David McKeown | The Catholic University of America (original) (raw)

Papers by David McKeown

Environmental Science & Technology, Dec 13, 2006

Journal of Non-Crystalline Solids, Feb 29, 2024

This report describes the results of testing specified by the Test Plans (VSL-08T1520-1 Rev 0 and... more This report describes the results of testing specified by the Test Plans (VSL-08T1520-1 Rev 0 and VSL-08T1510-1 Rev 0). The work was performed in compliance with the quality assurance requirements specified in the Test Plans. Results required by the Test Plans are reported. The test results and this report have been reviewed for correctness, technical adequacy, completeness, and accuracy.

The main objective of the work was to develop and select a glass formulation for vitrification te... more The main objective of the work was to develop and select a glass formulation for vitrification testing of the actual waste sample of LAW AP-101 at Battelle - Pacific Northwest Division (PNWD). Other objectives of the work included preparation and characterization of glasses to demonstrate compliance with contract and processing requirements, evaluation of the ability to achieve waste loading requirements, testing to demonstrate compatibility of the glass melts with melter materials of construction, comparison of the properties of simulant and actual waste glasses, and identification of glass formulation issues with respect to contract specifications and processing requirements.

This work was conducted under a quality assurance program compliant with 10 CFR 830 Subpart A, NQ... more This work was conducted under a quality assurance program compliant with 10 CFR 830 Subpart A, NQA-1 (2000), and DOE Order 414.1C. This program is supplemented by a Quality Assurance Project Plan for ORP work that is conducted at VSL [1-21]. Test and procedure requirements by which the testing activities are planned and controlled are defined in this plan. The program is supported by VSL standard operating procedures that were used for this work [1-22]. Exceptions are the X-ray absorption spectroscopy measurements and the tin Mössbauer spectroscopy measurements, which were performed at outside facilities and are of research quality. The requirements of DOE/RW-0333P are not applicable to this work.

Physical Review, Dec 31, 1988

Journal of Nuclear Materials, 2017

Mo-containing high-level nuclear waste borosilicate glasses were investigated as part of an effor... more Mo-containing high-level nuclear waste borosilicate glasses were investigated as part of an effort to improve Mo loading while avoiding yellow phase crystallization. Previous work showed that additions of vanadium decrease yellow phase formation and increases Mo solubility. X-ray absorption spectroscopy (XAS) and Raman spectroscopy were used to characterize Mo environments in HLW borosilicate glasses and to investigate possible structural relationships between Mo and V. Mo XAS spectra for the glasses indicate isolated tetrahedral Mo 6+ O 4 with MoO distances near 1.75 Å. V XANES indicate tetrahedral V 5+ O 4 as the dominant species. Raman spectra show composition dependent trends, where MoO symmetrical stretch mode frequencies ( 1) are sensitive to the mix of alkali and alkaline earth cations, decreasing by up to

Journal of Non-Crystalline Solids, 2004

Sulfur can be the waste-loading limiting constituent for vitrification of sulfur-bearing radioact... more Sulfur can be the waste-loading limiting constituent for vitrification of sulfur-bearing radioactive wastes due to low solubility in silicate melts. Methods to improve sulfur loading would benefit from improved understanding of the structural aspects of sulfur incorporation in borosilicate and other glasses. To this end, sulfur XANES spectra were collected for eight crystalline standards and twenty-four glasses, including borosilicate, phosphate,

Geobiology, Feb 27, 2023

Non‐crystalline silica mineraloids are essential to life on Earth as they provide architectural s... more Non‐crystalline silica mineraloids are essential to life on Earth as they provide architectural structure to dominant primary producers, such as plants and phytoplankton, as well as to protists and sponges. Due to the difficulty in characterizing and quantifying the structure of highly disordered X‐ray amorphous silica, relatively little has been done to understand the mineralogy of biogenic silica and how this may impact the material properties of biogenic silica, such as hardness and strength, or how biosilica might be identified and differentiated from its inorganic geological counterparts. Typically, geologically formed opal‐A and hyalite opal‐AN are regarded as analogs to biogenic silica, however, some spectroscopic and imaging studies suggest that this might not be a reasonable assumption. In this study, we use a variety of techniques (X‐ray diffraction, Raman spectroscopy, and scanning electron microscopy) to compare differences in structural disorder and bonding environments of geologically formed hydrous silicas (Opal‐A, hyalite, geyserite) and silica glass versus biogenic silicas from an array of organisms. Our results indicate differences in the levels of structural disorder and the Raman‐observed bonding environments of the SiO2 network modes (D1 mode) and the Q‐species modes (~1015 cm−1) between varieties of biogenic silicas and geologically formed silicas, which aligns with previous studies that suggest fundamental differences between biogenic and geologically formed silica. Biosilicas also differ structurally from one another by species of organism. Our mineralogical approach to characterizing biosilicas and differentiating them from other silicas may be expanded to future diagenesis studies, and potentially applied to astrobiology studies of Earth and other planets.

