An exploration of benchtop X‐ray emission spectroscopy for precise characterization of the sulfur redox state in cementitious materials

Abstract

The evolution of sulfur chemistry in cements is best known in the bailiwick of failure mechanisms via sulfate attack, but is equally important for its contributions to the reduction capacity of cementitious materials often used for immobilizing nuclear waste streams destined for long‐term storage, for example, cementitious waste forms (CWF). The total reduction capacity of CWFs, encompassing contributions from both S and Fe reductants, and its implications toward radionuclide immobilization is most often studied by destructive wet chemistry methods requiring acid digestion in the presence of Ce(IV) and subsequent titration and colorimetric interpretation. Here, we investigate a similarly analytical but nondestructive alternative, benchtop high resolution wavelength‐dispersive X‐ray fluorescence spectroscopy, most commonly known as X‐ray emission spectroscopy (XES), for probing the bulk sulfur oxidation state distribution. We present here an initial investigation of S XES, including an improved experimental protocol for lab XES of inhomogeneous samples, both as a complement to the Ce(IV) test and for new scientific opportunities that it enables for observing changes in sulfur chemistry. We discuss future improvements and opportunities, including: (1) the practical challenges associated with coordinating XES and Ce(IV) liquid extraction for a more comprehensive perspective on reduction capacity and for a high‐precision evaluation of uncertainties in the Ce(IV) test; and (2) new opportunities, due to the nondestructive nature of XES, for controlled evolution studies aimed at elucidating specific chemical responses of CWFs exposed to invasive gas or liquid species or to accelerated aging by radiative dose or thermal treatment.

Laboratory X-ray Emission Spectrometer for Phosphorus Kα and Kβ Study of Air-Sensitive Samples

“…To minimize potential errors from finite sample size, sample homogeneity, or analyzer irregularities, we follow the methods of Abramson, et al 25 . This is done by taking short two-minute camera exposures at each position while stepping individual samples by ~0.44 mm through the illumination region and summing the resulting spectra.…”

“…Recent work has, however, emphasized that some of the above issues surrounding access can be addressed with high-throughput laboratory XES for analytical application [25][26][27][28][29][30] . This includes laboratory-based study of the P Kα and Kβ XES of several systems 21,29,31,32 with successful determination of oxidation state distribution, ligand identity, and bonding of various phosphorus compounds.…”

“…Building on these prior developments and addressing the added complexity of air-sensitivity of P-rich systems, we report here the design and commissioning of a new laboratory-based XES spectrometer that simultaneously measures the P Kα (core-to-core) and P Kβ (valence-to-core) XES while permanently installed in a research-grade controlled-atmosphere glovebox. This instrument uses the small footprint 'Dispersive Rowland Refocusing' (DRR) geometry 33 seen to have synchrotron-level performance across many studies despite the use of only a low-powered, low-brilliance conventional x-ray tube source 25,33,34 . We propose that the integration of compact laboratory based XES instruments with controlled atmosphere gloveboxes holds high scientific and analytic potential, well beyond the present case of phosphorus.…”

Atomic spectrometry update: review of advances in the analysis of metals, chemicals and materials

“…Analysis was achieved using ICP-MS with an assortment of internal standards introduced as a mixture on-line rather than being spiked into individual samples. The internal standards used were 45 Sc for those elements with a mass up to and including 59 Co, 72 Ge for the analytes 62 Ni to 95 Mo, 125 Te for 101 Ru to 137 Ba, 175 Lu for the analytes 189 Os to 197 Au and 209 Bi for analytes with a higher mass. Any polyatomic interferences were removed through the introduction of helium at a ow rate of 5 mL min −1 into the collision/reaction chamber.…”

“…A paper by Abramson et al employed X-ray emission spectroscopy for precise characterization of the S redox state in cementitious materials and, in particular, cementitious waste forms. 137 Cementitious waste forms are containers used to retain radionuclides and so it is important to identify their reductive capacity because many of the contents, e.g. pertechnetates (Tc VII ) are less soluble if reduced to Tc IV .…”