Dual Nature of Polyethylene Glycol-Based Aqueous Biphasic

Sep 4, 2008 - Center for Green Manufacturing, Department of Chemistry, and ... The Queen's UniVersity of Belfast, Belfast, BT9 5AG, Northern Ireland, ...
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Ind. Eng. Chem. Res. 2008, 47, 7390–7396

Dual Nature of Polyethylene Glycol-Based Aqueous Biphasic Extraction Chromatographic (ABEC) Resins: Uptakes of Perchlorate versus Mercury(II) Meghna Dilip,†,# Scott T. Griffin,†,§ Scott K. Spear,‡ Christiaan Rijksen,| He´ctor Rodrı´guez,| and Robin D. Rogers*,†,| Center for Green Manufacturing, Department of Chemistry, and Alabama Institute for Manufacturing Excellence, The UniVersity of Alabama, Tuscaloosa, Alabama 35487, and QUILL, School of Chemistry and Chemical Engineering, The Queen’s UniVersity of Belfast, Belfast, BT9 5AG, Northern Ireland, U.K.

Polyethylene glycol (PEG)-grafted aqueous biphasic extraction chromatographic (ABEC) resins have been shown to remove both mercury(II) cations (Hg2+) or perchlorate anions (ClO4-) from aqueous media under different conditions. Water destructuring (chaotropic) anions such as perchlorate (∆Ghyd ) -214 kJ/mol) partitioned to ABEC resin in the presence of water structuring (kosmotropic) anions, with increased affinity for the resin as the kosmotropic nature of the anion increased (Cl- < SO42- < PO43-), and could be easily stripped using water. However, while the partitioning of ClO4- to the resin was possible only in the presence of a kosmotropic salt; Hg2+ was retained from the resin only in the absence of such salts (i.e., from pure water) and stripped from the resin when eluted with kosmotropic salt solutions. Here, the different mechanisms for uptake of these ions by an ABEC resin are explored. 1. Introduction Aqueous biphasic systems (ABS) are two-phase systems that can be formed above a critical concentration or temperature by mixing aqueous solutions of (a) two polymers (polymer/polymer ABS), for example, polyethylene glycol (PEG) and dextran,1,2 (b) a polymer with a water structuring, kosmotropic salt (polymer/salt ABS), for example, PEG and K3PO4,3,4 or (c) a chaotropic, water destructuring salt, such as an ionic liquid, and a kosmotropic salt (salt/salt ABS), for example, 1-butyl-3methylimidazolium chloride and K3PO4.5,6 Both phases in an ABS are essentially made up of water, which is especially attractive for large scale industrial separations. Therefore, ABS systems are generally regarded as environmentally benign,7 offering an alternative to volatile organic compounds currently being used in liquid/liquid (l/l) separations. The relative ability of various salts to separate amphiphilic molecules from aqueous salt solution depends on such factors as ionic charge, hydration radius, ability to structure water, and specific interactions between the molecule and salt.3 These socalled salting-out effects can also be related to the ion’s lyotropic number or position in the Hofmeister series, which can be used as a measure of relative hydrophobicity of an ion.8 Ananthapadmanabhan and Goddard showed that the lower the lyotropic number of the anion, the more effective it was in the formation of an ABS.4 Berggren et al. demonstrated that the partitioning of a salt to the relatively more hydrophobic upper phase in a PEG/dextran system was enhanced with an increase in hydrophobicity of the anion or cation in accordance with the Hofmeister series.9 * To whom correspondence should be addressed. Phone: +1-205348-4323. Fax: +1-205-348-0823. E-mail: [email protected]. † Center for Green Manufacturing, Department of Chemistry, The University of Alabama. ‡ Alabama Institute for Manufacturing Excellence, The University of Alabama. | The Queen’s University of Belfast, Belfast, BT9 5AG, Northern Ireland, U.K. § Current address: Cytec Industries, Inc., 1937 West Main Street, Stamford, CT 06904. # Current address: Department of Chemistry, Worcester State College, Worcester, MA 01602.

