Polymer-Supported Bifunctional Amidoximes for ... - ACS Publications

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Polymer-Supported Bifunctional Amidoximes for the Sorption of Uranium from Seawater Spiro D. Alexandratos,* Xiaoping Zhu, Marc Florent, and Remy Sellin Department of Chemistry, Hunter College of the City University of New York, 695 Park Avenue, New York, New York 10065, United States The Ph.D. Program in Chemistry, The Graduate Center of the City University of New York, New York, New York 10016, United States S Supporting Information *

ABSTRACT: Bifunctional amidoxime fibers are synthesized from commercially available polyacrylonitrile as sorbents for the recovery of U(VI) from seawater. The purpose of introducing a second ligand is to enhance the affinity of the amidoxime (AO) ligand via two possible mechanisms: by acting directly upon the AO to increase its ability to ion exchange or providing additional coordination sites to the uranyl ion. Amines are chosen as the second ligand since they have a high affinity for U(VI) at ppm levels from synthetic seawater. Diethylenetriamine (DETA) is utilized as the amine because it is sufficiently flexible to interact with AO and able to bind metal ions. Along with AO and AO-DETA fibers, the primary NH2 moieties were modified with phosphonic acid ligands (AO-phon-DETA). All fibers have high affinities (>99% sorption) for the uranyl ion when initially present at 0.90 ppm in synthetic seawater. When contacted with actual seawater at the Pacific Northwest National Laboratory for 20.8 days, the loadings for the fibers are 124 μg/g (AO), 535 μg/g (AO-DETA), and 789 μg/g (AO-phon-DETA). For comparison, a polyethylene fiber on which was grafted polyacrylonitrile and then converted to AO at the Oak Ridge National Laboratory gave a uranyl loading of 2610 μg/g at the same contact time, perhaps due to its optimized support structure and/or the presence of diamidoxime groups. FTIR spectra indicate that the effect of DETA is through a hydrogen bonding interaction with the AO to facilitate dissociation of the acidic N−O−H. Continuing research on optimizing the loading onto the bifunctional fibers emphasizes the effect of phosphorus capacity, amine ligand, and synthesis conditions.

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INTRODUCTION

loading twice as high as stacks of nonwoven fabric due to better contact between seawater and the ligand.11 Bifunctional polymers display greater ionic selectivity and kinetics: for example, triethyl/trihexyl-ammonium ligands were immobilized on cross-linked polystyrene beads and the former (hydrophilic) ligand increased access of TcO4− to the latter (selective) ligand.12 The access/recognition mechanism has been proposed for a number of bifunctional polymers.13 This is now extended by preparing polymers in which the second ligand enhances the selectivity of a ligand that is at least moderately selective via two possible mechanisms: it can act upon the first ligand in a way that affects its binding mechanism (such as ion exchange) or it can provide additional coordination to the targeted metal ion. Since our target is U(VI) and amidoxime (AO) is known to have an affinity for it, amines are chosen as the second ligand since they have a high U(VI) affinity at ppm levels from synthetic seawater.14 Diethylenetriamine (DETA) is utilized as the amine because it is sufficiently flexible to interact with AO and may provide

The amount of uranium in seawater is estimated at 4.5 billion tons1 and its recovery would avoid the deleterious effect of terrestrial mining on the environment. However, the uranium is present at 3.3 ppb with other competing ions (Ca2+, Mg2+, Na+, K+, etc.) present at levels that are orders of magnitude greater. Its recovery would thus require a selective adsorbent with a ligand that operates best at a pH of 8. Amidoxime immobilized onto different supports2,3 displays promising selectivity for the harvesting of an economically significant amount of U(VI).4 Nonetheless, the low uranium concentration and the occurrence of competing ions remain significant challenges to its recovery. This is evident by a study of a graphene oxide/ amidoxime hydrogel that has a U(VI) adsorption capacity of 398.4 mg g−1 from aqueous solution (i.e., no competing ions present);5 in contrast to this, the highest value achieved to date for immobilized amidoxime from seawater is 5.4 mg/g.6 A braided amidoxime-grafted fiber loads 1.5 mg U(VI) /gfiber from seawater at a 30 day contact time.7 Though it performs comparatively well, a higher capacity and more rapid sorption rate will improve chances for its commercial deployment. Increasing surface area8 and sorbent hydrophilicity9 are approaches to enhancing metal ion sorption into polymers. Hydrophilic comonomers such as acrylic acid increase the uranyl sorption rate and capacity.10 Braided fabric has a uranyl © XXXX American Chemical Society

