their contacts with nitrates. In total, 10.1 g-N-NO₃ g^-1-Cu was reduced over the Cu-LTA-modified cathode in a three-electrode system with high N₂ selectivity (92.1 %). Compared to purely microporous zeolites, mesoporous LTA has a higher capacity for Cu(II) removal and nitrate reduction. The mesoporous structure allows easy access to the inner active sites with low diffusion resistance. The low Tafel slope and high current density confirm the high activity of the mesoporous Cu-LTA, making it a promising and efficient material for the removal and reuse of heavy metal ions.

Centimeter-Long Microbial Electron Transport for Bioremediation Applications

Microbial bioremediation based on nano- to micrometer-scale electron transport has been intensively studied during the past decade, but its application can be hindered by a deficiency of suitable electron acceptors or slow mass transportation at contaminated sites. Microbial long-distance electron transport (LDET), which can couple spatially separated redox reactions across distances in natural environments, has recently emerged at centimeter-length scales. LDET explains a range of globally important biogeochemical phenomena and overcomes the drawbacks of conventional bioremediation by directly linking distant electron donors and acceptors. Here, we highlight recent research outcomes in examining, characterizing, and engineering LDET, and describe how LDET can be exploited to develop advanced technologies for the bioremediation of soils and sediments.

Biochar enhances bioelectrochemical remediation of pentachlorophenol-contaminated soils via long-distance electron transfer

The soil bioelectrochemical system (SBES) is a promising biotechnology for the remediation of contaminated soils. However, the effective distance of pollutant removal in the SBES was usually limited in a few centimeters near the electrode surface. In this study, we used biochar as the model conductor to construct a conductive network with microbes in the soil matrix to extend the effective distance of pollutant removal in the SBES. Pentachlorophenol (PCP) was used as the representative contaminant to probe long-distance electron transfer facilitated by the networks. The removal of PCP and microbial community analyses at different distances toward the electrode were monitored. The results showed that PCP transformation in the SBES without biochar amendment mainly occurred within 4 cm around the electrode. However, the effective distance of ∼ 16 cm toward the electrode could be achieved for efficient PCP degradation in the SBES amended with highly conductive biochar. Microbial community analysis confirmed the establishment of bacteria-biochar networks, where Desulfitobacterium and Geobacter were enriched and spatially distributed in the biochar-amended SBES. The results demonstrate that long-distance electron transfer can be achieved in the biochar-amended soil matrix, and shed light on the development of bioelectrochemical strategy for efficient organic pollutant degradation in soils.

Elucidating the negative effect of denitrification on aromatic humic substance formation during sludge aerobic fermentation

Humic substance (HS), as an aromatic compound, is the core product of aerobic fermentation. Denitrification-dependent degradation of aromatic compounds have been repeatedly observed in environment. However, few studies have elucidated the relationship between denitrification and aromatic HS during sludge aerobic fermentation. This study was conducted to investigate the effect of enhanced denitrification on aromatic HS formation. On the 24th day of sludge aerobic fermentation, five tests (CK, Run1, Run2, Run3 and Run4) were executed, and nitrate concentrations were adjusted to 480 ± 20, 500 ± 20, 1000 ± 20, 1500 ± 20 and 2000 ± 20 mg/kg with potassium nitrate, respectively. Analytical results demonstrated that nitrate addition increased denitrifying genes abundance and enhanced denitrification, which further reduced aromatic HS formation (p < 0.05). Especially in Run3, the concentrations of HS and humic acid on the 52nd day dramatically decreased by 12.9 % and 34.2 % in comparison with those on the 31st day. High-throughput sequencing revealed that enhanced denitrification effectively stimulated the metabolism of denitrifying microorganisms with aromatic-degrading capability. Co-occurring network analysis indicated that some keystone taxa of denitrification aromatic-degrading microorganisms involved in the conversion of nitrate to nitrite were the most crucial for enhancing denitrification and reducing aromatic HS formation.

Nutrient conversion and recovery from wastewater using electroactive bacteria

Wastewater is widely recognized as a sink of active nitrogen and phosphorus, and the recovery of both nutrients as fertilizers is widely studied in recent years. Electroactive bacteria increasingly attract attentions in this area because they are able to produce an electric field in microbial electrochemical systems to concentrate ammonium and phosphate for recovery. Importantly, these unique bacteria are able to convert nitrate and nitrite directly to ammonium, maximizing the active nitrogen species capable of recovery. Ferric ions produced by electroactive bacteria can be precipitated with phosphate to recover as vivianite in neutral wastewaters. All these processes employed electroactive bacteria as both nitrate and iron reducer and bioelectric field generator. The mechanism as well as technologies are summarized, and the challenges to further improve their performance are discussed.

Nitrification plays a key role in N2O emission in electric-field assisted aerobic composting

Nitrous oxide (N₂O) emission is a serious environmental problem in composting. Previous studies have indicated that electric field assistance results in lower N₂O emissions in aerobic composting; however, the exact mechanisms involved in electric-field assisted aerobic composting (EAAC) are not clear. In this study, the biological N transformation processes and the N-associated genes were investigated. The results demonstrated that electric field application inhibited nitrification, weakened the nitrifying functional genes (the hao and nxrA genes declined maximally by 86% and 86.8%, respectively), and increased the N₂O consumption-related gene (nosZ) by a maximum factor of 2.76 compared with that in CAC. The correlation analysis demonstrated that nitrification was the main source of N₂O emission in EAAC. The findings imply that EAAC is a promising process for mitigating N₂O emission at the source during aerobic composting.