Ion-exchange membrane bioelectrochemical reactor for removal of nitrate in the biological effluent from a coking wastewater treatment plant

Enhanced •Cl generation by introducing electrophilic Cu(II) in Co3O4 anode for efficient total nitrogen removal with hydrogen recovery in urine treatment

Urine is a nitrogen-containing waste, but can be used as an attractive alternative substrate for H2 recovery. However, conventional urea oxidation reaction is subject to complex six-electron transfer kinetics and requires alkaline conditions. Herein, an efficient method of enhancing •Cl generation by introducing electrophilic Cu(II) into Co3O4 nanowires anode was proposed, which realized the highly efficient TN removal and H2 production in urine treatment under neutral conditions. The key mechanism is that the electrophilic effect of Cu(II) attracts electrons from the oxygen atom, which causes the oxygen atom to further attract electrons from Co(II), reducing the charge density of Co(II). Electrophilic Cu(II) accelerates the difficult conversion step of Co(II) to Co(III), which enhances the generation of •Cl. The generated •Cl efficiently converts urea to N2, while the electron transport promotes H2 production on the CuO@CF nanowires cathode. Results showed that the steady-state concentration of •Cl was increased to about 1.5 times by the Cu(II) introduction. TN removal and H2 production reached 94.7% and 642.1 μmol after 50 min, which was 1.6 times and 1.5 times that of Co3O4 system, respectively. It was also 2.3 times and 2.1 times of RuO2, and 3.3 times and 2.5 times of Pt, respectively. Moreover, TN removal was 11.0 times higher than that of without •Cl mediation, and H2 production was 4.3 times higher. More importantly, excellent TN removal and H2 production were also observed in the actual urine treatment. This work provides a practical possibility for efficient total nitrogen removal and hydrogen recovery in urine wastewater treatment.

Boosting photocatalytic reduction of nitrate to ammonia enabled by perovskite/biochar nanocomposites with oxygen defects and O-containing functional groups

Photocatalytic ammonia synthesis from waste nitrate has emerged as a promising strategy in water treatment; however, the conversion and selectivity still remain a great challenge. Herein, recyclable magnetic perovskite (LaFeO₃)/biochar nanocomposites were successfully synthesized by the co-pyrolysis of the lotus biomass and Fe/La salts without extra organic complexants. Results showed that the lotus interacted with the iron ions (Fe³⁺) and the lanthanum ions (La³⁺) changing the surface and structural characteristics of catalysts. Oxygen defects of LaFeO₃ were enhanced due to biomass introduction, which accelerated the separation of electron-hole pairs. On the other hand, Fe/La salts participated in the modification process of the biochar surface during the carbonization, which promoted the exposure of oxygen-containing functional groups and aromatic structures facilitating the nitrate adsorption. Notably, the redox-active quinone/phenol groups on the biochar surface contributed to the photogenerated electrons exchange favoring the ammonium ion (NH₄⁺) selectivity as direct electron donor. Nitrate conversion reached 98% and ammonia selectivity reached 97% over the LaFeO₃/biochar photocatalyst under visible light irradiation, when the mass ratio of lotus and Fe/La salts was optimized. Our findings may potentially provide a green and cost-effective way for ammonia recovery from nitrate contaminants.

Three-dimensional biofilm electrode reactors (3D-BERs) for wastewater treatment

Three-dimensional biofilm electrode reactors (3D-BERs) are highly efficient in refractory wastewater treatment. In comparison to conventional bio-electrochemical systems, the filled particle electrodes act as both electrodes and microbial carriers in 3D-BERs. This article reviews the conception and basic mechanisms of 3D-BERs, as well as their current development. The advantages of 3D-BERs are illustrated with an emphasis on the synergy of electricity and microorganisms. Electrode materials utilized in 3D-BERs are systematically summarized, especially the critical particle electrodes. The configurations of 3D-BERs and their integration with wastewater treatment reactors are introduced. Operational parameters and the adaptation of 3D-BERs to varieties of wastewater are discussed. The prospects and challenges of 3D-BERs for wastewater treatment are then presented, and the future research directions are proposed. We believe that this timely review will help to attract more attentions on 3D-BERs investigation, thus promoting the potential application of 3D-BERs in wastewater treatment.

