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Hydrogen peroxide assisted photochemical degradation of ethylenediaminetetraacetic acid
Advances in Environmental Research 7 Ž2002. 197᎐202
Hydrogen peroxide assisted photochemical degradation of ethylenediaminetetraacetic acid Airton Kunz a , Patricio Peralta-Zamoraa,U , Nelson Duran ´ b a
Laboratorio Ambiental e de Materiais, Departamento de Quımica, Uni¨ ersidade Federal de Parana, ´ de Quımica ´ ´ ´ C.P. 19081, CEP 81531-990, Curitiba-PR, Brazil b Biological Chemistry Laboratory, Physical Chemistry Department, Instituto de Quımica, Uni¨ ersidade Estadual de ´ Campinas, C.P. 6154, CEP 13083-970, Campinas-SP, Brazil Accepted 25 August 2001
Abstract In the present work the hydrogen peroxide-assisted photochemical degradation of ethylenediaminetetraacetic acid ŽEDTA. is reported. The effect of pH and H 2 O 2 :EDTA molar ratio on the efficiency of the EDTA degradation was evaluated by using a 2 2 factorial design. Working at optimized experimental conditions ŽpH of 2 and H 2 O 2 :EDTA molar ratio of 10., using a microwave-activated photochemical reactor and monitoring the EDTA degradation by total organic carbon analysis, mineralization ratios higher than 90% were observed at reaction times of 6 min. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: EDTA; Photochemical degradation; Hydrogen peroxide; Microwaves
1. Introduction The close correlation between environmental pollution and industrial activity is an extensively documented reality of recent decades ŽBolivar et al., 2000; Dayan et al., 2000; Vohra and Davis, 2000.. Many efforts have been dedicated to the study of alternative technologies, that are able to minimize the deleterious effect caused by this anthropic source. Among several options, the development of processes with the capacity to transform the great number of toxic compounds that surge as industrial effluents into harmless compounds, is one of the most immediate alternatives ŽWeber and LeBoeuf, 2000.. With this objective, many processes
U
Corresponding author. Tel.: q55-41-361-3297; fax: q5541-361-3186. E-mail address: [email protected] ŽP. PeraltaZamora..
have been proposed over the years. In this context, photochemical processes are important, mainly due to their capacity to degrade recalcitrant and toxic species in relatively short reaction times ŽHoffmann et al., 1995; Lisenbigler et al., 1995; Mills and Le Hunte, 1997; Legrini et al., 1993.. An important part of the photochemical processes explored as remediation alternatives is represented by heterogeneous systems based on the use of ultraviolet light and semiconductor metallic oxides. However, the viability of this kind of process is debatable, mainly due to the extreme difficulty encountered in the separation of the solid photocatalysts at the end of the process. From this point of view, the use of homogeneous systems appears to be a good alternative. Advanced oxidation processes based on the UV-H 2 O 2 system have shown high efficiency in the degradation of several compounds of environmental relevance ŽBenitez et al., 2000; Galindo and Kalt, 2000; Bolton and Cater, 1994.. Ethylenediaminetetraacetic acid ŽEDTA. is a strong
1093-0191r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 0 9 3 - 0 1 9 1 Ž 0 1 . 0 0 1 2 6 - 5
A. Kunz et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 197᎐202
198
chelating agent extensively used in industrial applications such as metal plating, water softening, photography, textile and paper manufacture ŽDavis and Green, 1999.. Processes that involve the removal of metal oxides from heat transfer surfaces, such as in boilers or nuclear reactors, also use this complexing agent, due to their ability to complex metals ŽMadden et al., 1997.. EDTA exhibits a serious environmental impact due to its low biodegradability by conventional biological and physicochemical methods for the treatment of wastewater ŽNortemann, 1999; Tucker et al., 1999.. Recently it was demonstrated that EDTA is ultimately biodegraded under practical industrial wastewater treatment conditions, in a study involving a finishing plant dedicated to treatment of the effluent from a paper mill ŽKaluza et al., 1998.. With high metal affinity, EDTA forms very stable metal complexes over a broad pH range, decreasing metal bioavailability in some natural and unpolluted mediums ŽBergers and Groot, 1994.. In this work the H 2 O 2-assisted photochemical degradation of EDTA was evaluated, using a microwave-activated ultraviolet source. Preliminary optimization studies were carried out by factorial design, while high performance liquid chromatography and total organic carbon analysis were used to evaluate kinetically the efficiency of the proposed process. The results reported here correspond to a preliminary stage of a large project with the objective to study advanced oxidation processes to degrade EDTA from a nuclear plant.
