Urea degradation by electrochemically generated reactive chlorine species: products and reaction pathways

Trace Br- Inhibits Halogenated Byproduct Formation in Saline Wastewater Electrochemical Treatment

The electrochemical technology provides a practical and viable solution to the global water scarcity issue, but it has an inherent challenge of generating toxic halogenated byproducts in treatment of saline wastewater. Our study reveals an unexpected discovery: the presence of a trace amount of Br- not only enhanced the electrochemical oxidation of organic compounds with electron-rich groups but also significantly reduced the formation of halogenated byproducts. For example, in the presence of 20 μM Br-, the oxidation rate of phenol increased from 0.156 to 0.563 min-1, and the concentration of total organic halogen decreased from 59.2 to 8.6 μM. Through probe experiments, direct electron transfer and HO• were ruled out as major contributors; transient absorption spectroscopy (TAS) and computational kinetic models revealed that trace Br- triggers a shift in the dominant reactive species from Cl2•- to Br2•-, which plays a key role in pollutant removal. Both TAS and electron paramagnetic resonance identified signals unique to the phenoxyl and carbon-centered radicals in the Br2•--dominated system, indicating distinct reaction mechanisms compared to those involving Cl2•-. Kinetic isotope experiments and density functional theory calculations confirmed that the interaction between Br2•- and phenolic pollutants follows a hydrogen atom abstraction pathway, whereas Cl2•- predominantly engages pollutants through radical adduct formation. These insights significantly enhance our understanding of bromine radical-involved oxidation processes and have crucial implications for optimizing electrochemical treatment systems for saline wastewater.

Electrocatalysts for Inorganic and Organic Waste Nitrogen Conversion

Anthropogenic activities have disrupted the natural nitrogen cycle, increasing the level of nitrogen contaminants in water. Nitrogen contaminants are harmful to humans and the environment. This motivates research on advanced and decarbonized treatment technologies that are capable of removing or valorizing nitrogen waste found in water. In this context, the electrocatalytic conversion of inorganic- and organic-based nitrogen compounds has emerged as an important approach that is capable of upconverting waste nitrogen into valuable compounds. This approach differs from state-of-the-art wastewater treatment, which primarily converts inorganic nitrogen to dinitrogen, and organic nitrogen is sent to landfills. Here, we review recent efforts related to electrocatalytic conversion of inorganic- and organic-based nitrogen waste. Specifically, we detail the role that electrocatalyst design (alloys, defects, morphology, and faceting) plays in the promotion of high-activity and high-selectivity electrocatalysts. We also discuss the impact of wastewater constituents. Finally, we discuss the critical product analyses required to ensure that the reported performance is accurate.

In situ cleaning of foulants by gas scouring on the membrane-electrode in an electro-membrane bioreactor

Low-maintenance membrane cleaning is essential for the stable operation of membrane bioreactors. This work proposes an in-situ electrical-cleaning method using an electro-MBR. When the applied bias was transiently increased, the membrane flux recovered rapidly because of the scouring effect from gas evolution reactions. The exfoliation of the cake layer induced by gas scouring played a major role in mitigating membrane fouling, recovering the transmembrane pressure (TMP) by 88.6 % under optimal conditions. Membrane modules did not require replacement during the operation period due to the efficacy of electrical cleaning, with the TMP varying between 37.5 % and 62.5 % of the ultimate pressure requiring change of the membrane modules. Despite the increase in power consumption of 0.66 Wh·m-3 due to the additional applied bias, there was no need for chemical additives or manual maintenance. Therefore, the electrical cleaning method enhanced the service life and reduced the maintenance costs of the electro-MBR.

Concomitant urea stabilization and phosphorus recovery from source-separated fresh urine in magnesium anode-based peroxide-producing electrochemical cells

In this study, we investigated the recovery of nitrogen (N) and phosphorus (P) from fresh source-separated urine with a novel electrochemical cell equipped with a magnesium (Mg) anode and carbon-based gas-diffusion cathode. Recovery of P, which exists primarily as phosphate (PO43-) in urine, was achieved through pH-driven precipitation. Maximizing N recovery requires simultaneous approaches to address urea and ammonia (NH3). NH3 recovery was possible through precipitation in struvite with soluble Mg supplied by the anode. Urea was stabilized with electrochemically synthesized hydrogen peroxide (H2O2) from the cathode. H2O2 concentrations and resulting urine pH were directly proportional to the applied current density. Concomitant NH3 and PO43- precipitation as struvite and urea stabilization via H2O2 electrosynthesis was possible at lower current densities, resulting in urine pH under 9.2. Higher current densities resulted in urine pH over 9.2, yielding higher H2O2 concentrations and more consistent stabilization of urea at the expense of NH3 recovery as struvite; PO43- precipitation still occurred but in the form of calcium phosphate and magnesium phosphate solids.

Synergistic catalysis of TiO2/WO3 photoanode and Sb-SnO2 electrode with highly efficient ClO• generation for urine treatment

Urine is the major source of nitrogen pollutants in domestic sewage and is a neglected source of H2. Although ClO• is used to overcome the poor selectivity and slow kinetics of urea decomposition, the generation of ClO• suffers from the inefficient formation reaction of HO• and reactive chlorine species (RCS). In this study, a synergistic catalytic method based on TiO2/WO3 photoanode and Sb-SnO2 electrode efficiently producing ClO• is proposed for urine treatment. The critical design is that TiO2/WO3 photoanode and Sb-SnO2 electrode that generate HO• and RCS, respectively, are assembled in a confined space through face-to-face (TiO2/WO3//Sb-SnO2), which effectively strengthens the direct reaction of HO• and RCS. Furthermore, a Si solar panel as rear photovoltaic cell (Si PVC) is placed behind TiO2/WO3//Sb-SnO2 to fully use sunlight and provide the driving force of charge separation. The composite photoanode (TiO2/WO3//Sb-SnO2 @Si PVC) has a ClO• generation rate of 260% compared with the back-to-bake assembly way. In addition, the electrons transfer to the NiFe LDH@Cu NWs/CF cathode for rapid H2 production by the constructed photoelectric catalytic (PEC) cell without applied external biasing potential, in which the H2 production yield reaches 84.55 μmol h-1 with 25% improvement of the urine denitrification rate. The superior performance and long-term stability of PEC cell provide an effective and promising method for denitrification and H2 generation.

Urea catalytic oxidation for energy and environmental applications

Urea is one of the most essential reactive nitrogen species in the nitrogen cycle and plays an indispensable role in the water-energy-food nexus. However, untreated urea or urine wastewater causes severe environmental pollution and threatens human health. Electrocatalytic and photo(electro)catalytic urea oxidation technologies under mild conditions have become promising methods for energy recovery and environmental remediation. An in-depth understanding of the reaction mechanisms of the urea oxidation reaction (UOR) is important to design efficient electrocatalysts/photo(electro)catalysts for these technologies. This review provides a critical appraisal of the recent advances in the UOR by means of both electrocatalysis and photo(electro)catalysis, aiming to comprehensively assess this emerging field from fundamentals and materials, to practical applications. The emphasis of this review is on the design and development strategies for electrocatalysts/photo(electro)catalysts based on reaction pathways. Meanwhile, the UOR in natural urine is discussed, focusing on the influence of impurity ions. A particular emphasis is placed on the application of the UOR in energy and environmental fields, such as hydrogen production by urea electrolysis, urea fuel cells, and urea/urine wastewater remediation. Finally, future directions, prospects, and remaining challenges are discussed for this emerging research field. This critical review significantly increases the understanding of current progress in urea conversion and the development of a sustainable nitrogen economy.

