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Exhaustive conversion of inorganic nitrogen to nitrogen gas based on a photoelectro-chlorine cycle reaction and a highly selective nitrogen gas generation cathode Yan Zhang, Jinhua Li, Jing Bai, Zhaoxi Shen, Linsen Li, Ligang Xia, Shuai Chen, and Baoxue Zhou Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04626 • Publication Date (Web): 29 Dec 2017 Downloaded from http://pubs.acs.org on December 30, 2017
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Exhaustive conversion of inorganic nitrogen to nitrogen gas based on a
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photoelectro-chlorine cycle reaction and a highly selective nitrogen gas
3
generation cathode
4
Yan Zhang a, Jinhua Li a, Jing Bai a**, Zhaoxi Shen a, Linsen Li a, Ligang Xia a, Shuai Chen a,
5
Baoxue Zhou a,b∗∗
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a
7
No. 800 Dongchuan Rd, Shanghai 200240, China.
8
b
9
Education, Shanghai 200240, PR China
School of Environmental Science and Engineering, Shanghai Jiao Tong University
Key Laboratory of Thin Film and Microfabrication Technology, Ministry of
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∗ Corresponding author. School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, PR China. ** Corresponding author. Email addresses:
[email protected] (B. Zhou) ;
[email protected] (J. Bai)
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ABSTRACT:
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A novel method for the exhaustive conversion of inorganic nitrogen to nitrogen gas
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was proposed in this paper. The key properties of the system design included an
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exhaustive photoelectrochemical cycle reaction in the presence of Cl−, in which Cl•
15
generated from oxidation of Cl− by photoholes selectively converted NH4+ to nitrogen
16
gas and some NO3− or NO2−. The NO3− or NO2− was finally reduced to nitrogen gas
17
on a highly selective Pd-Cu-modified Ni foam (Pd-Cu/NF) cathode to achieve
18
exhaustive conversion of inorganic nitrogen to nitrogen gas. The results indicated
19
total nitrogen removal efficiencies of 30 mgL-1 inorganic nitrogen (NO3−, NH4+,
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NO3−:NH4+=1:1 and NO2−:NO3−:NH4+=1:1:1) in 90 min were 98.2%, 97.4%, 93.1%
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and 98.4%, respectively, and the remaining nitrogen was completely removed by
22
prolonging the reaction time. The rapid reduction of nitrate was ascribed to the
23
capacitor characteristics of Pd-Cu/NF that promoted nitrate adsorption in the presence
24
of an electric double layer, eliminating repulsion between the cathode and the anion.
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Nitrate was effectively removed with a rate constant of 0.050 min-1, which was 33
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times larger than that of Pt cathode. This system shows great potential for inorganic
27
nitrogen treatment due to the high rate, low cost and clean energy source.
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Keywords: exhaustive cycle process, Pd-Cu/Ni foam, photoelectro-chlorine cycle
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reaction, WO3 nanoplate array, inorganic nitrogen removal
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1. INTRODUCTION
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With the rapid development of industrialization processes, a large amount of inorganic
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nitrogen (ammonia, nitrate and nitrite) wastewater has entered natural water, leading
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to potential threats to humans and ecosystems, such as methemoglobinemia1 and
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eutrophication2. Diverse technologies have been developed to treat the nitrogen
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contamination, such as ion exchange,3,
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chlorination6 and electrochemical processes.7 In most cases, it is difficult to
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completely convert inorganic nitrogen to nitrogen gas in one-directional conversion
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pathway: ammonia is easily oxidized to nitrate, while nitrate is always converted to
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ammonia, resulting in low removal efficiency of the total nitrogen. The biological
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method can convert inorganic nitrogen to nitrogen gas by nitrification and
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denitrification, but it requires precise control of the carbon source to give a certain
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C/N ratio, which is a bottleneck for the treatment of wastewater with high nitrogen
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contents.
