Driving Role of Zinc Oxide Nanoparticles with Different Sizes and Hydrophobicity in Metabolic Response and Eco-Corona Formation in Sprouts (Vigna radiata).

Zinc oxide nanoparticles (ZnO NPs) cause biotoxicity and pose a potential ecological threat; however, their effects on plant metabolism and eco-corona evolution between NPs and organisms remain unclear. This study clarified the molecular mechanisms underlying physiological and metabolic responses induced by three different ZnO NPs with different sizes and hydrophobicity in sprouts (Vigna radiata) and explored the critical regulation of eco-corona formation in root-nano systems. Results indicated that smaller-sized ZnO inhibited root elongation by up to 37.14% and triggered oxidative burst and apoptosis. Metabolomics confirmed that physiological maintenance after n-ZnO exposure was mainly attributed to the effective stabilization of nitrogen fixation and defense systems by biotransformation of the flavonoid pathway. Larger-sized or hydrophobic group-modified ZnO exhibited low toxicity in sprouts, with 0.89-fold upregulation of citrate in central carbon metabolism. This contributed to providing energy for resistance to NP stress through amino acid and carbon/nitrogen metabolism, accompanied by changes in membrane properties. Notably, smaller-sized and hydrophobic NPs intensely stimulated the release of root metabolites, forming corona complexes with exudates. The hydrogen-bonded wrapping mechanism in protein secondary structure and hydrophobic interactions of heterogeneous functional groups drove eco-corona formation, along with the corona evolution intensity of n-ZnO > s-ZnO > b-ZnO based on higher (α-helix + 3-turn helix)/ß-sheet ratios. This study provides crucial insight into metabolic and eco-corona evolution in bionano fates.

An eco-friendly self-assembled catalyst preparation and study of tetracycline degradation: Performance, mechanism to application.

Chloromethyl styrene resin can undergo specific chemical modifications and is an excellent adsorbent material for treating difficult-to-degrade substances in wastewater. In this study, chloromethyl styrene resin will be used as a carrier, and polystyrene chloromethyl resin (PS-Cl) was converted into PS-NH2 by amino modification. The self-assembly of cobalt-based metal-organic framework (CoMOF) was induced on the surface of PS-NH2 by using a novel preparation technique. The performance of the prepared PS-NH2@CoMOF self-assembled catalysts with core-shell-like structures in degrading the target pollutant, tetracycline (TC), was evaluated. The catalysts effectively induced rapid OH radical production from H2O2, had a degradation rate of as high as 88.3 % for 20 mg/L TC solution, and were highly stable and adaptable to aqueous environments. Free radicals and intermediates in the catalytic degradation process were detected by electron paramagnetic resonance and high-performance liquid chromatography mass spectrometry, and possible catalytic degradation pathways were analyzed. The catalytic dissociation behavior of H2O2 in the presence of different catalysts was studied in depth and compared with that of similar metal-organic framework materials through density-functional theory calculations. Results demonstrated the excellent performance of the PS-NH2@CoMOF catalysts. Finally, the catalysts' potential for use in practical engineering applications was evaluated with a flow column experimental model, and the results were more than satisfactory. Therefore, the use of the catalysts to degrade TC has great potential.

Modulatory Role of Biochar Properties and Environmental Risk of Heavy Metals by Co-Pyrolysis of Fenton Sludge and Biochemical Sludge.

Recent studies have reported that Fenton sludge and biochemical sludge contain high concentrations of toxic substances and heavy metals (HMs), whereas improper treatment can pose serious threats to environmental safety. Pyrolysis is considered an efficient technology to replace conventional sludge treatment. This study investigated the pyrolysis and kinetic processes of Fenton sludge and biochemical sludge, revealed the physicochemical properties of sludge biochar, and highlighted the role of co-pyrolysis in sludge immobilization of HMs and environmental risks. Results showed that Fenton sludge and biochemical sludge underwent three stages of weight loss during individual pyrolysis and co-pyrolysis, especially co-pyrolysis, which increased the rate of sludge pyrolysis and reduced the decomposition temperature. The kinetic reaction indicated that the activation energies of Fenton sludge, biochemical sludge, and mixed sludge were 11.59 kJ/mol, 8.50 kJ/mol, and 7.11 kJ/mol, respectively. Notably, co-pyrolysis reduced the activation energy of reactions and changed the specific surface area and functional group properties of the biochar produced from sludge. Meanwhile, co-pyrolysis effectively immobilized Cu, Pb, and Zn, increased the proportion of metals in oxidizable and residual states, and mitigated the environmental risks of HMs in sludge. This study provided new insights into the co-pyrolysis properties of sludge biochar and the risk assessment of HMs.

