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Excessive emissions of ammonia and organic pollutants have been a challenge for conventional wastewater treatment. Chlorine oxide radical-mediated advanced oxidation processes (ClO•-AOPs) have recently garnered significant interest for their benefits, such as ease of operation and high selectivity. This review presents the latest insights into the removal mechanism of ammonia and organic pollutants by ClO•, especially focusing on the similarities and distinctions between representative radicals. ClO• has a redox potential of 1.5 V and is formed via photo/photoelectrochemical and electrochemical reactions, whose concentration is significantly higher than that of Cl• and OH•. ClO• shows selective reactivity toward compounds with electron-rich moieties at higher second-order rate constants, and the major degradation mechanism is electron transfer. The reaction kinetics, free radical quenching, electron paramagnetic resonance, and radical probes are reviewed. ClO• is not easily consumed by water matrix components (carbonate and dissolved organic matter (DOM)), and its proper potential can abate the chlorinated disinfection byproducts after treatment. Lastly, we offer recommendations for further investigation of ClO• in the fields of photo/photoelectrocatalysis and electrocatalysis, as well as emphasize essential topics for future research. [Display omitted] [ABSTRACT FROM AUTHOR]

Nitrogen-doped hollow core-mesoporous shelled carbon spheres (NHMCSs) are successfully synthesized via a simple, scalable hydrothermal treatment of hollow mesoporous carbon spheres (HMCSs) with ammonia water as the nitrogen source. This strategy not only efficiently introduces nitrogen atoms into the carbon framework, but also causes the fracture of some spheres and increases the shell thickness of hollow spheres. The subsequent pyrolysis at 900 °C in Ar results in the transformation of pyrrolic-N to graphitic-N. The resulting NHMCS-900 has a high BET specific surface area (648 m 2 g −1 ), large pore volume (1.06 cm 3 g −1 ), hierarchical mesoporosity (2–8, 43.9 nm), uniform shells and large hollow cores. The electrochemical experiments indicate that NHMCS-900 as a metal-free electrocatalyst possesses the improved ORR activity with a close four-electron reaction pathway (∼3.88) and a peroxide yield lower than 10% over a wide potential range in alkaline solution. Contrary to commercial Pt/C, NHMCS-900 is unaffected by the methanol crossover effect and exhibits excellent long-term durability with 97.0% of the initial current density after 5.56 h of continuous operation. The excellent activity of NHMCS-900 is mainly attributed to the high relative content of pyridinic-N and graphitic-N. Besides, the superior activity is also ascribed to its unique structural properties, promoting fast and efficient mass transfer and providing inner and outer surfaces with N-relative active sites for ORR. [ABSTRACT FROM AUTHOR]

Nitrogen-doped hollow mesoporous carbon spheres (NHCSs) were successfully prepared via a simple, scalable hydrothermal method, followed by thermal treatment at 650 °C in ammonia atmosphere and subsequent high temperature annealing at 800–1000 °C in nitrogen, respectively. The resulting NHCSs with a particle diameter of ∼150 nm and a shell thickness of ∼20–25 nm have high specific surface areas (738–820 m 2 g −1 ), large pore volume (0.50–0.56 cm 3 g −1 ), bimodal pores system (3.9 and 51.6 nm) and adjustable N-doping levels (3.6–7.8 at.%) depending on the pyrolysis temperature. The unique structure of hollow spheres for NHCSs with uniform mesopores throughout the shells can both promote fast mass transfer and provide inner and outer surfaces with high density of N-related active sites, thus improving the reaction kinetics. The NHCS-1000 (annealed at 1000 °C) exhibited not only comparable ORR activity with a direct four-electron reaction pathway in terms of onset and half-wave potentials, and limiting current densities, but superior long-term durability and methanol-tolerance to commercial Pt/C catalyst, because it has the highest relative content of graphitic-N and pyridinic-N groups, indicating their crucial roles in ORR. The NHCS-1000 with excellent ORR performance is of potential to replace Pt/C catalyst for ORR in practical applications. [ABSTRACT FROM AUTHOR]

