Synergistic degradation performance and mechanism of 17β-estradiol by dielectric barrier discharge non-thermal plasma combined with Pt–TiO2

Accepted Manuscript Synergistic degradation performance and mechanism of 17β-estradiol by dielectric barrier discharge non-thermal plasma combined with Pt-TiO2 Ying Chen, Lei Sun, Zebin Yu, Li Wang, Guoliang Xiang, Shungang Wan PII: DOI: Reference:

S1383-5866(15)30128-3 http://dx.doi.org/10.1016/j.seppur.2015.07.061 SEPPUR 12480

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Separation and Purification Technology

Received Date: Revised Date: Accepted Date:

15 May 2015 23 July 2015 25 July 2015

Please cite this article as: Y. Chen, L. Sun, Z. Yu, L. Wang, G. Xiang, S. Wan, Synergistic degradation performance and mechanism of 17β-estradiol by dielectric barrier discharge non-thermal plasma combined with Pt-TiO2, Separation and Purification Technology (2015), doi: http://dx.doi.org/10.1016/j.seppur.2015.07.061

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Synergistic degradation performance and mechanism of 17β-estradiol by dielectric barrier discharge non-thermal plasma combined with Pt-TiO2 Ying Chena, Lei Sun a, Zebin Yu a, *, Li Wanga, Guoliang Xianga, Shungang Wanb a

School of environment, Guangxi University, Nanning 530004, P. R. China

b

Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, P.R. China

Abstract A synergistic system of dielectric barrier discharge non-thermal plasma (DBD) combined with Pt-TiO2 was developed to investigate the degradation performance of 17βestradiol (E2) by making full utilization of the UV light formed from the discharge zone. The Pt-TiO2 was prepared by impregnation-reduction method. The results of EDS analysis indicated that Pt was load on TiO2, the atomic percentage of Pt and amount loaded were 0.27% and 0.91 at%, respectively. The removal efficiency and degradation yields were obviously increased from 72.0% to 98.9% and 329.2×10−6 to 468.1×10-6 g/kW•h by addition Pt-TiO2 to DBD system as the peak voltage, E2 concentration, pH value, and photocatalyst additive amount were 12 kV, 400 μg/L, 5.6, and 50 mg/L, respectively. Results demonstrated that the UV light formed could be utilized to strength the degradation performance of E2. The results also elucidated that the degradation process followed the first-order kinetic model, and the experimental parameters had significant effect on the first-order kinetic constants and removal efficiency of E2. Furthermore, six intermediate products were identified and their proposed structures were provided. The degradation mechanisms revealed that the active species of hydroxyl radicals and ozone had a key function in the degradation of E2 in the synergistic system. *

Corresponding author. Tel./Fax: + 86–0771–3270672 E-mail address: [email protected] (Z. Yu)

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Keywords: 17β-Estradiol, Dielectric barrier discharge, Pt-TiO2, Photoelectric synergism, Degradation mechanisms. 1. Introduction Endocrine-disrupting compounds (EDCs) are chemicals that have negative effects on the endocrine systems of humans and wildlife. Among the EDCs, 17β-estradiol (E2), which mainly pertains to the principal, natural, and most potent estrogen, is ubiquitous in aquatic environments. E2 may enter the aquatic environment from contraceptive pill residues, hormone replacement therapy residues, and human excretion [1]. E2 has been experimentally reported to exhibit a degree of estrogenic activity for human estrogen receptors at 41 ng/L, and can even cause vitellogenin production in male fish at 1.0 ng/L [2]. Even worse, E2 can be found in drinking water treatment plant influents [3] because existing conventional water treatment plants are incapable of removing these emerging contaminants [4]. Therefore, a sustainable, effective, and economical technical process to remove E2 from aqueous solutions is necessary. Advanced oxidation processes (AOPs) are effective methods for E2 removal, which have been employed to treat E2 in aqueous solutions. AOPs include photo-Fenton [2], ozonation [1, 5], photocatalysis [6, 7], and dielectric barrier discharge non-thermal plasma (DBD) [8]. In our previous study, 100% removal efficiency (RE) was achieved at an E2 concentration of 100 µg/L with a peak voltage 12 kV and pH 5.6 after 30 min of plasma treatment [8]. In addition, some researchers investigated the intensity and emission spectra of ultraviolet (UV) light formed during the pulsed discharge process at various experimental conditions in detail. Huang et al. [9] studying contribution of UV light to the decomposition of toluene in DBD/photocatalysis system, observed that UV light at the wavelength of 315, 337, 365, 391 nm emitted from plasma. Magureanu et al. [10] observed that the intensity and emission spectrum of UV light formed at 20 kV were less than 4000 a.u. and 400 nm, respectively.

