Nitrate Removal Efficiency Research Articles

Construction of electron-deficient Co on the nanoarrays enhances absorption and direct electron transfer to accelerate electrochemical nitrate reduction

Electrochemical nitrate reduction is a promising remediation strategy for nitrate-contaminated wastewater treatment, in which nitrate adsorption is a prerequisite step in the overall process. Herein, the iron-induced cobalt phosphide was grown in situ on porous nickel foam (Fe-CoP/NF) for the electrochemical nitrate reduction. Structural characterization verified doping of Fe and the uniform nanotube arrays of Fe0.03CoP/NF. Remarkably, the Fe0.03CoP/NF exhibited a high efficiency nitrate removal efficiency (99.3 %) and excellent ammonia selectivity (100 % selectivity and 0.485 mg·h−1cm−2 NH3 yield rate). Both experimental and theoretical results reveal that Fe doping alters the local charge distribution of the Co active centers to form electron-deficient Co. The Co electron-deficient regions were constructed due to the difference in electronegativity between Co and Fe. Furthermore, the formation of electron-deficient centers facilitates the reduction of charge transfer resistance. In particular, Fe0.03CoP/NF maintained an excellent conversion efficiency of nitrate to N2 (99.8 %) with 60 mM Cl−, and the selectivity of N2 is maintained above 99.1 % during long-term operation. This system possesses a low electrical consumption of 1.79 kWh·molN−1. This study designed an enhanced electrocatalyst through enhanced nitrate absorption and direct electron transfer strategies, thus providing a promising and low-power consumption approach for addressing nitrate pollution.

Dual application of lotus stem in fabricating a bioreactor for the remediation of nitrate, plumbum, and carbamazepine in industrial wastewater

Addressing the treatment of complex industrial wastewater continues to be a substantial challenge within the realm of water treatment. A biofilm reactor was engineered, integrating lotus stem biochar, lotus stem biological filler, and the ZW5 strain for enhanced wastewater treatment. The removal efficiencies of nitrate (NO3−-N), calcium (Ca2+), chemical oxygen demand (COD), phosphorus (PO43--P), plumbum (Pb2+), and carbamazepine (CBZ) were 98.46, 81.08, 91.79, 96.63, 97.75, and 99.26% when HRT was 3 h, pH was 7.0, Pb2+, CBZ, and influent salinity contents were 15.0, 3.0, and 1500 mg L−1. The reactor effectively fixed the microorganisms and provided a good microenvironment and niche, which was conducive to the adsorption of Pb2+ and CBZ by microorganisms and biominerals. Furthermore, the modified biochar in the reactor filler facilitated microbial metabolism and enabled indirect electron transfer. Microorganisms altered their dominant community structure in response to Pb2+ and CBZ exposure. Microorganisms maintained efficient activity through synergy and direct electron transfer through nanowires. The reactor serves as a model for pollutant removal from complex industrial wastewater, leveraging the multifaceted use of lotus stems and microbial-induced calcium precipitation.

Structure-activity relationship between crystal plane and pyrite-driven autotrophic denitrification efficacy: Electron transfer and metagenome-based microbial mechanism

Pyrite-driven autotrophic denitrification (PAD) has been recognized as a promising treatment technology for nitrate removal. Although the occurrence of PAD has been found in recent years, there is a knowledge gap about effects of crystal plane of pyrite on the performance and mechanism of PAD system. Here, this study investigated the effects of crystal planes ({100}, {111} and {210}) of single-crystal pyrite on denitrification performance, electron transfer, and microbial mechanism in PAD system. The removal efficiency of nitrate in B-{210} reached 100%, which was 1.67-fold and 2.86-fold higher than that of B-{100} and B-{111}, respectively. X-ray photoelectron spectroscopy and electrochemical results indicated that Fe-S bonds of pyrite with {210} crystal plane were more susceptible to breakage by Fe3+ oxidation assault, and leaching microbially available Fe2+ and sulfur intermediates to drive autotrophic denitrification. Metagenomic results suggested that community of functional pyrite-driven denitrifiers varied in response to crystal plane, and abundances of N-S transformation and EET-related microbes and genes in B-{210} notably up-regulated compared to B-{100} and B-{111}. In addition, this work proposed a dual-mode for electron transfer pathway during pyrite oxidation and nitrogen transformation in PAD system. In B-{210}, Fe(II)- and sulfur-driven denitrifiers obtained electron after pyrite oxidation-dissolution, and the enrichment of pyrite-oxidizing bacteria in B-{210} could enhance the electron transfer from pyrite through electron shuttles. This work highlighted that stronger surface reactivity and electron shuttle effect in B-{210} enhanced electron transfer, leading to favorable PAD performance in B-{210}. Overall, this study provides novel insights into the structure-activity relationship between the crystal plane structure of pyrite and denitrification activity in PAD system.

