The deep challenge of nitrate pollution in river water of China

Nitrous oxide dominates greenhouse gas emissions from hydropower's reservoirs in China from 2020 to 2060

China is ambitious to increase its hydropower share to mitigate climate changes. The greenhouse gas (GHG) emissions from hydroelectric reservoirs may hinder the climate goal. The spatio-temporal patterns of such emissions under future climate changes at the national scale are not clearly addressed. In this study, we evaluate these emissions from 79 hydroelectric reservoirs across China (61.22 % of the national hydropower generation) in 2020, covering carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), using the G-res (Greenhouse Gas Reservoir) tool and Integrated Model to Assess the Global Environment-Dynamic Global Nutrient Model (IMAGE-DGNM). A random forest (RF) model is also used to project the emissions in the period of 2020 to 2060 under Shared Socioeconomic Pathway (SSP) scenarios. The results indicate that the carbon intensity (CI) and areal flux varied largely. The reservoirs located in low-altitude areas and older reservoirs generally have higher CIs. N2O contributed with more than 80 % of the total GHG emission, in which the NH4+ concentration is a key factor influencing the N2O emissions. The projection shows that these emissions will increase by 1.30 %, 6.63 %, and 17.33 % in 2060 compared to 2020 under the SSP119, SSP245, and SSP585 scenarios, respectively, in which CH4 has the largest growth. Finally, implications toward reduction in such emissions are discussed.

Insights on nitrate pollution-induced intestinal dysfunction in turbot (Scophthalmus maximus) revealed by integrated dynamic metabolomics and transcriptomics

Nitrate pollution in aquatic ecosystems has attracted global attention and has toxic effects on marine organisms. However, the precise molecular mechanisms underlying nitrate toxicity in the fish gut remain obscure. To this end, turbot were subjected to nitrate exposure (200 mg/L NO3-N) for 0, 10, 20, and 30 days to explore nitrate toxicity and metabolic mechanisms in the gut by employing a multi-omics analysis integrating metabolomics with transcriptomics. The metabolomics analysis showed that nitrate exposure resulted in significant changes in the intestinal metabolite network, implying that the intestinal metabolism of turbot was impaired. Metabolites Pathway Analysis (MetPA) results revealed that the metabolic pathways significantly impacted by nitrate exposure included amino-acid metabolism pathways, such as phenylalanine, tyrosine, and tryptophan biosynthesis, phenylalanine metabolism, arginine biosynthesis, D-glutamine and D-glutamate metabolism, and aminoacyl-tRNA biosynthesis. Additionally, network interaction analysis between key differential metabolites (DMs) and differentially expressed genes (DEGs) identified seven essential amino acids associated with this process. Short Time-series Expression Miner (STEM) analysis determined that six distinct temporal expression patterns exhibited dynamic changes in DMs, mainly enriched in the metabolism of carbohydrates and lipids, indicating an increased energy demand to withstand nitrate stress. Multi-omics analysis revealed that sustained nitrate stress can interfere with protein digestion and absorption, alter collagen anabolism and specific composition of the extracellular matrix (ECM), and ultimately disrupt intestinal homeostasis. Our findings enhance our understanding of nitrate toxicity in fish and offer insights that can improve nitrate management in marine ecosystems.

Light-regulated dentification and dissimilatory nitrate reduction by nano-bio electric syntrophic consortium

Microbial-oriented nitrogen recycling is a vital strategy for nitrogen pollution control in the treatment of low C/N wastewater. However, the deficient electron donors in water body limits the reactive nitrogen recovery. Herein, we design a nano-bio electric syntrophic consortium for light-regulated dentification and dissimilatory nitrate reduction to ammonium (DNRA) without organic carbon sources input. Using Fe0 coupons as the sole electron donor, the extracellular electron uptake rate of a model denitrifier (Pseudomonas aeruginosa PAO1) is enhanced by coculturing it with an electroactive bioregulator, Shewanella oneidensis MR-1 (MR-1), thereby achieving an average nitrate removal rate of 63.8 ± 0.1 mg N/d/L with ammonium recovery efficiency of 27.1 ± 0.2 % under illumination. Notably, in situ self-assembled FeS nanoparticles via a bottom-up Fe0 biocorrosion approach are observed on the outer membrane and periplasmic space of MR-1. Under illumination, native MtrCOmcA-CymA protein complex and FeS nanoparticles act in electron conduits to facilitate transmembrane photoelectron uptake of MR-1 for microbial DNRA process. Biochemical and transcriptomic analyses reveal that the NADH generation, chemotaxis moving and energy-taxis of MR-1 hybrids strength the driving force for microbial DNRA process. Overall, we demonstrate that the constructed FeS-assisted coculture, as an emerging model of electric syntrophy, could support the solar-triggered nitrogen metabolism from non-phototrophic microbes. Given that Fe0 biocorrosion is a facile route to MR-1 growth, this nano-bio system also affords a promising pathway for low C/N wastewater treatment and reactive nitrogen recovery via DNRA process.

A Cu0.76Co2.24O4/γ-Cu2(OH)3Cl composite catalyst for efficient neutral nitrate reduction

The electrocatalytic nitrate reduction reaction (eNO3-RR) is an environmentally friendly process that converts nitrate wastewater into high-value ammonia (NH3). However, the multi-step electron and proton transfer in this reaction leads to slow kinetics and competitive reactions, making it challenging to achieve energy-efficient performance. Herein, a Cu0.76Co2.24O4/γ-Cu2(OH)3Cl (CCOC) composite has been prepared as an electrocatalyst for the eNO3-RR. The CCOC catalyst demonstrated an outstanding NH3 yield rate of 10.71 mg h-1 cm-2 and a remarkable faradaic efficiency (FE) of 95.9% in a 0.5 M Na2SO4 neutral solution containing 0.1 M NO3-, surpassing most reported catalysts under neutral conditions. In situ investigations demonstrated that Cu0.76Co2.24O4 with high-valent Cuδ+ and Coδ+ significantly enhances H2O dissociation and proton production while also promoting the adsorption of NO3- and *NH intermediates. These properties contribute to the high NH3 selectivity and activity observed under neutral conditions. This work demonstrates Cu0.76Co2.24O4/γ-Cu2(OH)3Cl as a promising candidate for the sustainable and efficient production of NH3 through the eNO3-RR, offering new insights into efficient nitrate reduction in neutral environments.

Inorganic nitrogen and phytoplankton dynamics in a subtropical reservoir during low-flow winter periods: Implications for nitrogen management

The dynamics of nutrient cycling in inland waters, particularly during algal blooms, play a critical role in shaping aquatic ecosystems. However, the interaction between phytoplankton and inorganic nitrogen during low-flow periods remains poorly understood. This study presents high-frequency monitoring of ammonium (NH4-N), nitrate (NO3-N), nitrite (NO2-N) and phytoplankton communities in a subtropical river-type reservoir during the low-flow winter period, characterized by blooms dominated by cryptophytes and green algae. Our results revealed that NH4-N concentrations exhibited a diurnal pattern of decreasing during the day and increasing during the night, which was negatively correlated with total algal biomass at the intraday fluctuation scale (coefficient = -0.378, p = 0.028), indicating strong algal uptake of ammonium during daytime. NO3-N and NO2-N concentrations, however, did not show clear diurnal co-varied patterns with algae. On the day-to-day scale, the external nitrogen inputs resulting from rainfall contributed to the changes, particularly after extended dry periods. We observed low NH4-N concentrations and total algal biomass during the end of algal bloom. However, 3-4 days later ammonium thrived, followed by another algal bloom. Algal bloom occurrences caused large diurnal fluctuations in reservoir NH4-N concentrations (daily differences >5 μmol L-1), resulting the maximum nighttime NH4-N flux reaching up to five times the minimum daytime flux. Our study highlights the advantages of high-frequency synchronous monitoring of nutrient-algae dynamics to understand their interactions, and the importance of ammonium control on preventing algal blooms during low-flow winter periods.

γ-Al2O3 selectively adsorbs transition group metals from contaminated waters to produce bi-metallic catalysts for efficient nitrate reduction

Metals present in industrial or mining wastewater are hazardous to human health and the environment, and their remediation is costly. In this study, we used γ-Al2O3 to capture transition metals (e.g., Pd, Cu, Ni, and Co) from wastewater and used the spent adsorbent as catalysts for nitrate reduction. Based on a pH-dependent surface-mediated redox reaction mechanism, Pd and Cu ions are selectively adsorbed onto γ-Al2O3 at ambient temperature without involving the use of chemical reductants, thus transforming these pollutants into active catalytic sites. We particularly focused on Pd/Cu bimetallic catalysts, finding that the Pd and Cu ion concentrations in contaminated water impacted both nitrate reduction efficiency and nitrogen selectivity. The best nitrate reduction occurred with 0.74 wt% Pd and 0.66 wt% Cu on γ-Al2O3, achieving high nitrate reactivity and ∼81 % selectivity for N2 formation over 10 cycles. Ni and Co, which have smaller redox potential differences than Pd and Cu, showed limited interference with Pd and Cu adsorption, allowing γ-Al2O3 to form Pd-Cu bimetallic catalysts selectively in batch and column tests. Overall, metal-based catalysts can be fabricated under ambient conditions while remediating metal-contaminated wastewater, thereby producing functional products (e.g., hydrogenation catalysts), which is consistent with circular economy principles.

