Inorganic Electron Donors Research Articles

Adaptation mechanisms of Alcanivorax facilitating its predominance in marine environments

IntroductionAlcanivorax, a typical alkane-degrading bacterium, has demonstrated the ability to utilize inorganic electron donor in some reports. However, a comprehensive analysis of its potentiality to utilize inorganic electron donor is still lacking.MethodsIn this study, genomic and phylogenetic analyzes were used to explore the potential oxidative capacity of inorganic compounds in Alcanivorax. And its functions were verified through physiological experiments.ResultsThe sulfur oxidation-related genes sqr and tsdA are prevalent and have various evolutionary origins. Potential genes for CO oxidation were present in 39 strains, whereas genes associated with iron, hydrogen, and ammonia oxidation were either rare or absent. The physiological functions of Sqr and TsdA were confirmed in six representative strains under heterotrophic conditions. Adding thiosulfate enhanced Alcanivorax growth. However, Alcanivorax bacteria perform sulfide detoxification through Sqr rather than by gaining energy via sulfide oxidation Although no strain was confirmed to be chemoautotrophs, we discovered that the two clades, A. xenomutans and A. profundimaris, can grow under conditions with very low organic matter.DiscussionThe ability to utilize inorganic compounds as a supplementary energy source and adapt to carbon oligotrophic growth may contribute to the prevalence of Alcanivorax in marine ecosystems.

Oxidation of sulfur, hydrogen, and iron by metabolically versatile Hydrogenovibrio from deep sea hydrothermal vents.

Chemolithoautotrophic Hydrogenovibrio are ubiquitous and abundant at hydrothermal vents. They can oxidize sulfur, hydrogen or iron, but none are known to use all three energy sources. This ability though would be advantageous in vents hallmarked by highly dynamic environmental conditions. We isolated three Hydrogenovibrio strains from vents along the Indian Ridge, which grow on all three electron donors. We present transcriptomic data from strains grown on iron, hydrogen or thiosulfate with respective oxidation and autotrophic CO2 fixation rates, RubisCO activity, SEM, and EDX. Maximum estimates of one strain's oxidation potential were 10, 24, and 952mmol for iron, hydrogen and thiosulfate oxidation and 0.3, 1, and 84mmol CO2 fixation, respectively, per vent per hour indicating their relevance for element cycling in-situ. Several genes were up- or downregulated depending on the inorganic electron donor provided. Although no known genes of iron-oxidation were detected, upregulated transcripts suggested iron-acquisition and so far unknown iron-oxidation-pathways.

Enhanced nitrogen removal and greenhouse gas reduction via activated carbon coupled iron-based constructed wetlands

Constructed wetlands (CWs) have been widely used to remove nitrogen in the effluent from sewage plants. Still, the removal capacity of nitrate (NO3−-N) in CWs is unsatisfactory due to the insufficient electron donor. This study explored the role of activated carbon (AC) in nitrogen removal and greenhouse gas (GHG) emissions reduction in different iron-based CWs. Long-term experimental results showed that pyrite and sponge iron-based CWs had higher NO3−-N removal rates (98.18 % and 94.10 %). Compared with the control group, AC promoted their total nitrogen (TN) removal efficiency (78.87–87.59 %) and reduced GHG emissions (CO2: 3.79–4.11 %, N2O: 2.97–8.78 %, CH4: 9.93–10.73 %). Moreover, AC not only promoted the values of electron transfer system activity (ETSA), the activity of nitrate reductase and nitrite reductase, but also significantly increased the microbial diversity and abundance of the substrate layer in pyrite/SI- based CWs. The relative abundance of the dominant bacteria in CWs with pyrite coupling with AC (PC-CW) was the highest (nirS: 77.60 %, nosZ: 88.98 %) compared with the other CWs. The higher relative abundance of Dechloromonas and Thiobacillus in PC-CW indicated that the composite substrates of pyrite and AC promoted the extracellular electron transfer of denitrifying microorganisms. This study provides new insights into optimizing inorganic electron donors to improve the removal of NO3−-N and the reduce of GHG emissions in CWs for low C/N wastewater treatment.