Novel sulfide-driven denitrification methane oxidation (SDMO) system based on SBR-MBfR and EGSB-MBfR

Traditional denitrification process consumes external organic carbon leading to an increase in treatment costs and carbon emission. A novel sulfide-driven denitrification methane oxidation (SDMO) system, which could simultaneously utilize CH4 and H2S in biogas as electron donors for denitrification to reduce the cost and carbon emission, has been successfully operated in two kinds of reactors SBR-MBfR and EGSB-MBfR for ∼ 200 days in this study. The performance and microbial community were explored meanwhile machine learning approach have been used for predicting the complex effects on SDMO’s denitrification efficiency. Result shows SBR-MBfR’s superior performance with the maximum nitrate removal efficiency > 90 % while the denitrification rates and biological activity were also better than EGSB-MBfR. The autotrophic denitrification (Auto-D) contributed 5 % much more for nitrate removal in SBR-MBfR than EGSB-MBfR while denitrification anaerobic methane oxidation (DAMO) was more prominent in EGSB-MBfR. Nineteen machine learning models were used for operation data training and random forest regressor was demonstrated to be the optimal model for predicting nitrate removal efficiency in SDMO. According to random forest regressor’s analysis, HRT presented the highest importance in affecting SDMO nitrate removal efficiency. After the introduction of biogas for domestication, Auto-D bacteria Thiobacillus was the dominant bacteria in both systems occupying > 20 % and the abundance would increase gradually from ∼ 20 % to ∼ 60 % as H2S content rising. A hidden positive correlation between DAMO bacteria Candidatus Methylomirabilis and Thiobacillus was uncovered by mantel test analysis here, implying the integrating of Auto-D and DAMO process for nitrate removal. This study not only provides insights into the novel SDMO technology but also offers a potential advancement in simultaneously low-carbon wastewater treatment and biogas utilization.

Calcium regulates nitrogen removal in constructed wetlands: The interactions between dissolved organic matter and microbial response

Limited carbon bioavailability in low C/N tailwater aggravated the difficulty of nitrogen (N) removal. Calcium (Ca) can regulate carbon transformation, but the effects of Ca on N removal in constructed wetlands (CWs) remain elusive. To fill the gap, this study constructed two types of Ca-based CWs, and the results revealed that the removal efficiencies of nitrate (NO3−-N) and total nitrogen (TN) in Ca-based CWs increased by 9.83–22.25 % and 8.59–18.72 %, respectively, compared to control group. The proportion of protein-like DOM and P/H index (represent bioavailability of DOC) increased in Ca-based CWs, supporting the growth of heterotrophic denitrifiers and facilitated NO3−-N and TN removal. In addition to directly enhancing complete denitrification, Ca-based CWs also optimize the N removal pathway. Moreover, Ca enhanced complexity and robust of the microbial network, increased microbial communication, thereby jointly facilitating N removal. This study highlighted Ca improved N removal by collaborating the interactions of DOM transformation and microbial activity, which provided an abundant and cheap substrate for the treatment of low C/N wastewater by CWs

Efficient removal of nitrate and ammonium by strain Pseudomonas poae EH-E3 under acidic pH and low-temperature conditions

The combined effect of low temperature and an acidic environment may restrict the growth and metabolic activity of microorganisms involved in nitrogen removal. To overcome poor biological nitrogen removal efficiency during low-temperature treatment of acidic wastewater, a novel pure strain of Pseudomonas poae EH-E3 was successfully isolated and screened based on its strong heterotrophic nitrification and aerobic denitrification capacity. Strain EH-E3 could completely eliminate inorganic nitrogen sources of nitrate within 30h and ammonium within 20h at 15 °C and pH 5.0, with corresponding removal efficiencies for total nitrogen of 95.13% and 91.35%, respectively. Nitrogen balance testing indicated that strain EH-E3 removed 43.25% of ammonium, 45.72% of nitrate, and 51.20% of nitrite in the form of gaseous products. Enzyme activity assays revealed that the specific activities of ammonia monooxygenase, nitrate reductase, and nitrite reductase were 0.533, 0.956, and 0.011 U/mg proteins, respectively. These findings suggest that strain EH-E3 has strong potential for use in low-temperature treatment of acidic wastewater.

