A Mini-Fluidic UV Photoreaction System for Bench-ScalePhotochemical StudiesMengkai 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 ofSciences, 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 photochemicalstudies. While ensuring a high accuracy in UV fluence measurements, the MUPS can also increase the maximal available fluencerate (FR) by ∼100-fold (i.e., similar to the practical FRs existing in engineering applications), as compared to the commonly usedquasi-collimated beam apparatus, and measure sample absorbance online. Photolysis experiments with two chemical actinometers(KI/KIO3 and 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 absorbancemeasurements greatly facilitate the determination of photochemical parameters (e.g., rate constants and quantum yields).

■ INTRODUCTION

Ultraviolet (UV) photoreactions have been widely used forchemical synthesis, pollutant degradation, and water andwastewater disinfection. Bench-scale UV photoreactions areimportant for detailed kinetic and mechanism studies.1−3 Asthey are distinct from conventional “dark” chemical reactions,the determination of photoreaction kinetic parameters requiresaccurate quantification of both irradiance and exposure time.Most bench-scale photoreactions are conducted using a quasi-collimated beam apparatus (qCBA) or a batch cylindricalreactor.4−6 Under quasi-collimated beams, the irradiance has arelatively homogeneous distribution and thus can be measuredeasily by a UV radiometer as opposed to the case for a batchcylindrical reactor. In a qCBA, samples can be exposed tovarious fluences simply by changing the exposure time. Becausethe beams are almost parallel, the irradiance and the fluencerate (FR) are essentially the same (henceforth, we will use theterm FR rather than irradiance).However, limited by its optical construction, the maximal FR

available in a qCBA is much lower than the average FR inpractical UV reactors, which largely limits the qCBA applicationwith regard to tests that need high FRs. In a qCBA, the FRdistribution varies to some extent across the dish and along the

depth of the sample solution, which can present difficulties foraccurate FR determination. Hence, the FR (measured by aradiometer) at the center of the sample surface has to bemultiplied 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 introducingan enlarged uncertainty into the FR measurements.Fluidic reaction systems have been used widely in chemical

studies (e.g., microfluidic devices and stopped-flow technol-ogy),8,9 with distinct merits of fast online analysis and easilyadjustable sample treatment capacity. However, to date, veryfew fluidic devices have been developed in the UV field. In thisstudy, we report a novel bench-scale UV photoreactionapparatus, namely a mini-fluidic UV photoreaction system(MUPS). Through optimal design of the optical structure, amuch broader FR range can be achieved for the MUPS than forthe qCBA, with the maximal FR being similar to the practical

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

Letter

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© XXXX American Chemical Society A DOI: 10.1021/acs.estlett.5b00207Environ. Sci. Technol. Lett. XXXX, XXX, XXX−XXX

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values existing in engineering applications. Several typicalphotochemical experiments were conducted to evaluate thisnewly developed photoreaction system.

■ EXPERIMENTAL SECTIONAs illustrated in Figure 1a, the core part of the MUPS was asegmented cylindrical quartz photoreactor, which housed

axially a 105 W low-pressure high-output UV lamp and aprotection quartz sleeve (23 mm outside diameter) in thecenter. A polytetrafluoroethylene (PTFE) mini-tube (∼75%transmittance at 254 nm, 2 mm inside diameter) was coiledaround the outer surface of the quartz photoreactor. The tubecoils should avoid the end-lamp regions (i.e., the last 5 cm ateach end of the lamp arc length) where the FR distribution(details shown in Figure S1) is nonuniform. A test watersample was pumped through the PTFE tube using a peristalticpump to receive the desired UV exposure. To maintain a stablelamp output, the photoreactor chamber was filled withdeionized water whose temperature was controlled by a waterrecirculator. To obtain various FRs in the PTFE tube, thephotoreactor was specially fabricated to comprise threesegments of different diameters (30, 50, and 80 mm).Moreover, an adjustable ballast was utilized to fine-tune thelamp output. The exposure length of the mini-tube was presetbefore 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 DR5000UV−vis spectrophotometer (SPM) for online absorbancemeasurements 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 andplaced on the outer surface of the photoreactor (Figure 1a,b).This detector has a 360° response to photons and a maximalmeasurement error of 3%.11 Furthermore, two chemicalactinometers (i.e., KI/KIO3 and atrazine) were used for furthervalidation of the FR and fluence measurements.Table 1 compares 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 tothe practical values existing in engineering applications) forbench-scale photochemical studies, as opposed to the qCBA(maximal FR of approximately 0.1−0.25 mW/cm2). The reasonlies in the fact that in the MUPS, the sample can be placed nearthe lamp (approximately 1.5−4.0 cm), while in the qCBA, thesample has to be placed at least ∼30 cm from the lamp toobtain nearly parallel UV beams. In fact, the water sampleusually flows through a UV reactor at a distance ofapproximately 0−4 cm from the sleeve surface. Moreover, thecombinational use of three segments of different diameters andan adjustable ballast can readily deliver an FR over the broadrange of 0−25.3 mW/cm2 to avoid an overly short or longexposure time. Besides, the design of a closed fluidicphotoreactor avoids sample evaporation loss and allows forvarying sample treatment capacities.Through optical structure modifications, the MUPS achieves

