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Dissolution of Pentelic Marble at Alkaline pH Malvina G. Orkoula†,‡ and Petros G. Koutsoukos*,†,‡ Department of Chemical Engineering, University of Patras, GR-26500 Patras, Greece, and Institute of Chemical Engineering and High-Temperature Chemical Processes, P.O. Box 1414, GR-26500 Patras, Greece Received March 27, 2000. In Final Form: May 31, 2000 The kinetics of dissolution of powdered Pentelic marble from two different sites located on Mount Pentelicon of Attica, Dionysos and Cave, were investigated. The results were compared with those of synthetically precipitated calcite, the main inorganic constituent (>98%) of the tested marbles. The dissolution process at solution undersaturation near equilibrium in all cases showed a linear dependence of the rates of dissolution on the relative solution undersaturation. The dissolution was found to be polynuclear, surface diffusion controlled. Morphological examination of the specimens showed that dissolution proceeds through formation of steps, cracks, pores, and layerwise disintegration, corroborating the conclusion from the kinetics results. Powdered marble is suggested to be an excellent model system for the investigation of marble dissolution as it exhibited remarkable similarities with the dissolution of whole marble blocks.
Introduction Damage of historical monuments due to environmental factors has been recognized over the past century and has triggered scientific research aiming at the protection of the marble structure of the monuments. Acid rain and sulfur dioxide have been held responsible for the deterioration of the marble and limestone artwork.1,2 Aside from acidity, wet precipitation by itself is believed to contribute to dissolution even at neutral or alkaline pH. The dissolution of marble has been the subject of numerous investigations over wide pH and undersaturation ranges. The dissolution at acidic pH has attracted most of the research attention3-9 due to its connection with environmental pollution by SO2 and NOx. At low pH, control of the process by transport phenomena has been reported due to the extremely fast surface reaction rates. The corresponding experiments were done at very large solution undersaturation values, and in several cases the experimental conditions were largely varied. At these conditions, rather large uncertainties are introduced in the measured kinetic parameters. Compton and Unwin9 employed a channel-flow cell, in which they attained high fluid flow and they reported a first-order dissolution kinetics. The dissolution of calcium carbonate in alkaline pH values has been reported to be surface reaction controlled.3,10-14 However, in these studies either the solution * Corresponding author: Phone: +30 61 997265. Fax: +30 61 993255. E-mail:
[email protected]. † University of Patras. ‡ Institute of Chemical Engineering and High-Temperature Chemical Processes. (1) Amoroso, G. G.; Fassina, V. Stone Decay and Conservation, 1st ed.; Elsevier: Amsterdam, 1983. (2) Camuffo, D. Microclimate for Cultural Heritage, 1st ed.; Elsevier: Amsterdam, 1998. (3) Sjoberg, E. L. Stockholm Contrib. Geol. 1978, 32, 1. (4) Plummer, L. N.; Wigley, T. M. L.; Parkhurst, D. L. Am. J. Sci. 1978, 278, 179. (5) King, C. V.; Liu, C. L. J. Am. Chem. Soc. 1933, 55, 1928. (6) Lund, K.; Fogler, H. S.; McCune, C. C.; Ault, J. W. Chem. Eng. Sci. 1975, 30, 825. (7) Sjoberg, E. L.; Rickard, D. T. Geochim. Cosmochim. Acta 1984, 48, 485. (8) Compton, R. G.; Daly, P. J. J. Colloid Interface Sci. 1984, 101, 159. (9) Compton, R. G.; Unwin, P. R. Philos. Trans. R. Soc. London 1990, A330, 1.
