Experimental Assessment of Photon Fluence Rate Distributions in a

Feb 21, 2017 - The performance of a medium-pressure (MP) mercury lamp photoreactor is strongly influenced by the spatial photon fluence rate (PFR) dis...
0 downloads 6 Views 736KB Size
Subscriber access provided by UNIV OF NEW ENGLAND

Article

Experimental assessment of photon fluence rate distributions in a medium-pressure UV photoreactor Mengkai Li, Zhimin Qiang, Chen Wang, James R. Bolton, and Ernest R. Blatchley Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b06298 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

Environmental Science & Technology

1 2

Experimental assessment of photon fluence rate distributions

3

in a medium-pressure UV photoreactor

4 5

Mengkai Li,†,‡ Zhimin Qiang,‡ Chen Wang,‡ James R. Bolton,§

6

Ernest R. Blatchley III*,†ǁ

7 8 9



Lyles School of Civil Engineering, Purdue University, West Lafayette, Indiana 47907,

10

United States

11



12

Eco-Environmental Sciences, University of Chinese Academy of Sciences, Chinese

13

Academy of Sciences, 18 Shuang-qing Road, Beijing 100085, China

14

§

15

Edmonton, AB T6G 2W2, Canada.

16

ǁ

17

Lafayette, Indiana 47907, United States

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

Department of Civil and Environmental Engineering, University of Alberta,

Division of Environmental & Ecological Engineering, Purdue University, West

18 19 20

*Corresponding author: ph. 1-765-494-0316; fax 1-765-4940395; email

21

[email protected]; Lyles School of Civil Engineering, Purdue University, 550

22

Stadium Mall Drive, West Lafayette, IN 47907 USA.

1

ACS Paragon Plus Environment

Environmental Science & Technology

23

ABSTRACT: The performance of a medium-pressure (MP) mercury lamp

24

photoreactor is strongly influenced by the spatial photon fluence rate (PFR)

25

distributions which are wavelength-dependent. To address this issue, PFR

26

distributions in an MP lamp photoreactor were measured using a 360-degree response

27

micro-fluorescent silica detector (MFSD). To accurately express the optical behavior

28

in an MP photoreactor, PFR, MFSD response PFR (PFRMFSD), and effective

29

germicidal PFR (PFRGER) were defined and compared. The measured axial and radial

30

PFRMFSD values agreed well with the corresponding results from a simulation model

31

(UVCalc®). The PFR and PFRGER were obtained from the measured PFRMFSD by

32

using correction factors calculated by the UVCalc®. Under identical UV transmittance

33

(254 nm) conditions (75% and 85%), the weighted average PFRGER values were

34

13.3−18.7% lower than the corresponding PFR values, indicating that PFRGER, rather

35

than PFR should be used in MP photoreactor design to meet disinfection standards.

36

Based on measured lamp output, medium absorption spectrum, MFSD response, and

37

microbial deoxyribonucleic acid response spectrum, the detailed relationships

38

between the PFR, PFRMFSD, and PFRGER were elucidated. This work proposes a new

39

method for the accurate description of wavelength-dependent PFR distributions in MP

40

photoreactors, thus providing an important tool for the optimal design of these

41

systems.

42 43

Key words: photon fluence rate distribution; medium-pressure lamp; UV

44

photoreactor; micro-fluorescent silica detector (MFSD)

45

2

ACS Paragon Plus Environment

Page 2 of 31

Page 3 of 31

Environmental Science & Technology

46

■ INTRODUCTION

47

Ultraviolet (UV) photoreactors are commonly used for disinfection and advanced

48

oxidation processes in water and wastewater treatment.1,2 Medium-pressure (MP)

49

mercury (Hg) lamps are often preferentially chosen for large-scale applications

50

because their high output power leads to a relatively small photoreactor size.3-6

51

Moreover, their polychromatic output can be advantageous, as compared with

52

(essentially) monochromatic low-pressure (LP) Hg lamps, if the lamp output spectrum

53

correlates well with the absorption spectrum of the target microorganism or chemical

54

pollutant. Examples of applications in which these advantages have been

55

demonstrated include UV disinfection,5,7 UV photolysis of nitrosodimethylamine,4

56

and UV/H2O2 advanced oxidation process.4

57

Fluence is the master variable that governs the performance of UV photoreactors.

58

An essential prerequisite for fluence calculations or measurements is the accurate

59

quantification of fluence rate (FR) distributions. The FR distributions in LP

60

photoreactors have been well described. For these applications, numerical simulation

61

models have been developed, including the point source summation, line source

62

integration, extensive source with volumetric emission, and UVCalc® models.8-15

63

Moreover, several measurement technologies, such as the spherical actinometry,16,17

64

SIC detector18,19 and micro-fluorescent silica detector (MFSD),20,21 have been

65

demonstrated for the determination of FR distributions in LP photoreactors.

66

For MP photoreactors, the FR distributions are more complex than those of LP

67

lamp reactor in several respects: 1) the FR at a test point is attributable to photons of

3

ACS Paragon Plus Environment

Environmental Science & Technology

68

all wavelengths represented in the MP lamp output spectrum, while only a single

69

wavelength (i.e., 254 nm) should be considered for LP lamps; 2) because of the

70

absorption spectra (or UV transmittance (UVT) spectra) of the media through which

71

UV radiation is transmitted, the FR contribution of each wavelength will demonstrate

72

a spatial variation; 3) the target microorganism or chemical pollutant has also a

73

characteristic absorption spectrum (or quantum yield), which makes the reaction rate

74

wavelength-dependent; and 4) a detector (or sensor) has inevitably a unique response

75

spectrum, and therefore does not respond uniformly to photons of all wavelengths in

76

the output spectrum of an MP lamp.

