Environ. Sci. Technol. 1999, 33, 2489-2491
Design and Use of Redox-Recyclable Organometallic Extractants for the Cationic Radionuclides 137Cs+ and 90 2+ Sr from Waste Solutions J E N N I F E R F . C L A R K , †,‡ R E B E C C A M . C H A M B E R L I N , * ,‡ KENT D. ABNEY,‡ AND S T E V E N H . S T R A U S S * ,†
FIGURE 1. Structures of the lipophilic organometallic extractants I- and II-.
Chemical Science and Technology Division (CST-11), Mail Stop J514, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, and Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523
processes have been developed for the extraction of 137Cs+ and 90Sr2+ from processing waste or waste simulants (4, 5), but there are fewer effective strategies for the recovery of these radionuclides from the extractants in a minimal volume of secondary waste and for the reuse of the extractants.
Two new redox-active organometallic extractants for the redox-recyclable extraction and recovery (R2ER) of the cationic radionuclides 137Cs+ and 90Sr2+ from aqueous waste simulants, Na[Fe(η5-C5H5)(η5-(3)-1,2-C2B9H9(n-C12H25)2)] (Na+I-) and Na[Fe(η5-C5H5)(η5-(3)-1,2-C2B9H7(n-C12H25)2-9,12Br2)] (Na+II-), were synthesized and investigated. When diluted with toluene or xylenes, they extracted Cs+ from aqueous waste simulants such as (i) 1 mol L-1 NaOH + 1 mol L-1 NaNO3, (ii) 1 mol L-1 HNO3, and/or (iii) 1 mol L-1 NaCl, with D(Cs) values that ranged from 5 to 20; the corresponding D(Sr) values were e0.04. The addition of poly(ethylene glycol) as a co-diluent increased D(Sr) values to as high as 33 but decreased D(Cs) values by a factor of 2-10, depending on conditions. When separated from the raffinate and exposed to air, the extractant-containing organic phase released g88% of its radioactivity in the form of a solid precipitate of minimal volume. When a small amount of water was present during this procedure, the extractant-containing organic phase released g99% of its radioactivity to the new aqueous phase. When an oxidized dilute extractant was rereduced and used for a second R2ER cycle, the D(Cs) value was unchanged.
Experimental Section
Introduction 137Cs+
90Sr2+
The soluble, high-yield fission products and are present in nuclear processing waste and contaminated groundwater in many locations (1). They are contained in wastes originating from both military and civilian nuclear fuel cycles and in the United States are stored in both highly acidic and alkaline aqueous media. As high-activity γ- and/ or β-emitters with half-lives of ca. 30 years, they account for 97% of the penetrating radiation and 98% of the thermal energy in high-level waste 30 years after fuel irradiation (2). Removal of 137Cs+ and 90Sr2+ reduces the exposure risk and cost of processing these wastes and minimizes the overall volume of high-level waste for final disposal (2, 3). Viable * Corresponding author telephone: (505)667-1841 (R.M.C.) and (970)491-5104 (S.H.S.); e-mail:
[email protected] (R.M.C.) and
[email protected] (S.H.S.). † Colorado State University. ‡ Los Alamos National Laboratory. 10.1021/es9810585 CCC: $18.00 Published on Web 06/09/1999
We are investigating the use of redox-active transition metal-containing extractants for the separation and recovery of specific pollutant ions (6-8). Redox-recyclable extraction and recovery (R2ER) cycles have been developed for aqueous TcO4- (6, 7) and Hg2+ (8); these processes allow for recovery of the target ions in a minimal volume and recycle of the extractant materials. Additional criteria satisfied by these R2ER cycles are high selectivities and relatively large capacities for the target ions. In this paper, we report an R2ER cycle for 137Cs+ and 90Sr2+ that is based on the new extractants Na[Fe(η5-C5H5)(η5-(3)-1,2-C2B9H9(n-C12H25)2)] (Na+I-) and Na[Fe(η5-C5H5)(η5-(3)-1,2-C2B9H7(n-C12H25)2-9,12-Br2)] (Na+II-), shown in Figure 1. During the extraction step of the cycle, the anionic iron(II) complexes I- and II-, which contain one cyclopentadienide ligand, Cp- and one substituted dicarbolide ligand, (Dc′)2-, are functionally similar to well-studied cobalt(III) dicarbollide extractants (4). The key advance in the R2ER cycle is the facile oxidation/deactivation of I- or IIto neutral I or II and the concomitant facile recovery of the radionuclides. This innovation is possible because the Fe(II) center in Fe(Cp)(Dc′)- can be reversibly oxidized under mild conditions (e.g., the E1/2 value for unsubstituted [Fe(Cp)(Dc)]0/- is -0.08 V vs SCE in acetonitrile (9)), unlike the Co(III) center in Co(Dc′)2- complexes. The complex Fe(Cp)(Dc), prepared by Hawthorne in 1968 (10), was the only mixed Cp/Dc complex of iron reported in the literature before this work.
