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Removal of Nitrate by Photocatalytic Denitrification Using Nonlinear Optical Material Guoshuai Liu, Shijie You, Ming Ma, Hong Huang, and Nanqi Ren Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b03455 • Publication Date (Web): 12 Sep 2016 Downloaded from http://pubs.acs.org on September 12, 2016

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Environmental Science & Technology Article

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Revised manuscript for: Environmental Science & Technology Submission date: 2016-09-09

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Removal of Nitrate by Photocatalytic Denitrification Using Nonlinear Optical

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Material

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Guoshuai Liu, Shijie You *, Ming Ma, Hong Huang, Nanqi Ren

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State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, Harbin 150090, P. R. China.

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Corresponding author: * Shijie You P. O. Box 2603#, No. 73, Huanghe Road, Nangang District, Harbin, 150090, China. Tel.: +86–451–86282008; Fax: +86–451–86282110 E–mail: [email protected]

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ACS Paragon Plus Environment

Environmental Science & Technology

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ABSTRACT

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Removal of nitrate from water has been receiving growing attention in water treatment. In this study,

38

we report the photocatalytic denitrification (PCDN) by nonlinear optical (NLO) material, i. e. lithium

39

niobate (LiNbO3). The hydrothermally synthesized LiNbO3 powder could achieve efficient

40

denitrification in water, evidenced by 98.4% nitrate removal and 95.8% nitrogen selectivity at

41

reaction time of 120 min and pH−neutral condition. Based on the first−order kinetics of PCDN, the

42

kinetic constant for LiNbO3 is almost three times as that of conventional TiO2 (P25) under the same

43

conditions. As suggested by the hole scavenger experiments, the LiNbO3 should proceed with

44

photocatalytic reduction of nitrate through direct heterogeneous interaction with electrons at the

45

conduction band of LiNbO3. This may represent a different mechanism from P25, where nitrate is

46

mainly reduced by CO2•− radicals generated by the holes at valence band. The unique second

47

harmonic generation (SHG) effects of NLO materials enable them to produce more electrons and

48

minimize the electron−hole recombination, which improves the efficiency and stability of PCDN

49

process. The current study provides a proof−of−concept demonstration of NLO photocatalytic

50

material for more effective nitrate removal in water treatment.

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■ INTRODUCTION

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Due to the global increase of agricultural activities, there is an increasing concern over nitrate

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pollution in regions where groundwater serves as the main source of drinking water.1-3 Despite less

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direct toxicity of nitrate ions, they can be potentially transformed to nitrite in human body, a harmful

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species that may cause methemoglobinemia or “blue baby” syndrome.4-6 Many governments and

65

organizations have set the upper limit of nitrate concentration in drinking water, for example, 10

66

mgN L−1 by US Environment Protection Agency (EPA) and China EPA, 11.3 mgN L−1 by European

67

Community, and 10 mgN L−1 by Ministry of Environment, Japan.6-9

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Removing nitrate from groundwater has been a technical challenge for quite a long time. As a

69

soluble and stable anion, nitrate can be removed by several technologies like ion exchange, reverse

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osmosis, chemical/electrochemical denitrification and biological denitrification.10-13 However, wide

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applications of these methods are impeded by their low efficiency, high cost and operational

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complexity. Additionally, the treatment of drinking water by bioprocess must be exercised with

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caution due to the potential risks caused by bacterial and pathogenic organisms.14 It is well known

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that the photocatalytic oxidation has been investigated extensively for its capability of producing

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highly oxidative •OH, but little attention has been paid to photocatalytic reduction of oxidative

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pollutants like nitrate or perchlorate in water. Photocatalytic denitrification (PCDN) has emerged as a

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promising approach to achieve this goal, since it was first reported by Schlögl and co−workers in

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1999.15 During PCDN process activated by light irradiation, the photocatalyst generates electrons

79

(eCB-) in conduction band (CB) and holes (hVB+) in valence band (VB) of semiconductor. Then, the

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nitrate is reduced through direct interaction with eCB- or reaction with reductive CO2•− radicals

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produced from the reaction between hVB+ and hole scavengers (e. g. formic acid).16-18 According to

82

the literatures, the latter mechanism generally dominates the PCDN for several materials like

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conventional TiO2, ZnO, ZnS, CdS, and SrTiO3.19-24 However, there remain challenges for PCDN

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mediated by CO2•− radicals for several reasons. First, it is difficult to control the formation of CO2•− 3




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radicals due to the dependence on hole scavenger used. Second, the PCDN commonly proceeds with

86

the formation of undesired nitrite or ammonium and hence poor N2 selectivity. This may be

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associated with the high valence band potential for the formation of •OH radicals that may lead to

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re−oxidation of nitrite or ammonium to nitrate. Lastly, the overall performance is impaired by

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recombination of electron−hole, as is often encountered in photocatalytic oxidation.