77

Because of the complex optical behavior described above, experimental

78

measurements of polychromatic FR distributions are beneficial, not only for a

79

validation of numerical FR distribution models, but also for a direct determination of

80

FR distributions that can be used to assess the performance of MP photoreactors.

81

However, to date, no such measurements have been reported. Important barriers to

82

this approach include a lack of an appropriate detector, one that requires a 360-degree

83

UV response (FR detector) and that can function properly when subjected to high

84

fluence rates, as observed in MP photoreactors.

85

In our previous work, an MFSD was developed for in-situ measurements of FR

86

distributions in an LP photoreactor.21,22 The key features of this 360-degree response

87

detector, such as fast response, water-resistance and small volume (0.07 mm3), are

88

particularly suitable for FR distribution measurements in aqueous solutions. In

89

addition, Ge-doped silica, the sensitive window of MFSD, has a high resistance to UV

4

ACS Paragon Plus Environment

Page 4 of 31

Page 5 of 31

Environmental Science & Technology

90

irradiation, and its response spectrum spans a wavelength range of 200−275 nm.

91

Therefore, it was hypothesized that the MFSD would also function well for FR

92

distribution measurements in an MP photoreactor.

93

Note that for a photochemical or photobiological reaction, the local reaction rate

94

is proportional to the local absorbed photon flux.23 Moreover, the MFSD, a

95

photoelectric detector, should obey the photoelectric effect, indicating that the electric

96

current induced by the photoelectric effect is proportional only to the (local) photon

97

flux incident on the sensitive window. Therefore, for purposes of this study, it is more

98

appropriate to use photon fluence rate (i.e., PFR, units of einstein m−2 s−1) rather than

99

energy-based FR (units of mW cm−2) for description of the spatial distributions of

100

radiant energy.

101

In this study, experiments were conducted to measure the in-situ PFR

102

distributions in an MP photoreactor by use of the MFSD. Three aqueous media,

103

namely deionized (DI) water and aqueous solutions of copper sulfate (CS) and

104

potassium acid phthalate (PP), were used to examine the effect of medium absorption

105

spectrum. The experimental results were compared with a PFR simulation model

106

(UVCalc®). In addition, numerical analyses were conducted to elucidate the detailed

107

relationships between PFR, MFSD response PFR (PFRMFSD), and effective germicidal

108

PFR (PFRGER) in an MP photoreactor based on measured lamp output, MFSD

109

response, and microbial deoxyribonucleic acid (DNA) response spectrum. This work

110

will improve our understanding of the complex optical behavior that characterizes MP

5

ACS Paragon Plus Environment

Environmental Science & Technology

111

photoreactors, and thus is useful for the optimal design of polychromatic UV

112

photoreactors.

113

■ EXPERIMENTAL SECTION

114

Measurement platform. Figure 1 represents a schematic diagram of the

115

experimental platform for measuring the PFR distributions in a single-lamp MP

116

photoreactor. A cylindrical quartz-wall UV photoreactor (inner diameter = 190 mm

117

and inner length = 1000 mm) wrapped in a black cotton cloth was used; UV

118

reflectance from the air/quartz/air interfaces of this photoreactor has previously been

119

estimated at approximately 8%.24 An MP Hg lamp (arc length = 100 mm, outside

120

diameter = 11 mm; Foshan Comwin Light and Electricity Co., Ltd., China) with a

121

quartz sleeve (outside diameter = 23 mm) was housed in the center of the cylindrical

122

quartz photoreactor. The total input power of the lamp was 500 W with a UVC

123

(200−300 nm) efficiency of 17.5%.

6

ACS Paragon Plus Environment

Page 6 of 31

Page 7 of 31

Environmental Science & Technology

124 125

Figure 1. Schematic diagram of the experimental setup for measuring photon fluence

126

rate (PFR) distributions.

127

An MFSD (1.0 mm length × 0.3 mm outside diameter), whose fluorescence was

128

transmitted by an optical fiber housed in a long, thin and hollow stainless-steel tube

129

and then amplified by a multimeter, was fixed onto a two-dimensional guideway with

130

its vertical axis oriented parallel to the lamp axis. This configuration allowed the

131

detector to move either parallel (displacement precision of 1 mm) or perpendicular

132

(displacement precision of 0.01 mm) to the lamp axis. The MFSD was calibrated at

133

254 nm as described previously.21 Because of high PFR and high temperature in the

134

gap between the MP lamp and the sleeve, a reference detector for monitoring the lamp

135

output fluctuation was not used in this study.20, 25

7

ACS Paragon Plus Environment

Environmental Science & Technology

136

Measurement process. Test solutions included DI water (UVT254 = 100%) and

137

aqueous solutions of CuSO4 (CS) and potassium acid phthalate (PP) with

138

concentrations adjusted to yield UVT254 values of 85% and 75%. The use of DI water

139

as a test medium represents a simplified condition, in which the effect of medium

140

absorption spectrum could be eliminated; as such, the difference in the PFRMFSD

141

distributions between MP and LP photoreactors was attributable only to the

142

polychromatic output of the MP lamp and the MFSD response spectrum. For a fixed

143

UVT254 value, measurement data in the two test solutions (i.e., CS and PP) could be

144

used to investigate the effect of medium absorption spectrum on the MP photoreactor

145

performance.

