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Ind. Eng. Chem. Res. 2006, 45, 30-33
Plasma-Chemical Destruction of Trilaurylamine Issued from Nuclear Laboratories of Reprocessing Plants D. Moussa,†,‡ J. L. Brisset,*,‡ E. Hnatiuc,§ and G. Decobert| Socie´ te´ des Traitements en Milieu Ionisant, 1 route de la Noue, 91196 Gif-sur-YVette, France, Laboratoire d’Electrochimie, UFR Sciences, UniVersite´ de Rouen, 76821 Mont-Saint-Aignan, France, Technical UniVersity Gheorghe Asachi, Iasi, Romania, and AREVA/COGEMA/BUT, 2 rue Paul Dautier, 78141 Velizy-Villacoublay, France
Trilaurylamine (TLA) and dodecane (i.e., hydrogenated tetrapropylene HTP) are used in laboratories of nuclear fuel reprocessing control laboratories to extract specific radionuclides such as plutonium at a certain valence. The spent solvents issued from laboratories are generally not compatible by nature and activity with the effluent pathway installed on site to treat the major industrial solvent. Spent TLA is a radioactive liquid waste and must be mineralized separately before conditioning. A new process is proposed and studied with pure TLA. It requires the strongly oxidizing species (HO•, HO2•, and O3) created in a humid air plasma of the gliding arc (i.e., “glidarc”) type at atmospheric pressure. Two kinds of reactors were studied, using batch and pump-recycled targets, respectively. 1. Introduction Trilaurylamine (i.e., TLA, (C12H25)3N)) is an efficient actinide extractant used in plutonium analysis for PUREX process phase monitoring.1,2 For its industrial use, this solvent is diluted with hydrogenated tetrapropane, which is a mixture of dodecane isomers. Such compound is therefore a mixture of radioactive liquid wastes which is not compatible by nature and activity with the effluent and must be degraded before conditioning. This paper deals with an original method for degrading the mentioned solvent that involves a humid air quenched plasma at atmospheric pressure and low temperature. This plasma is provided by a gliding arc discharge reactor which allows the use of significant power sources (a few kilowatts). Therefore, this device is attractive for industrial applications and/or for fundamental investigation for its low cost and the high specific energy (500-700 J‚L-1) delivered to the target, about 60 times those relevant to corona discharges.3 Electric discharges in humid air are convenient sources of active species, among which the OH and NO radicals are prominent. They were identified by emission spectrometry and quantified.4 These key species are responsible for the chemical properties of the discharge induced in an aqueous solution. NO is a precursor for the nitrous and nitric acids formed at the surface of the liquid phase and hence are associated with the acid effect of the plasma. OH is a more powerful oxidizing species than nitrate ions and is able to react at the liquid surface with a suitable target molecule or to form the oxidizing dimer H2O2. Both chemical properties are readily used for pollutant abatement of aqueous solutions. 2. Experimental Section The gliding arc (or “glidarc”) device is described elsewhere.5 It was proposed by Czernichowski et al.6,7 few years ago for gas treatment and especially for the removal of volatile organic carbides. It proved to be efficient for gas treatments.6 More recently we developed and adapted the device to the abatement †
Socie´te´ des Traitements en Milieu Ionisant. Universite´ de Rouen. § Technical University Gheorghe Asachi. | AREVA/COGEMA/BUT. ‡
of pollutants dispersed in aqueous solutions8 and to spent solvent degradation.9 The gliding arc discharge device involves two diverging electrodes (80 mm long; electrode to liquid distance, 25 mm) disposed around a humid air blowing nozzle connected to a suitable power source, generally a high-voltage (HV) transformer. An arc forms at the minimum gap and its feet move along the electrodes to the tip, due to the air flux effect, until it breaks into a plume when it is short-circuited by a new arc. All batch treatments were performed with a HV transformer supplied by Aupem-Sefli (maximum voltage 10 kV and 0.1 A in open conditions). For circulating treatments, the reactor was modified by replacing the generator with a standard HV transformer (6 kV) fitted with a special control unit with two auxiliary electrodes that delivers an increased current intensity (0.7-1 A). The TLA degradation kinetics was monitored by UV-visible absorption spectrometry after extraction with chloroform. The blue complex obtained by shaking 10 mL of the chloroform solution with cobalt(II) nitrate (500 mg) and potassium thiocyanate (500 mg) was analyzed at the absorption peak (625 nm) with a Techcomp UV 8500 spectrophotometer. Gas phase chromatography (GPC) measurements were performed with a Varian Star 3400 apparatus involving a Restek megabore polydimethylsiloxane-polydiphenylsiloxane 95/5 capillary column (internal diameter 0.32 mm; length 30 m) and using nitrogen as the carrier gas. 2.1. Batch Treatment. The batch treatment was carried out in a 1 L water-cooled glass reactor (Figure 1) filled with TLA (40 mL) disposed on water (785 mL). The TLA degradation kinetics was spectrometrically monitored after extraction of the TLA-Co(II)-SCN blue complex with chloroform. However, the formation of a solid phase readily occurred and slowed the reaction. Formed gases were monitored by a COSMA Cristal 500 COCO2 measuring IR cell and by Dra¨ger disposable tubes with sampling in the outlet flow of the reactor, before a trap for the exhaust fumes filled with (2 M) sodium hydroxide solution (250 mL). Carboxylic acids were extracted in aqueous phase by shaking the bulk organic phase with 1 M sodium hydroxide solution. The resulting aqueous phase was acidified with (2 M) H2SO4
10.1021/ie0503850 CCC: $33.50 © 2006 American Chemical Society Published on Web 12/02/2005
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3. Results and Discussion
Figure 1. Batch treatment reactor.
