Kinetics and Mechanism of Wet-Air Oxidation of Nuclear-Fuel

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Kinetics and Mechanism of Wet-Air Oxidation of Nuclear-Fuel-Chelating Compounds Souley Bachir,*,† Stephane Barbati,† Maurice Ambrosio,† and Paul Tordo‡ Laboratoire de Chimie et Environnement, 3 place Victor Hugo, Case 29, 13331 Marseille, France, and Laboratoire de Chimie, Biologie et Radicaux Libres, avenue Normandie-Niemen, 13397 Marseille, France

In this paper, we examine the wet-air oxidation (WAO) of four nuclear-fuel-chelating compounds [diethylenetriaminepentaacetic acid (DTPA), ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), and thenoyltrifluoroacetone (TTA)]. The study focuses on the kinetics and mechanism of the degradation process. Identification of the free-radical intermediates was performed via the spin-trapping technique. The reaction of each compound studied was found to occur in two steps, with a partial order of one with respect to TOC for both steps. Among the compounds studied, TTA shows the most difficulty when degrading. Hydroxyl, hydroxycarbonyl, and nitrogenous radicals are some of those formed during the degradation process. Schematic reaction pathways, which can explain the formation of some mineral byproducts, are proposed on the basis of these radicals. Introduction Wet-air oxidation is an attractive treatment technique for industrial wastewater. This process involves the combustion of pressurized organic matter at relatively high temperatures (150-350 °C). Water is an excellent solvent for dissolving reagents, substrates, and byproducts. It is also a wonderful heat-transfer medium. Many papers have demonstrated the applicability of WAO to several industrial wastewaters.1-8 Today, this technique has reached rather extensive commercial applications. Despite this fact, however, fundamental information about the kinetics and reaction mechanisms is scarce and often contradictory. A better understanding of the reactions taking place in the WAO process is necessary in order to optimize waste degradation. Many model compounds have been studied with this perspective in mind. We can quote the work of Day et al.,9 Willms et al.,10 Thomas et al.,11 Rivas et al.,12 Hao et al.,13 Shende et al.,14 and Krisner.15 Most of the compounds studied have a partial order of one with respect to the initial substrate and a variable order with respect to the oxidant. The free-radical mechanism appears to account for the reactions, and many authors have proposed a number of free-radical reaction pathways without, however, identifying the nature of the radical species formed in the reaction medium. In this paper, the kinetics and mechanism of WAO of four organic compounds (Table 1) are reported. These compounds are commonly used in the extraction and purification phases of nuclear fuel (uranium, thorium, and plutonium) and therefore form the major part of the organic matter in the wastewater resulting from these extraction and purification processes. The study of their degradation via wet-air oxidation allows for testing of the applicability of this technology to the treatment of this type of wastewater with a view toward * Author to whom correspondence should be addressed. Address: P.O. Box 5371, Succ. B, Montreal (QC), H3B 4P1 Canada. E-mail: [email protected]. † Laboratoire de Chimie et Environnement. ‡ Laboratoire de Chimie, Biologie et Radicaux Libres.

destroying all of the organic matter and reducing the global volume of waste before the storage phase. Few papers have been published on the destruction of these chelating compounds by WAO. However, we can quote the works of Moriya et al.16 and Rowbotton et al.17 These authors have reported the oxidation of some organic complexants using the wet-peroxide oxidation (WPO) process. In the present investigation, we have focused our attention on the degradability of the substrates and the identification of the free-radical intermediates formed during the degradation process. The reaction pathways are also discussed. Material and Analytical Methods Equipment. The tests were carried out in a laboratory batch facility schematized in Figure 1. This installation includes (i) a 500-mL autoclave equipped with an agitator and a thermocouple that is placed in an electrical oven and kept hermetically sealed by 12 screws; (ii) liquid and gas sample lines equipped with a cooling system and a condensation pot; (iii) a gas inlet system composed of two lines, the oxidizing mixture (50% N2/50% O2) line and the low-pressure nitrogen line to purge the reactor before each experiment; and (iv) a safety system with an electrovalve and a rupture disk. The opening of the electrovalve (which allows the disposal of the experimental system) is activated when the pressure or temperature reaches the threshold set by the operator. Experimental Protocol. The autoclave was loaded with 300 mL of an aqueous solution of the substrate, and nitrogen was bubbled into the vessel to drive out the air. After the oven was programmed, agitation was started (500 rpm), and the mixture was heated to the desired reaction temperature. The oxidant (enriched air containing 50% O2 and 50% N2) was then allowed into the vessel at a predetermined level pressure. This moment was considered as the beginning of the reaction (t0). Liquid samples were withdrawn periodically during the experiment for physicochemical analysis.

