Ind. Eng. Chem. Res. 2004, 43, 3223-3227
3223
KINETICS, CATALYSIS, AND REACTION ENGINEERING Decomposition of Ethylenediaminetetraacetic Acid by Supercritical Water Oxidation Hyeon-Cheol Lee,†,‡ Jung-Hyun In,† Kyung-Yub Hwang,‡ and Chang-Ha Lee*,† Department of Chemical Engineering, Yonsei University, Seoul 120-749, Korea, and Water Environment & Remediation Research Center, KIST, Seoul 136-791, Korea
Ethylenediaminetetraacetic acid (EDTA) was decomposed by supercritical water oxidation (SCWO) in a tubular plug-flow reactor. The effect of the oxidant amount on the decomposition rate and efficiency was more significant at lower temperature. Also, excess oxidant played a key role in decreasing the activation energy for EDTA decomposition as CODCr. The nitrogen from EDTA was found to transform into NO3--N by thermal decomposition, while a portion of the nitrogen of EDTA and NO3--N was transformed into NH4+-N and finally converted to N2 gas in the SCWO process. The decomposition kinetics of EDTA as CODCr in the SCWO process was described by a global rate expression. Introduction Ethylenediaminetetraacetic acid (EDTA) has numerous applications in industry, such as metal plating, water softening, photography, textile, and paper manufacture, because it can control the action of different metal ions through complexation and work as a strong chelating agent.1,2 Indeed, it has been used extensively in processes related to the removal of metal oxides from heat-transfer surfaces because of its ability to complex metals.3 Moreover, EDTA is used as an important decontaminating agent in the nuclear industry. Because its presence in decontamination wastes is able to cause complexation of the radioactive cations, it can hardly be removed by various treatment processes such as chemical and photochemical degradation, ion exchange, ultrasonic treatment, etc.1,3,4 EDTA is not easily biodegradable,5 scarcely degradable by chlorine, hardly retained by activated carbon filters, and resistant to ozone treatment.6 EDTA degradation has been attempted with UV/oxidants, radiolysis, heterogeneous photocatalysis, and radiophotocatalysis.7,8 Although advanced oxidation processes or advanced oxidation technologies are promising treatments owing to their ability to destroy a large variety of organic pollutants,9 complete decomposition is difficult because second pollutants are produced and the reaction time is inefficiently too long. Consequently, our work sought to apply the supercritical water oxidation (SCWO) process to the decomposition of EDTA as a way of minimizing secondary pollutants and to improving the decomposition efficiency. The properties of water, such as density, viscosity, diffusivity, and its static dielectric constant, are changed dramatically by slight variations of temperature and * To whom correspondence should be addressed. Tel.: (822)2123-2762. Fax: (822)312-6401. E-mail:
[email protected]. † Yonsei University. ‡ KIST.
