Nitrate Removal Rate Research Articles

The global significance of abiotic factors affecting nitrate removal in woodchip bioreactors

Woodchip bioreactor (WBR) is one of the green infrastructures in the agriculture system to reduce nitrate from agricultural drainage and stormwater. A lot of abiotic factors have been reported that affect nitrate removal lacking a comprehensive understanding. In this study, we studied eight important abiotic factors, including media age, hydraulic retention time (HRT), influent nitrate concentration (Cin), temperature, dissolved organic carbon (DOC), dissolved oxygen (DO), pH, and effective porosity (ρe) of WBR-filling materials. Based on a database that included 10,179 sets of data from 63 peer-reviewed articles, the nitrate removal rate (NRR) and nitrate removal efficiency (NRE) corresponding to the eight abiotic factors by different categories were comprehensively reported. According to this database, this study found the optimal range of abiotic factors for NRR and NRE in WBR were different. Regarding NRR, the optimal range of media age, HRT, Cin, temperature, effluent DOC, DO, pH, and ρe were in the first year, 0–5 h, 10–20 mg L−1, 20–25 °C, 0–5 mg L−1, 0–0.5 mg L−1, 7–8, and 0.6–0.7, respectively. For NRE, the optimal range of media age, HRT, Cin, temperature, effluent DOC, DO, pH, and ρe were in the first year, 500–3000 h, 0–10 mg L−1, 10–15 °C, >50 mg L−1, 0–0.5 mg L−1, 4–5, and 0.4–0.5, respectively. Moreover, the principal component analysis (PCA) indicated the field studies' principal components were different from laboratory studies. Furthermore, the structural equation modeling (SEM) also revealed the causal relationship of the eight abiotic factors on NRR and NRE is totally different. Lessons from this study can be incorporated into DNBR designs, especially improving nitrate removal rates by optimizing different abiotic factors. It also provides insights regarding the contributions of different abiotic factors for NRR and NRE independently and comprehensively.

Biological treatment of ironworks wastewater with high-concentration nitrate using a nitrogen gas aerated anaerobic membrane bioreactor

Difficulty in treating ironworks wastewater with high concentrations of nitrate is a major issue in steel manufacturing. We developed an N2 gas aerated anaerobic membrane bioreactor (AnMBR) and further assessed its nitrate removal capacity at the bench (0.026 m3) and pilot (4 m3) scales using a comprehensive microbiome survey. Both scale AnMBRs were fed with 5,300–8,000-mg/L nitrate-concentrated ironworks wastewater and methanol. At an early operation phase, nitrate and total organic carbon (TOC) increased in treated wastewater, which coincided with dynamic shift of the sludge microbiome. Thereafter, the accumulated nitrate was degraded in the absence of nitrite, in parallel with the TOC decrease and/or depletion, implying the presence of denitrification with methanol oxidation. At this phase, Hyphomicrobium nitrativorans capable of completely reducing nitrate predominated the microbiome. The increase in hydrolyzing and fermentative bacteria suggested their involvement in degradation of the decayed sludge biomass. H. nitrativorans, collaborating with high-molecular compound metabolizers, contributed importantly to the achievement of the nitrate removal rate of 1.1 ± 0.1 kg NO3-N/m3/day for pilot scale. Compared based on the complete (e.g., >99.9 %) removal, the achieved rate was comparable with the previous records (2.0–2.5 kg NO3-N/m3/day). Nevertheless, the supplied nitrate concentrations were remarkably higher than the previous ones (650–1,500 mg/L). These highlight that the N2 gas aerated AnMBR is widely applicable for the effective treatment of the nitrate-concentrated industrial wastewater with the considerably small footprint.

Co-occurrence of autotrophic and heterotrophic denitrification in electrolysis assisted constructed wetland packing with coconut fiber as solid carbon source.