Nitrate pollution in urban runoff: A comprehensive risk assessment for human and ecological health

Nitrate pollution in urban runoff poses significant environmental and public health risks, with its impact varying across different land use types. This study investigates nitrate concentrations in runoff from residential, commercial, industrial, and traffic zones in Tehran, Iran, using Event Mean Concentration (EMC) analysis and Monte Carlo simulations to assess both ecological and human health risks. The results indicate that industrial and traffic zones exhibit the highest nitrate concentrations, reaching 58.13 mg/L, significantly exceeding regulatory thresholds. Ecological risk assessments highlight the potential for aquatic system degradation, while health risk evaluations reveal hazard index (HI) values surpassing the safe limit (HI > 4), particularly in industrial and high-traffic areas. These findings underscore the need for targeted mitigation strategies, including the implementation of green infrastructure and stricter pollution control measures, to improve urban water quality and reduce associated risks.

The In Situ Assembly of an Equipotential Cathode for Nitrite Enrichment Enabling Electrochemical Nitrate Reduction to N2

Electrocatalytically reducing NO3- to N2 is of great significance for environmental remediation and global nitrogen cycling. However, it is currently hindered by low N2 selectivity since adsorbate N-intermediates are hard to migrate and couple each other during the N-N coupling step. Herein, an in situ assembly strategy was taken to attach Pd@Cu2O nanoparticles with CuO nanowire arrays to form an equipotential cathode CuO-Pd@Cu2O, which optimized N2 selectivity to 91%, much higher than that of directly loaded Pd-Cu cathode (55%). Theoretical calculations combined with in situ spectroscopies demonstrated that the equipotential cathode can shield the electric field and enrich NO2- intermediate inside. Meanwhile, a unique reaction pathway was revealed that the enriched NO2- can directly couple with *N and also tune the Pd d-band center, avoiding the hurdles in N-N coupling. The approach here provides a new perspective in cathode design and a mechanistic understanding for the N-N coupling reaction.

Enhanced removal of carbamazepine by microalgal-fungal symbiotic systems in the presence of Mn(II): Synergistic mechanisms and microbial community dynamics

Microalgal-fungal symbiotic systems (MFSS) have emerged as a promising approach for wastewater treatment, yet the mechanisms driving reactive oxygen species (ROS) generation and pharmaceutical pollutant removal remain underexplored. This study investigates the synergistic interactions within MFSS and their role in Mn(II) oxidation, with a focus on enhancing carbamazepine (CBZ) degradation and microbial community dynamics. The results reveal that microalgal-fungal interactions inhibit Fe-S cluster activity, disrupting electron transport chains and promoting extracellular superoxide production. This superoxide surge directly accelerates Mn(II) oxidation, while Mn(III) and ROS drive synergistic effects to amplify CBZ removal efficiency. Notably, system-specific variations in superoxide generation were observed across different MFSS configurations, determining their degradation performance. Water quality factors, such as microbial community complexity and nitrate concentration, play crucial roles in CBZ degradation in natural water systems. High-throughput sequencing reveals dynamic shifts in bacterial and eukaryotic communities, highlighting their synergistic interactions in pollutant degradation. Temporal and spatial changes in microbial community structure suggest that the system evolves into a more adaptive configuration during pollutant treatment, enhancing long-term stability. These findings advance the mechanistic understanding of ROS-mediated pollutant degradation in MFSS and provide actionable strategies for optimizing bioremediation systems in engineered and natural water environments.

Differential effects of environmentally relevant concentrations of ibuprofen on denitrification and nitrous oxide emissions in river sediments

The increasing presence of ibuprofen in aquatic ecosystems poses significant challenges to their biogeochemical functions, including nitrogen transformations. In this study, we employed 15N-labeling techniques to investigate the effects of environmentally relevant concentrations of ibuprofen (0-10,000 ng L-1) on denitrification and the associated nitrous oxide (N2O) emissions in river sediments over a 60-day period. The results revealed a hump-shaped response in denitrification rates to ibuprofen addition across a range of nitrate concentrations (1-60 mg N L-1), with rates peaking near 200 ng L-1 ibuprofen, followed by inhibition at certain higher concentrations, leading to a reduction of up to 25.8 % compared to the treatment without ibuprofen. Kinetic analysis showed that the maximum denitrification rate followed the same hump-shaped trend, despite a decrease in nitrate affinity with increasing ibuprofen concentrations. The abundance of denitrifying bacteria mirrored the pattern observed in denitrification rates across different ibuprofen concentrations. However, increasing ibuprofen concentrations consistently accelerated N2O production rates. Microbial analysis suggests that the increase in N2O production genes was faster than for reduction genes, while the decrease was slower with increasing ibuprofen concentrations. This study highlights the hump-shaped response of denitrification rates and the consistent increase in N2O emissions induced by ibuprofen, offering insights for developing environmental management strategies to mitigate ibuprofen and nitrogen pollution, as well as reducing N2O emissions.

Nitrate induced hepatic fibrosis in tadpoles of Bufo gargarizans by mediating alterations in toll-like receptor signaling pathways

The nitrate pollution has become an increasingly serious environmental problem worldwide, and the toxic effects of elevated nitrate levels in the environment on aquatic animals remain to be elucidated. The purpose of the present study was to investigate the mechanisms of liver injury to tadpoles after exposure to nitrate from embryonic to metamorphic climax and to assess the recovery process of liver function after cessation of exposure. In the group with continuous nitrate exposure, the livers and thyroid of tadpoles showed remarkably histological lesions, of this with structural disorganization of the hepatocytes, cellular atrophy, and fibrosis, as well as significant reduction in the follicular and colloidal area of the thyroid. Meanwhile, the expression levels of genes related to inflammatory signaling pathways, such as TLR2, TLR6 and NF-κB, were significant elevated. After termination of exposure at Gs23, liver damage (histologic, ultrastructural, and molecular levels) was almost completely recovered, whereas thyroid gland damage was irreversible. Overall, this study shed light on the harmful effects of nitrate pollution on amphibian health and emphasizes the importance of controlling nitrate emissions in the environment.

Nitrogen, sulfur, iron, and microbial communities co-shape the seasonal biogeochemical behaviors of As and Sb in coastal tidal flat wetlands associated with rivers

Arsenic (As) and antimony (Sb) are affected by complex biogeochemical processes in coastal ecosystems. However, the influence of N, S, Fe, and microbial communities on the biogeochemistry of As and Sb in coastal tidal flat wetlands remain uncertain, particularly when rivers flow into these areas. This study combined diffusive gradients in the thin-film technique with high-throughput sequencing to investigate the release and vertical distribution of As and Sb in river and coastal tidal flat wetland sediments. The results indicated a distinct stratification phenomenon in the As release at depths ranging from 20 mm to -150 mm. At river sites, the release of As occurred in the upper layer (above -40 mm), with peak values of 4.3 and 9.3 μg/L at HS and SY sites in summer, respectively, likely due to anaerobic ammonium oxidation. In the lower layer (below -40 mm), both As and Sb were released, and this was possibly due to Fe reduction. However, at the coastal tidal flat sites, the release of As and Sb may have been driven by anaerobic ammonium oxidation, dissimilatory nitrate reduction to ammonium, sulfate reduction, and Fe reduction. At the river sites, As exhibited increased activity during the summer, and the residual forms were converted more easily into mobile forms. Sb remained relatively stable during both winter and summer. Conversely, both As and Sb primarily existed in residual forms and exhibited higher stability during summer in the coastal tidal flat sites. The microbial phyla Nitrospirota (3.6-7.0 %) and Acidobacteriota (9.5-10.2 %) were more prevalent at the river sites, whereas Desulfobacterota (8.8-12.0 %) and Firmicutes (0.13-27.9 %) were more prevalent at the coastal tidal flat sites. The bacterial genera involved in the N, S, and Fe transformation processes differed between the two sites, and they were primarily Thiobacillus, Limnobacter, and Sulfurovum at the river sites and Sva1033, Anaeromyxobacter, and Sva0485 at the coastal tidal flat sites. In this study, the microorganisms that mediated N, S, and Fe complex processes at various depths in the sediment-water interface were decoupled to elucidate the effect of these processes on the biogeochemical behavior of As and Sb as they move from rivers to coastal tidal flat wetlands.

Synergistic Effect of Boron Doping and Porous Structures on Titanium Dioxide for Efficient Photocatalytic Nitrate Reduction to Nitrogen in Pure Water

Photocatalytic reduction of nitrate to N2 holds great significance for environmental governance. However, the selectivity of nitrate reduction to N2 is influenced by sacrificial agents and the kinds of cocatalysts (such as Pt and Ag). The presence of unconsumed sacrificial agents can aggravate environmental pollution, while noble metal-based cocatalysts increase application costs. Herein, the porous boron-doped TiO2 (B-TiO2) was successfully synthesized by using the sol-gel method with Amberlite IRA-900 as a template. The incorporation of 33% boron into TiO2 (33% B-TiO2) achieved a 100% nitrate (20 ppm) conversion rate and 94.5% N2 selectivity without the need for sacrificial agents and cocatalysts during the nitrate reduction process. The catalyst's unique multistage porous structure not only enhances the light absorption ability but also significantly provides abundant surface adsorption sites. Additionally, theoretical studies show that boron doping effectively modulates the band structure of TiO2 and increases the electron density at the Ti surface active sites, both of which are essential for achieving a high nitrate reduction efficiency. This work emphasizes the synergistic effect between morphology control and electronic structure regulation in promoting the photocatalytic reduction of nitrate to nitrogen, providing valuable insights for the photocatalytic treatment of nitrate wastewater.