10.1021/ie000818t CCC: $20.00 © 2001 American Chemical Society Published on Web 03/16/2001

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Figure 1. Experimental setup. Table 1. Formula of the Substrates compounds

chemical formula

% of carbon

nitrilotriacetic acid (NTA) ethylenediaminetetraacetic acid (EDTA) diethylenetriaminepentaacetic acid (DTPA) trifluorothenoylacetone (TTA)

(HOOCCH2)3N (HOOCCH2)2NCH2CH2N(CH2COOH2)2 (HOOCCH2)2NCH2CH2N(CH2COOH)CH2CH2N(CH2COOH2)2

37.69 41.09 42.27 43.24

Substrates and Analytical Methods. The compounds studied are ethylenediaminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), diethylenetriaminepentaacetic acid (DTPA), and thenoyltrifluoroacetone (TTA). These compounds were obtained from Fluka. The 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) used for radical trapping was obtained from Aldrich and distilled before use. Aqueous solutions were prepared with MilliQ-filtered (Millipore) water. The following apparati were used for the chemical analysis of the WAO effluents: (i) a TOC 5050 Shimadzu analyzer for determining total organic carbon (TOC), (ii) a high-performance liquid chromatography (HPLC) instrument for determining acetic and formic acids and ammonium ions, and (iii) a DIONEX DX 100 ionic chromatograph for determining mineral anions (SO42-, NO3-, NO2-, F-). The ESR spectra were recorded on a Bruker ESP 300 spectrometer equipped with a NMR gaussmeter for magnetic-field calibration. An HP 5350B microwavefrequency counter was used to determine the g factor. Samples were taken from the WAO reactor and mixed very quickly with the spin trap before being frozen in liquid nitrogen. The trap used was an aqueous solution of 5,5-dimethyl-1-pyrroline-1-oxide (0.2 M). The samples were kept at room temperature for the acquisition of ESR spectra. Kinetic Study The chemical reaction performed during the WAO process is the oxidation of organic carbon by molecular oxygen, following the global pathway

C + O2 f CO2 The kinetics of this reaction can be described in two different ways. (i) First, the evolution of the initial substrate as well as the formation and/or destruction of the intermediates formed can be characterized. Li et al.18 had reported a model based on Scheme 1 Scheme 1

where A ) all initial and relatively unstable intermediate organic compounds, B ) refractory intermediates (acetic acid), and C ) oxidation end products. This model assumes that A is easily oxidized and that B includes refractory intermediates. Its application requires the effective knowledge of all of the intermediates formed by the initial substrate and their respective formation and destruction constants (k2 and k3) under the same conditions. (ii) Second, the evolution of the global TOC can be characterized, and the appearance and/or destruction of the refractory intermediates can be disregarded. In the framework of this study, we have chosen this second method, because our work focuses on the reduction of the global organic matter rather than the specific constituents of the waste. The global rate equation assumes the general form shown in eq 1

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Figure 2. Evolution of the degradation yield of TTA with respect to the agitation speed. (Operating conditions: initial substrate concentration ) 5 gL-1, reaction temperature ) 300 °C, oxygen partial pressure ) 4.5 MPa, reaction time ) 10 min.)

r)-

d[TOC] ) k[TOC]R[O2]dβ dt

Figure 3. Apparent kinetic constant of nitrilotriacetic acid degradation.

