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Nanocomposites of Ag3PO4 and PhosphorousDoped Graphitic Carbon Nitride for Ketamine Removal Changsheng Guo, Miao Chen, Linlin Wu, Yingying Pei, Chunhua Hu, Yuan Zhang, and Jian Xu ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00295 • Publication Date (Web): 10 Apr 2019 Downloaded from http://pubs.acs.org on April 10, 2019

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ACS Applied Nano Materials

1

Nanocomposites of Ag3PO4 and Phosphorous-Doped Graphitic Carbon Nitride for

2

Ketamine Removal

3

Changsheng Guo, †, ※ Miao Chen, †, ‡, ※ Linlin Wu, † Yingying Pei, † Chunhua Hu, ‡ Yuan

4

Zhang, † Jian Xu †, *

5

†

6

Research Academy of Environmental Sciences, Beijing, 100012, China

7

‡

8

Education, School of Resources Environmental and Chemical Engineering, Nanchang

9

University, Nanchang, 330031, China

State Key Laboratory of Environmental Criteria and Risk Assessment, Chinese

Key Laboratory of Poyang Lake Environment and Resource Utilization, Ministry of

10

1

ACS Paragon Plus Environment

ACS Applied Nano Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

11

ABSTRACT

12

As one of the most abused illicit drugs, ketamine (KET) has been widely detected in

13

different water environment around the globe, which necessitates the development of

14

effective approaches for KET removal from water. In the present study, several novel

15

Ag3PO4/P-g-C3N4 heterojunction composites were successfully constructed using in-situ

16

growth method, and the samples were characterized by a serious of instruments. The

17

synthesized samples were deployed for KET degradation. Results showed that

18

Ag3PO4/P-g-C3N4 (1:1) exhibited the most excellent photocatalytic degradation

19

performance on KET with the pseudo-first-order rate constant of 0.0326 min-1 at neutral

20

pH value, which was 3- and 6-fold faster than Ag3PO4 and P-g-C3N4, respectively. The

21

elevated photocatalytic performance of Ag3PO4/P-g-C3N4 was attributed to the

22

synergistic effects of high charge separation capacity and the Z-scheme heterojunction

23

structure. Low concentrations of dissolved organic matter, nitrate or bicarbonate

24

accelerated the KET degradation by Ag3PO4/P-g-C3N4, but high levels of these

25

constitutes would inhibit the KET degradation. The scavenging experiments revealed that

26

photogenerated superoxide radicals and holes were the main reactive species in the KET

27

removal. Twelve degradation intermediates of KET over Ag3PO4/P-g-C3N4 were

28

identified and the possible degradation pathway was proposed. Demethylation,

29

dehydrogenation, hydroxylation deamination, ring open and Na-modification were the

30

major pathways for KET degradation. The Ag3PO4/P-g-C3N4 also exhibited relatively 2


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good photocatalytic performance on KET degradation in surface water and secondary

32

effluent.

33

KEYWORDS: Ag3PO4/P-g-C3N4; ketamine; degradation mechanism; intermediate;

34

pathway

35

1. INTRODUCTION

36

Abuse of illicit drugs and their incomplete elimination in the sewage treatment

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plants (STPs) have led to their frequent detection in different types of aquatic

38

environments.

39

“club drug” worldwide for the purpose of entertainment, in addition, KET is also

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prescribed as anesthetic drugs used in humans and animals, and as antidepressant to

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relieve the symptom of depression. 4, 5 It was frequently detected in effluents and aquatic

42

environments such as surface waters,

43

groundwater.

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was up to 341 ng·L-1 and 206 ng·L-1 respectively in Taiwan.

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KET concentration ranged from 1.5 ~ 16.3 ng·L-1 within seasonal variations. 10 The KET

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concentration in STPs influents in England was up to 447.3 ng·L-1.

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conventional water treatment processes including biodegradation, hydrolysis, photolysis

48

and sorption could not effectively eliminate KET from water,

49

techniques to degrade KET is urgent and essential.

50

1-3

7, 8

As a typical illicit drug, ketamine (KET) has been widely used as the

6

hospital wastewaters

1

and even drinking and

For instance, the concentration of KET in rivers and hospital effluents 6, 9

In Beijing urban rivers

1, 6, 11, 12

11

Because the

developing new

Advanced oxidation processes (AOPs) are commonly employed to degrade illicit 3



13-15

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51

drugs in water.

