Solid-State NMR Identification and Quantification of Newly Formed Aluminosilicate Phases in Weathered Kaolinite Systems

Abstract

The weathering of a specimen kaolinite clay was studied over the course of 369 d via solid-state 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy and high-field 27Al MAS NMR. The chosen baseline solution conditions (0.05 mol kg-1 of Al, 2 mol kg-1 of Na+, 1 mol kg-1 of NO3-, 1 mol kg-1 of OH-, and pH approximately 13.8) approximate those of solutions leaking from waste tanks at the Hanford Site in Richland, WA. Nonradioactive Cs and Sr cations were added to this synthetic tank waste leachate (STWL) solution at concentrations of 10(-3), 10(-4), and 10(-5) molal (m) to represent their radionuclide counterparts. The transformations of silicon- and aluminum-containing solid phase species were monitored quantitatively by using NMR spectroscopy, with the resulting spectra directly reporting the influence of the initial Cs and Sr on formation and transformation of the neo-formed solids. At the lowest concentration of Cs and Sr employed (10(-5) m in each cation) peaks consistent with the formation of zeolite-like minerals were detected via 29Si and 27Al MAS NMR as early as 33 d. At concentrations of 10(-3) m in each cation, new silicon species are not detected until 93 d, although neophases containing four-coordinate aluminum were detectable at earlier reaction times via 27Al MAS NMR. At the highest magnetic field strengths employed in this NMR study, deconvolutions of resonances detected in the tetrahedral region of the 27Al MAS spectra yielded multiple components, indicating the existence of at least four new aluminum-containing phases. Two of these phases are identified as sodalite and cancrinite through comparison with diffuse-reflectance infrared (DRIFT) spectra and powder X-ray diffraction (XRD) results, while a third phase may correlate with a previously detected aluminum-rich chabazite phase. All measurable solid reaction products have been quantified via their 27Al MAS resonances acquired at high magnetic field strengths (17.6 T), and the quantitative nature of the 27Al NMR data shows that cancrinite growth increases while sodalite reaches a steady state with respect to total aluminum in the solid phases. The data also relate the coupling of Cs sorption to the ripening of feldspathoid phases in this heterogeneous system as a function of time, and illustrate the important influence of co-contaminants on the environmental reaction kinetics studied here.

Coupling Sorption to Soil Weathering During Reactive Transport: Impacts of Mineral Transformation and Sorbent Aging on Contaminant Speciation and Mobility

“…These secondary phases are capable of taking up both Cs and Sr from solution as they form. Characterization of reaction products by 27 Al and 29 Si magic angle spinning (MAS) NMR experiments conducted across a range of field strengths at PSU and EMSL have proven extremely useful in determining the time-dependent formation and the relative mass fractions of neophases [4,5]. EXAFS spectroscopic results also indicate that precipitation of SrCO 3 (s) can occur during clay and sediment reaction with STWL if its solubility is exceeded [6].…”

“…Details of experimental conditions and solution-phase chemistry are reported in [2]. 27 Al and 29 Si NMR Studies of Reacted Sediments: Results from 29 Si and 27 Al magic-angle spinning (MAS) NMR experiments on specimen clay and homogeneous nucleation samples have helped to quantify the time-dependent change in the amount of various Al-bearing secondary solids during the course of reaction under STWL conditions [2,7,9,10]. One-dimensional 29 Si and 27 Al MAS NMR experiments have been run on unreacted, STWL-reacted, and AAO-extracted HF and RG sediments at all three Cs/Sr cocontaminant concentrations.…”

Final Report: Caustic Waste-Soil Weathering Reactions and Their Impacts on Trace Contaminant Migration and Sequestration

“…At PNNL, NMR spectrometers were used with magnetic field strengths of 17.6, 18.8, and 21.1 T (corresponding to 'H resonance frequencies of 750, 800, and 900, MHz respectively). 29 Si and 2 7~1 magic-angle spinning (MAS) NMR spectroscopy have provided structural and kinetic information in weathered kaolinite and montmorillonite specimen clay systems (Chorover et al, 2003;Choi et al, 2005b;Crosson et al, 2006). By performing MAS NMR experiments at a range of magnetic field strengths, we have extracted full sets of relative populations, isotropic chemical shifi values, and quadrupolar parameters for tetrahedrally coordinated aluminum sites in four new phases formed by the weathering of kaolinite in an STWL for up to 369 days (Crosson et al, 2006).…”

“…29 Si and 2 7~1 magic-angle spinning (MAS) NMR spectroscopy have provided structural and kinetic information in weathered kaolinite and montmorillonite specimen clay systems (Chorover et al, 2003;Choi et al, 2005b;Crosson et al, 2006). By performing MAS NMR experiments at a range of magnetic field strengths, we have extracted full sets of relative populations, isotropic chemical shifi values, and quadrupolar parameters for tetrahedrally coordinated aluminum sites in four new phases formed by the weathering of kaolinite in an STWL for up to 369 days (Crosson et al, 2006). The kinetics of secondary phase formation can then be mapped based on total aluminum speciation or the separate formation kinetics of all four phases (e.g., see Fig.…”

Collboration: Interfacial Soil Chemistry of Radionuclides in the Unsaturated Zone

“…29 Si and 27 Al magic-angle spinning (MAS) NMR spectroscopy have provided structural and kinetic information in weathered kaolinite and montmorillonite specimen clay systems (Chorover et al, 2003;Choi et al, 2005b;Crosson et al, 2006). By performing MAS NMR experiments at a range of magnetic field strengths, we have extracted full sets of relative populations, isotropic chemical shift values, and quadrupolar parameters for tetrahedrally coordinated aluminum sites in four new phases formed by the weathering of kaolinite in an STWL for up to 369 days (Crosson et al, 2006). The kinetics of secondary phase formation can then be mapped based on total aluminum speciation or the separate formation kinetics of all four phases (e.g., see Fig.…”