pressure at its near-critical point (Tc ) 374 °C and Pc ) 221 bar).10 In addition, the static dielectric constant of supercritical water (SCW) is close to that of nonpolar solvents because of its minuscule value (supercritical condition < 2; atmospheric condition ≈ 80).11,12 Also, hydrogen bonding decreases with a decrease in the water density.13,14 Also, in the SCW process, a reaction can occur in a single phase and mass-transfer resistance becomes negligible because organic compounds and gases become miscible.15,16 As a result of these factors, the SCWO process is applied to the decomposition of chemically stable compounds such as dioxin, chlorinated hydrocarbons, and others.17,18 Hazardous organic compounds are completely oxidized and converted to carbon dioxide (CO2) and water (H2O) using an oxidant.19 In general, the desired decomposition and removal efficiency can be achieved in a few seconds or minutes by way of SCWO.17-20 Because EDTA in the wastewater from the power plant is mixed with metal-complex EDTA, the decomposition results of EDTA by using SCWO can provide further fundamental data for the complete treatment of wastewater. In this study, experiments in the decomposition of EDTA were conducted with the supercritical range of water (387-500 °C and 250 bar) in a tubular plug-flow reactor. Decomposition efficiencies of EDTA were compared by varying the amount of hydrogen peroxide (H2O2) as an oxidant. In addition, the decomposition mechanism of EDTA was studied under various subcritical conditions (250 °C and 250 bar) with and without oxidant. From the results, a kinetic model was proposed for the decomposition of EDTA using SCWO. Experimental Section The SCWO experiments with EDTA were conducted in a high-pressure tubular plug-flow reactor. The preheaters and reactor, made from stainless steel tubes, were immersed in a molten salt bath, where the reaction temperature was kept at a constant within (1 °C. After
10.1021/ie049952u CCC: $27.50 © 2004 American Chemical Society Published on Web 05/08/2004
3224 Ind. Eng. Chem. Res., Vol. 43, No. 13, 2004 Table 1. Decomposition of EDTA under Various Conditionsa pressure (bar)
temp (°C)
residence time (s)
H2O2 amount (%)b
efficiency as CODCr (%)
250 250 250 250 250 250 250 250 250 250 250 250 230 280
387 387 387 417 417 417 450 450 450 500 500 500 450 450
35.1-88.9
100 200 400 100 200 400 100 200 400 100 200 400 200 200
76.8-88.6 81.9-93.4 88.6-97.3 80.6-91.2 83.7-93.2 89.1-96.6 89.5-96.2 90.6-97.1 92.8-98.5 95.8-98.4 96.4-99.0 97.5-99.6 89.0-94.2 91.7-98.9
23.6-60.2
19.2-48.3
15.9-40.1
16.9-42.5 23.1-58.2
a The initial feed concentration is 5000 mg/L of EDTA (corresponding to 3063 mg/L as CODCr). b Based on a stoichiometric amount.
Figure 1. Conversion of EDTA with the feed flow rate, supplied H2O2 amount, and temperature at 250 bar.
two high-pressure pumps (Lab alliance prep. 100; Pmax ) 408 bar) delivered the oxidant and the EDTA solution to the preheaters, the mixture was supplied to the coiled reactor. The treated solution was cooled instantly in a condenser. It was then depressurized to atmospheric pressure through a back-pressure regulator. The total flow rates of the influents (oxidant and EDTA solutions) were adjusted to 2.0, 3.0, 4.0, and 5.0 mL/min, with each corresponding to a residence time of 15.9-88.9 s in the reactor. Because the residence time was obtained from the ratio of the reaction tube volume to the volumetric flow rate, the density and volume properties of the SCW were taken into consideration in the calculations of its flow rate. The theoretical values for density and volumes at various conditions were derived from a steam table.21 The experimental temperature range in the SCWO was 387-500 °C at pressures of 230-250 bar. In addition, to study the decomposition mechanism of EDTA, the thermal decomposition of EDTA without oxidant was performed at 250-500 °C and 250 bar. The percentage of hydrogen peroxide supplied to the reactor was based on the stoichiometric demand of oxygen for the complete oxidation of EDTA, which needs to convert from the carbon content in the feed to carbon dioxide. The amount of hydrogen peroxide used was changed from 100% to 400% depending on the stoichiometric demand. The samples were analyzed with a high-performance liquid chromatography (HPLC; Younglin, ACME 9000LC) with an isocratic pump and a symmetry 5 µm C-18 (Waters) column. The dichromate method was applied to measure CODCr in the liquid samples. Results and Discussion Decomposition Efficiency. As shown in Figure 1, EDTA was converted completely at 250 °C and more than 100% H2O2. In addition, EDTA was decomposed easily by way of the thermal effect without any need for oxidant. Furthermore, the efficiency of thermal decomposition steeply improved under supercritical conditions as opposed to subcritical conditions. Furthermore, without any oxidant, the efficiency of thermal decomposition steeply improved under supercritical
