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Degradation of DEET and Caffeine under UV/ Chlorine and Simulated Sunlight/Chlorine Conditions Peizhe Sun, Wan-Ning Lee, Ruochun Zhang, and Ching-Hua Huang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b02287 • Publication Date (Web): 21 Nov 2016 Downloaded from http://pubs.acs.org on November 21, 2016

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Environmental Science & Technology

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Degradation of DEET and Caffeine under UV/Chlorine and Simulated

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Sunlight/Chlorine Conditions

4 Peizhe Suna,b,*, Wan-Ning Leea, Ruochun Zhangc and Ching-Hua Huang a,*

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a

School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia

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30332, United States b

School of Environmental Science and Engineering, Tianjin University, Tianjin 300072, China c

Institute of Surface-Earth System Science, Tianjin University, Tianjin 300072, China

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*

Corresponding Authors Phone: 404-894-7694; Fax: 404-358-7087;

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E-mail: [email protected] (Ching-Hua Huang).

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Phone: 404-358-4858; Fax: 404-358-7087;

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E-mail: [email protected] (Peizhe Sun).

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Revised manuscript submitted to

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Nov. 16, 2016

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(Manuscript word account: 5600 + 1800 + 300)

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1 ACS Paragon Plus Environment


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ABSTRACT

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Photo-activation

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photochemically stable chemicals accumulated in swimming pools. This study investigated the

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degradation of two such chemicals, N,N-diethyl-3-methylbenzamide (DEET) and caffeine, by low

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pressure ultraviolet (UV) light and simulated sunlight (SS) activated free chlorine (FC) in different water

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matrices. Both DEET and caffeine were rapidly degraded by UV/FC and SS/FC, but exhibited different

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kinetic behaviors. The degradation of DEET followed pseudo-first-order kinetics, whereas the

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degradation of caffeine accelerated with reaction. Mechanistic study revealed that, under UV/FC, ·OH

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and Cl· were responsible for degradation of DEET, whereas ClO· related reactive species (ClOrrs),

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generated by the reaction between FC and ·OH/Cl·, played a major role in addition to ·OH and Cl· in

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degrading caffeine. Reaction rate constants of DEET and caffeine with the respective radical species were

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estimated. The imidazole moiety of caffeine was critical for the special reactivity with ClOrrs. Water

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matrix such as pH had stronger impact on the UV/FC process than the SS/FC process. In saltwater matrix

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under UV/FC and SS/FC, the degradation of DEET was significantly inhibited, but the degradation of

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caffeine was much faster than that in non-salty solutions. The interaction between Br- and Cl- may play an

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important role in the degradation of caffeine by UV/FC in saltwater. Reaction product analysis showed

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similar product patterns by UV/FC and SS/FC, and minimal formation of chlorinated intermediates and

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disinfection byproducts.

of

aqueous

chlorine

could

promote degradation


of chlorine-resistant

and

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INTRODUCTION

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Chlorination is widely used for drinking water and recreational water disinfection. In the U.S., most

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swimming pool water is sanitized with free chlorine (FC), which is commonly introduced in the form of

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calcium hypochlorite or sodium hypochlorite. However, chlorination in pools has several significant

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drawbacks, including formation of chlorinated disinfection byproducts (DBPs) such as monochloramine

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(NH2Cl) and chloroform, and poor disinfection efficacy against chlorine-resistant microorganisms such as

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Giardia.1-3 To mitigate these drawbacks and associated health risks, ultraviolet (UV) light has been

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increasingly applied in swimming pool settings as a secondary disinfection process, because of its

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effectiveness to control chloramines and chlorine-resistant microorganisms.4-7 The application of UV can

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also lower the demand for residue chlorine in pool water and thus decrease the DBP formation potential.

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As a result, it is anticipated that the number of pools in the U.S. to be treated by a combination of UV and

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FC sequentially and/or simultaneously to increase over time.

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The simultaneous application of UV and FC (i.e. the UV/FC process) has also been proposed as a

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novel advanced oxidation process (AOP) for drinking water and wastewater treatment.8-10 FC, under

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irradiation at germicidal UV wavelength, mainly decomposes to yield hydroxyl radical (·OH) and

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chlorine radical (Cl·),11 both of which have high reactivity towards organic molecules. Several studies

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have shown that micropollutants, such as herbicides, pharmaceuticals and DBPs, were successfully

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removed under UV/FC conditions by reacting with ·OH or/and Cl·.8,10,12,13 The quantum yield (Φ) of FC at

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254 nm was reported closed to 1.0 mole·Einstein-1, which is higher than other UV-based AOPs, such as

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UV/H2O2 (Φ ≈ 0.5 mole·Einstein-1) and UV/persulfate (Φ ≈ 0.7 mole·Einstein-1).11,13,14 If irradiated at a

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higher wavelength (> 320 nm), such as under sunlight exposure, photolysis of OCl- yields ground-state

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atomic oxygen (O(3p)), which is further converted to ozone (O3) by reacting with dissolved oxygen in

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water.15-17 The sunlight/FC process also has been suggested to inactivate pathogenic microorganisms for

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drinking water.16



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In swimming pool settings, the photo-activation of FC is expected to occur in indoor chlorinated pools

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installed with UV apparatus and in outdoor chlorinated pools. Formation of DBPs is a primary health

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concern of chlorination; thus, several studies have investigated the DBP formation potential under UV/FC

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conditions and compared that under chlorination alone. Representative DBP precursors relevant for