A correlation has also been made between the Gibbs free energy of hydration (∆Ghyd) of the ions and their salting-out ability.10 The more negative the free energy of hydration of a particular anion (i.e., more kosmotropic, or water structuring), the greater the salting-out ability. Thus in a PEG/salt ABS, the PO43- anion, with a ∆Ghyd of -2773 kJ/mol, is a better saltingout agent than the SO42- anion with ∆Ghyd of -1090 kJ/mol.11,12 On the other hand, ions with small negative or positive values of ∆Ghyd (more chaotropic or water destructuring) exhibit a salting-in ability. For example, the previously investigated chaotropic anions perrhenate (∆Ghyd ) -234 kJ/mol)13 and pertechnetate (∆Ghyd ) -251 kJ/mol)12 partition to the polymerrich phase in a PEG/salt ABS.14 Table 1 presents the ∆Ghyd of selected anions and cations that are mentioned in this work. An ABS may be conveniently characterized using a phase diagram constructed from the component compositions of polymer, salt, and water required to induce phase separation. A typical phase diagram is shown in Figure 1. The binodal curve indicated in the figure marks the boundary between the monophasic region and the biphasic region.15 The compositions above the binodal curve are heterogeneous, whereas for system compositions below the binodal curve a single homogeneous phase exists. Tie-lines connect the points on the binodal corresponding to the compositions of the phases in equilibrium. Along a tie-line, only the relative volume of the phases (Vr) changes, and this may be exploited toward the recovery of products while using ABS extraction protocols. The tie-line length (TLL) can be used as a measure of the phase divergence, and is calculated by elemental trigonometry. The phase divergence has been shown to be proportional to the chemical Table 1. ∆Ghyd of Selected Anions and Cations in Order of Their Relative Chaotropicity/Kosmotropicity12 anions 3-

PO4 SO42ClTcO4ReO4ClO4-

∆Ghyd (kJ/mol)

cations

∆Ghyd (kJ/mol)

-2773 -1090 -347 -251 -23413,14 -214

2+

-1992 -1766 -1763 -375 -304 -292

10.1021/ie800841j CCC: $40.75  2008 American Chemical Society Published on Web 09/04/2008

Ni Hg2+ Cd2+ Na+ K+ NH4+

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Figure 1. Typical phase diagram of an ABS composed of PEG and (NH4)2SO4 showing a binodal curve (solid line, 9), some illustrative tielines (dashed lines,b), and generic volume ratios Vr (calculated as the quotient of the salt-rich phase over the polymer-rich phase) of phases along a tie-line (adapted from ref 27).

potential difference between the phases and solute preference for one phase over the other increases with phase divergence.16 Despite the advantages over traditional organic/water separations, ABS have not found wider acceptance. Some of the probable reasons include that the stripping of partitioned solutes in l/l mode is difficult, and considerable loss of components occurs because of their mutual presence in each phase. In addition, when compared to traditional organic/water systems, the separation of the phases is slow in an ABS, making the use of these systems more time-consuming.17 With the above listed disadvantages in mind, the aqueous biphasic extraction chromatographic (ABEC) resin was developed to transform the liquid/liquid (l/l) polymer/salt ABS into a solid/liquid (s/l) affinity separation system by anchoring PEG to a solid support, such as divinylbenzene cross-linked polystyrene copolymer (Merrifield’s resin).18 The major advantages of the ABEC resin are the uncomplicated loading of the solute from a salt solution, followed by stripping the partitioned solute by simple elution with water.17 While phase diagrams provide an effective method for characterizing l/l ABS systems, they cannot be constructed for analogous s/l ABEC resins. Although the amount of PEG within the resin may be determined, the amount of PEG actually taking part in phase separation cannot be deduced directly. Nonetheless, the similarities between ABEC resins and ABS have been demonstrated, and the resins have been successfully used for the uptake of several anions and metal cations.19-24 ABEC resins and PEG/salt ABS have been previously shown to have several features in common: 1. Solutes that partition to the PEG-rich phase in a PEG/salt ABS also partition onto the ABEC resin from salt solutions of high enough concentration.20,21,25 2. The ABS partitioning and ABEC uptake of these solutes increases with the concentration of the phase-forming salt.20,25 3. The relative uptake of solutes in both systems increases according to the relative ability of the kosmotropic salt used to salt-out PEG. The more negative the free energy of hydration of a particular anion (more kosmotropic or water structuring), the greater the salting-out ability.20,25 4. In both cases, for a given set of conditions, the uptakes increase as the molecular weight of the PEG increases.24 On the other hand, important differences between ABS and ABEC systems have been observed:

