Kinetics and mechanisms studies on dimethyl phthalate degradation in aqueous solutions by pulse radiolysis and electron beam radiolysis

Radiation Physics and Chemistry 80 (2011) 420–425

Contents lists available at ScienceDirect

Radiation Physics and Chemistry journal homepage: www.elsevier.com/locate/radphyschem

Kinetics and mechanisms studies on dimethyl phthalate degradation in aqueous solutions by pulse radiolysis and electron beam radiolysis Ming-Hong Wu a, Ning Liu a, Gang Xu a,n, Jing Ma a, Liang Tang a, Liang Wang a, Hai-Ying Fu b a b

School of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China Shanghai Institute of Applied Physics, Chinese Academy of Sciences, Shanghai 201800, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 4 August 2010 Accepted 27 October 2010

The kinetics and mechanisms of hydroxyl radical/hydrated electron reactions with dimethyl phthalate (DMP) were investigated using pulse radiolysis and electron beam radiolysis techniques. The bimolecular rate constants for the reaction of hydroxyl radical and hydrated electron with DMP were measured to be 3.4  109 M  1 s  1 and 1.6  1010 M  1 s  1 under pulse radiolysis experiments, respectively. The major products after radiation were elucidated by LC/MS/MS and ion chromatography analysis. It was found that DMP degradation had different mechanisms in oxidative and reductive conditions: hydroxyl radical attacked aromatic ring of DMP leading to the fracture of benzene ring, formed a series of byproducts which were completely mineralized while hydrated electron attacked the ester group of DMP, formed the product of phthalic acid in reductive conditions. & 2010 Elsevier Ltd. All rights reserved.

Keywords: Dimethyl phthalate Pulse radiolysis Electron beam radiolysis Hydroxyl radical Hydrated electron

1. Introduction As one of the phthalate esters (PAEs), dimethyl phthalate (DMP) is typically applied in cellulose ester-based plastics, such as cellulose acetate and butyrate (Staples et al., 1997). DMP is a component of coating, food packing, cosmetics, lubricants, decorative cloths and other products (Baikova et al., 1999). As a result of its wide and large quantities use in industry, DMP has been recognized as a significant environmental pollutant, which has been detected in various environmental samples, such as surface waters, freshwaters, mineral waters, seawaters, urban lakes, sediments and landfill leachate (Gledhill et al., 1980; Mersiowsky, 2002; Montuori et al., 2008; Ogunfowokan et al., 2006; Penalver et al., 2000; Zeng et al., 2009). The concerns on the environmental healthy, particularly the physiological and biochemical effect on organisms of DMP have been recognized for many years (Wang et al., 2004). It has been reported that DMP and its intermediates are suspected to be the reason of functional disturbances in the liver and nervous systems of animals (Wang et al., 2008; Yuan et al., 2008a). Known as the endocrine-disrupting chemical, it may have the possibility of promoting chromosome injuries in human leucocytes and interfering with the reproductive systems and normal development of animals and humans (Jobling et al., 1995; Lottrup et al., 2006). Therefore, the US Environmental Protection Agency has listed DMP as a priority pollutant (US EPA, 1992).

n

Corresponding author. E-mail address: [email protected] (G. Xu).

0969-806X/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2010.10.008

Advanced oxidation and reduction processes (AO/RPs) are alternatives to traditional treatments and have been developed for the removal of many compounds (Ikehata et al., 2006; Zhang et al., 2007). AO/RPs typically involve the formation of hydroxyl radicals as oxidizing species and hydrated electrons as reducing species. Both of these species can be utilized in the degradation of organic contaminants present in drinking water or wastewater (Song et al., 2008a). The processes based on the generation of hydroxyl radicals have been applied to the degradation of DMP, such as Fenton’s reaction (Zhao et al., 2004), UV/H2O2 process (Xu et al., 2009), TiO2-UV and Fe(VI)-TiO2-UV photocatalyst (Ding et al., 2008; Yuan et al., 2008b), ozone-based oxidation (Chang et al., 2009; Chen et al., 2008), denitrifying degradation (Liang et al., 2007) and many combinations of them (Wang et al., 2009; Zhou et al., 2007). However, ionizing radiation can produce both oxidizing and reducing species. Radiolysis of water produces three highly reactive species, viz. hydroxyl radicals (dOH), hydrated  electrons (eaq ), hydrogen radicals (dH), in addition to the less reactive H2 and H2O2 (see Eq. (1), where the numbers in parentheses are the G-values (mmol J  1); Buxton et al., 1988).  +(0.06)dH + (0.05)H2 H2O-(0.28)dOH+ (0.27)eaq + (0.07)H2O2 + (0.27)H +