conditions changed with dissolution and/or they were done at conditions far from equilibrium, resulting in fast kinetics. Additional important information has been obtained by real-time measurements using atomic force microscopy (AFM).15,16 It was thus shown that crystal defects are important for the dissolution process. Further work is needed to couple atomic scale observations with kinetic experiments of dissolution in order to describe dissolution processes.17 In the present work, the kinetics of dissolution were measured at conditions of constant undersaturation.18 The investigation was done at constant, alkaline pH 8.25 and at conditions near equilibrium, i.e., at very low undersaturations. These conditions are compatible with a nonpolluted environment in areas in which calcitic soils and geological formations are present. Powdered samples were used for the investigation of marble dissolution since we could thus have sufficiently high total surface areas as contrasted to marble slabs, in which dissolution is very slow because of the very low total surface area available to undersaturated solution. Experimental Section The solids investigated included powdered Pentelic marble and synthetically prepared calcite. Freshly cut marble slabs from two different quarries in the area of Pentelicon mountain, Dionysos and Cave (the latter is considered as the quarry from which the marbles of the Parthenon were produced), were subjected to milling. Calcite crystals were prepared by mixing equal volumes of 0.1 M CaCl2 and Na2CO3 solutions. The precipitated solid was filtered off, washed with distilled water, and dried overnight at 120 °C. The solids were characterized by powder X-ray diffraction (Philips PW1830/40), and their morphology was examined by scanning electron microscopy (SEM, (10) Plummer, L. N.; Wigley, T. M. L. Geochim. Cosmochim. Acta 1976, 40, 191. (11) Sjoberg, E. L. Geochim. Cosmochim. Acta 1976, 40, 441. (12) Morse, J. W.; Berner, R. A. Am. J. Sci. 1972, 272, 840. (13) Compton, R. G.; Pritchard, K. L. Philos. Trans. R. Soc. London 1990, A330, 47. (14) Morse, J. W. In Reviews in Mineralogy; Reeder, R. J., Ed.; Miner. Soc. of America: Blacksburg, VA, 1983; Vol. 11, p 227. (15) Hillner, P. E.; Gratz, A. J.; Manne, S.; Hansma, P. K. Geology 1992, 20, 359. (16) Hillner, P. E.; Manne, S.; Gratz, A. J.; Hansma, P. K. Ultramicroscopy 1992, 42-44, 1387. (17) Gratz, A. J.; Manne, S.; Hansma, P. K. Science 1991, 251, 1343. (18) Tomson, M. B.; Nancollas, G. H. Science 1978, 200, 1059.
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JEOL JSM-5200). The two Pentelic marble samples consisted exclusively of calcite (>98%). The specific surface areas were measured by N2 adsorption (Micromeritics, Gemimi III 2375) and were found to be 1.70 m2/g for Dionysos marble powder, 2.94 m2/g for Cave marble powder, and 0.31 m2/g for calcite. The powder dissolution experiments were done in undersaturated solutions prepared from standardized stock calcium chloride and sodium bicarbonate solutions as described in detail elsewhere.19 All experiments were done at 25 °C, pH 8.25, and ionic strength 0.1 M adjusted by NaCl. The master variable used for monitoring the dissolution process was H+ activity, measured by a combination glass-Ag/AgCl electrode. The dissolution was initiated with the introduction of carefully weighed amounts of the solids. pH changes as small as 0.005 units in the solution triggered the addition of titrant solutions from two mechanically coupled burets to the working solution. The titrants consisted of CaCl2, NaHCO3, NaCl as inert electrolyte, and HCl to adjust the solution pH. The concentrations of the titrants were calculated from the mass balance for each component so that the constancy of the solution composition is ensured. Assuming that the working undersaturated solution contains x1M CaCl2, x2M NaHCO3, x3M NaOH (used for the pH adjustment), and x4M NaCl (for the ionic strength adjustment), the titrant solutions should contain y1M CaCl2, y2M NaHCO3, y3M HCl, and y4M NaCl, in order to keep the concentration of all components constant. If the total volume of the undersaturated solution is V and the volume added by each buret dV, for constant calcium concentration
x1 )
x1V + y1dV + dm V + 2dV
(1)
where dm are the moles of calcium released from the dissolution of calcium carbonate. From eq 1 it follows
y1 ) 2x1 -
dm dV
(2)
Let dm/dV ) c, which is called the effective dissolution constant, a measure of the amount of solid released by dissolution per unit volume and is determined by preliminary experiments. Similarly, for constant carbonate concentration in solution, it should be
y2 ) 2x2 - c
(4)
The minus sign corresponds to the fact that, for the working solution pH adjustment to the value 8.25, NaOH addition is needed while HCl is needed for balancing hydroxyl release during dissolution. Finally the inert electrolyte content of the titrants is
y4 ) 2x4
Samples were withdrawn during the dissolution process at regular intervals and filtered, and the filtrates were analyzed for calcium by atomic absorption spectrometry (Perkin-Elmer AAnalyst 300) in order to confirm the constancy of the solution composition. In all experiments the analysis showed that throughout the course of the dissolution process the calcium concentration remained constant to within (5%, over a period of 24 h of dissolution. The rates of dissolution were calculated from the traces of titrant solutions added to the working solution with time. A typical curve showing the amount of dissolved matter as a function of time is presented in Figure 1. The rate of dissolution decreased with time. The reported rates of dissolution were taken at 2% dissolution with respect to the added solid, assuming that the specific surface area of the dissolving solids remains constant. This assumption is valid for a small extent of dissolution (CaOH2+ surface complexes are predominant,20 it was considered that two proton equivalents are withdrawn per mole of CaCO3 dissolved. Thus the HCl concentration in the titrants is
y3 ) 2(-x3) + 2c
Figure 1. Characteristic curve of dissolved matter with time.
