A Mini-Fluidic UV Photoreaction System for Bench-Scale Photochemical Studies

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A Mini-Fluidic UV Photoreaction System for Bench-Scale Photochemical Studies

Mengkai Li,

^†

Zhimin Qiang,*

^,^†

James R. Bolton,

^‡

Jiuhui Qu,

^†

and Wentao Li

^†

†Key Laboratory of Drinking Water Science and Technology, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, 18 Shuang-qing Road, Beijing 100085, China

‡Department of Civil and Environmental Engineering, University of Alberta, Edmonton, AB T6G 2W2, Canada

*^S Supporting Information

ABSTRACT: A mini-fluidic ultraviolet (UV) photoreaction system (MUPS) has been developed for bench-scale photochemical studies. While ensuring a high accuracy in UVfluence measurements, the MUPS can also increase the maximal availablefluence rate (FR) by∼100-fold (i.e., similar to the practical FRs existing in engineering applications), as compared to the commonly used quasi-collimated beam apparatus, and measure sample absorbance online. Photolysis experiments with two chemical actinometers (KI/KIO₃and atrazine) demonstrate that the MUPS can easily be applied to photochemical studies in both low (<100 mJ/cm²) and high (≥100 mJ/cm²)fluence ranges with accurate quantifications of FR and exposure time; in addition, online absorbance measurements greatly facilitate the determination of photochemical parameters (e.g., rate constants and quantum yields).

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INTRODUCTION

Ultraviolet (UV) photoreactions have been widely used for chemical synthesis, pollutant degradation, and water and wastewater disinfection. Bench-scale UV photoreactions are important for detailed kinetic and mechanism studies.¹⁻³ As they are distinct from conventional “dark” chemical reactions, the determination of photoreaction kinetic parameters requires accurate quantiﬁcation of both irradiance and exposure time.

Most bench-scale photoreactions are conducted using a quasi- collimated beam apparatus (qCBA) or a batch cylindrical reactor.⁴⁻⁶Underquasi-collimated beams, the irradiance has a relatively homogeneous distribution and thus can be measured easily by a UV radiometer as opposed to the case for a batch cylindrical reactor. In a qCBA, samples can be exposed to variousﬂuences simply by changing the exposure time. Because the beams are almost parallel, the irradiance and the ﬂuence rate (FR) are essentially the same (henceforth, we will use the term FR rather than irradiance).

However, limited by its optical construction, the maximal FR available in a qCBA is much lower than the average FR in practical UV reactors, which largely limits theqCBA application with regard to tests that need high FRs. In a qCBA, the FR distribution varies to some extent across the dish and along the

depth of the sample solution, which can present diﬃculties for accurate FR determination. Hence, the FR (measured by a radiometer) at the center of the sample surface has to be multiplied by various correction factors, including Petri factor, reﬂection factor, divergence factor (DF), and water factor (WF), to derive the average FR within the reaction medium.⁷ Each of these factors has an associated error, thus introducing an enlarged uncertainty into the FR measurements.

Fluidic reaction systems have been used widely in chemical studies (e.g., microfluidic devices and stopped-flow technology),^8,9 with distinct merits of fast online analysis and easily adjustable sample treatment capacity. However, to date, very fewfluidic devices have been developed in the UVfield. In this study, we report a novel bench-scale UV photoreaction apparatus, namely a mini-fluidic UV photoreaction system (MUPS). Through optimal design of the optical structure, a much broader FR range can be achieved for the MUPS than for theqCBA, with the maximal FR being similar to the practical

Received: April 13, 2015 Revised: August 31, 2015 Accepted: September 2, 2015

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Environ. Sci. Technol. Lett.XXXX, XXX, XXX−XXX

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values existing in engineering applications. Several typical photochemical experiments were conducted to evaluate this newly developed photoreaction system.

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EXPERIMENTAL SECTION

As illustrated in Figure 1a, the core part of the MUPS was a segmented cylindrical quartz photoreactor, which housed

axially a 105 W low-pressure high-output UV lamp and a protection quartz sleeve (23 mm outside diameter) in the center. A polytetraﬂuoroethylene (PTFE) mini-tube (∼75%

transmittance at 254 nm, 2 mm inside diameter) was coiled around the outer surface of the quartz photoreactor. The tube coils should avoid the end-lamp regions (i.e., the last 5 cm at each end of the lamp arc length) where the FR distribution (details shown in Figure S1) is nonuniform. A test water sample was pumped through the PTFE tube using a peristaltic pump to receive the desired UV exposure. To maintain a stable lamp output, the photoreactor chamber was ﬁlled with deionized water whose temperature was controlled by a water recirculator. To obtain various FRs in the PTFE tube, the photoreactor was specially fabricated to comprise three segments of diﬀerent diameters (30, 50, and 80 mm).

