Photocatalytic degradation of 17α-ethinylestradiol (EE2) in the presence of TiO2-doped zeolite

Accepted Manuscript Title: Photocatalytic degradation of 17␣-ethinylestradiol (EE2) in the presence of TiO2 -doped zeolite Author: Zhong Pan Elizabeth A. Stemmler Hong Je Cho Wei Fan Lawrence A. LeBlanc Howard H. Patterson Aria Amirbahman PII: DOI: Reference:

S0304-3894(14)00497-X http://dx.doi.org/doi:10.1016/j.jhazmat.2014.06.040 HAZMAT 16049

To appear in:

Journal of Hazardous Materials

Received date: Revised date: Accepted date:

25-4-2014 10-6-2014 18-6-2014

Please cite this article as: Z. Pan, E.A. Stemmler, H.J. Cho, W. Fan, L.A. LeBlanc, H.H. Patterson, A. Amirbahman, Photocatalytic degradation of 17rmalpha-ethinylestradiol (EE2) in the presence of TiO2 -doped zeolite, Journal of Hazardous Materials (2014), http://dx.doi.org/10.1016/j.jhazmat.2014.06.040 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Photocatalytic degradation of 17α-ethinylestradiol (EE2) in the presence of TiO2-doped zeolite

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Zhong Pana, Elizabeth A. Stemmlerb, Hong Je Choc, Wei Fanc, Lawrence A. LeBlancd, Howard H. Pattersone, and Aria Amirbahmana,*

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Department of Civil and Environmental Engineering, University of Maine, Orono, ME 04469, USA b Department of Chemistry, Bowdoin College, Brunswick, ME 04011, USA c Department of Chemical Engineering, University of Massachusetts Amherst, Amherst, MA 01003, USA d School of Marine Sciences, University of Maine, Orono, ME 04469, USA e Department of Chemistry, University of Maine, Orono, ME 04469, USA

* corresponding author at E-mail: [email protected], Tel: +1-207-581-1277, Fax: +1-207581-3888

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Highlights

We developed a novel low-silica zeolite-TiO2 photocatalyst for EE2 degradation

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The catalyst showed a higher EE2 photodegradation efficiency than unsupported TiO2

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Oxidative degradation pathways initiated by hydroxyl radicals were predominant

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ABSTRACT

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Current design limitations and ineffective remediation techniques in wastewater treatment plants

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have lead to concerns about the prevalence of pharmaceutical and personal care products (PPCPs) in receiving waters. A novel photocatalyst, TiO2-doped low-silica X zeolite (TiO2-LSX), was

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used to study the degradation of the pharmaceutical compound, 17α-ethinylestradiol (EE2). The catalyst was synthesized and characterized using XRD, BET surface analysis, SEM-EDAX, and

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ICP-OES. The effects of different UV light intensities, initial EE2 concentrations, and catalyst

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dosages on the EE2 removal efficiency were studied. A higher EE2 removal efficiency was

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attained with UV-TiO2-LSX when compared with UV-TiO2 or UV alone. The EE2 degradation process followed pseudo-first-order kinetics. A comprehensive empirical model was developed to describe the EE2 degradation kinetics under different conditions using multiple linear regression analysis. The EE2 degradation mechanism was proposed based on molecular calculations, identification of photoproducts using HPLC-MS/MS, and reactive species quenching experiments; the results showed that oxidative degradation pathways initiated by hydroxyl radicals were predominant. This novel TiO2-doped zeolite system provides a promising application for the UV disinfection process in wastewater treatment plants. Keywords

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Photodegradation; 17α-ethinylestradiol; TiO2-doped zeolite; UV; hydroxyl radical.

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1. Introduction A large number of various pharmaceuticals and personal care products (PPCPs), consumed by both humans and animals, are not completely assimilated; instead, they are excreted

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unchanged or as metabolites, back into aquatic environment. The widespread occurrence of PPCPs in the environment has been extensively reported, leading to increased global awareness

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and concern [1-3]. One such substance, the synthetic estrogen 17α-ethinylestradiol (EE2; Figure

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1), an endocrine-disrupting chemical, is ranked in the top 100 of priority PPCPs [4]. EE2 is a primary component in contraceptive pills and postmenopausal hormonal supplements and is

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more recalcitrant than natural estrogen with respect to its removal during water treatment [5]. It is frequently detected in wastewater treatment plants (WWTPs) and is discharged into receiving

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waters due to incomplete removal during the treatment process [6]. Even in trace amounts (partsper-billion level), EE2 can induce the feminization of male fish and alter the reproductive

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potential of the fish population [7]. Given the potential for adverse effects of EE2 on ecosystems, human health and drinking water safety, and the inefficient EE2 removal from WWTPs, it is

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essential to explore effective techniques for EE2 treatment within WWTPs. Enhanced photolysis is a promising approach for EE2 removal from water [6, 8-10]. TiO2 has been extensively and successfully used as a photocatalyst due to its high photocatalytic activity, desirable properties and low cost. However, principal limitations of unsupported nano-sized TiO2 particles are their facile aggregation in suspension that leads to smaller effective surface area and lower catalytic efficiency, and the difficulties encountered in its effective separation via filtration [11, 12]. To circumvent these shortcomings, attempts to immobilize catalytic TiO2 nanoparticles on a support or carrier, including glass beads [13], fiberglass [14], silica [15], clay [16], and zeolites [17, 18] have been made.

