The crystallization of calcium carbonate
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The Crystallization of Calcium Carbonate
II. Calcite Growth Mechanism ~ G. H. NANCOLLAS AND ~-V[.~/[. R E D D Y
Chemistry Department, State University of New York at Buffalo, Buffalo, New York
Hale Waihona Puke sults, however, since the implied assumption of homogeneous nucleation is of doubtful validity (1). Nucleation is likely to occur on impurity particles which offer available sites for crystM growth. Both the size and size distribution of the particles vary during spontaneous precipitation experiments. In an earlier publication (1), preliminary results for the kinetics of growth of calcite crystals were reported. The present study is an extension of this work over a range of temperatures enabling the calculation of the energy of activation of the crystal growth process. Parallel photomicrographic studies of the growing crystals have yielded important information concerning concomitant surface structure changes.
824
Journal of Colloid and Interface Science, Vol. 37, No. 4, December1971
CALCITE GROWTH MECHANISM runs had an average edge length of 10 t~m and an estimated surface area of 0.3 m~/gm. Supersaturated calcium carbonate solutions were prepared by the procedure described earlier (1). A Coming Model 12 pH meter and Sargent Combination Electrode (no. 8-30072-15) with a Sargent. 5'Iodel SP recorder allowed continuous measurement of hydrogen ion activity during a crystal growth run. Before and after each run, the electrode system was standardized with NBS buffers prepared according to Bates (10). In all crystal growth experiments, magnetic stirring was employed with a Teflon-coated stirring bar; the speed of stirring was measured using a strobe light. Calcium carbonate seed crystals and supersaturated solutions were separated during the growth experiment by filtering 10 ml of the inoculated supersaturated solution through a Mitlipore filter (no. BDWP01390). Analysis of the total calcium ion concentration in the filtrate was performed using a Perkin-Elmcr Model 303 Atomic Absorption Spectrometer. A computer program was employed to calculate ionic concentrations in solution during crystal growth using the experimental total calcium ion concentrations and hydrogen ion activities. Calculation of ionic concentrations were made as described previously (1) using literature values for carbonic acid-dissociation constants (11) and making allowance for calcium cm'bonate, bicarbonate, and hydroxide ion pairs (12-14). Activity coefficients were calculated using the extended form of the Debye-H~ekel equation suggested by Davies (15). A Unitron Series N microscope with a Polaroid Camera was used for photomieroseopy of calcite crystals.
The mechanism of growth of calcium carbonate crystals is important in desalination technology because of the formation of calcium carbonate scale on heat transfer surfaces (2). In addition, calcium carbonate crystallization may also play a major role in carbonate buffering of seawater (3). In seawater itself, the effect of the various ionic constituents upon the rate of calcium carbonate crystallization may explain the anomalous existence of metastable calcium carbonate polymorphs (3-4). Spontaneous precipitation of calcium carbonate from supersaturated solutions and the effect of inhibiting additives upon the rate of precipitation have been studied by previous workers (59). Certain additives, such as the polyphosphates, markedly reduce the rate of calcium carbonate precipitation. Such spontaneous crystallization processes do not permit reliable kinetic analysis to be made of the re1l~art I in this series, The C r y s t a l l i z a t i o n of Calcium Carbonate, I, Isotopic Exchange and Kinetics (1).
