Metal Ion Separations in Aqueous Biphasic ... - ACS Publications

Chapter 5

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 6, 2016 | http://pubs.acs.org Publication Date: February 11, 1999 | doi: 10.1021/bk-1999-0716.ch005

Metal Ion Separations in Aqueous Biphasic Systems and Using Aqueous Biphasic Extraction Chromatography Jonathan G. Huddleston, Scott T. Griffin, Jinhua Zhang, Heather D. Willauer, and Robin D. Rogers 1

Department of Chemistry, The University of Alabama, Tuscaloosa, AL 35487

Polyethylene glycol-based aqueous biphasic systems (ABS) and the complementary aqueous biphasic extraction chromatographic (ABEC) resins are capable of selectively removing metal ions from complex solutions, such as the radioactive Hanford tank waste supernates. These aqueous separations methods have the potential to eliminate the use of volatile organic solvents in many separations, yet their utility in more conventional solvent extraction processes has until recently received scant attention. This paper reviews the nature of ABS and ABEC separations, categorizes the types of possible metal ion separations, and discusses where these techniques may find practical application. The relationship between the liquid/liquid ABS separations and the chromatographic ABEC separations is discussed in detail. "Greening the Chemical Industry." Current thinking in industry, academe, and government acknowledges that the sustained growth, profitability, and technological development of the chemical industry may only be achieved by stressing the importance of the health and safety of employees, consumers, and the general public and by fully embracing the idea of a continuing responsibility for environmental stewardship (7). To achieve this, establishedproducts and processes may have to be reengineered and new products and processes, to be adopted at all, will have to be environmentally benign at the outset. There will be pressure to eliminate the generation of toxic waste and secondary waste products during a chemical process. In this context, separations processes will have an increasingly important role in a situation in which the costs of raw discharge and effluent treatment must inevitably rise. Corresponding author. ©1999 American Chemical Society

Bond et al.; Metal-Ion Separation and Preconcentration ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

79


80 Uniquely amongst currently available waste treatment options, separations processes offer the potential for closed processing and product recycling. However, liquid/liquid separations processes seem problematic in this context since the vast majority currently involve the use of toxic and flammable volatile organic compounds (VOCs) (2). Nevertheless, the widespread adoption, in general where distillation is not an option, of liquid/liquid extraction using VOCs testifies to its unique advantages as a unit operation in separations processing. Liquid/liquid extraction can be adapted to the selective separation of a wide variety of solutes through compatibility with a range of diluents and extractants. Extraction kinetics are usually rapid, thus enabling high throughput and large scales of operation. Additionally, with suitably designed multi-stage contactors, extractions may be optimized for high selectivity and efficiency. Frequently, the back extraction or stripping of the organic phase may be accomplished easily, either by oxidation/reduction or by manipulation of the charge state of the solute through a pH change. However, the need to utilize VOCs as diluents in these processes brings with it a number of significant disadvantages. Costs of diluents and extractants may be high and there is significant capital cost associated with the safe engineering of unit operations involving volatile and flammable solvent systems. Disposal of spent diluent and extractants will also incur significant cost and be increasingly impacted by environmental regulations. Aqueous Alternatives to Solvent Extraction. Despite their forty-year history (3), little attention has been paid by the chemical engineering community to the existence of a class of liquid/liquid extraction systems whose nature is entirely aqueous. These so-called aqueous two-phase systems or aqueous biphasic systems (ABS) are formed when certain water-soluble polymers are combined with one another or with certain inorganic salts at specific concentrations in aqueous solution (3). ABS have been shown to be effective for the liquid/liquid extraction of a wide variety of solutes including biological macromolecules and particles (5-5) and, more recently, for the extraction of metal ions and small organic molecules (6-23). The metal ion extractions may be of three types (6-9). The metal ion may be extracted into the PEG-rich phase without the addition of any extractant (10-15). The extraction may proceed through the formation of negatively charged inorganic anionic complexes (16) or by the use of a water-soluble organic complexant (17-21). The latter extraction technology heightens interest in the partitioning of small organic molecules in ABS. However, such extractions may find important applications in their own right (22,23). ABS retain all the practical advantages of traditional liquid/liquid extraction schemes, and also possess a number of unique advantages, due, in large part, to their wholly aqueous nature. ABS based on polyethylene glycol (PEG) are virtually non-toxic and the components are inexpensive bulk commodities. The physical properties of such systems are sufficiently close to those of traditional liquid/liquid extraction systems, that for the most part, common plant may be used in the engineering design of the extraction process (24). PEG/salt-based ABS are formed when aqueous solutions of high molecular weight PEG are salted-out by specific salt solutions producing two immiscible, but wholly aqueous phases. The equilibrium phase diagram of a typical aqueous two-


