Environ. Sci. Technol. 2002, 36, 1861-1867
Selective Separation of Hydroxide from Alkaline Nuclear Tank Waste by Liquid-Liquid Extraction with Weak Hydroxy Acids C. KEVIN CHAMBLISS,† TAMARA J. HAVERLOCK, PETER V. BONNESEN, NANCY L. ENGLE, AND BRUCE A. MOYER* Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6119
Recovery and recycle of caustic reagents in industrial processes offer potential means of pollution prevention, as investigated herein for particular needs related to the cleanup of alkaline nuclear waste. Specifically, the recovery of hydroxide from alkaline media by liquid-liquid extraction can be effected utilizing weak hydroxy acids, as demonstrated for NaOH utilizing a series of lipophilic fluorinated alcohols and alkylated phenols dissolved in 1-octanol. Extraction efficiency follows the expected order of acidity of the hydroxy acids, the phenols being the most efficient extractants among the compounds tested. After extraction, NaOH is effectively recoverable from the organic phase upon contact with water. The weakest hydroxy acids are the most efficiently stripped, NaOH recovery being nearly quantitative in a single contact. In competitive extraction experiments, good selectivity for hydroxide recovery over other anions such as nitrate and chloride was demonstrated. Since the order of extraction favors larger anions, the exceptional preference for hydroxide implies that the extraction occurs by deprotonation of the hydroxy acids in a cation-exchange process. Stripping therefore occurs by hydrolysis to regenerate the neutral hydroxy acid, liberating NaOH to the aqueous phase. Since hydroxide equivalents rather than actual hydroxide ions are transferred to the solvent, the process is termed “pseudohydroxide extraction.” Hydroxide recovery from a simulant of alkaline nuclear tank waste (Hanford DSSF simulant) was also demonstrated in repeated extraction and stripping cycles.
Introduction The selective separation of hydroxide ion from an aqueous solution containing various anions is a problem of both fundamental and practical importance in the field of separation science. Anion selectivity in phase-transfer processes typically follows the Hofmeister series (1), with an overall size bias favoring the partitioning of large, weakly hydrated anions to a nonaqueous phase. A more quantitative assessment of anion selectivity is provided by comparing values of * Corresponding author telephone: (865)574-6718; fax: (865)5744939; e-mail:
[email protected]. † Present address: Department of Chemistry and Biochemistry, Baylor University, Waco, TX. 10.1021/es011124u CCC: $22.00 Published on Web 03/08/2002
2002 American Chemical Society
the standard molar Gibbs energy of transfer (∆G°tr) from water to a particular solvent (2). The values of ∆G°tr for the transfer of F-, Cl-, and NO3- from water to 1,2-dichloroethane (DCE) are respectively 65, 52, and 32 kJ mol-1 (3). Thus, the transfer of these ions from water to DCE is highly unfavorable overall but becomes less so with increasing anion size. Moreover, the additional driving force needed for extraction of the small anion F- in competition with NO3- is sizable. Interestingly, no ∆G°tr data have been reported for hydroxide ion in a current comprehensive tabulation (3). However, hydroxide ion has a comparable thermochemical radius to that of the small fluoride ion (both given as 0.133 nm) (3) and is expected to behave similarly (4, 5). Therefore, virtually all anions are expected to be more extractable than hydroxide, and the selective and efficient separation of hydroxide accordingly poses a challenging scientific problem. From the environmental perspective, the ability to separate hydroxide from aqueous mixtures represents value in the area of pollution prevention. Sodium hydroxide and other caustic chemicals are used in large quantities in processes such as those found in the paper, aluminum, and nuclear industries. Inasmuch as such processes create large caustic waste streams destined for disposal, economical methods for the recovery or recycle of bulk caustic reagents can enable significant reduction in waste volume and environmental risk. Our own research has been motivated by needs of the U.S. Department