1999 American Chemical Society
We determined that Na+Fe(Cp)(Dc)- is too soluble in water to be used as an effective R2ER extractant. Therefore, we developed the synthetic procedure shown below for the preparation of substituted derivatives (10): 1, Na(Hg); 2, Li2(Dc′); 3, O2
Fe(Cp)(C6H6)+ 98 Fe(Cp)(Dc′) The greenish-brown, viscous oils I and II were prepared in this manner using lipophilic didodecyl-substituted Dc′ ligands (10). The presence of the bromine atoms in II improved the resistance to oxidation by adventitious oxygen during the extraction step of the R2ER cycle (E1/2 values for the I0/- and II0/- couples are -0.32 and -0.15 V vs SCE, respectively, in CH2Cl2 (9)). The pink, activated extractants Na+I- and Na+II- were formed by shaking a 0.05 mol L-1 xylene or toluene solution VOL. 33, NO. 14, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2489
TABLE 1. D(Cs) and D(Sr) Values for Extractions with Dicarbolide Extractants at 24 ( 1 °C exp
extractanta
diluentb
aq waste simulantc
atmosphere
[PEG], Md
D(Cs)
D(Sr)
1 2 3 4 5 6 7 8 9 10 11e 12e
Na+INa+IINa+IINa+IINa+IINa+IINa+IINa+IINa+IINa+IIH+IIIH+III-
XYL TOL TOL TOL TOL TOL TOL TOL TOL TOL DEB DEB
NaOH/NaNO3 NaOH/NaNO3 NaOH/NaNO3 NaOH/NaNO3 NaOH/NaNO3 HNO3 HNO3 HNO3 HNO3 NaCl HNO3 NaOH
N2 N2 air N2 air N2 air N2 air N2 air air
0 0 0 0.1 0.1 0 0 0.1 0.1 0 0.002 0.002
20 5.0 5.4 0.25 0.30 6.9 0.21 2.7 0.56 17 13 5.2
e0.4 e0.4 e0.4 1.2 1.4 e0.4 e0.4 33 3.9 e0.4 0.15 0.83
a I- ) Fe(η5-C H )(η5-(3)-1,2-C B H (n-C H ) )-; II- ) Fe(η5-C H )(η5-(3)-1,2-C B H (n-C H ) -9,12-Br )-; III- ) Co((3)-1,2-C B H (C H ) ) -. The 5 5 2 9 9 12 25 2 5 5 2 9 7 12 25 2 2 2 9 9 6 13 2 2 extractant concentration was 0.050 M in all cases. b XYL ) xylenes; TOL ) toluene; DEB ) diethylbenzene. c NaOH/NaNO3 ) 1 M NaOH plus 1.5 M NaNO3; HNO3 ) 1 M HNO3; NaCl ) 1 M NaCl; NaOH ) 1 M NaOH. d PEG ) poly(ethylene glycol). e These data are from ref 4. Extractant IIIis not oxygen-sensitive.
of I or II, respectively, with an equal volume of 1 mol L-1 NaOH containing the reductants Na2S2O4 (4 equiv) and/or metallic zinc (1 g/10 mL) for 18 h. In some experiments, the organic phase also contained 0.1 M poly(ethylene glycol) (PEG, 400 MW), which can form a complex with Sr2+ and enhance its removal from the aqueous phase (4). After centrifugation, the organic phase was used to extract 137Cs+ and 85Sr2+ from aqueous waste simulants. Three simulants were used with toluene solutions of Na+II-: 1 mol L-1 HNO3; 1 mol L-1 NaOH/1.5 mol L-1 NaNO3; and 1 mol L-1 NaCl. Only the second simulant was used with xylene solutions of Na+I-. Each simulant was spiked with tracer amounts of the γ-emitters 137Cs+ and 85Sr2+ to provide an activity level of 10 µCi mL-1 for each radionuclide before extraction. Aliquots of the organic phases containing the activated extractants were shaken for 2 min with equal volumes of the aqueous simulants. After phase disengagement by centrifugation for 1 min, the equilibrium activity in each phase was measured by γ-spectrometry using a Ge(Li) counter. Distribution coefficients, D(Cs) and D(Sr), were calculated using the formula D(M) ) [cpm]org/[cpm]aq (cpm ) counts per minute/ mL of sample, corrected for background).