81 phase system based on PEG monomethylether-5000 (M-PEG-5000) and (NH^SCU is shown in Figure 1. Mixture compositions to the left of the binodal curve are monophasic whilst systems to the right form biphasic systems. Mixtures having the overall compositions Β on the tie lines ABC, form phases having the compositions indicated by the nodes A and C. It can be seen that the light phase (A) is composed primarily of PEG and the lower, heavy phase (C), primarily of (NH4) S0 . The figure also shows that systems lying on longer tie lines form phases of increasingly divergent composition. It is from this difference in the compositions of the two phases that the selectivity of the system arises. A variety of salts may be used to form ABS with PEG (25), a number of which are shown in Table I. The ability of salts to form ABS with PEG has been related (6) to their Gibbs free energy of hydration (Table II; AGhyd equivalent to AhydG i in reference 26). Both the cation and anion contribute fo this effect but the anion dominates. The more negative the AGhyd of an ion, the greater its salting-out effect for PEG. AGhyd of the salt seems to be the most important factor in determining the choice of salt used to form the biphase. The more negative the AGhyd of the salt, the lower will be the concentration of PEG and salt required to form a biphasic system. For particular combinations of anions and cations, the important factor appears to be their combined free energy of hydration which seems to be simply additive in its effect on water structure and salting-out of PEG. In cases where metal salts are present in complex matrices (10,11% such as concentrated solutions of NaOH as in the Hanford waste tanks (11% the effectiveness of the separation may be qualitatively estimated from the AGhyd of the matrix ions and that of the ions to be extracted. It has been demonstrated, for example, that chaotropic ions with small negative AGhyd, such as TeCV, partition quantitatively to the PEG-rich phase of a suitable ABS (11). In an ABS prepared by mixing equal volumes of 3.5 M (NH4) S0 and 40 % w/w PEG-2000, sodium salts having anions with AGhyd equal to about -310 kJ/mol, have a distribution value close to 1 (10). The presence of sodium halide salts having anions with -AGhyd < 310 kJ/mol (CI", Br", Γ) in this ABS, prefer the PEG-rich phase and have the effect of depressing the distribution ratio of the pertechnetate anion. Sodium salts with anions having -AGhyd greater than this (e.g., F") add to the salting-out effect and thus increase the pertechnetate distribution ratios. Sodium salts of HCO3" (AGhyd -310 kJ/mol) have a negligible effect on the distribution of Tc(V in this ABS (10).


2

4

c a

2

c

4

Overcoming the Perceived Disadvantages of ABS. Traditional solvent extraction processes often involve the back extraction of the extracted species into a fresh aqueous phase following the initial forward extraction into the organic phase (27). This is often a relatively straightforward step involving, for example, reduction or change in pH to create a charged species with enhanced solubility in the aqueous phase. Following this, spent solvent is recycled, usually after regeneration by distillation or secondary extraction (27). Currently, this appears much more difficult to achieve with ABS. Back extraction steps may be more difficult to design as molar concentrations of certain salts are required to maintain a biphasic system and recycling is hampered by difficulties associated with preparing the PEG-rich phase for recycling.



82

Ο

5

10

15

Weight Percent (NH ) S0 4

2

20

4

Figure 1. Equilibrium composition of an ABS composed of M-PEG-5000 and (NH4)2S0 showing the binodal curve bounding the points AADCC, the tie lines (ABC) connecting the nodes (A,C), and also a line (ODB) from which the System Stability (ST) may be derived (see text). 4


83

Table I. A Selection of Salts Forming ABS Univalent Divalent Anions Anions NaOH ZnS0 Na C0 KOH Alum K2CO3 RbOH Na Se0 (NH4) C0 CsOH Rb C0 Na Cr0 NaF Li S0 Na Mo0 Na(formate) Na S0 Na W0 (NH4) S0 K HP0 Rb S0 Na S0 Cs S0 Na Si0 MgS0 Na S A1 (S0 ) Na (succinate) FeS0 Na (tartrate) CuS0 1-hydroxy ethane-1,1,-diphosphonic acid. 2

2

2


3

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2

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4

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with PolyethyleneGlycol (25) Tetravalent Trivalent Anions Anions Na P0 Na Si0 K P0 Na (HEDPA) Na V0 Na (citrate) (MLO^citrate)

3

3

2

2

4

1

Table II. The Gibbs Free Energies* of a Selection of Anions and Cations Anion Cation AGhyd (kJ/mol) AGhyd (kJ/mol) -245 Cs-170 Re0 " -285 -220 Rb Γ -285 -250 BrNH4 K -305 Cl" -270 -385 Na -310 HC0 -510 -345 Li F -1825 Fe -345 OH -1880 Zn -1110 Se0 " -1920 -1120 Cu Cr0 Mg -1940 S0 -1145 -5450 S0 " -1230 Al -1280 s-1300 C0 " PO -2835 Adapted from reference 26. 4