(1)

Creating oxidative or reductive conditions using radiolysis makes it an excellent approach for clarifying reaction mechanisms. The oxidative and reductive behavior of DMP in aqueous solutions has been investigated by steady-state and pulse radiolysis. The main objective of this study is to determine the rate constant and the possible reaction pathways for the reaction of hydroxyl radical/hydrated electron with

M.-H. Wu et al. / Radiation Physics and Chemistry 80 (2011) 420–425

DMP. In this study, transient spectra observed by pulse radiolysis give a better understanding of the characteristic of the intermediate species. Product studies of DMP degradation using electron beam irradiation in aerated solutions were made to provide further insight into the mechanisms occurring under oxidative and reductive conditions.

2. Methods and materials 2.1. Materials DMP standard was obtained from Sigma ( 499% purity). Tert-butyl alcohol (t-BuOH) was of HPLC grade and also purchased from Sigma. All solutions were prepared using triply distilled water and all experiments were carried out at room temperature and natural pH levels. The experiment solutions were purged with high purity N2O (99.999%) for hydroxyl radical experiments, or with high purity N2 (99.999%) to remove dissolved oxygen for hydrated electron experiments. 2.2. Pulse radiolysis and electron beam radiolysis Pulse radiolysis experiments were carried out with 10 ns pulses of 10 MeV electrons from a linear electron accelerator in the Institute of Applied Physics and the pulse dose used was 20 Gy. Pulse dosimetry was performed with a 0.1 mol dm  3 KSCN solution using e ¼7600 M  1 cm  1 for (SCN)d2  at 480 nm. Analysis was performed with a 300 W xenon lamp, shining perpendicularly through a quartz cuvette having an optical path length of 10 mm. To study only the reactions of hydroxyl radical, solutions were pre-saturated with N2O, which quantitatively converts hydrated electrons and hydrogen atoms to hydroxyl radicals via reactions (2) and (3) (Buxton et al., 1988). To achieve hydrated electrons reactions, solutions are pre-saturated with N2 in the presence of 0.10 M t-BuOH, which can be converted into relatively inert t-BuOH radicals by scavenging hydroxyl radicals and hydrogen atoms (see Eq. (4) and Eq. (5); Buxton et al., 1988).  eaq + N2O+H2O-N2 + OH  + dOH, k2 ¼9.1  109 M  1 s  1 6

1

H+ N2O-N2 + OH, k3 ¼2.1  10 M

d

OH+ (CH3)3COH- CH2(CH3)2COH+H2O, k4 ¼6.0  108 M  1 s  1

(2) (3)

d

(4)

and the eluent was a mixture of Na2CO3 (3.2 mM) and NaHCO3 (1.0 mM) in gradient conditions at a flow rate of 0.70 mL min  1. The injection volume was 10 mL. A Hybrid Quadrupole-TOF LC/MS/MS analysis was also performed to identify the degradation products, which were consisted of an HPLC (Agilent 1100) Pump and a Q-STAR XL (AB Sciex) Mass Spectrometer with a turbo-spray ionization source. A C18 column (150 mm  4.6 mm) was employed with a mobile phase of acetonitrile/water (30:70, v/v) at a flow rate of 0.8 mL min  1. Mass spectra were operated in both positive and negative mode. The injection volume was 50 mL.