Efficient Hydrogen Generation and Total Nitrogen Removal for Urine Treatment in a Neutral Solution Based on a Self-Driving Nano Photoelectrocatalytic System

Urine is the main source of nitrogen pollution, while urea is a hydrogen-enriched carrier that has been ignored. Decomposition of urea to H2 and N2 is of great significance. Unfortunately, direct urea oxidation suffers from sluggish kinetics, and needs strong alkaline condition. Herein, we developed a self-driving nano photoelectrocatalytic (PEC) system to efficiently produce hydrogen and remove total nitrogen (TN) for urine treatment under neutral pH conditions. TiO2/WO3 nanosheets were used as photoanode to generate chlorine radicals (Cl•) to convert urea-nitrogen to N2, which can promote hydrogen generation, due to the kinetic advantage of Cl-/Cl• cyclic catalysis. Copper nanowire electrodes (Cu NWs/CF) were employed as the cathode to produce hydrogen and simultaneously eliminate the over-oxidized nitrate-nitrogen. The self-driving was achieved based on a self-bias photoanode, consisting of confronted TiO2/WO3 nanosheets and a rear Si photovoltaic cell (Si PVC). The experiment results showed that hydrogen generation with Cl• is 2.03 times higher than in urine treatment without Cl•, generating hydrogen at 66.71 μmol h-1. At the same time, this system achieved a decomposition rate of 98.33% for urea in 2 h, with a reaction rate constant of 0.0359 min-1. The removal rate of total nitrogen and total organic carbon (TOC) reached 75.3% and 48.4% in 2 h, respectively. This study proposes an efficient and potential urine treatment and energy recovery method in neutral solution.

Electric conversion treatment of cobalt-containing wastewater

Long-term accumulation of cobalt-containing wastewater may also pollute groundwater and cause a large amount of loss of valuable metals. Therefore, the comprehensive utilization of cobalt-containing wastewater must be realized, especially as cobalt itself is a very important strategic resource. This paper proposes a membrane electroconversion method to separate cobalt ions from cobalt-containing wastewater and prepare cobalt hydroxide. In addition, the electrolysis process was optimized, and single-factor experiments such as the initial concentration, cobalt ions, current density, temperature etc., and economic calculations such as current efficiency were explored. The electrolysis product was calcined as the precursor to obtain the oxide Co₃O₄, and the calcination experiment was also optimized. In this concentration range, more than 90% of cobalt can be recovered within 2 h.

Efficacy of inorganic nitrogen removal by a salt-tolerant aerobic denitrifying bacterium, Pseudomonas sihuiensis LK-618

An aerobic denitrifying bacterium, stain LK-618, was isolated from lake sediment surface and the efficacy of inorganic nitrogen removal was tested. Stain LK-618 identified as Pseudomonas sihuiensis by 16S rRNA sequencing analysis. Trisodium citrate was found to be the ideal carbon source for this strain. When an initial nitrogen sources of approximately 50 mg/L nitrate, ammonium, or nitrite was solely selected as the nitrogen source, the nitrogen removal efficiencies were 91.4% (3.86 mg/L/h), 95.07% (2.47 mg/L/h) and 97.7% (2.41 mg/L/h), respectively. Nitrogen balance analysis revealed that 55.12% NO₃^--N was removed as N₂. Response surface methodology (RSM) analysis demonstrated that the optimal Total Nitrogen (TN) removal ratio for strain LK-618 was under C/N ratio of 12.63, shaking speed of 52.06 rpm, temperature of 28.5 °C and pH of 6.86. In addition, strain LK-618 could tolerate NaCl concentrations up to 20 g/L, and its most efficient denitrification capacity was presented at NaCl concentrations of 0-10 g/L. Therefore, strain LK-618 has potential application on the removal of inorganic nitrogen from saline wastewater under aerobic conditions.