(1)

where k is the real constant rate of the reaction, which obeys the Arrhenius law

( )

k ) A exp

-Ea RT

(2)

Tests were carried out with an initial substrate concentration of 5 g L-1. The molar amounts of organic carbon are 9.86 × 10-3 mol L-1 for NTA, 7.03 × 10-3 mol L-1 for EDTA, 5.37 × 10-3 mol L-1 for DTPA, and 9.73 × 10-3 mol L-1 for TTA. The oxygen partial pressure used for each test is 4.5 MPa (9 MPa of air pressure). This value of oxygen partial pressure allows the dissolved oxygen concentration to be present in excess compared to the organic carbon concentration for a given temperature according to the Henry’s Law. The results of preliminary experiments demonstrated that a 500 rpm agitation speed is sufficient to instantaneously dissolve the gaseous oxygen and to minimize the mass transfer, which affects the equilibrium between liquid and gaseous phases (Figure 2). The order degeneracy method is used to determine the partial order with respect to TOC concentration. In this context, eq 3 represents the global rate equation of the reaction

r)-

d[TOC] ) K[TOC]R dt

(3)

where K ) A exp(-Ea/RT)[O2]dβ and, if β f 0, Kexp ) A exp(-Ea/RT) The integration of this equation yields

1-

(

[TOC]0 ) Kexpt for R ) 1 ln [TOC]

)

[TOC]0 [TOC]

(4)

R-1

) Kexpt(1 - R)[TOC]0R-1 for R * 1 (5)

Experiments and Results. The experimental data show that the TOC concentration decreases with time in a two-step process (Figures 3 and 4). The first step involves a rapid destruction and ends with a substantial abatement of TOC during the first 20 min of reaction

Figure 4. Apparent kinetic constant of the ethylenediaminetetraacetic acid.

time. This rapid TOC reduction step corresponds to the degradation of the initial substrate to CO2 and H2O and to other organic byproducts, which can more or less be further oxidized. A second step follows during which the global reaction rate decreases. In this slow step, the degradation process is governed mainly by the reactivity of the intermediates formed during the rapid step, which have reached a sufficient level to affect the global reaction rate. Each of these steps exhibits a partial first order with respect to TOC, with very variable apparent constants, however, with respect to the initial substrate (Table 2). Lin19 and Shende14 have observed this process of twostep kinetics in other organic substrates. Concerning the partial order with respect to dissolved oxygen concentration (β), S. Perissoud20 demonstrated that the value of β tends toward zero when oxygen is present in large excess and mass transfer is minimized. On the basis of these results, we can estimate the relative activation energies (Er) during the rapid step. Although the Er values computed are not the real activation energies of the oxidation rate, they nevertheless allow for a comparison of the respective degradability of the substrates during the first step of the degradation process. Indeed, during this step, the global degradation process is governed by the intrinsic oxidation reaction, and the oxidation mainly concerns the initial substrate. These relative activation energies can

Ind. Eng. Chem. Res., Vol. 40, No. 8, 2001 1801 Table 2. Apparent Kinetic Constants of the Studied Compoundsa substrate TTA EDTA DTPA NTA

a

first step

second step

reaction temp (°C)

K1

R2

K2

R2

300 280 260 300 280 260 300 280 260 280 260 240

0.1135 0.0411 0.0364 0.2987 0.125 0.0629 0.2047 0.1148 0.0401 0.1284 0.0734 0.0334

0.999 0.906 0.936 0.997 0.999 0.933 0.986 0.999 0.939 0.929 0.936 0.951

0.0114 0.011 0.0166

0.950 0.993 0.968

0.0309 0.032 0.0929 0.0609 0.0242 0.0699 0.046 0.0084

0.994 0.97 1 0.982 0.990 0.999 0.968 0.918

Relative standard deviation ) 2%. Figure 6. Evolution of TOC and byproduct proportions in the TTA WAO. (The data are expressed in TOC equivalent units. The % TOC is computed vs the initial TOC concentration and the % C byproducts vs the TOC at the time t).

