Photodegradation of 4-alkylphenols using BiVO4 photocatalyst under irradiation with visible light from a solar simulator

Applied Catalysis B: Environmental 46 (2003) 573–586

Photodegradation of 4-alkylphenols using BiVO4 photocatalyst under irradiation with visible light from a solar simulator Shigeru Kohtani a,∗ , Masaya Koshiko b , Akihiko Kudo c,d , Kunihiro Tokumura a , Yasuhito Ishigaki a , Akira Toriba b , Kazuichi Hayakawa a , Ryoichi Nakagaki a a

c

Graduate School of Natural Science and Technology, Kanazawa University, 13-1 Takara-machi, Kanazawa 920-0934, Japan b Faculty of Pharmaceutical Sciences, Kanazawa University, 13-1 Takara-machi, Kanazawa 920-0934, Japan Department of Applied Chemistry, Faculty of Science, Science University of Tokyo, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan d Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation (CREST, JST), 4-1-8 Honcho, Kawaguchi-shi, Saitama 332-0012, Japan Received 28 April 2003; received in revised form 1 July 2003; accepted 15 July 2003

Abstract A visible-light-driven photocatalyst, BiVO4 , has been applied for degradation of a series of linear 4-n-alkylphenols under irradiation from a solar simulator. Degradation rates become faster with increasing alkyl chain length. Half-life of 4-n-nonylphenol (4-n-NP) is 18 min, which is about eight times shorter than that of phenol. The rate law of disappearance for heptyl-, octyl- and nonylphenols exhibits zero-order kinetics. The amount of adsorption on BiVO4 surface is much larger for longer hydrophobic alkylphenols. It is thus concluded that BiVO4 is suitable for degradation of hydrophobic alkylphenols such as nonyl- and octylphenols nominated as endocrine disruptors. Degradation of branched 4-nonylphenol (mixture of isomers) has also been examined, and disappearance of its estrogenic activity was confirmed by means of the yeast two-hybrid assay. A gas chromatography–mass spectroscopy (GC–MS) analysis after solid-phase extraction indicates that the major product is cis,cis-4-alkyl-6-oxo-2,6-hexadienoic acids, and the minor products are 4-alkylcatechols and 4-(1-alkenyl)phenols. © 2003 Elsevier B.V. All rights reserved. Keywords: BiVO4 ; Photocatalysis; Degradation; Alkylphenol; Endocrine disruptor; Solar simulator; Gas chromatography–mass spectroscopy; Estrogenic activity; Yeast two-hybrid assay; Adsorption

1. Introduction Phenols with long alkyl chain such as nonylphenol (NP) and octylphenol (OP) have been used for source materials of industrial, household, and commercial applications, especially, non-ionic nonylphenolpolyethoxylates (NPEOs) and octylphenolpolyethoxylates (OPEOs) surfactants. These alkylphenols are sus∗ Corresponding author. Tel.: +81-76-234-4483; fax: +81-76-234-4484. E-mail address: [email protected] (S. Kohtani).

pected of disturbing the hormonal controls of fresh water species and affecting their reproduction in aquatic environments [1–5]. It has been reported that NP and OP are formed by biodegradation of NPEOs and OPEOs in sewage treatment [6,7], and widely distributed in water of rivers and lakes as well as aquatic sediments in some countries [8]. The risk assessments of NP were conducted by the Ministry of the Environment of Japan, the report of which warns us of arising the endocrine disrupting effects on fresh water species like medaka (Oryzias latipes) [4,5].

0926-3373/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/S0926-3373(03)00320-5

