High NO Research Articles

Long-term variations and trends of tropospheric and ground-level NO2 over typical coastal areas

Tropospheric and ground-level levels of nitrogen dioxide (NO2) may have changed due to environmental policies and technological changes. Here, multiyear ground-based measurement (2014–2022) and satellite-derived retrieval (October 2004 to May 2023) datasets of NO2 concentrations are used to study the long-term ground-level and tropospheric NO2 patterns and trends at four time scales (interannual, quarterly, monthly, and hourly) over the three regions with the highest global NO2 concentrations. Several tropospheric NO2 hotspots are observed over industrialized and highly populated coastal zone areas. Over the past 18 years, these hotpots have generally decreased due to the implementation of strong and effective environmental protection policies. A significant trend reversal was observed in 2011 in East-central China. The maximum values of tropospheric NO2 occurred in winter over East-central China, Western Europe and East-central USA, with concentrations of 13.010, 4.243 and 4.149 × 1015 molecules cm−2, respectively. The Bohai Sea exhibited disproportionately high tropospheric NO2 levels, suggesting it as a significant contributor to regional nitrogen pollution. Ground-level NO2 shows similar temporal patterns to tropospheric concentrations, with over 82.8 % of sites in East-central China exceeding 30 μg m−3, compared to 12.8 % in Western Europe and 3.8 % in East-central USA. The analysis of diurnal variations in ground-level NO2 concentrations shows a significantly decrease during the daytime compared to nighttime, reflecting that the peak occurs at approximately 08:00 and 20:00 LT, while the lowest concentrations at around 12:00 LT. These insights clarify the impacts of environmental policies on NO2 levels and identify critical areas for targeted pollution control, enhancing our understanding of NO2 dynamics in high-pollution regions globally.

Nitrogen migration and transformation during re-suspension and photo-induction in landscape water replenished by reclaimed water.

Sediment re-suspension plays a crucial role in releasing endogenous nitrogen and greenhouse gases in shallow urban waters. However, the impacts of repeated re-suspension and photo-induced processes on migration and transformation from endogenous nitrogen, as well as the emission of greenhouse gases, remain unclear. This study simulated three conditions: re-suspension (Rs), re-suspension combined with ultravioletirradiation (Rs + UV), and ultraviolet irradiation (UV). The findings revealed that both repeated sediment re-suspension and exposure to UV light altered the characteristics of surface sediments. Decrease of convertible nitrogen in sediments, leading to the release of ion-exchangeable nitrogen (IEF-N) into NH4+-N and NO3--N, influenced greenhouse gas production differently under various conditions. The study observed the highest concentration of dissolved N2O in under UV irradiation, positively correlated with NO2--N and NO3--N. Re-suspension increased the turbidity of the overlying water and accelerated nitrification, resulting in the highest NO3--N concentration and the lowest dissolved N2O concentration. Additionally, in the Rs + UV dissolved N2O maintained the higher concentrations than in Rs, with greatest amount of N conversion in surface sediments, and a 59.45% reduction in IEF-N. The production of N2O during re-suspension was mainly positively correlated with NH4+-N in the overlying water. Therefore, this study suggest that repeated re-suspension and light exposure significantly influence nitrogen migration and transformation processes in sediment, providing a theoretical explanation for the eutrophication of water and greenhouse gas emissions.

A new quantitative method via on-line mass spectrometer: Application in biomass char/NO reaction

