Article pubs.acs.org/IC
Cite This: Inorg. Chem. 2017, 56, 13214-13227
Pertechnetate-Induced Addition of Sulfide in Small Olefinic Acids: Formation of [TcO(dimercaptosuccinate)2]5− and [TcO(mercaptosuccinate)2]3− Analogues Kimberly M. Reinig,† Rachel Seibert,‡ Daniel Velazquez,‡ Jakob Baumeister,† Firouzeh Najafi Khosroshahi,† Wei Wycoff,§ Jeff Terry,‡ John E. Adams,† Carol A. Deakyne,† and Silvia S. Jurisson*,† †
Department of Chemistry and §NMR Core Facility, University of Missouri, Columbia, Missouri 65211, United States Department of Physics, Illinois Institute of Technology, Chicago, Illinois 60616, United States
‡
S Supporting Information *
ABSTRACT: Technetium-99 (99Tc) is important to the nuclear fuel cycle as a long-lived radionuclide produced in ∼6% fission yield from 235U or 239Pu. In its most common chemical form, namely, pertechnetate (99TcO4−), it is environmentally mobile. In situ hydrogen sulfide reduction of pertechnetate has been proposed as a potential method to immobilize environmental 99TcO4− that has entered the environment. Reactions of 99TcO4− with sulfide in solution result in the precipitation of Tc2S7 except when olefinic acids, specifically fumaric or maleic acid, are present; a water-soluble 99Tc species forms. NMR (1H, 13C, and 2D methods) and X-ray absorption spectroscopy [XAS; near-edge (XANES) and extended fine structure (EXAFS)] studies indicate that sulfide adds across the olefinic bond to generate mercaptosuccinic acid (H3MSA) and/or dimercaptosuccinic acid (H4DMSA), which then chelate(s) the 99Tc to form [99TcO(MSA)2]3−, [99TcO(DMSA)2]5−, or potentially [99TcO(MSA)(DMSA)]4−. 2D NMR methods allowed identification of the products by comparison to 99Tc and nonradioactive rhenium standards. The rhenium standards allowed further identification by electrospray ionization mass spectrometry. 99TcO4− is essential to the reaction because no sulfide addition occurs in its absence, as determined by NMR. Computational studies were performed to investigate the structures and stabilities of the potential products. Because olefinic acid is a component of the naturally occurring humic and fulvic acids found in soils and groundwater, the viability of in situ hydrogen sulfide reduction of environmental 99TcO4− as an immobilization method is evaluated.
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INTRODUCTION Technetium is a group 7 element situated below manganese and above rhenium and has no stable isotopes. Its redox chemistry is complex, with known oxidation states ranging from 1− to 7+. The most well-known radioisotope of technetium is 99mTc from its long-time use in diagnostic nuclear medicine, with compounds for myocardial stress/rest tests and bone scans among the approved radiopharmaceuticals. A second technetium radioisotope is of importance environmentally, namely, 99Tc, because of its long half-life (t1/2 = 2.1 × 105 years) and ∼6% yield from the thermal neutron fission of 235U and 239Pu. Technetium exists predominantly as one of two species in the environment, either pertechnetate (TcO4−) or technetium(IV) oxide (TcO2). 99 Tc has entered the environment from the nuclear era, and the pertechnetate anion, the most common chemical form under oxic conditions, is mobile. Hanford, WA, was one of the larger sites used for nuclear weapons development for the Manhattan Project, with several tons of nuclear fuel reprocessed to isolate 239 Pu. Underground storage tanks were used to contain the 54 million gallons of radioactive waste generated during reprocessing, and some of the tanks have leaked, with more © 2017 American Chemical Society
than 1 million gallons of radioactive waste seeping into the vadose zone (soil above the water table).1 Because of the close proximity of the Hanford Site to the Columbia River, 99Tc is of significant environmental concern. Several studies aimed at remediating pertechnetate in the vadose zone through in situ stabilization have been reported,2−7 with immobilization of environmental pertechnetate by reduction to an insoluble technetium(IV) species as a common theme. In situ immobilization of technetium species requires either direct or indirect reduction of technetium(VII), the latter by reduction of surface iron(III) in minerals common in soils (e.g., goethite, hematite), which would then reduce the technetium. One potential method to immobilize metal ions involves in situ gaseous hydrogen sulfide reduction, which can act both directly and indirectly on technetium(VII).8 Sulfides, such as sodium sulfide and hydrogen sulfide, react with pertechnetate in solution and precipitate technetium(IV) oxide or insoluble technetium Received: August 4, 2017 Published: October 10, 2017 13214
DOI: 10.1021/acs.inorgchem.7b02001 Inorg. Chem. 2017, 56, 13214−13227
Article
Inorganic Chemistry sulfides, namely, TcS2 and Tc2S7.3 Equations 1−4 show the potential reactions under aqueous conditions.
