gypsum overgrowths passivate calcite to acid attack

JOURNAL OF COLLOID AND INTERFACE SCIENCE 192, 207–214 (1997)ARTICLE NO. CS974978

Gypsum Overgrowths Passivate Calcite to Acid Attack

Jonathan Booth,* Qi Hong,* Richard G. Compton,* ,1 Keith Prout,† and Robin M. Payne‡

*Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QZ, United Kingdom; †InorganicChemistry Laboratory, Oxford University, South Parks Road, Oxford OX1 3QR, United Kingdom; and ‡Zeneca

Pharmaceuticals, Hurdsfield Industrial Estate, Macclesfield, Cheshire SK10 2NA, United Kingdom

Received March 4, 1997; accepted May 5, 1997

and the (010) plane of CaSO4r2H2O. The latter has beenThe dissolution of calcite (CaCO3 ) in aqueous solution at pH previously characterized by Hall (16) using AFM.

below ca. 5–6 is known to proceed via the direct reaction ofprotons at the solid surface. However, exposure of the mineralto sulfuric acid is shown to lead to the rapid formation of an EXPERIMENTALoverlayer of calcium sulfate (gypsum) which completely passi-vates the surface against further reaction and dissolution. The Channel flow experiments were conducted using the celloverlayer is nucleated instantaneously on CaCO3 surfaces. Scan-

previously described and characterized (12–14, 17–19).ning electron and atomic force microscopy show that stronglyThis comprises a rectangular duct cut in a Perspex blockadherent overgrowths are encouraged by a lattice matching be-and closed by a cover plate. The latter consists of a crystaltween the cleavage (211) plane of calcite and the (010) plane ofof calcite (semioptical grade Iceland Spar, Roger TaylerCaSO4r2H2O. q 1997 Academic Press

Minerals, Surrey) cast into a block of Araldite resin (Ciba-Key Words: calcite dissolution; gypsum crystallization; atomicforce microscopy; channel flow cell; calcium carbonate; calcium Geigy, resin CY219 and hardener HY219). The cover platesulfate. is either used directly so as to study a freshly cleaved crystal

surface (see below) or else is polished smooth by the useof diamond lapping compounds (Engis Ltd., Kent) of pro-gressively finer grit size down to 0.25 mm. The finishedINTRODUCTIONsurface was rinsed to remove organic impurites and ensuresurface reproducibility and then used essentially as described

The dissolution of calcium carbonate (calcite) has majorbefore (17) except as noted below. A flat platinum microdisk

environmental, geological, and biological consequences.electrode (Bioanalytical Systems, West Lafayette, USA)

First, the erosion of buildings, monuments, and statues in-was positioned a short distance downstream of the crystal,

duced by acid rain raises serious conservation issues (1–3);again flush with the cover plate surface. This was employed

second, the ionic composition of sediments, soils, and marinefor amperometric detection; the associated (pseudo) refer-

and freshwater systems are sensitive to calcite dissolution/ence electrode (a silver wire) was positioned upstream of

precipitation processes (4–8) which, third (9–11), maythe crystal while a platinum gauze counter electrode was

also have medical implications. Usually below pH 5–6 dis-located downstream.

solution occurs primarily through direct reaction of aqueousThe cell is assembled by mating together the Perspex

protons at the CaCO3 surface (12–14). However, in thisblock and cover plate and sealing by means of an O-

paper we seek to demonstrate that the exposure of calcite toring around the channel and a silicone rubber gasket. The

strong sulfuric acid leads to the rapid formation of an over-precise geometry of assembled cells was measured to

layer of calcium sulfate (gypsum) which can completely0.001 cm with a traveling microscope. The cell is plumbed

passivate the surface against further reaction and which isinto a flow system which consists of a glass reservoir (500

nucleated instantaneously on CaCO3 surfaces. Scanningml capacity ) and several meters of 1.5-mm bore PTFE

electron and atomic force microscopy are used to reveal thattubing through which solution runs to waste. The solution

strongly adherent overgrowths are encouraged by a latticeis deoxygenated in the reservoir and the rate of flow is

matching between the cleavage plane of calcite defined bycontrolled by gravity feed through delivery via one of

the Miller indices (211) for a rhombohedral unit cell (15)several calibrated capillaries capable of passing a totalvolume flow rate range of 1003 to 0.5 cm3 s01 . Thermo-

1 To whom correspondence should be addressed. statting of experiments is achieved by locating the cell and

207 0021-9797/97 $25.00Copyright q 1997 by Academic Press

All rights of reproduction in any form reserved.

