Biomedical Engineering Reference
In-Depth Information
6.2
Synthetic Strategies for Amorphous Calcium Carbonate
Calcium carbonate is one of the most important biological and industrial materials
because of its abundance in nature and its wide applications in industry. The poly-
morphs of calcium carbonate include three anhydrous crystalline phases (aragonite,
vaterite, and calcite) and three metastable forms (monohydrate, hexahydrate and
ACC), in which ACC is the least stable polymorph and has been widely found
in organisms. The stabilization of ACC was commonly achieved by using various
additives, including biological macromolecules [ 49 ], magnesium [ 50 ], and synthetic
polymers [ 51 ]. It is well known that ACC has important functions in biominerals
and biomineralization processes. For example, it can be used as structural skeletal
elements and temporary storage deposits. Moreover, a variety of calcium carbonate
biominerals are now thought to be crystallized via an amorphous precursor pathway.
Therefore, the ACC plays a key role in the biomineralization of CaCO 3 and how to
synthesize the ACC in the lab is emerging as a hot research field.
In this chapter, we select several typical synthetic approaches commonly used to
synthesize ACC to give facilities for readers to grasp the various strategies reported
so far.
6.2.1
Direct-Mixing Method
6.2.1.1
The Na 2 CO 3 -NaOH-CaCl 2 Aqueous Reaction System
Amorphous precipitates of CaCO 3 were prepared by reaction in a mixed solu-
tion of Na 2 CO 3 -NaOH with a CaCl 2 solution within pH value in the range of
11.2
13.0 at an ambient temperature of 278 K [ 52 ]. Equivolumes of aqueous
solutions of NaOH (0.2 mol L 1 )andNa 2 CO 3 (0.1 mol L 1 ) were mixed with
mechanical stirring, to obtain mixed solutions. The mixed solutions and an aqueous
solution of CaCl 2 (0.1 mol L 1 ) were kept in a refrigerator at 278 K for 1 h.
Equimolar CaCl 2 solution with respect CO 3 2 in the mixed solutions was added
rapidly to the mixed solutions with mechanical stirring at an ambient temperature of
278 K. The obtained product was filtered immediately and washed with acetone. The
precipitates were dried in a vacuum desiccator for 1 day. On heating, the obtained
ACC loses the absorbed water until ca. 530 K and subsequent crystallization to
calcite (Fig. 6.1 ).
pH
6.2.1.2
The Na 2 CO 3 -MgCl 2 -CaCl 2 Aqueous Reaction System
Magnesium can certainly be expected to influence the biogenic CaCO 3 ,suchas
incorporating into the calcite lattice, inducing the formation of aragonite, stabilizing
the ACC and so on. Interestingly, we have come to learn that all known biogenic
Search WWH ::




Custom Search