One of the most common crystal habits of the thermodynamically stable polymorph of calcium carbonate, calcite, is the rhombohedral one, which exposes {10.4} faces. When calcite is precipitated in the presence of Li+ ions, dominantly {00.1} faces appear together with the {10.4}, thus generating truncated rhombohedrons. This well-known phenomenon is explored in this work, with the aim of obtaining calcite crystals with smooth {00.1} faces. In order to achieve this objective, the formation of calcite was examined in precipitation systems with different c(Ca2+)/c(Li+) ratios and by performing an initial high-power sonication. At the optimal conditions, a precipitate consisting of thin, tabular {001} calcite crystals and very low content of incorporated Li+ has been obtained. The adsorption properties of the tabular crystals, in which the energetically unstable {00.1} faces represent almost all of the exposed surface, were tested with model dye molecules, calcein and crystal violet, and compared to predominantly rhombohedral crystals. It was found that the {00.1} crystals showed a lower adsorption capability when compared to the {10.4} crystals for calcein, while the adsorption of crystal violet was similar for both crystal morphologies. The obtained results open new routes for the usage of calcite as adsorbing substrates and are relevant for the understanding of biomineralization processes in which the {00.1} faces often interact with organic macromolecules.
Seashells are a calcium-carbonate-based material that can be converted into valuable advanced functional materials. Seashells are also a waste material from aquaculture. They are produced in millions of tonnes per year and represent an environmental issue. They uniquely contain an intraskeletal organic matrix rich in carboxylate groups that so far has not been exploited or has been even removed, when they were used as calcium carbonate substitutes. The intraskeletal organic matrix allows for a so far never reported covalent functionalization. Such a process strengthens the surface functionalization with respect to adsorption and, most importantly, opens up the possibility for the functionalization of the biogenic calcium carbonate with a wide variety of molecules by means of organic chemistry reactions. As a proof of concept, powdered waste oyster shells were covalently functionalized with a fluorescent probe. The impact of this research can be terrific in the valorization of CaCO 3 from biogenic wastes providing advanced functional products tailored for individual applications. Moreover, its consequences on the environment and society will epitomize a perfect example of a circular economy.
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