Superior nitrate and chromium reduction synergistically driven by multiple electron donors: Performance and the related biochemical mechanism

Nitrate and Cr(VI) are the typical and prevalent co-contaminants in the groundwater, how to synchronously and effectively diminish them has received growing attention. The most problem that currently limits the nitrate and Cr(VI) reduction technology for groundwater remediation is with emphasis on exploring the optimal electron donors. This study investigated the feasibility of utilizing the synergistical effect of inorganic electron donors (pyrite, sulfur) and inherently limited organics to promote synchronous nitrate and Cr(VI) removal, which meets the requirement of naturally low-carbon and eco-friendly technologies. The NO3−-N and Cr(VI) removal efficiencies in the pyrite and sulfur involved mixotrophic biofilter (PS–BF: approximately 90.8 ± 0.6% and 99.1 ± 2.1%) were substantially higher than that in a volcanic rock supported biofilter (V–BF: about 49.6% ± 2.8% and 50.0% ± 9.3%), which was consistent with the spatial variations of their concentrations. Abiotic and biotic batch tests directly confirmed the decisive role of pyrite and sulfur for NO3−-N and Cr(VI) removal via chemical and microbial pathways. A server decline in sulfate production correlated with decreasing COD consumption revealed that there was sulfur disproportionation induced by limited organics. Metagenomic analysis suggested that chemoautotrophic microbes like Sulfuritalea and Thiobacillus were key players responsible for sulfur oxidation, nitrate and Cr(VI) reduction. The metabolic pathway analysis suggested that genes encoding functional enzymes related to complete denitrification, S oxidation, and dissimilatory sulfate reduction were upregulated, however, genes encoding Cr(VI) reduction enzymes (e.g. chrA, chrR, nemA, and azoR) were downregulated in PS-BF, which further explained the synergistical effect of multiple electron donors. These findings provide insights into their potential cooperative interaction of multiple electron donors on greatly promoting nitrate and Cr(VI) removal and have implications for the remediation technology of nitrate and Cr(VI) co-contaminated groundwater.

Mixotrophic aerobic denitrification enhanced by inorganic electron donor: Novel insights into iron valence and nirS-type denitrifying bacterial community model

Nitrogen pollution was the main factor leading to reservoir eutrophication. The removal of low-concentration nitrate pollutants in water source reservoirs is an international problem to be solved. Insufficient organic electron donors in natural reservoir are a crucial limiting factor for in situ denitrification. Microbial mixotrophic aerobic denitrification using iron as the assisted electron donor is a potential pathway to treat micropolluted water bodies. The iron/activated carbon (IAC) was used to be an assisted electron provider to advance the mixotrophic aerobic denitrification performance of denitrifying bacterial community in micro-polluted reservoir water. Upon adding IAC into the reactors, the total nitrogen (TN) reduction capacity improved >41.10 %. A slight increase in organic matter removal efficiency observed in the IAC reactors compared to the control reactors. High-throughput sequencing revealed that IAC altered the richness, and co-occurrence of denitrifying bacterial community, further advancing nitrate transformation and removal. The relative abundant of Thauera, Dechloromonas, Cupriavidus, Alicycliphilus, and Azoarcus increased and presented predominant in the IAC reactors. Meanwhile, network analysis, niche width model, neutral community model, and species abundance distribution supported this finding. The Jinpen reservoir (JP; 34°32'38" N, 107°53'53" E, located in Xi’an, North China) and Xikeng reservoir (XK; 34°32'38" N, 107°53'53" E, located in Shenzhen, South China) raw water treatment experiments demonstrated excellent TN (> 60 %) and organic matter removal efficiencies (> 44 %).

Mixotrophic aerobic denitrification facilitated by denitrifying bacterial-fungal communities assisted with iron in micro-polluted water: Performance, metabolic activity, functional genes abundance, and community co-occurrence