+

+

+

+

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3

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2+

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2

2+

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4 2-

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Covalent attachment of PEG onto a solid support has overcome many of these limitations since forward extraction conditions may be achieved by addition of salt and back extraction conditions simply by adding water (9-12,28). Thus, a simple process may be envisaged in which the metal ion is extracted into the covalently bonded PEG-rich phase, under the appropriate conditions of salt, pH, etc. Recovery is achieved simply by elution with water, thus overcoming most of the limitations of ABS operation associated with the recycling of the polymer-rich phase. It will be shown in the present chapter that applications developed in ABS can be directly


84 transferred to ABEC. The distribution of solutes between the liquid phases of ABS and the liquid and solid phase of ABEC is directly comparable under identical salt solution conditions. Experimental Complete details of the experimental methods involving partitioning of pertechnetate may be found in reference 28. (NH^SCU, K C 0 , K P 0 , NaOH, and PEG were of reagent grade and obtained from Aldrich (Milwaukee, WI, USA). NHL^TcC^ was obtained from Isotope Products Laboratories (Burbank, California, USA) and was diluted with deionized water to an activity of 0.06 - 0.08 μΟ/μΙ, for use in the experiments. Ultima Gold Scintillation Cocktail (Packard Instrument Co., Downers Grove IL, USA) and a Packard Tri-Carb 1900 TR Liquid Scintillation Analyzer were used in the standard liquid scintillation assays. Polymer and salt stock solutions were prepared on a weight percent or molar basis and the compositions quoted refer to preequilibrium stock solution concentrations. Distribution ratios were determined by mixing 1 mL of a 40% w/w PEG solution with salt stock solution of known concentration. Mixtures were then vortex mixed for 2 min and centrifuged (2000 χ g) for 2 min. Mixtures were then spiked with the metal ion tracer and centrifuged (2 min, 2000 χ g) and then vortexed for 2 min. The coexisting phases were then disengaged by further centrifugation (2 min, 2000 χ g). Aliquots were removed from each phase for liquid scintillation analysis. Since equal aliquots of the phases were analyzed, the distribution of the metal ion tracer in the phases could be defined as:


2

3

3

4

^ _ Activity in counts per minute PEG - rich phase Activity in counts per minute salt -richphase For the determination of the distribution of pertechnetate to solid-phase conjugated PEG, EIChroM Iodine resin (ABEC-5000, 100-200 mesh) was used (EIChroM Industries, Darien, IL). Full details of the preparation of this resin may be found in reference 28. Salt solutions were prepared in the same way as for the ABS experiments. The distribution ratio for the metal ions onto the ABEC resin was determined as follows. 15 to 20 mg of resin was added to each of two vials and the weight recorded. 1 mL of a Tc0 -spiked salt solution was added to each adsorbent containing tube. The tubes were briefly centrifuged and mixed by magnetic stirring for 30 min. The adsorbent was separated from the supernatant by centrifugation (2000 χ g, 2 min). 100 μΕ of adsorbent free supernatant was transferred to a vial containing liquid scintillation cocktail and counted. The distribution of pertechnetate was calculated using: -

4

^

_A -A A j

w

f

f

contact volume (mL) wt. of resin (g) * dwcf

where Ai is the activity in counts per minute in the solution prior to contact with the


85

adsorbent and A/ is the activity in counts per minute after contact with the adsorbent. The contact volume is the total volume of the adsorbate containing solution used and dwcf is the dry weight conversion factor of the resin. The latter is determined from the weight of the resin dried to constant weight at 110 °C in an oven.


Results and Discussion Extraction of Metal Ions in ABS. The distribution of metal ion species within ABS is controlled by their preference for hydration in one phase or the other. The degree to which the ion behaves as a chaotrope or a chosmotrope and the difference in hydrogen bond orientation and water structure between the two phases, one rich in salt and the other rich in PEG, seem to be the dominant factors in the resulting distribution. Pertechnetate, in common with a number of similar metal anions, partitions preferentially to the PEG-rich phase because, in classical terms, it is a soft anion having large size and low charge density. It is these properties which are placed on a quantitative footing through the calculation of the free energy of hydration. The most important application to date of ABS for metal ion extraction involves the quantitative recovery of the pertechnetate anion. This is of importance since the short lived Tc (tm = 6 h) is used in the majority of medical procedures involving radioisotopes (29,30). Additionally, high levels of Tc04~ are produced as by products of nuclear fission where its long half-life and environmental mobility give rise to some concern (57). However, Tc0 " is also useful as a system probe to determine the factors of importance in controlling the distribution of metal anions in ABS, since its distribution may easily be followed by liquid scintillation counting. Thus, the effects on the relative composition of the phases, and thus on metal ion partitioning, of various ABS parameters, such as concentration and molecular mass of PEG, salt type and concentration, or the presence of matrix ions, may be determined by study of pertechnetate distribution ratios. Figure 2 shows the distribution of Tc0 " at 25 °C in six different ABS formed with PEG-2000 and M-PEG-5000 and with the salts potassium carbonate, ammonium sulfate, and sodium hydroxide. As the salt concentration is increased at a fixed concentration of PEG (40% w/w), the phase incompatibility increases as the compositions of the phases diverges from a theoretically identical composition at the critical point. Consideration of Figure 1 shows that the concentration of PEG rises rapidly in the top phase with a concomitant decrease in the concentration of the phase-forming salt. For the lower, salt-rich, phase, the reverse takes place. As a consequence, the solute (Te0 ) becomes unequally distributed between phases of increasingly divergent composition, as shown in Figure 2. In general, higher molecular weights of PEG are salted-out by a given salt at lower concentrations of PEG and salt. Also, with only a few exceptions (25), PEG of defined molecular weight is salted-out at lower concentrations of PEG and salt by salts having more negative AGhyd- Thus, DT is lower over the range of salt concentrations used for PEG-2000/(NH4) SO than for PEG-2000/K CO and D is also lower for M-PEG5000/(NH4) SO than M-PEG-5000/K CO . Additionally the M-PEG-5000 systems produce higher D s than the PEG-2000 systems. This exactly reflects the idea that 99m