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Feng et al. [11] also demonstrated that the strong emission spectra in the discharge zone were observed at 337.13, 357.69, and 380.49 nm, and the intensity of UV light was less than 2500 a.u.. As mentioned above, the UV light with wavelength less than 400 nm could be generated in a discharge system, which can be used as a lamp-house to induce the photocatalysis of TiO2 [12]. However, the UV light generated from the discharge zone is not fully utilized for the degradation of pollutants. Thus, the RE of E2 can be further enhanced by adding a photocatalyst in the DBD system. A synergistic system of pulsed discharge plasma coupled with TiO 2 photocatalyst has been developed to enhance the RE and degradation yield of organic compounds in water [1317]. When the TiO2 photocatalyst is introduced into the pulsed discharge plasma system, the UV light generated during the pulse discharge process facilitates the dissipation of electrons and holes to generate electrons and positive hole (e-/h+) pairs in TiO2 [18]. The e-/h+ pairs can initiate a series of chemical reactions that mineralize pollutants [19]. However, one of the most important factors that hampers the photocatalytic reaction process is the recombination of e- and h+ [20]. Thus, the deposition of metals onto TiO2 has been widely used to solve the electron–hole recombination problem [21]. TiO2 doped with noble metal Pt has been intensively investigated to increase the photoactivity of TiO2. Zhao et al. [22] showed that the metal-modified TiO2 may increase the number of electron traps on the catalyst surface and attract more pollutants. The proposed mechanism included facilitating e -/h+ separation and promoting interfacial electron transfer [23-25]. Moreover, Fan et al. [26] reported that the PtTiO2 photocatalytic activity in an acidic medium is higher than that in a basic medium. Our previous study showed that the solution pH quickly decreased to an acidic value for single DBD [8], which was beneficial in increasing the photocatalytic performance of Pt-TiO2. However, only a number of studies have reported in detail the pulsed discharge plasma system combined with the Pt-TiO2 photocatalyst for degradation of E2.

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In this study, a synergistic system of DBD combined with a photocatalyst was developed to decompose E2. This paper aims to investigate the enhancement of RE and degradation yield of E2 through addition of Pt-TiO2 to fully utilize the UV light generated from the discharge zone. The effects of the key experimental parameters, such as peak voltage, initial E2 concentration, initial pH value, and Pt-TiO2 additive amount on the RE of E2 and the kinetics rate constant are demonstrated in detail. Furthermore, possible degradation mechanisms of E2 in the synergistic system were tentatively proposed. 2. Materials and methods 2.1 Materials E2 (purity≥96%) was obtained from Dr. Ehrenstorfer and used without further purification. Stock solutions of E2 were prepared at 40 mg/L in acetonitrile. TiO2 (80% anatase, 20% rutile, 50 m2/g) was supplied by Degussa. Methanol, acetonitrile and n-hexan (HPLC grade) were purchased from Sigma-Aldrich. All other chemicals were of chemical grade and obtained from Merck. 2.2 Preparation of Pt-TiO2 The Pt-TiO2 photocatalysts were prepared by impregnation-reduction method described in a previous study [27]. This method was modified in the present study. Commercial TiO 2 was first kept at 673 K under air for 1 h. After cooling to room temperature, TiO2 was suspended in water at a concentration of 12.5 g/L and sonicated for 20 min. An H2PtCl6 solution was then added dropwise, and the obtained suspension was further stirred for 30 min. Afterward, a KBH4 solution at l g/L was added dropwise with constant stirring, and the resulting mixture was allowed to react for 10 min. The product was filtered, washed three times with H2O, and then dried at 105 °C. The product obtained was labeled Pt-TiO2. The theoretical nominal amount of the Pt loaded on TiO2 was 1.0 at.%. 2.3 Experimental procedures