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swimming pools such as human bodily fluid (e.g., sweat and urine), amine compounds and

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micropolllutants were investigated.18-22 For example, Weng et al. reported an elevated formation of

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dichloromethylamine, dichoroacetonitrile and cyanogen chloride in pool water under UV/FC conditions

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than chlorination alone.18 Trichloronitromethane formation from simple amines and polyamines was also

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reported to increase up to 15 folds by UV/FC compared with chlorination alone.21 Ben et al. investigated

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DBP formation from the antiseptic triclosan under UV, FC and UV/FC conditions, which showed that

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chloroform formation was notably enhanced under UV/FC conditions.22 Wang et al. conducted full-scale

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and pilot-scale UV/FC tests of drinking water treatment.23 The results showed minimal trihalomethane

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and haloacetic acid formation during UV/FC treatment, while haloacetonitriles were produced rapidly.

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On the other hand, photo-activated FC holds great promise to degrade micropollutants such as various

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pharmaceuticals and personal care products (PPCPs) that are introduced by swimmers into pool

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water.10,13,24-26 The concentrations of PPCPs (e.g. analgesics, antibiotics, stimulants, UV-filters, insect-

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repellants, etc.) in swimming pools are reported in the range of ng·L-1 to µg·L-1.27-33 Based on the data

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available to date, N,N-diethyl-3-toluamide (DEET) and caffeine are among the highest concentration

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PPCPs frequently detected in swimming pools, likely because both chemicals are utilized at a large

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quantity and are resistant to chlorine oxidation and sunlight photolysis.27,28 It has been reported that DEET

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was detected at concentrations over 2 µg·L-1,28 and caffeine up to 1.54 µg·L-1,27 in pools.

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To date, potential degradation of DEET and caffeine by photo-activated FC in swimming pool relevant

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conditions has not been investigated in detail, and thus the focus of this study. The degradation of these

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two chemicals under UV/FC and simulated sunlight/FC (SS/FC) conditions in different water matrices

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(from deionized water to saltwater) was investigated. Specifically, the objectives of this study were to

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elucidate the radical reactions involved in light-activated FC conditions for micropollutant degradation, 4 ACS Paragon Plus Environment

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measure the reaction rate constants of radical species with DEET and caffeine, assess water matrix effects,

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and identify transformation products and potentially harmful byproducts.

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MATERIALS AND METHODS Chemicals and reagents. Sources of chemicals, reagent and synthetic saltwater are provided in the Supporting Information (SI) Text S1 and Table S1.

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Experimental setup. The UV/FC conditions were created by spiking aliquots of NaOCl solution into

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target water matrix (30 mL) in a quartz reactor (50 mL) that was irradiated by a 4-W low pressure UV

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lamp (Philips Co., Netherlands) emitting primarily 254 nm UV light in a photoreactor. The experimental

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setup is illustrated in SI Figure S1. The UV fluence rate was measured to be 3.86×10-6 Einstein·L-1·s-1

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using potassium ferrioxalate as chemical actinometer. To create simulated sunlight/FC (SS/FC)

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conditions, a bench-scale collimated-beam apparatus was employed (SI Figure S2), which was equipped

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with a 300-W Xenon lamp (PerkinElmer, PE300BF). The spectrum emission (SI Figure S3) of the lamp

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was characterized by a spectroradiometer (Spectral Evolution, SR-1100). The reaction solution (50 mL)

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was put into a glass beaker (150 mL) which was placed on a stir plate, normal to the incident light. To

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monitor reaction progress, sample aliquots were taken from the reactor periodically, quenched with

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sodium thiosulfate and analyzed by suitable analytical methods.

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Chemical analysis. FC stock solutions were standardized iodometrically.34 The decay of FC during

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reactions was measured with the DPD (N,N-diethyl-p-phenylenediamine) method.35 Chloride, chlorite

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and chlorate were measured using a DIONEX ion chromatography system. DEET and caffeine were

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routinely measured by an Agilent HPLC-DAD system equipped with a C8 column (4.6 × 150 mm, 5 µm,

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Agilent, Eclipse XDB-C8). The mobile phase was a mixture of 40% H2O and 60% methanol isocratically.

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DEET and caffeine were measured at 220 nm and 270 nm, respectively. For identification of

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transformation products, LC/MS (Aglient LCMS 1100 system) was applied using scan mode (m/z 50–300)

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with the LC method previously described.27 The detailed LC/MS method is described in SI Text S2.



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To detect the presence of halogenated DBPs in samples after UV/FC and SS/FC treatment, 50-mL

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samples were extracted using 3-mL methyl tertiary-butyl ether (MTBE). Then the extracts were

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transferred to 2-mL vials for analysis by gas-chromatography coupled with electron capture detector (GC-

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ECD) as detailed in SI Text S3.

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Kinetic Modeling. The UV/FC and SS/FC processes were modeled using the Simbiology application

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in Matlab 2014b. Around one hundred elementary reactions (SI Table S2) were considered with rate

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constants obtained from literature.14,36,37 This model takes into account the effects of most inorganic ions,

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including chloride, sulfate, carbonate species, and bromide on the UV/FC and SS/FC processes. The

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concentrations of radical species were calculated at the end of 2 min reaction time. Preliminary test runs

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showed that the concentrations of major radicals reached pseudo-steady-state within 2 min reaction time.

