Effect of low temperature on abiotic and biotic nitrate reduction by zero-valent Iron

Simultaneous removal of nitrogen, phosphorus, and organic matter from oligotrophic water in a system containing biochar and construction waste iron: Performances and biotic community analysis

The issue of combined pollution in oligotrophic water has garnered increasing attention in recent years. To enhance the pollutant removal efficiency in oligotrophic water, the system containing Zoogloea sp. FY6 was constructed using polyester fiber wrapped sugarcane biochar and construction waste iron (PWSI), and the denitrification test of simulated water and actual oligotrophic water was carried out for 35 days. The experimental findings from the systems indicated that the removal efficiencies of nitrate (NO3--N), total nitrogen (TN), chemical oxygen demand (COD), and total phosphorus (TP) in simulated water were 88.61%, 85.23%, 94.28%, and 98.90%, respectively. The removal efficiencies of actual oligotrophic water were 83.06%, 81.39%, 81.66%, and 97.82%, respectively. Furthermore, the high-throughput sequencing data demonstrated that strain FY6 was successfully loaded onto the biological carrier. According to functional gene predictions derived from the Kyoto Encyclopedia of Genes and Genomes (KEGG) database, the introduction of PWSI enhanced intracellular iron cycling and nitrogen metabolism.

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.

Coupling of polyhydroxybutyrate and zero-valent iron for enhanced treatment of nitrate pollution within the Permeable Reactive Barrier and its downgradient aquifer

Permeable Reactive Barriers (PRBs) have been utilized for mitigating nitrate pollution in groundwater systems through the use of solid carbon and iron fillers that release diverse nutrients to enhance denitrification efficiency. We conduct laboratory column tests to evaluate the effectiveness of PRBs in remediating nitrate pollution both within the PRB and in the downgradient aquifer. We use an iron-carbon hydrogel (ICH) as PRB filler, which has different weight ratios of polyhydroxybutyrate (PHB) and microscale zero-valent iron (mZVI). Results reveal that denitrification in the downgradient aquifer accounts for at least 19.5 % to 32.5 % of the total nitrate removal. In the ICH, a higher ratio of PHB to mZVI leads to higher contribution of the downgradient aquifer to nitrate removal, while a lower ratio results in smaller contribution. Microbial community analysis further reveals that heterotrophic and mixotrophic bacteria dominate in the downgradient aquifer of the PRB, and their relative abundance increases with a higher ratio of PHB to mZVI in the ICH. Within the PRB, autotrophic and iron-reducing bacteria are more prevalent, and their abundance increases as the ratio of PHB to mZVI in the ICH decreases. These findings emphasize the downgradient aquifer's substantial role in nitrate removal, particularly driven by dissolved organic carbon provided by PHB. This research holds significant implications for nutrient waste management, including the prevention of secondary pollution, and the development of cost-effective PRBs.

Microbially driven Fe-N cycle: Intrinsic mechanisms, enhancement, and perspectives

The iron‑nitrogen (FeN) cycle driven by microbes has great potential for treating wastewater. Fe is a metal that is frequently present in the environment and one of the crucial trace elements needed by microbes. Due to its synergistic role in the microbial N removal process, Fe goes much beyond the essential nutritional needs of microorganisms. Investigating the mechanisms behind the linked Fe-N cycle driven by microbes is crucial. The Fe-N cycle is frequently connected with anaerobic ammonia oxidation (anammox), nitrification, denitrification, dissimilatory nitrate reduction to ammonium (DNRA), Feammox, and simultaneous nitrification denitrification (SND), etc. Although the main mechanisms of Fe-mediated biological N removal may vary depending on the valence state of the Fe, their similar transformation pathways may provide information on the study of certain element-microbial interactions. This review offers a thorough analysis of the facilitation effect and influence of Fe on the removal of nitrogenous pollutants in various biological N removal processes and summarizes the ideal Fe dosing. Additionally, the synergistic mechanisms of Fe and microbial synergistic N removal process are elaborated, covering four aspects: enzyme activity, electron transfer, microbial extracellular polymeric substances (EPS) secretion, and microbial community interactions. The methods to improve biological N removal based on the intrinsic mechanism were also discussed, with the aim of thoroughly understanding the biological mechanisms of Fe in the microbial N removal process and providing a reference and thinking for employing Fe to promote microbial N removal in practical applications.

