Environ. Sci. Technol. 2007, 41, 3747-3751
FeCl3/NaNO2: An Efficient Photocatalyst for the Degradation of Aquatic Steroid Estrogens under Natural Light Irradiation LIANZHI WANG,† FEIFANG ZHANG,† R E N H U A L I U , * ,‡ T O N Y Y . Z H A N G , § XINGYA XUE,† QING XU,† AND X I N M I A O L I A N G * ,†,‡ Dalian Institute of Chemical Physics, the Chinese Academy of Sciences, Dalian 116023, P .R. China, School of Pharmacy, East China University of Science and Technology, Shanghai 200237, P .R. China, and Eli Lilly and Company, Indianapolis, Indiana 46285
Assistance and acceleration of the environment’s selfremediation of pollutants represent an important and longstanding goal for environmental chemistry communities. Here, a degradation route using a combination of a nitrite and a ferric salt as the photocatalyst is presented for catalytically removing 17β-estradiol (E2), estriol (E3), and 17Rethynylestradiol (EE2) in water under mimicked natural environmental conditions, i.e., in the phytotron. After a 1 day reaction, 86.6% of the estrogen E2 was degraded. Extending the incubation time to 30 days, more than 99.9% E2 was removed and a very small quantity of malonic acid observed as the residual organic compound, and estrogenic activity was determined. The results showed that the estrogenic activities of the intermediate products are negligible and that there is no secondary risk associated with increased the estrogenic activity. The degradation system demonstrated that FeCl3/NaNO2 is an efficient photocatalyst which is active on natural light irradiation. This work highlights a promising development for in situ treatment of pollutants in natural-environment conditions.
Introduction Self-remediation of environmental pollutants is an integral part of nature's evolution. The natural environment is, therefore, able to maintain the ecological balance by eradicating offenders through biological degradation or chemical decomposition. However, the rate of the degradation processes for many man-made chemicals under natural environmental conditions are too slow to be within a practical time frame (1). As a result, assistance and acceleration of the environment’s self-remediation of pollutants represent an important and long-standing goal for research communities as their potential impact on creating a sustainable future for mankind (2). However, in spite of the extensive efforts to achieve this goal, progress in this field to date has been hampered by the paucity of catalysts that are effective for in * Address correspondence to either author. Phone: 0086+41184379519 (X. L.); 021-64251830 (R. L.). Fax: 0086+411-84379539 (X. L.); 021-64251830 (R. L.); E-mail:
[email protected] (X. L.). † Dalian Institute of Chemical Physics. ‡ East China University of Science and Technology. § Eli Lilly and Company. 10.1021/es0625778 CCC: $37.00 Published on Web 04/18/2007
2007 American Chemical Society
situ degradation of persistent environmental pollutants. Specifically, the challenge in performing the degradation reactions exists in being amenable to natural-environment conditions. We report herein that the combination of a nitrite and a ferric salt can catalytically remove 17β-estradiol (E2), estriol (E3), and 17R-ethynylestradiol (EE2) (Figure 1) in water under natural environmental conditions (natural light, air, ambient pressure, and temperature). Estrogens are an important class of steroidal compounds that have found widespread use as hormone replacement therapeutic agents, contraceptives, and farm animal meat/ milk production regulators. They may also elicit a variety of adverse effects on both humans and animals by intervening the normal functions of their endocrine pathways (3-9). The significant quantities of these estrogen pollutants found in the global environment and the growing knowledge about their adverse effect have aroused intense interests among the global scientific communities (10-19). Development of a catalyst system that is capable of degrading these estrogen pollutants under natural environmental conditions seems