Low-dosage nitrate pollutants can contribute to eutrophication in surface water bodies, such as lakes and reservoirs. This study employed assembled denitrifying bacterial-fungal communities as bio-denitrifiers, in combination with zero-valent iron (ZVI), to treat micro-polluted water. Immobilized bacterial-fungal mixed communities (IBFMC) reactors demonstrated their ability to reduce nitrate and organic carbon by over 43.2 % and 53.7 %, respectively. Compared to IBFMC reactors, IBFMC combined with ZVI (IBFMC@ZVI) reactors exhibited enhanced removal efficiencies for nitrate and organic carbon, reaching the highest of 31.55 % and 17.66 %, respectively. The presence of ZVI in the IBFMC@ZVI reactors stimulated various aspects of microbial activity, including the metabolic processes, electron transfer system activities, abundance of functional genes and enzymes, and diversity and richness of microbial communities. The contents of adenosine triphosphate and electron transfer system activities enhanced more than 5.6 and 1.43 folds in the IBFMC@ZVI reactors compared with IBFMC reactors. Furthermore, significant improvement of crucial genes and enzyme denitrification chains was observed in the IBFMC@ZVI reactors. Iron played a central role in enhancing microbial diversity and activity, and promoting the supply, and transfer of inorganic electron donors. This study presents an innovative approach for applying denitrifying bacterial-fungal communities combined with iron enhancing efficient denitrification in micro-polluted water.

Hexavalent Chromium Detoxification by Biochars: Influences of Organic and Inorganic Electron Donors

Hexavalent Chromium Detoxification by Biochars: Influences of Organic and Inorganic Electron Donors

Elucidating distinct roles of chemical reduction and autotrophic denitrification driven by three iron-based materials in nitrate removal from low carbon-to-nitrogen ratio wastewater

Effective nitrate removal is a key challenge when treating low carbon-to-nitrogen ratio wastewater. How to select an effective inorganic electron donor to improve the autotrophic denitrification of nitrate nitrogen has become an area of intense research. In this study, the nitrate removal mechanism of three iron-based materials in the presence and absence of microorganisms was investigated with Fe2+/Fe0 as an electron donor and nitrate as an electron acceptor, and the relationship between the iron materials and denitrifying microorganisms was explored. The results indicated that the nitrogen removal efficiency of each iron-based material coupled sludge systems was higher than that of iron-based material. Furthermore, compared with the sponge iron coupled sludge system (60.6%–70.4%) and magnetite coupled sludge (56.1%–65.3%), the pyrite coupled sludge system had the highest removal efficiency of TN, and the removal efficiency increased from 62.5% to 82.1% with time. The test results of scanning electron microscope, X-ray photoelectron spectroscopy and X-ray diffraction indicated that iron-based materials promoted the attachment of microorganisms and the chemical reduction of nitrate in three iron-based material coupled sludge systems. Furthermore, the pyrite coupled sludge system had the highest nitrite reductase activity and can induce microorganisms to secrete more extracellular polymer substances. Combined with high-throughput sequencing and PICRUSt2 functional predictive analysis software, the total relative abundance of the dominant bacterial in pyrite coupled sludge system was the highest (72.06%) compared with the other iron-based material systems, and the abundance of Blastocatellaceae was relatively high. Overall, these results suggest that the pyrite coupled sludge system was more conducive to long-term stable nitrate removal.

Anaerobic cyanides oxidation with bimetallic modulation of biological toxicity and activity for nitrite reduction

Cyanide is a typical toxic reducing agent prevailing in wastewater with a well-defined chemical mechanism, whereas its exploitation as an electron donor by microorganisms is currently understudied. Given that conventional denitrification requires additional electron donors, the cyanide and nitrogen can be eliminated simultaneously if the reducing HCN/CN− and its complexes are used as inorganic electron donors. Hence, this paper proposes anaerobic cyanides oxidation for nitrite reduction, whereby the biological toxicity and activity of cyanides are modulated by bimetallics. Performance tests illustrated that low toxicity equivalents of iron-copper composite cyanides provided higher denitrification loads with the release of cyanide ions and electrons from the complex structure by the bimetal. Both isotopic labeling and Density Functional Theory (DFT) demonstrated that CN−-N supplied electrons for nitrite reduction. The superposition of chemical processes reduces the biotoxicity and enhances the biological activity of cyanides in the CN−/Fe3+/Cu2+/NO2– coexistence system, including complex detoxification of CN− by Fe3+, CN− release by Cu2+ from [Fe(CN)6]3−, and NO release by nitrite substitution of −CN groups. Cyanide is the smallest structural unit of C/N-containing compounds and serves as a probe to extend the electron-donating principle of anaerobic cyanides oxidation to more electron-donor microbial utilization.