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Nonlinear optical (NLO) materials have drawn an increasing research attention due to their great

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promise for applications in modulator and holographic memory.25-27 It is only recently that NLO

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materials find their applications in photocatalysis.25 The NLO material has a unique property of

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spontaneous polarization screened by either free electrons and holes, or ions and molecules adsorbed

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on the surface.28 The internal dipolar field creates the charged surface, which then triggers the

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photogenerated charge carriers to move in the opposite direction (the dipole moment value of

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approximately 7.0×10−5 C cm−2). 29 This can mitigate the problems of electron−hole recombination,

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which improves photocatalytic activity and stability compared to conventional semiconductors. As a

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specific NLO photocatalyst, LiNbO3 has been demonstrated capable of splitting water to produce

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hydrogen as a result of NLO property and negative conduction band value.30 From a thermodynamic

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point of view, if the photocatalyst had proper Fermi level and the conduction band value could be

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sufficiently negative, the directly electron−driven PCDN by NLO photocatalyst would be expected.

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Herein, we report for the first time the nitrate removal via PCDN by NLO material, i. e. typically

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LiNbO3. First, the crystalline structure and morphology of LiNbO3 powders were characterized using

104

X−ray diffraction (XRD) and scanning electron microscopy (SEM). Second, the band structure and

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density of states (DOS) were calculated theoretically using density functional theory (DFT),

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followed by estimation of band gap and band position. Third, the performances of LiNbO3 and

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commercial TiO2 (P25) were examined and compared for photocatalytic reduction of nitrate in water.

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Last, the possible mechanisms for PCDN by LiNbO3 were discussed.

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■ MATERIALS AND METHODS

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All the chemicals used were of analytical reagents grade. The TiO2 used is commercially available

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Degussa P25 (Germany) with a surface area of ca. 50 m2 g-1, which is made of two phases of anatase

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(80%) and rutile (20%).

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Preparation of NLO LiNbO3 Crystals. In a typical synthesis, LiOH, Nb2O5, sodium dodecyl sulfate

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(SDS) were dissolved in 400 mL deionized water (DI−water) under continuous stirring conditions.

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The resulting white suspension was then transferred to an autoclave to alcoholize at 260 oC for 24 h.

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The precipitated powders were filtered and washed three times with ethyl alcohol to remove residual

118

solvent. Following drying at 50 oC for 12 h, the LiNbO3 powders were obtained.

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Characterization. The Powder X−ray diffraction (XRD) measurement was conducted using

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Bruker D8 Advance X−ray diffract meter with Cu−Kα radiation. The acceleration voltage and the

121

applied current were 40 kV and 40 mA, respectively. The morphology of as-prepared samples was

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characterized by using an FESEM−4800 field emission scanning electron microscope (SEM, Hitachi,

123

Japan). The UV−vis diffuse reflectance spectrum (DRS) of the catalyst powders was obtained using a

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UV−vis spectrophotometer (Cary 500 Scan Spectrophotometers, Varian, U.S.). BaSO4 was used as a

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reflectance standard in the UV−visible diffuse reflectance experiment. In order to investigate the

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polarity of LiNbO3, the second harmonic generation (SHG) measurement was carried out using the

127

Kurtz−Perry method. A common non−linear optical material KH2PO4 (KDP) was chosen as a

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reference. The light source was a Q−switched 1064 nm Nd: YAG laser, which could produce a

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pulsed infrared beam to irradiate the samples. The Brunauer–Emmett–Teller (BET) surface area and

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porosity of CNM were measured using accelerated surface area and Porosometry system (ASAP

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2020, GlobalSpec. Inc., U.S.). Zeta potential of the LiNbO3 samples was recorded on zeta potential

132

analyzer (Nano–Z, Malvern Corp., U.S.) at step–wise change of pH values adjusted by HCl (1.0 M)

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and NaOH (1.0 M).

134

Theoretical Calculations. All the calculations were performed using the periodic density 5




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functional theory (DFT) package of Cambridge Serial Total Energy Package (CASTEP) codes. The

136

core electrons were treated with the ultrasoft pseudo−potential. The exchange−correlation effects of

137

valence electrons were described through the generalized gradient approximation (GGA), k−point set

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as (3×3×2) and the energy cutoff was set as 450 eV in the Brillouin zone integration. Its convergence

139

criteria were set as follows: the force on the atoms was less than 0.01 eV Å−1, the stress on the atoms

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was less than 0.02 GPa, the atomic displacement was less than 5×10−4 Å, and the energy change per

141

atom was less than 5×10−6 eV. Based on the optimized crystal structure, the electronic structure and

142

the optical properties were then calculated.