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In recent years, the photoelectrocatalytic (PEC) technique has drawn great attention
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because it offers an eco-friendly route to the degradation of pollutants and generation
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of electricity by using sunlight8-12. Wang13 developed a PEC system by using a
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p-GaInP2 photoelectrode for nitrate-N reduction. Xiao14 et al. treated ammonia-N
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wastewater by using TiO2 nanotube arrays. However, most of the studies only realized
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one-directional conversion of inorganic nitrogen, and the destruction rate remained
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low.
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Herein, we propose the exhaustive PEC cycle system for rapid removal of inorganic
4
biochemical treatments5, breakpoint
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nitrogen in a solution containing Cl−, achieving the simultaneous reduction of NO3− or
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NO2− and oxidation of NH4+. Under illumination, Cl− was oxidized to chlorine
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radicals (Cl•) by photoholes, and Cl• selectively transformed NH4+ into nitrogen gas
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and small amounts of NO3− or NO2− (Eqs.2-4). Meanwhile, NO3− or NO2− in solution
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can be reduced to nitrogen gas on cathode (Eqs.5-6). Finally, inorganic nitrogen was
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completely converted to N2. The introduction of Cl− expanded the radical reaction to
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the whole solution, and Cl• reacts with NH4+ more rapidly than •OH. In addition, the
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Cl− underwent a catalytic cycle in the process15, avoiding the addition every run.
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h++Cl−→Cl
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2NH4++6Cl→N2+6Cl−+8H+
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NH4++8Cl+3H2O→NO3−+10H++8Cl−
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NH4++6Cl+2H2O→NO2−+8H++6Cl−
(minor)
(4)
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NO3−+3H2O+5e−→2N2+6OH−
(major)
(5)
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NO3−+7H2O+8e−→NH4++10OH−
(minor)
(6)
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However, NO3− or NO2− were generated in the process, resulting in incomplete
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nitrogen removal. The removal rate is also unsatisfactory because NO3− (or NO2−) is
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difficult to be adsorbed on the cathode surface due to the repulsion between like
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charges. Moreover, nitrate is easily converted to the byproduct NH4+. To overcome
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these drawbacks, a Pd-Cu modified Ni foam cathode was designed to effectively
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reduce the anions. A highly electronically conductive nickel foam with a 3D structure
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was selected as the cathode substrate16, 17 because of high diffusion efficiency. Cu and
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Pd
18, 19
(1) (major) (minor)
(2) (3)
are considered to be efficient promoters for nitrate reduction. The Pd-Cu/NF
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cathode has the characteristics of a capacitor, and nitrate easily adsorbs to the cathode
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due to the positive charge of its surface, thereby facilitating nitrate reduction.
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Furthermore, Pd-Cu selectively enhances the reduction of nitrate to nitrogen gas
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following the adsorption process, inhibiting the formation of NH4+.
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Based on this idea, we proposed a novel exhaustive cycle using Cl− as the medium
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and Pd-Cu/NF as the cathode to convert inorganic nitrogen to N2 under solar light.
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The Pd-Cu/NF cathode was attained by electrodeposition, and a WO3 nanoplate array
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film was selected as the photoanode for its visible light response and nontoxicity 20.
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Using this system, inorganic nitrogen removal was carried out by investigating the
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effects of different operation parameters (pH, applied potential and Cl− concentration),
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the final products and the degradation pathways. The results showed that the removal
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efficiency of inorganic nitrogen was greatly promoted and we hope this paper
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provides an economical and efficient method for sustainable wastewater treatment.
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2. MATERIALS AND METHODS
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Chemicals and Materials. Ni foam (3 mm thin) was purchased from Kunshan
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Jiayisheng Electronics Co., Ltd. NaNO3, NaNO2, (NH4)2SO4 and NaCl were
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purchased from Sinopharm Chemical Reagent Co., Ltd. and used as the sources of
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NO3−-N, NO2−-N, NH4+-N and Cl−. Unless otherwise indicated, all chemicals were of
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analytical reagent, and deionized water was produced by a Milli-Q ultrapure water
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system.
