Cd2+ uptake by calcite, solid-state diffusion, and the formation of solid-solution: Interface...

Geochimica ef Cosmochimica Acfa Vol. 56, pp. 1941-1954 Copyright 0 1992 Per@mon Prew Ltd. Printed in U.S.A.

0016-7037/92jM.o0 + .@I

Cd*+ uptake by caicite, solid-state diffusion, and the formation of solid-solution: Interface processes observed with near-surface sensitive techniques (XI’S, LEED, and AES)

SUSAN L. STIPP, I** MICHAEL F. HOCHELLA JR.,* GEORGE A. PARK&~ and JAMES 0. LECKIE’ ‘Environment Program, Department of CiviI En~n~~n~ Stanford University, Stanford, CA 94305, USA

%epartment of Geology, Stanford University, Stanford, CA 94305, USA 3Department of Applied Earth Sciences, Stanford University, Stanford, CA 94305, USA

(Received July i 7, 199 1; accepted in revised&m February 1.4, 1992)

Ahstraet-Cadmium uptake by calcite from aqueous solution was studied using techniques sensitive to the near-surface: X-ray photoelectron spectroscopy (XPS) , low-energy electron diffraction ( LEED 1, and Auger electron spectroscopy (AES). These techniques allowed direct observations of structure and bonding en~ronmen~ at the calcite surface. The results indicate that the main processes involved in cadmium uptake by calcite are adsorption and solid-state diffusion into the crystal, which leads eventually to the formation of solid-solution.

Pure calcite crystals were cleaved from precleaned Iceland spar and were briefly exposed to aqueous solutions containing various concentrations of Cd *+ CO:-, ClO;, and/or Cl-. Some Cd*+ was radio- , labelled. LEED results demonstrate that the calcite surface is atomically ordered, even after hydration and cadmium uptake. y-scintillation data from crystals exposed briefly to solutions of ‘@‘Cd*+ indicate that surface uptake ranged from the equivalent of about 1 to 4 monolayers. XPS analyses in the first 2 hours after exposure detected Cd within the top 30 A, but crystals stored in air or in ultra-high vacuum showed a decrease in Cd surface inanition with time such that after two days, Cd was barely detectable in the near-surface region. In other experiments, LEED verified the crystallinity of otavite ( CdC03) grown epitaxially over the ( 101 } cleavage faces of calcite, and XPS showed almost no Ca in the near-surface on scans taken immediately alter precipitation; but after storage for a month in ultrahigh vacuum, binding energy shifts and the presence of a Ca peak strongly suggested the development of solid-solution by diffusion through the solid. No Cd enrichment was observed at sites of surface defects using AES, indicating that solid-state diffusion into the mineral surface was not accomplished simply by migration along microfractures alone.

This work suggests that solid-state diffusion may play a role in the rate and extent of uptake of certain trace metals from solution and probably leads to the formation of ~tid-~lution in calcite and other carbonate minerals. It also suggests that the process of diffusion into the solid mineral host should be considered in hydrogeochemical models that intend to simulate and predict trace-metal mobility in carbonate terrains.

IN~ODU~ON

PROCE.%ES THAT TAKE PLACE at the interface between pore fluids and porous media a&x% the character and composition of both. Attempts at describing such processes by hydrogeochemical models must be based on sound understanding of the chemical mechanisms involved. In the past, considerable mechanistic insight has been derived indirectly from mac- roscopic properties observed in kinetic and ~uilib~um solution-chemistry studies ( SPOSITO, 1986). Now, however, techniques are available that allow direct observation of surface composition and structure, thus providing an excellent opportunity to check present interface-process models, and to correct, modify, or extend them. This paper describes surface processes that occur on calcite in contact with aqueous solutions containing cadmium using several surface-sensitive techniques.

Calcite was chosen as the study solid for several reasons: ( 1) it is a common constituent of soils, sediments, and rocks

* Presenf oddest: ~~rnent de Chimie Mix&ale, Analytique et Appliquke, Universit6 de Genke, 30 Quai E.-Anserrnet, CH- 12 11 Gengve 4, Switzerland.

