Decoration of boron nanoparticles on a graphene sheet for ammonia production from nitrate

Clean water and sanitation are two of the most important challenges worldwide and the main source for freshwater is groundwater. Nowadays, water is polluted by human activities. Concern about the presence of nitrates (NO₃^-) in groundwater is increasing day-by-day due to the intensive use of fertilizers and other anthropogenic sources, such as sewage or industrial wastewater discharge. Thus, the main solution available is to remove NO₃^- from groundwater and transfer it back to a usable nitrogen source. Electrochemical reduction of NO₃^- to ammonia (NH₃) under ambient conditions is a highly desirable method and it needs an efficient electrocatalyst. In this work, we synthesized a composite of amorphous boron with graphene oxide (B@GO) as an efficient catalyst for the nitrate reduction reaction. XRD and TEM analysis revealed an amorphous boron decoration on the graphene oxide sheet, and XPS confirmed that no bonding between boron and carbon occurs. In B@GO, a stronger defect carbon peak was observed than in GO and there was a random distribution of boron particles on the surface of the graphene nanosheets. Amorphous boron exhibits a higher bond energy, more reactivity, and chemical activity toward nitrate ions, which could be due to the lone pair present in the B atoms and could also be due to the edge oxidized B atoms. B@GO has a high number of active sites exposed leading to excellent nitrate reduction performance with a faradaic efficiency of 61.88% and good ammonia formation rate of 40006 μg h^-1 m_cat^-1 at -0.8 V versus reversible hydrogen electrode.

Simultaneous denitrification and antibiotic degradation of low-C/N-ratio wastewater by a three-dimensional biofilm-electrode reactor: Performance and microbial response

Three-dimensional biofilm-electrode reactors (3D-BERs) were fabricated and used to simultaneously remove nitrate and metronidazole (MNZ) from low-C/N-ratio wastewater. The results showed that 1 mg/L MNZ significantly promoted nitrate removal. After MNZ was added to the reactor, the removal efficiencies of total nitrogen (TN) and NO₃^--N increased significantly from 18.97% and 52.09% to 71.63% and 99.98% within 6 h, respectively. The MNZ-removal kinetics conformed to a pseudo-first-order model, and the removal rate constant reached a maximum value of 0.853 h^-1, which was 4.1 and 2.8 times higher than that of pure microorganisms and pure electrochemical reactors, respectively. This indicated that the 3D-BERs constructed in this study were capable of simultaneous MNZ degradation and denitrification. In the presence of nitrate, six MNZ-degradation intermediates were identified, and four MNZ transformation pathways were proposed, including cleavage of hydroxyethyl groups, reduction of nitro groups, N-denitration, and deprotonation of side-chain hydroxyl groups. High-throughput sequencing revealed that the reactor was rich in various MNZ-degraders and denitrifiers, such as Hydrogenophaga, Methylomonas, Crenohrix, Dechloromonas, and Methylophilus. A function prediction analysis of nitrogen metabolism showed that the 3D-BER reactor with MNZ had higher denitrification activity than the other reactors tested. It was speculated that the intermediates produced by MNZ could act as carbon sources allowing denitrifying bacteria to perform denitrification, which made a nonnegligible contribution to the removal of nitrogen.

Impact of Periodic Polarization on Groundwater Denitrification in Bioelectrochemical Systems

