Dissolution of CaCO 3 (1014) Surface
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ABSTRACT Atomic force microscopy (AFM) has been used to study dissolution of the cleaved CaCO3(1014) surface in clean and impurity containing aqueous environments. In the clean solution, dissolution was found to occur by retreat of steps and creation of rhombohedral pits on a surface. Dissolution is anisotropic with two different step velocities differing by a factor of 2.3, resulting from different atomic step structures. Dissolution is partially changed after adding impurities in the solution via rounding of the fastest dissolution comer of rhombohedral pits and slowing down the step velocity in that direction. The role of impurity on dissolution is discussed in terms of preferential adsorption of impurities on kink sites. INTRODUCTION
CaCO 3 (calcite) is among the most common reactive phases in nature, and dissolution and precipitation of calcite play substantial roles in environmental contaminant transport and remediation. It has been found that CaCO3 could affect the contaminant migration by interacting with impurities through sorption and co-precipitation[1]. It is important, therefore, to understand the way the impurities interact with the surface and affect the dissolution. Although reactions at the solid-solution interface are far more common in nature, they receive much less attention in the surface science community than those that occur at the solid vacuum interface, partially due to the complexity of the process. In the case of dissolution, it is controlled by two processes: surface reaction and diffusion through a boundary layer above the surface[2,3]. The co-existence of more than one processes greatly complicates the understanding and the quantification. It is desirable, therefore, to separate the two processes and to examine one at a time. The separation between interfacial interaction and boundary layer diffusion can be achieved by controlling the rate of solution flow. A flowing solution effectively reduces the thickness of the boundary layer. When the flow rate is high enough so that diffusion length in solution (Vt-hD-, where D is the diffusion constant and 'r is the residence time) is greater than the thickness of the boundary layer, the dissolution is controlled mainly by the surface reaction. Therefore, investigation of dissolution under a high flow rate condition allows us to examine the role of interfacial interaction on dissolution. The objective of this work is to use AFM as an in situ, real time, and high resolution probe to study the surface dissolution. A particular goal was a mechanismic understanding of the effect 409 Mat. Res. Soc. Symp. Proc. Vol. 355 01995 Materials Research Society
of impurities on dissolution. After a brief description of experimental details, we will present AFM images of CaCO 3(1014) surface under solution which show the effect of dissolution on surface morphology. These images provide direct visual information of the dissolution process at the nanometer to micron scale. We will also examine the effect of impurities on dissolution by showing the change of
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