Pertechnetate-Induced Addition of Sulfide in Small Olefinic Acids: Formation of [TcO(dimercaptosuccinate)₂]^5- and...

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.