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.

Formation of oxidation byproducts during electrochemical treatment of simulated produced water

Electrochemical oxidation (EO) can effectively remove recalcitrant organic contaminants from produced water (PW) but the formation of toxic oxidation byproducts (OBPs) is an unintended consequence. This study has rigorously investigated the OBPs formation during the EO treatment of a simulated PW containing phenol - a common organic contaminant existing in PW, as a model contaminant. In the absence of ammonia, free chlorine was generated from Cl- oxidation to serve as the main oxidant for phenol oxidation. During the EO process, 2,4,6-trichlorophenol and 2,6-dichlorobenzoquinone were identified as the critical intermediates that led to the formation of carbonaceous OBPs (C-OBPs). Some C-OBPs like chloroform (TCM), chloral hydrate (CH), and trichloroacetic acid (TCAA) reached their peak concentrations of 15 - 180 μM that were then reduced to 1 - 115 μM via volatilization and/or electrochemical reduction. When ammonia was present, nitrogenous OBPs (N-OBPs) were formed with the peak levels of 1 - 10 μM at the chlorination breakpoint (when ammonia was completely removed) that were subsequently reduced below 1 uM via volatilization and/or hydrolysis. It was observed that ammonia significantly decreased the formation of both C-OBPs and chlorate due to the consumption of free chlorine. A higher current density accelerated OBPs formation rates with different effects on volatile and non-volatile OBPs. The results of this study will enhance our understanding of OBPs formation precursors and mechanisms during electrochemical process and help develop strategies for proper control of OBPs to achieve safer electrochemical wastewater treatment.

Enhanced oxidation of urea by pH swing during chlorination: pH-dependent reaction mechanism

Urea reacts with chlorine to form chlorinated ureas (chloroureas), and fully chlorinated urea (tetrachlorourea) is further hydrolyzed into CO2 and chloramines. This study found that the oxidative degradation of urea by chlorination was enhanced by the pH swing, wherein the reaction proceeded under an acidic pH (e.g., pH = 3) in the first stage, and the solution pH was subsequently increased to a neutral or alkaline value (e.g., pH > 7) in the second-stage reaction. The degradation rate of urea by pH-swing chlorination increased with increasing chlorine dose and pH during the second-stage reaction. The pH-swing chlorination was based on the opposite pH dependence of sub-processes comprising urea chlorination. The formation of monochlorourea was favored under acidic pH conditions; however, the subsequent conversion into di- and trichloroureas was favored under neutral or alkaline pH conditions. The deprotonation of monochlorourea (pKa = 9.7 ± 1.1) and dichlorourea (pKa = 5.1 ± 1.4) was suggested to be responsible for the accelerated reaction in the second stage under increased pH conditions. pH-swing chlorination was also effective in degrading urea at low concentrations (micromolar levels). In addition, the total nitrogen concentration significantly decreased during the degradation of urea because of the volatilization of chloramines and the release of other gaseous nitrogen compounds.

Chitosan Supports Boosting NiCo2O4 for Catalyzed Urea Electrochemical Removal Application

Currently, wastewater containing high urea levels poses a significant risk to human health. Else, electrocatalytic methodologies have the potential to transform urea present in urea-rich wastewater into hydrogen, thereby contributing towards environmental conservation and facilitating the production of sustainable energy. The characterization of the NiCo2O4@chitosan catalyst was performed by various analytical techniques, including scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). Furthermore, the activity of electrodes toward urea removal was investigated by several electrochemical techniques. As a function of current density, the performance of the modified NiCo2O4@chitosan surface was employed to remove urea using electrochemical oxidation. Consequently, the current density measurement was 43 mA cm-2 in a solution of 1.0 M urea and 1.0 M KOH. Different kinetic characteristics were investigated, including charge transfer coefficient (α), Tafel slope (29 mV dec-1), diffusion coefficient (1.87 × 10-5 cm2 s-1), and surface coverage 4.29 × 10-9 mol cm-2. The electrode showed high stability whereas it lost 10.4% of its initial current after 5 h of urea oxidation.

Handleable TiO2-coated zeolitic material for photodecomposition of caffeine boosted by urine matrix

The photocatalytic decomposition of caffeine under UV-light irradiation was observed for the first time in a matrix of synthetic urine using granules of hydrogenated and iron-exchanged natural zeolite, coated with two loadings of TiO₂. A natural clinoptilolite-mordenite blend was used to prepare photocatalytic adsorbents coated with TiO₂ nanoparticles. The performance of the obtained materials was tested in the photodegradation of caffeine, a water contaminant of emerging concern. The photocatalytic activity was better in the urine matrix, due to the formation of surface complexes on the TiO₂ coating, cation exchange performed by the zeolite support, and use of the carrier electrons in the reduction of ions, affecting recombination of the electrons and holes during photocatalysis. The composite granules maintained photocatalytic activity for at least four cycles, with more than 50% of caffeine removal in the synthetic urine matrix.

Efficient electrochemical generation of active chlorine to mediate urea and ammonia oxidation in a hierarchically porous-Ru/RuO2-based flow reactor

The electrochemical chlorination of urea to CO₂ and N₂ end-products, via active-chlorine-mediated oxidation under nearly neutral conditions, is an effective treatment for medium-concentrated urea-containing wastewater. Herein, we design a novel flow reactor integrated with three-dimensional hierarchically porous Ru/RuO₂ architectures anchored on a Ti mesh. The hierarchically macroporous electrode can create sufficient exposure of catalytically active sites and facilitate the microscopic mass transport and diffusion inside the active layer, thereby contributing to the increased removal efficiency of urea-N and ammonia-N. The combined results of electrochemical measurements, UV-visible spectrometry and in situ Raman spectrometry, show that the OCl^- species produced by chlorine evolution reaction (CER) are the main active constituents for removing urea-N. Theoretical calculations reveal thLTWAat the Ru/RuO₂ possesses a moderate Cl binding strength, lower theoretical overpotentials of CER and a higher conductivity, compared with pure RuO₂. On this basis, we assemble a circular flow reactor with the hierarchically porous electrodes in a two-electrode system to obtain an enhanced microfluidic process, which during 9 days of uninterrupted operation, at a high electrolysis current of 500 mA, achieve a total nitrogen removal of 92.6% and an energy consumption of 7.94 kWh kg^-1 N, demonstrating the promising application of the novel process.