A novel quantitative analysis method via on-line mass spectrometer (OMS) was developed to calculate the mass flows of the reactants and products for the reaction between NO and biomass char in micro-fluidized bed. The experimental results showed that the quantitative method could accurately calculate the mass flow rate of each gas even though there were ion currents overlapping at mass-to-charge ratio (m/z) = 12,16, and 28 when CO, CO2 and N2 were simultaneously present in OMS. Furthermore, the method facilitated the quantitative analysis of gaseous reactants and products, thereby enabling the assessment of mass conversions of carbon (C) and nitrogen (N) in the NO reduction reaction. It was observed that CO was the predominant carbon-containing product in the reaction involving biomass char and NO at 900 °C, with over 96 % of nitrogen in NO participating in the reduction reaction being converted to N2. This study also revealed that increasing the fluidization number, NO concentration, and temperature in the micro-fluidized bed could significantly accelerate the conversion processes of C and N in the reduction of NO by biomass char. The peak mass flow rates of CO, CO2, and N2 were found to increase correspondingly with higher NO concentrations and temperature levels. The NO reduction process over sawdust char was compared to that of corncob char, showing a higher maximum NO reduction efficiency. The reaction order with respect to NO concentration was determined to be 0.82 for corncob char and 0.68 for sawdust char. Additionally, the apparent activation energies were determined to be 92.35 kJ/mol for corncob char and 125.60 kJ/mol for sawdust char, respectively.

Exploring dissolved N2O characteristics and unearthing indirect N2O emission factors in the shallow groundwater of paddy and upland fields

Indirect emissions of nitrous oxide (N2O) stemming from nitrogen (N) leaching in agricultural fields constitute a significant contributor to atmospheric N2O. Groundwater nitrate (NO3−-N) pollution is severe in the Ningxia Yellow River Irrigation Area (NYRIA), coupled with high NO3−-N leaching, exacerbates the risk of indirect N2O emissions from groundwater. Over two years of field observations, this study investigated the characteristics and interannual variations of dissolved N2O (dN2O) concentrations and indirect N2O emission factors (EF5g) in shallow groundwater. The research focused on three typical farmlands in the NYRIA, each subjected to six levels of N fertilizer application. The mean dN2O concentrations in the groundwater of paddy, corn and vegetable fields were 5.17, 8.40 and 16.35 μg N·L−1, respectively. Notably, the dN2O concentrations in the shallow groundwater of upland fields exceeded those in paddy fields, with maximum levels in vegetable fields nearly an order of magnitude higher. Elevated N application significantly increased dN2O concentrations across various farmlands, showing statistically significant variation. However, differences in EF5g-A and EF5g-B within the same farmland were negligible. Denitrification was the primary process contributing to N2O production in groundwater, with nitrification also played a crucial role in upland fields. Factors such as NO3−-N, NH4+-N, dissolved oxygen (DO), and pH critically influenced N2O production. EF5g-B, which considers the NO3−-N consumption during denitrification processes in groundwater, was deemed more appropriate than EF5g-A for assessing the indirect N2O emission in the NYRIA. The EF5g of agricultural fields exhibited minimal sensitivity to N input but was significantly affected by other factors, such as the planting pattern. The study revealed the rationality of adopting EF5g-B in assessing indirect N2O emissions, providing valuable insights for N management strategies in regions with high NO3−-N leaching. Minimizing N fertilizer application while ensuring crop yield, especially in upland fields, is beneficial for reducing N2O emissions.

Changes in the limiting nutrients and dominant phytoplankton communities of three major European rivers: Response to catchment lithologies and human activities

Catchment lithologies and human activities can cause variations in riverine carbon (C), nitrogen (N), and phosphorus (P) loads, and the ratio of these nutrients may affect the primary productivity of rivers and their phytoplankton communities. We conducted a comparative study of biogeochemical variations in three major European rivers—the Rhine, Danube, and Seine—which reveals the impacts of different carbonate outcrops and human impacts (agricultural activities and sewage input) on the changes in the limiting nutrients and phytoplankton community compositions of flowing water systems. N limitation was observed in the Rhine and Danube owing to their low average NO3– loads (4.04–6.45 mg/L). Although the extensive outcrops of carbonate rocks in the Seine catchment result in high average HCO3– concentrations of ∼ 4.68 mmol/L, the elevated inorganic C input did not match the high average NO3– loads (∼19.38 mg/L), driven by intensive agricultural activity and urban sewage input, demonstrating a potential C limitation on productivity. Our results show that the productivity of the Seine River, determined by the chlorophyll-a concentration, was significantly related to the dissolved inorganic carbon (DIC) concentration (coefficient of determination: R2 = 0.68, P < 0.05). Our results demonstrate that changes in DIC content can significantly alter the dominant phytoplankton species in the water column. The high inputs of DIC and aqueous CO2(aq) to the Seine lead to Bacillariophyta proportions that are 3.96–8.81 % and 19.87–22.56 % higher than those of the Rhine and Danube in the wet and dry seasons, respectively. This can be explained by the difference in the efficiency of carbon concentrating mechanisms (CCMs) among phytoplankton species. Therefore, to mitigate river eutrophication and improve the quality of the aquatic environment, we suggest that future water–carbon management in the flowing water systems with abundant nutrients should consider C-limitation alleviation and its potential effect for reducing the abundance of harmful algae.