conditions as opposed to subcritical conditions. However, because the products converted from EDTA without oxidant remained in liquid as various low molecular materials, the CODCr concentration of the solution treated even at severe SCW condition without oxidant was almost the same as that of the feed at 3063 mg/L within the tested range. Because the oxidant was supplied to the reactor through a separate line, the CODCr concentration of the feed solution did not change at the experimental condition until the feed mixed with the oxidant in the reactor. Therefore, the decomposition efficiency of EDTA in SCWO should be presented by total organic compounds such as CODCr because of the initial conversion effect before addition of the oxidant. Table 1 shows the effects of temperature, residence time, pressure, and supplied H2O2 on EDTA decomposition (5000 mg/L of EDTA; 3063 mg/L as CODCr). The decomposition efficiency increased markedly with an increase in the temperature and the amount of H2O2. The decomposition efficiency approached 99.6% at 400% H2O2, 500 °C, and 250 bar within 40.1 s. In addition, even with a low oxidant amount at 500 °C, a decomposition of higher than 99% was achieved for the same residence time (Figure 2). As shown in Table 1, the effects of the temperature on decomposition were significant at 100% H2O2. However, the increments of decomposition percent of EDTA as CODCr with each temperature at 100%, 200%, and 400% were 9.8% (from 88.6% to 98.4%), 5.6% (from 93.4% to 99.0%), and 2.3% (from 97.3% to 99.6%), respectively. The role of temperature in the improvement of the decomposition efficiency decreased with an increase in the oxidant concentration because excess oxidant could contribute to the decomposition of EDTA at a relatively lower temperature condition. Destruction Behaviors of Nitrogen in EDTA. It was reported that nitrogen in an organic compound was transformed to another form of nitrogen during SCWO.22 Thus, in this work, an analysis of NO3--N and NH4+-N was made to investigate this transformation of the nitrogen in EDTA. Figure 3 shows that the concentration of NO3--N was higher than 150 mg/L without oxidant at 250 °C. When the oxidant was supplied to the reaction, the NO3--N concentration rapidly decreased to below 40 mg/L at 250 °C. Furthermore, under SCWO conditions, the concentration of NO3--N decreased to below 6.5 mg/L. Also,
Ind. Eng. Chem. Res., Vol. 43, No. 13, 2004 3225
Figure 2. Decomposition efficiencies of EDTA as CODCr with a temperature of 450 °C and a residence time at 250 bar and oxidant of 200% H2O2.
Figure 4. Residual NH4+-N concentration in EDTA (5000 mg/ L) with residence time, temperature, and H2O2 amount at 250 bar.
N and NH4+-N under subcritical conditions. Moreover, during SCWO, the nitrogen in EDTA was mainly transformed into NH4+-N by a thermal effect. So, a significant decrease of NO3--N and an increase of NH4+-N was observed in the SCWO process. However, the effect of the oxidant concentration on the transformation of nitrogen was not significant. Reaction Kinetics. A study was done on the reaction kinetics for decomposition of EDTA as CODCr using SCWO. For the purpose of engineering design, it is usually sufficient to develop a global rate model to express the reduction of waste components in SCWO. Thus, in the development of kinetic models, the global rate equation was adopted for the overall oxidation reaction.25-29 The following equation was applied for the decomposition rate of EDTA as CODCr in the SCWO process using hydrogen peroxide:
Figure 3. Residual NO3--N concentration in EDTA (5000 mg/ L) with residence time, temperature, and H2O2 amount at 250 bar.
