[Show abstract][Hide abstract] ABSTRACT: Long-term predictions on the mobility and the fate of radionuclides and contaminants in cementitious waste repositories require a molecular-level understanding of the geochemical immobilization processes involved. In this study, the use of X-ray absorption spectroscopy (XAS) for chemical speciation of trace elements in cementitious materials will be outlined presenting two examples relevant for nuclear waste management. The first example addresses the use of XAS on powdered cementitious materials to determine the local coordination environment of Sn(IV) bound to calcium silicate hydrates (C-S-H). Sn K-edge XAS data of Sn(IV) doped C-S-H can be rationalized by corner sharing binding of Sn octahedra to Si tetrahedra of the C-S-H structure. XAS was further applied to determine the binding mechanism of Sn(IV) in the complex cement matrix. The second example illustrates the potential of emerging synchrotron-based X-ray micro-probe techniques for elucidating the spatial distribution and the speciation of contaminants in highly heterogeneous cementitious materials at the micro-scale. Micro X-ray fluorescence (XRF) and micro-XAS investigations were carried out on Co(II) doped hardened cement paste. These preliminary investigations reveal a highly heterogeneous spatial Co distribution. The presence of a Co(II)-hydroxide-like phase Co(OH)2 and/or Co-Al layered double hydroxide (Co-Al LDH) or Co-phyllosilicate was observed. Surprisingly, some of the initial Co(II) was partially oxidized and incorporated into a Co(III)O(OH)-like phase or a Co-phyllomanganate.
[Show abstract][Hide abstract] ABSTRACT: The uptake of selenate (SeVIO42−) or selenite (SeIVO32−) by hardened cement paste (HCP) and important constituents of the cement matrix such as calcium silicate hydrate (C–S–H), portlandite (CH), ettringite (AFt) and monosulfate (AFm) was investigated using X-ray absorption spectroscopy (XAS). The XAS measurements were conducted on samples with Se loadings ranging between 1200 and 8800 ppm. X-ray absorption near edge structure (XANES) spectroscopy shows that redox reactions do not influence uptake processes in the cementitious systems. The EXAFS (extended X-ray absorption fine structure) spectra of Se(IV) and Se(VI) bound to CH, AFt, AFm and C–S–H are similar to those of SeO42− and SeO32− in solution, indicating a “solution-like” coordination environment upon uptake by the cement minerals. Similarly, the spectra of Se(IV)- and Se(VI)-treated HCP samples reveal the absence of backscattering atoms at short distances. These results suggest that the coordination sphere of the SeO42− and SeO32− entities is maintained upon immobilization by HCP and cement minerals and non-specific interactions dominate at the given Se loadings.
Cement and Concrete Research 01/2006; 36(1-36):91-98. DOI:10.1016/j.cemconres.2005.05.003 · 3.85 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Synchrotron-based micro-X-ray fluorescence (micro-XRF) and micro-X-ray absorption spectroscopy (micro-XAS) were used in the present study to obtain spatially resolved micro-scale information on elemental composition, trace element distribution, chemical speciation and oxidation state and/or mineral phase distribution within historical iron artefacts dating from the Iron Age to early Medieval Times. Large area two-dimensional trace element distribution maps and oxidation state maps with micrometer spatial resolution were required to answer open archaeological questions in the context of ancient metal processing. The first set of examples was focusing on historical weapons and included two ancient iron sword blades. The micro-XRF maps revealed a distinct, highly correlated distribution pattern of trace elements such as As, Ni, Cu and Zn. Accordingly, the number of used raw materials could be determined unambiguously and key information concerning the used ancient smithing technique were gained. Further, the ability to record—in a fast manner—large area maps with high spatial resolution (‘elemental screening’) led to the discovery of a hitherto unknown enhanced occurrence of selected trace elements (Cu, Zn, and Au) at the blade surface. Complementary investigations by high resolution scanning electron microscopy were able to localize these trace metals within a carbon-rich matrix may be pointing towards an artifact induced during preservation. A second set of investigated artefacts is dealing with smithing waste products and related historical processing techniques and conditions. Synchrotron-based micro-XRF and micro-XAS were used to determine the structural composition as well as the spatial variation of the predominant Fe oxidation state and corresponding crystallographic phases. The study revealed the presence of distinct domains of Fe0, FeIIO (wustite), and α-FeIIIOOH (goethite), separated by sharp domain boundaries. These findings help to gain new insights concerning the nature and origin of used raw materials as well as regarding employed processing techniques during historic iron fabrication and weapon manufacturing.The study demonstrates the potential of oxidation state and mineral phase mapping based on energy selective micro-XRF maps and spectroscopic phase identification. Such a spatially resolved recording of the chemical speciation is based on X-ray absorption spectroscopy. This analytical technique is exclusive to synchrotron light sources. However, the steadily increasing number of available synchrotron-based X-ray microprobes allows nowadays for more routine utilization of such micro-XAS techniques.