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RESULTS AND DISCUSSION

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Photodecomposition of FC. The photodecomposition of FC was investigated under UV and SS

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irradiation in 10 mM phosphate buffer solution (PBS) at pH 5–9. The FC centration versus time was in

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good agreement with first-order kinetics (SI Figure S4). Therefore, the observed rate constants (kobs) were

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obtained from the FC concentration profiles. As shown in Figure 1, FC decomposed at similar rates (kobs =

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0.06–0.07 min-1) under UV irradiation across the pH range, whereas kobs significantly increased with the

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increase of solution pH under SS irradiation. The photodecomposition rate of FC can be expressed by eqn.

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1, where [ClT] is the total concentration of FC; and kHOCl and kOCl- are the photodecomposition rate

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constants of HOCl and OCl– species, respectively.

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−

d[ClT ] = kobs [ClT ] = k HOCl[ HOCl] + kOCl− [OCl − ] dt

(1)

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The eqn. 2 and 3 express kHOCl and kOCl- as a function of average quantum yield ( Φ , mole·Einstein-1,

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defined as the weighted average quantum yield across the wavelength 290 nm to 400 nm), absorbance (ε,

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M-1·cm-1), fluence rate (I, Einstein·L-1·s-1) and light pathlength (l, cm). The speciation of FC (i.e. [HOCl]

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and [OCl–]) was calculated based on the pKa (7.5) and pH values. Therefore, kHOCl and kOCl- were obtained 6 ACS Paragon Plus Environment

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by fitting eqn. 1 with experimental data in Figure 1 using Matlab curve-fitting toolbox. Finally, the

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quantum yield values were calculated by eqn. 2 and 3: Φ HOCl = Φ OCl − = 1.08±0.03 at UV irradiation (254

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OCl HOCl − nm) because kHOCl ≈ kOCl- and ε 254 nm ≈ ε 254 nm ; Φ HOCl = 0.432±0.03 and Φ OCl = 0.945±0.06 at SS

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irradiation.

−

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 k HOCl = Φ HOCl ⋅   

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 k OCl − = Φ OCl − ⋅   

∑λ ε λ

HOCl

OCl −

∑λ ε λ

 I λ  ⋅ l ⋅ 2.303  

(2)

 I λ  ⋅ l ⋅ 2.303  

(3)

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For UV irradiation, the quantum yields measured in this study are within the range previously

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reported (Φ = 0.9–1.5 at 254 nm).11,38 For SS irradiation, although quantum yields of FC were not

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available for the sunlight-simulator used in this study, the values can be compared with those determined

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by Cooper et al.,39 who measured the quantum yields of HOCl/OCl– at multiple wavelengths from 240 to

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435.8 nm. While the quantum yields determined in this study are the weighted average across the

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wavelength of 290 to 400 nm, Φ is likely largely contributed by the quantum yields at the wavelengths

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that overlap the most with FC species’ absorbance (i.e. around 345 nm, SI Figure S3). Indeed, the average

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quantum yields (0.566 at pH 7, calculated based on the speciation of FC) in this study are comparable to

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the values (0.61 at 334.1 nm and 0.55 at 365.0 nm) determined by Cooper et al. at pH 7.39 With the

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determined quantum yields, the dynamic model built using the Simbiology application in Matlab was able

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to predict photodecomposition of FC and radical production under UV and SS irradiation.

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Degradation of DEET and caffeine under UV/FC conditions. The degradation of DEET and

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caffeine was first investigated in DI water matrix under UV, FC and UV/FC conditions. Results showed

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that both DEET and caffeine degraded rapidly by UV/FC, whereas they were resistant to UV alone and

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FC alone (Figure 2). Based on the quantum yields and molar absorbances of DEET and caffeine measured

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in previous studies,27 the first-order rate constants of their photolysis under UV conditions employed in

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this study are calculated to be 0.0028 min-1 and 0.0076 min-1, respectively, which are negligible compared



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with the reactions under UV/FC conditions. DEET and caffeine are also known to be resistant to FC,27

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which is part of the reason why they were detected at high concentrations in chlorinated indoor swimming

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pools.27 Therefore, the degradation of DEET and caffeine by UV/FC was mainly due to the reactions with

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reactive species generated by photo-decomposed FC.

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The degradation of DEET by UV/FC followed pseudo-first-order kinetics (Figure 2A). However, the

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degradation of caffeine deviated from first-order kinetics and accelerated as reaction proceeded (Figure

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2B). Additionally, the degradation of caffeine was significantly faster than that of DEET. Indeed, more

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than 85% of parent caffeine was degraded by UV/FC after 3 min of reaction compared with the removal

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of less than 25% of parent DEET. These differences in kinetics suggested that the major reaction

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mechanisms contributed to the degradation of DEET and caffeine were likely different. Moreover, it is

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noteworthy that non-first-order degradation of organic compounds under UV/FC has not been reported in

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previous literature. Therefore, efforts were made to investigate the reactive species responsible for the

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degradation of DEET and caffeine under UV/FC conditions, with the aim to elucidate reaction

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mechanisms.