Fig. 1. Schematic representation of the photochemical reactor.
drate ŽSigma, 99%. was used as aqueous solutions of 1 mmol ly1. Hydrogen peroxide 30% Žwrw. ŽMalinckrodt. was used as received.
2.2. Photochemical process The photochemical treatment was carried out in a microwave-activated photochemical reactor Žmodel UV LAB 䊛 EL., kindly donated by UMEX ŽDresden, Germany.. The inner component of the reactor was fabricated with synthetic quartz of high UV-light permeability, while the external one, which contained mercury vapor at low pressure, was constructed with quartz of low permeability ŽFig. 1.. Samples of EDTA Ž15 ml., with pH values adjusted between 2 and 4, were added in adequate amounts of hydrogen peroxide and submitted to irradiation cycles of 1.5 min, using a commercial microwave oven. The efficiency of the photochemical process was evaluated by measuring the total organic carbon content ŽShimadzu TOC-5000A total organic carbon analyzer., while the EDTA degradation was determined by high performance liquid chromatography ŽShimadzu, LC-10AD., according to the instrumental conditions reported by Bergers and Groot Ž1994.:
2. Experimental 2.1. Reagents Ethylenediaminetetraacetic acid disodium salt dihy-
Column: C18 ŽLichrospore 100, RP-18 Merck.;
Table 1 Experimental design to study the effect of pH and H 2 O 2 :EDTA ratio on the EDTA degradation efficiency wEDTA: 15 ml Ž1.0 mmol ly1 ., time: 1.5 min.x Parameter
Level Žy.
Central point Ž0.
Level Žq.
pH H 2 O 2 :EDTA ratio
2 10
3 20
4 30
Experiment
pH
Ratio
TOC reduction Ž%.
1 2 3 4 5 6
y q y q 0 0
y y q q 0 0
58 51 57 54 52 54
Main effects: pH: y5 "1, Ratio: 1 "1; combined effect: pH= Ratio: 2 " 1.
A. Kunz et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 197᎐202
Mobile phase: methanol:formicrformiate buffer Ž10:90 vrv.; UV-detector: 258 nm.
3. Results and discussion 3.1. Preliminary optimization The preliminary optimization study, designed to verify the effect of pH and H 2 O 2 :EDTA ratio on the EDTA degradation efficiency, was carried out by the factorial design given in Table 1. The H 2 O 2 :EDTA ratio was initially assayed between the levels of 10 and 30, because according to the current literature, they are the ratios usually used in photochemical degradation processes assisted by hydrogen peroxide ŽInce, 1999.. From the results shown in Table 1, it is possible to observe that when the pH is changed from the low to the high level, the degradation efficiency is decreased by nearly 5 percentage points Žmain effect: y5.. This result implies that for high degradation rates the lower pH level must be used. When the H 2 O 2 :EDTA ratio is changed between the two selected levels the degradation efficiency is enhanced by approximately 1 percentage point Žmain effect: 1.. However, as the determinations show a typical estimated standard deviation of approximately 1 percentage point Žsee result of the duplicated central point., we can conclude that, between the limits of the experimental design, this experimental parameter does not show any significant effect on the EDTA degradation efficiency. The standard deviation of the TOC determination
was estimated from the unexpressive effects calculated in the factorial design. It is very common in this kind of experimental design to use these criteria to obtain an estimate value of the error of the determination ŽBarros Neto et al., 1995.. In Table 1 it is possible to observe that the effect of the H 2 O 2 :EDTA ratio is negligible Ž1 percentage point.. If we admit that this variable does not influence the efficiency of the degradation process, we can also admit that the difference between the experiments 1᎐3 and 3᎐4 are due to errors in the analytical methodology. If we use these differences, together with the results obtained in the duplicate point, we can calculate an estimated error of approximately 1%. From the geometric interpretation presented in Fig. 2 it is possible to clearly observe two tendencies toward a maximal degradation capacity of the photochemical system. Even when the tendencies are discrepant in relation to the H 2 O 2 :EDTA ratio effect, it is very easy to observe that both point toward the low pH level. In view of these preliminary observations, subsequent photochemical experiments will be carried out at a pH of 2 and a low H 2 O 2 :EDTA ratio.