Isolation and characterization of a salt-tolerant denitrifying bacterium Alishewanella sp. F2 from seawall muddy water

A salt-tolerant denitrifying bacterium strain F2 was isolated from seawall muddy water in Dalian City, Liaoning Province, China. Strain F2 was identified by morphological observations, physiological and biochemical characteristics and 16 S rDNA identification. The salt tolerance of strain F2 was verified and the factors affecting the removal ability of strain F2 to nitrous nitrogen (NO2-N) and nitrate nitrogen (NO3-N) in saline conditions were investigated. Strain F2 was identified as Alishewanella sp., named Alishewanella sp. F2. Strain F2 can tolerate NaCl concentrations up to 70 g/L, and its most efficient denitrification capacity was observed at NaCl concentrations of 0-30 g/L. In the medium with NaCl concentrations of 0-30 g/L, strain F2 exhibited high removal efficiencies of NO2-N and NO3-N, with the removal percentages for both NO2-N and NO3-N of approximately 99%. In saline conditions with 30 g/L NaCl, the optimum culture pH, NaNO2 initial concentrations and inoculation sizes of strain F2 were 8-10, 0.4-0.8 g/L and 5-7%, respectively. Strain F2 was highly effective in removing NO2-N and NO3-N in saline conditions, and it has a good application potential in saline wastewater treatment.

Highly active and durable carbon electrocatalyst for nitrate reduction reaction

The development of a new class of carbon electrocatalysts for nitrate reduction reaction (NRR) that have high activity and durability is extremely important, as currently reported metallic electrocatalysts show a main drawback of low stability owing to leaching and oxidation. Herein, we demonstrate that a unique N-doped graphitic carbon-encapsulated iron nanoparticles can be utilized as a promising NRR electrocatalyst. The resulting Fe(20%)@N-C achieves a better nitrate removal proportion of 83.0% (attained in the first running cycle) compared to the efficiencies of other reference catalysts, including those with lower entrapped Fe content. The nitrogen selectivity is 25.0% in the absence of Cl^- and increases to 100% when supplemented with 1.0 g L^-1 NaCl. More importantly, there is no statistically significant difference (at a 95% confidence interval) regarding the removal percentage recorded over 20 cycles for the Fe(20%)@N-C cathode. We propose that the iron nanoparticles could attenuate the work function on the neighboring carbon atoms, which are the reactive sites for NRR, and that the graphitic shells hinder the access of the electrolyte, thus protecting the iron particles from dissolution and oxidation. Testing with the real industrial wastewater further demonstrates the superiority of Fe(20%)@N-C cathode towards NRR, as evidenced by efficient removal of nitrate available in the biological effluent from a local coking wastewater treatment plant.

Efficient purification and chemical energy recovery from urine by using a denitrifying fuel cell

Urine is a major biomass resource, and its excessive discharge would lead to severe aquatic nitrogen pollution and even eutrophication. In this study, we designed an innovative denitrifying fuel cell (DFC) under illumination to purify urine and convert its chemical energy into electricity. The central ideas include the following: 1) on the anode, chlorine radicals (Cl) and hydroxyl (HO) radicals were induced to react with amine or ammonia in urine into N₂, and to mineralize organics into CO₂, respectively; 2) on the cathode, NO₂^- or NO₃^- generated in the cell was selectively reduced to N₂ and tiny NH₄⁺ by Pd/Au/NF; 3) NH₄⁺ was further oxidized to N₂ by Cl according to process 1), then the total nitrogen (TN) was ultimately removed by a continuous redox loop between anode and cathode; 4) the separation and migration of charges were strengthened by a self-bias poly-Si/WO₃ photoanode. Result indicated that the DFC showed an efficient yield of electricity and almost completely N-removing properties: power density of 2.24 mW cm^-2, total nitrogen and total organic carbon (TOC) removal efficiency, respectively 99.02% and 50.76% for artificial urine; and power density of 2.51 mW cm^-2, TN and TOC removal efficiency, respectively 98.60% and 54.55% for actual urine. The study proposes a potential and environment-friendly approach by using novel DFC to purify urine and generate electricity.

Microalgae cultivation for phenolic compounds removal

Microalgae are promising sustainable and renewable sources of oils that can be used for biodiesel production. In addition, they contain important compounds, such as proteins and pigments, which have large applications in the food and pharmaceutical industries. Combining the production of these valuable products with wastewater treatment renders the cultivation of microalgae very attractive and economically feasible. This review paper presents and discusses the current applications of microalgae cultivation for wastewater treatment, particularly for the removal of phenolic compounds. The effects of cultivation conditions on the rate of contaminants removal and biomass productivity, as well as the chemical composition of microalgae cells are also discussed.

