Photocatalytic degradation of diethylstilbestrol in water with nano-semiconductor catalysts

Zhou Jianke, Yue Qiang, Li Luhua, Li Jingxia
(Research Center of Physics and Chemistry Analysis, Hebei University; Hebei Province Key Laboratory of Analytical Science and Technology, Baoding, 071002)

Received on Apr. 11, 2005.

Abstract Photocatalytic degradation of diethylstilbestrol (DES) was carried out in the presence of nanosized TiO2, ZnO and TiO2-ZnO with UV. The degradation rate was strongly influenced by the catalyst concentration, pH value and lightsource distance. Compared with TiO2 and ZnO, DES was decomposed more efficiently using nano-TiO2-ZnO photocatalyst. An initial DES concentration of 10mgL-1 was completely degraded with TiO2-ZnO after 60min under the optimum conditions. The photodegradation reactions were found to follow the first order kinetics law, and the apparent degradation rate constant was 0.0704min-1.

Diethylstilbestrol (DES) is a estrogenic drug that is approximately 10 times as potent as estradiol-17b which is the most potent endogenous estrogen [1]. It was used rather commonly in the 1940-1960s to maintain pregnancies in USA. But after 20 years, it was discovered that women who took DES while pregnant and their daughters faced a higher risk of breast cancer, infertility, vaginal adenosis, and abnormalities of the fallopian tubes, cervix, and uterus [2-3]. In the past few years a number of papers have highlighted the potentially dangerous consequences to human and wildlife of the presence of estrogens in the aquatic environment [4-6]. In contrast to natural estrogen, estrogenic drugs such as DES are more stable and remain in the body longer than natural estrogen [7]. These estrogens are difficult to be totally removed from wastewater by the traditional treatment. So it is necessary to develop technology for its elimination and decomposition as soon as possible. Recently, there are many investigations on the degradation of estrogens in environment using heterogeneous photocatalysis technique [8-10]. But there are few articles focusing photocatalytic degradation of DES. In the present work, photodegradation of DES in aqueous suspensions is investigated using nanosized TiO2, ZnO and TiO2-ZnO as photocatalyst. The aim of this work was to investigate the different photocatalytical behaviour of these catalysts and the influence on the degradation of various parameters, such as photocatalyst concentration, pH value and lightsource distance.

1.1 Reagents
DES (99%) was obtained from Acros Organics (Sweden). Photocatalysts were supplied by Hebei University Research Center of Nano-Material. Methanol was HPLC grade, all other chemicals used were of analytical grade and were used without further purification. Water used was doubly distilled.
1.2 Photodegradation procedure
Photodegradation was conducted in a photoreactor made by ourselves. The UV lamp (25w, 254nm) was in the center of the photoreactor, keeping parallel with sample container. DES solution of different concentration, containing precise amount of photocatalysts, was continuously stirred with a magnetic bar. The pH of the sample solution was adjusted with 0.1N HCl or NaOH depending on the desired values. In the majority of the experiments, temperature was kept at 25¡À1ºC with an electric fan.
1.3 Analyses
Samples were taken at given time intervals and filtered through a 0.45mm membrane filter in order to remove photocatalysts before injecting. The progress in the photodegradation of DES was followed with a HPLC (Shimadzu LC-10AT, Japan) equipped with a UV-VIS detector (Shimadzu SPD-10A, Japan) at 240nm. A separation column was a Diamonsil-C18 column (250¡Á0.46mm, 5mm, Dikma Technologies). The mobile phase was CH3OH/H2O mixture (80%/20% v/v) at a flow rate of 1.0mlmin-1. The injection volume was 20ml. The retention time of DES was 5.6min.

2.1 Chromatogram of DES
Nanosized TiO2 and ZnO were used commonly as photocatalyst to photodegrade organic pollutants [11-13]. And a considerable increase of the photocatalytic activity has been reported by combination TiO2 with ZnO [14]. Coupled semiconductor photocatalysts may increase the photocatalytic efficiency by increasing the charge separation and extending the photo-responding range. At the same time, their physical and optical properties are greatly modified [15]. In all our studies, DES was degraded using TiO2, ZnO and TiO2-ZnO catalysts to compare their photocatalytic activities. Nano-TiO2-ZnO was more efficiently, Fig.1 is the typical chromatogram of DES in UV and UV/TiO2-ZnO system after illumination for 30min.