Enhanced nitrate removal efficiency and microbial response of immobilized mixed aerobic denitrifying bacteria through biochar coupled with inorganic electron donors in oligotrophic water

The nitrogen removal characteristics and microbial response of biochar-immobilized mixed aerobic denitrifying bacteria (BIADB) were investigated at 25 °C and 10 °C. BIADB removed 53.51 ± 1.72 % (25 °C) and 39.90 ± 4.28 % (10 °C) nitrate in synthetic oligotrophic water. Even with practical oligotrophic water, BIADB still effectively removed 47.66–53.21 % (25 °C), and 39.26–45.63 % (10 °C) nitrate. The addition of inorganic electron donors increased nitrate removal by approximately 20 % for synthetic and practical water. Bacterial and functional communities exhibited significant temperature and stage differences (P < 0.05), with temperature and total dissolved nitrogen being the main environmental factors. The dominant genera and keystone taxa exhibited significant differences at the two temperatures. Structural equation model analysis showed that dissolved organic matter had the highest direct and indirect effects on diversity and function, respectively. This study provides an innovative pathway for utilizing biochar and inorganic electron donors for nitrate removal from oligotrophic waters.

Novel candidate taxa contribute to key metabolic processes in Fennoscandian Shield deep groundwaters.

The continental deep biosphere contains a vast reservoir of microorganisms, although a large proportion of its diversity remains both uncultured and undescribed. In this study, the metabolic potential (metagenomes) and activity (metatranscriptomes) of the microbial communities in Fennoscandian Shield deep subsurface groundwaters were characterized with a focus on novel taxa. DNA sequencing generated 1270 de-replicated metagenome-assembled genomes and single-amplified genomes, containing 7 novel classes, 34 orders, and 72 families. The majority of novel taxa were affiliated with Patescibacteria, whereas among novel archaea taxa, Thermoproteota and Nanoarchaeota representatives dominated. Metatranscriptomes revealed that 30 of the 112 novel taxa at the class, order, and family levels were active in at least one investigated groundwater sample, implying that novel taxa represent a partially active but hitherto uncharacterized deep biosphere component. The novel taxa genomes coded for carbon fixation predominantly via the Wood-Ljungdahl pathway, nitrogen fixation, sulfur plus hydrogen oxidation, and fermentative pathways, including acetogenesis. These metabolic processes contributed significantly to the total community's capacity, with up to 9.9% of fermentation, 6.4% of the Wood-Ljungdahl pathway, 6.8% of sulfur plus 8.6% of hydrogen oxidation, and energy conservation via nitrate (4.4%) and sulfate (6.0%) reduction. Key novel taxa included the UBA9089 phylum, with representatives having a prominent role in carbon fixation, nitrate and sulfate reduction, and organic and inorganic electron donor oxidation. These data provided insights into deep biosphere microbial diversity and their contribution to nutrient and energy cycling in this ecosystem.

Research on the performance and kinetics of nitrogen and phosphorus removal in composite ecological floating beds

The water purification efficiency of the traditional ecological floating beds (EFB) is unstable, and their ability to remove nitrogen and phosphorus is easily influenced by factors such as environmental temperature and microbial quantity. This study introduces inorganic electron donors and biofilm carrier materials into the EFB system, constructing Iron-Carbon Composite EFB (ICEFB), Sponge Iron Composite EFB (SIEFB), Zeolite Composite EFB (ZEFB), and Bio-ball Composite EFB (BEFB). The experiment investigates the water purification performance of each EFB at different temperatures and examines the water purification capabilities of the sponge iron-zeolite composite system through column experiments. Additionally, the nitrogen and phosphorus removal kinetics of the EFB system and the nitrogen and phosphorus removal processes of the filler system (Sponge iron-Clinoptilolite filling system) under column experiments are analyzed. The results indicate that the nitrogen and phosphorus removal performance of SIEFB and ICEFB, which are coupled with inorganic electron donors and biofilm carrier fillers, surpasses that of ZEFB and BEFB. Under the conditions of 20–30 °C, SIEFB and ICEFB achieve maximum removal rates of 99.33% and 99.27% for NH4+-N, and 64.42% and 52.22% for TN, respectively. The maximum removal rates and removal loads for TP reach 89.35%, 80.92%, and 10.73 g/(m3∙d), 9.54 g/(m3∙d), respectively. Additionally, column experiments demonstrate that the addition of sponge iron enhances the phosphorus removal rate of the zeolite filling system. The nitrogen and phosphorus removal processes of the four EFB systems can be well-fitted by utilizing a first-order kinetic model based on the Arrhenius equation and the Monod kinetic model. Anticipating that this research could further perfect the application of ecological floating bed technology in water treatment, and contribute to the restoration efforts of water environments.