Novel mixotrophic denitrification biofilter for efficient nitrate removal using dual electron donors of polycaprolactone and thiosulfate

Novel mixotrophic denitrification biofilter for efficient nitrate removal using dual electron donors of polycaprolactone and thiosulfate

Assessment of Nitrate Reduction by Microbes in Artificial Groundwater Medium

There are significant reasons for nitrate contamination in groundwater (Delhi, India): sewage, runoff from landfill sites, nitrogenous chemical fertilisers, and pesticides from agricultural lands. The highest recorded concentration of nitrate in Delhi’s groundwater is reported to be 1500 mg/l. Consumption of high nitrate through water may pose serious health problems in humans, especially children (below five years). The study’s primary objective was to isolate and identify nitrate-remediating microbes from the nitrate-contaminated site Okhla Barrage, located on the Yamuna River in Delhi, India. A total of 11 different strains were isolated from this site. Among these four strains exhibited 40%–50% remediation efficiency at a nitrate concentration of 1000 mg/l. Molecular characterisation revealed that these four strains, Enterobacter aerogenes, E. coli K12, <i>Klebsiella oxytoca</i> and <i>Lelliottia amnigena</i>, belong to the Enterobacteriaceae family. This study assessed the nitrate remediation potential of isolated microbes in groundwater with 1000 and 1500 mg/l nitrate concentrations. By using a 2% inoculum, the microbes were incubated anaerobically at room temperature for ten days. Nitrate concentrations were monitored every 48 hours. <i>Lelliottia</i>, <i>E. coli</i>, and <i>Enterobacter</i> reduced nitrate (1500 mg/l) by approximately 42%, 24%, and 29%, respectively, while <i>K. oxytoca</i> showed minimal reduction. <i>L. amnigena</i> exhibited superior nitrate removal efficiency compared to other strains. According to the reported data, these strains are known to reduce nitrate concentrations of 620 mg/l. However, our findings demonstrate a remarkable nitrate remediation capacity of 1500 mg/l, showcasing a novel contribution to this study. Further detailed analysis for condition optimisation and association of microbe-microbe could be more helpful.

Simultaneous removal of nitrate, copper, carbamazepine, and calcium from micropolluted water by fulvic acid through promotion of denitrification and microbial-induced calcium precipitation: Performance and mechanism

In this paper, a study of the denitrification strain Cupriavidus sp. W12 was conducted to remove copper (Cu2+), carbamazepine (CBZ), and calcium (Ca2+) by microbial-induced calcium precipitation (MICP) after adding fulvic acid (FA). After the addition of 20 mg/L FA, the removal efficiencies of nitrate (NO3–-N), Cu2+, CBZ, and Ca2+ reached 100.0 %, 98.7 %, 96.6 %, and 73.6 %, correspondingly and there was no accumulation of nitrite (NO2–-N). FA stimulated the growth of strain W12, improved electron transfer activity, and facilitated the conversion of gaseous nitrogen. The research revealed that FA might enhance microbial activity and result in a more dense and porous structure of the biological precipitate. Cu2+ and CBZ were removed by co-precipitation and adsorption. As the initial report of FA promoting MICP to remove complex pollutants, this paper offers a theoretical foundation for NO3–-N, Cu2+, CBZ, and Ca2+ remediation in micropolluted water.

Fabrication of FeCu/CC Composite for Efficient Removal of Nitrate for Aqueous Solution Via Electrochemical Reduction Process

