A Strategy for Separating and Recovering Aqueous Ions: Redox

This can be compared with 290 mL/g (87 mL/mmol of cationic sites) for ... Nevertheless, the data indicate that redox-recyclable anion exchange is a vi...
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Anal. Chem. 1998, 70, 757-765

A Strategy for Separating and Recovering Aqueous Ions: Redox-Recyclable Ion-Exchange Materials Containing a Physisorbed, Redox-Active, Organometallic Complex C. Kevin Chambliss, Matthew A. Odom, Christine M. L. Morales, Charles R. Martin,* and Steven H. Strauss*

Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523

A series of anion-exchange materials were prepared by adsorption of the dark-green organometallic salt HEP+NO3or HEP+ReO4- dissolved in organic solvents onto three different silica gels (HEP ) 1,1′,3,3′-tetrakis(2-methyl2-hexyl)ferrocene). Adsorption isotherms showed that the amount of HEP+ salt adsorbed depended on the choice of counteranion, solvent, surface area, and pore size diameter of the silica gel. After drying the HEP+NO3-/ SiO2 and HEP+ReO4-/SiO2 solid materials, the organometallic salts did not desorb into the aqueous phase when the solids were treated with aqueous solutions containing NaNO3 and/or HNO3. The HEP+NO3-/SiO2 materials functioned as redox-recyclable ion exchangers. Treatment of the materials with aqueous waste simulants containing KReO4, NaNO3, and HNO3 resulted in NO3-/ReO4- ion exchange as follows: HEP+NO3-/SiO2(s) + ReO4-(aq) h HEP+ReO4-/SiO2(s) + NO3-(aq). The distribution coefficient for one of the new materials was 100 mL/g (440 mL/mmol of HEP+) for an aqueous waste simulant containing ReO4- and 1.0 M HNO3. This can be compared with 290 mL/g (87 mL/mmol of cationic sites) for Reillex-HPQ, a commercial non-redox-recyclable ionexchange resin which has been studied for ReO4- and TcO4- extraction. The higher distribution coefficient per millimole of cationic sites suggests that HEP+NO3-/SiO2 is more selective for ReO4- than Reillex-HPQ under these conditions. The recovery of adsorbed ReO4- was accomplished by treating the exchanged materials with aqueous ferrocyanide, which caused the reduction of adsorbed HEP+ to adsorbed HEP and concomitant release of the adsorbed counterions ReO4- and NO3-. Reactivation of HEP/SiO2 to HEP+NO3-/SiO2 was accomplished with aqueous ferric nitrate. Five complete extraction-deactivation/(ReO4- recovery)-reactivation cycles (duty cycle time 94 min) consistently showed a slow decrease in distribution coefficient (∼20% over five cycles). Nevertheless, the data indicate that redox-recyclable anion exchange is a viable concept and that redox-recyclable ionexchange materials with improved stability should be S0003-2700(97)00567-2 CCC: $15.00 Published on Web 02/15/1998

© 1998 American Chemical Society

considered as viable alternatives to traditional anionexchange resins in the future. Ion-exchange chromatography is a mature technology.1-6 When it is used for the remediation of toxic and/or radioactive aqueous waste streams, however, minimization of the volume of secondary waste (e.g., the “strip” solution) destined for permanent disposal is still a significant challenge. This is especially problematic whenever hazardous but dilute waste streams are concerned because, in general, the volume of secondary waste per mole of contaminant recovered is inversely proportional to the concentration of contaminant in the primary waste.7 A good example of a difficult separation problem is the pretreatment of nuclear process waste stored at Hanford, WA8-10 or at Oak Ridge, TN.11,12 These waste streams contain high concentrations of many inorganic and organic ions (ionic strengths exceeding 5 M are common), but the concentrations of radioactive ions are low (∼5 × 10-5 M for 90Sr2+, 99TcO4- (pertechnetate), and 137Cs+). The (1) Dasgupta, P. K. Anal. Chem. 1992, 64, 775A. (2) Peters, R. W.; Shem, L. In Emerging Separation Technologies for Metals and Fuels, Proceedings of a Symposium; Lakshmanan, V. I., Bautista, R. G., Somasundaran, P., Eds.; Minerals, Metals & Materials Society: Warrendale, PA, 1993; pp 3-64. (3) Kawasaki, M.; Omori, T.; Hasegawa, K. Radiochim. Acta 1993, 63, 53, and references therein. (4) Ashley, K. R.; Ball, J. R.; Pinkerton, A. B.; Abney, K. D.; Schroeder, N. C. Solvent Extr. Ion Exch. 1994, 12, 239, and references therein. (5) Abe, M. Ion Exch. Solvent Extr. 1995, 12, 381. (6) Streat, M. Ind. Eng. Chem. Res. 1995, 34, 2841. (7) Tedder, D. W. Sep. Purif. Methods 1992, 21, 23. (8) Babad, H.; Fulton, J. C.; DeFigh-Price, B. C. A Strategy for Resolving HighPriority Hanford Site Radioactive Waste Storage Tank Safety Issues; report WHC-SA-1661-FP, Westinghouse Hanford Co.: Richland, WA, 1993. (9) Barker, S. A.; Thornhill, C. K.; Holton, L. K. Pretreatment Technology Plan; report WHC-EP-0629, Westinghouse Hanford Co.: Richland, WA, 1993. (10) Committee on Separations Technology and Transmutation Systems, Board on Radioactive Waste Management, Commission on Geosciences, Environment, and Resources, National Research Council. Nuclear Wastes: Technologies for Separations and Transmutation; National Academy Press: Washington, DC, 1996. (11) Bostick, W. D.; Osborne, P. E.; Del Cul, G. D.; Simmons, D. W. Treatment of Aqueous Solutions Contaminated with Technetium-99 Originating from Uranium Enrichment Activities: Final Report; report K/TCD-1120, Oak Ridge K-25 Site, Oak Ridge, TN, 1995. (12) Beck, D. E.; Osborne, P. E.; Bunch, D. H.; Fellows, R. L.; Sellers, G. F.; Shoemaker, J. L.; Bowser, K. T.; Bostick, D. T. Removal of Technetium-99 from Simulated Oak Ridge National Laboratory Newly-Generated Liquid LowLevel Waste; report K/TCD-1141, Oak Ridge K-25 Site, Oak Ridge, TN, 1995.

