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VUV/UV/chlorine as an enhanced advanced oxidation process for organic pollutant removal from water: Assessment with a novel mini-fluidic VUV/UV photoreaction system (MVPS) Mengkai Li, Zhimin Qiang, Pin Hou, James R. Bolton, Jiuhui Qu, Peng Li, and Chen Wang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00133 • Publication Date (Web): 17 May 2016 Downloaded from http://pubs.acs.org on May 21, 2016

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Environmental Science & Technology

1 2 3

VUV/UV/chlorine as an enhanced advanced oxidation process

4

for organic pollutant removal from water: Assessment with a

5

novel mini-fluidic VUV/UV photoreaction system (MVPS)

6 7

Mengkai Li,†,‡ Zhimin Qiang,,†,‡ Pin Hou,§ James R. Bolton,‖ Jiuhui Qu,†,‡ Peng Li,§

8

Chen Wang†,‡

9 10

†

Key Laboratory of Drinking Water Science and Technology, Research Center for

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Eco-Environmental Sciences, Chinese Academy of Sciences, 18 Shuang-qing Road,

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Beijing 100085, China.

13 14 15

‡

University of Chinese Academy of Sciences, 19 Yu-quan Road, Beijing 100049,

China. §

School of Chemical and Environmental Engineering, China University of Mining and

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Technology, Beijing 100083, China.

17

‖

18

Edmonton, AB T6G 2W2, Canada.

Department of Civil and Environmental Engineering, University of Alberta,

19 20 21



Corresponding author phone: +86 10 62849632; fax: +86 10 62923541; e-mail:

[email protected] (Z. M. Qiang)

22

1

ACS Paragon Plus Environment


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ABSTRACT: Vacuum ultraviolet (VUV) and ultraviolet (UV)/chlorine processes are

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regarded as two of many advanced oxidation processes (AOPs). Because of the

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similar cost of VUV/UV and UV lamps, a combination of VUV and UV/chlorine (i.e.,

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VUV/UV/chlorine) may enhance the removal of organic pollutants in water but

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without any additional power input. In this paper, a mini-fluidic VUV/UV

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photoreaction system (MVPS) was developed for bench-scale experiments, which

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could emit both VUV (185 nm) and UV (254 nm) or solely UV beams with a nearly

30

identical UV photon fluence. The photon fluence rates of UV and VUV output by the

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MVPS were determined to be 8.88 × 104 and 4.93 × 105 einstein m2 s1,

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respectively. The VUV/UV/chlorine process exhibited a strong enhancement

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concerning the degradation of methylene blue (MB, a model organic pollutant) as

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compared to the total performance of the VUV/UV and UV/chlorine processes,

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although the photon fluence of the VUV only accounted for 5.6% of that of the UV.

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An acidic pH favored MB degradation by the VUV/UV/chlorine process. The

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synergistic mechanism of the VUV/UV/chlorine process was mainly ascribed to the

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effective use of OH for pollutant removal through formation of longer-lived

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secondary

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VUV/UV/chlorine process, as an enhanced AOP, can be applied as a highly effective

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and energy-saving technology for small-scale water and wastewater treatment.

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Key words: Vacuum ultraviolet (VUV); UV/chlorine; mini-fluidic photoreaction

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system; advanced oxidation process; synergistic effect

radicals

(e.g.,



OCl).

This

study

demonstrates

2


that

the

new

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■ INTRODUCTION

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Advanced oxidation processes (AOPs) involving the generation of hydroxyl

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radicals (OH) can effectively degrade recalcitrant organic constituents in water and

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wastewater treatment.1 The combined use of ultraviolet and free chlorine

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(UV/chlorine), two commonly applied disinfection methods, has been regarded as an

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effective AOP for inactivation of water-borne pathogens and decomposition of

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hazardous organic compounds.26 Many studies have reported the fast degradation of

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organic pollutants by the UV/chlorine process, which involves a series of chain

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reactions with the formation of both OH and chlorine radicals (Cl) as highly reactive

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species:4,6,7

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HOCl  H + OCl , pKa = 7.5 (25 oC)

(1)

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HOCl   OH  Cl 

(2)

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OCl  O + Cl

(3)

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O  H2O   OH + OH

(4)

hv254

hv254

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Vacuum ultraviolet (VUV) covers a wavelength range from 100 to 200 nm.

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Photo-oxidation with VUV is also considered as an AOP because these photons can be

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absorbed by water with a high absorption coefficient (e.g., 1.8 cm–1 at 185 nm) to

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produce OH:8

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H2O   OH  H  hv185

(5)

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It is known that low-pressure (LP) mercury lamps are typically designed to emit only

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254 nm UV light (useful for disinfection), whereas the simultaneously emitted 185 nm

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VUV light is purposely absorbed by the lamp wall made of Ti-doped quartz to avoid

3



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the generation of ozone. However, if a high-purity synthetic quartz is used,9,10 both

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185 nm VUV and 254 nm UV (i.e., VUV/UV) beams can penetrate the lamp wall.

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Direct VUV photolysis of water can produce an appreciable amount of OH for

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oxidative decomposition of organic pollutants without any additional power input. As

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a result, the VUV/UV process has been applied to enhance the disinfection of

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pathogens and the removal of dissolved organic matter and various micropollutants in

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water and wastewater.1115 For UV-based AOPs (e.g., UV/chlorine), if a VUV/UV LP

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lamp (using synthetic quartz) is used as the light source, a higher efficiency can be

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expected than for a conventional UV LP lamp.

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Because of the low penetration depth of VUV in air (about 50 mm), the

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traditional quasi-collimated beam apparatus is inapplicable to the study of bench-scale

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VUV reactions in aqueous solution, except that a few researchers have attempted to

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improve its design by placing the whole apparatus in a N2-filled box.16,17 Hence, most

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previous studies were conducted in annular batch reactors with a VUV/UV lamp (10–

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500 W) situated in the center. Moreover, these batch reactors were usually operated in

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a recirculating mode for sample collection at various reaction times (or photon

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fluences).1820

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However, batch VUV/UV reactors have some problems in bench-scale studies.