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Degradation using a heterogeneous TiO2 photocatalyst has been extensively studied to remove organic pollutants from environments [9–21]. Titanium oxide is apparently an excellent photocatalyst to degrade NP and NPEOs and to mineralize those to CO2 and H2 O, though it works only under UV light irradiation [15–21]. The CO2 mineralization using TiO2 arises from the positive valence band edge position (ca. +3.0 V versus NHE at pH 0) compared to the redox potential of OH/H2 O (E0 = +2.8 V versus NHE) [9,10]. On the other hand, Brand et al. reported 90% decomposition of OPEOs after 24 h of solar irradiation using homogeneous Fe(III)-aquo complex [22], where OH radicals are produced from Fe(OH)2+ [23]. They further claimed that the photocatalyst is able to degrade OP under solar light. The same procedure has also been applied for degradation of NPEOs: the photoproducts are identified by an LC-APCI-MS detection method [24]. We have recently reported that a heterogeneous visible-light-driven BiVO4 photocatalyst shows the ability to degrade linear alkyl 4-n-nonylphenol (4-nNP) under irradiation from a solar simulator [25]. The usage of BiVO4 with a monoclinic scheelite structure aims at complete water splitting into H2 and O2 [26–29]. The photocatalyst exhibits a 2.4 eV band gap: the absorption band edge extends up to ca. 520 nm. Although overall water splitting was not observed, photocatalytic O2 evolution by oxidation of H2 O occurs in AgNO3 aqueous solution under visible light (>420 nm). BiVO4 is therefore expected to possess high oxidation activity for degradation of organic pollutants under solar light. We have demonstrated that the photocatalytic activity of BiVO4 for degradation of 4-n-NP is comparable to that of TiO2 in air-saturated solution and the former becomes greater than the latter in O2 -satureated solution [25]. However, CO2 mineralization yield is quite low because of insufficient oxidation power against photoproducts. This may be due to the valence band edge of BiVO4 located at negative potential (ca. +2.4 V versus NHE at pH 0) [25,27] compared to the redox potential of OH/H2 O. The present study was undertaken to examine alkyl-chain length dependence of the photocatalytic degradation of 4-n-alkylphenols, to elucidate the degradation mechanism, and to prove the disappearance of estrogenic activity of endocrine disrupting NP

using BiVO4 under solar light. Analysis of photoproducts was performed by means of gas chromatography– mass spectroscopy (GC–MS) after a solid-phase extraction. Biological evaluation of estrogenic activities for irradiated NP solutions was carried out by the yeast two-hybrid system [30].

2. Experimental 2.1. Materials 2.1.1. Alkylphenols Purchased alkylphenols were used as received: Phenol (Wako Chemicals, >99.2%), 4-ethylphenol (Kanto Reagents, >97.0%), 4-n-propylphenol (Kanto Reagents, 99.4%), 4-n-butylphenol (Wako Chemicals, >98.0%), 4-n-pentylphenol (Kanto Reagents, 99.7%), 4-n-hexylphenol (Kanto Reagents, 98.8%), 4-n-heptylphenol (Kanto Reagents, 99.4%), 4-n-octylphenol (Kanto Reagents, 98.8%), 4-tert-octylphenol (Kanto Reagents, 97.9%), 4-n-nonylphenol (Kanto Reagents, 99.5%), and 4-nonylphenol (Kanto Reagents, mixture of isomers, 97.3%). 4-Ethylresorcinol (Tokyo Kasei, >98%). 2.1.2. 4-(1-Propenyl)phenol 4-(1-Propenyl)phenol, a candidate for photoproducts from 4-propylphenol, was synthesized by demethylation of trans-anethole. A three-necked flask containing an argon inlet glass tube and a rubber septum was evacuated and then filled with argon gas. Positive argon pressure was maintained throughout the following sequence. Using a syringe, 15 ml of dry tetrahydrofuran (Wako Chemicals, 99.5%) and 2.7 ml (0.0145 mol) of diphenylphosphine (Tokyo Kasei, >90%) were added through the septum. The resulting solution was stirred and cooled with an ice bath, and 10 ml (0.016 mol) of cold 1.6 M n-butyllithium-hexane solution (Nacalai Tesque, 15% in hexane) was added by a syringe over ca. 3 min. The solution was allowed to warm to room temperature over 30 min before 1.63 g (0.011 mol) of trans-anethole (Tokyo Kasei, 98.0%) was added. The mixture was stirred at room temperature for 5 h. The reaction mixture was then poured into 100 ml of vigorously stirred water, and 5 ml of 10% aqueous NaOH was added. Alkali-insoluble impurities