A review of autotrophic denitrification for groundwater remediation: A special focus on bioelectrochemical reactors

Groundwater is an important resource that can help in climate change adaptation. However, the pollution of these aquifers with nitrate is a widespread problem of growing concern. Biological denitrification using inorganic electron donors shows significant advantages in treating nitrate-polluted groundwater where organic matter presence is negligible. However, mass transfer limitations and secondary contamination seem to be the major hinderance to spread the use of these technologies. This could be solved by the use of bioelectrochemical systems (BES), which emerge as an attractive technology to solve these problems due to the reported low energy demand and high denitrification rates. However, technical and operational issues must be considered to replicate these results at full-scale. This review summarizes the biological basis of autotrophic denitrification and the key aspects of its application in bioelectrochemical systems. In addition, an estimation of the capital costs required for the implementation of a BES considering different population sizes and initial nitrate concentration in the groundwater is made.

Simultaneous nitrate and chromium removal mechanism in a pyrite-involved mixotrophic biofilter.

Microbially mediated NO3--N and Cr(VI) reduction is being recognized as an eco-friendly and cost-effective remediation strategy. Iron sulfide mineral, as a natural inorganic electron donor, has a strong influence on NO3--N and Cr(VI) transformation, respectively. However, little is known about the simultaneous nitrate and chromium removal performance and underlying mechanism in an iron sulfide mineral-involved mixotrophic biofilter. This study demonstrated that the NO3--N and Cr(VI) removal efficiencies were stable at 62 ± 8% and 56 ± 10%, and most of them were eliminated in the 0-100-mm region of the biofilter. Cr(VI) was reduced to insoluble Cr(III) via microbial and chemical pathways, which was confirmed by the SEM-EDS morphology and the XPS spectra of biofilm and pyrite particles. SO42- was as a main byproduct of pyrite oxidation; however, the bacterial SO42- reduction synchronously occurred, evidenced by the variations of TOC and SO42- concentrations. These results suggested that there were complicated and intertwined biochemical relations between NO3--N/Cr(VI)/SO42-/DO (electron acceptors) and pyrite/organics (electron donors). Further investigation indicated that both the maximal biomass and greatest denitrifiers' relative abundances in microbial sample S1 well explained why the pollutants were removed in the 0-100-mm region. A variety of denitrifiers such as Pseudoxanthomona, Acidovorax, and Simplicispira were enriched, which probably were responsible for both NO3--N and Cr(VI) removal. Our findings advance the understanding of simultaneous nitrate and chromium removal in pyrite-involved mixotrophic systems and facilitate the new strategy development for nitrate and chromium remediation.

Tree-based ensemble machine learning model for nitrate reduction by zero-valent iron

The use of zero-valent iron (ZVI) for nitrate reduction has long been a topic of interest owing to its excellent reactivity compared to other inorganic electron donors. However, there is currently a lack of comprehensive and sophisticated models that can effectively integrate and analyze the dynamic data generated by this type of experiment. Such modeling is necessary to facilitate information mining and improve our understanding of the underlying mechanisms. Taking advantage of available published experimental data, a gradient-boosting decision tree and extreme gradient boosting were applied to 288 data points collected from peer-reviewed publications and compared with conventional non-tree-based models, i.e., artificial neural networks (ANN) and support vector regression (SVR). First, a predictive analysis was performed for the rate constant and the percentages of ammonium and nitrogen conversion using seven predictors related to ZVI properties, reactant concentration, and experimental parameters. Both tree-based models accurately predicted the target variables with comparable performance (R2 = 0.88–0.97) and outperformed ANN and SVR (R2 = 0.60–0.89). In addition, SHapley Additive exPlanations analysis revealed that ZVI properties (44 % average contribution) and experimental condition (68.5 % average contribution) were the key variables affecting the nitrate reduction rate constant and conversion product, respectively. Aerobic conditions were favorable for the conversion of nitrate to ammonium, with anoxic conditions being beneficial for the selectivity of nitrogen gas conversion. This study demonstrates a promising approach using machine learning with complex cross-system data for predicting the performance of water treatment and for mining valuable insights into the process by model interpretation. This method presents a new approach to revolutionize data analysis and modeling in water remediation, which can simplify the experimental operational burden.