Analytical Chemistry, Vol. 70, No. 4, February 15, 1998 757

Figure 1. Representation of the complete redox-recyclable extrantion-recovery cycle. The species EXTR is a neutral, redox-active, transition-metal-containing molecule, solid, or polymer. The species EXTR* is either the oxidized, cationic transition-metal species (for the extraction of anions from aqueous media) or the reduced, anionic transition-metal species (for the extraction of cations from aqueous media). In the case of solvent extraction, the species EXTR, EXTR*, and (ION)EXTR* are present in the water-immiscible organic phase. In the case of ion-exchange chromatography, the species EXTR, EXTR*, and (ION)EXTR* are present in the stationary solid phase.

Solutions of HEP in water-immiscible organic solvents did not extract 99TcO4- from aqueous waste simulants, but organic solutions of the salt HEP+NO3- extracted >99% 99TcO4- even though [NO3-]/[99TcO4-] > 105 in the initial aqueous waste simulant (an example of Hofmeister separation22-26). The activation (oxidation of HEP to HEP+) and deactivation (reduction of HEP+ to HEP) steps in the repeatable cycle were accomplished using aqueous Ce(NH4)2(NO3)6 and solid iron powder, respectively.16 The complete cycle is summarized in the simplified equations below (i.e., simplified in the sense that HEP+TcO4-(org) is really a mixture of ∼10-1-10-3 M HEP+NO3-(org) and only ∼10-5 M HEP+TcO4-(org) and the solid, secondary waste contains TcO2(s) in addition to Fe(TcO4)3(s)):

extraction technologically and scientifically challenging extraction and recovery of these radionuclides from high-ionic-strength matrixes is under investigation in many laboratories. There is also an urgent need to remove 99TcO4- from contaminated groundwater in several locales, including Hanford, WA, Oak Ridge, TN, and Paducah, KY.13-15 We are developing a new paradigm in separation science, the use of hydrophobic, redox-recyclable metal complex extractants for the removal and recovery of ionic contaminants from acidic, neutral, and alkaline aqueous solutions.16,17 A scheme showing the complete extraction-deactivation/recovery-reactivation cycle is shown in Figure 1. In a previous paper,16 we reported a liquidliquid extraction-recovery process for radioactive pertechnetate, a nuclear fission product of environmental concern.18-20 The primary aqueous waste simulant contained 1 M NaOH, 1.5 M NaNO3, but only 10-5 M 99TcO4-. The secondary waste produced was a mixture of solids containing >99% of the radioactivity originally in the primary waste but with a total volume less than 1% of the primary waste volume.16 Recent improvements may allow the secondary waste volume to be reduced to only 0.03% of the primary waste volume.21 The deactivated form of the redoxrecyclable anion extractant was the hydrophobic organometallic complex 1,1′,3,3′-tetrakis(2-methyl-2-hexyl)ferrocene (HEP):16 (13) Del Cul, G. D.; Bostick, W. D.; Trotter, D. R.; Osborne, P. E. Sep. Sci. Technol. 1993, 28, 551. (14) Toran, L. In Groundwater Contamination and Control; Zoller, U., Ed.; Marcel Dekker: New York, 1994; p 437. (15) Tagami, K.; Uchida, S. Radioisotopes 1995, 44, 528. (16) Clark, J. F.; Clark, D. L.; Whitener, G. D.; Schroeder, N. C.; Strauss, S. H. Environ. Sci. Technol. 1996, 30, 3124. (17) Gash, A. E.; Spain, A. L.; Dorhout, P. K.; Strauss, S. H. Manuscript in preparation. (18) Schroeder, N. C.; Morgan, D.; Rokop, D. J.; Fabryka-Martin, J. Radiochim. Acta 1993, 60, 203. (19) Garten, C. T., Jr. Environ. Int. 1987, 13, 311. (20) Wildung, R. E.; McFadden, K. M.; Garland, T. R. J. Environ. Qual. 1979, 8, 156. (21) Gansle, K. M.; Clark, J. F.; Schroeder, N. C.; Strauss, S. H., unpublished work, Los Alamos National Laboratory, 1996-1997.

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HEP+NO3-(org) + TcO4-(aq waste) f HEP+TcO4-(org) + NO3-(aq waste) deactivation/recovery 3HEP+TcO4-(org) + Fe(s) f 3 HEP(org) + Fe(TcO4)3(s) reactivation HEP(org) + Ce4+(aq) + NO3-(aq) f HEP+NO3-(org) + Ce3+(aq) This new type of process, extraction and recovery with redoxrecyclable extractants, accomplishes three things simultaneously: (1) it has a high selectivity and a large capacity for the target ion; (2) it allows for the efficient reuse (recycle) of the extractant; (3) it allows for the recovery of the target ion in a minimal volume of secondary waste. In the past, the primary focus of investigators studying extraction has been increasing selectivity and/or capacity. In recent years, however, tougher environmental regulations27 and the high initial cost of new, more effective, and more selective extractants have made the reuse of the extractant an increasingly important issue. In addition, the cost of permanent disposal is directly related to the volume of the secondary waste. For these reasons, the reuse of extractant materials and the minimization of secondary waste volume must become the focus of scientific efforts in the near future. (22) Bucher, J.; Diamond, R. M.; Chu, B. J. Phys. Chem. 1972, 76, 2459. (23) Collins, K. D.; Washabaugh, M. W. Q. Rev. Biophys. 1985, 18, 323. (24) Marcus, Y.; Kamlet, M. J.; Taft, R. W. J. Phys. Chem. 1988, 92, 3613. (25) Hara, H.; Ohkubo, H.; Sawai, K. Analyst 1993, 118, 549. (26) Moyer, B. A.; Bonneson, P. V. Physical Factors in Anion Separation. In Supramolecular Chemistry of Anions; Bianchi, A., Bowman-James, K., GarciaEspan ˜a, E., Eds.; VCH: New York, 1997. (27) Kirschner, E. M. Chem. Eng. News 1994, (June 20), p 13.