99

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T C

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Tc



86

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I ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι ι I 0

1

2

3

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5

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Molar Concentration of Salt Stock Solution Figure 2. The distribution of Tc0 ' in ABS composed of M-PEG-5000 or PEG2000 and the salts K C 0 , (NH^SO* and NaOH. 4

2

3


87 the divergence in composition of the phases of the system is responsible for the unequal distribution of the solutes and that this is greater for higher molecular weights of PEG and more strongly salting-out agents at defined salt and PEG concentration. The data in the lower portion of Figure 3 shows the distribution of Tc0 " in ABS prepared from 40% w/w M-PEG-5000 to which equal volumes of M S 0 salt solutions (where M is either Na , NH4 , Rb , or Cs*) having the salt concentrations shown in the figure were added. It is apparent that the distribution ratio of Tc0 " depends upon AGhyd of the cation so that the more negative the AGhyd of the cation, the higher the distribution ratio of the Tc0 ". This is clarified in Figure 4 where the distribution ratio of Tc0 " is shown in relation to the AGhyd of the cation. Since the systems are all composed of 1.4 M M S0 , the AGhyd of the anion is constant. Thus, the distribution ratio of Tc0 " is dependent on AGhyd of the phase-forming cation, increasing with cations having increasingly negative AGhydIt must be conceded that in Figure 4, it is the experimental determination of AGhyd (as given in reference 26) which is used and not, as in the other figures the theoretically calculated value (26): This requires some justification. Two aspects of the solvation of ions are specifically accounted for in the calculation of AGhyd, the free energy of cavity formation and the free energy associated with the électrostriction of water in the hydration shell of the ion due to its electrical field. Ion dependent features included in this model of ionic hydration energy are the charge on the ion and the ionic radius, which for Rb and NFL*" are practically identical (r = 0.149 and 0.148 nm, respectively) and thus, the calculated AGhyd for these ions is the same (-285 kJ/mol). Thus, as mentioned, the experimental values (Rb = -275 kJ/mol, NKU = -285 kJ/mol) have been preferred in Figure 4. From this argument, it follows that partition in ABS should prove to be an excellent method for the estimation of AGhyd of ionic solutes which would include in the estimate all factors involved in the aqueous solvation of the solute under the conditions used, such as speciation, complex formation, and specific hydrogen bonding effects. It would thus be of some interest to compare estimates so obtained to theoretical estimates and to estimates obtained by other experiments. For instance, it is interesting to note that the free energy of hydration of H* is given as 1015 kJ/mol (26) (-1050 kJ/mol, experimental). Considering the value of -310 kJ/mol for an anion having even distribution in the PEG-2000/(NH4) SO system mentioned above, this implies a preference of the H* ion for the lower phase. In turn, this implies that the pH of the lower phase of an ABS will be less than that of the upper phase which has been found, experimentally, to be the case (52). We are currently investigating these aspects of metal ion partitioning in ABS. The effect of changing the anion may be examined analogously by the partition of Tc0 " in systems composed of different salt anions but in which the cation is held constant. This is shown in the lower part of Figure 5 where the distribution ratio of Tc0 " increases with increasingly negative free energies of hydration of the anion in the order Se0 " < S0 " < C0 . These effects of AGhyd of cation and anion may be combined to predict the distribution ratio of Tc0 " in systems composed of salts differing in both anion and cation type where the system is also composed of a defined molecular weight of 4

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M = Na' (-365kJ/mol)

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M = NH (-285kJ/moI)

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M = Rb (-275 kJ/mol) M = Cs (-250kJ/mol)

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40% w/w M-PEG-5000 1

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Molar Concentration of M2SO4 Salt Stock Solution Figure 3. The distribution (D or D ) of Tc(V in ABS composed of M-PEG5000 or to ABEC-5000 with increasing concentrations (M) of salts of the form M2SO4; where M is Na NFL^, Rb , or Cs . The Gibbs free energies of hydration indicated for the cations are experimental values from reference 26 (see text). w

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380 360 -(Gibbs Free Energy of Hydration of the ABS Cation) kJ/mol 300

320

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Figure 4. The distribution of pertechnetate (D or Dw) of TcCV in ABS composed of M-PEG-5000 or to ABEC-5000 in the presence of different sulfate salts (1.4 M) in relation to the Gibbs free energy of hydration (experimental valuesfromreference 26) of the ABS cation.


