- Email: [email protected]
Degradation of microcystin-LR toxin by Fenton and Photo-Fenton processes
Toxicon 43 (2004) 829–832 www.elsevier.com/locate/toxicon
Degradation of microcystin-LR toxin by Fenton and Photo-Fenton processes Erick R. Bandalaa,*, Dolores Martı´nezb, Evaristo Martı´nezc, Dionysios D. Dionysioud b
a Instituto Mexicano de Tecnologı´a del Agua, Paseo Cuauhna´huac 8532, Jiutepec, Morelos 62550, Me´xico Division de Estudios de Posgrado, Facultad de Ingenierı´a-Campus Morelos, Universidad Nacional Auto´noma de Me´xico, Paseo Cuauhna´huac 8532, Jiutepec, Morelos 62550, Me´xico c Varian de Me´xico, Concepcio´n Be´istegui 109, Col. Del Valle, 03100 Me´xico DF, Me´xico d Department of Civil and Environmental Engineering, University of Cincinnati, 765 Baldwin Hall, P.O. Box 210071, Cincinnati, OH 45221-0071, USA
Received 17 November 2003; accepted 11 March 2004
Abstract This study reports a laboratory investigation of the degradation of microcystin-LR using Fenton (Fe2þ þ H2O2) and PhotoFenton processes. The effect of hydrogen peroxide concentration on the Fenton reaction rate was investigated at constant Fe2þ concentrations. It was observed that at low concentrations of hydrogen peroxide (0.25– 0.5 mM), the extent of microcystin-LR degradation was low, even after prolonged reaction time (up to 600 min). Higher H2O2 concentrations (2.5– 5 mM) resulted in higher degradation rates that yielded microcystin-LR degradation as high as 60% in approximately 180 min. However, the highest degradation efficiency of the toxin was achieved during the Photo-Fenton process in which UV radiation was involved. In the Photo-Fenton process, the removal efficiency of microcystin-LR reached 84% in the first 25 min and 100% in approximately 35 – 40 min of irradiation. These results are encouraging for the application of efficient UV-based advanced oxidation technologies for toxin removal from drinking water sources. q 2004 Elsevier Ltd. All rights reserved. Keywords: Microcystin-LR; Fenton; Photo-Fenton; Photocatalysis; Toxins; H2O2 Fe2þ; Fe3þ; Ferrous; Ferric; Hydrogen peroxide; Drinking water; Treatment; Advanced oxidation; AOT
1. Introduction Microcystins are a group of at least 70 hepatotoxic peptides produced primarily by freshwater cyanobacteria belonging to the genera Microcystis, Anabaena, Nostoc and Oscillatoria. The most common of this family of congeners is microcystin-LR (Sivonen, 1998). The presence of these toxins in water bodies has caused illness and death of wild and domestic animals worldwide (Sivonen and Jones, 1999), health problems to humans in different regions of the world (Carmichael, 1994), as well as human fatalities in a particular case in 1996. The latter concerns the death of more than 60 hemodialysis patients in Brazil due to hepatic * Corresponding author. Tel./fax: þ 52-777-329-3664. E-mail address: [email protected] (E.R. Bandala). 0041-0101/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.toxicon.2004.03.013
failure after they received water contaminated with microcystin at lethal dose concentrations (Jochimsen et al., 1998). The potential of microcystin and other cyanobacterial toxins to cause both acute and chronic toxicity has increased pressure to ensure their removal from sources of potable water (WHO, 1998). Microcystins are chemically very stable and are only slowly decomposed by acid, alkali or boiling (Harada et al., 1996). In field conditions, chemical or biological degradation of microcystins is very slow (Cousins et al., 1996). The removal of microcystins in drinking water has been evaluated using most of the commonly used treatment strategies. While chlorine at relatively high concentrations was found to be effective for degrading microcystin, in general it was demonstrated that conventional water treatment methods are not sufficient (Gajdek et al., 2001;
830
E.R. Bandala et al. / Toxicon 43 (2004) 829–832
Lawton et al., 2003). Advanced treatment methods such as ozonation and activated carbon filtration were found to be more effective (Himberg et al., 1989). Nevertheless, many other advanced treatment methods proved to have only limited efficiency for the degradation or chemical modification of microcystins (Lawton and Robertson, 1999). The use of TiO2 photocatalysis has been established in the last two decades as a useful technology for the complete destruction of a wide range of organic pollutants (Blake, 2001). Many reports have demonstrated that photocatalytic oxidation using TiO2 is a feasible technology for the degradation of microcystin-LR (Lawton et al., 2003; Robertson et al., 1997; Cornish et al., 2000; Liu et al., 2002; Shepard et al., 1998, 2002; Lawton and Robertson, 1999). Fenton reagent (Fe2þ þ H2O2), which like TiO2 photocatalysis, is based on the generation of hydroxyl radicals, has also been tested and demonstrated to be a promising alternative for the degradation of microcystin-LR (Gajdek et al., 2001; Yuan et al., 2002). When the Fenton process is complemented with radiation (ultraviolet, visible or both), the resulting process, known as Photo-Fenton, exhibits enhanced reaction rates for the degradation of organic pollutants (Bauer et al., 1999) including phenols, chlorinated aromatics, and pesticides (Caceres et al., 2003). In addition, the Photo-Fenton process is usually effective even at neutral pH whereas the Fenton reagent is limited only to acidic pH (Bauer et al., 1999). So far, only few studies have dealt with the degradation of microcystin with advanced oxidation technologies and in particular with Fenton reagent. Consequently, this study is focused on the application of Fenton and Photo-Fenton processes for the destruction of microcystin-LR in water.