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Zeolites are microporous, aluminosilicate minerals with unique, uniform and well-defined pore and channel size (3-8 Å). They possess a large specific surface area and high sorption capacity. These properties render zeolites ideal for supporting TiO2 nanocrystals for wastewater

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treatment [19]. Zeolite-supported TiO2 has previously been used as a photocatalyst in the degradation of a wide array of water-borne contaminants. Enhanced photocatalytic activity of

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zeolite-supported TiO2 relative to unsupported TiO2 in the degradation of various pollutants has

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been observed [19-22]. In this work, a TiO2-modified zeolite photocatalyst was developed by anchoring TiO2 onto low-silica X zeolite (LSX) by solid-state dispersion. We chose LSX as a

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support for TiO2, since this catalyst possesses high ion exchange and sorption capacities, and a relatively large pore size. The photoactivity of a TiO2-based photocatalyst supported on zeolite is

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higher for a lower Si:Al ratio zeolite [23, 24]; however, the photocatalytic activity of the LSX-

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TiO2 catalyst with respect to the degradation of PPCPs has not been well investigated. The

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respect to EE2 degradation.

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purpose of this paper is to investigate the photocatalytic activity of the TiO2-LSX catalyst with

2. Experimental methods  2.1. Synthesis of TiO2-LSX

LSX was synthesized using the method developed by Kühl [25] with some modifications. Initial molar composition used in the synthesis was SiO2: Al2O3: Na2O: K2O: H2O = 2.2: 1: 5.005: 1.495: 110.5. In a typical LSX synthesis, 1.924 g of NaAlO2 was dissolved in 16.758 g of distilled water with stirring, followed by additions of 2.266 g of NaOH and 1.678 g of KOH. After the solution cooled down to room temperature, 4.988 g of Na2SiO3 was added slowly. A viscous gel was formed after stirring for about 5 min. Subsequently, the gel was transferred into an autoclave for crystallization that was performed at 70 ˚C for 24 h. The obtained samples were 4 Page 5 of 35

washed with distilled water and dried at 100 oC for 12 h. TiO2-LSX was prepared by a solidstate dispersion method. At first, TiO2 (P25 Degussa), LSX and ethanol were thoroughly mixed in an agate mortar. Ethanol completely evaporated during mixing. Finally, the samples were

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dried at 110 ˚C overnight, followed by calcination at 450 ˚C for 6 h. TiO2 content used in the synthesis was ~7 wt.% (see Table S1 in SI for details). Reagents used in this study are listed in

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SI.

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2.2. Characterization of TiO2-LSX

Characterization of TiO2-LSX catalyst included X-ray diffraction (XRD), BET analysis,

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elemental analysis using the ICP-OES, and SEM-EDAX. Details of characterization methods are given in SI.

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2.3. Photodegradation experiments

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Photodegradation experiments were conducted in a Rayonet photochemical chamber reactor

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(Model RPR-100; Southern New England Ultraviolet Company, USA) equipped with RPR2537A˚ lamps emitting radiation in the UV-C range (~254 nm). The intensity of the UV lamp

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source was measured by the ferrioxalate actinometry method [26]. Potassium ferrioxalate was used as the chemical actinometer and the ferrozine method was used for determination of dissolved Fe(II) [27]; the method is described in detail in SI. All experiments were conducted in a 10-3 M carbonate buffer solution at pH = 8.1. A known quantity of catalyst was added to a 150 mL quartz vessel, following which 50 mL of EE2 solution was added. The suspensions were mixed using a magnetic stir bar throughout the experiments. At various time intervals, the quartz vessel was removed from the UV chamber, 3-5 mL of solution were passed through a 0.45 µm syringe filter, and the change in the fluorescence intensity of the filtrate was measured.

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2.4. Quenching experiments Quenching experiments were conducted using different quenchers to probe the role of various reactive oxygen species (·OH radicals, holes, O2·− radicals, and H2O2) on the EE2

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degradation pathway. Experiments in the absence of dissolved O2 were conducted by purging the

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TiO2-LSX suspensions with N2 for 1 h prior to UV irradiation. The suspensions were continuously purged with N2 throughout the photodegradation process. Isopropanol (IPA; 2mM)

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was used as a scavenger for both ·OH radicals and holes; 2 mM benzoquinone (BQ) and 500 mg

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L-1 catalase were used as scavengers for O2·− radicals and H2O2, respectively [28, 29]. 2.5. Synchronous-scan fluorescence spectroscopy (SSFS)

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EE2 degradation was monitored by SSFS. SSFS was used to permit the selective and sensitive production of EE2 signals without prior chromatographic separation. The instrument

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used was a Jobin Yvon Fluorolog-3 spectrofluorometer with emission and excitation

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monochromators, a 400W Xenon lamp source, and a photomultiplier tube. The SSFS technique

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entails scanning both the excitation and emission monochromators over the range of 250-400 nm while maintaining a constant wavelength difference of Δλ = 30 nm between the maximum emission and excitation for EE2. Prior to the SSFS analysis, calibration checks of the xenon lamp source and water Raman peak were performed. 2.6. HPLC-Chip/nanoESI-Q-TOF MS/MS analysis EE2 photodegradation products and pathways were analyzed using a 6530 High Performance Liquid Chromatographic-Chip Quadrupole Time-of-Flight Mass Spectrometer (HPLC-Chip/QTOFMS; Agilent Technologies, Santa Clara, CA). Details of the HPLC-MS/MS methods are provided in SI.