• Nitrogen-containing wastewater fuel cells for TN removal and energy recovery are reviewed. • Photoanode or cathode materials for strengthening Cl•/ClO• generation or NO 3 −-N reduction are discussed. • Exhaustive TN removal system through Cl•/ClO• oxidation of NH 3 N/NH 4 +-N and NO 3 − reduction is proposed. • Several typical actual nitrogen-containing wastewater fuel cells are designed. • This review provides a new idea for the resource treatment of nitrogen-containing wastewater. The excess nitrogen discharge into water bodies has resulted in extensive water pollution and human health risks, which has become a critical global issue. Moreover, nitrogenous wastewater contains considerable chemical energy contributed by organic pollutants and nitrogenous compounds. Therefore, the treatment of various kinds of nitrogen-containing wastewater for nitrogen removal and energy recovery is of significance. Biological methode and advanced oxidation processes (AOPs) are the main methods for nitrogen removal. However, biological treatment is easily inhibited by high-salinity, high ammonia nitrogen (NH 3 -N/NH 4 +-N), nitrite and toxic organics in wastewater, which limits its application. AOPs mainly induce in situ generation of highly reactive species, such as hydroxyl radical (HO•), sulfate radical (SO 4 •−) and chlorine radicals (Cl•, ClO•, Cl 2 •−), for nitrogen removal. Nevertheless, HO• shows low reactivity and N 2 selectivity towards NH 3 -N/NH 4 +-N oxidation, and SO 4 •− also demonstrates unsatisfactory NH 3 -N/NH 4 +-N removal. It has been shown that Cl•/ClO• can efficiently remove NH 3 -N/NH 4 +-N with high N 2 selectivity. The generation of Cl•/ClO• can be triggered by various techniques, among which the PEC technique shows great potential due to its higher efficiency for Cl•/ClO• generation and eco-friendly approach for pollutants degradation and energy recovery by utilizing solar energy. Cl•/ClO• oxidation of NH 3 -N/NH 4 +-N and nitrate nitrogen (NO 3 −-N) reduction can be strengthened through the design of photoanode and cathode materials, respectively. Coupling with this two pathways, an exhaustive total nitrogen (TN) removal system is designed for complete TN removal. When introducing the mechanism into photocatalytic fuel cells (PFCs), the concept of nitrogen-containing wastewater fuel cells (NFCs) is proposed to treat several typical types of nitrogen-containing wastewater, achieving high-efficiency TN removal, organics degradation, toxic chlorate control, and energy recovery simultaneously. Recent research progress in this field is reviewed, summarized and discussed, and in-depth perspectives are proposed, providing new ideas for the resource treatment of nitrogen-containing wastewater. [Display omitted] [ABSTRACT FROM AUTHOR]

Generally, the loading of oxygen evolution catalyst (OEC) must be carefully controlled to an extremely thin level or the photo-generated holes will mainly charge the catalyst rather than oxidizing water. To overcome the extreme sensitivity of charge transport to catalyst thickness, we proposed the iron valence state regulating on hematite to negatively shift the surface defect state, thereby suppressing the severe OEC charging. In detail, a fluorine layer (FeO x F y) was inserted between Fe 2 O 3 and OEC (ZIF-67) to reduce the oxidation state of iron atoms, forming a case where only part of Co(II) can be oxidized (build a rapid Co(II)/Co(III) cycle). Even in the case of thicker OEC (~10 nm), Fe 2 O 3 @F@ZIF-67 exhibits increased photocurrent density of 2.56 mA cm−2 at 1.23V RHE (~170% higher than Fe 2 O 3 @ZIF-67), accompanied by a rapid decrease in sensitivity to OEC loading. Above pretreatment not only relaxed the catalyst process requirements, but also provide new insights into interfacial catalysis process. Image 1 • Electronegativity of fluorine atoms reduce the oxidation state of surface iron atoms. • Negatively shifted interface trap states cause Co(II) to be partially oxidized. • Charge transfer of treated hematite is no longer sensitive to catalyst loading. • Interface pre-treatment plus thickness post-treatment achieve efficient water oxidation. [ABSTRACT FROM AUTHOR]

