Nitrate-Contaminated Water Remediation Supported by Solid Organic Carbon and ZVI-Combined System

Simultaneous removal of nitrate, nitrobenzene and aniline from groundwater in a vertical baffled biofilm reactor

The challenge of simultaneous removal of nitrobenzene (NB), aniline (AN) and nitrate from groundwater in a single bioreactor is mainly attributed to the persistence of AN to degradation with anoxic denitrification conditions. In this work, simultaneous removal of NB (100 μM), AN (100 μM) and nitrate (1 mM) was achieved within 8 h with a COD/N ratio of 8 in a vertical baffled biofilm reactor (VBBR). By setting DO concentration at 0.4-0.5 mg L^-1 to create a micro-aerobic condition, NB removal rate was accelerated without accumulation of AN, and AN could serve as electron donors for denitrification after ring cleavage. High-throughput sequencing showed that biofilm was predominated by denitrifiers (Luteimonas, Planctomyces, Thiobacillus, Thauera and so on) and NB-degrading bacteria (Pseudomonas), and biodiversity varied vertically along the height of the reactor. A dominantly anaerobic pathway for reducing NB to AN was identified by PICRUSt analysis, as the predicted genes involved in aerobic transformation of NB were several magnitudes lower than those in the anaerobic pathway. This study provided a new insight to the role of oxygen in robust bioremediation groundwater contaminated with NB, AN and nitrate.

Recent advances in the source identification and remediation techniques of nitrate contaminated groundwater: A review

Researchers have long been committed to identify nitrate sources in groundwater and to develop an advanced technique for its remediation because better apply remediation solution and management of water quality is highly dependent on the identification of the NO₃^- sources contamination in water. In this review, we systematically introduce nitrate source tracking tools used over the past ten years including dual isotope and multi isotope techniques, water chemistry profile, Bayesian mixing model, microbial tracers and land use/cover data. These techniques can be combined and exploited to track the source of NO₃^- as mineral or organic fertilizer, sewage, or atmospheric deposition. These available data have significant implications for making an appropriate measures and decisions by water managers. A continuous remediation strategy of groundwater was among the main management strategies that need to be applied in the contaminated area. Nitrate removal from groundwater can be accomplished using either separation or reduction based process. The application of these processes to nitrate removal is discussed in this review and some novel methods were presented for the first time. Moreover, the advantages and limitations of each approach are critically summarized and based on our own understanding of the subject some solutions to overcomes their drawbacks are recommended. Advanced techniques are capable to attain significantly higher nitrate and other co-contaminants removal from groundwater. However, the challenges of by-products generation and high energy consumption need to be addressed in implementing these technologies for groundwater remediation for potable use.

Effect of addition sites on bioaugmentation of tea polyphenols-NZVI/PE composite packing: Nitrogen removal efficiency and service life

Although efficient improvement of the nitrogen removal from wastewater by adding iron was achieved in wastewater process, the influence mechanism of addition sites is unclear. The study was based on the A/O-MBR treating simulated domestic wastewater, and tea polyphenol-nano zero-valent iron/polyethylene packing (TP-NZVI/PE) was added into the anoxic tank, aerobic tank and membrane effluent end of the process, respectively. The effect of the different addition sites on the nitrogen removal performance of A/O-MBR was investigated. Combine with the corrosion rate of NZVI on the packing surface to optimize TP-NZVI/PE addition site. The enhancement mechanism of TP-NZVI/PE under different addition site was explored through the calculation of the materials balance (carbon, nitrogen, phosphorus). The results showed that the pollutant removal of A/O-MBR was significantly increased with the TP-NZVI/PE added. In particular, the TP-NZVI/PE was added into the aerobic tank, and the pollutant removal rate was increased 31.71% (TN) and 53.00% (total phosphorus), respectively. Meanwhile, the service life of TP-NZVI/PE in the aerobic tank was 66 days. The anti-oxidation and dispersion of NZVI was improved with the encapsulation of tea polyphenols and support of packing, and it also played a certain slow-release effect, so that the service life of NZVI was further prolonged in aerobic condition. Combined with the material balance analysis, the result showed that the environmental structure made diversity in the aerobic tank by added the TP-NZVI/PE, and the simultaneous nitrification and denitrification process was achieved. The dependence of the denitrification process on the carbon source was greatly reduced. Besides, it promoted the adsorption and chemical precipitation process of the system for phosphor pollutant and achieved the denitrifying phosphorus removal performance.

