Nitrite Research Articles

Reduction with zinc — Impact on the determination of nitrite and nitrate ions using microfluidic paper-based analytical devices

We used a microfluidic paper-based analytical device (μPAD) to investigate the influence that zinc reduction exerts on the determination of nitrite and nitrate ions in natural water samples. The μPAD consists of layered channels for the reduction of nitrate to nitrite with zinc powder and the subsequent detection of nitrite with Griess reagent. The amount of zinc, number of layers, and reaction time for the reduction were optimized to obtain an intense signal for nitrate. Initially, the sensitivity to nitrate corresponded to 55% that of nitrite, which implied an incomplete reduction. We found, however, that zinc decreased the sensitivity to nitrite in both the μPAD and spectrophotometry. The sensitivity to nitrite was decreased by 48% in spectrophotometry and 68% in the μPAD following the reaction with zinc. One of the reasons for the decreased sensitivity is attributed to the production of ammonia, as we elucidated that both nitrite and nitrate produced ammonia via the reaction with zinc. The results suggest that the total concentration of nitrite and nitrate must be corrected by constructing a calibration curve for nitrite with zinc, in addition to developing curves for nitrate with zinc and for nitrite without zinc. Using these calibration curves, the absorbance at different concentration ratios of nitrite and nitrate ions could be reproduced via calculation using the calibration curves with zinc for nitrite and nitrate. Eventually, the developed μPAD was applied to the determination of nitrite and nitrate ions in natural water samples, and the results were compared with those using a conventional spectrophotometric method. The results of the μPAD are in good agreement with those of conventional spectrophotometry, which suggests that the μPAD is reliable for the measurement of nitrite and nitrate ions in natural water samples.

Efficient nitrite determination by electrochemical approach in liquid phase with ultrasonically prepared gold-nanoparticle-conjugated conducting polymer nanocomposites.

An electrochemical nitrite sensor probe is introduced herein using a modified flat glassy carbon electrode (GCE) and SrTiO3 material doped with spherical-shaped gold nanoparticles (Au-NPs) and polypyrrole carbon (PPyC) at a pH of 7.0 in a phosphate buffer solution. The nanocomposites (NCs) containing Au-NPs, PPyC, and SrTiO3 were synthesized by ultrasonication, and their properties were thoroughly characterized through structural, elemental, optical, and morphological analyses with various conventional spectroscopic methods, such as field-emission scanning electron microscopy, energy-dispersive X-ray spectroscopy, high-resolution transmission electron microscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, and Brunauer-Emmett-Teller method. The peak currents due to nitrite oxidation were characterized in detail and analyzed using conventional cyclic voltammetry (CV) as well as differential pulse voltammetry (DPV) under ambient conditions. The sensor response increased significantly from 0.15 to 1.5mM of nitrite ions, and the sensor was fabricated by coating a conducting agent (PEDOT:PSS) on the GCE to obtain the Au-NPs/PPyC/SrTiO3 NCs/PEDOT:PSS/GCE probe. The sensor's sensitivity was determined as 0.5μA/μM∙cm2 from the ratio of the slope of the linear detection range by considering the active surface area (0.0316cm2) of the flat GCE. In addition, the limit of detection was determined as 20.00 ± 1.00 µM, which was found to be satisfactory. The sensor's stability, pH optimization, and reliability were also evaluated in these analyses. Overall, the sensor results were found to be satisfactory. Real environmental samples were then analyzed to evaluate the sensor's reliability through DPV, and the results showed that the proposed novel electrochemical sensor holds great promise for mitigating water contamination in the real samples with the lab-made Au-NPs/PPyC/SrTiO3 NC. Thus, this study provides valuable insights for improving sensors for broad environmental monitoring applications using the electrochemical approach.

