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• 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]

• 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]

• Efficient degradation of N-containing organic wastewater via chlorine oxide radical was proposed. • Chlorine oxide radical was generated via a photoelectrochemical system. • N-ammonia was oxidized directly into N 2 with the reaction of chlorine oxide radical. N-containing organic wastewater is harmful to human health and ecosystem while its degradation remains a huge challenge for decarburization and denitrification simultaneously. In this paper, we proposed a new photoelectrochemical method of chlorine oxide radicals production to oxidize mixture wastewater of phenol and N-ammonia into N 2 and CO 2. The chlorine oxide radicals were generated via a rapid reaction of HClO with hydroxyl radicals or chlorine free radicals on a synergistic dual anode, in which hydroxyl radicals and chlorine free radicals generated on WO 3 side, and HClO generated on Sb-SnO 2 side, respectively. Phenol degradation rate on WO 3 -Sb/SnO 2 was 2.54 and 4.79 times than that on WO 3 and Sb/SnO 2. Meanwhile, N-ammonia removal rate on WO 3 -Sb/SnO 2 was 5.69 and 20 times greater than that on WO 3 and Sb/SnO 2 , respectively. The effects of potential, Cl− concentration and initial pH were examined and the optimal conditions were potential 2.0 V vs Ag/AgCl, 0.05 M NaCl, pH = 5. The free radical quenching experiments and electron-spin resonance (ESR) confirmed that chlorine oxide radicals played an important role in the degradation of N-ammonia and phenol. Reasonable pathways of phenol in the WO 3 -Sb/SnO 2 system were also proposed. This work provides an exhaustive, novel, environmental-friendly photoelectrochemical system for N-containing organic wastewater treatment. [ABSTRACT FROM AUTHOR]

• A photoelectrocatalytic system based on carbonate radical reactions is constructed. • Carbonate radical is generated from HCO 3 − by the initiating of photocatalysis. • Organic pollutants degradation and electricity generation can be greatly enhanced. In this paper, we propose an efficient simultaneous refractory organics degradation and electricity generation method for carbonate-containing wastewater based on carbonate radical reactions initiated by a BiVO 4 -Au/PVC (PVC: photovoltaic cell) system. In the system, nanoporous BiVO 4 film and Au modified PVC were used as photoanode and photocathode, respectively. HCO 3 − was used as the electrolyte. Carbonate radicals, which have lower recombination rates than hydroxyl radicals and strong oxidation abilities, can be generated easily by the capture reaction of hydroxyl radicals with HCO 3 −, which is one of the most abundant anions in the aquatic environment. The results show that the removal ratios of rhodamine b, methyl orange and methylene blue in the system increased sharply to 77.98 %, 89.15 % and 93.2 % from 18.23 %, 21 % and 23.14 % (BiVO 4 -Pt/ SO 4 2-), respectively, after 120 min. Meanwhile, the short-circuit current density is up to 2.19–2.41 times larger than the traditional system. Other common ions in natural water minimally affected the properties of the new system. The excellent performance could be ascribed to large amounts of carbonate radicals in the system, which have great potential for efficient carbonate-containing wastewater treatment and energy recovery. [ABSTRACT FROM AUTHOR]