Initial Nitrate Concentration Research Articles

Nitrate Removal by Zero-Valent Metals: A Comprehensive Review

Nitrate is a widespread water contaminant that can pose environmental and health risks. Various conventional techniques can be applied for the removal of nitrate from water and wastewater, such as biological denitrification, ion exchange, nanofiltration, and reverse osmosis. Compared to traditional methods, the chemical denitrification through zero-valent metals offers various advantages, such as lower costs, simplicity of management, and high efficiencies. The most utilized material for chemical denitrification is zero-valent iron (ZVI). Aluminium (ZVA), magnesium (ZVM), copper (ZVC), and zinc (ZVZ) are alternative zero-valent metals that are studied for the removal of nitrate from water as well as from aqueous solutions. To the best of our knowledge, a comprehensive work on the use of the various zero-valent materials that are employed for the removal of nitrate is still missing. Therefore, in the present review, the most recent papers concerning the use of zero-valent materials for chemical denitrification were analysed. The studies that dealt with zero-valent iron were discussed by considering microscopic (mZVI) and nanoscopic (nZVI) forms. For each Fe0 form, the effects of the initial pH, the presence or absence of dissolved oxygen, the initial nitrate concentration, the temperature, and the dissolved ions on the nitrate removal process were separately evaluated. Finally, the different materials that were employed as support for the nanoparticles were examined. For the other zero-valent metals tested, a detailed description of the works present in the literature was carried out. A comparison of the various features that are related to each considered material was also made.

Modified Hazelnut Shells as a Novel Adsorbent for the Removal of Nitrate from Wastewater

The aim of the study was to prepare a novel adsorbent by chemical modification of hazelnut shells and evaluate its potential for the nitrate removal from model solutions and real wastewater. The characterization of the novel adsorbent, i.e., modified hazelnut shell (MHS) was performed. The adsorbent characterization included the analysis of elemental composition and the surface characteristics analysis by scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR). The adsorption experiments (batch technique) were performed to investigate the effects of adsorbent concentration, contact time, initial nitrate concentration, and solution pH. The nitrate removal efficiency increased with the increase in MHS concentration and decreased with the initial nitrate concentration. MHS was found to be effective in nitrate removal over a wide pH range (from 2 to 10), and the highest amount of nitrate adsorbed was 25.79 mg g−1 in a model nitrate solution. Depending on the aqueous medium (model solutions or real wastewater samples), it was shown that both Langmuir and Freundlich adsorption isotherm models can be used to interpret the adsorption process. It was found that the kinetics are well described by a pseudo-second order model and the nitrate adsorption process can be controlled by chemisorption. The intraparticle diffusion model has been used to identify an adsorption-controlled process by diffusion mechanisms. Adsorption/desorption experiments in column confirmed that MHS could be successfully used in multiple cycles (at least three), indicating the potential of MHS as an alternative to costly commercial adsorbents for the removal of nitrates from wastewater.

Nitrate adsorption onto surface-modified red mud in batch and fixed-bed column systems: equilibrium, kinetic, and thermodynamic studies.

This research aimed to develop a novel composite as a low-cost adsorbent for the removal of nitrate ion from aqueous solutions. The characterization of this composite (composition of red mud with dimethyldioctadecylammonium bromide (DDAB)) was performed by XRF, XRD, FTIR, and BET analyses. The most influential variables on nitrate adsorption, including contact time, solution acidity, adsorbent amount, and temperature were studied. The maximum amount of nitrate adsorbed onto the prepared adsorbent was obtained at pH 5.5 and contact time 30min. The heterogeneous adsorption occurred during the uptake of nitrate. The results of kinetic study revealed that intra-particle diffusion was the major limitation for nitrate adsorption rate. The values of thermodynamic parameters illustrate the non-spontaneous, associative, and exothermic adsorption process. Increasing the temperature enhances the tendency of the process to non-spontaneously. Research on fixed-bed column has been done under different initial nitrate concentrations. The adsorption capacity of nitrate was increased with an increase in the initial concentration of nitrate. The results of column data were successfully explained using the Thomas and Yoon-Nelson models.