3.2. Degradation kinetics Working at optimized experimental conditions, the kinetics of the degradation process were evaluated. From Fig. 3, it is possible to observe that the degradation process, measured as the reduction of total organic carbon content, proceeded very quickly in the presence of H 2 O 2 and UV-light, reaching a degradation ratio higher than 90% at reaction times of 6 min. Under the same conditions, the presence of UV-light alone induced a degradation of approximately 50%, while the action of microwaves or the combination of H 2 O 2 and microwaves did not show any degradation capacity. In a first analysis, the elimination of 50% of the TOC content, in the absence of the oxidant agent ŽH 2 O 2 ., appears to be an inexplicable result. However, if we consider that the photochemical reactor provides radiation of wavelength lower than 200 nm, the mineralization of EDTA can be explained due to its reaction with the hydroxyl radical produced from the photolysis of water wEq. Ž1.x. H 2 O q h Ž - 200 nm. ª H ⭈q HO⭈
Fig. 2. Geometric representation of the factorial design.
199
Ž1.
The efficient mineralization of aqueous samples containing organochloride compounds by photolysis has been reported in the recent literature ŽLopez et al., 2000.. Even when a small portion of organic carbon remained after a photochemical treatment of 6 min, the HPLC analysis indicated a complete EDTA degradation at reaction times of approximately 45 s ŽFig. 4..
200
A. Kunz et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 197᎐202
Fig. 3. EDTA degradation as a function of the treatment. EDTA: 15 ml Ž1.0 mmol ly1 ., H 2 O 2 :EDTA ratio: 10, pH: 2.0.
Two transient species appeared during the initial stages of the photochemical process Ž Rt of 4.9 and 13.7 min., species that are completely degraded at higher reaction times. An unidentified impurity Ž Rt: 5.3 min. remained practically unchanged during the photochemical process, probably representing the 10% of non-degraded TOC content. If we consider the electrical power of the microwave system Ž1050 W hy1 ., the time required for a degrada-
tion of 90% Ž6 min., the sample volume Ž15 ml. and the initial EDTA concentration Ž1 mmol ly1 ., we can calculate an experimental ‘electrical energy per mass’ ŽEEM. ŽBolton et al., 1996. value of approximately 21 kW h gy1 wEq. Ž2.x However, considering that the commercial availability of high capacity microwaveactivated photochemical reactors exists Ž see Žhttp.www.umex.de.., this parameter can be decreased to 0.3 kW h gy1 by only extending the reactor capacity
Fig. 4. Schematic representation of the chromatographic response.
A. Kunz et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 197᎐202
to 1 l. Taking into account that classical heterogeneous photocatalytic processes, applied under our experimental conditions, show typical EEM values of approximately 4 kWh gy1, the proposed procedure appears as an economically viable degradation alternative. EEMs Wr Žw EDTA x 0 = MW= SVr1000= DE r100 = 60rRT .
Ž2.
Where: wEDTAx 0 : MW: SV: DE: RT: W:
201
tochemical degradation of EDTA at reaction times lower than 10 min represents a remarkable result. One of the main disadvantages of the system is the necessity for artificial irradiation sources. However, with the recent commercial availability of low-cost continuous photochemical reactors which permit the application of intense UV-radiation at angles of 360⬚, the economical viability of the process has been significantly increased. References
initial EDTA concentration Ž1 = 10y3 mol ly1 .; molecular weight of EDTA Ž372.24 g moly1 .; sample volume Žml.; degradation efficiency Ž%. at RT; reaction time Žmin.; UV-source power ŽkW hy1 , 1.05..
When the samples were submitted to the high reaction time Ž6 min, in cycles of 1.5 min. the temperature was raised to approximately 85 ⬚C. Isothermal conditions were not maintained in the experiments due to the negligible effect of temperature on the EDTA content Žsee Fig. 3.. It is important to highlight that, as shown in Fig. 3, the microwave and the microwave᎐H 2 O 2 systems do not cause any significant mineralization of the EDTA molecule. This fact was confirmed by using HPLC and UV-vis spectrophotometry. The microwave source is used only to excite the mercury vapor in the reactor and produce ultraviolet light. For this reason, the UVrH 2 O 2rmicrowave system corresponds to a conventional photochemical process assisted by hydrogen peroxide, with the unique difference that, in this case, the work involves the use of a photochemical reactor of high photonic efficiency. Our group has carried out several studies of EDTA degradation with conventional reactors Žunpublished results .. By using these systems the degradation velocity is significantly lower, but the mechanistic aspects are practically the same as those observed in the proposed system.