146

In the central cross-section of the photoreactor, the detector was moved

147

perpendicular to the lamp axis (i.e., from the sleeve to the reactor wall) to measure the

148

radial PFR distributions at preselected test points in various test media. Then, to

149

determine the axial PFR distributions, the detector was moved parallel to the lamp

150

axis at radial distances of 30.0 and 60.0 mm away from the lamp surface. The detector

151

readings were collected and stored at a 2-second interval by a data acquisition switch

152

unit (34972A, Agilent, USA). At each test point, the detector remained in place for 0.5

153

min and a total of 15 detector readings were collected. The last 10 stable readings

154

were then averaged as the reported PFR value.

155

Chemicals, analytical methods, and PFR simulation model. CS and PP

156

reagents were of analytical grade and purchased from Sigma-Aldrich (St. Louis, MO,

157

USA). UVT spectra were measured by a spectrophotometer (2600, Shimadzu, Japan)

8

ACS Paragon Plus Environment

Page 8 of 31

Page 9 of 31

Environmental Science & Technology

158

in a 1.0 cm path-length quartz cell. A PFR model (UVCalc® version 1B, Bolton

159

Photosciences Inc., Edmonton, Alberta, Canada) was employed to calculate the PFR

160

distributions at each wavelength under each UVT condition. This model employs the

161

multiple segment source summation (MSSS) approximation and accounts for

162

refraction and reflection at the air/quartz/water interfaces.26 With a known relative

163

output spectrum of the MP lamp and a UVT spectrum of the test medium, the PFR

164

distributions in the MP photoreactor could be obtained. The model-simulated PFR

165

distributions were compared with the MFSD measured data, and then further used to

166

calculate the correction factors for the conversions among PFR, PFRGER and PFRMFSD.

167

Details of the calculations are presented below.

168

The relationships between PFR, PFRMFSD, PFRGER. To accurately quantify the

169

performance of an MP photoreactor, it is necessary to clarify the relationships among

170

three key variables, namely PFR, PFRMFSD, and PFRGER as defined previously. An

171

illustration of these relationships is presented in Figure 2. MP lamps emit

172

polychromatic UV and visible radiation at wavelengths ranging from 200 to 500 nm.

173

The PFR at a test point in an MP photoreactor is usually defined as the unweighted

174

sum of the PFR values contributed by all photons of across the most effective

175

wavelength range of 200−300 nm for UV disinfection and UV-based AOPs. For

176

purposes of model simulations, this wavelength range (i.e., 200−300 nm) was divided

177

into 20 bands (i.e., the band number n = 20). Hence, band1 represented 200−205 nm,

178

and band20 represented 296−300 nm. A vector-based analysis with multiple vector

9

ACS Paragon Plus Environment

Environmental Science & Technology

Page 10 of 31

179

components representing the behaviors of various wavelength bands was performed in

180

this study, which is applicable to polychromatic lamp calculations.

181 182 183

Figure 2. Illustration of the relationships between PFR, PFRMFSD, and PFRGER.

184 185

[P], P, and [pn] were defined as the vector lamp power, scalar lamp power, and

186

fraction of the lamp power from every wavelength band, respectively (see eq 1).

187

[PFR], A and [an] were defined as the vector PFR, scalar PFR, and fraction of the PFR

188

contributed by each wavelength band (within 200−300 nm) at a test point in an MP

189

photoreactor, respectively (see eq 1). Note that A and [an] will be different at each test

190

point in the photoreactor. By using UVCalc®, [PFR] can be obtained with a known [P]

191

and a determined medium absorption spectrum:

192

UVCalc [ P] = P[ pn ]  →[PFR] = A[an ] = A[a1 , a2 , a3......an ] UVT,200-300nm

(1)

20

193

∑ [a ] = 1

(2)

n

n =1

194

Likewise, [PFRMFSD], B and [bn]T were defined as the vector PFRMFSD, scalar

195

PFRMFSD, and fraction of PFRMFSD from each wavelength band (i.e., eq 3), 10

ACS Paragon Plus Environment

Page 11 of 31

Environmental Science & Technology

196

respectively. Note that the relative response spectrum of MFSD was normalized to

197

1.00 at 254 nm (i.e., b11 = 1), and the sum of bn ( ∑ [bn ] ) was greater than 1.

20

n =1

198

 b1  b   2  [bn ] =  b3    ......  bn 

(3)

199

Moreover, [PFRGER], C and [cn]T were defined as the vector PFRGER, scalar

200

PFRGER, and fraction of PFRGER from each wavelength band (i.e., eq 4), respectively.

201

Note that the relative response fraction of microorganism (DNA) was normalized to

202

1.00 at 254 nm (i.e., c11 = 1), and the sum of cn ( ∑ [cn ] ) was greater than 1.

20

n =1

203

204

 c1  c   2  [cn ] =  c3    ......  cn 

(4)

As a result, the [PFRMFSD] can be calculated as follows:

205

[PFR MFSD ] = [PFR][bn ] = A[ an ][bn ] = A[ a1 , a2 , a3 ......an ]    b1  b   2   b3  ......    bn 

(5)

206

[PFR MFSD ] = A[ a1b1 , a 2 b2 , a 3b3 ,......a n bn ]

(6)

207

Because the MFSD reading (i.e., B) is linearly dependent on the photons received and

208

the MFSD response spectrum, one has:

209 210

B = A(a1b1 + a2b2 + a3b3 + ......anbn ) By algebra: 11

ACS Paragon Plus Environment

(7)

Environmental Science & Technology

211

A 1 = = CFPFR / MFSD B (a1b1 + a2b2 + a3b3 + ......anbn )

Page 12 of 31

(8)

212

Since the correction factor, PFR/PFRMFSD (or CFPFR/MFSD), can be obtained by

213

combining numerical simulations (e.g., UVCalc®) with a known MFSD response

214

spectrum and the MFSD reading, the PFR value (i.e., A) can then be calculated using

215

eq 8. The scalar PFRGER can be calculated analogously as follows:

216

A 1 = = CFGER / MFSD C (a1c1 + a2c2 + a3c3 + ......an cn )

(9)

217

In addition, the vector [PFR] and [PFRGER] are equal to the products between A and

218

[an] and between C and [cn], respectively.