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First, the LP lamp emits both VUV (185 nm) and UV (254 nm) beams, so it is

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difficult to distinguish the contribution of each wavelength. Especially for the

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VUV/UV-based AOPs, the UV component often plays an important role in OH

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generation. Hence, to accurately assess the effect of the VUV component, a special

4


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apparatus is needed that can output an identical UV photon fluence for both VUV/UV

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and UV tests. If a VUV/UV LP lamp and a UV LP lamp are used alternately in the

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same apparatus, one cannot ensure an identical UV photon fluence of the two lamps

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because many factors (e.g., power input, ballast quality, and operational conditions)

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may influence the lamp output. Moreover, because the VUV only comprises a small

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fraction of the total lamp output (i.e., about 58% of the UV output) and has a low

94

transmittance in air, quartz sleeves are not used in many batch VUV/UV reactors so as

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to maximize the VUV output. However, without protection of the quartz sleeve that

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helps stabilize the lamp surface temperature, the fluctuation in lamp output can induce

97

an enlarged uncertainty in bench-scale kinetic studies. In addition, it is difficult to

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determine accurately the photon fluence because of the non-uniform distribution of

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photon fluence rate (PFR) in batch VUV/UV reactors. Therefore, it is desirable to

100

have a new bench-scale VUV/UV apparatus for relevant photoreaction studies.

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In this study, a mini-fluidic VUV/UV photoreaction system (MVPS) was

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developed for bench-scale photochemical experiments, in which a lamp could deliver

103

either the combined VUV/UV (i.e., 185/254 nm) beams or the sole UV beam with a

104

nearly identical UV photon fluence. A UV-Vis spectrophotometer was installed in the

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MVPS to provide online absorbance measurements or spectral scans of water samples.

106

Based on the newly developed MVPS, this study further explored qualitatively the

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impact of VUV on the UV/chlorine process with regard to the photodegradation

108

kinetics of organic pollutants in aqueous solution. Methylene blue (MB) was selected

109

as a model pollutant because its concentration can be easily measured at 664 nm with

5



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the online spectrophotometer in the MVPS. The measurement at this wavelength

111

could avoid the interference of HOCl/OCl. Besides, a synergistic mechanism of the

112

VUV/UV/chlorine process was proposed and experimentally assessed. This study

113

demonstrates that the VUV/UV/chlorine process, as an enhanced AOP, can remove

114

organic pollutants effectively in water and wastewater treatment.

115

■ EXPERIMENTAL SECTION

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Mini-fluidic VUV/UV Photoreaction System (MVPS). To overcome the

117

problems of batch VUV/UV reactors, a new apparatus, namely MVPS, was designed

118

and fabricated for bench-scale photochemical studies, as illustrated in Figure 1. The

119

core part of the MVPS was a cylindrical and jacketed quartz-wall photoreactor (length

120

= 300 mm, outer diameter = 40 mm). Cooling water (i.e., deionized water) was

121

recirculated through the outer chamber between two quartz walls to control a constant

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temperature of the photoreactor, thus ensuring stable VUV and UV outputs. An 8 W

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cold-cathode LP mercury lamp (arc length = 200 mm, Wanhua Co., Zhejiang, China),

124

manufactured with a high-purity synthetic quartz wall to emit both VUV (185 nm)

125

and UV (254 nm) beams, was installed along the central axis of the cylindrical

126

photoreactor.

127

A straight synthetic quartz tube [VUV/UV tube, high VUV transmittance (i.e., 60%

128

per millimeter)] and a straight Ti-doped quartz tube (UV tube, no VUV transmittance),

129

both of 2 mm inner diameter and 100 mm length, were installed parallel to the lamp at

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the same radial distance (about 5 mm) to the lamp surface (Figure 1). By this design,

131

water sample in the UV tube could only receive the sole UV exposure, while that in 6


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the VUV/UV tube could receive the combined VUV/UV exposure. Furthermore, a

133

nearly identical UV photon fluence could be achieved in the VUV/UV and UV tubes

134

because both had a high UV (254 nm) transmittance and the same distance to the lamp

135

surface. Note that the two tubes should avoid the end-lamp regions (i.e., the last 50

136

mm at each end of the lamp arc) where the PFR distribution is usually non-uniform.23

137

Nitrogen gas was flushed through the inner chamber of the photoreactor, which

138

avoided the absorption of VUV by the interior air to produce ozone and thus

139

maximized the VUV output. A micro-fluorescent silica detector (MFSD) was installed

140

at about 5 mm to the lamp surface, which could monitor online any fluctuation in the

141

UV output in each experiment.23

142

Figure 1

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A UV-Vis spectrophotometer (UV2600, Shimadzu, Japan) with a continuous

144

injection cell was connected to the VUV/UV and UV tubes to provide online

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spectrophotometric analysis. The water sample was pumped by a peristaltic pump

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sequentially through the exposure portion of the MVPS (i.e., the VUV/UV or UV tube)

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and through the spectrophotometer, and then recirculated to receive additional

148

exposure until a desired photon fluence was reached. It should be pointed out that for

149

a certain exposure time, only a part of water sample could receive the VUV/UV or

150

UV irradiation, while the remaining part was in the dark. Hence, by defining a

151

reduction equivalent exposure time (tree, s) as the total reaction time (t, s) multiplied

152

by the ratio of the exposure volume of the operation tube (πr2L, m3) to the total

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sample volume (V, m3), the VUV (Fp,o,VUV, einstein m2) or UV (Fp,o,UV, einstein m2)

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photon fluence can be readily calculated as follows:24

 r 2h

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t ree 

156

Fp,o,UV  Ep,o,UVtree

(7)

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Fp,o,VUV  Ep,o,VUVtree

(8)

V

(6)

t

158

where r and h are the radius and length of the VUV/UV (or UV) tube (m),

159

respectively; and Ep,o,UV and Ep,o,VUV are the UV PFR in the UV or VUV/UV tube and

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the VUV PFR in the VUV/UV tube (einstein m2 s1), respectively. Hence, the total

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exposure photon fluence (Fp,o, einstein m2) in the VUV/UV tube is equal to the sum

162

of Fp,o,VUV and Fp,o,UV. In other words, tree can be regarded as the exposure time when

163

the whole sample receives the exposure with the same UV or VUV/UV PFR(s) in the

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relevant tube.

