Ultrasound-Enhanced Dissolution of UO2 in Supercritical CO2

Mar 29, 2002 - Generation of Nitrous Acid by Ultrasound Irradiation in the Organic Solution Consisting of Tri- n -butylphosphate, Nitric Acid and Wate...
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Ind. Eng. Chem. Res. 2002, 41, 2282-2286

SEPARATIONS Ultrasound-Enhanced Dissolution of UO2 in Supercritical CO2 Containing a CO2-Philic Complexant of Tri-n-butylphosphate and Nitric Acid Youichi Enokida Research Center for Nuclear Materials Recycle, Nagoya University, Nagoya 4648603, Japan

Samir Abd El-Fatah and Chien M. Wai* Department of Chemistry, University of Idaho, Moscow, Idaho 83844

Application of ultrasound can significantly enhance the dissolution of UO2 powders placed on small glass beads in supercritical fluid CO2 using a CO2-soluble tri-n-butylphosphate (TBP)/ HNO3/H2O complexant as an extractant. The amount of UO2 dissolved in CO2 for a 20-min dynamic extraction at 323 K and 15 MPa can be increased by an order of magnitude with sonification compared to experiments without sonification. The application of ultrasound probably contributes to the transfer of locally concentrated UO2(NO3)2‚2TBP from the surface of glass beads into supercritical fluid CO2. This ultrasound-aided process for direct dissolution of UO2 in supercritical fluid CO2 that requires no aqueous solutions and organic solvents may have important applications for reprocessing of spent nuclear fuels and for treatment of certain nuclear wastes. Introduction Extraction of uranium from solid matrixes is required for initial production of nuclear fuels, reprocessing of spent fuels, and decontamination of uranium wastes.1-3 Conventionally, leaching with strong acids such as nitric acid is widely used in these processes, but the treatment and recycling of waste acids are costly and their potential impact on the environment is a major concern.1,2 Supercritical fluid extraction (SFE) techniques have been reported for recovering uranium from its oxides and from environmental samples using fluorinated β-diketones and tri-n-butylphosphate (TBP) as extractants.4-6 This new extraction technology appears promising for the effective processing of uranium with a marked reduction in waste generation because no aqueous solutions and organic solvents are involved and phase separation can be easily achieved by depressurization.7,8 Recently, a CO2-soluble TBP/HNO3/H2O complexant was found to be effective for dissolution and extraction of UO2 and U3O8 in supercritical fluid CO2 (SF-CO2).9,10 The nature of this complexant system in SF-CO2 is not totally understood. The TBP/HNO3/H2O complex system probably dissolves UO2 by oxidation of U(IV) in solid UO2 to U(VI) as UO22+ followed by the formation of UO2(NO3)2‚2TBP in SF-CO2. UO2(NO3)2‚2TBP is highly soluble in SF-CO2, exceeding 0.45 mol L-1 in CO2 at 313 K and 20 MPa.11 It is the most soluble metal complex in SF-CO2 so far reported in the literature. The recovery * To whom correspondence should be addressed. Tel: (208) 885-6787. Fax: (208) 885-6173. E-mail: [email protected].

of uranium as UO2(NO3)2‚2TBP is currently of commercial use in the Purex process, and utilization of TBP/ HNO3/H2O in SF-CO2 seems feasible from an engineering point of view. One technical problem that needs to be solved is enhancement of the UO2 dissolution rate, which is necessary for effective reprocessing in real applications because of the narrow space available in the spent fuels. Ultrasound has been shown to promote the conversion of nitric acid to nitrous acid12,13 and to aid in the removal of solutes from solid matrixes.14 Sonification may significantly improve the dissolution of UO2 in SF-CO2 using TBP/HNO3/H2O as an extractant because oxidation and diffusion processes are involved in the dissolution. In this paper, we report our recent experimental results regarding ultrasoundenhanced dissolution of UO2 with TBP/HNO3/H2O in SF-CO2. Experimental Section The supercritical fluid apparatus used for our experiments is illustrated in Figure 1. Pressurized CO2 (99.9%, Praxair, San Carlos, CA) was introduced from a cylinder to the experimental system via a syringe pump (model 260D, ISCO Inc., Lincoln, NE) with a controller (series D, ISCO Inc.). An ultrasound cleaner (Fisher Scientific FS30, Pittsburgh, PA) with a heater was used. Two different sizes of stainless steel cells were used, a 6.94mL cell for the extractant (i.e., TBP/HNO3/H2O in SFCO2) and a 3.74-mL cell for the UO2 dissolution. The volumes were measured gravimetrically using water. A restrictor made of poly(ether ether ketone) (PEEK) with 0.005 in. i.d. was used for sample collection.