As a promising AOP technique, photocatalytic oxidation has been

52

extensively used to eliminate persistent pollutants in water.

53

(g-C3N4) is a catalytic material that was able to remove tetracycline,

54

bisphenol A, 21 and other organic pollutants. 22-24 However, the degradation efficiency of

55

organic contaminants over g-C3N4 was limited by the property of its low sunlight

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utilization capability and high recombination rate of photogenerated holes (h+) and

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electrons (e-). 25, 26 Phosphorus-doped g-C3N4 (P-g-C3N4) could broaden the visible light

58

absorption region with the band energy of 2.55 eV. 27 In addition, to make full use of the

59

whole solar spectrum and elevate the charge transfer efficiency, heterojunctions are

60

generally prepared to modulate the light absorption property.

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electron mediator a heterojunction could separate the photoinduced holes and electrons

62

into two photo-systems, which could help isolate the reduction and oxidation reaction

63

sites and enhance the photocatalytic performance.

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catalyst, which has drawn much concern because it has high oxidative capability and O2

65

evolution under solar light illumination.

66

was not stable in solutions, which was photochemically decomposed or self-corroded

67

under conditions without sacrificial chemicals. 33, 36 Combination of Ag3PO4 and g-C3N4

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therefore would be an option to overcome the above shortcomings.

69

hybrid composite could enhance the stability of the composite in the degradation of

70

methylene blue. 38 Our previous studies also proved that heterojunctions may prevent the

33-35

30-32

16-18

Graphitic carbon nitride

28, 29

19

phenols,

20

For instance, with an

Ag3PO4 is a visible-light driven

However, in practical applications Ag3PO4

4


37

Ag3PO4@g-C3N4

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71

light corrosion and show superior photocatalytic activities because of their effective

72

charge separation capacity and enhanced specific surface areas.

73

knowledge, the heterojunction composite of Ag3PO4/P-g-C3N4 has not been synthesized

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previously.

39, 40

To the best of our

75

In this work, several Ag3PO4/P-g-C3N4 hybrid materials with different mass ratios

76

were prepared via thermal polymerization coupled with in situ precipitation method. The

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optimal composite was applied to eliminate KET, and the impact of parameters including

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pH, bicarbonate (HCO3-), nitrate (NO3-) and dissolved organic matter (DOM) in the

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degradation process was investigated. The reaction mechanism, intermediates and

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possible pathways of KET during the photocatalytic degradation were proposed as well.

81

As far as we know, it is the first time that the hybrid composite of Ag3PO4/P-g-C3N4 was

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deployed to eliminate illicit drugs under visible light irradiation.

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2. MATERIALS AND METHODS

84

2.1 Chemicals and reagents

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Melamine, silver nitrate (AgNO3), urea, dibasic sodium phosphate (Na2HPO4),

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ammonium monohydric phosphate ((NH4)2HPO4), sodium hydroxide (NaOH), sodium

87

bicarbonate

88

ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), 1,4-benzoquinone (BQ),

89

5,5-diemthyl-1-pyrroline N-oxide (DMPO) and sodium azide (NaN3) were of analytical

90

grade and obtained from Sinopharm Chemical Reagent (Shanghai, China). Humic acid

(NaHCO3),

hydrochloric

acid

(HCl),

5


ethanol,

isopropanol

(IPA),


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91

(HA), nitroblue tetrazolium (NBT) and terephthalic acid (TA) were of analytical grade

92

and obtained from Sigma-Aldrich (St. Louis, MO, USA). HPLC grade reagents

93

(methanol, acetonitrile and formic acid) were obtained from Fisher (Poole, UK).

94

Ketamine was purchased from Cerilliant Corporation (Round Rock, TX, USA). A

95

Milli-Q system (Millipore, MA, USA) was used to produce Milli-Q water. The reagents

96

were used directly without further purification.

97

2.2 Preparation of Ag3PO4/P-g-C3N4

98

The method to synthesize P-g-C3N4 was similar to the previous publication.

