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Oxidative paraben removal with chlorine dioxide: Reaction kinetics and mechanism
Separation and Purification Technology xxx (xxxx) xxxx
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Oxidative paraben removal with chlorine dioxide: Reaction kinetics and mechanism ⁎
Qianhui Maoa,b,c, Qi Lia, Huimin Lia, Shoujun Yuanb, , Jibiao Zhanga,
⁎
a
Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China Department of Municipal Engineering, School of Civil and Hydraulic Engineering, Hefei University of Technology, Hefei 230009, China c Shandong Shengkai Architectural Design Consulting Ltd, Yantai 264000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Parabens Chlorine dioxide Kinetics Degradation products Reaction pathway
This study investigated the reactivity and mechanism of paraben oxidization by chlorine dioxide. Degradation kinetics of methylparaben (MeP) and propylparaben (PrP) followed pseudo-first-order kinetics, and were studied in a pH range of 4.0–7.0 at 15 ± 1 °C. The kapp values were 84.3 M−1·s−1 and 83.2 M−1·s−1 for MeP and PrP, respectively, when pH value was 5.0 and the temperature was 15 ± 1 °C. The rate constants depended on the solution pH, temperature, humic acid, and HCO3– concentrations. At pH values of 4–5.0, kapp increased slowly, while at a pH value of 7.0, kapp increased rapidly. The reaction rate constants increased with increasing temperature. Furthermore, the reaction rate was inhibited by humic acid addition, while the reaction rate increased after addition of HCO3–. Two main degradation products were identified by GC–MS and the reaction pathways were proposed.
1. Introduction Parabens, the esters of 4-hydroxybenzoic acid, offer several advantages including highly efficient activity against the growth and reproduction of bacteria over a wide pH range [1]. These advantages led to the wide use of parabens as antimicrobial preservatives in cosmetics, food packing, and pharmaceuticals [2–4]. Methylparaben (MeP) and propylparaben (PrP) (chemical structure shown in Fig. 1) are the most commonly used parabens and are often used together in products [5]. As a result, both have been found in surface water, groundwater, and drinking water [6–8]. MeP in effluent water (treated water already used for drinking water supply) from drinking water treatment plant was at 21–80 ng L−1. The concentrations of MeP and PrP in the Pearl River Delta reached 1.062 μg L−1 and 3.142 μg L−1, respectively [9]. Furthermore, the concentrations of MeP and PrP in raw water reached 30 μg L−1 and 20 μg L−1, respectively [10]. Many studies have reported estrogenic activity of parabens and this estrogenic effect increases with increasing chain length of the ester group [11,12]. A potential relationship between breast cancer and paraben application has also been suggested since these compounds have been found in the human breast [13,14]. Among the oxidants, chlorine dioxide (ClO2) has been increasingly employed as disinfectant in water treatment systems due to its antibacterial and anti-viral properties [15,16]. It has been shown that ClO2 ⁎
can be used as an alternative to chlorine and it is effective for the reduction of chlorinated products [17–20]. As a powerful oxidant (Ea = 0.936), ClO2 can remove many organic and inorganic pollutants [21,22]. Previous studies reported oxidative degradation via ClO2 of a number of pharmaceutical contaminants such as diclofenac, estrogenic 17α-ethinylestradiol, roxithromycin, sulfonamide antibiotics, pyrazolone pharmaceuticals and fluoroquinolones [15,23–26]. Considering the increasing use of ClO2 for water supply treatment, it is very important to investigate the reactions of ClO2 with the most extensively consumed pharmaceutical drugs, that are regularly detected in surface water. Furthermore, the fate of ClO2 oxidized parabens remains unclear. Recently, the removal of parabens by ozone, and their photodegradation with TiO2, Fe(III)-citrate, and a H2O2/UV system have been reported [2,27–29]. In the H2O2/UV system, the rate constant for the reaction between the hydroxyl radical and butylparaben depended on solution pH and temperature. However, second-order rate constants for the reaction of ozone with PrP strongly decreased in acidic solution compared to alkaline solution. From a mechanistic point of view, numerous oxidation products, produced by different oxidants also have been reported [2,30]. However, the fate of parabens oxidized by ClO2 disinfection process is unclear and the kinetic study has not been reported concerning parabens with ClO2. This study (1) determined the kinetics for the reaction of ClO2 with
Corresponding authors. E-mail addresses: [email protected] (S. Yuan), [email protected] (J. Zhang).
https://doi.org/10.1016/j.seppur.2019.116327 Received 26 May 2019; Received in revised form 14 November 2019; Accepted 17 November 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Qianhui Mao, et al., Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116327
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2.3. Analytical methods
MeP
The paraben concentrations were determined using an Agilent 1200 series high-performance liquid chromatograph (HPLC) equipped with a UV detector at 254 nm. Parabens and their decomposition products were separated using an HC-C18 (4.6 × 150 mm, 5 µm) column (Agilent, USA) equipped with an ECS C18 (4.6 × 10 mm, 5 µm) guard column (Elite, China). A water-methanol mixture (3/7, v/v) was used for the mobile phase at a flow rate of 1.0 mL min−1. The major byproducts that formed during paraben degradation were analyzed using a gas chromatograph-mass spectrometer (GC–MS, PerkinElmer Clarus 680 + Clarus SQ 8T, USA). Each sample was prepared via the following process: 5 mL of the sample was withdrawn from the reactor and freeze-dried for 12 h. Then, the sample was redissolved with 1 mL DMF and filtered through 0.20 µm mixed cellulose ester membranes; a volume of 100 µL of the derivatization reagent BSTFA was added to replace the acidic hydrogen with non-polar trimethylsilyl groups. Finally, after 1 h of incubation at 70 °C, the derivatized sample was analyzed using GC–MS. Separation was accomplished using an HP-5Ms capillary column (Agilent, 30.0 m × 250 µm × 0.25 µm). The GC oven temperature was set to 60.0 °C for 1 min, and then increased to 300 °C at 15 °C·min−1 which was held for 5 min. The injector temperature was set to 250 °C, and the injection volume was 1.0 µL. The temperature of both ion source and interface were set to 250 °C. Helium was used as carrier gas. The flow rates and bypass of the gas were 1.0 mL·min−1 and 20.0 mL·min−1, respectively. The electron ionization voltage was 70 eV. The concentration of the ClO2 stock solution was standardized prior to application by iodometric titration with a standard sodium thiosulfate solution. The total organic carbon (TOC) content was measured with a TOC analyzer (TOC-VCPN, Shimadzu, Japan). Samples were filtered through a polysulfone membrane filter (0.22 μm).