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Electrode preparation. Prior to the synthesis, the Ni foam was carefully cleaned
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through consecutive sonication in H2SO4 (1.0 M), acetone and deionized water to
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surface
oxidized
layer. The
Pd-Cu/NF
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remove
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co-electrodeposition of Pd and Cu using potentiostatic technique. A solution
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composed of 2 mM PdCl2, 4 mM CuSO4·5H2O and 0.1 M HCl was used as the
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deposition electrolyte. The Ni foam, Pt foil and a saturated calomel electrode (SCE)
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were used as the working electrode, counter electrode and reference electrode,
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respectively. The deposition voltage was controlled at −1.0 V vs. SCE for 30 min by
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using an electrochemical workstation (CHI 660C, Chenhua Instrument Co., Ltd.,
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China). Then, the Pd-Cu/NF was cleaned with deionized water and subsequently dried
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at 60°C under vacuum. For comparison, Pd/NF and Cu/NF electrodes were prepared
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via a similar method in the absence of CuSO4·5H2O or PdCl2 in the deposition
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electrolyte.
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The WO3 photoanode was prepared according to the method in our previous report21,
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and the detailed procedure is provided in the supporting information.
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Experimental Setup. The degradation of inorganic nitrogen was carried out in a
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single 50 mL electrolytic cell. During the reaction, the light source was a 350 W Xe
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lamps with an AM 1.5 filter (light intensity, 100 mW cm−2). Unless otherwise
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specifically mentioned, 40 mL of synthetic wastewater containing 0.05 mol L-1
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Na2SO4 and 0.05 M NaCl was treated at pH 5.0. NaOH or H2SO4 (0.5 M) was used to
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adjust the pH of the solution. The immersed area of both the Pd-Cu/NF cathode and
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the WO3 anode was 4 cm2 (2 cm×2 cm). The distance between the two electrodes was
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maintained at 1.5 cm. An electrochemical workstation was employed as the power
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source, and a potential of -1.2 V vs SCE was applied.
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Analytical Methods. The surface morphologies of Pd-Cu/NF and WO3/FTO
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electrodes were studied by a scanning electron microscope (SEM, Zeiss
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SUPRA55-VP) equipped with an energy-dispersive X-ray spectrometer. The
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crystallinities of the Cu/NF, Pd/NF and Pd–Cu/NF electrodes were identified by X-ray
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diffraction (XRD, Rigaku D-Max B). The composition of the prepared electrode was
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determined by X-ray photoelectron spectroscopy (XPS, AXIS UltraDLD).
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Electron-spin resonance (ESR) spectra were obtained on a Bruker EMX-8/2.7. Laser
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Flash Photolysis experiments were performed on a laser flash photolysis spectrometer
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(LP920) to detect Cl2•−.
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Linear sweep voltammetry (LSV) was conducted at the Pd-Cu/NF and NF electrodes
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in solutions containing varying amounts of NaNO3 and 0.05 M NaCl with a sweep
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rate of 10 mV s−1. Nitrate and nitrite concentrations were measured by ion
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chromatography (Dionex, USA), and ammonia was determined by Nessler reagent
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using a UV-visible spectrophotometer (752N, INESA, Shanghai) at 420 nm. The
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amount of N2 was measured in closed system with a gas chromatograph
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(GC-2010Plus, SHIMADZU) equipped with a thermal conductivity detector.
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3. RESULTS AND DISCUSSION
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Characterization of the Pd-Cu/NF electrode. Figure 1 shows SEM images of the
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NF and Pd–Cu/NF electrodes, respectively. A cross-linked grid structure with
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abundant and regular superficial wrinkles was observed for the NF in Figure 1a,
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providing a high porosity and large specific surface area. After deposition of Pd-Cu,
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the NF turned from gray to black (Figure S2). The image in Figure 1b showed
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electrodeposited materials uniformly covered NF, though some agglomeration were
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observed. The particle size was approximately 150 nm, and this increases the surface
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area and provides interspaces for penetration of the electrolyte. The elemental
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distribution of the Pd-Cu/NF electrode was obtained by EDX. The elemental mapping
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revealed the K-edge signals of Pd, Cu, and Ni (Figure S3). The observed even
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distribution of Pd, Cu and Ni confirmed the uniform deposition of the Pd-Cu array.
