Efficient and Selective Extraction of 99mTcO4â from Aqueous Media

Subscriber access provided by University of Rochester | River Campus & Miner Libraries

Letter

Efficient and Selective Extraction of 99mTcO4– from Aqueous Media using Hydrophobic Deep Eutectic Solvents Timothy E. Phelps, Nakara Bhawawet, Silvia S Jurisson, and Gary A. Baker ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03950 • Publication Date (Web): 15 Oct 2018 Downloaded from http://pubs.acs.org on October 15, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Sustainable Chemistry & Engineering

Efficient and Selective Extraction of 99mTcO4– from Aqueous Media using Hydrophobic Deep Eutectic Solvents Tim E. Phelps†, Nakara Bhawawet†, Silvia S. Jurisson*,†and Gary A. Baker*,† †Department

of Chemistry, University of Missouri, 601 S. College Ave., Columbia MO

65201 Email of corresponding authors: [email protected] (S.S.J.); [email protected] (G.A.B.)

ABSTRACT: Extraction of trace pertechnetate (99mTcO4–) from aqueous media using hydrophobic deep eutectic solvents (DESs) is reported for the first time. The hydrophobic DESs

studied

comprise

a

1:2

molar

ratio

of

a

tetraalkylammonium

or

tetraalkylphosphonium halide mixed with a monocarboxylic acid (i.e., saturated fatty acid). Quantitative (>99%) removal of tracer levels of

99mTcO – 4

from an aqueous phase in the

presence of a large excess of competing anions (e.g., Cl–, NO3–, HCO3–, H2PO4–, SO42–) within 5–60 min at 25 C without the aid of a formal extracting agent (e.g., ionophore) demonstrates an efficient and selective extraction. At infinite dilution of pertechnetate, very high distribution ratios in excess of 103 are observed. Attempts to strip technetium from the DES extractant phase by reduction with SnCl2 are also reported. Overall, these

ACS Paragon Plus Environment

1

ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 25

outcomes provide the basis for considering hydrophobic DESs as an alternative platform for 99TcO4– waste management.

KEYWORDS: Deep eutectic solvent, Extraction, Pertechnetate, Perrhenate, Oxyanions

INTRODUCTION Technetium-99 (99Tc, –, t1/2 = 2.11 × 105 years), a radionuclide produced in ~6% yield during artificial nuclear fission (e.g., during the production of weapon-grade plutonium,

239Pu),

is responsible for high levels of radioactive waste and continues to be

a major concern at nuclear waste sites due to the release of pertechnetate (Tc(VII)O4–), a chemically stable, toxic, and mobile anion, into the environment.1,

2

To date, several

strategies have been pursued to immobilize or remove TcO4– from aqueous media, including precipitation by reduction to insoluble technetium dioxide (TcO2),3, 4 extraction with environmentally unfriendly (e.g., toxic, high waste, or non-recyclable) chemicals,5-9 and ion-exchange methods.10-15 Deep eutectic solvents (DESs) represent an intriguing, potentially sustainable, and unexplored opportunity in this regard. DESs are fluids comprised of components self-associating via complex, dynamical, and correlated hydrogen-bonding networks to produce a eutectic mixture with a melting point below that of its individual components.16-19 Although a typical DES consists of a 1:2 molar ratio mixture of hydrogen-bonding accepter (HBA) and hydrogen-bonding donor (HBD) species (e.g., choline chloride coupled with urea: a standard DES referred to as reline),


2

Page 3 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


unconventional DESs including halide-free examples20 and hydrophobic (waterimmiscible) versions have recently emerged as well. For example, Kroon and co-workers demonstrated hydrophobic DESs as extractants for the recovery of volatile fatty acids21 and transition metal ions (e.g., Co2+)22 from diluted aqueous solutions, with Tereshatov et al. expanding this strategy to encompass indium extraction.23 Very recently, hydrophobic terpene-based eutectic systems have also been investigated by Coutinho and co-workers and tested for metal separations.24, 25 In the present communication, we demonstrate for the first time the efficient and selective extraction of trace

99mTcO – 4

from aqueous solutions using hydrophobic DESs.