Aiming at the problems of lack of carbon sources for nitrogen removal and low phosphorus removal efficiency of constructed wetlands (CWs) in treating wastewater treatment plant (WWTP) effluent, an electrolysis assisted constructed wetland (E-CW) with coconut fiber as substrate and solid carbon sources was constructed. The synthetic secondary effluent was used as the influent of the E-CW with a wastewater treatment capacity of 140Ld-1. The total nitrogen (TN) and the total phosphorus (TP) removal efficiency of the E-CW with coconut fiber treating WWTP effluent were 69.4% and 93.3%, respectively, which were 54.3% and 88.2% higher than those of CW with coconut fiber and no electrolysis. The removal efficiency of TN was 39.9% higher than that of E-CW with gravel. The current intensity had significant effect on nitrogen removal efficiency and the release of carbon sources from coconut fiber. When current intensity increased from 0.25 A to 1.00 A, the TN removal efficiency and nitrate removal rate increased by 21.1% and 0.21mgL-1 h-1, respectively, and the volatile fatty acids (VFAs) released from coconut fiber increased by 57.7mgL-1. The 16S rRNA high-throughput sequencing results indicated that the main functional nitrogen-removing microbes were Hydrogenophaga, Thauera, Rhodanobacteraceae_norank, Xanthobacteraceae_norank, etc. Multiple paths including autotrophic denitrification with hydrogen and Fe2+ as electron donors and heterotrophic denitrification were achieved in the system. Meanwhile, the main functional lignocellulose degradation microbes were enriched in the system, including Cytophaga_xylanolytica_group, and Caldilineaceae. Because electrolysis created a favorable environment for them to release carbon sources from coconut fiber. This study provided a new perspective for advanced nutrients removal of WWTP effluent in CWs.

Effect of hydraulic retention time on the electro-bioremediation of nitrate in saline groundwater

Bioelectrochemical systems (BES) have proven their capability to treat nitrate-contaminated saline groundwater and simultaneously recover value-added chemicals (such as disinfection products) within a circular economy-based approach. In this study, the effect of the hydraulic retention time (HRT) on nitrate and salinity removal, as well as on free chlorine production, was investigated in a 3-compartment BES working in galvanostatic mode with the perspective of process intensification and future scale-up. Reducing the HRT from 30.1 ± 2.3 to 2.4 ± 0.2 h led to a corresponding increase in nitrate removal rates (from 17 ± 1 up to 131 ± 1 mgNO3−-N L−1d−1), although a progressive decrease in desalination efficiency (from 77 ± 13 to 12 ± 2 %) was observed. Nitrate concentration and salinity close to threshold limits indicated by the World Health Organization for drinking water, as well as significant chlorine production were achieved with an HRT of 4.9 ± 0.4 h. At such HRT, specific energy consumption was low (6.8·10−2 ± 0.3·10−2 kWh g−1NO3−-Nremoved), considering that the supplied energy supports three processes simultaneously. A logarithmic equation correlated well with nitrate removal rates at the applied HRTs and may be used to predict BES behaviour with different HRTs. The bacterial community of the bio-cathode under galvanostatic mode was dominated by a few populations, including the genera Rhizobium, Bosea, Fontibacter and Gordonia. The results provide useful information for the scale-up of BES treating multi-contaminated groundwater.

Denitrification Performance in Packed-Bed Reactors Using Novel Carbon-Sulfur-Based Composite Filters for Treatment of Synthetic Wastewater and Anaerobic Ammonia Oxidation Effluent.

To avoid nitrate pollution in water bodies, two low-cost and abundant natural organic carbon sources were added to make up the solid-phase denitrification filters. This study compared four novel solid-phase carbon-sulfur-based composite filters, and their denitrification abilities were investigated in laboratory-scale bioreactors. The filter F4 (mixture of elemental sulfur powder, shell powder, and peanut hull powder with a mass ratio of 6:2.5:1.5) achieved the highest denitrification ability, with an optimal nitrate removal rate (NRR) of 723 ± 14.2 mg NO3–-N⋅L–1⋅d–1 when the hydraulic retention time (HRT) was 1 h. The HRT considerably impacted effluent quality after coupling of anaerobic ammonium oxidation (ANAMMOX) and solid-phase-based mixotrophic denitrification process (SMDP). The concentration of suspended solids (SS) of the ANAMMOX effluent may affect the performance of the coupled system. Autotrophs and heterotrophs were abundant and co-existed in all reactors; over time, the abundance of heterotrophs decreased while that of autotrophs increased. Overall, the SMDP process showed good denitrification performance and reduced the sulfate productivity in effluent compared to the sulfur-based autotrophic denitrification (SAD) process.