4. Conclusions The H 2 O 2-assisted photochemical process discussed here represents an excellent alternative for the degradation of EDTA. If we consider that the almost complete removal of EDTA by activated sludge systems can be achieved only with retention times greater than 12 days Žvan Ginkel et al., 1997., the complete pho-
Barros Neto, B., Spacino, I.S., Bruns, R.E., 1995. Planejaao de Experimentos. UNICAMP, Campmento e Otimizac¸˜ inas, p. 87. Benitez, F.J., Beltran-Heredia, J., Acero, J.L., Rubio, F.J., 2000. Contribution of free radicals to chlorophenols decomposition by several advanced oxidation processes. Chemosphere 41, 1271᎐1277. Bergers, P.J.M., Groot, A.C., 1994. The analysis of EDTA in water by HPLC. Water. Res. 28, 639᎐642. Bolivar, J.P., Garcia-Tenorio, R., Vacal, F., 2000. Radioecological study of an estuarine system located in the south of Spain. Water. Res. 34, 2941᎐2950. Bolton, J.R., Cater, S.R., 1994. Homogeneous photodegradation of pollutants in contaminated water: an introduction. In: Helz, G.R., Zepp, R.G., Crosby, D.G. ŽEds.., Aquatic and Surface Photochemistry. Lewis, Boca Raton, FL, pp. 467᎐490. Bolton, J.R., Bircher, K., Tumas, W., Tolman, C.A., 1996. Figures-of-merit for the technical development and application of advanced oxidation processes. J. Adv. Oxidat. Calcul. 1, 13᎐17. Davis, P., Green, D.L., 1999. Photocatalytic oxidation of cadmium-EDTA with titanium dioxide. Environ. Sci. Technol. 33, 609᎐617. Dayan, A., Zalmanovich, D.S., Ullmann, A., 2000. Measurements of solid contaminant emission rates from boiling pools. Chem. Eng. Sci. 55, 2203᎐2212. Galindo, C., Kalt, A., 2000. UVrH 2 O 2 oxidation of azo dyes in aqueous media: evidence of a structure-degradability relationship. Dyes Pigments 42, 199᎐207. Hoffmann, M., Martin, S.T., Choi, W., Bahnemann, W., 1995. Environmental applications of semiconductor photocatalysis. Chem. Rev. 95, 69᎐96. Ince, N.H., 1999. ‘Critical’ effect of hydrogen peroxide in photochemical dye degradation. Water. Res. 33, 1080᎐1084. Kaluza, U., Klingelhofer, P., Taeger, K., 1998. Microbial ¨ degradation of EDTA in an industrial wastewater treatment plant. Water. Res. 32, 2843᎐2845. Legrini, O., Oliveros, E., Braun, A.M., 1993. Photochemical processes for water treatment. Chem. Rev. 93, 671. Lisenbigler, L., Lu, G., Yates, J.T., 1995. Photocatalysis on TiO 2 surfaces: principles, mechanism, and selected results. Chem. Rev. 95, 735᎐758. Lopez, J.L., Garcıa, ´ F.S., Gonzalez, ´ M.C. et al., 2000. Hydroxyl radical initiated photodegradation of 4-chloro-3,5-dinitrobenzoic acid in aqueous solution. J. Photochem. Photobiol. A: Chem. 137, 177᎐184.
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Madden, T.H., Datye, A.K., Fulton, M., Prairie, M.R., Majumdar, S.A., Stange, B.M., 1997. Oxidation of metal-EDTA complexes by TiO 2 photocatalysis. Environ. Sci. Technol. 31, 3475᎐3481. Mills, A., Le Hunte, S., 1997. An overview of semiconductor photocatalysis. J. Photochem. Photobiol. A: Chem. 108, 1᎐35. Nortemann, B., 1999. Biodegradation of EDTA. Appl. Microbiol. Biotechnol. 51, 751᎐759. Tucker, M.D., Barton, L.L., Thomson, B.M., Wagener, B.M.,
Aragon, A., 1999. Treatment of waste containing EDTA by chemical oxidation. Waste Manage. 19, 477᎐482. van Ginkel, C.G., Vandenbroucke, K.L., Stroo, C.A., 1997. Biological removal of EDTA in conventional activatedsludge plants operated under alkaline conditions. Biores. Technol. 59, 151᎐155. Vohra, M.S., Davis, A.P., 2000. TiO 2 -assisted photocatalysis of lead-EDTA. Water. Res. 34, 952᎐964. Weber Jr., W.J., LeBoeuf, E.J., 2000. Processes for advanced treatment of water. Water. Sci. Technol. 40, 11᎐19.