Enhanced nitrogen removal from micro-polluted water by Fe (II)/Fe (III) cycling under intermittent aeration

The potential of iron-carbon materials (FeC) as inorganic electron donors for aerobic denitrification of micro-polluted source water was explored. For this purpose, a raw water denitrification experiment was conducted using FeC under intermittent aeration condition. Then, magnetite was added as a biological carrier to construct another system, FeC and magnetite intermittent aeration reaction system, (FCMI). The findings revealed that FCMI notably removed the pollutants with the removal efficiencies of nitrate, total nitrogen, total phosphorus, and chemical oxygen demand reaching 95.63%, 88.62%, 90.48%, and 61.46%, respectively. Cycling between aerobic and anaerobic environments facilitated the formation of Fe (II)/Fe (III) cycle within FeC materials. Microbial profiling revealed that Proteobacteria and Actinobacteria comprised 97.14% of the total abundance in FCMI. Iron-reducing bacteria and aerobic denitrifying bacteria were found to thrive on the surface of magnetite materials. The abundance of enzymes related to denitrification under intermittent aeration exceeded those under continuous aeration levels by 8–19%. This study verifies the ability of iron electrons to cycle under intermittent aeration condition and proposes a novel strategy for in-situ denitrification of micro-polluted source water.

(Invited) Reshaping Resource Flows Via Electrochemically Driven Processes: Opportunities for Anode Co-Valorization Combined with Scalable Water and CO2 Electrolysis

Understanding and advancing the water-energy-climate nexus is key to mitigating the immense threats of climate change and solving many of the related environmental issues we face today. Due to the rapid decrease in the cost of renewable energy, it is now practical to design devices that use renewable electrons in electrochemical processes to drive the transformation of CO2 and other waste feedstock (wastewater, food waste, biomass) into high-value products while also recovering important resources such as water, nutrients, and energy. Nevertheless, the bulk of current electrolyzer designs rely on the traditional oxygen evolution reaction (OER) to extract electrons from water, which has been expensive, location limited, high-risk, and generates low value (O2) product. In light of these limitations, several OER alternatives have been proposed to decrease energy consumption and/or generate high-value products that can offset operation costs and improve sustainability. These reactions can tackle a range of objectives including raw chemical generation (e.g.H2O2), waste oxidation, and molecule upgrade. By coupling worthwhile oxidation reactions at the anode with cathode electrosynthesis, industries can leverage existing infrastructure for purification, distribution, and waste-management, and even produce feedstock chemicals required for parallel processes housed at the same facility. Employing such co-valorization tactics can indeed address current drawbacks of the OER by increasing revenues and the value proposition of the overall process.In this presentation, we discuss state-of-the-art electrolysis processes that use alternative anode reactions to improve the economic viability and scalability of water or CO2 electrolysis. In particular, we will deliver a quantitative comparison of a wide range of inorganic and organic electron donors that be used in the anode to lower energy costs and/or produce value-added products. This includes the prospect of upgrading abundant waste streams such as N- containing urea and ammonia into green energy carriers. In addition, we assess the use of various transition-metal based and molecular catalysts and discuss the feasibility of using low-grade water sources such as seawater and wastewater as electrolytes. Through this wide-ranging analysis, we will include an example study for large-scale electrolysis in California, USA, provide long-term perspectives on OER substitutes for anode co-valorization, and deliver insight on future research directions. In the latter portion of the presentation, we will also discuss recent progress towards the development of cutting-edge integrated electrochemical-biological technologies that can be applied in diverse environmental and chemical applications including wastewater treatment, water reuse, desalination, remediation, and CO2 capture and conversion. Overall, the culmination of these studies can help guide future R&D efforts towards safe and efficient water, nutrient, and energy recovery.