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Elucidation of reactive species under UV/FC conditions. To minimize the impact from pH changes

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during reaction, the experiments were re-conducted in 10 mM PBS at pH 7. The degradation (Figure 3)

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showed the same trends as those conducted in DI water. The initial and final pH and FC concentrations

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were measured, which showed negligible changes within the short reaction time (< 5 min). It is

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commonly believed that the photodecomposition of FC under UV irradiation at 254 nm primarily

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produces ·OH/O·– and Cl· (eqn 4). Because O·– fast reacts with H2O to generate ·OH (1.8×106 M-1·s-1)27

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and has low reactivity with organic compounds (compared with ·OH),27,40 O·– is likely much less

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important than ·OH and Cl· in the UV/FC system. In addition, considering that FC was introduced as

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NaOCl aqueous solution which contained a small amount of Cl– due to thermodecomposition of

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HOCl/OCl–, some amount of Cl2·– and ClOH·– were also produced as secondary radicals via the reaction

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between Cl· or ·OH with Cl– (eqn. 5 and 6). Another secondary radical, ClO·, was generated from attack

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of FC by radical species (eqn. 7) and underwent self-scavenging reaction (eqn. 8). Therefore, five major 8 ACS Paragon Plus Environment

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reactive species (·OH, Cl·, Cl2·–, ClOH·– and ClO·) were considered and their concentrations (Table 1)

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under UV/FC conditions in 10 mM PBS without DEET or caffeine were simulated by dynamic model.

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hν • HOCl / OCl − → OH / O •− + Cl •

(4)

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Cl • + Cl −  → Cl2•−

(5)

OH + Cl − ← → ClOH •−

(6)

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HOCl / OCl − + •OH / Cl •  → ClO •

(7)

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ClO • + ClO •  → Cl 2 O2

(8)

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•

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Because DEET and caffeine are known to react rapidly with ·OH (kapp at (4.6–7.5)×109 M-1·s-1 and

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(5.9 – 6.9)×109 M-1·s-1, respectively),40-42 efforts were made to create conditions under which the

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contributions of chlorinated radicals were able to be compared. Eqn. 5 and 6 suggest that increase of Cl–

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will enhance the production of Cl2·– and ClOH·– by consuming ·OH and Cl·. As shown in Table 1, under

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conditions with 7 mg·L-1 FC, the addition of 0.2 M NaCl increased the concentrations of Cl2·– and ClOH·–

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by approximately three orders of magnitudes, and decreased the concentrations of Cl· slightly. The

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concentrations of ·OH, however, was not affected because of the fast backward reaction of eqn. 6

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(6.1×109 s-1). Experiments showed that, at 7 mg·L-1 FC, the degradation of DEET and caffeine was not

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enhanced in spite of the significant increase of Cl2·– and ClOH·– with 0.2 M NaCl (Figure 3), which

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suggested that the contributions of Cl2·– and ClOH·– were negligible. However, possibility existed that the

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contribution from the increase of Cl2·– and ClOH·– might exactly compensate the decrease of Cl·, thus

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leading to little change in degradation rate. For that, further quantitative evaluation was conducted to

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discern the contribution of Cl· and the results are discussed in the later section of determination of

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second-order rate constants.

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Under UV/FC conditions, the source and major scavenger of ·OH and Cl· was FC itself, meaning that

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both the production and consumption rates of ·OH and Cl· varied proportionally with the change of FC

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concentrations. Therefore, the simulated concentrations of ·OH and Cl· varied within a narrow range at



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the FC concentrations of 7, 14, and 28 mg·L-1 (Table 1, without addition of NaCl). In contrast, Cl2·–,

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ClO· and ClOH·– were produced from radicals reacting with FC (including Cl– in FC stock) and not

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scavenged by FC. Therefore, the concentrations of Cl2·–, ClO· and ClOH·– increased proportionally with

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the increase of FC concentration (Table 1). As shown in Figure 3A, the degradation of DEET was not

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affected by the increase of FC, indicating that the important radicals were ·OH and Cl·, rather than Cl2·–,

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ClO· and ClOH·–. As for caffeine (Figure 3B), the degradation was significantly enhanced by the increase

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of FC. If Cl2·– and ClOH·– were unimportant radicals for the degradation of caffeine as proposed above,

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ClO· was very likely a major radical which degraded caffeine under UV/FC conditions. As suggested by

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Wu et al., ClO· was responsible for the degradation of certain organic pollutants under UV/FC

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conditions.43

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Overall, the study results thus far strongly supported the hypothesis that the degradation of DEET was

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mainly due to ·OH and Cl·, whereas caffeine was degraded by ·OH, ClO·, and possibly by Cl·, under

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UV/FC conditions.

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Determination of second-order rate constants. Applying competition kinetic method (SI Text S4),

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the second-order rate constant between DEET and ·OH (kDEET/·OH) was determined to be 6.7×109 M-1·s-1,

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in agreement with the literature values, under UV/H2O2 conditions using nitrobenzene (NB) as a probe

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compound. To measure the second-order rate constant between DEET and Cl· (kDEET/Cl·), as suggested by

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Fang et al.,40 NB and benzoic acid (BA) were employed to quantify the concentrations of ·OH and Cl· in

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the UV/FC system, because they are inert to UV irradiation and FC oxidation. NB only reacts with ·OH,

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whereas BA reacts with both ·OH and Cl·. Therefore, in a solution containing NB, BA and DEET, the

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concentrations of ·OH and Cl· were calculated based on the degradation of NB and BA. Combining with

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the measured value of kDEET/·OH, kDEET/Cl· was determined to be 3.8×109 M-1·s-1 (calculation detailed in SI

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Text S5). Applying the obtained kDEET/·OH and kDEET/Cl·, the dynamic model in Simbiology successfully

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predicted the degradation of DEET under UV/FC conditions at different FC initial concentrations (Figure

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3A).