Investigation of black phosphorus anodic catalyst for electrolysis: Degradation of organics via a perchlorate-free oxidant activation

This study investigated the potential of a novel fabricated black phosphorus (BP) nanoparticle electrode as an alternative to noble metal-based catalysts for application in electrolysis. The BP electrode was compared with other conventional catalysts (boron-doped diamond (BDD) and a dimensional stable electrode (DSA)) under different electrolyte conditions for the generation of specific oxidants (e.g., OH^•, HOCl, OCl^-, SO₄^{• -}) in the bulk phase during electrolysis. In the presence of sulfate-based electrolyte, results on the electrochemical oxidation showed that the BP not only resulted in an 8-fold increase in the current efficiency compared to DSA, but also reduced energy consumptions by approximately 30-fold. Moreover, electrolysis using certain electrodes (i.e., BDD) under high current densities in the presence of chlorine-based electrolyte has been reported to be hazardous to the water system due to the generation of toxic chlorine oxyanions (i.e., perchlorate), which necessitates the operation of a post-treatment process. Likewise, application of the BDD electrode was confirmed to produce perchlorate under high current densities, while no by-product was generated by electrolysis with the BP electrode. Finally, multiple degradation pathways for selective water treatment was monitored under oxidation with the BP electrode. To the best of our knowledge, this study is the first to apply the novel fabricated BP electrode as the anodic catalyst for the treatment of a water system.

Reactive species conversion into 1O2 promotes substantial inhibition of chlorinated byproduct formation during electrooxidation of phenols in Cl--laden wastewater

The generation of chlorinated byproducts during the electrochemical oxidation (EO) of Cl^--laden wastewater is a significant concern. We aim to propose a concept of converting reactive species (e.g., reactive chlorines and HO^• resulting from electrolysis) into ¹O₂ via the addition of H₂O₂, which substantially alleviates chlorinated organic formation. When phenol was used as a model organic compound, the results showed that the H₂O₂-involving EO system outperformed the H₂O₂-absent system in terms of higher rate constants (5.95 × 10^-2 min^-1vs. 2.97 × 10^-2 min^-1) and a much lower accumulation of total organic chlorinated products (1.42 mg L^-1vs. 8.18 mg L^-1) during a 60 min operation. The rate constants of disappearance of a variety of phenolic compounds were positively correlated with the Hammett constants (σ), suggesting that the reactive species preferred oxidizing phenols with electron-rich groups. After the identification of ¹O₂ that was abundant in the bulk solution with the use of electron paramagnetic resonance and computational kinetic simulation, the routes of ¹O₂ generation were revealed. Despite the consensus as to the contribution of reaction between H₂O₂ and ClO^- to ¹O₂ formation, we conclude that the predominant pathway is through H₂O₂ reaction with electrogenerated HO^• or chlorine radicals (Cl^• and Cl₂^•-) to produce O₂^•-, followed by self-combination. Density functional theory calculations theoretically showed the difficulty in forming chlorinated byproducts for the ¹O₂-initiated phenol oxidation in the presence of Cl^-, which, by contrast, easily occurred for the Cl^•-or HO^•-initiated phenol reaction. The experiments run with real coking wastewater containing high-concentration phenols further demonstrated the superiority of the H₂O₂-involving EO system. The findings imply that this unique method for treating Cl^--laden organic wastewater is expected to be widely adopted for generalizing EO technology for environmental applications.

Synergistic Nanowire-Enhanced Electroporation and Electrochlorination for Highly Efficient Water Disinfection

Conventional water disinfection methods such as chlorination typically involve the generation of harmful disinfection byproducts and intensive chemical consumption. Emerging electroporation disinfection techniques using nanowire-enhanced local electric fields inactivate microbes by damaging their outer structures without byproduct formation or chemical dosing. However, this physical-based method suffers from a limited inactivation efficiency under high water flux due to an insufficient contact time. Herein, we integrate electrochlorination with nanowire-enhanced electroporation to achieve a synergistic flow-through process for efficient water disinfection targeting bacteria and viruses. Electroporation at the cathode induces sub-lethal damages on the microbial outer structures. Subsequently, electrogenerated active chlorine at the anode aggravates these electroporation-induced injuries to the level of lethal damage. This sequential flow-through disinfection system achieves complete disinfection (>6.0-log) under a very high water flux of 2.4 × 10⁴ L/(m² h) with an applied voltage of 2.0 V. This disinfection efficiency is 8 times faster than that of electroporation alone. Further, the specific energy consumption for the disinfection by this novel process is extremely low (8 × 10^-4 kW h/m³). Our results demonstrate a promising method for rapid and energy-efficient water disinfection by coupling electroporation with electrochlorination to meet vital needs for pathogen elimination.

Electrochemical oxidation of pharmaceuticals in synthetic fresh human urine: Using selective radical quenchers to reveal the dominant degradation pathways and the scavenging effects of individual urine constituents

Electrochemical oxidation of fresh human urine is a promising method to prevent pharmaceuticals from being discharged into the environment. Here, we evaluate the importance of electro-generated oxidants and direct anodic oxidation for degradation of four pharmaceutical (cyclophosphamide (CP), carbamazepine (CBZ), sulfamethoxazole (SMX) and ibuprofen (IBP)) accounting for the scavenging effect of urine constituents using boron-doped diamond (BDD) and IrO₂ electrodes. Allyl alcohol and tert-butanol were used as selective quenchers for adsorbed and dissolved radicals, respectively. In electrolyte containing only chloride and pharmaceuticals, we found that CBZ and SMX are primarily oxidized by electro-generated Cl₂ in the fluid boundary layer , and CP and IBP are primarily oxidized by physisorbed ^•OH or chemisorbed chlorine (IrO₃-Cl). Regarding the effects of other fresh urine constituents, urea, creatinine, and uric acid quench the dissolved reactive chlorine species (Cl^•/Cl₂^•‒, HOCl, Cl₂, etc.). However, SO₄^2‒ shows no effect on pharmaceutical degradation while H₂PO₄^‒ and citrate ions quench IrO₃-Cl resulting in a mixed kinetic and mass-transfer limiting oxidation of pharmaceuticals on IrO₂. Citrate ions only quench the dissolved oxidants (surface adsorbed radicals are the dominant oxidants) leading to the pharmaceutical degradation limited by the mass transfer of pharmaceutical to BDD surface. This work provides an understanding of the significance of various pathways for pharmaceutical degradation, scavenging effect of urine constituents, and strategies for rapid pharmaceutical degradation in human urine.

Organic removal from coal-to-chemical brine by a multistage system of adsorption-regeneration and electrochemically driven UV/chlorine processes

Treatment of coal-to-gas brine (CGB) is a great challenge since it contains elevated inorganic salts and a high level of toxic and bio-accumulative organics. In this study, CGB treatment was conducted by adsorptionregeneration and electrochemically driven UV/chlorine (E-UV/Cl₂) processes. LS-109D macroporous resin was optimal adsorbent primarily due to unique pore structure, which preferably adsorbed the aromatic fluorescent components with quenching Cl∙ effect and low molecular weight acids recalcitrant to ∙OH. The E-UV/Cl₂ process outperformed the UV photolysis process and electrochemical advanced oxidation processes (EAOPs) for oxidation of organic compounds due to its full utilization of Cl^- in CGB to produce highly active oxidation agents. Thanks to the synergy between process units in organic matter removal, dissolved organic carbon (DOC) of CGB was reduced from 163.41 mg/L to 26.58 mg/L by the multistage system. Furthermore, the CGB with characteristics of high fluorescence and molecular weight (MW) distribution was converted to effluent with low fluorescence and MW distribution. The exhausted LS-109D was regenerated by ultrasound-assisted hot water elution at 363 K. After pretreated by ozonation, the eluate can be easily treated by biological process. The study suggests that the multistage system can provide an effective treatment option for removing organics from CGB.