a low variance in the FR output, which is important for theaccuracy of fluence quantifications. First, by recirculating waterof a constant temperature through the photoreactor chamber tomaintain a stable mercury vapor pressure in the lamp, theMUPS has a FR variance lower than that of the qCBA whoseFR varies with room temperature. Second, in a transversesection of the photoreactor, the distance from the lamp centerto any PTFE tube center is virtually constant (Figure 1b), so auniform FR distribution can be obtained along the tube asopposed to a nonuniform FR distribution over the watersample surface in the Petri dish of the qCBA (Figure 1c). Third,the PTFE tube has an inner diameter as small as 2 mm (and aneven smaller diameter can be selected), so a small FR variancewithin the tube cross section can be expected. In the tube crosssection, the FR variance is impacted by both the UVabsorbance of the water sample (i.e., WF) and the divergenceof UV beams (i.e., DF), which can be expressed as follows:

= −′

− ′

alWF

1 10ln(10)

al

(1)

=+ ′D

D lDF

(2)

where a is the absorption coefficient (1/cm) of the watersample, l′ is the “effective” optical path length (cm) because ofthe nonparallel UV beams in the MUPS (as opposed to thosein the qCBA), and D is the distance (cm) from the UV lamp tothe PTFE tube. Because the l′ in the MUPS is considerablysmaller than the sample thickness in the qCBA, the former canallow a larger variance in sample absorbance while the WF iskept nearly constant during photolysis. However, the smaller D

Figure 1. Schematic diagrams of (a) the mini-fluidic UV photoreactionsystem (MUPS), (b) the MUPS transverse section, and (c) the fluencerate (FR) distribution in the Petri dish of qCBA.

Table 1. Comparison of the Performance of the qCBA andMUPS

qCBA MUPS

lamp output variable with room temperature stableFR range (mW/cm2) 0−0.25 0−25.3exposure time longer shortersample volume range smaller largersample thickness (cm) 1.0−2.0 0.05−0.20Petri factor 0.7−0.9 nonedivergence factor 0.94−0.99 0.88−0.99aaCalculated with eq 2 by assuming l′ = 2 mm (the most unfavorablescenario).

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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 high fluence ranges. In the low-fluence (i.e., <100 mJ/cm2) operation mode, the water sample was pumpedsequentially through the PTFE tube for UV exposure and anonline UV−vis SPM for absorbance measurements. On thebasis of the MFSD-measured FR value (E0, mW/cm2), a presetexposure length (L, cm) of the PTFE tube, and the flow rate(Q, mL/s), the fluence (F, mJ/cm2) can be calculated asfollows:

π=F Er LQ0

2

(3)

where r is 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 exposurepart of the PTFE tube and the online SPM and thenrecirculated to receive the UV irradiation again until a desiredexposure time was reached. Note that for a certain exposuretime, only a part of the sample received the UV irradiation,while the remaining part was in the dark. If a reductionequivalent exposure time (tree, s) is defined as the total exposuretime (t, s) multiplied by the ratio of the exposure volume of thetube (πr2L, mL) to the total sample volume (V, mL), thefluence can be readily calculated:

π=tr LV

tree

2

(4)

=F E t0 ree (5)

In other words, tree can be regarded as the exposure time whenthe whole sample simultaneously receives the UV irradiationwith 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) weredetermined 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 onlineSPM in the MUPS at 222 nm and the ultraperformance liquidchromatography−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 μmparticle size). Milli-Q water (Millipore) was used in allexperiments and analytical determinations.