The component structures of the three hydrophobic DESs were varied by the choice of HBA cation (trihexyltetradecylphosphonium, [P14,666+] or tetraoctylammonium, [N8888+]) and fatty acid as HBD species (hexanoic or decanoic acid), combined in a 1:2 (HBA:HBD) molar ratio (Figure 1). We note that the DES comprising 1:2 [N8888][Br]:[DecA] (denoted DES B in this communication) has already been reported and characterized previously.21 The appearance (Figure S1) and NMR characterization (Figures S2–4) of the synthesized DESs are provided in the Supporting Information (SI). Proton NMR characterization confirms that the identities and ratios of the individual constituents are retained upon formation of the eutectic, as expected. The extraction efficiency, effects of competing anions, and conditions for testing back extraction conditions are discussed in the following section. RESULTS AND DISCUSSION


3


Page 4 of 25

In order to study the remediation of TcO4– from aqueous systems using hydrophobic DESs, desired volumes of aqueous media containing a wide range of anion matrices (i.e., 0.15 M Cl–, NO3–, H2PO4–, SO42–, I–, or ReO4– at pH 5 or 0.15 M HCO3– at pH 8) were spiked with radiotracer (sub-nanomolar) levels of

99mTcO – 4

and then thoroughly

vortexed with specified volumes of a hydrophobic DES. Samples were mixed for a specified period of time adequate to achieve equilibrium and then centrifuged to facilitate phase separation. Technetium-99m (99mTc, , t1/2 = 6.0 h) was employed as a surrogate for 99Tc

to optimize extraction parameters. Aliquots of each phase (DES and aqueous) were

assayed by counting the 99mTc 140 keV γ-ray using a NaI(Tl) well counter detector (Ortec Model 4890). We note that pH 5 and 8 are both within the range of natural waters, normally considered to be pH 4–9, depending on environmental conditions.26-28 As shown in Figure 2, quantitative extraction of

99mTcO – 4

(>99%) by the hydrophobic DESs is

demonstrated when using equivolume (1:1, v/v) mixtures of DES and aqueous phase containing a large excess of common anions, such as Cl– and NO3–. The distribution ratios (DTc), calculated as the ratio of the activity (counts) of

99mTc

in the DES to that of the

aqueous phase at 25 ± 1 °C (given by eq S1 in the SI), are presented in Figure S5 (SI). Figure 3 shows the distribution ratios of

99mTcO – 4

in the presence of HCO3–, Cl–,

NO3–, H2PO4–, and SO42– competing ions when employing a 1:50 (v/v) ratio of DES to aqueous phase. Notably, the extraction efficiency for

99mTcO – 4

is only minimally affected

by competing anions in most cases and is highly competitive with existing methods.6, 8, 9


4

Page 5 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Before discussing these results further, it is important to appreciate that a number of complex ion features, including ion size, hydration energies, hard-soft-acid-base (HSAB) electrostatic interactions, and ion-pairing all likely play pivotal roles in the observed extraction behavior.21, 22 Originating from the Hofmeister series, which orders ions on their ability to stabilize or destabilize proteins and membranes, ions can be categorized according to their kosmotropicity or chaotropicity. Ions having strong interactions with water molecules (i.e., water-structuring ions such as PO43–, CO32–, SO42–, S2–) have large negative standard molar Gibbs free energies of hydration (ΔhydG°) and are termed kosmotropes.29 On the other hand, chaotropic ions have weak or unfavorable interactions with water and are “structurebreakers”. Kosmotropes are generally small and highly charged, while chaotropes are large and possess low charge. In practice, all multivalent ions are highly hydrated and are, therefore, highly kosmotropic. Within this context, the partitioning of the charge-diffuse TcO4– to the hydrophobic DES phase is likely driven by its more chaotropic nature relative to other ions in the effluent and the sacrificial halide anion “preloaded” in the DES. This is possible because TcO4– (like other oxyanions) has less hydration energy due to its smaller charge-to-radius ratio. Returning to Figure 3, it is apparent that quantitative