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1200

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1250 1300 1350 1250 1300 -(Gibbs Free Energy of Hydration of the ABS Anion) kJ/mol

Figure 5. The distribution of pertechnetate (D or Dw) of TcCV in ABS composed of M-PEG-5000 or to ABEC-5000 in the presence of different sulfate salts (1.4 M) in relation to the Gibbsfreeenergy of hydration (from reference 26) of the ABS anion.


91 PEG. This is illustrated in Figure 6, which shows the distribution ratio of TcCV in several different ABS. The molecular weight and concentration of PEG used to form these systems is held constant at 40% w/w M-PEG-5000, but the salt type and concentration is varied as indicated in the figure. DT is plotted against the total AGhyd of the salt solution added to form the ABS. This is simply calculated as the sum of AGhyd of the ions comprising the salt multiplied by their concentration to give the total AGhyd of the salt stock solution in kJ/L. A good correlation is observed between the total AGhyd of the stock salt solution and DiePrediction of Metal Ion Partitioning from ABS Characteristics. It seems likely that it will be possible to predict the distribution of metal ions in PEG-salt ABS simplyfromknowledge of AGhyd of the solute and the total AGhyd of the salt solution used to form the ABS. This seems to rely on the fact that AGhyd of the salt, determines the cloud point of PEG solutions and therefore the increasing divergence of the phase compositions of ABS. Since the cloud point of PEG is also dependent on the molecular weight of PEG (33), it should be possible to take this factor into account to improve the scope of the prediction to encompass systems composed of different molecular weights of PEG. It should be noted, however, that certain salts do not appear to follow the expected lyotropic sequence and thus these relationships cannot be universally applied. Examples of such salts (25) include CaCk and AICI3 which do not form ABS, certain sulfates having multivalent cations, such as magnesium and zinc, and lithium salts (when compared to similar salts of other alkali metal cations) which do not form ABS at the expected concentrations of PEG and salt. In biological application of ABS, it is usual to relate solute partitioning to the tie line length (TLL - see Figure 1) (34). TLL is simply the orthogonal sum of the difference in PEG (or salt) concentrations between the two coexisting phases, denoted by the nodes A and C, in Figure 1 and has the units w/w %.


c

2

TLL = ^A(PEG) + (hSALTf The logarithm of the distribution coefficient is usually found to be a linear function of the TLL, at least close to the critical point (35), for a defined molecular weight of PEG and a particular salt. This requires considerable expenditure of effort to determine the appropriate phase diagrams and tie line relationships such as that shown in Figure 1. Similarly, the distribution of partitioned species between the phases of an ABS may be described in terms of the difference in salt (or PEG) concentrations between the phases (Asalt or APEG). Figure 7 shows the distribution of TcCV in several PEG-2000/NaOH ABS in relation to AfNaOH] (the difference in NaOH concentration between the salt-rich and PEG-rich phases). The data shown in Figure 7 fit the relationship: log D = -0.174 + 0.67S(A[NaOH]) Tc


92

4

ΙΟ Downloaded by UNIV OF CALIFORNIA SAN DIEGO on April 6, 2016 | http://pubs.acs.org Publication Date: February 11, 1999 | doi: 10.1021/bk-1999-0716.ch005

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Figure 6. The distribution of pertechnetate (D or Dw) of TcCV in ABS composed of M - P E G - 5 0 0 0 or to A B E C - 5 0 0 0 in the presence of various concentrations of different salts (as shown) in relation to the total Gibbs free energy of hydration of the salt (calculated as 2[M ]»AG t + [R "] AGan where [ M ] and [R '] represent the concentrations of the cation and anion, and AG represents the Gibbs free energy of hydration of the ion (as calculated in reference 26). +

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93

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Figure 7. The distribution of Tc(V in relation to the salt concentration difference between the top and bottom phases of a PEG-2000/NaOH ABS.