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Chemicals and Analysis. Milli-Q water (Advantage A10, Millipore, USA) was

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used in all experiments and analytical determinations. MB and uridine were purchased

167

from Sigma-Aldrich (St. Louis, USA), whose concentrations were determined by

168

absorbance measurements at 664 and 262 nm using the online spectrophotometer,

169

respectively. The concentration of methanol (Sigma-Aldrich) was determined by gas

170

chromatography equipped with a flame ionization detector (GC-FID, Shimadzu 2010,

171

Japan). The concentration of H2O2 (Beijing Chemical Reagents Co., Beijing, China)

172

was determined by the titanium oxysulfate (TiOSO4, Fluka) method with a detection

173

limit of 0.01 mg L1.25 Active chlorine (HOCl/OCl–) was prepared freshly from a

174

stock NaOCl solution (1015% by weight, Sigma-Aldrich), and its concentration was 8


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measured by the N,N-diethyl-p-phenylenediamine colorimetric method on a UV-Vis

176

spectrophotometer (Hach DR5000, USA).26 To scavenge OH, 5 mM tert-butanol

177

(TBA, Sigma-Aldrich) or 0.5 mM nitrobenzene (NB, Sigma-Aldrich) was added to

178

relevant reaction solutions in advance.

179

Experimental Procedures. Each experiment was initiated after warming up the

180

lamp for about 15 min until the MFSD signal reached a constant value. Uridine27

181

(0.12 mM) or methanol28,29 (95 mM) was used as the chemical actinometer to

182

determine the Fp,o.UV and Fp,o.VUV in the MVPS, respectively. To assess the

183

enhancement on MB degradation by the VUV/UV/chlorine process as compared to

184

the UV/chlorine process, an unbuffered MB solution (5 mg L1) was spiked with 25

185

mg L1 (as active chlorine) NaOCl and immediately pumped through the VUV/UV

186

and UV tubes to receive the irradiation. The solution pH decreased approximately

187

from an initial value of 6.4 to a final value of 5.5 during the course of reaction. To

188

examine the effect of pH, the MB solution was buffered at either pH 5 (with

189

phosphate) or 10 (with borate) and then subjected to photodegradation in the

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VUV/UV/chlorine and UV/chlorine processes. According to its pKa at 25 oC (i.e., 7.5),

191

active chlorine at pH values of 5 and 10 consist of 99.7% and 0.3% HOCl,

192

respectively. The MB concentration was measured by the online spectrophotometer at

193

a time interval of 0.5 min during photodegradation experiments that lasted for 10 min

194

each.

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■ RESULTS AND DISCUSSION

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Determination of Fp,o,UV and Fp,o,VUV in the MVPS. A uridine solution (0.12 mM)

197

was used as the chemical actinometer to determine the Fp,o,UV in the MVPS. Figure 2

198

shows the logarithmic decay of uridine with reaction time in each tube in the presence

199

or absence of TBA (i.e., OH scavenger). The fit lines indicate that each reaction could

200

be well expressed by pseudo-first-order kinetics. In the UV tube, TBA addition had

201

little impact on the reaction rate constant because of the negligible OH generation

202

during the direct UV photolysis of uridine. In the VUV/UV tube, because VUV

203

photons preferred to react with water (with a high concentration of 55.6 M) than with

204

uridine, the direct VUV photolysis of uridine was insignificant. The total uridine

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decay was ascribed to both the direct UV photolysis and the oxidation by OH

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generated from the direct VUV photolysis of water. As a result, the uridine decay in

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the VUV/UV tube was obviously faster than in the UV tube (Figure 2). After

208

scavenging OH with TBA in the VUV/UV tube, the reaction rate constant of uridine

209

(2.74  104 s1) was close to that in the UV tube (2.51  104 s1), demonstrating that

210

the UV and VUV/UV tubes had nearly identical Fp,o,UV values. Figure 2

211 212 213 214

Based on the UV photolysis rate constant of uridine determined above, the Fp,o,UV (einstein m2) could be calculated as follows: Fp,o,UV 

ku t ln(10)  0.1   λ   u

(9)

215

where ku (s1) is the time-based pseudo-first-order rate constant for UV photolysis of

216

uridine; and  (M1 cm1) and Φu are the decadic molar absorption coefficient and

217

quantum yield of uridine at an exposure wavelength (), respectively. The 254 and Φu 10


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values were reported to be 8,775 M1 cm1 and 0.020, respectively.27 Therefore, the

219

UV PFR (i.e., Ep,o,UV) was calculated to be 8.88 × 104 einstein m2 s1 (or a fluence

220

rate of 41.8 mW cm2) at 254 nm from eqs 6 and 7 (r = 1 mm, h = 100 mm, V = 44

221

mL).

222

In addition, a methanol solution (95 mM) was used as another chemical

223

actinometer to determine the Fp,o,VUV in the VUV/UV tube. Figure 3 shows that the

224

VUV photolysis of methanol could be well expressed by zero-order kinetics, and the

225

rate constant of methanol degradation (kMeOH) was 7.0 × 108 M s1. Hence, the VUV

226

photon flux could be calculated with eq 10:28

227

qVUV 

k MeOHV 0.946( H 2 O f H 2 O   MeOH f MeOH )

(10)

228

where qvuv is the absorbed VUV photon flux (einstein s1); ΦH2O and ΦMeOH are the

229

total quantum yields of OH generated from VUV photolysis of water (ΦH2O = 0.375)

230

and methanol (ΦMeOH = 1), respectively;30 fH2O and fMeOH are the incident photon

231

fractions absorbed by water (fH2O = 0.9962) and methanol (fMeOH = 0.0038),

232

respectively; and the factor of 0.946 refers to the production of methanol by the

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disproportionation reaction of hydroxymethyl radicals which slows down the OH–

234

induced degradation of methanol.28 The VUV PFR (i.e., Ep,o,VUV) at the outer surface

235

of the VUV/UV tube was equal to the qvuv divided by the cross-sectional area of the

236

tube [i.e., 4.93 × 105 einstein m2 s1 (or a fluence rate of 3.19 mW cm2)], which

237

accounted for 5.6% of the UV PFR. With the Ep,o,VUV available, the Fp,o,VUV could be

238

readily calculated with eq 8.