Biogenic sulfide by sulfur disproportionation enhances nitrate removal and reduces N2O production during sulfur autotrophic denitrification

Sulfur autotrophic denitrification (SADN) is regarded as a cost-effective bioremediation technology for nitrate-contaminated water. Nevertheless, the low bioavailability of sulfur is a major challenge that hinders nitrogen removal efficiency. A sulfur autotrophic disproportionation (SADP) process was proposed to convert sulfur to biogenic sulfide, greatly increasing the availability of electron donors. Throughout the 201-day laboratory-scale test, it was observed that the SADP process achieved desirable performance with 198.87 ± 39.8 mg S/L biogenic sulfide production per day, which could provide sufficient electron donors for the SADN process in treatment of 671.22 ± 134.40 mg N/L/d nitrate. Microbial community analysis confirmed the presence and dominancy of sulfur-disproportionating bacteria (SDB) (e.g., Desulfocaspa sp. taking up to 8.27% of the entire microbial community), while Thiobacillus was the most dominant genus of sulfur oxidizing bacteria (SOB), accounting for 87.32% of the entire community. Further experiments revealed that the addition of chemical and biogenic sulfides enhanced the nitrate removal rate of the SADN process by a factor of 1.31 and 1.34, respectively. Additionally, biogenic sulfide was found to be the most effective nitrous oxide (N2O) mitigator, reducing emission by 82% and 95% in denitrification and denitritation processes, respectively. The results demonstrated that the integrated SADP and SADN processes was a more effective and carbon-neutral alternative in treatment of nitrate-contaminated water.

Electrocatalytic conversion of nitrate to ammonia on the oxygen vacancy engineering of zinc oxide for nitrogen recovery from nitrate-polluted surface water

Nitrate pollution in surface water poses a significant threat to drinking water safety. The integration of electrocatalytic reduction reaction of nitrate (NO3RR) to ammonia with ammonia collection processes offers a sustainable approach to nitrogen recovery from nitrate-polluted surface water. However, the low catalytic activity of existing catalysts has resulted in excessive energy consumption for NO3RR. Herein, we developed a facile approach of electrochemical reduction to generate oxygen vacancy (Ov) on zinc oxide nanoparticles (ZnO1-x NPs) to enhance catalytic activity. The ZnO1-x NPs achieved a high NH3-N selectivity of 92.4% and NH3-N production rate of 1007.9 [Formula: see text] h-1 m-2 at -0.65 V vs. RHE in 22.5 mg L-1NO3--N, surpassing both pristine ZnO and the majority of catalysts reported in the literature. DFT calculations with in-situ Raman spectroscopy and ESR analysis revealed that the presence of Ov significantly increased the affinity for the NO3- (nitrate) and key intermediate of NO2- (nitrite). The strong adsorption of NO3- on Ov decreased the energy barrier of potential determining step (NO3- →∗NO3) from 0.49 to 0.1 eV, boosting the reaction rate. Furthermore, the strong adsorption of NO2- on Ov prevented its escape from the active sites, thereby minimizing NO2- by-product formation and enhancing ammonia selectivity. Moreover, the NO3RR, when coupled with a membrane separation process, achieved a 100% nitrogen recycling efficiency with low energy consumption of 0.55 kWh molN-1 at a flow rate below 112 mL min-1 for the treatment of nitrate-polluted lake water. These results demonstrate that ZnO1-x NPs are a reliable catalytic material for NO₃RR, enabling the development of a sustainable technology for nitrogen recovery from nitrate-polluted surface water.

Improving future agricultural sustainability by optimizing crop distributions in China

Improving agricultural sustainability is a global challenge, particularly for China's high-input and low-efficiency cropping systems with environmental tradeoffs. Although national strategies have been implemented to achieve Sustainable Development Goals in agriculture, the potential contributions of crop switching as a promising solution under varying future climate change are still under-explored. Here, we optimize cropping patterns spatially with the targets of enhancing agriculture production, reducing environmental burdens, and achieving sustainable fertilization across different climate scenarios. Compared with current cropping patterns, the optimal crop distributions under different climate scenarios consistently suggest allocating the planting areas of maize and rapeseed to the other crops (rice, wheat, soybean, peanut, and potato). Such crop switching can consequently increase crop production by 14.1%, with accompanying reductions in environmental impacts (8.2% for leached nitrogen and 24.0% for irrigation water use) across three representative Shared Socio-economic Pathways from 2020 to 2100. The sustainable fertilization rates vary from 148-173 kg N ha-1 in 2030 to 213-253 kg N ha-1 in 2070, significantly smaller than the current rate (305 kg N ha-1). These outcomes highlight large potential benefits of crop switching and fertilizer management for improving China's future agricultural sustainability.

Nitrate sources and transformation in surface water and groundwater in Huazhou District, Shaanxi, China: Integrated research using hydrochemistry, isotopes and MixSIAR model

Global water resources affected by excessive nitrate (NO3-) have caused a series of human health and ecological problems. Therefore, identification of NO3- sources and transformations is of pivotal significance in the strategic governance of widespread NO3- contaminant. In this investigation, a combination of statistical analysis, chemical indicators, isotopes, and MixSIAR model approaches was adopted to reveal the hydrochemical factors affecting NO3- concentrations and quantify the contribution of each source to NO3- concentrations in surface water and groundwater. The findings revealed that high groundwater NO3- concentration is concentrated in the southwestern region, peaking at 271 mg/L. NO3- concentration in the Wei River and Yuxian River exhibited an increase from upstream to downstream, but in the Shidi River and Luowen River, its concentration was highest in the upstream. Groundwater NO3- has noticeable correlation with Na+, Ca2+, Mg2+, Cl-, HCO3-, TDS, EC, and ORP. In surface water, NO3- level is significantly correlated with NH4+ and ORP. Major sources of NO3- in surface and groundwater comprise manure & sewage and soil nitrogen. Source contribution for surface water was calculated by MixSIAR model to obtain soil nitrogen (57.7%), manure & sewage (23.8%), chemical fertilizer (12%), and atmospheric deposition (6.4%). In groundwater, soil nitrogen and manure & sewage accounted for 19% and 63.8% of nitrate sources, respectively. Both surface water and groundwater exhibited strong oxidation, with nitrification the primary process. It is expected that this study will provide insights into the dynamics of NO3- and contribute to the development of effective strategies for mitigating NO3- contaminant, leading to sustainable management of water resources.

Enhancing low-temperature nitrification biofilter with Acinetobacter harbinensis HITLi7T for efficient ammonia nitrogen removal in engineering applications

Low temperature has always been a significant limitation for the biological removal of ammonia nitrogen (NH3-N) from water. Acinetobacter harbinensis HITLi7T (HITLi7T) was used to enhance the low-temperature nitrification biofilter (LTNB) with a treatment capacity of 20,000 m3/d. At 2 °C, with an empty bed contact time of 3 h, the LTNB achieved NH3-N removal levels of 1.2 ∼ 1.5 mg/L. The nitrifying bacteria (Nitrosomonas, Nitrosospira, Nitrospira and Candidatus_Nitrotoga) were significantly enriched. PICRUSt2 and FAPROTAX revealed the nitrification pathway of NH3-N conversion to hydroxylamine, then to nitrite, and finally to nitrate. The high co-occurrence of HITLi7T with the nitrifying bacteria suggested that HITLi7T might also promote the enrichment of nitrifying bacteria. Life cycle assessment showed that LTNB was an economical and environmentally friendly method for NH3-N removal. These results indicated that HITLi7T enhanced the nitrification performance of biofilters, improved the cold tolerance of nitrifying bacteria, and had potential for practical applications.

Multi-evidences investigation into spatiotemporal variety, sources tracing, and health risk assessment of surface water nitrogen contamination in China

A comprehensive understanding of nitrogen pollution status, especially the identification of sources and fate of nitrate is essential for effective water quality management at the local scale. However, the nitrogen contamination of surface water across China was poorly understood at the national scale. A dataset related to nitrogen was established based on 111 pieces of literature from 2000 to 2020 in this study. The spatiotemporal variability, source tracing, health risk assessment, and drivers of China's surface water nitrogen pollution were analyzed by integrating multiple methods. These results revealed a significant spatiotemporal heterogeneity in the nitrogen concentration of surface water across China. Spatially, the Haihe River Basin and Yellow River Basin were the basins where surface water was seriously contaminated by nitrogen in China, while the surface water of Southwest Basin was less affected. Temporally, significant differences were observed in the nitrogen content of surface water in the Songhua and Liaohe River Basin, Pearl River Basin, Southeast Basin, and Yellow River Basin. There were 1%, 1%, 12%, and 46% probability exceeding the unacceptable risk level (HI＞1) for children in the Songhua and Liaohe River Basin, Pearl River Basin, Haihe River Basin, and Yellow River Basin, respectively. The primary sources of surface water nitrate in China were found to be domestic sewage and manure (37.7%), soil nitrogen (31.7%), and chemical fertilizer (26.9%), with a limited contribution from atmospheric precipitation (3.7%). Human activities determined the current spatiotemporal distribution of nitrogen contamination in China as well as the future development trend. This research could provide scientifically reasonable recommendations for the containment of surface water nitrogen contamination in China and even globally.