Strategies for optimizing urea removal to enable portable kidney dialysis: A reappraisal

BACKGROUND

Portable hemodialysis has the potential to improve health outcomes and quality of life for patients with kidney failure at reduced costs. Urea removal, required for dialysate regeneration, is a central function of any existing/potential portable dialysis device. Urea in the spent dialysate coexists with non-urea uremic toxins, nutrients, and electrolytes, all of which will interfere with the urea removal efficiency, regardless of whether the underlying urea removal mechanism is based on urease conversion, direct urea adsorption, or oxidation. The aim of the current review is to identify the amount of the most prevalent chemicals being removed during a single dialysis session and evaluate the potential benefits of an urea-selective membrane for portable dialysis.

METHODS

We have performed a literature search using Web of Science and PubMed databases to find available articles reporting (or be able to calculate from blood plasma concentration) > 5 mg of individually quantified solutes removed during thrice-weekly hemodialysis sessions. If multiple reports of the same solute were available, the reported values were averaged, and the geometric mean of standard deviations was taken. Further critical literature analysis of reported dialysate regeneration methods was performed using Web of Science and PubMed databases.

RESULTS

On average, 46.0 g uremic retention solutes are removed in a single conventional dialysis session, out of which urea is only 23.6 g. For both urease- and sorbent-based urea removal mechanisms, amino acids, with 7.7 g removal per session, could potentially interfere with urea removal efficiency. Additionally for the oxidation-based urea removal system, plentiful nutrients such as glucose (24.0 g) will interfere with urea removal by competition. Using a nanofiltration membrane between dialysate and oxidation unit with a molecular weight cutoff (MWCO) of ~200 Da, 67.6 g of non-electrolyte species will be removed in a single dialysis session, out of which 44.0 g are non-urea molecules. If the membrane MWCO is further decreased to 120 Da, the mass of non-electrolyte non-urea species will drop to 9.3 g. Reverse osmosis membranes have been shown to be both effective at blocking the transport of non-urea species (creatinine for example with ~90% rejection ratio), and permissive for urea transport (~20% rejection ratio), making them a promising urea selective membrane to increase the efficiency of the oxidative urea removal system.

CONCLUSIONS

Compiled are quantified solute removal amounts greater than 5 mg per session during conventional hemodialysis treatments, to act as a guide for portable dialysis system design. Analysis shows that multiple chemical species in the dialysate interfere with all proposed portable urea removal systems. This suggests the need for an additional protective dialysate loop coupled to urea removal system and an urea-selective membrane.

Iron Phosphide Precatalyst for Electrocatalytic Degradation of Rhodamine B Dye and Removal of Escherichia coli from Simulated Wastewater

Electrocatalysis using low-cost materials is a promising, economical strategy for remediation of water contaminated with organic chemicals and microorganisms. Here, we report the use of iron phosphide (Fe2P) precatalyst for electrocatalytic water oxidation; degradation of a representative aromatic hydrocarbon, the dye rhodamine B (RhB); and inactivation of Escherichia coli (E. coli) bacteria. It was found that during anodic oxidation, the Fe2P phase was converted to iron phosphate phase (Fe2P-iron phosphate). This is the first report that Fe2P precatalyst can efficiently catalyze electrooxidation of an organic molecule and inactivate microorganisms in aqueous media. Using a thin film of Fe2P precatalyst, we achieved 98% RhB degradation efficiency and 100% E. coli inactivation under an applied bias of 2.0 V vs. reversible hydrogen electrode in the presence of in situ generated reactive chlorine species. Recycling test revealed that Fe2P precatalyst exhibits excellent activity and reproducibility during degradation of RhB. High-performance liquid chromatography with UV-Vis detection further confirmed the electrocatalytic (EC) degradation of the dye. Finally, in tests using Lepidium sativum L., EC-treated RhB solutions showed significantly diminished phytotoxicity when compared to untreated RhB. These findings suggest that Fe2P-iron phosphate electrocatalyst could be an effective water remediation agent.

Highly efficient removal of total nitrogen and dissolved organic compound in waste reverse osmosis concentrate mediated by chlorine radical on 3D Co3O4 nanowires anode

Reverse osmosis concentrate (ROC) from wastewater reclamation has posed significant disposal challenges due to its highly concentrated NH₃-N, chloride ion and bio-refractory organics, and developing technologies for their removal are essential. Herein, we developed an efficient electrochemical system to remove total nitrogen and dissolved organic compound (DOC) simultaneously mediated by chlorine radical (Cl•), which is generated by activation of chloride ion existing in ROC on an inexpensive, three-dimensional Co₃O₄ nanowires. Results showed that the total nitrogen and total organic carbon removal were 98.2% and 56.9% in 60 min for synthetic ROC with 56 mg/L of NH₃-N and 20 mg/L of DOC. The utilization of Co₃O₄ nanowires enhanced NH₃-N degradation by 2.58 times compared with Co₃O₄ nanoplates, which were 1.69 and 17.5 times these of RuO₂ and Pt. We found that structural Co³⁺/Co²⁺ acts as cyclic catalysis to produce Cl• via single-electron transfer, which convert NH₃-N to N₂ and lead to faster DOC degradation. This architecture provides abundant catalytic sites and sufficient accessibility of reactants. Small amount of nitrate generated by oxidation of NH₃-N was further reduced to N₂ on Pd-Cu/NF cathode. These findings provide new insights for utilization of waste Cl^- and development of novel electrochemical system for ROC disposal.

Onsite Graywater Treatment in a Two-Stage Electro-Peroxone Reactor with a Partial Recycle of Treated Effluent

The efficacy of an uncoupled electro-peroxone (E-peroxone) prototype reactor system for the treatment of synthetic graywater is determined. The two-stage E-peroxone process integrates ozonation with the in situ production of hydrogen peroxide (H₂O₂) in a first stage reactor before ozone (O₃) is converted via the peroxone reaction to a hydroxyl radical (^•OH). The two-stage prototype reactor system allows for the generation of H₂O₂ via cathodic oxygen reduction in the first-stage reactor before mixing with O₃ in the second-stage reactor. This approach prevents the degradation of polytetrafluoroethylene (PTFE) coated carbon cathodes by ^•OH that takes place in a single well-mixed reactor that combines electrochemical peroxide generation with O₃. The dosage of H₂O₂ into the second-stage reactor is optimized to enhance graywater treatment. Under these conditions, the uncoupled E-peroxone system is capable of treating synthetic graywater with an initial chemical oxygen demand (COD₀) of 358 mg O₂/L, a total organic carbon (TOC₀) of 96.9 mg/L, a biochemical oxygen demand (BOD₀) of 162 mg O₂/L, and a turbidity of 11.2 NTU. The two-stage electro-peroxone system can reduce the initial COD₀ by 89%, the TOC₀ by 91%, BOD₀ by 86%, and the turbidity by 95% after 90 min of treatment. At this performance level, the reactor effluent is acceptable for discharge and for use in nonpotable applications such as toilet-water flushing. A portion of the effluent is recycled back into the first-stage reactor to minimize water consumption. Recycling can be repeated consecutively for four or more cycles, although the time required to achieve the desired H₂O₂ concentration increased slightly from one cycle to another. The two-stage E-peroxone system is shown to be potentially useful for onsite or decentralized graywater treatment suitable for arid water-sensitive areas.