High humidity and NO2 co-exposure exacerbates allergic asthma by increasing oxidative stress, inflammatory and TRP protein expressions in lung tissue

Allergic asthma is a chronic inflammatory airway disease with a high mortality rate and a rapidly increasing prevalence in recent decades that is closely linked to environmental change. Previous research found that high humidity (HH) and the traffic-related air pollutant NO2 both aggregated allergic asthma. Their combined effect and mechanisms on asthma exacerbation, however, are unknown. Our study aims to toxicologically clarify the role of HH (90%) and NO2 (5 ppm) on allergic asthma. Ninety male Balb/c mice were randomly assigned to one of six groups (n = 15 in each): saline control, ovalbumin (OVA)-sensitized, OVA + HH, OVA + NO2, OVA + HH + NO2, and OVA + HH + NO2+Capsazepine (CZP). After 38 days of treatment, the airway function, pathological changes in lung tissue, blood inflammatory cells, and oxidative stress and inflammatory biomarkers were comprehensively assessed. Co-exposure to HH and NO2 exacerbated histopathological changes and airway hyperresponsiveness, increased IgE, oxidative stress markers malonaldehyde (MDA) and allergic asthma-related inflammation markers (IL-1β, TNF-α and IL-17), and upregulated the expressions of the transient receptor potential (TRP) ion channels (TRPA1, TRPV1 and TRPV4). Our findings show that co-exposure to HH and NO2 disrupted the Th1/Th2 immune balance, promoting allergic airway inflammation and asthma susceptibility, and increasing TRPV1 expression, whereas CZP reduced TRPV1 expression and alleviated allergic asthma symptoms. Thus, therapeutic treatments that target the TRPV1 ion channel have the potential to effectively manage allergic asthma.

Effect of Physico-Chemical Properties Induced by N, P Co-Doped Biomass Porous Carbon on Nitrous Oxide Adsorption Performance

The adsorption and enrichment of greenhouse gases on biomass porous carbon is a promising approach. Herein, a simple type of nitrogen and phosphorus co-doped biomass porous carbon (NPPC), which was derived from fast-growing eucalyptus bark, was reported via one-step method of carbonization and activation, and the nitrous oxide (N2O) adsorption performance and the adsorption mechanism of the NPPCs were also investigated. The results showed that NPPC-800-2 demonstrated a high specific surface area (1038.48 m2∙g−1), abundant micropores (0.31 cm3∙g−1), and enriched content of N and P (4.17 wt.% and 0.62 wt.%), which also exhibited a high N2O adsorption capacity of 0.839 mmol∙g−1. Moreover, the addition of N enhanced the surface polarity of carbon, thereby altering its pore structure. And P doping induced changes in the structural orientation of carbon, resulting in an increased presence of N-P functional groups. This finding reveals that the eucalyptus bark-based N and P co-doped porous carbon shows great potential for wide applications in N2O capture and provides effective guidance for the design and development of waste biomass adsorbent.

