Surface Analysis of Ground Calcium Carbonate Filler Treated with

Apr 27, 2001 - Differentiating calcium carbonate polymorphs by surface analysis techniques-an XPS and TOF-SIMS study. Ming Ni , Buddy D. Ratner. Surfa...
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Ind. Eng. Chem. Res. 2001, 40, 2445-2451

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Surface Analysis of Ground Calcium Carbonate Filler Treated with Dissolution Inhibitor Peter Pang,† Yves Deslandes,‡ Stephane Raymond,‡ Gerry Pleizier,‡ and Peter Englezos*,† Department of Chemical & Biological Engineering and Pulp and Paper Centre, University of British Columbia, 2216 Main Mall, Vancouver, BC, V6T 1Z4 Canada, and Institute for Chemical Process and Environmental Technology, National Research Council, Building M-12, Ottawa, ON, K1A 0R6 Canada

This work demonstrates that the solubility of ground calcium carbonate (GCC) decreased when GCC was treated with phosphate-containing chemical inhibitors. The extent of inhibition of the dissolution process was found to increase with inhibitor dosage until a saturation point was reached, beyond which further addition of inhibitor did not have any further effect. The mechanism of the inhibition was investigated by conducting surface analysis of the treated GCC. X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy, and energydispersive X-ray analysis were employed to confirm the presence of phosphate on the surface of the treated GCC. Scanning electron microscopic pictures revealed that the treated GCC particles had a different surface morphological pattern than the untreated GCC particles. It is proposed that the inhibition was brought about by the precipitation of calcium phosphate phases such as hydroxyapatite on the GCC surface. 1. Introduction Calcium carbonate (CaCO3) finds extensive use as a filler in papermaking. Apart from the general benefits of using a mineral filler such as improved brightness and opacity, paper filled with CaCO3 has better aging resistance because of its buffering ability. In addition, there is an economic incentive to use CaCO3 because of the ability to produce cheap precipitated calcium carbonate on-site at the paper mill.1-3 The application of CaCO3 in acidic papermaking such as newsprint production has been limited by the extensive decomposition of the filler under acidic conditions. The pH strongly influences the solubility of CaCO3. A change in pH from alkaline to acidic conditions causes a dramatic increase in the filler solubility by several orders of magnitude. Not only does the dissolution increase the furnishes cost, but it also buffers the wet end pH to approximately 8.5, which can cause darkening of the mechanical pulp. In addition, dissolved calcium has been shown to have a detrimental effect on the efficiency of chemical additives such as retention aids.4 Inhibition of the dissolution of CaCO3 has been studied extensively in the field of geochemistry. Various inorganic and organic compounds, which are termed inhibitors, have been shown to have an inhibitory effect on the rate of CaCO3 dissolution. It is generally believed that this inhibitory effect is due to adsorption of the inhibitor on the reactive regions of the CaCO3 surface such as kinks, which retards the rate of dissolution and precipitation of CaCO3. As a result, most of the dissolution inhibitors are also known to inhibit the growth of CaCO3 crystals from a system that is oversaturated with respect to CaCO3. Inhibitors that have been reported * To whom correspondence should be addressed. Telephone: (604) 822-6184. Fax: (604) 822-6003. E-mail: [email protected]. † University of British Columbia. ‡ National Research Council.