Efficient electrocatalytic nitrate reduction via boosting oxygen vacancies of TiO2 nanotube array by highly dispersed trace Cu doping

Nitrate pollution of water bodies is a serious global-scale environmental problem. The electrocatalytic nitrate reduction reaction (NO3RR) using Cu-based electrodes allows excellent nitrate removal; however, its long-term operation results in copper leaching, leading to health risks. This study proposes a strategy for efficient nitrate removal through the activation of oxygen vacancies on a highly dispersed Cu-doped TiO2 nanotube array (Cu/TNTA) cathode via electrodeposition. The mechanism underlying the activation of oxygen vacancies and enhancement in charge transfer at the cathode–pollutant interface upon trace (0.02 wt%) Cu doping is elucidated by electron paramagnetic resonance analysis, UV–visible diffuse reflection spectroscopy, and Raman spectroscopy. The Cu/TNTA-300 exhibits a nitrate removal rate of 84.3% at 12 h; the electrode activity did not decrease after 10 cycles, and no Cu2+ was detected in the solution. Electrochemical characterization and density functional theory calculations demonstrate that Cu doping promotes efficient charge transfer between nitrate and the electrode and reduces the energy barrier of the nitrate reduction reaction. This work provides a platform for novel design of cathodes for use in efficient and safe electrocatalytic nitrate reduction in environmental water bodies.

Nitrate Removal from Groundwater by Heterotrophic and Electro-Autotrophic Denitrification

A heterotrophic and autotrophic denitrification (HAD) system shows satisfactory performance for groundwater with nitrate contamination. In this study, an HAD system combining solid-phase heterotrophic denitrification and electrochemical hydrogen autotrophic denitrification (SHD-EHD) was developed for the treatment of nitrate-contaminated groundwater, in which polycaprolactone (PCL) was used as the carbon source to enhance the nitrate removal performance and prevent secondary pollution of the electrochemical hydrogen autotrophic denitrification (EHD) system. The denitrification performance, microbial community structure and nitrogen metabolism were investigated. The results showed that a high nitrate removal rate of 99.04% was achieved with an influent nitrate concentration of 40 mg/L, a current of 40 mA and a hydraulic retention time (HRT) of 4 h. By comparing the performance with the EHD system, it was found that the HAD system with PCL promoted the complete denitrification and reduced the accumulation of NO2−-N. Analysis of the microbial community structure identified the key denitrifying bacteria: Dechloromonas, Thauera and Hydrogenophaga. A comparison of microbial communities from SHD-EHD and solid-phase heterotrophic denitrification (SHD) demonstrated that electrical stimulation promoted the abundance of the dominant denitrifying bacteria and the electroactive bacteria. Analysis of the nitrogen metabolic pathway revealed that the conversion of NO to N2O was the rate-limiting step in the overall denitrification pathway. The SHD-EHD developed in this study showed great potential for groundwater nitrate removal.

Coupled sulfur and electrode-driven autotrophic denitrification for significantly enhanced nitrate removal

Elemental sulfur (S0)-based autotrophic denitrification (SAD) has gained intensive attention in the treatment of secondary effluent for its low cost, high efficiency, and good stability. However, in practice, the supplementary addition of limestone is necessary to balance the alkalinity consumption during SAD operation, which increases water hardness and reduces the effective reaction volume. In this study, a coupled sulfur and electrode-driven autotrophic denitrification (SEAD) process was proposed with superior nitrate removal performance, less accumulation of sulfate, and self-balance of acidity-alkalinity capacity by regulating the applied voltage. The dual-channel electron supply from S0 and electrodes made the nitrate removal rate constant k in the SEAD process 3.7-5.1 and 1.4-3.5 times higher than that of the single electrode- and sulfur-driven systems, respectively. The S° contributed to 75.3%-83.1% of nitrate removal and the sulfate yield during SEAD (5.67-6.26 mg SO42−/mg NO3−-N) was decreased by 17%-25% compared with SAD. The S0 particle and electrode both as active bio-carriers constructed collaborative denitrification communities and functional genes. Pseudomonas, Ralstonia and Brevundimonas were the dominant denitrifying genera in S0 particle biofilm, while Pseudomonas, Chryseobacterium, Pantoea and Comamonas became dominant denitrifying genera in the cathode biofilm. The narG/Z/H/Y/I/V, nxrA/B, napA/B, nirS/K, norB/C and nosZ were potential functional genes for efficient nitrate reduction during the SEAD process. Metagenomic sequencing indicated that S0 as an electron donor has greater potential for complete denitrification than the electrode. These findings revealed the potential of SEAD for acting as a highly efficient post denitrification process.