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The DBD discharge reactor used for the present study was described in our previous study [8]. A series of experiments were firstly conducted to investigate the difference of degradation performance of E2 for single DBD system, DBD system with added TiO2 and PtTiO2, respectively. In each experiment, the volume of the aqueous solution, initial pH, E2 initial concentration, photocatalytic additive amount, and peak voltage were fixed at 500 mL, 5.6, 400 μg/L, 50 mg/L, and 12 kV for 30 min unless otherwise specified, respectively. All experiments were performed firstly in the dark for 30 min so E2 can achieve adsorption equilibrium on the Pt-TiO2 surface, then further treatment by plasma. Meanwhile, in order to avoid the agglomeration of photocatalyst, magnetic stirrer was employed to continuously stir the solution throughout the experimental process. Furthermore, the effect of experimental parameters on the removal efficiency of E2 and kinetic constants was further investigated in detail by varying one of parameters while others parameters fixed at constant in DBD system with added Pt-TiO2. The specific experimental parameters were listed in Table 1. In addition, the degradation mechanism of E2 was investigated by determining the intermediates formed in the synergistic system when the E2 concentration, Pt-TiO2 additive amount, and peak voltage were 5 mg/L, 50 mg/L, and 12 kV, respectively. The temperature of the solution was slowly increased from room temperature to approximately 40 °C during the discharge treatment that lasted for 30 min without cooling the reactor during the DBD treatment process. The gas component above the surface of the aqueous solution in the reaction tank was ambient air, and it was not forced to flow. 2.4 Analytical methods The morphologies of TiO2 and Pt-TiO2 were analyzed by field emission scanning electron microscopy (FE-SEM, S-5500, Hitachi), whereas the element compositions of the catalyst surfaces were analyzed by energy dispersive spectroscopy (EDS). The crystal composition of the powders was analyzed by X-ray diffraction (XRD, D/MAX-2500 V,

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Rigaku Co., Japan) at 40 kV voltage and 100 mA current. Data were collected at a scan rate of 2 deg/min. The light adsorption performance was determined by UV-vis diffuse reflectance spectroscopy (DRS, JASCOISV-722, JASCO Co., Japan). The E2 and intermediates produced were first filtered through glass filters, and the solution was then subjected to extraction by solid-phase extraction (SPE). The SPE procedure used for extraction was described in our previous study [8]. The extract concentration of E2 was determined using HPLC (Agilent 1260 Series) equipped with an Agilent Eclipse XDBC18 column (250 mm× 4.6 mm) at room temperature. The wavelength of the UV detector was set at 280 nm, and a well-defined peak was observed at a retention time of 6.8 min for E2. The mobile phase was 42% acetonitrile and 58% water, and the flow rate was 1.0 mL/min. The intermediate products of E2 were determined by UPLC-QTOF-MS (Xevo G2-S, Waters Corporation, Milford, MA, USA) equipped with a Waters Acquity BEH C18 column (100 mm × 2.1 mm, 1.7 μm) at 40 °C. The mobile phase of eluent A (water) and eluent B (acetonitrile) was allowed to flow at a rate of 0.4 mL min−1. The B linear solvent gradient was as follows: 0–4 min, 20%–30%; 4–10 min, 30%–55%; 10–17 min, 55%–85%; 17–17.5 min, 85%–20%. The injection volume was 1 μL. The mass spectrometry conditions were as follows: MS detection was performed using a XEVO G2-S QTOF MS System, and the ionization mode was negative electrospray (ESI−). The source temperature was set at 100 °C, the desolvation temperature was kept at 400 °C, and the desolvation gas flow was 400 L/h. The RE and degradation yield of E2 were calculated according to Eqs . (1) and (2) [8, 28]:

 (%) 

Y=

C0  Ct 100 C0

C0  V  10-6 100  P  t

(1)

(2)

where η (%) and Y (g/kW•h) are the RE of E2 and the degradation yield, respectively. C0 6

and Ct (µg/L) are the initial concentration and residual concentration at different discharge time t (h), respectively. V (L) is the volume of the aqueous solution in the reaction tank, and P (kW) is the average power dissipated in the discharge. Investigating the degradation rate of E2 is important. The plasma degradation of E2 generally follows the first-order kinetics model as described in Eq. (3) [29].