highly desirable. Although a large number of degradation methods for environmental pollutants have been developed, the conditions of these methods are mostly beyond the conditions of the natural environment, necessitating harsh conditions, e.g., elevated temperatures and/or pressure, UV light radiation, and stoichiometric oxidizing agents and other auxiliary agents including organic solvents, and buffer agents. These catalyst systems are, therefore, incompetent to in situ degrade the pollutants in the environment. We envision that the exploitation of natural elements, i.e., dioxygen from air and light from the sun, to degrade the environmental pollutants might provide a new approach toward achieving these goals. While the assumption is an attractive one in theory, practical execution of this degradation strategy was hindered by the lack of efficient routes. An intriguing possibility for execution of strategy is photocatalytic degradation by sunlight. In most cases, however, the important photocatalyst, such as TiO2based or Photo-Fenton degradation, was carried out under UV-light irradiation (20-27). Only a few these catalysts are reported to be visible light efficient for the degradation of environmental pollutants (28-32). We are particularly interested in seeking to discover the compounds that not only have catalytic activity under natural light irradiation but also are ubiquitous in nature because they are more ecofriendly and cheaper. We were fortunate to realize the confluence of several events. First, we discovered a highly efficient aerobic catalytic system for the degradation of trichlorophenol using a nitrite as the mediator (33-36). Soon after, we uncovered the fact that a combination of a nitrite and FeCl3 is capable of oxidizing a variety of alcohols under ambient atmosphere and room temperature (37). Most notably nitrites and Fe(III) are both ubiquitous in nature (38). With this in mind, we attempted to use FeCl3/NaNO2 as the catalyst for possible degradation of estrogenic pollutants under natural environmental conditions (ambient temperature, pressure, and sunlight). Thus we designed the degradation model with NaNO2, FeCl3, air, and mimic sunlight as the reaction conditions using 17β-estradiol (E2) as a prototypical estrogen pollutant.
Materials and Methods Photocatalytic Oxidation of Estrogens. 15 mL of the test solution was put into a test tube with a stopper. Desired volume of FeCl3 solution and NaNO2 solution were added. The test tubes were put into the phytotron which was VOL. 41, NO. 10, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Formula of 17β-estradiol (E2), estriol (E3), and 17rethynylestradiol (EE2).
FIGURE 3. Removal of E2(1.8 × 10-5 M, 15 mL) with FeCl3 in aqueous solution under sunlight irradiation: (a) The purity E2 solution (b) The mole ratio of E2/ FeCl3 was 1:0.5, (c) The mole ratio of E2/ FeCl3/ was 1:1.
FIGURE 2. Removal of E2 (1.8 × 10-5 M, 15 mL) with NaNO2 and FeCl3 in aqueous solution under sunlight irradiation: (a) The mole ratio of E2/ FeCl3/NaNO2 was 1:0.1:0.1, (b) The mole ratio of E2/ FeCl3/NaNO2 was 1:0.2:0.2, (c) The mole ratio of E2/ FeCl3/NaNO2 was 1:0.5:0.5, (d) The mole ratio of E2/ FeCl3/NaNO2 was 1:1:1. irradiated with mimic sunlight at 22 °C for 16 h followed by incubation dark at 18 °C for 8 h-dark during each 24 h cycle. The light intensity was 12000-15000 Lux. Analysis of the Treated Solution. Residual estrogens were determined by high performance liquid chromatography (HPLC) equipped with an ODS column (250 mm × 4.6 mm) at 30 °C and a Waters 474 fluorescence detector (excitation wavelength, 280 nm; emission wavelength, 320 nm.) The mobile phase was an acetonitrile/water mixture (4/6, v/v). The flow rate was set to 1.0 mL/min for E2 and EE2, 0.8 mL/min for E3. The injection volume is 8 µL. Under these conditions, the retention time of E2, EE2, and E3 are 11.5, 15.5, and 5.3 min, respectively. GC-MS analysis was performed using an Agillent 6890 GC-MS. The carrier was helium, and a DB-5 capillary column (30 m × 0.25 mm) was used. GC injection volume