3. Results and discussion 3.1. Hydroxyl radical reactions Fig. 1 presents the typical transient absorption spectra of DMP reacting with hydroxyl radical in N2O-saturated 0.5 mM DMP aqueous solutions. This spectrum shows a characteristic absorption from 300 to 350 nm with a maximum absorption at 320 nm. According to the previous report, the transient life-time and maximum absorption in the range 300–350 nm were characteristic of hydroxyclohexadienyl radicals resulting from the attack of hydroxyl radical to the aromatic ring (Merga et al., 1994; Merga et al., 1996), this absorption range 300–350 nm of the intermediates was attributed to the corresponding DMP hydroxyclohexadienyl adducts. The rate constant for the reaction of hydroxyl radical with a specific functional group could be determined by monitoring the growth kinetics as a function of substrate concentration (Mezyk et al., 2007; Song et al., 2009). The growth kinetics for DMP hydroxyclohexadienyl adduct were monitored at 320 nm by pulse radiolysis with initial DMP concentrations (C0) ranging from 0.12 mM to 1.23 mM, as inset in Fig. 2. The bimolecular radical rate constant for hydroxyl radical reaction with DMP was determined by plotting exponential curves to the pseudo-first-order growth kinetics (inset in Fig. 2) and fitting the pseudo-first-order rate constants (kobs) as a function of the initial concentrations of DMP (Fig. 2). A fit linear equation (Eq. (6)) was obtained. From the slope of the line, the bimolecular reaction rate constants of 3.4  109 M  1 s  1 for reaction of hydroxyl radical with DMP was determined. The obtained rate constants for hydroxyl radical reaction with DMP is comparable to that with benzene

H+ (CH3)3COH-dCH2(CH3)2COH+H2, k5 ¼5.0  105 M  1 s  1 (5)

Steady-state experiments were performed using a GJ-2-II electron accelerator with beam energy of 1.8 MeV at Applied Radiation Institute of Shanghai University. Samples were placed in radiation field about 30 cm distance from the source. The experiments were carried out mainly at absorbed doses of 1– 20 kGy and the dose rate was kept 0.045 kGy s  1. 2.3. Analytical procedures The loss of DMP was followed using a HPLC system (Aglient 1200) equipped with a C18 column (150 mm  4.6 mm) and an auto-sampler with the volume injection set to 10 mL. A VW detector monitored at 224 nm. The mobile phase was a mixture of acetonitrile and water (40:60, v/v) at a flow rate of 1.0 mL min  1. The organic acids produced from DMP electron beam radiolysis were determined by Ion Chromatography (IC-Metrohm MIC advanced) equipped with a METROSEP A SUPP 5–250 (5 mm particle size, 250 mm  4 mm) column. The determination of these organic acids was achieved on hydrophilic anion exchange column,

0.006

Absorbance

d

d

s

1

d

421

1 µs 10 µs 20 µs

0.004

0.002

0.000 300

350

400 Wavelength (nm)

450

500

Fig. 1. Transient absorption spectra obtained upon hydroxyl radical oxidation of DMP in N2O-saturated aqueous solutions.

M.-H. Wu et al. / Radiation Physics and Chemistry 80 (2011) 420–425

(7.5–7.8  109 M  1 s  1) from previous studies (Buxton et al., 1988), further supporting the initial formation of hydroxyclohexadienyl radical in our proposed mechanism. The measured value is accordant with the hydroxyl radical reaction being dominating at the benzene ring, with hardly any contribution from the abstraction of hydrogen atom on COOCH3  side chain. kobs ¼(3.4  109) C0 + 4.6  105 R2 ¼ 0.9967

(6)

3.2. Hydrated electron reactions The transient absorption spectra of hydrated electron with DMP were obtained under pulse radiolysis in aqueous solutions deoxygenated by N2 in the presence of t-BuOH. In pulse radiolysis, it was found that a strong absorption spectrum with a very short life-time appears in a broad length region from 450 to 760 nm (see Fig. 3). This short life-time and broad region strong absorption spectrum with maximum absorption around 700 nm were supposed to indicate hydrated electron. As shown in Fig. 3 and the inset, after

7 [DMP] 1.23 0.98 0.61 0.25 0.12

5

0.010 0.008 0.006 0.004 0.002

4

kobs ¼ (1.6  1010) C0 +2.1  106 R2 ¼0.9823 Absorbance

Rate constant (106 s-1)

6

-0.002 -0.5

0.0

0.5

1.0

1.5

2.0 2.5

Time (µs) 2 1 0.2

0.0

0.4

0.6 [DMP] (mM)

0.8

1.0

1.2

Fig. 2. Second-order rate constant determined for the reaction of hydroxyl radical with DMP monitoring growth at 320 nm. Inset: the growth kinetics for DMP transient products were monitored at 320 nm with initial DMP concentrations ranging from 0.12 to 1.23 mM.