Nickel phosphide on Ni foam as anode and peroxymonosulfate as the chemical oxidizer for effective direct urea fuel cell

The direct urea fuel cell (DUFC) is a low cost and competitive approach for contemporaneous urine or urea-contaminated wastewater treatment and electricity generation. However, the lack of efficient urea oxidation reaction (UOR) electrocatalysts and suitable electron acceptors remains a challenge for practical applications. Here, we developed a DUFC system using Ni₂P@Ni foam as the anode and peroxymonosulfate (PMS) as the chemical oxidizers. The Ni₂P@Ni foam anode showed a high oxidation activity for UOR with an onset potential of 0.30 V vs. Ag/AgCl and Tafel slope of 34.4 mV/dec. PMS with high theoretical potential improved the cell voltage to 1.43 V. A power density of DUFC up to 4.91 mW/cm² was achieved using PMS at room temperature, which was approximately twice as high as using H₂O₂ (2.38 mW/cm²). Ni^II/Ni^III was the redox active species on the Ni₂P anode in the DUFC process, and Ni^II was electrochemically oxidized to Ni^III, which reverted to Ni^II by urea reduction. When real human urine was used as the fuel, a power density of 4.46 mW/cm² can be achieved at room temperature. This DUFC with high cell performance showed potential application in urea wastewater treatment.

Dimensionally stable anode (Doped-MMO) mediated electro-oxidation and multi-response optimization study for remediation of urea wastewater

In the present study, the potential application of novel doped-MMO (Ti/IrO₂/Ta₂O₅/SnO₂-Sb₂O₄) anodes as an alternative source to costly electrodes have been visualized for the EO treatment of urea. Parametric optimization for the treatment of urea through the EO process by doped-MMO has been done successfully. The high R² values of both responses i.e. % Degradation and energy consumption for quadratic suggested by BBD under RSM advocates a good correlation between predicted and experimental data. The maximum % Degradation and energy consumption at optimized were found to be 91.2%, 51.53 kWh m^-3 for urea respectively. Additionally, efforts were made to minimize treatment time further by implementing a dual effect, namely photo-electrocatalysis. The anode was found to be relatively stable even after 120 runs. The analysis of treated urea solution was verified in terms of total organic carbon (TOC) 90.0% reduction. The average operating cost of the electro-oxidation treatment process is determined to be 1.91 $ m^-3. The results of this study demonstrate the potential of doped-MMO as a promising concept for the treatment of wastewater that can be successfully applied in real life.

Descriptive Role of Pt/PtOx Ratio on the Selective Chlorine Evolution Reaction under Polarity Reversal as Studied by Scanning Electrochemical Microscopy

This study investigated competing chlorine evolution reaction (ClER) and oxygen evolution reaction (OER) on Pt electrodes under variable polarity reversal intervals (±16.7 mA cm^-2, 30-600 s) in the context of distinctive roles of Pt(0) and PtO_x on the surface in dilute (0.1 M) NaCl solutions. The substrate generation/tip collection mode of scanning electrochemical microscopy (SECM) quantified the current efficiency (CE) of ClER with a large tip-to-substrate distance (>500 μm) to avoid intervention of bubbles and spatial variations. Surface interrogation SECM using [Ru(NH₃)₆]^2+/3+ coupled with X-ray photoelectron spectroscopy (XPS) identified the Pt⁴⁺-enriched surface of PtO_x with a bilayer structure to give more efficient regeneration of Pt(0) under the shorter reversal interval. The in situ SECM complemented bulk electrolysis and XPS to demonstrate that ClER on Pt(0) and OER on PtO_x primarily determine the CE of ClER, in agreement with a kinetic model. The descriptive role of surface Pt/PtO_x ratio rationalized the enhanced selectivity for ClER upon the polarity switching, being independent on a scaling relationship. The current reversal (not allowed to IrO₂ electrodes) also alleviated calcareous scale deposit in the electrolyte with hardness.

Selective oxidation of pharmaceuticals and suppression of perchlorate formation during electrolysis of fresh human urine

Urine comprises only a small (~1%) volumetric fraction of municipal wastewater, but represents a dominant source of pharmaceuticals, many of which may pass through conventional wastewater treatment and pose risks to aquatic ecosystems. Point-source treatment of source-separated urine presents a unique opportunity to degrade pharmaceuticals before dilution with wastewater, and electrochemical advanced oxidation processes are one increasingly investigated option. However, they often lead to the formation of oxidation byproducts including chlorate, perchlorate at very high concentrations. Here, we show that the high urea content of fresh human urine suppresses the formation of oxychlorides by inhibiting formation of HOCl/OCl^‒ during electrolysis, while still enabling pharmaceutical degradation due to the slow rate of urea oxidation by ^•OH. This results in improved performance compared to equivalent treatment of hydrolyzed aged urine. This electrochemical oxidation scheme is shown to degrade the model contaminants cyclophosphamide and sulfamethoxazole with surface-area-to-volume-normalized pseudo-first-order rate constants greater than 0.08 cm/min in authentic fresh human urine. It results in ~100 × decrease in pharmaceutical concentrations in 2 h while generating ~1000 × lower oxychloride byproduct concentrations in synthetic fresh urine than synthetic hydrolyzed aged urine matrixes. Importantly, this proof-of-principle shows that simple and safe electrochemical methods can be used for point-source-remediation of pharmaceuticals in fresh human urine (before storage and hydrolysis), without formation of significant oxychloride byproducts.

Photolysis of N-chlorourea and its effect on urea removal in a combined pre-chlorination and UV254 process

Urea is one of the most important nitrogenous organic pollutants in water, and its removal attracts attention because of a growing concern related to water eutrophication. Urea has previously been considered to be largely unaffected by the UV-chlorine process. However, N-chlorourea, an intermediate of urea chlorination, has been shown to absorb ultraviolet radiation, and as such its photolysis is possible. Experiments were conducted to quantify the kinetics of N-chlorourea degradation under UV₂₅₄ irradiation. The results showed that about 92% of N-chlorourea was degraded under UV₂₅₄ irradiation. Ammonia and nitrate were detected as the primary nitrogen containing products of the photolysis of N-chlorourea. Solution pH ranging from 3.0 to 7.5 influenced the distribution of these products but not on the degradation rate. Based on these data, a possible pathway of photodegradation of N-chlorourea under UV₂₅₄ is proposed. The degradation of urea was also achieved by the photolysis of N-chlorourea during the combined pre-chlorination and UV₂₅₄ process. Insights gained in this study may be useful for exploring the potential of combined pre-chlorination and UV₂₅₄ process on urea removal in water treatment.