Moreover, an adjustable ballast was utilized to ﬁne-tune the lamp output. The exposure length of the mini-tube was preset before experiments were conducted with some blocking units (e.g., a transmittance slit or an opaque sheltering the tube).

After exposure, the sample was delivered to a Hach DR5000 UV−vis spectrophotometer (SPM) for online absorbance measurements at a desired wavelength.

In the MUPS, the FR in the PTFE tube could be directly determined by using a microﬂuorescent silica detector (MFSD),¹⁰ which was inserted into a short PTFE tube and placed on the outer surface of the photoreactor (Figure 1a,b).

This detector has a 360°response to photons and a maximal measurement error of 3%.¹¹ Furthermore, two chemical actinometers (i.e., KI/KIO₃and atrazine) were used for further validation of the FR andﬂuence measurements.

Table 1compares the performance between the qCBA and the MUPS. The principal merit of the MUPS is that it is

capable of delivering high FRs (up to 25.3 mW/cm², similar to the practical values existing in engineering applications) for bench-scale photochemical studies, as opposed to the qCBA (maximal FR of approximately 0.1−0.25 mW/cm²). The reason lies in the fact that in the MUPS, the sample can be placed near the lamp (approximately 1.5−4.0 cm), while in theqCBA, the sample has to be placed at least ∼30 cm from the lamp to obtain nearly parallel UV beams. In fact, the water sample usually flows through a UV reactor at a distance of approximately 0−4 cm from the sleeve surface. Moreover, the combinational use of three segments of different diameters and an adjustable ballast can readily deliver an FR over the broad range of 0−25.3 mW/cm² to avoid an overly short or long exposure time. Besides, the design of a closed fluidic photoreactor avoids sample evaporation loss and allows for varying sample treatment capacities.

Through optical structure modifications, the MUPS achieves a low variance in the FR output, which is important for the accuracy offluence quantifications. First, by recirculating water of a constant temperature through the photoreactor chamber to maintain a stable mercury vapor pressure in the lamp, the MUPS has a FR variance lower than that of theqCBA whose FR varies with room temperature. Second, in a transverse section of the photoreactor, the distance from the lamp center to any PTFE tube center is virtually constant (Figure 1b), so a uniform FR distribution can be obtained along the tube as opposed to a nonuniform FR distribution over the water sample surface in the Petri dish of theqCBA (Figure 1c). Third, the PTFE tube has an inner diameter as small as 2 mm (and an even smaller diameter can be selected), so a small FR variance within the tube cross section can be expected. In the tube cross section, the FR variance is impacted by both the UV absorbance of the water sample (i.e., WF) and the divergence of UV beams (i.e., DF), which can be expressed as follows:

= −

′

− ′

WF 1al 10 ln(10)

al

(1)

= + ′ D D l

DF (2)

where a is the absorption coefficient (1/cm) of the water sample,l′is the“effective”optical path length (cm) because of the nonparallel UV beams in the MUPS (as opposed to those in theqCBA), andDis the distance (cm) from the UV lamp to the PTFE tube. Because the l′ in the MUPS is considerably smaller than the sample thickness in theqCBA, the former can allow a larger variance in sample absorbance while the WF is kept nearly constant during photolysis. However, the smallerD Figure 1.Schematic diagrams of (a) the mini-fluidic UV photoreaction

system (MUPS), (b) the MUPS transverse section, and (c) theﬂuence rate (FR) distribution in the Petri dish ofqCBA.

Table 1. Comparison of the Performance of the qCBA and MUPS

qCBA MUPS

lamp output variable with room temperature stable

FR range (mW/cm²) 0−0.25 0−25.3

exposure time longer shorter

sample volume range smaller larger

sample thickness (cm) 1.0−2.0 0.05−0.20

Petri factor 0.7−0.9 none

divergence factor 0.94−0.99 0.88−0.99^a

aCalculated witheq 2by assumingl′= 2 mm (the most unfavorable scenario).