as described in Section 2.4. In order to quench the Fenton reaction, 0.025 ml of a 0.1 g/l catalase solution were added once the sample was taken to remove hydrogen peroxide as reported by Caceres et al. (2003). Three different hydrogen peroxide concentrations (0.25, 0.5 and 5 mM) and two Fe2þ concentrations (0.25 and 2.5 mM) were tested. In experiments dealing with low concentrations of Fenton reagent (0.25 and 0.5 mM of H2O2 and 0.25 mM of Fe2þ), the initial concentration of microcystin-LR was approximately 4 mM. For all experiments, pH was adjusted to below 5.0 using sulfuric acid 1.0 M. 2.3. Photo-Fenton reaction In experiments dealing with the Photo-Fenton process, a 36 W UV lamp (WHI-36W-PLL, Philips Lighting Company) with a spectral output at 365 nm, was used. The reaction mixture was transferred into 11 borosilicate glass screw-cap 2-ml vials and exposed to UV radiation until reaching the desired irradiation time. After that, each sample was immediately analyzed for microcystin-LR concentration using HPLC. As in the case of the Fenton process, catalase solution was used to quench the reaction after sampling. Three levels of initial concentration (0.1, 0.25 and 0.5 mM) of hydrogen peroxide were tested. In all the illuminated experiments, the initial Fe2þ concentration was 0.25 mM. The initial concentration of microcystin-LR was approximately the same as that used in the Fenton process. As in the case of Fenton process, the solution pH in the Photo-Fenton experiments was also below 5.0. 2.4. Analytical methods
2. Experimental methods 2.1. Chemicals Microcystin-LR was isolated from field samples of a cyanobacterial bloom from the Valle de Bravo dam in Me´xico using a method previously described by Harada (1996). Microcystin solutions were prepared in Milli-Q grade water (conductivity , 1 mmho/cm). Hydrogen peroxide (Aldrich, 30% stabilized) and FeSO4·7H2O (Baker) were ACS reagent grade and were used as received. Catalase (Sigma, 2200 UA/mg) was used to stop Fenton reaction in samples before analysis.
Analysis of microcystin-LR was performed by HPLC using a Hewlett Packard 1050 liquid chromatograph equipped with a UV diode array detector. For microcystinLR quantification a reference standard (Sigma, MicrocystinLR from Microcystis aeruginosa, purity 95%, lot number 110 £ 1672) was used. The analysis was performed using a C18 Supelcosil LC-18.5 mm (Supelco) column. The injection volume was 75 ml and the flow rate of the mobile phase was 1.0 ml/min. The mobile phase was a mixture of Milli-Q water (A), ammonium acetate (10 mM; pH 7) (B), methanol (C), and acetonitrile (D). Table 1 shows Table 1 Mobile phase gradient used for HPLC analysis of microcystin-LR
2.2. Fenton reaction
Time (mm)
B (%)
C (%)
D (%)
All the reactions were carried out in the dark at room temperature. After the reagents were mixed, samples of the reaction mixture were taken at appropriate time intervals (each 15 min for dark Fenton process and 5 min for the Photo-Fenton experiments) and analyzed directly by HPLC
0.0 15 20 24 25
95 93 60 60 93
5 5 5 5 5
0 2 35 35 2
E.R. Bandala et al. / Toxicon 43 (2004) 829–832
831
the gradient and composition of the mobile phase used for HPLC analysis. All chromatograms were analyzed at 238 nm.