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For caffeine, its second-order rate constant with ·OH (kCAF/·OH) was determined to be 6.4×109 M-1·s-1,

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in agreement with the literature values, using the competition kinetic method similarly (SI Text S4). To

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further elucidate the contribution of Cl· to the overall degradation of caffeine, a solution containing NB,

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BA and caffeine was exposed to UV with 14 mg·L-1 FC. The degradation of all three compounds was

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monitored (SI Figure S5). Based on the degradation of NB and BA, the steady-state concentration of

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Cl· was calculated to be 6.3×10-14 M. Assuming the reaction between caffeine and Cl· (kCAF/Cl·) was close

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to the diffusion-limit rate, i.e. ~5×1010 M-1·s-1, a small, non-negligible amount of caffeine’s degradation

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could be contributed by Cl· (SI Figure S5). Therefore, the contribution of Cl· cannot be excluded. On the

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other hand, there lacked an experimental method for the quantification of ClO· in the literature. Therefore,

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this study first attempted to employ Simbiology to simultaneously estimate kCAF/Cl· and kCAF/ClO· by fitting

251

the experimental data with the dynamic model. It was found that the calculated concentrations of Cl· and

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ClO· remained approximately constant even with the degradation of caffeine because the major scavenger

253

of Cl· was FC and the major scavenger of ClO· was ClO· itself (eqn. 8). However, the constant radical

254

concentrations resulted in a linear relationship between ln(C/C0) and reaction time, therefore failing to

255

capture the curved degradation kinetic profile of caffeine.

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Since the reactions involving Cl· have been well documented, the limited kinetic information of the

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reactions involving ClO· is likely responsible for the discrepancy between modeling results and

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experimental data. Indeed, two ClO· combining to form one Cl2O2 is the only reaction with known

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reaction rate constant (eqn. 8, k = 2.5×109 M-1·s-1),37 whereas Cl2O2 was proposed to convert back to ClO·,

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whose rate constant is not available.44 Therefore, the model may not estimate the concentration of

261

ClO· accurately. If the backward reaction, from Cl2O2 to ClO·, was sufficiently fast, the major scavenging

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effect of ClO· would be no longer from ClO· itself. Therefore, for modeling purpose, this study treated

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ClO· and Cl2O2 as a single species, named ClOrrs (ClO· related reactive species). The nominal

264

concentration of ClOrrs is defined to be the concentration of ClO· in the dynamic model without

265

considering the sink of ClO·. Experimental results under the UV/FC condition with 7 mg·L-1 FC were



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used to estimate kCAF/Cl· and kCAF/ClOrrs because it had the largest dataset. Simbiology gave the estimated

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kCAF/Cl· and kCAF/ClOrrs of 1.46×1010 M-1·s-1 and 1,361 M-1·s-1, respectively, and successfully captured the

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curved degradation kinetic profile of caffeine. Applying the values of kCAF/·OH, kCAF/Cl· and kCAF/ClOrrs, the

269

degradation of caffeine under UV/FC conditions at 14 mg·L-1 and 28 mg·L-1 FC were also in good

270

agreement with model prediction (Figure 3B). Furthermore, SI Figure S6 depicts the contribution of each

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radical species to the degradation of caffeine based on the simulated concentrations of radical species and

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reaction rate constants (i.e. kCAF/·OH, kCAF/Cl· and kCAF/ClOrrs). Results showed that caffeine was primarily

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degraded by ClOrrs under UV/FC conditions.

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Although the sinks of ClOrrs under UV/FC conditions were not considered in the present model, in

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the solution containing caffeine, the major sink of ClOrrs is expected to be caffeine, which suggested that

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a lower initial concentration of caffeine must result in a faster degradation rate. This hypothesis was

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confirmed by the experiments conducted at 4, 10 and 20 µM caffeine (SI Figure S7). As for Cl·, however,

278

caution should be taken when applying kCAF/Cl· in other systems. Because only a small amount of

279

caffeine’s degradation was due to Cl· (see SI Figure S6), the estimated kCAF/Cl· value may be subject to a

280

relatively large error. Indeed, by varying kCAF/Cl· from 3×109 to 3×1010 M-1·s-1, degradation of overall

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caffeine under UV/FC was not significantly affected (SI Figure S8).

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Reactive moieties of caffeine. Caffeine is comprised of 1,3-dimethyluracil (DMU) and 1-

283

methylimidazole (MIM). Both moieties are commonly present in biomolecules and industrial materials.

284

Therefore, further efforts were made to identify which moiety of caffeine contributed to the unique

285

reaction with ClO·. The degradation of DMU and MIM was studied in DI water matrix with 7 mg·L-1 FC

286

under UV irradiation (SI Figure S9). Control experiments were performed under UV alone and FC alone,

287

which showed negligible degradation of DMU and MIM within reaction time (~3 min). As shown in SI

288

Figure S9, the degradation of DMU clearly showed first-order kinetic behavior (R2 > 0.999) whereas the

289

degradation of MIM accelerated with reaction progress similarly to the degradation of caffeine. These


Page 13 of 29


290

results suggested that the imidazole moiety of caffeine reacted with ClO· and contributed to the unique

291

kinetics under UV/FC and SS/FC conditions.