Nitrate contamination is a common problem in groundwater around the world. Nitrate can be cathodically reduced in bioelectrochemical systems using autotrophic denitrifiers with low energy investment and without chemical addition. Successful denitrification was demonstrated in previous studies in both microbial fuel cells and microbial electrolysis cells (MECs) with continuous current flow, whereas the impact of intermittent current supply (e.g., in a fluidized-bed system) on denitrification and particularly the electron-storing capacity of the denitrifying electroactive biofilms (EABs) on the cathodes have not been studied in depth. In this study, two continuously fed MECs were operated in parallel under continuous and periodic polarization modes over 280 days, respectively. Under continuous polarization, the maximum denitrification rate reached 233 g NO₃^--N/m³/d with 98% nitrate removal (0.6 mg NO₃^--N/L in the effluent) with negligible intermediate production, while under a 30 s open-circuit/30 s polarization mode, 86% of nitrate was removed at a maximum rate of 205 g NO₃^--N/m³/d (4.5 mg NO₃^--N/L in the effluent) with higher N₂O production (6.6-9.3 mg N/L in the effluent). Conversely, periodic polarization could be an interesting approach in other bioelectrochemical processes if the generation of chemical intermediates (partially reduced or oxidized) should be favored. Similar microbial communities dominated byGallionellaceaewere found in both MECs; however, swapping the polarization modes and the electrochemical analyses suggested that the periodically polarized EABs probably developed a higher ability for electron storage and transfer, which supported the direct electron transfer pathway in discontinuous operation or fluidized biocathodes.

N-Doped Graphene as an Efficient Metal-Free Electrocatalyst for Indirect Nitrate Reduction Reaction

N-doped graphene samples with different N species contents were prepared by a two-step synthesis method and evaluated as electrocatalysts for the nitrate reduction reaction (NORR) for the first time. In an acidic solution with a saturated calomel electrode as reference, the pyridinic-N dominant sample (NGR2) had an onset of 0.932 V and a half-wave potential of 0.833 V, showing the superior activity towards the NORR compared to the pyrrolic-N dominant N-doped graphene (onset potential: 0.850 V, half-wave potential: 0.732 V) and the pure graphene (onset potential: 0.698 V, half-wave potential: 0.506 V). N doping could significantly boost the NORR performance of N-doped graphene, especially the contribution of pyridinic-N. Density functional theory calculation revealed the pyridinic-N facilitated the desorption of NO, which was kinetically involved in the process of the NORR. The findings of this work would be valuable for the development of metal-free NORR electrocatalysts.

Wheat straw decomposition stage has little effect on the removal of inorganic N and P from wastewater leached through sand-straw mixes

Wheat straw amendment to sandy soil can remove nitrogen (N) and phosphorus (P) from wastewater but it is unclear whether prior decomposition affects removal. Sand mixed with finely ground wheat straw at 12.5 g straw kg^-1 was placed in leaching columns. Wastewater was added either immediately after mixing with straw (fresh straw) or after the sand-straw mix had been incubated moist for 7 or 14 days (7D or 14D straw). Sand alone was considered as control. Leaching was carried out 4, 8 or 16 days after addition of wastewater and inorganic N and P were analysed after leaching in both leachate and sand. In the amended treatments, nitrate and available P in the sand-straw mix were not detectable throughout the experiment. On day 16, inorganic N in the sand-straw mix was highest in fresh straw where it was three-fold higher than in 14D straw and 30% higher than in sand alone and 7D straw on day 16. Straw decomposition stage had no consistent effect on microbial biomass N and P. Released CO₂ was lower in 14D straw than in fresh straw and 7D straw. With straw amendment, > 95% of inorganic N added with wastewater was removed compared to 40-50% with sand alone. Inorganic P leaching was reduced by about 30% compared to sand alone on day 16. In conclusion, wheat straw addition reduced leaching of N compared to sand alone, but the decomposition stage of the straw had little effect on the removal of N and P from wastewater.