Determination of major drive of ozone formation and improvement of O3 prediction in typical North China Plain based on interpretable random forest model

O3 pollution in China has become prominent in recent years, and it has become one of the most challenging issues in air pollution control. We used data on atmospheric pollutants and meteorology from 2019 to 2021 to build an interpretable random forest (RF) model, applying this model to predict O3 concentration in 2022 in five cities in the Southwest North China Plain. The model was also used to identify and explain the influence of various factors on O3 formation. The correlation coefficient R2 between the predicted O3 concentration and observed O3 concentration was 0.82, the MAE was 15.15 μg/m3, and the RMSE was 20.29 μg/m3, indicating that the model can effectively predict O3 concentration in the studying area. The results of correlation analysis, feature importance, and the driving factor analysis from SHapley Additive exPlanations (SHAP) model indicated that temperature (T), NO2, and relative humidity (RH) are the top three features affecting O3 prediction, while the weights of wind speed and wind direction were relatively low. Thus, O3 in the southwestern North China Plain may mainly come from the formation of local photochemical activities. The dominant factors behind O3 also varied in different seasons. In spring and autumn, O3 pollution is more likely to occur under high NO2 concentration and high-temperature conditions, while in summer, it is more likely to occur under high-temperature and precipitation-free weather. In winter, NO2 is the dominant factor in O3 formation. Finally, the interpretable RF model is used to predict future O3 concentration based on features provided by Community Multiscale Air Quality (CMAQ) and Weather Research & Forecast (WRF) model, and the simulation performance of CMAQ on O3 concentration is enhanced to a certain extent, improving the prediction of future O3 pollution situations and guiding pollution control.

Hourly near-ground NO2 concentration retrieval from geostationary satellite observations

Abstract. Nitrogen dioxide (NO2) is an important contributor to the formation of acid rain, photochemical smog and aerosol particles, which seriously endangers public health. At present, remote sensing of polar-orbiting satellites is a conventional means to obtain large-scale NO2 distribution, but it cannot capture the rapid change because of long revisit periods. The Advanced Himawari Imager (AHI) on the Himawari-8 geostationary satellite has the advantage of high time resolution, which makes it possible to realize near-real-time atmospheric monitoring. Here, based on the absorption characteristics of NO2 in infrared radiation, hourly near-surface NO2 concentrations are retrieved based on the brightness temperature from AHI and auxiliary information such as meteorology and aerosol. The results of 10-fold cross-validation show that NO2 estimations from satellite are in good agreement with in-situ measurements, and their determination coefficient (R2) can reach 0.79. Due to different emission and atmospheric diffusion conditions at different time, the model performance presents a diurnal variation of high accuracy in the noon and afternoon but low accuracy in the morning. Based on the retrieval dataset, it is found that high NO2 concentrations are mainly concentrated in the densely populated and industrial areas such as the North China area. In addition, NO2 pollution mainly occurs in autumn and winter, and the average NO2 concentration in winter is about 1.63 times that in summer in 2021. This study provides a new insight for satellite retrieval of NO2, which is of great significance for real-time pollution monitoring and public health protection.

Constructing multiple active sites of Bi2WO6 for efficient photocatalytic NO removal and NO2 inhibition

Insufficient photocatalytic NOx oxidation capacity will lead to excessive toxic by-products (NO2), which still seriously restrict its practical application. Herein, the multiple active sites (Cl, Bi0 and OVs) modified Bi2WO6 has been established by in-situ synthesis method. The synergistic effect of multiple active sites greatly increased the photo-electric properties of the Bi2WO6, resulting in the boosting photocatalytic NOx deep oxidation to nitrate. The visible light driven OVs-Bi@BWO-Cl catalyst exhibited a highest NO conversion efficiency (67.3 %) with extremely low NO2 concentration (17.9 ppb). The synergism of structural regulation from the OVs with Cl-doping and surface plasmon resonance of Bi0 significantly enhanced light absorption and provided a fast charge transport channel, improving the separation efficiency of photo-generated carriers. The in-situ DRIFTS and density functional theory (DFT) results shown that the synergistic effect by multiple active sites could enhance the adsorption and activation of reactants to accelerate the processes of H2O/O2-to-ROS and decrease the energy barrier of NO removal to promote deep oxidation of NO-to-NO3−. This work can provide ideas for the design and preparation of the catalyst for safe photocatalytic environment purification.