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Figure 1
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The XRD patterns of the NF, Pd/NF, Cu/NF and Pd-Cu/NF were investigated, and the
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results are presented in Figure 2a. Pure NF had strong peaks at 2θ angles of 44.5°,
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51.8° and 76.4°. A typical Pd peak was observed at 40.1° in the Pd/NF and Pd-Cu/NF
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cathodes18. However, the characteristic diffraction lines of Cu were not observed. The
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Cu3Pd phase has two strong peaks at 2θ=42° and 48°, which are close to the Ni
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peaks22 and may be responsible for the absence of Cu peaks. The low crystallinity of
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the copper phase may be another reason.23 Moreover, the observed Ni peak of
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Pd-Cu/NF was in accordance with that of NF, and an obvious peak shift was observed,
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indicating that the Pd-Cu film synthesized on the NF by electrodeposition was
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bimetallic and composed of an alloy rather than separate metals.
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The detailed elemental composition and valence states were characterized by XPS
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measurements. The spectra of Pd-Cu/NF (Figure 2b) shows the characteristic peaks
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of Pd and Cu elements. As illustrated in Figure 2c, the binding energies of the
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spin-orbit coupling (Pd 3d3/2 and Pd 3d5/2) appeared at 340.5 eV and 335.2 eV,
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respectively, which can be assigned to metallic (Pd0)
24, 25
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was located at 342.2 eV and 337.0 eV, attributed to the presence of Pd2+. Figure 2d
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presents the Cu 2p spectrum of the Pd-Cu/NF electrode, which was deconvoluted into
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three different peaks at 931.8, 934.4, and 942.7 eV, corresponding to Cu0, Cu+, and
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Cu2+, respectively26.
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Figure 2
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Nitrate and nitrite reduction on the Pd-Cu/NF cathode. The performance of
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Pd-Cu/NF cathode in NO3− removal was compared with that of Pd/NF, Cu/NF, NF
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and Pt cathodes, and the results are shown in Figure 3a. Obviously, Pd-Cu/NF
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cathode showed the best performance. The results showed that only 3.3 mgL-1 nitrate
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was reduced when Pt was used as cathode. This is because Pt is good for H2 evolution
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in acidic solutions, which suppresses the reduction of NO3−. For the NF cathode,
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nitrate reduction was slow, with a conversion yield of 6.3 mgL-1 after 90 min, while
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yields of only 18.2 mg L-1 and 20.6 mg L-1 were obtained for the Cu/NF and Pd/NF
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cathodes. The rate data were fitted with a pseudo-first-order rate equation as follows:
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ௗሾேைయష ሿ
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The apparent rate constants for the Pt, NF, Cu/NF, Pd/NF and Pd-Cu/NF electrodes
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were 0.0015, 0.002, 0.010, 0.012 and 0.050 min-1, respectively, as shown in Figure S4.
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These results indicated that the Pd-Cu/NF had the fastest reduction rate, which was 33
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times greater than that of Pt. Figure S5 shows the ability of the Pd-Cu/NF cathode to
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perform nitrite reduction. The results showed that 30 mg L-1 of nitrite was removed
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completely within 60 min. Some nitrate was generated at the beginning and then was
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removed. This was because some nitrite was oxidized to nitrate by the photoholes27.
ௗ௧
= −ܭሾܱܰଷି ሿ
(7)
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This result indicated that nitrite can be removed either through direct reduction
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(NO2−→N2 and NH4+) or indirect reduction (NO2−→NO3−→N2 and NH4+) in the
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exhaustive cycle process. The produced NH4+ could be further converted to nitrogen
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gas by chlorine free radicals. To investigate the formation of N2 in the system, we
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performed the nitrate reduction in a closed-batch reactor (Figure S6). NO3− was
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completely removed in 90 min, while the NO2− formation was very low throughout
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the experiment (Figure S6b). The results showed the concentrations of N2 was about
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28.7 mg L-1 at 90 min, indicating that N2 was the main gaseous product.