Special Issue: Uranium in Seawater Received: October 6, 2015 Revised: March 28, 2016 Accepted: March 29, 2016

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DOI: 10.1021/acs.iecr.5b03742 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Functionalized fibers: diethylenetriamine (DETA), aminomethylphosphonic acid (phon-DETA), aminomethylphosphinic acid (phinDETA), amidoxime-diethylenetriamine (AO-DETA), amidoxime-aminomethylphosphonic acid (AO-phon-DETA), and amidoxime (AO).

Characterization. Nitrogen capacities were determined via the Kjeldahl method after oxidizing 0.100 g of dry fiber with concentrated H2SO4 containing copper sulfate and potassium sulfate. The ammonia distilled from the flask was bubbled through 0.100 M HCl, and 50 mL aliquots (triplicate runs) were titrated with standardized 0.100 M NaOH; the nitrogen capacity was calculated from the titration by difference in the amount of HCl consumed.16 Phosphorus capacities were determined (in triplicate) after mineralizing 0.020 g fiber in concentrated H2SO4 and copper sulfate followed by reaction with ammonium vanadate−molybdate. The intensity of yellow coloration was measured at 470 nm on a Spectronic 21D (Milton Roy). The P content (mg) was calculated from a standard curve and converted to mmol P/gpolymer.17 Percent solids, the ratio of dry to wet fiber weight, was determined by Buchner-drying fiber for 5 min to remove excess water (wet weight) and then oven-drying at 110 °C for 12 h (dry weight). FTIR spectra were recorded on a PerkinElmer 650 spectrophotometer. Loading of Uranyl onto AO and AO-DETA Fibers. Fiber (0.05 g wet) was contacted with 20 mL of 50 ppm U(VI) in synthetic seawater for 1 week. The solution was then removed and a fresh 20 mL U-containing seawater solution was added and shaken for 1 week. The U-loaded fiber was washed with water and dried in a vacuum oven at 60 °C, and its FTIR was spectrum recorded. Uranyl Sorption. The extent of uranyl sorption from synthetic seawater18 was determined via batch experiments. Fibers were first conditioned in synthetic seawater to a constant pH. Uranium seawater solution was prepared from 1000 mg/L uranium standard solution whose pH was made neutral with solid NaOH and then diluted with seawater to 0.90, 6.0, and 50 mg U/L. Seawater-conditioned fibers were Buchner-dried and 0.050 g samples were contacted with 5.0 mL of uranium-spiked synthetic seawater solutions at 23 °C for 17 h (a long enough time to ensure equilibrium, as confirmed by sorption percentages ≥ 95%). Uranyl levels were analyzed with an inductively coupled plasma atomic emission spectrometer (PerkinElmer 7000); percent sorption was calculated from the concentration difference before and after equilibration; it is reproducible in triplicate runs to ±2% of the values reported.

additional coordination sites to the uranyl ion. Polyacrylonitrile is the polymer support because it can be readily substituted with both AO and amines and is available commercially. Furthermore, it is of interest to modify the amines with phosphorus acid ligands by reaction with phosphorous acid or sodium hypophosphite since aminomethylphosphinates have a high U(VI) affinity from acidic solutions.15 AO, DETA, AODETA, aminomethylphosphonic acid (phon-DETA, AO-phonDETA) and aminomethylphosphinic acid (phin-DETA, AOphin-DETA) are thus immobilized on polyacrylonitrile (PAN) fiber (Figure 1).