Coupled processes involving ammonium inputs, microbial nitrification, and calcite dissolution control riverine nitrate pollution in the piedmont zone (Qingshui River, China)

Rivers in agricultural countries widely suffer from diffuse nitrate (NO3−) pollution. Although pollution sources and fates of riverine NO3− have been reported worldwide, the driving mechanisms of riverine NO3− pollution associated with mineral dissolution in piedmont zones remain unclear. This study combined hydrogeochemical compositions, stable isotopes (δ18O–NO3−, δ15N–NO3−, δ18O–H2O, and δ2H–H2O), and molecular bioinformation to determine the pollution sources, biogeochemical evolution, and natural attenuation of riverine NO3− in a typical piedmont zone (Qingshui River). High NO3− concentration (37.5 ± 9.44 mg/L) was mainly observed in the agricultural reaches of the river, with ~15.38 % of the samples exceeding the acceptable limit for drinking purpose (44 mg/L as NO3−) set by the World Health Organization. Ammonium inputs, microbial nitrification, and HNO3-induced calcite dissolution were the dominant driving factors that control riverine NO3− contamination in the piedmont zone. Approximately 99.4 % of riverine NO3− contents were derived from NH4+-containing pollutants, consisted of manure & domestic sewage (74.0 % ± 13.0 %), NH4+-synthetic fertilizer (16.1 % ± 8.99 %), and soil organic nitrogen (9.35 % ± 4.49 %). These NH4+-containing pollutants were converted to HNO3 (37.2 ± 9.38 mg/L) by nitrifying bacteria, and then the produced HNO3 preferentially participated in the carbonate (mainly calcite) dissolution, which accounted for 40.0 % ± 12.1 % of the total riverine Ca2+ + Mg2+, also resulting in the rapid release of NO3− into the river water. Thus, microbial nitrification could be a new and non-negligible contributor of riverine NO3− pollution, whereas the involvement of HNO3 in calcite dissolution acted as an accelerator of riverine NO3− pollution. However, denitrification had lesser contribution to natural attenuation for high NO3− pollution. The obtained results indicated that the mitigation of riverine NO3− pollution should focus on the management of ammonium discharges, and the HNO3-induced carbonate dissolution needs to be considered in comprehensively understanding riverine NO3− pollution in piedmont zones.

Long-term variation in exposure to NO2 concentrations in the city of Naples, Italy: Results of a citizen science project

The objective of this study is to evaluate long-term changes in the level of exposure to NO2 among the population living in the urban area of Naples (south Italy). This has been achieved by integrating data from the regional reference monitoring network with information collected during the citizen science initiative called ‘NO2, NO grazie!’ conducted in February 2020 and coordinated by the Non-Governmental Organisation ‘Cittadini per l'aria’. This citizen science campaign was based on low-cost passive samplers (Palmes tubes), providing the ability to obtain unprecedented high-resolution NO2 levels. Using a Land Use Random Forest (LURF), we extrapolated the experimental data obtained from the citizen science campaign and evaluated the changes in population exposure from 2013 to 2022 and the uncertainty associated with this assessment was quantified. The results indicate that a large proportion of the inhabitants of Naples are still exposed to high NO2 concentrations, even if strict emission containment measures are enforced. The average levels remain higher than the new interim and air quality targets suggested by the World Health Organisation. The implementation of co-created citizen science projects, where NGO and citizens actively participate alongside scientists, can significantly improve the estimation and the interpretation of official reference data.

Interaction of physical activity and low-level air pollution on cardiovascular disease: a prospective cohort study in UK Biobank.