Nitrate removal by a novel aerobic denitrifying Pelomonas puraquae WJ1 in oligotrophic condition: Performance and carbon source metabolism

Reducing nitrate contamination in drinking water has become a critical issue in urban water resource management. Here a novel oligotrophic aerobic denitrifying bacterium, Pelomonas puraquae WJ1, was isolated and purified from artificial lake sediments. For the first time, excellent aerobic denitrification capabilities were demonstrated. At a carbon-to‑nitrogen ratio of 5.0, strain WJ1 achieved 100.0 % nitrate removal and 84.92 % total nitrogen removal within 24 h, with no nitrite accumulation. PCR amplification and sequencing confirmed the presence of the denitrification genes napA, nirS, and nosZ in the strain. The nitrogen balance demonstrated that approximately 74.95 % of the initial nitrogen was eliminated as gaseous products under aerobic conditions. Furthermore, carbon balance analysis showed that most electron donors from strain WJ1 were directed towards oxygen, with limited availability for nitrate reduction. A combination of bio-ECO analysis and network modeling indicated that strain WJ1 has robust metabolic capabilities for diverse carbon sources and exhibits high adaptability to complex carbon environments. Overall, Pelomonas puraquae WJ1 removed approximately 45.89 % of the nitrates in raw water, demonstrating significant potential for practical applications in oligotrophic denitrification.

Assessment of surface and groundwater quality in the Ctalamochita River basin, Argentina: hydrogeochemical characteristics and exploratory data analysis

Freshwater and groundwater are important resources for the drinking water supporting agricultural and livestock activities in the Córdoba province, Argentina. The aim of this study was to assess the physicochemical and microbiological quality of surface water (n = 14) and groundwater (n = 17) sites in the middle-lower basin of the Ctalamochita River (Córdoba, Argentina) for human and animal consumption. A total of 18 physicochemical and five microbiological parameters were evaluated to determine the hydrogeological characteristics of both water resources and their suitability for human and animal consumption using the Water Quality Index (WQI). The results indicated that Na+ and HCO3- were the dominant cation and anion, respectively, in both water resources. Physicochemical and microbiological parameters values were compared with national and international guidelines. The WQI showed that groundwater samples exhibited poorer quality when compared to surface waters for human consumption, primarily due to elevated concentrations of major ions and the presence of total coliforms and Pseudomonas aeruginosa. Meanwhile, the WQI for animal consumption indicated that both surface and groundwater samples were suitable for this purpose. The Piper diagram showed that most of the surface water samples were classified as Na+-Cl--HCO3-, while groundwater samples were classified as Na+-HCO3-. This classification highlights the hydrogeochemical differences between the two water resources. The Gibbs diagram indicates that the chemical composition of both surface and groundwater sources is primarily controlled by processes of rock-water interaction and evaporation. The findings of this study will facilitate the development of a proactive plan to safeguard and sustain water resources in the middle-lower basin of the Ctalamochita River. This can be achieved through the implementation of preventive strategies and the introduction of innovative policies.

Enterobacter sp. HIT-SHJ4 isolated from wetland with carbon, nitrogen and sulfur co-metabolism and its implication for bioremediation

Both autotrophic and heterotrophic denitrification are known as important bioprocesses of microbe-mediated nitrogen cycle in natural ecosystems. Actually, mixotrophic denitrification co-driven by organic matter and reduced sulfur substances are also common, especially in hypoxic environments such as estuarine sediments. However, carbon, nitrogen and sulfur co-metabolism during mixotrophic denitrification in natural water ecosystems has rarely been reported in detail. Therefore, this study investigated the co-metabolism of carbon, nitrogen and sulfur using samples collected from four distinct natural water ecosystems. Results demonstrated that samples from various sources all exhibited the ability for co-metabolism of carbon, nitrogen and sulfur. Microbial community analysis showed that Pseudomonas and Paracoccus were dominant bacteria ranging from 65.6% to 75.5% in mixotrophic environment. Enterobacter sp. HIT-SHJ4, a mixotrophic denitrifying strain which owned the capacity for co-metabolism of carbon, nitrogen and sulfur, was isolated and reported here for the first time. The strain preferred methanol as its carbon source and demonstrated remarkable efficiency for removing sulfide and nitrate with below 100 mg/L sulfide. Under weak acid conditions (pH 6.5-7.0), it exhibited enhanced capability in converting sulfide to elemental sulfur. Its bioactivity was evident within a temperature from 25 °C to 40 °C and C/N ratios from 0.75 to 3. This study confirmed the widespread presence of microbial-mediated synergistic carbon, nitrogen and sulfur metabolism in natural aquatic ecosystems. HIT-SHJ4 emerges as a novel strain, shedding light on carbon, nitrogen and sulfur co-metabolism in natural water bodies. Furthermore, it also serves as a promising candidate microorganism for in-situ ecological remediation, particularly in dealing with contamination posed by nitrate, sulfide, and organic matter.

Pre-anoxic electro-stimulation enhanced simultaneous nitrification-denitrification in single-stage electrolysis-integrated sequencing batch biofilm reactor

Simultaneous nitrification-denitrification (SND) is a promising nitrogen removal process. However, total nitrogen (TN) removal is limited due to unsatisfactory denitrification. This study demonstrated that short-time (1 h) pre-anoxic electro-stimulation significantly enhanced SND efficiency in the aerobic phase by promoting the proliferation of mixotrophic and heterotrophic denitrifiers. SND and TN removal efficiencies at the optimal electric current (EC) (0.02 A) were 85.6 % and 93.9 %, which were 39.1 % and 17.2 % higher than control. Microbial community analysis indicated that the abundance of mixotrophic and heterotrophic denitrifiers significantly increased. H2 generated in the electro-stimulation process induced the proliferation of mixotrophic denitrifiers. The weak EC (0.02 A) promoted the activity and growth of heterotrophic denitrifiers by accelerating electron transfer. They concurrently mediated heterotrophic denitrification to enhance SND efficiency. PICRUSt2 analysis revealed that the abundance of denitrifying genes dramatically surged. This study provides new insights into applying electrolysis to achieve advanced SND while minimizing electricity consumption.

Groundwater geochemistry using modified integrated water quality index (IWQI) and health indices with special emphasis on nitrates and heavy metals in southern parts of Tirupati, South India

Groundwater is particularly vulnerable to pollution in places with a high population density and extensive human usage of the land, especially in southern parts of Tirupati, India. To assess this, 60 bore-well samples were obtained and assessed for physical specifications, ion chemistry, and heavy metals during the pre- and post-monsoon seasons 2022. The current investigation employed a modified integrated water quality index (IWQI), conventional graphical and human health risk assessment (HHRA) of nitrates and heavy metals to know the groundwater chemistry and its detrimental health effects on humans. The major ions were analyzed using American public health association (APHA) standards, whereas heavy metals were analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES). Additionally, pH Redox Equilibrium and C (PHREEQC), a geochemical model written in C programming language was employed to determine the saturation indices of mineral facies and ArcGIS 10.3.1 was used for spatial distribution patterns of IWQI. Then, the HHRA of nitrates and heavy metals was performed using United States environmental protection agency (US EPA) guidelines. The noteworthy outcomes include elevated levels of Ca2+, Mg2+, Cl-, NO3-, Cu, Fe, Mn, and Pb, demonstrating rock-water interaction, silicate weathering, Ca-Mg-HCO3 followed by mixed water facies, dissolution/precipitation, reverse exchange, and anthropogenic contamination are the major controlling processes in groundwater of southern Tirupati, India. The modified IWQI reveals that most groundwater samples (38%) fall under the bad quality class, with (47%) in the poor quality class and only (15%) classified as medium quality class in pre- and post-monsoon seasons. Elevated IWQI were observed in all directions except in the east, which is suitable for drinking. Moreover, the major hazard quotient (HQ) and hazard index (HI) for nitrates (NO3-) and heavy metals like copper (Cu), iron (Fe), manganese (Mn), lead (Pb) and zinc (Zn) are above the critical value of 1, revealing potential risk to humans, especially infants, followed by children and adults, entailing the instantaneous implementation of proper remedial measures and stringent policies to reduce the risk associated with groundwater pollution in the southern parts of Tirupati.

Elucidating the sources and transformation of nitrate in the Xianyang-Xi'an segment of the Weihe River basin, Northwest China

Urban rivers worldwide have been increasingly threatened by nitrate (NO3-) pollution. The Xianyang-Xi'an segment of the Weihe River, located in the loess plateau with serious soil erosion, has been highly urbanized and with intensive agricultural activities. Tracing the sources and transformations of NO3- is particularly challenging for this watershed which has multiple N sources and variable environmental factors. In this study, integrating antecedent studies with multiple stable isotopes and MixSIAR models, these river basins can be categorized into three classes: (1) urban areas, sewage, and manure were the predominant sources of NO3- in the Weihe River's mainstream, accounting for 73.4 ± 12.8%; (2) suburban areas, sewage and manure (Fenghe River, 58.0 ± 14.0%; Bahe River, 53.9 ± 15.0%) were recognized as the main sources of NO3-; (3) and the rural areas, ammonium nitrogen fertilizers were identified as the primary source of NO3- in the Heihe and Laohe Rivers. In addition, nitrification dominated the mainstream of the Weihe, Fenghe, and Bahe Rivers, while neither denitrification nor nitrification was evident in the Heihe and Laohe Rivers. In conclusion, this study is important for the improvement of surface water quality of rivers with different land use types and the development of targeted water environment management.