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2.7. Molecular calculations Molecular calculations were performed to facilitate the understanding of EE2 degradation mechanisms. The theoretical structure, electronic distribution, and molecular orbitals of EE2 in

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three different systems (EE2 alone, EE2-LSX, and EE2-TiO2) in the neutral, positive mono-

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cation, and negative mono-anion forms were determined by the Density Functional Theory (DFT) with hybrid B3LYP functional with Becke exchange [30] and Lee–Yang–Parr correlation [31]

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The 6-31+G (d,p) basis set was used in the calculations.

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using Gaussian 09. Atomic charges were calculated from Mulliken [32, 33] population analysis.

The zeolite model with the molecular formula of (HO)3Si-O-Al(OH)3 was chosen to

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represent LSX for the calculations [34]. The TiO2 model consisted of one TiO2 molecule. The relative stability and chemical reactivity of active sites of EE2 were examined by the calculation

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of DFT-based reactivity descriptors including global hardness and softness, and local descriptors

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such as Fukui function f(r). Global hardness is a tool to determine the resistance to change in the

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electron distribution or charge transfer in a molecule, and is equal to the gap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) using the frozen-core approximation [35-37].  Global softness is a global index associated with the polarizability in the molecule [35]. Fukui function, an important local DFT indicator on the basis of the frontier molecular orbital theory, is used for comparing reactive sites within the same molecule [35]. More details on the molecular calculations are given in SI.

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3. Results and discussion 3.1. X-ray diffraction (XRD) patterns

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Figure 2 shows the powder X-ray diffraction (XRD) patterns of LSX, TiO2-LSX, and pure TiO2. The XRD pattern of LSX sample corresponds well with FAU framework structure [38],

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indicating a highly crystalline faujasite zeolite is obtained in the synthesis (also see Table S1; Si/Al = 1.14). The characteristic diffraction peaks of TiO2 (P25) can be observed at 2θ = 25.4˚

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corresponding to (101) plane of Anatase in the XRD pattern of TiO2-LSX, indicating TiO2 has

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been impregnated within the sample. A small portion of Rutile was also observed in the TiO2 sample as the appearance of the peak at 2θ = 27.8˚ corresponding to (110) plane of Rutile, which

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is overlapped with the ones of LSX. The presence of a weak diffraction peak at 2θ = 28.1˚ reveals a small amount of Na−P zeolite in the sample [39].  The XRD pattern of TiO2-LSX shows

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the same diffraction peaks and relative intensity from the zeolitic phase as the LSX sample,

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3.2. BET measurements

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suggesting impregnation of TiO2 does not cause significant changes in the zeolite structure [39].

The measured surface area and pore volumes show relatively small differences between the LSX and TiO2-LSX catalysts (Table 1), suggesting that there is no TiO2 particle aggregation on the zeolite surface [12]. The micropore volume of TiO2-LSX decreases slightly as compared to that of zeolite alone, suggesting that the micropores are not blocked by TiO2 deposition [40].  3.3. SEM-EDAX

The surface morphology of the catalysts was investigated by SEM and EDAX. Figure S1A illustrates the morphology of LSX as uniform spherical crystals, while pure TiO2 (Figure S1B) is characteristic of heterogeneous nanoparticles. Analysis of the TiO2-LSX (Figure S1C and D) shows zeolite surface coverage by large heterogeneous TiO2 nanoparticles. This conclusion is 8 Page 9 of 35

further supported by EDAX spectra (Figure S2), where the spectrum for LSX shows characteristic zeolite composition peaks, while the spectrum for TiO2-LSX is dominated by signals from Ti. Elemental maps for Al, Si and Ti (data not shown) show regions of high Ti

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intensity where Si and Al signals are less intense, suggesting that these areas may be ascribed to regions where TiO2 has been deposited. Collectively, these images support the conclusion that

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TiO2 was well dispersed on the external surface of LSX, in agreement with our assumption based

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on the BET data. 3.4. Control experiments

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To quantify EE2 adsorption onto the TiO2-LSX catalyst or the reaction vessel, a 50-mL suspension of 10 ppm EE2 and 0.5 g L-1 TiO2-LSX was agitated in the dark. Sample analysis

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showed that after 60 min of mixing, ~10% removal of EE2 was observed, suggesting that EE2

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loss due to adsorption without exposure to UV light is relatively negligible (Figure S3). Figure

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S3 also shows additional control experiments, where the extent of EE2 photodegradation by direct UV photolysis and in the presence of the TiO2 catalyst were studied. For the UV alone

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experiment, a ∼60% removal of EE2 was achieved after 60 min. Under the same UV intensity, the presence of 0.5 g L-1 TiO2 resulted in a ∼90 % removal of 10 ppm EE2 after 60 min, and a 50% enhancement over the direct UV photolysis. The EE2 degradation kinetics with both UV alone and UV-TiO2 followed a pseudo first-order rate law. 3.5. Photocatalytic activity of TiO2-LSX Photodegradation kinetics of EE2 were studied in the presence of the TiO2-LSX catalyst by varying the catalyst concentration (Figure 3A), EE2 initial concentration (Figure 3B) and UV intensity (Figure 3C). The degradation rates under all conditions followed pseudo first-order kinetics, 9 Page 10 of 35

−

d[EE2] = kapp[EE2] dt

(1)

where, kapp is the apparent first-order rate constant (min-1). Comparing the photodegradation

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efficiencies, 0.1 g L-1 of the TiO2-LSX catalyst that contains only ~0.007 g L-1 of TiO2 brings about the same kapp as does the 0.5 g L-1 of the TiO2 only catalyst (Table 2). EE2 degradation rate

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increased with increasing TiO2-LSX concentration and UV intensity, reflected in the increasing

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values of kapp (Table 2). However, the EE2 degradation rate decreased with increasing EE2 initial concentration, corresponding to decreasing values of kapp (Table 2).