Iron-based multi-carbon composite and Pseudomonas furukawaii ZS1 co-affect nitrogen removal, microbial community dynamics and metabolism pathways in low-temperature aquaculture wastewater

Aerobic denitrification is the key process in the elimination of nitrogen from aquaculture wastewater, especially for wastewater with high dissolved oxygen and low carbon/nitrogen (C/N) ratio. However, a low C/N ratio, especially in low-temperature environments, restricts the activity of aerobic denitrifiers and decreases the nitrogen elimination efficiency. In this study, an iron-based multi-solid carbon source composite that immobilized aerobic denitrifying bacteria ZS1 (IMCSCP) was synthesized to treat aerobic (DO > 5 mg/L), low temperature (<15 °C) and low C/N ratio (C/N = 4) aquaculture wastewater. The results showed that the sequencing batch biofilm reactor (SBBR) packed with IMCSCP exhibited the highest nitrogen removal performance, with removal rates of 95.63% and 85.44% for nitrate nitrogen and total nitrogen, respectively, which were 33.03% and 30.75% higher than those in the reactor filled with multi-solid carbon source composite (MCSC). Microbial community and network analysis showed that Pseudomonas furukawaii ZS1 successfully colonized the SBBR filled with IMCSCP, and Exiguobacterium, Cellulomonas and Pseudomonas were essential for the nitrogen elimination. Metagenomic analysis showed that an increase in gene abundance related to carbon metabolism, nitrogen metabolism, extracellular polymer substance synthesis and electron transfer in the IMCSCP, enabling denitrification in the SBBR to be achieved via multiple pathways. The results of this study provided new insights into the microbial removal mechanism of nitrogen in SBBR packed with IMCSCP at low temperatures.

Promoting aerobic denitrification in reservoir water with iron-activated carbon: Enhanced nitrogen and organics removal efficiency, and biological mechanisms

Carbon scarcity limits denitrification in micropolluted water, especially in drinking water reservoirs. Therefore, a Fe-activated carbon (AC) carrier was used in this study to enhance the nitrogen removal capacity of aboriginal denitrification in drinking water reservoirs under aerobic conditions. Following carrier addition, total nitrogen (TN) and permanganate index (CODMn) removal efficiencies reached 81.89% and 72.66%, respectively, and were enhanced by 40.45% and 39.65%. Nitrogen balance analysis indicated that 77.86% of the initial TN was converted into gaseous nitrogen. Biolog analysis suggested that the metabolic activity of denitrifying bacteria was substantially enhanced. 16S rRNA gene sequencing indicated that organic degradation bacteria, hydrogen-consuming, Fe-oxidizing, and Fe-reducing denitrifying bacteria (e.g., Arenimonas, Hydrogenophaga, Zoogloea, Methylibium, and Piscinibacter) evolved into the dominant species. Additionally, napA, nirS, nirK, and nosZ genes were enriched by 3.17, 6.68, 0.40, and 6.70 folds, respectively, which is conducive to complete denitrification. These results provide a novel pathway for the use of Fe-AC to promote aerobic denitrification in micropolluted drinking water reservoirs.

Pollutants reduction via artificial mixing in a drinking water reservoir: Insights into bacterial metabolic activity, biodiversity, interactions and co-existence of core genera