239

Figure 3 11



240

VUV/UV/chlorine Process. Figure 4 shows the degradation of MB in the

241

UV/chlorine and VUV/UV/chlorine processes with pH uncontrolled (pH decreased

242

approximately from 6.4 to 5.5 over the reaction course). The fit lines indicate that all

243

reactions followed pseudo-first-order kinetics. Only a slight degradation was found if

244

the MB solution was exposed to UV or active chlorine alone (kU = 9.0 m2 einstein1,

245

kCl = 3.17  104 s1). However, a notably higher photon fluence-based reaction rate

246

constant (kV/U = 268 m2 einstein1) was observed in the VUV/UV process because of

247

the additional advanced oxidation by OH. After spiking active chlorine, the photon

248

fluence-based reaction rate constant increased considerably to 352 m2 einstein1 (kU/Cl)

249

in the UV/chlorine process and to 1,180 m2 einstein1 (kV/U/Cl) in the

250

VUV/UV/chlorine process.

251

Figure 4

252

Since little MB was degraded under UV exposure alone and the direct VUV

253

photolysis of MB was also negligible, the degradation of MB in the

254

VUV/UV/chlorine process could arise from only two parts: (1) oxidation by active

255

chlorine and radicals (e.g., OH and Cl) generated from active chlorine exposed to

256

UV (254 nm), and (2) oxidation by OH generated from VUV (185 nm) photolysis of

257

water. The former part was equivalent to the MB degradation by UV/chlorine (i.e., in

258

the UV tube), considering that the UV photon fluence was nearly identical in both

259

tubes. The latter part was equivalent to the MB degradation by VUV/UV (i.e., in the

260

VUV/UV tube) without active chlorine addition. Therefore, in theory, the photon

261

fluence-based rate constant of MB degradation in the VUV/UV/chlorine process

12


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(kV/U/Cl) should be equal to the sum of those in the UV/chlorine and VUV/UV

263

processes (i.e., kU/Cl + kV/U). Nevertheless, the kV/U/Cl (1,180 m2 einstein1) was

264

actually 1.9 times the sum of kV/U (268 m2 einstein1) and kU/Cl (352 m2 einstein1),

265

manifesting a significant enhancement of the MB degradation by the combined

266

VUV/UV/chlorine process. Similar tests were also performed by substituting active

267

chlorine with H2O2, whereas the MB degradation rate constant in the VUV/UV/H2O2

268

process (kV/U/H = 271 m2 einstein1) was almost equal to the sum of those in the

269

UV/H2O2 (kU/H = 77 m2 einstein1) and VUV/UV (kV/U = 185 m2 einstein1) processes

270

(Figure S1).

271

Effect of Active Chlorine Dose. The effect of active chlorine dose on MB

272

degradation by VUV/UV/chlorine, UV/chlorine, VUV/UV/H2O2, and UV/H2O2 is

273

shown in Figure 5a. As the active chlorine dose was raised from 0 to 50 mg L1, both

274

kV/U/Cl and kU/Cl increased obviously with the former increasing relatively faster. By

275

contrast, both kV/U/H and kU/H increased slowly with increasing H2O2 dose at a similar

276

pace. To quantitatively assess the enhancement of the combined process (i.e.,

277

VUV/UV/chlorine or VUV/UV/H2O2) on MB degradation as compared to the total

278

performance of the two relatively simpler processes [i.e., UV/chlorine (or UV/H2O2)

279

plus VUV/UV], an enhancement factor (R, with its subscript denoting a chemical

280

added) was defined as follows:

281

RCl 

282

RH 

 kV/U/Cl   kV/U  kU/Cl  kV/U/H   kV/U  kU/H

1

(11)

1

(12)

13



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283

Figure 5b shows that spiking active chlorine to the VUV/UV process yielded

284

obviously higher R values than spiking H2O2. Moreover, the RCl kept increasing with

285

increasing active chlorine dose until reached a maximum value (i.e., 0.9) at 25 mg L1

286

active chlorine, thereafter it decreased at a higher active chlorine dose (50 mg L1). By

287

contrast, the RH kept increasing with increasing H2O2 dose, but its increase was much

288

slower than that of RCl. It is well known that H2O2 competes for •OH,7 and the

289

self-recombination of •OH resulting from VUV photolysis of water (to form additional

290

H2O2) could further exacerbate this scavenging effect in the VUV/UV/H2O2 process.

291

However, in the UV/chlorine process, the quantum yield of active chlorine (to

292

produce radicals) increased with increasing active chlorine dose.5 Figure 5

293

294

Effect of pH. MB degradation by UV/chlorine and VUV/UV/chlorine was

295

comparatively examined at pHs 5 and 10. Without irradiation, the solution pH only

296

showed a slight impact on MB degradation by active chlorine (Figure S2). However,

297

after exposure to UV or VUV/UV light, the solution pH could significantly impact

298

MB degradation (Figure 6). Specifically, under UV exposure alone, MB was degraded

299

more quickly at pH 10 (kU = 62 m2 einstein1) than at pH 5 (kU = 13 m2 einstein1).

300

By contrast, the VUV/UV process removed MB more effectively at pH 5 (kV/U = 365

301

m2 einstein1) than at pH 10 (kV/U = 245 m2 einstein1). This could be attributed to

302

two opposite aspects. On the one side, when abundant •OH was generated by VUV

303

photolysis of water, the following reactions could occur:3133

304



OH  H+ + O

(13) 14


Page 15 of 36




OH +  OH  H 2 O 2

(14)

306

H 2 O 2  H + + HO 2 

(15)

307

HO2    OH  H 2O + O 2

(16)

305

308 309

Reactions 13, 15 and 16 would shift toward the right with increasing pH. Because OH was consumed to produce O and O2, which have lower oxidation potentials

•

310

than OH, the MB degradation would be retarded correspondingly. On the other side,

311

the pH increase promoted the deprotonation of two side amine moieties in the MB

312

structure (Figure S3), thus facilitating the electrophilic attack of oxidative radicals.

313

The net effect came out to be that MB degradation by VUV/UV slowed down at pH

314

10 as compared to that at pH 5. Figure 6

315 316

For the UV/chlorine process, its photon fluence-based rate constant at pH 5 (kU/Cl

317

= 517 m2 einstein1) was about 3.3 times that at pH 10 (kU/Cl = 155 m2 einstein1),

318

which could arise from the dissociation of HOCl to OCl– (pKa = 7.5, 25 oC). It was

319

reported that under UV exposure, HOCl has a higher quantum yield for radical

320

production than does OCl–,4,7 thus the pH decrease enhanced MB degradation by

321

UV/chlorine. For the VUV/UV/chlorine process, the photon fluence-based rate

322

constant (i.e., kV/U/Cl) at pH 5 was also significantly higher (2.9 times) than that at pH

323

10. Overall, the synergistic effect of the VUV/UV/chlorine process was stronger at pH

324

5 (RCl = 0.4) than at pH 10 (RCl = 0.1).

