Nucleation, Growth and Evolution of Hydroxyapatite Films on Calcite
- PDF / 28,442,762 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 54 Downloads / 206 Views
Nucleation, Growth and Evolution of Hydroxyapatite Films on Calcite Sonia Naidu1, Jeremy M. Blair2, George W. Scherer3 1,2 Department of Chemical and Biological Engineering, Princeton University, Eng. Quad. E226, Princeton NJ 08544, USA 3 Department of Civil and Environmental Engineering, Princeton University, Eng. Quad. E-319, Princeton NJ 08544, USA ABSTRACT Marble, a non-porous stone composed of calcite, is subject to acid rain dissolution due to its relatively high dissolution rate. With the goal of preventing such damage, we have investigated the deposition of films of relatively insoluble hydroxyapatite (HAP) on marble. This paper investigates the factors that affect the nucleation and growth kinetics of HAP on marble. A mild, wet chemical synthesis route, in which diammonium hydrogen phosphate (DAP) salt was reacted with marble, alone and with cationic and anionic precursors under different reaction conditions, was used to produce inorganic HAP films on the mineral surface. Film nucleation, growth and metastable phase evolution were studied, using techniques such as scanning electron microscopy (SEM) and grazing incidence X-ray diffraction (GID). The onset of nucleation, and the growth rate of the film, increased with cationic (calcium) and anionic (carbonate) precursor additions. The calcium and phosphate precursors also influenced metastable phase formation, introducing a new phase. INTRODUCTION The motivation behind this work is to develop a surface protective treatment that is resistant to acid attack, is structurally compatible with calcite, involves relatively simple chemistry, and does not noticeably alter the aesthetics of the stone. Hydroxyapatite (HAP) was chosen due its low solubility product, Ksp (~10-59) [1] and dissolution rate, Rdiss (~10-14 moles•cm-2•s-1 at pH 5.6) [2]. HAP also has a similar crystal structure and a lattice match to calcite [3]. This structural compatibility is expected to favor nucleation of HAP and permit the formation of a coherent, epitaxial layer of HAP on the surface of calcite. A mild treatment based on reacting a solution of diammonium hydrogen phosphate (DAP) with calcite under ambient temperature and pressure to produce the mineral HAP was developed by Kamiya et al. [4]: 10CaCO3 + 5(NH4)2HPO4 → Ca10(PO4,CO3)6(OH,CO3)2 + 5(NH4)2CO3 + 3CO2 + 2H2O (1) This chemistry has been successfully applied to restore the mechanical properties of thermally damaged limestone [5-8] and marble [9, 10]. An investigation of the acid resistance of DAP-treated marble showed that HAP films were partially protective, but incomplete growth and residual porosity degraded the coating’s ability to protect the stone from acid dissolution [11]. The objectives of this paper are to investigate the factors that affect nucleation and growth of calcium phosphate (CaP) films on calcite. To study growth kinetics, film coverage was evaluated for different reaction conditions. Phase evolution was studied by distinguishing between phases using differences in crystal morphology and identifying correspondin