1. Raising the temperature of a PEG-based ABS leads to increased solute partitioning corresponding to the resulting increase in phase divergence, while a similar elevation in temperature leads to a decrease in solute partitioning to ABEC resin.26 2. The amount of salt needed to induce phase separation in an ABS is lower than that needed to lead to observable partitioning of chaotropic solutes to ABEC resin. This difference has led to the assumption that the distribution of solutes in ABS is based on partitioning into preexisting solvent domains.27 The similarities observed between ABEC and ABS led to the conclusion that the separation of solutes occurs via a saltingout mechanism with preferential solute retention in one phase over the other in both the l/l and s/l modes. However, we have recently observed that unlike the chaotropic ClO4-, the ion Hg2+ exhibits partitioning to ABEC in the absence of kosmotropic salt and is not retained in the presence of kosmotropic salt. Given the environmental burden and bioaccumulative nature of both Hg2+ and ClO4-, we decided to explore whether an ABEC separation process could remove both ionic species from freshwater sources.28-31 In the present work, the dual nature of ABEC resins is investigated, and similarities and differences between these systems and ABS are discussed. 2. Experimental Methods 2.1. Materials. ABEC resins of two different PEG molecular weights, ABEC-5000 (200-300 mesh) and ABEC-2000 (60-100 mesh) were obtained from Eichrom Industries (Darien, IL). NaOH was purchased from EM Industries (Gibbstown, NJ). All other nonradioactive chemicals were purchased from Aldrich (Milwaukee, WI). All chemicals were of reagent grade and were used as received. All kosmotropic salt solutions used in forming an ABS were prepared in water on a molar basis. Water was deionized (resistivity ) 18 MΩ cm) using commercial deionization and polishing systems. Radioactive 203HgCl2 tracer was obtained from American Radiolabeled Company Inc. (St. Louis, MO). Other radiotracers (63NiCl2, NH499TcO4, 59FeCl3, 109CdCl2, 152EuCl3, 22NaCl, 60 CoCl2, 137CsCl, and 204TlCl) were obtained from Amersham Life Science Inc. (Arlington Heights, IL). The radiotracers were used as received or diluted with deionized water to an activity of ∼0.03 µCi/µL for batch uptake and column runs. 2.2. Conditioning ABEC Resins and Dry Weight Conversion Factors. The ABEC resins contain 60-80% water, depending on how they are treated. To ensure constant water content in all experiments, the resins were conditioned just prior to normal use as described elsewhere.18,21 This was carried out by placing approximately 3 g of resin in a 5 cm diameter Bu¨chner funnel containing a Whatman no. 2 qualitative filter disk. The resins were washed by passing several portions of deionized water over them and dried with air that was bubbled through deionized water, before use. The hydrated mass of the resin was converted to its dry weight equivalent using a dry weight conversion factor (dwcf) obtained by gravimetric analysis and application of eq 1 dwcf )

weight of dry resin, g ( weight of hydrated resin, g )

(1)

2.3. Batch and Chromatographic Studies: Electrode Method for Perchlorate. A particular amount of ABEC-2000 or ABEC-5000 resin (∼100 mg) was accurately measured into a vial. To this vial, 5 mL (contact volume) of a solution of known concentration of ClO4- (as NaClO4), prepared in the

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kosmotropic salt solution of choice, was added and mixed gently in a rotary stirrer for 1 h. After this solution was allowed to stand for 1 h, 3 mL of the supernatant solution was carefully pipetted out, and the ClO4- concentration was determined using a Detect Ion ClO4- selective electrode (Nico Scientific Inc., Huntingdon Valley, PA) and a calibration chart previously determined from prepared ClO4- standards in each particular strength of salt solution. The dry weight distribution coefficients (Dw) were calculated using eq 2 Dw )

[

i f CClO - CClO 4 4 f CClO 4

]

×

[ m ×Vdwcf ]