AbstractThe electrochemical nitrate reduction (ENR) for the conversion of nitrate to nitrogen gas is a significant strategy to remediate the eutrophication in surface water. However, the exploitations and practical applications of suitable electrocatalysts still face great challenges. In this study, FeCu bimetal microparticles were deposited onto surface of carbon cloth (CC) to fabricate the FeCu/CC electrocatalyst for the removal of nitrate. Among obtained catalysts, the sample with Fe/Cu molar ratio of 1 : 15 (Fe1Cu15/CC) exhibits the excellent electroreduction efficiency for nitrate removal (R(NO3−), >90 %) but an ineffective selectivity of N2 formation (S(N2), ~23 %). After the anchor of reduced graphite oxide (rGO) to surface of Fe1Cu15/CC, at the optimized condition without the assistance of chlorine oxidation, R(NO3−) and S(N2) of rGO/Fe1Cu15/CC catalyst reach 94.2 % and 72.8 %. After 5 day endurance test, the values of R(NO3−) and S(N2) are still 82.9 % and 62.8 %, demonstrating the promising durability of this catalyst electrode. Meanwhile, the potential application of rGO/Fe1Cu15/CC was explored for the treatment of simulated wastewater discharged by urban wastewater treatment plant, the concentration of total nitrogen (TN) decreases from 15–1.17 mg/L, and this value is lower than TN threshold limit in Class IV of Chinese surface water quality standard.

Exploring the effects of polyethylene and polyester microplastics on biofilm formation, membrane Fouling, and microbial communities in Modified Ludzack-Ettinger-Reciprocation membrane bioreactors

Microplastics (MPs) inevitably enter wastewater treatment plants (WWTPs), yet their impacts remain poorly understood. This study investigates the effects of MPs on system performance and membrane fouling in a Modified Ludzack-Ettinger (MLE)-Reciprocation Membrane Bioreactor (rMBR), an energy-efficient alternative to conventional membrane bioreactors. Additionally, the study examines changes in microbial community induced by different types and shapes of MPs—polyethylene (PE) pellets and polyester (PES) fibers— as well as biofilm formation on MPs, using next-generation sequencing. Results revealed that transmembrane pressure (TMP) increased 2–3 times faster in the presence of PE pellets, while TMP remained stable during the PES stage, implying that MP type and shape could influence biofouling behaviors. Furthermore, enhanced nitrate removal was observed in the aerobic tank due to denitrifying biofilm formation on MPs. However, PES MPs reduced nitrate removal efficiency from 99.6 ± 0.3 % to 90.9 ± 7.9 % and decreased the relative abundance of denitrifying bacteria. Numerous taxa showed affinity to PE pellets, including some pathogens, e.g., Norcadia and Mycobacterium. Notably, an uncultured phylum Candidatus Saccharibacteria dominated in membrane biofilm and MPs, reaching up to 37 % relative abundance. This study is the first to explore how different types and shapes of MPs affect membrane bioreactor systems, particularly with respect to microbial community structure and biofilm formation. The findings offer new insights into the influence of MPs on wastewater treatment processes and highlight the significance of the uncultured phylumCandidatus Saccharibacteriain membrane fouling.

Efficient and sustainable removal of linear alkylbenzene sulfonate in a membrane biofilm: Oxygen supply dosage impacts mineralization pathway

Linear alkylbenzene sulfonate (LAS) can be thoroughly mineralized within sufficient oxygen (O2), but which is energy intensive and may causes serious foaming problem. Although cometabolism can achieve efficient LAS removal within a wide range of O2 dosages, how O2 dosage systematically affects LAS metabolic pathway is still unclear. Here, membrane aerated biofilm reactor (MABR) enabled accurate O2 delivery and bulk dissolved oxygen (DO) control. MABR achieved efficient removal of LAS (>96.4 %), nitrate (>97.8 %) and total nitrogen (>96.2 %) at the three target DO conditions. At high DO condition (0.6 mg/L), LAS was efficiently removed by aerobic mineralization (predominant) coupled with aerobic denitrification biodegradation with the related functional enzymes. Pseudomonas, Flavobacterium, Hydrogenophaga, and Pseudoxanthomonas were dominant genus contributing to four possible LAS aerobic metabolic pathways. As O2 dosage reduced to only 29.7 % of the demand for LAS mineralization, O2 facilitated LAS activation, benzene-ring cleavage and a portion of respiration. NO3--N respiration-induced anaerobic denitrification also contributed to ring-opening and organics mineralization. Desulfomicrobium and Desulfonema related two possible anaerobic metabolic pathways also contributed to LAS removal. The findings provide a promising strategy for achieving low-cost high LAS-containing greywater treatment.