Preparation and study of straw porous biochar with aromatic ring structure for adsorption performance and mechanism toward TNT red water.

2,4,6-Trinitrotoluene (TNT) production processes generate a substantial amount of toxic wastewater. Therefore, it is crucial to identify efficient and sustainable methods for treating this wastewater. This paper explores the application of sustainable biomass-derived carbon produced from rice straw for the adsorption of 2,4,6-trinitrotoluene (TNT) red water. The rice straw-derived biochar (SBC) materials were synthesized by two-step reactions through hydrothermal carbonization and chemical activation with KOH. Characterization of the fabricated biochar was conducted using various techniques. Here, the chemical oxygen demand (COD) was used as an evaluation index for adsorption efficiency. The adsorption kinetics showed a good fit with the pseudo-second-order model, and the adsorption equilibrium was achieved in 30 min. The biochar's high surface area (1319 m2/g) and large pore volume (1.058 cm3/g) gave it a large adsorption capacity. The Langmuir model exhibited better correlation for equilibrium data analysis, with a maximum adsorption capacity of 173.9 mg/g at 298 K. The SBC was found to have a high removal effect over a wide pH range (from 1 to 13) and showed remarkable stability after undergoing five desorption-adsorption cycles using ethanol and acetone as eluent. The results provide a simple and low-cost method for the efficient treatment of TNT red water.

Evaluating the inhibitory effects of Nitrobenzene short-term stress on denitrification performance: Electron behaviors, bacterial and fungal community.

Denitrifying system is a feasible way to remove nitro-aromatic compounds (NACs) in wastewater. However, the toxicity and mechanisms of NACs to denitrification remain unknown. This study investigated effects of nitrobenzene (NB, a typical NAC) on denitrification in short term. Results showed that NB in 10-50 mg/L groups decreased NO3--N removal efficiency by 9%-24%, but increased nitrous oxide (N2O) generation by 6-17fold. Mechanistic research indicated that NB could deteriorate electron behaviors and disturbed enzyme activities of microbial metabolism and denitrification, leading to a decline in denitrification performance. Structural equation modeling revealed that N2O reductase activity was the core factor in predicting denitrification performance at exposure of NB, with the indirect effects of NADH and electron transport system activity. High-throughput sequencing analysis demonstrated that NB had made an alteration on both bacterial and fungal community structure, as well as their interactions.

Mechanism of quinone mediators modified polyurethane foam for enhanced nitrobenzene reduction and denitrification.

The nitrobenzene (NB) reduction and denitrification performance of the immobilized biofilm (I-BF) reactors based on 9,10-anthraquinone-2-sulfonyl chloride (ASC) modified polyurethane foam (PUF-ASC) carriers were investigated. Experiments demonstrated that the quinone mediators enhanced NB reduction and denitrification performance. The NB reduction rates increased by 1.46, while the NO3--N removal rates increased by 1.55 times in the PUF-0.1ASC system. The quinone mediators promote extracellular polymeric substances (EPS) secretion. Electrochemical tests indicated that quinone mediators enhanced the electron transfer of biofilm systems. NADH generation was accelerated and microbial electron transport system activity (ETSA) was promoted. The abundance of genera with electrochemical activity, NB degradation and denitrification ability (Pseudomonas sp., Diaphorobate sp., and Acinetobacter sp.) increased. Metabolic pathways relating to NO3--N and NB reduction were uploaded. In conclusion, electron acquisition by NO3--N and NB was facilitated, bacterial community structure and metabolic pathways were affected by the quinone mediators.

Sequentially modified carbon felt for enhanced p-nitrophenol biodegradation through direct interspecific electron transfer.