325

Effect of NB. Because NB reacts very fast with OH (kNB,OH = 3.9  109 s1) but

326

hardly reacts at all with reactive chlorine species (e.g., Cl),4 the contributions of OH

15



327

and the reactive chlorine species to MB degradation could be clarified by adding NB

328

as an OH scavenger. In the presence or absence of 0.5 mM NB, the photon

329

fluence-based rate constants of MB degradation by VUV/UV, UV/chlorine and

330

VUV/UV/chlorine at pHs 5 and 10 are listed comparatively in Table 1. As expected,

331

under VUV/UV exposure, little MB was degraded with NB addition in both acidic

332

and alkaline conditions. Furthermore, in the presence of NB, the photon fluence-based

333

rate constants for MB degradation by UV/chlorine and VUV/UV/chlorine also

334

decreased. However, a notable portion of MB was still degraded, because NB could

335

not scavenge the reactive chlorine species generated in the two processes. In the

336

VUV/UV/chlorine process, it can be calculated that the relative contributions of OH

337

and the reactive chlorine species to MB degradation were 63% and 35% at pH 5, and

338

67% and 29% at pH 10, respectively. The direct photolysis only contributed to 2% and

339

4% of MB degradation at pHs 5 and 10, respectively.

340

Table 1

341

Synergistic Mechanism of VUV/UV/chlorine. In most AOPs (e.g., UV/H2O2 and

342

UV/O3), reactive species are distributed quite uniformly in aqueous solution.

343

Nevertheless, for the VUV/UV process, because of the low penetration depth of the

344

185 nm VUV in water, a majority of reactive species (i.e., OH and H) are formed in

345

the region near the quartz sleeve (i.e., the near-field region), whereas much less

346

reactive species are formed in the region with a larger distance from the quartz sleeve

347

(i.e., the far-field region). Hence, in the near-field region, the OH and H with a high

16


Page 16 of 36

Page 17 of 36


348

local concentration tend to induce the formation of H2O2 (eq 14) and H2 (eq 17)34

349

arising from their self-recombination reactions.

H  H  H2 , kH = 1  1010 M1 s1

350

(17)

351

To test the formation of H2O2, Milli-Q water was exposed to VUV/UV irradiation

352

in the MVPS, and 0.59 mg L1 H2O2 was detected at a photon fluence of 2.28 × 102

353

einstein m2 (Figure 7). Imoberdorf and Mohseni12 also reported that up to 0.32 mM

354

H2O2 was generated in an annular VUV/UV reactor after a long exposure time of 40

355

min. By contrast, with addition of 25 mg L-1 active chlorine to the VUV/UV process

356

(i.e., VUV/UV/chlorine), the formation of H2O2 was greatly inhibited until the

357

depletion of active chlorine (Figure 7). Thereafter, H2O2 started to appear and reached

358

0.21 mg L1 at a photon fluence of 2.28 × 102 einstein m2. Accordingly, active

359

chlorine was consumed much faster in the VUV/UV/chlorine process than in the

360

UV/chlorine process. This implies that in the VUV/UV/chlorine process, the abundant

361



OH in the near-field region could further react with HOCl/OCl– to produce secondary

362

radicals such as OCl (eqs 18 and 19)4 instead of forming H2O2 (eq 14). These

363

secondary radicals could live longer and migrate for a larger distance to attack MB

364

molecules available in the far-field region of the bulk solution. Hence, through

365

addition of active chlorine, the OH generated from VUV photolysis of water was

366

mostly used for pollutant removal rather than ending up in the ineffective H2O2. This

367

may explain the synergistic effect observed in the VUV/UV/chlorine process.

368 369



OH  HOCl   OCl + H2O

(18)



OH  OCl   OCl + OH 

(19)

17



Page 18 of 36

Figure 7

370 371

The effect of active chlorine dose is also manifested in Figure 5. When the active

372

chlorine dose increased from 0 to 25 mg L1, the primary radicals (OH) could react

373

with HOCl/OCl– to produce the secondary radicals (e.g., OCl) as aforementioned,

374

thus causing a rapid increase in the RCl due to their larger migration distance (Figure

375

5b). However, when the active chlorine dose further increased to 50 mg L1, the RCl

376

decreased to some extent. This probably arises from three aspects: 1) at a high active

377

chlorine concentration, the primary OH in the near-field region was completely

378

consumed by MB and HOCl/OCl, so the secondary radicals already reached a

379

maximum concentration; 2) the secondary radicals had a relatively weaker oxidation

380

potential than OH (e.g., OCl vs. OH); and 3) with the availability of abundant

381

HOCl/OCl, some tertiary reactive species (e.g., Cl2) might be formed, which could

382

only react slowly with MB.

383

In addition, the synergistic effect of the VUV/UV/chlorine process could also

384

arise from the VUV photolysis of HOCl/OCl– (eq 20), although VUV photons react

385

much faster toward H2O. Table 1 shows that in the presence of NB, the kV/U/Cl was 2.9

386

and 2.5 times the kU/Cl at pH 5 and 10, respectively. This implies that after completely

387

scavenging

388

VUV/UV/chlorine process than in the UV/chlorine process, which led to a much

389

faster MB degradation.

390



OH, a higher Cl concentration could be expected in the

185 HOCl     OH  Cl 

hv

(20)

18


Page 19 of 36


391

Discussion on MVPS. The MVPS is a new bench-scale apparatus for

392

fundamental photoreaction studies. Although it cannot solely deliver VUV (185 nm)

393

irradiation, it can output VUV/UV and UV beams with a nearly identical UV photon

394

fluence in the two test tubes through special design of the optical structure, thus

395

enabling studies on the net effect of VUV irradiation. Hence, the MVPS provides a

396

powerful tool to compare quantitatively the treatment efficiencies of UV- and

397

VUV/UV-based AOPs in the presence or absence of an assistant chemical (e.g., active

398

chlorine), where UV often plays an important role in pollutant removal by forming

399

reactive species and by direct photolysis as well. In addition, the two test tubes (i.e.,

400

VUV/UV and UV tubes) are installed close to the lamp (about 5 mm to the lamp

401

surface), which gives rise to a high UV PFR [8.88 × 10-4 einstein m2 s1 (or a fluence

402

rate of 41.8 mW cm2) at 254 nm] and a short reaction time correspondingly. Further

403

assisted with an online spectrophotometer, the MVPS can quickly determine the

404

reaction kinetics under VUV/UV exposure, with a significant reduction in both

405

experimental workload and operational error.