Figure 5. Correlation of the apparent constants with the reaction temperature.

be determined graphically by plotting ln(Kexp) versus 1/T.

ln(Kexp) ) -Er/RT + ln(A) ) -Ea/RT + C

(6)

Figure 5 shows the correlations obtained for the four compounds studied. The Er values thus computed are 125 kJ mol-1 for TTA, 104 kJ mol-1 for DTPA, 99 kJ mol-1 for EDTA, and 78 kJ mol-1 for NTA. These Er values confirm the degradability order of the four compounds observed in a previous study.21 Thenoyltrifluoroacetone requires the greatest value of Er, which means that this compound is the least degradable substrate. During the second step, the global degradation rate is the result of the partial degradation rates of all intermediates that have accumulated. The proportions of these intermediates vary according to the structure of the initial compound and the reaction temperature. This translates into apparent constants that greatly vary depending on the compound and the reaction temperature. The organic byproducts identified are methanol and formic acid in the effluent from nitrilotriacetic acid (NTA); methanol and acetic and formic acids in the effluents from ethylenediaminetetraacetic acid (EDTA) and diethylenetriaminepentaacetic acid (DTPA); and methanol, acetone, and acetic and formic acids in the effluent from thenoyltrifluoroacetone (TTA). Figure 6 shows the evolution in byproduct proportions (expressed in percentages of organic carbon at time t in the liquid

effluent) for the case of TTA at 300 °C. We observe that the proportion of acetic acid increases with time up to 40 min. This formation of acetic acid contributes considerably to the decrease in the global degradation yield. The mineral byproducts found in the effluent from thenoyltrifluoroacetone are fluorine and sulfate ions (F-, SO42-). The amine compounds yield mainly ammonium ions, which constitute 90-98% of the initial organic nitrogen in the final effluent. Nitrites and nitrates are found in very small proportions. In the best cases, we found no more than 1% of the initial organic nitrogen. These observations concur with other results found in the scientific literature.22-25 ESR Study of Reaction Intermediates The radical intermediates formed during WAO reactions have been identified by electron spin resonance (ESR) spectroscopy. This technique is a prime method for investigating molecules such as free radicals.26 ESR spectroscopy is based on the interaction between an external magnetic field and molecules with a nonzero electronic magnetic moment. The technique is appropriate for direct detection and identification of free radicals. However, its application can be rather difficult for direct detection of free radicals because of their extremely high chemical reactivity, which means that they have too short a lifespan to be detected. The ESR spin-trapping technique is therefore used to overcome this detection dilemma.27 Here, reactive free radicals join with adequate reagents (spin trap) to yield persistent addition products, spin adducts (Scheme 2). Scheme 2

Spin adducts are longer-living free radicals and can readily be observed by ESR spectroscopy at room temperature. Their very fine splitting (resulting from the interaction between the magnetic moment of the

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Figure 7. ESR spectra of hydrothermal oxidation of EDTA. (Instrument parameters: microwave power ) 10 mW, modulation frequency ) 100 kHz, modulation amplitude ) 0.1 mT, gain ) 6.0 × 105, time constant ) 10 ms, scan range ) 10.0 mT, scan time ) 240 s, 10 accumulations). Reconstituted spectra D: 86% of DMPO-OH (aN )1.50 mT; aH )1.50 mT), 14% of DMPO-C(R2)OH (aN ) 1.53 mT, aH ) 2.36 mT). Reconstituted spectra D′: 30% of DMPO-OH (aN ) 1.50 mT, aH ) 1.50 mT), 70% of DMPO-NR2 (aN 1.65 mT, aN0 ) 0.34 mT, aH ) 2.26 mT).

unpaired electron and the nonzero magnetic moment of surrounding adduct nuclei) provide information that can help in identifying the original radical. Nitrones and nitroso compounds are the most commonly used spin traps. For both of these traps, the adduct is a nitroxidefree radical that is formed by covalent reaction of the original radical with the spin trap (Scheme 3). Scheme 3