PrP
Fig. 1. The chemical structures of MeP and PrP.
MeP and PrP; (2) investigated the effects of pH value, temperature, humic acid, and HCO3– on the reaction of ClO2 with MeP and PrP; (3) identified the degradation products via GC–MS; and (4) proposed the reaction mechanisms for the degradations of MeP and PrP based on the identified byproducts and the underlying kinetic model. The results clarify the fate and behavior of parabens during water disinfection by ClO2 and provide helpful information about the safety of water treatment in the case of paraben micropollution.
2. Materials and methods 2.1. Materials Methylparaben (MeP, 98%), propylparaben (PrP, 99%), Sodium chlorite (NaClO2, 90% purity), and methanol (HPLC grade) were purchased from Aladdin Industrial Corporation (Shanghai, China). Stock solutions of MeP and PrP (100 μM) were prepared in deionized water and stored at 4 °C until further use. A pure solution of ClO2 was generated from gaseous ClO2 by slowly adding diluted H2SO4 to a NaClO2 solution. Impurities such as chlorine were removed from the N2 gas stream by a NaClO2 scrubber. The gaseous ClO2 was introduced into ultra-pure water and stored in a brown bottle at 4 °C in the dark to slow its decomposition. N,N-dimethylformamide (DMF, 99.9%) was purchased from CNW Technologies (Germany) and the derivatization reagent N,O-bis (trimethylsilyl)-trifluoroacetamide (BSTFA, 99.0%) was purchased from Tokyo Chemical Industry Co., Ltd. (Japan). Other reagents were of analytical grade and were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
3. Results and discussion 3.1. Kinetic for parabens and ClO2 reactions In response to a large excess of ClO2 relative to the fixed initial concentration of MeP and PrP (5 μM), the reaction exhibited a pseudofirst order dependence on the MeP and PrP concentration as demonstrated by the linearity of the time-course plot of ln([paraben]t/ [paraben]0). Fig. 2 presents the pseudo-first order kinetic plot of the reaction between MeP, PrP, and ClO2 and the correlation coefficients R2 were > 0.985. For each experiment, the following equations can be written:
2.2. Paraben degradation experiments All experiments were performed in 250-mL batch reactors equipped with small stirring bars to ensure homogeneous reaction conditions at 15 ± 1 °C. The solutions were maintained at a pH of approximately 5.0 using 20 mM acetate buffer. In the kinetics experiment, both MeP and PrP (5 μM) degradation by ClO2 were studied under pseudo-first-order conditions with at least 10-fold excess of ClO2 (50–100 μM). A 1.0-mL sample was collected at regular time intervals during the reaction and was immediately quenched with 10 µL sodium thiosulfate (0.1 M). The working volume was 100 mL. To investigate the effects of pH, temperature, humic acid, and HCO3– on the reaction of ClO2 with both parabens, 5 μM of each paraben and 50 μM ClO2 (100 mL) were used. 20 mM acetate buffer (pH 4.0–6.0) and 20 mM phosphate buffer solution (pH 7.0–9.0) were used to maintain the respective solution pH. The variation of the solution pH at the initial and final points of the experiment remained below 0.1. For the investigation of the effect of temperature, the temperature of the reaction solution (varying from 5 ± 1 to 30 ± 1 °C) was maintained using a water or ice bath. The effect of the humic acid concentration (1.0–15 mg L−1) on the oxidation was evaluated by adding humic acid stock solution (0.5 g L−1) to the reaction system. To identify the transformation products, the experiments were performed with 100 μM parabens and 400 μM ClO2 without pH adjustment. All experiments were conducted in duplicate and all experiments with MeP and PrP were performed separately.
−
d[paraben] = kobs [paraben] dt
(1)
or
ln
[paraben]t = −kobs t [paraben]0
(2)
where kobs represents the pseudo-first-order kinetic constant, and for each experiment, the kobs could be calculated according to the slope of the relationship between ln([paraben]t/[paraben]0) and time. [paraben] represents the concentrations of MeP and PrP. With increasing ClO2 concentration, the kobs for MeP increased from 4.22 × 10−3 s−1 to 9.03 × 10−3 s−1 and the kobs for PrP increased from 4.16 × 10−3 s−1 to 1.03 × 10−2 s−1. The reaction order relative to ClO2 can be obtained by representing the kobs variation as a function of the ClO2 concentration. The insert in Fig. 2 shows that the pseudofirst-order kinetic constant is proportional to the ClO2 concentration with a R2 > 0.973. These results indicate that the kinetics for the reaction of parabens with ClO2 follows a second-order reaction, firstorder relative to the MeP and PrP concentration, and first-order relative to the ClO2 concentration. The equation can be written as: 2
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0.010
-1
obs
0.005
0
20
40
60
80
100
ClO (ȝM) 2
[ClO2]0=50
ȝM R2=0.985
0.000
-1
0
ȝM R2=0.985
2
60
80
100
-2 [ClO2]0=50 [ClO2]0=75
-3
ȝM R2=0.992 ȝM R2=0.988
[ClO2]0=87.5 ȝM R2=0.994
[ClO2]0=100 ȝM R2=0.993
0
40
[ClO2]0=62.5 ȝM R2=0.991
[ClO2]0=87.5 ȝM R2=0.994
-3
20
ClO (ȝM) 2
[ClO2]0=62.5 ȝM R2=0.989 [ClO2]0=75
0.005
k
ln([PrP]t[PrP]0)
0.000
-1
-2
R2=0.973
(s )
-1
(s ) obs
0.010
0
R2=0.989
k
ln([MeP]t[MeP]0)
0
[ClO2]0=100 ȝM R2=0.993
4
6
8
0
2
7LPe (PLn)
4
6
8
7LPe (PLn)
Fig. 2. The Peseudo-first-order kinetic plots of the reactions between parabens and ClO2 (pH = 5.0, [paraben]0 = 5 μM, T = 15 ± 1 °C).