Ln

C0  kt Ct

(3)

where k is the first-order rate constant (min−1). The values of k and the half-life degradation time (t1/2=0.693/k) were calculated based on the fittings of the first-order model. 3. Results and discussion 3.1 Characterization of TiO2 and Pt-TiO2 photocatalysts As shown in Fig. 1a, the sizes of pure TiO2 particles are uniform. After TiO2 was doped with metallic Pt, Pt-TiO2 still had high uniformity in terms of size (Fig. 1b). This result suggests that the deposition of Pt particles does not damage the TiO2 structure. Moreover, the analysis of EDS spectrum (Fig. 1c) of Pt-TiO2 samples showed that the peaks can be only assigned to Ti, O, and Pt, which also demonstrated that Pt was effectively loaded on TiO2. The composition analysis result confirmed that the atomic percentage of Pt was 0.27%, and the nominal amount of Pt loaded on TiO2 was 0.91 at%, which is close to the theoretical values (1.0 at.%). The formation of TiO2 and Pt-TiO2 was confirmed by XRD, as shown in Fig. 2. The diffraction patterns of TiO2 and Pt-TiO2 are almost consistent (Fig.2a). This result suggests that Pt particles do not change the crystalline structure of TiO 2. However, no characteristic diffraction peak is observed for Pt based on the XRD results. The reflectance spectra of TiO2 and Pt-TiO2 are illustrated in Fig. 2b, and the spectrum of TiO2 consists of a single absorption peak below 400 nm. Compared with that of pure TiO2, the absorption of PtTiO2 was strengthened by doping Pt on the TiO2 surface in the UV zone. 3.2 Comparison of the degradation performance of E2 7

The RE values of E2 for TiO2 and Pt-TiO2 with and without pulse discharge are shown in Fig. 3. The results show that the maximum RE was 4.5% after 30 min of adsorption for TiO2 without pulse discharge. Moreover, the contribution of adsorption onto TiO2 is negligible. The contribution of adsorption onto Pt-TiO2 followed the same trend as that of TiO2. Fig. 3 further shows that the RE of E2 can be enhanced by adding TiO2 into the pulsed discharge system, unlike in single DBD; the RE increases from 72.0% to 95.0%. The degradation process can be described by the first-order kinetic equation. The first-order kinetic rate constant k gradually increases from 0.0426 to 0.1001 min-1, and the half-life decreases from 16.3 to 6.9 min (Table 2). In addition, the E2 degradation yield increases from 329.2×10 −6 g/kW•h to 439.4×10−6 g/kW•h. The DBD process can effectively degrade E2, which has been discussed in our previous study [8]. The degradation of E2 is caused by the production various active species, such as O3 and •OH, because of the electron impact dissociation of oxygen and water molecules, followed by subsequent chemical reactions during the plasma discharge process in humid air [30]. These active species can be dissolved in the solution and further decompose the pollutants by oxidation processes [31]. Unlike in the single DBD process, in the DBD process, photocatalysis is induced by UV light generated in the plasma discharge zone when TiO2 particles are present in the solution [13, 18]. The reaction occurred close to the water surface, and strong oxidizing compounds, such as holes (h+) and • OH, were produced. Therefore, TiO2 in the discharge system can induce photocatalysis, which enhances the RE and degradation yield of E2. Pt-TiO2 was added into the solution to further boost the degradation performance of E2 in the DBD process. As shown in Fig. 3, Pt-TiO2 shows better photocatalytic performance than TiO2 in the DBD process, and the RE and degradation yield increases from 95.0% to 98.9% and from 439.4×10−6 to 468.1×10−6 g/kW•h, respectively. The first-order kinetic rate constant k gradually increases from 0.1001 min−1 to 0.1446 min−1, and the half-life decreases

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from 6.9 min to 4.8 min. These increases can be attributed to noble metal nanoparticles such as Pt, which is one of the most effective electron traps, because a Schottky barrier is formed at the metal-semiconductor contact. Pt-doped TiO2 enhances the charge separation in the semiconductor. Thus, the energy gap increases and the chances of electron-hole pair recombination are decreased. The surface deposition of small Pt clusters on TiO 2 contributed to the acceleration of O2•− formation because the charge separation increases and the chances of electron-hole pair recombination are decreased. The varied functions of Pt clusters (Pt n) are shown in Eqs. (4)-(5) [32]. Ptn+e-→Ptn-

(4)

Ptn-+O2→O2•-+ Ptn

(5)