10.1021/ie010761q CCC: $22.00 © 2002 American Chemical Society Published on Web 03/29/2002

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Figure 1. Schematic diagram of the experimental system for UO2 dissolution in SF-CO2: 1, CO2 cylinder; 2, syringe pump; 3, ligand cell; 4, sample cell; 5, ultrasound device with a water bath; 6, T-shaped joint; 7, collection vial; 8, heater for the PEEK restrictor. Table 1. Composition of the TBP/HNO3 Complex complex no.

molecular ratio for TBP:HNO3:H2Oa

TBP vol.,b mL

HNO3 vol.,b mL

1 2 3

1:0.7:0.7 1:1.0:0.4 1:1.8:0.6

5 5 5

0.815 1.30 5.00

a Based on Karl-Fischer analysis and acid-base titration of the TBP phase. b Initial volume of TBP and 15.5 M nitric acid used for complex preparation.

The ligand cell placed upstream of the sample cell was kept in a static mode for 10 min to allow complete mixing of TBP/HNO3/H2O with SF-CO2 by application of ultrasound before dynamic extraction for all experiments. The sample cell was pressurized to the same pressure as the ligand cell with SF-CO2. The dynamic extraction process was started by opening the valve separating the two cells, as well as the inlet and outlet valves shown in Figure 1. Samples were collected at 2-min intervals in chloroform (density ) 1.472 g mL-1) or in n-dodecane (density ) 0.749 g mL-1) during a dynamic extraction of 20 min. The flow rate of the supercritical fluid was between 0.5 and 0.8 mL min-1 in all of the experiments. To increase the surface area of the sample, 5 g of granular glass beads (60-80 mesh; density ) 2.3 g mL-1) was used. A certain amount (21 or 7.2 mg) of UO2 (Alfa Division, Danvers, MA) was mixed with the glass beads, and the coated beads were transferred to fill the internal volume of the dissolution cell. For each experiment, 3 mL of a TBP/HNO3/H2O complex was used as the extractant for uranium. Backextraction was performed by shaking the collected sample (in 7 mL of organic solvent) with 3 mL of deionized water for 3 min, followed by twice washing the organic phase with 3 mL of deionized water. The combined aqueous phase was collected in a 10-mL volumetric flask. The pH of the aqueous solution was measured with a pH meter (Orion model 701A, Cambridge, MA), and the uranium content was analyzed spectrophotometrically with Arsenazo-I at a wavelength of 594 nm.15 Absorption spectra were measured and recorded using a UV-Vis spectrophotometer (Cary 1E, Varian Inc., Palo Alto, CA). The TBP/HNO3/H2O extractant was prepared by adding 5 mL of TBP (density ) 0.979 g mL-1) with different volumes of concentrated nitric acid (69.5%; density ) 1.42 or 15.5 mol L-1) in a glass tube with a stopper. The mixture was shaken vigorously on a wrist action mechanical shaker for 5 min followed by centrifuging for 2 h. After centrifugation, 3 mL of the TBP phase was used for the experiments. Table 1 shows the ratios of TBP/HNO3/H2O for the three different extractants prepared and used in this study. The concentra-

Figure 2. UO2 dissolution in SF-CO2 containing TBP/HNO3/H2O with and without ultrasound application under 323 K and 15 MPa. Amount of reagents: 21 mg of UO2 (18.5 mg of U); 3 mL of TBP/ HNO3/H2O. All fitted curves were obtained by the least-squares method and approached 100% recovery.