in the literature include phosphate-containing chemicals,5-11 magnesium ions,12-14 oxalate ions,15 carboxylic acids,16,17 and other organic compounds.6,8,18,19 Various patents relating to the use of phosphate compounds for manufacturing acid-tolerant CaCO3 filler have been discussed.20-22 Ain and Laleg recently conducted several trials on a newsprint paper machine using a commercial acid-tolerant PCC.23 The wet end pH was stabilized at neutral with minimal PCC dissolution and pulp darkening. We have previously examined the inhibiting capability of phosphate, oxalate, magnesium, maleic acid, and succinic acid on the dissolution of papermaking-grade precipitated CaCO3 (PCC) filler in water.24 Phosphate was found to be the most effective inhibitor and was able to lower the solubility of PCC by 80%. The objectives of this work are, first, to examine the inhibiting capability of phosphate ions on the dissolution of ground CaCO3 (GCC) filler by monitoring the concentration of dissolved calcium, and second, to characterize the surface structure of GCC treated with the inhibitor in order to understand the inhibition mechanism. 2. Experimental Section 2.1. Dissolution Experiments. Ground calcium carbonate was obtained in the form of 70 wt % slurry from Columbia River Carbonates (Woodlands, WA). The average GCC particle size was measured to be 1.62 µm in diameter by the technique of light diffraction (Malvern Mastersizer 2000, Malvern Instruments, U.K.), and the specific surface area was calculated to be 4.44 m2/ g. According to the supplier, the GCC contains dispersing agent and less than 1% of MgCO3 and acid insolubles. A stock GCC suspension with a concentration of 100 g/L was first prepared by diluting 14.29 g of 700 g/L GCC thick stock suspension to 100 mL with deionized water (ELGASTAT UHQ II, Elga Ltd., Bucks, U.K.).

10.1021/ie000846b CCC: $20.00 © 2001 American Chemical Society Published on Web 04/27/2001

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The suspension was stirred for 30 min and left on a rotating device for 24 h before use. In a 2.5-L beaker, the required amount of phosphoric acid (Aldrich Chemical), which was used as a source of phosphate, was first added to approximately 2 L of deionized water. The pH of the water was then adjusted to 5.5 with 0.01 mol/L HCl or NaOH aqueous solution (Fisher Scientific). Afterward, the solution was stirred at 800 rpm for 20 min prior to the start of the dissolution experiment. Twenty milliliters of 100 g/L GCC stock suspension was then added to the solution to give an overall concentration of 1 g/L dilute GCC suspension (in the case of 10 g/L GCC, 200 mL was added). The diluted suspension was stirred at 800 rpm for 24 h. At regular time intervals, 30 mL of sample was collected and filtered through a 0.22-µm Millipore membrane filter (Bedford, MA). The Ca concentration of the filtrate was analyzed by atomic adsorption spectroscopy (GBC Scientific Equipment Inc., Arlington Heights, IL) using an air/ acetylene flame. After 24 h of mixing, the phosphate concentration was also determined as phosphomolybate complex by colorimetric determination25 (HP8452A diode array spectrophotometer, Hewlett Packard, Palo Alto, CA). All experiments were carried out at 25 ( 1 °C. The supersaturation ratio β with respect to any given phase of the solution was defined as β ) IAP/Ksp, where IAP and Ksp are the ionic activity product and the thermodynamic solubility product of the phase of interest, respectively. The Ksp values for octacalcium phosphate (OCP),26 Ca4H(PO4)3‚2.5H2O, and hydroxyapatite (HAP),27 Ca5(PO4)3OH, were taken as 10-49.3 and 10-58.5, respectively. The activity of each ion in the solution was calculated from the measured pH and the measured calcium and phosphate concentrations using the geochemical computer simulation program PHREEQCI, version 1.03.28 In some experiments, the 100 g/L GCC stock suspension was pretreated with H3PO4 prior to the dissolution experiment. For the pretreatment, H3PO4 was introduced during the preparation of the 100 g/L GCC suspension. The level of H3PO4 added was such that, when the pretreated GCC suspension was subsequently diluted to 1 g/L, the concentration of H3PO4 in the dilute suspension was in the same range as that used in the previous experiments. 2.2. Surface Analysis. During the dissolution experiment, samples were collected and filtered through a 0.22-µm Millipore membrane filter. The solids were washed with 100 mL of deionized water in order to remove the phosphate from the solution and were then dried in a vacuum desiccator for at least 24 h prior to surface analysis. The surface structure of the treated GCC was characterized by X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) with energy-dispersive X-ray analysis (EDX), scanning electron microscopy (SEM), and X-ray diffraction (XRD). X-ray photoelectron spectra were acquired using a Kratos AXIS HS X-ray photoelectron spectrometer (Kratos, Manchester, U.K.). Samples were analyzed using a monochromatic Al KR X-ray source (1486.6 eV) operated at 280 W (14 kV, 20 mA) and a 180° hemispherical analyzer with a three-channel detector. Charge compensation was achieved through the use of an electron flood gun. The spectrometer was run in fixed analyzer transmission (FAT) mode, with electrostatic