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3.6. Determination of quantum yield

The wavelength dependent quantum yield of a compound (EE2 in this case) is defined as the

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number of moles of the compound photodegraded divided by the number of moles of photons

Number of EE2 molecules destroyed Number of photons of light absorbed

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φ=

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absorbed by the compound (mol einstein-1) [41, 42].

(2)

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Under direct photolysis at λ = 254 nm, the EE2 degradation rate may be expressed as [42]:

d[EE2] = φ 254 I254 = 2.303 φ 254 I0, 254ε254 l[EE2] dt

(3)

where,  ε254  (M-1 cm-1) is the molar extinction coefficient of EE2 at 1330 M-1 cm-1, I0,254 (einstein min-1) is the initial light intensity at 1.26 × 10-5 einstein min-1 as determined by ferrioxalate actinometry, and l is the cell path length. From Eqs.1 and 3,

φ 254 =

k app

(4)

2.303 ε254 I0, 254 l 10

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The quantum yield of EE2 in aqueous solution in the presence of TiO2-LSX decreased with an increasing UV intensity (Table 2). This can be explained by the fact that a higher light intensity with a higher photon generation flux enhances photon absorption by EE2, thus reducing

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its quantum yield. Compared to the quantum yield of EE2 under UV direct photolysis and UVTiO2 (0.004 and 0.009 mol einstein-1 respectively; Table 2), the quantum yield of EE2 in the

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presence of the TiO2-LSX catalyst was significantly higher (e.g., 0.022 mol einstein-1 with UV

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intensity of 1.26×10-5 einstein min-1; Table 2). The observed decrease in kapp with an increase in the initial EE2 concentration (Table 2) may be attributed to the fact that for a given photon flux,

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photon availability per mole of substrate decreases as the substrate concentration increases.

effect [43, 44].

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3.7. EE2 photodegradation kinetic model

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Therefore, high EE2 concentrations inhibit photon absorption by EE2, producing an inner filter

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The overall EE2 photodegradation rate expression in the presence of TiO2-LSX consists of

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the UV direct photolysis rate, and the TiO2-LSX-catalyzed photolysis rate, as follows [6, 45]:

(

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d [EE2] a b = kapp[EE2] = kUV I 0m + k s I 0n [TiO 2 - LSX ]0 [EE2]0 [EE2] dt

(5)

where, kUV  is the rate constant for the direct UV photolysis, and ks is the photodegradation rate constant in the presence of the TiO2-LSX catalyst. The constants, a, b, m, and n are the reaction orders. EE2 adsorption to the catalyst was not included, since it was negligible (Figure S3). To obtain the [EE2]0 reaction order b (Eq.5), experiments were carried out at various initial EE2 concentrations in 5-30 ppm range in the presence of constant TiO2-LSX concentration 0.5 g L-1 with UV intensity of 1.26×10-5 einstein min-1. A linear regression plot of ln kapp  versus ln[EE2]0 was used to obtain the reaction order b = −0.76 (Figure S4A). Similarly, the slope and 11 Page 12 of 35

intercept of the linear regression plot of ln ka,UV versus ln I0  (Figure S4B) gave m = 0.90 and kUV = 2994, respectively. To determine the constants ks, n and a (Eq.5), apparent pseudo-first-order photodegradation rate constants with UV alone (ka,UV) and in the presence of the TiO2-LSX

−0.76

                                                                                                                                 

(6)

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= k app − k a ,UV = k s I 0n [TiO 2 - LSX

]0a [EE2 ]0−0.76

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k

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k a ,UV = kUV I 0m [EE2 ]0

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catalyst (k’app) were introduced as,

(7)

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From the experiments with different TiO2-LSX amounts and I0 = 1.26×10-5 einstein min-1, ln k’app versus ln[TiO2-LSX] were fit linearly, and a was determined to be 0.60 from the slope

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(Figure S4C). From the linear regression plot of ln k’app versus ln I0  (Figure S4D), ks  and n were

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determined to be 187 and 0.48, respectively.

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By substituting kUV, ks, and the reaction orders into Eq.5, the pseudo-first-order rate

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expression for the EE2 photocatalytic decomposition in the presence of TiO2-LSX catalyst is obtained:

−

(

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d [EE2] 0.60 −0.76 = 2994 × I 00.90 + 187 × I 00.48 [TiO 2 - LSX ]0 [EE2]0 [EE2] dt

(8)

Figures 3A-C show the agreement between the experimental observations and the kinetic model (Eq.8) for EE2 photodecomposition. The close agreement suggests that Eq.8 can be used to predict the degradation rate constant under various operational conditions such as UV light intensity, initial EE2 concentration and catalyst dose, providing a practical and useful tool in the course of system and process design.

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3.8. Molecular calculations Global and local descriptors, two principal elements in the determination of system stability and molecular reactivity, were applied to elucidate the EE2 photodegradation mechanism.