In this paper we report that (1) salts of HEP+ can be adsorbed onto silica gels using organic solvents and will not desorb into the aqueous phase when the dried materials are treated with various aqueous waste simulants, (2) NO3- coadsorbed with HEP+ will rapidly and selectively exchange with ReO4- in aqueous solution (note that ReO4- is a suitable solvent-extraction28 and ionexchange4,29 surrogate for TcO4-), and (3) adsorbed HEP+ and adsorbed HEP can be rapidly interconverted using water-soluble redox reagents. This new type of selective, redox-recyclable, ionexchange material can be used in a closed cycle similar to that described above for our new liquid-liquid extraction process16 but with the added advantage that no organic solvents are used once the metal complex is loaded onto the silica gel (i.e., the aqueous waste stream never comes in contact with organic solvents). In addition, observation 1 demonstrates that some redox-recyclable ion-exchange materials may be made by simple adsorption instead of the more difficult and more expensive potential alternative of covalently grafting redox-active metal complexes onto solid supports. EXPERIMENTAL SECTION Materials and Reagents. The compounds HEP+NO3- and HEP+ReO4- were prepared by literature procedures.16,30 The reagents and solvents Fe(NO3)3 (Fisher), K4Fe(CN)6 (Fisher), K3Fe(CN)6 (Fisher), HNO3 (Mallinckrodt), KReO4 (Aldrich), Ce(NH4)2(NO3)6 (Aldrich), toluene (Fisher), dichloromethane (Fisher), and chlorobenzene (Aldrich) were reagent grade or better and were used as received. Distilled water was purified and deionized (to 18 MΩ) with a Barnstead Nanopure water purification system. The three silica gels, which were used as received, were Merck grade 10180 (40 Å, 750 m2/g (40)), Merck grade 60 (60 Å, 550 m2/g (60)), and Merck grade 10184 (100 Å, 300 m2/g (100)). All three were 70-230 mesh. Adsorption Isotherms. Solutions of known concentrations of green HEP+X- (X- ) NO3-, ReO4-) in 25 mL of the three organic solvents were treated with 0.100 g of one or more of the three silica gels. Each solution was shaken at 25 °C. The concentration of HEP+X- in the organic solvent during the course + HEP+X-(solv) + SiO2(s) h HEP X /SiO2(s) white green green

of the adsorption process was monitored by UV-visible spectroscopy using a Perkin-Elmer Lambda 3B spectrophotometer. The amount of HEP+X- adsorbed per gram of silica gel is equal to (∆C)(Vsol)/m, where ∆C, Vsol, and m are the change in concentration of HEP+X-, the total volume of solution, and the mass of silica gel, respectively. Plots of adsorbed millimoles of HEP+X- versus total millimoles of HEP+X- show saturation behavior typical of a Langmuir isotherm,31 as shown for a typical experiment in Figure 2. Plots such as these were used to determine the maximum (28) Rohal, K. M.; Van Seggen, D. M.; Clark, J. F.; Van Egeren, M. K.; Chambliss, C. K.; Strauss, S. H.; Schroeder, N. C. Solvent Extr. Ion Exch. 1996, 14, 401, and references therein. (29) Ashley, K. R.; Cobb, S. L.; Ball, J. R.; Abney, K. D.; Schroeder, N. C. Solvent Extr. Ion Exch. 1995, 13, 353. (30) Clark, J. F.; Clark, D. L.; Gansle, K. M.; Chambliss, C. K.; Clapsaddle, B. J.; Miller, S. M.; Anderson, O. P.; Strauss, S. H. Manuscript in preparation. (31) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; Wiley: New York, 1990; p 421 ff.

Figure 2. Adsorption isotherm for the loading of HEP+ReO4dissolved in toluene onto 100-Å pore diameter SiO2 (100). The maximum loading is 0.261(2) mmol of HEP+ReO4-/g of SiO2, which corresponds to ∼105% monolayer coverage.

amount of HEP+X- that could be loaded onto a silica gel under a given set of conditions. Ion-Exchange Materials. Several redox-recyclable ionexchange materials were prepared using either toluene or dichloromethane solutions of HEP+X- and the three 70-230-mesh silica gels 40, 60, and 100. In a typical loading, 5 g of 100 were shaken at 25 °C for 24 h with 100 mL of a 0.0133 M toluene solution of HEP+ReO4-. The dark-green solid material HEP+ReO4-/100 was collected by filtration and dried under vacuum for several hours. This procedure resulted in a loading of 0.261(2) mmol of HEP+ReO4-/g of 100 (five measurements, (σ in the least significant digit is shown in parentheses), which is therefore the maximum loading possible for this silica gel, this organometallic salt, and this solvent. Batch Kd′ and Kd* Values. The distribution coefficient Kd is a figure of merit for selectivity of an ion-exchange material (i.e., selectivity increases as Kd increases). At small (mmol of ReO4-)/ (mmol of NO3-) ratios, it can be defined as follows (V is the volume of solution that is being treated with the ion-exchange material and [Re]i and [Re]f are the initial and final concentrations, respectively, of all soluble rhenium species (i.e., ReO4- plus HReO4)):

Kd )

mmol of ReO4- adsorbed/g of HEP+NO3-/SiO2 mmol of ReO4- remaining per/mL of solution

)

{[ReO4-]i - [ReO4-]f}V {mass of HEP+NO3-/SiO2}[ReO4-]f

The units of Kd are milliliter per gram. An alternative figure of merit, defined as Kd′, has the same units as Kd but is based on the substitution of [Re] for [ReO4-]:

Kd′ )

{[Re]i - [Re]f}V {mass of HEP+NO3-/SiO2}[Re]f

Note that Kd′ is based on actual experimental measurements, Analytical Chemistry, Vol. 70, No. 4, February 15, 1998

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whereas Kd is not (i.e., ICP-AES analysis measures [Re]f, not [ReO4-]f). A third figure of merit, Kd*, is the selectivity per millimole of cationic sites on the material:

Kd* ) Kd′

{

}

mass of HEP+NO3-/SiO2 mmol of adsorbed HEP+

The units of Kd* are milliliter per millimole. Note that Kd* is equivalent to Kd′ divided by the ion-exchange capacity of the material. The aqueous solutions treated in our study consisted of varying concentrations of KReO4 (0.002-0.003 M), HNO3 (0-1.0 M), and NaNO3 (0-1.0 M). For batch Kd′ experiments, aliquots of the aqueous solutions were shaken at 25 °C with 1.00 g of HEP+NO3-/SiO2. The ratio (mmol of total rhenium)/(mmol of HEP+) was never more than 0.037. The small amount of rhenium in the system ensured that the amount of adsorbed NO3- was essentially constant. After a period of time, the mixture was filtered through No. 4 Whatman filter paper. The concentrations of rhenium in the aqueous solution before ([Re]i) and after the extraction step ([Re]f) were determined using a Perkin-Elmer P400 inductively coupled plasma atomic emission spectrometer (ICP-AES) equipped with a high-salt nebulizer (the Re emission at 221.426 nm was monitored). Calibration curves, which were linear in concentration over the range 1.00-0.0500 mM, were constructed using known concentrations of KReO4 in each particular aqueous solution studied (i.e., matrix matching was used for all experiments). One standard was reanalyzed for at least every five samples during the course of data collection. For each sample, five readings of the ICP-AES intensity were recorded and averaged. For each aqueous solution, at least two separate extractionanalysis experiments were performed, and average values of Kd′ were calculated. Preparation of Ion-Exchange Columns and Column Kd′ Values. The ion-exchange materials were dry-packed32 into 8 mm i.d. × 43 mm long glass columns equipped with No. 7 Ace-Thred connectors. Typically, 1.00 g of material was used for each column. Aqueous solutions were pumped through the columns (3.5 mL/min) using a Fluid Metering Model QSY pump equipped with a low-flow isolation kit and 1/16-in.-i.d. TFE tubing. In each Kd′ experiment, the aqueous solution was recirculated through the column until the concentration of all rhenium species (ReO4plus HReO4) was constant. Initial and final concentrations of rhenium were determined by ICP-AES as described above. RESULTS AND DISCUSSION The New Concept. The two common techniques for selectively removing anions from aqueous media are liquid-liquid solvent extraction and ion-exchange chromatography.33,34 After we demonstrated that the HEP+/0 redox couple could be used for the redox-recyclable solvent extraction and recovery of 99TcO4and ReO4- from aqueous nuclear waste simulants,16 we turned our attention to developing liquid-solid redox-recyclable anionexchange processes (recall that ReO4- is a suitable solvent(32) Siekierski, S. In Extraction Chromatography; Braun, T., Ghersini, G., Eds.; Elsevier: Amsterdam, 1975; pp 55-56. (33) Thornton, J. D. Science and Practice of Liquid-Liquid Extraction; Clarendon Press: Oxford, U.K., 1992; Vols. 1 and 2. (34) Dasgupta, P. K. Anal. Chem. 1992, 64, 775A.

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extraction28 and ion-exchange4,29 surrogate for TcO4-). In addition to the general advantages of redox-recyclable extraction and recovery already discussed, no organic solvents would need to be used for an anion-exchange process using a redox-active solid material. An effective ion-exchange material must have a high capacity and selectivity for the target ion. Two other important design criteria for redox-recyclable ion-exchange materials are that the cationic moieties (1) must not desorb from the solid support into the aqueous waste stream and (2) must not be reduced or otherwise decomposed by the aqueous waste stream. One obvious way to prevent desorption is to use a water-insoluble polymeric material containing the redox-active moieties. In one example of this approach, Porter and co-workers reported that polypyrrole (PPY)-coated glassy-carbon spheres could be partially oxidized (i.e., doped) to produce a solid material for the separation and preconcentration of dansyl amino acids.35 However, this material had a relatively low ion-exchange capacity (anion exchange occurred at less than 5% of the cationic moieties present in the fully doped PPY film). Furthermore, we found that fully doped poly-N-methylpyrrole is not sufficiently selective for ReO4in the presence of a large excess of NO3- (typical nuclear process waste, which can be either highly acidic or highly alkaline, has a [NO3-]/[99TcO4-] > 105).36 In addition, we found that fully doped poly-N-methylpyrrole was not stable over a wide pH range.36 Another potential class of redox-active polymers are ferrocenecontaining polymers.37 In 1986 we reported that electroactive ionomers containing ferrocene can be used as electroreleasing materials.38 However, aromatic compounds with R-hydrogen atoms, including n-alkyl-substituted ferrocenes such as poly(vinylferrocene), are prone to irreversible overoxidation (e.g., R-methylene groups can be oxidized to R-keto groups).39 Furthermore, monoalkylated ferricenium ions, including partially oxidized poly(vinylferrocene), are prone to nucleophile-induced decomposition reactions.40-43 The propensity of ferricenium ions to undergo nucleophile-induced decomposition decreases with decreasing Fe(Cp′)2+/0 reduction potential, which decreases as more alkyl groups are incorporated into the Cp′ rings.40-43 Other investigators have used the ferrocene/ferricenium couple to extract and/or transport ions.44 For these reasons, suitable ferrocene-based redox-recyclable ion-exchange polymers should contain pendant ferrocenes that are polysubstituted with tertiary alkyl groups. Rather than synthesize such polymers, we took a simpler and less expensive (35) Deinhammer, R. S.; Porter, M. D.; Shimazu, K. J. Electroanal. Chem. 1995, 387, 35. (36) Chambliss, C. K.; Martin, C. R.; Strauss, S. H., unpublished work, Colorado State University, 1995. (37) Pittman, C. U., Jr.; Rausch, M. D. Pure Appl. Chem. 1986, 58, 617. (38) Ghatak-Roy, A. R.; Martin, C. R. Anal. Chem. 1986, 58, 1574. (39) March, J. Advanced Organic Chemistry, 4th ed.; Wiley-Interscience: New York, 1992; p 1183 ff. (40) Prins, R.; Korswagen, A. R.; Kortbeek, A. G. T. G. J. Organomet. Chem. 1972, 39, 335. (41) Holecek, J.; Handlir, K.; Klikorka, J.; Bang, N. D. Collect. Czech. Chem. Commun. 1974, 44, 1379. (42) Kochetkova, N. S.; Materikova, R. B.; Belousov, Y. A.; Salimov, R. M.; Babin, V. N. J. Organomet. Chem. 1982, 235, C21. (43) Huang, W. H.; Jwo, J.-J. J. Chin. Chem. Soc. 1991, 38, 343. (44) (a) Beer, P. D.; Smith, D. K. Prog. Inorg. Chem. 1997, 46, 1. (b) De Santis, G.; Fabbrizzi, L.; Licchelli, M.; Monichino, A.; Pallavicini, P. J. Chem. Soc., Dalton Trans. 1992, 2219.