219

■ RESULTS AND DISCSSSION

220

Spectra. Figure 3a shows the UVT spectra of the CS and PP solutions. The

221

concentrations of both solutions were adjusted to yield UVT254 values of 75% or 85%,

222

as indicated above. Note that in the short-wavelength region of 200−250 nm, the UVT

223

values of the PP solution were greater than those of the CS solution; while in the

224

long-wavelength region of 250−300 nm, the opposite was true.

12

ACS Paragon Plus Environment

Page 13 of 31

Environmental Science & Technology

1.0

(a)

UVT

0.8 0.6 0.4 PP, UVT254 = 85% CS, UVT254 = 85% PP, UVT254 = 75% CS, UVT254 = 75%

0.2 0.0

0.05

226 227 228

DNA MFSD

1.2

Relative response

225

(b)

1.0

MP lamp 0.04

Normalized at 254 nm

0.8

0.03 0.02

0.6 0.4

229

0.2

230

0.0

0.01

Relative lamp output

1.4

0.00 200

225

250

275

300

325

350

231

Wavelength (nm) 232 233

Figure 3. UVT spectra of copper sulfate (CS) and potassium acid phthalate (PP)

234

solutions (a) and MFSD response, DNA response, and MP lamp output spectra

235

(Foshan Comwin) (b).

236 237

The MFSD response, DNA response, and MP output spectra are shown in Figure

238

3b. These results indicate that the MFSD does not respond uniformly across all 13

ACS Paragon Plus Environment

Environmental Science & Technology

239

wavelengths to output radiation from the MP lamp. Measureable responses by the

240

MFSD are evident only for the range of 200−275 nm, with peak response at 248 nm.

241

For the DNA response spectra, peaks were evident at approximately 210 nm and 260

242

nm. Wavelengths above 300 nm are not effectively absorbed by DNA and are

243

generally inefficient for disinfection. The MP lamp output spectrum was obtained

244

from the manufacturer and measured using a spectroradiometer. The principal

245

emission wavelength is greater than 350 nm; output in the 200−300 nm accounted for

246

only 17.5% of the total output for this lamp. In fact, this low efficiency of UV output

247

has been regarded as a main demerit of MP lamps as a source of UVC radiation.

248

PFRMFSD distributions in DI water. Because the MFSD does not respond

249

uniformly to all wavelengths emitted from an MP lamp, it can only measure the

250

PFRMFSD distributions in the MP photoreactor. Figure 4a illustrates the measured and

251

modeled radial PFRMFSD distributions in the central cross-section of the photoreactor

252

filled with DI water. When the detector was moved from 10 to 70 mm from the sleeve

253

surface, the PFRMFSD decreased from 12 × 10−5 to 2.6 × 10−5 einstein m−2 s−1. The

254

modeled results agreed well with the measurement results across this range of radial

255

distances.

14

ACS Paragon Plus Environment

Page 14 of 31

Page 15 of 31

Environmental Science & Technology

(a)

Modeled PFRMFSD Measured PFRMFSD

20

-5

-2

-1

PFRMFSD (x10 einstain m s )

25

15

10

5

0 0

20

40

60

80

Radial distance from the sleeve surface (mm) 256

10

259

-5

260 261 262 263

Modeled PFRMFSD (30 mm rad. dist.) Measured PFRMFSD (30 mm rad. dist.)

(b)

-1 -2

258

PFRMFSD (x10 einstein m s )

257

8

Modeled PFRMFSD (60 mm rad. dist.) Measured PFRMFSD (60 mm rad. dist.)

6

4

2

0 264

0

10

20

30

40

50

60

70

80

Axial distance from the lamp center (mm) 265 266

Figure. 4. Measured and model simulated radial (a) and axial (b) PFRMFSD

267

distributions in DI water.

268

The measured and modeled axial PFRMFSD distributions in the MP photoreactor

269

filled with DI water are illustrated in Figure 4b. The detector was moved from the 15

ACS Paragon Plus Environment

Environmental Science & Technology

270

lamp center to the lamp end along two lines parallel to the lamp axis, at radial

271

distances of 30.0 mm and 60.0 mm from the lamp surface. The measured and

272

modeled PFR values decreased when approaching the lamp end. The model

273

simulations generally agreed with the measured data; however, for the test points

274

along the vertical line 30.0 mm away from the lamp surface, the model

275

underestimated measured PFRMFSD values in the near lamp-end region (i.e., 30−70

276

mm from the lamp center). One possible explanation for this behavior was reflection

277

at the water/air interface at the top liquid level in the photoreactor, as indicated by

278

Fresnel’s Law. This would result in an increased measured PFRMFSD in the near

279

lamp-end region, which was not been considered by the model. Smaller effects would

280

be found far from the liquid level and near wall region (i.e., the 60.0 mm radial

281

distance to the lamp surface). Similar effects have been reported in an LP photoreactor,

282

and disappeared in an air medium.21

283

PFRMFSD distributions in different media. Figure 5 shows PFRMFSD

284

distributions in the photoreactor filled with CS and PP solutions. Because of the

285

additional effect of UVT spectra, the PFRMFSD simulation was more complex than that

286

in DI water. Model simulation results again agreed well with the measured results in

287

both media for both UVT conditions, indicating robustness of the MP PFR model

288

simulation. Despite the same UVT254 value, the PFR distributions in CS and PP were

289

different, which can yield differences in photoreactor performance. At present, most

290

UV facilities equipped with either LP or MP lamps use UVT254 as the principal

291

control parameter. These results indicate that for polychromatic systems, including

16

ACS Paragon Plus Environment

Page 16 of 31

Page 17 of 31

Environmental Science & Technology

292

MP photoreactors, the UVT spectrum of the treated water and its potential variation

293

range should also be considered in UV photoreactor design.