41

99

Ag3PO4/P-g-C3N4 composites were prepared by a facile in situ coprecipitation approach

100

at ambient temperature. In a typical process, 50 mL Milli-Q water and 50 mL ethanol

101

were mixed well in a beaker, then a certain quality of P-g-C3N4 were added, and the

102

suspension was sonicated for half an hour. Different amounts of AgNO3 were dissolved

103

in above suspension and magnetically stirred for 30 min in the dark. Then, 50 mL

104

Na2HPO4 solution at different concentrations was dropwise added in the mixture which

105

was vigorously stirred. With magnetically stirring for one more hour, the precipitates in

106

the solution were collected by centrifugation, rinsed by Milli-Q water and ethanol

107

respectively for 3 times, and dried under vacuum at 60oC for 24 h. The collected yellow

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powder was Ag3PO4/P-g-C3N4 hybrid composite. The materials were denoted as A/CN

109

(X), where X (1:10, 1:5, 1:2, 1:1, 2:1, 5:1, 10:1) represented the mass ratios of Ag3PO4 to

110

P-g-C3N4 in the Ag3PO4/P-g-C3N4 composites. Ag3PO4 catalyst was also synthesized 6


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111

according to the above steps without adding P-g-C3N4. The preparation of

112

Ag3PO4/P-g-C3N4 composite was shown in Scheme 1. The dosage of reagents used was

113

shown in Supporting Information Table S1.

114 115 116

Scheme 1. Illustration of the preparation of Ag3PO4/P-g-C3N4 composite. 2.3 Characterization

117

An X-ray diffractometry (XRD, Rigaku D/Max-2500) using a radiation of Cu Kα

118

(λ= 0.15406 nm) was used to determine the samples’ crystality. The morphology and the

119

particle size of the samples was analyzed by a transmission electron microscopy (TEM,

120

JEM-100CXII), high resolution transmission electron microscopy (HRTEM, JEM-2100F)

121

and scanning electron microscopy (SEM, Hitachi, s-4800) coupled with the

122

energy-dispersive X-ray spectroscopy (EDX, Oxford Aztec X-MaxN 80). The Fourier

123

transform infrared (FT-IR) spectra was recorded with a spectrometer (Nicolet

124

5SX-FTIR). UV-visible diffuse reflectance spectra (UV-vis DRS) were analyzed by a

125

UV-vis spectrophotometer (Hitachi, U-3010) with BaSO4 as the reference. The chemical 7



126

states and surface compositions of photocatalysts were identified by an X-ray

127

photoelectron spectroscopy (XPS, PHI Quantera SXM). The N2 adsorption-desorption

128

isotherms were obtained by an automatic analyzer (BET, Tristar Ⅱ 3020M).

129

Photoluminescence (PL) spectrum was investigated by a fluorescence spectrophotometer

130

(Hitachi, F-4500) with the excitation wavelength at 360 nm. The photocurrent tests were

131

conducted by the electrochemical workstation (Chenhua, CHI 660E, China) with a 300 W

132

Xenon lamp (Institute of Electric Light Source, Beijing).

133

2.4 Photocatalytic degradation experiments

134

The photocatalytic degradation was conducted in an XPA-7 photochemical reactor

135

(Xujiang Machinery Factory, Nanjing, China) at room temperature. In a typical

136

degradation procedure, 0.05 g photocatalyst and 50 mL KET aqueous solution were

137

added in a quartz tube under the visible-light irradiation by an 800 W Xenon lamp

138

(Institute of Electric Light Source, Beijing) with a 420 nm cut-off filter. Before

139

irradiation, the suspension in the quartz tube was stirred magnetically in darkness for half

140

an hour to reach the adsorption/desorption equilibrium between catalyst and KET. An

141

aliquot of 0.5 mL reaction solution was withdrawn at specific time intervals, filtered

142

through a 0.22 μm membrane filter (JinTeng, Tianjin) and ready for analysis. The control

143

experiments were conducted without photocatalysts. Detailed information on

144

instrumental analysis was provided in Supporting Information.

145

2.5 Analysis of hydroxyl radical (·OH) and superoxide radical (·O2-) 8


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TA and NBT were used as probe molecules to measure the generation of ·OH

147

and ·O2-over Ag3PO4/P-g-C3N4 composite in aqueous solution, respectively. ·OH could

148

react with TA to produce 2-hydroxyterephthalic acid, which was a highly fluorescent

149

product. 42 The fluorescence intensity of 2-hydroxyterephthalic acid is proportional to the

150

number of ·OH generated in the system. 43 NBT had absorption peak at the wavelength of

151

259 nm. ·O2- could react with NBT to generate insoluble purple formazan, which couldn’t

152

show the absorption peak at 259 nm.