Physical Review B, 1992

X-ray-absorption near-edge structure (XANES) data were collected for Zn in sphalerite (ZnS), and ... more X-ray-absorption near-edge structure (XANES) data were collected for Zn in sphalerite (ZnS), and for Cu and Fe in chalcopyrite (CuFeS~), where all three cations are in nearly identical coordination environments. The data have similar features, except near the edge, where the edge maximum decreases in amplitude, while a pre-edge feature appears and increases in amplitude from Zn to Cu to Fe. The pre-edge feature was previously assigned to a 1s-to-3d atomic transition for Cu and Fe in the chalcopyrite structure. XANES calculations were performed for all three edges. The multipleand single-scattering contributions to the calculated edge spectra were scaled down to better fit the experimental data. The conclusions from the calculations indicate that the pre-edge feature in the experimental Cuand Fe-edge data for chalcopyrite can be due to interference effects from the atomic structure surrounding the absorber, but cannot exclude the possibility that the pre-edge feature is due to atomic bound-state transitions of the absorber.

Journal of Non-Crystalline Solids, 2005

Silver K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine str... more Silver K-edge X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data were collected and analyzed to characterize silver (Ag) environments in borosilicate and Ca-aluminoborate glass formulations developed as potential candidates for the immobilization of certain nuclear wastes. Silver is found in some nuclear waste streams and must be encapsulated in glass during waste vitrification processes. A related concern deals with phase separation within these glasses and whether colloidal silver would be present in the glass melt, which could present processing issues, or in the waste glass product. Characterization of the silver environments provides useful information for optimizing the silver incorporation ability of such glasses. Data were also gathered on four crystalline standards: Ag-foil, Ag 2 O, argentojarosite (AgFe 3 (SO 4) 2 (OH) 6), and AgO. XANES data indicate Ag + as the dominant species in the glasses. XANES and EXAFS data show that the average Ag environment in the Ca-aluminoborate glass is different compared with those in the two borosilicate glasses investigated. EXAFS analyses show that Ag in the borosilicate glasses is coordinated by two oxygens in a similar environment to that in crystalline Ag 2 O, except that the associated AgO distances are approximately 0.10 Å longer in the glass. Silver in the Ca-aluminoborate glass may be within one highly disordered site, or possibly, several different sites, where the average AgO distance, coordination number, and Debye-Waller factor are larger than those determined for the borosilicate glasses. Despite their relatively high silver contents, there is no evidence from XANES or EXAFS of colloidal silver in the glasses investigated.

Journal of Non-Crystalline Solids, 2022

This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit:

Journal of Nuclear Materials, 2014

High-level waste (HLW) glasses containing bismuth, phosphorus, and chromium were investigated usi... more High-level waste (HLW) glasses containing bismuth, phosphorus, and chromium were investigated using Raman and X-ray absorption spectroscopy (XAS). The novel and practically important occurrence of foaming on cooling of these melts is associated with P and Cr from the HLW. In response, glasses were synthesized where Bi 2 O 3 and P 2 O 5 contents were varied independently. Relationships between P and Cr were found, where as P 2 O 5-content increases, chromate Cr-O stretch Raman modes diminish intensity, while Cr XAS shows that Cr reduces, from 50% Cr 6+ + 50% Cr 3+ to nearly 20% Cr 6+ + 80% Cr 3+ , explaining the chromate mode behavior. In the most P 2 O 5-rich glass, the chromate Cr-O distance increases by approximately 0.10 Å, which may indicate bonding between CrO 4 and PO 4 tetrahedra, similar to that in chromophosphates. The presence of chromo-phosphate domains in HLW melts can be linked to oxygen generation as a source of the foaming.

Journal of Non-Crystalline Solids, 2008

X-ray absorption spectroscopy (XAS) was used to characterize the tin (Sn) environments in four bo... more X-ray absorption spectroscopy (XAS) was used to characterize the tin (Sn) environments in four borosilicate glass nuclear waste formulations, two silicate float glasses, and three potassium aluminosilicate glasses. Sn K-edge XAS data of most glasses investigated indicate Sn 4+ O 6 units with average Sn-O distances near 2.03 Å. XAS data for a float glass fabricated under reducing conditions show a mixture of Sn 4+ O 6 and Sn 2+ O 4 sites. XAS data for three glasses indicate Sn-Sn distances ranging from 3.43 to 3.53 Å , that suggest Sn 4+ O 6 units linking with each other, while the 4.96 Å Sn-Sn distance for one waste glass suggests clustering of unlinked Sn 4+ O 6 units.

Journal of Non-Crystalline Solids, 2003

X-ray absorption data were collected and analyzed to characterize the manganese environments in b... more X-ray absorption data were collected and analyzed to characterize the manganese environments in borosilicate glass formulations to be used for immobilization of nuclear wastes. Mn can become a significant constituent in some radioactive wastes, because of the use of Mn-compounds in waste pretreatment processes. Sixteen borosilicate glasses were investigated, which were synthesized to simulate the Mn environments in the anticipated waste glasses, where MnO concentrations range from 0.4 to 13.6 wt%. The X-ray absorption near edge structure (XANES) for all glasses investigated indicate that most of the manganese within these samples is divalent. The extended X-ray absorption fine structure (EXAFS) analysis results for the glasses show average Mn-O distances near 2.07 A A, coordination numbers between 4.3 and 5.2, and large first-shell Debye-Waller factors. The EXAFS findings indicate that Mn 2þ in borosilicate glass is most likely within a distribution of environments that include 4-and 5-coordinated sites. EXAFS data and fitting results also show that the average manganese environments in these glasses are statistically invariant with respect to composition as well as to synthesis conditions.