Master of Science
Longstaffe, Fred J.
Interlayer cation composition and cation hydration enthalpy can potentially affect the excess hydrogen-yield and δ2H of smectitic clays. Complexities arise from condensed atmospheric water adsorbed on the clay, as it constitutes a potential additional source of hydrogen when attempting to measure the δ2H of structural OH groups. To approach this problem, the δ2H of different cation-saturated, dried forms of six Clay Minerals Society Source Clay smectites were measured. A modified sample drying and on-line High-Temperature-Conversion-Elemental-Analysis (TCEA) Continuous-Flow-Isotope-Ratio-Mass-Spectrometry protocol, facilitated isotopic measurements. A stronger interlayer cation hydration enthalpy (Ca2+>Na+>K+), caused higher residual (post-degassing) adsorbed water contents, which produced poorer smectite δ2H reproducibility. Drying K-saturated smectites, 4 hours at 220˚C, produced the lowest adsorbed water contribution and the most reproducible and possibly ‘accurate’ δ2H for smectite hydroxyl hydrogen. Notwithstanding limited rehydration potential, the TCEA method provided the lowest measurement error for hydroxyl δ2H and facilitated greater sample throughput than classical methods.
Summary for Lay Audience
Some clay minerals, such as smectites, are expandable and can fit positively charged ions (cations) between 2:1 layers in what is called the interlayer, similar to the filling between pieces of bread in a sandwich. Each of these cations has different abilities to hold atmospheric water vapour around itself in the interlayer. Prior to measurement of the hydrogen isotope composition (δ2H) of hydrogen in the sheets of the clay mineral (OH hydrogen), hydrogen from adsorbed water must first be removed by heating. Accurate measurement of the δ2H of OH hydrogen is important for understanding the history of water movement in clay-rich rocks that may be selected for underground nuclear waste storage (water that remains in the rock for millennia is best). The δ2H of six dry smectites from the Clay Minerals Society Source Clay Project were measured to investigate how water adsorbed by cations in the interlayer, and still remaining after heating of the smectite at 220°C, can change δ2H. A modified sample drying and on-line High-Temperature-Conversion-Elemental-Analysis (TCEA) Continuous-Flow-Isotope-Ratio-Mass-Spectrometry procedure was used for δ2H measurements. The TCEA bakes the clay mineral at 1450°C until all hydrogen is removed, and the Mass Spectrometer measures the δ2H of this hydrogen. The cation that held water most strongly was Ca, followed by Na, and then K. Therefore, smectites with only Ca or Na in the interlayer held onto more adsorbed water, even after being heated at 220°C for several hours, before being baked at 1450°C for δ2H analysis. When this excess adsorbed water remained, it mixed with the OH hydrogen during baking at 1450°C and changed the measured δ2H from what would be characteristic of OH hydrogen only. In contrast, excess adsorbed water was removed from smectites containing K in the interlayer by heating at 220°C prior to isotopic analysis. In summary, for smectites containing only interlayer K, heating for 4 hours at 220˚C prior to isotopic analysis produced the lowest adsorbed water contribution and the most reproducible and possibly ‘accurate’ δ2H for smectite OH hydrogen.
Kanik, Nadine J., "Impact of interlayer cation composition and strongly bound water on smectite ẟ²H, as determined by a modified TCEA method" (2020). Electronic Thesis and Dissertation Repository. 7472.
Available for download on Wednesday, December 07, 2022