406

Compared with a cylindrical batch VUV/UV reactor, the MVPS has a much

407

shorter optical path-length (2 mm inner diameter, and even a thinner tube can be

408

selected). Because the background UV absorbance of water sample induces a PFR

409

attenuation along the radial distance of the lamp according to the Lambert-Beer Law,

410

a smaller variance of UV PFR (i.e., Ep,o,UV) within the cross-section of the test tube in

411

the MVPS can be reasonably expected than that in a batch reactor. Moreover, by

412

recirculating water of a constant temperature through the outer chamber of the

19



413

photoreactor (to maintain a stable mercury vapor pressure in the lamp) and online

414

monitoring the lamp output with an MFSD as well, a stable UV output is ensured

415

during each experiment. Therefore, the UV photon fluence in the MVPS can be

416

determined more accurately than that in a batch reactor.

417

Note that the MVPS adopts an operation mode of intermittent light exposure, that

418

is, tree is only a part of the total reaction time (t). For some complex reactions whose

419

photo-reaction (dependent of light) and dark-reaction (independent of light) rates are

420

comparable, intermittent light exposure may induce a certain error in kinetic

421

measurements. Under this circumstance, the dark-reaction rate needs to be measured

422

separately by a control experiment, and then the overall kinetic parameters obtained

423

from the MVPS should be corrected based on relevant kinetic model calculations. In

424

the present study, because the rate constant of MB degradation by active chlorine in

425

the dark was much smaller than those of photo-reactions (Figure 4), intermittent light

426

exposure would not induce a problem.

427

Discussion on VUV/UV/chlorine. The low VUV output has long been regarded

428

as the principal limitation for its practical applications to water treatment.9 Hence, a

429

majority of research efforts have been made to enhance the VUV output, such as

430

increasing the VUV transmittance of the quartz used for lamp wall and sleeve, raising

431

the total power of the lamp, and inventing new light sources for VUV irradiation.

432

However, unfortunately, to date, no significant progress has been made. This study,

433

for the first time, demonstrates that the treatment efficiency of the VUV/UV process

434

can be simply but greatly improved by addition of active chlorine. Because the new

20


Page 20 of 36

Page 21 of 36


435

VUV/UV/chlorine process is easy to apply, highly efficient, and energy saving, it has

436

a promising prospect for small-scale water and wastewater treatment.

437

■ ASSOCIATED CONTENT

438

Supporting Information. Three figures are provided. This material is available free of

439

charge via the Internet at http://pubs.acs.org.

440

■ AUTHOR INFORMATION

441

Corresponding Author

442

*

Phone: (+86)-10-62849632; fax: (+86)-10-62923541; e-mail: [email protected].

443

Notes

444

The authors declare no competing financial interest.

445

■ ACKNOWLEDGEMENTS

446

This work was financially supported by the National Natural Science Foundation of

447

China (51408592, 51525806, 51221892) and the National Geographic Air and Water

448

Conservation Fund (GEFC23-15).

449

■ REFERENCES

450

(1)

Keen, O. S.; McKay, G.; Mezyk, S. P.; Linden, K. G.; Rosario-Ortiz, F. L.

451

Identifying the factors that influence the reactivity of effluent organic matter with

452

hydroxyl radicals. Water Res. 2014, 50, 408419.

453

(2)

Watts, M. J.; Linden, K. G. Chlorine photolysis and subsequent OH radical

454

production during UV treatment of chlorinated water. Water Res. 2007, 41 (13),

455

28712878. 21



456

(3)

Weng, S. C.; Blatchley, E. R. Ultraviolet-induced effects on chloramine and

457

cyanogen chloride formation from chlorination of amino acids. Environ. Sci. Technol.

458

2013, 47 (9), 42694276.

459

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Fang, J. Y.; Fu, Y.; Shang, C. The roles of reactive species in micropollutant

460

degradation in the UV/free chlorine system. Environ. Sci. Technol. 2014, 48 (3),

461

18591868.

462

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Feng, Y. G.; Smith, D. W.; Bolton, J. R. Photolysis of aqueous free chlorine

463

species (HOCl and OCl-) with 254 nm ultraviolet light. J. Environ. Eng. Sci. 2007, 6

464

(3), 277284.

465 466 467

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Jin, J.; El-Din, M. G.; Bolton, J. R. Assessment of the UV/chlorine process as

an advanced oxidation process. Water Res. 2011, 45 (4), 18901896. (7)

Wang, D.; Bolton, J. R.; Andrews, S. A.; Hofmann, R. Medium pressure UV

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combined with chlorine advanced oxidation for trichloroethylene destruction in a

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model water. Water Res. 2012, 46 (15), 46774686.

470 471 472 473 474 475

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Weeks, J. L.; Meaburn, G. M. A.; Gordon, S. Absorption coefficients of liquid

water and aqueous solutions in far ultraviolet. Radiat. Res. 1963, 19 (3), 559567. (9)

Zoschke, K.; Bornick, H.; Worch, E. Vacuum-UV radiation at 185 nm in water

treatment - A review. Water Res. 2014, 52, 131145. (10) Oppenländer, T. Photochemical Purification of Water and Air. Wiley-VCH: Weinheim, Germany, 2003; pp. 368.

476

(11) Buchanan, W.; Roddick, F.; Porter, N.; Drikas, M. Fractionation of UV and

477

VUV pretreated natural organic matter from drinking water. Environ. Sci. Technol.

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2005, 39 (12), 46474654.

(12) Imoberdorf, G.; Mohseni, M. Degradation of natural organic matter in surface water using vacuum-UV irradiation. J. Hazard. Mater. 2011, 186 (1), 240246.