(2)

i f where C ClO and C ClO are the initial and final concentrations 4 4 of ClO4 respectively, V is the contact volume (mL), m is the mass of the ABEC resin (g), and dwcf is the aforementioned dry weight conversion factor. The Dw values reported here are the average of at least two measurements and are typically accurate to (5%. For each chromatographic study, ∼2 g of ABEC resin were slurried in water and packed into a 1 × 20 cm glass column (Fisher Scientific, Norcross, GA). A small piece of glass wool or a Fisher-brand diffusion disk was placed on top of the resin bed to prevent disturbing the top of the resin bed from the addition of load solution. The column was equilibrated/ preconditioned with ∼25 mL of water and of an aqueous solution of kosmotropic salt of the same type and concentration as the load solution, at an approximated flow rate of 5 mL/min. Load solutions of 100 mL containing a parts per million (ppm) quantity of ClO4- prepared in a molar solution of kosmotropic salt (e.g., 25 ppm ClO4- prepared in 3 M (NH4)2SO4) were loaded onto the column followed by 25 mL of a stronger saltingout solution without ClO4- as the rinse solution (e.g., 3.5 M (NH4)2SO4). The ClO4- was then stripped using 100 mL of deionized water. Fractions of approximately 5 mL were collected throughout the chromatographic run in previously weighed 20 mL scintillation vials. The vials with liquid sample were weighed. The density of the eluent solutions was measured using the mass of solution in a 1 mL volumetric flask. Measurements were carried out in triplicate, and the average of the measurements was taken as the density. Using mass and density measurements, the exact volume of the fractions was calculated. Fractions thus collected were analyzed for ClO4- using the same procedure as outlined in the batch studies, using an ion selective electrode. The resins were not reused unless otherwise noted. In such cases, the resins were reconditioned before subsequent use. This was done by washing them with 50 mL of water in the column mode and re-equilibrating with 25 mL of kosmotropic salt solution of the same strength as the load solution. 2.4. Batch and Chromatographic Studies: Radiometric Method for Mercury. The batch contacts for Hg2+ were carried out using radiotracer techniques. Aqueous 203HgCl2 (2 µL of ∼0.03 µCi/µL activity) was added to 2.2 mL of the solution of interest and mixed, and a 100 µL aliquot was removed for analysis. One milliliter of the spiked solution (contact volume) was added to two separate samples of conditioned resin of known mass (typically 15-30 mg). The samples were then gently stirred for 30 min, followed by 2 min of centrifugation (to ensure that the resin remained in contact with the solution). The solution was stirred for another 30 min, after which it was filtered through a 0.45 µm pipet-tip filter. A 100 µL aliquot was then removed for radioanalysis. 203Hg2+ is a γ-emitting

isotope, and the solution activities were analyzed by using a Packard Cobra II Auto-Gamma counting system (Packard Instrument Co, Inc., Meriden, CT). Analysis of the uptake by ABEC resins was carried out by measurement of the weight distribution ratios (Dw) calculated in a manner similar to that of ClO4- but using the activities of 203 Hg2+ as a measure of concentration, as in eq 3 Dw )

[

Ai203Hg - Af203Hg f

A203Hg

]

×

[ m ×Vdwcf ]

(3)