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In view of the reduction characteristics, the LSV of Pd-Cu/NF and NF were measured.
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The LSV of the NF electrode in a 0.05 M chloride solution showed an obvious rise in
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the current at 1.1 V (Figure S7a), which was attributed to hydrogen evolution. After
196
the addition of nitrate, a current increase appeared at 1.3 V, resulting from nitrate
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reduction. A higher current was observed on the Pd-Cu/NF cathode under identical
198
scan operations (Figure S7b), demonstrating its prominent nitrate reduction capability.
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Meanwhile, a current peak was observed for the Pd-Cu/NF electrode at 0.5 V, which
200
was due to the adsorption of nitrate28. However, defined peaks were not observed for
201
the NF cathode. These results indicated that the Pd-Cu/NF cathode easily captured
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nitrate for reduction. This was probably because the Pd-Cu/NF cathode had the
203
characteristics of a capacitor and formed an electric double layer, which was
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conducive to the adsorption of nitrate on the cathode29.
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Figure 3
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Ammonia oxidation in the exhaustive cycle process. Figure 3b shows the
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degradation curves of NH4+-N for different processes. We found NH4+ was not
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degraded by the PC-chlorine system, and 3.0 mg L-1 of NH4+ was removed by the
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EC-chlorine process within 90 min. A total of 9.4 mg L-1 of NH4+ was removed in
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PEC process because bias potential can inhibit the recombination of photogenerated
211
electrons, thereby increasing the efficiency. However, the NH4+ rapidly decreased in
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the exhaustive cycle process, and about 99% removal efficiency was achieved in 90
213
min. The variations in the NH4+ and NO3− concentrations are shown in Figure S8. In
214
the conversion process, nitrite was hardly detected, and the nitrate first increased and
215
then decreased to zero, proving that NH4+ was converted to N2. To clarify the role of
216
Cl•, experiments were carried out with specific probe scavengers (tert-butyl alcohol
217
reacts with both •OH and Cl•, while nitrobenzene reacts with only •OH)30. Figure S9
218
showed that NH4+ degradation rate significantly decreased after the addition of
219
t-BuOH, revealing that both OH and Cl were responsible for NH4+ oxidation. In
220
addition, the removal efficiency was slightly inhibited by the addition of NB,
221
suggesting that Cl• played a crucial role. To further confirm the conjecture, ESR
222
technique with DMPO as trapping agent was used to determine the radical species. As
223
shown in Figure 3c, quartet peaks of DMPO-OH• with 1:2:2:1 intensity were
224
detected in the absence of Cl−, while an eleven-line ESR spectra were obtained in the
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PEC-chlorine system. Previous studies demonstrated that a seven-line ESR spectra
226
corresponds to DMPO-X31, which indicated that both •Cl and OH• were generated in
227
the process. The transient absorption spectrum of PEC-chlorine system showed a
228
typical absorption at λ=340 nm (Figure 3d), which corresponding to Cl2•− radical32.
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Generally, the Cl2•− radical is formed through the reaction between Cl• and Cl−, which
230
also indicating that Cl• was generated.
231
A separated dual-cell PEC-chlorine system with a cation exchange membrane was
232
used to investigate the side-reactions (Figure S10). In cathode chamber, nitrate is
233
reduced to N2 and NH4+ (Figure S10b). However, in anode chamber, there is a slight
234
decrease of nitrate, and it cannot detect ammonium because of the presence of
235
chloride ions (Figure S10c). We then monitored the pH changes in the system and
236
found the pH of the solutions increased in cathode and decreased in anode chamber,
237
which was indicative of the water decomposition (Figure S10d).