was 1 µL, the split ratio was 50:1, and the oven program was held at 50 °C for 5 min, increase by 3 °C /min to 200 °C, then 10 °C /min to 300 °C, and held at 300 °C for 30 min. Electron impact ionization was at 70 eV. The scanning of m/z was from 30 to 500. Before GC-MS analysis, the solution was concentrated 300-fold by evaporation under vacuum and eluted with acetonitrile (2 mL). Then was dried by nitrogen stream and eluted with pyridine (100 µL). The samples were derivatized by adding 100 µL of the derivatization (N-methyl-N-trimethylsilyl trifluoroacetamide, MSTFA), and sample was incubated for 30 min (160 rpm) at 37 °C, then put it in Lab for 2 h at ambient temperature. Evaluation of Estrogenic Activities. The estrogenic activities of the test solutions were tested using the recombinant yeast-based estrogen assay as reported in the literature (39, 40). Recombinant yeast cells were kindly provided by J.P. Sumpter from Brunel University, Uxbridge, UK. The yeast was grown overnight (18-20 h) at 37 °C on a selective medium (SC medium, prepared following the procedures of the literature (39, 40). In performing the assay, exponentially growing overnight cultures were diluted with SC medium to 3748
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an OD600 nm of 0.25. Then 995 µL of cell suspension and 5 µL of test sample solution were added to 96-well plate to incubate at 30 °C with vigorous orbital shaking (130 rpm) on a titer plate shaker for 2 h, and then the cell density of the culture was measured at 600 nm wavelength. Fifty µL test cultures were transferred to a new 96-well plate; after addition of 120 µL of Z-buffer and 20 µL chloroform, the assays were carefully mixed (vortex 25 s) and preincubated for 5 min at 30 °C. The enzyme reaction was started by adding 40 µL o-NPG (13.3 mM, dissolved in Z-buffer). The assays were incubated at 30 °C on a titer plate shaker. The reactions were terminated by the addition of 100 µL Na2CO3 (1 M). After centrifugation at 12 000 g for 15 min (Sigma Laborzentrifugen 2K15, Germany), 200 µL of the supernatant was transferred into a new 96-well plate and the OD420 nm was determined. The solution was concentrated 20-fold by evaporated under vacuum and eluted with dimethyl sulfoxide (DMSO, 1 mL). The treated solution was diluted by 8 orders of magnitude according to the residual E2 after 2 days.
Results and Discussion Photocatalytical Degradation of E2 in the Presence of Fe(III) and NaNO2. According to the requirement of model mimic, we selected the reaction concentration of FeCl3 and NaNO2 both at their environmental concentration level (29). The reactions were monitored by HPLC/fluorescence analysis. Data shown in Figure 2 demonstrates the effectiveness of this unique combination of these environment factors for degrading prototypical estrogens, E2. The efficiency of this system was quite astounding considering the demand on the reaction kinetics due to low reactant concentrations (E2 concentration: 1.8 × 10-5 M). For example, with highly dilute NaNO2 (1.8 × 10-5 M) and FeCl3 (1.8 × 10-5 M), 86.6% of the estrogen E2 was degraded only within 1 day (Figure 2, curve d). To further explore the potential of NaNO2 and FeCl3 in catalytic degradation of E2, we lower the use amount of NaNO2 and FeCl3. As shown in Figure 2, when 50 mol %, 20 mol %, and 10 mol % NaNO2 and FeCl3 were used, the removals of E2 were 53.4, 15.6, and 3.3%, respectively, after 1 day reaction. As the use amount of NaNO2 and FeCl3 decreases, the rate of E2 removal decreased correspondingly. Interestingly, although only a 3.3% decrease was observed when catalytic amount of NaNO2 (10 mol %) and FeCl3 (10 mol %) was used after the 1 day reaction, the degradation level reached 38.8% prolonging the reaction time to 13 days. These results clearly implicated the roles of NaNO2 and FeCl3 as active mediators in the degradation of estrogens.
FIGURE 4. Removal of E2(1.8 × 10-5 M, 15 mL) with NaNO2 in aqueous solution under sunlight irradiation: (a) The purity E2 solution (b) The mole ratio of E2/ NaNO2 was 1:1.
