Role of Mass Transport in the Deposition, Growth, and Transformation of Calcium Carbonate on Surfaces at High Supersaturation

Abstract

We demonstrate how combined in-situ measurements and finite element method modeling can provide new insight into the relative contribution of mass transport to the growth of calcium carbonate on two model surfaces, glass and gold, under high-supersaturation conditions relevant to surface scaling. An impinging jet-radial flow system is used to create a high-supersaturated solution at the inlet of different cells: an optical microscope cell presenting a glass surface for deposition and quartz crystal microbalance (QCM) and in-situ IR spectroscopy cells, both presenting a gold surface. The approach described is quantitative due to the well-defined mass transport, and both time-lapse optical microscopy images and QCM data are analyzed to provide information on the growth kinetics of the calcite crystals. Initially, amorphous calcium carbonate (ACC), formed in solution, dominates the deposition process. At longer times, the growth of calcite is more significant and, on glass, is observed to consume ACC from the surface, leading to surface regions depleted of ACC developing around calcite microcrystals. On Au, the mass increase becomes linear with time in this region. Taken together, these microscopic and macroscopic measurements demonstrate that calcite growth has a significant component of mass transport control at high supersaturation. Finite element method (FEM) simulations of mass-transport-limited crystal growth support the strong mass transport contribution to the growth kinetics and further suggest that the observed growth must be sustained by more than just the Ca²⁺ and CO₃ ^2- in solution, with dissolution/direct attachment of ACC and/or ion pairs also contributing to the growth process.

Figure 1

Schematic of the solution delivery system and flow cells used. (a) Solution path, (b) microscopy flow cell, (c) QCM flow cell, and (d) ATR-IR flow cell. Red lines indicate the solution direction.

Figure 2

Time-lapse images recorded with the in-situ optical microscopy set up at 50, 225, 500, 600, 800, and 990 s. Regions labeled “a” highlight the initial heterogeneous nucleation of calcite. Regions labeled “b” and “c” indicate phase change nucleation, and regions marked “d” show depletion. The Supporting Movie, from which these frames are taken, is supplied separately.

Figure 3

Estimated crystal size over time evaluated from in-situ optical images.

Figure 4

Time-lapse pictures recorded with the in-situ optical microscopy set up. ACC dissolution process in the presence of overlap of the diffusion field for two closely spaced growing calcite crystals (a) and for a single growing calcite crystal (b).

Figure 5

Deposition of calcium carbonate, using the QCM flow cell set up, as a function of time. The red dashed line shows the limiting behavior for a constant flux that characterizes the deposition process at longer times.

Figure 6

FE-SEM images of the QCM crystal after deposition. The images were taken underneath the nozzle (a) and at the edge of the gold-coated surface (b). Deposits were predominantly rhombohedral calcite, but a few more spherical particles, likely vaterite, were observed (c).

Figure 7

In-situ IR spectra (a) and fitted integrated intensities (b) showing appearance of the ν₂ mode from ACC (red) and then vaterite (black) over time.

Figure 8

FEM simulation of convection-diffusion in flow cells. R_{calcite,glass} = 7.5 μm. Inlet equilibrated with ACC. (a) Relationship between flow cell and FEM domain, (b) enlargement of FEM domain with boundaries labeled, with simulated velocity magnitude overlaid, (c) CO₃^2– concentration, (d) enlarged region of (c) showing calcite and ACC regions. Flux direction marked with streamlines. Note that (c) is reflected in the vertical axis relative to the geometry of (a), (b), and (d).

Figure 9

Schematic of calcite growth from ACC-saturated solution at three different stages in the deposition process (see text for details). CBL indicates the concentration boundary layer.