Multifunctional MOF: Cold/hot adapted sustainable oxidase-like MOF nanozyme with ratiometric and color tonality for nitrite ions detection

Integrating multiple functionalities into a single entity is highly important, especially when a broad spectrum of application is required. In the present work, we synthesized a novel manganese-based MOF (denoted as UoZ-6) that functions as a cold/hot-adapted and recyclable oxidase nanozyme (Km 0.085 mM) further developed for ratiometric-based colorimetric and color tonality visual-mode detection of nitrite in water and food. Nitrite ions promote the diazotization process of the oxTMB product, resulting in a decay in the absorbance signal at 652 nm and the emergence of a new signal at 461 nm. The dual-absorbance ratiometric platform for nitrite ion detection functions effectively across a wide temperature range (0 °C to 100 °C), offering a linear detection range of 5–45 μM with a detection limit of 0.15 μM using visual-mode. This approach is sensitive, reliable, and selective, making it effective for detecting nitrite ions in processed meat and water.

Mimicking lightning-induced electrochemistry on the early Earth

To test the hypothesis that an abiotic Earth and its inert atmosphere could form chemically reactive carbon- and nitrogen-containing compounds, we designed a plasma electrochemical setup to mimic lightning-induced electrochemistry under steady-state conditions of the early Earth. Air-gap electrochemical reactions at air-water-ground interfaces lead to remarkable yields, with up to 40 moles of carbon dioxide being reduced into carbon monoxide and formic acid, and 3 moles of gaseous nitrogen being fixed into nitrate, nitrite, and ammonium ions, per mole of transmitted electrons. Interfaces enable reactants (e.g., minerals) that may have been on land, in lakes, and in oceans to participate in radical and redox reactions, leading to higher yields compared to gas-phase-only reactions. Cloud-to-ground lightning strikes could have generated high concentrations of reactive molecules locally, establishing diverse feedstocks for early life to emerge and survive globally.

Effects of Different Extraction Methods on the Structural and Functional Properties of Soluble Dietary Fibre from Sweet Potatoes.

In this study, hot water treatment (WT), ultrasonic treatment (UT), ultrasonic-sodium hydroxide treatment (UST), ultrasonic-enzyme treatment (UET), and ultrasonic-microwave treatment (UMT) were used to treat sweet potatoes. The structural, physicochemical, and functional properties of the extracted soluble dietary fibres (SDFs) were named WT-SDF, UT-SDF, UST-SDF, UET-SDF, and UMT-SDF, respectively. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), thermal properties, and Brunauer-Emmett-Teller (BET) analysis were employed. The structural results indicated that the UST-SDF exhibited the best thermal stability, highest crystallinity, and maximum specific surface area. Moreover, compared to hot water extraction, ultrasonic extraction, or ultrasonic extraction in combination with other methods, enhanced the physicochemical and functional properties of the SDF, including extraction yield, water-holding capacity (WHC), oil-holding capacity (OHC), glucose adsorption capacity (GAC), glucose dialysis retardation index (GDRI), sodium cholate adsorption capacity (SCAC), cholesterol adsorption capacity (CAC), nitrite ion adsorption capacity (NIAC), and antioxidant properties. Specifically, the UST-SDF and UMT-SDF showed better extraction yield, WHC, OHC, GAC, CAC, SCAC, and NIAC values than the other samples. In summary, these results indicate that UST and UMT could be applied as ideal extraction methods for sweet potato SDF and that UST-SDF and UMT-SDF show enormous potential for use in the functional food industry.

Enhanced electrochemical detection of nitrite and nitrate in water using Chitosan-Copper phthalocyanine nanocomposite

In this investigation, we study the behavior of a hybrid nanomaterial system combining chitosan-based tetrasulfonated copper phthalocyanine catalyst (Chit/Ts-CuPc) modified carbon screen-printed electrodes (CSPE) for the determination of nitrite and nitrate. Under optimal experimental conditions defined by cyclic voltammetry (CV) and square wave voltammetry (SWV) surveys, the functionalized CSPE exhibited significant catalytic activity towards nitrite and nitrate with a considerable enhancement of the cathodic-anodic peak currents. The Chit/Ts-CuPc nanocomposite enabled the sensors to have a broad linear detection range, which allows quantifying analytes at various concentrations, from 40 to 100 µM for nitrite and 1 to 10 µM for nitrate. Low detection limits (3Sb/m) have been achieved, which are 0.06 µM and 0.05 µM for nitrite and nitrate ions, respectively. Moreover, the proposed modified electrode showed its applicability by measuring the concentration of nitrite and nitrate ions with interfering species and in real water samples.