292

pH effect on the degradation of DEET and caffeine under UV/FC conditions. The experiments

293

were conducted at pH 5–9 in 10 mM PBS. The increase of pH resulted in substantial decrease in the

294

degradation rate of DEET (Figure 4A). As discussed above, the degradation of DEET was primarily due

295

to ·OH and Cl·, which were produced by the photodecomposition of FC. Under UV/FC conditions, the

296

production rates of ·OH and Cl· were the same across pH 5–9 (Figure 1). However, at higher pH, more

297

FC was in the form of OCl–, which reacted with ·OH and Cl· at higher rates than HOCl. Therefore, high

298

pH resulted in stronger scavenging effect. The lower ·OH and Cl· at higher pH also resulted in the

299

decrease removal of caffeine at 90 sec (Figure 4B). The plots with ln(C/C0) vs. time are shown in SI

300

Figure S10A. However, the estimated ClOrrs under different pH conditions were nearly the same

301

according to the dynamic model. Because the degradation of caffeine was predominantly due to ClOrrs,

302

the current model cannot explain the significant decrease of removal at higher pH. One possibility is that

303

the reactions involving ClO· were affected by aqueous pH, leading to the change of ClOrrs concentration

304

and speciation. More research is needed to discern the mechanism further.

305

Saltwater matrix. In saltwater swimming pools, it was expected that components such as halides

306

may significantly impact the overall degradation of DEET and caffeine under UV/FC and SS/FC

307

conditions. Synthetic saltwater (mimicking seawater) was employed in this study with the recipe shown in

308

SI Table S1. Control experiments showed there was negligible degradation of DEET or caffeine in the

309

saltwater matrix within 5 min (data not shown). The first-order degradation rates of DEET by UV/FC in

310

PBS and in saltwater were compared. As shown in Figure 5A, substantial inhibition of degradation rate

311

was observed in the saltwater matrix, which was expected because anions in saltwater, such as HCO3–,

312

Br– and Cl–, had strong scavenging effects on radical species. In particular, Figure 3A shows that a high

313

concentration of Cl– inhibited the degradation of DEET under UV/FC conditions.

314

For caffeine, surprisingly, the removal was greatly enhanced in saltwater matrix (Figure 5B). Almost

315

complete removal of caffeine was achieved in 60 sec in saltwater, compared with only around 40% 13 ACS Paragon Plus Environment


Page 14 of 29

316

removal in PBS. To elucidate which saltwater components contributed to enhanced degradation of

317

caffeine, a screening test was conducted by adding NaBr, Na2SO4, NaCl and NaHCO3 individually or

318

combined into DI water matrix at concentrations comparable to those in the synthetic saltwater.

319

Negligible impact was observed when the saltwater components were added individually, whereas the

320

combining of (NaCl + NaBr), (NaBr + NaHCO3) and (NaCl + NaBr + NaHCO3) significantly enhanced

321

the degradation of caffeine under UV/FC conditions (SI Figure S11). Clearly, Br– played an essential role

322

through interacting with other anions. In particular, SI Figure S11 shows that Br– and Cl– had great

323

synergy in degrading caffeine under UV/FC conditions. To further confirm this interaction, the

324

degradation of caffeine by UV/FC was monitored in the solutions containing 0.85 mM NaBr with

325

different amounts of NaCl. The increase of NaCl concentration resulted in significant increase of caffeine

326

removal (Figure 6).

327

FC is known to rapidly transform to HOBr/OBr– in bromide-rich waters.45 In synthetic saltwater

328

matrix, the concentration of Br– (0.85 mM) was significantly higher than FC (7 mg·L-1 as Cl2 or 0.1 mM).

329

Therefore, DEET and caffeine were actually degraded under UV/HOBr conditions. In analogy to the

330

UV/FC process, the photodecomposition of HOBr/OBr- yielded ·OH and Br·.46,47 Therefore, highly

331

reactive intermediate radicals, such as ClBr·–, BrOH·–, BrO·, Br2·– and CO3·–, generated from the reactions

332

of Br· with saltwater anions likely contributed to the enhanced degradation of caffeine. In particular,

333

ClBr·– may play an important role in saltwater matrix because the degradation of caffeine was greatly

334

enhanced with the coexistence of Br– and Cl– (Figure 6). However, further study is needed to confirm the

335

contribution of these reactive intermediate radicals, as the radical reactions in saltwater matrix are much

336

more complicated than those in DI water matrix.

337

Degradation of DEET and caffeine under SS/FC conditions. In analogy to the UV/FC conditions,

338

the degradation of DEET and caffeine was also studied under SS/FC conditions in DI water, PBS and

339

saltwater matrices for comparison.

340

In DI water (Figure 2), the degradation kinetics of DEET followed pseudo-first-order pattern under

341

SS/FC whereas the degradation of caffeine accelerated as the reaction proceeded, which was consistent 14 ACS Paragon Plus Environment

Page 15 of 29


342

with the results under UV/FC conditions. These shared features of degradation kinetics indicate that the

343

major reactive species are likely the same under SS/FC and UV/FC conditions.

344

In buffered systems (pH 5–9, Figure 4A), the increase of pH resulted in substantial decrease of

345

degradation rate for DEET, although the change was less pronounced under SS/FC conditions than under

346

UV/FC conditions. The smaller degradation rate difference between acidic and basic pHs under SS/FC

347

conditions may be attributed to two reasons. First, the photodecomposition of FC under SS was faster at

348

higher pH (Figure 1), leading to a higher production rate of ·OH and Cl· at higher pH. Second, the

349

photodecomposition of OCl– at wavelength > 300 nm generated ground state oxygen atoms (O(3P)),15

350

which further produced ozone (O3) by reacting with dissolved oxygen.17 Although the reaction rate

351

between DEET and O3 is low (0.12 M-1 s-1),48 O(3P) is expected to be a highly reactive species which may

352

degrade DEET.