Hydrogen peroxide assisted photochemical degradation of ethylenediaminetetraacetic acid Airton Kunz a , Patricio Peralta-Zamoraa,U , Nelson Duran ´ b a
Laboratorio Ambiental e de Materiais, Departamento de Quımica, Uni¨ ersidade Federal de Parana, ´ de Quımica ´ ´ ´ C.P. 19081, CEP 81531-990, Curitiba-PR, Brazil b Biological Chemistry Laboratory, Physical Chemistry Department, Instituto de Quımica, Uni¨ ersidade Estadual de ´ Campinas, C.P. 6154, CEP 13083-970, Campinas-SP, Brazil Accepted 25 August 2001
Abstract In the present work the hydrogen peroxide-assisted photochemical degradation of ethylenediaminetetraacetic acid ŽEDTA. is reported. The effect of pH and H 2 O 2 :EDTA molar ratio on the efficiency of the EDTA degradation was evaluated by using a 2 2 factorial design. Working at optimized experimental conditions ŽpH of 2 and H 2 O 2 :EDTA molar ratio of 10., using a microwave-activated photochemical reactor and monitoring the EDTA degradation by total organic carbon analysis, mineralization ratios higher than 90% were observed at reaction times of 6 min. 䊚 2002 Elsevier Science Ltd. All rights reserved. Keywords: EDTA; Photochemical degradation; Hydrogen peroxide; Microwaves
1. Introduction The close correlation between environmental pollution and industrial activity is an extensively documented reality of recent decades ŽBolivar et al., 2000; Dayan et al., 2000; Vohra and Davis, 2000.. Many efforts have been dedicated to the study of alternative technologies, that are able to minimize the deleterious effect caused by this anthropic source. Among several options, the development of processes with the capacity to transform the great number of toxic compounds that surge as industrial effluents into harmless compounds, is one of the most immediate alternatives ŽWeber and LeBoeuf, 2000.. With this objective, many processes
U
Corresponding author. Tel.: q55-41-361-3297; fax: q5541-361-3186. E-mail address: [email protected] ŽP. PeraltaZamora..
have been proposed over the years. In this context, photochemical processes are important, mainly due to their capacity to degrade recalcitrant and toxic species in relatively short reaction times ŽHoffmann et al., 1995; Lisenbigler et al., 1995; Mills and Le Hunte, 1997; Legrini et al., 1993.. An important part of the photochemical processes explored as remediation alternatives is represented by heterogeneous systems based on the use of ultraviolet light and semiconductor metallic oxides. However, the viability of this kind of process is debatable, mainly due to the extreme difficulty encountered in the separation of the solid photocatalysts at the end of the process. From this point of view, the use of homogeneous systems appears to be a good alternative. Advanced oxidation processes based on the UV-H 2 O 2 system have shown high efficiency in the degradation of several compounds of environmental relevance ŽBenitez et al., 2000; Galindo and Kalt, 2000; Bolton and Cater, 1994.. Ethylenediaminetetraacetic acid ŽEDTA. is a strong
1093-0191r02r$ - see front matter 䊚 2002 Elsevier Science Ltd. All rights reserved. PII: S 1 0 9 3 - 0 1 9 1 Ž 0 1 . 0 0 1 2 6 - 5
A. Kunz et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 197᎐202
198
chelating agent extensively used in industrial applications such as metal plating, water softening, photography, textile and paper manufacture ŽDavis and Green, 1999.. Processes that involve the removal of metal oxides from heat transfer surfaces, such as in boilers or nuclear reactors, also use this complexing agent, due to their ability to complex metals ŽMadden et al., 1997.. EDTA exhibits a serious environmental impact due to its low biodegradability by conventional biological and physicochemical methods for the treatment of wastewater ŽNortemann, 1999; Tucker et al., 1999.. Recently it was demonstrated that EDTA is ultimately biodegraded under practical industrial wastewater treatment conditions, in a study involving a finishing plant dedicated to treatment of the effluent from a paper mill ŽKaluza et al., 1998.. With high metal affinity, EDTA forms very stable metal complexes over a broad pH range, decreasing metal bioavailability in some natural and unpolluted mediums ŽBergers and Groot, 1994.. In this work the H 2 O 2-assisted photochemical degradation of EDTA was evaluated, using a microwave-activated ultraviolet source. Preliminary optimization studies were carried out by factorial design, while high performance liquid chromatography and total organic carbon analysis were used to evaluate kinetically the efficiency of the proposed process. The results reported here correspond to a preliminary stage of a large project with the objective to study advanced oxidation processes to degrade EDTA from a nuclear plant.
