ARTICLE pubs.acs.org/est
Soft X-ray Spectromicroscopy of Cobalt Uptake by Cement Rainer D€ahn,†,‡,* Marika Vespa,^,† Tolek Tyliszczak,§ Erich Wieland,† and David K. Shuh‡ †
Laboratory for Waste Management, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland Actinide Chemistry Group, Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States § Advanced Light Source Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States ‡
bS Supporting Information ABSTRACT: Scanning transmission X-ray microscopy was used to investigate the speciation and spatial distribution of Co in a Co(II)-doped cement matrix. The aim of this study was to improve the understanding of the heavy metals immobilization process in cement on the molecular level. The Co-doped cement samples hydrated for 30 days with a Co loading of 5000 mg/kg were prepared under normal atmosphere to simulate conditions used for cement-stabilized waste packages. Co 2p3/2 absorption edge signals were used to determine the spatial distributions of the metal species in the Co(II)-doped cement. The speciation of Co was determined by collecting near-edge X-ray absorption fine structure spectra. On the basis of the shape of the absorption spectra, it was found that Co(II) is partly oxidized to Co(III). The correlation, respectively the anticorrelation with elements such as Al, Si, and Mn, show that Co(II) is predominantly present as Co-hydroxide-like phase as well as Co-phyllosilicate, whereas Co(III) tends to be incorporated only into a CoOOH-like phase. Thus, this study suggests that thermodynamic calculations of Co(II)-immobilization by cementitious systems should take into consideration not only the solubility of Co(II)-hydroxides but also Co(III) phases.
’ INTRODUCTION The safe disposal of hazardous and radioactive wastes in deep geological repositories is a challenging task. Although new technologies focusing on radioactive waste minimization will reduce the waste arising in the future, strategies ensuring safe waste immobilization are still needed. The radionuclide release from a deep geological repository can be controlled and reduced by a suitable choice of the engineered and geological barriers. Cement-based materials play an important role in multibarrier concepts developed worldwide for the disposal of low- and intermediate-level radioactive wastes. In Switzerland, for example, it is planned to dispose of cement-stabilized radioactive wastes arising from electricity production in nuclear power plants as well as medicine, industry, and research in a deep geological repository. Hardened cement paste (HCP) is used to condition and stabilize the waste materials and to construct the engineered barrier systems (container, backfill, and liner materials) of repositories for radioactive waste.1 Therefore, a mechanistic understanding of the processes governing the binding and retention of heavy metals in cement systems is essential for long-term predictions of the environmental impact of cement-stabilized waste forms. From a chemical standpoint, HCP is an extremely heterogeneous material with discrete minerals in the nano- to micrometer size range, which makes investigations with the scanning transmission X-ray microscope (STXM) ideal. The HCP material consists of mainly calcium (aluminum) silicate hydrates (C-(A)-S-H), portlandite (calcium hydroxide), calcium aluminates, and ferrites. Interaction of groundwater with the waste matrices in the repository can result in the release of radionuclides. The released radionuclides can then be transported through engineered barrier systems (cement) to the surrounding host rock. As a consequence, immobilization processes, such as r 2011 American Chemical Society
the precipitation of newly formed solid phases, can considerably retard the release of radionuclides. The immobilization potential of HCP originates from its selective binding properties for metal cations and anions (e.g., ref 2). In the approach toward molecular-level descriptions of uptake mechanisms in HCP, macroscopic studies using radiotracers have been combined with spectroscopic techniques. The former method yields kinetic and thermodynamic information on uptake processes, whereas the latter method provides corresponding information on the chemical and structural environment of species on the molecular level. For example, bulk X-ray absorption spectroscopy (XAS) has successfully been applied to gain a molecular-level understanding of the interaction of Ni, Zn, Sn, Se, I, and U with HCP and individual cement minerals, for example, C-S-H, which is the most abundant mineral in the cement matrix.3-9 Several recent studies have employed an approach of combining conventional spectroscopic and microscopic techniques, for example, scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) and backscattered electron (BSE) imaging, with synchrotron-based micro X-ray fluorescence (micro-XRF) and micro-XAS to gain spatially resolved microscale knowledge on immobilization processes in heterogeneous waste matrices.10-13 BSE-imaging allows typical constituents of cement phases to be identified based on gray-level contrast and the morphology. In the next step, micro-XRF mapping is used to study the elemental distribution in the same region of interest as previously imaged by SEM and to identify Received: November 3, 2010 Accepted: December 21, 2010 Revised: December 21, 2010 Published: January 25, 2011 2021
dx.doi.org/10.1021/es103630t | Environ. Sci. Technol. 2011, 45, 2021–2027
Environmental Science & Technology regions of interest that then can be analyzed by micro-XAS. However, the micro-XAS studies carried out to date were limited to a microscale spatial resolution (∼1 1 μm2), which is orders of magnitudes higher than the spatial resolution of SEM (a few nanometers). STXM is the only available technique combining imaging and spectroscopy with a spatial resolution on the nanometer scale applicable to environmental systems (such as cement and soils). Other techniques such as transmission electron microscopy (TEM) combined with electron energy loss spectroscopy (EELS) have the disadvantage of causing severe radiation damage in environmental systems.14,15 Recently, Vespa et al.11 investigated the uptake of Co(II) by cement using micro-XRF/XAS. Co is an important contaminant in waste materials generated in medical applications and industrial processes. For example, 60Co is a key gamma-ray source and is extensively used as a tracer and radiotherapeutic agent.16 Coradioisotopes may also be associated with irradiation-activitated metallic components from nuclear power plants and can, therefore, be present in cement-stabilized radioactive waste. In this case, a mechanistic understanding of Co-immobilization is of major importance for predicting the long-term fate of Co in the cementitious near-field of repositories for radioactive waste. In connection with the disposal of hazardous industrial and municipal waste, molecular-level information on the speciation of Co will allow a more detailed assessment of the leachability of the heavy metal to be performed, for example, from landfills and contaminated soils into aquifers. The investigations of Vespa et al.11 revealed that Co(II) is partly oxidized to Co(III) in samples prepared under atmospheric conditions. The study further showed that it is not possible with a microscale beam to distinguish whether Co(II) is predominantly present as a pure Co-hydroxide-like phase and/or Co-phyllosilicate, and whether Co(III) tends to be incorporated into a pure CoOOH-like phase and/or Co-phyllomanganate. The study concluded that either a mixture of different Co(II) and Co(III) phases has formed in the Codoped cement samples and/or that the size of the Co(II)/ Co(III)-forming phases is smaller than the beam size (5 5 μm2), thereby resulting in an average signal on the length scale probed. The objective of the present study was to investigate the Co speciation in Co(II)-doped cement on the nanometer scale. The hydration process was started by adding a Co(II) salt solution to the unhydrated cement. Hydration was carried out under normal atmospheric conditions to simulate real conditions under which cementitious waste packages are usually produced. The Co(II)doped cement samples were investigated using a scanning transmission X-ray microscope (STXM) coupled with near-edge X-ray absorption fine structure (NEXAFS) spectroscopy. STXM is well-suited to address questions about the heterogeneity of cementitious materials because it provides elementspecific quantitative mapping of individual chemical species in an energy range of 130 to 2100 eV17,18 at environmentally relevant concentrations (i.e., a few hundred mg/kg) with a spatial resolution of down to 30 nm.19-23 STXM imaging combined with NEXAFS spectroscopy can provide chemical information at this spatial scale with a high spectral resolution (