MeP
1600
7.40×102
600
kapp (M-1s-1)
kapp (M-1s-1)
800
kapp
400
200
1.70×103
PrP
1200
kapp
800
400 4.24
24.7
1.32×102
0
9.33
32.3
4
5
1.84×102
0
4
5
6
pH
7
6
7
pH
Fig. 3. Effects of pH values on the rate constants of the reactions between parabens and ClO2 ([paraben]0 = 5 μM, [ClO2]0 = 50 μM, T = 15 ± 1 °C).
ln([PrPP]t/[PrP]0)
-1
0
PrP
6
ln([MeP]t/[MeP]0)
6
0
MeP
-1
-2
-2 5°C 10°C 15°C 23.5°C 29°C
5
-4 0
3
6
9
Time (min)
-4 0
3
6
9
Time (min)
4
y=32.05-7.95x R2=0.990
4
5°C 10°C 15°C 25°C 30°C
-3
5
lnkapp
lnkapp
-3
y=29.40-7.23x R2=0.989
3 3 3.3
3.4
3.5
3.6
3.3
1000/T (K-1)
3.4
3.5
3.6
1000/T (K-1)
Fig. 4. The Arrhenius plots of ln kapp vs. 1/T. (pH = 5.0, [paraben]0 = 5 μM, [ClO2]0 = 50 μM).
−
d[paraben] = kapp [paraben][ClO2] dt
t1/2 =
(3)
with
kapp = kobs /[ClO2]
ln 2 kobs
(5)
where kapp (M−1·s−1) represents the second-order kinetic constant. For each experiment kapp was determined by Eq. (4); [ClO2] represents the
(4) 3
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1.0
1.0
0.6 0.4
0.6 0.4
0.2
0.2
0.0
0.0
0
2
4
6
8
PrP
0 mg/L 1 mg/L 5 mg/L 10 mg/L 15 mg/L
0.8
[PrP]t/[PrP]0
0.8
[MeP]t/[MeP]0
MeP
0 mg/L 1 mg/L 5 mg/L 10 mg/L 15 mg/L
10
0
2
4
6
8
10
Time (min)
Time (min)
Fig. 5. Effects of humic acid on the reactions between parabens and ClO2 (pH = 7.0, [paraben]0 = 5 μM, [ClO2]0 = 50 μM, T = 15 ± 1 °C).
MeP
1.0 0 mg/L 20 mg/L 60 mg/L 100 mg/L
0.6 0.4
0.6 0.4
0.2
0.2
0.0
0.0 0
2
4
6
8
0 mg/L 20 mg/L 60 mg/L 100 mg/L
0.8
[PrP]t/[PrP]0
[MeP]t/[MeP]0
0.8
PrP
1.0
10
0
2
4
6
8
10
Time (min)
Time (min)
Fig. 6. Effects of HCO3– on the reactions between parabens and ClO2 (pH = 7.0, [paraben]0 = 5 μM, [ClO2]0 = 50 μM, T = 15 ± 1 °C).
TOC
1.5
0.4
0.6
0.2
0.3
0.0
0.0 0
10
20
30
40
50
60
[PrP]t/[PrP]0
0.9
4.2
pH
0.6
[PrP]t/[PrP]0
3.6
3.0
5
1.5
pH
0.6
1.2
0.4
0.9
0.2
0.6
0.0
0.3 0
Time (min)
1.8
TOC
0.8
pH 1.2
PrP
1.0
4.8
10
20
30
40
50
60
4
pH
1.8
TOC (Ct/C0)
[MeP]t/[MeP]0
0.8
[MeP]t/[MeP]0
TOC (Ct/C0)
MeP
1.0
3
2
Time (min)
Fig. 7. The mineralization processes of the reactions between ClO2 and parabens ([paraben]0 = 100 μM, [ClO2]0 = 400 μM).
3.2. Effect of different water parameters on MeP and PrP by ClO2 oxidation
concentration of ClO2; and t1/2 represents the half-life of MeP and PrP. The values of kapp were determined as 84.3 M−1·s−1 and 83.2 M−1·s−1 for MeP and PrP, respectively, at pH 5.0 and with a concentration of ClO2 at 50 μM. When ClO2 concentration was 50 μM, the experimental t1/2 values of MeP and PrP were calculated by Eq. (5) as 164 s and 166 s, respectively. However, during the chlorination of both parabens, 50% of four parabens were eliminated with the total chlorine concentration of 7.45 mg L−1 at about 210 s [31].
3.2.1. Effects of pH on MeP and PrP oxidation A number of studies indicated that pH values significantly influence the reaction of organic compounds and ClO2 [32–35]. As shown in Fig. 3, for pH values ranging from 4 to 5, kapp increased slowly, which kapp increased rapidly with further increasing pH values. As the pH values increased from 4 to 7, the kapp for MeP increased from 4
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2.0x108
Relative abundance
1.0x108 5.0x107
9.41 min MeP
substitution or addition, which is the case with chlorine or hypochlorite [39]. With increasing OH– concentration, ClO2 exists in various forms, and the increase of its oxidation property can be explained in many ways [40]. In addition, the reaction of ClO2 with tetracycline antibiotics, methiocarb, and azo dye amaranth showed a similar trend and the kapp values increased gradually with increasing pH value [20,36,40].
MeP
12.18 min P3
1.5x108
11.18 min P1
0.0 9.0 1.5x108
9.5
10.0
1.0x108
10.73 min PrP
10.5
11.0
11.5
12.0
12.5
13.0 PrP
12.98 min P4
3.2.2. Effect of temperature on MeP and PrP oxidation Solution temperature is an important parameter for the reaction between ClO2 and parabens. As shown in the insert of Fig. 4, when the temperature increased from 5 °C to 30 °C, the reaction rate of MeP and PrP clearly increased. Moreover, the kapp for MeP increased from 31.0 M−1·s−1 to 2.97 × 102 M−1·s−1 and the kapp for PrP increased from 26.7 M−1·s−1 to 2.33 × 103 M−1·s−1. According to Fig. 4, lnkapp and 1/T showed a good linear relationship and the Arrhenius formula was used to determine the activation energy (Ea, kJ mol−1):
12.25 min P2
5.0x107 0.0 10.5
11.0
11.5
12.0
12.5
13.0
13.5
Time (min) Fig. 8. The GC–MS chromatograms of the paraben samples treated by ClO2.