3.1. Batch Treatment. 3.1.1. TLA Degradation. The degradation rate of TLA is very low (Figure 3, curve A). The overall kinetic law is zero-order with a very slow reaction rate (k ≈ 1 g‚h-1). This limited reaction rate may be related to a couple of reasons: (i) the occurrence of long saturated carbon chains which prevent the hydroxyl radical attack near the nitrogen atom, and (ii) the formation of trilaurylammonium salts in plasma induced acidic medium. These salts make the organic phase waxy, and thus limit or prevent the diffusion of the plasma active species. 3.1.2. Carbon Monoxide and Dioxide Formation Monitoring (Figure 4). We monitored the formation of final products CO and CO2 by means of an infrared sensor cell. The resulting kinetic rates are slow. For treatment times t shorter than 8 h, the whole of the CO2 emission flow is constant (0.016 mol‚h-1), which accounts for a zero-order process. This suggests a chemical oxidation process occurring at a constant effective plasma/TLA interface. The CO emission is more important for t e 0.5 h (maximum flow 0.023 mol‚h-1) and then decreases to 0.0053 mol‚h-1. At the beginning of the treatment, the ammonium salts are present in small quantities, so the TLA surface is quite more volatile and reactive toward the plasma. Organic vapors may therefore react with CO2 according to complex plasma-chemical processes similar to those reported13,14 and may be written for TLA as follows:
147CO2 + 2C36H75N f 219CO + 75H2O + N2 f 219CO + 75H2O + N2
Figure 2. Pump recycled target reactor.
and saturated with NaCl. Then the acids were extracted with ether, esterified by boron trifluoride-methanol reagent according to the Methalfe method,10 and analyzed by GPC. 2.2. Pump Recycled Target. We developed a recycling device to improve the kinetic rate in view of industrial application. The improved reactor allows us to get an enlarged electrode gap and an increased energy provided to the discharge. TLA (60 mL) was treated in the presence of 1 M sodium hydroxide (580 mL) to prevent the formation of poorly soluble trilaurylammonium salts, due to plasma acidic effects. The device also involves the recycling of the liquid target to prevent local overheating and a flat spraying nozzle which injects the liquid in the plasma trail, just under the electrodes (Figure 2), to improve the plasma/TLA contact. A complementary decantation flask is involved to maintain a good contact with the plasma by recycling only the TLA enriched part of the liquid target. The analytical monitoring methods are the same as for batch treatment.
For 1-8 h treatments, the CO emission also follows a zeroorder kinetics with he average rate of 0.0053 mol‚h-1. The zeroorder feature agrees with the TLA disappearing kinetics (Figure 3), since the relevant concentration is an approximately linear decreasing function of the treatment time. For longer treatment times, both kinetics get slower due to an important ammonium salt accumulation in a thin organic layer. This layer is thus broken up by the impinging gas flow. The resulting decrease of the effective surface between the plasma and the organic phase must also contribute to the oxidation slowing down. 3.1.3. Oxidation Byproducts and Carbon Balance. In addition to the formation of carbon oxides as final products, the degradation of TLA involves intermediate carboxylic acids CnH2n+1COOH, 2e ne 11, which were identified by GPC (Figure 5). Undecanoic (C10H21COOH) and dodecanoic (C11H23COOH) acids are by far the main components of the acid fraction, due to the preferential attack of the HO• radical at hydrogen atoms on carbon near the nitrogen atom.11,12 An infrared spectrum of the neutral residue after treatment recorded for qualitative analysis purpose also shows the presence of amides (Figure 6). A proposed mechanism is given in Figure 7. The relevant balance for a 12-h treatment is the following: CO, 3.14%; CO2, 7%; RCO2H, 1.9%; remaining TLA, 62%; not found, 25.9%. The unidentified carbon fraction (25.9%) must be heavy compounds with amide and/or amine oxide functions shown by the infrared spectrum (Figure 6). Polymer compounds may be also present because plasma reactions involve radicals that may polymerize. 3.2. Treatment with Pump Recycled Target. The recycling of the organic phase with NaOH solution ensures a continuous washing and prevents trilaurylammonium salts from forming. This enables the target to remain fluid during the treatment.
