Jointly published by Akadémiai Kiadó, Budapest and Springer, Dordrecht
React.Kinet.Catal.Lett. Vol. 92, No. 1, 3−9 (2007) 10.1007/s11144-007-5045-0
RKCL5045 UV-LIGHT INDUCED PHOTODEGRADATION OF BISPHENOL A IN WATER: KINETICS AND INFLUENCING FACTORS
Beibei Wang, Feng Wu*, Peixia Li and Nansheng Deng School of Resources and Environmental Science, Hubei Key Laboratory of Biomass-Resources Chemistry and Environmantal Biotechnology, Wuhan University, Wuhan, 430079, P.R. China
Received May 17, 2007, accepted May 29, 2007
Abstract In this work, photodegradation of bisphenol A (BPA) in water was investigated under an UV sterilization lamp (λ = 254 nm). The photodegradation kinetics of 5.0 ~ 50.0 mg/L BPA in aqueous solutions was found to follow first-order law. The influencing factors such as the pH value, light source, different water media and salinity on photodegradation of BPA were studied in detail. Keywords: Bisphenol A, UV-light, photodegradation
INTRODUCTION Bisphenol A (BPA), a suspected endocrine disruptor, has been widely used as the monomer for the production of epoxy resins and polycarbonate plastics. BPA has an estrogenic activity and a slight toxicity. It could be released from polycarbonate flasks and lacquer coatings [1-4]. As for the degradation and treatment of BPA, some studies have been focused on Advanced Oxidation Processes (AOPs) like the Fenton’s reagents oxidation and photooxidation [5, 6]. _________________________ * Corresponding author. Tel: 86-27-68778511; Fax: 86-27-68778511 E-mail:
[email protected] 0133-1736/2007/US$ 20.00. © Akadémiai Kiadó, Budapest. All rights reserved.
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The direct photolysis technology under UV-light has a high removal efficiency of organic compounds in water [8]. In this study, we investigated the kinetics of BPA photodegradation and the influencing factors on the degradation of BPA. The results of this work may provide useful information to understand the degradation processes of BPA under UV-light condition. Additionally, as some toxic organic pollutants, such as phenol, have been treated by UV-light irradiation, this work will also be of importance for demonstrating the application of direct UV-light photolysis technology to treat water or wastewater containing BPA. EXPERIMENTAL Reagents Chemically pure BPA was purchased from Damao Chemicals Co. (Tianjin, PRC) and used without purification. Acetonitrile was HPLC grade (Lingfeng Chemical Reagent Co., Shanghai, PRC). All reagents were of analytical grade. Lake water was collected from East Lake, Wuhan, P.R.C. Sea water was collected from Xiamen, P.R.C, and tap water was collected from the local water supply system in Wuhan, P.R.C. Photochemical reaction Irradiation under UV light was performed with sterilization lamps with a wavelength of 254 nm (Gucun Illumination Instrument Co., Shanghai, PRC) in a photochemical reaction chamber. The lamps were hung over the quartz tubes (o.d.1.5 cm, length 11.5 cm) at a fixed distance of 7.0 cm. The degradation of BPA under sunlight was carried out in 10 mL Pyrex tubes (o.d.1.5 cm, length 11.5 cm) without stirring and blowing air. At different time intervals during the irradiation, samples were collected and the remaining BPA was determined by HPLC-UV.
Analysis The UV absorbance spectra of BPA solutions was recorded with a spectrophotometer UV-1601 (Shimadzu, Japan). BPA in water was detected by HPLC [Shimadzu LC-6A pump, HP Zorbax SB-C18 column (4.6 mm × 150 mm, 5 µm)]. The data are presented as mean values from triplicate experiments. The errors are below 5%.
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RESULTS AND DISCUSSION The influence of the initial concentration of BPA Figure 1 shows the relation between the initial degradation velocity of BPA and the initial concentration of BPA in solutions of pH 7.0 and 10.0, respectively. The photodegradation kinetics of BPA is found to follow the first-order law in the solutions at both pH values. The kinetic equation is v = 0.0199 c (R = 0.9921) at pH 7.0, while v = 0.129 c (R = 0.9869) at pH 10.0.
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pH = 10.0 pH = 7.0
6
V (mg/L/min)
5 4 3 2 1 0
0
10
20 30 c BPA (mg/L)
40
50
Fig. 1. The initial photodegradation rates of BPA in water at pH 7.0 and 10.0, respectively. [BPA]0 = 5.0 ~ 50.0 mg/L. when the pH value was 7.0, the kinetics equation is v = 0.0199 c (R = 0.9921) while when the pH value was 10.0, v = 0.129 c (R = 0.9869)
The effect of the pH value Figure 2 shows that when the pH value varied from 5.0 to 9.0, the degradation rate did not change significantly. However, in general, the total trend was a continuous increase with increasing pH values, especially when the pH value was above 10.0. After radiation for 20 min, the degradation efficiency of BPA was nearly 7.1% at pH 3.0, while the removal efficiency of BPA was 89.7% at pH 10.0.