99mTcO – 4

extraction is

maintained for a 1:50 (v/v) extraction performed in the presence of excess competing anions having more favorable ΔhydG° (and less favorable HSAB interactions), namely HCO3–, Cl–, H2PO4–, and SO42– which have reported ΔhydG° values of –335, –340, –465, and –1080 kJ mol–1, respectively.30 Although excess NO3– (ΔhydG° = –300 kJ mol–1)30 slightly suppressed 99mTcO – extraction 4

using a 1:50 (v/v) system, this was not entirely surprising as high NO3–

concentrations were previously reported to lower

99TcO – 4

extraction efficiency.8 Given


5


Page 6 of 25

these results, it is safe to assert that that these hydrophobic DES media are selective for TcO4– over common anions since a large excess of competing anions in the aqueous phase is not detrimental to

99mTcO – 4

extraction efficiency (note that, for the 1:50 (v/v) system,

0.15 M of competing anion represents an amount 8- to 10-fold higher than the number of moles of DES used in the extraction). Conversely, anions with less favorable ΔhydG°, namely ReO4– and I– (–235 kJ mol–1 31 and –275 kJ mol–1,30 respectively), serve as positive controls and suppress

99mTcO – 4

extraction when exceeding a 1:5 (v/v) mixture for perrhenate (ReO4–) or a 1:10 (v/v) mixture for iodide (Figure 4). The perrhenate anion is commonly employed as a nonradioactive Group 7 structural analog of TcO4– due to its comparable ion size and properties (i.e., hydrated ionic radii of 2.6 and 2.5 Å; surface charge ionic densities of 1.2 and 1.3 Å–2, and ΔhydG° of –330 and –251 kJ mol–1 for ReO4– and TcO4–, respectively32. Not surprisingly, both localize in the thyroid, as does iodide.33 The distribution ratios obtained for

99mTcO – 4

from an aqueous solution of

0.15 M ReO4– using DES A as extracting phase reveal that DTc falls off monotonically as the volumetric ratio of DES to aqueous phase decreases (Figure S6, SI). This behavior is fully expected because the ReO4– is a surrogate for TcO4– and is effectively competing with radiological tracer levels of the pertechnetate, ultimately exceeding the capacity of the DES phase to uptake tetra-oxo anions. Unexpectedly, however, the formation of a precipitate was evident after mixing aqueous ReO4– and I– with DESs B and C, for all v/v ratios. In order for

99mTcO – 4

to partition

from the aqueous phase into the opposing DES phase, the charge balance of both phases must be


6

Page 7 of 25 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


maintained, with the most likely mechanism being anion exchange with the slightly more kosmotropic halide of the DES (i.e., ΔhydG° = –340 and –315 kJ mol–1 for Cl– and Br–, respectively,30 compared to a ΔhydG° of –251 kJ mol–1 for TcO4–): TcO4–aq + C+X–DES ↔ X–aq + C+TcO4–DES

(1)

where the subscripts aq and DES denote the aqueous and DES phases, respectively, and X– is a halide provided by the DES (for example, for DES B, C+X– denotes [N8888][Br]). We note that, although transfer of an ion-pair (such as Na+TcO4–) directly to the DES phase cannot be ruled out, it is expected to be a trivial, with anion exchange being the dominant mode of partitioning. Given the logical assumption that anion exchange is operative here and that this is also the likely mode for ReO4– and I– uptake by the DES, the observation of a precipitate indicates ion pairing and that the concomitant [N8888+][ReO4–] and [N8888+][I–] salts are poorly soluble in water. The formation of [N8888+][ReO4–] and [N8888+][I–] ion pairs is a manifestation of the stronger soft-soft electrostatic interactions between these anions and the bulky, hydrophobic [N8888+] cation, in contrast with weaker associations observed for the smaller, less polarizable (harder) anions Cl–, NO3–, H2PO4–, HCO3– and SO42–.34 It should be recognized that similar formation of insoluble [N8888+][TcO4–] is also anticipated during

99TcO – 4

extraction, however, the

Efficient and Selective Extraction of 99mTcO4â from Aqueous Media

Recommend Documents

Efficient and Selective Extraction of 99mTcO4â from Aqueous Media