To investigate theassociations of physical activity (PA), low-level air pollution, and interaction on cardiovascular diseases (CVD) incidence based on the UK Biobank. PA was measured by the International Physical Activity Questionnaire and five air pollutants were estimated using Land Use Regression. All association estimates were based on Cox regression. Dose-response relationship was explored by restricted cubic spline, while multiplicative and additive interaction were examined by Pinteraction and relative excess risk due to interaction (RERI). As deviating proportional hazards assumption, we analyzed data as follow-up < 4years and ≥ 4years, separately. PA with 1000-4000 Metabolic Equivalent Task (MET) min/week showed the strongest protective impact on CVD incidence, while only low-level nitrogen dioxides (NO2) showed negative impact among five air pollutants and was considered for further analysis. Multiplicative interaction between PA and NO2 was observed during ≥ 4years follow-up (Pinteraction = 0.049) while not during < 4years (Pinteraction = 0.290). Positive additive interactions were found for high PA and low NO2 (< 20 μg/m3) group (RERI: 0.07, 95% confidence intervals: 0.02-0.11) during < 4years, and for moderate PA with NO2 at 40- μg/m3 (0.07, 0.02-0.13) and < 20 μg/m3 (0.07, 0.02-0.12), while high PA showed similar results with NO2 at 40-, 20- and < 20 μg/m3 during ≥ 4years. PA about 1000-4000 METs min/week showed the lowest CVD risk. Possibility of interaction with PA and NO2 is more likely to present with the increase in follow-up duration. We call for the optimal thresholds of PA, and exploring interaction thoroughly by considering types of PA.

Benthic clade II-type nitrous oxide reducers suppress nitrous oxide emissions in shallow lakes

Shallow lakes, recognized as hotspots for nitrogen cycling, contribute to the emission of the potent greenhouse gas nitrous oxide (N2O), but the current emission estimates for this gas have a high degree of uncertainty. However, the role of N2O-reducing bacteria (N2ORB) as N2O sinks and their contribution to N2O reduction in aquatic ecosystems in response to N2O dynamics have not been determined. Here, we investigated the N2O dynamics and microbial processes in the nitrogen cycle, which included both N2O production and consumption, in five shallow lakes spanning approximately 500 km. The investigated sites exhibited N2O oversaturation, with excess dissolved N2O concentrations (ΔN2O) ranging from 0.55 ± 0.61 to 53.17 ± 15.75 nM. Sediment-bound N2O (sN2O) was significantly positively correlated with the nitrate concentration in the overlying water (p < 0.05), suggesting that nitrate accumulation contributes to benthic N2O generation. High N2O consumption activity (RN2O) corresponded to low ΔN2O. In addition, a significant negative correlation was found between RN2O and nir/nosZ, showing that bacteria encoding nosZ contributed to N2O consumption in the benthic sediments. Redundancy analysis indicated that benthic functional genes effectively reflected the variations in RN2O and ∆N2O. qPCR analysis revealed that the clade II nosZ gene was more sensitive to ΔN2O than the clade I nosZ gene. Furthermore, four novel genera of potential nondenitrifying N2ORB were identified based on metagenome-assembled genome analysis. These genera, which are affiliated with clade II, lack genes responsible for N2O production. Collectively, benthic N2ORB, especially for clade II-type N2ORB, harnesses N2O consumption activity leading to low N2O emissions from shallow lakes. This study advances our knowledge of the role of benthic clade II-type N2ORB in regulating N2O emissions in shallow lakes.

Microbial communities overwhelm environmental controls in explaining nitrous oxide emission in acidic soils

Intensively fertilized acidic soils are global hotspots of nitrous oxide (N2O) emissions, contributing to net agronomic greenhouse gas outcomes. Identifying the key drivers of soil N2O emissions is hampered by the synergistic or antagonistic effects of multiple factors. Within a framework based on the predominant role of microbial communities producing N2O, the N2O emissions are affected either by proximal regulators: temporary soil property fluctuations affect N2O production transcriptionally, or by distal regulators: persistent genetic rearrangements in local microbial communities. The proximal regulators, individually or together, may spontaneously impact distal regulators. Here, we use acidic soils from tea (Camellia sinensis L.) plantations on a broader geographic scale as a model system. Based on amplicon sequencing and properties of 195 acidic (average pH = 5.0) soils, we determined the importance of proximal and distal regulation to N2O emissions. Microbial phylogenetic diversity as a distal regulator overwhelms mineral N content as a proximal regulator in explaining high N2O emissions. Low-abundance, diverse prokaryotic communities (e.g., Acidothermu) and specialized denitrifying fungal communities (e.g., Fusarium) were associated with high N2O emissions. Revisiting the impact of proximal regulators on distal regulators revealed that soil pH is the sole proximal regulator influencing the prokaryotic rare taxa that correlated with high N2O emissions. When considering proximal regulators together (here soil properties compiled as soil fertility index), the microbial diversity were independent of soil fertility. The microbial assembly was dominated by stochastic processes. Consequently, proximal regulators have a limited impact on distal regulators of N2O emissions from acidic soils. In conclusion, the framework underscored the importance of in situ microbial communities as distal regulators in explaining high N2O emissions from acidic soils.