AID JCIS 4978 / 6g2e$$$541 07-28-97 15:32:46 coidas

208 BOOTH ET AL.

FIG. 1. AFM images recorded using a Park Scientific Instruments Model SFM-BD2 in air. (a) An unreacted calcite crystal showing carbonate ionsas the main feature; these have a minimum separation of 5 A in excellent agreement with the known separation in the crystal bulk (33). The calciumions are ‘‘buried’’ in between the larger carbonate ions. The z scale in this figure is 0–2.5 A. (b) A calcite surface after exposure to sulfuric acid (seetext) and covered with crystals of gypsum.

07-28-97 15:32:46 coidas

209GYPSUM OVERGROWTHS PASSIVATE CALCITE TO ACID ATTACK

water of resistivity ú107 V cm and AnalaR grade reagents.Argon was supplied by the British Oxygen Co.

RESULTS AND DISCUSSION

The dissolution kinetics of calcium carbonate, as crystalsof calcite, was kinetically followed by means of a channelflow cell experiment (20) in which an amperometric detectoris located downstream of the dissolving interface so as tomeasure the consumption of protons passing over theCaCO3/water boundary. It is known (12) that at low pH thedissolution flux depends in a first-order manner on the H/

concentration at the reactive surface,

j /mol cm02 s01 Å k1[H/]surface [1]FIG. 2. A representative current/ time transient measured for a polishedcalcite crystal of length 0.57 cm, crystal-electrode separation of 0.27 cm,and cell depth of 1 mm. A solution flow rate of 0.037 cm3 s01 was employed.

where k1 Å 0.043 cm s01 . This has led us to investigate thesituation where the products of the acid-induced dissolutionare relatively insoluble so that the possibility exists that they

about 1 m of the preceding tubing within an air thermostat,may deposit at the formerly reactive surface and inhibit fur-

allowing temperature control to 25 { 0.57C.ther dissolution. Such a possibility exists for the case of

A Topometrix TMX 2010 Discoverer atomic force micro-calcite dissolving in sulfuric acid given the solubility prod-

scope, operating in contact mode, was almost invariably em-ucts of 2.5 1 1005 M 2 (CaSO4r2H2O) (21) and 4.8 1 1009

ployed to image the surfaces of solid substrates although aM 2 (CaCO3) (22). This was confirmed by atomic force

Park Scientific Instruments Model SFM-BD2 found occa-microscopy (AFM). Figure 1a shows a calcite crystal, im-

sional use. A commercial Topometrix liquid cell was used,aged in air before exposure to acid while Fig. 1b shows a

without modification, for in situ AFM imaging. The rates ofcorrespondingly fully passivated surface after exposure to a

dissolution of the surfaces of single crystals of calcite weresolution containing 0.1 M H2SO4. Comparison of the figures

measured using the following procedure. First, a single crys-shows that the atomically resolved and clean surface is re-

tal was freshly cleaved and imaged in air. Solution was thenplaced by a substantial overgrowth of ca. micrometer-sized

flowed into the liquid cell and after ca. 10 ml had passedcrystallites of gypsum.

through the cell the flow was stopped. Images of the dissolv-We next studied the dissolution of semi-optical grade

ing surfaces were then recorded continuously, at ca. 50-s(12) Iceland Spar crystals using the channel flow cell

intervals, corresponding to a scan rate of 10 Hz and a resolu-(CFC) technique. The dissolution media employed were

tion of 200 1 200 data points per image. For quantitativegenerated by adding HCl to a 2.0 M aqueous solution of

experiments a scan area of 20 1 20 mm was employed. InLi2SO4 so as to produce solutions that were ca. 0.1 M

all cases two sorts of experiments were undertaken: first, theHSO0

4 . Standard electrochemical methods showed that therecording of conventional topographical images and, second,the monitoring of the absolute z piezo voltage during scan-ning. The latter was recorded by scaling down the z piezovoltage using a potential divider and feeding it to an externalinput channel of the electronic control unit of the AFMwhich permitted an absolute voltage map of the scanned areato be generated. Calibration of the voltage then permittedreal drops or increases in height to be measured. This calibra-tion was achieved by imaging a test grid, with a knownprecisely defined thickness of 2400 A, and measuring the zpiezo voltage changes between topographical maxima andminima. Using this procedure a voltage–height ‘‘conversionconstant’’ of 0.308 V/mm was calculated. In this way theheight of the crystal could be measured throughout the disso-lution period in the presence of aqueous acid. FIG. 3. A schematic diagram showing the difference between (a) in-

stantaneous and (b) progressive nucleation of overgrowths.Solutions were made up using triply distilled deionized