481

(13) Li, W. Z.; Lu, S. G.; Chen, N.; Gu, X. G.; Qiu, Z. F.; Fan, J.; Lin, K. F.

482

Photo-degradation of clofibric acid by ultraviolet light irradiation at 185 nm. Water

483

Sci. Technol. 2009, 60 (11), 29832989.

484 485 486 487

(14) Chen, J.; Zhang, P. Y.; Liu, J. Photodegradation of perfluorooctanoic acid by 185 nm vacuum ultraviolet light. J. Environ. Sci.-China 2007, 19 (4), 387390. (15) Kim, I.; Tanaka, H. Photodegradation characteristics of PPCPs in water with UV treatment. Environ. Int. 2009, 35 (5), 793802.

488

(16) Duca, C.; Imoberdorf, G.; Mohseni, M. Novel collimated beam setup to study

489

the kinetics of VUV-induced reactions. Photochem. Photobiol. 2014, 90 (1), 238240.

490

(17) Wang, D.; Oppenländer, T.; El-Din, M. G.; Bolton, J. R. Comparison of the

491

disinfection effects of vacuum-UV (VUV) and UV light on bacillus subtilis spores in

492

aqueous suspensions at 172, 222 and 254 nm. Photochem. Photobiol. 2010, 86 (1),

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176181.

494

(18) Collivignarelli, C.; Sorlini, S. AOPs with ozone and UV radiation in drinking

495

water: Contaminants removal and effects on disinfection byproducts formation. Water

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Sci. Technol. 2004, 49 (4), 5156.

497

(19) Echigo, S.; Yamada, H.; Matsui, S.; Kawanishi, S.; Shishida, K. Comparison

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between O3/VUV, O3/H2O2, VUV and O3 processes for the decomposition of

499

organophosphoric acid triesters. Water Sci. Technol. 1996, 34 (9), 8188.

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500

(20) Han, W. Y.; Zhang, P. Y.; Zhu, W. P.; Yin, J. J.; Li, L. S. Photocatalysis of

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p-chlorobenzoic acid in aqueous solution under irradiation of 254 nm and 185 nm UV

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light. Water Res. 2004, 38 (19), 41974203.

503 504

(21) Qiang, Z. M.; Li, M. K.; Bolton, J. R. Development of a tri-parameter online monitoring system for UV disinfection reactors. Chem. Eng. J. 2013, 222, 101107.

505

(22) Li, M. K.; Qiang, Z. M.; Wang, C.; Bolton, J. R.; Lian, J. F. Development of

506

monitored tunable biodosimetry for fluence validation in an ultraviolet disinfection

507

reactor. Sep. Purif. Technol. 2013, 117, 1217.

508

(23) Li, M. K.; Qiang, Z. M.; Li, T. G.; Bolton, J. R.; Liu, C. L. In situ

509

measurement of UV fluence rate distribution by use of a micro fluorescent silica

510

detector. Environ. Sci. Technol. 2011, 45 (7), 30343039.

511

(24) Li, M. K.; Qiang Z. M.; Bolton J. R.; Qu, J. H.; Li, W. T. A mini-fluidic UV

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photoreaction system for bench-scale photochemical studies. Environ. Sci. Technol.

513

Lett. 2015, 2 (10), 297301.

514

(25) Giannakis, S.; Vives, F. A. G.; Grandjean, D.; Magnet, A.; De Alencastro, L. F.;

515

Pulgarin, C. Effect of Advanced Oxidation Processes on the micropollutants and the

516

effluent organic matter contained in municipal wastewater previously treated by three

517

different secondary methods. Water Res., 2015, 84, 295306.

518

(26) Eaton, A. D.; Franson, M. A. H. American Public Health Association, Standard

519

Methods for the Examination of Water and Wastewater. 19th ed.; American Public

520

Health Association: Washington, DC, 1995.

521

(27) Scholes, M. L.; Schuchmann, M. N.; von Sonntag, C. Enhancement of

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radiation-induced base release from nucleosides in alkaline-solution - Essential role of

523

the O•– radical. Int. J. Radiat. Biol. 1992, 61 (4), 443449.

524

(28) Oppenländer, T.; Schwarzwalder, R. Vacuum-UV oxidation (H2O-VUV) with a

525

xenon excimer flow-trough lamp at 172 nm: Use of methanol as actinometer for VUV

526

intensity measurement and as reference compound for OH-radical competition

527

kinetics in aqueous systems. J. Adv. Oxi. Technol. 2002, 5 (2), 155163.

528

(29) Jin, S.; Mofidi, A. A.; Linden, K. G., Polychromatic UV fluence measurement

529

using chemical actinometry, biodosimetry, and mathematical techniques. J. Environ.

530

Eng.-ASCE 2006, 132 (8), 831841.

531

(30) Duca, C. Effect of water matrix on vacuum UV process for the removal of

532

organic micropollutants in surfacewaer. Ph.D. thesis, University of British Columbia,

533

Vacouver, Canada, 2015.

534

(31) Ratpukdi, T.; Siripattanakul, S.; Khan, E. Mineralization and biodegradability

535

enhancement of natural organic matter by ozone-VUV in comparison with ozone,

536

VUV, ozone-UV, and UV: Effects of pH and ozone dose. Water Res. 2010, 44 (11),

537

3531−3543.

538

(32) Kruithof, J. C.; Kamp, P. C.; Martijn, B. J. UV/H2O2 treatment: A practical

539

solution for organic contaminant control and primary disinfection. Ozone Sci. Eng.

540

2007, 29 (4), 273‒280.

541

(33) Ma, C. M.; Hong, G. B.; Chen, H. W.; Hang, N. T.; Shen, Y. S. Photooxidation

542

contribution study on the decomposition of azo dyes in aqueous solutions by

543

VUV-based AOPs. Int. J. Photoenergy 2011, 2011, 1‒8.

25



544

(34) Imoberdorf, G., Mohseni, M. Modeling and experimental evaluation of

545

vacuum-UV photoreactors for water treatment. Chem. Eng. Sci. 2011, 66 (6), 1159‒

546

1167.

547

26


Page 26 of 36

Page 27 of 36


548

FIGURE CAPTIONS

549

Figure 1.

(MVPS).

550 551

Schematic diagram of the mini-fluidic VUV/UV photoreaction system

Figure 2.