In addition to the rate constants for hydroxyl radical/hydrated electron reaction with DMP, steady-state experiments were also carried out to investigate the stable products formed in these reactions. These experiments were performed using electron beam radiolysis, with products assigned using LC/MS/MS and IC under N2O-saturated (oxidative) or N2-saturated containing 0.1 M t-BuOH (reductive) conditions. In the presence of N2O, hydroxyl radical is the key active species responsible for the degradation of DMP. Hydrated electron is the main species in the irradiated DMP aqueous solutions containing t-BuOH. The samples were run on the HPLC to determine concentrations of DMP after electron beam radiolysis. Under oxidative and reductive conditions, as noticed in Fig. 5, DMP were rapidly degraded by electron beam irradiation, and at an absorbed dose of 20 kGy, more than 99% of the initial concentrations removed in

16

0.05 Absorbance

Absorbance

0.04

310 nm 640 nm

0.04

0.03

0.03 0.02 0.01 0.00 -0.3 0.0 0.3 0.6 0.9 1.2 1.5 1.8

0.02

[DMP] 0.06 0.12 0.25 0.37 0.49 mM

14

Time (µs)

0.05 µs 0.1 µs 1.0 µs

0.01

Rate constant (106 s-1)

0.05

(7)

3.3. Steady-state irradiations by electron beam

0.000

3

the decay of hydrated electron, a new transient species at 310 nm and long life-time was observed. Because hydrated electron can reduce a solute to produce the solute anion as a good nucleophile (Enomoto and LaVerne, 2008), the new transient species can be ascribed to the DMP anion. The rate constant for hydrated electron with DMP was measured by directly monitoring the change in the absorption of hydrated electron at 640 nm as a function of C0, as shown in Fig. 4. The decay curves with C0 ranging from 0.06 to 0.49 mM (see inset in Fig. 4) were fitted to pseudo-first-order exponential kinetics, from which the second-order linear plots shown in Fig. 4 were obtained. The slope of the fit line (Eq. (7)) is the bimolecular rate constant for the reaction of hydrated electron with DMP, and the obtained value is 1.6  1010 M  1 s  1. Determinations conducted the growth kinetics at 310 nm with the similar rate constant. The hydrated electron rate constant for DMP is about three orders magnitude times faster than for benzene (0.7– 1.3  107 M  1 s  1; Buxton et al., 1988). This difference between DMP and benzene indicates that most of the hydrated electron reactions must be transferred to the side chain other than to aromatic ring forming the corresponding DMP radical anions in ester groups.

12

0.05 0.04 0.03 0.02

Absorbance

422

0.01

10 8

0.0

6

0.3 0.6 Time (µs)

0.9

0.00 1.2

4 2

0.00

0.0 300

350

400

450

500

550

600

650

700

0.1

0.2

0.3

0.4

0.5

[DMP] (mM)

Wavelength (nm) Fig. 3. Transient absorption spectra obtained upon hydrated electron reduction of DMP in N2-saturated aqueous solutions containing t-BuOH. Inset: time profiles recorded at 310 and 640 nm in the pulse radiolysis.

Fig. 4. Second-order rate constant determined for the reaction of hydrated electron with DMP monitoring decay at 640 nm. Inset: the decay kinetics for hydrated electron was monitored at 640 nm with initial DMP concentrations ranging from 0.06 to 0.49 mM.

M.-H. Wu et al. / Radiation Physics and Chemistry 80 (2011) 420–425

423

0.10

0.5 0.08

Formic acid Acetic acid Oxalic acid

Concentration (mM)

[DMP] (mM)

0.4

Oxidative conditions Reductive conditions

0.3

0.2

0.06

0.04

0.1

0.02

0.0

0.00

0

5

10

15

0

20

5

10 Dose (kGy)

Absorbed dose (kGy) Fig. 5. Change of DMP concentrations as a function of absorbed doses under oxidative and reductive conditions.