94

More generally, the following relationship may describe the distribution of Tc0 " in PEG/salt ABS: 4

log D =a + b(Asalt)


Tc

where a and b are constants for a given salt. This suggests that the driving force for the distribution of a solute in an ABS results from the concentration difference of the phase-forming salt between the phases, or more generally,fromthe divergence of the phase compositions. Although A[salt] may be used to predict the distribution of Tc0 ' in ABS, the coefficients a and b must be determined for each salt. Additionally, a phase diagram must be constructed for each salt system employed. Other methods for predicting solute distribution in ABS have been proposed including the "system stability" (ST) (24), which measures the distance of a particular system from the origin of the phase diagram relative to the distance of the binodal curve from the origin. This method of determining the system stability is shown in Figure 1 where constructing the line OB from the origin to the overall system composition results in a point D where the line intersects the binodal curve. The ratio BD/BO is defined as the system stability. Under most practical circumstances, this measure should be strongly correlated with TLL. However, at extremes of the phase diagram, that is, at high or low volume ratio, it is likely to deviate somewhat from this relationship. The distribution of Tc0 " in relation to the system stability is shown in Figure 8 for biphasic systems formed with PEG-2000 and three different salts (NaOH, ( N H ) 2 S 0 , and KsP0 ). The system stability is closely correlated to the observed distribution coefficient of Tc0 ", despite the great differences in the relative position of the binodal curves for these PEG/salt systems. The data shown in Figure 8fitthe relationship: 4

4

4

4

4

4

\ogD = 0.725 + 3.82STTc

where ST is the system stability ratio defined above. As mentioned earlier, changing the molecular weight of PEG used to form an ABS, or changing the temperature, or the salt used, changes the relative position of the binodal curve on the phase diagram and thus the phase incompatibility and the solute distribution ratio. System stability and D j have therefore been examined in systems composed of different molecular weights of PEG and formed at different equilibrium temperatures. These systems, shown in Figure 9, were PEG2000/(NH4) SO and PEG-3400/(NH ) SO at temperatures of 25 °C and 50 °C. An almost identical relationship is observed for these systems as was observed for the systems shown in Figure 8. It appears that ST may be used to describe the distribution of Tc0 " in ABS composed of different salts and molecular weights of PEG and at different equilibrium temperatures. As a measure of the relative divergence of the phase compositions of different biphasic systems and as a predictive tool for determining the partitioning behavior of solutes, system stability is most useful where detailed knowledge of the phase c

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System Stability (ST)

Figure 9. The distribution of TcCV in relation to the System Stability (see text) for ABS composed of different molecular mass PEG fractions at different temperatures in the presence of (NEL^SC)^



97

diagram beyond a simple coexistence curve is not available. However, it is necessary to know the overall composition of the equilibrium system. The present methodology, in which ABS are prepared from stock solutions of PEG and salt, provided that two phases are formed, requires knowledge of the phase diagram but not the tie line relationships, or the overall equilibrium composition of the partitioning system. In this case, an approximate correlation of the partitioning behavior with an increase in added salt concentration may be derived from a reformulation of the binodal curve in terms of the slope of the logarithm of the PEG concentration (% w/w) versus the salt concentration (M). The intercept of this line with the molar concentration of the salt gives a parameter Mo which may be used to plot distribution data for different salt systems as M/Mo (37). It appears that the latter two parameters (ST and M/Mo) may be independent of PEG molecular weight for metal ions and small organics, that is, where excluded volume effects (38) are relatively unimportant. This correlation is currently being investigated further. Nevertheless, understanding and predicting the performance of liquid/liquid ABS in metal ion partitioning has enabled the facile application of ABEC resins to these same problems as illustrated below. Comparison of ABS and ABEC Processes for Metal Ion Extraction. Figures 3 to 6 show the distribution of TcCV between the aqueous phases of an ABS composed of M-PEG-5000 and a variety of salts and between a solid phase bearing covalently attached M-PEG-5000 in contact with similar salt containing aqueous phases. Since the water content of these materials is very high (dwcf < 0.2) the distribution values for the ABEC resin may be expected to be almost an order of magnitude higher than for the corresponding ABS. This is because the distribution value for the chromatographic system is expressed in terms of the resin dry weight unlike the ABS distribution coefficient. In addition, Figure 2 shows that DT is higher at constant salt concentration for ABS formed with M-PEG-5000 than with PEG-2000 by some smaller additional factor. Since the ABEC resin used here is derivatized with M PEG-5000, a similar increment in distribution ratio may be expected over PEG2000/salt systems. Nevertheless, Figures 3-6 show that the ABEC solid phase has a consistently higher uptake (Dw) for TcCV than would be predicted from the distribution (DT ) in the corresponding liquid/liquid ABS. The reasons for this are not clear, but may, perhaps, be related to the density of the PEG ligands on the polymeric adsorbent surface. This matter is currently under investigation. Apart from this apparent increase in the distribution ratio for the ABEC system over the corresponding ABS, the other factors of importance in determining the distribution of TcCV in ABS seem to be exactly reproduced in relation to the distribution of Tc0 " to the ABEC resin. Figure 3 shows the effect of varying the cation in ABS systems composed of M-PEG-5OOO/M2SO4 on Dx . Also shown in the figure is the Dw of TcCV in contact with the ABEC resin under identical conditions. Allowing for the previously discussed difference in magnitude of the distribution ratios, the behavior of the ABEC system closely follows the behavior of the M-PEG-5000 system in its uptake of this anion. Figure 4 shows the distribution ratio of TcCV in relation to the AGhyd of the cation for both ABEC and ABS systems for different cations present as 1.2 M c