32 Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006
Figure 3. TLA degradation kinetics. (A) Batch treatment with low current. (B) Pump recycled target treatment with high current. For a better view to compare the respective rates, the batch treatment plot was shifted (the real initial amount is 33 g).
Figure 4. CO and CO2 emission kinetics for batch TLA treatment.
Figure 5. Chromatogram of the acidic fraction recovered after TLA batch treatment (raw signal).
treatment, i.e., 14 g‚h-1 (Figure 3, curve B). These results confirm that recycling allows an important increase in the kinetic rate and is therefore a major improvement for the development of the technique for industrial applications. 4. Conclusion
Figure 6. Infrared spectrum of the neutral fraction recovered after TLA batch treatment, showing amide presence (CdO stretch band at 1673 cm-1).
Additionally, the device is fitted with a flat spraying nozzle that ensures a better contact between the plasma and the target. Also, a higher current intensity provides more active species. The degradation rate is thus 14 times higher than for batch
A new process using wet air gliding arc plasma is proposed to mineralize TLA. This plasma device was originally developed for gas treatments and later for treatment of organic liquids. It is now adapted to the degradation of a heavy extracting and complexing agent, trilaurylamine, which is also a surfactant. Thus, this plasma technique finds a place to complete the review on the degradation of recalcitrant surfactants by advanced oxidation processes by Ikehata and Gamal El-Din.15 The TLA treatments obey a zero-order degradation kinetics in accordance with surface oxidation processes, but the batch treatment is very slow. The rate can be largely improved by recycling and continuously washing the organic phase by sodium hydroxide solution, so that it reaches 14 g‚h-1, which falls in the range of industrial specifications. The exposure to the plasma yields CO2, CO, carboxylic acids, and heavy organic compounds
Ind. Eng. Chem. Res., Vol. 45, No. 1, 2006 33
nitrogen, and carbon) and to improve the liquid phase degradation and the CO2 yield at the plasma-liquid interface. Literature Cited
Figure 7. Proposed mechanism to explain carboxylic acid formation from TLA oxidation.
involving amide and amine oxide functions identified by IR spectroscpopy. The plasma-treated solvents undergo oxidation reactions, cracking into volatile compounds and high conversion into CO as described for several other hydrocarbons.13,14 The cracking and conversion into CO are predominant processes without stirring when organic vapors are significantly concentrated above the liquid phase. Mechanisms taking into account the chemical properties of the main gaseous species formed in the discharge (i.e., HO• and NO) are proposed to explain the formation of carboxylic acids. Work is in progress to complete the balance sheet for all the elements present in the treated solvents (i.e., phosphorus,
(1) Schimmack, W.; Auerswald, K.; Bunzl, K. EnViron. Radioact. 2001, 53 (1), 41-57. (2) Bunzl, K.; Kracke, W. J. Radioanal. Nucl. Chem., Lett. 1994, 186, 401-413.. (3) Proce´ de´ s e´ lectriques de mesure et de traitement des polluants; Hnatiuc, E., Ed.; Tec & Doc: Paris, 2002. (4) Benstaali, B.; Boubert, P.; Cheron, B. G.; Addou, A. Brisset, J. L. Plasma Chem Plasma Process. 2002, 22, 553-571. (5) Czernichowski, A. Pure Appl. Chem. 1994, 66 (6), 1301-1310. (6) Brethes-Dupouey, S.; Peyrous, R.; Held, B. Eur. Phys. J.: Appl. Phys. 2000, 11, 43-58. (7) Fridman, A. A.; Petrousov, A.; Chapelle, J.; Cormier, J. M.; Czernichowski, A.; Lesueur, H.; Stevefelt, J. J. Phys. III Fr. 1994, 4, 14491465. (8) Benstaali, B.; Moussa, D.; Addou, A.; Brisset, J. L. Eur. Phys. J.: Appl. Phys. 1998, 4, 171-179. (9) Moussa, D.; Brisset, J. L.; Bargues, S. Destruction d’un solvant organique par plasma froid. Fr. Patent 98.13439, 1998. (10) Metcalfe, L.; Schmitz, A. A. Anal. Chem. 1951, 33, 363. (11) Atkinson, R. Chem. ReV. 85, 1985, 69-201. (12) Schuchmann, M. N.; von Sonntag, C. J. Chem. Soc., Perkin Trans. 1984, 2, 699-704. (13) Czernichowski, A.; Czernichowski, P.; Wesolowska, K. Plasmacatalytical partial oxidation of various carbonaceous feeds into synthesis gas. Presented at the Second International Conference Fuel Cell Science, Engineering & Technology, Rochester, NY, USA, June 14-16, 2004. (14) Maezono I.; Chang J. S. IEEE Trans. Ind. Appl. 1990, 26, 651655. (15) Ikehata, K.; Gamal El-Din, M. Ozone: Sci. Eng. 2004, 26, 327343.
ReceiVed for reView March 28, 2005 ReVised manuscript receiVed October 8, 2005 Accepted November 3, 2005 IE0503850