The holes transfer efficiency is a key issue in the process of photoelectrocatalytic water splitting. Here, we found that the oxygen concentration and distribution in hematite evidently affected the bulk and surface holes transfer, and thus influenced the performance of photoelectrocatalytic water splitting. Two macro-changes on oxygen were observed during the thermal treatment. The first-stage led to a lower bulk O/Fe ratio of only 1/1 and the generation of adsorbed oxygen, obtaining porous nanorods (NRs) with a ~1.4 nm FeO x layer. Bulk charge recombination centers and surface deep traps caused a lower bulk and surface charge transfer. The second-stage led to a bulk O/Fe ratio of nearly 3/2 and single high crystallization NRs. Major increases in charge transfer constant from 0.27 to 4.5 s−1 and in photocurrent density (vs.1.23 V RHE) from 0.0235 to 1.20 mA/cm2 were observed, which demonstrates the higher holes transfer efficiency. Furthermore, the connection of oxygen migration and charge transfer was derived. This work could provide effective guidance for further optimization of semiconductor photoanodes. Image 1 • Oxygen-changes in hematite evidently affected photoelectrocatalytic water splitting. • Two changes on oxygen concentration and distribution were observed by annealing. • Oxygen deficiency of bulk was showed in first stage, causing charge recombination. • Surface deep traps were formed due to excessive absorbed oxygen on the surface. • High crystallization was obtained by the second stage for superior holes transfer. [ABSTRACT FROM AUTHOR]

• Efficient method to generate •Cl for electrocatalytic urine treatment was proposed. • Electrophilic Cu(Ⅱ) was introduced in Co 3 O 4 nanowire anode. • Cu(II) attracted electrons from O-Co(II) structure to reduce charge density of Co(II). • Electrophilic Cu(II) promoted conversion of Co(II) to Co(Ⅲ) for •Cl generation. • Efficient TN removal and H 2 recovery were reached in urine treatment. Urine is a nitrogen-containing waste, but can be used as an attractive alternative substrate for H 2 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 Co 3 O 4 nanowires anode was proposed, which realized the highly efficient TN removal and H 2 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 N 2 , while the electron transport promotes H 2 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 H 2 production reached 94.7% and 642.1 μmol after 50 min, which was 1.6 times and 1.5 times that of Co 3 O 4 system, respectively. It was also 2.3 times and 2.1 times of RuO 2 , 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 H 2 production was 4.3 times higher. More importantly, excellent TN removal and H 2 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. [Display omitted] [ABSTRACT FROM AUTHOR]

• Sulfate radicals photocatalytic fuel cell (PFC) is proposed for treatment of sulfate-rich organic wastewater. • Sulfate can be converted in situ to sulfate radicals on the surface of photoanode. • The PFC system shows excellent performance in organic degradation and electricity generation. • A self-bias photoanode was also design to accelerate charges separation and electron transfer. Sulfate is a primary component of industrial wastewater and the existing wastewater treatment methods cannot effectively recover the chemical energy of pollutants. Sulfate radicals (SO 4 −•), as a strong oxidant, have obvious advantages in the degradation of refractory pollutants compared with hydroxyl radicals (HO•). In this study, a novel photocatalytic fuel cell (PFC) is first proposed for the treatment of sulfate-rich organic wastewater, in which sulfates can be converted in situ easily into sulfate radicals to degrade organics in wastewater while generating electricity. Sulfate radicals dominating the oxidation reaction of the PFC system are confirmed by the free-radical capture reaction. Furthermore, the results of the present study show that various model compounds and actual wastewater can be used as substrates in the PFC system. For instance, methyl orange, a typical refractory organic, exhibits excellent electricity generation with a short-circuit current density of 3.72 mA cm−2, open-circuit voltage of 1.32 V, and a maximum power-density output of 1.68 mW cm−2. Meanwhile, the degradation efficiency of methyl orange reached 74.2% after 4 h in the PFC system. The PFC system used self-bias photoanode consisting of a front photoanode of WO 3 and a rear photocathode of Si photovoltaic cell (Si PVC), which could accelerate the speed of charge separation and increase the electron transfer rate of the PFC cell. Therefore, the proposed PFC system provides great potential for resourceful utilization of sulfate-rich organic wastewater and chemical energy recycling. [ABSTRACT FROM AUTHOR]