Figure 1. Dissolved Ca concentration profile of GCC dissolution in the presence of phosphate inhibitor.

magnification. The analyzer chamber pressure was in the range of 10-9-10-8 Torr. For TEM observations, samples were dispersed in deionized water using a sonicator for 4 min. A small drop of the suspension was deposited onto a TEM copper grid (300 mesh) coated with a carbon (Formvar) film. After the water was evaporated, the grid was examined using a Philips CM20 transmission electron microscope at 200 kV. Images were recorded with a Gatan On-Axis Slow-Scan CCD Camera model 679 camera (Gatan, Pleasanton, CA). Energy-dispersive analysis spectra were recorded using a Link eXL II Energy Dispersive X-ray Analysis instrument, mounted on the TEM (Oxford Instruments, Oxford, U.K.). For SEM scans (Hitachi, model S-2300), samples were attached to stubs without further treatment and were gold-plated prior to analysis. Powder X-ray diffraction patterns were obtained from powder samples sprinkled onto a clean silicon wafer. The spectra were recorded using a Scintag XDS 2000 system (Scintag, Cupertino, CA), operated in reflection in the θ, θ configuration. A Cu radiation source operated at 45 kV and 35 mA was used. The detector system was a scintillator counter. 3. Results and Discussion 3.1. Inhibition of GCC Dissolution. When GCC alone was dissolved in water, it was observed that the pH rose rapidly to a peak as high as 9.6 after 30 min before falling back gradually to an equilibrium value of 8.3 after 15 h. However, in the presence of phosphate (0.002 mol/L), the initial rapid rise of pH was suppressed because of the buffering capability of H3PO4. Instead, the pH rose steadily to an equilibrium value of 7.9. Figure 1 compares the dissolution profiles of untreated GCC and GCC treated with 0.002 mol/L phosphate. The dissolution kinetics of untreated GCC at concentrations of 1 and 10 g/L were found to be slightly different, but both samples reached the same equilibrium concentration after 15 h. It is important to point out that the initial Ca concentration just before the dissolution took place was different. This difference was due to the fact that the amount of GCC suspension added from the 100 g/L stock suspension to the 2 L of water was 10 times more in the experiment for 10 g/L GCC than for 1 g/L GCC , as seen from the experimental procedure. The equilibrium Ca concentrations of un-

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Figure 2. Effect of phosphate dosage on the equilibrium Ca concentration. The equilibrium time was 24 h.

Figure 3. XPS analysis of (a) untreated GCC only and (b) GCC treated with 0.01 mol/L H3PO4.

treated 1 and 10 g/L GCC suspensions were (6.33 ( 0.06) × 10-4 and (6.2 ( 0.2) × 10-4 mol/L, respectively. As expected, the equilibrium Ca concentration should be independent of the amount of excess GCC present in the system. The speciation of an open CaCO3-H2OCO2 system was computed using PHREEQCI. The equilibrium concentration of Ca was calculated to be 4.93 × 10-4 mol/L, in agreement with the experimental results. In the presence of 0.002 mol/L phosphate, the dissolution showed an opposite trend. At first, a large amount of GCC was dissolved because of the buffering action of H3PO4, which maintained the pH of the system close to neutral. With time, the Ca concentration

Figure 4. TEM scans of (a) untreated GCC; (b), (c) GCC treated with 0.01 mol/L H3PO4. The dotted circle in part c indicates the area taken for EDX analysis.