Remediation of arsenic- and nitrate-contaminated groundwater through iron-dependent autotrophic denitrifying culture

Groundwater contamination with arsenic and nitrate poses a pressing concern for the safety of local communities. Bioremediation, utilizing Fe(II)-oxidizing nitrate reducing bacteria, shows promise as a solution to this problem. However, the relatively weak environmental adaptability of a single bacterium hampers practical application. Therefore, this study explored the feasibility and characteristics of a mixed iron-dependent autotrophic denitrifying (IDAD) culture for effectively removing arsenic and nitrate from synthetic groundwater. The IDAD biosystem exhibited stable performace and arsenic resistance, even at a high As(III) concentration of 800 μg/L. Although the nitrogen removal efficiency of the IDAD biosystem decreased from 71.4% to 64.7% in this case, the arsenic concentration in the effluent remained below the standard (10 μg/L) set by WHO. The crystallinity of the lepidocrocite produced by the IDAD culture decreased with increasing arsenic concentration, but the relative abundance of the key iron-oxidizing bacteria norank_f_Gallionellaceae in the culture showed an opposite trend. Metagenomic analysis revealed that the IDAD culture possess arsenic detoxification pathways, including redox, methylation, and efflux of arsenic, which enable it to mitigate the adverse impact of arsenic stress. This study provides theoretical understanding and technical support for the remediation of arsenic and nitrate-contaminated groundwater using the IDAD culture.

Sources, fate and influencing factors of nitrate in farmland drainage ditches of the irrigation area

Global irrigation areas face the contradictory challenges of controlling nitrate inputs and ensuring food-safe production. To prevent and control nitrate pollution in irrigation areas, the study using the Yellow River basin (Ningxia section) of China as a case study, employed nitrogen and oxygen dual isotope tracing and extensive field investigations to analyze the sources, fate, and influencing factors of nitrate in agricultural drainage ditches. The results of source tracing of nitrate showed that annual proportions of nitrate sources entering the Yellow River in the ditches are as follows: for manure & sewage, fertilizer, and natural sources, the ratios are 33%, 35%, and 32% overall. The results of nitrate fate showed that nitrates derived from nitrate fertilizer exhibit a lower residual rate in drainage ditches (ecological ditches) compared to ammonium fertilizer, which can undergo self-ecological restoration within one year. The results of influencing factors showed that crops with high water and nutrient requirements, such as vegetables, the nitrate pollution and environmental harm resulting from "exploitative cultivation" are five times more than normal cultivation practices in dryland and paddy fields, especially winter irrigation without crop interception exacerbates the leaching of nitrate from the soil. Therefore, nitrate management in irrigation areas should focus on preventing and controlling "exploitative cultivation" and losses during winter irrigation, while appropriately adjusting the application ratio of ammonium nitrogen fertilizers. The results of the study can guide strategies to mitigate nitrate pollution in irrigated areas such as livestock farming, fertilizer application, irrigation management, ditch optimization, and crop cultivation.

Effects of season and water quality on community structure of planktonic eukaryotes in the Chaohu Lake Basin

Introduction

Analyzing the correlation between planktonic eukaryotic communities (PECs) and aquatic physicochemical parameters (APPs) provides important references for predicting the impact of climate change and human activities on aquatic ecosystems.

Methods

To assess the influence of seasons and APPs on PEC structures in lakes and rivers, we utilized high-throughput sequencing of the 18S rRNA gene to analyze PEC structures in a lake and seven rivers in the Chaohu Lake Basin and analyzed their correlations with APPs.

Results

Our results revealed that PEC structure was significantly affected by season, with the highest α-diversity observed in summer. Furthermore, we identified several APPs, including water temperature, conductivity, dissolved oxygen, pH, phosphate, total phosphorus, trophic level index (TLI), nitrate, ammonia nitrogen, and total nitrogen, that significantly influenced PEC structures. Specifically, we found that Stephanodiscus hantzschii, Simocephalus serrulatus, Cryptomonas sp. CCAC_0109, Pedospumella encystans, Actinochloris sphaerica, Chlamydomonas angulosa, Gonyostomum semen, Skeletonema potamos, Chlamydomonas klinobasis, Pedospumella sp., and Neochlorosarcina negevensis were significantly correlated to TLI, while Limnoithona tetraspina, Theileria sp., and Pseudophyllomitus vesiculosus were significantly correlated to the water quality index (WQI). However, our random forest regression analysis using the top 100 species was unable to accurately predict the WQI and TLI.

Discussion

These results provide valuable data for evaluating the impact of APPs on PEC and for protecting water resource in the Chaohu Lake Basin.

Copper-based electro-catalytic nitrate reduction to ammonia from water: Mechanism, preparation, and research directions

Global water bodies are increasingly imperiled by nitrate pollution, primarily originating from industrial waste, agricultural runoffs, and urban sewage. This escalating environmental crisis challenges traditional water treatment paradigms and necessitates innovative solutions. Electro-catalysis, especially utilizing copper-based catalysts, known for their efficiency, cost-effectiveness, and eco-friendliness, offer a promising avenue for the electro-catalytic reduction of nitrate to ammonia. In this review, we systematically consolidate current research on diverse copper-based catalysts, including pure Cu, Cu alloys, oxides, single-atom entities, and composites. Furthermore, we assess their catalytic performance, operational mechanisms, and future research directions to find effective, long-term solutions to water purification and ammonia synthesis. Electro-catalysis technology shows the potential in mitigating nitrate pollution and has strategic importance in sustainable environmental management. As to the application, challenges regarding complexity of the real water, the scale-up of the commerical catalysts, and the efficient collection of produced NH3 are still exist. Following reseraches of catalyst specially on long term stability and in situ mechanisms are proposed.

Enzymatic Mesoporous Metal Nanocavities for Concurrent Electrocatalysis of Nitrate to Ammonia Coupled with Polyethylene Terephthalate Upcycling

Electrochemical upcycling of waste pollutants into high value-added fuels and/or chemicals is recognized as a green and sustainable solution that can address the resource utilization on earth. Despite great efforts, their progress has seriously been hindered by the lack of high-performance electrocatalysts. In this work, bimetallic PdCu mesoporous nanocavities (MCs) are reported as a new bifunctional enzymatic electrocatalyst that realizes concurrent electrocatalytic upcycling of nitrate wastewater and polyethylene terephthalate (PET) plastic waste. Abundant metal mesopores and open nanocavities of PdCu MCs provide the enzymatic confinement of key intermediates for the deeper electroreduction of nitrate and accelerate the transport of reactants/products within/out of electrocatalyst, thus affording high ammonia Faradic efficiency (FENH3) of 96.6% and yield rate of 5.6 mg h-1 mg-1 at the cathode. Meanwhile, PdCu MC nanozymes trigger the selective electrooxidation of PET-derived ethylene glycol (EG) into glycolic acid (GA) and formic acid with high FEs of >90% by a facile regulation of potentials at the anode. Moreover, concurrent electrosynthesis of value-added NH3 and GA is disclosed in the two-electrode coupling system, further confirming the high efficiency of bifunctional PdCu MC nanozymes in producing value-added fuels and chemicals from waste pollutants in a sustainable manner.

Anthropogenic impacts and quantitative sources of nitrate in a rural-urban canal using a combined PMF, δ15N/δ18O-NO3-, and MixSIAR approach

Nitrate (NO3-) pollution in irrigation canals is of great concern because it threatens canal water use; however, little is known about it at present. Herein, a combination of positive matrix factorization (PMF), isotope tracers, and Mixing Stable Isotope Analysis in R (MixSIAR) was developed to identify anthropogenic impacts and quantitative sources of NO3- in a rural-urban canal in China. The NO3- concentration (0.99-1.93 mg/L) of canal water increased along the flow direction and was higher than the internationally recognized eutrophication risk value in autumn and spring. The inputs of the Fuhe River, NH4+ fertilizer, soil nitrogen, manure & sewage, and rainfall were the main driving factors of canal water NO3- based on principal component analysis and PMF, which was supported by evidence from δ15N/δ18O-NO3-. According to the chemical and isotopic analyses, nitrogen transformation was weak, highlighting the potential of δ15N/δ18O-NO3- to trace NO3- sources in canal water. The MixSIAR and PMF results with a <15% divergence emphasized the predominance of the Fuhe River (contributing >50%) and anthropogenic impacts (NH4+ fertilizer plus manure & sewage, >37%) on NO3- in the entire canal, reflecting the effectiveness of the model analysis. According to the MixSIAR model, (1) higher NO3- concentration in canal water was caused by the general enhancement of human activities in spring and (2) NO3- source contributions were associated with land-use patterns. The high contributions of NH4+ fertilizer and manure & sewage showed inverse spatial variations, suggesting the necessity of reducing excessive fertilizer use in the agricultural area and controlling blind wastewater release in the urban area. These findings provide valuable insights into NO3- dynamics and fate for sustainable management of canal water resources. Nevertheless, long-term chemical and isotopic monitoring with alternative modeling should be strengthened for the accurate evaluation of canal NO3- pollution in future studies.