Lateral flow assay with dual-emission nitrogen-doped carbon dots for ratiometric fluorescent determination of nitrite in food

Nitrite ion (NO2–), renowned for its potent antiseptic properties and color-preserving abilities, potentially leads to carcinogenesis and hypertension if consumed excessively. Herein, a ratiometric fluorescent probe exhibits specific response to NO2– using nitrogen-doped carbon dots (N-CDs) with a low detection limit (LOD) as low as 52 nM and a low quantification limit (LOQ) of 173 nM. Initially, the emission peaks at 495 nm and 415 nm decrease as result of the internal filtration effect (IFE). With further increasing NO2– concentration, the reaction between NO2– and N-CDs forms an azoic compound (N-CDs@NO2–), which promotes the instant quenching of the emission peak at 415 nm through fluorescence resonance energy transfer (FRET). Moreover, portable N-CDs/LFASs devices with smartphone are successfully applied to visually identify NO2– with the exceptional LOD, achieving excellent recoveries and high precision in real sample. Therefore, this newly N-CDs/LFASs offers a reliable means for the NO2– detection, showcasing potential applications.

Research on Extraction Process and Physicochemical Properties of Dietary Fiber

The attention of dietary fiber has been increasing in recent years. Dietary fiber can regulate blood lipid, blood glucose and intestinal flora, prevent colon cancer and diabetes and plays an active role in promoting human health. The advantages and disadvantages of extraction methods including chemical, fermentation, enzymatic method and multi methods assisted at home and abroad were analyzed. The physicochemical properties of dietary fiber, functional properties and antioxidant capacity were summarized. The physicochemical properties of dietary fiber such as water holding, expansibility, fat and oil holding capacity, the functional properties of dietary fiber including adsorption of glucose, cholesterol, nitrite ion, sodium cholate, cation exchange, adsorption of heavy metals and antioxidant capacity such as DPPH radical and hydroxyl radical scavenging ability were studied，which provided theoretical basis for the development and application in health products and food.

Electrochemical sensing of nitrate ions at Cu-immobilized gold electrode in neutral medium

The Cu-immobilized Au surface was electro-catalytically employed for nitrate ions reduction in a neutral solution. According to cyclic voltammetric studies, the Cu/Au electrode has an improved electro-catalytic impact on converting nitrate ions into nitrite ions, governed by a two-electron transfer system. This modified electrode demonstrated good performance in real sample analysis along with good sensitivity (3.2518 $ \mu A \mu M^{-1} cm^{-2}$), long linear detection range (1.2 to 111.4 $\mu M$), a very lower limit of detection (0.85 $\mu M$), an excellent reproducibility. Thus, to determine nitrate ions in an aqueous media, an electrochemical sensor based on a Cu/Au electrode is suggested. GUB JOURNAL OF SCIENCE AND ENGINEERING, Vol 10(1), 2023 P 75-85

Exploring a maize-derived dietary fiber-phenolic acid complex with prebiotic effects

The structural, functional, and prebiotic properties of three maize-derived cell wall dietary fiber-phenolic acid complexes (CWDFPC1, CWDFPC2, and CWDFPC3) were investigated. The results showed that all three CWDFPCs had similar proximate composition and XRD pattern (type I). However, there were significant differences in the phytochemical profiles of their phenolic compounds (PC). Although the testa was the primary source of bound PC (BPC) in all three CWDFPCs, CWDFPC2 had the highest BPC content (15.41 mg GAE/g) and exhibited the greatest antioxidant activity in vitro (DPPH and ABTS assays). The water holding capacity of CWDFPC1 (6.53 g/g) and CWDFPC3 (6.86 g/g) was higher than CWDFPC2 (4.84 g/g), and three CWDFPCs had similar nitrite ion adsorption capacity, bile adsorption capacity, and cation-exchange capacity. After 48 h of in vitro fecal fermentation, CWDFPC2 produced more short-chain fatty acids (46.33 mM) compared to CWDFPC1 and CWDFPC3 (40.26 mM and 44.20 mM, respectively).