Rational Surface and Interfacial Engineering of IrO2 /TiO2 Nanosheet Arrays toward High-Performance Chlorine Evolution Electrocatalysis and Practical Environmental Remediation

The chlorine evolution reaction (CER) is a critical and commercially valuable electrochemical reaction in industrial-scale utilization, including the Chlor-alkali industry, seawater electrolysis, and saline wastewater treatment. Aiming at boosting CER electrocatalysis, hybrid IrO₂ /TiO₂ nanosheet arrays (NSAs) with rational surface and interfacial tuning strategies are proposed in this study. The IrO₂ /TiO₂ NSAs exhibit superb CER electrocatalytic activity with a low overpotential (44 mV) at 10 mA cm^-2 , low Tafel slope of 40 mV dec^-1 , high CER selectivity (95.8%), and long-term durability, outperforming most of the existing counterparts. The boosting mechanism is proposed that the aerophobic/hydrophilic surface engineering and interfacial electronic structure tuning of IrO₂ /TiO₂ are beneficial for the Cl^- mass-transfer, Cl₂ release, and Volmer-Heyvrosky kinetics during the CER. Practical saline wastewater treatment by using the IrO₂ /TiO₂ NSAs electrode is further conducted, demonstrating it has a higher p-nitrophenol degradation ratio (95.10% in 60 min) than that of other electrodes. The rational surface and interfacial engineering of IrO₂ /TiO₂ NSAs can open up a new way to design highly efficient electrocatalysts for industrial application and environmental remediation.

High chlorine evolution performance of electrochemically reduced TiO2 nanotube array coated with a thin RuO2 layer by the self-synthetic method

Recently, reduced TiO2 nanotube arrays via electrochemical self-doping (r-TiO2) are emerging as a good alternative to conventional dimensionally stable anodes (DSAs) due to their comparable performance and low-cost. However, compared with conventional DSAs, they suffer from poor stability, low current efficiency, and high energy consumption. Therefore, this study aims to advance the electrochemical performances in the chlorine evolution of r-TiO2 with a thin RuO2 layer coating on the nanotube structure (RuO2@r-TiO2). The RuO2 thin layer was successfully coated on the surface of r-TiO2. This was accomplished with a self-synthesized layer of ruthenium precursor originating from a spontaneous redox reaction between Ti3+ and metal ions on the r-TiO2 surface and thermal treatment. The thickness of the thin RuO2 layer was approximately 30 nm on the nanotube surface of RuO2@r-TiO2 without severe pore blocking. In chlorine production, RuO2@r-TiO2 exhibited higher current efficiency (∼81.0%) and lower energy consumption (∼3.0 W h g-1) than the r-TiO2 (current efficiency of ∼64.7% of and energy consumption of ∼5.2 W h g-1). In addition, the stability (ca. 22 h) was around 20-fold enhancement in RuO2@r-TiO2 compared with r-TiO2 (ca. 1.2 h). The results suggest a new route to provide a thin layer coating on r-TiO2 and to synthesize a high performance oxidant-generating anode.

Optimal design of an electrochemical reactor for blackwater treatment

Electrolysis of blackwater for disinfection and nutrient removal is a portable and scalable technology that can lessen the need for cities to construct large-scale wastewater treatment infrastructure and enable the safe onsite reuse of blackwater. Several systems for treating wastewater from single toilets are described in the literature, but there are few examples of systems designed to use electrolysis to treat blackwater from nearby toilets, which is a situation more common in densely packed urban living environments. In order to scale a single toilet electrolysis system to one that could service multiple toilets, computational fluid dynamic analysis was used to optimize the electrochemical reactor design, and laboratory and field-testing were used to confirm results. Design efforts included optimization of the reactor shape and mixing to improve treatment efficiency, as well as automated cleaning and salt injection to reduce maintenance and service requirements. PRACTITIONER POINTS: Design of a reverse polarity mechanism to enable in situ electrode cleaning and improve long-term electrode performance. Optimization of a hopper design and drainpipe location to collect and remove flaking precipitates and mitigate maintenance issues. Design of an automated salt injection system to guarantee sufficient chloride levels for producing adequate chlorine residuals for consistent disinfection.

An approach to the photocatalytic mechanism in the TiO2-nanomaterials microorganism interface for the control of infectious processes

The approach of this timely review considers the current literature that is focused on the interface nanostructure/cell-wall microorganism to understand the annihilation mechanism. Morphological studies use optical and electronic microscopes to determine the physical damage on the cell-wall and the possible cell lysis that confirms the viability and microorganism death. The key parameters of the tailoring the surface of the photoactive nanostructures such as the metal functionalization with bacteriostatic properties, hydrophilicity, textural porosity, morphology and the formation of heterojunction systems, can achieve the effective eradication of the microorganisms under natural conditions, ranging from practical to applications in environment, agriculture, and so on. However, to our knowledge, a comprehensive review of the microorganism/nanomaterial interface approach has rarely been conducted. The final remarks point the ideal photocatalytic way for the effective prevention/eradication of microorganisms, considering the resistance that the microorganism could develop without the appropriate regulatory aspects for human and ecosystem safety.

Enhanced photoelectrocatalytic degradation of bisphenol a by BiVO4 photoanode coupling with peroxymonosulfate

Peroxymonosulfate (PMS) was introduced into a photoelectrocatalytic (PEC) system with a bismuth vanadate (BiVO₄) photoanode to enhance the PEC oxidation of bisphenol A (BPA). With the addition of 5 mM PMS, the degradation efficiency of 10 mg/L BPA was significantly improved from 24.2% to 100.0% within 120 min and the side reaction of O₂ evolution was avoided at a potential as low as 0.25 V. The electron spin resonance and radicals quenching results suggested that photogenerated holes instead of SO₄^•- and OH were primarily responsible for the BPA degradation. To further explore the role of PMS, a photocatalytic fuel cell with the structure of BiVO₄ (photoanode)|10 mg/L BPA|proton exchange membrane (separator)|5 mM PMS|Pt (cathode) was constructed and demonstrated that PMS played a key role as electrons acceptor instead of the precursor of SO₄^•-. The PEC tests including open-circuit potential, linear sweep voltammetry and electrochemical impedance spectroscopy indicated that a more efficient separation of photogenerated charges was achieved in the PEC process with the help of PMS, thus generating more photogenerated holes for enhanced BPA degradation. This work may provide a novel way to enhance the separation of photogenerated charges at the photoanode.

Photoelectrochemical degradation of perfluorooctanoic acid (PFOA) with GOP25/FTO anodes: Intermediates and reaction pathways

Perfluorooctanoic acid (PFOA) have been widely studied due to their persistence, bioaccumulation and possible toxic effects. In this work, we investigated a photoelectrochemical (PEC) system consisting of a graphene oxide-titanium dioxide (GOP25) anode coated on fluorine-doped tin oxide (FTO) glass for removal of PFOA in an aquatic environment. The GOP25/FTO anode was fabricated and well characterized. Nearly complete decomposition of 0.5 mg/L PFOA was achieved after 4 h of PEC treatment with an initial pH of 5.3 and a current density of 16.7 mA cm^-2. The presence of graphene oxide (GO) on the TiO₂ anode could enhance its electrochemical performance, thereby leading to increased decomposition efficiency. A total of 18 PFOA transformation products, including short-chain perfluoroalkyl acids, are reported in this work, and 13 products were observed for the first time. Four possible routes of PFOA decomposition, namely, decarboxylation followed by oxidation, defluorination, hydroxylation and Cl atom substitution, were determined. The presence of chlorinated byproducts in the system indicated that reactive chlorine species contributed to PFOA degradation.