210 BOOTH ET AL.

FIG. 4. A SEM image showing the formation of gypsum crystallites on the (211) cleavage surface of a calcite crystal.

speciation in the solution was such that there were negligi- surfaces ultimately became fully passivated with respect toacid-induced dissolution within a period of ca. 1000 s. Sec-ble quantities of free protons and that the diffusion coeffi-ond, the initial current flowing showed that for a completelycient of HSO0

4 in this medium was 1.2 1 1005 cm2 s01 .unpassivated lapped surface the analog of Eq. [1] in theIn a typical experiment a 10 mm diameter microdisk elec-medium studied wastrode was located in a CFC 3.5 mm downstream of a

crystal (23) exposed to the dissolution medium and usedto monitor the amount of HSO0

4 surviving passage over j(HSO04 ) /mol cm02 s01 Å 2k2[HSO0

4 ]surface , [2]the reacting interface through measurement of the trans-port limited current for the reduction of HSO0

4 (24) . The where j is the reaction flux of HSO04 , k2 Å 0.0013 {

use of a microdisk detector electrode in this experiment, 0.0005 cm s01 , and the factor of 2 arises since 2 mol ofrather than a conventionally dimensioned electrode,

HSO04 are consumed at the interface for each CaCO3 unit

avoided the passage of inconveniently large currents.which reacts as

Figure 2 shows a typical transient revealing an increaseof the microdisk current with time suggesting that the calcite

CaCO3 / 2HSO04 r Ca2/ / 2SO20

4 / H2CO3. [3]surface progressively becomes less reactive toward the acidsolution so that a greater concentration of HSO0

4 survivesThird, the shape of the current transient reflects the kineticspassage over the crystal surface until when the surface fullyand mechanism of the formation of the passivating gypsumpassivates a maximum detector signal is observed. Standardovergrowth. Modeling in terms of Eq. [2] permits the deduc-modeling procedures (12, 25) were used to describe thetion of the change in k2 with time as the lapped surfacemass transport regime linking the interface and the detectorbecomes blocked. We assume that the dissolution flux sim-electrode and gave three insights.ply reflects the fraction of the calcite surface, S , covered byFirst, the magnitude of the long time currents, and their

flow rate dependence, suggested that lapped (23) calcite CaSO4 and so is unavailable for reaction with HSO04 . Then,



FIG. 5. Ex situ AFM image recorded using a Topometrix Discoverer TMX 2000 microscope of a lapped calcite (211) surface after exposure to 0.1M HSO0

4 /2.0 M Li2SO4 for 90 s.

by analogy with models developed for the growth of elec- plane of a calcite crystal which has been reacted with 0.1trodeposited metal films (26, 27), the time ( t) dependence M HSO0

4 /2.0 M Li2SO4 for a period of just 60 s. Theof S is given by formation of gypsum crystals overgrowing the smooth

calcite surface is evident and two observations can benoted. The crystallites are of comparable size consistentk2( t) /k2( t Å 0) Å 1 0 S( t) Å exp(0k3t

n) , [4]with the concept that they have all started growing at thesame instant as hinted by the CFC data. Also the crystal-where the exponent n reflects whether in the case of interest

the passivation arises from the growth of nuclei formed in- lites are all aligned in a defined direction relative to theunit cell of the (211) cleavage plane. This suggests astantaneously (n Å 2) or progressively (n Å 3) as schemati-

cally differentiated in Fig. 3. The rate constant k /s0n partly very specific interaction between the substrate and theovergrowth. We consider this aspect below. Second, AFMreflects the rate of growth of the enlarging nuclei, partly the

number of nuclei per unit area, and, for n Å 3, partly the studies were conducted initially in an ex situ mannerwhich confirmed the conclusions of the SEM study,rate of nucleation. Data recorded for a wide range of CFC

geometries and flow rates (28) were found to be consistent namely that lapped crystals were effectively instantane-ously nucleated with gypsum nuclei aligned in a fixedwith Eq. [4] using n Å 2 which suggests that the passivating

layer is formed from the coalescence of instantaneously direction (Fig. 5 ) . Analogous studies using cleaved sam-ples of CaCO3 showed preferred sites for the formationformed gypsum nuclei (29).

