Organic coprecipitates with calcite: NMR spectroscopic evidence

Dissolved organic ligands are well known to interact strongly with the calcite surface, altering precipitation and dissolution rates, crystal morphology, and possibly the ability of calcite to sequester metal contaminants. We show, using NMR spectroscopic techniques, that some of the citrate molecules present in a solution of precipitating calcite are incorporated structurally into the calcite crystal. Calcite grown by a seeded constant-addition method contains approximately 1 wt % coprecipitated citrate and yields ¹³C{¹H} cross-polarization magic-angle spinning NMR spectra that contain broad peaks for citrate plus a signal from carbonate. Results from ¹³C{¹H} heteronuclear correlation NMR experiments show that citrate is located in close spatial proximity to carbonate groups. In addition, calcite/citrate coprecipitates contain about 0.4 wt% excess water, which is not present as fluid inclusions, and some of which occurs as rigid structural water. These results suggest that water and hydrogen-bonding interactions play a role in the interface between included organic molecules and the calcite host. Additional NMR data obtained for calcite coprecipitates of aspartic and glutamic acids suggest they are also incorporated structurally but at concentrations much lower than for citrate, whereas no evidence was found for phthalate incorporation.