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Experimental Setup and Procedures. The PCDN experiments were performed in a stirred batch

144

reactor. Before initiating the reaction by UV light, 250 mg of catalyst powders were added into the

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reactor containing 660 mL water (catalyst concentration of 380 mg L−1) and the solution was then

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sonicated for 15 min. The solution was first irradiated for 30 min before adding nitrate. The nitrate

147

concentration was approximately 50 mg L-1 (0.8 mmol L−1) and the hole scavengers (i. e. formic acid,

148

KI and humic acid) were added at a concentration of 1.0 mmol/L, giving the solution with final pH

149

of 6.8±0.2. A 110 W high−pressure Hg lamp was employed as a 365 nm UV light source. The

150

samples were withdrawn periodically and the powder catalysts were removed by filtration through

151

0.2 µm cellulose acetate membrane. The nitrogen species of interest (NO3−, NO2−) were determined

152

using the chromatography of ions (LC−10A, Shimadzu, Japan). The concentration NH4+ ions were

153

measured by using standard Nessler’s reagent colorimetry method. The total nitrogen (TN) was

154

analyzed on the total organic carbon analyzer (VSCN8, Shimadzu, Japan).

155

Analyses. The photocatalytic selectivity toward N2 was calculated according to

156 −

157

S(N2 ) =

−

−

−

−

+

[NO3 ]0 − [NO3 ]t − [NO2 ]t − [NH4 ]t [NO3 ]0 − [NO3 ]t

158 6


× 100%

(1)

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where S(N2) is the N2 selectivity, [NO3−]0 and [NO3−]t (mmol L-1) the nitrate concentration at time 0

160

and t, [NO2−]t and [NH4+]t the nitrite and ammonium concentration at time t.

161 162

■ RESULTS AND DISCUSSION

163

Characterization of LiNbO3 Crystal. The XRD profiles (Figure 1A) were indexed to the

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characteristic peaks of LiNbO3 powders, which were in good agreement with the trigonal structure

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(PDF 20−631). The sharp and intense diffraction peaks indicate the high crystalline of the samples.

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SEM was utilized to characterize the morphology and crystal size of the prepared LiNbO3 samples.

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As shown in inserted images of Figure 1A, the LiNbO3 powders have uniform morphology involving

168

plenty of regular rectangular blocky structures with 800−nm length, 200−nm width and 150−nm

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thickness, respectively. The isotherm of LiNbO3 samples revealed a reversible type−II adsorption

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and small hysteresis upon desorption of gas from the pores, representing the adsorption with strong

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adsorbate–adsorbent interactions (BET surface area of 48.98 m2 g−1, Figure S1 in Supporting

172

Information). The uniform morphology, nano−structure and relatively large surface area should be

173

beneficial for interfacial adsorption of nitrate ions and photocatalytic reduction reaction.

174 175

Figure 1 (A, B, C, D)

176 177

LiNbO3 crystal has a highly symmetry trigonal structure (point group: 3 m; space group: 161,

178

R3c), containing 6 Li atoms, 6 Nb ions and 18 O atoms (Figure 1B). As seen from the band structure

179

(Figure 1C) and density of states (DOS, Figure 1D), the Fermi energy level is 0 eV. The conduction

180

band and valence band are located above and below the Fermi level, respectively. LiNbO3 crystal

181

contains an indirect band gap, corresponding to the location of valence band maximum (VBM) and

182

conduction band minimum (CBM) at different k−point.31-33 In indirect transition semiconductors, the

183

emission of excited electrons has to be coordinated by phonon generation, such that the activity and 7




184

lifetime can be maintained. The band gap of pure LiNbO3 was theoretically calculated to be 3.285 eV

185

between VBM and CBM (Figure 1C). As shown in DOS curves (Figure 1D), the bottom of

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conduction band is mainly attributed to 4d orbital electron of Nb and 2p orbital electron of O, while

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the top of valence band is composed of 2p orbital electron of O and a small amount of 4d orbital

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electron of Nb. The deep energy level of valence is mainly attributed to the s orbital of Li, s and 2p

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orbital electron of Nb, and s orbital electron of O. The absorption edge of LiNbO3 were determined

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by the electrons excited from 2p orbit of O to 4d orbit of Nb, and this results in photocatalytic

191

activity of LiNbO3 excited by UV−light.