To directly confirm this inference two separate studies of these nuclei (Fig. 6 ) . In this case etch pits are appar-ent and these appear to be essential prior to the nucle-were conducted. First, an ex situ study was made using

scanning electron microscopy (SEM) on samples of ation step. AFM studies employing the usual 0.1 Mlapped (23) Iceland Spar crystals which had been exposed HSO0

4 /2.0 M Li2SO4 medium were restricted to ex situmeasurements because the changed hydrodynamic regimeto 0.1 M HSO0

4 /2.0 M Li2SO4 for various periods of timeup to 2500 s. Figure 4 shows an SEM image of a (211) led to the formation of CO2 bubbles over the duration of


212 BOOTH ET AL.

FIG. 6. Ex situ AFM image recorded using a Topometrix Discoverer TMX 2000 microscope of a cleaved calcite (211) surface after exposure to0.1 M HSO0

4 /2.0 M Li2SO4 for 90 s.

the experiments. However, in situ AFM experiments using0.01 M HSO0

4 /2.0 M Li2SO4 were viable and the absoluteheight of a surface of a cleaved crystal monitored as afunction of time by measuring averaging (over the entiresurface imaged) the magnitude of the voltage applied tothe z piezo ceramic (see Experimental ) . Calibration ofthe latter with a grid of known pitch permits an immediateconversion into height. We have found this approach tobe much more reliable than the usual method based onthe subtraction of successive images since height in theseimages is recorded relative to the lowest point in thatparticular image and so is only meaningful if this is afixed point between successive images. Figure 7 showsthe absolute height measured relative to the mean initialsurface location monitored using the z piezo voltage. Theplot passes through a clear minimum; initially this averageheight decreases for a period of minutes but then in-creases. We again attribute this to dissolution occurring

FIG. 7. Variation in the mean z piezo voltage as deduced using in situprior to gypsum overgrowth.AFM images for a 10 1 10 mm area of cleaved calcite for the first 1500 s

In conclusion, we address the issue implicit in Figs. 4 of exposure to a flowing aqueous solution of 0.01 M HSO04 /2.0 M Li2SO4.

and 5 concerning the epitaxial overgrowth of the gypsum These data were obtained using a Topometrix Discoverer TMX 2000 micro-scope.layer. Examination of the known crystal structures of gyp-



FIG. 8. (a) Projection of the calcite crystal structure onto the (211) plane. For clarity, only the uppermost layer is shown. (b) Projection of thegypsum crystal structure onto the (010) plane. For clarity, only the uppermost layer is shown and water molecules have been excluded. (c) Diagramshowing ‘‘lattice matching’’ of the the crystal planes depicted in (a) and (b).

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214 BOOTH ET AL.

14. Barwise, A. J., Compton, R. G., and Unwin, P. R., J. Chem. Soc., Fara-sum (30) and calcite (31) enables the comparison of lat-day Trans. 86, 137 (1990).tice spacings within the former with those exposed on the

15. Alternatively the calcite structure can be described by a hexagonal unitcleavage (211) plane of calcite (Fig. 8a ) . Figure 8b shows cell. In this convention the indices of the cleavage plane are ( I0IV 4).the (010 ) plane of gypsum studied by Hall (16) . Both 16. Hall, C., and Cullen, D. C., AIChE J. 42, 232 (1996).