Depositing noble metallic Pt nanoparticles on the TiO2 surface increases the quantum yields during photodestruction reaction. Thus, 98.9% of E2 was degraded with a significantly higher degradation yield (468.1×10−6 g/kW•h) in this study, thereby indicating that the DBD process with Pt-TiO2 is an effective and energy-efficient approach for E2 degradation. 3.3 Effect of peak voltage Figure 4 shows the effect of peak voltage on the RE of E2. As shown in Fig. 4, RE increased with increasing peak voltage. The RE is 72.0% at a peak voltage of 10 kV after 30 min of treatment, and increases to 83.9% and 99.3% at peak voltages of 11 and 12 kV, respectively. The first-order kinetic rate constant and half-life are 0.0432, 0.0605, and 0.1446 min−1 and 16.0, 11.5, and 4.8 min at peak voltages of 10, 11 and 12 kV, respectively (Table 3). The peak voltage directly affects the production of activated species and UV [33]. Thus, the intensity of discharge electric field increases with increasing applied peak voltage, and more active species (O3, •O, •H, •OH) are generated. In addition, a stronger UV radiation was also generated at a higher peak voltage. This radiation can promote the formation of photogenerated electrons and holes in Pt-TiO2. The photogenerated electrons and holes can

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directly or indirectly degrade E2. Thus, E2 degradation is promoted. Furthermore, the electrons and holes promote the generation of reactive oxygen species, such as •OH and O2•-, on the catalyst when the Pt-TiO2 surface is irradiated. The generation of these species can also improve the RE of E2 [34]. The final RE does not significantly change at a peak voltage of 13 kV compared with that at a peak voltage of 12 kV because the concentration of the generated oxidation species is enough to decompose E2 at a peak voltage of 12 kV. 3.4 Effect of initial E2 concentration The RE of E2 (Fig. 5) and first-order kinetic rate constant (Table 3) depends on the E2 initial concentration. The RE of E2 was 99.3% at an E2 initial concentration of 200 μg/L after 30 min of treatment, and then the value abruptly decreased to 73.7% when the E2 initial concentration is increased to 1000 μg/L. In addition, as listed in Table 3, the first-order kinetic rate constant k gradually decreased from 0.1600 min−1 to 0.0481 min−1 and the half-life increased from 4.3 min to 14.4 min when the initial concentration was increased from 200 μg/L to 1000 μg/L. Gao et al. [8] also found similar results for the degradation of E2 using non-thermal plasma. This result could be attributed to the active species produced in the DBD system that were maintained at specific concentration levels under steady discharge at fixed peak voltage [28]. However, more intermediate products were produced when the E2 initial concentration was further increased. A competition to consume active species occurs between the degradation of E2 and the decomposition of the intermediate products. Thus, RE decreased when the initial concentration of E2 was further increased at a fixed peak voltage. 3.5 Effect of initial pH As shown in Fig. 6a, the maximum RE of E2 was obtained at pH 2. The RE decreased from 97% to 92% when the pH was changed from 2 to 11 within 30 min. The first-order kinetic rate constant k reached a value of 0.2227 min-1, and the half-life was 3.1 min at the initial pH of 2. However, when the initial pH was increased from 5 to 11, the first-order

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kinetic rate constant k gradually decreased from 0.1613 min-1 to 0.0765 min-1 and the half-life increased from 4.3 to 9.1 min. Furthermore, the solution pH decreased to 1.9 and 2.8 when the initial pH was 2, and then increased to more than 5 after 15 min of treatment. The pH then remained stable afterward (Fig.6b). The drastic decrease in pH value can be explained by the formation and accumulation of strong acids (e.g., HNO 3) during the plasma discharge process according to a previous study [8]. The RE of E2 significantly increased at pH values between 2 and 11 within 15 min (Fig.6a), especially for the initial pH of 2. These results may be due to the fact that the initial pH is one of the most important factors that influence the degradation of organic compounds in DBD [8], ozone oxidation process [1], as well as heterogeneous photocatalysis [35]. First, plasma generates several reactive species such as O 3, •OH, and H2O2 [8] that cooperate in the degradation of E2 molecules. The oxidation-reduction potential of these reactive species is lower in a basic solution than in an acidic solution [28]. Second, O3 is generated in the gas plasma zone and can transfer to the liquid phase to degrade pollutants [29]. Different reactions take place when the solution pH is varied. The direct ozonation oxidation pathway will be the dominant route at acidic or neutral solution conditions. However, the radical-type chain reaction of ozone will be promoted in a basic solution and accelerates the transformation of ozone into •OH radicals [36]. Furthermore, •OH radicals can be scavenged by OH - [27]. Thus, the reaction between •OH and OH - is given by •OH+OH-→O•-+ H2O