153

photocatalytic experiment, except that KET solutions were substituted with 50 mL 5×10-4

154

M terephthalic acid solution (which was dissolved in 2×10-3 M NaOH solution to

155

guarantee its solubility)

156

withdrawn samples at given time were detected by fluorescence spectrophotometer

157

excited at 315 nm and UV-vis spectrophotometer (Shimadzu, UV1800, Japan),

158

respectively. Electron spin resonance (ESR) technique was conducted to further verify

159

the presence of ·OH and ·O2-, with detailed information provided in Text S3.

160

3. RESULTS AND DISCUSSION

161

3.1 Characterization of the samples

162

3.1.1 XRD analysis

45

44

The experimental procedures were similar to the

and 50 mL 1.25×10-6 M NBT solution,

44

respectively. The

163

Figure 1a illustrates the XRD spectra of the P-g-C3N4, Ag3PO4 and

164

Ag3PO4/P-g-C3N4 photocatalysts. For P-g-C3N4, the diffraction peaks at 13.1o and 27.5o

165

were corresponded to the (100) and (002) diffraction planes of g-C3N4, respectively 9



46

166

(JCPDS87-1526).

For Ag3PO4, the crystal structure was consistent with the

167

body-centered cubic phase (JCPDS06-0505).

168

showed characteristic peaks in accordance with g-C3N4 and Ag3PO4 peaks. The

169

characteristic peaks at 21.1o, 29.9 o, 33.3 o, 36.8o, 42.7o, 48.0o, 52.9o, 55.2o, 57.3o, 61.8o,

170

66.1o, 70.1o, 72.1o and 73.9o were indexed to the (110), (200), (210), (211), (220), (310),

171

(222), (320), (321), (400), (330), (420), (421) and (332) diffraction planes, respectively.

172

38

173

increasing amount of Ag3PO4 particles. The absence of diffraction peaks of P-g-C3N4 or

174

Ag3PO4 of the synthesized materials suggested the low percentage of P-g-C3N4 or

175

Ag3PO4 in the Ag3PO4/P-g-C3N4 nanocomposites.

47

The Ag3PO4/P-g-C3N4 photocatalyst

The P-g-C3N4 peak intensities decreased while the Ag3PO4 peak increased with the

10


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176 177

Figure 1. XRD patterns (a), FT-IR spectra (b) and UV-vis DRS spectra (c) of the

178

as-prepared samples; Plots of (ahv)1/2 versus hv for the band gap energy of samples (d).

179

3.1.2 SEM and EDX analysis

180

SEM images of P-g-C3N4, Ag3PO4, and Ag3PO4/P-g-C3N4 composites are presented

181

in Figure S1. P-g-C3N4 exhibited the structure of porous mesoporous, and Ag3PO4

182

materials were orbicular with the diameter size of 100 ~ 200 nm. As shown in Figure

183

S1b-e, Ag3PO4 particles also exhibited the spherical and smooth morphology, suggesting

184

its successful dispersion on P-g-C3N4 materials surface by ion exchange. The elemental

185

mapping scanning of A/CN (1:1) with different EDX elemental distribution maps were 11



186

shown in Figure S2. Ag and P elements were dispersed on the g-C3N4 surface, in

187

addition, EDX elemental maps illustrated the different elements distribution in the

188

composite of Ag3PO4/P-g-C3N4. As shown in Figure S2b-e, the elements of C, O, P, N,

189

and Ag were uniformly dispersed on the surface of the obtained photocatalysts, implying

190

that P-g-C3N4 and Ag3PO4 were tightly combined.

191

3.1.3 TEM analysis

192 193

Figure 2. TEM images of the synthesized composites. (a) A/CN (1:5); (b) A/CN (1:2); (c)

194

A/CN (1:1); (d) A/CN (2:1); (e) A/CN (10:1) and (f) Ag3PO4.