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Figure 5. EDX analysis of (a) untreated GCC and (b) GCC treated with 0.01 mol/L H3PO4.

decreased gradually and reached an equilibrium value of (0.7 ( 0.1) × 10-4 mol/L. The equilibrium total phosphate concentration after 24 h of mixing was found to be (3.67 ( 0.08) × 10-4 mol/L. On the basis of the measurements at t ) 1 h, the initial values of log βHAP and log βOCP were estimated to be 10.0 and 3.8, respectively (calculations were done using [Ca]1hr ) 9.0 × 10-4 mol/L, [P]1hr ) 0.002 mol/L, and pH ) 7.1). It is clear that the solution was supersaturated with respect to OCP and HAP. Hence, the gradual decrease in Ca concentration was due to precipitation of solid phases containing calcium and phosphorus (Ca-P), either as segregated crystals or at the GCC surfaces. Mass balance calculations indicate that the amount of phosphate precipitated from solution after 24 h was 1.63 × 10-3 mol/g GCC. It is well-known that calcium phosphate precipitates in various forms depending on the chemical concentration, composition, pH, and temperature. Under neutral

to slight basic conditions and at ambient temperature, calcium phosphate first precipitates from solution as amorphous calcium phosphate.26,29 This metastable precursor then dissolves, and a more stable intermediate crystalline phase, octacalcium phosphate, Ca4H(PO4)3‚2.5H2O, is formed, which further transforms into the thermodynamically stable hydroxyapatite, Ca5(PO4)3OH. The transformation is influenced by the experimental conditions and takes place generally within hours of reaction.26,29-32 Thus, it is likely to occur within the time scale of our experiments. As will be discussed later, transmission electron microscopy of the treated GCC samples indicated that the Ca-P phases were present not as segregated crystals but rather as a layer on the GCC surface. The GCC particles present in the system can serve as crystallization seeds for heterogeneous nucleation of the Ca-P phases on the GCC. Phosphate, which has been shown to adsorb onto CaCO3 particles,7,9,10,33 can form complexes with the surface Ca and precipitate to form Ca-P solid phases. In their NMR analysis, Hinedi et al.34 showed that an apatite-like phase formed on the CaCO3 surface only when there was a sufficient amount of adsorbed phosphate. On the other hand, with a low surface phosphate concentration, the adsorbed phosphate was not in the form of any calcium phosphate solid phase. As the Ca-P solid phases form on the GCC surfaces, they block the dissolution sites and retard the GCC dissolution process. As more Ca-P phases precipitate on the GCC surface, the dissolution eventually terminates. Because calcium phosphate, in general, has a much lower solubility product than CaCO3, the dissolved Ca will continue to precipitate out until equilibrium is reached, with the equilibrium point depending on the chemistry of the precipitated Ca-P solid phases. Hence, it is expected that the equilibrium concentration of Ca should be governed by the solubility of the Ca-P solid phases. The speciation of an open Ca5(PO4)3OHH2O-CO2 system was computed by PHREEQCI, and the equilibrium Ca concentration was calculated to be 1.19 × 10-4 mol/L, which is comparable with the experimental results. The inhibition mechanism has been attributed to the adsorption of phosphate ions onto the dissolution sites.5,7 However, the phosphate concentration used in this work is at least 3 orders of magnitude higher than that used in previous studies, and thus, it is sufficient to induce precipitation of calcium phosphate on the GCC particles and inhibit dissolution. Hence, it is conceivable that the dominant inhibition mechanism is the direct formation of Ca-P solid phases on the GCC particles. The inhibitory effect was also studied by pretreating a 100 g/L GCC stock suspension with phosphate for 24 h prior to the dissolution experiments. As shown in Figure 1, when the pretreated suspension was diluted to 1 g/L GCC, the treated GCC was found to exhibit resistance to dissolution. Because the pretreated GCC was believed to be coated with the precipitated Ca-P phases during the 24-h pretreatment stage, the gradual increase in the Ca concentration can be attributed to the dissolution of the Ca-P solid phases into the water. Likewise, it is expected that the equilibrium concentration of Ca was controlled by the chemistry of the Ca-P phases. On the basis of this result, it is possible, in practice, to design a pretreatment stage to treat CaCO3 filler with phosphate before it is added to the pulp suspension.