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EXPERIMENTAL SECTION Polyacrylonitrile fiber (CTF 525) was provided by Sterling Fibers, Inc. (Florida, USA). Reagent grade diethylenetriamine, sodium hypophosphite, phosphorous acid, sodium carbonate, concentrated HCl, paraformaldehyde, hydroxylamine hydrochloride, and uranium standard solution were purchased from Sigma-Aldrich or Thermo Fisher (USA) and used as received. Synthetic seawater was purchased from Ricca Chemical Company (Texas, USA). Amination of Polyacrylonitrile (DETA). Four samples of PAN (2.0 g each) were stirred with 50 mL of diethylenetriamine (DETA) and 10.0 g of sodium carbonate in 200 mL of water at 70 °C, 80 °C, 90 °C, and 100 °C for 3 days, then washed with water. Phosphonation of DETA (Phon-DETA). DETA fiber (2.0 g wet weight) was refluxed with 12.0 g of phosphorous acid, 4.5 g of paraformaldehyde, 10 mL of concentrated HCl and 100 mL of water for 17 h. After being cooled, the fibers were washed with water and conditioned in a glass frit funnel with 1 L each of 1.0 M NaOH, water, and 1.0 M HCl and water. Phosphination of DETA (Phin-DETA). The procedure is as with Phon-DETA except for the use of 8.0 g of sodium hypophosphite and 3.0 g of paraformaldehyde. Amidoximation (AO, AO-DETA, AO-Phon-DETA). PAN, DETA, and Phon-DETA (2.0 g) were added to 20 g of hydroxylamine hydrochloride, 100 mL of H2O, and 11.0 g of Na2CO3 to give a solution of pH 6. The mixture was stirred at 80 °C for 17 h. The products were washed with water and conditioned as above. B


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Figure 2. Reaction of polyacrylonitrile with diethylenetriamine.

Figure 3. FTIR spectra of diethylenetriamine immobilized onto PAN fibers as a function of reaction temperature: (black, top) 90 °C; (pink, mid) 80 °C; (blue, bottom) 70 °C.

Figure 4. Conversion of DETA-80 (top) and DETA-90 (bottom) to aminomethylphosphonic acids.

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RESULTS AND DISCUSSION

Assuming linearity for these values and the extent of reaction, there is 35% conversion to DETA at 70° and 75% conversion at 80 °C. (To emphasize: these are first approximations since differences in unit weight have not been taken into account; however, it is relevant that the reaction of amidoxime with PAN proceeds in an approximately linear manner over the temperature range of 50−80 °C.19) The nitrogen capacity for PAN fiber reacted with DETA at 100 °C is 11.9 mmol/g and shows a carbonyl band due to carboxylic acid from basecatalyzed hydrolysis of the −CN; PAN reacted with DETA at reflux had a yet lower nitrogen capacity (7.96 mmol/g) indicating further hydrolysis at the higher temperature to the carboxylic acid. Phosphorylation of DETA. DETA-70, -80, and -90 were phosphorylated to the aminomethylphosphonic acids (Figure 4). FTIR spectra show the characteristic PO stretch at 1154 cm−1 and bands assigned to P−O(H) at 916 and 1033 cm−1. The phosphorus capacities were 0.63, 0.89, and 2.56 mmol/g on DETA-70, -80, and -90, respectively, as the nitrogen capacities decreased to 15.45, 13.60, and 7.56 mmol/g. Since

Amination of PAN. PAN is aminated with DETA (Figure 2) at temperatures of 70 °C, 80 °C, 90 °C, and 100 °C. FTIR spectra (Figure 3) do not show carboxylic acid formation since the CO stretching band at 1659 cm−1 is as expected for an amide and lower than that for a carboxylic acid (1710 cm−1). The band at 1549 cm−1 is due to the primary amine group from DETA. FTIR spectra show that amination at the nitrile increases with reaction temperature (Figure 3). The nitrile groups are partially aminated when reacted at 70 and 80 °C, and reacted to ≥95% at 90 °C. The −CN band at 2243 cm−1 is evident and the change in its intensity can be used with the corresponding nitrogen capacities to estimate the extent of reaction: 18.4 mmol/g for PAN fiber (no reaction), 17.3 at 70 °C (DETA70), 16.1 at 80 °C (DETA-80), and 15.3 at 90 °C (DETA-90). The extent of reaction can be approximated since the calculated value for complete conversion to the triamine is 15.5 (as found for DETA-90) and the value for no conversion is 18.4. C


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Figure 5. FTIR spectra of DETA-90 (black; bottom within 1200−1600 cm−1) and phon-DETA-90 (red; top within 1200−1600 cm−1).