195

TEM images are shown in Figure 2. The pure Ag3PO4 showed an irregular

196

spherical structure with a diameter about 50 ~ 200 nm (Figure 2f). For all 12


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197

Ag3PO4/P-g-C3N4 composite, P-g-C3N4 (color in dark grey) exhibited a thin and lamellar

198

structure without steadfast outline. Ag3PO4 particles (color in black) with a similar size

199

were uniformly deposited on the P-g-C3N4 surface. The results indicated that the

200

P-g-C3N4 catalysts could be regarded as the supporting structure to bound Ag3PO4

201

particles in the Ag3PO4/P-g-C3N4 composites. Ag nanoparticles with diameter of 2 ~ 10

202

nm could be observed on the Ag3PO4/P-g-C3N4 surface, which could serve as the center

203

to capture e- from conductive band (CB) of Ag3PO4 and h+ from valence band (VB) of

204

P-g-C3N4. HRTEM image of A/CN (1:1) was shown in Figure S3, which verified the

205

presence of metallic Ag and formation of hybrid heterojunction on Ag3PO4/P-g-C3N4

206

composite. Ag3PO4 and P-g-C3N4 had large direct-contact areas, and the intimate contact

207

between them resulted in the formation of heterojunction structure. This structure could

208

promote the stability of the composites and favor the charge transfer in the heterojunction

209

system, which could elevate the photocatalytic performance of Ag3PO4/P-g-C3N4 by

210

facilitating the separation efficiency of photo-induced e--h+ pairs.

211

metallic Ag in Ag3PO4/P-g-C3N4 composite could be further proved by XPS results.

212

3.1.4 FT-IR analysis

49

The presence of

213

FT-IR spectra of synthesized photocatalysts are shown in Figure 1b. The peak at 559

214

cm-1 in the Ag3PO4 spectrum was attributed to the O=P-O stretching vibration, and the

215

1005 cm-1 peak was corresponding to the P-O-P bending vibration in PO43-. 48 Except for

216

Ag3PO4, the peaks of other samples at 1633 cm-1 ~ 1223 cm-1 were ascribed to the typical 13



217

stretching vibration of C=N and C-N, and the absorption peak at 817 cm-1 corresponded

218

to the triazine units of g-C3N4. 50 For the composites containing P-g-C3N4, the wide peaks

219

ranging from 3500 ~ 3000 cm-1 could be assigned to the stretching vibrations of NH2 or

220

NH groups.

221

peaks at 817 cm-1, 1406 cm-1 and 1649 cm-1 decreaed with the increasing contents of

222

Ag3PO4 in a series of Ag3PO4/P-g-C3N4 composites. All characteristic peaks of g-C3N4

223

and Ag3PO4 were found in the A/CN (2:1, 5:1, 10:1) composites, indicating the

224

successful synthesis of Ag3PO4/P-g-C3N4 composite by photo-deposition and ion

225

exchange measures.

226

3.1.5 UV-vis DRS analysis

20

The intensity of the peaks at 559 cm-1 and 1005 cm-1 increased while the

227

UV-Vis DRS spectra illustrating the optical properties of the samples are presented

228

in Figure 1c. The absorption edge of P-g-C3N4, Ag3PO4 and A/CN (1:1) were

229

approximately 481 nm, 550 nm and 561 nm, respectively, indicating their excellent light

230

absorption capacity. The absorption intensity of Ag3PO4/P-g-C3N4 composites increased

231

with the increasing Ag3PO4 ratio, which could enhance the composites photocatalytic

232

activity under visible light irradiation. In comparison to P-g-C3N4, the visible light

233

absorption region had been observably enhanced by the interaction between Ag3PO4 and

234

P-g-C3N4 in the composites, indicating Ag3PO4/P-g-C3N4 hybrids were responsive to

235

visible light with high photocatalytic activities.

236

The band gap energy of the photocatalysts is evaluated with the formula (1): 51 14


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αhv = A(hv-Eg)n/2

237

(1)

238

where α is the absorption coefficient, h is Planck constant, v is the light frequency, A is a

239

constant and Eg is band gap energy. n is decided by the optical transition types of

240

semiconductors. n=1 and n=4 are expression of the direct and indirect transition

241

semiconductor, respectively. According to the plots of (αhv)1/2 versus hv in Figure 1d, the

242

band gap of Ag3PO4, P-g-C3N4 and A/CN (1:1) were calculated to be 2.14 eV, 2.49 eV

243

and 2.16 eV, respectively. The doping of phosphorus on the g-C3N4 caused the different

