Synthesis and Adsorbing Properties of Tabular {001} Calcite Crystals

Abstract

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.

Role of Hydrodynamics, Li+ Addition and Transformation Kinetics on the Formation of Plate-Like {001} Calcite Crystals

“…At that, stirring with a magnetic bar caused an abrasive action on the glass, thus causing the predominant nucleation of metastable solid phases, like calcium oxalate trihydrate or vaterite [45]. Similarly, the ultrasonic irradiation leads to the preferential nucleation of metastable solid phases as well, as a consequence of a higher energy input in the supersaturated solutions of slightly soluble calcium salts, like carbonates or oxalates [38,46] (US data shown in Table S3 was reported previously [1].…”

“…The initial magnetic stirring was achieved by 20 mm diameter stirrer rotating at 400 rpm. The sonication was initiated in the NaHCO 3 solution before addition of CaCl 2 /LiCl solution, by using Branson Sonifier 250 (20 kHz frequency) [1]. The applied power output was set at 8 W. The ultrasonic horn (diameter 5 mm) was immersed in the center of the reaction vessel, 5 cm above the bottom.…”

“…Calcium carbonate crystals, particularly those of calcite, are interesting model systems for studying the interfacial interactions on crystal planes, namely, for any detailed analysis of crystal growth kinetics and mechanisms, or investigation of basic molecular interactions that may happen at the solid-liquid interfaces. To perform this investigation, crystal seed material uniform in size and with well exposed crystallographic planes is necessary [1][2][3][4][5][6]. Calcite crystals of different morphologies may be precipitated from aqueous solutions at different technological or environmental conditions or during the biomineralization processes.…”

“…They can be found in the brittle stars (echinoderms) in form of micro lens arrays in light receptor systems [17,18]. The plate-like calcite crystals can be produced in vitro as well, by calcium carbonate precipitation in the presence of Li + ions [1,[19][20][21]. At that, the expression of the {001} faces is enhanced by higher c(Li + )/c(Ca 2+ ) ratios [19,[21][22][23][24][25][26].…”

Biological Crystallization

“…However, in presence of Li + the tabular {00.1} faces appear together with the {10.4}, thus generating truncated rhombohedrons. The paper of Matijaković et al [3] explores the morphological aspects and adsorbing properties of model organic substances of the {10.4} versus {00.1} faces, which are relevant for the understanding of biomineralization processes, in which the {00.1} faces often interact with organic macromolecules and open new routes for the usage of calcite as adsorbing substrate with applications in the environment.…”