Endogenous pollution due to long periods of hypolimnetic anoxia in stratified reservoirs has become a worldwide concern, which can threaten metabolic activity, biodiversity, water quality security, and ultimately human health. In the present study, an artificial mixing system applied in a drinking water reservoir was developed to reduce pollutants, and the biological mechanism involved was explored. After approximately 44 days of system operation, the reservoir content was completely mixed resulting in the disappearance of anoxic layers. Furthermore, the metabolic activity estimated by the Biolog-ECO microplate technique and biodiversity was enhanced. 16S rRNA gene sequencing indicated a great variability on the composition of bacterial communities. Co-occurrence network analysis showed that interactions among bacteria were significantly affected by the proposed mixing system. Bacteria exhibited a more mutualistic state and >10 keystone genera were identified. Pollutants, including nitrogen, phosphorus, organic matter, iron, and manganese decreased by 30.63-80.15 %. Redundancy discriminant analysis revealed that environmental factors, especially the temperature and dissolved oxygen, were crucial drivers of the bacterial community structure. Furthermore, Spearman's correlation analysis between predominant genera and pollutants suggested that core genus played a vital role in pollutant reduction. Overall, our findings highlight the importance and provide insights on the artificial mixing systems' microbial mechanisms of reducing pollutants in drinking water reservoirs.

Enhanced nitrogen removal of micropolluted source waterbodies using an iron activated carbon system with siliceous materials: Insights into metabolic activity, biodiversity, interactions of core genus and co-existence

Aerobic denitrification technology can effectively abate the nitrogen pollution of water source reservoirs. In this study, 40% siliceous material was used as the carrier to replace the activated carbon in Fe/C material to enhance denitrification and purify water. The removal efficiency of new material for target pollutants were nitrate nitrogen (95.68%), total phosphorus (68.23%) and chemical oxygen demand (46.20%). Aerobic denitrification of water samples and anaerobic denitrification of sediments in three systems jointly assisted nitrogen removal. In a reactor with new material, diversity and richness of denitrifying bacterial communities were enhanced, and the symbiotic structure of aerobic denitrifying bacteria was more complex (Bacillus and Mycobacteria as the dominant bacteria); the microbial distribution better matched the Zif and Mandelbrot models. This system significantly increased the abundance of key enzymes in water samples. The new material effectively removed pollutants and represents a promising and innovative in-situ remediation method for reservoirs.

Autotrophic denitrification using Fe(II) as an electron donor: A novel prospective denitrification process

As a newly identified nitrogen loss pathway, the nitrate-dependent ferrous oxidation (NDFO) process is emerging as a research hotspot in the field of low carbon to nitrogen ratio (C/N) wastewater treatment. This review article provides an overview of the NDFO process and summarizes the functional microorganisms associated with NDFO from different perspectives. The potential mechanisms by which external factors such as influent pH, influent Fe(II)/N (mol), organic carbon, and chelating agents affect NDFO performance are also thoroughly discussed. As the electron-transfer mechanism of the NDFO process is still largely unknown, the extensive chemical Fe(II)-oxidizing nitrite-reducing pathway (NDFO_chem) of the NDFO process is described here, and the potential enzymatic electron transfer mechanisms involved are summarized. On this basis, a three-stage electron transfer pathway applicable to low C/N wastewater is proposed. Furthermore, the impact of Fe(III) mineral products on the NDFO process is revisited, and existing crusting prevention strategies are summarized. Finally, future challenges facing the NDFO process and new research directions are discussed, with the aim of further promoting the development and application of the NDFO process in the field of nitrogen removal.

Enhanced Cd(II) immobilization in sediment with zero-valent iron induced by hydrogenotrophic denitrification

In this study, an integrated system of Fe⁰ and hydrogenotrophic microbes mediated by nitrate (nitrate-mediated bio-Fe⁰, NMB-Fe⁰) was established to remediate Cd(II)-contaminated sediment. Solid phase characterization confirmed that aqueous Cd(II) (Cd(II)_aq) was successfully immobilized and enriched on iron surface due to promoted iron corrosion driven by hydrogenotrophic denitrification and subsequent greater biomineral production such as magnetite, lepidocrocite and green rust. Compared to a Cd(II)_aq removal of 21.1% in overlying water of the nitrate-mediated Fe⁰ (NM-Fe⁰) system, the NMB-Fe⁰ system obtained a much higher Cd(II)_aq removal of 83.1% after 7 d remediation. The leaching test and sequential extraction results also showed that the leachability of Cd(II) decreased by 75.9% while the residual fraction of Cd(II) increased by 185.7% in comparison with untreated sediment. Besides, the Cd(II)_aq removal raised with the increase of nitrate concentration and Fe⁰ dosage, further revealing the promotion effect of nitrate on Cd(II) removal by bio-Fe⁰. This study highlighted the involvement of bio-denitrification in the remediation of Cd(II)-contaminated sediment by Fe⁰ and provided a new insight to enhance its reactivity and applicability for Cd(II) immobilization.