Fig. 1. Schematic representation of the photochemical reactor.
drate ŽSigma, 99%. was used as aqueous solutions of 1 mmol ly1. Hydrogen peroxide 30% Žwrw. ŽMalinckrodt. was used as received.
2.2. Photochemical process The photochemical treatment was carried out in a microwave-activated photochemical reactor Žmodel UV LAB 䊛 EL., kindly donated by UMEX ŽDresden, Germany.. The inner component of the reactor was fabricated with synthetic quartz of high UV-light permeability, while the external one, which contained mercury vapor at low pressure, was constructed with quartz of low permeability ŽFig. 1.. Samples of EDTA Ž15 ml., with pH values adjusted between 2 and 4, were added in adequate amounts of hydrogen peroxide and submitted to irradiation cycles of 1.5 min, using a commercial microwave oven. The efficiency of the photochemical process was evaluated by measuring the total organic carbon content ŽShimadzu TOC-5000A total organic carbon analyzer., while the EDTA degradation was determined by high performance liquid chromatography ŽShimadzu, LC-10AD., according to the instrumental conditions reported by Bergers and Groot Ž1994.:
2. Experimental 2.1. Reagents Ethylenediaminetetraacetic acid disodium salt dihy-
Column: C18 ŽLichrospore 100, RP-18 Merck.;
Table 1 Experimental design to study the effect of pH and H 2 O 2 :EDTA ratio on the EDTA degradation efficiency wEDTA: 15 ml Ž1.0 mmol ly1 ., time: 1.5 min.x Parameter
Level Žy.
Central point Ž0.
Level Žq.
pH H 2 O 2 :EDTA ratio
2 10
3 20
4 30
Experiment
pH
Ratio
TOC reduction Ž%.
1 2 3 4 5 6
y q y q 0 0
y y q q 0 0
58 51 57 54 52 54
Main effects: pH: y5 "1, Ratio: 1 "1; combined effect: pH= Ratio: 2 " 1.
A. Kunz et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 197᎐202
Mobile phase: methanol:formicrformiate buffer Ž10:90 vrv.; UV-detector: 258 nm.
3. Results and discussion 3.1. Preliminary optimization The preliminary optimization study, designed to verify the effect of pH and H 2 O 2 :EDTA ratio on the EDTA degradation efficiency, was carried out by the factorial design given in Table 1. The H 2 O 2 :EDTA ratio was initially assayed between the levels of 10 and 30, because according to the current literature, they are the ratios usually used in photochemical degradation processes assisted by hydrogen peroxide ŽInce, 1999.. From the results shown in Table 1, it is possible to observe that when the pH is changed from the low to the high level, the degradation efficiency is decreased by nearly 5 percentage points Žmain effect: y5.. This result implies that for high degradation rates the lower pH level must be used. When the H 2 O 2 :EDTA ratio is changed between the two selected levels the degradation efficiency is enhanced by approximately 1 percentage point Žmain effect: 1.. However, as the determinations show a typical estimated standard deviation of approximately 1 percentage point Žsee result of the duplicated central point., we can conclude that, between the limits of the experimental design, this experimental parameter does not show any significant effect on the EDTA degradation efficiency. The standard deviation of the TOC determination
was estimated from the unexpressive effects calculated in the factorial design. It is very common in this kind of experimental design to use these criteria to obtain an estimate value of the error of the determination ŽBarros Neto et al., 1995.. In Table 1 it is possible to observe that the effect of the H 2 O 2 :EDTA ratio is negligible Ž1 percentage point.. If we admit that this variable does not influence the efficiency of the degradation process, we can also admit that the difference between the experiments 1᎐3 and 3᎐4 are due to errors in the analytical methodology. If we use these differences, together with the results obtained in the duplicate point, we can calculate an estimated error of approximately 1%. From the geometric interpretation presented in Fig. 2 it is possible to clearly observe two tendencies toward a maximal degradation capacity of the photochemical system. Even when the tendencies are discrepant in relation to the H 2 O 2 :EDTA ratio effect, it is very easy to observe that both point toward the low pH level. In view of these preliminary observations, subsequent photochemical experiments will be carried out at a pH of 2 and a low H 2 O 2 :EDTA ratio.