In Situ Electrochemical Generation of Reactive Chlorine Species for Efficient Ultrafiltration Membrane Self-Cleaning

Reactive membranes based on hydroxyl radical generation are hindered by the need for chemical dosing and complicated module and material design. Herein, we utilize an electrochemical approach featuring in situ generation of reactive (radical) chlorine species (RCS) through anodization of chloride ions for membrane self-cleaning. A hybridized carbon nanotube (CNT)-functionalized ceramic membrane (h-CNT/CM), possessing high hydrophilicity, permeability, and conductivity, was fabricated. Using carbamazepine (CBZ) as a probe, we confirmed the presence of RCS in the electrified h-CNT/CM. The rapid and complete degradation of CBZ in a single-pass ultrafiltration indicates a high localized RCS concentration within the three-dimensional porous CNT interwoven layer. We further demonstrate that the electrogeneration of RCS is a critical prestep for free chlorine (HClO and ClO^-) formation. The self-cleaning efficiency of the membrane after fouling with a model organic foulant (alginate) was assessed using an electrified cross-flow membrane filtration system. The fouled h-CNT/CM exhibits a near complete water flux recovery following a short (1 min) self-cleaning with an applied voltage of 3 or 4 V and feed solutions of 100 or 10 mM sodium chloride, respectively. Considering the superior performance of the RCS-mediated self-cleaning compared to conventional membrane chemical cleaning using sodium hypochlorite, our results exemplify an effective strategy for in situ electrogeneration of RCS to achieve a highly efficient membrane self-cleaning.

Generation and application of reactive chlorine species by electrochemical process combined with UV irradiation: Synergistic mechanism for enhanced degradation performance

Saline wastewater originates from many industries, containing a large amount of salt (NaCl) and other toxic and harmful organic matter, which have a great impact on the soil and groundwater. However, the treatment of saline wastewater is a serious problem because organic contents are hard to degrade with the high salinity by the common water treatment technologies. Herein, an electrochemical process coupled with ultraviolet (UV) irradiation was proposed for the saline wastewater treatment. High efficiency of p-nitrophenol (p-NP) and ammonia degradation were contributed from the in situ electrochemical produced active chlorine and photo-induced chlorine radicals. Under the optimal conditions (0.10 A, 0.05 M NaCl, and pH 6.00), approximately 98.91% p-NP was removed after 60 min with the rate constant of 7.521 × 10^-2 min^-1 in the electrochemical process, and 28.99% mineralization rate was obtained, while with the synergistic effect of UV and electrochemistry, approximately 100% of p-NP removal (k = 9.331 × 10^-2 min^-1) was achieved by 30 min treatment and about 83.70% of p-NP can be mineralized to CO₂ after 60 min. The study on the synergistic mechanism of enhanced degradation performance illustrated that Cl with high oxidation capacity played an important role in the p-NP oxidation. Besides, based on the chlorine radical reactions, this method was also effectively applied to remove ammonia nitrogen (92.00% removal of total nitrogen in 100 min) for nitrogen-containing wastewater. Thus, this study offers a promising approach for the treatment of saline industry wastewater.

Exhaustive denitrification via chlorine oxide radical reactions for urea based on a novel photoelectrochemical cell

Urea is a major source of nitrogen pollution in domestic sewage and its denitrification is difficult since it is very likely to be converted into ammonia or nitrate instead of expected N₂. Herein, we propose an exhaustive denitrification method for urea via the oxidation of amine/ammonia-N with chlorine oxide radical, which induced from a bi-functional RuO₂//WO₃ anode, and the highly selective reduction of nitrate-N on cathode in photoelectrochemical cell (PEC). Under illumination, the WO₃ photoanode side promotes the quantities hydroxyl and reactive chlorine radical, and these radicals are immediately combined to stronger chlorine oxide radical by RuO₂ side, which obviously enhances the efficiency and speed of the urea oxidation. Synchronously, the over-oxidized nitrate can be selectively reduced by Pd and Au nanoparticles on the surface of cathode. Eventually, exhaustive denitrification is realized by the circulative reaction. Experimental observations and theoretical calculation revealed that chlorine oxide radical promoted significant denitrification of urea with an efficiency of 99.74% in 60 min under the optimum condition. The removal rate constant of the RuO₂//WO₃ anode was 3.08 times than that of single WO₃ anode and 2.64 times than that of single RuO₂ anode, confirming the chlorine oxide radical had stronger ability on denitrification than reactive chlorine radical. Also, the bi-functional anode contributed to best current efficiencies, utilizing the energy availably. This work proposes a promising method of exhaustive denitrification for urea.

Urea removal strategies for dialysate regeneration in a wearable artificial kidney

The availability of a wearable artificial kidney (WAK) that provides dialysis outside the hospital would be an important advancement for dialysis patients. The concept of a WAK is based on regeneration of a small volume of dialysate in a closed-loop. Removal of urea, the primary waste product of nitrogen metabolism, is the major challenge for the realization of a WAK since it is a molecule with low reactivity that is difficult to adsorb while it is the waste solute with the highest daily molar production. Currently, no efficient urea removal technology is available that allows for miniaturization of the WAK to a size and weight that is acceptable for patients to carry. Several urea removal strategies have been explored, including enzymatic hydrolysis by urease, electro-oxidation and sorbent systems. However, thus far, these methods have toxic side effects, limited removal capacity or slow removal kinetics. This review discusses different urea removal strategies for application in a wearable dialysis device, from both a chemical and a medical perspective.

Effect of reactor configuration on the kinetics and nitrogen byproduct selectivity of urea electrolysis using a boron doped diamond electrode

Electrochemical systems have emerged as an advantageous approach for decentralized management of source-separated urine with the possibility of recovering or removing nutrients and generating energy. In this study, the kinetics and byproduct selectivity of the electrolytic removal of urea were investigated using a boron doped diamond working electrode under varied operational conditions with a primary focus on comparing undivided and divided reactors. The urea removal rate in the undivided and divided reactors was similar, but the divided reactor had an increased required cell voltage needed to maintain the equivalent current density. The current efficiency was similar for 0.1, 0.25, and 0.5 A (33.3, 83.3, 167 mA/cm²), suggesting no interference from competing reactions at higher potentials. In a divided reactor, increasing the anolyte pH reduced the urea removal rate presumably from hydroxyl radical scavenging by hydroxide. Further, for all divided reactor experiments, the final pH was less than 1, suggesting that the transport of protons across the ion exchange membrane to the cathode was slower than the oxidation reactions producing protons. The nitrogen byproduct selectivity was markedly different in the undivided and divided reactors. In both reactors, nitrate (NO₃^-) formed as the main byproduct at the anode, but in the undivided reactor it was reduced at the stainless steel cathode to ammonia. In the presence of 1 M chloride, the urea removal kinetics improved from the generation of reactive chlorine species, and the byproduct selectivity was shifted away from NO₃^- to presumably chloramines and N₂. Overall, these results indicate that the electrochemical reactor configuration should be carefully considered depending on the desired outcome of treating source-separated urine (e.g., nitrogen recovery, H₂ generation).