Strengthen the purification of eutrophic water and improve the characteristics of sediment by functional ecological floating bed suspended calcium peroxide and sponge iron jointly

To overcome the shortcomings of conventional ecological floating bed (CEFB) in purifying landscape water, this study constructed a functional ecological floating bed (FEFB) through the suspension of calcium peroxide (CP) and sponge iron (SI) jointly below the CEFB. The purification effect of water quality and influence of sediment were compared in control check, CEFB, and FEFB systems, which were loaded the same sediment and reclaimed water in a field experiment. Results showed that the FEFB suspended with CP and SI had evident purification effect on the quality of landscape water supplied with reclaimed water and can maintain stably the nutrient status of the water body at mesotrophic levels and low turbidity. The FEFB promoted the degradation of humus, thus eliminating the chroma risk in water body caused by the decay of plants from the CEFB. Moreover, the FEFB can control the sediment mass produced, reduce the total nitrogen (TN) mass of sediment, and decrease the transformable TN (TTN) content in the sediment. The FEFB enhanced the stability of phosphorus (P) in the sediment, where the relative content of Ca-P and stable P reached 42.18% and 64.27%, respectively. To sum up, the FEFB suspended with SI and CP can not only effectively control the eutrophication and sensory index of landscape water but also change the TTN content and P forms in sediment, making the sediment more stable. Thus, the FEFB provides an innovative approach to reduce endogenous nutrient release for landscape water along with recharging with reclaimed water.

Roles of zero-valent iron in anaerobic digestion: Mechanisms, advances and perspectives

With the rapid growth of population and urbanization, more and more bio-wastes have been produced. Considering organics contained in bio-wastes, to recover resource from bio-wastes is of great significance, which can not only achieve the resource recycle, but also protect the environment. Anaerobic digestion (AD) has been proved as one of the most promising strategies to recover bio-energy from bio-wastes, as well as to realize the reduction of bio-wastes. However, the conventional interspecies electron transfer is sensitive to environmental shocks, such as high ammonia, organic pollutants, metal ions, etc., which lead to instability or failure of AD. The recent findings have proved that the introduction of zero-valent iron (ZVI) in AD system can significantly enhance methane production from bio-wastes. This review systematically highlighted the recent advances on the roles of ZVI in AD, including underlying mechanisms of ZVI on AD, performance enhancement of AD contributed by ZVI, and impact factors of AD regulated by ZVI. Furthermore, current limitations and outlooks have been analyzed and concluded. The roles of ZVI on underlying mechanisms in AD include regulating reaction conditions, electron transfer mode and function of microbial communities. The addition of ZVI in AD can not only enhance bio-energy recovery and toxic contaminants removal from bio-wastes, but also have the potential to buffer adverse effect caused by inhibitors. Moreover, the electron transfer modes induced by ZVI include both interspecies hydrogen transfer and direct interspecies electron transfer pathways. How to comprehensively evaluate the effects of ZVI on AD and further improve the roles of ZVI in AD is urgently needed for practical application of ZVI in AD. This review aims to provide some references for the introduction of ZVI in AD for enhancing bio-energy recovery from bio-wastes.

Synthesis of self-renewing Fe(0)-dispersed ordered mesoporous carbon for electrocatalytic reduction of nitrates to nitrogen