3.2. Degradation kinetics Working at optimized experimental conditions, the kinetics of the degradation process were evaluated. From Fig. 3, it is possible to observe that the degradation process, measured as the reduction of total organic carbon content, proceeded very quickly in the presence of H 2 O 2 and UV-light, reaching a degradation ratio higher than 90% at reaction times of 6 min. Under the same conditions, the presence of UV-light alone induced a degradation of approximately 50%, while the action of microwaves or the combination of H 2 O 2 and microwaves did not show any degradation capacity. In a first analysis, the elimination of 50% of the TOC content, in the absence of the oxidant agent ŽH 2 O 2 ., appears to be an inexplicable result. However, if we consider that the photochemical reactor provides radiation of wavelength lower than 200 nm, the mineralization of EDTA can be explained due to its reaction with the hydroxyl radical produced from the photolysis of water wEq. Ž1.x. H 2 O q h Ž - 200 nm. ª H ⭈q HO⭈
Fig. 2. Geometric representation of the factorial design.
199
Ž1.
The efficient mineralization of aqueous samples containing organochloride compounds by photolysis has been reported in the recent literature ŽLopez et al., 2000.. Even when a small portion of organic carbon remained after a photochemical treatment of 6 min, the HPLC analysis indicated a complete EDTA degradation at reaction times of approximately 45 s ŽFig. 4..
200
A. Kunz et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 197᎐202
Fig. 3. EDTA degradation as a function of the treatment. EDTA: 15 ml Ž1.0 mmol ly1 ., H 2 O 2 :EDTA ratio: 10, pH: 2.0.
Two transient species appeared during the initial stages of the photochemical process Ž Rt of 4.9 and 13.7 min., species that are completely degraded at higher reaction times. An unidentified impurity Ž Rt: 5.3 min. remained practically unchanged during the photochemical process, probably representing the 10% of non-degraded TOC content. If we consider the electrical power of the microwave system Ž1050 W hy1 ., the time required for a degrada-
tion of 90% Ž6 min., the sample volume Ž15 ml. and the initial EDTA concentration Ž1 mmol ly1 ., we can calculate an experimental ‘electrical energy per mass’ ŽEEM. ŽBolton et al., 1996. value of approximately 21 kW h gy1 wEq. Ž2.x However, considering that the commercial availability of high capacity microwaveactivated photochemical reactors exists Ž see Žhttp.www.umex.de.., this parameter can be decreased to 0.3 kW h gy1 by only extending the reactor capacity
Fig. 4. Schematic representation of the chromatographic response.
A. Kunz et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 197᎐202
to 1 l. Taking into account that classical heterogeneous photocatalytic processes, applied under our experimental conditions, show typical EEM values of approximately 4 kWh gy1, the proposed procedure appears as an economically viable degradation alternative. EEMs Wr Žw EDTA x 0 = MW= SVr1000= DE r100 = 60rRT .
Ž2.
Where: wEDTAx 0 : MW: SV: DE: RT: W:
201
tochemical degradation of EDTA at reaction times lower than 10 min represents a remarkable result. One of the main disadvantages of the system is the necessity for artificial irradiation sources. However, with the recent commercial availability of low-cost continuous photochemical reactors which permit the application of intense UV-radiation at angles of 360⬚, the economical viability of the process has been significantly increased. References
initial EDTA concentration Ž1 = 10y3 mol ly1 .; molecular weight of EDTA Ž372.24 g moly1 .; sample volume Žml.; degradation efficiency Ž%. at RT; reaction time Žmin.; UV-source power ŽkW hy1 , 1.05..
When the samples were submitted to the high reaction time Ž6 min, in cycles of 1.5 min. the temperature was raised to approximately 85 ⬚C. Isothermal conditions were not maintained in the experiments due to the negligible effect of temperature on the EDTA content Žsee Fig. 3.. It is important to highlight that, as shown in Fig. 3, the microwave and the microwave᎐H 2 O 2 systems do not cause any significant mineralization of the EDTA molecule. This fact was confirmed by using HPLC and UV-vis spectrophotometry. The microwave source is used only to excite the mercury vapor in the reactor and produce ultraviolet light. For this reason, the UVrH 2 O 2rmicrowave system corresponds to a conventional photochemical process assisted by hydrogen peroxide, with the unique difference that, in this case, the work involves the use of a photochemical reactor of high photonic efficiency. Our group has carried out several studies of EDTA degradation with conventional reactors Žunpublished results .. By using these systems the degradation velocity is significantly lower, but the mechanistic aspects are practically the same as those observed in the proposed system.