Polyamide nylon 6 as a potential carrier of nitrate anions in aqueous environments

Νitrates and microplastics may co-exist in terrestrial and aquatic ecosystems implying potential interaction and, subsequently affecting their environmental fate and the ecosystem quality. However, studies highlighting the interaction between these substances do not exist or are very limited. In this study, batch-type experiments were carried out using aqueous solutions (20 mL, pH 6) to investigate the effect of different parameters such as the polyamide nylon 6 (PN6) dose (0.2, 1.0, 1.5, and 2.0 g), initial nitrate concentration (2, 25, 50, 100, and 150) and contact time on the sorption capacity of PN6 for nitrate anions. The experimental data revealed that microplastic addition results in a remarkable decrease of nitrates in solution (qmax = 3.5 mg/g), mostly during the first 3 days of experimentation. In addition, the FTIR spectra of PN6 aliquots prior and after sorption indicated that the sorption occurs basically via electrostatic attraction between the negatively-charged nitrate anion and positively charged moieties on the PN6 surface, assuming that PN6 microplastics may act as potential carriers of nitrates in aqueous environments, and affect their transport in the geosphere and in living organisms.

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

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

Simultaneous electrocatalytic removal of inorganic nitrogen compounds in groundwater: Modeling and mechanistic studies

The use of biological and chemical methods for the simultaneous removal of inorganic nitrogen compounds containing nitrate, ammonia, and nitrite in groundwater typically cannot achieve synchronous oxidation and reduction in an undivided reactor. In the present study, with the strict regulation of the oxidation reaction in the anode zone and reduction reaction in the cathode zone, simultaneous removal of nitrate, ammonia, and nitrite was achieved in an undivided electrochemical system. Our results indicated that ammonia and nitrite had been completely removed, and the concentration of nitrate was reduced to 8.58 mg N/L with initial nitrate, ammonia, and nitrite concentrations of 50.00, 10.00 and 5.00 mg N/L, respectively. Cyclic voltammetry (CV) and potentiostatic electrolysis revealed that the electrode surface reaction process, that is, nitrate, ammonia, and nitrite in the system, could be converted into nitrogen via ClO− generated from Cl− produced from the electrochlorination process during multiple cycle attenuation. Moreover, both CV and Density functional theory (DFT) calculations revealed the inhibitory effect of coexisting ammonia and nitrite on the electrochemical reduction of nitrate on the Cu cathode. This strategy opens a paradigm for the simultaneous efficient electrocatalytic removal of nitrate, nitrite, and ammonia in an undivided single reactor.

Nitrate removal from synthetic and real groundwater by electrocoagulation: effect of operating parameters and electrolytes

ABSTRACT The present study elaborates the electrocoagulation process for the removal of nitrates from synthetic and real groundwater. Electrocoagulation treatment has a positive effect on the quality of groundwater in general and the removal of nitrate in particular, which is one of the most potent pollutants found in groundwater. Moreover, it is supposed that nitrate removal mechanisms are likely to be dependent on operational parameters like applied current, electrode material, initial concentration of nitrates, and pH of the solution, etc. Therefore, in the present study various process parameters have been optimised in terms of nitrate removal efficiency. Experiments were performed using batch process at different initial concentration of nitrates (100–500 mg/L), pH (6.0–12), stirring speed (100–500 rpm), inter-electrode distance (0.5–2 cm) and electrolysis time (30–180 min). Further, the effect of co-existing ions using KNO3, Ca(NO3)2 and Mg(NO3)2 in the presence of NaCl and Na2SO4 was investigated. Maximum 98%, Nitrate removal efficiency was obtained at inter-electrode distance 1 cm, agitation speed 300 rpm, electrolyte concentration of 1.1688 (g/L) NaCl, current 1.5 A and time 180 min for initial nitrate concentration of 100 (mg/L). Isotherm and kinetics models have been studied and it was observed from the present investigation that the process follows pseudo-second-order kinetics. The Freundlich isotherm model simulations match satisfactorily with the experimental observations. Further, optimised parameters were used to remove nitrates from real groundwater and 92.5% removal efficiency was attained.

Optimization of the electrochemical reduction process and ORP effects in nitrate removal.