238
Mechanism of inorganic nitrogen removal. Based on the above experiment, a
239
reasonable schematic mechanism for the removal of inorganic nitrogen in the
240
PEC-chlorine system was proposed (Scheme 1). Nitrate and nitrite were reduced at
241
the cathode. The adsorption of nitrate to the cathode was the first step and Pd-Cu/NF
242
electrode has the characteristics of a capacitor, which could quickly adsorb nitrate or
243
nitrite (Eq.8 and 11) through the positive charge. After adsorption on Cu site, NO3−
244
was reduced to NO2− and then further reduced to N2 on the Pd sites (Eqs. 9-12). The
245
reduction intermediate was nitrite, and a small amount of nitrite was observed during
246
monitoring of the nitrate reaction. In addition, nitrate could be directly reduced to N2.
247
Nitrate reduction was accelerated at a much higher rate on the Pd-Cu/NF cathode than
248
on the other studied electrodes.
249
NO3−+Cu ↔ Cu-NO3−ads
(8)
250
Cu-NO3−ads +H2O+2e−→NO2−+Cu+2OH−
(9)
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Cu-NO2−ads+5H2O+6e−→NH3+7OH−
(10)
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NO2−+Pd ↔ Pd-NO2−ads
(11)
253
2Pd-NO2−ads+4H2O+6e−→N2 (g) +2Pd+8OH−
(12)
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Next, NH4+ in the solution were oxidized at the photoanode. Under irradiation,
255
photogenerated holes (h+) were generated at the WO3 and could oxidize water to HO•,
256
while Cl• is generated by direct hole oxidation or through a transient adduct of Cl−
257
with the OH• (Eqs. 13-17). The Cl• will react with Cl− that can form Cl2•− and further
258
to HOCl/OCl− (Eqs. 18-19). It is known that HO•, Cl• and HClO are strong oxidants
259
that can react with NH4+ (Eqs. 20-21). Cl• is a selective oxidant and its reactivity is
260
higher than that of HO• in the oxidation of NH4+ (Eqs.22-26). Cl• readily undergoes
261
rapid addition, hydrogen abstraction, and direct electron transfer reactions with NH4+,
262
foaming a series of radicals (NH2• and •NHCl), and chloramines are generated as
263
reaction intermediates. In addition, the chloramines can be further oxidized to produce
264
gaseous nitrogen. It shows that a cyclic reaction between Cl− and Cl• was formed after
265
the complete oxidation of nitrogen to nitrogen gas. It should be noted that small
266
amount of NO3− was formed, indicating NH4+ was excessively oxidized. HO• is
267
theoretically preferable for the oxidation of NH4+ into NO3− and possible pathway
268
were shown in Eqs. 27-30. NH2• can be transformed to NH2OH by HO•. Then,
269
NH2OH can be oxidized to NO2−, and further oxidized to NO3−. However, the
270
generated NO3− could be reduced to nitrogen gas at the Pd-Cu/NF cathode to achieve
271
exhaustive treatment of inorganic nitrogen through cycle reaction.
272
WO3 + hv → h+ + e−
(13)
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H2O + h+ → HO• + H+
(14)
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Cl−+ h+ → Cl•
(15)
275
HO• +Cl− → ClOH•−
(16)
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ClOH•−+H+→ Cl•+H2O
(17)
277
Cl• + Cl−→ Cl2•−
(18)
278
Cl2•− + H2O→HOCl+Cl−+H++e−
(19)
279
NH4+ + Cl• → NH2• + Cl− + 2H+
(20)
280
NH2• + HO-Cl →NH2Cl + HO•
(21)
281
NH2Cl + Cl• → •NHCl+ Cl− + H+
(22)
282
•NHCl + HO-Cl → NHCl2 + HO•
(23)
283
NHCl2+ NH2Cl→N2+3H++3Cl−
(24)
284
NHCl2+H2O→NOH +2H+ +2Cl−
(25)
285
NHCl2 + NOH → N2 + HClO + H+ + Cl−
(26)
286
NH2• + HO•→NH2OH
(27)
287
NH2OH + OH• → NO2−→NO3−
(28)
288
HClO ↔ ClO−+H+
(29)
289
4ClO−+NH4+→NO3−+ H2O+4Cl−+2H+
(30)
290
Scheme 1.