Table 1. Maximum Loading of HEP+X- onto Silica Gels (X- ) NO3-, ReO4-) loadingc

Figure 3. Adsorption isotherms for the loading of HEP+NO3dissolved in toluene (TOL), chlorobenzene (CLB), and dichloromethane (DCM) onto 40-Å pore diameter SiO2 (40).

approach to ferrocene-containing anion-exchange materials. This new approach involved the adsorption of hydrophobic ferricenium salts onto high-surface-area inert supports. It is superior to approaches involving substituted-ferrocene-containing polymers because the ring substituents do not have to contain a polymerizable functionality. Instead, the ring substituents only have to render the ferricenium ions stable with respect to overoxidation and nucleophilic attack and render the ferricenium salts insoluble in the aqueous waste streams of concern. Note that many investigators have studied the adsorption of salts of lipophilic cations to silica gel for chromatographic purposes.45 However, in no case was the physisorbed cation a redox-active species. Therefore, the preparation and use of ionexchange columns that can be activated and deactivated by simple redox reactions represents a new concept in the extraction and recovery of anions from aqueous media. Adsorption of HEP+X- Salts onto Silica Gels. In order to demonstrate the feasibility of this new approach, HEP+NO3- and HEP+ReO4- were adsorbed onto several high-surface-area silica gels. We had earlier demonstrated that HEP+NO3- is sufficiently hydrophobic so that it does not partition between organic solvents and either aqueous 1 M HNO3 or 1 M NaOH/1.5 M NaNO3.16 We now report that HEP+NO3- and HEP+ReO4- adsorbed onto silica gels do not desorb into either aqueous 1 M HNO3 or 0.025 M K3Fe(CN)6. The new ion-exchange materials were prepared by treating silica gels with organic solutions of HEP+NO3- or HEP+ReO4-. The three silica gels used, 100, 60, and 40, were 70-230 mesh and had 100-, 60-, and 40-Å pore diameters, respectively (300, 550, and 750 m2/g surface areas, respectively). The three solvents used were toluene, chlorobenzene, and dichloromethane. Several adsorption isotherms are shown in Figures 2 and 3. Table 1 lists the maximum loading of the organometallic salts in millimole adsorbed salt per gram of silica gel used or per gram of HEP+X-/ (45) (a) Zaporozhets, O. A.; Nadzhafova, O.; Zubenko, O. Y.; Ishchenko, A. I.; Trachevskii, V. B.; Sukhan, V. V. Ukr. Khim. Zh. 1995, 61, 64. (b) Biernat, J. F.; Konieczka, P.; Tarbet, B. J.; Bradshaw, J. S.; Izatt, R. M. Sep. Purif. Methods 1994, 23, 77. (c) Flieger, A. Chem. Anal. 1993, 38, 483. (d) Sahoo, S. K. Talanta 1991, 38, 789. (e) Przeszlakowski, S.; Kocjan, R. Chromatographia 1983, 17, 266. (f) Battistoni, P.; Bompadre, S.; Fava, G.; Gobbi, G. Talanta 1983, 30, 15.

silica gela 100 60 40 40 40 40 60 100

anion

solventb

mmol of adsorbed HEP+X-/g of SiO2

mmol of adsorbed HEP+X-/g of HEP+X-/SiO2

ReO4ReO4ReO4NO3NO3NO3NO3NO3-

TOL TOL TOL TOL DCM CLB CLB CLB

0.261(2) 0.363(2) 0.370(2) 0.415(5) 0.18(1) 0.37d 0.35d 0.254(7)

0.214(2) 0.279(2) 0.283(2) 0.328(5) 0.16(1) 0.30d 0.28d 0.219(7)

a 40, Merck grade 10180, 40 Å (pore diameter), 750 m2/g (surface area); 60, Merck grade 60, 60 Å, 550 m2/g; 100, Merck grade 10184, 100 Å, 300 m2/g. b TOL, toluene; DCM, dichloromethane; CLB, chlorobenzene. c One estimated standard deviation in the least significant digit is shown in parentheses. d This value is for a single measurement only.

SiO2 prepared. The isotherm for 100 and HEP+ReO4- dissolved in toluene (Figure 2) shows typical Langmuir saturation behavior,31 suggesting that adsorption was complete once a monolayer of HEP+ReO4- had formed. The “footprint” of HEP+ReO4- (i.e., the area of the formula unit projected onto a surface) was estimated to be ∼200 Å2 from the dimensions of the HEP+ ion in the structure of HEP+NO3-, which was determined by X-ray crystallography (van der Waals radii were included in this calculation).30 Using this estimate, the surface coverage calculated for 0.261 mmol of adsorbed HEP+ReO4-/g of 100 is 105%, in good agreement with presumed monolayer coverage. The surface coverages for 60 and 40 loaded with HEP+ReO4- in toluene are 79 and 59%, respectively, suggesting less than monolayer coverage. It is possible that the smaller pores of these two silica gels prevent the relatively large HEP+ ions from accessing 100% of the manufacturer-specified, BET-determined surface area; since the footprint of HEP+ on the SiO2 surface is ∼10 Å × 18 Å, the effective diameter of a silica gel pore that was originally 40 Å in diameter could be less than 20 Å, or even less than 10 Å, once some HEP+ReO4- adsorbs to the pore walls. The pore size dependence of HEP+NO3- loading from chlorobenzene was very similar. The loading of HEP+X- from toluene solution onto 40 was marginally greater for X- ) NO3- (0.415(5) mmol/g) than for X) ReO4- (0.370(2) mmol/g). More significantly, the loading of HEP+NO3- onto 40 increased as the polarity and/or dielectric constant of the solvent decreased from dichloromethane (0.18(1) mmol/g) to chlorobenzene (0.37 mmol/g) to toluene (0.415(5) mmol/g), as shown in Figure 3 (here and elsewhere, (σ in the least significant digit is shown in parentheses). There are several possible reasons for the latter behavior,46 including solventdependent equilibria involving free ions, ion pairs, and ion clusters. Whatever the cause or causes of the solvent effect, ion-exchange materials with loadings listed in Table 1 were reproducibly prepared as described above for further use in this study. Distribution Coefficients. Samples of the three ion-exchange materials HEP+NO3-/100, HEP+NO3-/60, and HEP+NO3-/40 (46) Iler, R. K. The Chemistry of Silica; Wiley-Interscience: New York, 1979.