Electrodialysis ion-exchange membrane bioreactor (EDIMB) to remove nitrate from water: Optimization of operating conditions and kinetics analysis

Nitrate pollution has become a worldwide problem. In this study, we remove nitrate from water by electrodialysis ion-exchange membrane bioreactor (EDIMB) and enabling simultaneous nitrate enrichment and denitrification. In this reactor, nitrate migrated from the water chamber to the biological chamber via electrodialysis and was degraded by microorganisms. The effects of voltage and biomass concentration on the reactor performance were examined, and the kinetics data of the water chamber and biological chamber were fitted. The experimental results showed that the migration of nitrate in the water chamber conformed to the first-order model, and the constructed zero-Michaelis-Menten model described changes in nitrate concentration in the biological chamber. Furthermore, when the inflow nitrate concentration was 40 mg N/L, 5 V was the best voltage, and 3.00 g VSS/L was the best biomass concentration. The nitrate removal rate in the water chamber was 98.94%, and there was no accumulation of nitrate or nitrite in the biological chamber. Compared with traditional ED processes, the nitrate removal efficiency was 8.86% higher, and the current efficiency was 22.14% higher. The total organic carbon (TOC) of the water chamber was only 1.43 mg C/L, which proves that the structure of the EDIMB confined the denitrifying bacteria and organic carbon donors in the biological chamber and avoided secondary pollution in the water chamber. Microbial community analysis showed that Thauera (66.06%) was the dominant bacterium in the EDIMB system, and Azoarcus (9.81%) was a minor denitrifying genus.

Rice washing drainage (RWD) embedded in poly(vinyl alcohol)/sodium alginate as denitrification inoculum for high nitrate removal rate with low biodiversity

Immobilization technology with low maintenance is a promising alternative to enhance nitrate removal from water. In this study, washing rice drainage (RWD) was immobilized by poly(vinyl alcohol)/sodium alginate (PVA/SA) to obtain RWD-PVA/SA gel beads as inoculum for denitrification. When initial nitrate concentration was 50 mg N/L, nitrate was effectively removed at rates of 50–600 mg/(L∙d) using acetate as carbon source (C/N = 1.25). Arrhenius activation energy (Ea) of nitrate oxidoreductase was 28.64 kJ/mol for the RWD-PVA/SA gel beads. Temporal and spatial variation in microbial community structures were revealed along with RWD storage and in the reactors by Illumina high-throughput sequencing technology. RWD-PVA/SA gel beads has a simple (operational taxonomic units (OTUs) 〈100). Dechloromonas, Pseudomonas, Flavobacterium and Acidovorax were the most four dominant genera in the denitrification reactors inoculated with RWD-PVA/SA gel beads. This study provides an inoculum for denitrification with high nitrate removal performance and simple microbial community structures.

Influence of iron species on the simultaneous nitrate and sulfate removal in constructed wetlands under low/high COD concentrations

Nitrate and sulfate are crucial factors of eutrophication and black and odorous water in the surface water and thus have raised increasing environmental concerns. Constructed wetlands (CWs) are the last ecological barrier before effluent enters the natural water body. To explore the simultaneous removal of nitrate and sulfate, the CW microcosms of CW-Con (with quartz sand), CW-ZVI (quartz sand and zero-valent iron), CW-Mag (quartz sand and magnetite), CW-ZVI + Mag (quartz sand, ZVI and magnetite) groups were set up under the low (100 mg/L)/high (300 mg/L) chemical oxygen demand (COD) concentration. Under the high COD condition, CW-ZVI group showed the best performance in nitrate (97.1%) and sulfate (96.9%) removal. Under the low COD concentration, the removal content of nitrate and sulfate in CW-ZVI group was better than CW-Mag group. The reason for this result was that zero-valent iron (ZVI) could be the electron donor for nitrate and sulfate reduction. Meanwhile, ZVI promoted chemical denitrification under high COD concentration according to PCA analysis. In addition, the produced sulfides inhibited the relative abundance of denitrifying bacteria, resulting in the lowest nitrate removal rate in CW-Mag group with sufficient electron donors. This study provided an alternative method to enhance simultaneous sulfate and nitrate removal in CWs.