291
Effect of the applied potential. The applied potential is an important parameter for
292
controlling the rate of reactions. Figure S11 shows the variations in the NH4+, NO3−
293
and TN concentrations under different potentials ranging from -0.5 to -1.5 V (vs.
294
SCE). As shown in Fig. S11a, the nitrate reduction rate increased with increasing
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potential within 0.5-1.2 V. The enhancement of the nitrate reduction was attributed to
296
the occurrence of nitrate reduction at a certain potential. The optimal nitrate removal
297
was identified to occur at -1.2 V. Meanwhile, a decrease in nitrate removal observed at
298
-1.5 V could be attributed to competition with hydrogen ions (Eq. 31). In addition,
299
minimal nitrite was detected in this process. As shown in Figure S11c, the trend in TN
300
removal was similar to that of the nitrate concentration, and the highest efficiency was
301
achieved at -1.2 V.
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2ܪା + 2݁ ି → ܪଶ
303
Effect of the initial pH. The solution pH is also a critical factor examining the
304
removal of nitrate by Pd-Cu/NF cathode were conducted at pH 3.0–9.0, and the
305
results are shown in Figure S12. The reduction of nitrate exhibited similar tendencies
306
at the various initial pH values. Complete nitrate reduction was observed at pH 5,
307
while 25.7 mg L-1 of nitrate was degraded by Pd-Cu/NF at pH 3. The decrease in
308
nitrate reduction was attributed to the competition of H+ with nitrate for the active
309
sites of cathode. The removal decreased when the pH was increased to 9, because
310
WO3 is unstable under alkaline conditions. The NH4+ concentration increased sharply
311
from pH 5 to 9, as observed in Figure S12b. One reason was that a high pH value
312
promotes a disproportion reaction to form chlorate, resulting in the loss of chloride
313
ions from solution33. The TN removal also revealed that moderately acidic conditions
314
(pH=5) were favorable for the reduction of nitrate (Figure S12c).
(31)
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Figure 4
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Effect of the Cl− concentration. To remove nitrogen completely, Cl− was added to
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oxidize NH4+ to N2. Figure 4a shows the influence of chloride concentration on the
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removal of nitrate and the removal efficiency slightly increased in the presence of Cl−.
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Without the Cl−, the concentration of nitrate declined from 30 to 1.57 mgL-1 within 90
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min. However, Cl− ions have a strong influence on the NH4+ generation. NH4+
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generation significantly decreased with increasing Cl− concentration in Figure 4b.
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There was still 6.93 mg L-1 NH4+-N in solution after 90 min treatment in the absence
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of Cl−, while the remaining was only 2.58 mg L-1 when 0.02 M Cl− was present, and
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nearly no NH4+ was detected with further increasing the Cl− concentration to 0.05 M
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or more. The NH4+ could be effectively oxidized to N2 with the help of Cl, resulting
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in a great improvement in the TN removal. As shown in Figure S13, the TN removal
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efficiency increased from 72.9% to 87.4% and 98.2% with the increase in the Cl−
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concentration from 0 to 0.05 M, respectively. When the concentration was further
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increased to 0.07 M, no significant improvement was observed. Based on the results
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above, the optimal Cl− concentration for the removal of nitrate was approximately
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0.05 M, which gave a high TN degradation efficiency. The exhaustive cycle system
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was also applied to treat nitrate wastewater with high concentration. Although the
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efficiencies decrease with increasing initial concentration, the absolute amount of
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nitrate removal increase. It showed that 90 mgL-1 nitrate can be removed (86.2%) in
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this system after 90 min treatment (Figure S14).