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The band gap (Eg, eV) of LiNbO3 can also be estimated according to 34

193

194

α hv = A(hv − Eg )

n

2

(2)

195 196

where α is the optical absorption coefficient, A the proportionality constant, and hν the photonic

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energy. n is determined by the type of optical transition of a semiconductor (n=1.0 for direct

198

transition and n=4.0 for indirect transition). Thanks to indirect transition for LiNbO3, substituting

199

n=4 to Eq. (2) gives the Eg of 3.5 eV (Figure S2 in Supporting Information). The band gap estimated

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from experimental data is 0.215 eV, a value lower than that theoretically calculated from electronic

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structure, as a consequence of the well−known LDA−GGA underestimation.35

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PCDN Performance of LiNbO3. We examined the feasibility of using LiNbO3 to reduce NO3– in

203

the presence of formic acid (FA) serving as hole scavenger. The typical TiO2 (Degussa, P25)

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photocatalyst was also investigated for comparison under the same conditions. When 0.8 mmol L−1

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NO3– and 1.0 mmol L−1 FA were mixed in the absence of photocatalyst, there was no observation of

206

NO3– reduction under both dark and irradiation conditions (Table S1 in Supporting Information),

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which excluded the possibility of direct redox reaction between NO3– and FA. The concentration of

208

TN, NO3–, NO2– and NH4+, together with the N2 selectivity were monitored during PCND by LiNbO3 8


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and P25 based on the same catalyst loading of 380 mg L−1. As shown in Figure 2A, the LiNbO3 was

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able to decrease NO3– concentration from initial 0.8 mmol L−1 to 0.013 mmol L−1 within a reaction

211

time of 120 min, accounting for NO3– removal efficiency as high as 98.4%. The NO3– removal

212

corresponded well to the first−order kinetics with kinetic constant (k) of 0.038 min−1 (Figure S3 in

213

Supporting Information). The NH4+ was always maintained at a low level, indicated by the maximum

214

concentration below 0.01 mmol L−1. The NO2– was first accumulated to 0.17 mmol L−1 at 15 min,

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followed by subsequent decrease to 0.0011 mmol L−1 at the end of 120 min. In the initial stage of the

216

reaction, the NO2– was accumulated from the rapid reduction of nitrate at relatively high

217

concentration. It was noticed that the TN was a little bit higher than NO3−, and such difference

218

should result from the formation of intermediate species like N2O and N2O5 during nitrate reduction.

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Given their low concentration, their impact is presumably insignificant. On the basis of the fraction

220

of aqueous nitrogen (0.0241 mmol L−1, 3%) in total nitrogen (0.8 mmol L−1) and Eq. 1, the

221

N2−selectivity was increased with the reaction time, reaching the maximum of 95.8% at the end of

222

reaction. On the contrary, as illustrated in Figure 2B, the P25 exhibited only 48.5% nitrate removal

223

with final concentration of 0.42 mmol L−1 (k of 0.011 min−1, Figure S3 in Supporting Information).

224

The kinetic constant for LiNbO3 was approximately 345% larger than that for P25. Meanwhile, more

225

NO2– (0.19 mmol L−1, 23%) and NH4+ (0.0483 mmol L−1, 6%) accumulation in the reacted solution

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was observed, corresponding to decease in N2 selectivity from 94.3% to 38.1% within a time of 120

227

min. These results clearly suggest that LiNbO3 is more active toward NO3– reduction and more

228

selective toward N2 formation than P25 during the PCDN process.

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The stability of the LiNbO3 was also evaluated by the experiments of catalyst recycling. As

230

shown in Figure S4 in Supporting Information the photocatalytic activity of the LiNbO3 decreased

231

slightly, indicated by 3.2% loss of nitrate removal after five−cycle experiments. The decrease in

232

activity should be due to the mass loss of catalyst during operation. These results reveal a good

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stability of LiNbO3 during PCDN. 9




234

Figure 2 (A, B)

235 236 237

Mechanisms of PCDN. During PCDN process, the nitrate may be reduced by (i) reductive CO2•−

238

radicals produced from the reaction between hVB+/•OH and hole scavengers, (ii) electrons generated

239

at CB, as well as (iii) hydrogen produced from water splitting at CB.

240

Formic acid (FA) is a common hole scavenger to produce CO2•− radicals through oxidation by

241

•OH radicals produced from water oxidation at hole. To identify the role of CO2• radicals, we

242

performed the experiments by selecting two types of hole scavenger, i. e. FA and KI as probe agent.