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Figure 6. SEM scans of (a), (b) untreated GCC; (c), (d) GCC treated with 0.01 mol/L H3PO4. Table 1. XPS Analysis: Atomic Composition of Untreated GCC and GCC Treated with 0.01 mol/L H3PO4a atomic composition (atomic %)

untreated GCC

GCC + H3PO4

carbon as C-C (284 eV) carbon as C-C-O (286 eV) carbon as CO3 (289 eV) oxygen calcium aluminum silicon phosphorus magnesium

8.8 12.8 58.0 10.9 4.1 5.3 0.1

7.8 2.5 8.3 57.6 12.0 3.5 4.3 4.1 -

a At least two areas were analyzed on each sample, and the average values are reported.

The relationship between the phosphate dosage and the inhibiting effect was also examined. As seen in Figure 2, the extent of inhibition increases with increasing phosphate concentration and reaches a plateau. The equilibrium Ca concentration was reduced by as much as 90% with the addition of phosphate. The optimum phosphate concentrations for 10 and 1 g/L GCC suspensions were approximately 0.005 and 0.003 mol/L, respectively. In addition, it was found that similar results were obtained when the phosphate was added either directly to the dilute GCC suspension (1 g/L) or to the stock GCC suspension (100 g/L). The optimal dosage could correspond to the level of phosphate required to form enough Ca-P solid phases to fully cover the dissolution sites and inhibit the dissolution. 3.2. X-ray Photoelectron Spectroscopy. The XPS survey spectra of GCC and GCC treated with 0.01 mol/L phosphate are shown in Figure 3. The atomic composition (in atomic %) calculated from the spectra using the

software provided with the instrument is tabulated in Table 1. For untreated GCC, there is only one type of C atom, which is linked to three O atoms (CO3) and has a peak corresponding to 289 eV. Judging from the stoichiometry, this C peak should have an intensity similar to that of the Ca peak. As shown in Table 1, the intensity ratio of C as CO3 to Ca is 0.8. Carbon peaks such as C-C (284 eV) and C-C-O (286 eV) were also detected and were due to surface contamination from C-containing materials originally present in the sample. One possible source of these adventitious carbon contaminants could be a dispersing agent such as sodium polyacrylate. The presence of Al, Si, and Mg in the GCC spectra was due to impurities such as clay present in the sample. The XPS analysis clearly confirmed the presence of phosphate only on the surface of treated GCC. The stoichiometric Ca/P ratio calculated from the intensity ratio was approximately 3 and did not match the value for octacalcium phosphate or hydroxyapatite. However, it is important to point out that, if the Ca-P phases do not fully cover the entire surface of the GCC particles and/or are thinner than a few nanometers in thickness, it is not possible to determine the stoichiometry of the Ca-P phases solely from the atomic composition. The reason for this is that XPS probes approximately the top 7 nm of the surface. Hence, if the layer of the Ca-P phases is too thin, the atomic composition obtained could originate from the Ca-P phases, as well as the uncovered GCC and the GCC beneath the Ca-P phases. This is supported by the observation of the intense C peak at 289 eV, which indicated that some CaCO3 was still being detected.

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Figure 7. XRD patterns of (a) untreated GCC and (b) GCC treated with 0.01 mol/L H3PO4.