Nitrate contaminated water resources, a huge environmental problem for countries, including our own. This research aims to evaluate the efficiency of the electrochemical process for the removal of nitrate from the aqueous solution by direct and alternating current. Experiments were done by the Taguchi method, including electrode array, time exposure, voltage, pH, and the initial concentration of nitrate. Minitab 17 and SPSS 18 software were utilized to design and data analysis. In detail, the connection of the electrodes was monopolar, and the direct current (DC) has a greater efficiency as alternating current (AC) in the nitrate removal (p > 0.001). In AC, the nitrate initial concentration, voltage, electrodes material, contact time, and pH have the most effect on the nitrate removal. The optimized conditions for nitrate removal include Al-Al electrode, pH of 7, a voltage of 30 V, and nitrate initial concentration of 100 mg/L with the contact of time 150 min. Selection of appropriate materials for electrodes construction, adequate voltage, and removal of intervention agents from aqueous solution led to greater removal efficiencies. The optimized conditions that can reduce the aqueous solution ORP with high speed and the electrochemical reactor efficiency for nitrate removal were increased. PRACTITIONER POINTS: Electrochemichal (EC) process is evaluated for treatment of nitrate wastewater for the first time. High removal efficiency (over 95%) from nitrate wastewater was obtained. Minimizing energy consumption and maximizing nitrate removal were simultaneously achieved in the integrated single EC process.

Synthesis and performance of bimetallic poly vinyl alcohol beads in electrocatalytic denitrification

Research in the field of nanoscience and nanotechnology has witnessed significant progress in the last few years. The synthesis of nanoparticles and microparticles for the improvement of functionality of several processes has paved ways for innovation into uninvestigated methods of developing these particles. Significance should be given to such mechanisms which enable easy handling and reusability of the formed micro/nanoparticles. In the presented research a unique and novel method for synthesis of such particles immobilized on the surface of poly vinyl alcohol (PVA) beads has been discussed. The use of silver (Ag) and palladium (Pd) as noble metals has been done for coating the PVA beads; to form a bi-metallic catalyst. These beads have been applied in the denitrification process of synthetic nitrate solution, having initial nitrate concentration (NO3 -N) of 20 mg/L, carried out in a divided reactor cell of 120 mL volume, at 35⁰C for 1.5 h. About 75 % removal of nitrate was obtained. Removal of nitrate took place by destruction of nitrate and its subsequent conversion into nitrite, ammonical nitrogen, and nitrogen gas. The usage of Pd-Ag-PVA beads in the work has been unexplored in the past and the current work provides insights to behold application of catalysed electrochemical reactions for denitrification process on a large-scale.

Photodegradation of Azithromycin in Aqueous System Under VIS/UV Irradiation

Antibiotics, such as azithromycin, have become one major group of emerging contaminants in aquatic systems. It is urgent to explore efficient, safe, and economic methods to removal azithromycin in aquatic environment. This paper described the photolysis of azithromycin and its affecting factors (initial concentration of azithromycin, pH, nitrate and Fe (III)-oxalate complexes) under visible and UV light irradiation. The results indicated ultraviolet has stronger ability to degrade azithromycin than visible light. The photodegradation efficiencies of azithromycin decreased with the increase of its initial concentration under UV irradiation, as well as pH value. The maximum degradation rates occurred in the initial concentration of 10 mg∙L-1 and pH value of 4, which was 53.1% and 82.6%, respectively. The addition of nitrate can promote the photodegradation of azithromycin but Fe (III)-oxalate complexes had the opposite effect. The achievements proved photodegradation is a promising technology to remove azithromycin from water environment.

Reducing Nitrate from Water Using Lanthanum-modified Adsorbent: Optimization, Thermodynamics, Kinetics, and Isotherms

Lanthanum-modified commercial activated carbon (LMAC) adsorbent was synthesized, characterized, and then applied for reducing nitrate from aqueous solutions under various conditions. The extent of nitrate removal depended on four factors: temperature, the aqueous solution pH, initial nitrate concentration, and contact time. The Taguchi approach was used as the method of design for the experiments. Under optimal conditions ( T= 300°C, pH=3, C0=10 ppm, and t= 210 min), the removal percentages and capacity of nitrate adsorption were found to be 71.31%, and 1.43 for activated carbon (AC) and 93.31% and 1.87 for LMAC, respectively. Thermodynamic parameters, including the enthalpy, Gibbs free energy, and entropy, indicated the spontaneous and exothermic nature of the adsorption process. Various isotherms and first and second-order kinetic models were applied to investigate the adsorption process. The pseudo-second-order kinetic model and Langmuir isotherm could well describe the adsorption process.

Introduction of nitrogen doped graphene nanosheets as efficient adsorbents for nitrate removal from aqueous samples.