In electrocatalytic reduction of nitrates to nitrogen, key issues are electrode activity, sustainable materials, preparation methods and cost. Herein, lignin, Fe³⁺ ion, and non-ionic surfactant were combined with evaporation-induced self-assembly (EISA) to prepare zero-valent Fe-dispersed ordered mesoporous carbon (OMC) electrode materials denoted as Fe#OMC. The method developed for preparing Fe-coordinated OMC material avoids the use of toxic phenols, aldehyde reagents and metal doping compounds. When synthesized Fe#OMC samples were applied as electrode materials for the electrocatalytic reduction of nitrate in aqueous solutions, maximum nitrate nitrogen removal was as high as 5373 mg N·g^-1 Fe from aqueous solutions containing 400 mg·L^-1 NO₃^--N, while nitrogen selectivity was close to 100%, exceeding catalytic performance of comparable materials. Active hydrogen produced by electrolysis of water during the reaction re-reduced Fe ions formed in the OMC material and stabilized Fe#OMC electrode performance and recycle. The Fe#OMC electrode is self-renewing with respect to its Fe zero-valent state, is simple to prepare from sustainable materials and is effective for electrocatalytic reduction of nitrate or nitrogen-containing compounds in water.

Enhancing nitrate removal efficiency of micro-sized zero-valent iron by chitosan gel balls encapsulating

The activity of micro-sized zero-valent iron (MZVI) material for nitrate removal in neutral pH and low C/N ratios water needs to be improved. In this study, micro-sized zero-valent iron@chitosan (MZVI@CS) material was synthesized through embedding MZVI particles into chitosan (CS) gel by sol-gel method, and was used for deep removal of NO₃^--N in the absence of organic carbon sources and neutral pH. The NO₃^--N removal rate of MZVI@CS was 0.37 mg-N·L^-1·d^-1 (dosage of 1%, initial pH = 7, 25 °C, initial nitrate concentration = 15 mg-N·L^-1), which was 11.33 times higher than that of MZVI. The apparent activation energy (E_a) of MZVI@CS with nitrate was 38.23 kJ·mol^-1. MZVI@CS can remove nitrate effectively at a low concentration (15 mg-N·L^-1). A stable denitration rate (0.37-2.28 mg-N·L^-1·d^-1) could be maintained under weak acidic, neutral and alkaline conditions (pH = 5-9). More than 80% of reduced nitrate was converted to N₂, and only a small amount was converted to NH₄⁺ or NO₂^-. The gel structure of MZVI@CS eliminated the agglomeration between MZVI particles while the forming of Fe-CS chelates reduced the formation of iron oxide and solved the problems of passivation, hence successfully strengthened the NO₃^--N removal efficiency of MZVI. Therefore MZVI@CS has great application potential in NO₃^--N deep removal of water bodies with neutral pH and low C/N ratios.

Use of sponge iron dosing in baffled subsurface-flow constructed wetlands for treatment of wastewater treatment plant effluents during autumn and winter

Sponge iron (SI) is widely used in water treatment. As effluents from wastewater treatment plant (WWTP) require advanced treatment methodology, three forms of constructed wetlands (CWs): wetlands with sponge iron (SI), copper sulfate modified sponge iron (Cu/SI), and sponge iron coupled with solid carbon sources (C/SI), have been investigated in this paper for the removal effects of organic matter and nutrients in WWTP effluents, and the corresponding mechanisms have been analyzed. The results showed the effect of baffled subsurface-flow constructed wetland (BSFCW) with SI dosing to purify the WWTP effluents after the stable operation. The water flow of this BSFCW is the repeated combination of upward flow and downward flow, which can provide a longer treatment pathway and microbial exposure time. The average removal rates of total inorganic nitrogen (TIN) were 27.80%, 30.17%, and 44.83%, and the average removal rates of chemical oxygen demand (COD) were 19.96%, 23.73%, and 18.38%. The average removal rates of total phosphorus (TP) were 85.94%, 82.14%, and 83.95%. Cu/SI improved the dissolution of iron, C/SI improved denitrification, and a winter indoor temperature retention measure was adopted to increase the effectiveness of wetland treatment during the winter months. After comprehensively analyzing X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and two-dimensional numerical simulation diagrams, a plausible conjecture that microbes use electrons from SI for autotrophic denitrification is presented. Moreover, the stress effect of wetlands dosed with SI on plants decreased stepwise along the course since C/SI used on wetlands had less impact on plant stress.