3. Results and discussion 3.1. Fenton process Fig. 1 shows the results for dark Fenton experiments for microcystin-LR degradation. When low concentrations of Fenton reagent were used for the two microcystin-LR concentrations tested, very low degradation of toxin was achieved under these conditions. The highest value for microcystin-LR degradation was determined to be lower than 20% after more than 600 min of reaction when 0.5 mM of H 2O2 and 0.25 mM of Fe2þ were used. During the control experiments, those without iron or without hydrogen peroxide, no toxin decomposition was observed. The results described above are in agreement with those reported by Gajdek et al. (2001) from experiments performed in the absence of iron or in the absence of H2O2. These authors observed microcystin-LR degradation only when the initial concentrations of H2O2 and Fe2þ reached values of 5.0 and 0.5 mM, respectively. However, in the present study, microcystin-LR degradation was observed at reagent concentrations lower than those reported by Gajdek et al. (2001). Further, increases in the concentration of Fenton reagent components resulted in a considerable increase in the degradation rate of the toxin as can also be seen in Fig. 1. When 5 mM of H2O2 and 2.5 mM of Fe2þ were used, the extent of toxin degradation reached 61% within 180 min. Most of the toxin degradation was observed during the first 15 min of the reaction as expected due to the initial surge of hydroxyl radicals as soon as the components of the Fenton reaction (Fe2þ and H2O2) were brought into contact.
Fig. 2. Degradation of microcystin-LR by Photo-Fenton process using different concentrations of hydrogen peroxide. The initial Fe2þ concentration was 0.25 mM.
3.2. Photo-Fenton process The use of radiation considerably improved the degradation rates of microcystin-LR. The results of the Photo-Fenton process are presented in Fig. 2. Most of the toxin was removed within the first 25 – 30 min of irradiation time. Toxin removal efficiencies close to 100% were observed after 35– 40 min of irradiation. As shown in Fig. 2, increase in the concentration of H2O2 from 0.1 to 0.5 mM did not have an important influence on the reaction rate. Nevertheless, comparing the results of Fig. 1, it is clear that the Photo-Fenton process is much more efficient than the conventional dark Fenton process for the degradation of microcystin-LR. The results obtained in this work are also comparable with those reported by Gajdek et al. (2001) using the dark Fenton process. These authors achieved 100% degradation of the toxin within 20 min of reaction time, when using 15 mM of H2O2 (30 times the highest H2O2 concentration used in the present study). Comparison between Figs. 1 and 2 shows important differences in the degradation rate of microcystin-LR during dark Fenton process at two different concentrations of H2O2 and Fe2þ and one set of conditions of the Photo-Fenton process. It is clear that Fenton process with low H2O2 and Fe2þ was the slower degradation process. As the reagent concentration increases, the degradation kinetics become more interesting as it can be seen for Fenton reaction using H2O2 ¼ 5 mM and [Fe2þ] ¼ 2.5 mM. Finally, when UV irradiation is included in the experiments, the results are very promising because the degradation rate increases by several times with respect to the previous experiments.
4. Conclusions
Fig. 1. Microcystin-LR degradation under different concentration of Fenton reagent (H2O2 and Fe2þ).
Fenton and Photo-Fenton processes were found to effectively degrade microcystin-LR in water samples. In general, results from this work agree with those of several previous reports dealing with the degradation of this toxin using other advanced oxidation technologies. The results
832
E.R. Bandala et al. / Toxicon 43 (2004) 829–832
obtained in this work are encouraging and proved that Fentonlike processes, such as UV-based Photo-Fenton reagent can be a good alternative to remove cyanobacterial toxins and other kinds of xenobiotics from contaminated water, especially when such water may be used as a source of drinking water. In particular, Photo-Fenton technology can be an attractive technology since it can be easily scaled-up and has ‘green features’. For example, it includes only UV radiation and environmentally friendly components such as iron and hydrogen peroxide. In this work, it was demonstrated that Photo-Fenton reagent could be comparable (i.e. at least from a feasibility prospective) to other hydroxyl-radical based technologies, such TiO2 photocatalysis, which is the most widely AOT process that has been investigated for the destruction of algal toxins. While direct comparison between TiO2 photocatalysis and the Photo-Fenton process is very difficult at this stage, the results obtained in this work indicate that the Photo-Fenton process is effective and more studies in this subject should continue in the future to optimize the process and gain better understanding of the degradation reaction mechanism. Some key advantages of the Photo-Fenton process that can be further examined is the use of solar radiation instead of UV light emitted from commercial sources, since it is known that the Photo-Fenton process can utilize both UV and visible light wavelengths (Bauer et al., 1999; Zhao et al., 2000).
Acknowledgements This work was partially financed by the Instituto Mexicano de Tecnologı´a del Agua (IMTA), Mexico and by the University of Cincinnati Research Council. D. Martı´nez thanks to Consejo Nacional de Ciencia y Tecnologı´a Mexico for financial support. The authors thank Mrs. Adriana Sa´nchez Lo´pez for correcting the English.