both conditions. The degradation profiles of DMP were fitted by exponential decay kinetics, to give absorbed doses needed for 50% degradation of 0.5 and 1.0 kGy for oxidative and reductive conditions, respectively. These values are consistent with the reported initial G-value of 0.59 mmol J  1 for hydroxyl radicals in oxidative conditions (LaVerne and Pimblott, 1993; Song et al., 2008b) and of 0.27 mmol J  1 for hydrated electron in reductive conditions (Eqs. (1) and (4)) (Buxton et al., 1988), which further supports the fact that absorbed dose needed for 50% degradation of DMP under oxidative conditions is about two times faster than that in reductive conditions. LC/MS/MS analyses revealed a number of byproducts at detectable levels. According to the total ion chromatogram and the corresponding mass spectra, the decomposition products of DMP from electron beam irradiation were determined. Under the oxidative conditions, one product with m/z of 163 was observed, corresponding to the loss of 31 mass units (ester group) to DMP, and DMP was further determined by comparing the profiles of this product with that of DMP standards. The other major product with m/z of 179 corresponds to the addition of 16 mass units of DMP. This intermediate is in agreement with hydroxylation of the aromatic ring of DMP forming dimethyl hydroxyphthalate (DMHP) and such products have been reported in photolytic degradation of DMP (Chen et al., 2008; Yuan et al., 2008b). Additionally, two separate products with m/z of 109 were observed, and according to the masses of protonated molecule, [M+ H] + , 1, 4-benzoquinone and 1, 2-benzoquinone were proposed. However, under the reductive conditions, the product with m/z of 187 is proposed to be the result of phthalic acid (PA) loss of a water molecule and then addition of a potassium ion. PA is further confirmed by comparing the LC/MS/MS profiles of PA standard with that of this product. From the analyses of IC, many organic acids produced in DMP electron beam radiolysis were detected under oxidative conditions. Formic acid, acetic acid, oxalic acid three short-chain aliphatic carboxylic acids were identified by the IC of standard organic acids, and some other acids still need to be further determined. Fig. 6 shows the concentrations of these three determined acids. It was obvious that the concentration of formic acid reached a maximum point, and then declined with the increase in absorbed doses. On the contrary the concentrations of acetic acid and oxalic acid increased with the increase in absorbed doses. In addition, there was no acetic acid produced when absorbed dose was less than 2 kGy and the concentration of oxalic acid had a sharp increase after 5 kGy. It was presumed that the aromatic ring of DMP could be

15

20

Fig. 6. Determined short-chain aliphatic carboxylic acids produced in DMP degradation by electron beam irradiation in oxidative condition.

COOCH3

COOCH3

1

COOCH3

OH 2

COOCH3

OH OH CH3OOC 3

4

COOCH3 OH

HO OH

6

7

HCOOCH3 5

CH3OH 8

HCOOH 9

OH COOH O

O

HOOCCOOH 12

O 11 10 O Organic acids

CO2 + H2O

Scheme 1. A proposed radiolytic degradation pathway of DMP with  OH under oxidative conditions.

fractured around absorbed dose of 5 kGy. Whereas, no organic acids were observed by IC analysis under reductive conditions, suggested that no aliphatic carboxylic acids were produced from DMP radiolytic degradation under reductive conditions.

3.4. Mechanism of DMP degradation in oxidative/reductive conditions Based on the products identified under steady-state irradiations by electron beam and the transient absorption spectrum of DMP observed using pulse radiolysis in the present study, the mechanism of DMP radiolytic degradation under various conditions are proposed. Under the oxidative conditions, as shown in Scheme 1, electrophilic hydroxyl radicals played key roles in the DMP radiolytic degradation.