Reinforcement of denitrification in a biofilm electrode reactor with immobilized polypyrrole/anthraquinone-2,6-disulfonate composite cathode

In biofilm electrode reactors (BER), good nitrate removal performance can be achieved through cooperation of heterotrophic and hydrogen autotrophic denitrification under low carbon/nitrogen conditions. In this study, we proposed a more multifunctional composite cathode, which combine immobilized anthraquinone-2,6-disulphonic disodium salt (AQDS) with polypyrrole (PPy) by electrochemical polymerization-doping method. The nitrate removal performance in BER with PPy/AQDS composite cathode was obviously improved, the nitrate removal rate (4.96 mg/L·h) was almost 2.0 times higher than the control BER system, and relatively stabled nitrate removal efficiency (≥90.0%) was also achieved even as the COD/N of 2.50. Compared with the bare graphite felt, PPy/AQDS coating cathode showed much better electrocatalytic activities, which was more advantageous for in situ production of H2 to support hydrogen autotrophic denitrification process. The PPy-bound AQDS could also act as electron intermediaries, which is beneficial to greatly promote indirect electron process between the denitrifiers and nitrate. Moreover, the PPy/AQDS composite layer formed many particles for improving the specific surface area and bio-attachment site for bacterial attachment, which was conducive for the proliferation of microorganisms and denitrification efficiency. The ratio of biofilm and electrode of PPy/AQDS biocathode was 0.32 ± 0.08, which was 2.46 times than bare electrode (0.13 ± 0.06). Furthermore, enrichment of specific denitrifiers and enhancement of denitrifying enzyme activity was obtained using PPy/AQDS treated electrode, the much higher relative abundance of Thauera of PPy/AQDS biocathode was 1.58 times to the application of bare graphite felt.

Wastewater substrate disinfection for cyanobacteria cultivation as tertiary treatment.

Cultivation of microalgae or/and cyanobacteria in nutrient-rich wastewaters offers an opportunity for enhancing sustainability of tertiary wastewater treatment processes via resources/energy recovery/production, mitigation of emitted GHGs and provision of added value products. However, maintaining a monoculture in wastewater-media constitutes a significant challenge to be addressed. In this regard, the present work assesses the efficiency of the low-cost wastewater substrate disinfection techniques of filtration, use of NaClO, H2O2 or Fe(VI), as a preliminary treatment stage upstream a cyanobacteria cultivation photobioreactor. The growth rate of cyanobacterium Synechococcus elongatus PCC 7942, and nitrate and phosphate removal rates, were experimentally assessed in cultivation setups with biologically treated dairy wastewater that had been subjected to a single or a synergetic couple of disinfection techniques. The results showed that filter thickness has a greater effect on disinfection efficiency than filter pore size. Furthermore, the disinfection efficiency of Fe(VI), which was produced on-site by electrosynthesis via a Fe0/Fe0 cell, was greater than that of NaClO and H2O2. Filtration at ≤ 1.2-μm pore size coupled with chemical disinfection led to unhindered Synechococcus elongatus PCC 7942 growth and efficient nitrate and phosphate removal rates, at dosages, in terms of Concentreation-Time (CT) product,of CT ≥ 270mgmin L-1 for NaClO and CT ≥ 157mgmin L-1 for Fe(VI). The coagulation action of Fe(III) species that result from Fe(VI) reduction and the oxidation action of Fe(VI) can assist in turbidity, organic compounds and phosphorous removal from wastewater media. Moreover, the residual iron species can assist in Synechococcus elongatus PCC 7942 harvesting and may enhance photosynthesis rate by increasing light transfer efficiency. Thus, a filtration configuration coupled with chemical disinfection, preferably using ferrates, downstream of sedimentation tank of a secondary biological wastewater treatment stage is proposed as a necessary, efficient and low-cost disinfection technique for full-scale scale implementation of cyanobacteria cultivation as tertiary wastewater processes.

Ni-nanoparticle-modified Cu nanowires for enhanced electrocatalytic nitrate removal

Copper (Cu)-based materials have been extensively studied for nitrate (NO3 −) removal as an inexpensive and abundant electrocatalyst for water purification through the nitrate reduction reaction (NO3RR). However, it typically suffers from nitrite (NO2 −) accumulation due to high selectivity toward nitrite formation. To address this issue, the authors herein report a strategy of modifying copper nanowires with nickel (Ni) nanoparticles to improve the NO3RR performance. The nickel nanoparticles both facilitate electron transfer from nickel to copper and enhance the conversion of nitrite, thereby improving the overall removal of nitrate with a minimal yield of nitrite. Through a facile liquid-phase deposition process, the loading amount of nickel nanoparticles can be easily tailored by simply changing the concentration of precursors, and the best copper/nickel molar ratio for nitrate removal performance is 20. At this ratio, the material simultaneously delivers a high nitrate removal rate of 92.2% and low nitrite selectivity of only 2.2% at −0.9 V against the reversible hydrogen electrode, accompanied with superior stability for a continuous NO3RR. This study thus offers an efficient, stable and low-cost copper–nickel bimetallic catalyst for NO3RR.