counting (LSC). A Centrifan compact evaporator/concentrator (KD Scientific, St. Louis, MO) was used for concentrating 99Tc-containing samples. All water (H2O) used was 18 MΩ and was degassed with argon prior to use. Except where noted, chemicals were reagent grade, obtained from either Aldrich Chemical Co. or Fisher Scientific, and were used without further purification. Sodium sulfide in solution was used in place of hydrogen sulfide gas and was prepared by adding the crystalline solid to water or buffer that had been purged with argon for at least 2 h prior to use. C18 reversed-phase silica gel (230−400 mesh) was purchased from Aldrich Chemical Co. A CEM Discover SP microwave system with ActiVent Technology (CEM Corp., Matthews, NC) was used for the microwave synthesis of (Na/NH4)5[ReO(meso-DMSA)2] (detailed below). Measurements. Radio-TLC (TLC = thin-layer chromatography) strips (Saturation pads, Analtech, Newark, DE) were counted using a Bioscan 200 Imaging Scanner (gas ionization detector), or by cutting the TLC strips and counting them in a Tri-Carb 2900TR Liquid Scintillation Analyzer (PerkinElmer, Waltham, MA). All yields were determined by 99Tc radio-TLC using an acetone−H2O (50:50, v/v) mobile phase. NMR spectroscopy was run on a Bruker DRX500 or a Bruker DRX600 spectrometer at 25 °C in D2O. 4,4-Dimethyl-4silapentane-1-sulfonic acid was used as an internal reference. IR spectra were obtained as oil mulls on a Thermo Nicolet Nexus 670 FT-IR instrument. (Na/NH4)5[ReO(meso-DMSA)2]. Synthesis of Na5[ReO(mesoDMSA)2] was accomplished following modification of a literature procedure.13 meso-2,3-Dimercaptosuccinic acid (DMSA; 134.8 mg, 0.74 mmol) was dissolved in 2 mL of H2O. Stannous chloride dihydrate (SnCl2·2H2O; 83.5 mg, 0.37 mmol) dissolved in 2 mL of H2O was added, followed by the dropwise addition of saturated sodium bicarbonate (NaHCO3) to pH 7. Ammonium perrhenate (NH4ReO4; 100 mg, 0.37 mmol) dissolved in 2 mL of H2O was then added to the reaction mixture, which was stirred at room temperature for 30 min and then filtered. The resulting brown solution was dried in vacuo, yielding silver-orange crystals (146 mg, 0.26 mmol, 70%). Liquid chromatography−electrospray ionization mass spectrometry (LC−ESI-MS) and NMR (1H and 13C) were used to confirm product formation. 1H NMR [500 MHz, D2O, 27 °C, δ (ppm)]: 4.43 (2H, s), 4.465, 4.49 (2H, d), 4.505 (2H, s). FT-IR (KBr, ν/cm−1): 910, 935, 953 (ReO st, three isomers). (Na/NH4)3[ReO(MSA)2]. Method 1: Mercaptosuccinic acid (MSA; 111.1 mg, 0.74 mmol) was dissolved in 2 mL of H2O. NH4ReO4 (100 mg, 0.37 mmol) dissolved in 2 mL of H2O was added, and saturated NaHCO3 was added dropwise to pH 7. SnCl2·2H2O (83.5 mg, 0.37 mmol) was added to the mixture as a solid. The reaction mixture was stirred at room temperature for 30 min and then filtered. The resulting brown solution was dried in vacuo, yielding silverbrown crystals. Method 2: MSA (104.59 mg, 0.69 mmol) was dissolved in 2 mL of H2O. In a separate vial, NH4ReO4 (37.45 mg, 0.14 mmol) was dissolved in 2 mL of H2O. The two solutions were combined in a microwave reaction vessel. Saturated NaHCO3 was added dropwise until the pH reached 7. SnCl2·2H2O (62 mg) was added to the mixture. The microwave vessel was immediately capped and heated at 125 °C for 2 min with stirring (200 W max). The reaction mixture was cooled to room temperature and then filtered to separate the brown precipitate from the dark-brown supernatant. The supernatant was dried in vacuo and then washed with acetone (5 mL × 3). The presence of multiple isomers precluded isolation of a solid and quantification. FT-IR (KBr, ν/cm−1): 910, 920, 923 (ReO st, multiple isomers). 13C NMR [500 MHz, D2O, 27 °C, δ (ppm)]: 45.64, 46.02, 46.65, (CH2); 48.63, 55.50, 55.73, 55.97 (CH); 182.95, 183.32, 183.66, 195.68, 195.87, 197.09 (COOH). ESI-MS (m/z): 497.19, 499.16 (calcd for 185/187 ReO9C8H8S2 [M−]); 249.50, 251.37 (calcd for 185/187ReO9C8H7S2 [M2−]). (Na/NH4)5[99TcO(meso-DMSA)2] and (Na/NH4)3[99TcO(MSA)2]. The synthesis of Na5[99TcO(meso-DMSA)2] has been reported;12 a modification of this procedure was used. SnCl2·2H2O (31 mg, 0.14 mmol) dissolved in 1 mL of H2O was added to a solution of the appropriate mercaptosuccinic acid (DMSA, 51 mg, 0.28 mmol; MSA, 104 mg, 0.98 mmol) dissolved in 2 mL of H2O. A NaHCO3 solution
TcO4 − + 3e− + (n + 2)H 2O + 4H+ ↔ TcO2 ·nH 2O (1) −
−
2TcO4 + 7HS + H 2O ↔ Tc 2S7 + 9OH
−
(2)
2TcO4 − + 7H 2S + 2H+ ↔ Tc 2S7 + 8H 2O
(3)
2TcO4 − + 7S2 − + 8H 2O ↔ Tc 2S7 + 16OH−
(4)
Liu et al. demonstrated the effectiveness of using Na2S to reduce and precipitate technetium as Tc2S7 over a wide pH range from aqueous solutions under oxic and anoxic conditions.3 Exposed to air, Tc2S7 only formed under acidic conditions (∼80% at pH 1), with decreasing immobilization yield as the pH increased and