Synthesis of gamma aluminum oxide nano-powder for adsorptive removal of nitrate from aqueous solutions

Nitrate pollution of surface and groundwater could be a worldwide issue resulting in the threat to the environment and public health. The removal of nitrate from water was carried out using gamma aluminum oxide (γ-Al2O3) nano powder. The nano powder was synthesized from aluminum oxide powder using the aqueous-based reflux method and applied for nitrate removal. The surface properties of the activated γ-Al2O3 were characterized using FTIR, XRD, and BET surface area analysis. Analysis of variance (ANOVA) was used to analyze nitrate removal to determine the appropriate regression model, which was found to be the quadratic model with a coefficient of determination (R2 = 0.99). The adsorption process was observed to be dependent on initial nitrate concentration, adsorbent dose, and time. The optimized nitrate removal efficiencies were 97.01% using a dose of 2.26 gL−1, an initial nitrate concentration of 10 mgL−1, and a contact time of 45 min. Additionally, the kinetics of nitrate ion adsorption was discussed based on pseudo-first order and pseudo-second order kinetic models. The experimental data fitted well with the pseudo-second order kinetic model, indicating that the adsorption mechanism is chemisorption. The equilibrium adsorption of nitrate experimental data followed the both Freundlich and Langmuir isotherm (R2 = 0.95), implying a homogeneous nature of the γ-Al2O3 surface sites. These results suggest further consideration of the practical application of γ-Al2O3 for nitrate treatment for domestic purposes.

Ferrihydrite regulated tire-wear microplastics biofilm for enhanced nitrogen transformation in surface water

Tire-wear microplastics (T-MPs) are increasingly present in surface water, which can intact with ferrihydrite and influence biofilm formation on T-MPs. However, the interaction of T-MPs and ferrihydrite on biofilm formation and its effect on nitrogen transformation in surface water is still unknown. In this paper, four sets of experiments were set up in simulated surface water inoculated with precultured bacteria, respectively, A: simulated surface water, B: addition of ferrihydrite, C: addition of T-MPs, and D: addition of both T-MPs and ferrihydrite. Characterization results showed that co-exposure of T-MPs and ferrihydrite increased roughness and decreased hydrophobicity in the surface of T-MPs, and resulted in higher biomass and EPS values in biofilm of T-MPs with ferrihydrite. Under co-exposure, the nitrate removal efficiency for 12 h was increased by 32.37 % and 25.74 % comparing with single ferrihydrite and T-MPs, and the accumulation and removal of nitrite as well as the production of higher concentrations of nitrous oxide indicated that the co-exposure contributed to the denitrification and stimulated nitrogen transformation. Furthermore, α-diversity index characterized a rich microbial community, especially the enrichment of functional microorganisms Firmicutes and Thauera, which were altered to enhance nitrogen transformation function (glutamine synthetase, nitronate monooxygenase). Ferrihydrite promoted complete denitrification process in the system and strengthened the role of nitrogen transformation microorganisms in the T-MPs biofilm. The abundance of key enzymes and functional genes (napA, nrtC, nirB, nosZ) increased with co-exposure T-MPs and ferrihydrite, and genes associated with nitrate and nitrite reduction (narG, narI) exhibited higher abundance in biofilm of T-MPs compared to the surrounding water. This research provided important insights into the effects of T-MPs co-exposure interactions with ferrihydrite on nitrogen transformation in surface water.

Nutrient removal performance and the nitrogen-sulfur conversion pathways in sulfur-iron based biofilter under acidic/alkaline conditions

The nitrate and phosphate removal in sulfur based (Vp) and sulfur-iron based autotrophic denitrification systems (Sp) with sponge iron as the iron source was investigated under different pH (6–9). In this study, Sp (>98 %) and Vp (>97 %) had higher nitrate removal efficiency at pH 7–8 and the highest phosphate removal efficiency (94.9 % ± 3.57 %) at pH 6. The flow cytometry detection revealed that the cell integrity was damaged due to the formation of iron crusts at pH 6, which resulted in irreversible inhibition in Sp. Electron transfer and nitrogen transformation showed that compared with the sustained inhibition of denitrification at pH 6, the inhibition at pH 9 occurred mainly in the initial 0–2 h due to the enhanced flow of electrons toward the formation of biological element sulfur (Sbio0). Coupled sponge iron promoted the adsorption and reduction of NO2−/N2O and reduced N2O emissions. Metatranscriptomics analysis showed that significant upregulation of key gene expression for nitrogen and sulfur metabolism in Sp, with complex regulatory mechanisms enhancing system stability and performance. Acidic conditions reduced the abundance of denitrification and sulfur oxidation functional genes. However, alkaline conditions (pH of 9) mainly caused the decreased abundance of denitrification functional genes, which was favorable for accumulation of Sbio0. This study provides a theoretical basis for the stable operation and regulation of the sulfur-iron based autotrophic denitrification process.