Shift of bacterial community and denitrification functional genes in biofilm electrode reactor in response to high salinity

High salinity suppresses denitrification by inhibiting microorganism activities. The shift of microbial community and denitrification functional genes under salinity gradient was systematically investigated in a biofilm electrode reactor (BER) and biofilm reactor (BR) systems. Denitrification efficiency of both BER and BR was not significantly inhibited during the period of low salinity (0-2.0%). As the salinity increased to 2.5%, BER could overcome the impact of high salinity and maintained a relatively stable denitrification performance, and the effluent NO₃^--N was lower than 1.5 mg/L. High salinity (>2.5%) impoverished microbial diversity and altered the microbial community in both BER and BR. However, two genera Methylophaga and Methyloexplanations were enriched in BER due to electrochemical stimulation, which can tolerate high salinity (>3.0%). The relative abundance of Methylophaga in BER was almost 10 times as much as in BR. Paracoccus is a hydrogen autotrophic denitrifier, which was obviously inhibited with 1.0% NaCl. The hetertrophic denitrifiers were primarily responsible for the nitrate removal in the BER compared to the autotrophic denitrifiers. The abundance and proportion of denitrifying functional genes confirmed that main denitrifiers shift to salt-tolerant species (nirK-type denitrifiers) to reduce the toxic effects. The napA (2.2 × 10⁸ to 6.5 × 10⁸ copies/g biofilm) and nosZ (2.2 × 10⁷ to 4.4 × 10⁷ copies/g biofilm) genes were more abundant in BER compared to BR's, which was attributed to the enrichment of Methylophaga alcalica and Methyloversatilis universalis FAM5 in the BER. The results proved that BER had greater denitrification potential under high salinity (>2.0%) stress at the molecular level.

Nutrient conversion and recovery from wastewater using electroactive bacteria

Wastewater is widely recognized as a sink of active nitrogen and phosphorus, and the recovery of both nutrients as fertilizers is widely studied in recent years. Electroactive bacteria increasingly attract attentions in this area because they are able to produce an electric field in microbial electrochemical systems to concentrate ammonium and phosphate for recovery. Importantly, these unique bacteria are able to convert nitrate and nitrite directly to ammonium, maximizing the active nitrogen species capable of recovery. Ferric ions produced by electroactive bacteria can be precipitated with phosphate to recover as vivianite in neutral wastewaters. All these processes employed electroactive bacteria as both nitrate and iron reducer and bioelectric field generator. The mechanism as well as technologies are summarized, and the challenges to further improve their performance are discussed.

Continuous flow, large-scale, microbial fuel cell system for the sustained treatment of swine waste

Microbial fuel cells (MFCs) have long held the promise of being a cost-effective technology for the energy-neutral treatment of wastewater. However, successful pilot-scale demonstrations for this technology are still limited to very few. Here, we present a large-scale MFC system, composed of 12 MFCs with a total volume of 110 L, successfully treating swine wastewater at a small educational farm. The system was operated for over 200 days in continuous mode with hydraulic residence time of 4 hr. Very stable electrochemical and waste treatment performance was observed with up to 65% of chemical oxygen demand (COD) removed and a maximum treatment rate of 5.0 kg COD/m³ .day. Robust microbial enrichment was performed and adapted to metabolize and transform a diversity of compounds present. The Net Energy Recovery (NER = 0.11 kWhr/kg COD) is not only competitive with conventional cogeneration processes, but is in fact sufficient to sustain the operational energy requirements of the system. PRACTITIONER POINTS: This study demonstrates the design and operation of a large-scale microbial fuel cells (MFC) system for continuous treatment of swine wastewater. The system achieved a high chemical oxygen demand removal rate within a short hydraulic residence time. This study moves one-step closer to applying MFC technology for real wastewater treatment.

Bioelectrochemical treatment and recovery of copper from distillery waste effluents using power and voltage control strategies

Copper recovery from distillery effluent was studied in a scalable bioelectro-chemical system with approx. 6.8 L total volume. Two control strategies based on the control of power with maximum power point tracking (MPPT) and the application of 0.5 V using an external power supply were used to investigate the resultant modified electroplating characteristics. The reactor system was constructed from two electrically separated, but hydraulically connected cells, to which the MPPT and 0.5 V control strategies were applied. Three experiments were carried out using a relatively high copper concentration i.e. 1000 mg/L followed by a lower concentration i.e. 50 mg/L, with operational run times defined to meet the treatment requirements for distillery effluents considered. Real distillery waste was introduced into the cathode to reduce ionic copper concentrations. This waste was then recirculated to the anode as a feed stock after the copper depletion step, in order to test the bioenergy self-sustainability of the system. Approx. 60-95% copper was recovered in the form of deposits depending on starting concentration. However, the recovery was low when the anode was supplied with copper depleted distillery waste. Through process control (MPPT or 0.5 V applied voltage) the amount and form of the copper recovered could be manipulated.