1941

( BERNER, 197 1); also, it is known to sorb trace metals from solution (PICKERING, 1983; KAUSHANSKY and YARIV, 1986; PAPADOPOULOS and ROWELL, 1988) and, as a result, it is of interest both environmentally and geologically; (2) an ex- tensive base of info~ation describing pure calcite and solid- solutions already exists in the literature (NANCOLLAS and REDDY, 1971; PLUMMER and BUSENBERG, 1982; REEDER, 1983; STIPP and HOCHELLA, 1991; and many others); (3) most reactions in the carbonate system proceed at a rate conducive to laboratory-time experiments ( PLUMMER et al., 1978, 1979; BUSENBERG and PLUMMER, 1986; and others); (4) optical-quality Iceland spar is a ready source of large, chemically pure, single crystals; (5) laboratory synthesis of crystalline calcite is relatively easy.

Cadmium is not naturally abundant at the Earth’s surface, but during the past two centuries, anthropogenic activities (zinc mining, el~op~tin~ paint and plastic rnan~a~u~~ and application of some types of fertilizer) have increased concentrations in air, water, and soils to dangerous levels. It is taken up by plants and concentrated in animals high in the food chain, leading to diseases of the liver, kidneys, and pancreas (FRIBERG et al. 197 1; FULKERSON and GOELLER, 1973; COPENHAVER et al., 1973; NRIAGU, 1980, 1981).

1942 S. L. Stipp et al.

Studies in the field ( PAT~H~NEE~AM, 1978; STIPP, 1984) and in the laboratory ( FLJJ~NO et al., 1976; MCBRIDE, 1980; Lo- RENS, 1981; DAVIS et al., 1987; ZACHARA et al., 1991) indicate that substantial amounts of cadmium are removed from solutions in contact with calcite.

Cadmium is also interesting from a chemical point of view, particularly in its relationship to Ca and calcite. Cd*+, the only common valence state in aqueous solution, has an ionic radius that is about 2% smaller than that of Ca*+ (COTTON and WILKINSON, 1976), and the two metals have similar electronic configumtions (full d orb&ah, while vaIence results from loss of the highest energy pair of s electrons). As expected, otavite (CdCOs) has unit cell dimensions (a = 4.93 and c = 16.27 A; BORODEN et al., 1979) that are very close to those ofcalcite (CaCO,; a = 4.99 and c = 17.06 A; REEDER, 1983). Because of these properties, one would expect the formation of a nearly ideal solid-solution.

In this work, we have used X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy ( AES), and low- energy electron diffraction ( LEED ) , along with standard solution chemistry techniques to study the interesting chemical properties and interaction of Cd2+ and calcite. We have at- tempted to reveaf the mechanisms involved in the uptake of a trace metal from solution and its inco~ration into a crys- tahine solid. Building on the basis provided by the detailed surface spectroscopic characterization of pure calcite ( STIPP and HOCHELLA, 1991), we have been able to observe the uptake of Cd”+ and the precipitation of an otavite overlayer on a calcite template. Although solid-state diffision is not generally considered to play an important role in the processes leading to trace-metal uptake from solution at ambient temperatures (LAHANN and SIEBERT, 1982; DAVIS et al., 1987) we show here that Cd quickly diffuses from the surface into the bulk of the calcite crystal, while at the same time, Ca diffuses outward in an approach to solid-solution formation.

BACKGROUND

Teehniqws

X-ray photoelectron spectroscopy. XPS uses X-rays of known wavelength to eject electrons from the core and valence levels of atoms in the near-surface of a solid (for a review of XPS, see HOCHELLA, 1988.) Photoelectrons can be ejected without energy loss from a depth of several tens of Angstroms from oxides. From data presented by SEAH and DENCH ( 1979) and HOCHELLA and CARIM ( 1988), we have estimated that roughly 95% of ejected electrons in the energy range of interest originate from a depth of less than 90 %, in calcite. The probability of electron escape without energy loss falls off ex~nent~lly with the depth of origin, resulting in a heavy biasing of the sampling depth toward the surface. Thus, an element in a monolayer on the surface may be visible, whereas the same amount of material somewhere in the near-surface may not. The relative sensitivities to XPS analysis of various elements in this study are listed in Table I. It is clear from these data that XPS has good sensitivity for Cd relative to the other elements of interest.

Photoelectrons have kinetic energies that are equal to the energy of the ejecting photon minus the energy by which the electron was bound to its atom of origin (the small contri-

Table 1: Kelative XPS and AES detection sensitivity for various elements of interest in this study (leofield, 1976 and Davis et al., 1976). Nomenclature for the XPS peaks, i.e. Is, Zp;, etc. is based on the energy level of the photoelectron’s origin. Nomenclature for the AES peals, i.e. (KLL), (LMM), etc. is based on the three atomic levels that are involved in an Auger electron transition. For details, see Hochella (1988).