Synergistic denitrification and Mn cycle driven by sulfur autotrophy significantly enhanced nitrogen removal

Due to the low C/N and high NO3−-N content in tailwater of wastewater treatment plants, the need for further denitrification treatment attracted wide attention. To achieve deep denitrification of tailwater with low C/N and high NO3−-N, a novel sulfur-manganese carbonate ore denitrification (SMCD) biological reactor was constructed. The denitrification performance of SMCD reactor was significantly higher than that of sulfur denitrification and manganese carbonate ore denitrification reactors. Across the study, the removal efficiencies of TN and NO3−-N were above 90%, and without NO2−-N accumulation in the SMCD reactor. Quantitative results of functional genes showed that SMCD reactor were conducive to realize complete denitrification. Besides, manganese autotrophic denitrification (MAD) process occurred in the SMCD reactor. By analyzing the nitrogen as well as SO42- and Mn(Ⅱ) concentrations along the height, it was found that the conversion between Mn(II) and Mn(IV) was stable during Stage Ⅲ (C/N=1). The microbiota in the SMCD reactor was distinctly different from that in the controls. Thiobacillus, Thermomonas, and an unclassified member in family cvE6 were the dominant members of SMCD reactor. Besides, Thermomonas, Dokdonella, and Simplicispira were identified as potential Mn oxidizers. The SMCD reactor exhibited a significant synergistic denitrification effect by coupling sulfur autotrophic denitrification and MAD process, which provided an effective solution for the deep denitrification of low C/N tailwater.

Hydrochemical and microbial community characteristics and the sources of inorganic nitrogen in groundwater from different aquifers in Zhanjiang, Guangdong Province, China

Groundwater from different aquifers in the Zhanjiang area suffers from different degrees of nitrogen pollution, which poses a serious threat to the health of urban and rural residents as well as the surrounding aquatic ecological environment. However, neither the water chemistry and microbial community characteristics in different aquifer media nor the sources of inorganic nitrogen pollution have been extensively studied. This study integrated water quality parameters, dual isotopes (δ15N–NO3− and δ18O–NO3−), and 16S rRNA data to clarify the hydrochemical and microbial characteristics of loose rock pore water (LRPW), layered bedrock fissure water (LBFW), and volcanic rock pore fissure water (VRPFW) in the Zhanjiang area and to determine inorganic nitrogen pollution and sources. The results show that the hydrochemistry of groundwater in different aquifers is complex and diverse, which is mainly affected by rock weathering and atmospheric precipitation, and the cation exchange is strong. High NO3− concentration reduces the richness of the microbial community (VRPFW). There are a large number of bacteria related to nitrogen (N) cycle in groundwater and nitrification dominated the N transformation. A quarter of the samples exceeded the relevant inorganic nitrogen index limits specified in the drinking water standard for China. The NO3− content is highest in VRPFW and the NH4+ content is highest in shallow loose rock pore water (SLRPW). In general, NO3−/Cl−, dual isotope (δ15N–NO3− and δ18O–NO3−) data and MixSIAR quantitative results indicate manure and sewage (M&S) and soil organic nitrogen (SON) are the main sources of NO3−. In LRPW, as the depth increases, the contribution rate of M&S gradually decreases, and the contribution rate of SON gradually increases. The results of uncertainty analysis show that the UI90 values of SON and M&S are higher. This study provides a scientific basis for local relevant departments to address inorganic nitrogen pollution in groundwater.