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HOMO and LUMO energies are two important quantum mechanical indices, determining the

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electronic band gap and the reactivity of a compound. According to the “Maximum Hardness Principle”, harder molecules usually possess greater HOMO-LUMO gaps, rendering them more

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stable and less reactive [36]. As such, global hardness and softness calculations are a useful means for comparison of EE2 reactivity in the presence of different catalysts. Our molecular

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calculations show that the HOMO-LUMO gap decreases in the order of EE2 alone ≈ EE2-LSX >

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EE2-TiO2, suggesting that the TiO2 catalyst reduces the global hardness and promotes the global softness compared to EE2 alone (Table S2).

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The local descriptor Fukui function calculations were employed to assign the most vulnerable

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sites of EE2 with respect to ·OH radicals attack, and to allow the determination of intermediate

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byproducts and EE2 degradation pathways. The magnitude of the Fukui function of an atom indicates its reactivity, with a larger Fukui function value indicating a higher reactivity of the atom [35, 46]. The Fukui function values for the EE2 molecule (Table S2) suggest that C2 followed by C10 (Figure 1) are the most susceptible sites to ·OH radical attack in the presence of the catalyst.

3.9. NanoESI-LC-MS/MS analysis of EE2 photodegradation pathways An EE2 standard sample and a set of photodegraded samples subjected to photodegradation in the presence of TiO2-LSX for 5 to 60 min were analyzed by LC-MS/MS in positive ion mode using a chip-based chromatographic/nanoESI platform, coupled with a high resolution Q-TOF mass analyzer. Extracted ion chromatograms for EE2 (Figure S5) showed signals for EE2 that 13 Page 14 of 35

decreased exponentially as a function of photolysis time. Total ionization chromatograms (TICs; Figure S6) showed the evolution of EE2-derived photoproducts, most of which eluted from the column earlier than EE2, indicating the production of products with higher polarity or smaller

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molecular size.

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Under the conditions used for analysis, many photodegradation products were not chromatographically resolved into individual compound peaks in the TICs. In order to more

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clearly determine the number of individual compounds detected in each chromatographic run, the compound chromatograms (ECCs) for products with masses between 250-400 Da (Figure 4)

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were extracted. The exact mass measured for each extracted compound was then used to assign a

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molecular formula. Furthermore, the collected abundances for all compound-associated ions can be used to provide semi-quantitative data based upon the ESI signal intensity for each extracted

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compound. The retention times, exact mass measurements, assigned elemental formulas, and

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relative compound intensities for approximately 70 observed photolysis products were

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summarized (data not shown).

The MS-level analysis provides clear evidence that photolysis with the TiO2-LSX results in significant EE2 degradation. The complexity of the mixture of over 70 photoproducts, where multiple isomeric products are observed, precludes a detailed product analysis in the context of this work; however, the structural features observed for a few products relevant to the mechanism proposed for EE2 degradation are summarized (Table S3). EE2 photocatalytic degradation may be initiated by ·OH radical attack or direct hole oxidation through mechanisms I and II (Scheme I). MS-level analysis of the photolysis data showed three products (Compounds 1, 2, and 3; Table S3) that appeared with m/z = 313.18, corresponding to the putative products P1 and P2 (Scheme I). The earliest eluting compound (Compound 1; Figure 4A) was detected with 14 Page 15 of 35

highest abundance after 5 min of photolysis, and decreased in an exponential fashion upon further photolysis (Figure 4B). The MS/MS spectrum of Compound 1 (Figure 4D) closely resembled that of EE2, with the former showing sequential losses of water and ethylene and an

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abundant, low mass peak at m/z = 123.05. This later peak, reflecting an addition of one oxygen atom relative to the peak detected in the spectrum of EE2, provides strong support for a structure

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where one additional oxygen atom has been added to the A-ring of EE2. Assuming addition at

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carbon-2 (C2; Figure 1), the compound was identified as P1 (Scheme I). The two remaining m/z = 313.18 isomers (Compounds 2 and 3) are more strongly retained by the HPLC column,

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suggesting that they are less polar than P1. Both isomers are detected with maximum intensity following 40 min of photolysis (Figure 3) and yielded MS/MS spectra that are quite similar to

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each other and distinct from that observed for Compound 1 (P1). While putative P1 was

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extensively fragmented upon CID with 20 eV (Figure 4D), the [M+H]+ precursor was still

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abundant for Compounds 2 and 3 (Figure 4E). Product ions in the MS/MS spectrum of Compound 3 showed peaks resulting from losses of H2O and CO, consistent with a hydroxyl-

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1,4-estradien-3-one structure. Other product ions result from losses of C4H4O with and without loss of H2O. The loss of C4H4O is a characteristic fragmentation observed when ethinyl/hydroxyl substitution is present at carbon-17 of the D-ring (Figure 1) [47, 48]. Collectively, the MS/MS spectra and chromatographic retention support the identification of these two isomers as epimeric forms of P2 (Scheme I). We considered the formation of isomers resulting from additions at secondary carbons of the B- and C-rings (Figure 1), but discounted these possibilities because facile water losses would be expected to dominate the MS/MS spectra for those structures. The exact mass measurements (Table S3) and MS/MS spectra (data not shown) for other compounds detected in this study provide strong support for the production of more highly

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oxidized products derived from EE2. As one measure of this trend, the molecular formulas of EE2-derived photoproducts were analyzed, and the level of compound oxidation increases with photolysis time was observed (Figure S7). The molecular formula analysis (data not shown )

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revealed that ten of the 70 detected compounds showed an increase in the number of carbons (21 or 22 carbons) relative to EE2 (20 carbons). Furthermore, MS/MS analysis showed that these

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eight products and an additional four compounds yielded MS/MS spectra showing an abundant

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peak resulting from loss of CH3OH or CH3OH and water. The loss of methanol, characteristic of methyl esters, provides evidence to suggest that methanol, added to solubilize EE2, was

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incorporated in some photolysis products.

