Technetium stabilization in Portland cement and bentonite clay barriers by thiourea

In this paper we elaborated the prospective approach for the immobilization of technetium in bentonite clay and Portland cement. It includes thiourea (Tu) addition for Tc effective speciation modification from mobile Tc(VII) to immobile Tc species. Based on the resulting compound structure analysis of Tu with technetium, the reductive mechanism of immobilization was proved. A new technetium complex structure with Tu was described by single crystal XRD analysis, where technetium was obtained in the oxidation state + 3. The complex is stable due to the coordination binding of Tc to the sulfur atoms of the Tu molecules and to the chlorine atom, effective charge transfer along the S–Tc bond, and intramolecular hydrogen bonds. XANES spectra characteristics of technetium (VII, IV, III) reference compounds and samples containing Tc–Tu species in a cement matrix and bentonite clay indicates the presence of technetium in bentonite clay in the form of its tetravalent oxidation state during the Fourier transformation. Graphical abstract