Keeping Nitrogen Use in China within the Planetary Boundary Using a Spatially Explicit Approach

Nitrogen (N) supports food production, but its excess causes water pollution. We lack an understanding of the boundary of N for water quality while considering complex relationships between N inputs and in-stream N concentrations. Our knowledge is limited to regional reduction targets to secure food production. Here, we aim to derive a spatially explicit boundary of N inputs to rivers for surface water quality using a bottom-up approach and to explore ways to meet the derived N boundary while considering the associated impacts on both surface water quality and food production in China. We modified a multiscale nutrient modeling system simulating around 6.5 Tg of N inputs to rivers that are allowed for whole of China in 2012. Maximum allowed N inputs to rivers are higher for intensive food production regions and lower for highly urbanized regions. When fertilizer and manure use is reduced, 45-76% of the streams could meet the N water quality threshold under different scenarios. A comparison of "water quality first" and "food production first" scenarios indicates that trade-offs between water quality and food production exist in 2-8% of the streams, which may put 7-28% of crop production at stake. Our insights could support region-specific policies for improving water quality.

Environmental perchlorate, thiocyanate, and nitrate exposures and bone mineral density: a national cross-sectional study in the US adults

Associations of perchlorate, thiocyanate, and nitrate exposures with bone mineral density (BMD) in adults have not previously been studied. This study aimed to estimate the associations of individual and concurrent exposure of the three chemicals with adult BMD. Based on National Health and Nutrition Examination Survey (NHANES, 2011-2018), 1618 non-pregnant adults (age ≥ 20 years and 47.0% female) were included in this study. Survey-weighted linear regression models were used to estimate individual urinary perchlorate, thiocyanate, and nitrate concentrations with lumbar spine BMD and total BMD in adults. Then, weighted quantile sum (WQS) regression and Bayesian kernel machine regression (BKMR) models were conducted to evaluate associations of co-occurrence of the three chemicals with adult BMD. In all participants, nitrate exposure was inversely associated with lumbar spine BMD (β =  - 0.054, 95%CI: - 0.097, - 0.010). In stratification analyses, significant inverse associations were observed in female and participants older than 40 years old. In WQS regressions, significant negative associations of the weighted sum of the three chemicals with total and lumbar spine BMD (β =  - 0.014, 95%CI: - 0.021, - 0.007; β =  - 0.011, 95%CI: - 0.019, - 0.004, respectively) were found, and the dominant contributor was nitrate. In the BKMR models, non-linear dose-response associations of nitrate exposure with lumbar spine and total BMD were observed. These findings suggested that environmental perchlorate, thiocyanate, and nitrate exposure may reduce adult BMD and nitrate is the main contributor.

Emerging contaminants in water environments: progress, evolution, and prospects

This article employs bibliometric tools like VOSviewer, Bibliometrix, and CiteSpace for a comprehensive visual analysis of 1,612 documents on Emerging Contaminants in Waters from the Web of Science database. The objective is to elucidate the historical development, research hotspots, and trends in international studies of this field, offering valuable insights and guidance for future research directions. The analysis reveals a consistent increase in publications from 2003 to 2023, with the United States, China, and Spain being the most prolific contributors. A detailed examination of keyword co-occurrence and cluster analysis shows a predominant focus on themes such as pollutant detection, risk assessment, and biogeochemical cycling. Furthermore, the study underscores the significance of forming interdisciplinary networks among authors and institutions, highlighting its critical role in enhancing the quality and innovation of scientific research. The findings of this study not only chart the progression and focal points of research in this domain but also underscore the pivotal role of international collaboration, serving as an indispensable reference for shaping future research trajectories and fostering global cooperation.

Characterizing the water resource-environment-ecology system harmony in Chinese cities using integrated datasets: A Beautiful China perspective assessment

Integrated management and synergistic improvement of the water system is a topic of widespread concern. This study innovatively integrates three functions of quality assessment, synergy evaluation, and driving influence determination to establish a systematic framework assessing water system harmony. A case study of 336 Chinese cities is further performed by combining multi-scale and multi-source datasets. The results show China's water system quality has improved from 2015 to 2022. Development in the water resource, environment, and ecology subsystems have been differentiated, with 0.05 %, 4.33 %, and -1.64 % changes, respectively. The degradation of water ecology and the weak synergy with the other two subsystems have limited China's water system harmony. Water environment improvement played a contributive role in improving the water system quality. The contribution structure of water resources, environment, and ecology has shifted towards equilibrium in recent years. We found and highlighted the north-south differentiation of water system harmony in Chinese cities. The Beijing-Tianjin-Hebei and its surroundings, the Yangtze River Delta, and the middle reaches of the Yangtze River are identified as priority regions for water system harmony improvement. The primary contribution of this study is to propose an assessing concept of water resource-environment-ecology system harmony, establish well-structured assessment methods, and integrate the multiple data sources. The novel methods and findings, including the indicator system, application of data mining and decomposing methods, and the city-level water system harmony map, deconstruct and quantify the complex and diverse water system, supporting clearer and more efficient water management policymaking.

Algal Extracellular Organic Matter Induced Photochemical Oxidation of Mn(II) to Solid Mn Oxide: Role of Mn(III)-EOM Complex and Its Ability to Remove 17α-Ethinylestradiol

Photosensitizer-mediated abiotic oxidation of Mn(II) can yield soluble reactive Mn(III) and solid Mn oxides. In eutrophic water systems, the ubiquitous algal extracellular organic matter (EOM) is a potential photosensitizer and may have a substantial impact on the oxidation of Mn(II). Herein, we focused on investigating the photochemical oxidation process from Mn(II) to solid Mn oxide driven by EOM. The results of irradiation experiments demonstrated that the generation of Mn(III) intermediate was crucial for the successful photo oxidization of Mn(II) to solid Mn oxide mediated by EOM. EOM can serve as both a photosensitizer and a ligand, facilitating the formation of the Mn(III)-EOM complex. The complex exhibited excellent efficiency in removing 17α-ethinylestradiol. Furthermore, the complex underwent decomposition as a result of reactions with reactive intermediates, forming a solid Mn oxide. The presence of nitrate can enhance the photochemical oxidation process, facilitating the conversion of Mn(II) to Mn(III) and then to solid Mn oxide. This study deepens our grasp of Mn(II) geochemical processes in eutrophic water and its impact on organic micropollutant fate.

Electrocatalytic reduction of nitrate to ammonia by Pd/In modified Nickel foam electrode in aqueous solution

Nitrate pollution in surface water and ground water has drawn wide attention, which has brought challenges to human health and natural ecology. Electroreduction of nitrate to NH3 in waste water was a way to turn waste into wealth, which has attracted interest of many researchers. Using Nickel foam as substrate, we prepared Pd/In bimetallic electrode (NF-Pd/In) according to a two-step electrodeposition method. There are many irregularly shaped particles in the size range of 10 nm-100 nm accumulated on the surface of prepared NF-Pd/In electrode, which could supply high specific area and more active sites for nitrate electroreduction. FESEM-EDS, XRD and XPS analysis confirmed the uniform distribution of Pd and In on the surface of prepared NF-Pd/In electrode, with a mass ratio of 4.5/1. Above 96% of 100 mg/L NO3--N was removed and 95% of NH3 selectivity was reached after 5 h of reaction under -1.6 V vs. Ag/AgCl sat. KCl when using 0.05 mol/L of Na2SO4 as electrolyte. High concentration of NaCl (0.05 mol/L) in the test solution dramatically decreased the NH3 selectivity because the produced NH3 could be further oxidized to N2 by the formed HClO from Cl-. EIS tests indicated that the prepared NF-Pd/In electrode showed much lower electrode resistance than NF due to the adsorptive property and electrocatalytic ability for nitrate removal. Density functional theory (DFT) calculations indicated that the presence of In could promote the conversion of NO3- to *NO3 during the process of nitrate electroreduction to NH3. Circulating tests demonstrated the stability of prepared NF-Pd/In electrode.

Nitrate Sensor with a Wide Detection Range and High Stability Based on a Cu-Modified Boron-Doped Diamond Electrode

This paper describes an electrochemical sensor based on a Cu-modified boron-doped diamond (BDD) electrode for the detection of nitrate-contaminated water. The sensor utilizes the catalytic effect of copper on nitrate and the stability of the BDD electrode. By optimizing the electrolyte system, the linear detection range was expanded, allowing the sensor to detect highly concentrated nitrate samples up to 100 mg/L with a low detection limit of 0.065 mg/L. Additionally, the stability of the sensor was improved. The relative standard deviation of the current responses during 25 consecutive tests was only 1.03%. The wide detection range and high stability of the sensor makes it suitable for field applications and the on-site monitoring of nitrate-contaminated waters.