-

XPS line relative

sensitivity

-

H not detectable c 18 1.0 0 1s 2.9 Cl 2~s 1.6 C8 2P) 3.4 Cd Sdg 11.9

Au 4fs 9.8

-

AES line relative

sensitivity

not detectable (KLL) 1.0 (KLL) 2.6 (LMM) 5.0 (LMM) 2.2

(MNN) 5.0

bution of the spectrometer work function is discussed in HOCHELLA, 1988). Exact binding energies vary with the environment and/or oxidation state ofthe parent atom. Atoms directly on the surface of a solid have a different electronic configuration than those in the near-surface region, and this difference sometimes results in slight binding energy shifts for the ejected photoelectrons. Sometimes, the shift may be observed as distinct assymetric shoulders on the main peaks of XPS spectra, but occasionally, photoelectron peak components from two or more environments sufficiently overlap that the observed peak width simply appears to increase. Both of these effects can provide information about the surface bonding structure.

Photopeak intensities for individu~ elements from different samples can rarely be compared directly because it is difhcuh to maintain constant instrument and sample config- urations; however, element intensity ratios from one sample can be compared directly to those of another. Intensity ratios are unaffected by binding energy shifts, so they often provide a separate, independent set of information that is useful for comparison with interpretations made only from shifts of binding energy.

Bombardment of an insulating solid with X-rays causes positive charge to be induced on the sample surface by pho- toeiectron escape and this results in lower photoelectron kinetic energies (higher apparent binding energies). The magnitude ofthis shift varies from sample to sample, which com- plicates direct binding energy comparisons. Therefore, to obtain chemical information for insulators, it is essential to determine the absolute binding energies. A discussion of various methods of charge referencing is presented by SWIFT (1982), SWIFT et al. (1983), HOCHELLA (1988), and STIPP and HOCHELLA ( 199 1). The method chosen for this study makes use of very small amounts of gold evaporated onto the surface (about 0.5 to S Au atoms/A’). Determination of the binding energy shift for A~Q from its known value (84.0 eV) allows calculation of absolute binding energies for

Cd’+ solid-state diffision in calcite 1943

all constituent elements of the insulating sample. Details of the method are described in STIPP and HOCHELLA ( 199 1).

Low-energy electron d@kmion. In the LEED technique, a beam of low-energy electrons impact a sample normal to its surface. If the top few monolayers are ordered, elastically backscattered electrons form a diffraction pattern on a flu- orescent screen above the sample. Thus, the first important information to be gained is verification of a crystalline or amorphous surface. Then, if a pattern is observed, it can be used to determine the size and shape of the surface unit cell (for more details, see HOCHELLA et al., 1989, 1990).

Auger electron ~~ctro~co~~. In AES, an electron beam is used to generate Auger electrons. These, like phot~le~ns, can escape without energy loss only &om very shallow depths; the ejected electrons have kinetic energies characteristic of their element of origin (for a review of AES, see HOCHELLA, 1988). The same electron beam can be rastered over the sample surface in scanning mode to produce a scanning electron microscopy (SEM) image useful for locating a desired site for AES analysis. The electron beam can be focused to provide an analysis diameter varying from several microns to a few hundred Angstroms. Unfortunately, the electron beam can cause electron-stimulated desorption resulting in a change in surface composition; minimi~ng beam exposure and accelerating vdtage yields results with less sutiace modification. The detection sensitivities of AES to various elements important to this study are listed in Table 1; it is clear that AES has good sensitivity for Cd at the surface.

PMCC!SM?S

Ions can be sorbed (i.e., taken out of solution and attached to a foreign surface) by true adsorption, by absorption or diffusion into the solid, by surface precipitation (to form a single, distinct, adherent phase), or by coprecipitation (to form an adherent precipitate which might be a solid-solution inco~mting a second sorbate or even a mixture of precipitates). “Adsorption is the cumulation of matter at the interface between the aqueous solution phase and a solid adsorbent without the development of a three-dimensional, molecular arrangement” ( SPOSITO, 1986). A&orbed species may simply accumulate at the surface by coulombic attraction to surface charge, or they may react with surface functional groups forming a specific chemical bond, or they may exchange with a constituent of the solid to occupy a normal site in the surface structure of the adsorbent. Each of these modes has been suggested to explain experimental measurements of the extent or rate of uptake, or of changes in surface charge as a function of the concentration of the adsorbing metal ion, pH, ionic strength, and/or time (PARKS, 1990).

