Chapter 3
Development of Process Chemistry for the Removal of Cesium from Acidic Nuclear Waste by Calix[4]arene-crown-6 Ethers Peter V. Bonnesen, Tamara J. Haverlock, Nancy L . Engle, Richard A. Sachleben, and Bruce A. Moyer Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6119
A solvent suitable for extracting cesium from acidic nitrate media, such as that stored at the U . S . Department of Energy's Idaho National Engineering and Environmental Laboratory (INEEL), has been developed. The solvent possesses good chemical stability, displays excellent cesium selectivity, and provides good extraction and stripping performance with satisfactory phase-coalescence behavior. The calix[4]arene-crown-6 ether used in this solvent (1,3alt-bis-n-octyloxycalix[4]arene-benzo-crown-6) was selected from a series of mono- and bis-crown-6 derivatives of 1,3-alternate calix[4]arenes that were shown to possess good stability to a simulant of I N E E L ' s Sodium Bearing Waste (SBW). Calixarene -benzocrown ethers possessing an alkyl substituent at the 4-position of the benzocrown, such as calix[4]arene-bis-tert-octylbenzo -crown-6, were shown to be much more susceptible to nitration by the S B W simulant or by 4 M nitric acid than calixarene-benzo crown ethers without the alkyl substituent. The cesium distribution behavior for a solvent comprised of l,3-alt-bis-n-octyloxy -calix[4]arene-benzo-crown-6 at 0.01 M in an aliphatic diluent modified with a fluorine-containing alcohol was shown to be stable over the course of a 60 day continuous contact with the S B W simulant at 25 °C. Toward process development, cesium distribution ratios on extraction (SBW simulant), scrubbing (50 m M nitric acid), and stripping (1 m M nitric acid) operations demonstrated a functional solvent cycle using a solvent of this composition augmented by 0.001 M trioctylamine (TOA). Without this lipophilic amine, stripping is inefficient. The presence of T O A in the solvent had no adverse effect on the Cs distribution behavior during extraction and scrubbing operations, nor did T O A negatively impact the Cs/Na and Cs/K selectivity ratios.
© 2000 American Chemical Society
27
Introduction Mono- and bis-crown-6 derivatives of 1,3-alternate calix[4]arenes possess high extractive strength for cesium (1-15) and, accordingly, are currently being investigated for possible use in separating radiocesium from nuclear wastes (1,3,4,6,14-18). These calixarene-crown ethers also possess excellent Cs/Na (>10 ) and C s / K (> 10 ) selectivities; however, they are currently quite expensive (> $100/g). Thus, for separation methods such as supported liquid membranes and solvent extraction employing calixarene-crown ethers to be economically viable, the consumption of these materials needs to be minimal. Ways to minimize the consumption of the calixarene-crown ether include optimizing the calixarene-crown ether's extracting power and reducing its loss (and therefore replacement costs) by a careful combination of good chemistry and engineering. A s discussed at length elsewhere (77), we have been examining solvent extraction in our separations-technology development, considering this separation method to be a proven high-throughput technique readily adaptable to remote operation for nuclear-waste cleanup. Process-suitable solvents for solvent extraction generally require high flash-point, low polarity, aliphatic hydrocarbon diluents (19). Unfortunately, the solubility and extractive power of many calixarene-crown ethers in such diluents is low. Toward a solution to this problem, we discovered that good solubility and Cs extraction power could be achieved when lipophilic calixarenecrown ethers such as calix[4]arene-bis-te^octylbenzo-crown-6 (1, Figure 1) were combined with alkylphenoxy alcohol-based solvating components, or modifiers, in the kerosene diluent (18). In particular, a solvent composed of 1 at 0.01 M with the modifier l-(l,l,2,2-tetrafluoroethoxy)-3-(4-ferr-octylphenoxy)-2-propanol (Figure 2, O R N L "Cs-3") at 0.20 M in Isopar® L diluent was shown to be effective for removing cesium from alkaline nitrate waste stored at the U.S. Department of Energy's (DOE's) Westinghouse Savannah River Site (77), affording cesium distribution ratios above 12 for the first contact between pristine solvent and actual Savannah River Site High Level Waste (20). Thus, good extraction power at low (0.01 M ) extractant concentration helps to minimize sol vent-inventory costs. In addition, replacement costs of 1 should be small, as it is chemically stable to alkaline media and is of sufficient lipophilicity that losses to aqueous phases are negligible (77). With respect to good engineering, the use of centrifugal contacting equipment allows rapid turnover of the solvent, minimal solvent inventory, and because of the short contact times, minimal exposure of the solvent to high radiation fields, thus decreasing losses of the (calixarene-crown ether) extractant by radiolytic degradation (21-23). Solvent and extractant losses due to entrainment are also minimized due to the forces developed in centrifugal contactors that separate the solvent and aqueous phases. Rapid turnover of the solvent means the most efficient use of the solvent possible, making it feasible to reduce the solvent inventory to just a few hundred gallons for a waste throughput equivalent to that of a full-scale plant capable of processing the 230,000 k L of alkaline radioactive waste stored at D O E ' s Hanford Site (23). In this paper, we report the results of our initial efforts toward developing a process solvent suitable for extracting cesium from acidic high nitrate nuclear waste, such as that stored in waste tanks at the D O E ' s Idaho National Engineering and Environmental Laboratory (INEEL). With the change from alkaline to nitric acid 4
2
1-(1,1,2,2-tetrafluoroethoxy)-3[4-(f-octyl)phenoxy]-2-propanol Figure 2. Solvent modifier ORNL "Cs-3 " used in this study.
29 waste came new concerns with respect to the chemical stability of the solvent. On the alkaline-side it was discovered that, whereas the calixarene-crown ether extractant calix[4]arene-bis-ter/-octylbenzo-crown-6 (1) possessed good long-term chemical stability to base (such as the 1.9 M free hydroxide present in the Savannah River simulant), the Cs-3 modifier did not. (A new class of modifiers that possess excellent alkaline stability have recently been developed.) In contrast, on the acid-side it was discovered that the Cs-3 modifier possessed good stability to the nitric acid (about 1.26 M ) present in the I N E E L waste simulant, but that 1 did not, instead becoming nitrated at an unacceptably fast rate. A n explanation of the factors contributing to nitration susceptibility, along with an evaluation of calixarene-crown ethers and process solvents that promise to be suitable for an acid-side cesium extraction process, is presented below.
Experimental Materials A l l salts and solvents were reagent grade and were used as received, except trioctyl amine, which was distilled prior to use. Distilled, deionized water was obtained from a Barnstead Nanopure filtering system (resistivity 18 Μ Ω ) and was used to prepare all aqueous solutions. Nitric and hydrochloric acids were Ultrex II grade (J.T. Baker). Isopar® L isoparaffinic diluent (lot# 0306 10967) was obtained from Exxon Chemical Company, Houston, Texas. The Sodium Bearing Waste (SBW) simulant was received from Dr. Donald J. Wood at D O E ' s Idaho National Engineering and Environmental Laboratory (INEEL). The C s radiotracer used for spiking the waste simulants was obtained as C s C l in 1M HC1 from Amersham (Arlington Heights, IL) and was used as received. The N a radiotracer was obtained as N a C l in water from Isotope Products Laboratories (Burbank, C A ) and was used as received. The calixarene crown ethers examined in this work are shown in Figure 1. Calix[4]arene-bis-tert-octylbenzo-crown-6 (1, (18)), l,3-alt-bis-«-octyloxycalix[4]arene 4'-teri-octylbenzo-crown-6 (3, (24)), l,3-alt-bis-«-octyloxycalix[4]arene 5'nitro-4'-tert-octylbenzo-crown-6 (4, (24)), bis-«-octyloxycalix[4]arene-crown-6 (5, (5)), l,3-alt-bis-«-octyloxycalix[4]arene-benzo-crown-6 (6, (24)), and bis-«-octyloxycalix[4]arene-dibenzocrown-6 (7, (25)), were prepared as described previously. Calix[4]arene-bis-5'-nitro-4'-feri-octylbenzo-crown-6 (2), was prepared in the same manner used to prepare 4. Calix[4]-bis-l,2-benzo-crown-6 (8) was purchased from A C R O S Organics. The modifier l-(l,l,2,2-tetrafluoroethoxy)-3-(4-^-octylphenoxy)-2-propanol (Figure 2, O R N L "Cs-3") was prepared as previously described (17). 1 3 7
1 3 7
2 2
2 2
Batch-Equilibrium Experiments Using Cs-137 Tracer As a general procedure, batch-equilibrium liquid-liquid contacting experiments were performed in polypropylene or Teflon® F E P tubes. A n exception is the initial
30 solvent stability experiments between the S B W simulant and the solvent containing calixarene-crown ether 1, which were performed in deionized water-rinsed borosilicate glass vials with black-phenolic screw caps containing polyethylene inserts. In all contacting experiments, equal volumes of aqueous and organic phases were contacted for 30 min (or longer for stability experiments) at 25.0 ± 0.2 °C by end-over-end rotation at 35 ± 5 R P M using a Glass-Col® laboratory rotator placed inside a 25 °C constant temperature airbox. The vials were then centrifuged for three to five minutes at 2869 χ g in a refrigerated centrifuge maintained at 25 ± 1 °C (Sanyo M S E Mistral 2000R) to ensure complete phase separation. Aliquots of each phase were removed for analysis, and the C s activity in each phase determined by standard gamma(v)-counting techniques using a Packard® Cobra Quantum Model 5003 gamma counter equipped with a 3" Nal(Tl) crystal through-hole type detector. Distribution ratios were determined as the ratio of the C s activity in the organic phase to the ^ C s activity in the aqueous phase at equilibrium, and are reproducible to within ± 5%. 1 3 7
1 3 7
7
Nitric Acid Stability Experiments The stability of selected calixarene crown ethers 1, 3, 5, 6, and 8 to nitration by nitric acid was measured by contacting 20 m M deutero-chloroform solutions of each calixarene-crown with an equal volume of 4.0 M nitric acid in Nalgene Teflon® F E P 10-mL centrifuge tubes by end-over-end rotation as described above. Aliquots of the chloroform phase were periodically withdrawn, and the proton N M R (Bruker M S L 400 operating at 400.13 M H z for proton) spectrum recorded and compared with the corresponding spectra of both the pristine solution prior to contact, and the solution one hour after contact (some resonances shift upon extraction of nitric acid, but no nitration was observed in any case after 1 hour contact). A relaxation delay ( D ) of 20 seconds was employed to ensure the complete relaxation of all protons selected for integration. The resonances selected for intregration included the protons on the tertoctyl group for 1 and3, the aromatic protons on the benzo-crown portion of 1, 3, 6, and 8, and the aromatic protons on the "belt" of the calixarene for 5, for both the parent calixarene-crown, and the nitrated product. The amount of nitrated calixarenecrown ether present as a function of time relative to the starting calixarene-crown ether was determined by comparison of the integration areas of the respective relevant peaks. Peak assignments for the nitrated calixarene-crowns were confirmed based on comparison to authentic samples: nitration of 1 gave only 2, and nitration of 3 gave only 4 (as the nitrated products), under these nitration conditions. 0
Elemental Analyses by Inductively Coupled Argon Plasma (ICAP) Spectroscopy
Instrumentation I C A P analyses were performed using a Thermal Jarell Ash (Franklin, M A ) IRIS Inductively Coupled Argon Plasma Optical Emission Spectrometer (ICAP/OES), equipped with a charge-injection device (CID) capable of recording atomic emission lines in the wavelength range 177 to 780 nm.