Nitrogen and phosphorus removal from wastewater by sand with wheat straw

Wheat straw amendment to sandy soil has the potential to remove nutrients from wastewater. This study investigated the ability of wheat straw to remove inorganic nitrogen (N) and phosphorus (P) from wastewater when mixed into sand at different rates. Wastewater from a sewage treatment plant was added to sand alone and amended with different wheat straw rates 2.5, 5, 7.5, 10, and 12.5 g wheat straw kg^-1 so that the sand was covered with about 15 cm of wastewater. Leaching was carried out after 4, 8, and 16 days and inorganic N and P were analysed after leaching in both the leachate and sand, as well as N₂O and CO₂ release. In the amended sand, nitrate was about fourfold lower throughout the experiment compared to sand alone. Ammonium was twofold higher than sand alone at 12.5 g straw kg^-1 throughout the experiment and on day 16 also at ≥ 5 g straw kg^-1. Leachate inorganic N concentration was up to 70-fold higher in sand alone than in amended soils irrespective of straw rate. On day 16, P leaching was about threefold lower and P retention was 40% higher in all amended treatments than sand alone. The redox potential in sand alone was higher than with straw amendments. With straw amendment, the release of CO₂ per day was six times higher than with sand alone and increased with straw rates, but very little N₂O and CH₄ was released throughout the experiment. It can be concluded that amendment of sand with wheat straw can remove large proportions of inorganic N and P from wastewater, even at low straw rates. Likely mechanisms for retention are dissimilatory nitrate reduction and subsequent binding of ammonium to straw for N, and binding to the straw and microbial uptake for P.

Enhancing the Electricity Generation and Nitrate Removal of Microbial Fuel Cells With a Novel Denitrifying Exoelectrogenic Strain EB-1

Microbial fuel cells (MFCs) have been tentatively applied for wastewater treatment, but the presence of nitrogen, especially nitrate, induces performance instability by changing the composition of functional biofilms. A novel denitrifying exoelectrogenic strain EB-1, capable of simultaneous denitrification and electricity generation and affiliated with Mycobacterium sp., was isolated from the anodic biofilm of MFCs fed with nitrate containing medium. Polarization curves and cyclic voltammetry showed that strain EB-1 could generate electricity through a direct electron transfer mechanism with a maximum power density of 0.84 ± 0.05 W m⁻². Additionally, anodic denitrification, as a concurrent metabolism, was demonstrated with an efficient removal rate of 0.66 ± 0.01 kg N m⁻³ d⁻¹ at a COD/N ratio of 3.5 ± 0.3. Importantly, voltage output was not negatively influenced by nitrate, indicating that the concurrent process of nitrate removal and electricity generation was a limitation of the electron donor rather than an inhibition of the system. Furthermore, various organic materials were successfully utilized as anode donors for strain EB-1, and demonstrated the exciting performances in terms of simultaneous denitrification and electricity generation. Mycobacterium sp. EB-1 thus expands the diversity of exoelectrogens and contributes to the potential applications of MFC for simultaneous energy recovery and wastewater treatment.