Identification of aerobic-denitrifying Psychrobacter cryohalolentis strain F5-6 and its nitrate removal at low temperature

Biological denitrification is suppressed when temperature is lower than 10 °C. In this study, Psychrobacter cryohalolentis F5-6 was isolated from Harbin region and showed high aerobic-denitrifying rate at low temperature. It contained high percentage of unsaturated fatty acids (87.32%) and short chain saturated acid (3.4% C10:0 and 4.0% C12:0 3-OH). Acetate was the best carbon source for the strain F5-6 performing denitrification. Aerobic denitrification (napA gene) and nitrate assimilation (nasA gene) were the nitrate metabolism processes for the strain F5-6. Its biological nitrate removal (BNR) rate at 10 °C showed fit with Monod equation (R2 = 0.9808). The maximum BNR rate was 23.71 mg NO3−-N g−1DW h−1 and KS was 116.96 mg L−1. The optimal denitrifying condition was pH 7.5, 15 °C with shaking speed of 180 rpm and C/N of 2. Results of this study supplied a bacterial resource for improving nitrate removal from low C/N water at low temperature.

Rhamnolipids as Green Stabilizers of nZVI and Application in the Removal of Nitrate From Simulated Groundwater.

Environmental contamination caused by inorganic compounds is a major problem affecting soils and surface water. Most remediation techniques are costly and generally lead to incomplete removal and production of secondary waste. Nanotechnology, in this scenario with the zero-valent iron nanoparticle, represents a new generation of environmental remediation technologies. It is non-toxic, abundant, cheap, easy to produce, and its production process is simple. However, in order to decrease the aggregation tendency, the zero-iron nanoparticle is frequently coated with chemical surfactants synthesized from petrochemical sources, which are persistent or partially biodegradable. Biosurfactants (rhamnolipids), extracellular compounds produced by microorganisms from hydrophilic and hydrophobic substrates can replace synthetic surfactants. This study investigated the efficiency of a rhamnolipid biosurfactant on the aggregation of nanoscale zer-valent iron (nZVI) and its efficiency in reducing nitrate in simulated groundwater at pH 4.0. Two methods were tested: 1) adding the rhamnolipid during chemical synthesis and 2) adding the rhamnolipid after chemical synthesis of nZVI. Scanning electron microscopy field emission, X-ray diffractometry, Fourier transform infrared spectroscopy, thermogravimetric analysis, Dynamic Light Scattering, and zeta potential measurements were used to characterize bare nZVI and rhamnolipid-coated nZVI. The effects of the type of nZVI and initial NO3 concentration were examined. Nanoscale zer-valent iron with the addition of the rhamnolipid after synthesis achieved the best removal rate of nitrate (about 78%), with an initial nitrate concentration of 25 mg L−1. The results suggest that nZVI functionalized with rhamnolipids is a promising strategy for the in situ remediations of groundwater contaminated by NO3, heavy metal, and inorganic carbon.

Bioelectrochemical reactor improved by assembling anode with rice husk for treating nitrate-contaminated groundwater

To maximize the hydrogenotrophic denitrification and minimize the negative impact of anode by-products, a rice husk-enhanced anode bioelectrochemical reactor (RABER) was constructed by assembling anode with rice husk, and the denitrification performances and mechanisms involved were investigated. It was demonstrated that rice husk can be used as a slow-release carbon source and microbial carrier in RABER and weakened the anodic by-products. RABER obtained the highest contribution of hydrogenotrophic denitrification as 33.7% and achieved the fastest nitrate removal rate of 0.27 mg-N/(L·d) at 200 mA/m2. The relative lower accumulation of dissolved oxygen (1.99 ± 0.11 mg/L) and hydroxyl radicals were obtained at 200 mA/m2. Moreover, stable extracellular polymeric substances and higher activities of denitrification-related enzymes, i.e., around 2.78-fold for nitrate reductase and 1.96-fold for nitrite reductase those of 0 mA/m2, promoted denitrification in RABER at 200 mA/m2. At an optimal current density of 200 mA/m2, the coexistence of cellulose-degrading bacteria (Cellvibrio), aerobic denitrifier (Brevundimonas), anaerobic denitrifier (Flavobacterium), and hydrogenotrophic denitrifier (Alicycliphilus) was achieved, and the inorganic carbon generated by heterotrophic denitrifiers and electrochemical mineralization was sufficient to meet the demands of hydrogenotrophic denitrifiers, conductive to the heterotrophic‑hydrogenotrophic synergistic denitrification. This study provided a theoretical basis for the enhancement of hydrogenotrophic denitrification in BER as well as minimizing the side effects of the anode and providing a new strategy for the disposal of agricultural waste.