(6)

Third, electron transfer is enhanced by Pt-TiO2 at an acidic condition [26]. Pt-TiO2 first promotes the formation of O2•− [Eq.(5)], and the •OH radical is then produced at an acidic condition, as shown in Eqs. (7)-(10) [32]. O2•-+2H++e-→H2O2

(7)

H2O2+e-→•OH+ OH•-

(8)

11

O2•-+H+→HO2•

(9)

HO2•+ H2O→H2O2+•OH

(10)

In addition, the influence of pH on the ionization of the reactant compounds and products may be an important factor to consider. The molecule of E2 becomes neutral in an acidic medium and starts to become negatively charged at alkaline conditions [35]. Thus, E2 molecules are difficult to be adsorbed onto the Pt-TiO2 surface, which is negatively charged at alkaline conditions. 3.6 Effect of Pt-TiO2 amount The effect of the photocatalyst amount on RE was determined, and the results show that the optimal Pt-TiO2 additive amount is 50 mg/L. At this value, RE reaches 98.9% (Fig.7). Meanwhile, the first-order kinetic rate constant k (Table 3) is 0.1446 min-1, and the half-life is 4.8 min. In addition, when the amount of Pt-TiO2 is less than 50 mg/L, the UV light produced in the DBD process significantly enhances the decomposition of E2 with increasing photocatalyst amount. Although the amount of Pt-TiO2 is larger than 50 mg/L, the RE decreases with increasing photocatalyst amount. These results may be explained by the aggregation of Pt-TiO2 particles, thereby reducing the contact surface between the solution and catalyst surface sites. The light produced during the DBD process will have a hard time infiltrating the solution given its opacity. Meanwhile, when the photocatalyst amounts exceed the optimum level, the photocatalyst may prevent the light from exciting the photocatalyst, and a screen effect occurs [6, 18, 19, 35]. Photocatalysis is induced by UV, and the amount of produced radicals decreases, thereby decreasing RE. 3.7 Degradation mechanism of E2 The intermediates produced during the degradation of E2 (5 mg/L) in the DBD system with the addition of Pt-TiO2 were determined. The total ion chromatography (TIC) of E2 degradation is shown in Fig. 8. Six main degradation products were detected at retention

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times of 2.5, 2.9, 3.7, 4.6, 5.5, and 14.0 min after 15 min of treatment. Meanwhile, the peak at 6.8 min is assigned to E2. In addition, several relatively weak peaks are observed aside from peaks 1 to 6 in the TIC of the mass spectra for E2 (Fig. 8). These peaks, which could not be identified, may be the unknown intermediates that were produced in the DBD system when E2 was oxidized by Pt-TiO2. Table 4 lists the experimental and theoretical masses (m/z), the error between these values in mDa and ppm, the double bond equivalent (DBE), the proposed elemental composition, and the proposed structure of the six degradation productions. A tolerance of 5 ppm in error between the measured and calculated accurate masses was considered in majority of the cases to guarantee the correct assignment of the molecular formula of major ions [6, 37]. Based on the obtained intermediates, several possible degradation pathways of E2 are elucidated in Fig. 9. E2 is oxidized by •OH and O3 to form 2-hydroxyestradiol (P2 (1)). 2Hydroxyestradiol was detected as an important degradation product, whose estrogenic activity decreases almost 18 times than that of E2 [38]. The formation of P2 (1) has two pathways [8]. For pathway 1, E2 could be attacked by •OH at the C2 atom with the highest 2FED2HOMO+2FED2LUMO value at the phenol moiety [6, 39] to form P2 (1). For pathway 2, E2 is directly oxidized by O3, and the two oxygen atoms are eliminated from the E2 ring to form P2 (1). P2 (1) is then further oxidized to form P3 by O 3 oxidation [1]. P3 is directly oxidized by O3, and the two oxygen atoms are then eliminated from the P3 ring to form P4. This pathway can be described by P2 (1)→P3→P4. The same degradation mechanism was also proposed by a previous study [1]. P4 is attacked by •OH to form P5, and the ring opening mechanism occurs to form dicarboxylic acids [6]. In addition, E2 can be initially oxidized by h+ on Pt-TiO2. The first electron extracted by h+ from the E2 molecule is at the C10 atom with the highest 2FED2HOMO [6, 39]. Given the rearrangement at the phenol moiety of the structure, the resonance structure is formed and the site at the C10 atom of E2 is active and prone to the