Effects of nitrite exposure on the oxidative stress, immune response and intestine microbiota of Procambarus clarkii

Nitrite (NIT) is a toxic nitrogenous compound that adversely affects living organisms. This study utilized pathological, metagenomic, and transcriptomic analyses to examine the toxic effects and damage mechanisms of NIT exposure on the intestine of Procambarus clarkii. Findings revealed that excessive NIT led to pathological changes in the crayfish intestine, an imbalance in intestinal flora, and an increase in harmful bacteria such as Enterobacteriaceae, Shewanella, and Vibrionaceae. Additionally, NIT exposure caused various physiological changes, including oxidative stress, immune imbalance, metabolic disorders, excessive autophagy, and apoptosis, leading to inflammatory damage in the intestine. Moreover, NIT exposure disrupted crayfish energy metabolism by reducing ATP content and disturbing energy homeostasis. The inhibition of the intestinal immune system is speculated to disrupt intestinal flora homeostasis and promote the overgrowth of opportunistic pathogenic bacteria, increasing the risk of crayfish infections. This study provides new insights into the antioxidant and immune defense mechanisms against NIT in crayfish and may aid in developing strategies to enhance disease and stress resistance in crustaceans.

Fabrication of Highly Sensitive and Selective Nitrite Colorimetric Sensor Based on the Enhanced Peroxidase Mimetic Activity of Using Acetic Acid Capped Zinc Oxide Nanosheets.

Nitrite ions (NO2-), as one of the leading type-A inorganic-anion, showing significant-effects in the aquatic environment and also to humans health. Whereas, the higher uptake causes detrimental threat to human health leading to various chronic diseases, thus demanding efficient, reliable and convenient method for its monitoring. For this purpose, in the present research study we have fabricated the mimetic nonozyme like catalyst based colorimetric nitrite sensor. The acetic acid capped Zinc Oxide (ZnO) nanosheets (NSs) were introduce as per-oxidase mimetic like catalyst which shows high efficiency towards the oxidative catalysis of colorless tetramethylbenzidine (TMB) to oxidized-TMB (blue color) in the presence of Hydrogen-peroxide (H2O2). The present nitrite ions will stimulate the as formed oxidized-TMB (TMBox), and will caused diazotization reaction (diazotized-TMBox), which will not only decreases the peak intensity of UV-visible peak of TMBox at 652nm but will also produces another peak at 446nm called as diazotized-TMBox peak, proving the catalytic reaction between the nitrite ions and TMBox. Further, the prepared colorimetric sensor exhibits better sensitivity with a wider range of concentration (1 × 10-3-4.50 × 10-1 µM), lowest limit of detection (LOD) of 0.22 ± 0.05 nM and small limit of quantification (LOQ) 0.78 ± 0.05 nM having R2 value of 0.998. Further, the colorimetric sensor also manifest strong selectivity towards NO2- as compared to other interference in drinking water system. Resultantly, the prepared sensor with outstanding repeatability, stability, reproducibility, re-usability and its practicability in real water samples also exploit its diverse applications in food safety supervision and environmental monitoring.

Structure, physicochemical and adsorption properties of insoluble dietary fiber from Lycium barbarum residue: A potential functional factor

This study was designed to explore the dietary fiber potential of Lycium Barbarum residue, aiming to ascertain its viability as a sustainable dietary fiber source. The study examined the extraction of water-insoluble dietary fiber (LBIDF) from Lycium barbarum residue (LBR) using the enzymatic method. The comparison involved analyzing the monosaccharide composition, microstructure, physicochemical properties, and adsorption properties of LBR and LBIDF. The results indicated that LBIDF possesses the ideal physicochemical properties compared to LBR. Through instrumental analysis, it has been demonstrated that LBIDF possesses a specific structure and functional group of fibers, making it suitable for processing below 300 °C. Furthermore, LBIDF exhibited an optimal functional capacity for cholesterol (41.749 mg/g vs. 31.464 mg/g in LBR), bile acid (30.623 mg/g vs. 19.875 mg/g in LBR), nitrite ion (1159.394 μmol/g vs. 783.261 μmol/g in LBR), glucose (3.552 mmol/g vs. 2.664 mmol/g in LBR), and glucose dialysis retardation index (6.43% vs. 13.52% in LBR). LBIDF can inhibit the digestion of fat and starch in vitro. These findings implied that dietary fiber samples from LBR may offer potential as functional food components.