where Ai203Hg is the activity of the solution before contact with the resin, Af203Hg is the activity of the solution after contact, and the other variables correspond to the ones used in eq 2. The Dw values reported here are the average of at least two measurements and are typically accurate to (5%. All other metal ion uptakes of γ-emitting isotopes (22Na+, 59 Fe3+, 60Co2+, 204Tl+, 109Cd2+, 137Cs+, and 152Eu3+) were performed in a similar manner. The β-emitting isotopes, 99TcO4and 63Ni2+, were measured by diluting the sampled aliquots in Ultima Gold Scintillation Cocktail and counting β-decay using a Packard Tri-Carb 1900 TR liquid scintillation analyzer (Packard Instrument Co., Meriden, CT). In a typical chromatographic study, a small 1 × 20 cm glass column (Fisher Scientific, Norcross, GA) was slurry packed with ∼2.0 g of ABEC-2000 resin. A small piece of glass wool or a Fisher-brand diffusion disk was placed on top of the resin bed to prevent disturbing the top of the resin bed from the addition of eluents. A load solution made of deionized water with 0.04 ppm “cold” Hg2+ (as the chloride salt) was spiked with 0.32 µCi of 203HgCl2 tracer. After elution of 25 mL of the load solution under gravity flow, the column was rinsed with 5 mL of deionized water. Stripping was carried out using 25 mL 3.5 M (NH4)2SO4. The eluate was collected in small fractions (5-25 drops) into preweighed gamma tubes. After the fractions were collected, the tubes with sample were weighed. The densities of the eluent solutions were measured using the mass of the solution in a volumetric flask. The densities were calculated as the average of three different measurements. Using these values and the mass of the samples, the volumes were calculated. After determination of the volumes, 100 µL aliquots were removed and analyzed radiometrically for mercury content. 3. Results and Discussion 3.1. Batch Studies of Perchlorate Partitioning onto ABEC. Batch partitioning of ClO4- was carried out using two different resins, ABEC-2000 and ABEC-5000 (differing in the molecular weight of PEG attached to the resin support), and one of three different aqueous salt solutions, K3PO4, (NH4)2SO4, and NaCl. The data (Figure 2, for ABEC-5000) indicate that ClO4- (∆Ghyd ) -214 kJ/mol), similar to previously studied chaotropic anions such as TcO4- and ReO4-,14,32 partitions onto ABEC resins with Dw values ranging from 10 to greater than 1000 depending on the concentration and type of kosmotropic salt used.14,33 In general, the uptake of ClO4- is higher than previous results for TcO4- (data reproduced from earlier studies, but with same resin ABEC-5000), in keeping with the relative chaotropicity of these anions (for TcO4-, ∆Ghyd ) -251 kJ/mol).14,16 In addition, in comparison to uptakes of TcO4- in ABEC-5000 using the same salt (NH4)2SO4, it can be noted that the slopes for ClO4- are relatively flat. This result is in keeping with the higher chaotropicity of ClO4-, where a lower

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Figure 2. Dry weight distribution ratios (Dw) of ClO4- to ABEC-5000 from aqueous NaCl (9), (NH4)2SO4 (2), and K3PO4 (b) solutions, at 25 °C. Dw values for TcO4- from (NH4)2SO4 (O) are included for comparison.27

Figure 3. Dry weight distribution ratios (Dw) of ClO4- to ABEC-2000 (b) and ABEC-5000 (9) as a function of ClO4- concentration, at 25 °C. The kosmotropic salt solution was 3 M (NH4)2SO4.

concentration of salt causes maximum uptakes. The uptakes for both TcO4- and ClO4- are equal at higher concentrations, but those for TcO4- are much lower at lower concentrations of (NH4)2SO4. As previously observed for chaotropic anions, the Dw values for ClO4- increased with increasing kosmotropic salt concentration and were higher at a given salt concentration for kosmotropes of more negative ∆Ghyd, that is, Dw increases in the order of the anions’ salting-out ability: PO43- > SO42- > Cl- (∆Ghyd values provided in Table 1).12,16 Cations may play a similar role, although their effects are typically much less significant when compared to that of the anions. ABEC resins and ABS generally behave in a similar manner with respect to their response to the type of kosmotropic anion and its concentrations. Interestingly, however, noteworthy uptakes of ClO4- from aqueous NaCl in ABEC-5000 were observed (Figure 2), although the chloride anion is not very kosmotropic and brine does not form an ABS at any concentration with PEG-5000 at room temperature. Generally, lower concentrations of kosmotropic salts are needed to cause comparable distribution ratios in ABEC when compared to the corresponding l/l ABS.17 This behavior seems to indicate that PEG may be ordered well below the point at which phase separation is visually discernible.27 In the current study, partitioning using a less kosmotropic anion such as Cl- is comparable to using lower concentrations of a more kosmotropic anion. It should be noted that there is no uptake of ClO4- from water alone; a fact used to strip ClO4- from the resin. Figure 3 illustrates the effect of the molecular weight of the grafted PEG polymer in the ABEC resin on the partitioning of ClO4-. Higher uptakes were obtained using ABEC-5000 than ABEC-2000, as previously observed for other chaotropic ions.33 Here also, the behavior of ABEC is similar to l/l ABS where

Figure 4. Chromatograms using an ABEC-5000 column loaded with 25 ppm NaClO4 spiked in 3 M (NH)4SO4, rinsed with 3.5 M (NH4)2SO4, and stripped with water (black symbols); and a second chromatogram under the same conditions obtained after the first run and washing with 50 mL of water (gray symbols). The second chromatogram has been offset by 4 units on the Y-axis.