Received February 8, 1971, accepted April 2, 1971
Stable supersaturated solutions of calcium carbonate have been prepared at 10, 25, and 40°C by careful control of experimental conditions. Upon the addition of seed crystals of calcite, the crystallization kinetics have been followed by monitoring the calcium and hydrogen ion concentrations. After an initial growth surge, the process follows the kinetic equation established previously in which the rate of crystallization is proportional to the product of the concentrations, [Ca+2][COa-2]. The activation energy for crystal growth is 11.0 :i: 1.0 kcal mole-1, and the rate is independent of the stirring rate. Photomicrographic evidence is presented in support of the suggestion that the initial surge ia the growth curves results from additional nucleation at the surface of the added calcite crystals and in the bulk of the supersaturated solution. INTRODUCTION
II. Calcite Growth Mechanism ~ G. H. NANCOLLAS AND ~-V[.~/[. R E D D Y
Chemistry Department, State University of New York at Buffalo, Buffalo, New York
Hale Waihona Puke sults, however, since the implied assumption of homogeneous nucleation is of doubtful validity (1). Nucleation is likely to occur on impurity particles which offer available sites for crystM growth. Both the size and size distribution of the particles vary during spontaneous precipitation experiments. In an earlier publication (1), preliminary results for the kinetics of growth of calcite crystals were reported. The present study is an extension of this work over a range of temperatures enabling the calculation of the energy of activation of the crystal growth process. Parallel photomicrographic studies of the growing crystals have yielded important information concerning concomitant surface structure changes.
824
Journal of Colloid and Interface Science, Vol. 37, No. 4, December1971
CALCITE GROWTH MECHANISM runs had an average edge length of 10 t~m and an estimated surface area of 0.3 m~/gm. Supersaturated calcium carbonate solutions were prepared by the procedure described earlier (1). A Coming Model 12 pH meter and Sargent Combination Electrode (no. 8-30072-15) with a Sargent. 5'Iodel SP recorder allowed continuous measurement of hydrogen ion activity during a crystal growth run. Before and after each run, the electrode system was standardized with NBS buffers prepared according to Bates (10). In all crystal growth experiments, magnetic stirring was employed with a Teflon-coated stirring bar; the speed of stirring was measured using a strobe light. Calcium carbonate seed crystals and supersaturated solutions were separated during the growth experiment by filtering 10 ml of the inoculated supersaturated solution through a Mitlipore filter (no. BDWP01390). Analysis of the total calcium ion concentration in the filtrate was performed using a Perkin-Elmcr Model 303 Atomic Absorption Spectrometer. A computer program was employed to calculate ionic concentrations in solution during crystal growth using the experimental total calcium ion concentrations and hydrogen ion activities. Calculation of ionic concentrations were made as described previously (1) using literature values for carbonic acid-dissociation constants (11) and making allowance for calcium cm'bonate, bicarbonate, and hydroxide ion pairs (12-14). Activity coefficients were calculated using the extended form of the Debye-H~ekel equation suggested by Davies (15). A Unitron Series N microscope with a Polaroid Camera was used for photomieroseopy of calcite crystals.
The mechanism of growth of calcium carbonate crystals is important in desalination technology because of the formation of calcium carbonate scale on heat transfer surfaces (2). In addition, calcium carbonate crystallization may also play a major role in carbonate buffering of seawater (3). In seawater itself, the effect of the various ionic constituents upon the rate of calcium carbonate crystallization may explain the anomalous existence of metastable calcium carbonate polymorphs (3-4). Spontaneous precipitation of calcium carbonate from supersaturated solutions and the effect of inhibiting additives upon the rate of precipitation have been studied by previous workers (59). Certain additives, such as the polyphosphates, markedly reduce the rate of calcium carbonate precipitation. Such spontaneous crystallization processes do not permit reliable kinetic analysis to be made of the re1l~art I in this series, The C r y s t a l l i z a t i o n of Calcium Carbonate, I, Isotopic Exchange and Kinetics (1).
Received February 8, 1971, accepted April 2, 1971
Stable supersaturated solutions of calcium carbonate have been prepared at 10, 25, and 40°C by careful control of experimental conditions. Upon the addition of seed crystals of calcite, the crystallization kinetics have been followed by monitoring the calcium and hydrogen ion concentrations. After an initial growth surge, the process follows the kinetic equation established previously in which the rate of crystallization is proportional to the product of the concentrations, [Ca+2][COa-2]. The activation energy for crystal growth is 11.0 :i: 1.0 kcal mole-1, and the rate is independent of the stirring rate. Photomicrographic evidence is presented in support of the suggestion that the initial surge ia the growth curves results from additional nucleation at the surface of the added calcite crystals and in the bulk of the supersaturated solution. INTRODUCTION