■ RESULTS AND DISCUSSIONThe KI/KIO3 actinometer12 was used to determine the incidentfluence in the MUPS (for the low-fluence operation mode) bymeasuring the I3

− absorbance at 352 nm with an SPM. Whenthe actinometer solution passed through the PTFE tube, theincident photon flux could be determined from the yield of theI3− 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, toobtain the desired fluences (1.9, 3.5, 4.5, and 5.5 mJ/cm2), theexposure time was varied by adjusting the tube exposure lengthand the sample flow rate according to eq 3 (details listed inTable S1). Figure 2 shows that at a fixed FR, the fluencemeasured by the KI/KIO3 actinometer was linearly propor-tional to the exposure time; meanwhile, at various FRs, nearlyidentical values were measured by the KI/KIO3 actinometer fora desired fluence. This is reasonable because the second law of

photochemistry stipulates that the extent of a photochemicalreaction must be proportional to the number of absorbedphotons. This result also demonstrates that the MUPS candeliver an accurate fluence in the broad FR range of 0.034−25.3mW/cm2.In addition, it should be noted that a certain discrepancy

existed between the fluences measured by the MFSD and bythe KI/KIO3 actinometer (slope of 0.93), which could beprimarily ascribed to their different calibration methods. Thequantum yield of the KI/KIO3 actinometer was determined bya uniform sources tunable laser facility at the National Instituteof Standards and Technology of the United States12 (0.71 ±0.02; relative error of ∼3%), while the MFSD was calibrated bya UV radiometer which had been standardized by the NationalInstitute of Metrology of China10 (relative error of ∼8%). Froma practical viewpoint, although either the MFSD or the KI/KIO3 actinometer can be used to determine the FR in theMUPS, the former is more convenient to use.Accurate fluence determination depends on the accurate

measurements of both FR and exposure time. Because thePTFE 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 uniformresponse to UV beams with various incident angles. This isimpossible for a conventional UV detector with a flat responsewindow, but feasible for the MFSD that has an omnidirectionalresponse to photons.11 Furthermore, for fast photochemicalreactions, it is difficult to determine accurately a short exposuretime (e.g., <1 s) in the qCBA. However, very short exposuretimes can be accurately achieved in the MUPS by combinedadjustment of the FR, tube exposure length, and sample flowrate [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 detectedpesticide in natural waters and a commonly used chemicalactinometer, as well. The atrazine solution (10 mg/L) wasrecirculated through the MUPS with its absorbance monitoredonline at 222 nm (A222) by using the SPM. For UV photolysisof a compound with a low solution absorbance (<0.03), thereaction usually proceeds following the fluence-based pseudo-first-order kinetics (Text S1):

=C C k Fln( / )0 f (6)

Figure 2. Comparison of the fluences determined by the KI/KIO3actinometer and MFSD.

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ε= Φ′

kq l

VE

ln(10)f

p,0

0 (7)

where C0 and C are the initial and real-time concentrations of atest compound (M), respectively, kf is the fluence-basedpseudo-first-order rate constant (cm2/mJ), ε is the decadicmolar absorption coefficient at 254 nm [1/(M·cm)], V is thesolution volume (L), and qp,0 is the incident photon flux(einstein/s).The change of the atrazine solution absorbance under UV

irradiation was attributed to both atrazine degradation andbyproduct formation. Hence, the solution absorbance wasdeconvoluted to extract the atrazine absorbance (Text S2). In aparallel experiment, samples were collected at preselectedexposure times to analyze the residual atrazine concentrationsby UPLC−MS/MS. Figure 3 shows that the atrazine

degradation curves were nearly identical as tested at two FRvalues (0.58 and 25.3 mW/cm2) and analyzed by two methods(SPM and UPLC−MS/MS). Plotting ln(Aaz/Aaz,0) or ln(C/C0)versus fluence yielded a linear slope of 0.00064 ± 0.00002 cm2/mJ (i.e., kf of atrazine). It demonstrates that the MUPS canquickly determine the reaction rate constant through onlinemonitoring of the photolysis process, thus reducing the analysisworkload.The MUPS can also be applied to determine quickly the

quantum yield (Φ) of a test compound. According to the kfexpression (eq 6), by separately conducting UV photolysisexperiments of a test compound (atrazine, denoted with asubscript “az”) and a reference compound (uridine, denotedwith a subscript “ud”) and comparing their degradation rateconstants, we can readily determine the Φaz. Figure S3 showsthat a plot of ln(Aud/Aud,0) versus fluence yielded a linear slopeof 0.00068 cm2/mJ (i.e., kf,ud). Then, from the known opticalparameters (εud = 9131 [1/(M·cm)]; εaz = 3413 [1/(M·cm)],and Φud = 0.02)13 and the measured kf,az (0.00064 ± 0.00002cm2/mJ), the Φaz was 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 a qCBA.14,15

In summary, the newly developed MUPS has distinct meritsof accurate exposure time and FR measurements, practicalengineering FR outputs, online absorbance measurements, andfast determination of photochemical parameters. In addition,the novel design of its optical structure makes this apparatussimple and inexpensive to fabricate, easy to operate and

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

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acs.estlett.5b00207.