424

M.-H. Wu et al. / Radiation Physics and Chemistry 80 (2011) 420–425

Hydroxyl radicals are initially added to the electron-deficient aromatic ring of DMP forming the detected DMHP (2), which is supported by the pulse radiolysis of DMP. This is followed by loss of COOCH3  side chain, leading to the subsequent formation of product 5 and the isomers products 3 and 4. On the one hand, product 5 is hydrolyzed into methanol (8) and formic acid (9). Subsequently oxalic acid (12) is formed by a series of reactions. On the other hand, a further hydroxylation and loss of COOCH3  side chain result in the formation of hydroquinone (6) and pyrocatechol (7), which has been further proved by the results reported from Lin et al. (2009). It is known that compounds 6, 7 and benzoquinones (10 and 11) are the byproducts of phenol oxidation (Liu and Jiang, 2005), and benzoquinones are easily transformed into organic acids by the aromatic ring opening observed throughout the radiolysis process, which is confirmed by the results reported in the literature. Finally, all the products are mineralized into carbon dioxide and water under radiolytic degradation. Under the reductive conditions, nucleophile hydrated electrons initially transfer to ester groups of DMP leading to the generation of DMP ester radical anions (DMPd  ), which has been determined using pulse radiolysis in the present work. For there are many H + (Eq. (1)) produced in irradiated DMP aqueous solutions, DMPd will easily hydrolyze into monomethyl phthalate, which may undergo a further addition of hydrated electron and then hydrolysis into PA. Since there are no oxidative species in reductive conditions, PA cannot transform into carbon dioxide and water ultimately. However, PA is easily degraded and completely mineralized by other methods, such as biodegradation (Gu et al., 2009), photocatalytic degradation (Kaneco et al., 2006; Yuan et al., 2008b) and undergoing decarboxylation to form benzoic acid in thermal conditions (Onwudili and Williams, 2007). Therefore, DMP radiolytic degradation in reductive conditions could be used as a pretreatment technique. These results present that the importance of different reactive species produced in the radiolysis for attacking different sites of the solute under different conditions. The attack of hydroxyl radicals on the aromatic ring becomes dominating in comparison with the attack of the side chain under oxidative conditions. On the contrary, under the reductive conditions, hydrated electrons prefer to transfer to the side chain in contrast to the aromatic ring of DMP. Moreover, the intermediate products from DMP degradation under an oxidative condition are more abundant than those under a reductive condition.

4. Conclusions The rate constants and mechanisms for the reaction of hydroxyl radical and hydrated electron with DMP in aqueous solutions have been investigated using pulse radiolysis and electron beam radiolysis. The bimolecular rate constants for the reaction of hydroxyl radical and hydrated electron with DMP are 3.4  109 M  1 s  1 and 1.6  1010 M  1 s  1 determined in pulse radiolysis, respectively. Hydroxyl radicals were found to attack aromatic ring while hydrated electrons attacked the ester group of DMP. Byproducts from DMP radiolytic degradation under both oxidative and reductive conditions were determined by LC/MS/MS and IC analysis including dimethyl hydroxyphthalate, benzoquinone and shortchain aliphatic carboxylic acids in oxidative conditions, phthalic acid in reductive conditions. It was found that the possible reaction pathways could be influenced by oxidative and reductive conditions. All the products were completely mineralized into carbon dioxide and water ultimately under oxidative conditions.

Acknowledgments The authors thank the National Natural Science Foundation of China (Nos. 40973073, 40830744), National Key Technology R&D

Program in the 11th Five year Plan of China (Nos. 2008BAC32B03, 2009BAA24B04), Shanghai Leading Academic Discipline Project (No. S30109) and Shanghai Municipality Natural Science Foundation (Nos. 09ZR1411300, 09XD1401800) for financial support of this study.