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attack of •OH and •OOH radicals [40, 41]. Therefore, compounds P2 (2) and P6 can be formed by the direct attack of •OOH and •OH radicals, respectively. Compound P6 can be formed in the reaction route of E2→P6, and compound 10ɛ-17β-dihydroxy-1,4-estradieno-3one (P2(2)) can be produced following pathways E2→P2(2) and E2→P6→P2(2). Similar reaction routes in the hole-oxidation reactions have been reported by previous studies [6, 39]. In particular, Ohko et al. [39] indicated that P2(2) and P6 produced during the reactions did not exhibit any potent estrogenic activity in the treated water. 4. Conclusions The synergistic effect of plasma and photocatalyst significantly enhanced the RE and degradation yield of E2 in an aqueous solution. The degradation of E2 can be significantly improved by adding Pt-TiO2 to the DBD system because the UV light produced during the plasma process can be utilized. The optimum peak voltage, E2 concentration, solution pH value, and added amount of Pt-TiO2 were 12 kV, 400 μg/L, 2 and 50 mg/L, respectively, and the RE was improved to 98% after 30 min of treatment. Meanwhile, the degradation process can be described using a first-order kinetic model. Furthermore, the degradation mechanism was demonstrated based on six main intermediates identified, and hydroxyl radicals and ozone may be significant active species during this process. All results indicated that DBD combined with Pt-TiO2 is a feasible treatment technology for degradation of E2 in aqueous solutions. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21367002, 51368004, and 51208492), Guangxi Natural Science Foundation Program of China (No. 2014GXNSFAA118294, 2014GXNSFBA118058), Innovation Project of Guangxi Graduate Education of China (No.YCSZ2014044), and Undergraduate Training Programs for Innovation and Entrepreneurship of China (Guangxi University) (No.

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16

Figure captions Fig. 1 SEM images of TiO2 (a), Pt-TiO2 (b) and EDS spectrum (c). Fig. 2 XRD patterns (a) and UV–vis diffuse reflectance spectra (b) of TiO2 and Pt-TiO2. Fig. 3 RE of E2 in different degradation systems (E2 initial concentration 400 μg/L, pH 5.6, peak voltage 12 kV, photocatalyst additive amount 50 mg/L). Fig. 4 Effect of peak voltage on the RE of E2 (E2 initial concentration 400 μg/L, pH 5.6, PtTiO2 additive amount 50 mg/L). Fig. 5 Effect of initial E2 concentration on the RE of E2 (peak voltage 12 kV, pH 5.6, Pt-TiO2 additive amount 50 mg/L). Fig. 6 Effect of initial pH values on the RE of E2 (a) and change of pH values during the reaction (peak voltage 12 kV, E2 initial concentration 400 μg/L, Pt-TiO2 additive amount 50 mg/L). Fig. 7 Effect of Pt-TiO2 additive amount on the RE of E2 (peak voltage 12 kV, E2 initial concentration 400 μg/L, pH 5.6). Fig. 8 Total ion chromatography (TIC) spectra after treatment times of 15 and 30 min. (peak voltage 12 kV, E2 initial concentration 5 mg/L, initial pH 7.0, Pt-TiO2 additive amount 50 mg/L). Fig. 9 Proposed degradation mechanism of E2.

17

Tables Table 1 The experimental conditions for DBD system with added Pt-TiO2 Batch

Peak voltage (kV)

E2 concentration (μg/L)

Initial pH value

Pt-TiO2 additive amount (mg/L)

Treatment (min)

Batch 1

10 11 12 13

400 400 400 400

5.6 5.6 5.6 5.6

50 50 50 50

30 30 30 30

Volume of solution (mL) 500 500 500 500

Batch 2

12 12 12 12

200 400 800 1000

5.6 5.6 5.6 5.6

50 50 50 50

30 30 30 30

500 500 500 500

Batch 3

12 12 12 12

400 400 400 400

2 5 7 11

50 50 50 50

30 30 30 30

500 500 500 500

Batch 4

12 12 12 12

400 400 400 400

5.6 5.6 5.6 5.6

0 25 50 100

30 30 30 30

500 500 500 500

18

time

Table 2 First-order kinetic constants and degradation yield for different systems

First-order kinetic constants Process

-3

-1

2

Y (10-6g/kW h)

k×10 (min )

t1/2 (min)

