Stable strontium (Sr) isotopes 86Sr, 87Sr, and 88Sr in water and solid phases are routinely used by geoscientists to track environmental processes, such as rock weathering and flow of water in geothermal fields. The difference in the 87Sr/86Sr ratios can be used to distinguish between carbonate- and silicate-weathering in soils and rocks. Stable isotope fractionation of Sr takes place during precipitation of calcium carbonate minerals, due to kinetic effects. Studies on mass-dependent adsorption of Sr isotopes are nearly non-existent. Previous studies on other elements indicate that both enrichment or depletion of specific isotopes can take place during adsorption onto mineral surfaces. We use batch and column adsorption experiments, ab initio density functional theory calculations, and reactive transport models to quantify and predict isotopic fractionation during reactive flow of Sr-containing waters. We performed laboratory experiments to quantify isotope-specific adsorption of Sr onto iron (Fe) oxide Fe2O3, aluminum (Al) oxide Al2O3, and clay minerals kaolinite and montmorillonite. We used stock solutions enriched in one Sr isotope: 86, 87, or 88. We analyzed solutions pre- and post-adsorption, using an inductively coupled plasma mass spectrometer (ICP-MS) configured for isotopic ratio analysis. These batch experiments show that strontium isotopic fractionation can take place during adsorption on all four examined mineral surfaces. The affinity of each examined isotope varied, depending on the chemical make-up of the surface. Surface-area-normalized adsorption of Sr isotopes on Al2O3 oxide was the highest among the examined substrates with the most pronounced difference in uptake between the three examined Sr isotopes. For both Al2O3 and kaolinite surfaces the affinity of three Sr isotopes varied as 86&gt;87&gt;88. For Fe2O3, surface-area-normalized uptake was the lowest, with 86Sr and 87Sr isotopes having about the same affinity for the surface, while 88Sr uptake was lower than 86Sr and 87Sr uptake. We conclude that the isotope-specific adsorption depends on the reactive surface sites involved in this partitioning reaction. We will present reactive transport modeling results to evaluate the impact of mineral surface area and distribution on competitive isotope adsorption for different minerals under flow conditions.

Sr stable isotope fractionation during adsorption on Fe and Al oxides and clay minerals

Macromolecular Chemistry: The Second Century

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ACS Spring 2020 National Meeting & Expo

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.