of Energy (U.S. DOE) regarding the treatment and disposal of highly radioactive alkaline “tank wastes” stored at various U.S. DOE sites (69). Excluding water, the bulk composition of the alkaline tank wastes is dominated by sodium hydroxide, nitrate, nitrite, and aluminate salts; the radionuclide content represents but a tiny fraction ( 3b > 3c > 3d (30a). Comparison of the data for compounds 4a-c illustrates “fine-tuning” of acidity within a single class of hydroxy acids by variation of the electron-withdrawing ability of a substituent attached to a particular position. In compounds 4ac, the substituent -X in the core molecule p-CH3-C6H4C(CF3)(X)-OH was respectively varied as -CH3, -H, and -CF3. The electron-withdrawing effect increases in the order -CH3 < -H < -CF3, and accordingly, the resultant acidity of the alcohol as reflected by the amount of sodium extracted into the solvent phase ([Na]extr, Table 2) increases in that order (0.18 < 0.29 < 0.40). Not surprisingly, the stripping efficiency for these compounds (and also phenols 3a-d) follows the VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Compound Key for Hydroxy Acid Extractants
opposite trend of the extraction efficiency. In that the extraction efficiency of 4b and 4c is impressive, they are considered promising for practical applications, though the methyl group attached to the 4-position of the aromatic ring should probably be replaced by more bulky alkyl groups for increased lipophilicity (see below). Additional extractant design principles may be deduced by comparing the observed extraction efficiencies for linear and aryl ether alcohol extractants. For example, the importance of the proximity of electron-withdrawing fluorine atoms in relation to the hydroxy proton in the structure of linear alcohol extractants can be evaluated by comparing data for fluorononanols 1a-c. For this series, where only one methylene group separates the fluoroalkyl and the hydroxyl moieties, the equilibrium concentration of sodium in the organic phase following extraction from 7 M NaOH is 1864
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essentially the same (0.29 M). This observation suggests that substitution of hydrogen for fluorine at the terminal carbon atom (i.e., HF2C- vs F3C-) has negligible effect on the extraction behavior of this class of compounds. In contrast, fluorononanol 1c, with three methylene groups separating the fluoroalkyl and hydroxyl moieties, displays greatly reduced sodium extraction strength (0.058 M sodium in the organic phase). Similarly, the necessity of a strongly electron-withdrawing -CF3 group attached to the R-carbon (with respect to the -OH group) of aryl ether alcohol extractants is realized by comparing extraction data for 2a and 2b. The observed concentration of sodium in the organic phase is 0.28 M when compound 2a is employed for extraction from 7 M NaOH. Not surprisingly, the structurally similar 2b, which does not contain a -CF3 group, was not very effective. The extraction
FIGURE 2. Equilibrium isotherms for the extraction of sodium salts by hydroxy acids. The aqueous phase contains either NaOH (A-C) or NaNO3 (D). Reference lines indicate slopes (m) of 1 or 2. Conditions: 0.2 M hydroxy acid in 1-octanol, 1:1 volume ratio, 25 °C.
TABLE 2. Extraction and Stripping Results for Recovery of Hydroxide Ion from Aqueous 7.0 M NaOHa compd
[Na]extrb (M)
[Na]stripc (M)
% strip
[base]stripd (M)
[base]strip/ [Na]strip
1a 1b 1c 2a 2b 3a 3b 3c 3d 4a 4b 4c 1-octanol
0.29 0.29 0.058 0.28 0.087 0.31 0.29 0.22 0.049 0.18 0.29 0.40 0.050
0.29 0.28 0.058 0.28 0.087 0.26 0.26 0.22 0.048 0.18 0.29 0.39 0.050
100 97 100 100 100 82 91 100 98 100 100 96 100
0.30 0.30 0.056 0.30 0.087 0.26 nde nd 0.056 0.19 0.28 0.36 0.056
1.0 1.1 0.97 1.1 1.0 1.0 nd nd 1.2 1.1 0.97 0.92 1.1
a See text for experimental details. b The equilibrium concentration of sodium in the organic phase following extraction. The concentration of fluorinated alcohol or alkylated phenol in 1-octanol was 0.2 M; O/A ratio ) 1. c The equilibrium concentration of sodium in the aqueous strip phase; O/A ratio ) 1. d The equilibrium concentration of titratable base in the aqueous strip phase. e nd, not determined.