Introducing and developing new kinds of adsorbents are always a significant challenge in water treatments. In this work, for the first time, graphene oxide (GO), nitrogen-doped graphene oxide (ND-GO), highly nitrogen-doped graphene oxide (HND-GO), and 3D high nitrogen-doped graphene oxide (3D-HND-GO) were synthesized and comparatively evaluated in the removal of nitrate content of tap and underground waters. The removal of the target analyte was performed through a batch adsorption approach, and the factors influencing its removal efficiency (i.e., initial pH of the sample, primary concentrations of nitrate, amount of adsorbent, and contact time) were evaluated through a central composite design (CCD) and response surface methodology (RSM). Based on the results, 3D-HND-GO showed the highest removal efficiency in comparison with the other mentioned nanoparticles. The nitrate removal using this adsorbent was modeled successfully so that R 2, adjusted R 2, and predicted R 2 values were 0.9717, 0.9508, and 0.9010, respectively. In addition, the optimal removal condition was achieved using the Nelder-Mead non-linear optimization algorithm as follow: the initial concentrations of nitrate (expressed as nitrogen): 15.0mg/mL, the amount of the adsorbent: 2.0mg/mL; pH of the sample: 3.0; and the contact time: 20.0min. Under this optimal condition, the actual removal result (92.5 ± 4.0%) was in good agreement with the expected value (94.8 ± 5.1%). Additional studies were also performed to comprehensibly evaluate the adsorption activity of the adsorbent (e.g., kinetic, isotherm, and desorption parameters). The adsorption isotherm complied with the Langmuir model illustrating the considerable mono-layer adsorption capacities for the target ions with qm of 8.7mg/g. The adsorption process was indicated to obey a pseudo 2nd order kinetic model, with the rate-limiting step for the adsorption phase. This study revealed which 3D-HND-G leads to improved yield in the nitrate ions elimination, particularly at acidic media, which was related to the enhanced dispersibility and larger surface area. The adsorbent was further successfully used for treating tap and underground water samples. At the present moment, research as grown to modify 3D-HND-G in orders to increase the potentiality for industrial applications. The online version contains supplementary material available at 10.1007/s40201-021-00741-7.

Reverse osmosis treatment of condensate from ammonium nitrate production: Insights into membrane performance

Ammonium nitrate is an important fertilizer and industrial explosive. The production of ammonium nitrate entails the generation of a large volume of condensate laden with nitrogen that must be treated before environment discharge. Results in this study show that through appropriate membrane selection, over 90% rejection of ammonium nitrate can be achieved by reverse osmosis (RO) filtration. Using RO (which is highly compact and efficient) to enrich ammonium nitrate in the condensate would significantly reduce the size of the evaporation separator for ammonia recovery. The results also highlight the importance of membrane selection for this application. Results reported here suggest that a low pressure RO membrane (e.g. ESPA2) is more suitable for the dilute condensate while a high pressure RO membrane (e.g. SW30) is recommended for the concentrated condensate to ensure adequate ammonia and nitrate rejection. Ammonia and nitrate rejections were dependent on key operating parameters including applied pressure (or water flux), temperature, feed solution pH, and initial ammonium nitrate concentration in the condensate. The impact of operating conditions on ammonia and nitrate rejections was more profound for low pressure (thus high flux) than high pressure RO membrane. An extended filtration experiment shows no evidence of membrane fouling. Results from this study are useful to the integration of a compact RO system to ammonium nitrate manufacturing for pollution prevention and improving product yield.

New silica based adsorbent material from rice straw and its in-flow application to nitrate reduction in waters: Process sustainability and scale-up possibilities

This paper shows a particular example to move to a sustainable circular economical process from valorization of rice straw ashes by developing a green synthesis for obtaining a useful sub-product. This strategy can palliate negative effects of the agriculture waste practices on the environment and also the obtained silica reduced nitrate content in waters. It is demonstrated that the silica synthesis developed at lab was scalable more than a hundred times with good results. Adsorption studies of nitrate in standards and real well waters at lab scale and scaling-up provided similar results. Adsorption values near to 15 mg/g for nitrate standards and 8.5 mg/g for well water were obtained until achieving the initial nitrate concentration. Experimental breakthrough curves fitted to Thomas model, which gave similar results for adsorption capacities. The adsorption capacity was checked with that obtained by a commercial resin, providing improved results. The method at large scale was compared with industrial traditional methods and green adsorbents.