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value of the MUPS induces a relatively larger variance in its DF (Table 1).

The MUPS adopted two different operation modes for the low and highfluence ranges. In the low-fluence (i.e., <100 mJ/

cm²) operation mode, the water sample was pumped sequentially through the PTFE tube for UV exposure and an online UV−vis SPM for absorbance measurements. On the basis of the MFSD-measured FR value (E₀, mW/cm²), a preset exposure length (L, cm) of the PTFE tube, and theﬂow rate (Q, mL/s), the ﬂuence (F, mJ/cm²) can be calculated as follows:

= π F E r L

0 Q

2

(3) where ris the inner radius of the PTFE tube (cm).

In the high-fluence (i.e.,≥100 mJ/cm²) operation mode, the water sample was pumped sequentially through the exposure part of the PTFE tube and the online SPM and then recirculated to receive the UV irradiation again until a desired exposure time was reached. Note that for a certain exposure time, only a part of the sample received the UV irradiation, while the remaining part was in the dark. If a reduction equivalent exposure time (t_ree, s) is defined as the total exposure time (t, s) multiplied by the ratio of the exposure volume of the tube (πr²L, mL) to the total sample volume (V, mL), the fluence can be readily calculated:

= π

t r L

V t

ree 2

(4)

=

F E t_{0 ree} (5)

In other words,t_reecan be regarded as the exposure time when the whole sample simultaneously receives the UV irradiation with an FR equal to that in the PTFE tube.

All chemicals were of analytical grade or higher. The concentrations of KI and uridine (Sigma-Aldrich) were determined by measuring the absorbance at 352 and 262 nm, respectively, with a Hach DR5000 SPM. The atrazine (Sigma- Aldrich) concentration was determined by both the online SPM in the MUPS at 222 nm and the ultraperformance liquid chromatography−tandem mass spectrometry (UPLC−MS/

MS; 1290 Inﬁnity LC, 6420 Triple Quad LC/MS; Agilent) coupled with an SB-C18 column (2.1 mm×50 mm, 1.8 μm particle size). Milli-Q water (Millipore) was used in all experiments and analytical determinations.

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RESULTS AND DISCUSSION

The KI/KIO₃actinometer¹²was used to determine the incident fluence in the MUPS (for the low-fluence operation mode) by measuring the I₃⁻absorbance at 352 nm with an SPM. When the actinometer solution passed through the PTFE tube, the incident photonflux could be determined from the yield of the I₃⁻product. The tests were conducted at four FR values (i.e., 25.3, 5.4, 0.58, and 0.034 mW/cm²). At each selected FR, to obtain the desiredfluences (1.9, 3.5, 4.5, and 5.5 mJ/cm²), the exposure time was varied by adjusting the tube exposure length and the sample flow rate according to eq 3(details listed in Table S1). Figure 2 shows that at a fixed FR, the fluence measured by the KI/KIO₃ actinometer was linearly proportional to the exposure time; meanwhile, at various FRs, nearly identical values were measured by the KI/KIO₃actinometer for a desiredfluence. This is reasonable because the second law of

photochemistry stipulates that the extent of a photochemical reaction must be proportional to the number of absorbed photons. This result also demonstrates that the MUPS can deliver an accurateﬂuence in the broad FR range of 0.034−25.3 mW/cm².

In addition, it should be noted that a certain discrepancy existed between the ﬂuences measured by the MFSD and by the KI/KIO₃ actinometer (slope of 0.93), which could be primarily ascribed to their diﬀerent calibration methods. The quantum yield of the KI/KIO₃actinometer was determined by a uniform sources tunable laser facility at the National Institute of Standards and Technology of the United States¹²(0.71 ± 0.02; relative error of∼3%), while the MFSD was calibrated by a UV radiometer which had been standardized by the National Institute of Metrology of China¹⁰(relative error of∼8%). From a practical viewpoint, although either the MFSD or the KI/

KIO₃ actinometer can be used to determine the FR in the MUPS, the former is more convenient to use.