Bacterial community and nitrate removal by simultaneous heterotrophic and autotrophic denitrification in a bioelectrochemically-assisted constructed wetland

To enhance nitrate removal in constructed wetlands (CWs), a bioelectrochemically-assisted CW (BECW) integrating a three-dimensional biofilm-electrode reactor (3D-BER) into the CW was evaluated for the effectiveness of combined autotrophic and heterotrophic denitrification in the presence of organic matter and applied current. The effects of COD/N ratios on nitrate removal were investigated, and the bacterial communities in the granular active carbon (GAC) and graphite felt (GF) in the reactor's cathode region were compared. The highest NO₃^--N and TN removal efficiencies of 91.3±7.2% and 68.8±7.9% were obtained at the COD/N ratio of 5. According to the results of high-throughput sequencing analysis, sample GAC was enriched with a high abundance of Pseudomonas (17.29%) capable of autotrophic and heterotrophic denitrification, whereas autotrophic bacteria Thiobacillus (43.94%) was predominant in sample GF. The synergy between heterotrophic and autotrophic denitrification bacteria is believed to cause the high and stable nitrogen removal performance.

Bioelectrochemical nitrogen removal as a polishing mechanism for domestic wastewater treated effluents

Addition of an external carbon source is usually necessary to guarantee a sufficiently high C/N ratio and enable denitrification in wastewater treatment plants (WWTPs). Alternatively, denitrification processes using autotrophic microorganisms have been proposed i.e., with the use of H₂ as electron donor or with the use of cathodic denitrification in bioelectrochemical systems (BES), in which electrons are transferred directly to a denitrifying biofilm. The aim of this work was to investigate and demonstrate the feasibility of applying an easy-to-operate BES as a polishing mechanism for treated secondary clarified effluent from a municipal WWTP, containing low levels of organic matter, buffer capacity and low concentrations of remaining nitrate. In the proposed system, nitrogen removal rates (0.018-0.121 Kg N m^-3 d^-1) increased with the nitrogen loading rates, suggesting that biofilm kinetics were not rate limiting. The lowest energy consumption for denitrification was 12.7 kWh Kg N^-1, equivalent to 0.021 kWh m^-3 and could be further reduced by 14% by adding recirculation circuits within both the anode and cathode.

Opportunities for groundwater microbial electro-remediation

Groundwater pollution is a serious worldwide concern. Aromatic compounds, chlorinated hydrocarbons, metals and nutrients among others can be widely found in different aquifers all over the world. However, there is a lack of sustainable technologies able to treat these kinds of compounds. Microbial electro‐remediation, by the means of microbial electrochemical technologies (MET), can become a promising alternative in the near future. MET can be applied for groundwater treatment in situ or ex situ, as well as for monitoring the chemical state or the microbiological activity. This document reviews the current knowledge achieved on microbial electro‐remediation of groundwater and its applications.

Bioelectrochemical denitrification on biocathode buried in simulated aquifer saturated with nitrate-contaminated groundwater

Nitrate contamination in aquifers has posed human health under high risk because people still rely on groundwater withdrawn from aquifers as drinking water and running water sources. These days, bioelectrochemical technologies have shown a great number of benefits for nitrate remediation via autotrophic denitrification in groundwater. This study tested the working possibility of a denitrifying biocathode when installed into a simulated aquifer. The reactors were filled with sand and synthetic groundwater at various ratios (10, 50, and 100 %) to clarify the effect of various biocathode states (not-buried, half-buried, and fully buried) on nitrate reduction rate and microbial communities. Decreases in specific nitrate reduction rates were found to be correlated with increases in sand/medium ratios. A specific nitrate reduction rate of 322.6 mg m(-2) day(-1) was obtained when the biocathode was fully buried in an aquifer. Microbial community analysis revealed slight differences in the microbial communities of biocathodes at various sand/medium ratios. Various coccus- and rod-shaped bacteria were found to contribute to bioelectrochemical denitrification including Thiobacillus spp. and Paracoccus spp. This study demonstrated that the denitrifying biocathode could work effectively in a saturated aquifer and confirmed the feasibility of in situ application of microbial electrochemical denitrification technology.