increasing phase incompatibility is observed with increasing molecular weight and hydrophobicity of polymer.17 Figure 3, however, also shows that with increasing concentration of ClO4-, the Dw values decrease, indicating a reduced capacity for the anion. To confirm this observation, additional studies were conducted to determine the capacity of the ABEC resin for ClO4-. 3.2. Column Studies of Perchlorate Uptake: Capacity and Reuse of ABEC Resin. To be able to predict the efficiency and performance of ABEC resins in large scale continuous systems, small columns containing ABEC resin were studied to calculate capacities. A three-step continuous procedure was used, consisting of loading the resin with ClO4- (load), rinsing the resin to remove the excess unbound ClO4- (rinse), and finally stripping of the bound ClO4- (strip). Eluate fractions were collected and concentrations of ClO4- were analyzed. A column packed with 2 g of ABEC-5000 was loaded with a 3 M (NH4)2SO4 solution containing 25 ppm NaClO4. Figure 4 (black symbols) shows that all of the ClO4- was retained by the ABEC resin during the loading. During the rinse with a higher concentration (3.5 M) of kosmotropic (NH4)2SO4 solution, any interstitial ClO4- was washed out. Stripping the column with water resulted in two peaks corresponding to the removal of ClO4-. The two peaks were tentatively assigned to the sodium salt and the ammonium salt (vide infra). The capacity of the ABEC-5000 resin was determined to be 0.018 mmol of ClO4per gram of resin. The same resin was reused (Figure 4, gray symbols), after washing with 50 mL of deionized water and re-equilibrating with 3 M (NH4)2SO4. The capacity of the resin during reuse with a fresh 25 ppm load solution was calculated to be 0.016 mmol of ClO4- per gram of resin. The recovery of ClO4- was found to be ∼99% in both use and reuse, indicating almost complete stripping. 3.3. Column Studies of Perchlorate Uptake: Cation Effects. It was expected that the ClO4- would be released completely from the resin as soon as water was added. However, Figure 5 (chromatogram B) clearly shows that not all the ClO4is removed from the resin at the same time. After the initial strip with water, the perchlorate concentration in the eluate rapidly decreased. Two different cations are present in the load and rinse solutions (Na+ and NH4+), and it was hypothesized that these could have different effects on the stripping performance. To

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Figure 5. Effect of cation on retention and stripping of ClO4- using an ABEC-2000 with load solutions of 25 ppm KClO4 in 3 M (NH4)2SO4 (gray symbol), 25 ppm NaClO4 in 3 M (NH4)2SO4 (black symbol), and 25 ppm NaClO4 in 1.5 M Na2SO4 (open symbol) as load. The chromatograms have been progressively offset by 4 units on the Y-axis.

Figure 6. Dry weight distribution ratios (Dw) for various metal ions to ABEC-5000 from water (black bars) and from 3.5 M (NH4)2SO4 (gray bars), at 25 °C.

confirm this, a column was prepared using a common cation (Na+) in both the kosmotropic salting-out agent and the chaotropic solute that was to be partitioned onto the resin. When Na2SO4 (at a lower concentration because of its lower solubility) was used as the kosmotropic salt and NaClO4 was used as the ClO4- source in the load solution, only one peak was observed during the stripping step, as expected (Figure 5, chromatogram C). This peak appeared at the same elution volume as the second peak obtained in the earlier experiments utilizing (NH4)2SO4, suggesting that the first peak in chromatogram B (Figure 5) corresponds to NH4ClO4, while the second peak corresponds to NaClO4. It should be noted that Na+ and NH4+ have different ∆Ghyd (Table 1), and each ion can have different interactions with the PEG polymer (i.e., coordination vs hydrogen bonding), resulting in different elution times. A third chromatogram using KClO4 as the ClO4- source and (NH4)2SO4 as the kosmotropic salt was investigated (Figure 5, chromatogram A). In this case, the ClO4- was all stripped with the peak position directly corresponding to the peak assigned to NH4ClO4 in chromatogram B. Both K+ and NH4+ have very similar ∆Ghyd (Table 1). 3.4. Uptake of Mercury and Other Metal Ions from Water. The uptake of several metal ions onto ABEC-5000 from 3.5 M (NH4)2SO4 and from water are shown in Figure 6. It is clear that these metal ions partition onto ABEC resins in the absence of a kosmotropic salt and without the addition of any added extractant; in fact, the addition of the kosmotropic (NH4)2SO4 significantly reduces the Dw values. While chaotropic ions have been shown to partition onto ABEC resins through a salting-out mechanism,14 kosmotropic species, including many metal cations, tend to stay in the salt-rich phase and do not partition onto ABEC resins.14,33,34 It is typically only possible