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(trans-anethole, etc.) were removed by washing the basic aqueous phase with diethyl ether. The aqueous alkaline layer was then cooled in an ice bath, and acidified with concentrated HCl. During this acidification, the solution became milky white suspension which was then extracted with diethyl ether. The ether layer was washed with water and saturated aqueous NaCl, dried over anhydrous sodium sulfate, and filtered. Removal of diethyl ether under reduced pressure provided a residue. Purification was carried out by column chromatography (SiO2 , 9% ethyl acetate in hexane) to yield 0.2 g of white crystal (13% yield); mp: 95–96 ◦ C (90 ◦ C [31]); mass spectrum (EI) m/z: 134 (M+ ); 500 MHz 1 H NMR (CDCl3 ) δ: (ppm): 1.84 (d, J = 6.6 Hz, 3H), 4.76 (s, 1H), 6.07 (m, 1H), 6.33 (d, J = 6.6 Hz, 1H), 6.7–7.2 (m, 4H). 2.1.3. BiVO4 BiVO4 powder was synthesized in an aqueous medium as reported [25]. X-ray diffraction patterns of the synthesized BiVO4 indicated a monoclinic scheelite structure as reported [27,28]. A diffuse reflectance spectrum of the BiVO4 powder showed visible light absorption in 400–550 nm. BET surface area of BiVO4 was determined to be 0.2 m2 /g. 2.2. Irradiation with visible light from a solar simulator Irradiation experiments were carried out using a solar simulator (Oriel 81192) equipped with an AM2D air mass filter. The output from a 1000 W xenon arc lamp of the solar simulator was corrected to approximate the solar spectrum when the sun is at a zenith angle of 60.1◦ . The light intensity was measured to be 24 mW/cm2 by a thermopile sensor (Coherent 210). The prepared concentration of all alkylphenols in NaOH alkaline solution (pH ∼13) was the same value of 2 × 10−4 M. Prior to irradiation, 25 ml of the solution containing 0.1 g of BiVO4 was kept at 25 ◦ C overnight in the dark so as to attain adsorption equilibria of alkylphenols on BiVO4 . The sample solutions were then placed in a cylindrical quartz cell (45 mm i.d. × 50 mm) and stirred during irradiation. After centrifugation, concentrations of alkylphenols were determined by a reverse-phase HPLC system (a Waters 510 pump, a Shimadzu CTO-10AS column oven, a Otsuka Electronics MCPD 3600 multi-channel

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photodetector) equipped with a Supelco TPR-100 column (4.6 mm i.d. × 150 mm, 25 ◦ C). A mobile phase was acetonitrile/water containing 0.1% trifluoroacetic acid at a flow rate of 0.6–1.0 ml/min. Chromatograms were monitored at 280 nm. 2.3. Product analysis Alkylphenol solutions (20 ml, 2 × 10−4 M, pH ∼13) containing 0.1 g of BiVO4 were stirred in a 50 ml round-bottom flask and irradiated with a 300 W xenon arc lamp (VIX-300V) through a Toshiba L-42 glass filter (>400 nm). The irradiated solutions were centrifugated when 4-n-alkylphenols were decomposed by 80–90%. The supernatants were neutralized by addition of hydrochloric acid. Analytes were extracted from the solution by solid-phase extraction (SPE) with a GL-Pak Carbograph cartridge (carbon graphite, 150 mg, 3 ml) held on a SPE extraction manifold (Waters) which was connected to a vacuum pump (ULVAC MDA-020C). Conditioning was achieved by applying 2 ml of acetone and 2 ml of distilled water. After passage of 20 ml of the irradiated solution through the SPE cartridge, the cartridge was washed with distilled water and then dried under reduced pressure for ca. 30 min. Elution of the analytes was made by passage of 2 ml of acetone, and the eluent was concentrated to a final volume of ca. 0.2 ml under a stream of nitrogen. Analyses were carried out on a GC–MS instrument (Hewlett-Packard 6890/5973) equipped with a chiral capillary column (Supelco ␤-DEX 225, 30 m × 0.25 mm i.d., film thickness 0.25 ␮m). Helium carrier gas was used with linear velocity of 30 cm/s. A sample solution (1 ␮l) was injected with splitless mode. The injector temperature was 250 ◦ C, and the column temperature was kept at 50 ◦ C for the first 2 min and then increased at the rate of 5 ◦ C/min to 250 ◦ C. The mass spectrometer was set to scan mass units 50–550 for electron impact ionization with a source temperature of 230 ◦ C. Evolution of CO2 gas during irradiation was followed by gas chromatography as reported previously [25]. After irradiation, 0.2–0.3 ml of 2.5 M sulfuric acid was added to the sample solution through the rubber septum by use of a syringe. When thermal equilibrium was attained at 25 ◦ C, CO2 evolved in head space was collected by a gas-tight syringe through the rubber septum and analyzed using a gas chromatograph (Shi-