tion of H2O in the organic phase was measured by KarlFischer titration (Aquacounter AQ-7, Hiranuma, Japan). The concentration of HNO3 in the organic phase was measured with a titrator (COM-450, Hiranuma, Japan) with a 0.1 N NaOH solution after adding a large excess amount of deionized water. Results and Discussion The solubility of TBP‚(HNO3)1.8‚(H2O)0.6 in SF-CO2 was found to be 2.8% by mole at 323 K and 13.7 MPa.16 The complex TBP‚(HNO3)1.8‚(H2O)0.6 is miscible with SF-CO2 at 15 MPa. The other two complexes, TBP‚ (HNO3)1‚(H2O)0.4 and TBP‚(HNO3)0.7(H2O)0.7, are expected to be more soluble because they contain less HNO3. In addition, the ligand cell was treated with ultrasound as described in the Experimental Section. Therefore, all of the TBP/HNO3/H2O solution in our experiments was homogeneously mixed with SF-CO2 in the ligand cell and was expected to remain so as it moved into the sample cell in our experiments. The average residence time for SF-CO2 in the ligand cell was estimated to be about 11 min. The concentration of TBP/ HNO3/H2O in SF-CO2 entering the sample cell was expected to decrease with a decay constant, 0.091 min-1, which was the reciprocal number of the average residence time. The space available for the fluid in the sample cell was calculated to be 1.3 mL based on the known internal volume of the cell and the weight and density of the glass beads. The average residence time for the supercritical fluid was estimated to be about 2 min, which is much shorter than that in the ligand cell. Because the collection vial was changed every 2 min, the amount of uranium recovered in each collection vial represented the amount of uranium dissolved during the 2-min interval of the dynamic extraction process. The effect of applying ultrasound to UO2 dissolution at 323 K and 15 MPa is illustrated in Figures 2 and 3 as well as in Table 2. For the experiments with 21 mg of UO2 (or 18.5 mg of U), the total amount of U recovered in 20 min was small in the absence of ultrasound, about 0.8 mg for complexant no. 1 (TBP:HNO3:H2O ) 1:0.7: 0.7), 1.0 mg for complexant no. 2 (TBP:HNO3:H2O )

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Table 2. Recovery Amount of U in a 20-min Dynamic Extraction

a

run

TBP/HNO3/H2O

ultrasound application

1 2 3 4 5 6 7 8

TBP‚(HNO3)0.7‚(H2O)0.7 TBP‚(HNO3)1.0‚(H2O)0.4 TBP‚(HNO3)1.8‚(H2O)0.6 TBP‚(HNO3)0.7‚(H2O)0.7 TBP‚(HNO3)0.7‚(H2O)0.7 TBP‚(HNO3)1.0‚(H2O)0.4 TBP‚(HNO3)1.8‚(H2O)0.6 TBP‚(HNO3)0.7‚(H2O)0.7

off off off off on on on on

initial amount of U, mg

recovery amount in collecting tubes (E),a mg

first-order rate constant (λ), min-1

18.5 18.5 18.5 6.3 18.5 18.5 18.5 6.3

0.8 (4.3%) 1.0 (5.4%) 1.1 (5.9%) 0.8 (13%) 14.2 (76.8%) 15.5 (83.8%) 16.6 (89.7%) 4.6 (73%)

0.0034 ( 0.0005 0.0042 ( 0.0006 0.0047 ( 0.0006 0.0081 ( 0.0004 0.077 ( 0.004 0.096 ( 0.004 0.106 ( 0.003 0.077 ( 0.005

The values in parentheses show the recovery efficiency, E, defined by the ratio of the recovery amount divided by the initial amount.

initial UO2) after 20 min of dynamic extraction with ultrasound application. This is slightly lower than the percentage of dissolution observed in the experiment with 21 mg of initial UO2. In all four cases, the dissolution efficiency was increased by an order of magnitude with the application of ultrasound in this SFCO2 system with different compositions of the TBP/ HNO3/H2O complexant. The ultrasound-aided dissolution data can be fitted into the equation

E ) 100(1 - e-λt)

Figure 3. Effect of the initial amount on UO2 dissolution in SFCO2 containing TBP/HNO3/H2O with and without ultrasound application under 323 K and 15 MPa. Amount of reagents: 21 mg of UO2 (18.5 mg of U) or 7 mg of UO2 (6.3 mg of U); 3 mL of TBP/HNO3/H2O. All fitted curves were obtained by the leastsquares method and approached 100% recovery.