For our spin-trapping experiments, we used 5,5-dimethyl-1-pyrrolin-1-oxide (DMPO) to characterize the intermediate free radicals formed during the wet oxidation of EDTA. This nitrone is better able to spin radicals derived from trap oxygen than nitroso compounds. Two distinct solutions of DMPO were prepared: the first in pure water and the second in a phosphate buffer. The phosphate buffer was stirred for 6 h in the presence of DTPA (1 mM). The degradation conditions were as follows: initial substrate concentration ) 5 g L-1, reaction temperature ) 280 °C, and oxygen partial pressure ) 3 MPa. For each experiment, 100 µL of the sample was mixed with the same quantity of the trapping solution before the mixture was frozen in liquid nitrogen. The samples were withdrawn from the WAO reactor at four distinct times: before the oxidant was introduced, immediately after the oxidant was introduced, 2 min after the

oxidant was introduced, and 10 min after the oxidant was introduced. Results and Discussion. Figure 7 shows the sets of spectra obtained. These spectra depend on spintrapping conditions. Spectra A, B, and C were obtained with the DMPO buffer solution, whereas spectra A′, B′, and C′ were obtained with the DMPO pure-water solution. Spectra D and D′ are the reconstituted spectra made using the computer software.28 The reconstitution of a given spectra is based on the hyperfine coupling constants (hfc’s) of each adduct. The software written by Duling et al.28 contains a reconstitution database that allows us to determine the different signals constituting each spectrum. Thus, when experiments were carried out with DMPO in a phosphate buffer solution (pH ) 6.5), the ESR spectra showed the superposition of two signals corresponding to two different spin adducts. (i) The major signal (86%) has the hyperfine coupling constants aN ) 1.5 mT and aH ) 1.5 mT, which are characteristic of the DMPO-OH (Scheme 4, left) spin adduct. (ii) The second signal (14%) has higher hfc values, aN ) 1.53 mT and aH ) 2.36 mT, which are characteristic of DMPO spin adducts with an R-hydroxycarbonyl (Scheme 4, center). When experiments were carried out with the DMPO aqueous solution, the ESR spectra also showed the superposition of two signals. (i) The major signal has the hfc of a nucleus with a nuclear spin of 1, which could correspond to a coupling with a nitrogen nucleus (0.34 mT). This indicates that the trapped radical is Ncentered (Scheme 4, right). (ii) The minor signal (30%) corresponds to the DPMPO-OH spin adduct. Scheme 4

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The formation of hydroxyl radicals (OH•) results from the reaction of dissolved oxygen on the organic compound, a reaction that yields hydroperoxide. The degradation of this hydroperoxide produces OH• and R2CH-O• radicals. The R2N• radical results from the breakage of C-N bonds. This breakage is accompanied by the formation of R2CH-O• and/or R•. However, the contribution of R• radicals to the reconstituted spectra is very poor. The R2N• radical can be affected by two possible reactions. The first is the addition of molecular oxygen, which could further produce nitrite and nitrate ions (Scheme 5).

Scheme 8

In the termination phase, the radical species react to form stable compounds with reduced hydrocarbon chains. These compounds can also be formed by intramolecular rearrangement (Scheme 9). Scheme 9

Scheme 5

No trace of nitrous oxide (NOx) was detected in the gaseous phase, however. Nitrates and nitrites found in the effluent from the degradation of nitrogenous compounds constitute less than 1% of the initial nitrogen present. These inorganic ions are found only as traces at reaction temperatures above 300 °C. Otherwise, a reaction with water could occur that results in the formation of ammonium ions and gaseous nitrogen (Scheme 6). Scheme 6

The free-radical chain mechanism occurs as soon as the oxidizing mixture is introduced and the molecular oxygen absorbs the energy in the reaction medium. Once begun, the reaction process progresses very rapidly, ending with the formation of more or less refractory byproducts such as acetic acid and methanol. The series of initiation, propagation, and termination stages occurs essentially during the rapid first step. During the slower second step, other types of reaction pathways can be considered. Conclusion