R

DBD

42.63

16.3

0.993

329.2

DBD+TiO2

100.1

6.9

0.996

439.4

DBD+Pt-TiO2

144.62

4.8

0.988

468.1

19

Table 3 Effect of experimental parameters on the first-order kinetic constants using DBD system with added PtTiO2 First-order kinetic constant Specific Value k×10-3 Parameters t1/2 (min) R2 (min-1) 10 kV 43.2 16.0 0.977 11 kV 60.5 11.5 0.981 Peak voltage 12 kV 144.6 4.8 0.988 13 kV 158.9 4.4 0.986

E2 concentration

200 μg/L 400 μg/L 800 μg/L 1000 μg/L

160.0 144.6 62.4 48.1

4.3 4.8 11.1 14.4

0.993 0.988 0.967 0.949

Initial pH value

2 5 7 11

222.7 161.3 88.0 76.5

3.1 4.3 7.9 9.1

0.992 0.985 0.965 0.997

Pt-TiO2 additive amount

0 mg/L 25 mg/L 50 mg/L 100 mg/L

42.6 86.3 144.6 94.9

16.3 8.0 4.8 7.3

0.993 0.990 0.988 0.985

20

Table 4 Characteristic of E2 and the produced intermediates in the DBD system with the addition of Pt-TiO2 Mass (m/z) Production

P1 (E2)

Rt (min)

Error

Formula

DBE Experimental

Theoretical

mDa

PPM

6.8

C18H23O2

271.1696

271.1698

-0.2

-0.7

7.5

2.9

C18H23O3

287.1639

287.1647

-0.8

-2.8

7.5

Proposed structure

P2(1) or

P2

5.5

C18H23O3

287.1642

287.1647

-0.5

-1.7

7.5 P2(2)

P3

3.7

C18H21O3

285.1480

285.1491

-1.1

-3.9

8.5

P4

4.6

C18H21O4

301.1421

301.1440

-1.9

-6.3

8.5

P5

14.0

C18H24O7

353.2108

353.2117

-0.9

-2.5

9.5

P6

2.5

C18H23O4

303.1580

303.1596

-1.6

-5.3

7.5

21

Fig. 1

(a)

(b)

10000

(c)

8000 Counts (cps)

Ti

Element At% O 70.22 Ti 29.51 Pt 0.27

6000 4000 O

2000

Pt 0 0

2

4 Energy (Kev)

22

6

8

Fig. 2

2.0 (a) A

(b)

Intensity(a.u.)

R

A R

A

RR

R

1.5

R R

Intensity (a.u.)

TiO2

R

Pt-TiO2

1.0

Pt-TiO2

0.5 TiO2

20

30

40

50

60

70

80

0.0 300

400

500

600

Wavenumber (nm)

2 theta

23

700

800

Fig. 3

Removal efficiency (%)

100 80 60 TiO2 adsorption Pt-TiO2 adsorption DBD DBD+TiO2 DBD+Pt-TiO2

40 20 0 0

5

10

15 20 t (min)

24

25

30

Fig. 4

Removal efficiency (%)

100 80 60 40

10 kV 11 kV 12 kV 13 kV

20 0 0

5

10

15

t (min)

25

20

25

30

Fig. 5

Removal efficiency (%)

100 80 60 40

200 μg/L 400 μg/L 800 μg/L 1000 μg/L

20 0 0

5

10

15 20 t (min)

26

25

30

Fig. 6 12

(a)

(b) pH=2 pH=5 pH=7 pH=11

10

80

8

60 pH

Removal efficiency (%)

100

40

pH=2 pH=5 pH=7 pH=11

20

6 4 2

0

0

0

5

10

15 20 Time (min)

25

0

30

27

5

10

15 20 Time (min)

25

30

Fig. 7

Removal efficiency (%)

100 80 60 0 mg/L 25 mg/L 50 mg/L 100 mg/L

40 20 0 0

5

10

15 20 t (min)

28

25

30

Fig. 8

29

Fig. 9

30

Highlights ► Synergistic system of DBD combined with Pt-TiO2 photocatalyst was developed. ► 17β-Estradiol was effectively degraded by the synergistic system. ► The degradation process follows the first-order kinetic model. ► Six intermediate products were detected and proposed structures were provided. ► Hydroxyl radicals and ozone were main active species in the synergistic system.

31