Electrochemical degradation of acetaminophen in urine matrices: Unraveling complexity and implications for realistic treatment strategies

Urine has an intricate composition with high concentrations of organic compounds like urea, creatinine, and uric acid. Urine poses a formidable challenge for advanced effluent treatment processes following urine diversion strategies. Urine matrix complexity is heightened when dealing with pharmaceutical residues like acetaminophen (ACT) and metabolized pharmaceuticals. This work explores ACT degradation in synthetic, fresh real, and hydrolyzed real urines using electrochemical oxidation with a dimensional stable anode (DSA). Analyzing drug concentration (2.5 - 40 mg L−1) over 180 min at various current densities in fresh synthetic effluent revealed a noteworthy 75% removal at 48 mA cm−2. ACT degradation kinetics and that of the other organic components followed a pseudo-first-order reaction. Uric acid degradation competed with ACT degradation, whereas urea and creatinine possessed higher oxidation resistance. Fresh real urine presented the most challenging scenario for the electrochemical process. Whereas, hydrolyzed real urine achieved higher ACT removal than fresh synthetic urine. Carboxylic acids like acetic, tartaric, maleic, and oxalic were detected as main by-products. Inorganic ionic species nitrate, nitrite, and ammonium ions were released to the medium from N-containing organic compounds. These findings underscore the importance of considering urine composition complexities and provide significant advancements in strategies for efficiently addressing trace pharmaceutical contamination.

Atomistic understanding of Ti3C2 MXene membrane performance for separation of nitrate ions from aqueous solutions

Water desalination, which is a reliable method for providing drinking water and a suitable solution, as well as the membrane filtration method in wastewater treatment, has increased significantly in recent years. In this research, the separation of nitrite and nitrate ions from aqueous solutions was done using the MXene membrane of the Ti3C2 type using molecular dynamics simulation. In this study, various parameters, such as pore size MXene structure, characteristics of cavities, applied pressure, and flux were investigated. To investigate the removal of toxic pollutants from water, water flux, potential mean force, distribution of water molecules, and density were investigated. The results showed that the amount of penetration through the membrane increased with the increase in pressure. It was observed that by applying pressure to the system, the number of water molecules accumulated in front of the membrane decreases because they quickly pass through the membrane, which indicates the positive effect of increasing pressure on the separation rate of molecules. The permeability of this membrane was several times higher than the existing membranes in the industry. So that Mexene membranes, which consist of at least two layers, can repel ions with 100% success.

Controllable synthesis of cross-linked 3D ZnO nanosheet toward a nitrite electrochemical sensor

A 3D cross-linked ZnO (cl-ZnO) nanosheets were prepared via a simple one-pot hydrothermal approach, and then characterized by a series of modern advanced technique including XRD, SEM, TEM and XPS. This nanomaterial was used to modify the glass carbon electrode (GCE), obtaining an electrochemical sensor (cl-ZnO/GCE) of which the electrochemical properties were studied using various electrochemical techniques such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The sensor exhibited remarkably enhanced electrocatalytic activity for nitrite ions (NO2-) in comparison to that of the commercially available ZnO-modified GCE. Two electrochemical techniques, including chronoamperometry (CA) and linear sweep voltammetry (LSV) were adopted to test the determination performance of the sensor for nitrite, it exhibited a good linear relationship within the range of 0.8 μmol·L-1 - 0.462 mmol·L-1 and 0.608 - 7.84 mmol·L-1 evaluated by CA, the limit of detection (LOD) and sensitivity for NO2- were 0.26 μmol·L-1 (S/N = 3) and 824.6 µA (mmol·L-1)-1cm-2, respectively. For LSV, the two linearity responses of 0.95 μmol·L-1- 0.515 mmol·L-1 and 0.667 mmol·L-1- 5.41 mmol·L-1, high sensitivity of 1336.1 µA (mmol·L-1)-1cm-2 with low LOD of 0.32 μmol·L-1 were gained. This sensor can be employed to detect trace amount of NO2- in ham sausage and pickled vegetables successfully, supporting an insight and strategy for more sensitive nitrite electrochemical sensor.