+

The formation of NH4 is the major reaction affecting the nitrogenous radical. In most WAO experiments involving nitrogenous compounds, the initial nitrogen is found essentially in the form of NH4. This reaction confirms the active intervention of water in WAO reactions. General Reaction Pathways. Wet-air oxidation is a complex process. The reaction starts with an attack from the dissolved oxygen on one of the weakest C-H bonds of the substrate, entailing the formation of hydroperoxides. This initiation stage requires excitation of the molecular oxygen. Indeed, in its fundamental state, oxygen is characterized by a spectroscopic triplet state that cannot react with most organic molecules, which are found in a singulet state. The heat of the reaction medium destabilizes this fundamental state and forms a singlet state that has superior reactivity. The activated oxygen thus formed reacts with the hydrocarbon substrate to create hydroperoxide and hydroxyl radicals (Scheme 7). This type of initiation is the basis of most oxidation in organic chemistry. Scheme 7

The reaction propagation phases are composed of the destruction and/or recombination stages of the hydroperoxide and free radicals (Scheme 8).

The compounds studied are well degraded by the WAO process. The degradation is generally partial and ends with the formation of some light byproducts, thus contributing to a decrease in the global degradation yield. The reaction rate follows a partial first order with respect to TOC, with a variable value of the apparent constant K, however. The identification of hydroxyl, hydroxycarbonyl, and nitrogenous free radicals confirms the occurrence of a radical chain mechanism, which had been assumed by many authors. The selection of other spin-trap compounds will enable the identification of a number of free radicals that can be formed during the reaction process. Literature Cited (1) Dietrich, M. J.; Randall, T. L.; Canney, P. Wet Air Oxidation of Hazardous Organics in Wastewater. Environ. Prog. 1985, 4 (3), 174. (2) Hao, O. J.; Phull, K. K.; Chen, J. M.; Davis, A. P. Factors affecting Wet Air Oxidation of TNT red Water: Rates Studies. J. Hazard. Mater. 1993, 34, 51. (3) Robinson, J. M.; Foy, B. R.; Dell’Ocro, P. C.; Anderson, G. K.; Atencio, J. H.; Breshears, W. D.; Brewer, G. B.; Eaton, H. K.; McFarland, R. D.; McInroy, R. E.; Sedillo, M. A.; Wilmanns, E. G.; Buelow, S. J. Destruction of Nitrates, Organics, and Ferrocyanides by Hydrothermal Proceessing. Waste Management Symposium, Tucson, AZ, Feb 28-Mar 4, 1993. (4) Shende, R. V.; Mahajani, V. V. Catalytic Wet Oxidation of Aqueous Stream Containing Anthraquinone and Phthalocyanine Class Reactive Dyes: Ecofriendly Technology. Indian J. Technol. 1995, 2, 129. (5) Mishra, V. S.; Mahajani, V. V.; Joshi, J. B. Wet Air Oxidation. Ind. Eng. Chem. Res. 1995, 34, 2.