strength of 2a is similar to that observed for 1a and 1b above, suggesting that the effect of an ether linkage and a single -CF3 group in proximity to the -OH functionality is comparable to that of multiple fluorination starting at the β-carbon on linear fluorinated alcohols. The data presented for 3a may be used to compare the new fluorinated extractant 2a with a structurally similar phenol. While solvents containing 3a are impressive with
respect to sodium extraction, only 82% of the extracted sodium is recovered as the hydroxide salt in a single contact, indicating that the overall efficiency for hydroxide recovery is somewhat diminished as compared to tested fluorinated extractants. It may therefore be concluded that 3a is approaching an upper limit of acidity in terms of efficient recovery of sodium hydroxide. As was demonstrated with phenols 3b-d, the effective acidity of 3a could be decreased by moving the bulky alkyl substituents on the aromatic ring closer to the hydroxyl group (30). However, none of the phenolic species investigated possessed as good a combined extraction and stripping performance as the better-performing fluoro alcohols 1a, 1b, 2a, and 4a-c. To evaluate extraction selectivity for hydroxide under competitive conditions, the extractants 1a, 2a, and 3a at 0.2 M in 1-octanol were equilibrated with aqueous solutions containing high concentrations of competitive anions. Experiments employed solutions containing 1.75 M NaOH (typical of alkaline tank waste) plus either 3.5 M NaCl or 3.5 M NaNO3. A third aqueous solution was a simulant of a Hanford tank waste referred to as AW-101 Double-Shell Slurry Feed (DSSF) (21). As shown in Table 3, the selectivity for hydroxide separation was very good over other anions. Extraction from the DSSF waste simulant gave very high selectivities over Al(OH)4- ion with selectivity factors ranging from 280 (1a) to 650 (2a). Selectivity factors exceeded 20 for the key anion NO3-. Since Figure 2 shows that the nitrate extraction is largely due to the background salt extraction by 1-octanol, it may be expected that anion selectivities will improve with use of higher, more practical concentrations of extractant. VOL. 36, NO. 8, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3. Equilibrium Organic-Phase Concentrations of Extracted Ions and Hydroxide Selectivity Factorsa compd
[Na+]orgb (M)
Aqueous Phase: 1.75 M NaOH, 3.5 M NaCl [OH-]org,effc (M)
[Cl-]orgd (M)
rOH-/Cl-e
1a 2a 3a
8.3 × 10-2 9.9 × 10-2 1.8 × 10-1
8.6 × 10-2 1.2 × 10-1 2.0 × 10-1
6.4 × 10-3 6.9 × 10-3 1.2 × 10-2
28 36 37
compd
[Na+]orgb (M)
1a 2a 3a
8.6 × 10-2 1.1 × 10-1 1.8 × 10-1
Aqueous Phase: 1.75 M NaOH, 3.5 M NaNO3 [OH-]org,effc (M) [NO3-]orgd (M)
compd
[Na+]orgb (M)
[K+]orgb (M)
[Al3+]orgb (M)
1a 2a 3a
1.2 × 10-1 1.5 × 10-1 1.7 × 10-1
4.6 × 10-3 6.3 × 10-3 7.1 × 10-3
2.0 × 10-4 1.0 × 10-4 2.0 × 10-4
9.0 × 10-2 1.0 × 10-1 1.9 × 10-1
8.5 × 10-3 9.3 × 10-3 1.6 × 10-2
Aqueous Phase: DSSF Simulant [OH-]org,effc [Cl-]orgd [NO3-]orgd (M) (M) (M) 1.3 × 10-1 1.5 × 10-1 2.0 × 10-1
3.0 × 10-3 3.0 × 10-3 4.2 × 10-3
rOH-/NO3-e
7.8 × 10-3 7.5 × 10-3 1.2 × 10-2
22 24 26
re Na+/K+
OH-/Al(OH)4-
OH-/Cl-
OH-/NO3-
3.5 3.2 3.3
280 650 460
2.5 3.0 2.9
35 42 38
a See text for experimental details. b Determined by ICAP-AES. c Determined by titration with standardized HCl. These values represent effective hydroxide equivalents extracted and are not meant to imply actual extraction of hydroxide ion into the organic phase. d Determined by ion chromatography. e The selectivity factor, defined as the ratio of distribution ratios for two ions (e.g., ROH-/Cl- ) DOH-/DCl-).
