Environ. Sci. Technol. 2004, 38, 296-299
Photoproduction of Hydroxyl Radicals in Aqueous Solution with Algae under High-Pressure Mercury Lamp X I A N L I L I U , †,‡ F E N G W U †, A N D N A N S H E N G D E N G * ,† Department of Environmental Science, Wuhan University, Wuhan 430072, People’s Republic of China, and Huangshi Polytechnic College, Huangshi 435003, People’s Republic of China
Photoproduction of hydroxyl radicals (•OH) could be induced in aqueous solution with algae (Nitzschia hantzschiana, etc.) and (or not) Fe3+ under high-pressure mercury lamp with an exposure time of 4 h. •OH was determined by HPLC using benzene as a probe. The photoproduction of •OH increased with increasing algae concentration. Fe3+ could enhance the photoproduction of •OH in aqueous solution with algae. The results showed that the photoproduction of •OH in algal solution with Fe3+ was greater than that in algal solution without Fe3+. The light intensity and pH affected the photoproduction of •OH in aqueous solution with algae with/without Fe3+. The photoproduction of •OH in aqueous solution with algae and Fe3+ under 250 W was greater than that under 125 W HPML. The photoproduction of •OH in algal solution (pH ranged from 4.0 to 7.0) with (or not) Fe3+ at pH 4 was the greatest.
Introduction There is growing concern in the development of new methodologies for the degradation of water pollutants in recent years. Algal photocatalysis is a possible alternative/ complementary technology for the destruction of aquatic environmental pollutants. Zepp et al. (1) studied the influence of algae on photolysis rates of chemicals in water; the results showed that algae could accelerate the sunlight-induced transformation of nonionic organic chemicals, and some results indicate that the photosynthetic apparatus of the algae was probably not involved in the algae transformation. In our previous study, the photodegradation of 17Rethynylestradiol (EE2) and 17β-estradiol(E2) in aqueous solution exposed to high-pressure mercury lamp (HPML) or UV light was observed and could be accelerated by algae (Nitzschia hantzschiana, etc.). Photodegradation rate increased with increasing algae concentration (2). The above photocatalytic effects were described in various articles (3). Almost all opinions among these studies focused on the photocatalytic effect caused by H2O2 in water. Zepp et al (4) studied the rate of decomposition and photoproduction of hydrogen peroxide (H2O2) by algae in water. Their results suggested that algae had an important influence on the environmental concentration of H2O2. The results indicated * Corresponding author phone: + 86-27-8768-6550; fax: +8627-8721-5893; e-mail:
[email protected]. † Wuhan University. ‡ Huangshi Polytechnic College. 296
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 1, 2004
that exposure of algal suspensions to light resulted in a buildup of H2O2. The rate of formation increased with increasing light exposure and with increasing algae concentration. The production of hydrogen peroxide, both photosynthetically and nonphotosynthetically, has been studied in algae by Collen also (5). It was thought that the production of H2O2 in algae was light-dependent with exponentially increasing production with increased light intensity. Collen et al. studied the in vivo measurement of H2O2 production in the brown alga Fucus evanescens using 2′,7′-dichlorohydrofluorescein diacetate. As far as we have been able to ascertain, there are no published studies on the determination of photoproduction of hydroxyl radicals (•OH) in algae aqueous solution (with or without Fe3+). It is thought that the •OH can effectively degrade most of the organic pollutants (6, 7). A considerable attempt has been made to investigate the degradation of organic pollutants such as chlorophenols, 17R-ethynylestradiol (EE2), etc. (8-12). Both sunlight and UV light were found to be effective for these reactions (8). In this paper, benzene was used as a probe (13-15) to determine the photoproduction of •OH in aqueous solution with algae under HPML(λ G 313 nm). The selected typical reaction was the formation of phenol from benzene.
Experimental Section Chemicals and Reagents. HCl, FeCl3‚6H2O, CaCl2‚2H2O, Ca(H2PO4)2‚2H2O, CaSO4‚H2O, CuSO4‚5H2O, H3BO3, KCl, K2HPO4, KH2PO4, MgSO4‚7H2O, MnCl2‚4H2O, MnSO4‚4H2O, MoO3, NaOH, NaCl, Na2CO3, NaHCO3, Na2MoO4‚2H2O, NaNO3, Na2SiO3‚9H2O, (NH4)2SO4, ZnSO4‚7H2O, ascorbic acid, ferric citrate, benzene, phenol, and acetonitrile were analytical grade. Double-distilled water was used in these experiments. Preparation of Algae. The algae Nitzschia hantzschiana, Chlorella vulgaris, and Anabaena cylindrica used in the experiments were obtained from the Wuhan Hydrobiology Institute of Chinese Academy of Sciences (Wuhan, P. R. China). The algae were grown in culture medium at 25 °C using a 24 h light cycle (in a culturing box equipped with daylight lamps; light intensity, 2000LX). For N. hantzschiana, the culture medium consisted of 120 mg L-1 NaNO3, 70 mg L-1 MgSO4‚7H2O, 40 mg L-1 K2HPO4, 80 mg L-1 KH2PO4, 20 mg L-1 CaCl2‚2H2O, 10 mg L-1 NaCl, 100 mg L-1 Na2SiO3‚9H2O, 2 mg L-1 MnSO4‚4H2O, 5 mg L-1 ferric citrate, 2.86 mg L-1 H3BO3, 1.81 mg L-1 MnCl2‚ 4H2O, 0.22 mg L-1 ZnSO4‚7H2O, 0.079 mg L-1 CuSO4‚5H2O, 0.039 