Mn-C micro-electrolysis filler for nutrient removal in constructed wetlands: Performance evaluation and biochemistry processes

Insufficient provision of electron donors for denitrification often hinders nitrogen removal performance. While using a combination of iron and carbon can mitigate this problem, the surface passivation and limited electrical potential difference between Fe and C electrodes restricted the electron transfer efficiency. In this study, a manganese‑carbon carrier (Mn-C) was prepared and used for micro-electrolysis (Mn-C-ME) in conjunction with biological denitrification. Four constructed wetlands (CWs), including three Mn-amended CWs filled with low/middle/high content of Mn-C and one control, were established to investigate nutrient removal performance and underlying mechanisms under low C/N conditions. The results showed that Mn-amended CWs significantly enhanced total nitrogen removal from 31.01% to an optimal 91.06%. Nitrate was directly reduced by Mn(0), while Mn-C-ME generated inorganic electron donors ([H]/H2 and Mn2+) in situ for denitrification. The use of Mn ores oxidized ammonium and immobilized residual Mn2+ produced by Mn-C-ME. High-efficiency total phosphorus removal confirmed the formation of Mn-P precipitation. Moreover, the addition of Mn-C and Mn ores resulted in a noticeable difference in microbial community structures. Autotrophic denitrifying bacteria were abundant and positively correlated with Mn-oxidizing bacteria (MnOB) suggesting Mn(II)-driven autotrophic denitrification occurred. The high proportions of proteins, along with the detection of Mn2+ in extracellular polymeric substances, indicated biofilms might regulate the release of Mn2+ and supply a medium for Mn cycling, thus improving electron utilization. Our findings provide a new strategy to enhance nitrogen removal by coupling with multiple Mn-containing substrates.

Iron-carbon micro-electrolysis simultaneously enhanced nutrient and heavy metal removal in constructed wetlands for purifying polluted groundwater with variable hydraulic loadings

Heavy metal and nitrate pollution, as a primary water quality issue in shallow groundwater, is posing a severe threat to public health from drinking groundwater. In this study, iron-carbon (Fe-C) fillers were applied in constructed wetlands (CWs) to enhance the nutrient and heavy metal removal simultaneously, while the interaction of effective nitrate reduction and heavy metals mitigation under variable influent hydraulic loadings was also investigated. Groundwater is not conducive to biological heterotrophic denitrification due to its low dissolved organic matter content. Fe-C fillers can form plentiful microscopic galvanic cells and act as inorganic electron donors to promote the autotrophic denitrification process. The results showed that Fe-C micro-electrolytic CWs showed excellent simultaneous removal of nutrients and heavy metals. The nitrate nitrogen (NO3–-N) removal rate was as high as 75.77% due to the increased autotrophic denitrification process, while the removal of total nitrogen (TN) and total phosphorus (TP) could reach 73.72 % and 89.04% with a low nitrous oxide (N2O) emission of 12.12 μg m-2h−1. Besides, the removal rates of chromium (Cr) and lead (Pb) reached 98.96% and 97.09%, respectively. However, the presence of heavy metals might inhibit denitrification by competing with nitrite for electrons and generating free radicals, which decreased the denitrification by 10.02–39.05% particularly with the increasing of the hydraulic loading. On the other hand, heavy metals promoted TP removal by forming metal phosphate precipitates and complexations with phosphorus. This study provides comprehensive insights into processes and mechanism of simultaneous nutrient and heavy metal removal in CW.

Aerobic Denitrification Enhanced by Immobilized Slow-Released Iron/Activated Carbon Aquagel Treatment of Low C/N Micropolluted Water: Denitrification Performance, Denitrifying Bacterial Community Co-occurrence, and Implications.

The key limiting factors in the treatment of low C/N micropolluted water bodies are deficient essential electron donors for nitrogen removal processes. An iron/activated carbon aquagel (IACA) was synthesized as a slowly released inorganic electron donor to enhance aerobic denitrification performance in low C/N micropolluted water treatment. The denitrification efficiency in IACA reactors was enhanced by more than 56.72% and the highest of 94.12% was accomplished compared with those of the control reactors. Moreover, the CODMn removal efficiency improved by more than 34.32% in IACA reactors. The Illumina MiSeq sequencing consequence explained that the denitrifying bacteria with facultative denitrification, iron oxidation, and iron reduction function were located in the dominant species niches in the IACA reactors (e.g., Pseudomonas, Leptothrix, and Comamonas). The diversity and richness of the denitrifying bacterial communities were enhanced in the IACA reactors. Network analysis indicated that aerobic denitrifying bacterial consortia in IACA reactors presented a more complicated co-occurrence structure. The IACA reactors presented the potential for long-term denitrification operation. This study affords a pathway to utilize IACA, promoting aerobic denitrification during low C/N micropolluted water body treatment.