The widely applied aromatic nitration in modern industry leads to toxic p-nitrophenol (PNP) in environment. Exploring its efficient degradation routes is of great interests. In this study, a novel four-step sequential modification procedure was developed to increase the specific surface area, functional group, hydrophilicity, and conductivity of carbon felt (CF). The implementation of the modified CF promoted reductive PNP biodegradation, attaining 95.2 ± 0.8% of removal efficiency with less accumulation of highly toxic organic intermediates (e.g., p-aminophenol), compared to carrier-free and CF-packed biosystems. The constructed anaerobic-aerobic process with modified CF in 219-d continuous operation achieved further removal of carbon and nitrogen containing intermediates and partial mineralization of PNP. The modified CF promoted the secretion of extracellular polymeric substances (EPS) and cytochrome c (Cyt c), which were essential components to facilitate direct interspecies electron transfer (DIET). Synergistic relationship was deduced that glucose was converted into volatile fatty acids by fermenters (e.g., Longilinea and Syntrophobacter), which donated electrons to the PNP degraders (e.g., Bacteroidetes_vadinHA17) through DIET channels (CF, Cyt c, EPS) to complete PNP removal. This study proposes a novel strategy using engineered conductive material to enhance the DIET process for efficient and sustainable PNP bioremediation.

Activated sludge process enabling highly efficient removal of heavy metal in wastewater.

Activated sludge process was a low-cost alternative method compared to the conventional physicochemical process for the treatment of heavy metal-containing wastewater. In the present study, the removal efficiency of Pb2+, Cu2+, and Ni2+ from wastewater by a sequencing batch reactor (SBR) activated sludge system was investigated, and the mechanism was revealed by static adsorption experiment of activated sludge. The results showed that the activated sludge in the SBR system was effective in removing Pb2+ and Cu2+ from wastewater at 10 mg·L-1 initial concentration, with a removal efficiency of 83.1 ~ 90.0% for Pb2+ and 74.3 ~ 80.6% for Cu2+, respectively. However, the removal efficiency for Ni2+ was only 0 ~ 6.2%. Static adsorption experiments showed that the adsorption capacity of activated sludge for three heavy metals was shown as Pb2+ > Cu2+ > Ni2+. When the initial concentration was 20 mg·L-1, the equilibrium adsorption capacity of activated sludge for Pb2+, Cu2+, and Ni2+ was 18.35 mg·g-1, 17.06 mg·g-1, and 8.37 mg·g-1, respectively. The main adsorption mechanisms for Pb2+ and Cu2+ were ligand exchange, electrostatic adsorption, and surface organic complexation processes, but Ni2+ removal mechanism mainly included electrostatic adsorption and surface organic complexation processes, showing that Ni2+ removal was inhibited in the presence of Pb2+ and Cu2+. The physicochemical properties and microbial diversity of activated sludge were greatly affected by the heavy metals in the SBR system, and genus Rhodobacter was found to be dominant bacteria enabling resistance to heavy metal ions.

The mechanism of efficient photoreduction nitrate over anatase TiO2 in simulated sunlight.

Revealing the activation mechanism of nitrate (NO3-) reduction is crucially important to design high-efficiency photocatalysts for NO3- removal. In this work, the performance of photoreduction NO3- has been thoroughly studied over different crystalline phases TiO2. Photodegradation rate of NO3- over anatase TiO2 is found to be higher than that of rutile TiO2 at simulated sunlight, which can achieve high NO3- conversion of 94% and 100% nitrogen selectivity (original concentration of 50 mg/L NO3--N) at reaction time of 4 h. With the aid of in situ Fourier Transform Infrared (FTIR) and density functional theory (DFT) calculations, the possible reaction paths of photoreduction NO3- over anatase TiO2 was verified from theory and practice sides. NO3- was adsorbed on surface Ti site to form bridging nitrate (M - O)2NO model. Then, monodentate nitrite (M-O-N-O) model was the dominant intermediate in the reduction process of NO3-. This study presents a new opinion of photoreduction NO3- reaction paths.

Response of immobilized denitrifying bacterial consortium to tetracycline exposure.

Tetracycline (TC) as one of the most widely used antibiotics commonly exists in aquaculture tail water and piggery wastewater, causing risks to human. However, the response of immobilized anaerobic denitrifying bacterial consortium to TC exposure lacks systematic research. In this study, the denitrification performance and the compositional shift of extracellular polymeric substances (EPS) and microbial community under TC stress were investigated. The inhabitation effect of TC on nitrate reduction of the immobilized bacterial consortium became evident at high concentrations (50 mg/L and 100 mg/L). Nitrite reduction was more sensitively inhibited than nitrate reduction. The inhabitation effect was mainly due to the fact that TC damaged cell membranes and subsequently effect the intracellular enzymes activities relating to denitrification (NAR and NIR activities). About 50% of TC can be removed by the immobilized bacterial consortium under all tested TC concentrations. Three-dimensional excitation-emission matrix (3D-EEM) results implied that the tryptophan like substances of EPS were obviously quenched with increasing TC concentration. EPS played an important role in TC removal. The denitification performance of the immobilized bacterial consortium under TC stress was attributed to the genera Paraccoccus, Pseudoxanthomonas, Diaphorobacter and Pseudomonas. Initial TC concentration obviously affected the microbial communities. This study may facilitate the management of aquaculture tail water and piggery wastewater contaminated with nitrate and antibiotics.