243

For P25, the ECBM and EVBM is −0.4 V and +2.8 V (unless stated otherwise all the potentials were

244

reported versus standard hydrogen electrode, SHE; pH 6.8), respectively, of which the EVBM is

245

sufficiently high for water oxidation to produce •OH radicals responsible for oxidation of FA to

246

CO2•− radicals, a strong reducing intermediate species (•OH + HCOO− → CO2•− + H2O, E0CO2/CO2•− =

247

−1.8 V).35 This makes it thermodynamically possible to proceed with reduction of NO3− to NO2−

248

(E0NO3−/NO2− = +0.94 V), N2 (E0NO3−/N2 = +1.25 V) and NH4+ (E0NO3−/NH4+ = +1.203 V).20-24 When KI

249

was used as hole scavenger, the I− was oxidized to I2 by holes. Owing to the lack of reductive

250

radicals, both nitrate conversion efficiency and N2−selectivity were found to decrease by nearly more

251

than 50% compared to FA. Since ECBM of −0.4 V is also not enough for direct reduction of nitrate,

252

the nitrate reduction by P25 is mainly attributed to the formation of CO2•− radicals, which is

253

consistent well with the electron paramagnetic resonance (EPR) measurement reported in prior

254

studies.35, 36

−

255

For LiNbO3, the nitrate conversion efficiency and N2−selectivity were observed to differ

256

slightly for FA and KI, and both values were much higher than that for P25 (Table 1). Taking into

257

account more relevance of hole scavenger to oxidation at valence band, the independence of PCDN

258

on hole scavenger suggests the nitrate reduction promoted by conduction band of LiNbO3. For the 10


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NLO property of LiNbO3, the internal dipolar field induces a charged surface and energy

260

band−bending at pH of 7.0, the EVB value is in the range of 0−0.7 V is insufficient for water

261

oxidation to produce •OH radicals,28 and thus it appears unlikely for CO2•− radicals to be responsible

262

for PCDN even FA was used as hole scavenger. Although the EVBM of 0.7 V can oxidize I− to I2

263

(E0I2/I− = +0.56 V), I2 cannot reduce nitrate in water. Unlike P25, however, it is interesting to note that

264

the use of KI has negligible impact on the PCDN performance of LiNbO3 (Table 1). Based on these

265

phenomena, we conclude that the PCDN for LiNbO3 should be dominated by the reactions involving

266

conduction band (CB) via either interaction with electrons or hydrogen produced from water splitting.

267

The former pathway is thermodynamically possible resulting from the ECB of LiNbO3 as negative as

268

−3.5 V to −2.8 V,28 by which the nitrate can be directly reduced by electrons at low potential.

269

Additionally, the negative ECBM can also drive water reduction to form hydrogen, which may serve as

270

reductant to mediate nitrate reduction. To examine whether H2 may have a role in PCDN, the H2

271

production catalyzed by LiNbO3 was investigated and compared to the cases with and without

272

addition of NO3−. Following 120−min reaction, the amount of H2 yielded was 0.048 mmol and 0.025

273

mmol for the cases in the absence and presence of NO3− (0.8 mmol L−1), respectively, indicating

274

0.025 mmol H2 consumed for nitrate reduction (Table S2 in Supporting Information). If such portion

275

of H2 is assumed to completely convert NO3− to N2, the contribution of H2 to PCDN is estimated to

276

be 1.9%. This means that the role of H2 is very limited and more than 98% NO3− is reduced directly

277

by electrons at conduction band of LiNbO3.

278

With regard to N2 selectivity, for conventional photocatalyst like P25 with higher potential of

279

VBM (+2.8 V),37-41 the N2 selectivity of nitrate reduction may be depressed because both NO2− and

280

NH4+ may possibly be re−oxidized to NO3− by •OH radicals produced from water oxidation in the

281

hole. In contrast, there is no concern over •OH radical formation for LiNbO3 due to impossibility of

282

water oxidation at lower potential of 0.7 V for VBM than that of +2.8 V for •OH radicals. In this case,

283

the nitrate and nitrite can be reduced to nitrogen by electrons with much less accumulation of nitrite 11




284

and ammonium. This may provide one possible explanation for the reason why much higher N2

285

selectivity can be attained for LiNbO3 than that for P25.

286 287

Table 1

288 289

The NLO Property of LiNbO3. It is well known that the photocatalytic efficiency is crucially

290

impaired by recombination of photo−generated electrons/holes. One of the important properties of

291

NLO materials is the second harmonic generation (SHG). The SHG effect will promote the

292

separation of photo−electron/hole pairs, 28-30 producing more photo−electrons for PCDN process.

293

Figure 3A shows the SHG response of P25, LiNbO3, and KH2PO4 (KDP, a typical NLO material)

294

reference irradiated by a 1064 nm Nd: YAG laser. The SHG intensity obtained for LiNbO3 was

295

almost 4.5 times of that for KDP, whereas no SHG response was observed for P25. According to the

296

literature, the LiNbO3 has a single 180°−domain structure (spontaneous polarization Ps parallel to the

297

crystallographic Z−axis) and exhibits a strong photovoltaic effect along the Z−axis direction,28

298

resulting in a strong SHG effect to produce surface polarization, which is beneficial for

299

transportation of photo−generated carriers to the surface.