References Bauer, R., Waldner, G., Fallmann, H., Hager, S., Klare, M., Malato, S., Maletzky, P., 1999. The foto-fenton reaction and the TiO2/ UV process for wastewater treatment: novel developments. Catal. Today 53(1), 131 –144. Blake, D.M., 2001. Bibliography of work on the photocatalytic removal of hazardous compounds from water and air, Technical Report NREL/TP-510-31319, November 2001, National Renewable Energy Laboratory. Caceres, J., Malato, S., Fernandez-Alba, A., 2003. Analytical Evaluation and Advanced Oxidation Processes Optimization in Solar Pilot-Plants, Editorial CIEMAT, Madrid, Spain, (in Spanish). Carmichael, W.W., 1994. The toxins of cyanobacteria. Sci. Am. 270, 78 –86. Cornish, B.J.P.A., Lawton, L.A., Robertson, P.J.K., 2000. Hydrogen peroxide enhanced photocatalytic oxidation of microcystin-LR using titanium dioxide. Appl. Catal. B 25, 59–67.
Cousins, I.T., Bealing, D.J., James, H.A., Sutton, A., 1996. Biodegradation of microcystin-LR by indigenous mixed bacterial populations. Water Res. 30, 481 –485. Gajdek, P., Lechowski, Z., Bochnia, T., Kepczynski, M., 2001. Decomposition of microcystin-LR by Fenton oxidation. Toxicon 39, 1575–1578. Harada, K.I., 1996. Chemistry and Detection of Microcystins. In: Watanabe, M.F., Harada, K.I., Carmichael, W.W., Fujuki, H. (Eds.), Toxic Microcystis, CRC Press, New York. Harada, K.I., Tsuji, K., Watanabe, M.F., Kondo, F., 1996. Stability of microcystins from cyanobacteria-III. Effect of pH and temperature. Phycologia 35, 83–88. Himberg, K., Keijola, A.M., Hiisvirta, L., Pyysalo, H., Sivonen, K., 1989. The effect of water treatment process on the removal of hepatotoxins from Microcystis and Oscillatoria cyanobacteria: a laboratory study. Water Res. 23, 83 –88. Jochimsen, E.M., Carmichael, W.W., An, J.S., Cardo, D.M., Cookson, S.T., Holmes, C.E.M., Antunes, M.B. de C., Filho, D.A., de Melo Lyra, T.M., Barreto, V.S.T., Azevedo, S.M.F.O., Jarvis, W.R., 1998. Liver failure and death after exposure to microcystins at a hemodialysis center in Brazil. N. Engl. J. Med. 338, 873–878. Lawton, L., Robertson, P.K.J., 1999. Physico-chemical treatment methods for the removal of microcystins (cyanobacterial epatotoxins) from potable waters. Chem. Soc. Rev. 28, 217–224. Lawton, L., Robertson, P.K.J., Cornish, B.J.P.A., Marr, I.L., Jaspars, M., 2003. Processes influencing surface interaction and photocatalytic destruction of microcystins on titanium dioxide photocatalysts. J. Catal. 213, 109 –113. Liu, I., Lawton, L., Cornish, B., Robertson, P.K.J., 2002. Mechanistic and toxicity studies of the photocatalytic oxidation of microcystin-LR. J. Photochem. Photobiol. A: Chem. 148, 349–354. Robertson, P.K.J., Lawton, L., Mu¨nch, B., Rouzade, J., 1997. Destruction of cyanobacterial toxin by semiconductor photocatalysis. Chem. Commun. 4, 393–394. Shepard, G.S., Stokestro¨m, S., De Villiers, D., Engelbrecht, W.J., Sydenham, E.W., Wessels, G.F.S., 1998. Photocatalytic degradation of cyanobacterial microcystins toxins in water. Toxicon 32(12), 1895–1901. Shepard, G.S., Stockenstro¨m, S., de Villiers, D., Engelbrecht, W.J., Wessels, G.F.S., 2002. Degradation of microcystin toxins in a falling film photocatalytic reactor with immobilized titanium dioxide catalyst. Water Res. 36, 140 –146. Sivonen, K., 1998. Toxins Produced by Cyanobacteria. In: Miraglia, M., Van Egmond, H., Brera, C., Gilbert, J. (Eds.), Mycotoxins and Phycotoxins—Development in Chemistry, Toxicology and Food Safety, USA, pp. 547 –568. Sivonen, K., Jones, G., 1999. Cyan bacterial toxins. In: Chorus, I., Batram, J. (Eds.), Toxic Cyanobacteria in Water: A Guide to their Public Health Consequences, Monitoring and Management, E and FN Spon, London, p. 41. World Health Organization, 1998. Guidelines for Drinking Water Quality, second ed., Health Criteria and Other Supporting Information, Geneva, Addendum to vol. 2. Yuan, B., Hu, J., Fu, M., 2002. Removal of cyanobacterial microcystin-LR by ferrate oxidation-cuagulation. Toxicon 40, 1129–1134. Zhao, J., Xie, Y., Chen, F., Wang, H., 2000. Photoassisted degradation of dyes in the presence of Fe3þ and H2O2 under visible irradiation. J. Photochem. Photobiol A: Chem. 136, 235 –240.