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3.10. Photocatalysis mechanism

The overall photocatalytic degradation process of EE2 under UV-TiO2-LSX can be

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expressed as step 1 in Table 2. Step 2 (Table 3) of EE2 photocatalysis by the TiO2-LSX catalysts

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involves the photogeneration of the strongly reducing electron (ecb−) and strongly oxidizing hole

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(hvb+) pairs via the electronic excitation of TiO2. When UV light with energy equal or higher than the band-gap energy of the catalyst is introduced, valence band to conduction band transition initiates, promoting an electron from filled valence band to the vacant conduction band and leaving a hole alone in the valence band [49]. The separation of electron-hole pairs should be maintained to perform the photocatalytic function of TiO2. The primary steps 3-9 (Table 3) are the separation and trapping of electrons and holes to reducing sites and oxidizing sites on the surface of TiO2-LSX after the introduction of UV light, followed by generation of reactive species such as ·OH radicals, O2·− radicals, and H2O2. Zeolites possess a high oxygen-attraction capacity and inhibit back-electron transfer reactions [19]. Therefore, the electron-rich zeolite surface can serve as an electron sink and a hole 16 Page 17 of 35

scavenger, facilitating electron transfer from the TiO2 conduction band to the zeolite surface during the UV photolysis [19] that diminishes the extent of the electron-hole recombination. The electron acceptor, molecular oxygen in this case, captures the electrons from the excited TiO2 to

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produce O2·− radicals (step 4, Table 3). The O2·− radicals are labile and further react to generate ·

OH radicals, according to steps 5-7 and step 9 (Table 3). In addition, the presence of zeolites

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enhances the reaction of the holes at the TiO2-LSX surface with water, generating a higher yield

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of ·OH radicals and H2O2 (steps 3, 8-9, Table 3). Holes, as strongly oxidizing species, can also directly oxidize EE2 on the catalyst surface (step 11, Table 3). Most organic compounds can be

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decomposed to CO2 and H2O by the attack of radicals that possess a high oxidizing power [50].  These reactive species are strong enough to break C−C, C=C, C≡C, C=O, O−H bonds in EE2

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molecule and its intermediates to form CO2 and inorganic ions [21]. Zeolites may stabilize the

d

intermediate species formed from oxygen and water as the result of the reaction with

te

photogenerated electrons and holes on the TiO2-LSX surface [51].   Our HPLC-MS/MS analysis suggests that ·OH radicals are the main reactive oxidants in the

Ac ce p

EE2 photodegradation in the presence of UV-TiO2-LSX due to the prevalence of the hydroxylated intermediates (Scheme 1). Figure 5 shows that the oxidative pathway by ·OH radicals appears to be the dominant step due to significant quenching by IPA. When IPA donates an electron to a ·OH radical, the resultant alkoxy radical may release a second electron to the TiO2 conduction band. Therefore, as a known current-doubling effect trigger, IPA scavenges ·OH radicals to suppress the reaction, and at the same time, it may supply electrons to the conduction band to produce more O2·− radicals to accelerate the O2·− radical-based oxidation reaction [28]. If the O2·− radicals-based oxidation mechanism dominated in the EE2 degradation, the currentdoubling phenomenon by IPA would enhance photodegradation. However, as shown in Figure 5,

17 Page 18 of 35

O2·− radical-based oxidation has a limited role in the EE2 degradation, since BQ has a significantly smaller inhibitory effect on EE2 photodegradation rate as opposed to other quenchers.

ip t

Figure 5 shows that the quenching effect of catalase is as pronounced as that of the IPA,

cr

suggesting that H2O2 is a significant contributor to EE2 photodegradation. Figure 5 also shows that when dissolved O2 is excluded from the suspension, the EE2 photodegradation rate

us

decreases drastically. The absence of O2 inhibits the generation of H2O2 and the resulting yield of ·

OH radicals (steps 4-6 and 9, Table 3). The suppression of photodegradation rate in the absence

M

(nanoseconds) electron-hole recombination [52].

an

of dissolved O2 may also result from the inhibition of ·OH radicals production by fast

4. Conclusion

d

In this work, a novel photocatalyst, TiO2-LSX, was successfully synthesized to study its

te

efficacy with respect to EE2 degradation. The TiO2-LSX system exhibited an enhancement

Ac ce p

factor of 5.6 over the UV alone and 2.3 over TiO2 alone. In addition, quantum yield calculations demonstrate that photodegradation of EE2 in the presence of TiO2-LSX is more efficient than UV photolysis and UV-TiO2 oxidations. The results in this study suggest that the UV-TiO2-LSX system can be effectively used for the remediation of water contaminated with EE2. This novel technology can potentially be applied to UV disinfection treatment process in water and wastewater treatment plants, and can fill in a current gap of insufficient treatment of PPCPs in these plants.

18 Page 19 of 35

Supporting Information Solution preparation, actinometry experiment, characterization techniques, details of HPLCMS/MS and molecular calculation methods, LC-MS/MS results discussion and supporting tables

cr

ip t

and figures referenced in this study.

Acknowledgements

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This work was supported by the USGS-WRRI grant program (G11AP20083) and the National Science Foundation CHE-1126657 (E.A.S.). Zhong Pan acknowledges China Scholarship

an

Council for its financial support. Scott Collins assisted in generating the SEM-EDAX data, and

M

Brenda Hall and Gordon Bromley assisted in the HF dissolution of the zeolite. Molecular

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calculations were performed at the University of Maine Supercomputer Center.