Microbial polychlorinated biphenyl dechlorination in sediments by electrical stimulation: The effect of adding acetate and nonionic surfactant

The necessity for developing an efficient and cost-effective in situ bioremediation technology for sediments contaminated with polychlorinated biphenyls (PCBs) has prompted the application of low-voltage electrical fields to anaerobic digestion systems. Here we show that the use of a sediment-based bio-electrochemical reactor (BER) poised at a potential of -0.50V (vs. a standard calomel electrode, SCE) substantially enhanced the reduction of 2,3,4,5-tetrachlorobiphenyl (PCB 61) when acetate was added as a carbon source. The addition of surfactant Tween 80 to the BER further accelerated the PCB 61 transformation. The comparative study of closed- and open-circuit reactors demonstrated the enrichment conditions affecting the bacterial community structure, the dominant dechlorination metabolisms, and thus the extent, the rate and the products of the reduction of PCBs. The dominant bacterial dechlorinators detected in the BERs in the presence of acetate and Tween 80 are Dehalogenimonas, Dehalobacter, Sulfuricurvum, Dechloromonas and Geobacter, which should be responsible for PCB dechlorination. This study improves understanding of the key factors influencing dechlorination activity in sediment-based BERs polarized at a low potential, as well as the metabolic mechanisms dominating in the PCB dechlorination process.

Fabrication and electrolyte characterization of uniaxially-aligned anion conductive polymer nanofibers

We report the anion transport properties of anion conductive polymer nanofibers fabricated using an electrospinning method. The aligned nanofibers were prepared to evaluate the anion conductivity of the nanofibers. The aligned nanofibers had 10-15 times higher conductivity (up to 160 mS cm^-1 at 90 °C and 95%RH) and lower activation energy (23-25 kJ mol^-1) than the corresponding membranes, even though the nanofibers showed lower water uptake than the corresponding membranes. The anion conductivity measurement of nanofibers with different IEC values and anion species revealed that the dependency of anion conductivity on these factors was smaller in the nanofibers than in the corresponding membranes. These results indicate that effective ion transport pathways were formed in the nanofibers due to the phase separation and the polymer chain orientation along the nanofiber axis. These nanofibers are expected to be useful for future applications in alkaline fuel cells, air batteries, and other energy- and environment-related devices regardless of the ion species.

Effective biodegradation of nitrate, Cr(VI) and p-fluoronitrobenzene by a novel three dimensional bioelectrochemical system

p-Fluoronitrobenzene (p-FNB) was degraded in a novel three dimensional bioelectrochemical system (3D BES) and potentially utilized as carbon source for achieving both nitrate (NO3(-)−N) and Cr(VI) reduction. For single NO3(-)−N and Cr(VI) removal, 200 mg L(-1) NO3(-)−N and 100 mg L(-1) Cr(VI) could be almost completely converted to N2 and Cr(III) at current 50 mA. For single p-FNB degradation, 100 mg L(-1) p-FNB was completely degraded at current 50 mA. The critical current for defluorination was 40 mA, and the intermediate product p-fluoroaniline (p-FA) tended to decrease when current was higher than 40 mA. When NO3(-)−N, Cr(VI), and p-FNB were both coexisted in this system, the average NO3(-)−N, Cr(VI), and p-FNB removal efficiencies slightly decreased with addition of carbon source. Without carbon source, NO3(-)−N and Cr(VI) removal rates reached 34.45% and 41.38% with 91.02% p-FNB degradation, proving that NO3(-)−N and Cr(VI) could be reduced by degrading p-FNB in the BES.

Simultaneous nitrification and denitrification using a polypyrrole/microbial cellulose electrode in a membraneless bio-electrochemical system

In this study, the feasibility of ammonium and total nitrogen removal from aqueous solution using a simultaneous nitrification and denitrification process was studied in a membraneless bio-electrochemical system with a novel electrode.