Dissolution Kinetics of Epitaxial Cadmium Carbonate Overgrowths on Dolomite

Abstract

Thin films grown on mineral surfaces can immobilize toxic metals in natural systems, but the effects of epitaxy and film thickness on dissolution rates of the overgrowth are typically unknown. To explore these effects, otavite (CdCO3) films were grown on dolomite (104) surfaces from aqueous solutions containing [Cd] = [Ca] = [CO3] = 0.2 mM for 1–48 h and then dissolved in deionized water. Films of various thicknesses and strain states were obtained by varying the growth reaction time. Growth for up to 3 h produced strained thin films with Cd coverages of <4.3 ± 0.6 equivalent monolayers (ML) (corresponding to a thickness of up to 17 Å), whereas reaction times of up to 48 h produced thicker films with as many as 114 ± 14 equivalent ML of Cd. Intrinsic dissolution rates were measured as a function of otavite film thickness by a combination of synchrotron X-ray fluorescence, specular X-ray reflectivity, and atomic force microscopy. The initial dissolution rates for unstrained films were comparable to those reported for synthetic otavite powders, whereas the thinnest films (<3.1 ± 1.1 ML) dissolved at a 50% slower rate, indicating that epitaxial strain effectively enhanced their stability. We discuss potential reasons for this difference.

Effect of pH on the Formation of Gibbsite-Layer Films at the Muscovite (001)–Water Interface

“…Such a difference in thermodynamic stability was shown recently for thin otavite (CdCO 3 ) overgrowths on dolomite (Ca,Mg(CO 3 ) 2 ) compared to bulk otavite. 73 We conducted dissolution experiments The derived electron-density profile after reaction in DIW (Al DIW ) is similar to that obtained before the reaction (Figure 9). The best-fit model profile maintains three sharp peaks, indicative of the first layer of the gibbsite film, located at similar heights (Tables S10 and S11).…”

“…The inferred strain in the gibbsite films resulting from the epitaxy with the muscovite substrate implies that these thin layers of gibbsite may have a different thermodynamic stability from bulk gibbsite. Such a difference in thermodynamic stability was shown recently for thin otavite (CdCO 3 ) overgrowths on dolomite (Ca,Mg­(CO 3 ) 2 ) compared to bulk otavite . We conducted dissolution experiments to test the relative stability of the gibbsite films.…”

Carbonate mineral controls the transport of Cd from tailings to surrounding soils: An example from Cd-rich Niujiaotang Zn mine in Guizhou Province, Southwest China

“…Cadmium has a similar ion radio to Ca, therefore, Cd can substitute Ca in the dolomite crystal lattice to form (Cd, Ca) Mg(CO 3 ) 2 (Goldsmith, 1972). Cadmium incorporated into dolomite is also through otavite (CdCO 3 ) surface precipitate (Callagon et al, 2017;La Plante et al, 2018). As a result, dolomite is widely used to remove Cd in the environmental media (Mohammadi et al, 2015;Liu et al, 2021;Zhang and Liu, 2022).…”