19 Page 20 of 35

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[37] C. Lee, W.T. Yang, R.G. Parr, Local softness and chemical reactivity in the molecules CO, SCN− and H2CO J. Mol. Struc-Theochem., 163 (1988) 305-313. [38] M.M. Treacy, J.B. Higgins, Collection of simulated XRD powder patterns for zeolites, Access Online via Elsevier, 2001. [39] M. Ogura, Y. Kawazu, H. Takahashi, T. Okubo, Aluminosilicate species in the hydrogel phase formed during the aging process for the crystallization of FAU zeolite, Chem. Mater., 15 (2003) 2661-2667. [40] A.H. Alwash, A.Z. Abdullah, N. Ismail, Zeolite Y encapsulated with Fe-TiO2 for ultrasound-assisted degradation of amaranth dye in water, J. Hazard. Mater., 233-234 (2012) 184-193. [41] R.R. Chowdhury, P.A. Charpentier, M.B. Ray, Photodegradation of 17β-estradiol in aquatic solution under solar irradiation: kinetics and influencing water parameters, J. Photoch. Photobio. A. , 219 (2011) 67-75. [42] A. Leifer, The kinetics of environmental aquatic photochemistry: Theory and practice, ACS professional reference book (USA), 1988. [43] N. Modirshahla, M.A. Behnajady, Photooxidative degradation of Malachite Green (MG) by UV/H2O2: influence of operational parameters and kinetic modeling, Dyes Pigments, 70 (2006) 54-59. [44] N. Daneshvar, M.A. Behnajady, M.K.A. Mohammadi, M.S.S. Dorraji, UV/H2O2 treatment of Rhodamine B in aqueous solution: influence of operational parameters and kinetic modeling, Desalination, 230 (2008) 16-26. [45] M.A. Behnajady, N. Modirshahla, H. Fathi, Kinetics of decolorization of an azo dye in UV alone and UV/H2O2 processes, J. Hazard. Mater., 136 (2006) 816-821. [46] A.M. Vos, K.H. Nulens, F. De Proft, R.A. Schoonheydt, P. Geerlings, Reactivity descriptors and rate constants for electrophilic aromatic substitution: acid zeolite catalyzed methylation of benzene and toluene, J. Phys. Chem. B., 106 (2002) 2026-2034. [47] O.J. Pozo, P. Van Eenoo, K. Deventer, S. Grimalt, J.V. Sancho, F. Hernández, F.T. Delbeke, Collision-induced dissociation of 3-keto anabolic steroids and related compounds after electrospray ionization. Considerations for structural elucidation, Rapid Commun. Mass Sp., 22 (2008) 4009-4024. [48] M. Thevis, U. Bommerich, G. Opfermann, W. Schänzer, Characterization of chemically modified steroids for doping control purposes by electrospray ionization tandem mass spectrometry, J. Mass Spectrom., 40 (2005) 494-502. [49] W. Stumm, J.J. Morgan, Aquatic chemistry: chemical equilibria and rates in natural waters, John Wiley & Sons, 2012. [50] S. Fukahori, H. Ichiura, T. Kitaoka, H. Tanaka, Capturing of bisphenol A photodecomposition intermediates by composite TiO2–zeolite sheets, Appl. Catal. B-Environ., 46 (2003) 453-462. [51] H. Yahiro, T. Miyamoto, N. Watanabe, H. Yamaura, Photocatalytic partial oxidation of αmethylstyrene over TiO2 supported on zeolites, Catal. Today. , 120 (2007) 158-162. [52] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem.Rev., 95 (1995) 69-96.

22 Page 23 of 35

Table 1. BET analysis of the catalysts. Sample

BET surface areaa (m2 g-1)

Micropore volumeb (cm3 g-1)

Total pore volumec (cm3 g-1) 0.38 0.33 -

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cr

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TiO2-LSX 555.7 0.21 LSX 560.3 0.23 d TiO2 35-65 a BET surface area is calculated in the range of relative pressure from 0.1 to 0.25. b Micropore volume is calculated from t-plot method. c Total pore volume is calculated at a relative pressure of 0.95. d From the supplier.

23 Page 24 of 35

Table 2. Kinetic parameters for EE2 photodegradation. Variable

kapp (min-1)

Values

UV direct photolysisa UV-TiO2b TiO2-LSX concentration (g L-1)a

φ (mol einstein-1)

R2

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d

M

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cr

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0.018 0.004 0.991 0.043 0.009 0.952 0.01 0.025 0.005 0.988 0.05 0.040 0.009 0.981 0.1 0.045 0.016 0.975 0.2 0.053 0.019 0.981 0.5 0.100 0.022 0.987 Initial EE2 concentration 1 0.149 0.032 0.948 (mg L-1)c 2 0.141 0.030 0.963 5 0.143 0.031 0.989 10 0.100 0.022 0.987 20 0.053 0.019 0.957 30 0.038 0.012 0.973 -5 UV intensity 0.100 0.022 0.987 1.26×10 (einstein min-1)d 2.56×10-5 0.147 0.016 0.999 4.05×10-5 0.171 0.011 0.999 -5 5.32×10 0.203 0.010 0.994 a -5 -1 -1 UV intensity = 1.26×10 einstein min , [EE2]0 = 10 mg L , pH = 8.1. b UV intensity = 1.26×10-5 einstein min-1, [EE2]0 = 10 mg L-1, pH = 8.1, [TiO2] = 0.5 g L-1. c UV intensity = 1.26×10-5 einstein min-1, pH = 8.1, [TiO2-LSX] = 0.5 g L-1. d [EE2]0 = 10 mg L-1; [TiO2-LSX] = 0.5 g L-1; pH = 8.1.