Accurate fluence determination depends on the accurate measurements of both FR and exposure time. Because the PTFE tube coiled around the photoreactor is close to the lamp (1.5−4.0 cm), the UV beams are very divergent (nonparallel), which requires that the FR detector should have a uniform response to UV beams with various incident angles. This is impossible for a conventional UV detector with aflat response window, but feasible for the MFSD that has an omnidirectional response to photons.¹¹ Furthermore, for fast photochemical reactions, it is difficult to determine accurately a short exposure time (e.g., <1 s) in the qCBA. However, very short exposure times can be accurately achieved in the MUPS by combined adjustment of the FR, tube exposure length, and sample flow rate [e.g., 0.08, 0.14, 0.18, and 0.22 s at E₀ = 25.3 mW/cm² (Table S1)].

In the high-fluence operation mode, the MUPS was used to examine the photolysis of atrazine, a frequently detected pesticide in natural waters and a commonly used chemical actinometer, as well. The atrazine solution (10 mg/L) was recirculated through the MUPS with its absorbance monitored online at 222 nm (A₂₂₂) by using the SPM. For UV photolysis of a compound with a low solution absorbance (<0.03), the reaction usually proceeds following thefluence-based pseudo- first-order kinetics (Text S1):

= C C k F

ln( ₀/ ) _f ₍₆₎

Figure 2. Comparison of the ﬂuences determined by the KI/KIO₃ actinometer and MFSD.

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= Φε

′ k

q l VE ln(10)

f

p,0

0 (7)

whereC₀andCare the initial and real-time concentrations of a test compound (M), respectively, k_f is the fluence-based pseudo-first-order rate constant (cm²/mJ), ε is the decadic molar absorption coefficient at 254 nm [1/(M·cm)],Vis the solution volume (L), and q_p,0 is the incident photon flux (einstein/s).

The change of the atrazine solution absorbance under UV irradiation was attributed to both atrazine degradation and byproduct formation. Hence, the solution absorbance was deconvoluted to extract the atrazine absorbance (Text S2). In a parallel experiment, samples were collected at preselected exposure times to analyze the residual atrazine concentrations by UPLC−MS/MS. Figure 3 shows that the atrazine

degradation curves were nearly identical as tested at two FR values (0.58 and 25.3 mW/cm²) and analyzed by two methods (SPM and UPLC−MS/MS). Plotting ln(A_az/A_az,0) or ln(C/C₀) versusﬂuence yielded a linear slope of 0.00064±0.00002 cm²/ mJ (i.e., k_f of atrazine). It demonstrates that the MUPS can quickly determine the reaction rate constant through online monitoring of the photolysis process, thus reducing the analysis workload.

The MUPS can also be applied to determine quickly the quantum yield (Φ) of a test compound. According to the k_f expression (eq 6), by separately conducting UV photolysis experiments of a test compound (atrazine, denoted with a subscript “az”) and a reference compound (uridine, denoted with a subscript “ud”) and comparing their degradation rate constants, we can readily determine the Φaz. Figure S3shows that a plot of ln(A_ud/A_ud,0) versusﬂuence yielded a linear slope of 0.00068 cm²/mJ (i.e.,k_f,ud). Then, from the known optical parameters (εud= 9131 [1/(M·cm)];εaz= 3413 [1/(M·cm)], andΦud= 0.02)¹³and the measured k_f,az (0.00064±0.00002 cm²/mJ), theΦazwas readily determined to be 0.050±0.002, which agrees well with those reported by other researchers (0.046−0.050) using either a batch UV reactor or aqCBA.^14,15 In summary, the newly developed MUPS has distinct merits of accurate exposure time and FR measurements, practical engineering FR outputs, online absorbance measurements, and fast determination of photochemical parameters. In addition, the novel design of its optical structure makes this apparatus simple and inexpensive to fabricate, easy to operate and

maintain, and robust. Considering its accuracy, celerity, and simplicity, the MUPS can be expected to extend the photochemical studies.

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ASSOCIATED CONTENT

*^S Supporting Information

The Supporting Information is available free of charge on the ACS Publications websiteat DOI:10.1021/acs.estlett.5b00207.

One table, two sections of text, and threeﬁgures (PDF)

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AUTHOR INFORMATION Corresponding Author

*Telephone: +86-10-62849632. Fax: +86-10-62923541. E-mail:

[email protected].

Notes

The authors declare no competingﬁnancial interest.

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ACKNOWLEDGMENTS

This work was ﬁnancially supported by the National Natural Science Foundation of China (51290281, 51408592, and 51221892) and the People Programme (Marie Curie Actions) of the European Union’s Seventh Programme FP7/2007-2013 under a REA grant (318926).

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