Figure 6. Conversion of DETA-90 to aminomethylphosphinic acid (phin-DETA-90).

there are three nitrogens per ligand, the nitrogen capacity of the phosphonic acid from DETA-90 (phon-DETA-90) calculates to 2.52 mmol ligand, which is close to the phosphorus capacity indicating monophosphorylation of the primary amine sites. While a second phosphorylation may occur at the site of the first and the nitrogen analyses do not rule out 50% of the amine ligands being diphosphorylated, a comparison of the FTIR spectra of DETA-90 and phon-DETA-90 (Figure 5) indicates monophosphorylation due to the much diminished intensity of the −NH2 band at 1546 cm−1. To evaluate the effect of the phosphonate having sites of both intermediate and weak acid strength, DETA-90 was phosphorylated with sodium hypophosphite to give the phosphinic acid (phin-DETA-90, Figure 6). FTIR spectra show the PO band at 1158 cm−1 and the P−O(H) bands at 945 and 1042 cm−1. The phosphorus and nitrogen capacities (2.99 and 7.72 mmol/g) are comparable to those of the phosphonic acid. The DETA ligand content calculates to 2.57 mmol/g which is comparable to the phosphorus capacity. Uranyl Sorption by Phosphorylated DETA. The fibers were contacted with synthetic seawater spiked with 50, 6, and 0.9 mg/L U(VI) as tests for what can be expected with actual seawater containing uranium at the ppb level. The results are presented as percent sorption and are thus specific to the current conditions (50 mg of fiber contacted with 5 mL of solution) but are valid as relative measures of performance (Table 1). Contacting the fully functionalized DETA-90 with synthetic seawater shows 56%, 24%, and 20% U(VI) sorption from initial solutions of 50, 6, and 0.9 ppm U(VI), respectively; decreasing sorption with decreasing initial ppm indicates ineffectiveness at the ppb level. Phosphorylating the primary amine site increases affinity for the uranyl ion from ppm solutions. The phosphonic acid ligand itself has some selectivity for uranyl from artificial seawater (which consists mainly of 65 wt % NaCl, 16 wt % MgSO4, and 12 wt % MgCl2): 90% U(VI) is sorbed from a

Table 1. Percent Uranyl Sorption by DETA and Phosphorylated DETA Fibers from Synthetic Seawater Spiked with U(VI) fiber

50 mg/L U(VI)

6 mg/L U(VI)

0.90 mg/L U(VI)

DETA-90 phon-DETA-70 phon-DETA-80 phon-DETA-90 phin-DETA-90

56% 37% 49% 53%

24% 52% 61% 77% 22%

20% 54% 63% 78% 19%

solution at an initial level of 16.5 ppm;20 from actual seawater, where uranyl is 3 orders of magnitude lower, competition from the Mg2+, Ca2+, and Na+ present at far greater levels, may be too great to allow significant sorption of the uranyl (see below). The large decrease in affinity by phin-DETA-90 shows that the weak acid in the phosphonic acid is important to the uranyl affinity: all ions exchange with the acid of intermediate strength and it is the weak acid site that may then be available to bind the uranyl ion (Figure 7).

Figure 7. Binding of metal ions by the phosphinic and phosphonic acid ligands.

Monofunctional and Bifunctional Amidoxime Fibers. A polymer with only the AO ligand is the benchmark against which other polymers are compared for the sorption of uranium from seawater. The Sterling PAN fiber was thus reacted with hydroxylamine to give AO fiber. The FTIR spectrum (Supporting Information, Figure S1) shows the bands commonly associated with AO: the CN band at 1635 cm−1 and the N−O(H) band at 921 cm−1. It has a nitrogen capacity of 16.7 mmol/g. D