Study on the properties of denitrifying carbon sources from cellulose plants and their nitrogen removal mechanisms

Carbon sources of cellulose plants are promising materials that enhance the activities of denitrifying bacteria in the groundwater system. To further verify the denitrification performance of cellulose plants and the main factors of affecting the denitrifying system, six cellulose plants from agricultural wastes (wood chip, corn cob, rice husk, corn straw, wheat straw, and sugar cane) were selected for bioavailable organic matter leaching experiments, carbon denitrification experiments, functional bacteria identification, and analysis experiments. The results show that the extracts of cellulose plants contain a mixed carbon sources system including small molecular organic acids, sugars, nitrogen-containing organic components, and esters. The qPCR results showed that the denitrifying bacteria had obvious advantages compared to anaerobic ammonia-oxidizing bacteria during the stable period; the denitrification experiment showed that each of six cellulose plants removed more than 80% of nitrogen, and the denitrification rates reached 1.00-2.00 mg N cm^-3·d^-1. The supplement of cellulose plants promotes the metabolism rate of denitrifying bacteria, and the additional denitrifying bacteria have little effect on nitrate removal. In summary, the expected denitrification reaction occurred in the cellulose plant system, which is suitable as a carbon source material for water body nitrogen pollution remediation.

Geochemical stability of zero-valent iron modified raw wheat straw innovatively applicated to in situ permeable reactive barrier: N2 selectivity and long-term denitrification

The zero-valent iron (ZVI) modified wheat straw materials are widely used for treating groundwater by permeable reactive barrier (PRB). We report the performance of a field-scale PRB filled with ZVI modified wheat straw materials for nitrate (NO3-)-contaminated groundwater. In lab-scale PRB filled with ZVI modified wheat straw material, NO3- concentration entering the PRB was varied (27.80-59.86 mg L-1) according to the in situ NO3- contamination. A stable NO3- removal rate of 90% was achieved at a controlled hydraulic retention time of 22 days, together with a proportion of denitrifying bacteria up to 34.37%. The field-scale PRB filled with ZVI modified wheat straw material was successful at removing NO3- from groundwater (removal percentages ≥60%) at a groundwater flow rate of 0.01 m3 d-1. Monitoring of groundwater within this PRB provided evidences that the nitrogen gas (N2) selectivity increased with lower ammonia (NH4+) generated from ZVI reduction of NO3-, and few emission of NO2- present due to denitrification capacity in this PRB. The results are finally compared with the few others reported existing PRBs for nitrate-contaminated groundwater worldwide, and demonstrated that the ZVI modified wheat straw material would be an effective fillings for field PRB to remediate groundwater.

Modified solid carbon sources with nitrate adsorption capability combined with nZVI improve the denitrification performance of constructed wetlands

In this study, various modified agricultural wastes (modified canna leaves (MCL), modified rice straw (MRS) and modified peanut shells (MPS)) as solid carbon sources (SCSs) were used to remove nitrate in constructed wetlands (CWs). Then, modified SCSs combined with nZVI (SCSN) as co-electrons further enhanced both heterotrophic denitrification (HD) and autotrophic denitrification (AD) performance of CWs. The results showed that NO₃^--N removal efficiencies in CWs with SCSNs (75.3-91.1%) and in CWs with SCSs (63.3-65.5%) were significantly higher than that in CK-CW (47.0%). The presence of SCSs reduced the accumulation of NO₂^--N in CWs. Compared to the addition of SCSs, the addition of SCSNs decreased the effluent COD concentration in CWs, avoiding secondary pollution. In addition, the solid-phase denitrifiers Silanimonas and Thauera were enriched in MPS-CW. Thermomonas, an autotrophic denitrifying bacteria (ADB), and Azospira, a nitrate-reducing Fe (II) oxidation bacteria (NRFOB), exhibited high relative abundance in MPN-CW.