4. Conclusions The H 2 O 2-assisted photochemical process discussed here represents an excellent alternative for the degradation of EDTA. If we consider that the almost complete removal of EDTA by activated sludge systems can be achieved only with retention times greater than 12 days Žvan Ginkel et al., 1997., the complete pho-
Barros Neto, B., Spacino, I.S., Bruns, R.E., 1995. Planejaao de Experimentos. UNICAMP, Campmento e Otimizac¸˜ inas, p. 87. Benitez, F.J., Beltran-Heredia, J., Acero, J.L., Rubio, F.J., 2000. Contribution of free radicals to chlorophenols decomposition by several advanced oxidation processes. Chemosphere 41, 1271᎐1277. Bergers, P.J.M., Groot, A.C., 1994. The analysis of EDTA in water by HPLC. Water. Res. 28, 639᎐642. Bolivar, J.P., Garcia-Tenorio, R., Vacal, F., 2000. Radioecological study of an estuarine system located in the south of Spain. Water. Res. 34, 2941᎐2950. Bolton, J.R., Cater, S.R., 1994. Homogeneous photodegradation of pollutants in contaminated water: an introduction. In: Helz, G.R., Zepp, R.G., Crosby, D.G. ŽEds.., Aquatic and Surface Photochemistry. Lewis, Boca Raton, FL, pp. 467᎐490. Bolton, J.R., Bircher, K., Tumas, W., Tolman, C.A., 1996. Figures-of-merit for the technical development and application of advanced oxidation processes. J. Adv. Oxidat. Calcul. 1, 13᎐17. Davis, P., Green, D.L., 1999. Photocatalytic oxidation of cadmium-EDTA with titanium dioxide. Environ. Sci. Technol. 33, 609᎐617. Dayan, A., Zalmanovich, D.S., Ullmann, A., 2000. Measurements of solid contaminant emission rates from boiling pools. Chem. Eng. Sci. 55, 2203᎐2212. Galindo, C., Kalt, A., 2000. UVrH 2 O 2 oxidation of azo dyes in aqueous media: evidence of a structure-degradability relationship. Dyes Pigments 42, 199᎐207. Hoffmann, M., Martin, S.T., Choi, W., Bahnemann, W., 1995. Environmental applications of semiconductor photocatalysis. Chem. Rev. 95, 69᎐96. Ince, N.H., 1999. ‘Critical’ effect of hydrogen peroxide in photochemical dye degradation. Water. Res. 33, 1080᎐1084. Kaluza, U., Klingelhofer, P., Taeger, K., 1998. Microbial ¨ degradation of EDTA in an industrial wastewater treatment plant. Water. Res. 32, 2843᎐2845. Legrini, O., Oliveros, E., Braun, A.M., 1993. Photochemical processes for water treatment. Chem. Rev. 93, 671. Lisenbigler, L., Lu, G., Yates, J.T., 1995. Photocatalysis on TiO 2 surfaces: principles, mechanism, and selected results. Chem. Rev. 95, 735᎐758. Lopez, J.L., Garcıa, ´ F.S., Gonzalez, ´ M.C. et al., 2000. Hydroxyl radical initiated photodegradation of 4-chloro-3,5-dinitrobenzoic acid in aqueous solution. J. Photochem. Photobiol. A: Chem. 137, 177᎐184.
202
A. Kunz et al. r Ad¨ ances in En¨ ironmental Research 7 (2002) 197᎐202
Madden, T.H., Datye, A.K., Fulton, M., Prairie, M.R., Majumdar, S.A., Stange, B.M., 1997. Oxidation of metal-EDTA complexes by TiO 2 photocatalysis. Environ. Sci. Technol. 31, 3475᎐3481. Mills, A., Le Hunte, S., 1997. An overview of semiconductor photocatalysis. J. Photochem. Photobiol. A: Chem. 108, 1᎐35. Nortemann, B., 1999. Biodegradation of EDTA. Appl. Microbiol. Biotechnol. 51, 751᎐759. Tucker, M.D., Barton, L.L., Thomson, B.M., Wagener, B.M.,
Aragon, A., 1999. Treatment of waste containing EDTA by chemical oxidation. Waste Manage. 19, 477᎐482. van Ginkel, C.G., Vandenbroucke, K.L., Stroo, C.A., 1997. Biological removal of EDTA in conventional activatedsludge plants operated under alkaline conditions. Biores. Technol. 59, 151᎐155. Vohra, M.S., Davis, A.P., 2000. TiO 2 -assisted photocatalysis of lead-EDTA. Water. Res. 34, 952᎐964. Weber Jr., W.J., LeBoeuf, E.J., 2000. Processes for advanced treatment of water. Water. Sci. Technol. 40, 11᎐19.