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Table 2. Distribution Coefficients (Kd′ and Kd*) for HEP+NO3-/SiO2 and Reillex-HPQ Ion-Exchange Materials ion-exchange materiala

loadingb (mmol/g)

[HNO3](aq)c (M)

[NO3-](aq)c,d (M)

(10-2)Kd′ (mL/g)

(10-2)Kd* (mL/mmol of HEP+)

Reillex-HPQe HEP+NO3-/100 HEP+NO3-/100 HEP+NO3-/60 HEP+NO3-/40 HEP+NO3-/40 HEP+NO3-/40 HEP+NO3-/40 HEP+NO3-/40 HEP+NO3-/40 HEP+NO3-/40 HEP+NO3-/40 HEP+NO3-/40 HEP+NO3-/40 HEP+NO3-/40

3.3(1) 0.219(7) 0.219(7) 0.28h 0.30h 0.16(1) 0.16(1) 0.16(1) 0.16(1) 0.16(1) 0.16(1) 0.16(1) 0.16(1) 0.16(1) 0.16(1)

1.0 1.0 1.0 1.0 1.0 1.0 0.10 0.010 0.0010 0.10 0.010 0 0 0 0

1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.1 0.01 1.0 0.10 0.010 0.0010

2.9 1.0 1.0g 1.2 1.1 0.63 2.4 3.2 3.6 ∼20 >100 3.8 ∼20 >100 >100

0.87f 4.4 4.4g 4.3 3.8 3.9 15 20 22 ∼140 >700 24 ∼140 >700 >700

a See footnote a, Table 1, for descriptions. b Millimoles of HEP+NO - per gram of HEP+NO -/SiO or millimoles of cationic sites (methylpyridium 3 3 2 or pyridinium moieties) per gram of resin; one estimated standard deviation in the least significant digit is shown in parentheses. c Composition of the aqueous samples treated with the ion-exchange materials (each sample also contained 2.0-3.0 mM KReO4). d Total NO3- concentration (sum of [HNO3](aq) and [NaNO3](aq)). e Data from ref 29. f The dimensions for this Kd′ value are milliliters per millimole of cationic site (methylpyridium or pyridinium moieties). g Column Kd′ and Kd* values. h This value is for a single measurement only.

were treated with aqueous solutions of varying pH containing known concentrations of NO3- and ReO4-. After the mixtures were shaken at 25 °C and subsequently filtered, the final aqueous concentrations of all rhenium species, [Re]f (i.e., ReO4- plus any HReO4), were determined by ICP-AES. According to the following ion-exchange equilibrium, the difference between the initial and final amounts of all aqueous rhenium species is equal to the amount of ReO4- ion-paired with HEP+ on the solid ion-exchange material:

HEP+NO3-/SiO2(s) + ReO4-(aq) h HEP+ReO4-/SiO2(s) + NO3-(aq) The distribution coefficients Kd′ and Kd* (i.e., Kd′(ReO4-) and Kd*(ReO4-)) were calculated as described in the Experimental Section and are listed in Table 2. For each experiment, the ratio (mmol of total Re)/(mmol of HEP+) was never more than 0.037. The small amount of ReO4- in the system ensured that the amount of adsorbed NO3- was essentially constant. Control experiments proved that in the absence of adsorbed HEP+NO3- neither ReO4nor HReO4 adsorbed to the silica gels to any significant (i.e., measureable) extent. A shaking/equilibration time of 1 h was chosen after a series of experiments demonstrated that the ionexchange reactions had reached equilibrium within 10 min. All Kd′ and Kd* values for HEP+NO3-/40 with a loading of 0.16(1) mmol/g were determined using a single batch of this material. The first two entries in Table 2 can be used to compare HEP+NO3-/100 with the non-redox-recyclable commercial ion-exchange resin Reillex-HPQ, which has been studied for ReO4- and TcO4extraction by Ashley and co-workers.4,29 For an aqueous waste simulant containing ReO4- and 1.0 M HNO3, Kd′ and Kd* were 100 mL/g and 440 mL/mmol of HEP+, respectively, for HEP+NO3-/100 and 290 mL/g29 and 87 mL/mmol of cationic site, respectively, for Reillex-HPQ. (In the presence of aqueous 1 M HNO3, Reillex-HPQ contains both methylpyridinium and pyridinium cationic sites; the Kd* value at pH 0 for Reillex-HPQ in 762

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Table 2 is based on 3.3(1) mmol of cationic sites/g of resin.29) The Kd* value for HEP+NO3-/100 is 5 times larger than the Kd* value for Reillex-HPQ, an indication that the selectivity for ReO4over NO3- is greater for HEP+NO3-/100 than for Reillex-HPQ. The origin of this greater selectivity is not known at the present time and is currently under investigation. The lower Kd′ value for HEP+NO3-/100 is clearly a function of the much lower loading of HEP+NO3- per gram of HEP+NO3-/ 100 (i.e., 0.219(7) mmol/g HEP+NO3-/100 vs 3.3(1) mmol of cationic sites/g of Reillex-HPQ resin). As described above, we determined that a monolayer of HEP+NO3- covers essentially all of the BET surface area of 100 in HEP+NO3-/100. Therefore, further loading of HEP+NO3- onto HEP+NO3-/100 was not a viable way to increase Kd′ for this material. For this reason, we examined two silica gels, 60 and 40, with surface areas, and hence potential loadings, substantially larger than 100. The Kd′ value (aqueous phase, 1 M HNO3) for HEP+NO3-/60 is 20% larger than for HEP+NO3-/100, in reasonable agreement with the ∼28% larger loading of HEP+NO3-/60 compared with HEP+NO3-/100. Note that Kd* is essentially unchanged for these two materials, indicating that all of the HEP+NO3- loaded onto 60 has the same selectivity as HEP+NO3- loaded onto 100. This is not the case for HEP+NO3-/40, which has a Kd* value ∼15% lower than for HEP+NO3-/100 and HEP+NO3-/60. We conclude that some adsorbed HEP+NO3- on HEP+NO3-/40 is not exposed to the aqueous phase and hence cannot participate in the ion-exchange process (at least not on the time scale of our experiments). Therefore, as far as Kd′ is concerned, HEP+NO3-/60 is the most effective ion-exchange material of the three that we prepared. However, Kd′ is not the only parameter to be optimized for an effective recyclable ion-exchange material: duty cycle time, which will be discussed in a later section, is also important. As expected, Kd′ for HEP+NO3-/40 increased as the concentration of nitrate ion in the aqueous samples decreased. For example, at pH ∼6 (i.e., NaNO3 solutions prepared with 18-MΩ water and no added HNO3; see the last four entries in Table 2), Kd′ increased from 380 mL/g at [NO3-] ) 1.0 M to >10 000 mL/g

Table 3. Recovery of ReO4- from HEP+ReO4-/SiO2 Ion-Exchange Materials by Reduction of Adsorbed HEP+ with Aqueous Fe(CN)64- a time (min) 10 20 40 60 120

(final mmol of ReO4-(aq))/(initial adsorbed mmol of ReO4-) HEP+ReO4-/40 HEP+ReO4-/60 HEP+ReO4-/100 0.74 0.84 0.86 0.90 0.96

0.87 0.94 0.97 0.98 1.0

0.98 1.0 0.99 0.97 1.0

a Samples of HEP+ReO -/SiO were treated with 25 mM aqueous 4 2 K4Fe(CN)6 (Fe/Re g 10). After filtration, [Re]f in the filtrate was determined by ICP-AES.