4.24 M−1·s−1 to 7.40 × 102 M−1·s−1 and the kapp for PrP increased from 9.33 M−1·s−1 to 1.70 × 103 M−1·s−1. As the solution pH increases, the oxidation reduction potential of ClO2 also increases [36]. Moreover, ClO2 is relatively inert under acidic conditions and becomes more reactive under neutral and alkaline conditions [37,38]. However, in reactions of ClO2 as oxidant, mainly free radical electrophilic abstraction occurred, rather than oxidative
ln kapp = -
Ea + lnA RT
(6) −1
where R represents the molar gas constant (8.314 J (mol K) ), T represents the absolute temperature (K), and A represents the pre-exponential factor. According to Eq. (6), the activation energy of MeP is 66 kJ mol−1 and the Ea of PrP was 60 kJ mol−1. Compared to the reaction of HOCl with four parabens, with activation energies of
Fig. 9. The mass spectra and the proposed degradation products for the reactions between parabens and ClO2. 5
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and PrP oxidized by ClO2 increased by HCO3– addition. The removal efficiency of MeP increased from 76.4% to 90.2% with 100 mg L−1 of HCO3– after a reaction time of 5 min. Similar results were also obtained for PrP. This can be attributed to the following reaction [43]:
Table 1 The proposed degradation products for the reactions between parabens and ClO2. Retention time (min)
Name
Molecular formula
m/z
9.41
MeP
C8H8O3
209, 224
10.73
PrP
C10H12O3
193, 210
Chemical structure
HCO3– + %OH → HCO3% + HO–
11.18
P1
C8H8O4
193
12.25
P2
C10H12O4
193, 207
12.18
P3
C8H8O5
281
12.98
P4
C10H12O5
239, 281
(7) %
According to previous reports, OH is generated in the ClO2 reaction system [44]. When HCO3– was added to the system, HCO3· formed, which is a relatively strong oxidant [45,46]. Therefore, the reaction rate of parabens and ClO2 increased. 3.3. Mineralization of parabens during the oxidation reaction Fig. 7 shows the TOC concentrations during the oxidations of MeP and PrP. According to Fig. 7, MeP and PrP were completely removed after a reaction time of 1 h, while the concentrations of TOC remained almost unchanged. These results indicated that no mineralization occurred during the reaction and several organic intermediates had formed. Compared to the chlorination of parabens [31], the results were similar. Therefore, it is necessary to further identify the products of the oxidation of MeP and PrP by ClO2. 3.4. Oxidation product identification and proposed reaction pathways The oxidation products of MeP and PrP during ClO2 oxidation were analyzed by GC–MS. The chromatograms of the samples in scan mode are shown in Fig. 8, and three major peaks for MeP and PrP oxidation products were detected. According to mass analysis, the detected substances were two types of oxidation products (P1, P2, P3, and P4) and two corresponding parabens. The mass spectra and proposed chemical structures of the oxidation products are shown in Fig. 9. The main information about these compounds is listed in Table 1. According to the identified oxidation products, the reaction pathways of MeP and PrP oxidized by ClO2 were proposed as shown in Fig. 10. Hydroxylation was found to be a significant reaction in the oxidation of parabens by ClO2, mostly at the aromatic paraben ring. First, hydroxylation occurred on the 3C of parabens to yield P1 (MeP) and P2 (PrP); then, %OH continued to oxidize P1 and P2 to form P3 and P4, respectively. Similar products were also detected during ozone oxidation and UV photolysis of parabens [2,30]. P3 and P4 belong to the gallic acid ester compound, which is an antioxidant that is non-toxic and non-irritating to the skin and tissue [47]. At the same time, chlorination was also the important reaction pathway in the ClO2 reaction system [48]. Paraben degradation maybe involved chlorination reaction with the nucleophilic attack of chlorine, meanwhile degradation of the intermediates involving %OH radical attack at a halogenated site led to the formation of P1 and P2 by replacing one chlorine atom with an %OH group. Hydroxylation reaction of the intermediates promoted by %OH led to P3 and P4 formation. P1 and P2 degradation maybe involved chlorination reaction, meanwhile %OH radical attacked the intermediates and P3 and P4 formed.
36.53–47.81 kJ mol−1, the higher activation energy of the reaction of ClO2 with the two parabens indicated that the oxidation process is more difficult [31]. 3.2.3. Effect of humic acid on MeP and PrP oxidation The influences of different humic acid concentrations on the oxidation of MeP and PrP by ClO2 are illustrated in Fig. 5. Humic acid inhibited the removals of MeP and PrP by ClO2. In the presence of humic acid at concentrations of 0, 1, 5, 10, and 15 mg L−1, the removal efficiency of MeP decreased from 97% to 60%. Similarly, the removal efficiency of PrP decreased from 98% to 50%. During oxidation, humic acid competed with parabens for ClO2, which was originally used to oxidize parabens. Therefore, the oxidation of parabens by ClO2 was weakened and the reaction rates of MeP and PrP oxidized by ClO2 greatly decreased. The degradation of diclofenac by ClO2 was also significantly inhibited by humic acid [41]. Humic acid is a common molecular organic compound in sewage water; thus, its effects on the reaction system should be fully considered in the process of oxidation by ClO2.
4. Conclusions This study investigated the degradation kinetics of MeP and PrP by ClO2. The reaction between ClO2 and parabens were of second order, with first-order in parabens and ClO2, respectively. The kapp values were 84.3 M−1·s−1 and 83.2 M−1·s−1 for MeP and PrP, respectively, at pH 5.0 and with a ClO2 concentration of 50 μM. The reaction rate increased with increasing pH and temperature, respectively. The paraben degradation was significantly inhibited after the addition of humic acid, while the reaction rate increased after adding HCO3–. ClO2 treatment is an efficient method for paraben degradation, which leads to the rapid degradation of parabens within about 10 min. Due to the presence of intermediates during the reaction, the mineralization process of
3.2.4. Effect of HCO3– concentration on MeP and PrP oxidation It has been reported that HCO3– could improve the oxidation rate of organic compounds [42]. As shown in Fig. 6, the removal rates of MeP 6
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Fig. 10. The proposed reaction pathways for the reactions between parabens and ClO2.
parabens was low. Two main products were generated for each paraben via hydroxylation. Furthermore, both products were less toxic than the parent compound. Therefore, paraben degradation by ClO2 is an efficient and safe method.