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madzu GC-8A) equipped with a Porapak Q column (3.0 mm i.d. × 2 m, 100 ◦ C), a methanizer (Shimadzu MTN-1, 400 ◦ C), and a flame ionization detector (170 ◦ C). The injection port of the gas chromatograph was set at 170 ◦ C. N2 was used as the carrier gas. 2.4. Yeast two-hybrid assay Yeast two-hybrid assay system [30] was employed to confirm the decrease in estrogenic activities of NP (mixture of isomers) in BiVO4 suspended solution under irradiation from the solar simulator. In this system, estrogenic activities were evaluated by ␤-galactosidase activities expressed in yeast cells (Saccharomyces cerevisiae Y190) where the estrogen receptor, ER␣, and the coactivator, TIF2, interact when NP binds to the receptor. The cells were preincubated overnight at 30 ◦ C in SD medium free from tryptophan and leucine. After centrifugation of the irradiated solution, neutralization was made by the addition of hydrochloric acid. The culture medium (800 ␮l) in a 1.5 ml test tube was then mixed with 80 ␮l of the neutralized sample solution and incubated for 4 h at 27 ◦ C. Absorbance of the medium at 620 nm (A620 ) was checked by a microplate reader (ThermoBioAnalysis, Multiskan JX) before centrifugation for 5 min (14,000 rpm). The precipitated yeast cells were digested enzymatically by incubation with 600 ␮l of 1 mg/ml Zymolyase solution (in 0.1 M sodium phosphate, 10 mM KCl, 1 mM MgSO4 , pH 7.0) at 37 ◦ C for 20 min. The lyzate was mixed with 120 ␮l of 4 mg/ml ortho-nitrophenyl-␤-d-galactopyranoside solution (in 0.1 M phosphate buffer, pH 7.0) and reacted at 27 ◦ C for 20 min until a yellow color of ortho-nitrophenol was developed. Finally, 300 ␮l of stop solution (1 M Na2 SO4 ) was added to the lyzate which was then centrifuged for 5 min. Absorbance of the supernatant at 414 (A414 ) and 540 nm (A540 ) were measured on the microplate reader. The estrogenic activity was estimated by the following equation: activity (U) = 1000 ×

A414 − 1.75 × A540 . A620

3. Results and discussion All irradiation experiments were carried out in alkaline solution (pH ∼13), so that all of 4-alkylphenols

Table 1 Amounts of adsorption of 4-n-alkylphenols on BiVO4 determined by the reverse-phase HPLC 4-n-Alkylphenol

Amount of adsorption (␮mol/g)

4-n-Nonylphenol 4-n-Octylphenol 4-n-Heptylphenol 4-n-Hexylphenol 4-n-Pentylphenol 4-n-Butylphenol 4-n-Propylphenol 4-Ethylphenol Phenol

2.8 1.7 −0.1 −0.3 0.5 0.8 0.8 −0.6 0.6

± ± ± ± ± ± ± ± ±

0.6 0.6 0.4 1.2 0.4 1.3 0.9 0.5 1.1

Errors correspond to standard deviation from four independent samples. Prepared initial concentration of alkylphenols was 2 × 10−4 M.

are transformed into phenolate anions. This procedure will be advantageous for actual treatment of wastewater containing higher alkylphenols like NP and OP because the accumulated such higher alkylphenols in sludge of sewage treatment plants [32], caused by their highly hydrophobic property, can desorb from the sludge and dissolve into water. 3.1. Adsorption properties of 4-n-alkylphenols on BiVO4 surface The amounts of adsorption of 4-n-alkylphenols on BiVO4 are listed in Table 1. Here, prepared initial concentration of all 4-n-alkylphenols were 2.0 × 10−4 M. Although errors are considerably large, the values for 4-n-NP and 4-n-OP are more than 2 and 1 ␮mol/g, respectively. On the other hand, the amounts of adsorption for lower alkylphenols are less than 1 ␮mol/g. Thus, highly hydrophobic 4-n-NP and 4-n-OP are favorable to adsorb on BiVO4 surface, even though they are ionized in alkaline solution. Fig. 1 indicates adsorption isotherm of 4-n-NP fitted by the Langmuir equation: V = Vmax KC/(1 + KC), where V is the amount of adsorption on BiVO4 , Vmax the maximum value of adsorption, K the adsorption binding constant, and C the concentration of 4-n-NP in equilibrium. The fitting is well done with Vmax of 6.4 ± 1.3 ␮mol/g and K of (5.8 ± 2.0) × 103 M−1 . From the Vmax value and the specific surface area of BiVO4 (0.2 m2 /g), the area covered by one 4-n-NP molecule (σ) is calculated to be ca. 5 × 10−20 m2 . Such a small σ value