c

4

c


98 M2SO4.

The distribution ratio of TcCV is dependent on AGhyd of the phase-forming cation, increasing with cations having increasingly negative AGhyd in both the ABEC and ABS modes of operation. This similarity in partitioning behavior between the two modes of operation, ABS and ABEC, is again evident in Figure 5. Here the cation is held constant whilst the anion is changed. Under these circumstances the distribution ratio of TcCV increases for both ABEC and ABS with increasingly negative free energies of hydration of the anion in exactly the same order SeCU " < SO4 " < CO3 ". Finally, Figure 6 shows that the total AGhyd of the stock salt solution used to form the biphase or promote the ABEC interaction may be used to predict the distribution of the Tc(V anion with both the ABEC resin and in the ABS. Here the distribution ratio for TCO4" is shown for several different ABS and for the ABEC resin. The molecular weight and concentration of PEG used to form the ABS and the amount of added ABEC is held constant whilst the type and concentration of added salt is varied as shown. There is a strong relationship between both D T and D and the total free energy of hydration, calculated as the sum of the AGhyd of the ions comprising the salt multiplied by their concentration. Not only is their a good correlation between the total AGhyd of the salt stock solution and DT and Dw, but the partitioning response is very similar in both situations. This is a useful simplification and implies the facile translation of ABS partitioning experiences to the design and operation of partitioning operations utilizing the ABEC resin. 2


2

2

c

w

c

Conclusions Aqueous biphasic systems and aqueous biphasic extraction chromatography are applicable to many metal ion separations provided thefreeenergy of hydration of the target species is appropriate. This has been demonstrated comprehensively for the pertechnetate anion. Recently, the partition of small organic molecules and industrial dye molecules has also been demonstrated (22,23). This allows the selection and design of a range of specific extractants suitable for use in ABS and ABEC paralleling those found in conventional solvent extraction processes, thus, further extending the range of solutes which may be extracted through the application of this technology. The process engineer will then be faced with a real choice in process design in the field of liquid/liquid extraction, between the traditional organic solvent-based approach and a wholly aqueous alternative. The advantages of ABEC systems over liquid/liquid ABS lie principally in the facile retention and reuse of the extracting phase. The essential similarity, if not complete identity, of the two processes has been demonstrated. However, it must be expected that there will be differences between the two operations in practice, due to differences in kinetics and mass transfer arising from the essential physical differences between a liquid phase and a porous solid support. These processes represent a wholly aqueous liquid/liquid extraction scheme for the recovery of a wide range of inorganic and organic species. As such, they have great potential for application to a wide range of separations problems, in particular, they may find application where the necessity to replace VOCs with alternative aqueous processes is a major concern.


99 Acknowledgments This work is supported by the National Science Foundation (Grant CTS-9522159) and the Division of Chemical Sciences, Office of Basic Energy Sciences, Office of Energy Research, U.S. Department of Energy (Grant No. DE-FG02-96ER14673). Literature Cited


(1)

Technology Vision 2020, The U.S. Chemical Industry; American Chemical Society, American Institute of Chemical Engineers, The Chemical Manufacturers Association, The Council for Chemical Research, and The Synthetic Organic Chemical Manufacturers Association: Washington, DC, 1996. (2) Sekine, T.; Hasegawa, Y. Solvent Extraction Chemistry, Fundamentals and Applications; Marcel Dekker: New York, 1977. (3) Albertsson, P.-Å., Nature 1958, 182, 709. (4) Partitioning in Aqueous Two-Phase Systems; Walter, H.; Brooks, D. E.; Fisher, D., Eds.; Academic Press: Orlando, FL, 1985. (5) Aqueous Two-Phase Systems; Walter, H.; Johansson, G., Eds.; In Methods in Enzymology; Abelson, J. N.; Simon, M. I., Eds.; Academic Press: San Diego, CA 1994; Vol. 228. (6) Rogers, R. D.; Bond, A. H.; Bauer, C. B.; Zhang, J.; Griffin, S. T. J. Chromatogr., B:Biomed.Appl.1996, 680, 221. (7) Rogers, R. D.; Bond, A. H.; Bauer, C. B. Sep. Sci. Technol. 1993, 28, 101. (8) Rogers, R. D.; Bond, A. H.; Bauer, C. B.; Zhang, J.; Jezl, M. L.; Roden D. M.; Rein, S. D.; Chomko, R. R. In Aqueous Biphasic Separations: Biomolecules to Metal Ions; Rogers, R. D.; Eiteman Μ. Α., Eds.; Plenum: New York, 1995; p. 1. (9) Rogers, R. D.; Zhang, J. In Ion Exchange and Solvent Extraction; Marinsky, J. Α.; Marcus, Y., Eds.; Marcel Dekker: New York, 1997, Vol. 13; p. 141. (10) Rogers, R. D.; Zhang, J.; Griffin, S. T. Sep. Sci. Technol. 1997, 32, 699. (11) Rogers, R. D.; Bond, A. H.; Bauer, C. B.; Zhang, J.; Rein, S. D.; Chomko, R. R.; Roden, D. M. Solvent Extr. Ion. Exch. 1995, 13, 689. (12) Rogers, R. D.; Zhang, J.; Bond, A. H.; Bauer, C. B.; Jezl, M. L.; Roden, D. M. Solvent Extr. IonExch.1995, 13, 665. (13) Rogers, R. D.; Bond, A. H.; Zhang, J.; Bauer, C. B. Appl.Radiat.Isot. 1996, 47, 497. (14) Rogers, R. D.; Zhang, J. J. Chromatogr., B: Biomed.Appl.1996, 680, 231. (15) Rogers, R. D.; Bond, A. H.; Zhang, J.; Horwitz, E. P. Sep. Sci. Technol. 1997, 32, 867. (16) Rogers, R. D.; Bond, A. H.; Bauer, C. B. In Solvent Extraction in the Process Industries, Proceedings of ISEC '93; Logsdail D. H.; Slater, M . J., Eds.; Elsevier: London, 1993, Vol. 3; p. 1641. (17) Rogers, R. D.; Bond, A. H.; Bauer, C. B. Sep. Sci.Technol.1993, 28, 139. (18) Rogers, R. D.; Bauer, C. B.; Bond, A. H. J. Alloys Compd. 1994, 213/214, 305. (19) Rogers, R. D.; Bond, A. H.; Bauer, C. B. PureAppl.Chem. 1993, 65, 567.