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5. Author contribution statement Qianhui Mao: Conducting experiments, Data curation. Qi Li: Data Analysis. Huimin Li: Writing- Original draft preparation. Shoujun Yuan: Writing- Reviewing and Editing. Jibiao Zhang: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The study was supported by the Major Science and Technology Program for Water Pollution Control and Treatment (2012ZX07103004). We would like to thank Dr. Shuwen Yan and Dr. Lushi Lian from Prof. Weihua Song group for their help in the analysis process. References [1] M. Aguilar-Bernier, A.I. Bernal-Ruiz, F. Rivas-Ruiz, M.T. Fernández-Morano, M. de Troya-Martín, Contact sensitization to allergens in the Spanish standard series at Hospital Costa del Sol in Marbella, Spain: a retrospective study (2005–2010), Actas Dermo-Sifiliográficas 103 (2011) 223–228.
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8
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Oxidative paraben removal with chlorine dioxide: Reaction kinetics and mechanism ⁎
Qianhui Maoa,b,c, Qi Lia, Huimin Lia, Shoujun Yuanb, , Jibiao Zhanga,
⁎
a
Department of Environmental Science and Engineering, Fudan University, Shanghai 200433, China Department of Municipal Engineering, School of Civil and Hydraulic Engineering, Hefei University of Technology, Hefei 230009, China c Shandong Shengkai Architectural Design Consulting Ltd, Yantai 264000, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Parabens Chlorine dioxide Kinetics Degradation products Reaction pathway
This study investigated the reactivity and mechanism of paraben oxidization by chlorine dioxide. Degradation kinetics of methylparaben (MeP) and propylparaben (PrP) followed pseudo-first-order kinetics, and were studied in a pH range of 4.0–7.0 at 15 ± 1 °C. The kapp values were 84.3 M−1·s−1 and 83.2 M−1·s−1 for MeP and PrP, respectively, when pH value was 5.0 and the temperature was 15 ± 1 °C. The rate constants depended on the solution pH, temperature, humic acid, and HCO3– concentrations. At pH values of 4–5.0, kapp increased slowly, while at a pH value of 7.0, kapp increased rapidly. The reaction rate constants increased with increasing temperature. Furthermore, the reaction rate was inhibited by humic acid addition, while the reaction rate increased after addition of HCO3–. Two main degradation products were identified by GC–MS and the reaction pathways were proposed.
1. Introduction Parabens, the esters of 4-hydroxybenzoic acid, offer several advantages including highly efficient activity against the growth and reproduction of bacteria over a wide pH range [1]. These advantages led to the wide use of parabens as antimicrobial preservatives in cosmetics, food packing, and pharmaceuticals [2–4]. Methylparaben (MeP) and propylparaben (PrP) (chemical structure shown in Fig. 1) are the most commonly used parabens and are often used together in products [5]. As a result, both have been found in surface water, groundwater, and drinking water [6–8]. MeP in effluent water (treated water already used for drinking water supply) from drinking water treatment plant was at 21–80 ng L−1. The concentrations of MeP and PrP in the Pearl River Delta reached 1.062 μg L−1 and 3.142 μg L−1, respectively [9]. Furthermore, the concentrations of MeP and PrP in raw water reached 30 μg L−1 and 20 μg L−1, respectively [10]. Many studies have reported estrogenic activity of parabens and this estrogenic effect increases with increasing chain length of the ester group [11,12]. A potential relationship between breast cancer and paraben application has also been suggested since these compounds have been found in the human breast [13,14]. Among the oxidants, chlorine dioxide (ClO2) has been increasingly employed as disinfectant in water treatment systems due to its antibacterial and anti-viral properties [15,16]. It has been shown that ClO2 ⁎
can be used as an alternative to chlorine and it is effective for the reduction of chlorinated products [17–20]. As a powerful oxidant (Ea = 0.936), ClO2 can remove many organic and inorganic pollutants [21,22]. Previous studies reported oxidative degradation via ClO2 of a number of pharmaceutical contaminants such as diclofenac, estrogenic 17α-ethinylestradiol, roxithromycin, sulfonamide antibiotics, pyrazolone pharmaceuticals and fluoroquinolones [15,23–26]. Considering the increasing use of ClO2 for water supply treatment, it is very important to investigate the reactions of ClO2 with the most extensively consumed pharmaceutical drugs, that are regularly detected in surface water. Furthermore, the fate of ClO2 oxidized parabens remains unclear. Recently, the removal of parabens by ozone, and their photodegradation with TiO2, Fe(III)-citrate, and a H2O2/UV system have been reported [2,27–29]. In the H2O2/UV system, the rate constant for the reaction between the hydroxyl radical and butylparaben depended on solution pH and temperature. However, second-order rate constants for the reaction of ozone with PrP strongly decreased in acidic solution compared to alkaline solution. From a mechanistic point of view, numerous oxidation products, produced by different oxidants also have been reported [2,30]. However, the fate of parabens oxidized by ClO2 disinfection process is unclear and the kinetic study has not been reported concerning parabens with ClO2. This study (1) determined the kinetics for the reaction of ClO2 with
Corresponding authors. E-mail addresses: [email protected] (S. Yuan), [email protected] (J. Zhang).
https://doi.org/10.1016/j.seppur.2019.116327 Received 26 May 2019; Received in revised form 14 November 2019; Accepted 17 November 2019 1383-5866/ © 2019 Elsevier B.V. All rights reserved.
Please cite this article as: Qianhui Mao, et al., Separation and Purification Technology, https://doi.org/10.1016/j.seppur.2019.116327
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2.3. Analytical methods
MeP
The paraben concentrations were determined using an Agilent 1200 series high-performance liquid chromatograph (HPLC) equipped with a UV detector at 254 nm. Parabens and their decomposition products were separated using an HC-C18 (4.6 × 150 mm, 5 µm) column (Agilent, USA) equipped with an ECS C18 (4.6 × 10 mm, 5 µm) guard column (Elite, China). A water-methanol mixture (3/7, v/v) was used for the mobile phase at a flow rate of 1.0 mL min−1. The major byproducts that formed during paraben degradation were analyzed using a gas chromatograph-mass spectrometer (GC–MS, PerkinElmer Clarus 680 + Clarus SQ 8T, USA). Each sample was prepared via the following process: 5 mL of the sample was withdrawn from the reactor and freeze-dried for 12 h. Then, the sample was redissolved with 1 mL DMF and filtered through 0.20 µm mixed cellulose ester membranes; a volume of 100 µL of the derivatization reagent BSTFA was added to replace the acidic hydrogen with non-polar trimethylsilyl groups. Finally, after 1 h of incubation at 70 °C, the derivatized sample was analyzed using GC–MS. Separation was accomplished using an HP-5Ms capillary column (Agilent, 30.0 m × 250 µm × 0.25 µm). The GC oven temperature was set to 60.0 °C for 1 min, and then increased to 300 °C at 15 °C·min−1 which was held for 5 min. The injector temperature was set to 250 °C, and the injection volume was 1.0 µL. The temperature of both ion source and interface were set to 250 °C. Helium was used as carrier gas. The flow rates and bypass of the gas were 1.0 mL·min−1 and 20.0 mL·min−1, respectively. The electron ionization voltage was 70 eV. The concentration of the ClO2 stock solution was standardized prior to application by iodometric titration with a standard sodium thiosulfate solution. The total organic carbon (TOC) content was measured with a TOC analyzer (TOC-VCPN, Shimadzu, Japan). Samples were filtered through a polysulfone membrane filter (0.22 μm).