Degradation of microcystin-LR toxin by Fenton and Photo-Fenton processes Erick R. Bandalaa,*, Dolores Martı´nezb, Evaristo Martı´nezc, Dionysios D. Dionysioud b
a Instituto Mexicano de Tecnologı´a del Agua, Paseo Cuauhna´huac 8532, Jiutepec, Morelos 62550, Me´xico Division de Estudios de Posgrado, Facultad de Ingenierı´a-Campus Morelos, Universidad Nacional Auto´noma de Me´xico, Paseo Cuauhna´huac 8532, Jiutepec, Morelos 62550, Me´xico c Varian de Me´xico, Concepcio´n Be´istegui 109, Col. Del Valle, 03100 Me´xico DF, Me´xico d Department of Civil and Environmental Engineering, University of Cincinnati, 765 Baldwin Hall, P.O. Box 210071, Cincinnati, OH 45221-0071, USA
Received 17 November 2003; accepted 11 March 2004
Abstract This study reports a laboratory investigation of the degradation of microcystin-LR using Fenton (Fe2þ þ H2O2) and PhotoFenton processes. The effect of hydrogen peroxide concentration on the Fenton reaction rate was investigated at constant Fe2þ concentrations. It was observed that at low concentrations of hydrogen peroxide (0.25– 0.5 mM), the extent of microcystin-LR degradation was low, even after prolonged reaction time (up to 600 min). Higher H2O2 concentrations (2.5– 5 mM) resulted in higher degradation rates that yielded microcystin-LR degradation as high as 60% in approximately 180 min. However, the highest degradation efficiency of the toxin was achieved during the Photo-Fenton process in which UV radiation was involved. In the Photo-Fenton process, the removal efficiency of microcystin-LR reached 84% in the first 25 min and 100% in approximately 35 – 40 min of irradiation. These results are encouraging for the application of efficient UV-based advanced oxidation technologies for toxin removal from drinking water sources. q 2004 Elsevier Ltd. All rights reserved. Keywords: Microcystin-LR; Fenton; Photo-Fenton; Photocatalysis; Toxins; H2O2 Fe2þ; Fe3þ; Ferrous; Ferric; Hydrogen peroxide; Drinking water; Treatment; Advanced oxidation; AOT
1. Introduction Microcystins are a group of at least 70 hepatotoxic peptides produced primarily by freshwater cyanobacteria belonging to the genera Microcystis, Anabaena, Nostoc and Oscillatoria. The most common of this family of congeners is microcystin-LR (Sivonen, 1998). The presence of these toxins in water bodies has caused illness and death of wild and domestic animals worldwide (Sivonen and Jones, 1999), health problems to humans in different regions of the world (Carmichael, 1994), as well as human fatalities in a particular case in 1996. The latter concerns the death of more than 60 hemodialysis patients in Brazil due to hepatic * Corresponding author. Tel./fax: þ 52-777-329-3664. E-mail address: [email protected] (E.R. Bandala). 0041-0101/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.toxicon.2004.03.013
failure after they received water contaminated with microcystin at lethal dose concentrations (Jochimsen et al., 1998). The potential of microcystin and other cyanobacterial toxins to cause both acute and chronic toxicity has increased pressure to ensure their removal from sources of potable water (WHO, 1998). Microcystins are chemically very stable and are only slowly decomposed by acid, alkali or boiling (Harada et al., 1996). In field conditions, chemical or biological degradation of microcystins is very slow (Cousins et al., 1996). The removal of microcystins in drinking water has been evaluated using most of the commonly used treatment strategies. While chlorine at relatively high concentrations was found to be effective for degrading microcystin, in general it was demonstrated that conventional water treatment methods are not sufficient (Gajdek et al., 2001;
830
E.R. Bandala et al. / Toxicon 43 (2004) 829–832
Lawton et al., 2003). Advanced treatment methods such as ozonation and activated carbon filtration were found to be more effective (Himberg et al., 1989). Nevertheless, many other advanced treatment methods proved to have only limited efficiency for the degradation or chemical modification of microcystins (Lawton and Robertson, 1999). The use of TiO2 photocatalysis has been established in the last two decades as a useful technology for the complete destruction of a wide range of organic pollutants (Blake, 2001). Many reports have demonstrated that photocatalytic oxidation using TiO2 is a feasible technology for the degradation of microcystin-LR (Lawton et al., 2003; Robertson et al., 1997; Cornish et al., 2000; Liu et al., 2002; Shepard et al., 1998, 2002; Lawton and Robertson, 1999). Fenton reagent (Fe2þ þ H2O2), which like TiO2 photocatalysis, is based on the generation of hydroxyl radicals, has also been tested and demonstrated to be a promising alternative for the degradation of microcystin-LR (Gajdek et al., 2001; Yuan et al., 2002). When the Fenton process is complemented with radiation (ultraviolet, visible or both), the resulting process, known as Photo-Fenton, exhibits enhanced reaction rates for the degradation of organic pollutants (Bauer et al., 1999) including phenols, chlorinated aromatics, and pesticides (Caceres et al., 2003). In addition, the Photo-Fenton process is usually effective even at neutral pH whereas the Fenton reagent is limited only to acidic pH (Bauer et al., 1999). So far, only few studies have dealt with the degradation of microcystin with advanced oxidation technologies and in particular with Fenton reagent. Consequently, this study is focused on the application of Fenton and Photo-Fenton processes for the destruction of microcystin-LR in water.