Results and Discussion Distribution coefficients are listed in Table 1. For comparison, some data for the structurally similar but non-redox-active extractant H+Co(Dc′)2- (H+III-, (Dc′)2- ) (3)-1,2-C2B9H9(nC6H13)2)22-) are also listed (4). The complete R2ER cycle is shown in Figure 2. Overall, the data demonstrate that salts of I- and II- dissolved in water-immiscible organic solvents can extract appreciable amounts of Cs+ and Sr2+ from sodium- or proton-rich aqueous waste simulants having pH values from 0 to 14. In this regard, salts of I- and II- are comparable to salts of III-, which are proven extractants for Cs+ and Sr2+ (4). The highest D(Cs) values measured were 17-20 (experiments 1 and 10); the highest D(Sr) value was 33 (experiment 8). As with extractants H+III- and Na+III(4), the addition of PEG to the toluene phase decreased D(Cs) and increased D(Sr) when Na+II- was the extractant (cf. experiments 2 with 4 and 6 with 8). The extraction equilibria, which lie to the right because the hydration energies of two Na+ ions is greater than either two Cs+ ions or one Sr2+ ion, are shown below:
Na+II-(org) + Cs+(aq) h Cs+II-(org) + Na+(aq) 2Na+II-(org) + Sr2+(aq) h Sr2+(II-)2(org) + 2Na+(aq) The important advantage of R2ER extractants such as Iand II- over III- is that, following the extraction step of the 2490
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 33, NO. 14, 1999
FIGURE 2. R2ER cycle for the extraction and recovery of Cs+ using the extractant II-. The three species II, Na+II-, and Cs+II- inside the central circle are confined to the organic phase, and all the species outside the central circle are confined to the three aqueous phases. R2ER cycle, I- and II- can be readily and reversibly oxidized by one electron to I and II, respectively. Once oxidized, the ion pairing between, for example, Cs+ and II- in toluene would be replaced by ion pairing between Cs+ and a different anion (OH- in Figure 2), allowing (i) the separation of a simple salt of Cs+ from II and (ii) the recycling of II. Adventitious oxidation during the extraction step, however, can result in premature extractant deactivation and lower D values. For example, the presence of air during the extraction of 137Cs+ and 85Sr2+ from aqueous 1 M HNO3 with Na+II-/toluene decreased both D(Cs) and D(Sr) (cf. experiments 6 with 7 and 8 with 9; note that a decrease in D(Sr) for experiments 6 and 7 could not be confirmed because of the relatively low D(Sr) values). We tentatively conclude that O2 in the air caused the partial oxidation of II- in experiments 7 and 9, significantly decreasing D(Cs) and D(Sr) by decreasing the concentration of the activated (i.e., reduced) extractant. Interestingly, air had no effect when the aqueous waste simulant was 1 mol L-1 NaOH/1.5 mol L-1 NaNO3 (cf. experiments 2 with 3 and 4 with 5). This is probably due to the fact that O2 is a much stronger (and therefore faster acting) oxidant in acid than in base (E(O2/H2O) vs NHE is 1.23 V at pH 0 and 0.40 V at pH 14 (11)). The most significant aspect of this work was the deactivation/recycling of the extractant and the recovery of the radionuclides in a small volume of secondary waste. In one experiment, a 1.0-mL aliquot of γ-analyzed toluene from experiment 6 was exposed to air (but then kept tightly stoppered) for 2 weeks. The sample was then centrifuged to
separate the supernatant toluene phase from any solids that had formed, and a portion of the supernatant was counted again. Only 12% of the original 137Cs activity remained in solution. Therefore, most of the activity was present as a solid phase, presumably a mixture of NaOH and 137CsOH, both of which would be expected to have very limited solubility in water-saturated toluene. If the solid-phase secondary waste proves to be microcrystalline NaOH/CsOH, the Na:Cs ratio in the secondary waste and the volume of secondary waste would be only ∼1% and ∼0.1%, respectively, of their primary waste values. When an identical 1.0-mL aliquot of the toluene phase from experiment 6 was treated with 0.1 mL of water, exposed to air, shaken for just 3 min, and centrifuged, the toluene phase contained only 0.3% of its original 137Cs activity. The same result (i.e., 99.7% recovery) was obtained when a 1.0mL aliquot of the toluene phase from experiment 3 was similarly treated. It is possible that excess water is necessary for the complete oxidation of II-, as shown in