References Baikova, S.V., Samsonova, A.S., Aleshchenkova, Z.M., Shcherbina, A.N., 1999. The intensification of dimethylphthalate destruction in soil. Eurasian Soil Science 32 (6), 701–704. Buxton, G.V., Greenstock, C.L., Helman, W.P., Ross, A.B., 1988. Critical review of rate constants for the reactions of hydrated electrons, hydrogen atom and hydroxyl radicals (dOH/dO  ) in aqueous solutions. Journal of Physical and Chemical Reference Data 17 (2), 513–886. Chang, C.C., Chiu, C.Y., Chang, C.Y., Chang, C.F., Chen, Y.H., Ji, D.R., Yu, Y.H., Chiang, P.C., 2009. Combined photolysis and catalytic ozonation of dimethyl phthalate in a high-gravity rotating packed bed. Journal of Hazardous Materials 161 (1), 287–293. Chen, Y.H., Shang, N.C., Hsieh, D.C., 2008. Decomposition of dimethyl phthalate in an aqueous solution by ozonation with high silica zeolites and UV radiation. Journal of Hazardous Materials 157 (2–3), 260–268. Ding, X.J., An, T.C., Li, G.Y., Chen, J.X., Sheng, G.Y., Fu, J.M., Zhao, J.C., 2008. Photocatalytic degradation of dimethyl phthalate ester using novel hydrophobic TiO2 pillared montmorillonite photocatalyst. Research on Chemical Intermediates 34 (1), 67–83. Enomoto, K., LaVerne, J.A., 2008. Reactions of hydrated electrons with pyridinium salts in aqueous solutions. Journal of Physical Chemistry A 112 (48), 12430–12436. Gledhill, W.E., Kaley, R.G., Adams, W.J., Hicks, O., Michael, P.R., Saeger, V.W., LeBlane, G.A., 1980. An environmental safety assessment of butyl benzyl phthalate. Environmental Science & Technology 14 (3), 301–305. Gu, J.G., Han, B.P., Duan, S.S., Zhao, Z.Y., Wang, Y.P., 2009. Degradation of the endocrine-disrupting dimethyl phthalate carboxylic ester by Sphingomonas yanoikuyae DOS01 isolated from the South China Sea and the biochemical pathway. International Biodeterioration & Biodegradation 63 (4), 450–455. Ikehata, K., Naghashkar, N.J., Ei-Din, M.G., 2006. Degradation of aqueous pharmaceuticals by ozonation and advanced oxidation processes: a review. OzoneScience & Engineering 28 (6), 353–414. Jobling, S., Reynolds, T., White, R., Parker, M.G., Sumpter, J.P., 1995. A variety of environmentally persistent chemicals, including some phthalate plasticizers, are weakly estrogenic. Environmental Health Perspectives 103 (6), 582–587. Kaneco, S., Katsumata, H., Suzuki, T., Ohta, K., 2006. Titanium dioxide mediated photocatalytic degradation of dibutyl phthalate in aqueous solution—kinetics, mineralization and reaction mechanism. Chemical Engineering Journal 125 (1), 59–66. LaVerne, J.A., Pimblott, S.M., 1993. Yields of hydroxyl radical and hydrated electron scavenging reactions in aqueous solutions of biological interest. Radiation Research 135 (1), 16–23. Liang, D.W., Zhang, T., Fang, H.H.P., 2007. Denitrifying degradation of dimethyl phthalate. Applied Microbiology and Biotechnology 74 (1), 221–229. Lin, Y.X., Ferronato, C., Deng, N.S., Wu, F., Chovelon, J.M., 2009. Photocatalytic degradation of methylparaben by TiO2: multivariable experimental design and mechanism. Applied Catalysis B-Environmental 88 (1–2), 32–41. Liu, Y.J., Jiang, X.Z., 2005. Phenol degradation by a nonpulsed diaphragm glow discharge in an aqueous solution. Environmental Science & Technology 39 (21), 8512–8517. Lottrup, G., Andersson, A.M., Leffers, H., Mortensen, G.K., Toppari, J., Skakkebaeek, N.E., Main, K.M., 2006. Possible impact of phthalates on infant reproductive health. International Journal of Andrology 29 (1), 172–180. Merga, G., Rao, B.S.M., Mohan, H., Mittal, J.P., 1994. Reactions of OH and SO4  with some halobenzenes and halotoluenes: a radiation chemical study. Journal of Physical and Chemical Reference Data 98 (37), 9158–9164. Merga, G., Schuchmann, H.P., Rao, B.S.M., vonSonntag, C., 1996. OH radical-induced oxidation of chlorobenzene in aqueous solution in the absence and presence of oxygen. Journal of the Chemical Society-Perkin Transactions 2 (6), 1097–1103. Mersiowsky, N., 2002. Long-term fate of PVC products and their additives in landfills. Progress in Polymer Science 27 (10), 2227–2277. Mezyk, S.P., Neubauer, T.J., Cooper, W.J., Peller, J.R., 2007. Free-radical-induced oxidative and reductive degradation of sulfa drugs in water: absolute kinetics and efficiencies of hydroxyl radical and hydrated electron reactions. Journal of Physical Chemistry A 111 (37), 9019–9024. Montuori, P., Jover, E., Morgantini, M., Bayona, J.M., Triassi, M., 2008. Assessing human exposure to phthalic acid and phthalate esters from mineral water stored in polyethylene terephthalate and glass bottles. Food Additives and Contaminants 25 (4), 511–518. Ogunfowokan, A.O., Torto, N., Adenuga, A.A., Okoh, E.K., 2006. Survey of levels of phthalate ester plasticizers in a sewage lagoon effluent and a receiving stream. Environmental Monitoring and Assessment 118 (1–3), 457–480. Onwudili, J.A., Williams, P.T., 2007. Hydrothermal oxidation of di-n-butylphthalate: product distribution and reaction mechanisms. Energy & Fuels 21 (3), 1528–1533.