300

The more intensive SHG response suggests a stronger intrinsic polarity of the dipole moment

301

along the Z−axis direction. For LiNbO3 photocatalyst, the parade process of electrons and holes in

302

opposite directions along the Z−axis/[001] direction (the polarization vector direction) can also be

303

significantly facilitated.29 For these reasons, the LiNbO3 demonstrates a remarkable capability of

304

producing photo−electrons for effective reduction of NO3– in water.

305 306

Figure 3 (A, B)

307 308

Another advantage of NLO material is the prevention of electron/hole recombination. As 12


Page 13 of 25


309

revealed from the photocurrent of LiNbO3 and P25 in Figure 3B, LiNbO3 delivered much higher

310

photocurrent than P25, i. e. the transfer efficiency of photo−generated hole/electron pairs of LiNbO3

311

was much higher than that of P25. According to prior studies, the transfer of photo−generated

312

carriers can be promoted along Z−axis by the net dipole moments in the NLO crystal. The LiNbO3

313

possess a surface polarization at the value of approximately 7.0×10−5 C cm−2 below the Curie

314

temperature.29 The surface at which the polarization produces a positive potential is termed C+ face,

315

and the surface where the polarization produces a negative potential is termed the C− face (Figure S5

316

in Supporting Information). Depolarization fields acting to screen the surface potential draw

317

electrons and negatively charged species to the positive C+ face and holes to the negative C− face. In

318

this manner, the space charge regions are established on both surfaces, inducing downward band

319

bending at C+ face and upward bending at C− face. The internal polar field promotes their separation

320

along the field direction in Z−axis, which functions as a driving force for the photo−excited holes

321

and electrons to move toward their reduction and oxidation sites. Under the action of internal polar

322

field, the photo−induced electron and hole would transfer in the opposite direction, which enables

323

high−efficiency charge/hole separation. This can lead to remarkably improved photocatalytic activity

324

and stability compared to conventional semiconductors. The improved performance of LiNbO3

325

observed here is in good agreement with the previous studies, where the lifetime of generated

326

carriers can be increased by long photoluminescence up to 9 µs in lithium niobate.28

327

In addition, the LiNbO3 was shown to have an isoelectric point (IEP) of pH 8.4, meaning the

328

existence with positive charge at nearly pH−neutral condition (pH 6.8) due to the adsorption of

329

protons on the surface (Figure S6 in Supporting Information). The addition of nitrate (0.8 mmol L−1)

330

resulted in the negative shift of IEP as a result of electrostatic attraction, and thus the decrease in zeta

331

potential with respect to pH values. After polarization by instant irradiation, the NLO materials can

332

delocalize electron density and alter the reactivity of species adsorbed on it. The species chemisorbed

333

on the surface of NLO materials are bound for one order of magnitude longer than that on nonpolar 13




334

materials.42-44 For example, the polarized barium titanate thin films can absorb more ethanol

335

molecules at both C+ and C− face than depolarized. The adsorption of NO3− was only 5.1% before

336

polarization, and increased by almost three times (15.4%) after polarization (Figure S7 in Supporting

337

Information), which was the indicative of further enhancement in PCDN process.

338

Taken together, as an interesting feature of photocatalysis over NLO materials, LiNbO3 is

339

assumed to proceed with the PCDN in the following manner. The holes produced by LiNbO3 are

340

trapped by hole scavenger, and meanwhile the electrons are transferred onto the C+ face to react with

341

NO3−. Such a process is dependent on the separation of electrons and holes at the C+ and C− face due

342

to the dipole of the ferroelectric. The separation of carriers also promotes the redox reactions at

343

LiNbO3 surfaces to be spatially separated. In this case, the negatively charged NO3− is directly

344

reduced by electrons on the C+ face, which considerably improves the efficiency and selectivity of

345

PCDN. The possible mechanisms of PCDN are schematically illustrated in Scheme 1.

346 347

Scheme 1

348 349

Applications and Implications. PCDN represents a promising method for removing nitrate in

350

water. For PCDN, the energy level of the photocatalyst constitutes the key parameter with respect to

351

thermodynamic process. The traditional TiO2−based photocatalysts or their composites possess high

352

potential of valence band, but the conduction band remains insufficiently negative to drive direct

353

nitrate reduction. Producing CO2•− radicals necessitates adding the electron donor (e. g. hole

354

scavenger), which is impractical for drinking water treatment. Besides, the overall PCDN

355

performance is inherently impaired by the recombination of photo−carriers. To address these

356

problems, we herein developed a nonlinear optical LiNbO3 photocatalyst and demonstrated the

357

feasibility for nitrate removal in water. The LiNbO3 could achieve the overall nitrate removal of

358

98.4% and N2 selectivity of 95.8% at pH−neutral conditions, and these values are generally higher 14


Page 15 of 25


359

than most of results reported in the literatures as summarized in Table 2.