1:1.0:0.4), and 1.1 mg for complexant no. 3 (TBP:HNO3: H2O ) 1:1.8:0.6). There appears to be a positive correlation between the TBP:HNO3 ratio in the complexant and the dissolution efficiency, but the difference is small. After 20 min of dynamic extraction, we collected all of the glass beads from the extraction cell and found that the black UO2 powders remained on the surface of the glass beads for runs 1 and 4. For runs 2 and 3, no leftover UO2 powder was observed, and the glass beads were wetted by an organic solution. This organic solution was easily stripped from the glass beads with an aqueous nitric acid (1.6 M), and a yellow organic solution containing UO2(NO3)2‚2TBP was recovered. Thus, for runs 2 and 3, the UO2 powders were all dissolved and converted to UO2(NO3)2‚2TBP, but the local concentration of the uranyl complex probably was high enough for most of it to remain on the surface of the glass beads during the experimental time. With the application of ultrasound, the amount of uranium recovered from the collection solutions increased significantly as illustrated in Figure 2. The total amount of uranium recovered in 20 min of dynamic extraction was 14.2 mg for complexant no. 1, 15.5 mg for complexant no. 2, and 16.6 mg for complexant no. 3 for the experiments starting with an initial UO2 of 21 mg. These recoveries represent about 77%, 84%, and 90% dissolution of the initial UO2 in the SF-CO2 by complexant nos. 1-3, respectively. For the experiments starting with 7.2 mg of UO2 (or 6.3 mg of U), complexant no. 1 was able to dissolve 4.6 mg of U (or 73% of the

(1)

where E is the recovery efficiency in % (defined by the ratio of a recovery amount to the initial amount), λ is the recovery rate constant in min-1, and t is the extraction time in min. For all four cases with ultrasound application, good fittings of the data were obtained as shown by the solid curves in Figures 2 and 3. According to eq 1, the ultrasound-aided dissolution of UO2 with the TBP/HNO3/H2O extractants follows firstorder kinetics. The recovery rate constants λ are 0.077 ( 0.004, 0.096 ( 0.004, and 0.11 ( 0.003 min-1 for complexant nos. 1-3, respectively. There is a positive correlation of the dissolution efficiency with increasing TBP:HNO3 ratio in the extractant according to the λ values given in Table 2. The difference appears small within the experimental error. The ultrasound-aided dissolution rate constants can be converted to the dissolution half-lives from the relationship t1/2 ) 0.693/ λ. The calculated t1/2 for complexant no. 1 is about 9.0 min. This means given sufficient time (e.g., 5 t1/2 or about 1 h) about 97% of UO2 should be dissolved under the specific experimental conditions. For complexant no. 3, to dissolve about 97% of UO2 will take approximately 32 min under the same SFE conditions. The estimate is based on the assumption that the concentration of the TBP/HNO3/H2O extractant in the flowing SF-CO2 is constant. This can be achieved by using a second pump to deliver a constant amount of the extractant to the system. In our experimental design, a fixed amount (3 mL) of the extractant was loaded into the ligand cell and its concentration in the SF-CO2 stream would eventually decrease. The estimated times to achieve a 97% dissolution may not be accurate. The following chemical and physical steps are probably involved in our SF-CO2 process for the extraction of uranium from UO2 powders spiked on the surface of the glass beads: (a) convective and diffusive mass transport of TBP/HNO3/H2O in SF-CO2 to UO2 powder on the glass surface, (b) dissolution reaction of UO2 with TBP/HNO3/H2O in SF-CO2 and formation of UO2(NO3)2‚ 2TBP near or on the glass surface, and (c) convective and diffusive mass transport of UO2(NO3)2‚2TBP in SFCO2.