Treatment of dairy wastewater by immobilized microbial technology using polyurethane foam as carrier.

The development of dairy industry is accompanied by large volumes of wastewater production, which is threaten to human's health and the biosphere. In this study, synthetic dairy wastewater was treated by immobilized microbial technology using polyurethane foam as carrier. Batch experiments were conducted to determine the effects of different operational parameters, and an up-flow immobilized microbial reactor was built to investigate long-term performance of the system. Batch experiments showed that COD, TN and NH3-N dropped from 1932, 51.33 and 51.42 mg·L-1 to 75.3, 5.17 and 4.54 mg·L-1 after 48 h, respectively, at the optimum conditions (25 °C, pH 6.0). Besides, the reactor can remove 97.33% of COD, 96.46% of TN and 99.55% of NH3-N with HRT of 24 h, which the average volume load was 1.93 kg COD·(m3·d)-1. The analysis of microbial community determined that dominant bacteria at genus level were Acinetobacter, Fusibacter, Nannocystis and norank _f_NS9_marine_group.

Heavy metal-contained wastewater in China: Discharge, management and treatment.

A large amount of heavy metal-contained wastewater (HMW) was discharged during Chinese industry development, which has caused many environmental problems. This study reviewed discharge, management and treatment of HMW in China through collecting and analyzing data from China's official statistical yearbook, standards, technical specifications, government reports, case reports, and research paper. Results showed that industry wastewater discharged by an amount of about 221.6 × 108 t (in 2012), where emission of heavy metals including Pb, Hg, Cd, Cr(VI), T-Cr was around 388.4 t (in 2012). Heavy metal emission with wastewater in east China and central south China was observed to be graver than that in other areas. However, control of heavy metals in Pb and Cd in northwest China was more difficult compared with other areas. In terms of management, China's government has issued many wastewater discharge standards, strict management policies for controlling HMW discharge in recent years, resulting in reduced HMW discharge. In addition, main HMW treatment technology in China was chemical precipitation, and other technologies such as membrane separation, adsorption, ion exchange, electrochemical and biological methods were also occasionally applied. In the future, chemical industries will be concentrated in northwest China, therefore control of HMW discharge should be paid much more attention in those areas. In addition, more effective and environment-friendly heavy metal removal and regeneration technologies should be developed, such as biomaterials adsorbent.

Insight into the role of extracellular polymeric substances in denitrifying biofilms under nitrobenzene exposure.

Denitrifying biofilm promises to be very useful for remediation of nitro-aromatic compounds (NACs) and nitrates in wastewater. Little is known about the role of extracellular polymeric substances (EPS) in nitrobenzene (NB, a typical NAC) remediation, despite the indispensability of EPS for biofilm formation. Herein, the significance of the mechanistic role of EPS in the response of denitrifying biofilms to various levels of NB was investigated. The removal of NB was predominantly controlled via absorption, with little biodegradation during the short-term exposure. Specifically, NB was adsorbed by EPS, as shown by a total adsorption of 40.06% at the initial step, which declined to around 10.52% in the equilibrium stage, while sorption via cells gradually increased from 59.93% to 89.47% over the same period. The results suggested that EPS might act as an important reservoir for NB, which endows inner cells with increased adsorption ability. The presence of EPS might also alleviate the negative impacts of NB toxicity on inner cells, thus protecting microorganisms. This was indicated by the difference in denitrification performance and cell integrity between intact and EPS-free biofilms. High-throughput sequencing data demonstrated that EPS could maintain the stability of microbial communities under NB stress. The fluorescence quenching analysis further indicated that EPS formed stable complexes with NB mainly through hydrophobic interactions with protein-like fractions (tryptophan and tyrosine). Moreover, Fourier transform infrared spectroscopy identified that the hydroxyl, amino, carboxyl, and phosphate groups of EPS were the candidate functional groups binding with NB. Protein secondary structures were also significantly affected, resulting in a loose structure and enhanced hydrophobic performance for EPS. These results provide insights into the role of EPS in alleviating NB-caused cellular stress and the underlying binding mechanisms between NB and EPS.