One table, two sections of text, and three figures (PDF)

■ AUTHOR INFORMATIONCorresponding Author*Telephone: +86-10-62849632. Fax: +86-10-62923541. E-mail:qiangz@rcees.ac.cn.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis work was financially supported by the National NaturalScience Foundation of China (51290281, 51408592, and51221892) and the People Programme (Marie Curie Actions)of the European Union’s Seventh Programme FP7/2007-2013under a REA grant (318926).

■ REFERENCES(1) Sastre, F.; Fornes, V.; Corma, A.; Garcia, H. Selective, room-temperature transformation of methane to C1 oxygenates by deep UVphotolysis over zeolites. J. Am. Chem. Soc. 2011, 133 (43), 17257−17261.(2) Weng, S. C.; Blatchley, E. R. Ultraviolet-induced effects onchloramine and cyanogen chloride formation from chlorination ofamino acids. Environ. Sci. Technol. 2013, 47 (9), 4269−4276.(3) Deng, L.; Huang, C. H.; Wang, Y. L. Effects of combined UV andchlorine treatment on the formation of trichloronitromethane fromamine precursors. Environ. Sci. Technol. 2014, 48 (5), 2697−2705.(4) Duca, C.; Imoberdorf, G.; Mohseni, M. Novel collimated beamsetup to study the kinetics of VUV-induced reactions. Photochem.Photobiol. 2014, 90 (1), 238−240.(5) Bolton, J. R.; Stefan, M. I. Fundamental photochemical approachto the concepts of fluence (UV dose) and electrical energy efficiency inphotochemical degradation reactions. Res. Chem. Intermed. 2002, 28(7−9), 857−870.(6) Keen, O. S.; McKay, G.; Mezyk, S. P.; Linden, K. G.; Rosario-Ortiz, F. L. Identifying the factors that influence the reactivity ofeffluent organic matter with hydroxyl radicals. Water Res. 2014, 50,408−419.(7) Bolton, J. R.; Linden, K. G. Standardization of methods forfluence (UV dose) determination in bench-scale UV experiments. J.Environ. Eng. 2003, 129 (3), 209−215.(8) Thorsen, T.; Maerkl, S. J.; Quake, S. R. Microfluidic large-scaleintegration. Science 2002, 298, 580−584.(9) Nayak, R. K.; Peersen, O. B.; Hall, K. B.; Van Orden, A.Millisecond time-scale folding and unfolding of DNA hairpins usingrapid-mixing stopped-flow kinetics. J. Am. Chem. Soc. 2012, 134 (5),2453−2456.(10) Li, M. K.; Qiang, Z.; Li, T. G.; Bolton, J. R.; Liu, C. L. In situmeasurement of UV fluence rate distribution by use of a microfluorescent silica detector. Environ. Sci. Technol. 2011, 45 (7), 3034−3039.(11) Qiang, Z. M.; Li, M. K.; Bolton, J. R.; Qu, J. H.; Wang, C.Estimating the fluence delivery in UV disinfection reactors using a‘detector-model’ combination method. Chem. Eng. J. 2013, 233, 39−46.(12) Bolton, J. R.; Stefan, M. I.; Shaw, P. S.; Lykke, K. R.Determination of the quantum yields of the potassium ferrioxalate and

Figure 3. Atrazine degradation by UV photolysis as tested at two FRvalues and analyzed by two methods (SPM and UPLC−MS/MS).

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potassium iodide-iodate actinometers and a method for the calibrationof radiometer detectors. J. Photochem. Photobiol., A 2011, 222 (1),166−169.(13) Jin, S.; Mofidi, A. A.; Linden, K. G. Polychromatic UV fluencemeasurement using chemical actinometry, biodosimetry, and mathe-matical techniques. J. Environ. Eng. 2006, 132 (8), 831−841.(14) Canonica, S.; Meunier, L.; von Gunten, U. Phototransformationof selected pharmaceuticals during UV treatment of drinking water.Water Res. 2008, 42 (1−2), 121−128.(15) Beltran, F. J.; Ovejero, G.; Acedo, B. Oxidation of atrazine inwater by ultraviolet-radiation combined with hydrogen-peroxide.Water Res. 1993, 27 (6), 1013−1021.

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