Amorphous Calcium Carbonate: Preparation, Transformation And Application

In this dissertation, the reason for bio-mineral investigation, the progress in bio-mineralization and especially, the importance of amorphous calcium carbonate (ACC) in bio-mineralization have been reviewed. Although considerable progress has been made in the preparation and mechanism study of amorphous calcium carbonate, there are still lots of mysteries on the synthesis, stability and transforamtion behavior of amorphous calcium carbonate. Therefore, the present dissertation will focus on amorphous calcium carbonate (ACC). It was found that a small organic molecule can stabilize amorphous calcium carbonate. The role of organic molecule in stablization and crystallization manipulation and the mechanism in crystallization of amorphous calcium carbonate were studied. The application of polyacrylic acid (PAA) stabilized amorphous calcium arbonate for water treatment was investigated. Also, a proper dye was found for dyeing amorphous calcium carbonate and the shifting behavior of dye in crystallization was studied. The main results can be summarized as follows:1. It has been found that a low molecular weight organic molecule, 1,3-diamino-2-hydroxypropane-N,N,N',N'-tetraacetic acid, could stabilize amorphous calcium carbonate for at least three days in gas diffusion reaction. The transformation process from ACC to calcite crystals has been systematically studied. Nucleation sites and intermediates were both captured by time-dependent experiments. Two forms of ACC were found. One could form a close packed film on substrate and part of the nucleation occurred on the film. The other formed sphere-like conglomerations and dissolved from inside after nucleation occurred. The intermediates on substrate were found to be composed of fibres. A rod-dumbbell-sphere transformation phenomenon in crystallization transformation was observed. Selective adsorption and mesocrystal transformation mechanism are assumed to play key roles in the transformation process from ACC to calcite.2. Polyacrylic acid stabilized amorphous calcium carbonate was investigated for removal of toxic heavy metal ions from aqueous solutions. The maxium removal capacities for Cd²⁺, Pb²⁺, Cr³⁺, Fe³⁺and Ni²⁺ ions were found to be 514.62 mg g^-1, 1028.21 mg g^-1, 258.85 mg g^-1, 320.5 mg g^-1 and 537.2 mg g^-1, respectively. The feature of amorphous calcium carbonate in water treatment exhibits not only in high removal capacities, but also in decontamination of trace ions. A ca 83.0% on average removal percentage was found in treatment of trace radioactive Eu³⁺ ions polluted water. The decontamination of ACC on Cd²⁺ ions polluted solutions was systematically studied. A precipitation transformation mechanism caued by the high solubility of ACC was proposed to play key roles in such water treatment.3. 1,3-diamino-2-hydroxypropane-N,N,N',N'-tetraacetic acid stabilized amorphous calcium carbonate was synthesized via hydralization of dimethyl carbonate. The composition and crystallization was investigated. It was found that the compositions of such ACC were nearly the same and the proportion of organic molecule in ACC was quite low. Meanwhile, the crystallization behavior of such ACC was studied. It was found that such ACC almost transfromed into rod-like crystals, no mater how great differences exist in initially added organic molecules, indicating a very strong control effect on ACC transformation of organic molecule. A dye, 1,6,7,12-tetrachloro-3,4,9,10-perylene dianhydride, was used to stain amorphous calcium carbonate, resulting a complex of amorphous calcium carbonate and dye with high fluorescence intensity. It was found that such ACC-dye complex not only changed the emission wavelength, but also enhanced the fluorescence intensity greatly. The crystallization process of such dye-ACC was monitored by fluorescence microscopy, and it was found that the dye tranferred to the crystalline phase, indicating a prefer adsroption behavior on crystalline phase of selective adsroption indirectly.