as described in Section 2.4. In order to quench the Fenton reaction, 0.025 ml of a 0.1 g/l catalase solution were added once the sample was taken to remove hydrogen peroxide as reported by Caceres et al. (2003). Three different hydrogen peroxide concentrations (0.25, 0.5 and 5 mM) and two Fe2þ concentrations (0.25 and 2.5 mM) were tested. In experiments dealing with low concentrations of Fenton reagent (0.25 and 0.5 mM of H2O2 and 0.25 mM of Fe2þ), the initial concentration of microcystin-LR was approximately 4 mM. For all experiments, pH was adjusted to below 5.0 using sulfuric acid 1.0 M. 2.3. Photo-Fenton reaction In experiments dealing with the Photo-Fenton process, a 36 W UV lamp (WHI-36W-PLL, Philips Lighting Company) with a spectral output at 365 nm, was used. The reaction mixture was transferred into 11 borosilicate glass screw-cap 2-ml vials and exposed to UV radiation until reaching the desired irradiation time. After that, each sample was immediately analyzed for microcystin-LR concentration using HPLC. As in the case of the Fenton process, catalase solution was used to quench the reaction after sampling. Three levels of initial concentration (0.1, 0.25 and 0.5 mM) of hydrogen peroxide were tested. In all the illuminated experiments, the initial Fe2þ concentration was 0.25 mM. The initial concentration of microcystin-LR was approximately the same as that used in the Fenton process. As in the case of Fenton process, the solution pH in the Photo-Fenton experiments was also below 5.0. 2.4. Analytical methods
2. Experimental methods 2.1. Chemicals Microcystin-LR was isolated from field samples of a cyanobacterial bloom from the Valle de Bravo dam in Me´xico using a method previously described by Harada (1996). Microcystin solutions were prepared in Milli-Q grade water (conductivity , 1 mmho/cm). Hydrogen peroxide (Aldrich, 30% stabilized) and FeSO4·7H2O (Baker) were ACS reagent grade and were used as received. Catalase (Sigma, 2200 UA/mg) was used to stop Fenton reaction in samples before analysis.
Analysis of microcystin-LR was performed by HPLC using a Hewlett Packard 1050 liquid chromatograph equipped with a UV diode array detector. For microcystinLR quantification a reference standard (Sigma, MicrocystinLR from Microcystis aeruginosa, purity 95%, lot number 110 £ 1672) was used. The analysis was performed using a C18 Supelcosil LC-18.5 mm (Supelco) column. The injection volume was 75 ml and the flow rate of the mobile phase was 1.0 ml/min. The mobile phase was a mixture of Milli-Q water (A), ammonium acetate (10 mM; pH 7) (B), methanol (C), and acetonitrile (D). Table 1 shows Table 1 Mobile phase gradient used for HPLC analysis of microcystin-LR
2.2. Fenton reaction
Time (mm)
B (%)
C (%)
D (%)
All the reactions were carried out in the dark at room temperature. After the reagents were mixed, samples of the reaction mixture were taken at appropriate time intervals (each 15 min for dark Fenton process and 5 min for the Photo-Fenton experiments) and analyzed directly by HPLC
0.0 15 20 24 25
95 93 60 60 93
5 5 5 5 5
0 2 35 35 2
E.R. Bandala et al. / Toxicon 43 (2004) 829–832
831
the gradient and composition of the mobile phase used for HPLC analysis. All chromatograms were analyzed at 238 nm.
