Alkaline earth metal ions have significant impacts on natural systems and environmental processes. For example, magnesium is essential for the light harvesting reaction of photosystem I; the ratio of calcium to magnesium in a soil determines the tightness of a soil; adsorption of alkaline earth metal ions to mineral surfaces impacts the surface charge and subsequent adsorption of transition metals, oxyanions and natural organic matter; and the presence of precipitates such as limestone and dolomite often controls the buffering capacity of natural waters. Characterizing the adsorption and precipitation of alkaline earth metal ions in natural systems has been unexpectedly challenging. While surface complexation models have dramatically improved the ability to describe adsorption of transition metal cations as a function of pH, ionic strength and solution composition, the effects of ionic strength on adsorption of alkaline earth metal ions to oxide and hydroxide surfaces are often more dramatic and less understood compared to transition metal cations, and the structure and location of surface complexes within the interfacial region remains controversial. The interaction between alkaline earth metal ions during precipitation reactions is also of particular environmental interest. For example, while the presence of Mg2+ is known to inhibit calcite precipitation, the impacts vary depending on whether homogeneous nucleation or heterogeneous nucleation dominates. This presentation evaluates trends in adsorption of alkaline earth metal ions (Mg2+, Ca2+, Sr2+ and Ba2+) over a range of ionic strengths and oxide mineral characteristics to elucidate the impact of mineral properties such as dielectric constant and pHpzc and to distinguish between inner and outer sphere and monodentate and multidentate sorption complexes. Macroscopic experiments, surface complexation modeling and molecular modeling are used to explain differences in adsorption behavior and trends between low dielectric solids such as iron oxides and higher dielectric minerals such as manganese oxides. In moving from adsorption to precipitation, heteronucleation of calcite (and the impact of Mg2+ on heteronucleation) is shown to be a function of pH, degree of oversaturation, and ratio of calcium to carbonate. The results also highlight the need to characterize alkaline earth metal ion adsorption to explain heteronucleation precipitation processes.

Alkaline earth metal ion sorption processes: From adsorption to precipitation

249th National Meeting (2015)

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