Photolysis of uridine in the presence or absence of TBA (5 mM) in the VUV/UV and UV tubes.

552 553

Figure 3.

Photolysis of methanol (MeOH) in the VUV/UV tube.

554

Figure 4.

MB degradation by UV/chlorine and VUV/UV/chlorine. Conditions: [MB]0 = 5 mg L1 , [chlorine]0 = 25 mg L1.

555 556

Figure 5.

MB degradation by UV/chlorine, VUV/UV/chlorine, UV/H2O2, and

557

VUV/UV/H2O2 as a function of H2O2 or active chlorine dose: (a) the

558

photon fluence-based rate constants; and (b) the enhancement factor (R).

559

Conditions: [MB]0 = 5 mg L1.

560

Figure 6.

MB degradation by UV/chlorine and VUV/UV/chlorine as a function of

561

photon fluence at: (a) pH 5; and (b) pH 10. Conditions: [MB]0 = 5 mg L1,

562

[chlorine]0 = 25 mg L1.

563 564

Figure 7.

Chlorine consumption and H2O2 formation in the VUV/UV, UV/chlorine and VUV/UV/chlorine processes. Conditions: [chlorine]0 = 25 mg L1.

565

27



Page 28 of 36

Table 1. Pseudo-first-order Photon Fluence-based Rate Constants of MB Degradation (k, m2 einstein1) by VUV/UV, UV/chlorine and VUV/UV/chlorine

pH 5

pH 10

Process without NB

with NBa

without NB

with NBa

VUV/UV

365b

26

245

17

UV/chlorine

517

149

155

51

VUV/UV/chlorine

1222

432

447

130

a

[NB]0 = 0.5 mM.

b

Each number in this table is a result of the linear regression of 7‒20 experimental

data points (R2 > 99%).

28


Page 29 of 36


Figure 1. Schematic diagram of the mini-fluidic VUV/UV photoreaction system

(MVPS).

29



Page 30 of 36

ln ([uridine]/[uridine]0)

0.0

-0.2

-0.4

-0.6

-4

-1

kU = 2.51x10 s UV -4 -1 VUV/UV kV/U = 6.84x10 s -4 -1 UV+TBA kU = 2.42x10 s VUV/UV+TBA kV/U = 2.74x10-4 s-1

-0.8 0

200

400

600

800

1000

1200

t (s) 0

2

4

6

8

tree (s)

Figure 2. Photolysis of uridine in the presence or absence of TBA (5 mM) in the

VUV/UV and UV tubes.

30


Page 31 of 36


[MeOH] (mM)

98.5

98.0

97.5

slope = - 7.0 x10-5 97.0

0

2000

4000

6000

8000

10000

12000

t (s) 0

10

20

30

40

50

60

70

80

tree (s)

Figure 3. Photolysis of methanol (MeOH) in the VUV/UV tube.

31



Page 32 of 36

0.0 -0.5

ln ([MB]/[MB]0)

-1.0 -1.5 -2.0 chlorine kCl = 3.17 x 10-4 s-1 k'U = 9.0 m2 einstein-1 UV k'V/U = 268 m2 einstein-1 VUV/UV UV/chlorine k'U/Cl = 352 m2 einstein-1 VUV/UV/chlorine k'V/U/Cl = 1180 m2 einstein-1

-2.5 -3.0 -3.5 -4.0

0.0

0.5

1.0

1.5

2.0 -3

2.5

3.0

3.5

-2

Fp,o (x10 einstein m ) 0

100

200

300

400

500

600

t (s) 0.0

0.8

1.6

2.4

tree (s)

3.2

4.0

Figure 4. MB degradation by UV/chlorine and VUV/UV/chlorine. Conditions: [MB]0

= 5 mg L1 , [chlorine]0 = 25 mg L1.

32


Page 33 of 36


1500

VUV/UV/chlorine UV/chlorine VUV/UV/H2O2 UV/H2O2

(a)

RCl RH

(b)

1000

2

-1

k' (m einstein )

2000

500

0 1.4 1.2 1.0

R

0.8 0.6 0.4 0.2 0.0 0

10

20

30

40

50 -1

chlorine or H2O2 dose (mg L ) Figure 5. MB degradation by UV/chlorine, VUV/UV/chlorine, UV/H2O2, and

VUV/UV/H2O2 as a function of active chlorine or H2O2 dose: (a) the photon fluence-based rate constants; and (b) the enhancement factor (R). Conditions: [MB]0 = 5 mg L1.

33



Page 34 of 36

0.0

(a)

ln ([MB]/[MB]0)

-0.5 -1.0 -1.5 k'U = 13 m2 einstein-1 UV k'V/U = 365 m2 einstein-1 VUV/UV k'U/Cl = 517 m2 einstein-1 UV/chlorine 2 -1 VUV/UV/chlorine k'V/U/Cl = 1222 m einstein

-2.0 -2.5

(b)

0.0

ln ([MB]/[MB]0)

-0.5 -1.0 -1.5 k'U = 62 m2 einstein-1 UV k'V/U = 245 m2 einstein-1 VUV/UV k'U/Cl = 155 m2 einstein-1 UV/chlorine 2 -1 VUV/UV/chlorine k'V/U/Cl = 447 m einstein

-2.0 -2.5 0.0

0.5

1.0

1.5

2.0

-3

2.5

3.0

3.5

-2

Fp,o (x10 einstein m ) Figure 6. MB degradation by UV/chlorine and VUV/UV/chlorine as a function of

photon fluence at: (a) pH 5; and (b) pH 10. Conditions: [MB]0 = 5 mg L1, [chlorine]0 = 25 mg L1.

34



[chlorine], UV/chlorine [chlorine], VUV/UV/chlorine [H2O2], VUV/UV [H2O2], VUV/UV/chlorine

30

-1

[chlorine] (mg L )

25

0.8

0.6

-1

20

[H2O2] (mg L )

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15

0.4

10 0.2 5 0.0

0 0

4

8

12 -3

16

20

24

-2

Fp,o (x10 einstein m )

Figure 7. Chlorine consumption and H2O2 formation in the VUV/UV, UV/chlorine

and VUV/UV/chlorine processes. Conditions: [chlorine]0 = 25 mg L1.

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TOC /Abstract art

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Chlorine as an Enhanced Advanced ... - ACS Publications

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