25 Modeled PFRMFSD, UVT254 = 85% Measured PFRMFSD, UVT254 = 85%

20

Modeled PFRMFSD, UVT254 = 75% Measured PFRMFSD, UVT254 = 75%

-2

-1

PFRMFSD (einstein m s )

(a) 15 10 5 0

294 295 296 297 298 299

PFRMFSD (x10-5 einstein m-2 s-1)

25

(b) 20 15 10 5 0 0

20

40

60

80

300

Radial distance from the sleeve surface (mm) 301 302 303

Figure 5. Measured and model simulated radial PFRMFSD distributions in PP (a) and

304

CS (b) solutions.

17

ACS Paragon Plus Environment

Environmental Science & Technology

Page 18 of 31

305

Relationships between PFR, PFRMFSD, and PFRGER: Impact of MFSD and

306

DNA response spectra. PFRMFSD distributions provide relevant, but incomplete

307

information regarding photoreactor performance, including disinfection.

308

considering disinfection applications, PFR and PFRGER distributions in an MP

309

photoreactor may be more relevant than PFRMFSD distributions. Among these three

310

parameters, differences in MFSD and DNA spectra play the critical roles (see Figure

311

3b). In the discussion above, the scalar PFR (i.e., A) and PFRGER (i.e, C) value were

312

calculated by the CFPFR/MFSD and CFGER/MFSD based on the measured PFRMFSD (i.e.,

313

eqs 8 and 9). CFPFR/MFSD and CFGER/MFSD values in various media were calculated and

314

are shown in Figures S1a and S1b, respectively. It was found that the CFPFR/MFSD was

315

equal to 1.34 in DI water because the MFSD only responded across the wavelength

316

range 200−275 nm. The CFPFR/MFSD did not change when the test point moved from

317

the sleeve surface to the reactor wall, indicating that the PFR and PFRMFSD

318

demonstrated the same ratio at all points in the photoreactor. Likewise, CFGER/MFSD

319

was independent of the position of the test point. It was equal to 1.23 in DI water. The

320

calculated PFR and PFRGER distributions, as well as the PFRMFSD distribution in DI

321

water, are shown in Figure 6a. These results indicate that PFR values were highest

322

among the three parameters. Because of a greater response of DNA compared to that

323

of MFSD, the PFRGER values were slightly higher than PFRMFSD in all test points.

18

ACS Paragon Plus Environment

When

Page 19 of 31

Environmental Science & Technology

18

(a)

16

PFR PFRGER PFRMFSD

14 12 10 8 6

324 325 326 327 328

PFR ( x 10-5 einstein m-2 s-1)

4 2 10

(b)

PFR (PP) PFRGER (PP) PFRMFSD (PP)

6 4 2 0 200

329 330

PFR (CS) PFRGER (CS) PFRMFSD (CS)

8

(c)

Band5 (CS) Band5 (PP) Band11 (CS) and (PP) Band16 (CS) Band16 (PP)

160 80

331 60

332

40

333

20

334

0 0

335

10

20

30

40

50

60

70

Radial distance from the sleeve surface (mm)

336

Figure 6. Radial PFR, PFRMFSD, and PFRGER distributions in DI water (a) and CS and

337

PP solutions (UVT254 = 75%) (b), and radial PFR distribution of various wavelength

338

bands in PP and CS solutions (UVT254 = 75%) (c).

19

ACS Paragon Plus Environment

Environmental Science & Technology

339

For the CS and PP solutions, the CFPFR/MFSD increased with increasing distance

340

from the sleeve surface to the reactor wall. This is because the MFSD only responded

341

to radiation with wavelengths of 200−275 nm. Moreover, because the UVTs of both

342

CS and PP in wavelength range of 200−275 nm were lower than that of 275−300 nm

343

(see Figure 3a), the fraction of PFR in the wavelength range of 200−275 nm would

344

represent a smaller fraction of the PFR value (i.e., 200−300 nm) in the far-lamp region

345

than that in near-lamp region. Therefore, a higher CFPFR/MFSD was observed in the

346

far-lamp region than that in near-lamp region. In addition, CFGER/MFSD values

347

decreased first and increased with increasing distance from the sleeve surface. This

348

behavior was attributable to the complex response spectra of the MFSD and DNA.

349

The corrected PFR and PFRGER distributions, as well as the PFRMFSD distribution in

350

media with UVT254 of 85% and 75% are illustrated in Figures S2 and 6b, respectively.

351

Relationships between PFR, PFRMFSD, and PFRGER: Impact of medium

352

absorption spectrum. In the section above, differences among the PFR, PFRMFSD,

353

and PFRGER were discussed, which were impacted by the MFSD and DNA response

354

spectra, principally. However, differences between two media conditions were also

355

apparent. For solutions with UVT254 of 85% (i.e., Figure S2), all three PFR

356

parameters in PP were larger than those in CS. The differences between the CS and PP

357

for all three PFR parameters were greater in the near-lamp region than in the far-lamp

358

region. For the condition of UVT254 equal to 75% (i.e., 7c), all three PFR parameters

359

in PP in near-lamp region were greater than those in CS in near-lamp region, while

360

they are smaller in far-lamp region.