360 361

Table 2

362 363

In light of the above results, this study may provide a new strategy to design

364

NLO−material−based photocatalysis system toward PCDN for water treatment. First, LiNbO3 was

365

shown able to accomplish nitrate removal via direct interaction with electrons at C+ face of

366

conduction band (EVBM from −2.5 V to −3.8 V) rather than with intermediate radicals, making it

367

more efficient, more controllable and more viable than CO2•− radicals (E0CO2/CO2•− =−1.8 V). For this

368

reason, the performance of LiNbO3 appears to be less dependent on hole scavenger (Table 1).

369

Surprisingly, even when the complex humic acids serve as hole scavenger, the nitrate conversion

370

efficiency and N2−selectivity can be still maintained as high as 90.1% and 86.2% (Table S3 in

371

Supporting Information). This indicates a technological possibility of using humic substances or

372

natural organic matters (NOMs) present in surface or ground water to accompany in−situ PCDN

373

eliminating the need for addition of any hole scavenger. Second, the unique properties of NLO

374

materials can minimize the possibility of photo−carrier recombination during PCDN, which greatly

375

improves the efficiency and stability of the photocatalytic system during long−term operation. Third,

376

as an environmentally friendly and sustainable material, LiNbO3 can be produced by simple

377

hydrothermal methods that are available for scalable applications. These features make the NLO

378

LiNbO3 particularly attractive for application of removing nitrate in water treatment.

379

Notwithstanding, there continues to be significant interest in exploiting other types of NLO

380

photocatalysts and optimizing the operational conditions. Additionally, the second harmonic

381

generation (SHG) effect relies on the size distribution of the NLO materials, and thus much effort

382

will be needed to elucidate the correlation between the size distribution and SHG effect, and the way

383

they affect the PCDN efficiency in the future work. Last, it will be also of great interest to develop 15




384

theoretical methods to uncover the structure−function relationship of NLO materials when

385

interacting with target pollutants.

386 387

■ ASSOCIATED CONTENT

388

Supporting Information

389

Additional figures and tables. This material is available free of charge via the Internet at

390

http://pubs.acs.org.

391 392

■ AUTHOR INFORMATION

393

Corresponding author

394

* Shijie You

395

E–mail: [email protected] (S. J. You)

396 397

■ ACKNOWLEDGEMENTS

398

Project supported by the National Natural Science Foundation of China (No. 51378143, 51678184),

399

Innovative Foundation for Environmental and Ecological Research Center, State Key Laboratory of

400

Urban Water Resource and Environment (Grant No. 2015TS01), the Fundamental Research Funds

401

for the Central Universities (Grant No. HIT.BRETIII.201419). Many thanks go to Prof. Zhang Wei

402

and Ryan McCaffrey (University of Colorado, Boulder, U.S.) for their assistance to improve English

403

writing of this paper.

404 405

■ REFERENCES

406 407 408 409

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500 501 502 503 504

18


Page 19 of 25


505 506 507

Table Lists

508 509

Table 1 Effect of hole scavenger on NO3– conversion and N2 selectivity during PCND process by LiNbO3 and P25. P25

510 511 512 513 514

N2 selectivity (%)

NO3− conversion (%)

N2 selectivity (%)

FA

48.5

38

98.4

95.8

KI

25.5

18

96.2

93

Table 2 Comparison of PCDN performances for different photocatalysts Photocatalyst LiNbO3 P25 Pd–Cu/TiO2 TiO2/Ag Ni–Cu/TiO2 Cu/TiO2 Pd (5 wt.%)-Cu (0.6 wt.%)/AC Pd/CeO2 Pt/TiO2 Ag2O/P25

515 516 517

LiNbO3

NO3− conversion (%)

Scavenger

Experimental conditions [NO3−]0=0.8

-1

mmol L ; pH=7.0; HS: HCOOH; T: 120 min [NO3−]0=0.8 mmol L-1; pH=7.0; HS: HCOOH; T: 120 min [NO3−]0=0.8 mmol L-1; pH=7.0; HS: HCOOH; T: 60 min [NO3−]0=0.714 mmol L-1; pH=2.5; HS: HCOOH; T: 180 min [NO3−]0=0.714 mmol L-1; pH=2.5; HS: CH3COOH; T: 100 min [NO3−]0=0.714 mmol L-1; pH=2.5; HS: CH3COOH; T: 100 min [NO3−]0=3.2 mmol L-1; pH b; HS: H2; T: 100 min [NO3−]0=1.6 mmol L-1; pH=7.0; HS: H2; T: 300 min [NO3−]0=1.6 mmol L-1; pH=7.0; HS: HCOOH; T: 200 min [NO3−]0=2.0 mmol L-1; pH=7.0; HS: HCOOH; T: 240 min

NO3– reduced (%)

NO2– (%)

NH4+ (%)

N2 (%)

Ref.