PrP
Fig. 1. The chemical structures of MeP and PrP.
MeP and PrP; (2) investigated the effects of pH value, temperature, humic acid, and HCO3– on the reaction of ClO2 with MeP and PrP; (3) identified the degradation products via GC–MS; and (4) proposed the reaction mechanisms for the degradations of MeP and PrP based on the identified byproducts and the underlying kinetic model. The results clarify the fate and behavior of parabens during water disinfection by ClO2 and provide helpful information about the safety of water treatment in the case of paraben micropollution.
2. Materials and methods 2.1. Materials Methylparaben (MeP, 98%), propylparaben (PrP, 99%), Sodium chlorite (NaClO2, 90% purity), and methanol (HPLC grade) were purchased from Aladdin Industrial Corporation (Shanghai, China). Stock solutions of MeP and PrP (100 μM) were prepared in deionized water and stored at 4 °C until further use. A pure solution of ClO2 was generated from gaseous ClO2 by slowly adding diluted H2SO4 to a NaClO2 solution. Impurities such as chlorine were removed from the N2 gas stream by a NaClO2 scrubber. The gaseous ClO2 was introduced into ultra-pure water and stored in a brown bottle at 4 °C in the dark to slow its decomposition. N,N-dimethylformamide (DMF, 99.9%) was purchased from CNW Technologies (Germany) and the derivatization reagent N,O-bis (trimethylsilyl)-trifluoroacetamide (BSTFA, 99.0%) was purchased from Tokyo Chemical Industry Co., Ltd. (Japan). Other reagents were of analytical grade and were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).
3. Results and discussion 3.1. Kinetic for parabens and ClO2 reactions In response to a large excess of ClO2 relative to the fixed initial concentration of MeP and PrP (5 μM), the reaction exhibited a pseudofirst order dependence on the MeP and PrP concentration as demonstrated by the linearity of the time-course plot of ln([paraben]t/ [paraben]0). Fig. 2 presents the pseudo-first order kinetic plot of the reaction between MeP, PrP, and ClO2 and the correlation coefficients R2 were > 0.985. For each experiment, the following equations can be written:
2.2. Paraben degradation experiments All experiments were performed in 250-mL batch reactors equipped with small stirring bars to ensure homogeneous reaction conditions at 15 ± 1 °C. The solutions were maintained at a pH of approximately 5.0 using 20 mM acetate buffer. In the kinetics experiment, both MeP and PrP (5 μM) degradation by ClO2 were studied under pseudo-first-order conditions with at least 10-fold excess of ClO2 (50–100 μM). A 1.0-mL sample was collected at regular time intervals during the reaction and was immediately quenched with 10 µL sodium thiosulfate (0.1 M). The working volume was 100 mL. To investigate the effects of pH, temperature, humic acid, and HCO3– on the reaction of ClO2 with both parabens, 5 μM of each paraben and 50 μM ClO2 (100 mL) were used. 20 mM acetate buffer (pH 4.0–6.0) and 20 mM phosphate buffer solution (pH 7.0–9.0) were used to maintain the respective solution pH. The variation of the solution pH at the initial and final points of the experiment remained below 0.1. For the investigation of the effect of temperature, the temperature of the reaction solution (varying from 5 ± 1 to 30 ± 1 °C) was maintained using a water or ice bath. The effect of the humic acid concentration (1.0–15 mg L−1) on the oxidation was evaluated by adding humic acid stock solution (0.5 g L−1) to the reaction system. To identify the transformation products, the experiments were performed with 100 μM parabens and 400 μM ClO2 without pH adjustment. All experiments were conducted in duplicate and all experiments with MeP and PrP were performed separately.
−
d[paraben] = kobs [paraben] dt
(1)
or
ln
[paraben]t = −kobs t [paraben]0
(2)
where kobs represents the pseudo-first-order kinetic constant, and for each experiment, the kobs could be calculated according to the slope of the relationship between ln([paraben]t/[paraben]0) and time. [paraben] represents the concentrations of MeP and PrP. With increasing ClO2 concentration, the kobs for MeP increased from 4.22 × 10−3 s−1 to 9.03 × 10−3 s−1 and the kobs for PrP increased from 4.16 × 10−3 s−1 to 1.03 × 10−2 s−1. The reaction order relative to ClO2 can be obtained by representing the kobs variation as a function of the ClO2 concentration. The insert in Fig. 2 shows that the pseudofirst-order kinetic constant is proportional to the ClO2 concentration with a R2 > 0.973. These results indicate that the kinetics for the reaction of parabens with ClO2 follows a second-order reaction, firstorder relative to the MeP and PrP concentration, and first-order relative to the ClO2 concentration. The equation can be written as: 2
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0.010
-1
obs
0.005
0
20
40
60
80
100
ClO (ȝM) 2
[ClO2]0=50
ȝM R2=0.985
0.000
-1
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2
60
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100
-2 [ClO2]0=50 [ClO2]0=75
-3
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[ClO2]0=87.5 ȝM R2=0.994
[ClO2]0=100 ȝM R2=0.993
0
40
[ClO2]0=62.5 ȝM R2=0.991
[ClO2]0=87.5 ȝM R2=0.994
-3
20
ClO (ȝM) 2
[ClO2]0=62.5 ȝM R2=0.989 [ClO2]0=75
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k
ln([PrP]t[PrP]0)
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(s )
-1
(s ) obs
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k
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[ClO2]0=100 ȝM R2=0.993
4
6
8
0
2
7LPe (PLn)
4
6
8
7LPe (PLn)
Fig. 2. The Peseudo-first-order kinetic plots of the reactions between parabens and ClO2 (pH = 5.0, [paraben]0 = 5 μM, T = 15 ± 1 °C).