353

Interestingly, caffeine exhibited comparable degradation rates across pH 5–9 (Figure 4A). Based on

354

the same explanations for DEET, the comparable removal rates of caffeine may be due to higher

355

production of radicals at higher pH and reaction with O(3P). Furthermore, caffeine can react with O3 at a

356

relatively fast rate (650 M-1 s-1),49 which may also contribute to the degradation of caffeine at higher pH.

357

In general, by comparing UV/FC and SS/FC across pH 5–9, it is suggested that solution pH has stronger

358

impact on the UV/FC process than the SS/FC process.

359

In saltwater matrix (Figure 5), the degradation rates of DEET and caffeine under SS/FC were similar

360

to those under UV/FC conditions, implying similar mechanisms in both systems. However, as stated

361

above, due to the limited knowledge of photo-activated HOBr, further research is need to elucidate the

362

mechanisms for the enhanced degradation of caffeine in saltwater matrix under SS/FC conditions.

363

Product analysis. Photodecomposition products of chlorine. Based on all the reported reactions

364

under UV/FC and SS/FC conditions (SI Table S2), major final products of chlorine were expected to be

365

Cl–, ClO2– and ClO3–. Therefore, efforts were made to detect these three species after complete

366

photodecomposition of FC. Under both UV/FC and SS/FC conditions at pH 7, the initial FC was

367

transformed to Cl– and ClO3– by the molar ratio of 86% and 14%, respectively, whereas ClO2– was not 15 ACS Paragon Plus Environment


Page 16 of 29

368

detected. Although it has not been concluded how ClO3– is produced by photodecomposition of FC, it was

369

suggested that Cl2O2, generated from the combination of two ClO·, can rapidly hydrolyze to yield ClO2–

370

and ClO3–.44 Because ClO2– was unstable under light irradiation (SI Figure S12), ClO3– was the only

371

product of photodecomposed FC except for Cl– .

372

Degradation products of DEET and caffeine. The search for degradation products of DEET and

373

caffeine focused on two aspects: degradation intermediates and halogenated DBPs. LC/MS set at scan

374

mode from 50 to 300 m/z was employed to detect degradation intermediates. Multiple degradation

375

intermediates of DEET and caffeine (SI Table S3) were observed in the samples after treatment under

376

UV/FC and SS/FC conditions. For each compound, the same types of intermediates were detected under

377

either UV/FC or SS/FC conditions. On the basis of reaction mechanisms of ·OH and Cl·, DEET was

378

expected to be hydroxylated via either –OH addition or hydrogen abstraction, yielding M+14, M+16,

379

M+30, M+46 and M+48 intermediates (with one or multiple oxygen atoms added). The M+14 is the

380

predominant intermediate (based on peak area), which suggested that the degradation of DEET under

381

UV/FC was primarily via hydrogen abstraction, resulting in carbon-centered radical which further

382

transformed to ketone moiety through reacting with dissolved oxygen. Zhang and Lemley proposed the

383

transformation pathway of DEET under Fenton conditions, where DEET was primarily degraded by ·OH

384

to yield M+16 and M+32 as the major products.40 For caffeine, hydroxylated intermediates (e.g. M+32)

385

were also observed; however, the major degradation intermediates had smaller m/z than parent caffeine

386

(SI Table S3). Among the products of caffeine observed in this study, M+32 and M-53 were the only

387

intermediates reported in the literature that investigated the advanced oxidation of caffeine in water.46 In

388

addition, it should be noted that no chlorinated degradation intermediates of DEET or caffeine were

389

observed on LC/MS under either UV/FC or SS/FC.

390

To address the concern that the degradation of DEET and caffeine by UV/FC or SS/FC may produce

391

harmful DBPs, a screening test was performed, which analyzed 21 DBPs including trihalomethanes,

392

haloacetonitriles, haloacetamides, halonitroalkanes and other halogenated-DBPs (SI Text S3). The

393

method detection limits (MDLs) of DBPs were below 0.03 µg⋅L-1 expect for bromoform (MDL = 0.1 16 ACS Paragon Plus Environment

Page 17 of 29


394

µg⋅L-1). Samples containing 100 µM DEET or caffeine were treated with UV/FC or SS/FC in PBS or

395

saltwater matrices for 10 min. Results showed that none of the DBPs were at detectable concentrations in

396

any of these samples.

397

Environmental Implications. The human exposure to PPCPs in swimming pools is of increasing

398

concerns. This study demonstrates that persistent DEET and caffeine in chlorinated swimming pools can

399

be effectively removed under UV/FC and sunlight/FC conditions with minimal harmful byproduct

400

formation. Results in the present study can help explain several previous studies that investigated the

401

health risks associated with pool waters. For example, a recent study on the occurrence of PPCPs in

402

chlorinated swimming pools in Australia showed that the concentrations of caffeine were much lower in

403

outdoor swimming pools and in indoor pools equipped with UV processes.28 Liviac et al. tested

404

genotoxicity of recreational pools with different settings, in which outdoor pools and the pools with

405

UV/FC process had the lowest genotoxicity level.50 The authors attributed the benefits to the photolysis of

406

DBPs under UV and solar light. This study provides additional explanations that persistent chemicals in

407

chlorinated pools may be degraded by radicals generated under UV/FC and sunlight/FC conditions.