the NO3--N concentration reached 1.35 mg/L with an increase in the temperature and residence time. Figure 4 shows the concentration of NH4+-N as another form of nitrogen transformation. Also, the concentration of NH4+-N was below 20 mg/L during thermal decomposition at 250 °C. However, when the oxidant was supplied to the reaction, the NH4+-N concentration steeply increased to more than 100 mg/ L. In addition, Figure 4 shows that the NH4+-N concentration during SCWO increased with the temperature and residence time. In this study, the portion of nitrogen in EDTA transformed into NH4+-N and NO3--N was below 5%. It was expected that nitrogen in EDTA would be transformed into N2 gas during SCWO because the reaction temperature was too low for the formation of NOx.23,24 However, the results in terms of residual nitrogen compounds imply that the nitrogen in EDTA was mainly transformed into NO3--N by thermal decomposition. In the event of oxidant supply, some portion of nitrogen was transformed into both NO3--
-d[EDTA as CODCr] ) A exp(-Ea/RT) dt [EDTA as CODCr]a[H2O2]bτ (1) In eq 1, [EDTA as CODCr], [H2O2], and [H2O] represent the concentration of each component (mol/L). The reaction orders of EDTA as CODCr, H2O2, and H2O are represented by a, b, and c, respectively. Ea, R, T, A, and τ are the activation energy, gas constant, reaction temperature, Arrhenius preexponential factor, and reaction time, respectively. A reaction constant k can be defined by k ) A exp(-Ea/RT). In the SCW reaction, [H2O]c ≈ [H2O]0c because of excess water. Equation 1 can be solved analytically with the initial condition x ) 0 at τ ) 0 to provide the relationship between EDTA as CODCr and relevant process variables.
(1 - x)1-a - 1 ) (a - 1)k[EDTA as CODCr]0a-1 [H2O2]b)τ, for a * 1 (2) An integral method analysis was performed to fit the decomposition data of EDTA. The hydrogen peroxide concentration was assumed constant throughout the reaction because at least 100% H2O2 of the stoichiometric requirement was supplied to the solution. Table 1
3226 Ind. Eng. Chem. Res., Vol. 43, No. 13, 2004
Figure 5. Arrhenius plot of the reaction rate constant for decomposition of EDTA as CODCr in SCWO with temperature and H2O2 amount. Table 2. Values of the Reaction Rate Constants for Each Temperature temp (°C)
pressure (bar)
387 417 450 500 450 450
250 250 250 250 230 280
reaction rate constants (k, s-1) 100% H2O2 200% H2O2 400% H2O2 0.0306 0.0500 0.0830 0.1265
0.0361 0.0546 0.0873 0.1383 0.0794 0.0886
0.0456 0.0660 0.0990 0.1583
data sets led to reaction orders of a ) 2.048 for EDTA as CODCr and b ) 0.357 for H2O2.
rate ) k[EDTA as CODCr]2.048[H2O2]0.357
(3)
The rate constants at each temperature (387, 417, 450, and 500 °C) and supplied oxidant (100%, 200%, and 400% H2O2) are shown in Table 2. As expected, the reaction rate increased with an increase in the temperature and oxidant. Figure 5 shows the activation energy, depending on the supplied oxidant, obtained from the Arrhenius plot. As can be seen, the highest activation energy was 53.72 ( 0.74 kJ/mol at 100% H2O2, while the lowest was 47.00 ( 0.05 kJ/mol at 400% H2O2. Note that excess oxidant plays a role in decreasing the activation energy in EDTA oxidation. It was reported that the effect of pressure on SCWO is much smaller than that of temperature.30,31 In this study, Tables 1 and 2 show the effect of pressure (230, 250, and 280 bar) on the rate constant at 450 °C and 200% H2O2. The rate constants at pressures of 230, 250, and 280 bar were 0.0794, 0.0873, and 0.0886 s-1, respectively. The effect of pressure on the decomposition rate and efficiency in the SCWO was much smaller than the effect of the temperature and oxidant amount. It implies that the reaction temperature plays a key role in SCWO once the system reaches a supercritical condition. Conclusion EDTA was effectively oxidized by SCW with hydrogen peroxide in a plug-flow reactor. The effects of the oxidant amount and temperature on the decomposition rate and