Abstract 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 N 2 , and to mineralize organics into CO 2 , respectively; 2) on the cathode, NO 2 − or NO 3 − generated in the cell was selectively reduced to N 2 and tiny NH 4 + by Pd/Au/NF; 3) NH 4 + was further oxidized to N 2 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/WO 3 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. Graphical abstract Image 1 Highlights • An innovative concept of self-driven denitrified fuel cell (DFC) was proposed. • Simultaneous electricity generation and urine purification were emphasized. • Constituted poly-Si/WO 3 can accelerate the charge separation and electron transfer. • The continuous circulation could eliminate TN in urine by dual denitrification. [ABSTRACT FROM AUTHOR]

• Ammonia was effectively converted to N 2 gas in a self-driven PEC-chlorine system. • Cl radical is uniquely generated by the direct oxidation of photo-generated hole. • The oxidative force of photo-excited hole is tuned by regulating the VB edge. • Toxic byproducts chlorate and nitrate are controlled by BiVO 4 /WO 3 photoanode. The efficient removal of ammonia is a difficult issue in wastewater treatment because ammonia is easily converted to nitrate instead of N 2. The oxidation of ammonia by chlorine radical (Cl) is recognized as an effective method. However, the massive generation of toxic byproducts chlorate and nitrate pose great risk for its practical application due to the excessive oxidation capacity of hydroxyl radical. Herein, we propose a novel method to selectively generate Cl for efficient ammonia removal using BiVO 4 /WO 3 photoanode in a self-driven photoelectrocatalytic (PEC) system. Cl was predominantly produced by regulating the valence band edge of WO 3 though modifying BiVO 4 , which tuned the moderate oxidative force of hole to reduce OH generation and thereby inhibited the formation of chlorate and nitrate. The self-driven ammonia degradation was achieved by employing BiVO 4 /WO 3 and Si photovoltaic cells as composite photoanodes to improve light-absorption and electron-hole separation, thus enhancing Cl production. These results showed that 10 mg L−1 of ammonia-N was completely removed (99.3 %) in 120 min with 80.1 % of total nitrogen removal. Toxic byproducts chlorate and nitrate were inhibited by 79.3 % and 31 %, respectively, compared to WO 3. This work provides new insights to develop efficient, energy-saving and environment-friendly method for ammonia pollution treatment. [ABSTRACT FROM AUTHOR]

• A method of removal TN, organics, emerging pollutants for RO concentrate is proposed. • The method based on photoelectrocatalytic radical reaction system. • Inorganic salt in RO concentrate was used as electrolyte to generate free radicals. Reverse osmosis (RO) concentrate becomes a new knotty problem in water treatment for its high concentrated salts and complex pollutants. In this study, we presented a photoelectrocatalytic (PEC) chloride-radical (Cl) system to efficiently remove its pollutants including total nitrogen, natural organic matter and emerging pollutants simultaneously in taking advantage of pre-existing chloride and sulfate salts as electrolytes. NH 4 +-N, humic acid and emerging pollutants of nitrosodiethylamine and bisphenol A were used as model pollutants. A WO 3 | [Pd- Cu/NF] electrode couples were used in the PEC system. HO and Cl radicals were generated on the surface of WO 3 photoanode. Organics including humic acid and emerging pollutants were degraded by HO, and NH 4 +-N was efficiently oxidized to N 2 with the selective reaction of Cl and NH 4 +. NO 3 −-N overoxidized from NH 4 +-N was selectively reduced to N 2 on [Pd-Cu/NF] cathode. The results showed that the total nitrogen (TN) and total organic carbon (TOC) removal rate were 99.90% and 50.95% in 180 min for the model RO concentrate with 28 mg/L NH 4 +-N and 20 mg/L humic acid. The emerging pollutants including nitrosodiethylamine and bisphenol A performed removal rates of almost 100%. This study offers a novel, efficient and eco-friendly PEC system for deep purification of RO concentrate. [ABSTRACT FROM AUTHOR]