FIGURE 5. Removal of E2(1.8 × 10-5 M, 15 mL) with NaNO2 and FeCl3 in aqueous solution under different conditions.: (a) Sunlight irradiation (b) in the dark. To understand the role of FeCl3 and NaNO2 in present photocatalytic degradation system, we designed some control experiment (Figure 3 vs Figure 4). Control experiments revealed that ferric catalyst played a more dominant role in the system. For example, when stoichiometric amount of NaNO2 was reacted with E2 for 2 days in the absence of FeCl3, only 8.5% E2 degradation was achieved (Figure 4, curve b), yet the degradation level reached 84.9% when FeCl3 was used alone (Figure 3, curve c). Furthermore, the model process indicated that the degradation of E2 was prohibitively slow in the absence of NaNO2 and FeCl3. A slow degradation was observed when the model reaction was performed at the absence of light irradiation (Figure 5). The results suggest that FeCl3/NaNO2 acts as an active photocatalyst which is effective under visible light. The cooperative effect between NaNO2 and FeCl3 is quite distinctive. This observation demonstrates the importance of the interactions among environmental factors and their roles in the environmental degradation processes for pollutants. More than 99.9% E2 degradation was observed when stoichiometric amount of NaNO2 and FeCl3 were allowed to incubate with estrogen E2 for 30 days under the standard and mild reaction conditions. The degradation model was also applied to estrogen E3 and EE2. A similar degradation effect was observed when they were treated with NaNO2 and FeCl3 (Figure 6 and Figure 7). To the best of our knowledge, this is the first demonstration of the ability of NaNO2 and FeCl3 to efficiently degraded estrogen E2, E3, and EE2 in an aqueous environment mimicking natural conditions. As such, it is first reported that E3 and EE2 can be photocatalytically degraded under visible light irradiation. In other photocatalytic degradation protocols, the degradation of E3 and EE2 necessitated UV
FIGURE 6. Removal of E3(3.5 × 10-5 M, 15 mL) with NaNO2 and FeCl3 in aqueous solution under sunlight irradiation: (a) The mole ratio of E2/FeCl3/NaNO2 was 1:0.5:0.5, (b) The mole ratio of E2/ FeCl3/ NaNO2 was 1:1:1.
FIGURE 7. Removal of EE2(6.7 × 10-6 M, 15 mL) with NaNO2 and FeCl3 in aqueous solution under sunlight irradiation: (a) The mole ratio of E2/FeCl3/NaNO2 was 1:0.5:0.5, (b) The mole ratio of E2/ FeCl3/ NaNO2 was 1:1:1. light irradiation (21, 23-27). It is noteworthy that EE2, which is generally believed to be a recalcitrant substrate for degradation, is efficiently removed in present degradation system. Although the degradation rate of EE2 was relatively slow compared with E2/E3, the degradation level of EE2 reached 79.6% when incubated for a reaction time of 13 days (Figure 7, curve b). More significant is the fact that conditions of the model mimic those found in the natural environment: sunlight, air, ambient temperatures, atmospheric pressure, NaNO2, and FeCl3. The model process constitutes a proof of concept that estrogenic pollutants can be degraded in their original environmental positions without any biological factors participating. This newly developed degradation method is particularly attractive as current degradation systems to decontaminate polluted rivers/lakes require harsh conditions that are amenable to in situ treatment in a natural environment. The present model process highlights the importance of shutting off the points of entry of estrogens into the environment. Although many approaches exist, including the present degradation process in nature to the removal of the estrogens from the environment, estrogenic compounds are found in trace levels in wastewater of municipal sewage plants and, hence, in rivers. This is attributed to both natural estrogens such as 17β-estradiol(E2), estriol(E3), estrone(E1), and incessant anthropogenic activities, such as those excreted from human as a result of their use for contraception and hormone replacement therapy. VOL. 41, NO. 10, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 8. The Total ion current (TIC) of GC-MS analysis for residual organic compounds of E2 degradation after 2 d with 100 mol % NaNO2 and 100 mol % FeCl3 under mimic sunlight irradiation. The samples were derivatized by the derivatization (N-methyl-Ntrimethylsilyl trifluoroacetamide, MSTFA). (1) silanamine, N,1,1,1tetramethyl-N-[trimethylsilyl] which was produced by derivatization reagents. (2) propanedioic acid, bis(trimethylsilyl) ester. (3) bis(trimethylsilyl) estradiol. 4 and 5: bis(trimethylsilyl) 2 or 4-nitroestradiol.
FIGURE 10. Products formed after E2 degradation with FeCl3/NaNO2 catalyzed oxidation.