98.4

0.13

1.2

95.8

This study

52.5

23

6

38.1

This study

62

0

6.0

94

19

86

5.2

18

83

20

75

2.1

9.3

33

21

68

1.9

1.8

88

21

96.9

0.7

96.4

2.9

22

97.2

b

10

92

23

39

0

10

90

24

97.5

11.1

5.6

82.9

18

a. The electrolysis was carried out under potential control rather than current control. b. Data not reported.

518 519 520 521 522 523 19




524 525

Figure Captions

526 527 528 529 530

Figure 1 (A) The XRD pattern, (B) ball−stick model, (C) band structure, and (D) density of states of LiNbO3.

531

Figure 2 Time course of NO3–, NO2–, NH4+, and N2 selectivity during PCDN process using (A) LiNbO3 and (B) P25 under UV−light irradiation. The experiments were carried out at initial NO3– concentration of 0.8 mmol L–1, 1.0 mmol L–1 formic acid as hole scavenger and pH of 6.8±0.2.

532 533 534 535 536 537

Figure 3 (A) Oscilloscope traces of the SHG signals of LiNbO3 and the KDP reference irradiated by a Q−switched 1064 nm Nd: YAG laser; (B) Time course of photocurrent based on on−off cycles.

538 539 540 541

Scheme 1 Schematic illustrations of the photocatalytic reduction of NO3− by LiNbO3 and P25 photocatalyst.

542 543 544 545 546 547 548 549 550 551 552 20


Page 21 of 25


553 554

Figure 1 (A, B, C, D)

555 A C

(012)

A

6

Energy (eV)

2 0 -2 -4

(208)

(116) (211) (018) (214) (300)

(204)

(006) (113) (202)

(104) (110)

Intensity (a. u.)

4

-6

30

40 50 2 theta (degree)

60

B O

Nb

G

70 B 14 D DOS and pDOS (e/eV)

20

Li

s

12

GZ

XM p

d

RA Z Total

10 8 6 4 2 0 -19

-14

556 557 558 559 560 561 562 563 564 565 21


-9 -4 Energy (eV)

1

6



566 567

Figure 2 (A, B)

568 100

0.8

80 TN

0.6

60

NO3NO2-

0.4

40

NH4+

0.2

20

0 0

Concentration (mmol L-1)

B

30 60 90 120 Reaction time (min)

0 150

1

100

0.8

80 60

0.6 TN

0.4

NO3-

40

0.2

NO2-

20

NH4+

0 0

569 570

N2 selectivity (%)

1

30 60 90 120 Reaction time (min)

0 150

571 572 573 574 575 576 577 578 22


N2 selectivity (%)

Concentration (mmol L-1)

A

Page 23 of 25


579 580

Figure 3 (A, B)

581

Intensity (a. u.)

A P25

KH2PO4

LiNbO3

-4.5

-3

-1.5 0 1.5 Time (ms)

3

4.5

Current density (µA cm-2)

B 8 7 6

LiNbO3 On Off

5 4 3

P25

2 1 0 0

582

80

160 240 Time (min)

320

583 584 585 586 587 588

23


400



589 590

Scheme 1

591

-4

-3.5 V

C- face e-

e-

C+ face e-

e-

Potential (V vs SHE)

NO3-

e-

-3

e-

-2.8 V

N2

-2 -1 0 1 2 3

P25

CB

RCOOH

0V

e-

VB h+

h+

h+

H+

h+

h+

-0.4 V

e-

e-

e-

e-

CB

h+

0.7 V R• + CO2 +

H2

Eg=3.20 eV

NO3- , NO2-, NH4+, N2

H+ Polarization

VB

2.8 V

LiNbO3

h+

h+

OH-

h+

h+ h+

•OH

RCOOH

592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 24


CO2•-

Page 25 of 25


607 608

TOC Art

609 610 611 612

(Optional) The nonlinear optical material, LiNbO3, is first demonstrated capable of accomplishing photocatalytic denitrification with high conversion efficiency and high selectivity.

25


Removal of Nitrate by Photocatalytic Denitrification ... - ACS Publications

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