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at the Oak Ridge National Laboratory (ORNL AF1 adsorbent) gave a uranyl loading of 2610 μg/g at the same contact time. Consistent with what would be expected from the ppm results, phon-DETA-90 has low uranyl loading. The high loadings of Ca2+ and Mg2+ (26595 and 50076 μg/g, respectively) are noteworthy because they reflect the wellknown affinity of the phosphonic acid ligand for these ions;21 the lower loadings of the monovalent Na+ (1736 μg/g) and K+ (75 μg/g (not shown in Table 3)) are also consistent with the inherent affinities of the phosphonic acid ligand. Perhaps surprisingly, the AO fiber shows low uranyl loading but this is in line with research at ORNL on the importance of optimizing the support structure. Bifunctional AO-DETA-80 outperforms AO alone while comparison to AO-DETA-80* shows the effect of post-treatment of the fiber: elution with 0.02 M, rather than 1 M, NaOH results in a significant decrease in uranyl loading. This decrease is dependent on the concentration of uranyl ion since it is evident at the ppb level but not the ppm level (Table 2). A possible reason is given in the subsequent section on FTIR spectra but note that there is no elution effect on the loading of Mg2+ and Na+ which are present in much higher concentrations so it is not a matter of the polymer collapsing in the absence of 1 M NaOH elution. Lastly, AO-phon-DETA outperforms the other fibers in U(VI) loading. Again, the presence of the phosphonic acid ligand is evident in the high loading of calcium and magnesium ions. Note especially that results with synthetic seawater containing U(VI) at the ppm level all showed very high sorption levels; whether this is an effect of low vs high uranyl concentration on the kinetics will require further study. FTIR Spectra: Comparison of AO to SeawaterContacted AO. Water uptake of an AO membrane is 260% in deionized water and 200% in seawater22 thus indicating its hydrophilicity. AO is highly associated23 via hydrogen bonding and introducing DETA into the polymer can disrupt the strong hydrogen bonding among amidoxime moieties thus increasing metal ion sorption: U(VI) is complexed to a greater extent by Dipex with monoalkyl diphosphonate ligands than Diphonix with twice the acid groups24 and by the monoethyl ester of phosphoric acid compared to the diprotic phosphonic acid ligand.25 Comparing the ATR spectra of AO fibers before and after contact with uranyl-spiked synthetic seawater indicates that AO binds cations by an ion exchange mechanism where H+ at the N−OH of the amidoxime is replaced by the cation. A new band appears at 1103 cm−1 that is assigned to the N−O− that forms from proton exchange: electrons initially between NO−H shift toward oxygen as H+ is replaced by Mn+, increasing the N−O bond electron density resulting in an hypsochromic shift from 923 to 1103 cm−1 (Figure 8). The CN and N−OH bands are still evident at 1634 and 923 cm−1 since not all amidoxime groups ionize.

Bifunctional polymers can be prepared since contacting PAN with DETA below 90 °C partially converts the −CN to amide; the remaining nitriles can react with hydroxylamine to give the AO-DETA fiber. Reaction of hydroxylamine decreased the nitrogen capacity of DETA-70 from 17.3 to 14.9 mmol/g and that of DETA-80 from 16.1 to 15.4 mmol/g. FTIR spectra support the conversion of nitrile to amidoxime (Figure S2): the CN band at 2242 cm−1 disappears in all amidoximated fibers and the two bands associated with AO are evident (CN at 1636 cm−1 and N−O(H) at 920 cm−1). DETA is unchanged by the amidoximation and its NH2 band remains at 1560 cm−1. Partially functionalized DETA also allows the introduction of both aminomethylphosphonate and AO ligands by first phosphorylating the DETA and then reacting the remaining nitrile groups with hydroxylamine. The FTIR spectrum of the precursor shows the nitrile band (Figure S3) and the AOcontaining fiber (with phosphorus and nitrogen capacities of 0.89 and 13.6 mmol/g, respectively) shows the CN, N−O, PO, and P−O bands along with the absence of nitrile (Figure S4). Uranyl Sorption by Monofunctional and Bifunctional AO Fibers. Table 2 reports uranyl sorption from synthetic Table 2. Percent Uranyl Sorption by AO-Functionalized Fibers from Synthetic Seawater Spiked with U(VI) fiber