FIGURE 11. Estrogenic activity of aqueous solutions containing various amounts of E2. (a) E2 solution without photocatalytic treatment; (b) E2 solution treated photocatalytically for 2 days, and the mole ratio of E2/ NaNO2/FeCl3 were 1:0.5:0.5.
FIGURE 9. The Total ion current (TIC) of GC-MS analysis for residual organic compounds of E2 degradation after 30 d with 100 mol % NaNO2 and 100 mol % FeCl3 under mimic sunlight irradiation. The samples were derivatized by the derivatization (N-methyl-Ntrimethylsilyl trifluoroacetamide, MSTFA). (1) silanamine, N,1,1,1tetramethyl-N-[trimethylsilyl] which was produced by derivatization reagents. (2) propanedioic acid, bis(trimethylsilyl) ester. (6) Cyclotrisiloxane, hexamethyl which produced by column losing in the high temperature.
nitrified compounds in the natural environment might be removed by nitrites and Fe(III) salts in the aqueous environments, in a relatively rapid rate.
GC-MS Analysis of Intermediate Products and Terminated Products. To further probe the fate of estrogen E2 in the model process, we utilized gas chromatography-mass spectrum (GC-MS) to identify the intermediate products in the reaction process. When stoichiometric amounts of NaNO2 and FeCl3 were mixed with estrogen E2 under standard reaction conditions for 2 days, mass spectrometric analysis indicated the presence of low molecular weight organic acids (e.g., malonic acid) and nitroestrogens (i.e., 2-nitroestradiol and 4-nitroestadiol) as major chemical offspring (Figure 8). Of particular interest are the observations for both the nitrified E2 compounds. These compounds were further degraded with prolonged exposure as both 2- and 4-nitroestadiol disappeared and a very small quantity of malonic acid remained when the reaction time was extended to 30 days (Figure 9). The structures of the intermediate products formed after E2 degradation with FeCl3/NaNO2 catalyzed under natural conditions are shown in Figure 10. After a 30 day reaction, the GC-MS results indicated that detectable degradation productions do not contain N atoms, which indicates both 2- and 4-nitroestadiol to have been thoroughly degraded. On the other hand, the result also implies that some of these
Estrogenic Activity. Estrogenic activity is also one of the critical factors for evaluating the effectiveness of a new degradation system for estrogens. Toward this end, we selected recombinant yeast-based estrogen assay to determine the estrogenic activity of E2 to chart a time course, and the detection limit is 0.3 pg/L. We prepared an E2 solution which has been photocatalytically treated for 2 days. The removal of E2 was about 70%. The solution was concentrated 20-fold by evaporating it under vacuum and eluting it with dimethyl sulfoxide (DMSO, 1 mL). The treated solution, which contains about 30 µg/mL E2, was diluted by about 2-8 orders of magnitude, and the estrogenic activities of the diluted solutions were evaluated and plotted in Figure 11, curve b against the E2 concentrations in the diluted solutions. In the control experiment, the original E2 solution was treated to contain the same concentration of E2 with reacted solution. The activity-concentration relationship (dose-response curve) for E2 solution without photocatalytic treatment was investigated (Figure 11, curve a). The two curves are in good agreement. This result showed that the estrogenic activities of the intermediate products are negligible and that there is no secondary risk associated with increased the estrogenic activity as a result of the model degradation of E2 in water
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under the present experimental conditions. If the intermediate products in the treated solution have appreciable estrogenic activities, the plot should exhibit higher values than the original dose-response curve in a certain concentration region (20).
Acknowledgments We gratefully acknowledge the financial support from the National Natural Science Foundation of China (no. 20572110) and Key Project of Knowledge Innovation Program of Chinese Academy of Sciences (KJCX2-YW-H04). We thank Prof. Chen Jiping in Dalian Institute of Chemical Physics, Chinese Academy of Sciences for help with the experiment equipment and Prof. Wang Zijian in Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences for help in evaluating estrogenic activity.
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Received for review October 27, 2006. Revised manuscript received March 12, 2007. Accepted March 16, 2007. ES0625778 VOL. 41, NO. 10, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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