Environmental analysis of nitrobenzene using newly synthesized anisotropic gold nanostructures: Reaction kinetics and electrocatalytic activity

In this article, we established to produce anisotropic, colloidally stable gold nanostructures (AuNS) by rapidly mixing aqueous solutions of gold(III) chloride trihydrate (HAuCl4) and citric acid (CA) as dual reducing and capping agents at room temperature without the aid of a templating agent. Furthermore, the impact of various anions and shape-directing agents on the morphology and reaction kinetics of citric acid-capped gold nanostructures (CA-AuNS) was investigated. Additionally, the electrocatalytic activity of AuNS towards nitrobenzene (NB) was examined using a CA-AuNS modified glassy carbon (GC) electrode. The rate of reaction for the formation of CA-AuNS increased rapidly with higher concentrations of CA, indicating a strong dependence on the reductant’s concentration. This process followed first-order kinetics, suggesting that the reaction rate is directly proportional to the citric acid concentration. Consequently, optimizing CA levels can effectively control the synthesis rate and yield of CA-AuNS. The presence of nitrite ions led to rapid aggregation of gold nanostructures (AuNS) and a quick reaction time. Chloride ions increased the size of the AuNS. As bromide ion concentration increased, the reaction time slowed significantly, resulting in spherical gold nanoparticles. Both acetate ions and polyvinylpyrrolidone (PVP) inhibited the growth of AuNS. High-resolution transmission electron microscopy (HR-TEM) images showed the formation of various shapes, including spherical, hexagonal, pentagonal, and boat-like structures. In the presence of cetyltrimethylammonium bromide (CTAB), both complex formation between CTAB and HAuCl4 and the formation of spherical gold nanoparticles occurred. Upon addition of silver nitrate (AgNO3) to the colloidal solution, the color shifted to pale violet, and the morphology transitioned from anisotropic to irregular. However, when ascorbic acid (AA) was utilized, it proved ideal for maintaining the morphology of gold nanostructures (AuNS). The analysis of X-ray diffraction spectroscopy (XRD), cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and Fourier transform infrared spectroscopy (FT-IR) further confirmed the synthesized CA-AuNS. The FT-IR spectra of CA before and after the formation of AuNS indicated the participation of carboxylic acid (–COOH) and hydroxyl (–OH) groups in the reduction process of Au3+ metal ions. The GC/CA-AuNS electrode exhibits a larger electroactive surface area due to the coating of AuNS onto the electrode surface, enhancing the electrode’s electrical conductivity by providing additional active sites for electron transfer. In addition, the electrocatalytic activity of NB was investigated in this study. The GC/CA-AuNS electrode had better electrocatalytic activity than the unmodified GC electrode. As a result, with a LOD of 28.2 nM (S/N = 3), this electrode was chosen for sensitive NB determination.

Construction of Cyan Blue Fluorescence Probe based on Nitrogen-Doped Carbon Dots for Detecting Nitrite Ion in Ham Sausage.

Nitrite ion is one of the materials widely used in human life, and the accurate, sensitive and stable detection of nitrite ions is of great significance to people's healthy life. In this study, nitrogen-doped fluorescent carbon dots (N-CDs) for detecting nitrite salt solutions were prepared using citric acid monohydrate and Chrysoidin as precursors through a one-pot hydrothermal method. Under the condition of pH = 3, a noticeable quenching phenomenon occurred in the carbon dot solution with the increase in nitrite ion concentration. This quenching effect might be attributed to the diazonium effect. N-CDs have been successfully used as fluorescence probes for NO2- detection. NO2- can effectively quench the fluorescence intensity of N-CDs, providing a linear response to fluorescence quenching efficiency with respect to NO2- concentration within the range of 0-10µM and 10-30µM, and a detection limit of 52nM, showing high sensitivity. In addition, the probe was applied to the determination of NO2- in ham sausage samples with a detection limit of 0.67µM and recoveries in the range of 99.5-102.3%, the fluorescent probe showed satisfactory reliability.