Figure 7. Dry weight distribution ratios (Dw) for 203Hg2+ with ABEC5000 vs the concentration of NaX (X ) I- ((), Br- (2), Cl- (9)) at 25 °C.

to extract metal ions from aqueous solutions with ABEC resins using suitable extractants which coordinate to the metal ion and form more chaotropic complexes which can be extracted using either an ABS or ABEC resin.19 Therefore, it was surprising to note that metal ions, such as Hg2+, Cd2+, and Tl+, that would normally prefer the salt-rich phase of a l/l ABS, showed significant uptakes onto ABEC resin from water in the absence of salt or extractants. It is most likely that the mechanism of retention to the ABEC resin is not by salting-out, but rather by complexation of the metal ions to the grafted PEG chains of the ABEC resin. There are indeed several examples of PEG forming coordination complexes with metal ions,35-41 and PEG has been used as a synergistic extractant in, for example, the removal of strontium from aqueous media using cobalt dicarbollide.42 The most likely species of mercury for uptake to the ABEC resin is HgCl2 (the form of mercury added as the spike), which has a relatively high formation constant of Kf ) 13.2243 and which cannot be easily broken by addition of water. In high concentrations of (NH4)2SO4, however, formation of Hg(HSO4)+ (Kf ) 1.34) and Hg(HSO4)2 (Kf ) 2.4)44 are the major species in this ABS-type salt-rich environment; this species would not be able to coordinate with the PEG chains and would undoubtedly be very kosmotropic because of the sulfate coordination shell, therefore preferring the salt solution or salt-rich phase in an ABS. The result is that Hg2+ can be loaded onto an ABEC resin from water and stripped with (NH4)2SO4; the exact opposite of the usual ABEC load-strip procedure such as that described above for ClO4-. 3.5. Batch Uptakes of Mercury in Solutions of Low Halide Concentration. Batch uptakes of Hg2+ from aqueous solutions with increasing concentrations of sodium halide (Figure 7) also support our hypothesis that mercury is not retained through a salting-out mechanism but is rather retained as a metal-PEG complex. The Dw values are essentially invariant to the halide concentration and do not increase as would be expected in a salting-out mechanism.28 Moreover, the Dw of the mercury chloride tracer from aqueous solution without the addition of halide is in the same range as the Dw for mercury with the addition of 10-2 M NaCl (data not shown), strongly suggesting that mercury is complexed onto the PEG on the solidsupported ABEC resin as a mercury-chloride species. Figure 7 additionally illustrates the degree of preference of each metallohalogen complex to partition, in the order I- > Br- > Cl-. This is in keeping with the relative formation constants,44 of the mercury-halide complexes, HgCl2 (13.22) < HgBr2 (17.27) < HgI2 (23.82), and their relative hydrophobicities.

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removal and remediation from contaminated water sources but also argues for attention to the dual nature of polyethylene glycol as a chromatographic resin. Acknowledgment This research was supported by the Division of Chemical Sciences, Geoscience, and Bioscience, Office of Basic Energy Sciences, and Office of Science, U.S. Department of Energy (Grant DE-FG02-96ER14673). Literature Cited

Figure 8. Chromatogram using ABEC-2000 with a load solution of 0.04 ppm Hg2+ (as chloride spiked with 0.32 µCi 203HgCl2 tracer) in water, a water rinse, and a 3.5 M (NH4)2SO4 strip.