Removal of nitrate and phosphate from simulated agricultural runoff water by Chlorella vulgaris

Microalgae such Chlorella vulgaris can effectively absorb nitrate and phosphate from contaminated water. This work characterized nitrate and phosphate removal from simulated agricultural runoff using C. vulgaris. Statistically designed experiments were used to model the following responses: (1) algal growth; (2) nitrate removal; (3) phosphate removal; (4) protein in the algal biomass; (5) chlorophyll content of the biomass; (6) the biomass phenolics content; and (7) the free radical scavenging antioxidant activity of the biomass. These response were modelled for the following key experimental factors: initial nitrate concentration in the simulated runoff (1080–3240 mg L−1, as NaNO3), initial phosphate concentration (20–60 mg L−1, as K2HPO4), photoperiod (8–24 h of light/day) and culture duration (5–15 days). The validated models were used to identify the factor levels to maximize the various responses. Nitrate removal was maximized at 85.6% when initial nitrate and phosphate concentrations were 2322 mg L−1 and 38 mg L−1 (N:P atom ratio ≈ 125:1), respectively, with a 17.2 h daily photoperiod in a 13-day culture. Phosphate removal was maximized at 95% when the initial nitrate and phosphate concentrations were 1402 mg L−1 and 56.7 mg L−1 (N:P ≈ 51:1), respectively, with a 15.7 h daily photoperiod in a 14.7-day culture. At least ~14 h of a daily photoperiod and a ~11-day culture period were required to maximize all the studied responses. C. vulgaris is edible and may be used as animal feed. Nutritional aspects of the biomass were characterized. Biomass with more than 24% protein could be produced. Under the best conditions, the chlorophyll (potential food colorants) content of the biomass was 8.5% and the maximum level of total phenolics (antioxidants) in the biomass was nearly 13 mg gallic acid equivalent g−1.

Synthesis of Chitosan Stabilized Silver Nanoparticles and Evaluation of the in vitro Antibacterial Activity Against Xanthomonas oryzae pv. oryzae Causing Blight Disease of Rice

Bacterial blight is one of the most devastating rice diseases that cause huge economic loss worldwide. The cause of rice blight is Gram negative bacteria Xanthomonas oryzae pv. oryzae (X. oryzae pv. Oryzae). Since both silver nanoparticles and chitosan have antibacterial, antifungal and growth-stimulating effect, this work has focused on synthesizing chitosan stabilized silver nanoparticles (AgCSs) with small sizes and in vitro evaluating antibacterial activity against X. oryzae pv. oryzae bacteria. AgCSs were chemically synthesized by reducing silver nitrate by borohydride sodium in the presence of chitosan with optimization of the concentration of the reactants. AgCSs were characterized by UV/vis absorption spectra, field emission scanning electronic microscopy (FESEM), ImageJ software, zeta potential measurement, Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction. AgCSs have spherical configuration and narrow size distributions with different average sizes from 15 nm to 25 nm depending on the initial concentration of silver nitrate. All AgCSs colloidal systems were stable and exhibited no tendency for coagulation more than 5 months. It was the first time that chitosan-stabilized silver nanoparticles were assessed the in vitro antibacterial activity against bacterial blight VXO_281 strain. The disc diffusion method demonstrated that the smallest size silver nanoparticles (AgCS1) showed high antibacterial effect against the X. oryzae pv. oryzae VXO_281 strain with a concentration of more than 5 µg/mL and the inhibition zone was dose-dependent. The minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of AgCS1 against X. oryzae pv. oryzae VXO_281 were 2.5 µg/mL and 20 µg/mL, respectively.

Co-production of fucoxanthin, docosahexaenoic acid (DHA) and bioethanol from the marine microalga Tisochrysis lutea

The marine microalga Tisochrysis lutea is renowned for its ability to synthesize fucoxanthin and docosahexaenoic acid (DHA), which are nutritionally valuable high-value compounds. Although numerous studies in literature have assessed fucoxanthin and DHA production by T. lutea, very few studies have evaluated the feasibility of comprehensively utilizing biomass for co-production of these metabolites within the framework of biorefineries. To this end, the current study focused on the synthesis of fucoxanthin and DHA by cultivation of T. lutea under two different initial nitrate concentrations (1x: 882 µM, 3x: 2,646 µM) and three different light intensities (LL: 50 µmol/m2/s; ML: 100 µmol/m2/s; HL: 150 µmol/m2/s). The maximum fucoxanthin yield of 8.80 ± 0.30 mg/L (14.43 ± 0.52 mg/g) and DHA yield of 7.08 ± 0.02 mg/L (11.90 ± 0.14 mg/g) were achieved in the 3x HL culture at the end of 16 days of cultivation. Thereafter, a novel process of biphasic solvent extraction using ethanol/n-hexane/water (10:9:1 v/v/v) was utilized for co-extraction 97.96 ± 0.54% fucoxanthin and 74.11 ± 1.49% DHA from 3x HL biomass, and products were separated into two fractions. Fermentation of the residual biomass obtained from co-extraction resulted in a bioethanol yield of 48.49 ± 0.58 mg/g. Accordingly, the current study demonstrated the potential of T. lutea as a feedstock for biorefineries.