50 mg/L U(VI)

6.0 mg/L U(VI)

0.90 mg/L U(VI)

AO AO-DETA-80 AO-DETA-80*a AO-phon-DETA-80

82.5% 95.8% 95.8% 94.8%

96.6% 97.3% 97.8% 99.7%

100% 99.9% 99.3% 100%

a

Standard 1 M NaOH−H2O elution at room temperature replaced with 0.02 M NaOH−H2O.

seawater containing U(VI) at 50, 6.0, and 0.90 mg/L by AOfunctionalized fibers. The AO moiety displays a high U(VI) affinity, especially when compared to results in its absence (Table 1). When the uranyl ion is present at ppm levels in a high ionic strength solution, all fibers perform well. Replacing approximately 25% of the AO ligands with DETA in the bifunctional polymers has no detrimental effect on uranyl sorption and may even be beneficial. Eluting with 0.02 M NaOH in the room-temperature conditioning process in place of 1 M NaOH has no effect on uranyl sorption. Results from the 0.90 ppm solutions show quantitative sorption, and all fibers are equally good candidates for the actual seawater study. Uranyl Sorption from Actual Seawater. Table 3 reports the metal ion loadings onto the fibers from a 20.8 day contact with actual seawater at the Pacific Northwest National Laboratory (PNNL). For comparison, a polyethylene fiber onto which was grafted acrylonitrile and then converted to AO

Table 3. Uranyl Loading (μg/g) from Actual Seawater at PNNL by AO-Functionalized Fibera

a

fiber

Ca

Mg

V

Zn

Na

Ni

Fe

U

phon-DETA-90 AO AO-DETA-80 AO-DETA-80*b AO-phon-DETA-80

26595 2225 7354 5485 18635

50076 3435 10258 10743 31766

1158 426 1392 528 4277

4165 96 286 161 3436

1736 3476 2976 5507 2736

1596 24 198 95 1170

810 122 486 199 992

142 124 535 173 789

20.8 day contact time bStandard 1 M NaOH−H2O elution at room temperature replaced with 0.02 M NaOH−H2O. E


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Figure 8. FTIR spectrum of AO-fiber (red, top) and cation-exchanged AO fiber (black, bottom).

Figure 9. FTIR spectrum of AO-DETA (red) and cation-exchanged AO-DETA fiber (black).

seawater shows AO bands at 1108 (N−O−), 926 (N−OH), and 1635 (CN) cm−1 (Figure 9). The amine band originally at 1541 cm−1 is seen at 1560 cm−1 but it is unclear if this shift is sufficiently large to indicate the amine is involved in complexation. Along with the new band at 1108 cm−1 after contact, there is a large decrease in the CN band intensity at 1635 cm−1. This is consistent with N−O− formation upon cation exchange increasing electron density between the N−O (causing the hypsochromic shift to 1108) and also decreasing the electron density between the CN due to electron−electron repulsion resulting in its lower concentration and thus a less intense band; this must also result in a bathochromic shift and it may blend into the adjoining C−N(H2) band. This was examined further below.

A key issue is whether the uranyl ion is involved in cation exchange. It is reasonable to consider that one carbonate is removed in the exchange thus having uranyl bind to two N−O− sites. The manner in which AO binds U(VI) has been studied via density functional theory26 and X-ray crystallography.27 The FTIR spectra presented here do not identify the uranyl ion within the fiber but recent results (to be reported) in which the hydroxylamine reaction is done in a different solvent produces a FTIR spectrum with narrower bands, and, after contact with uranyl-spiked synthetic seawater, that spectrum does show bands assigned to the uranyl ion. FTIR Spectra: Comparison of AO-DETA to SeawaterContacted AO-DETA. The FTIR spectrum of AO-DETA (Figure S2) shows the same bands as AO at 1635 and 923 cm−1 along with an additional band at 1541 cm−1 due to the amine -NH2; contacting AO-DETA with uranyl-spiked synthetic F


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Figure 10. FTIR spectrum of 1.0 M NaOH-treated AO fiber.

Figure 11. FTIR spectrum of 0.10 M NaOH-treated AO fiber.