(5)
The titrant solutions in the two burets were as follows:
buret 1: y1 + y3 buret 2: y2 + y4 The restriction for the value of c is c e 2x1, 2x2. (19) Sabbides, Th. G.; Koutsoukos, P. G. In Mineral Scale Formation and Inhibition; Amjad, Z., Ed.; Plenum Press: New York, 1995; p 73. (20) Van Capellen, P.; Charlet, L.; Stumm, W.; Wersin, P. Geochim. Cosmochim. Acta 1993, 57, 3505.
Ω)
RCa2+,sRCO32-,s RCa2+,∞RCO32-,∞
(6)
Here R are the activities of the subscripted ions in the solution (subscript s) and at equilibrium (subscript ∞), respectively. The relative undersaturation, σ, is
σ ) 1 - Ω1/2 ) 1 - S
(7)
Ω was calculated taking into account all equilibria involved using the HYDRAQL code.19,21 It is interesting to note that the rate of dissolution of all calcitic materials tested decreased with time despite the fact that the driving force was kept constant (Figure 1). This abnormal behavior has been recently reported for the dissolution of calcium phosphate.22 The initial conditions of the experiment and the respective rates of dissolution measured are summarized in Table 1. The dependence of the dissolution rates, R, of calcite and Pentelic marble from two different sites (Dionysos and Cave) on σ is shown in Figure 2. (21) Papelis, C.; Hayes, K. F.; Leckie, J. O. In HYDRAQL: A program for the computation of the chemical equilibrium composition of aqueous batch systems including surface complexation modeling of ion adsorption at the oxide/solution interface; Technical Report No. 306; Stanford University Press: Stanford, CA, 1988. (22) Tang, R.; Nancollas, G. H. J. Cryst. Growth 2000, 212, 261.
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Figure 2. Kinetic data for the dissolution of Pentelic marble (Dionysos (2) Cave (b)) and synthetically precipitated calcite (9) at pH 8.25, 25 °C, and 0.1 M NaCl.
Figure 3. Kinetics of dissolution of calcite (9) and Pentelic marble (Dionysos (2) and Cave (b)) in 0.1 M NaCl, pH 8.25, for the polynuclear model. Table 1. Experimental Conditions for Calcium Carbonate Dissolution Experiments: Total Calcium, Cat, Total Carbonate, Ct ) 1.14 × 10-3 M, Relative Solution Undersaturation with Respect to Calcite, σ, and Measured Rates of Dissolution, R (×10-2 mg s-1 m2), for the Solids Tested (Subscript) at pH ) 8.25 and 25 °C Cat (10-3 M)
σ
Rcalcite
RDionysos
RCave
1.25 1.15 1.10 1.025 0.96 0.877
0.013 0.053 0.074 0.106 0.134 0.172
2.21 2.48 2.68 3.52 3.72 4.35
5.89 10.05 12.16 15.19 18.06 18.98
5.43 6.33 6.82 7.99 11.04 12.4
The mechanism of dissolution may be considered as a series of steps in which the growth units are detached from the surface, diffuse away from the active dissolution sites, desorb from the surface, and are finally transferred to the bulk solution.23 Volume diffusion was ruled out because according to this mechanism a linear dependence of the rate of dissolution on the relative solution undersaturation is predicted with a zero intercept.23 In our experiments, for all materials tested, despite the fact that they yielded a linear dependence of the rates of dissolution on the relative undersaturation, the fitted line failed to pass through the axes origin (Figure 2). It was therefore concluded that the dissolution of calcite and the calcitic marble powders tested was surface diffusion controlled. The same mechanism has been reported for dicalcium (23) Zhang, J. W.; Nancollas, G. H. In Reviews in Mineralogy; Hochella, M. F., White, A. F., Eds.; Mineralogical Society of America: Washington, DC, 1990; Vol. 23, p 364.