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(6) Levec, J. Wet oxidation processes for treating industrial wastewaters. Chem. Biochem. Eng. Q. 1997, 11 (1), 47. (7) Se`ve, E.; Antonini, G. Application de l’oxydation en voie humide au pre´traitement de graisses re´siduaires. De´ chets Sci. Tech. 1998, 11, 8. (8) Kolaczkowski, S. T.; Plucinski, P.; Beltran, F. J.; Rivas, F. J.; McLurgh, D. B. Wet air oxidation: A review of process technologies and aspects in reactor design. Chem. Eng. J. 1999, 73 (2), 143. (9) Day, D. C.; Hudgins, R. R.; Silverston, P. L. Oxidation of Propionic Acid Solutions. Can. J. Chem. Eng. 1973, 51, 733. (10) Willms, R. S.; Balinsky, A. M.; Reible, D. D.; Wetzel, D. M.; Harrison, D. P. Aqueous-Phase Oxidation: The Intrinsic Kinetics of Single Compounds. Ind. Eng. Res. 1987, 26, 148. (11) Thomas, J. W.; Taylor, J. E. High Pressure-Temperature Aqueous Oxidations. IV. A Kinetic Study of the Enolysation and Oxidation of Cyclohexanone in the Presence of Catalytic Metal Ions. Can. J. Chem. 1989, 67, 165. (12) Rivas, F. J.; Kolaczkowski, S. T.; Beltran, F. J.; McLurgh, D. B. Development of a model for the wet air oxidation of phenol based on a free radical mechanism. Chem. Eng. Sci. 1998, 53 (14), 2575. (13) Hao, O. J.; Phull, K. K. Oxidation of Nitrotoluenesulfonic Acid: Some Intermediates, Reaction Pathways and Byproduct Toxicity. Environ. Sci. Technol. 1993, 27, 1650. (14) Shende, R. V.; Mahajani, V. V. Kinetics of Wet Air Oxidation of Glyoxalic Acid and Oxalic Acid. Ind. Eng. Chem. Res. 1994, 33, 3125. (15) Krisner, E. Oxydation hydrothermale de l’acide ace´tique et de tributylphosphate en absence puis en pre´sence d’un catalyseur me´tallique: le cuivre. Ph.D. Dissertation, University of Provence, Marseille, France, 1998. (16) Moriya, Y.; Kurumada, N.; Todo, F.; Kuribayashi, H. Method of treating radioactive wastewater containing EDTA and other organic acids. Spectrum 88, Sept 11-15, 1988, Pasco, WA; p 304. (17) Rowbotton, K. T.; Wilkinson, J. N. R.; Conboy, T. M. Waste treatment. European Patent 0342 876 B1, 1989. (18) Li, L.; Chen, P.; Gloyna, E. F. Generalized Kinetic Model for Wet Oxidation of Organic Compounds. AIChE J. 1991, 37, 1687.

(19) Lin, S. H.; Ho, S. J.; Wu, C. L. Kinetic and performance characteristics of wet air oxidation of high-concentration of wastewater. Ind. Eng. Res. 1996, 35, 307. (20) Perissoud, S. Oxydation hydrothermale des liqueurs noires de papeterie: Pre´-Etude d’Engineering. DRT Dissertation, University of Provence, Marseille, France, 1998. (21) Bachir, S.; Ambrosio, M.; Federici, V.; Barnier, H. Optimization of the Degradation of Organic Complexants by Wet Air Oxidation. Analusis 1998, 26, 389. (22) Dubois, M. A. De´gradation des Re´sines e´changeuses d′ions par Oxydation par Voie Humide en Conditions sous-critique et supercritique. Ph.D. Dissertation, University of Provence, Marseille, France, 1994. (23) Chakchouk, M.; Deiber, G.; Foussard, J. N.; Debellefontaine, H. elimination de la pollution azote´e par l’oxydation catalytique a` l’oxyge`ne mole´culaire. Environ. Technol. 1995, 6, 645. (24) Copa, W. M.; Momont, J. A.; Lehmann, R. W. Wet Air Oxidation of Nitrogenous Organic Compounds. Proceedings of the Third International Symposium on Chemical Oxidation: Technology for the Nineties. Vanderbilt University, Nashville, TN, February 17-19, 1993; Eckenfelder, W. W., Jr., Browers, A. R., Roth, J. A., Eds.; Book 3, p 42. (25) Ito, M. M.; Kazuyuki, A.; Inoue, H. Wet Oxidation of Oxygen- and Nitrogen-Containing Organic Compounds Catalysed by Cobalt (III) Oxide. Ind. Eng. Res. 1989, 28, 894. (26) Tordo, P. Electron Paramagnetic Resonance. R. Soc. Chem. 1998, 16, 116. (27) Janzen, E. G.; Haire, D. L. Advances in Free Radical Chemistry; Tanner, D. D., Ed.; JAI Press: Greenwich, CT, 1990, 1, 253. (28) Duling, D. R.; Motten, A. G.; Mason, R. P. Generation and Evaluation of Isotropic ESR Spectrum Simulations. J. Magn. Reson. 1988, 77, 504.

Received for review September 14, 2000 Revised manuscript received January 30, 2001 Accepted February 3, 2001 IE000818T