244

band gap between the prepared P-g-C3N4 (2.49 eV) and typical g-C3N4 (2.70 eV). 52

245

3.1.6 XPS analysis

15



246 247

Figure 3. XPS survey spectra of P-g-C3N4 and Ag3PO4/P-g-C3N4 (a), high resolution C 1s

248

spectrum (b), N1s spectrum (c), O 1s spectrum (d), Ag 3d spectrum (e) and P 2p

249

spectrum (f) of Ag3PO4/P-g-C3N4 composite. 16


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250

The XPS result was shown in Figure 3. The elements of C, O, P, N, and Ag were

251

observed in the XPS survey spectrum of the A/CN (1:1) composite (Figure 3a). The high

252

resolution spectra of C 1s, N 1s, O 1s, Ag 3d and P 2p have been calibrated by the

253

standard carbon peak (284.8 eV). As illustrated in Figure 3b, the C 1s peak locating at

254

284.8 eV could be assigned to the C-C bonds or C-N bonds of the graphitic carbon in

255

P-g-C3N4.

256

present in the sample. 54 The N 1s spectrum (Figure 3c) can be deconvoluted to 3 peaks at

257

398.5, 399.6 and 400.9 eV. The first peak could be attributed to the tertiary nitrogen

258

groups (N-(C)3), 55 the second one could be attributed to the aromatic N atoms bonded to

259

the carbon C=N-C, 56 and the third one could be ascribed to amino group (N-H). 57 Figure

260

3d showed that the O 1s spectrum was divided into 2 peaks at 530.5 eV and 532.4 eV,

261

which were assigned to the oxygen in crystal lattice and oxygen absorbed in the

262

composite, respectively.

263

high-resolution Ag 3d spectrum (Figure 3e) were assigned to the Ag 3d5/2 and Ag 3d3/2

264

orbitals, respectively. 59 The peak at 367.8 eV could be further divided into 367.8 eV and

265

368.5 eV peaks, meanwhile, the 373.8 eV peak could be further deconvoluted into 373.8

266

eV and 374.8 eV peaks, respectively. The peaks at 367.8 eV and 373.8 eV could be

267

ascribed to the Ag+ in the Ag3PO4, 60 and peaks of 368.5 eV and 374.8 eV were assigned

268

to metallic Ag,

269

that the metallic Ag existed in the Ag3PO4/P-g-C3N4, which was formed during its

53

The peak at 288.0 eV was attached to the sp2-hybridizied carbon (N-C=N)

61

58

The two strong peaks at 367.8 eV and 373.8 eV by the

which was coincidence with previous studies.

17


62, 63

The result proved


270

synthesis process. The banding energy centered at 133.0 eV (Figure 3f) could be

271

attributed to the typical P-N coordination or the PO43- in the Ag3PO4/P-g-C3N4 composite.

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3.1.7 N2 adsorption-desorption analysis

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The specific surface area and corresponding pore size distribution curves of

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P-g-C3N4 and A/CN (1:1) catalyst are shown in Figure S4. The synthesized samples can

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be recognized as mesoporous materials due to its type IV isotherms. The

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Brunauer-Emmett-Teller (BET) surface area of P-g-C3N4, A/CN (1:1), and Ag3PO4 were

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31.23, 22.53 and 0.1404 m2·g-1, respectively. The specific surface area of Ag3PO4 was

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smaller than that of P-g-C3N4, and the combination of Ag3PO4 and P-g-C3N4 led to the

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decrease of specific surface area of A/CN (1:1) composite. The larger specific surface

280

area could result in the enhanced photocatalytic activity because more active sites could

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be provided.

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3.1.8 PL and photocurrent test

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PL emission spectra were deployed to evaluate the separation rate of photoinduced

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charge carriers in the synthesized materials and the results were presented in Figure S5a.

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All samples except A/CN (1:1) showed the emission peaks at around 469 nm, indicating

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that the combination of Ag3PO4 with P-g-C3N4 could improve the separation rate of the

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e--h+ pairs. As shown in Figure S5b, the composite of A/CN (1:1) possess a much higher

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photocurrent density and better stability than that of Ag3PO4 and P-g-C3N4, indicating the

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higher separation rate of photoinduced carriers. The enhanced photocurrent density can 18


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be ascribed to heterojunction structure and Ag nanoparticles formed on the A/CN (1:1)

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composite, which strengthened its photocatalytic activity remarkably.