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Fig. 1. Adsorption isotherm of 4-n-nonylphenol. The data were analyzed by the Langmuir equation: V = Vmax KC/(1 + KC).

may suggest that 4-n-NP adsorbs perpendicularly and a condensed monomolecular layer of 4-n-NP covers over the BiVO4 surface where nonyl group of 4-n-NP faces to the surface. In this circumstance, hydrophobic interaction between nonyl chain and the BiVO4 surface would be important rather than ionic interaction between phenolate moiety and surface charges such as Bi3+ or V5+ . Thus, the long alkyl chain attached to phenyl ring seems to act as an anchor on the more or less hydrophobic BiVO4 surface. 3.2. Alkyl chain dependence on degradation rates of 4-n-alkylphenols Fig. 2 shows time profiles of C/C0 under irradiation from the solar simulator, where C is the concentration of 4-n-alkylphenols at the irradiation time t and C0 is the concentration in the adsorption equilibrium on BiVO4 before irradiation. It is obvious that degradation rate becomes faster with increasing alkyl chain length. Thus, BiVO4 is suitable for degradation of hydrophobic higher alkylphenols such as NP and OP under solar light. Interestingly, C/C0 linearly decreases for higher alkylphenols such as heptyl-, octyl- and nonylphe-

nols. Namely, the zero-order kinetics is observed. For 4-n-NP, the linear dependence is observed for the several C0 in the range of 1–4 × 10−4 M. Assuming that Langmuir-type adsorption occurs on BiVO4 surface for all alkylphenols, the rate equation can be expressed as follows: −

dC kKC = kθ = dt 1 + KC

(1)

where k is the rate constant and θ the fractional coverage of alkylphenols on BiVO4 . Separation of variables (C and t) and integration between the limits C0 at time t = 0 and C at time t give the following equation [33]:

 C0 C α ln = βt (2) + 1− C C0 Here, the first and second terms of the Eq. (2) show the first- and zero-order decay kinetics, respectively. The constant α (=1/KC0 ) represents the relative contribution of the first-order decay component to the zero-order one. The constant β (=k/C0 ) is equivalent to the rate constant k. Fig. 3 indicates the plots of {α ln(C0 /C)+(1−(C/C0 ))} versus t for the degradation of all 4-n-alkylphenols fitted with the rate Eq. (2), where α is set to obtain the best correlation coefficients

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Fig. 2. Photocatalytic degradation of 4-n-alkylphenols in BiVO4 (0.1 g) suspended alkaline solution (25 ml, pH ∼13) under irradiation from the solar simulator (24 mW/cm2 ): (䉫) phenol, (䊉) 4-ethylphenol, (+) 4-n-propylphenol, (䊊) 4-n-butylphenol, (*) 4-n-pentylphenol, (×) 4-n-hexylphenol, (䉭) 4-n-heptylphenol, (䊐) 4-n-octylphenol, (䉬) 4-n-nonylphenol. Prepared initial concentration of 4-n-alkylphenols was 2 × 10−4 M.

(R2 ). The obtained α, β and R2 values are summarized in Table 2. For heptyl-, octyl- and nonylphenols, the α values are less than 0.01 so that the first-order decay component is negligible and therefore the zero-order kinetics dominates the degradations as mentioned previously. On the other hand, the degradations of other alkylphenols involve the first-order kinetics and the α values become larger on decreasing the alkyl chain length. Half-life (t1/2 ) except for phenol are deter-

mined by the following relation: t1/2 = (α ln 2 + 0.5)/β and listed in Table 2. In the case of phenol, the plots of ln(C0 /C) show good correlation with time as shown in Fig. 3, in which the second term in the Eq. (2) is negligible. The first-order kinetics is thus predominant in the degradation of phenol. Therefore, the degradation kinetics for 4-n-alkylphenols varies from the zero- to first-order depending on the number of methylene groups in the alkyl chain.

Fig. 3. Plots of {α ln(C0 /C) + (1 − (C/C0 ))} vs. t for the degradation of 4-n-alkylphenols fitted with the rate Eq. (2) (see text): (䉫) phenol, (䊉) 4-ethylphenol, (+) 4-n-propylphenol, (䊊) 4-n-butylphenol, (*) 4-n-pentylphenol, (×) 4-n-hexylphenol, (䉭) 4-n-heptylphenol, (䊐) 4-n-octylphenol, (䉬) 4-n-nonylphenol. The fitting parameters (α and β) for each 4-n-alkylphenol are listed in Table 2.