Appropriate stoichiometric ratios of dissolved organic carbon and nitrate can trigger a transition in nitrate removal in groundwater around plateau lakes, Southwest China

Increasing dissolved organic carbon (DOC) in groundwater as a carbon source for microorganisms that stimulate nitrate attenuation is considered a sustainable strategy to mitigate nitrate pollution in groundwater. However, little is known on the stoichiometric ratio of DOC and nitrate required in groundwater nitrate reduction processes, which has become an obstacle for evaluating the current status of DOC limitations on nitrate reduction. Here, the NO3--N and DOC concentrations in groundwater around 8 plateau lakes were investigated, and a microcosm experiment was performed to elucidate the effects of different DOC:NO3--N levels in groundwater on NO3--N reduction, and the current status of DOC limitations on groundwater NO3--N reduction around 8 lakes was further evaluated. The results indicated that nearly 41 % of the groundwater NO3--N concentrations exceeded the WHO threshold for drinking water (11.3 mg L-1) and 79 % of the groundwater DOC concentrations exceeded 5 mg L-1. The differences in groundwater NO3--N and DOC concentrations among the 8 lakes were controlled by the intensity of agricultural and human activities and hydrogeological background. The stoichiometric ratio of DOC:NO3--N regulated the NO3--N reduction process, and groundwater NO3--N accumulation rate appeared to become limited and sharply decreased when the DOC concentration was approximately 10 mg L-1 or when the DOC:NO3--N ratio was close to 1:1, and the DOC:NO3--N ratio threshold for limiting the NO3--N reduction process was approximately 2.25. Based on this threshold, >33 %-86 % of the groundwater samples around the 8 plateau lakes were strongly limited in the reduction of groundwater NO3--N due to a lack of sufficient DOC provides energy for heterotrophic microorganisms. Additionally, we highlight that the sustainable strategy of increasing DOC to stimulate groundwater NO3- attenuation should be combined with short-term strategies to jointly coordinate and control groundwater NO3- pollution.

Fecal transplantation of young mouse donors effectively improves enterotoxicity in elderly recipients exposed to triphenyltin

Triphenyltin (TPT) is a widely used biocide known for its high toxicity to various organisms, including humans, and its potential contribution to environmental pollution. The aging process leads to progressive deterioration of physiological functions in the elderly, making them more susceptible to the toxic effects of environmental pollutants. This study aimed to investigate the mitigating effect of fecal transplantation in young mice on the toxicological impairment caused by TPT exposure. For the study, 18-month-old mice were divided into four groups with six replicates each. The control group was fed a basal diet, the TPT group was exposed to 3.75 mg/Kg TPT, the feces group received fecal transplantation from 8-week-old young mice, and the combined group was exposed to 3.75 mg/Kg TPT after receiving fecal transplantation. Compared with the elderly control group, TPT induced significant upregulation of mRNA expression of pro-inflammatory factors (IL-1β, IL-6, TNF-α), while the anti-inflammatory factor gene IL-10 was significantly suppressed. The mRNA expression of intestinal barrier proteins (Claudin, Occludin, Muc2) was also significantly downregulated. However, fecal transplantation in young mice alleviated TPT-induced changes in inflammatory factors, ameliorated oxidative stress, and increased the activities of antioxidant enzymes (including SOD, CAT, GSH-Px). Further analysis using 16 s RNA showed that exposure to TPT led to changes in the composition of the intestinal flora. Untargeted metabolomics observations of feces from older mice revealed that exposure to TPT resulted in altered fecal metabolites. Fecal transplantation in young mice altered the microbiota of TPT-exposed older mice, especially by enhancing the levels of core probiotics. Similar beneficial effects were observed through untargeted metabolomics. Overall, this study highlights the potential benefits of young fecal transplantation in protecting the elderly from the toxicity of TPT, offering a promising approach to improve healthy aging.

The impact of environmental regulations on the upgrading of the industrial structure: Evidence from China

China's economy has transitioned into a phase of high-quality development, with enhancing its industrial structure becoming a critical objective. We gathered panel data from 30 major provinces in China from 2010 to 2020 and employed the fixed effects model to assess the actual influence of environmental regulations on industrial structure upgrading. Our empirical findings show that the impacts of various environmental regulations on industrial structure upgrading in China are significantly different. Mandatory environmental regulation demonstrates an inverted U-shaped nonlinear correlation with the upgrading level of the entire industrial structure. When the intensity of this regulation is low, it significantly accelerates industrial structure upgrading. As the intensity of this regulation rises, its effect on industrial structure upgrading is inhibitory. In contrast, induced environmental regulation exhibits a nonlinear U-shaped relationship with industrial structure upgrading and shows a nonlinear change trend of first decreasing and then rising. When the intensity of induced environmental regulation reaches a certain critical point and continues to increase, it will change from a negative influence on the upgrading of the industrial structure to a promoting effect. The further discussion of threshold regression and the robustness test also led to similar conclusions. The above research is conducive to the Chinese government's rational use of environmental regulation tools to promote industrial structure upgrading. It is also beneficial to developing countries, allowing them to learn from China's experience to improve the effectiveness of environmental regulation and boost their industrial development.

Harnessing the potential of ginkgo biloba extract: Boosting denitrification performance through accelerated electron transfer

Ginkgo biloba extract (GBE) had several effects on the human body as one of the widely used phytopharmaceuticals, but it had no application in microbial enhancement in the environmental field. The study focused on the impact of GBE on denitrification specifically under neutral conditions. At the identified optimal addition ratio of 2% (v/v), the system exhibited a noteworthy increase in nitrate reduction rate (NRR) by 56.34%, elevating from 0.71 to 1.11 mg-N/(L·h). Moreover, the extraction of microbial extracellular polymeric substance (EPS) at this ratio revealed changes in the composition of EPS, the electron exchange capacity (EEC) was enhanced from 87.16 to 140.4 μmol/(g C), and the transfer impedance was reduced within the EPS. The flavin, fulvic acid (FA), and humic acid (HA) provided a π-electron conjugated structure for the denitrification system, enhancing extracellular electron transfer (EET) by stimulating carbon source metabolism. GBE also improved electron transfer system activity (ETSA) from 0.025 to 0.071 μL O2/(g·min·prot) and the content of NADH enhanced by 22.90% while significantly reducing the activation energy (Ea) by 85.6% in the denitrification process. The synergy of improving both intracellular and extracellular electron transfer, along with the reduction of Ea, notably amplified the initiation and reduction rates of the denitrification process. Additionally, GBE demonstrated suitability for denitrification across various pH levels, enhancing microbial resilience in alkaline conditions and promoting survival and proliferation. Overall, these findings open the door to potential applications of GBE as a natural additive in the environmental field to improve the efficiency of denitrification processes, which are essential for nitrogen removal in various environmental contexts.

Deciphering the potential role of quorum quenching in efficient aerobic denitrification driven by a synthetic microbial community

Low efficiency is one of the main challenges for the application of aerobic denitrification technology in wastewater treatment. To improve denitrification efficiency, a synthetic microbial community (SMC) composed of denitrifiers Acinetobacter baumannii N1 (AC), Pseudomonas aeruginosa N2 (PA) and Aeromonas hydrophila (AH) were constructed. The nitrate (NO3--N) reduction efficiency of the SMC reached 97 % with little nitrite (NO2--N) accumulation, compared to the single-culture systems and co-culture systems. In the SMC, AH proved to mainly contribute to NO3--N reduction with the assistance of AC, while PA exerted NO2--N reduction. AC and AH secreted N-hexanoyl-DL-homoserine lactone (C6-HSL) to promote the electron transfer from the quinone pool to nitrate reductase. The declined N-(3-oxododecanoyl)-L-homoserine lactone (3OC12-HSL), resulting from quorum quenching (QQ) by AH, stimulated the excretion of pyocyanin, which could improve the electron transfer from complex III to downstream denitrifying enzymes for NO2--N reduction. In addition, C6-HSL mainly secreted by PA led to the up-regulation of TCA cycle-related genes and provided sufficient energy (such as NADH and ATP) for aerobic denitrification. In conclusion, members of the SMC achieved efficient denitrification through the interactions between QQ, electron transfer, and energy metabolism induced by N-acyl-homoserine lactones (AHLs). This study provided a theoretical basis for the engineering application of synthetic microbiome to remove nitrate wastewater.

Assessment of drinking water quality and health risk using water quality index and multiple computational models: a case study of Yangtze River in suburban areas of Wuhan, central China, from 2016 to 2021

Water quality, increasingly recognized for its significant impact on health, is garnering heightened attention. Previous studies were limited by the number of water quality indicators and the duration of analysis. This study assessed the drinking water quality and its associated health risk in suburban areas of Wuhan, a city in central China, from 2016 to 2021. We collected 368 finished water samples and 1090 tap water samples and tested these for 37 different indicators. The water quality was evaluated using the water quality index, with trends over time analyzed via the Mann-Kendall test. Furthermore, an artificial neural network model was employed for future water quality prediction. Our findings indicated that the water quality in rural Wuhan was generally good and had an improvement from 2016 to 2021. The qualification and excellent rates were 98.91% and 86.81% for finished water, and 97.89% and 78.07% for tap water, respectively. The drinking water quality was predicted to maintain satisfactory in 2022 and 2023. Additionally, principal component analysis revealed that the primary sanitary issues in the water were poor sensory properties, elevated metal contents, high levels of dissolved solids, and microbial contamination. These issues were likely attributable to domestic and industrial waste discharge and aging water pipelines. The health risks associated with the long-term consumption of this water have been steadily decreasing over the years, underscoring the effectiveness of Wuhan's ongoing water management efforts.