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pH = 3.0 pH = 5.0 pH = 7.0 pH = 9.0 pH = 10.0
5
cBPA(mg/L)
4 3 2 1 0 0
20 40 60 80 100 120 140 160 180
Time (min) Fig. 2. BPA concentration change with time in waters at different pH values. [BPA]0 = 5.0 mg/L
pH = 11.0 pH = 10.0 pH = 7.0 pH = 3.0
Absorbance
2.0 1.5 1.0 0.5 0.0 200
240
280
320
360
λ (nm)
Fig. 3. UV absorption spectra of 11.4 mg/L BPA in aqueous solution at different pH values
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From the UV-Vis absorption spectra in Fig. 3, almost no difference exists for the spectra at acidic, neutral and weak alkalic pH values (3.0 ~ 9.0). However, in strong alkalic solutions, the absorption spectra of BPA changes sharply in the wavelength region from 240 to 260 nm. This change is attributed to the ionization of BPA in aqueous solution at a pH above 10.0 (pKa = 9.6 ~ 10.2) [1]. The BPA phenolate anion originating from the deprotonization of the phenolic hydroxyl group makes the absorption spectra shift to longer wavelengths, which causes a significant increase of the absorbance at λ = 254 nm. Therefore, alkalic pH enhances the degradation of the BPA.
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cBPA(mg/L)
4 High pressure mercury Lamp Sunlight 30W UV sterilization lamp 60W UV sterilization lamp
3 2 1 0
0 20 40 60 80 100 120 140 160 180
Time(min)
Fig. 4. BPA concentration change with time under the radiation with different light sources. [BPA]0 = 5.0 mg/L, pH = 7.0
The photodegradation of BPA under different lights To study the influence of different light sources on the photodegradation efficiency of BPA, solutions containing 5.0 mg/L BPA at pH 7.0 were inradiated separately under irradiated sunlight, a high-pressure mercury lamp light (λ ≥ 365 nm), 30 W and 60 W UV lamp lights (λ = 254 nm). Figure 4 shows that after irradiation for 160 min, the removal efficiency of BPA in water from low to high were the high-pressure mercury lamp (5.95%), sunlight (10.6%), the 30 W UV lamp (74.8%) and 60 the W UV lamp (82.9%), respectively. This is because BPA in water has no absorption of light at wavelengths above 365 nm, but the high-pressure mercury lamp used in this work has illumination only at wavelengths above 365 nm. As sunlight is a complex light that has a wide range
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of wavelengths above 290 nm which could be absorbed by BPA, the degradation of BPA under sunlight was a little higher than under a high-pressure mercury lamp. The results also indicate that higher power makes the removal of BPA more efficient.
The degradation of BPA in different water media In order to simulate the degradation of BPA in different water media, BPA was added to waters of different sources (i.e. lake water, sea water and tap water). Figure 5 shows that the degradation rate of BPA was the slowest in tap water (lnc/c0 = -0.0136t), higher in sea water (lnc/c0 = -0.0149t), and highest in the east lake water (lnc/c0 = -0.0207t). The reason maybe the different pH values of the waters, tap water has a pH of 7.60, sea water 7.74, and lake water 8.23, respectively. However, in the previous section it was shown that with an increase in the pH value from 7.0 to 9.0, the degradation velocity was slightly increasing. Thus, the pH value may not be the main reason that affects degradation. Figure 5 also shows that the decline of the concentration of BPA in water was lower in the absence of NaCl than that in the presence of 1.0 µmol/L NaCl. This result is in accordance with the report by Sajiki et al. [8]. As a result, the reason of degradation efficiency of BPA in the sea water being higher than in tap water could be explained.
no NaCl -1 1 µmol l NaCl tap water sea water lake water
20.0
cBPA (mg/L)
16.0 12.0 8.0 4.0 0.0 0
20
40 60 80 Time (min)
100
120
Fig. 5. BPA concentration change with time in different water media. In tap water, sea water and lake water, [BPA]0 = 10.0 mg/L. In doubly distilled water with and without NaCl, [BPA]0 = 20.0 mg/L, pH = 7.0
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Zhan et al. reported that the direct photodegradation of BPA was very slow in a pure aqueous system, but it was rapid in nitrate solutions [9]. The east lake is a eutrophic water system, the concentration of NO3- in it is about 0.032 ~ 0.048 mmol/L. The relatively high concentration of NO3- could make the degradation of BPA faster as compared with the other two water media. Additionally, the organic substance present in the east lake can produce an active oxygen compound such as ·OH radicals and H2O2 by the photosensitization of humic substances which could also contribute to the enhancement of photodegradation of the BPA in the lake water. Generally, the reasons leading to different degeneration rate of BPA in different waters are complex. The pH value, the salinity, as well as the eutrophic degree or humic substances can play a combined role in the photochemical process.
CONCLUSION The photodegradation of BPA occurs under UV-light. When the initial concentration of BPA ranged from 5.0 to 50.0 mg/L, the photodegradation kinetics is found to follow the first-order law. The influencing factors of the pH value, light source and salinity all have effects on the photodegradation of BPA. High pH values benefit the photodegradation of BPA in aqueous solutions and the degradation efficiency of BPA could be enhanced at low NaCl concentration.
Acknowledgements. This work was financed by the Natural Science Foundation of PR China (No.40503016 and 20477031) and by Water Environment Research & Data Sharing Platform in the Middle Reaches of the Yangtse River (Wuhan University), No. WERDSPMYR-0601. We thank the anonymous referees’ comments on this manuscript. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9.
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