408

Although the scope of this study focused on swimming pool settings, photo-activated FC processes

409

have wide applications in drinking water and wastewater treatment as well. This study proposes a group

410

of new critical reactive species, ClOrrs, under photo-activated FC conditions. This new finding should be

411

considered in further research on photo-activated FC processes. Moreover, the observation that imidazole

412

moiety is highly reactive towards ClOrrs implies that imidazole-containing compounds, such as purine

413

and DNA, may be fast degraded under photo-activated FC processes and warrant more research.

414 415

ASSOCIATED CONTENT

416

Supporting Information. Text S1-S5, Tables S1−S4 and Figures S1−S13. This material is available free

417

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

418



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concentrates from recreational pools after various disinfection methods. Environ. Sci. Technol. 2010, 44

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546



547

0.12 UV SS Model fitting

-1

kobs (min )

0.10 0.08 0.06 0.04 0.02 0.00 4

548 549 550 551

5

6

7

8

9

10

pH

Figure 1. Photodecomposition rates of chlorine under UV and simulated sunlight (SS) irradiation at pH 5–9. Error bars represent standard deviations from duplicates. Reactions employed 7 mg⋅L-1 initial FC concentration in 10 mM PBS.


Page 24 of 29

Page 25 of 29


0.0

ln (C/C0)

-0.1

(A) -0.2 UV SS Chlorine UV/chlorine SS/chlorine

-0.3 -0.4 -0.5 0

1

2

3

4

5

6

Time (min)

552 0

ln (C/C0)

-1

(B) UV SS Chlorine UV/chlorine SS/chlorine

-2

-3

-4 0.0

553 554 555 556

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Time (min)

Figure 2. Degradation of (A) DEET and (B) caffeine under UV, simulated sunlight (SS), chlorine, UV/chlorine and SS/chlorine. Reactions employed 10 µM initial concentration of target compound and 7 mg⋅L-1 initial FC concentration in DI water matrix.

557



1

7 mg L-1 FC 0.1

7 mg L-1 FC+ 0.2 M NaCl 14 mg L-1 FC

0.0

0

-1

28 mg L FC Model prediction

-0.1

ln (C/C0)

ln (C/C0)

Page 26 of 29

-0.2

-1

-2

-0.3

-3

(A)

-0.4

-4

-0.5 0

50

100

561 562

150

200

250

300

350

0

20 40 60 80 100 120 140 160 180 200

Time (sec)

558 559 560

(B)

Time (sec)

Figure 3. Degradation of (A) DEET and (B) caffeine under UV irradiation with different concentrations of FC and NaCl. Dash lines represent model prediction results for conditions with different initial FC without NaCl. Reactions employed 10 µM initial concentration of target compound in 10 mM PBS at pH 7.

563 1.0

-1

k obs (min )

0.8

Removal at 90 sec (%)

UV/Chlorine SS/Chlorine

(A)

0.6 0.4 0.2 0.0

565 566 567 568

80

(B)

60 40 20

4

564

UV/Chlorine SS/Chlorine

100

5

6

7

8

9

10

4

5

6

pH

7

8

9

10

pH

Figure 4. (A) Degradation rates of DEET and (B) removal of caffeine under UV/chlorine and SS/chlorine conditions at pH 5–9. Reactions employed 10 µM initial concentration of target compound and 7 mg⋅L-1 initial FC concentration in 10 mM PBS. Error bars represent standard deviations from duplicates.

569


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120

(A)

PBS (pH 7.6) Saltwater (pH 7.6)

-1

kobs (min )

0.08

0.06

0.04

0.02


0.10

0.00

571 572 573 574

100

PBS (pH 7.6) Saltwater (pH 7.6)

80 60 40 20 0

UV/Chlorine

570

(B)

SS/Chlorine

UV/Chlorine

SS/Chlorine

Figure 5. (A) Degradation rates of DEET and (B) removal of caffeine under UV/chlorine and SS/chlorine conditions in phosphate buffer solution and synthetic saltwater. Reactions employed 10 µM target compound and 7 mg⋅L-1 FC initially. Error bars represent standard deviations from duplicates.

575 576


120 100 80 60 40 20 0 0

577 578 579 580

10

100

420

1000

NaCl concentration (mM)

Figure 6. Removal of caffeine at 60 sec under UV/FC conditions in PBS (pH 7.2) containing 0.85 mM NaBr and different amounts of NaCl. Reactions employed 10 µM caffeine and 7 mg⋅L1 FC initially. Error bars represent standard deviations from duplicates.



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581 582 583

584

Table 1. Simulated radical concentrations (in M) at different FC and NaCl concentrations at pH 7 under UV/FC conditions. FC (mg/L as Cl2)

Cl(M)

·OH

Cl·

Cl2·–

ClO·

ClOH·–

7

1.7×10-4

4.7×10-13

9.8×10-14

1.1×10-12

4.0×10-9

6.7×10-17

7

0.2

4.8×10-13

9.0×10-14

1.1×10-9

4.0×10-9

6.7×10-14

14

3.4×10-4

4.2×10-13

1.4×10-13

3.1×10-12

5.7×10-9

1.2×10-16

28

6.8×10-4

3.7×10-13

1.8×10-13

8.0×10-12

8.1×10-9

2.1×10-16

10 mM PBS water matrix, 3.86×10-6 Einstein·L-1·s-1 UV fluence rate, and 300 s simulation time.

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587


TOC Art:

588

589 590 591


Chlorine and Simulated

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