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The glass beads in the sample cell formed narrow pathways, and convective diffusion was limited compared with the normal bulk space. Usually in porous media like the pathways in the glass beads, the diffusion process is dominated by molecular diffusion.17 The concentration of UO2(NO3)2‚2TBP formed near the glass surface is locally very high because of surface interactions. When ultrasound is applied, a fast dissolution rate may result from an increase in the interfacial area between the adhered UO2(NO3)2‚2TBP and SF-CO2. Because the application of ultrasound leads to a vigorous agitation near the glass surface and can enlarge the effective diffusivity near the glass surface, the third step can be markedly enhanced. If the concentration of TBP/HNO3/H2O is low enough, step 1 could be the rate-controlling process. In our experiments, the amount of the TBP/HNO3/H2O complexant (3 mL) was in large excess relative to the chemical equivalent amount of U in the system (by about 30 times). Therefore, step 1 should not be a bottleneck for the SFE process. This is also supported by the fact that UO2(NO3)2‚2TBP was found to cover the surface of the glass beads after the experiment without ultrasound. Obviously, the complexant was able to dissolve UO2 without ultrasound, but diffusion of the product UO2(NO3)2‚2TBP in SF-CO2 was relatively slow because of the narrow spaces of the system. The dissolution of UO2 in aqueous nitric acid is known to consist of several steps that can be summarized as follows:18

(1) UO2 + 4HNO3 f UO2(NO3)2 + 2NO2 + 2H2O (2) 2NO2 + H2O f HNO3 + HNO2 (3) UO2 + 2HNO2 + 2HNO3 f UO2(NO3)2 + 2NO + 2H2O The net reaction can be described as

8 2 4 UO2 + HNO3 f UO2(NO3)2 + NO + H2O 3 3 3 The oxidation of UO2 described in the first step proceeds by way of electron transfer in the interface of solidliquid phases.18-21 Similar reactions probably would also occur for the dissolution of UO2 in the SF-CO2 system with the TBP/HNO3/H2O complex as an extractant. The ultrasound-aided dissolution rate constants given in Table 2 are correlated with the molecular ratio of HNO3 to TBP in the complexant. Figure 4 shows a plot of the λ values obtained from the experiments with 21 mg of initial UO2 and the three complexants with different HNO3:TBP ratios. The slope of the plot in this figure was determined to be 0.33, which is much smaller than the value (2.3) reported for UO2 dissolution in aqueous nitric acid.22 This probably can be attributed to the slow mass transfer in the narrow pathways near the surface of the glass beads. Further studies are currently in progress to understand all possible chemical reactions and physical mass transfers of uranium species in the UO2/TBP/HNO3/H2O system in SF-CO2. This study, nevertheless, provides support for a novel SF-CO2-based process for direct dissolution of UO2 that may have important applications for reprocessing of spent nuclear fuels and for treatment of nuclear wastes. Conclusions The application of ultrasound enhances extraction of uranium from UO2 powders placed on glass beads in

Figure 4. Logarithmic plots of rate constants versus molecular ratio of HNO3 to TBP in TBP/HNO3/H2O. Rate constants are given in Table 2 for runs 5-7.

SF-CO2 with a CO2-soluble TBP/HNO3/H2O complex. This large enhancement is probably brought about by the vigorous transportation of locally concentrated UO2(NO3)2‚2TBP from the surface of the glass beads into the SF-CO2. With sonification, a fast dissolution rate may result from an increase in the interfacial area between the adhered UO2(NO3)2‚2TBP and SF-CO2. This ultrasound-aided SF-CO2 dissolution process may have important applications for recovering uranium from UO2 trapped in narrow spaces such as in natural soil, sintered materials, and locally rough surfaces. Literature Cited (1) Benedict, M.; Pigford, T. H.; Levi, H. W. Nuclear Chemical Engineering; McGraw-Hill Publishing: New York, 1981; p 1008. (2) Wilson, P. D., Ed. The Nuclear Fuel Cycle; Oxford Science Publications: Oxford, U.K., 1996; p 342. (3) Cochran, R. G.; Tsoulfanidis, N.; Miller W. F. Nuclear Fuel Cycle: Analysis and Management; American Nuclear Society: La Grange Park, IL, 1999; p 381. (4) Lin, Y.; Brauer, R. D.; Laintz, K. E.; Wai, C. M. Supercritical Fluid Extraction of Lanthanides and Actinides from Solid Materials with a Fluorinated β-Diketone. Anal. Chem. 1993, 65, 2549. (5) Lin, Y.; Wai, C. M.; Jean, F. M.; Brauer, R. D. Supercritical Fluid Extraction of Thorium and Uranium Ions from Solid and Liquid Materials with Fluorinated β-Diketones and Tributylphosphate. Environ. Sci. Technol. 1994, 28, 1190. (6) Wai, C. M.; Wang, S. Supercritical Fluid Extraction: Metals as Complexes. J. Chromatogr. A 1997, 785, 369. (7) Phelps, C. L.; Smart, N. G.; Wai, C. M. Past, Present and Possible Future Applications of Supercritical Fluid Extraction Technology. J. Chem. Educ. 1996, 12, 1163. (8) Wai, C. M.; Waller, B. Dissolution of Metal Species in Supercritical FluidssPrinciples and Applications. Ind. Eng. Chem. Res. 2000, 39, 3837. (9) Tomioka, O.; Meguro, Y.; Iso, S.; Yoshida, Z.; Enokida, Y.; Yamamoto, I. New Method for the Removal of Uranium from Solid Waste with Supercritical CO2 Medium Containing TBP-HNO3 Complex. J. Nucl. Sci. Technol. 2001, 38, 461. (10) Samsonov, M. D.; Wai, C. M.; Lee, S. C.; Kulyako, Y.; Smart, N. G. Dissolution of Uranium Dioxide in Supercritical Fluid Carbon Dioxide. Chem. Commun. 2001, 1868-1869. (11) Carrott, M. J.; Waller, B. E.; Smart, N. G.; Wai, C. M. High Solubility of UO2(NO3)2‚2TBP complex in Supercritical CO2. J. Chem. Soc., Chem. Commun. 1998, 373. (12) Nikitenko, S. I.; Moisy, Ph.; Venault, L.; Madic, C. Kinetics of nitrous acid formation in a two-phase tri-n-butylphosphatediluent/aqueous nitric acid extraction sysytem under the effect of power ultrasound. Ultrason. Sonochem. 2000, 7, 135.