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Figure 5
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Effect of the ratio of NO3−, NO2− and NH4+. The effectiveness of the exhaustive
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cycle process was proven for NH4+ oxidation and NO3− reduction in the above study;
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however, its feasibility for the treatment of wastewater with different proportions of
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inorganic nitrogen remained to be confirmed. As shown in Figure 5a, the TN could be
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effectively removed under various ammonium and nitrate ratios. This indicates that
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the system can treat complex nitrogen-containing wastewater. However, the TN
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removal efficiency within 90 min was slightly reduced when the wastewater
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contained both nitrate and ammonium, especially in a ratio of 1:1 (93.1%). The
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variations in the NH4+ and NO3− concentrations are shown in Figure S15a. The results
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showed that the reduction rate of NO3− decreased when NH4+ was present. On one
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hand, NH4+ oxidation at the anode contributed a larger amount of NO3−. On the other
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hand, hydrogen ions were generated during NH4+ oxidation, which competed with
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nitrate for the active sites on cathode. Upon prolonging reaction time to 105 min,
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nearly 98% of the total nitrogen was removed in the exhaustive cycle process (Figure
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S15b). In addition, we found that this system can effectively treat wastewater
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containing inorganic nitrogen (NO3−, NO2−, and NH4+), and the TN removal
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efficiency was approximately 98.4% within 90 min (Figure S16).
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In addition to the inorganic nitrogen removal performance, the stability of the
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Pd-Cu/NF cathode is also important for its practical application. Five cycle runs of
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PEC treatments were performed by using the same Pd-Cu/NF cathode with the
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potential fixed at -1.2 V vs. SCE in a 30 mg L-1 nitrate solution containing 0.05 M
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chloride ions. Figure 5b showed that the nitrate removal was nearly maintained at the
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level of the fresh sample after five consecutive runs (97.7%) and the TN removal
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efficiency also keep at high level (94.2%) in Figure S17.
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ASSOCIATED CONTENT
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Additional figures as mentioned in the text.
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AUTHOR INFORMATION
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Corresponding Authors
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Email addresses:
[email protected] (B. Zhou);
[email protected] (J. Bai)
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Phone: (+86)21-54747351. Fax: (+86)21-54747351,
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ACKNOWLEDGEMENTS
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The authors would like to acknowledge the National Nature Science Foundation of
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China (No. 21776177, 51578332, 21576162) and SJTU-AEMD for support.
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Fig.1 (a) SEM images of the surface of Ni foam and (b) Pd-Cu/NF electrode. 110x50mm (300 x 300 DPI)
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Fig.2 (a) XRD patterns of the NF, Cu/NF, Pd/NF and Pd-Cu/NF electrodes. (b) The wide region scanning XPS spectrum of Ni foam and Pd-Cu/NF electrodes. XPS spectra for the narrow scan of (c) Pd 3d and (d) Cu 2p on Pd-Cu/NF electrode. 134x99mm (300 x 300 DPI)
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Fig.3 (a) NO3–-N removal efficiency on different cathodes. (b) NH4+-N degradation efficiency by electrochemical, photocatalysis, PEC and PEC-chlorine process, respectively. (c) ESR spectra of radicals trapped by DMPO in PEC system and PEC-chlorine system. (d) Transient absorption spectra under laser flash photolysis of aqueous solution in PEC and PEC-chlorine systems. Condition: Potential -1.2 V vs SCE, pH=5 and Cl– 0.05 M. 152x115mm (300 x 300 DPI)
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Scheme.1 Illustration of the inorganic nitrogen removal mechanism in the exhaustive photoelectrochemical cycle system. 94x55mm (300 x 300 DPI)
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Fig.4 Effect of Cl– concentration on (a) NO3–-N removal and (b) NH4+-N generation. Condition: potential -1.2 V vs SCE, pH=5 and 30 mg L-1 NO3––N. 119x49mm (300 x 300 DPI)
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Fig.5 (a) The removal of TN in different ratios of nitrate and ammonium (Cl– 0.05 M, pH=5, potential -1.2V vs SCE, 30 mg L-1 TN, 90 min treatment). (b) Nitrate degradation efficiency in exhaustive photoelectrochemical cycle system during five tests at 90 min intervals. Conditions: Cl– 0.05 M, potential 1.2 V vs SCE, pH=5, and 30 mg L-1 NO3––N. 119x48mm (300 x 300 DPI)
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Table of Contents 84x47mm (300 x 300 DPI)
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