20

ACS Paragon Plus Environment

Page 20 of 31

Page 21 of 31

Environmental Science & Technology

361

To explore the results in greater detail, a short-wavelength region (200−250 nm)

362

and a long-wavelength region (250−300 nm) were defined. PP had higher UVT values

363

in short-wavelength region and CS had higher UVT values in long-wavelength region

364

(Figure 3a). The PFR contributions of three representative wavelength bands, namely

365

220−225 nm (band5), 250−255 nm (band11), and 275−280 nm (band16) are

366

illustrated in Figure 6c. The PFRs for band11 of CS and PP were indistinguishable,

367

because the UVTs of both solutions were adjusted to 75% at 254 nm. The PFRMFSD

368

and PFRGER from band11 in CS and PP were the same too (see Figures S3 and S4),

369

because the MFSD and DNA responses were both normalized to 1 at 254 nm. In the

370

near-lamp region, because of the short path-length and higher UVT values in

371

short-wavelength region, the PFR of band5 in PP was higher than that for CS,

372

although it decreased rapidly with increasing path length. In the far-lamp region, the

373

PFRs of band5 in CS and PP were both zero, while the PFR of band16 in PP

374

decreased more quickly than in CS, which induced a higher PFR value in CS than that

375

in PP in far-lamp region. Similar results were observed with PFRMFSD and PFRGER

376

distributions (see Figure 6b). These results demonstrate that PFR (or PFRMFSD and

377

PFRGER) spatial distributions in various solutions (i.e., various UVT spectra) were

378

complex and would be impacted by path-lengths of various locations in the

379

photoreactor and transmittance ability of each wavelength.

380

Weighted average PFR, PFRMFSD and PFRGER. Table 1 shows the weighted

381

average PFR, PFRMFSD, and PFRGER values for various UVT254 values. The weighted

382

average values of all three PFR parameters for the 100% UVT case were higher than

21

ACS Paragon Plus Environment

Environmental Science & Technology

Page 22 of 31

383

those in other UVT cases, and the weighted average PFRGER in 100%, 85% CS, 85%

384

PP, 75% CS and 75% PP solutions were 18.7%, 14.0%, 13.7%, 14.2%, and 13.3%

385

lower than the corresponding values of PFR. Hence, it is important to emphasize the

386

application of PFRGER in UV disinfection, rather than PFR for MP photoreactors. For

387

identical values of UVT254, the weighted average PFRGER were different among the

388

various media, indicating that disinfection performance will depend on the absorption

389

spectra of the media, rather than only UVT254 as commonly used in LP photoreactors.

390

In addition, the weighted average values of all three parameters in CS solution with

391

85% UVT254 were lower than that in PP solution. However, they were higher than

392

those in PP solution with 75% UVT254. This is because at 75% UVT254, the PP

393

solution had higher PFR values in the near-lamp region and lower PFR values in PP in

394

far-lamp region than the CS solution (see Figure 6b), yet the PFR in the far-lamp

395

region was assigned a greater weight in the weighted average PFR calculation for a

396

larger radius (details of this calculation have been described previously24).

397

Table 1. Weighted average PFR, PFRGER, and PFRMFSD (einstein m−2 s−1) in

398

various media. Parameter

DI water (100%)

CS (85%)

PP (85%)

CS (75%)

PP (75%)

PFR

4.87

2.30

2.70

2.04

1.58

PFRGER

3.96

2.00

2.33

1.75

1.37

PFRMFSD

3.64

1.08

1.46

0.74

0.76

399

22

ACS Paragon Plus Environment

Page 23 of 31

Environmental Science & Technology

400

Potential applications. In recent years, many novel UV sources have been

401

developed and their potential applications in water treatment have been

402

investigated.27-32 An important attribute of UV sources is their emission spectrum.

403

Different wavelengths can induce different photochemical effects, including

404

disinfection. However, the traditional concept of PFR covering the UV range of

405

200−300 nm cannot completely describe process performance in polychromatic

406

systems. Therefore, it is necessary to account for the wavelength-dependent behavior

407

of these systems. The use the PFRGER in UV disinfection, which has been

408

demonstrated in this study, based on the DNA response spectrum (normalized 1 at 254

409

nm) for MP lamp or other non-LP UV lamps, represents such an approach. Ideally, the

410

same PFRGER from MP and LP lamps would induce the same inactivation

411

performance relative to a given target organism, while the same PFR would not. In

412

addition, the required UV disinfection fluence (or dose) of current disinfection

413

standards are based on the bench-scale quasi-collimated beam apparatus equipped

414

with an LP lamp (i.e., monochromatic exposure at 254 nm). By comparison, the

415

PFRGER may represent a more comprehensive description of fluence delivery to

416

photochemical targets than PFR in non-LP lamp when the DNA response spectrum is

417

normalized 1 at 254 nm.

418

This same logic could be useful for the UV-based photolysis or advanced

419

oxidation process when polychromatic lamps are applied. For example, since H2O2,

420

chlorine, and other chemicals excited by UV demonstrate unique absorption spectra,

421

the required fluence (product of PFR and exposure time) for target pollutant

23

ACS Paragon Plus Environment

Environmental Science & Technology

422

degradation should take account of the spectra of the UV sources. A definition of an

423

effective PFR (or fluence), such as the PFRGER (or germicidal fluence), could be

424

applied which would be obtained from eq 8 based on the absorption spectrum of the

425

chemical solution (e.g., H2O2 and chlorine). This would be more clearly to represent

426

UV performance for a given application than traditional methods that do not account

427

for wavelength-dependent effects. Hofman-Caris has also introduced a similar fluence

428

definition for the polychromatic lamp application.33

429

This study also involved application of a novel UV sensor (i.e., MFSD) in an MP

430

photoreactor. Because each detector has a unique response spectrum, calibration is

431

crucial for its application. Some UV detectors (or sensors) used for MP photoreactors

432

are calibrated at 365 nm because this wavelength is close to the centroid of the lamp

433

emission spectrum. In this study, the MFSD was calibrated at 254 nm. Meanwhile,

434

both MFSD and DNA response spectra were normalized 1 at 254 nm. Hence, the

435

PFRMFSD could be readily transferred to PFRGER. This method may lead to a new

436

detector calibration protocol for MP lamps.