3.3. Transmission Electron Microscopy with Energy Dispersive X-ray Analysis. TEM pictures of untreated and treated GCC are shown in Figure 4. Thin, sheetlike solid fragments were detected on the surface of treated GCC, as shown in Figure 4b. These fragments were found to attach to the GCC, as well as to the impure clay particles, but did not seem to fully cover the surface of the particles. In addition, it was found that a few fragments were completely isolated, which could be due to adhesion failure during the sampling procedure. A higher magnification of the attached fragment is shown in Figure 4c. Comparison of the EDX spectra of the area highlighted by a dotted line circle and that of the GCC itself confirmed the precipitation of Ca-P phases on the GCC surface (Figure 5). Although it is not possible to identify the form of calcium phosphate found on the GCC surface, it is clear from the scans that the fragments were not amorphous calcium phosphate, as this metastable phase is known to exist as spheroids of 20-100 nm in size.30-32 The Ca-P fragments were likely to be octacalcium phosphate or hydroxyapatite, as these two phases have been identified to have thin flakelike structures.30,32 3.4. Scanning Electron Microscopy. SEM scans clearly indicate a distinct difference in the surface appearance between the untreated and treated GCC particles (Figure 6). It is noted that the particles shown in the figure were aggregates of GCC particles rather than individual particles. It can be seen that the surface of the treated GCC particles was covered with some form of soft thin materials (Figure 6c,d). 3.5. X-ray Powder Diffraction. Figure 7a shows a characteristic pattern of GCC with intensities at d spacings corresponding to 0.303, 0.190, and 0.187 nm. The trace for treated GCC indicates the presence of Ca-P phases (Figure 7b). The sharp peak at 2θ ) 26° and the broad peak at 2θ ) 32° are characteristic of Ca-P phases.32,35,36 Although the exact composition of the Ca-P solids on the GCC surface is not known, it is still possible to speculate how about the role played by phosphate in the inhibition of the GCC dissolution. When GCC is initially added to an aqueous solution of H3PO4, it dissolves rapidly because of the acidic conditions. The rapid initial pH rise, which usually accompanies the

CaCO3 dissolution, is suppressed because of the strong buffering ability of H3PO4. This favors further the GCC dissolution. However, as the Ca concentration increases, amorphous calcium phosphate, which is the metastable phase of calcium phosphate, starts to precipitate when its solubility limit is exceeded. The GCC particles in the system provide a favorable place for the heterogeneous nucleation to occur. In addition, phosphate ions could adsorb onto the GCC and react with the surface Ca to initiate the nucleation of ACP.33 Subsequently, the amorphous calcium phosphate transforms into crystalline octacalcium phosphate and eventually hydroxyapatite. These crystalline phases, as well as the amorphous calcium phosphate formed on the GCC surface, might act as a barrier and hinder the dissolution. It appears that a protective layer of calcium phosphate covers the GCC surface. When Ca replenishment into the solution by dissolution is stopped, the dissolved Ca continues to precipitate as calcium phosphate until an equilibrium is reached, with the point of equilibrium depending on the solubility of the final form of calcium phosphate present on the GCC surface. 4. Conclusions Phosphate was found to inhibit the dissolution of ground calcium carbonate (GCC) filler. The optimum phosphate concentrations for 10 and 1 g/L GCC suspension were approximately 0.005 and 0.003 mol/L, respectively. Surface analysis by means of X-ray photoelectron spectroscopy and transmission electron microscopy with energy-dispersive X-ray analysis indicated the precipitation of calcium phosphate solid phases on the surface of treated GCC particles. With the use of scanning electron microscopy, the treated GCC particles were found to have a surface appearance different from that of the untreated GCC particles. It is proposed that the inhibition mechanism involves the formation of these thin calcium phosphate phases on the GCC surface, which then act as a barrier and block the dissolution sites. In practice, a phosphate-pretreatment stage for the CaCO3 filler can be applied to minimize the dissolution process and thus the detrimental impacts to the papermaking system. Acknowledgment We acknowledge support from the Network of Centres of Excellence for Mechanical Wood Pulps. We thank Columbia River Carbonates for supplying the ground calcium carbonate. We also thank Dr. Malcolm Smith of Pulp and Paper Centre, University of British Columbia, for his valuable comments. Literature Cited (1) Britt, K. W. Handbook of Pulp and Paper Technology, 2nd ed.; Van Nostrand Reinhold: New York, 1970. (2) Brown, A. Ground Calcium Carbonate Fillers for High Ash Content, High Strength Papers. TAPPI 1996 Papermakers Conference Proceedings, Tappi Press: Atlanta, GA, 1996. (3) Gill, R. A.; Scott, W. The Relative Effects of Different Calcium Carbonate Filler Pigments on Optical Properties. Tappi J. 1987, 70 (1), 93. (4) Gibbs, A.; Deng, Y.; Pelton, R. Flocculants for Precipitated Calcium Carbonate in Newsprint Pulps. Tappi J. 1997, 80 (4), 163. (5) Reddy, M. M.; Nancollas, G. H. Calcite Crystal Growth Inhibition by Phosphonates. Desalination 1973, 12, 61.