Some studies have interpreted the pH-dependence of divalent metal ion uptake by carbonates in terms of surface charge (SOMASUNDARAN and AGAR, 1967; FOXALL et al., 1979; SIFFERT and RMBEL, 1984; THOMPSON and POWNALL, 1989; CHARLET et al., 1990; STIPP and HOCHELLA, 199 1). In addition, many investigators have studied the rate and extent of reaction of divalent metals with calcite, invariably observing very rapid initial uptake followed by prolonged uptake at a much slower rate (papers dealing specifically with uptake of Cd( II) include: FLJJINO et al., 1976; KUMAGAI et

al., 1976; KITANO et al., 1978; MCBRIDE, 1979, 1980; Lo- RENS, 1981; DAVIS et al., 1987; ZACHARA et al., 1991.) “Fast” uptake is generally interpreted as adsorption, whereas the following “slow” uptake rates are assumed to represent precipitation or coprecipitation with the solid. MCBRIDE ( 1980) found evidence for adsorption by Cd’+/Ca*+ exchange during “fast” uptake in a study of cadmium sorption on calcite. DAVIS et al. ( 1987) presented a three-stage model in which Cd’+ adsorbs, then diffises into a “hydrated surface layer of disordered calcium carbonate” (alter the model of LAHANN and SIEBERT, 1982) which finally recrystallizes and de- hydrates, producing a (Ca,Cd, I-xt)COs solid-solution surface layer.

In this study, we are particularly interested in the possibiity that sorption of Cd’+ might be followed by migration of Cd into the solid, forming a solid-solution. Solid-solution is common in natural carbonates (see several references in REEDER, 1983; MORSE and MACKENZIE, 1990), and the properties of carbonate solid-solutions have been described ( MCINTIRE, 1963; WALTON, 1979; THORSTENSON and PLUMMER, 1977; LIPPMANN, 1980; GRESENS, 1981a,b; GLYNN et al., 1990, and references listed therein; and many others). Otavite and calcite, in particular, form a complete solid-solution series at elevated tem~mtu~ and pressure (CHANG and BRICE, 1971; BORODIN et al., 1979), as might be predicted by the similarity of size and electronic configuration of the Cd*+ and Ca*+ ions. While complete miscibility has not been demonstrated under ambient conditions, FUJINO et al. ( 1976) and KITANO et al. ( 1978) showed solid-solution for up to 10 wt% Cd in calcite, MCBRIDE ( 1980) implies its existence, and DAVIS et al. ( 1987) assume it.

The process of moving adsorbed Cd*+ from the surface into the sorbent is also feasible. Four basic mechanisms can be envisioned: simple interchange of two neighbouring atoms, migration of an atom through lattice interstices, translation into vacant lattice positions ( SEI~Z, 195 I ), or migration along microfmctur~. The first three represent diffusion through the solid-state: the last does not.

BR~~TTER et al. ( 1972) studied the solid-state diffusion of 4’Ca in calcite at elevated temperatures and estimated the diffusion coefficient, D, to be less than 5 X lo-” cm*/sec (1.8 X lo4 A*/h) at 800°C. LAHAV and BOLT ( 1964) studied the same system under ambient conditions and proposed 8.0 X lO_” cm*/sec (2.88 A2/h). Using this value and a very simple diffusion equation,

a = d2/D, (1)

where t represents time and d the depth of penetration, and assuming that Cd*’ moves at the same rate as 45Ca2+, we have calculated some rough estimates for diffusion into calcite (Table 2 ) . Solution flux to the surface, particle shape, and other boundary conditions have not been considered. How- ever, the calculations yield diffusion times for Cd migration that are on the order of only days to months through depths that exceed the limit of XPS and AES sensitivity. These ar- guments suggest that diffusion of Cd*+ from the surface into the bulk solid is possible and might be expected. Observations from the literature that might be interpreted as evidence that cation diffusion actually occurs will be described in the next

paragraphs.