3. Results and discussion 3.1. Fenton process Fig. 1 shows the results for dark Fenton experiments for microcystin-LR degradation. When low concentrations of Fenton reagent were used for the two microcystin-LR concentrations tested, very low degradation of toxin was achieved under these conditions. The highest value for microcystin-LR degradation was determined to be lower than 20% after more than 600 min of reaction when 0.5 mM of H 2O2 and 0.25 mM of Fe2þ were used. During the control experiments, those without iron or without hydrogen peroxide, no toxin decomposition was observed. The results described above are in agreement with those reported by Gajdek et al. (2001) from experiments performed in the absence of iron or in the absence of H2O2. These authors observed microcystin-LR degradation only when the initial concentrations of H2O2 and Fe2þ reached values of 5.0 and 0.5 mM, respectively. However, in the present study, microcystin-LR degradation was observed at reagent concentrations lower than those reported by Gajdek et al. (2001). Further, increases in the concentration of Fenton reagent components resulted in a considerable increase in the degradation rate of the toxin as can also be seen in Fig. 1. When 5 mM of H2O2 and 2.5 mM of Fe2þ were used, the extent of toxin degradation reached 61% within 180 min. Most of the toxin degradation was observed during the first 15 min of the reaction as expected due to the initial surge of hydroxyl radicals as soon as the components of the Fenton reaction (Fe2þ and H2O2) were brought into contact.
Fig. 2. Degradation of microcystin-LR by Photo-Fenton process using different concentrations of hydrogen peroxide. The initial Fe2þ concentration was 0.25 mM.
3.2. Photo-Fenton process The use of radiation considerably improved the degradation rates of microcystin-LR. The results of the Photo-Fenton process are presented in Fig. 2. Most of the toxin was removed within the first 25 – 30 min of irradiation time. Toxin removal efficiencies close to 100% were observed after 35– 40 min of irradiation. As shown in Fig. 2, increase in the concentration of H2O2 from 0.1 to 0.5 mM did not have an important influence on the reaction rate. Nevertheless, comparing the results of Fig. 1, it is clear that the Photo-Fenton process is much more efficient than the conventional dark Fenton process for the degradation of microcystin-LR. The results obtained in this work are also comparable with those reported by Gajdek et al. (2001) using the dark Fenton process. These authors achieved 100% degradation of the toxin within 20 min of reaction time, when using 15 mM of H2O2 (30 times the highest H2O2 concentration used in the present study). Comparison between Figs. 1 and 2 shows important differences in the degradation rate of microcystin-LR during dark Fenton process at two different concentrations of H2O2 and Fe2þ and one set of conditions of the Photo-Fenton process. It is clear that Fenton process with low H2O2 and Fe2þ was the slower degradation process. As the reagent concentration increases, the degradation kinetics become more interesting as it can be seen for Fenton reaction using H2O2 ¼ 5 mM and [Fe2þ] ¼ 2.5 mM. Finally, when UV irradiation is included in the experiments, the results are very promising because the degradation rate increases by several times with respect to the previous experiments.
4. Conclusions
Fig. 1. Microcystin-LR degradation under different concentration of Fenton reagent (H2O2 and Fe2þ).
Fenton and Photo-Fenton processes were found to effectively degrade microcystin-LR in water samples. In general, results from this work agree with those of several previous reports dealing with the degradation of this toxin using other advanced oxidation technologies. The results
832
E.R. Bandala et al. / Toxicon 43 (2004) 829–832
obtained in this work are encouraging and proved that Fentonlike processes, such as UV-based Photo-Fenton reagent can be a good alternative to remove cyanobacterial toxins and other kinds of xenobiotics from contaminated water, especially when such water may be used as a source of drinking water. In particular, Photo-Fenton technology can be an attractive technology since it can be easily scaled-up and has ‘green features’. For example, it includes only UV radiation and environmentally friendly components such as iron and hydrogen peroxide. In this work, it was demonstrated that Photo-Fenton reagent could be comparable (i.e. at least from a feasibility prospective) to other hydroxyl-radical based technologies, such TiO2 photocatalysis, which is the most widely AOT process that has been investigated for the destruction of algal toxins. While direct comparison between TiO2 photocatalysis and the Photo-Fenton process is very difficult at this stage, the results obtained in this work indicate that the Photo-Fenton process is effective and more studies in this subject should continue in the future to optimize the process and gain better understanding of the degradation reaction mechanism. Some key advantages of the Photo-Fenton process that can be further examined is the use of solar radiation instead of UV light emitted from commercial sources, since it is known that the Photo-Fenton process can utilize both UV and visible light wavelengths (Bauer et al., 1999; Zhao et al., 2000).
Acknowledgements This work was partially financed by the Instituto Mexicano de Tecnologı´a del Agua (IMTA), Mexico and by the University of Cincinnati Research Council. D. Martı´nez thanks to Consejo Nacional de Ciencia y Tecnologı´a Mexico for financial support. The authors thank Mrs. Adriana Sa´nchez Lo´pez for correcting the English.