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Table 2 Half-life (t1/2 ) and kinetic parameters (α and β) for the Eq. (2) on the degradation of 4-n-alkylphenols under irradiation from the solar simulator 4-n-Alkylphenol

t1/2 (min)a

α

β (×10−2 min−1 )

R2

Rate order

4-n-Nonylphenol 4-n-Octylphenol 4-n-Heptylphenol 4-n-Hexylphenol 4-n-Pentylphenol 4-n-Butylphenol 4-n-Propylphenol 4-Ethylphenol Phenol

18 26 21 34 60 59 65 98 133b

<0.01 <0.01 <0.01 0.03 0.08 0.23 0.20 0.35 1

2.80 1.91 2.42 1.55 0.93 1.12 0.98 0.76 0.52

0.987 0.996 0.996 0.987 0.986 0.997 0.996 0.997 0.991

Zero Zero Zero Zero Zero Zero Zero Zero First

+ first + first + first + first + first

R2 represents correlation coefficients for the plots in Fig. 3. a Obtained by the equation: t 1/2 = (α ln 2 + 0.5)/β. b Obtained by the equation: t 1/2 = ln 2/β, in which the second term of the Eq. (2) is neglected.

Fig. 4. Total ion chromatogram of photoproducts extracted from irradiated solution of 4-n-butylphenol after 85% decomposition under visible light (300 W Xe arc lamp, >400 nm). Detailed analytical conditions are described in the text.

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3.3. Products from 4-n-alkylphenols Evolution of CO and CO2 gas accompanied by mineralization of 4-n-alkylphenols has been investigated under irradiation from the solar simulator. However, only a trace amount of CO2 (mineralization yield <0.3%) was observed for all 4-n-alkylphenol solutions after 5 h irradiation. (The evolution of CO2 from 4-n-NP was not reported in our previous work [25]. It has become possible to detect such a trace amount of CO2 by the recent improvement of our apparatus.) Furthermore, no CO gas was detected for

all 4-n-alkylphenol solutions. These results indicate that almost all photoproducts are dissolved in solution and/or adsorbed on BiVO4 powders. The irradiated 4-n-alkylphenol solutions were analyzed by the GC–MS method after the solid-phase extraction as described in the experimental section. Here, a chiral GC capillary column was used for separation of chiral products. A total ion chromatogram observed for irradiated 4-n-butylphenol solution at 85% decomposition is illustrated in Fig. 4 as an example, where the peak due to 4-n-butylphenol appears at the retention time of 25.63 min. The major two peaks (solid

Fig. 5. Mass spectra of the chiral products extracted from irradiated 4-n-alkylphenol solutions containing BiVO4 photocatalyst (methylene number: n = 2–8). Retention time and m/z values (relative intensity) of the chiral products on the chiral capillary GC–MS are listed in Table 3.

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Table 3 Retention time and m/z values of the chiral products on the chiral capillary GC–MS Parent alkylphenols

Retention time (min)

Fragments (relative abundance)

4-Ethylphenol 4-n-Propylphenol 4-n-Butylphenol 4-n-Pentylphenol

27.02, 28.31, 29.83, 31.61,

4-n-Hexylphenol

33.32, 33.67

4-n-Heptylphenol

35.12, 35.44

4-n-Octylphenol

36.93, 37.19

55 (12), 69 (16), 83 55 (18), 69 (18), 80 41 (26), 55 (20), 69 43 (34), 55 (24), 69 196 (1) 43 (44), 55 (23), 69 210 (1) 41 (34), 55 (31), 57 195 (19), 224 (1) 43 (40), 55 (31), 57 209 (23), 238 (3)

27.93 28.96 30.33 32.08

(11), (13), (16), (17),

97 97 80 80

(71), (84), (18), (23),

111 (17), 125 (100), 139 (4), 154 (3) 111 (28), 125 (100), 139 (31), 168 (1) 97 (76), 111 (32), 125 (100), 139 (14), 153 (21), 182 (1) 97 (77), 111 (30), 125 (100), 139 (15), 153 (2), 167 (18),