Preparation of EDTA-modified magnetic attapulgite chitosan gel bead adsorbent for the removal of Cu(II), Pb(II), and Ni(II).

Attapulgite (ATP) has a unique porous layered chain structure and is an excellent environment-friendly decolorizer. However, its high viscosity makes it difficult to separate. First, it was combined with magnetic Fe3O4 nanoparticles to obtain Fe3O4-ATP that is easy to recycle and reuse. Then, Fe3O4-ATP nanoparticles were embedded in chitosan (CS) gel beads, and glutaraldehyde was used for cross-linking and curing. Lastly, ethylenediaminetetraacetic acid (EDTA) was successfully connected to Fe3O4-ATP/CS gel beads through amidation reaction, which further increased the capability of the adsorbent to adsorb heavy metal ions. The prepared magnetic composite adsorbent is a sphere with a rough surface and porous structure. The maximum adsorption capacity of the prepared Fe3O4-ATP/EDTA/CS adsorbent for Pb(II), Cu(II), and Ni(II) is 368.32, 267.94, and 220.31 mg g-1, respectively. After 5 times of repeated use, Fe3O4-ATP/EDTA/CS still showed good adsorption capacity for Cu(II), Pb(II), and Ni(II). Fe3O4-ATP/EDTA/CS has a large adsorption capacity for heavy metal ions, a wide pH applicability, good reuse, and rapid magnetic separation. In addition, Cu(II) loaded Fe3O4-ATP/EDTA/CS also showed good catalytic degradation performance for bisphenol A. This study prepared a new type of adsorbent with good adsorption performance and provided a promising method for the treatment of wastewater.

Domestic wastewater infiltration process in desert sandy soil and its irrigation prospect analysis.

Although domestic wastewater and its reclaimed water are alternative water resources in arid region, investigation of their negative effect must be done to prevent environmental pollution. In this paper, a short-term column experiment was conducted to simulate the infiltration process of wastewater in desert soil. Alfalfa was planted and irrigated with fresh water for control (CK), tertiary treated domestic wastewater (TTW), secondary treated domestic wastewater (STW) and raw domestic wastewater untreated (RW). The effect of wastewater application on desert soil, drainage and plant properties was evaluated. Experimental results demonstrated that the tested desert soil has no soil structure, organic matter, nor microbial community while possess high infiltration rate. The use of wastewater significantly improved plant growth, and the biomass of TTW, RW, STW were 5.5, 4.3, 2.9 times of CK. The infiltration rate of water in bare soil was high (high to low: TTW, CK, RW, STW), while plant growth reduced infiltration rate (ca. 40% with TTW and RW). Wastewater irrigation and plant growth decreased soil zeta potential, while increased formation of aggregates and bacterial abundance and diversity in soil. Top soil (0-30 cm) accumulation of nitrogen (N), phosphorus (P), organic matter and E. coli was evidenced and all could go down to deep soil and drainage with constant wastewater use. It was concluded that domestic wastewater had big potential in desert soil vegetation recovering and function restoration. Nevertheless, the N, salt, P and organic matter and E. coli in wastewater could give rise to desert soil and groundwater contamination if improper treatment was used.

Recent Progress in Heavy Metal Ion Decontamination Based on Metal-Organic Frameworks.

Heavy metals are inorganic pollutants which pose a serious threat to human and environmental safety, and their effective removal is becoming an increasingly urgent issue. Metal-organic frameworks (MOFs) are a novel group of crystalline porous materials, which have proven to be promising adsorbents because of their extremely high surface areas, optimizable pore volumes and pore size distributions. This study is a systematic review of the recent research on the removal of several major heavy metal ions by MOFs. Based on the different structures of MOFs, varying adsorption capacity can be achieved, ranging from tens to thousands of milligrams per gram. Many MOFs have shown a high selectivity for their target metal ions. The corresponding mechanisms involved in capturing metal ions are outlined and finally, the challenges and prospects for their practical application are discussed.

Application of Ti/IrO2 electrode in the electrochemical oxidation of the TNT red water.