Impact mechanism and performance enhancement of ultrasound on ZVI-anammox system

The zero-valent iron-anaerobic ammonium oxidation (ZVI-anammox) system has received widespread attention due to its excellent nitrogen removal performance and user-friendly operation. However, its disadvantages include a short service life, high ZVI consumption, and poor system stability. The use of ultrasound as a physical method is increasing in various water treatment processes. In this study, a series of batch tests were conducted to obtain the best ultrasonic parameter and explore the comprehensive effects of ultrasound on a ZVI-anammox system. The highest specific anammox activity of the ZVI-anammox system was found to be 2.88 mg total nitrogen/g of volatile suspended solids/h after ultrasonic treatment (0.2 w/mL, 5 min), which was 37.85% higher than a control group. Additionally, the service life of ZVI extended by 28.57% and the total nitrogen removal efficiency changed from 58.03-72.08 to 63.92-78.33% under ultrasonic irradiation. These phenomena were related to the mechanical force and cavitation of ultrasonic waves. Judging from the characteristics of sludge and ZVI, ultrasound can promote anammox sludge granulation, ease ZVI passivation, and enhance the stability of the entire system. This paper also briefly discusses the impact mechanisms of ultrasound on the ZVI-anammox system. In brief, ultrasound destroys the surface structure of ZVI and thus provides numerous attachment points for microorganisms that improve the performance of the entire system. The proposed ultrasound combined with ZVI is a novel method that has potential for use in large-scale engineering applications in actual sewage treatment after comprehensive analysis.

Autotrophic Fe-Driven Biological Nitrogen Removal Technologies for Sustainable Wastewater Treatment

Fe-driven biological nitrogen removal (FeBNR) has become one of the main technologies in water pollution remediation due to its economy, safety and mild reaction conditions. This paper systematically summarizes abiotic and biotic reactions in the Fe and N cycles, including nitrate/nitrite-dependent anaerobic Fe(II) oxidation (NDAFO) and anaerobic ammonium oxidation coupled with Fe(III) reduction (Feammox). The biodiversity of iron-oxidizing microorganisms for nitrate/nitrite reduction and iron-reducing microorganisms for ammonium oxidation are reviewed. The effects of environmental factors, e.g., pH, redox potential, Fe species, extracellular electron shuttles and natural organic matter, on the FeBNR reaction rate are analyzed. Current application advances in natural and artificial wastewater treatment are introduced with some typical experimental and application cases. Autotrophic FeBNR can treat low-C/N wastewater and greatly benefit the sustainable development of environmentally friendly biotechnologies for advanced nitrogen control.

Iron-Activated Carbon Systems to Enhance Aboriginal Aerobic Denitrifying Bacterial Consortium for Improved Treatment of Micro-Polluted Reservoir Water: Performances, Mechanisms, and Implications

Although many source waterbodies face nitrogen pollution problems, the lack of organic electron donors causes difficulties when aerobic denitrifying bacteria are used to treat micro-polluted water. Different forms of iron with granular activated carbon (AC) as carriers were used to stimulate aboriginal microorganisms for the purification of micro-polluted source water. Compared with the iron-absent AC system, targeted pollutants were significantly removed (75.76% for nitrate nitrogen, 95.90% for total phosphorus, and 80.59% for chemical oxygen demand) in the sponge-iron-modified AC system, which indicated that iron promoted the physical and chemical removal of pollutants. In addition, high-throughput sequencing showed that bacterial distribution and interaction were changed by ion dosage, which was beneficial for pollutant transformation and reduction. Microbial functions, such as pollutant removal and expression of functional enzymes that were responsible for the transformation of nitrate nitrogen to ammonia, were highly efficient in iron-applied systems. This study provides an innovative strategy to strengthen in situ remediation of micro-pollution in waterbodies.