mg L-1 Na2MoO4‚2H2O, and 20 mL L-1 soil extract. For C. vulgaris and A. cylindrica, the culture medium consisted of 200 mg L-1 (NH4)2SO4, 30 mg L-1 [Ca(H2PO4) 2‚2H2O + CaSO4‚H2O], 80 mg L-1 MgSO4‚7H2O, 100 mg L-1 NaHCO3, 25 mg L-1 KCl, 0.150 mL L-1 FeCl3(1%), 2.86 mg L-1 H3BO3, 1.81 mg L-1 MnCl2‚4H2O, 0.222 mg L-1 ZnSO4‚7H2O, 0.0177 mg L-1 MoO3(85%), 0.079 mg L-1 CuSO4‚5H2O, 0.5 mL L-1 soil extract; the medium was adjusted to pH 7.0-7.2 by using 0.1 M Na2CO3. The algae were cultured in axenic medium. When the condition were such that the algae were growing in a logarithmic growth phase and the density of algae was high (normally 12-14days), the algae were taken for use in experiments after being washed. Prior to illumination experiments, to remove colloidal ferric hydroxide particles that might have adsorbed on the algae cells, a modified version of the procedure (1) was used in experiments. This procedure involved washing the cells 10.1021/es034626b CCC: $27.50
2004 American Chemical Society Published on Web 11/18/2003
by gentle agitation for 30 min with 0.01 M aqueous ascorbic acid adjusted to pH 3.0. Then the algae were washed with double-distilled water 3-4 times. The resulting algae suspension was obtained in this way. The cell counting was carried out, and the density of algae (cells L-1) was calculated. Thus, the algae were prepared for next use. Different concentrations of algae were gained through diluting washed algae with double-distilled water. These experiments were carried out at a room temperature of 25 ( 2 °C. Photoreaction Procedure. The irradiation experiments were performed under HPML (λ G 313 nm, 250 W or 125 W, Wuhan Xinguang lamp Co. Ltd., China). The reactors were quartz tubes (each tube 8 cm length, 1.5 cm diameter, 1 mm wall thickness). The pH of the algal solutions ranged from 4 to 7 for all the experiments carried out in this work. The test tubes containing solutions were kept in the dark before and after irradiation. The aqueous solution of benzene and algae were mixed thoroughly and transferred into quartz tubes. An aliquot of a 0.01 M aqueous stock solution of benzene was added to the aqueous solution with algae. The concentration of benzene was 0.001 M in the sample (algal solution) after dilution. These tubes were capped. Then they were put into a box that was equipped with a high-pressure mercury lamp. The aqueous solution was irradiated through the wall of the quartz tube using HPML. The light intensities at the position of tubes were 38 000 Lux for 250 W HPML and 13 000 Lux for 125 W HPML, which were detected using a Digit Lux meter (TES 1332, Taiwan, China). Dark controls were carried out in parallel. At different time intervals (e.g., 0.5 or 1 h), each tube with benzene and algae (with Fe3+ or not) in aqueous solution was taken from out of the box. For the solution with algae, samples were centrifuged at 4000 rpm for 20 min. Then the phenol concentration of supernatant was analyzed using HPLC. With employment of the centrifugation procedures discussed elsewhere (16), the chemicals were sorbed less than 5% by the algae under these conditions. Also, the control experiments without algae were carried out. Analyses. The phenol concentrations were determined using HPLC. The formation of phenol from benzene was monitored at 280 nm using HPLC with LC spectrophotometer (Lambda-Max, model 481, Waters). The eluent was 40% acetonitrile at a flow rate of 0.8 mL min-1 using 150 × 4.6 mm Supelco C18 column. Standard solutions of phenol were prepared gravimetrically and used to calibrate and bracket the HPLC determination of phenol concentrations found from the oxidation of benzene added to the algae solution. It was thought that the •OH-mediated oxidation of benzene forms phenol with nearly 100% yield and the phenol concentration represented the concentration of photoproduction of hydroxyl radicals. Spectroscopic analysis was done on a UV-vis spectrophotometer (UV-1601, Shimadzu). Scavenging of •OH by high concentrations of benzene has been used to determine the •OH quantum yield. Aromatic hydroxylation is among one of the typical reactions of •OH and is used for the detection of •OH in the case of the Fenton reaction and of the photolysis of aqueous HNO2, NO3-, and NO2‚(17, 18). Benzene is very unreactive toward O2 (1∆g) (1921). The hydroxylation of benzene by •OH to produce phenol is a fairly selective process. Given the high reactivity of benzene with •OH (22-24), under the conditions of these experiments (0.001 M added benzene), virtually all of the •OH should have been scavenged by benzene. The effects of reactions between algae and hydroxyl radicals were ignored due to the complex reactions of reactive oxygen species. We regard the values determined as ‘net photoproduction’ values. This work mainly focused on the ‘net’ quantitation of hydroxyl radicals in aqueous solution with algae under high-pressure mercury lamp. The ‘net’ values could be regarded as an index
FIGURE 1. UV-vis absorption spectrum of an aqueous solution with 2.0 × 109 cells L-1 N. hantzschiana. of oxidation capacity of organic substances (e.g., benzene) by •OH. Further research on the reactions between algae and hydroxyl radicals needs to be carried out. Conversion of benzene to phenol was always limited to