The exact speciation of mercury halide complex with the addition of halide anions may vary depending upon the concentration of the NaX added to the system. Bulgariu and Bulgariu studied the speciation of Hg(II) halides in an ABS composed of PEG-1550 and (NH4)2SO4.44 They found that the speciation of mercury in the ABS largely depends on acidity of the solution, as well as on stability of the Hg(II) species, and may be represented as Hg2+ + 2X- a HgX(n-2)n

(4)

3.6. Chromatographic Study of the Uptake of Mercury from Water. A small column was packed with ABEC-2000 and loaded with a solution of 0.04 ppm of nonradioactive cold HgCl2 spiked with radioactive HgCl2 tracer in water. After elution of the load solution, the column was rinsed with deionized water (5 mL). Over 98% of the HgCl2 in the load solution was retained by the column. The loaded resin then was treated with 25 mL of 3.5 M (NH4)2SO4, thereby stripping 90% of the loaded HgCl2 from the column. This can be seen from the chromatogram shown in Figure 8. However, it was found that only 51% of the mercury was stripped within the first 4.5 mL. We can only estimate the capacity of the column from this data since the column only reached 7% breakthrough. However, from linear extrapolation, the capacity is estimated to be 0.08 mmol of HgCl2 per gram of ABEC-2000. 4. Conclusions ABEC resins have been successfully used to remove perchlorate from aqueous solutions with the use of additional kosmotropic salts. Batch studies showed that, in keeping with its degree of kosmotropicity and salting-out, distribution ratios of perchlorate were highest when K3PO4 was used as the kosmotropic salt. Chromatographic studies indicated that perchlorate can be loaded onto an ABEC column and then easily stripped with water. The columns have a moderate capacity for perchlorate, and there is potential for reuse of resin with little or no loss in capacity. On the other hand, Hg2+ can be removed directly from water by PEG-grafted ABEC resins. The Hg2+ appears to interact directly with the PEG polymer by complexation in its halide form. Uptakes of mercury were noted in the absence of kosmotropic salt and stripping was observed with (NH4)2SO4. This not only presents a very attractive option for mercury

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(35) Hines, C. C.; Reichert, W. M.; Griffin, S. T.; Bond, A. H.; Snowwhite, P. E.; Rogers, R. D. Exploring control of cadmium halide coordination polymers via control of cadmium(II) coordination sites utilizing short multidentate ligands. J. Mol. Struct. 2006, 796, 76. (36) Gokel, G. W.; Goli, D. M.; Schultz, R. A. Binding profiles for oligoethylene glycols and oligoethylene glycol monomethyl esters and an assessment of their abilities to catalyze phase-transfer reactions. J. Org. Chem. 1983, 48, 2837. (37) Rogers, R. D.; Jezl, M. L.; Bauer, C. B. Effects of poyethylene glycol on the coordination sphere of strontium in SrCl2 and Sr(NO3)2 complexes. Inorg. Chem. 1994, 33, 5682. (38) Rogers, R. D.; Bond, A. H.; Aguinaga, S.; Reyes, A. Complexation chemistry of bismuth(III) halides with crown ethers and polyethylene glycolssStructural manifestations of a steoreochemically active lone pair. J. Am. Chem. Soc. 1992, 114, 2967. (39) Rogers, R. D.; Bond, A. H.; Roden, D. M.; Reyes, A. Structural chemistry of poly(ethylene glycol) complexes of lead(II) nitrate and lead(II) bromide. Inorg. Chem. 1996, 35, 6964. (40) Rogers, R. D.; Bauer, C. B.; Bond, A. H. Crown ethers as actinide extractants in acidic aqueous biphasic systems: Partitioning behavior in solution and crystallographic analyses of the solid state. J. Alloys Compd. 1994, 213/214, 305. (41) Sari, N.; Kahraman, E.; Sari, B.; Ozgun, A. J. Synthesis of some polymer-metal complexes and elucidation of their structures. Macromol. Sci., Part A: Pure Appl. Chem. 2006, 43, 1227. (42) Chamberlin, R. M.; Abney, K. D. Strontium and cesium extraction into hydrocarbons using alkyl cobalt dicarbollide and polyethylene glycols. J. Radioanal. Nucl. Chem. 1999, 240, 547. (43) NIST Standard Reference Database 46, version 5.0; NIST Standard Reference Data: Gaithersburg, MD, 1998. (44) Bulgariu, L.; Bulgariu, D. Hg(II) extraction in a PEG-based aqueous two-phase system in the presence of halide ions I. Liquid phase analysis. Cent.-Eur. J. Chem. 2006, 4, 246.

ReceiVed for reView May 26, 2008 ReVised manuscript receiVed July 8, 2008 Accepted July 20, 2008 IE800841J