Ind. Eng. Chem. Res., Vol. 40, No. 11, 2001 2451 (6) Morse, J. W. Dissolution Kinetics of Calcium Carbonate in Seawater. V. Effects of Natural Inhibitors and the Position of the Chemical Lysocline. Am. J. Sci. 1974, 274, 638. (7) Reddy, M. M. Crystallization of Calcium Carbonate in the Presence of Trace Concentrations of Phosphorous-containing Anions. I. Inhibition by Phosphate and Glycerophosphate Ions at pH 8.8 and 25 °C. J. Cryst. Growth 1977, 41, 287. (8) Berner, R. A.; Westrich, J. T.; Graber, R.; Smith J.; Martens, C. S. Inhibition of Aragonite Precipitation from Supersaturated Seawater: A Laboratory and Field Study. Am. J. Sci. 1978, 278, 816. (9) Walter, L. M.; Hanor, J. S. Effect of Orthophosphate on the Dissolution Kinetics of Biogenic Magnesian Calcites. Geochim. Cosmochim. Acta 1979, 43, 1377. (10) Giannimaras, E. K.; Koutsoukos, P. G. The Crystallization of Calcite in the presence of Orthophosphate. J. Colloid Interface Sci. 1987, 116 (2), 423. (11) Dove, P. M.; Hochella, M. F., Jr. Calcite Precipitation Mechanisms and Inhibition by Orthophosphate: In Situ Observations by Scanning Force Microscopy. Geochim. Cosmochim. Acta 1993, 57, 705. (12) Reddy, M. M.; Wang, K. K. Crystallization of Calcium Carbonate in the Presence of Metal Ions. I. Inhibition by Magnesium Ion at pH 8.8 and 25 °C. J. Cryst. Growth 1980, 50, 470. (13) Zuddas, P.; Mucci, A. Kinetics of Calcite Precipitation from Seawater: I. A Classical Chemical Kinetics Description for Strong Electrolyte Solutions. Geochim. Cosmochim. Acta 1994, 58 (20), 4353. (14) Compton, R. G.; Brown, C. A. The Inhibition of Calcite Dissolution/Precipitation: Mg2+ Cations. J. Colloid Interface Sci. 1994, 165, 445. (15) Giannimaras, E. K.; Koutsoukos, P. G. Precipitation of Calcium Carbonate in Aqueous Solutions in the Presence of Oxalate Anions. Langmuir 1988, 4, 855. (16) Compton, R. G.; Pritchard, K. L.; Unwin, P. R.; Grigg, G.; Silvester, P.; Lees, M.; House, W. A. The Effect of Carboxylic Acids on the Dissolution of Calcite in Aqueous Solutions. J. Chem. Soc., Faraday Trans. I 1989, 85 (12), 4335. (17) Compton, R. G.; Brown, C. A. The Inhibition of Calcite Dissolution/Precipitation: 1,2-Dicarboxylic Acids. J. Colloid Interface Sci. 1995, 170, 586. (18) Suess, E. Interaction of Organic Compounds with Calcium Carbonate. I. Association Phenomena and Geochemical Implications. Geochim. Cosmochim. Acta 1970, 34, 157. (19) Inskeep, W. P.; Bloom, P. R. Kinetics of Calcite Precipitation in the Presence of Water-Soluble Organic Ligands. Soil Sci. Soc. Am. J. 1986, 50, 1167. (20) Passaretti, J. D. (Pfizer Inc.) Acid-Stabilized Calcium Carbonate, Process for its Production and Method for its Use in the Manufacture of Acidic Paper. U.S. Patent 5,156,719, Oct 20, 1992. (21) Wu, K. T. (ECC Inter. Inc.) Surface Modified Calcium Carbonate Composition and Uses Therefore. U.S. Patent 5,584,923, Dec 17, 1996.