A Novel ultrasensitive nitrite ion detection using tungsten trioxide-modified gold electrode

A novel ultrasensitive nitrite ion detection using tungsten trioxide/carbon (WO3/C) modified gold electrode has been studied. The WO3 was synthesized from tungsten metal through chemical methods with selective precipitation techniques. The visual color of the material obtained is bright yellow, indicating WO3 compound. The X-ray diffraction (XRD) diffractogram confirms WO3 compound in monoclinic shaped by showing of the 2θ at 23.205, 23.723, 24.384, 33.426, and 33.911. The characterization using Field Emission Scanning Electron Microscopy (FESEM) showed that the material obtained was a disk-shaped with diameter 150–250 nm and a thickness of 50 nm. The Energy Dispersive X-ray spectroscopy (EDX) also confirms WO3 compound with no impurities. The gold electrode was modified by dip immersion technique where the gold electrode was dipped in a mixture of WO3-carbon paste. The measurement of surface area by cyclic voltammetry shows that the WO3/C modified gold electrode has the highest surface area compared to unmodified gold and C-modified gold electrode, indicating it has more active electron transfer sites. The WO3/C modified gold electrode worked well at pH 7 for the detection of nitrite ion and gave the highest peak current (0.875 V) than the others. The measurement ranges from 1–200 mg/L with no interference from NO3−, SO42−, Zn2+, Fe2+, Pb2+, Cl−, Ca2+, and NH4+ ions. The sensitivity and the LOD of the electrode are 8.86 μA.L.mg−1.cm−2 and 0.8 µg/L, respectively. The proposed method not only exhibits identical precision and accuracy to the standard method in detecting nitrite ions in real samples but also offers the advantages of rapid analysis, uncomplicated pretreatment procedures, environmental friendliness, and economical operation. This performance indicates that the WO3/C modified gold electrode is feasible as an alternative electrode for nitrite ion detection.

Removal of metals and emergent contaminants from liquid digestates in constructed wetlands for agricultural reuse.

Given the increasing pressure on water bodies, it is imperative to explore sustainable methodologies for wastewater treatment and reuse. The simultaneous presence of multiples contaminants in complex wastewater, such as the liquid effluents from biogas plants, can compromise biological treatment effectiveness for reclaiming water. Vertical subsurface flow constructed wetlands were established as low-cost decentralized wastewater treatment technologies to treat the liquid fraction of digestate from municipal organic waste with metals, antibiotics, and antibiotic resistance genes, to allow its reuse in irrigation. Twelve lab-scale planted constructed wetlands were assembled with gravel, light expanded clay aggregate and sand, testing four different treating conditions (liquid digestate spiked with oxytetracycline, sulfadiazine, or ofloxacin, at 100 μg/ L, or without dosing) during 3 months. Physicochemical parameters (pH, chemical oxygen demand (COD), nutrients, metals, and antibiotics), the microbial communities dynamics (through 16S high-throughput sequencing) and antibiotic resistance genes removal (qPCR) were monitored in influents and effluents. Systems removed 85.8%-96.9% of organic matter (as COD), over 98.1% of ammonium and phosphate ions, and 69.3%-99.4% of nitrate and nitrite ions, with no significant differences between the presence or absence of antibiotics. Removal of Fe, Mn, Zn, Cu, Pb and Cr exceeded 82% in all treatment cycles. The treatment also removed oxytetracycline, sulfadiazine and ofloxacin over 99%, and decreased intl1, tetA, tetW, sul1 and qnrS gene copies. Nonetheless, after 3 months of ofloxacin dosing, qnrS gene started being detected. Removal processes relied on high HRT (14 days) and various mechanisms including sorption, biodegradation, and precipitation. Microbial community diversity in liquid digestate changed significantly after treatment in constructed wetlands with a decrease in the initial Firmicutes dominance, but with no clear effect of antibiotics on the microbial community structure. Removals above 85% and 94% were observed for Streptococcus and Clostridium, respectively. Results suggest that vertical subsurface flow constructed wetlands were a suitable technology for treating the liquid digestate to reuse it in irrigation agricultural systems, contributing to the circular bioeconomy concept. However, a more profound understanding of effective wastewater treatment strategies is needed to avoid antibiotic resistance genes dissemination.