437

The UV detector (or sensor) is also important for long-term monitoring of UV

438

fluence for an operating UV photoreactor. The results in Figure 6b indicate that due to

439

the different path-lengths and absorption spectra of the media (i.e., water to be

440

disinfected), the PFR constitution from various wavelengths changed among locations.

441

An online UV detector for long-term monitoring is usually installed on the reactor

442

wall or other position, which has a long path-length. When the water quality (medium

443

absorption spectrum) changes, the monitored PFR would be different from the real

24

ACS Paragon Plus Environment

Page 24 of 31

Page 25 of 31

Environmental Science & Technology

444

PFR or PFRGER, inducing an error on fluence monitoring. Therefore, this error should

445

be considered in long-term monitoring of polychromatic photoreactors.

446

■ ASSOCIATED CONTENT

447

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

448

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

449

(Figure S1) Radial CFPFR/MFSD and CFGER/MFSD distributions; (Figure S2) Radial PFR,

450

PFRMFSD, and PFRGER distributions with UVT254 of 85%; (Figure S3) Radial PFRMFSD

451

distribution of the various wavelength bands with UVT254 of 75%, and (Figure S4)

452

Radial PFRGER distribution of the various wavelength bands with UVT254 of 75%.

453

■ AUTHOR INFORMATION

454

Corresponding Author

455

*

456

Notes

457

The authors declare no competing financial interest.

458

■ ACKNOWLEDGEMENTS

Phone: 1-765-494-0316; e-mail: [email protected].

459

The authors gratefully acknowledge financial support from the Edward M. Curtis

460

Visiting Professorship of Purdue University, the National Natural Science Foundation

461

of China (21590814, 51525806, 51408592), and the National Geographic Air and

462

Water Conservation Fund (GERC23-15).

463

■ REFERENCES

464

(1)

Guo, M. T.; Huang, J. J.; Hu, H. Y.; Liu, W. J.; Yang, J., UV inactivation and 25

ACS Paragon Plus Environment

Environmental Science & Technology

465

characteristics after photoreactivation of Escherichia coli with plasmid: Health safety

466

concern about UV disinfection. Water Res. 2012, 46, (13), 4031-4036.

467

(2)

Rizzo, L.; Della Sala, A.; Fiorentino, A.; Li Puma, G., Disinfection of urban

468

wastewater by solar driven and UV lamp - TiO2 photocatalysis: effect on a multi drug

469

resistant Escherichia coli strain. Water Res. 2014, 53, 145-152.

470

(3)

Wang, D.; Bolton, J. R.; Hofmann, R., Medium pressure UV combined with

471

chlorine advanced oxidation for trichloroethylene destruction in a model water. Water

472

Res. 2012, 46, (15), 4677-4686.

473

(4)

Sharpless, C. M.; Linden, K. G., Experimental and model comparisons of low-

474

and medium-pressure Hg lamps for the direct and H2O2 assisted UV photodegradation

475

of N-nitrosodimethylamine in simulated drinking water. Environ. Sci. Technol. 2003,

476

37, (9), 1933-1940.

477

(5)

Beck, S. E.; Wright, H. B.; Hargy, T. M.; Larason, T. C.; Linden, K. G., Action

478

spectra for validation of pathogen disinfection in medium-pressure ultraviolet (UV)

479

systems. Water Res. 2015, 70, 27-37.

480

(6)

Huffman, D. E.; Gennaccaro, A.; Rose, J. B.; Dussert, B. W., Low- and

481

medium-pressure UV inactivation of microsporidia Encephalitozoon intestinalis.

482

Water Res. 2002, 36, (12), 3161-3164.

483

(7)

Guo, M. T.; Hu, H. Y.; Bolton, J. R.; El-Din, M. G., Comparison of low- and

484

medium-pressure ultraviolet lamps: Photoreactivation of Escherichia coli and total

485

coliforms in secondary effluents of municipal wastewater treatment plants. Water Res.

486

2009, 43, (3), 815-821.

26

ACS Paragon Plus Environment

Page 26 of 31

Page 27 of 31

Environmental Science & Technology

487 488 489

(8)

Jacob, S. M.; Dranoff, J. S., Light intensity profiles in a perfectly mixed

photoreactor. AIChE J. 1970, 16, (3), 359-363. (9)

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

490

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

491

Eng.-ASCE 2006, 132, (8), 831-841.

492

(10) Blatchley, E. R., Numerical modelling of UV intensity: Application to

493

collimated-beam reactors and continuous-flow systems. Water Res. 1997, 31, (9),

494

2205-2218.

495

(11) Irazoqui, H. A.; Cerda, J.; Cassano, A. E., Radiation profiles in an empty

496

annular photoreactor with a source of finite spatial dimensions. AIChE J. 1973, 19, (3),

497

460-467.

