Functional Group Adsorption on Calcite: I. Oxygen Containing and Nonpolar Organic Molecules

Considerable interest in calcite crystallization has prompted many studies on organic molecule adsorption. However, each study has explored only a few compounds, using different methods and conditions, so it is difficult to combine the results into a general model that describes the fundamental mechanisms. Our goal was to develop a comprehensive adsorption model from the behavior of a range of organic compounds by exploring how common functional groups interact with calcite and the effects of various side groups and hydrogen on adsorption. We used density functional theory, with semiempirical dispersion corrections (DFT-D2), to determine adsorption energy on calcite {10.4} for nonpolar (benzene, ethane, and carbon dioxide) and oxygen containing polar molecules (water, methanol, ethanol, phenol, formic acid, acetic acid, propanoic acid, benzoic acid, formaldehyde, acetaldehyde, propionaldehyde, benzaldehyde, dimethyl ether, acetone, and furan). From the adsorption energies, within the transition state theory approximation, we derived desorption temperature for each molecule. Then we used X-ray photoelectron spectroscopy (XPS) to determine the desorption temperature for four representative molecules and compared the experimental results with those predicted. Carboxylic acids (R-COOH) adsorb more strongly than water and alcohols (ROH), which in turn adsorb more strongly than the aldehydes (R-CHO). Attachment of a hydrogen atom or a side group changes adsorption behavior for hydroxyl and aldehyde functional groups but does not affect the carboxyl functional group significantly.