M.-H. Wu et al. / Radiation Physics and Chemistry 80 (2011) 420–425

Penalver, A., Pocurull, E., Borrull, F., Marce, R.M., 2000. Determination of phthalate esters in water samples by solid-phase microextraction and gas chromatography with mass spectrometric detection. Journal of Chromatography A 872 (1–2), 191–201. Song, W.H., Chen, W.S., Cooper, W.J., Greaves, J., Miller, G.E., 2008a. Free-radical destruction of b-lactam antibiotics in aqueous solution. Journal of Physical Chemistry A 112 (32), 7411–7417. Song, W.H., Cooper, W.J., Mezyk, S.P., Greaves, J., Peake, B.M., 2008b. Free radical destruction of b-blockers in aqueous solution. Environmental Science & Technology 42 (4), 1256–1261. Song, W.H., Xu, T.L., Cooper, W.J., Dionysiou, D.D., De La Cruz, A.A., O’Shea, K.E., 2009. Radiolysis studies on the destruction of microcystin-LR in aqueous solution by hydroxyl radicals. Environmental Science & Technology 43 (5), 1487–1492. Staples, C.A., Peterson, D.R., Parkerton, T.F., Adams, W.J., 1997. The environmental fate of phthalate esters: a literature review. Chemosphere 35 (4), 667–749. US EPA, 1992. and update. Code of federal regulations. 40 CFR, Part 136. USEPA. Wang, J.B., Zhou, Y.R., Zhu, W.P., He, X.W., 2009. Catalytic ozonation of dimethyl phthalate and chlorination disinfection by-product precursors over Ru/AC. Journal of Hazardous Materials 166 (1), 502–507. Wang, Y.L., Yin, B., Hong, Y.G., Yan, Y., Gu, J.D., 2008. Degradation of dimethyl carboxylic phthalate ester by Burkholderia cepacia DA2 isolated from marine sediment of South China Sea. Ecotoxicology 17 (8), 845–852.

425

Wang, Y.Y., Fan, Y.Z., Gu, J.D., 2004. Dimethyl phthalate ester degradation by two planktonic and immobilized bacterial consortia. International Biodeterioration & Biodegradation 53 (2), 93–101. Xu, B., Gao, N.Y., Cheng, H.F., Xia, S.J., Rui, M., Zhao, D.D., 2009. Oxidative degradation of dimethyl phthalate (DMP) by UV/H2O2 process. Journal of Hazardous Materials 162 (2–3), 954–959. Yuan, B.L., Li, X.Z., Graham, N., 2008a. Aqueous oxidation of dimethyl phthalate in a Fe(VI)-TiO2-UV reaction system. Water Research 42 (6–7), 1413–1420. Yuan, B.L., Li, X.Z., Graham, N., 2008b. Reaction pathways of dimethyl phthalate degradation in TiO2-UV-O2 and TiO2-UV-Fe(VI) systems. Chemosphere 72 (2), 197–204. Zeng, F., Wen, J.X., Cui, K.Y., Wu, L.N., Liu, M., Li, Y.J., Lin, Y.J., Zhu, F., Ma, Z.L., Zeng, Z.X., 2009. Seasonal distribution of phthalate esters in surface water of the urban lakes in the subtropical city, Guangzhou, China. Journal of Hazardous Materials 169 (1–3), 719–725. Zhang, S.J., Jiang, H., Li, M.J., Yu, H.Q., Yin, H., Li, Q.R., 2007. Kinetics and mechanisms of radiolytic degradation of nitrobenzene in aqueous solutions. Environmental Science & Technology 41 (6), 1977–1982. Zhao, X.K., Yang, G.P., Wang, Y.J., Gao, X.C., 2004. Photochemical degradation of dimethyl phthalate by Fenton reagent. Journal of Photochemistry and Photobiology A-Chemistry 161 (2–3), 215–220. Zhou, Y.R., Zhu, W.P., Liu, F.D., Wang, J.B., Yang, S.X., 2007. Catalytic activity of Ru/ Al2O3 for ozonation of dimethyl phthalate in aqueous solution. Chemosphere 66 (1), 145–150.