498

(12) Zazueta, A. L. L.; Destaillats, H.; Li Puma, G., Radiation field modeling and

499

optimization of a compact and modular multi-plate photocatalytic reactor (MPPR) for

500

air/water purification by Monte Carlo method. Chem. Eng. J. 2013, 217, 475-485.

501

(13) Li Puma, G.; Puddu, V.; Tsang, H. K.; Gora, A.; Toepfer, B., Photocatalytic

502

oxidation of multicomponent mixtures of estrogens (estrone (El), 17 beta-estradiol

503

(E2), 17 alpha-ethynylestradiol (EE2) and estriol (E3)) under UVA and UVC radiation:

504

Photon absorption, quantum yields and rate constants independent of photon

505

absorption. Appl. Catal. B-Environ. 2010, 99, (3-4), 388-397.

506

(14) Colina-Marquez, J.; Machuca-Martinez, F.; Li Puma, G., Radiation absorption

507

and optimization of solar photocatalytic reactors for environmental applications.

508

Environ. Sci. Technol. 2010, 44, (13), 5112-5120.

27

ACS Paragon Plus Environment

Environmental Science & Technology

509

(15) Li Puma, G.; Khor, J. N.; Brucato, A., Modeling of an annular photocatalytic

510

reactor for water purification: Oxidation of pesticides. Environ. Sci. Technol. 2004, 38,

511

(13), 3737-3745.

512

(16) Rahn, R. O.; Echols, S., Iodide/iodate chemical actinometry using spherical

513

vessels for radiation exposure as well as for monitoring absorbance changes.

514

Photochem. Photobiol. 2010, 86, (4), 990-993.

515

(17) Rahn, R. O.; Bolton, J.; Stefan, M. I., The iodide/iodate actinometer in UV

516

disinfection: determination of the fluence rate distribution in UV reactors. Photochem.

517

Photobiol. 2006, 82, (2), 611-615.

518

(18) Elyasi, S.; Taghipour, F., Simulation of UV photoreactor for degradation of

519

chemical contaminants: Model development and evaluation. Environ. Sci. Technol.

520

2010, 44, (6), 2056-2063.

521 522

(19) Elyasi, S.; Taghipour, F., General method of simulating radiation fields using measured boundary values. Chem. Eng. Sci. 2010, 65, (20), 5573-5581.

523

(20) Qiang, Z. M., Li, M. K., Bolton, J. R., Qu, J. H., Wang, C., Estimating the

524

fluence delivery in UV disinfection reactors using a ‘detector-model’combination

525

method. Chem. Eng. J. 2013, 233, 39-46..

526

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

527

of UV fluence rate distribution by use of a micro fluorescent silica detector. Environ.

528

Sci. Technol. 2011, 45, (7), 3034-3039.

529

(22) Li, M. K.; Qiang, Z. M.; Bolton, J. R., In situ detailed fluence rate

530

distributions in a UV reactor with multiple low-pressure lamps: Comparison of

28

ACS Paragon Plus Environment

Page 28 of 31

Page 29 of 31

Environmental Science & Technology

531

experimental and model results. Chem. Eng. J. 2013, 214, 55-62.

532

(23) Bolton, J. R.; Mayor-Smith, I.; Linden, K. G., Rethinking the concepts of

533

fluence (UV dose) and fluence rate: The importance of photon-based units - A

534

systemic review. Photochem. Photobiol. 2015, 91, (6), 1252-1262.

535

(24) Li, M. K.; Qiang, Z. M.; Bolton, J. R.; Ben, W. W., Impact of reflection on the

536

fluence rate distribution in a UV reactor with various inner walls as measured using a

537

micro-fluorescent silica detector. Water Res. 2012, 46, (11), 3595-3602.

538 539

(25) 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.

540

(26) Bolton, J. R., Calculation of ultraviolet fluence rate distributions in an annular

541

reactor: Significance of refraction and reflection. Water Res. 2000, 34, (13),

542

3315-3324.

543 544

(27) Song, K.; Mohseni, M.; Taghipour, F., Application of ultraviolet light-emitting diodes (UV-LEDs) for water disinfection: A review. Water Res. 2016, 94, 341-349.

545

(28) Jenny, R. M.; Jasper, M. N.; Simmons, O. D.; Shatalov, M.; Ducoste, J. J.,

546

Heuristic optimization of a continuous flow point-of-use UV-LED disinfection reactor

547

using computational fluid dynamics. Water Res. 2015, 83, 310-318.

548

(29) Bowker, C.; Sain, A.; Shatalov, M.; Ducoste, J. J., Microbial UV

549

fluence-response assessment using a novel UV-LED collimated beam system. Water

550

Res. 2011, 45, (5), 2011-2019.

551

(30) Crapulli, F.; Santoro, D.; Sasges, M. R.; Ray, A. K., Mechanistic modeling of

552

vacuum UV advanced oxidation process in an annular photoreactor. Water Res. 2014,

29

ACS Paragon Plus Environment

Environmental Science & Technology

553

64, 209-225.

554

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

555

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

556

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

557

176-181.

558

(32) Oppenlander, T.; Schwarzwalder, R., Vacuum-UV oxidation (H2O-VUV) with

559

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

560

VUV intensity measurement and as reference compound for OH-radical competition

561

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

562

(33) Hofman-Caris, C. H. M.; Harmsen, D. J. H.; Wols, B. A.; Beerendonk, E. F.;

563

Keltjens, L. L. M., Determination of reaction rate constants in a collimated beam

564

setup: The effect of water quality and water depth. Ozone Sci. Technol. 2015, 37, (2),

565

134-142.

30

ACS Paragon Plus Environment

Page 30 of 31

Page 31 of 31

Environmental Science & Technology

TOC Art

31

ACS Paragon Plus Environment