MeP
1600
7.40×102
600
kapp (M-1s-1)
kapp (M-1s-1)
800
kapp
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200
1.70×103
PrP
1200
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400 4.24
24.7
1.32×102
0
9.33
32.3
4
5
1.84×102
0
4
5
6
pH
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6
7
pH
Fig. 3. Effects of pH values on the rate constants of the reactions between parabens and ClO2 ([paraben]0 = 5 μM, [ClO2]0 = 50 μM, T = 15 ± 1 °C).
ln([PrPP]t/[PrP]0)
-1
0
PrP
6
ln([MeP]t/[MeP]0)
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0
MeP
-1
-2
-2 5°C 10°C 15°C 23.5°C 29°C
5
-4 0
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-4 0
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y=32.05-7.95x R2=0.990
4
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-3
5
lnkapp
lnkapp
-3
y=29.40-7.23x R2=0.989
3 3 3.3
3.4
3.5
3.6
3.3
1000/T (K-1)
3.4
3.5
3.6
1000/T (K-1)
Fig. 4. The Arrhenius plots of ln kapp vs. 1/T. (pH = 5.0, [paraben]0 = 5 μM, [ClO2]0 = 50 μM).
−
d[paraben] = kapp [paraben][ClO2] dt
t1/2 =
(3)
with
kapp = kobs /[ClO2]
ln 2 kobs
(5)
where kapp (M−1·s−1) represents the second-order kinetic constant. For each experiment kapp was determined by Eq. (4); [ClO2] represents the
(4) 3
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1.0
1.0
0.6 0.4
0.6 0.4
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0.2
0.0
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MeP
0 mg/L 1 mg/L 5 mg/L 10 mg/L 15 mg/L
10
0
2
4
6
8
10
Time (min)
Time (min)
Fig. 5. Effects of humic acid on the reactions between parabens and ClO2 (pH = 7.0, [paraben]0 = 5 μM, [ClO2]0 = 50 μM, T = 15 ± 1 °C).
MeP
1.0 0 mg/L 20 mg/L 60 mg/L 100 mg/L
0.6 0.4
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0.0
0.0 0
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[MeP]t/[MeP]0
0.8
PrP
1.0
10
0
2
4
6
8
10
Time (min)
Time (min)
Fig. 6. Effects of HCO3– on the reactions between parabens and ClO2 (pH = 7.0, [paraben]0 = 5 μM, [ClO2]0 = 50 μM, T = 15 ± 1 °C).
TOC
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2
Time (min)
Fig. 7. The mineralization processes of the reactions between ClO2 and parabens ([paraben]0 = 100 μM, [ClO2]0 = 400 μM).
3.2. Effect of different water parameters on MeP and PrP by ClO2 oxidation
concentration of ClO2; and t1/2 represents the half-life of MeP and PrP. The values of kapp were determined as 84.3 M−1·s−1 and 83.2 M−1·s−1 for MeP and PrP, respectively, at pH 5.0 and with a concentration of ClO2 at 50 μM. When ClO2 concentration was 50 μM, the experimental t1/2 values of MeP and PrP were calculated by Eq. (5) as 164 s and 166 s, respectively. However, during the chlorination of both parabens, 50% of four parabens were eliminated with the total chlorine concentration of 7.45 mg L−1 at about 210 s [31].
3.2.1. Effects of pH on MeP and PrP oxidation A number of studies indicated that pH values significantly influence the reaction of organic compounds and ClO2 [32–35]. As shown in Fig. 3, for pH values ranging from 4 to 5, kapp increased slowly, which kapp increased rapidly with further increasing pH values. As the pH values increased from 4 to 7, the kapp for MeP increased from 4
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2.0x108
Relative abundance
1.0x108 5.0x107
9.41 min MeP
substitution or addition, which is the case with chlorine or hypochlorite [39]. With increasing OH– concentration, ClO2 exists in various forms, and the increase of its oxidation property can be explained in many ways [40]. In addition, the reaction of ClO2 with tetracycline antibiotics, methiocarb, and azo dye amaranth showed a similar trend and the kapp values increased gradually with increasing pH value [20,36,40].
MeP
12.18 min P3
1.5x108
11.18 min P1
0.0 9.0 1.5x108
9.5
10.0
1.0x108
10.73 min PrP
10.5
11.0
11.5
12.0
12.5
13.0 PrP
12.98 min P4
3.2.2. Effect of temperature on MeP and PrP oxidation Solution temperature is an important parameter for the reaction between ClO2 and parabens. As shown in the insert of Fig. 4, when the temperature increased from 5 °C to 30 °C, the reaction rate of MeP and PrP clearly increased. Moreover, the kapp for MeP increased from 31.0 M−1·s−1 to 2.97 × 102 M−1·s−1 and the kapp for PrP increased from 26.7 M−1·s−1 to 2.33 × 103 M−1·s−1. According to Fig. 4, lnkapp and 1/T showed a good linear relationship and the Arrhenius formula was used to determine the activation energy (Ea, kJ mol−1):
12.25 min P2
5.0x107 0.0 10.5
11.0
11.5
12.0
12.5
13.0
13.5
Time (min) Fig. 8. The GC–MS chromatograms of the paraben samples treated by ClO2.