References Bauer, R., Waldner, G., Fallmann, H., Hager, S., Klare, M., Malato, S., Maletzky, P., 1999. The foto-fenton reaction and the TiO2/ UV process for wastewater treatment: novel developments. Catal. Today 53(1), 131 –144. Blake, D.M., 2001. Bibliography of work on the photocatalytic removal of hazardous compounds from water and air, Technical Report NREL/TP-510-31319, November 2001, National Renewable Energy Laboratory. Caceres, J., Malato, S., Fernandez-Alba, A., 2003. Analytical Evaluation and Advanced Oxidation Processes Optimization in Solar Pilot-Plants, Editorial CIEMAT, Madrid, Spain, (in Spanish). Carmichael, W.W., 1994. The toxins of cyanobacteria. Sci. Am. 270, 78 –86. Cornish, B.J.P.A., Lawton, L.A., Robertson, P.J.K., 2000. Hydrogen peroxide enhanced photocatalytic oxidation of microcystin-LR using titanium dioxide. Appl. Catal. B 25, 59–67.
Cousins, I.T., Bealing, D.J., James, H.A., Sutton, A., 1996. Biodegradation of microcystin-LR by indigenous mixed bacterial populations. Water Res. 30, 481 –485. Gajdek, P., Lechowski, Z., Bochnia, T., Kepczynski, M., 2001. Decomposition of microcystin-LR by Fenton oxidation. Toxicon 39, 1575–1578. Harada, K.I., 1996. Chemistry and Detection of Microcystins. In: Watanabe, M.F., Harada, K.I., Carmichael, W.W., Fujuki, H. (Eds.), Toxic Microcystis, CRC Press, New York. Harada, K.I., Tsuji, K., Watanabe, M.F., Kondo, F., 1996. Stability of microcystins from cyanobacteria-III. Effect of pH and temperature. Phycologia 35, 83–88. Himberg, K., Keijola, A.M., Hiisvirta, L., Pyysalo, H., Sivonen, K., 1989. The effect of water treatment process on the removal of hepatotoxins from Microcystis and Oscillatoria cyanobacteria: a laboratory study. Water Res. 23, 83 –88. Jochimsen, E.M., Carmichael, W.W., An, J.S., Cardo, D.M., Cookson, S.T., Holmes, C.E.M., Antunes, M.B. de C., Filho, D.A., de Melo Lyra, T.M., Barreto, V.S.T., Azevedo, S.M.F.O., Jarvis, W.R., 1998. Liver failure and death after exposure to microcystins at a hemodialysis center in Brazil. N. Engl. J. Med. 338, 873–878. Lawton, L., Robertson, P.K.J., 1999. Physico-chemical treatment methods for the removal of microcystins (cyanobacterial epatotoxins) from potable waters. Chem. Soc. Rev. 28, 217–224. Lawton, L., Robertson, P.K.J., Cornish, B.J.P.A., Marr, I.L., Jaspars, M., 2003. Processes influencing surface interaction and photocatalytic destruction of microcystins on titanium dioxide photocatalysts. J. Catal. 213, 109 –113. Liu, I., Lawton, L., Cornish, B., Robertson, P.K.J., 2002. Mechanistic and toxicity studies of the photocatalytic oxidation of microcystin-LR. J. Photochem. Photobiol. A: Chem. 148, 349–354. Robertson, P.K.J., Lawton, L., Mu¨nch, B., Rouzade, J., 1997. Destruction of cyanobacterial toxin by semiconductor photocatalysis. Chem. Commun. 4, 393–394. Shepard, G.S., Stokestro¨m, S., De Villiers, D., Engelbrecht, W.J., Sydenham, E.W., Wessels, G.F.S., 1998. Photocatalytic degradation of cyanobacterial microcystins toxins in water. Toxicon 32(12), 1895–1901. Shepard, G.S., Stockenstro¨m, S., de Villiers, D., Engelbrecht, W.J., Wessels, G.F.S., 2002. Degradation of microcystin toxins in a falling film photocatalytic reactor with immobilized titanium dioxide catalyst. Water Res. 36, 140 –146. Sivonen, K., 1998. Toxins Produced by Cyanobacteria. In: Miraglia, M., Van Egmond, H., Brera, C., Gilbert, J. (Eds.), Mycotoxins and Phycotoxins—Development in Chemistry, Toxicology and Food Safety, USA, pp. 547 –568. Sivonen, K., Jones, G., 1999. Cyan bacterial toxins. In: Chorus, I., Batram, J. (Eds.), Toxic Cyanobacteria in Water: A Guide to their Public Health Consequences, Monitoring and Management, E and FN Spon, London, p. 41. World Health Organization, 1998. Guidelines for Drinking Water Quality, second ed., Health Criteria and Other Supporting Information, Geneva, Addendum to vol. 2. Yuan, B., Hu, J., Fu, M., 2002. Removal of cyanobacterial microcystin-LR by ferrate oxidation-cuagulation. Toxicon 40, 1129–1134. Zhao, J., Xie, Y., Chen, F., Wang, H., 2000. Photoassisted degradation of dyes in the presence of Fe3þ and H2O2 under visible irradiation. J. Photochem. Photobiol A: Chem. 136, 235 –240.