Systematic XAS study on the reduction and uptake of Tc by magnetite and mackinawite

Abstract

The mechanisms for the reduction and uptake of Tc by magnetite (FeO) and mackinawite (FeS) are investigated using X-ray absorption spectroscopy (XANES and EXAFS), in combination with thermodynamic calculations of the Tc/Fe systems and accurate characterization of the solution properties (pH, pe, [Tc]). Batch sorption experiments were performed under strictly anoxic conditions using freshly prepared magnetite and mackinawite in 0.1 M NaCl solutions with varying initial Tc(vii) concentrations (2 × 10 and 2 × 10 M) and Tc loadings (400-900 ppm). XANES confirms the complete reduction of Tc(vii) to Tc(iv) in all investigated systems, as predicted from experimental (pH + pe) measurements and thermodynamic calculations. Two Tc endmember species are identified by EXAFS in the magnetite system, Tc substituting for Fe in the magnetite structure and Tc-Tc dimers sorbed to the magnetite {111} faces through a triple bond. The sorption endmember is favoured at higher [Tc], whereas incorporation prevails at low [Tc] and less alkaline pH conditions. The key role of pH in the uptake mechanism is interpreted in terms of magnetite solubility, with higher [Fe] and greater recrystallization rates occurring at lower pH values. A TcS-like phase is predominant in all investigated mackinawite systems, although the contribution of up to 20% of TcO·xHO(s) (likely as surface precipitate) is observed for the highest investigated loadings (900 ppm). These results provide key inputs for an accurate mechanistic interpretation of the Tc uptake by magnetite and mackinawite, so far controversially discussed in the literature, and represent a highly relevant contribution to the investigation of Tc retention processes in the context of nuclear waste disposal.

Framework Modifications and Dehydration Path of a Ag+-Modified Zeolite with STI Framework Type

“…The evolution of EXAFS spectra as a function of temperature was analyzed by Principal Component Analysis (PCA) 55 and iterative target-transformation factor analysis (ITFA) using the software package ITFA 56,57 . The PCA is used to identify whether a set of spectra can be represented as linear combination of a smaller number of independent components spectra, to derive the minimum number of components and finally ITFA is used to quantify the contribution of the components in each spectra 58 . Table 1 Crystal data and refinement parameters of Ag-stellerite (Ag-STI) at RT, 75 (RH = 0), 150 (RH = 0), 225 (RH = 0), 300 (RH = 0) and 400°C (RH = 0).…”

Mechanism unravelling for highly efficient and selective 99TcO4− sequestration utilising crown ether based solvent system from nuclear liquid waste: experimental and computational investigations

“…The E h value of the system and also an aqueous concentration of Tc( iv ) with TcO 2 ·1.6H 2 O(s) in equilibrium are significantly affected by high ionic strength, particularly in the case of 4.5 M MgCl 2 solutions. In the presence of concentrated brines and alkaline conditions (pH = pH max 9) Tc( vii ) is reduced to Tc( iv ) using magnetite, mackinawite 56 at pH = 8–9 in the presence of concentrated MgCl 2 and NaCl solutions. Here, we have obtained the extraction profile of TcO 4 − ions at different pH values of the medium (pH 1–14) ( Fig.…”

ROBL-II at ESRF: a synchrotron toolbox for actinide research

“…The high EXAFS performance on the detector side is supplemented by a unique set of inhouse data analysis tools, like wavelet plot analysis using Cauchy or FEFF-derived wavelets (Funke et al, 2005(Funke et al, , 2007, iterative transformation factor analysis (ITFA) (Rossberg et al, 2003(Rossberg et al, , 2009Yalçıntaş et al, 2016), also coupled with Monte Carlo (MC) simulations (Rossberg & Scheinost, 2005;Kirsch et al, 2011), the direct derivation of the radial pair distribution by Landweber iteration (Rossberg & Funke, 2010), and artificial intelligence approaches (Domaschke et al, 2014), in order to derive as much information as possible from this versatile technique. Fig.…”