Figure 4. Distribution coefficient (Kd*) pH dependence for extraction of ∼0.3 mM ReO4- from aqueous solutions containing 1.0 M NO3-. For comparison, Kd* for Reillex-HPQ at pH 0.0 (ref 13) is included as the square data point.

at [NO3-] ) 0.0010 M; at pH 2, Kd′ increased from 320 mL/g at [NO3-] ) 1.0 M to >10 000 mL/g at [NO3-] ) 0.010 M; at pH 1, Kd′ increased from 170 mL/g at [NO3-] ) 1.0 M to ∼2000 mL/g at [NO3-] ) 0.10 M. The initial pH of the aqueous samples treated with HEP+NO3-/ 40 was varied from 0 to 6 while the concentration of NO3- was held constant at 1.0 M with added NaNO3. The Kd* values (Table 2) for samples with pH 0, 1, 2, 3, and 6 were 390, 1500, 2000, 2200, and 2400 mL/mmol of HEP+. These results are plotted in Figure 4 (for comparison, Kd* for Reillex-HPQ at pH 0 is also included in the plot). As the pH was increased from 0 to 6, Kd* approached a limiting value of ∼2500 mL/mmol of HEP+. The origin of this behavior is unknown and is under continuing investigation. The decrease in Kd* could be due to the increase in [H3O+] or to the decrease in [Na+] or both. It is unlikely that the decrease in pH caused a significant change in [ReO4-] according to the equation

ReO4-(aq) + H3O+(aq) h HReO4(aq) + H2O(l)

because Raman spectroscopic data47 and isopiestic vapor pressure measurements48 indicate that ReO4- is not protonated to any measurable extent in water even when [H3O+] ) 6 M. Note that other reports claiming that Ka(HReO4) ) 18 M at µ ) 0,49 ∼1 M at µ ) 5 M,50,51 and ∼0.5 M at µ ) 3 M28 are not consistent with the Raman data (Ka(HReO4) ) [ReO4-][H3O+]/[HReO4]). Reduction/Oxidation of Adsorbed HEP+/HEP. The principal advantage of a redox-recyclable anion-exchange material is that the adsorbed anions can be recovered in a small volume of aqueous secondary waste by reducing the cationic sites to neutral (47) Covington, A. K.; Freeman, J. G.; Lilley, T. H. Trans. Faraday Soc. 1969, 65, 3136. (48) Boyd, G. E. Inorg. Chem. 1978, 17, 1808. (49) Bailey, N.; Carrington, A.; Lott, K. A. K.; Symons, M. C. R. J. Chem. Soc. 1960, 290. (50) Nakashima, T.; Lieser, K. H. Radiochim. Acta 1985, 38, 203. (51) Bibler, J. P.; Wallace, R. M. Determination of the Association Quotient for Pertechnetic Acid; report DPST-85-340; Savannah River Laboratory: Aiken, SC, 1985.

sites. In order to examine this step in the potential complete cycle shown in Figure 1, samples of HEP+ReO4-/SiO2 were treated with several aqueous reducing agents. Of the three reducing agents examined, Sn2+(aq), thiourea(aq), and Fe(CN)64-(aq), only the latter reduced green HEP+ReO4-/SiO2 to yellow-orange HEP/ SiO2 on a relatively short time scale (i.e., e2 h). The reductionrecovery reaction using ferrocyanide is as follows: 4HEP+ReO4-/SiO2(s) + Fe(CN)6 (aq) f dark green

HEP/SiO2(s) + ReO4-(aq) + Fe(CN)63-(aq) yellow-orange In a series of quantitative perrhenate recovery experiments, samples of HEP+ReO4-/100, HEP+ReO4-/60, and HEP+ReO4-/ 40 were shaken at 25 °C with a 10-fold or greater excess of 25 mM aqueous K4Fe(CN)6 (pH ∼6). The mixtures were filtered and examined for total rhenium by ICP-AES. The results are listed in Table 3. The ratios listed for each time interval are the average of duplicate measurements. All three materials released g96% of their adsorbed ReO4- ions within 2 h. Near-quantitative recovery of ReO4- was observed within 10 min for HEP+ReO4-/ 100, the material with the largest pore size. Near-quantitative recovery was also observed in less than 10 min when glass columns were dry packed with HEP+ReO4-/100 and 25 mM aqueous K4Fe(CN)6 was used to elute ReO4- from the column (the flow rate was 3.5 mL/min). Control experiments demonstrated that ReO4-(aq) was not reduced to a lower-valent rhenium species by ferrocyanide under these conditions, even after several days. Rapid reactivation of HEP/SiO2 was achieved by treating the yellow-orange materials with either aqueous Ce(NH4)2(NO3)6 or aqueous Fe(NO3)3. The oxidation/reactivation reaction using Fe3+(aq) is as follows:

HEP/SiO2(s) + Fe3+(aq) + NO3-(aq) f yellow-orange 2+ HEP+NO3-/SiO2(s) + Fe (aq) dark green

For example, when either 25 mM aqueous Ce(NH4)2(NO3)6 (pH 1.3) or 25 mM aqueous Fe(NO3)3 (pH 0) was pumped through a packed column of HEP/100, complete conversion to HEP+NO3-/ Analytical Chemistry, Vol. 70, No. 4, February 15, 1998

