Trace element incorporation in calcium carbonate precipitates: Characterization using synchrotron-based x-ray diffraction, x-ray fluorescence, and transmission x-ray microscopy

To better understand how substitution of foreign cations or oxyanions affects reactivity of the precipitated solid, distribution of the elements within the crystal and the overall extent of coprecipitation needs to be well characterized. This work focuses on coprecipitated calcium carbonate minerals that form as a result of carbon mineralization in subsurface environments or in industrial waste disposal sites such as mine tailings. There is motivation to understand how foreign elements are taken up in calcium carbonate precipitates as such environments are abundant in naturally occurring sources of toxic elements which are especially hazardous if released into our drinking water sources. To explore this, coprecipitation experiments involving zinc, cadmium, and arsenic were conducted, and precipitates were examined using synchrotron-based powder XRD and XRF analyses. Results showed changes to both morphology and polymorphism of calcium carbonate when formed in the presence of foreign components, and elemental maps revealed coprecipitation as a dominant method of uptake. Work presented here combines micro- and nano-scale characterizations of precipitated foreign-ion bearing calcium carbonates obtained from a suite of synchrotron- and laboratory-based imaging techniques including transmission x-ray microscopy (TXM), transmission electron microscopy (TEM), and scanning electron microscopy. By combining these techniques, we aim to elucidate the relationships between internal structural features and physical distribution of foreign elements within the observed features, and to understand the fate of these coprecipitated elements once exposed to extreme but relevant environmental conditions (such as acidic and Fe-rich solutions).