A novel electrochemical membrane filtration system operated with periodical polarity reversal for efficient resource recovery from nickel nitrate laden industrial wastewater

The economical and efficient removal of nickel nitrate from industrial wastewater remains a challenge. Herein, we developed an innovative electrochemical membrane filtration system that used a periodic polarity reversal process to adjust the acid-base environment near membrane interface for the recovery of nickel (II) and ammonia. The Ru based electrocatalytic layer could boost the selective reduction of nitrate to ammonia by generating atomic hydrogen, resulting in the precipitation of Ni2+ by the increasing pH at the membrane interface. Then, the precipitation of Ni(OH)2 could be effectively stripped and collected under the periodic polarity reversal process. In-situ interfacial measurements demonstrated that the polarity reversal process enabled a reversible transformation between strongly acidic (pH < 2) and alkaline (pH > 13) environments within a 200 µm range at the membrane interface. In continuous flow operation treating real industrial wastewater containing 96.7 mg-N L−1 nitrate and 135.0 mg L−1 Ni2+, the system demonstrated the capability to achieve 92.5 ± 2.6 % nitrate removal (with a recovery efficiency of 15.1 ± 1.9 g-NH3 kWh−1) and 99.7 ± 0.1 % Ni²⁺ removal (with a recovery efficiency of 24.9 ± 2.4 g-Ni kWh−1). Additionally, the specific treatment cost was approximately $0.17 m−3, attributed to the recovery of Ni(OH)₂ and ammonia. Furthermore, this system could deliver a significant economic benefit ($1.64 per m3) for treating a high concentration real wastewater (331.5 mg-N L−1 nitrate and 1496.3 mg L−1 Ni2+), outperforming traditional alkali precipitation and biological nitrification/denitrification processes. Overall, our study presents an economical and sustainable method for recovering valuable chemicals from wastewater containing heavy metals and inorganic nitrogen, potentially advancing cost-effective water treatment technologies.

Ferrous-driven efficient removal of nitrate and various heavy metals by Zoogloea sp. ZP7 under oligotrophic conditions: Kinetics and heavy metal stress responses

Denitrification in surface water is affected by its oligotrophic characteristics and heavy metal pollution. In this study, a heterotrophic denitrifying bacterium Zoogloea resiniphila ZP7, which can utilize ferrous (Fe2+) as an additional electron donor under oligotrophic conditions, was isolated. Strain ZP7 achieved a 98.5 % nitrate (NO3−-N) removal efficiency (NRE) within 12 h using sodium acetate as carbon sources, with a Fe2+ concentration of 10.0 mg L−1, pH of 7.0, carbon to nitrogen ratio of 2.0, and NO3−-N concentration of 5.01 mg L−1, and no nitrite residue was observed. The denitrification performance and the removing ability of heavy metal by strain ZP7 were studied when exposed to copper (Cu2+), zinc (Zn2+), and cadmium (Cd2+) alone or in combination. When 1.0 mg L−1 Cu2+, Zn2+, and Cd2+ were added simultaneously, the strain ZP7 could achieve 91.3 % NRE in 14 h, and the removal efficiencies of Cu2+, Zn2+, and Cd2+ were 68.1, 89.2, and 83.5 %, respectively. The results of fluorescence region integration showed that the response of strain ZP7 to different heavy metal system stresses was different. Moreover, the changes in the content of extracellular polymeric substances (EPS) and humic acids indicated that they contributed to the enhancement of the resistance of strain ZP7 to heavy metals. Various characterization results showed that EPS and bio‑iron oxides were beneficial to remove heavy metals. In future studies, strain ZP7 can be considered to be immobilized in biomaterials to treat contaminated oligotrophic water bodies to explore its application prospects in actual contaminated water bodies.