Spectrochimica Acta Part B Atomic Spectroscopy 10/2004; 59:1627-1635. DOI:10.1016/j.sab.2004.07.028 · 3.15 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: In this study, the immobilization mechanisms of Sn(IV) onto calcium silicate hydrates (C-S-H) and hardened cement paste (HCP) have been investigated by combining wet chemistry experiments with X-ray absorption spectroscopy (XAS). Evidence is presented which demonstrates the formation of a Sn(IV) inner-sphere surface complex on C-S-H with a CaO/SiO2 weight ratio of 0.7. Two possible structural models, implying a corner sharing between the Sn octahedra and Q1 or Q2b Si tetrahedra, have been developed based on the experimentally determined structural parameters. In HCP, the formation of a different type of Sn(IV) inner-sphere complex has been observed, indicating that C-S-H may not be the uptake-controlling phase for Sn(IV) in the cement matrix. An alternative structural model for Sn(IV) binding in HCP has been developed, assuming that ettringite is the uptake-controlling phase. At high Sn(IV) concentrations, Sn(IV) immobilization in HCP occurs due to the formation of CaSn(OH)6.
[Show abstract][Hide abstract] ABSTRACT: Under anoxic conditions zero-valent iron can react with water to produce hydrogen gas and magnetite or green rust, a highly reactive mineral phase that can induce reduction processes and thus control the speciation, the solubility, toxicity and the mobility of redox sensitive elements in (nuclear) waste repositories. In this study micro X-ray fluorescence (micro-XRF) and micro X-ray absorption spectroscopy (micro-XAS) were used to investigate the speciation of selenium that immobilized in the presence of Fe(0) and an anoxic synthetic groundwater solution. The selenium immobilization was accompanied by the formation of a green rust corrosion layer. Micro-XRF revealed that a Se-rich layer is present along the iron surfaces that were exposed to the Se(IV) solution. Micro-XAS experiments at the Se K-edge showed that Se(IV) was reduced to elemental Se(0). Thus., the reactivity of zero-valent and green rust should to be considered in assessing the long-term fate of selenium in nuclear waste repositories.
Journal de Physique IV (Proceedings) 03/2003; 104:417-420. DOI:10.1051/jp4:20030112 · 0.35 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The uptake of iodine species (I − /IO 3 −) by HCP (hardened cement paste) and a CSH (calcium silicate hydrate) phase under highly alkaline conditions has been investigated using X-ray absorption spectroscopy (XAS). The study was per-formed at the I K -edge (33.169 keV) instead of the I L 3 -edge (4.557 keV) to avoid interference with Ca (K -edge = 4.038 keV), a major element in HCP and CSH phases. The XANES (X-rays absorption near-edge structure) spectra revealed no changes in the formal oxidation state of iodide (I(−I) −) and iodate (I(V)O 3 −) upon uptake by HCP and CSH. The EXAFS (ex-tended X-ray absorption fine structure) oscillations from I − treated HCP and CSH samples were found to be extremely weak, limiting interpretation of the EXAFS data. The IO 3 − EXAFS spectra showed that the IO 3 − entity consisting of three oxygen atoms with a characteristic I−O distance (∼ 1.78 Å) is main-tained upon uptake by HCP and CSH. XANES further indicated that CSH is not the uptake-controlling phase in HCP.