Sulfur and iron cycles promoted nitrogen and phosphorus removal in electrochemically assisted vertical flow constructed wetland treating wastewater treatment plant effluent with high S/N ratio

Phosphate (PO₄^3--P) and nitrate (NO₃^--N) in the effluent of wastewater treatment plants are the predominant sources of eutrophication. In this study, a bench-scale electrochemically assisted vertical flow constructed wetland (E-VFCW) was developed, which exhibited favorable PO₄^3--P (89.7-99.4%), NO₃^--N (82.7-99.6%), and TN (51.9-93.7%) removal efficiency in tertiary wastewater treatment. In addition, little N₂O accumulation (0.32-2.19% of △NO₃^--N) was observed. The study further elucidated that PO₄^3--P was removed mainly in the anode chamber by co-precipitation (Fe⁽ⁿ⁺⁾OH-PO₄) and adsorption (FeOOH-PO₄) pathways. Multi-pathway of NO₃^--N reduction was proposed, with 13.9-30.2% of NO₃^--N predominantly eliminated in the anode chamber by ferrous-dependent NO₃^--N reduction bacteria. In the cathode chamber, electrons storage and resupply modes during S cycle exerted crucial roles in NO₃^--N reduction, which enhanced the resilience capabilities of the E-VFCW to shock loadings. Stoichiometric analysis revealed that 3.3-6.6 mmol e^-/cycle were stored in the form of S⁰, FeS, and FeS₂ in the E-VFCW under longer HRT or higher current density. However, the deposited S resupplied 19.6% and 28.3% of electrons for NO₃^--N reduction under shorter HRT (1 h) or lower current density (0.01 mA cm^-2). Moreover, ferrous-driven NO₃^--N-reducing or DNRA bacteria also promoted NO₃^--N elimination in the cathode chamber. These findings provide new insight into the coupling interactions among S, Fe and H cycles, as well as N and P transformations in electrochemically assisted NO₃^--N reduction systems.

Nitrate removal by combined heterotrophic and autotrophic denitrification processes: Impact of coexistent ions

In this current study, sawdust and zero-valent iron (Fe⁰) were used as co-electron donors to evaluate the effects of coexistent ions on the combined heterotrophic and autotrophic denitrification (HAD) processes. The results showed that HCO₃^- and SO₄^2- drastically enhanced nitrate removal. The promotion effect derived from both biological and chemical process by HCO₃^- and chemical process by SO₄^2-. However, Ca²⁺ ions would remarkably increase nitrate removal due to promoting the electron transfer and the metabolic activities of bacteria, whereas the Cu²⁺ ions inhibited the biological process due to the deleterious effect on bacteria. Meanwhile, Fe²⁺ and Fe³⁺ ions exhibited inhibition effect firstly because of their toxicity to bacteria and promotion subsequently due to their enhancement on Fe⁰ chemical denitrification. Moreover, byproducts such as nitrite, ammonium, dissolved organic carbon (DOC), etc. were also influenced by common ions.

Sustaining reactivity of Fe(0) for nitrate reduction via electron transfer between dissolved Fe(2+) and surface iron oxides

The mechanism of the effects of Fe(2+)(aq) on the reduction of NO3(-) by Fe(0) was investigated. The effects of initial pH on the rate of NO3(-) reduction and the Fe(0) surface characteristics revealed Fe(2+)(aq) and the characteristics of minerals on the surface of Fe(0) played an important role in NO3(-) reduction. Both NO3(-) reduction and the decrease of Fe(2+)(aq) exhibited similar kinetics and were promoted by each other. This promotion was associated with the types of the surface iron oxides of Fe(0). Additionally, further reduction of NO3(-) produced more surface iron oxides, supplying more active sites for Fe(2+)(aq), resulting in more electron transfer between Fe(2+) and surface iron oxides and a higher reaction rate. Using the isotope specificity of (57)Fe Mossbauer spectroscopy, it was verified that the Fe(2+)(aq) was continuously converted into Fe(3+) oxides on the surface of Fe(0) and then converted into Fe3O4 via electron transfer between Fe(2+) and the pre-existing surface Fe(3+) oxides. Electrochemistry measurements confirmed that the spontaneous electron transfer between the Fe(2+) and structural Fe(3+) species accelerated the interfacial electron transfer between the Fe species and NO3(-). This study provides a new insight into the interaction between Fe species and contaminants and interface electron transfer.