(18), 80 (22), 97 (83), 111 (33), 125 (100), 139 (20), 153 (2), 181 (21), (34), 69 (18), 80 (21), 97 (80), 111 (34), 125 (100), 139 (17), 153 (6), (30), 71 (29), 80 (21), 97 (85), 111 (38), 125 (100), 139 (18), 153 (4),

circles) at 29.83 and 30.33 min show the same integration areas and identical mass spectra (n = 4 in Fig. 5). These two peaks are merged into one when a non-chiral capillary column is used. Therefore, the analytes is a chiral compound, the structure of which will be discussed later. The product peaks at 28.85 min (open circle) and 31.67 min (asterisk) will be also discussed later. On the other hand, since the peak at 23.20 min is observed from a non-irradiated control solution, this peak is attributed to a contaminant in solution. Other peaks are still unidentified at present. Here, we focus on the chiral photoproduct in the irradiated 4-n-alkylphenol solutions. Fig. 5 shows mass spectra of the chiral products from 4-ethylphenol (n = 2) to 4-n-octylphenol (n = 8). Retention times and the m/z values in the mass spectra are listed in Table 3. The m/z values of molecular ions (M+ )

strongly suggest that all of these are O2 adducts of parent 4-n-alkylphenols. Mass peaks at m/z = 125, 139, and M + − 29 are common to all photoproducts. In addition, fragment patterns below m/z = 125 are very similar for all. Taking the chirality into account, the photoproducts are attributed to cis,cis-4-alkyl-6-oxo-2,6-hexadienoic acid (1) as depicted in Scheme 1. Because of the steric hindrance between carboxylic and formyl groups, these dienes are not planar, and exist in left- and right-handed structures. The mass peaks at m/z = M + − 29 can be explained by the loss of CHO group. The common fragment ions of m/z = 125 and 139 are formed by the fragmentation at X and Y positions of the alkyl chains, respectively. Scheme 2 illustrates a redox reaction resulting in the chiral compound formation. The first step is the

Scheme 1.

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Scheme 2.

one-electron oxidation of the 4-n-alkylphenoxylates to the corresponding phenoxyl radicals by the hole of BiVO4 . Since spin density at the 2-position of 4-alkylphenoxyl radical is considerably large [34], molecular oxygen dissolved in the solution can add to the 2-position of the phenoxyl radical to form the peroxy radical. The third step is characterized as a reductive rearrangement of the peroxy radical, which may involve the ring C–C bond cleavage and electron transfer from the conduction band of BiVO4 . Judging from peak intensities on the total ion chromatograms, these chiral products seem to be the most abundant products from 4-n-alkylphenols except for 4-n-OP and 4-n-NP. Peak intensities of the chiral products from 4-n-OP became quite small, and those from 4-n-NP were not detected in our GC–MS analysis. The absence of chiral compound in the photolysis mixture for 4-n-NP may be related to a long distance between the phenolate moiety and the BiVO4 surface, in which the long alkyl chain acts as a spacer. Therefore, electron transfer from the phenolate moiety to the BiVO4 surface could be depressed. In this situation, direct oxidation by a valence band hole might be important. The photoproducts indicated by the open circle peak on the GC–MS total ion chromatogram of 4-n-butylphenol (Fig. 4) and those from other 4-n-alkylphenol have to be considered. Mass spectra of the photoproducts from 4-n-propylphenol (n = 3)

to 4-n-nonylphenol (n = 9) are summarized in Fig. 6. All m/z values of the molecular ions suggest that these products are formed by dehydrogenation of the parent 4-n-alkylphenols. The structure of compounds are 4-(1-alkenyl)phenol (2) as depicted in Scheme 1. The common base fragment ions of m/z = 133 are generated by the bond cleavage at Z position and assigned to 4-hydroxystyrylmethyl cation (HOC6 H4 CH = CHCH+ 2 ). The presence of 4-(1-propenyl)phenol in the photolysis mixture for n = 3 is confirmed by the comparison with the authentic sample. The identification for photoproducts from other 4-n-alkylphenols as 4-(1-alkenyl)phenol is based upon the fact that fragment patterns below m/z = 133 are similar to that of 4-(1-propenyl)phenol. We next consider the photoproduct peak indicated by an asterisk on the GC–MS total ion chromatogram of 4-n-butylphenol (Fig. 4). Mass spectra of the photoproducts from 4-ethylphenols (n = 2) to 4-n-nonylphenol (n = 9) are summarized in Fig. 7. The m/z of molecular ions shows the addition of oxygen atom to the parent alkylphenols. A series of the products are 4-alkylchatechol (3) as shown in Scheme 1. The common fragment ions of m/z = 123 are due to 3,4-dihydroxybenzyl cation formed by the bond cleavage between ␣- and ␤-carbon on the alkyl chain as depicted in Scheme 1. This fragmentation takes place in the same manner as that of