100 (20) Rogers, R. D.; Bauer, C. B.; Bond, Α. Η. Sep. Sci. Technol. 1995, 30, 1203. (21) Rogers, R. D.; Bauer, C. B. J. Chromatogr., B: Biomed.Appl.1996, 680, 237. (22) Rogers, R, D.; Willauer, H. D.; Griffin, S. D.; Huddleston, J. G. J. Chromatogr. Β 1997, (in press). (23) Huddleston, J. G.; Willauer, H. D.; Boaz, K.; Rogers, R. D. J. Chromatogr. Β 1997, (in press). (24) Hustedt, H.; Kroner, K. H.; Kula, M.-R. In Partitioning in Aqueous Two-Phase Systems; Walter, H.; Brooks, D. E.; Fisher, D., Eds.; Academic Press: Orlando, FL, 1985; p. 529. (25) Ananthapadmanabhan, K. P.; Goddard, E. D. Langmuir 1987, 3, 25. (26) Marcus, Y. J. Chem. Soc., Faraday Trans. 1991, 87, 2995. (27) Robbins, L. A. In Handbook of Separation Techniques for Chemical Engineers; Schweitzer, P. Α., Ed.; McGraw-Hill: New York, 1996, 3rd Edition; p. 1-419. (28) Huddleston, J. G.; Griffin, S. T.; Zhang, J.; Willauer, H. D.; Rogers, R. D. In Aqueous Two-Phase Systems; Kaul, R., Ed.; Methods in Biotechnology; Walker, J. M., Ed.; Humana Press: Totowa, NJ, 1997; (in press). (29) Steigman, J.; Eckelman, W. C. The Chemistry of Technetium in Medicine; National Academy Press: Washington, DC, 1992. (30) Boyd, R. E. Radiochimica Acta 1982, 30, 123. (31) Jones, C. J. In Comprehensive Coordination Chemistry; Wilkinson, G.; Gillard, R. D.; McCleverty, J. Α., Eds.; Springer-Verlag: Berlin, 1987 Vol. 6; p. 881. (32) Eiteman, Μ. Α.; Gainer, J. L. Chem. Eng. Comm. 1991, 105, 171. (33) Bamberger, S. B.; Brooks, D. E.; Sharp, Κ. Α.; Van Alstine, J. M.; Webber, T. J. In Partitioning in Aqueous Two-Phase Systems. Theory, Methods, Uses, and Applications to Biotechnology; Walter, H.; Brooks, D. E.; Fisher, D., Eds.; Academic Press: Orlando, FL, 1985, p. 85. (34) Huddleston, J. G.; Lyddiatt, A. Appl. Biochem. Biotechnol. 1990, 26, 249. (35) de Belval, S.; le Breton, B.; Huddleston, J.; Lyddiatt, A. J. Chromatogr.B 1997, (in press). (36) Asenjo, J. Α.; Turner, R. E.; Mistry, S. L.; Kaul, A. J. Chromatogr., A. 1994, 668, 129. (37) Huddleston, J. G.; Rogers, R. D. 1997, (unpublished results). (38) Abbott, N. L.; Blankschtein, D.; Hatton, T. A. Bioseparation 1990,1,191.


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