4Cs+II-(org) + O2(org) + 2H2O(org) f 4II(org) + 4CsOH(s) Recovery of the radionuclides from the toluene/PEG extraction phases with air as the oxidant was not complete even in the presence of added water. A sample of the organic phase from experiment 8 was exposed to air and treated with water. After centrifugation, the organic phase still contained 93% of the original 137Cs activity and 42% of the original 90Sr activity. However, when the experiment was repeated with 1 equiv of the strong one-electron oxidant Ce(NH4)2(NO3)6 added to the water, only 2% of the original 137Cs activity and less than 5% of the original 90Sr activity remained in the organic phase. When the organic phases from experiments 5 or 9 were treated with 2 equiv of aqueous Ce(NH4)2(NO3)6, less than 0.1% of the original 137Cs and 90Sr activity remained in the organic phase. A sample of the toluene phase from experiment 3 was treated with aqueous Ce(NH4)2(NO3)6, rereduced, and used for a second extraction with the basic waste simulant. The D(Cs) value was essentially unchanged; 5.6 for the second extraction vs 5.4 for the first extraction. Therefore, the new extractants I- and II- are not only selective for Cs+ and Sr2+ in the presence of high concentrations of Na+ in acid, base,
and neutral solution, but II- can also be reversibly deactivated by oxidation, allowing for facile radionuclide recovery and efficient recycling of the extractant. This experiment demonstrates that II- and II are stable and maintain their activity in the presence of strong aqueous acid and base. This is an essential property of a potential R2ER extractant.
Acknowledgments This research was supported by the Laboratory-Directed Research and Development Program and the Civilian and Industrial Technology Program Office at Los Alamos National Laboratory (LANL) and by NSF Grant CTS-9726143. LANL is operated by the University of California for the U.S. Department of Energy under Contract W-7405-ENG-36.
Literature Cited (1) Commission on Geosciences, Environment, and Resources, National Research Council. Nuclear Wastes: Technologies for Separations and Transmutation; Report PB96-184247; National Academy Press: Washington, DC, 1996. (2) DOE Plan for Recovery and Utilization of Nuclear Byproducts from Defense Wastes; Report DOE/DP-0013-V-2, Vol. 2; U.S. DOE: Washington, DC, 1983. (3) Barker, S. A.; Thornhill, C. K.; Holton, L. K. Pretreatment Technology Plan; Report WHC-EP-0629; Westinghouse Hanford Co.: Richland, WA, 1993. (4) Chamberlin, R. M.; Abney, K. D. J. Radioanal. Nucl. Chem. 1999, 240, 547. (5) Behrens, E. A.; Sylvester, P.; Clearfield, A. Environ. Sci. Technol. 1998, 32, 101. (6) Clark, J. F.; Clark, D. L.; Whitener, G. D.; Schroeder, N. C.; Strauss, S. H. Environ. Sci. Technol. 1996, 30, 3124. (7) Chambliss, C. K.; Odom, M. A.; Morales, C. M. L.; Martin, C. R.; Strauss, S. H. Anal. Chem. 1998, 70, 757. (8) Gash, A. E.; Spain, A. L.; Dysleski, L. M.; Flaschenriem, C. J.; Kalaveshi, A.; Dorhout, P. K.; Strauss, S. H. Environ. Sci. Technol. 1998, 32, 1007. (9) Hawthorne, M. F.; Young, D. C.; Andrews, T. D.; Howe, D. V.; Pilling, R. L.; Pitts, A. D.; Reintjes, M.; Warren, L. F., Jr.; Wegner, P. A. J. Am. Chem. Soc. 1968, 90, 879. (10) Clark, J. F.; Chamberlin, R. M.; Abney, K. D.; Strauss, S. H. Manuscript in preparation. (11) Bard, A. J., Parson, R., Jordan, J., Eds. Standard Potentials in Aqueous Solution; Dekker: New York, 1985.
Received for review October 14, 1998. Revised manuscript received March 15, 1999. Accepted March 29, 1999. ES9810585
VOL. 33, NO. 14, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
2491