!I?able 2: Appraximate time required for diIfusion of 45Ca to various depths in calcite. CalcuIations ware made with equation 1 and D = 8.0 I lo-*’ cm2/sec (L&W and Bolt, 1964).

time depth (A)

3 hours 3 thickness of o*e monolayer of Cd203 on

the (101) cleavage surface. 12.6 hours 6

I

approximate depth of LEED analysis. 1.4 days 10

13 days

4 months

1.6 years

30

90

200

estimated XPS analysis depth (95% of signat) aa minimised by tilting sample with respect to analyser.

estimated XPS analysis depth (95% of signal) as maximized for untilted calcite samples.

Measurements of uptake from solution have been very useful in developing conceptual models for surfack. processes, but they and the sorption modelling upon which they are based are not sufficient for unequivocal identification of either sorption processes or surface complexes (fi@OSITO, 1986;

CHARLET et al., 1990). Surface-sensitive spectroscopic methods have been used to further clarify sorption processes on carbonates. ZACHARA et al. ( 1989), for example, used XPS to study the surface ofcalcite after exposure to solutions close to or exceeding saturation with respect to Zn( II) carbonates. They found no evidence for the formation of Z~COJ(~, or CaJntl,,C03 solid-solutions but did observe a single, new, Zn-containing phase, probably hydrozincite (Zn~(~O~)*(OH~),~~nt~athin~tingor~“~screte plate-like flecks.” ZACHARA et al. ( 199 I) studied the sorption of several divalent metals (Cd, Zn, Mn, Co, Ni, Ba, and Sr) on calcite. They observed “fast” and “slow” processes of up take and noted that about 20% of Zn, 70% of Mn, and 80% of Cd was not desorbable. The authors suggest that the sorbate “probably was incorporated into calcite by lattice penetration or recrystallization.” These results are consistent with a process whereby adsorbed atoms that have penetrated the crystal lattice at the surface move by solid-state diffusion away from the surface in response to the concentration gradient.

For a second example, BANCRO~ et al. ( 1977a,b, 1979)

and BROWN ( 1978) used XPS to observe uptake of Ba*+ and Pb2+ by calcite at various surface concen~tions ranging as low as a fraction of a monolayer. Their intensity ratio data (BANCROFT et al., 1977a) show a decrease in the surface concentration of Ba with respect to Ca that is proportional to the lapse of time between solution exposure and analysis. Although the authors explain this change as an artifact of carbon contamination on the surface, a decrease in surface Ba concentration is consistent with migmtion of the adsorbed metal into the solid.

In other studies, MUCCI et al. ( 1985) and MUCCI and MORSE ( 1985) used AES to study thin overgrowths (to.5 pm) of magnesian calcite on Iceland spar. They found that the composition of the overlayers precipitated from a supersaturated solution did not change even after several months

of exposure to a nearly saturated solution; and using depth profiling of the substrate, they observed that the slid-~lution interface with the underlying calcite crystal extended through several atomic layers.

FULGHUM et al. ( 1988) used XPS and SIMS (secondary ion mass spectroscopy) to differentiate surface adsorbed Pb2+ from that coprecipitated with calcite powder. A different pattern of Pb”’ distribution with depth was clear for the two sample types. However, a sample with surface-adsorbed Pb*+ that had been exposed for 20 min to a solution containing 5 X lo-’ M Pb(N03)2 showed lead concentrations in the interior of the particles that were more than an order of magnitude above background. Several reasons proposed for the discrepency include the possibility that the sample “has more Pb*+ adsorbed in internal pores”; that it results from artifacts associated with sputtering in the analytical technique; or that it comes from trace amounts of Pb in the sample-holding foil. The results are also consistent with the possibility that some Pb has diffused from the surface into the interior.

In this work, we have used XPS, AES, and I.&ED to demonstrate that otavite can precipitate onto cleavage surfaces of calcite to form crystalline overgrowths and that Cd*+, sorbed at submonolayer concentrations, can diffise into solid calcite to form solid solutions under conditions that would not permit recrystallization.

EXP~~ME~AL MATE~A~ AND ME33IODS

Materials

The importance of a “clean ” surface

It is difficult to prepare mineral surfaces that are completely free of adventitious material; but in analyses with near-surf&e techniques, it is advantageous and sometimes critical that contamination be kept to a minimum ( STIPP and HOCHELLA, 199 I ). If an adventitious layer is more than a few monolayers thick, XPS peak intensities for elements from the substrate are significantly attenuated. Unfortu- nately, this attenuation is a function of the photoelectron kinetic energy and leads to proMems with quantitative analysis. Likewise, LEED patterns may not be obtainable with more than a couple of monolayers of #ntamina~on. During this work, we used the techniques described in ST!PP and H-ELLA ( 199 1) and s~rnrna~~

Cd*+ solid-state diffusion in calcite 1945

below to maintain surface cleanliness, thus minimizing problems associated with contamination.