24 Page 25 of 35

Table 3. Photocatalysis mechanism. Step 1

Process hv, TiO2 -LSX

2

e-cb + O2 →O2⋅−

us

cr

O 2 ⋅− + H 2 O → OH − + HO 2 ⋅ O 2 ⋅− + HO 2 ⋅ + H + → H 2 O 2 + O 2 O 2 + 2e -cb + H + → H 2 O 2 2h +vb + 2H 2 O → 2H 2 O 2 + 2H + H 2 O 2 + e -cb → OH − + ⋅ OH ⋅

OH + EE2 → photocatalysis intermediates → CO2 + H 2 O h +vb + EE2 → photocatalysis intermediates → CO 2 + H 2 O

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M

11

h +vb + H 2 O → ⋅OH + H +

an

3 4 5 6 7 8 9 10

ip t

EE2+ O2 ⎯ ⎯ ⎯ ⎯⎯→CO2 + H2O ∗ 3.2ev TiO 2 + hv ⎯hv ⎯≥ ⎯ ⎯→ TiO2 (e -cb + h +vb )

25 Page 26 of 35

Figure captions Figure 1. Chemical structure of 17α-ethinylestradiol. Chemical formula: C20H24O2, and exact mass = 296.178.

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Figure 2. XRD patterns of LSX, TiO2-LSX, TiO2 Figure 3. Time-concentration plots for EE2 degradation in the UV-TiO2-LSX system. (A) I 0 = 1.26×10-5 einstein min-1, [EE2]0 = 10 ppm, [TiO2-LSX] = 0.01-0.5 g L-1; (B) I 0 = 1.26×10-5

us

cr

einstein min-1, [TiO2-LSX] = 0.5 g L-1. [EE2]0 = 5-30 ppm; (C) I 0 = 1.26×10-5 ~ 5.32×10-5 einstein min-1, [TiO2-LSX] = 0.5 g L-1, [EE2]0 = 10 ppm. The symbols are experimental data and the lines are model fits based on the rate expression in Eq.8.

d

M

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Figure 4. (A) Extracted ion chromatogram for the m/z 313.18, [M+H]+ ions from C20H24O3 compounds present in EE2 samples photodegraded for 5 min in the presence of TiO2-LSX. Numbers identify compounds with exact mass measurements that are reported in Table S3; (B) Intensity for Compound 37, measured as a function of photodegradation time; (C) Intensity for Compound 54, measured as a function of photodegradation time; (D) MS/MS spectrum for the m/z 313.18 ion from Compound 37 (putative P1); (E) MS/MS spectrum for the m/z 313.18 ion from Compound 54 (one putative epimer of P2); MS/MS spectra measured using a collision energy of 20 eV with N2 as the collision gas.

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Figure 5. Quenching effects with different quenchers. I 0 = 1.26×10-5 einstein min-1, [EE2]0 = 10 ppm, [TiO2-LSX] = 0.5 g L-1, pH = 8.1, [IPA] = 2 mM, [BQ] = 2 mM, [catalase] = 500 mg L-1.

26 Page 27 of 35

ip t cr us an M

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te

d

Figure 1; Pan et al., 2014

27 Page 28 of 35

•Anatase

TiO2 LSX

o Rutile

ip t cr

• o

an

•

us

Intensity (a.u.)

TiO -LSX 2

o

10

15

20

25

M

5

30

35

40

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Figure 2; Pan et al., 2014

d

2θ

28 Page 29 of 35

-1 0.01 g L 0.02 0.1 0.2 0.5

A 10

ip t

6

cr

4

2

0 0

10

20

30

40

Time (min)

B

60

an M

25

EE2 concentration (ppm)

50

5 ppm 10 20 30

30

20

d

15

te

10

5

Ac ce p

0

0

5

10

15

20

Time (min)

C

1.26×10-5 einstein min-1 2.52×10-5

10

4.05×10-5 5.32×10-5

8

EE2 concentration (ppm)

us

 

EE2 concentration (ppm)

8

6

4

2

0 0

5

10

15

20

Time (min)

29 Page 30 of 35

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Figure 3; Pan et al., 2014

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ip t cr us an M d te Ac ce p

Figure 4; Pan et al., 2014

31 Page 32 of 35

ip t

0.10

cr

0.06

us

kapp/min-1

0.08

an

0.04

0.00

BQ

Catalase

No quencher

te

Ac ce p

Figure 5; Pan et al., 2014

IPA

d

Nitrogen purge

M

0.02

32 Page 33 of 35

1 2

OH

3

HO

10

A 4

C 9

B 5

8

17 13

OH

D

16

14 15

ip t

12 11

h+vb

7 6

EE2

cr

OH

HO H

O

HO

us

OH

OH

-H OH

OH

OH

an

HO P1

HO

O

C20 H24O3 Exact Mass: 312.173

M

C20 H24O3 Exact Mass: 312.173 Mechanism I

P2

Mechanism II

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Scheme 1. Proposed mechanisms for EE2 photodegradation

33 Page 34 of 35

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Graphical Abstract (for review)

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