FTIR Spectra: Comparison of AO to AO-DETA. To test whether the decrease in CN band intensity could be due to an interaction with N−O−, AO was shaken with 1.0 and 0.10 M NaOH for 17 h at 23 °C. After removing excess NaOH, the fibers were dried and their FTIR spectra were taken (Figures 10 and 11). When treated with 1 M NaOH, the 1635 CN band disappears and a very strong band at 1415 cm−1 appears that is ascribed to neutralization of N−OH by NaOH to form N−O−. Decreasing the NaOH concentration to 0.10 M results in incomplete neutralization of N−OH given the diminished 1635 band along with the one at 1420 cm−1. The intensity of the CN band decreases significantly upon contact with NaOH, as also seen with cation-exchanged AO-DETA. There, however, the N−O− band was evident at 1108 cm−1 which leads to the

conclusion that position and intensity of that band depends upon the degree of conversion and the extent of ion pairing. In binding to Ga(III) from a basic solution,28 the N−O band was reported to shift from 935 to 950 cm−1; in the present case, contact with cations in pH 8 solution results in a shift to 1108 cm−1, and complete conversion with basic solutions of pH ≥ 13 (and no water-wash to permit any reconversion to N−O(H)) results is a shift to 1415 cm−1. The treatment of AO fibers with sodium or potassium hydroxide has been found to increase U(VI) sorption by 2−3 times over the value with no treatment.29 Alkaline treatment has been proposed to improve hydrophilicity, but it is now suggested that what it does is facilitate dissociation of the N− OH. Improved uranium sorption by AO-DETA is ascribed to G


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Figure 12. FTIR spectrum of AO eluted with 0.025 M NaOH (red, top) and AO-DETA (black, bottom).

the same mechanism due to interaction between the amidoxime and amine groups. This is supported by comparing the FTIR spectra of AO (Figure S1), AO-DETA (Figure S2), and AO eluted with 0.025 M NaOH (Figure 12): the spectra of AODETA and AO eluted with 0.025 M NaOH are almost indistinguishable. From this we propose that AO fibers with high uranyl affinity from seawater will require bifunctionality wherein the auxiliary group interacts with the amidoxime to facilitate dissociation of the acidic N−O−H.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

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The authors declare no competing financial interest.

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CONCLUSIONS The amidoxime ligand is the best ligand identified for the removal of U(VI) from seawater but it is not ideal: it has a significant affinity for the vanadyl ion in seawater and is capable of ion exchange with Group IA (Na+, K+) and IIA (Ca2+, Mg2+) cations present at far greater levels than uranyl. Ion exchange is evident in FTIR spectra from the N−O− band at 1108 cm−1. Results indicate that the affinity of AO for U(VI) can be enhanced by incorporating auxiliary diethylenetriamine ligands into the polymer; phosphorylating the primary amine sites may further enhance the affinity. This is the subject of ongoing study as is work to define the mechanism by which the triamine enhances uranyl binding. FTIR spectra support the mechanism that diethylenetriamine interacts with AO to favor N−OH dissociation which enhances uranyl sorption. However, it is also possible that DETA increases uranyl affinity by directly coordinating to the U(VI) after it ion exchanges onto the AO. FTIR spectra do not show evidence of DETA coordination, but a new synthesis protocol produces sharper FTIR spectra and results will be reported in a subsequent publication. Definitive EXAFS studies are scheduled at the Oak Ridge National Laboratory. At this point, it is evident that bifunctionality enhances uranyl sorption and will be important in a second-generation AO sorbent.

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FTIR spectra of AO (Figure S1), AO-DETA (Figure S2), phon-DETA (Figure S3), and AO-phon-DETA-80 (Figure S4) fibers (PDF)

ACKNOWLEDGMENTS We gratefully acknowledge support from the Department of Energy through Contract 120542 from the Nuclear Energy University Program administered by Battelle Energy Alliance, LLC. The actual seawater experiments were done at the Pacific Northwest National Laboratory under the supervision of Dr. Gary Gill, and we are grateful to him for his advice and the timely work done at PNNL.

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REFERENCES

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S Supporting Information *

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