efficiency were significant. The reaction temperature in the SCWO process plays a key role in increasing the decomposition rate and efficiency of EDTA as CODCr. However, the improvement of the decomposition reaction was not directly proportional to any temperature increase above the critical temperature. The variation of rate constants with pressure was negligible. Given the residual nitrogen compounds, the nitrogen in EDTA was transformed into NO3--N by thermal decomposition while some portion of nitrogen was transformed into NH4+-N with oxidant under subcritical conditions. Furthermore, during SCWO, the nitrogen in EDTA was mainly transformed into NH4+-N with an increase in the temperature. However, the ammonia yield from the destruction of EDTA was less than 5% of the total amount of nitrogen in EDTA. Most of the nitrogen in EDTA was expected to be converted to N2 gas at the SCWO process. The decomposition kinetics of EDTA in the SCWO process was well described by a global rate expression. The lowest activation energy evaluated was 47.00 ( 0.05 kJ/mol at 400% H2O2 at 500 °C and 250 bar. Acknowledgment This work was supported by the Korea Research Foundation Grant KRF-2003-041-D00170, and the authors thank the KRF. Literature Cited (1) Chitra, S.; Paramasivan, K.; Sinha, P. K.; Lal, K. B. Ultrasonic treatment of liquid waste containing EDTA. J. Clean. Prod. 2003, in press. (2) Rui, W. Performance of new liquid redox desulfurization system of heteropoly compound in comparison with that of iron chelate. Korean J. Chem. Eng. 2003, 20, 659-663. (3) Tucker, M. D.; Barton, L. L.; Thomson, B. M.; Wagener, B. M.; Aragon, A. Treatment of waste containing EDTA by chemical oxidation. Waste Manage. 1999, 19, 477-482. (4) Airton, K.; Patricio, P. Z.; Nelson, D. Hydrogen peroxide assisted photochemical degradation of ethylenediaminetetraacetic acid. Adv. Environ. Res. 2002, 7, 197-202. (5) Hinck, M. L.; Ferguson, J.; Puhaakka, J. Resistance of EDTA and DTPA to aerobic biodegradation. Water Sci. Technol. 1997, 35, 25-31. (6) Gilbert, E.; Hoffmann, G. S. Ozonation of ethylenediaminetetraacetic acid (EDTA) in aqueous solution, influence of pH value and metal ions. Water Res. 1990, 24, 39-44. (7) Rodriguez, J.; Mutis, A.; Yeber, M. C.; Freer, J.; Baeza, J.; Mansilla, H. K. Chemical degradation of EDTA and DTPA in a totally chlorine free (TCF) effluent. Water Sci. Technol. 1999, 40, 267-272. (8) Madden, T. H.; Datye, A. K.; Fulton, M.; Prairie, M. R.; Majumdar, S. A.; Stange, B. M. Oxidation of metal-EDTA complexes by TiO2 photocatalysis. Environ. Sci. Technol. 1997, 31, 3475-3481. (9) Emilio, C. A.; Jardim, W. F.; Litter, M. I.; Mansilla, H. D. EDTA destruction using the solar ferrioxalate advanced oxidation technology (AOT) comparison with solar photo-Fenton treatment. J. Photochem. Photobiol. A 2002, 151, 121-127. (10) Lee, Y. W. Supercritical fluid: Applications and technologies (I). News and Information for Chemical Engineers 2001, 19, 325-333. (11) Modell, M. Supercritical-Water-Oxidation. Standard handbook for hazardous wastes treatment and disposal; Freeman, H. M., Ed.; McGraw-Hill: New York, 8.11; 1989. (12) Konys, J.; Fodi, S.; Hausselt, J.; Schmidt, H.; Casal, V. Corrosion of high-temperature alloys in chloride-containg supercritical water oxidation systems. Corrosion 1999, 55, 45-51. (13) Uematsu, M.; Franf, E. U. Static dieletric constant of water and stream. J. Phys. Chem. 1980, 9, 1291.