4.24 M−1·s−1 to 7.40 × 102 M−1·s−1 and the kapp for PrP increased from 9.33 M−1·s−1 to 1.70 × 103 M−1·s−1. As the solution pH increases, the oxidation reduction potential of ClO2 also increases [36]. Moreover, ClO2 is relatively inert under acidic conditions and becomes more reactive under neutral and alkaline conditions [37,38]. However, in reactions of ClO2 as oxidant, mainly free radical electrophilic abstraction occurred, rather than oxidative
ln kapp = -
Ea + lnA RT
(6) −1
where R represents the molar gas constant (8.314 J (mol K) ), T represents the absolute temperature (K), and A represents the pre-exponential factor. According to Eq. (6), the activation energy of MeP is 66 kJ mol−1 and the Ea of PrP was 60 kJ mol−1. Compared to the reaction of HOCl with four parabens, with activation energies of
Fig. 9. The mass spectra and the proposed degradation products for the reactions between parabens and ClO2. 5
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and PrP oxidized by ClO2 increased by HCO3– addition. The removal efficiency of MeP increased from 76.4% to 90.2% with 100 mg L−1 of HCO3– after a reaction time of 5 min. Similar results were also obtained for PrP. This can be attributed to the following reaction [43]:
Table 1 The proposed degradation products for the reactions between parabens and ClO2. Retention time (min)
Name
Molecular formula
m/z
9.41
MeP
C8H8O3
209, 224
10.73
PrP
C10H12O3
193, 210
Chemical structure
HCO3– + %OH → HCO3% + HO–
11.18
P1
C8H8O4
193
12.25
P2
C10H12O4
193, 207
12.18
P3
C8H8O5
281
12.98
P4
C10H12O5
239, 281
(7) %
According to previous reports, OH is generated in the ClO2 reaction system [44]. When HCO3– was added to the system, HCO3· formed, which is a relatively strong oxidant [45,46]. Therefore, the reaction rate of parabens and ClO2 increased. 3.3. Mineralization of parabens during the oxidation reaction Fig. 7 shows the TOC concentrations during the oxidations of MeP and PrP. According to Fig. 7, MeP and PrP were completely removed after a reaction time of 1 h, while the concentrations of TOC remained almost unchanged. These results indicated that no mineralization occurred during the reaction and several organic intermediates had formed. Compared to the chlorination of parabens [31], the results were similar. Therefore, it is necessary to further identify the products of the oxidation of MeP and PrP by ClO2. 3.4. Oxidation product identification and proposed reaction pathways The oxidation products of MeP and PrP during ClO2 oxidation were analyzed by GC–MS. The chromatograms of the samples in scan mode are shown in Fig. 8, and three major peaks for MeP and PrP oxidation products were detected. According to mass analysis, the detected substances were two types of oxidation products (P1, P2, P3, and P4) and two corresponding parabens. The mass spectra and proposed chemical structures of the oxidation products are shown in Fig. 9. The main information about these compounds is listed in Table 1. According to the identified oxidation products, the reaction pathways of MeP and PrP oxidized by ClO2 were proposed as shown in Fig. 10. Hydroxylation was found to be a significant reaction in the oxidation of parabens by ClO2, mostly at the aromatic paraben ring. First, hydroxylation occurred on the 3C of parabens to yield P1 (MeP) and P2 (PrP); then, %OH continued to oxidize P1 and P2 to form P3 and P4, respectively. Similar products were also detected during ozone oxidation and UV photolysis of parabens [2,30]. P3 and P4 belong to the gallic acid ester compound, which is an antioxidant that is non-toxic and non-irritating to the skin and tissue [47]. At the same time, chlorination was also the important reaction pathway in the ClO2 reaction system [48]. Paraben degradation maybe involved chlorination reaction with the nucleophilic attack of chlorine, meanwhile degradation of the intermediates involving %OH radical attack at a halogenated site led to the formation of P1 and P2 by replacing one chlorine atom with an %OH group. Hydroxylation reaction of the intermediates promoted by %OH led to P3 and P4 formation. P1 and P2 degradation maybe involved chlorination reaction, meanwhile %OH radical attacked the intermediates and P3 and P4 formed.
36.53–47.81 kJ mol−1, the higher activation energy of the reaction of ClO2 with the two parabens indicated that the oxidation process is more difficult [31]. 3.2.3. Effect of humic acid on MeP and PrP oxidation The influences of different humic acid concentrations on the oxidation of MeP and PrP by ClO2 are illustrated in Fig. 5. Humic acid inhibited the removals of MeP and PrP by ClO2. In the presence of humic acid at concentrations of 0, 1, 5, 10, and 15 mg L−1, the removal efficiency of MeP decreased from 97% to 60%. Similarly, the removal efficiency of PrP decreased from 98% to 50%. During oxidation, humic acid competed with parabens for ClO2, which was originally used to oxidize parabens. Therefore, the oxidation of parabens by ClO2 was weakened and the reaction rates of MeP and PrP oxidized by ClO2 greatly decreased. The degradation of diclofenac by ClO2 was also significantly inhibited by humic acid [41]. Humic acid is a common molecular organic compound in sewage water; thus, its effects on the reaction system should be fully considered in the process of oxidation by ClO2.
4. Conclusions This study investigated the degradation kinetics of MeP and PrP by ClO2. The reaction between ClO2 and parabens were of second order, with first-order in parabens and ClO2, respectively. The kapp values were 84.3 M−1·s−1 and 83.2 M−1·s−1 for MeP and PrP, respectively, at pH 5.0 and with a ClO2 concentration of 50 μM. The reaction rate increased with increasing pH and temperature, respectively. The paraben degradation was significantly inhibited after the addition of humic acid, while the reaction rate increased after adding HCO3–. ClO2 treatment is an efficient method for paraben degradation, which leads to the rapid degradation of parabens within about 10 min. Due to the presence of intermediates during the reaction, the mineralization process of
3.2.4. Effect of HCO3– concentration on MeP and PrP oxidation It has been reported that HCO3– could improve the oxidation rate of organic compounds [42]. As shown in Fig. 6, the removal rates of MeP 6
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Fig. 10. The proposed reaction pathways for the reactions between parabens and ClO2.
parabens was low. Two main products were generated for each paraben via hydroxylation. Furthermore, both products were less toxic than the parent compound. Therefore, paraben degradation by ClO2 is an efficient and safe method.
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5. Author contribution statement Qianhui Mao: Conducting experiments, Data curation. Qi Li: Data Analysis. Huimin Li: Writing- Original draft preparation. Shoujun Yuan: Writing- Reviewing and Editing. Jibiao Zhang: Supervision. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements The study was supported by the Major Science and Technology Program for Water Pollution Control and Treatment (2012ZX07103004). We would like to thank Dr. Shuwen Yan and Dr. Lushi Lian from Prof. Weihua Song group for their help in the analysis process. References [1] M. Aguilar-Bernier, A.I. Bernal-Ruiz, F. Rivas-Ruiz, M.T. Fernández-Morano, M. de Troya-Martín, Contact sensitization to allergens in the Spanish standard series at Hospital Costa del Sol in Marbella, Spain: a retrospective study (2005–2010), Actas Dermo-Sifiliográficas 103 (2011) 223–228.
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