Prussian blue analogues-Derived Iron-Cobalt carbon fiber Binder-Free electrodes for electrocatalytic nitrate reduction

Developing a highly selective and efficient electrocatalytic system for nitrate reduction reaction (NO3RR) is of great significance but still remains challenging for environmental remediation and energy utilization. Herein, we constructed a binder-free cathode consisted of iron-cobalt Prussian blue analogues-based bimetal derived carbon fiber (FC-PBAFC) to enhance NO3RR performances. Significantly, high NO3−-N removal capacity of 169.1 mgN h−1 gcat-1, excellent ammonia Faradic efficiency (FE) of 83.2 ± 0.7 % and high ammonia selectivity of 99.3 ± 0.6 % were achieved by FC-PBAFC binder-free cathode. The excellent NO3RR performance was attributed to the rational polymer tethering strategy for the binder-free cathode construction. The introduction of polyacrylonitrile for electrospinning shaping essentially regulated the structural and compositional distribution of binder-free electrode during followed pyrolysis. The in-situ infrared spectroscopy and DFT calculation demonstrated that the Fe and Co regulation optimized the adsorption energy of key intermediate products with suppressing the side reactions. The bimetal binder-free cathode offers a promising strategy for selective and effective electroreduction of nitrate for ammonia synthesis.

In situ DRIFTS study and mechanism analysis of selective catalytic reduction of NOx by NH3 over ions doping MnCeOx/GR catalyst

The present study demonstrates an innovative approach of doping MnCeOx/GR catalyst with three transition metal ions, namely Fe3+, Co3+, and Ni2+. Subsequently, the catalyst exhibiting superior performance was carefully selected. Meanwhile, the micro-reaction process of MnCeOx/GR and ion-doped MnCeOx/GR catalysts were established by in situ DRIFTS. The phenomena occurred in the experiment process were thoroughly examined through the application of various characteristic analysis, for instance, XRD, Raman, BET, XPS, H2-TPR, and NH3-TPD to analyze that Co3+ doping had a good improvement of activity and N2 selectivity on MnCeOx/GR catalyst at low temperature. This was evidenced by the activity test experiments, which revealed that CoMnCeOx/GR catalyst has the highest NO conversion within 150–225 °C temperature range, surpassing 90 %. Our findings showed that Co3+ doping not only enhanced redox capability and surface acidity, but also raised the relative Mn4+, Ce3+, and Oα content compared to MnCeOx/GR catalyst, which eventually contributed that the reactant molecules can be more effectively adsorbed onto the catalyst, thereby enhancing activation of NH3 and O2, thus improving efficiency of CoMnCeOx/GR catalyst. Additionally, it was proved that the Eley-Rideal and Langmuir-Hinshelwood mechanisms co-existed on CoMnCeOx/GR catalyst in a competitive way by in situ DRIFTS test.

Advancing room temperature NO2 gas sensing performance through high-energy mechanical milling of Tin-dichalcogenides

Eco-friendly gas sensing devices with simple architecture, reduced cost, and high performance at room temperature are being sought to replace the traditional metallic oxide materials, aiming to address environmental concerns. Lamellae semiconducting materials have shown promising detection properties for this purpose. Here, we investigated the sensing response of SnS2, Sn(S0.5Se0.5)2, and SnSe2 tin-dichalcogenides prepared by high-energy mechanical milling. High RNO2/Rair response signals, from 102 to 106, for 2–100 ppm of NO2 were observed for temperatures between 30 °C and 300 °C. The materials were not sensitive to CO, while H2 detection could only be observed above 200 °C, implying high NO2 selectivity. Additionally, we investigated the influence of samples suspension in water and isopropanol on grain size and morphology. We found that isopropanol crystallizes amorphous selenium phase dispersed in the Sn-Se system and increase the agglomeration in the Sn(S0.5Se0.5)2 system. Deformed and defective particles were observed regardless the preparation methodology. This unique defect-rich morphology might increase surface reactivity for selective NO2 detection by physisorption, owing a high adsorption/desorption rate at room temperature.