(22) Wu, K. T. (ECC Inter. Inc.) Acid Resistant Calcium Carbonate Composition and Uses Therefore. U.S. Patent 5,593,488, Jan 14, 1997. (23) Ain, R. L.; Laleg, M. Mill Experiences with AT Precipitated Calcium Carbonate (PCC) in Papers Containing Mechanical Pulp. Pulp Paper Can. 1997, 98 (12), 172. (24) Pang, P.; Khoultchaev, K. K.; Englezos, P. Inhibition of the Dissolution of Papermaking Grade Precipitated Calcium Carbonate Filler. Tappi J. 1998, 81 (4), 188. (25) Jeffery, G. H.; Bassett, J.; Mendham, J.; Denny, R. C. Vogel’s Textbook of Quantitative Chemical Analysis, 5th ed.; Longman Scientific & Technical: Essex, U.K., 1989. (26) Christoffersen, J.; Christoffersen, M. R.; Kibalczyc, W.; Andersen, F. A. A Contribution to the Understanding of the Formation of Calcium Phosphates. J. Cryst. Growth 1989, 94, 767. (27) McDowell, H.; Gregory, T. M.; Brown, W. E. J. Res. Natl. Bur. Stand. (U.S.) 1977, 81A, 273. (28) Charlton, S. R.; Macklin, C. L.; Parkhurst, D. L. PHREEQCIsA Graphical User Interface for Geochemical Computer Program PHREEQC; U.S. Geological Survey Water-Resources Investigations Report 97-4222; U.S. Geological Survey: Washington, D.C., 1997. (29) Lundager, H. E.; Christensson, F. Precipitation of Calcium Phosphate at 40 °C from Neutral Solution. J. Cryst. Growth 1991, 114, 613. (30) Christoffersen, M. R.; Christoffersen, J.; Kibalczyc, W. Apparent Solubilities of Two Amorphous Calcium Phosphates and of Octacalcium Phosphate in the Temperature Range 30-42 °C. J. Cryst. Growth 1990, 106, 349. (31) Lazic, S. Microcrystalline Hydroxyapatite Formation from Alkaline Solutions. J. Cryst. Growth 1995, 147, 147. (32) Abbona, F.; Baronnet, A. A XRD and TEM Study on the Transformation of Amorphous Calcium Phosphate in the Presence of Magnesium. J. Cryst. Growth 1996, 165, 98. (33) Leckie, J.; Stumm, W. Phosphate Precipitation. In Water Quality Improvement by Physical and Chemical Processes; University of Texas Press: Austin, TX, 1970. (34) Hinedi, R.; Goldberg, S.; Chang, A. C.; Yesinowski, J. P. A 31P and 1H MAS NMR Study of Phosphate Sorption onto Calcium Carbonate. J. Colloid Interface Sci. 1992, 152, 141. (35) Iijima, M.; Kamemizu, H.; Wakamatsu, N.; Goto, T.; Doi, Y.; Moriwaki, Y. Effects of Ca Addition on the Formation of Octacalcium Phosphate and Apatite in Solution at pH 7.4 and 37 °C. J. Cryst. Growth 1998, 193, 182. (36) Feng, Q. L.; Wang, H.; Cui, F. Z. Kim; T. N. Controlled Crystal Growth of Calcium Phosphate on Titanium Surface by NaOH Treatment. J. Cryst. Growth 1999, 200, 550.

Received for review September 28, 2000 Revised manuscript received February 20, 2001 Accepted March 11, 2001 IE000846B