TABLE 4. Results for Multiple Cycles Using DSSF Simulant cycle
Solvent: 1-Octanol Containing 0.2 M 1a DNa [Na]extr (M) [Na]strip1 (M)
1 2 3 4
cycle 1 2 3 4
0.018 0.018 0.018 0.018
1 2 3 4
0.12 0.12 0.12 0.12
Solvent: 1-Octanol Containing 0.2 M 3a DNa [Na]extr (M) [Na]strip1 (M) [Na]strip2 (M) 0.031 0.031 0.032 0.030
cycle
0.12 0.12 0.12 0.12
0.21 0.21 0.21 0.21
0.16 0.16 0.16 0.16
0.051 0.052 0.047 0.038
Solvent: 1-Octanol Containing 0.2 M 4c DNa [Na]extr (M) [Na]strip1 (M) [Na]strip2 (M) 0.038 0.016 0.0094 0.0063
0.26 0.11 0.06 0.04
0.252 0.110 0.066 nd
0.007 0.001 0.001 nd
To demonstrate recyclability of the extractants, experiments were performed using solvent systems 1a, 3a, and 4c at 0.2 M in 1-octanol and equilibrated with the DSSF waste simulant containing 22Na tracer. Table 4 shows that both 1a and 3a solvent systems perform consistently through four cycles, although recovery of the hydroxide from the phenol (3a) required a second strip that was not needed with 1a. The constancy of the sodium distribution ratio implies complete regeneration of the solvent in the stripping steps. It also implies negligible loss of these hydroxy acids to the aqueous phases. In this regard, results for 4c show significant loss of extraction capacity on recycle, which strongly suggests insufficient lipophilicity. Further work on the lipophilicity issue is in progress. Although a detailed examination of the equilibria and organic-phase speciation is beyond the scope of this survey paper, the data obtained support an interpretation based on cation exchange (eq 2). 1-Octanol alone extracts alkali metal salts by simple ion-solvation principles (29, 31, 32), as may be seen from Figure 2 for sodium nitrate. Equilibrium 1866
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behavior is generally understood in terms of the solvated cation and anion in the organic phase, either completely dissociated or ion-paired according to the dielectric constant of the solvent and the ionic strength (29). The question arises as to whether the added hydroxy acids simply enhance this mechanism for sodium hydroxide extraction, since the greater acidity of these extractants would provide greater ability to solvate small anions (2, 4, 29). To explain the selectivity for hydroxide over nitrate in this way, one must postulate an anti-Hofmeister selectivity (1, 2), that is, one that becomes increasingly favorable with increasing anion charge density. However, the data in Table 3 clearly show that nitrate is preferred over chloride, implying a normal Hofmeister-type selectivity. Thus, the apparent selectivity for hydroxide is best interpreted as arising from cation exchange, a process that is pH driven and thus uniquely applicable to strongly basic anions. Spectroscopic experiments are in progress to address this issue more directly. The slope behavior observed for NaOH extraction in Figure 2 is consistent with the postulated process shown in eq 2. Since the aqueous sodium and hydroxide concentrations are equal in the experiment, the expression for the equilibrium constant may be expanded as follows:
log [Na+]org )
2log [Na+]aq + log [HL]org + log K + log ζ (3)
where K is the equilibrium constant and ζ is the ratio of molar activity coefficients gHL(g()2/gNaL. The expression predicts a slope of 2 for a condition far from saturation of the extractant. For 3a and 4c, the slopes indeed tend to 1.9 at the lowest concentrations after a minor (7%) correction for aqueous activity coefficients (33). For the other, weaker extractants, the background extraction of 1-octanol becomes important, and the slopes lie between 2 and 1 (the corresponding slope for 1-octanol alone). Also, as the organicphase ionic strength decreases, the ion pairs shown in eq 2 may dissociate, in which case the expansion of the equilibrium expression predicts a unit slope. It should be pointed out that, although the agreement of the model with the data is encouraging, it is equally consistent with an ion-pair extraction model. As discussed above, the evidence for cation exchange lies in the selectivity for hydroxide.
Reference to Figure 2 and Table 2 reveals a saturation effect, but one that is superstoichiometric. Namely, Table 2 shows that the organic-phase concentration of sodium upon extraction from NaOH solutions reaches concentrations greater than what can be accounted for by a 1:1 extraction of sodium (eq 1) and the background extraction of 1-octanol (0.050 M). The origin of this apparent synergism is presently unknown and is under continuing investigation. It may be due to polarization of 1-octanol or organic-phase water molecules by extracted Na+ ions, the same mechanism thought to account for the decreased auto-ionization constant for water in the presence of sodium salts (33). The synergism appears to be somewhat higher for the fluoro alcohols, as 1a and 1b performed similarly to 3b on extraction, yet phenol 3b is expected to be 1.5-2 pKa units more acidic (17, 18, 30). In summary, lipophilic fluoro alcohols and phenols can be used effectively for selective separation of NaOH from a variety of caustic waste streams. Factors influencing the efficiency of extraction and stripping include electronwithdrawing effects of fluorine-containing substituents and steric effects. Despite the higher cost, the fluoro alcohols may offer increased alkaline stability, as alkylphenols can degrade, most commonly by oxidation, when in prolonged contact with caustic solutions (34). Further research both to investigate the underlying extraction mechanism and to develop a practical process for the recovery of hydroxide from caustic media is in progress.
Acknowledgments This research was sponsored by the Environmental Management Science Program, Offices of Science and Environmental Management, U.S. Department of Energy, under Contract DE-AC05-00OR22725 with Oak Ridge National Laboratory, managed and operated by UT-Battelle, LLC. The participation of C.K.C. was made possible by an appointment to the Oak Ridge National Laboratory Postgraduate Program administered by the Oak Ridge Associated Universities. We thank Mark Klingshirn for experimental assistance with the ion chromatography studies.
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Received for review July 10, 2001. Revised manuscript received January 11, 2002. Accepted January 31, 2002. ES011124U
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