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Fig. 6. Mass spectra of 4-(1-alkenyl)phenol products (methylene number: n = 3–9). Retention time (min) on the chiral capillary GC–MS; n = 3: 27.00 min, n = 4: 28.85 min, n = 5: 30.75 min, n = 6: 32.83 min, n = 7: 34.77 min, n = 8: 36.68 min, n = 9: 38.57 min.

parent 4-n-alkylphenols whose major fragment ions of m/z = 107 are produced by the bond cleavage at the same position of the alkyl chains. The retention time of the sample from photolysis mixture for 4-ethylphenol does not coincide with that of the authentic 4-ethylresorcinol. This fact indicates that the hydroxylation takes place at the ortho-position with respect to the existing hydroxy group. Because this can be generalized, 4-alkylcatechols are formed on the photodegradation instead of 4-alkylresorcinols. 4-n-Alkylcatechol formation from 4-n-alkylphenols may be explained in terms of OH radical generation on the BiVO4 surface. From the study of degradation

of aromatic substances using TiO2 , the attack of OH radicals takes place on benzene ring at the ortho-, para-, or meta-positions depending on the nature of functional groups attached to the ring [9,10]. 3.4. Degradation of branched 4-nonylphenol (mixture of isomers) and disappearance of estrogenic activities 4-NP used for industrial products is usually a complex mixture of isomers with branched nonyl groups [35]. Degradation of the branched 4-NP and disappearance of its estrogenic activities are very important

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Fig. 7. Mass spectra of 4-alkylcatechol products (methylene number: n = 2–9). Retention time (min) on the chiral capillary GC–MS; n = 2: 28.47 min, n = 3: 29.93 min, n = 4: 31.67 min, n = 5: 33.63 min, n = 6: 35.55 min, n = 7: 37.56 min, n = 8: 39.39 min, n = 9: 41.12 min.

in actual water treatment. Fig. 8 shows degradation of the branched 4-NP and decrease in estrogenic activities under irradiation from the solar simulator. The branched 4-NP is completely degraded after 80 min irradiation, which is about two times slower than for linear 4-n-NP (Fig. 2). On the other hand, the estrogenic activities remained approximately constant for about 60 min, and then gradually decrease to zero around 140 min. Thus, complete disappearance of estrogenic activities is achieved. Such a delayed reduction in estrogenic activities may be caused by the production of other estrogenic compounds in the photocatalytic degradation process.

It is reasonable to assume that 4-(1-nonenyl)phenol (2) in Scheme 1 may possess some estrogenic activities because its molecular structure is similar to that of parent NP. According to quantitative analysis using high-resolution capillary GC [35], however, the most part (84.5%) of 4-NP mixture contains doubly-substituted structures at the benzyl (␣) position, i.e. ␣,␣-dimethyl and ␣-methyl/␣-ethyl or ␣-propyl. Since the content of ␣-monoalkylated phenols, i.e. ␣-monomethylated and ␣,␤-dimethylated phenols is only 15.5% [35], the residual estrogenic activities after disappearance of 4-NP may not be explained by the formation of 4-(1-nonenyl)phenol.

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Fig. 8. Photodegradation of branched 4-nonylphenol (䊊) and reduction in estrogenic activity (䊉) under the solar simulator irradiation (24 mW/cm2 ) of BiVO4 . Errors for estrogenic activity correspond to standard deviation from three samples.

4. Conclusion It is demonstrated that photodegradation using the visible-light-driven BiVO4 photocatalyst is useful for higher alkylphenols such as OP and NP because hydrophobic nature of long alkyl chain results in the large amounts of adsorption on BiVO4 . It is probable that the long alkyl chain attached to the phenol moiety acts as an anchor on BiVO4 surface. A series of photoproducts of linear 4-n-alkylphenols were identified by the GC–MS analysis after solid-phase extraction. Chiral cis,cis-4-alkyl-6-oxo-2,6-hexadienoic acids are mainly produced except for longer alkyl 4-n-OP and 4-n-NP. 4-Alkylcatechols are also detected by the GC–MS analysis, suggesting that OH radicals can be generated on BiVO4 surface under the visible light irradiation. In addition, a small amount of dehydrogenated 4-(1-alkenyl)phenols is identified. These reactions are summarized in Scheme 1. Reduction in estrogenic activities was also confirmed in the degradation of branched 4-NP (mixture of isomers) under irradiation from the solar simulator. The remaining estrogenic activities after degradation of branched

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