Magnetite-loaded rice husk biochar promoted the denitrification performance of Aquabacterium sp. XL4 under low carbon to nitrogen ratio: Optimization and mechanism

The removal of nitrate (NO₃^--N) under the low carbon to nitrogen (C/N) ratio is a widespread issue. Here in, a modified biochar (MRHB) was prepared by combining rice husk and magnetite to promote the denitrification performance of Aquabacterium sp. XL4 under low C/N ratio. In addition, when the modified H₂O₂ concentration was 0.6 mM, the dosage was 5.0 g L^-1, the C/N ratio was 1.5, and the pH was neutral, the nitrate removal efficiency is 97.9%. Fluorescence excitation-emission matrix spectra (3D-EEM) showed that the metabolism of strain XL4 was stable under optimal conditions. Furthermore, the results of flow cytometry (FC) showed that the amounts of intact cells with MRHB was excellent. The measurement of cytochrome c concentration, total membrane permeability (T_mp), electron transport system activity (ETSA), and cyclic voltammetry curve (CV) confirmed that the MRHB improved the electron transfer and membrane activity of strain XL4.

The promotion and inhibition effect of graphene oxide on the process of microbial denitrification at low temperature

This study found that graphene oxide (GO) improved microbial denitrification at low temperatures (~12 °C), and the optimal concentration was 10 mg/L as the removal rate of NO₃-N increased by 17%. At the optimal concentration, GO improved the electron transport system activity of the microbes and enhanced the activity of nitrate reductase and nitrite reductase while exhibited low microbial toxicity. The addition of GO increased the content of tightly bound extracellular polymeric substances (EPS). The results of fluorescence spectrometer indicated that GO accelerated the renewal of bound EPS (B-EPS). Fourier Transform infrared spectroscopy (FTIR) results showed that GO affected the secondary structure of the protein in B-EPS, making B-EPS more hydrophobic and promoting microbial aggregation. B-EPS affected by GO can promote the electron transfer process of microorganisms. However, high concentration (>25 mg/L) of GO may inhibit denitrification by competing for electrons, which was not conducive to denitrification thermodynamically.

Performances and mechanisms of simultaneous nitrate and phosphate removal in sponge iron biofilter

Sponge iron is a potential material for nitrogen and phosphate removal. To explore the performances and mechanisms of nitrogen and phosphate removal by sponge iron, a sponge iron biofilter was established. The results showed that nitrate was completely removed at HRT of 48 h without external carbon source and at HRT of 3 h with C/N ratio of 5. Furthermore, it was easy to achieve partial denitrification at HRT of 1 h with C/N ratio of 3. The mechanisms of nitrate removal were chemical reduction and nitrate dependent ferrous oxidation without external carbon source and heterotrophic denitrification with external carbon source. XPS result indicated that phosphate was removed by the formation of ferric phosphate precipitation. High throughput sequencing showed that iron-oxidizing bacteria Gallionellaceae was highly enriched in biofilter, accounting for 17.83%, which indicated that it was feasible to achieve autotrophic nitrate removal by sponge iron biofilter.

Simultaneous removal of nitrate and diethyl phthalate using a novel sponge-based biocarrier combined modified walnut shell biochar with Fe3O4 in the immobilized bioreactor

A novel biological carrier combining sponge and modified walnut shell biochar with Fe₃O₄ (MWSB@Fe₃O₄) was fabricated to achieve simultaneous removal of nitrate and diethyl phthalate (DEP). The optimal reaction conditions of the immobilized bioreactor were: carbon to nitrogen (C/N) ratio of 1.5, Fe²⁺ concentration of 20 mg L^-1, and hydraulic retention time (HRT) of 8 h. Under the optimal conditions and DEP concentration of 800 μg L^-1, the highest removal efficiency of DEP and nitrate in the immobilized bioreactor with the novel biological carrier were 67.87% and 83.97% (68.43 μg L^-1 h^-1 and 1.71 mg L^-1 h^-1), respectively. Scanning electron microscopy (SEM) showed that the novel biological carrier in this study carried more bio-sediments which is closely related to the denitrification efficiency. The gas chromatography (GC) data showed that the nitrogen production of the immobilized bioreactor (99.85%) was higher than that of another experimental group (97.84%). Fluorescence excitation-emission matrix (EEM) and Fourier transform infrared spectrometer (FTIR) indicated the immobilized bioreactor emerged more extracellular polymeric substances (EPS) which was related to favourable biological stability under the DEP environment. Moreover, according to high-throughput sequencing data, the Zoogloea sp. L2 responsible for iron-reduction and denitrification was the main strain in this immobilized bioreactor.