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photocurrent tests manifested the high separation rate of e--h+ pairs, contributing to the

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highest photocatalytic degradation efficiency of ketamine by A/CN (1:1) composite.

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3.2 Photocatalytic degradation of KET

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3.2.1 Degradation of KET over different synthesized samples

64

The PL and

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Figure 4. Degradation curves (a) and kinetic curves (b) of KET over different synthesized

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samples under visible light illumination.

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The photolytic degradation of KET was negligible without the catalyst (Figure 4a).

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With the catalysts, the loss of KET in the first 30 min in the dark was less than 10%,

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indicated that the adsorption of KET by synthesized samples can be ignored. The KET

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photocatalytic degradation over different samples fitted well with the pseudo-first-order

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kinetic equation:

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-ln(Ct/C0) = kt

(2)

19



Page 20 of 47

305

where k is the reaction rate constant, t is the reaction time, C0 is the initial concentration

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and Ct is the concentration at time t. The pseudo-first-order curves of KET degradation

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are illustrated in Figure 4b. The different mass ratios between P-g-C3N4 and Ag3PO4 in

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Ag3PO4/P-g-C3N4 composites could remarkably impact the degradation rate of KET.

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P-g-C3N4 showed the lowest photocatalytic performance on KET with a rate constant of

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0.0053 min-1, and Ag3PO4 had a rate constant of 0.0116 min-1. With the mass content of

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Ag3PO4 increasing, the photocatalytic performance of Ag3PO4/P-g-C3N4 increased,

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however, further increasing Ag3PO4 content (A/CN (2:1), A/CN (5:1) and A/CN (10:1))

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reduced the degradation rate. The A/CN (1:1) composite exhibited the most excellent

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performance on KET degradation, with a removal efficiency of 99.95% after 90 min

315

irradiation, while the removal efficiencies of KET over P-g-C3N4 and Ag3PO4 under the

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same condition were 41.64% and 64.50%, respectively. The degradation rate constant of

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KET over A/CN (1:1) composite was 0.0326 min-1, 6.16- and 2.82-fold faster than

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P-g-C3N4 and Ag3PO4, respectively. According to TEM result, when optimal amount of

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Ag3PO4 was successfully combined with P-g-C3N4, heterojunction structure could be

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formed, which could enhance the separation rate of photo-carriers and promote the

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photocatalytic performance of Ag3PO4/P-g-C3N4 composites.

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Ag3PO4 dispersed on the P-g-C3N4 surface could result in a lower interfacial charge

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transfer between them, which brings a relative low separation efficiency of photo-carriers

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on the composites and leads to a lower degradation rate of KET. 20


49

However, excessive

65

In the following

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experiment the A/CN (1:1) composite was used for KET degradation.

326 327

Figure 5. The photocatalytic degradation of KET by Ag3PO4/P-g-C3N4 composite at

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different pH values (a); and at different concentration of HCO3- (b); DOM (c) and NO3-

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(d).

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3.2.2 Effect of pH

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The influence of solution pH on the KET photocatalytic degradation was shown in

332

Figure 5a. KET degradation rate constants at pH 3, 5, 7, 9 and 11 were 0.0016 min-1,

333

0.0267 min-1, 0.0326 min-1, 0.0299 min-1, and 0.0159 min-1, respectively (Figure S6a).

334

Best removal efficiency of KET (88.89%) was achieved over Ag3PO4/P-g-C3N4 21



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composite at pH 7 after 60 min irradiation. Under strongly acidic (pH=3) or alkaline

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(pH=11) conditions, the KET degradation was significantly suppressed due to the

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inhibited formation of reactive species (RS).

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were more hydroxyl ions that could form ·OH, which could accelerate the degradation

339

efficiency compared with under acidic condition.

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condition, the ·OH would be eliminated, resulting in the low elimination efficiency. 68

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3.2.3 Role of bicarbonate (HCO3-)

66

It has been reported that at pH 9 there

67

However, under strong alkaline

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Figure 5b shows the KET degradation in the presence of HCO3- in the

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Ag3PO4/P-g-C3N4 system. The degradation of KET was enhanced with low concentration

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of HCO3- (

Nanocomposites of Ag3PO4 and Phosphorous ... - ACS Publications

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