Characteristics of a novel heterotrophic nitrification-aerobic denitrification yeast, Barnettozyma californica K1

Yeast strain K1, isolated from surface sediment, was identified as Barnettozyma californica. The strain showed efficient heterotrophic nitrification and aerobic denitrification (HN-AD) at initial ammonium, nitrite and nitrate concentrations of 14–140 mg/L. Additionally, the optimum carbon source for its growth and nitrogen removal activity was sucrose, followed by glucose, acetate and citrate. The maximum removal efficiencies of ammonium, nitrite and nitrate were 99.11%, 99.13% and 98.84% under 48 h of culture with sucrose at 140 mg/L nitrogen and the corresponding removal efficiencies of total nitrogen were 90.16%, 86.65% and 81.48%, respectively. The optimum conditions for the inorganic nitrogen removal and growth of strain K1 were a C/N of 18 and a salinity of 5-15 ppt. The amoA, nirK and nosZ genes of K1 were detected. All the results suggest that B. californica K1 is capable of HN-AD and has the potential to remove inorganic nitrogen from wastewater.

An Efficient Bioaggregate Reactor for Enhanced Denitrification of Sewage with Low Carbon/Nitrogen Ratio

AbstractThis study developed a bioaggregate reactor by using Pseudomonas strain GF2. The response surface method indicated that the optimal parameters were pH 7, a hydraulic retention time (HRT) of 6 h, and an initial nitrate concentration of 25 mg L−1. Nitrate removal reached a maximum of 99.19 % at a C/N ratio of 2. Gas chromatographic detection revealed that nitrogen gas was the main product (98.46 %). 3D fluorescence spectroscopy showed that, with increasing HRT, the amounts of fulvic acid‐like, tryptophan, and protein‐like substances increased, which were consistent with the change in microbial community. Fourier transform infrared spectroscopy confirmed the existence of OH, CH, C=O, and C=C. High‐throughput sequencing showed that Pseudomonas and Azospira were the dominant genera.

Research on the sensitivity of parameters and calibrates DNDC model for calculating emissionsfrom paddy rice cultivation activities

This study estimates the sensitivity of parameters to serve the adjustment and construction of standard parameters in the Decomposition - Denitrification (DNDC) model when calculating greenhouse gas emissions from rice cultivation on alluvial, alkali, acid-sulfate, and bleached soils of Red river delta, Vietnam. The research results show that the following parameters: bulk density, clay fraction, microbial activity index, soil organic carbon (SOC), temperature, conductivity, fertilization of urea, and porosity have a high sensitivity for the calculation of CH4 emissions; the following parameters: pH, drainage efficiency, depth of water retention layer, field capacity, initial nitrate and ammonium concentrations at surface soil, and rainfall have low sensitivity and less influence for the calculation of CH4 emissions; the wilt moisture and salinity index have no effect on CH4 emissions. In case of N2O emissions, the following parameters: amount of manure, clay fraction, microbial activity index, pH, initial nitrate concentration at surface soil, bulk density, SOC, field capacity, porosity and depth of water retention layer have a high sensitivity and more influence to N2O emissions; the following parameters: drainage efficiency, conductivity, initial ammonium concentration at surface soil, temperature, fertilisation of urea and rainfall have low sensitivity and less influence to N2O emissions; the wilting point and soil salinity index have no effect on N2O emissions. The results of model calibration show a good correlation between observed and simulated values in both spring and summer crops in 2018. The R2 of the spring and summer crops reach 0.86 and 0.79, the Nash-Sutclitffe efficiency index reaches 0.82 and 0.77 for CH4 emission; meanwhile, for N2O emission, these values are 0.62, 0.69, and 0.76, respectively. The study also found a model calibration parameter used to calculate greenhouse gas emissions from rice cultivation on alluvial, alkali, acid-sulfate, and bleached soils.