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(13) Venault, L.; Moisy, Ph.; Nikitenko, S. I.; Madic, C. Kinetics of nitrous acid formation in nitric acid solution under the effect of power ultrasound. Ultrason. Sonochem. 1997, 4, 195-204. (14) Trofimov, T. I.; Samsonov, M. D.; Lee, S. C.; Smart, N. G.; Wai, C. M. Ultrasound Enhancement of Dissolution Kinetics of Uranium Oxides in Supercritical Fluid Carbon Oxide. J. Chem. Technol. Biotechnol. 2001, 76, 1223. (15) Fritz, J. S.; Jonson-Richard, M. Calorimetric Uranium Determination with Arsenazo. Anal. Chim. Acta 1959, 20, 164. (16) Enokida, Y.; Suzuki, M.; Yamamoto, I. Vapor-liquid equilibrium of UO2(NO3)2‚2TBP and Supercritical Carbon Dioxide Mixture. Proceedings of Actinides 2001, Hayama, Japan, Nov 4-9, 2001. (17) Perry, R. H.; Green, D. W.; Malony, J. O. Perry’s Chemical Engineers’ Handbook; McGraw-Hill Professional Publishing: New York, 1997; p 2640. (18) Ikeda, Y.; Yasuike, Y.; Takashima, Y.; Park, Y.; Asano, Y.; Tomiyasu, H. 17O NMR Study on Dissolution Reaction of UO2 in Nitric Acid, Mechanism of Electron Transfer, J. Nucl. Sci. Technol. 1993, 30, 962.

(19) Ikeda, Y.; Yasuike, Y.; Nishimura, K.; Hasegawa, S.; Takashima, Y. Kinetic Studies on Dissolution of UO2 Powders in Nitric Acid. J. Nucl. Mater. 1995, 224, 266. (20) Zimmer, E.; Merz, E. Dissolution of Thorium-Uranium Mixed Oxides in Concentrated Nitric Acid. J. Nucl. Mater. 1984, 124, 64. (21) Shabbair, M.; Robins, R. G. Kinetics of the Dissolution of Uranium Dioxide in Nitric Acid. J. Appl. Chem. 1968, 18, 129. (22) Asano, Y.; Kataoka, M.; Tomiyasu, H.; Ikeda, Y. Kinetic Studies on Dissolution of UO2 Powders in Acid Solutions by Using Cerium(IV) or Chlorine Dioxide as Oxidants. J. Nucl. Sci. Technol. 1996, 33, 152.

Received for review September 10, 2001 Revised manuscript received February 6, 2002 Accepted February 10, 2002 IE010761Q