31 Contacting and Analytical Procedures The distribution behavior of the selected elements A l , B , Fe, Hg, K , and N a to selected solvents was investigated by contacting 3 m L of the solvent with an equal volume of the S B W simulant in 15-mL polypropylene centrifuge tubes for 30 minutes in the manner described above, in duplicate. To analyze for these metals using I C A P / O E S , the loaded extraction solvents were first completely stripped into an aqueous phase. This was accomplished by combining 2.0 mL of the loaded organic phase with an equal volume of 1,3-diisopropylbenzene (Aldrich), and contacting the resulting organic solution with 8.0 mL of 2% HC1 (Ultrex II grade) in 15-mL polypropylene centrifuge tubes as above. A second strip was then performed by taking 3.0 m L of the organic phase from the first strip (containing 1.5 m L of the original solvent) and contacting it with 6.0 mL of 2% HC1. The aqueous phases from the two strips, each representing a four-fold dilution of the metal ion concentration from the original solvent, were transferred directly to polypropylene tubes for I C A P analyses. The metal ion concentrations in the solvent were determined by multiplying the sum of the metal ion concentrations found in the first and second strip by four. The instrumental detection limit (IDL) for the elements was set equal to three times the standard deviation of a 2% HC1 blank based on 10 replicates. The detection limits for A l , B , Fe, Hg, K , and N a correspond to D < 8.4 χ 10" , D < 1.7 χ 1(T , D < 2.4 χ 10" , £ > < 5.0 χ 10" , D < 2.7 χ 10" , and £>Na < 3.8 χ 10" . The completeness of the metal ion stripping from the solvent using this method was validated by measuring the sodium distribution by N a tracer in polypropylene vials. The sodium distribution values obtained using the tracer method were only 10-14% higher than the values obtained using ICAP, which, given an experimental uncertainty of ± 5 % in the distribution ratio obtained using either method, is reasonably good agreement. The metal ion concentrations in the aqueous raffinate from the extraction and in the simulant itself were analyzed by diluting aliquots 100-fold with 2% HC1. The concentrations in the simulant and the raffinâtes for these metal ions were found to be essentially the same (within 5% experimental uncertainty) both to each other and to the concentrations listed in Table 1 for the data provided by I N E E L (26). The metal ion concentrations found in the feed were used in the calculation of the metal ion distribution ratios, assuming 100% mass balance. 6
A[
5
F e
4
Hg
4
B
5
6
K
2 2
Results and Discussion Initial Contacting Experiments with Sodium Bearing Waste Simulant I N E E L ' s Sodium Bearing Waste (SBW) comprises one class of nitric acidcontaining high-level nuclear waste that is being targeted for Cs removal (27). The composition of the S B W simulant provided to us by Dr. Donald J. Wood at the I N E E L is shown in Table I (26). The simulant contains a high concentration of nitrate (4.46 M ) , moderate concentrations of acid (1.26 M ) and sodium (1.32 M ) , and lower concentrations of many other metals, including potassium (0.138 M ) , the main competing ion for complexation of cesium by the calixarene-crown ether (15,17). A
32
Table 1. Composition of Simulated Sodium-Bearing Waste (SBW) Component +
M
Component
Acid (H )
1.26 χ ioo
Al
5.56 χ ί ο -
Β
1.40 χ ΙΟ"
Cd
2.05 χ 10-6
Ca
9.83 χ ΙΟ"
Ce (III)
fl
M
Κ
1.38 χ o-
Μη (II)
1.42 χ 0-2
Mo0 2-
1.49 χ 0-3
Na
1.32 χ
NO3-
4.46 χ
3.63 χ 10-4
Ni
1.63 χ 10-3
CI
3.52 χ 10-2
Pb
9.27 χ 0-4
Cr (III)
5.63 χ 10-3
P0 3-
Cs
7.52 χ 10-5
Sr
F
9.66 χ 10-2
SO4 -
3.86 χ 0-2
Fe (III)
2.40 χ 10-2
Zr
8.76 χ 0-3
Hg(II)
1.93 χ 10-3
1
2
4
2
4
1
o° o°