Via the thermal sintering, a nanocrystalline IrO2 coating was formed on the Ti substrate to successfully prepare a Ti/IrO2 electrode. Based on the electrochemical analysis, the prepared Ti/IrO2 electrode was found to have powerful oxidation effect on the organics in the TNT red water, where the nitro compound was oxidized through an irreversible electrochemical process at 0.6 V vs. SCE. According to the analysis of the nitro compound content, the UV-vis spectra, and the FTIR spectra of 2,4,6-trinitrotoluene (TNT) red water with electrolytic periods, the degradation mechanism of the dinitrotoluene sulfonate (DNTS) was developed. And the intermediates were characterized by UPLC-HRMS. The DNTS mainly occurred one electron transfer reaction on the Ti/IrO2 electrode. At the early stage of the electrolysis, the polymerization of DNTS was mainly dominated. The generated polymer did not form a polymer film on the electrode surface, but instead it promoted a further reduction. After electrolyzing for 30 h, all NO2 function group in the TNT red water was degraded completely.

Toxicological responses of Fe3O4 nanoparticles on Eichhornia crassipes and associated plant transportation.

This work investigated the interaction of Fe3O4 nanoparticles (NPs) with a floating water plant (Eichhornia crassipes). The effects of Fe3O4 NPs on E. crassipes physiology and root morphology as well as the migration and transformation of Fe3O4 NPs in plant were studied emphatically. Fe3O4 NPs (200 mg/L) showed significant growth inhibition on E. crassipes roots and leaves after a 21-day exposure, while dissolved Fe3+ ions and Fe3O4 bulk particles had no obvious effect on E. crassipes growth. Scanning electron microscopy showed that the roots of E. crassipes were significantly damaged, the root tips became thin and the root epidermis began to peel off after Fe3O4 NPs exposure. In addition, there was disordered cell arrangement and a destroyed elongation zone of the root tips. The physiology of E. crassipes was also affected. In particular, after exposure to Fe3O4 NPs (200 mg/L), a distinct decrease in chlorophyll content and catalase activity and an increase of malondialdehyde (MDA) content could be seen. Transmission electron microscopy and energy dispersive spectroscopy revealed that Fe3O4 NPs were present in the root epidermis, intercellular space and protoplasts, as well as in the leaf cytoplasm and chloroplasts. Vulcanization in the leaves was also found through diffraction analysis, which may be due to the small number of absorbed nanoparticles, some of which still existed in the original form in the leaves while others were reduced to FeS through interactions with plant components during translocation. These findings are helpful for better understanding the fate of NPs in aquatic plants. Moreover, it is important to evaluate the water environment safety of NPs.

Denitrification performance and microbial diversity of immobilized bacterial consortium treating nitrate micro-polluted water.

A heterotrophic denitrification process using bacterial consortium immobilized by polyurethane foams carriers to treat nitrate micro-polluted water was investigated. Nitrate reduction and nitrite accumulation were studied under several factors including initial COD/NO3--N concentration ratio, initial pH, initial NO2--N/NO3--N concentration ratio and inlet NO3--N concentration. Batch denitrification experiments showed that nitrate was completely removed at 5 h without nitrite accumulation under the optimum conditions of COD/NO3--N concentration ratio of 5.0-5.5 and initial pH of 7.2 ±â€¯0.1. High initial NO2--N/NO3--N ratio enhanced denitrification rate mainly by accelerating nitrite reduction. Denitrification processes followed zero-order reaction kinetics at different initial NO3--N concentrations and obtained higher denitrification rate at higher inlet nitrate. High-throughput sequencing results showed that microbial community structure differed between the surface and interior space of polyurethane foams carriers while the dominant population in the inner zone of carriers was Pseudoxanthomonas.

Microbial reduction of nitrate in the presence of zero-valent iron.

Microbial reduction of nitrate in the presence of zero-valent iron (ZVI) was evaluated in anoxic shake flasks to assess the feasibility of ZVI-facilitated biological nitrate removal. Nitrate was completely reduced within 3days in the presence of both ZVI and microorganisms (ZVI-M). In contrast, only 75% of the nitrate was reduced in the presence of ZVI but without microbial inoculum. Nitrate removal was affected by ZVI-M flasks initial pH, nitrate concentration and ZVI dosage. Nitrate removal in the inoculated ZVI flasks system could be divided into two phases: adaptation phase and log phase which could be described by first-order kinetic equations. The analysis of bacterial communities in the inoculated flasks in the absence and presence of ZVI, indicated that the addition of ZVI increased the relative abundance of Methylotenera spp., Alcaligenes eutrophus, Pseudomonas spp. which might play an important role in nitrogen removal. The presence of ZVI could enhance biological denitrification through four mechanisms: the biological reduction of nitrate with 1) electrons derived directly from ZVI; 2) with hydrogen released from ZVI; 3) with Fe2+ released from ZVI; and 4) with acetate generated by homoacetogens which utilize H2 released from ZVI.