Figure 4. SEM images of marble crystallites after dissolution: (a) original powder; (b) formation of cracks; (c) cavities; (d) layerwise disintegration.
phosphate dihydrate.24 The kinetic data were fitted to the polynuclear model:19,23,25-27
R ) kdσ2/3(-ln S)1/6 exp(-A/ln S)
(8)
Here kd is the dissolution rate constant and A a constant (24) Zhang, J. W.; Nancollas, G. H. In Advances in Industrial Crystallization; Garside, J., Davey, R. J., Jones, A. G., Eds., ButterworthHeinemann: Oxford, U.K., 1991, pp 47-62. (25) Christoffersen, J. J. Cryst. Growth 1980, 49, 29. (26) Hillig, W. B. Acta Metall. 1966, 14, 1868. (27) Nielsen, A. E. J. Cryst. Growth 1984, 67, 289.
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Figure 6. SEM image of the surface of a marble slab after dissolution.
Figure 5. SEM images of synthetic calcite crystals after dissolution: (a) original powder; (b) cavities; (c) layerwise disintegration.
related with the surface energy, γ, of the dissolving crystals:
A)
πa4γ2 3kT
(9)
Here a is the linear size of a growth unit, k Boltzmann’s constant, and T the absolute temperature. Rearranging eq 8 and taking logarithms of both sides yields
ln
A R ) ln kd ln S σ2/3(-ln S)1/6
(10)
The kinetic data obtained for all powdered materials tested were fitted in eq 10 as may be seen in Figure 3. The slope of the three lines was similar, reflecting the similar surface energy of the calcitic component of the materials. The different intercepts reflect the differences in solubility of the different materials. Pentelic marble contains metal impurities (Mn2+ and Mg2+) which cause the differentiation of its solubility in comparison with pure synthetic calcite. It has been suggested that surface texture changes may be used to distinguish between surface and bulk diffusion-
controlled dissolution.28 The presence of steps on the surface of calcite crystals is thus suggested to point to a surface-controlled process. The effect of dissolution on the morphology of crystallites was investigated by SEM. The morphology of the crystals of Pentelic marble powder and synthetic calcite before dissolution is shown in Figures 4a and 5a, respectively. Powdered Pentelic marble, both from Dionysos and Cave, presents few large crystals of average equivalent diameter in the range between 80 and 120 µm the surface of which is covered by tiny crystallites of diameter 1-3 µm. Synthetic calcite showed a more uniform size distribution of very well formed rombohedra ranging between 2 and 10 µm. Following dissolution, the crystallites showed markedly different morphology. The small crystallites, attached to the larger calcitic crystals in the marble powder, disappeared, suggesting that these crystallites dissolve at the first stages of dissolution. The remaining, large crystallites showed large cracks along the twinning boundaries (Figure 4b); cavities appeared toward the interior (Figure 4c,d) while layerwise disintegration was also observed (Figure 4d). Similar morphological characteristics were obtained from the experiments on the dissolution of calcite powder. Mass loss during dissolution takes place by layerwise exfoliation, and some pores develop as well (Figure 5b,c). Moreover the presence of steps shown in Figure 4c corroborated the conclusion from the kinetic analysis that the dissolution is controlled by surface diffusion.28 Similar morphological characteristics for the dissolution of calcite powder at neutral to alkaline conditions have been reported3. Finally, the relevance of the model experiments in which powdered materials were used with the behavior of whole marble slabs was examined. A small marble slab (2 × 5 × 10 mm) was subjected to dissolution at constant undersaturation, at σ ) 0.106. The morphology of the (28) Berner, R. A.; Morse, J. W. Am. J. Sci. 1974, 274, 108.
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surface of the dissolved specimen is shown in Figure 6. As may be seen the same morphological characteristics with the powder were observed (steps and cracks) suggesting that the dissolution of the whole slabs proceeds with the same mechanism. It should be noted however that the absolute rates of dissolution were very small because of the significantly lower surface area in contact with the undersaturated solution. Conclusions In the present work the dissolution of powdered Pentelic marble from two different sites, Dionysos and Cave, was studied in relation to their principal chemical constituent, calcite. Dissolution rates were accurately measured at conditions of constant undersaturation. The kinetic data were consistent with a polynuclear, surface diffusioncontrolled mechanism. The rate constants were found to
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be different for the three materials tested possibly reflecting differences in solubility, which are included in the rate constant. Crystal morphology after dissolution exhibited cracks in the grain boundaries and cavities toward the interior of the crystal, while layerwise removal of material was observed. These characteristics were more pronounced for the marble crystals. The presence of steps on the dissolved calcite crystals corroborated the suggestion of a surface-controlled mechanism. The dissolution pattern of whole marble slabs gave morphological features identical with those of the powdered samples suggesting that the results from powdered marble may represent well the dissolution of marble surfaces. Acknowledgment. Support of the present work by EU Contract No. ENV-CT98-0704 is greatly acknowledged. LA0004597