Boosting seawater denitrification in an electrochemical flow cell

Nitrogen compounds in current seawater treatment processes typically are converted to nitrate, threatening seawater quality and marine ecology. Electrochemical denitrification is a promising technique, but its efficiency is severely limited by the presence of excess chloride ions. In this work, a flow-through cell went through an on-demand chlorine-mediated electrochemical-chemical tandem reaction process was designed for efficient seawater denitrification. Equipped with ultrathin cobalt-based nanosheets as the cathode catalyst and commercial IrO2-RuO2/Ti as the anode, the newly designed flow-through cell achieved nitrate removal efficiency that was about 50 times greater than the batch cell and nearly 100 % N2 selectivity. Moreover, nitrite and ammonia can also be removed with over 93 % efficiency in total nitrogen (TN) removal. Furthermore, the concentration of active chlorine in the effluent could be adjusted within two orders of magnitude, enabling on-demand release of active chlorine. Finally, this flow-through cell reduced the TN of actual mariculture tailwater (40.1 mg N L-1 nitrate) to only 5.7 mg N L-1, meeting the discharge standard for aquaculture tailwater of Fujian, China. This work demonstrates the paradigm of deep denitrification from ultra-concentrated chlorine ion wastewater using an on-demand active chlorine-mediated electrochemical-chemical tandem reaction process.

Enhanced electrochemical nitrate removal from groundwater by simply calcined Ti nanopores with modified surface characters

Simple and convenient preparation method with high catalytic reduction activity is crucial for the remediation of nitrate contamination. In this study, the innovation for fabricating a nanoelectrode was developed by calcinating the anodized plate to alter the surface crystalline phase of the material. The prepared calcined Ti nano-pores (TNPs) electrode could effectively remove up to 95.1% nitrate from simulated groundwater at 30 mA·cm–2 electrolysis for 90 min, while under the same conditions, the removal efficiency of nano electrode prepared by conventional methods was merely 52.5%. SEM images indicated that the calcined-TNPs nanoelectrode was porous with different pore sizes. The higher nitrate removal efficiency of TNPs-500 (95.1%) than TNPs-400 (77.5%) and TNPs-550 (93.4%) may resulted from the positive nonlinear response of the larger ECSA, the improved electron transfer and suitable surface structure, and not the “anatase-to-rutile” of surface TiO2 nanotubes. After 90 min of electrolysis, by using RuO2 as an anode and adding 0.3 g·L–1 NaCl solution, 87.5% nitrate was removed, and the by-products (ammonia and nitrite) were negligible. Increased temperature and alkaline conditions can enhance the nitrate removal, while higher initial nitrate concentration only improved the nitrate removal slightly. Moreover, The TNPs-500 electrode also exhibited excellent nitrate removal performance in real groundwater with the efficiency at 82.9% and 92.1% after 90 and 120 min, which were 0.87 (removal efficiency=95.1%), 0.92 (removal efficiency=100%) of the efficiency for simulated groundwater, indicating the widely applicable conditions of the TNPs-500 electrode. This approach of surface bonded elements and structure modification through calcination significantly improves catalytic activity, and will guide the simple designing of functional nanostructured electrodes with wide application conditions.

The stratified response of heterotrophic, autotrophic or mixotrophic denitrification to enrofloxacin stress along different filling heights in up-flow fixed-bed bioreactors

Antibiotics frequently exist in nitrate wastewater and seriously threaten biological denitrification. This study investigated the impact of enrofloxacin (ENR) on autotrophic, heterotrophic, and mixotrophic denitrification processes at various filling heights. The experiments were conducted across four consecutive phases with ENR concentrations of 0, 0.1, 1, and 10 mg/L. As the influent ENR concentration increased, the denitrification performance of the FeS2-based autotrophic denitrification (PAD) system was significantly inhibited at different filling heights, with nitrate removal efficiency (NrE) dropping to 30.42 %. In contrast, the polycaprolactone (PCL)-based heterotrophic denitrification (PHD) and mixotrophic denitrification (PAD+PHD) systems only affected NrE in the lower layer, whereas the middle and upper layers maintained high NrE levels, largely unaffected by ENR stress, reaching up to 98.28 % and 94.02 % respectively. Sulfate reduction was more prominent in the upper and middle layers of the mixotrophic denitrification bioreactor (PHD system). Bacterial diversity indices initially increased and then decreased as ENR concentration rose from 0 to 10 mg/L, with the PHD system showing lower diversity compared to the PAD system. Redundancy analysis (RDA) revealed that the relative abundances of Proteobacteria and Actinobacteriota in the PAD system, and Bacteroidota and Firmicutes in the PHD system, were negatively correlated with ENR concentration. Overall, the PAD system experienced significant stress from ENR at various filling heights, whereas the upper-middle layer of the PHD system demonstrated greater resistance. This highlighted the impact of antibiotic contamination along with the performance of different filling heights and emphasized the need for tailored strategies in wastewater treatment.