Identification of surface water - groundwater nitrate governing factors in Jianghuai hilly area based on coupled SWAT-MODFLOW-RT3D modeling approach

A comprehensive understanding of the key controlling factors on NO3-N spatiotemporal distribution in surface and groundwater is of great significance to nitrogen pollution control and water resources management in watershed. Hence, the coupled SWAT-MODFLOW-RT3D model was employed to simulate nitrate (NO3-) fate and transport in Huashan watershed system. The model was calibrated using a combination of stream discharge, groundwater levels, NO3-N in-stream loading and groundwater NO3-N concentrations. The simulation revealed the significant spatiotemporal variations in surface water-groundwater nitrate interactions. The annual average percolation of NO3- from rivers to groundwater was 171.5 kg/km2 and the annual average discharge NO3- content from groundwater into rivers was 451.9 kg/km2 over the simulation period. The highest percolation of NO3- from rivers to groundwater occurred in April and the highest discharge NO3- content from groundwater into rivers occurred in July. Grassland and agriculture land contributed more nitrate contents in river water and groundwater compared to bare land and forest in the study area and the water exchange was the primary driving force for nitrate interactions in the surface water-groundwater system. Sensitivity analysis indicated that river runoff and groundwater levels were most influenced by the SCS runoff curve number f (CN2) and aquifer hydraulic conductivity (K), which, in turn, significantly affected nitrate transport. Regarding water quality parameters, the denitrification exponential rate coefficient (CDN) had the most pronounced impact on NO3-N in-stream loading and groundwater NO3-N concentrations. This study underscores the central role of surface-groundwater (SW-GW) interactions in watershed-scale nitrate research and suggests that parameters with higher sensitivity should be prioritized in analogous watershed modeling.

Identification of nitrate accumulation mechanism of surface water in a mining-rural-urban agglomeration area based on multiple isotopic evidence

The accumulation of nitrate (NO3-) in surface waters resulting from mining activities and rapid urbanization has raised widespread concerns. Therefore, it is crucial to develop a nitrate transformation information system to elucidate the nitrogen cycle and ensure sustainable water quality management. In this study, we focused on the main river and subsidence area of the Huaibei mining region to monitor the temporal and spatial variations in the NO3- content. Multiple isotopes (δD, δ18O-H2O, δ15N-NO3-, δ18O-NO3-, and δ15N-NH4+) along with water chemistry indicators were employed to identify the key mechanisms responsible for nitrate accumulation (e.g., nitrification and denitrification). The NO3- concentrations in surface water ranged from 0.28 to 7.50 mg/L, with NO3- being the predominant form of nitrogen pollution. Moreover, the average NO3- levels were higher during the dry season than during the wet season. Nitrification was identified as the primary process driving NO3- accumulation in rivers and subsidence areas, which was further supported by the linear relationship between δ15N-NO3- and δ15N-NH4+. The redox conditions and the relationship between δ15N-NO3- and δ18O-NO3-, and lower isotope enrichment factor of denitrification indicated that denitrification was weakened. Phytoplankton preferentially utilized available NH4+ sources while inhibiting NO3- assimilation because of their abundance. These findings provide direct evidence regarding the mechanism underlying nitrate accumulation in mining areas, while aiding in formulating improved measures for effectively managing water environments to prevent further deterioration.

Nitrate removal by anammox bacteria utilizing photoexcited electrons via inward extracellular electron transfer channel

Dissimilatory nitrate reduction to ammonium (DNRA) has been found to occur in some anammox bacteria species, and the DNRA metabolites (nitrite and ammonium) can further be removed to nitrogen from water. However, the activation of DNRA pathway of anammox bacteria is usually limited by the access to electron donors. Herein, we constructed a photosensitized hybrid system combining anammox bacteria (Candidatus Kuenenia stuttgartiensis and Candidatus Brocadia anammoxidans) with CdS nanoparticles semiconductor for energy-efficient NO3- removal. Such photosensitized anammox-CdS hybrid systems achieved NO3- removal with an average efficiency of 88% (the maximum of 91%) and a N2 selectivity of 72%, only with photoexcited electrons as donors. The DNRA-anammox metabolism of anammox bacteria was proved to responsible for NO3- removal via inward extracellular electron transfer channel. The greatly up-regulated genes encoding c-type cytochrome proteins (5 or 11 hemes) in the outer membrane, c-type cytochrome protein (4 hemes) and electron transport protein RnfA-E in the inner membrane, ferredoxin (2Fe-2S) in the cytoplasm and c-type cytochrome bc1 in anammoxosome membrane were supposed to play key roles in the inward extracellular electron transfer pathway. This work provides a novel insight into the design of the biotic-abiotic hybrid photosynthetic systems, and opens a new strategy for light-driven NO3- removal from the perspective of light energy input.

Nitrate transformation and source tracking of rivers draining into the Bohai Sea using a multi-tracer approach combined with an optimized Bayesian stable isotope mixing model

Excessive levels of NO3- can result in multiple eco-environmental issues due to potential toxicity, especially in coastal areas. Accurate source tracing is crucial for effective pollutant control and policy development. Bayesian models have been widely employed to trace NO3- sources, while limited studies have utilized optimized Bayesian models for NO3- tracing in the coastal rivers. The Bohai Rim is highly susceptible to ecological disturbances, particularly N pollution, and has emerged as a critical area. Therefore, identification the N fate and understanding their sources contribution is urgent for pollution mitigation efforts. In addition, understanding the influenced key driven factors to source dynamic in the past ten years is also implication to environmental management. In this study, water samples were collected from 36 major river estuaries that drain into the Bohai Sea of North China. The main transformation processes were analyzed and quantified the sources of NO3- using a Bayesian stable isotope mixing model (MixSIAR) with isotopic approach (δ15N-NO3- and δ18O-NO3-). The overall isotopic composition of δ15N-NO3- and δ18O-NO3- in estuary waters ranged from -0.8-19.3‰ (9.3 ± 4.6‰) and from -7.1-10.5‰ (5.0 ± 4.3‰), respectively. The main sources of nitrate in most river estuaries were manure & sewage, and chemical fertilizer, while weak denitrification and mixed processes were observed in Bohai Rim region. A temporal decrease in the nitrogen load entering the Bohai Sea indicates an improvement in water quality in recent years. By incorporating informative priors and utilizing the calculated coefficients, the accuracy of sourcing results was significantly improved. This study highlighted the optimized MixSIAR model enhanced its accuracy for sourcing analysis and providing valuable insights for policy formulation. Future efforts should focus on improving management strategies to reduce nitrogen into the bay.

Coupling the measures of pollution source control and water replenishment to improve water quality in the catchment scale of Qianshan River Basin

With the development of the economy, the problem of urban black odorous water bodies has become increasingly significant, having a serious impact on the environment. As important means of remediating aquatic environments, pollution source control and water replenishment are of great significance in improving water quality. This study takes the Qianshan River Basin in Zhuhai City as its study area to simulate their effects on the improvement of water quality. A coupled model of water quantity and quality in Qianshan River Basin was constructed using MIKE11to analyze the water quality compliance rate, with sewage interception rates of 85%, 90%, and 95%, and to investigate the effect of pollution source control on the improvement of the aquatic environment. Using different sewage interception rates, the amount of water replenishment was calculated in order to meet water quality standards, the water replenishment scheme was determined via river-specific and time-specific methods, and the model was used to analyze the replenishment effect of the scheme. The results show that increasing the sewage interception rate can significantly improve the COD compliance rate, and improve the NH3-N and TP compliance rate; however, the enhancement effect is not sufficiently significant. When a sewage interception rate of 95% is implemented, there are still five rivers with a low NH3-N compliance rate, and six rivers with low a TP compliance rate. Comparing the water replenishment effect under different sewage interception rates of 85% and 95%, the water replenishment program alongside a sewage interception rate of 95% can effectively improve the aquatic environment and the water quality essentially meets the standard under different rainfall conditions; this demonstrates that the program presented herein can be used as the aquatic environment remediation program of choice for the Qianshan River Basin.

Cyclic di-GMP inhibits nitrate assimilation by impairing the antitermination function of NasT in Pseudomonas putida

The ubiquitous bacterial second messenger cyclic diguanylate (c-di-GMP) coordinates diverse cellular processes through its downstream receptors. However, whether c-di-GMP participates in regulating nitrate assimilation is unclear. Here, we found that NasT, an antiterminator involved in nitrate assimilation in Pseudomonas putida, specifically bound c-di-GMP. NasT was essential for expressing the nirBD operon encoding nitrite reductase during nitrate assimilation. High-level c-di-GMP inhibited the binding of NasT to the leading RNA of nirBD operon (NalA), thus attenuating the antitermination function of NasT, resulting in decreased nirBD expression and nitrite reductase activity, which in turn led to increased nitrite accumulation in cells and its export. Molecular docking and point mutation assays revealed five residues in NasT (R70, Q72, D123, K127 and R140) involved in c-di-GMP-binding, of which R140 was essential for both c-di-GMP-binding and NalA-binding. Three diguanylate cyclases (c-di-GMP synthetases) were found to interact with NasT and inhibited nirBD expression, including WspR, PP_2557, and PP_4405. Besides, the c-di-GMP-binding ability of NasT was conserved in the other three representative Pseudomonas species, including P. aeruginosa, P. fluorescens and P. syringae. Our findings provide new insights into nitrate assimilation regulation by revealing the mechanism by which c-di-GMP inhibits nitrate assimilation via NasT.