A sustainable approach for bioremediation of secondary salinized soils: Studying remediation efficiency and soil nitrate transformation by bioaugmentation

Nitrate is the main nitrogen source for plant growth, but it can also pollute the environment. A major cause of soil secondary salinization is the rising level of nitrates in the soil, which poses a threat to the sustainability and fertility of global greenhouse soils. Herein, Bacillus megaterium NCT-2 was used as a microbial agent to remove nitrate by bioaugmentation, and the remediation efficiency of secondary salinized soil in different degrees was evaluated. The findings showed that the highest nitrate removal rate of 62.76% was in a medium degree of secondary salinized soil. Moreover, the results of 16S rRNA high-throughput sequencing and quantitative real-time PCR (qPCR) demonstrated that NCT-2 agent reduced the microbial diversity, increased the microbial community stability, and changed the composition and function of the microbial community were changed by NCT-2 agent in all districts soil. Further analysis demonstrated that the NCT-2 bacterial agent significantly increased the key enzyme genes of the assimilation pathway (nitrite reductase gene NasD, 87–404 times, and glutamine reduction enzyme gene GlnA, 13–52 times) and dissimilatory reduction to ammonium (DNRA) (nitrate reductase gene NarG, 14–56 times) in different degrees of secondary salinized soils. This proved that NCT-2 agent could promote the nitrate assimilation and the dissimilation and reduction to ammonium in secondary salinized soil. Thus, the current findings suggested that the NCT-2 agent has a significant potential for reducing excessive nitrate levels in secondary salinized soil. The remediation efficiency was related to the microbial community composition and the degree of secondary salinization. This study could provide a theoretical basis for the remediation of secondary salinized soil in the future.

A MXene-based multiple catalyst for highly efficient photocatalytic removal of nitrate

Photocatalytic removal of nitrate in wastewater has attracted wide attention because of its simple operation and environmental protection. However, the preparation of photocatalysts with high efficiency and high nitrogen selectivity is still a challenge. In this paper, TiO2 is grown in situ on Ti3C2 MXene by a simple calcination method and modified with silver particles. The presence of Ti3C2 reduces the recombination rate of photogenerated electrons and generates more photogenerated electrons. At the same time, the silver particles also increase the photoelectron density and further improve the carrier separation of the catalyst. Due to its unique structure and optical properties, the prepared photocatalyst shows an excellent nitrate removal rate under a high-pressure mercury lamp. At 500 mgN/L, the nitrate removal rate reaches 96.1%, and the nitrogen selectivity reaches 92.6%. Even after 5 cycles, the prepared photocatalyst still maintains a high nitrate photocatalytic removal efficiency (89%). The electron transfer path is verified by density functional theory calculations.

Nitrate Removal from Water Using Electrocoagulation with Activated Carbon

The present work studies the clarification of drinking water by removing Nitrate and Nitrite using electrocoagulation (EC) and the addition of activated carbon which produced from the almond peel (AP). The efficiency of coupling process studied by using different amounts of adsorbent material to identify the most appropriate dosage, for enhancing drinking water purification, The efficiency of the current density used in the removal of Nitrate and Nitrite was studied, energy consumption of the process was also determined. The coupling between electrocoagulation and the addition of the activated carbon showed that the addition of an appropriate dosage resulted in the enhancement of the removal rate of Nitrate and Nitrite especially at lower current densities, and resulted in a considerable reduction in the needed time compared to the conventional process that does not use activated carbon. The electrocoagulation with the addition of activated carbon achieved efficiency removal of approximately 58.5% for Nitrate and 98.9% for Nitrite. Based on the high removal efficiency, short contact time, and low energy consumption, the electrocoagulation with the addition of activated carbon method could be recommended instead of the conventional process. Furthermore, the results suggest that the productivity of the process is enhanced by adding a very small amount of NaCl as a supporting electrolyte.