Electrochemical treatment of phenol-containing wastewater by facet-tailored TiO2: Efficiency, characteristics and mechanisms

Electrochemical oxidation is widely used for water and wastewater treatment. Anodic material is crucial and the shape-tailored {001}-exposed TiO₂ has been proven to be an ideal electrode material for pollutant oxidation. In this work, the electrochemical treatment of wastewater containing typical p-substituted phenols by facet-tailored TiO₂ is studied in terms of efficiency, characteristics and mechanisms. Experimental results demonstrate that the anodic oxidation of p-substituted phenols becomes more difficult with the increasing Hammett's constant (σ) of phenols, while their degradation rates (k) increase continuously with the initial surface concentration (Γ). Phenols are degraded mainly by surface-bound ·OH and direct electron transfer on the TiO₂/Ti electrode, rather than by bulk-free ·OH suspended in the aqueous phase. Theoretical calculations reveal that the surface-bound ·OH-mediated oxidation mechanism is attributed mainly to the strong surface bond strength between shape-tailored TiO₂ and water molecule as well as the reactive ·OH. Such strong interactions are associated with the higher density of atomic steps, edges and kinks of low-coordinate surface atoms with a large number dangling bonds on the high-energy {001} polar facet. For practical treatment of real wastewater with different matrixes, the facet-tailored TiO₂/Ti electrode exhibits both a high efficiency and a fast kinetics. Our findings provide a new chance to degrade phenolic pollutants in wastewater and offer atomic-scale insights into the preparation, modification and application of TiO₂-based anodic materials for electrochemical water treatment.

Removal of urea from swimming pool water by UV/VUV: The roles of additives, mechanisms, influencing factors, and reaction products

To discover an applicable technology for urea abatement from swimming pool water (SPW), this study compared the performances of seven ultraviolet (UV)-based technologies on urea removal, including UV alone, UV coupled with hydrogen peroxide (UV/H₂O₂), sulfite (UV/Na₂SO₃), potassium persulfate (UV/K₂S₂O₈), a combination of UV and vacuum UV (UV/VUV), and UV/VUV in tandem with either H₂O₂ (VUV/H₂O₂) or potassium persulfate (VUV/K₂S₂O₈). Among them, UV and UV/Na₂SO₃ showed little removal ability, and UV/H₂O₂ removed only 12.8% of urea within 3-h experiments, while UV/VUV degraded 71.7% of urea without introducing substantial total dissolved solids (TDS). Therefore, UV/VUV was considered as a promising technology for further exploration. In comparison, although UV/K₂S₂O₈ exhibited higher urea removal than UV/VUV, it caused dramatic increases of TDS, which made the regulatory threshold for the TDS increment difficult to maintain. Within UV/VUV studies, some common components in SPW (e.g., cyanuric acid, humic acid, nitrate, and bicarbonate) inhibited the removal process, whereas chloride and sulfate facilitated it, while free chlorine at doses ≤ 3 mg-Cl₂/L and pH levels from 6.8 to 8.0 imposed little impact on urea degradation. Overall, UV/VUV degraded 40.0% and 22.2% of urea from tap water and SPW, respectively; both were lower than the efficiency observed in ultrapure water. As for reaction byproducts, urea phototransformation via UV/VUV yielded nitrate and ammonia as the key products with the mass balance of nitrogen element being met. However, the contents of organic carbon decreased at a rate slightly lower than urea degradation, suggesting that urea was mostly mineralized and slightly converted to unknown organic compounds. The results hence demonstrate that UV/VUV is an effective alternative for urea removal from SPW.

In Situ Photoelectrochemical Chloride Activation Using a WO3 Electrode for Oxidative Treatment with Simultaneous H2 Evolution under Visible Light

Reactive chlorine species (RCS) such as HOCl and chlorine radical species is a strong oxidant and has been widely used for water disinfection. This study investigated a photoelectrochemical (PEC) method of RCS production from ubiquitous chloride ions using a WO₃ film electrode and visible light. The degradation of organic substrates coupled with H₂ evolution using a WO₃ electrode was compared among electrochemical (EC), photocatalytic (PC), and PEC conditions (potential bias: +0.5 V vs Ag/AgCl; λ > 420 nm). The degradation of 4-chlorophenol, bisphenol A, acetaminophen, carbamazepine, humic acid, and fulvic acid and the inactivation of E. coli were remarkably enhanced by in situ RCS generated in PEC conditions, whereas the activities of the PC and EC processes were negligible. The activities of the WO₃ film were limited by rapid charge recombination in the PC condition, and the potential bias of +0.5 V did not induce any significant reactions in the EC condition. The PEC activities of WO₃ were limited in the absence of Cl^- but significantly enhanced in the presence of Cl^-, which confirmed the essential role of RCS in this PEC system. The PEC mineralization of organic compounds was also markedly enhanced in the presence of Cl^- where dark chemical chlorination by NaOCl addition induced a negligible mineralization. The H₂ generation was observed only at the PEC condition and was negligible at PC and EC conditions. On the other hand, the oxidation of chloride on a WO₃ photoanode produced chlorate (ClO₃^-) as a toxic byproduct under UV irradiation, but the visible light-irradiated PEC system generated no chlorate.

AgI loading BiOI composites with enhanced photodegradation efficiency for bisphenol A under simulated solar light

Bismuth oxyiodide (BiOI) is a narrow band gap semiconductor which can be driven by visible irradiation. In order to efficiently separate photo-generated carriers and utilization of visible light, a facile solvothermal approach was used to synthesize a novel AgI loading BiOI 3D hierarchical composite (AgI-BiOI). The AgI-BiOI with Ag and Bi molar ratio of 1:8 (AgI-BiOI (1-8)) showed great enhancement for photocatalytic degradation of bisphenol A (BPA) with pseudo-first degradation rate constant about 3.7 or 14.5 times than that of pristine BiOI or AgI under simulated solar light. This synergistic enhancement for BPA degradation on AgI-BiOI(1-8) is mainly ascribed to enhancing the light absorption intensity and accelerating photo-generated carriers separation due to the formation of AgI-BiOI heterojunction. Free radical quenching experiments proved that positive holes (h⁺) and superoxide (O₂^•-) radicals were dominantly responsible for the degradation of BPA rather than singlet oxygen (¹O₂) or hydroxyl radicals (•OH). The AgI-BiOI(1-8) hardly showed any ecotoxicity to C. elegans through lethal experiments. The luminance bacteria acute toxicity of degradation intermediates of BPA increased before 30 min then reduced significantly with reaction. The good durability and environmental-friendly characteristics make AgI-BiOI(1-8) catalyst to be a good solar light-driven candidate.

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.