17. Brown, C. A., Compton, R. G., and Narramore, C. A., J. Colloid Inter-Figs. 8a and 8b reveal parallel rows of cations and anions.face Sci. 160, 372 (1993).The cation–cation spacing in each case is 4.99 A, sug-

18. Compton, R. G., and Brown, C. A., J. Colloid Interface Sci. 165, 445gesting that overgrowth of the (010) gypsum plane on top (1994).of the calcite cleavage plane is favorable in that rows of 19. Compton, R. G., and Brown, C. A., J. Colloid Interface Sci. 170, 586

(1995).anions and cations can ‘‘match up’’ as illustrated in Fig.20. Compton, R. G., and Unwin, P. R., J. Electroanal. Interfacial Chem.8c. This explanation for the observed epitaxial overgrowth

205, 1 (1986).is confirmed by the gypsum crystallite morphology in21. Culberson, C. H., Latham, G., and Bates, R. G., J. Phys. Chem. 82,

Figs. 4 and 5 in which the observed dominant face is 2693, 1978.consistent with the (010) plane (32) . 22. Stark, J. G., and Wallace, H. G., ‘‘Chemistry Data Book,’’ 2nd ed.,

John Murray, London, 1982.23. Calcite crystals were used either as directly cleaved using a sharp

blade or after polishing with graded diamond laps (down to 0.25 mm)REFERENCESimmediately prior to use.

24. The hydrogen sulphate anion, HSO04 was found to undergo one electron

1. Schiavon, N., Ciavari, G., Schiavon, G., and Fabbri, D., Sci. Total reduction at a halfwave potential of ca. 00.47 V (vs pseudo Ag). TheEnviron. 167, 87 (1995). transport limited current was found to scale directly with [HSO0

4 ] and2. Elfving, P., Panas, I., and Lindquist, O., App. Surf. Sci. 78, 83 (1994). be consistent with the expected value for a microdisc electrode (ca.3. Haneef, S. J., Johnson, J. B., Thompson, G. E., and Wood, G. C., Cor- 0.20 mA at a 10 mm disk in a solution containing 0.1 M HSO0

4 /2.0 Mrosion Sci. 34, 497 (1993). Li2SO4) provided the (very minor) effects of convection under the

4. Compton, R. G., Pritchard, K. L., and Unwin, P. R., Freshwater Biol. flow conditions employed were corrected for as described in: Booth,22, 285 (1989). J., Compton, R. G., Cooper, J. A., Dryfe, R. A. W., Fisher, A. C., Da-

5. Morse, J. W., Marine Chem. 20, 91 (1986). vies, C. L., and Walters, M. K., J. Phys. Chem. 99, 10942 (1995).6. House, W. A., in ‘‘Research in Chemical Kinetics,’’ Vol. 1, p. 107. 25. Compton, R. G., Pilkington, M. B. G., and Stearn, G. M., J. Chem.

Elsevier, Amsterdam, 1993. Soc., Faraday Trans. I 84, 2155 (1988).7. Orton, R., and Unwin, P. R., J. Chem. Soc. Faraday Trans. 89, 3947 26. Evans, U. R., Trans. Faraday Soc. 41, 365 (1945).

(1993). 27. Abyaneh, M. Y., J. Electroanal. Chem. 387, 29 (1995). [See refer-8. Buhmann, D., and Dreybrodt, W., Chem. Geol. 48, 189 (1985). ences cited therein]9. Walker, M. M., and Katz, J. L., J. Dental Res. 63, 325 (1984). 28. Fixed flow rate transient experiments were performed using calcite

10. Hay, D. I., Schluckebier, S. K., and Moreno, E. C., Calcified Tissue single crystals with an exposed length between ca. 0.5 and 0.7 cm,Int. 39, 151 (1986). using solution flow rates in the range 0.001–0.04 cm3 s01 .

11. Fujita, Y., Yamamuro, T., Nakamura, T., Kotani, S., Ohtsuki, C., and 29. A mean value for k3 of 1.1 ({1.0) 1 1005 s02 was measured.Kokubo, T., J. Biomed. Mater. Res. 25, 991 (1991). 30. Pedersen, B. F., and Semmingsen, D., Acta Crystallogr. B 38, 1074

12. Compton, R. G., and Unwin, P. R., Philos. Trans. R. Soc. London, A (1982).330, 1 (1990). 31. Effenberger, H., Mereiter, K., and Zemann, J., Zeit. Kristallographie

13. Compton, R. G., Pritchard, K. L., Unwin, P. R., Grigg, G., Silvester, 156, 233 (1981).S., Lees, M., and House, W. A., J. Chem. Soc., Faraday Trans. I 85, 32. Cody, R. D., and Cody, A. M., J. Sedimentary Petrol. 58, 247 (1988).

33. Reeder, R. J., Rev. Mineral. 11, 1 (1983).4335 (1989).


gypsum overgrowths passivate calcite to acid attack

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