Ind. Eng. Chem. Res., Vol. 43, No. 13, 2004 3227 (14) Mitton, D. B.; Yoon, J. H.; Latanision, R. M. An overview of corrosion phenomena in SCWO systems for hazardous waste destruction. Zairyo to Kankyo 2000, 49, 130-137. (15) Connoly, J. F. Solubility of hydrocarbons in water near the critical temperature. Chem. Eng. J. 1966, 13, 11. (16) Japas, M. L.; Franck, E. U. High-pressure phase equlibria and PVT-data of the water-oxygen system including water-air to 673K and 250MPa. Phys. Chem. 1985, 89, 1268. (17) Sako, T.; Sugeta, T.; Otake, K.; Sato, M.; Tsugumi, M.; Hiaki, T.; Hongo, M. Decomposition of dioxin in fly ash with supercritical water oxidation. J. Chem. Eng. Jpn. 1997, 30, 744747. (18) Lee, G.; Nunoura, T.; Matsumura, Y.; Yamamoto, K. Comparison of the effects of the addition of NaOH on the decomposition of 2-chlorophenol and phenol in supercritical water and under supercritical water oxidation conditions. J. Supercrit. Fluids 2002, 24, 239-250. (19) Peter, K.; Eckhard, D. An assessment of supercritical water oxidation (SCWO); existing problems, possible solution and new reactor concepts. Chem. Eng. J. 2001, 83, 207-214. (20) Park, J. H.; Park, S. D. Kinetics of cellobiose decomposition under subcritical and supercritical water in continuous flow system. Korean J. Chem. Eng. 2002, 19, 960-966. (21) Wolfgang, W.; Alfred, K. Properties of water and steam; Springer: Berlin, 1998. (22) Killilea, W. R.; Swallow, K. C.; Hong, G. T. J. Supercrit. Fluids 1992, 5, 72. (23) Miller, J. A.; Bowman, C. T. Mechanism and modeling of nitrogen chemistry in combustion. Presented at the Fall meeting of the Western States section of the Combustion Institute, Dana Point, CA, Oct 1988.
(24) Webley, P. A.; Tester, J. W. Holgate, H. R. Oxidation kinetics of ammonia and ammonia-methanol mixtures in supercritical water in the temperature range 530-700 °C at 246 bar. Ind. Eng. Chem. Res. 1991, 30, 1745-1754. (25) Ruokang, L.; Phillip, E. S.; David, S. 2-Chlorophenol oxidation in supercritical water: Global kinetics and reaction products. AIChE J. 1993, 39, 178-187. (26) Jianli, Y.; Savage, P. E. Kinetics of catalytic supercritical water oxidation of phenol over TiO2. Environ. Sci. Technol. 2000, 34, 3191-3198. (27) Martino, C. J.; Savage, P. E. Total organic carbon disappearance kinetics for the supercritical water oxidation of monosubstituted phenols. Environ. Sci. Technol. 1999, 33, 1911-1915. (28) Takehiro, M.; Motonobu, G.; Akio, K.; Tsutomu, H. Supercritical water oxidation of a model municipal solid waste. Ind. Eng. Chem. Res. 2000, 39, 2807-2810. (29) Portela, J. R.; Nebot, E.; Ossa, E. M. Kintetic comparison between subcritical and supercritical water oxidation of phenol. Chem. Eng. J. 2001, 81, 287-299. (30) Meyer, J. C.; Marrone, P. A.; Tester, J. W. Acetic acid oxidation and hydrolysis in supercritical water. AIChE J. 1995, 41, 2108-2121. (31) Kabyemela, B. M.; Adschiri, T.; Malaluan, R. M.; Arai, K. Glucose and fructose decomposition in subcritical and supercritical water: Detailed reaction pathway, mechanisms and kinetics. Ind. Eng. Chem. Res. 1999, 38, 2888-2895.
Received for review January 14, 2004 Revised manuscript received March 18, 2004 Accepted April 2, 2004 IE049952U