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100 occured within 25 min. Even though Ce4+ is a much stronger oxidant than Fe3+ (E° values for the Ce4+/3+ and Fe3+/2+ couples are 1.76 and 0.77 V, respectively), the color change from yelloworange to dark green (i.e., the oxidation of HEP/100 to HEP+NO3-/100) occurred at about the same rate. Significantly, Fe3+ does not oxidize HEP under all conditions. For example, when solutions of HEP in organic solvents were shaken with acidic aqueous Fe(NO3)3, HEP(solv) was not oxidized to HEP+NO3-(solv)30 (in contrast, when identical solutions were shaken with aqueous Ce(NH4)2(NO3)6, HEP(solv) was oxidized to HEP+NO3-(solv)16). We conclude that HEP adsorbed onto silica gel has a lower HEP+/0 reduction potential than HEP dissolved in organic solvents. This may be because the hydrated surface of silica gel is a much more polar environment for HEP+ than the environment provided by organic solvents such as toluene, 2-nonanone, and dichloromethane, and the creation of a charged species requires less work in a more polar environment. The results reported here for ReO4- recovery and HEP/SiO2 reactivation are very encouraging as far as nuclear waste remediation is concerned. This new methodology should be applicable to 99TcO4- extraction and recovery using HEP/SiO2 or related redox-recyclable ion-exchange materials with little or no modification. The distinct advantage of this approach is that the 99TcO4anion would be recovered unchanged by redox deactivation of the cationic sites on the ion-exchange material (e.g., reduction of HEP+ to HEP with a mild reducing agent). In contrast, an elegant but more complicated procedure, the reduction of 99TcO4- by Sn2+(aq) to a lower-valent technetium species in the presence of solubilizing organic complexing agents, was necessary to recover technetium-99 from Reillex-HPQ loaded with 99TcO4-.52 Furthermore, it is noteworthy that Fe3+(aq) can be used instead of Ce4+(aq) to reactivate HEP/SiO2. Iron(III) salts are less expensive and less toxic and pose less of a disposal problem than cerium(IV) salts.53 Furthermore, if a way is found to regenerate and recycle the reactivation oxidant, less energy would be expended reoxidizing Fe2+(aq) to Fe3+(aq) than reoxidizing Ce3+(aq) to Ce4+(aq). Complete Extraction-Deactivation/Recovery-Reactivation Cycles. As described above, all three steps in the complete extraction-deactivation/recovery-reactivation cycle were examined independently. To test the materials under operational redoxrecyclable ion-exchange conditions, packed columns of HEP+ReO4-/100 were studied. This material was chosen to maximize loading and Kd′ and to minimize deactivation/recovery and reactivation times. The time required for one complete extraction-deactivation/recovery-reactivation cycle (i.e., the duty cycle) is an important parameter in terms of process development, and the performance of the ion-exchange material in the three steps must be considered collectively. For example, HEP+NO3-/ 60 has a larger Kd′ than HEP+NO3-/100 (see Table 2), but the recovery of ReO4- from HEP+ReO4-/60 is significantly slower than from HEP+ReO4-/100 (see Table 3). Therefore, it may be possible under operational conditions to extract and recovery more ReO4- (or more 99TcO4-) per unit time with a lower capacity but faster cycling material. (52) Schroeder, N. C. (Los Alamos National Laboratory), personal communication, 1997. (53) Arvela, P.; Kraul, H.; Stenback, F.; Pelkonen, O. Toxicology 1991, 69, 1.

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Table 4. Column Distribution Coefficients (Kd′) for Extraction of 0.3 mM ReO4- from 1.0 M HNO3 Using HEP+NO3-/100a Kd′ (mL/g of HEP+NO3-/SiO2) cycle

column 1

column 2

column 3

average

1 2 3 4 5

102 99 90 83 79

95 92 92 82 78

99 101 93 82 78

99 97 92 80 78

a Each cycle consisted of treating a packed column of HEP+NO -/ 3 100 with (i) the waste simulant to determine Kd′, (ii) 25 mM K4Fe(CN)6 + to reduce HEP NO3 /100 and recover ReO4 , and (iii) 25 mM Fe(NO3)3 in 1 M HNO3 to reoxidize HEP/100 to HEP+NO3-/100.

The first complete cycle began after HEP+ReO4-/100 was reduced to HEP/100 and reoxidized to HEP+NO3-/100. Thereafter, Kd′ was measured as described in the Experimental Section, the columns were deactivated and reactivated, as described above, and Kd′ was measured again. The complete cycles consisted of the following six steps (times for each step shown in parentheses; flow rate, 3.5 mL/min): (i) Kd′ measurement (50 min); (ii) flush with 1 M HNO3 (3 min); (iii) deactivation/recovery with Fe(CN)64(10 min); (iv) flush with 1 M HNO3 (3 min); (v) reactivation with Fe3+ (25 min); (vi) flush with 1 M HNO3 (3 min). The total duty cycle time was 94 min. Five complete cycles, giving five successive Kd′ values, were carried out for each of three packed columns. The successive Kd′ values are listed in Table 4. There was a reproducible, steady decrease in Kd′ over the five cycles, from ∼100 to ∼80 mL/g. One possible cause of this behavior is a slow degradation of HEP+ in the presence of NO3-. Ferricenium cations are known to undergo attack by nucleophilic anions.40-43 One proposed scheme, involving the reduction of 2 equiv of Fe(Cp′)2+ to Fe(Cp′)2 and the complete destruction of 1 equiv of Fe(Cp′)2+ to inorganic Fe3+, is as follows:40

3Fe(Cp′)2+ + 3X- f 2Fe(Cp′)2 + FeX3 + organic products We have recently shown that even NO3- is nucleophilic enough to cause HEP+ dissolved in organic solvents to decompose slowly in this manner.30 However, even if slow decomposition of one particular organometallic cation, HEP+, adsorbed on silica gel does occur, the data in Table 4 demonstrate that redox-recyclable anion exchange is a viable concept and that redox-recyclable ionexchange materials should be considered as viable alternatives to traditional anion-exchange resins in the future. In continuing work, we are studying adsorbents that can be used with alkaline as well as with acidic nuclear process waste (ion-exchange materials based on silica gel cannot, of course, be used with alkaline waste). We are also studying a variety of ferricenium and other oxidized organometallic cations which may be more stable in the presence of relevant nucleophilic anions such as NO3- and OH-. By analogy to the work presented here, note that ion-exchange materials based on adsorbed organometallic complexes MLn with suitable MLn0/- potentials should allow for the redox-recyclable extraction and recovery of cations such

as 90Sr2+ and 137Cs+ from nuclear process waste. Finally, it has not escaped our attention that suitable organometallic complexes adsorbed on conductive supports would be electrochemically recyclable ion-exchange materials. Such materials would be very advantageous because extraction-recovery processes would no longer require chemical reagents for redox deactivation or reactivation. ACKNOWLEDGMENT This research was supported by grants from the National Science Foundation (CHE-9308583, CHE-9628769, CTS-9423090),

the Department of Energy (DE-FG07-96ER1469), and the Oak Ridge Institute for Science and Engineering (fellowship for C.K.C.). We thank K. M. Gansle, Drs. B. A. Moyer and N. C.Schroeder, and Professors D. W. Grainger and K. R. Ashley for valuable discussions and assistance. Received for review June 2, 1997. Accepted November 4, 1997.X AC9705677 X

Abstract published in Advance ACS Abstracts, December 15, 1997.

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