Solids

Chemically pure otavite powder (designated OfP) was synthesized under clean conditions as described in STIPP et al., ( 1992). It was aged (stirred slowly, 60-120 rpm, in distilled, deionized water) for several months at 70°C and then several more months at 25°C before a portion was filtered, vacuum dried, and used in these experiments. SEM photographs showed euhedral, rhombohedral crystal form with particle dimensions ranging from 0.1 to 1.5 Frn and averaging about 0.5 pm. Powder XRD was used to confirm the structure of the synthetic material (see STIPP et al., 1992 for details and a photo).

Pure calcite powder (designated CUP) was obtained by aging Fisher Reagent Grade calcite at room temperature for several months with pure CO, bubbling in several fresh rinses of distilled, deionized water (using a-method adapted from PLUMMER and BUSENBERG, 1982). Vacuum-dried samples yielded surface areas on the order of 0.2 m2/g (BET analyses).

The calcite, single-crystal, cleavage fragments (designated CuX) were prepared from carefully cleaned, opticalquality Iceland spar (Chihuahua, Mexico; Ward’s Scientific) according to the method described in STIPP and HOCHELLA ( 1991). These samples were on the order of 8 mm* and 2.5 mm thick with only { 10 1 } faces exposed. Surface area was estimated with the aid of a light microscope and scale.

Several freshly prepared, single-crystal, calcite cleavage fragments (about 10 mm* by 2 mm thick) were cut in such a way that neither face had touched anything but air. They were immersed in a solution that initially was saturated both with pure CO2 and with CdC03. The flask was plugged with a rubber stopper and set aside. Slow degassing through the seal over the following weeks and months allowed slight supersaturation with respect to CdCGs, and precipitation of a cloudy overgrowth began on the cleavage surfaces that had been shiny when fresh. Samples were removed periodically for analysis. In this article, they are designated C&O.

Solutions

The water used for making all solutions was freshly deionized with a MilliQ (MQ) system (resistivity of 15 MII/cm or more) and then processed in a glass still with all-glass tubing. In order to prevent hydrocarbon contamination, neither the distilled MilliQ water (DMQ) nor the solutions were ever allowed to touch plastic vessels, rubber stoppers, flexible tubing, or surgical gloves. All solutions were made and stored in glassware that had been washed with laboratory detergent (Alconox), rinsed in deionized water, then soaked in 10% nitric acid for at least 10 h and rinsed several times with MQ and then DMQ water. Glass vessels were used for water and for all solutions because plasticware is known to alter cadmium concentrations ( STIPP et al., 1992). Where solution concentrations were low enough that loss of Cd2+ to container walls might significantly alter concentrations, the glassware was preconditioned overnight with a separate ahquot of the same concentration. The CO2 used for sparging was 99.99% pure, and the N2 was 99.997% pure.

Solutions at saturation with CdCO, were drawn with a glass syringe (metal needle and coupling removed) from the synthetic otavite storage vessel (described previously) and hltered through a micropore filter attached to the syringe. Several other Cd-bearing solutions were made by weight from Cd(C14)* - 6HzO and/or CdC12 (Baker Re- agent-Grade). ‘09Cd, with a half-life of 420 days, in a solution acidified with 5 X lo-’ M HCI, was used to spike some of the solutions. Most Cd uptake analyses were performed radiometrically. ‘OsCd is a y-emitter, allowing scintillationcounting of solids as well as solutions. Some total Cd concentrations were determined using graphite furnace atomic absorption ( Perkin-Elmer 403) and others, with a cadmium ion-selective electrode (Orion).

In order to understand the processes that are important to the uptake of Cd*+ from solution, we needed samples where the dominant process was clearly identifiable. The objective was not to study the solution conditions at various points along reaction pathways, but to choose conditions at limits where one process would clearly dom-

inate over all others. When equilibrium calculations were necessary, such as for the determination of pH or solution concentrations for ensuring undersaturation with respect to CdCOs, association and solubihty constants for the Cd-C02-H20 system presented by STIPP et al. ( 1992) and for the Ca-C&-H20 system presented by PLUMMER and BUSENBERG ( 1982) were used in HYDRAQL, an aqueous spe- ciation model (PAPELIS et al., 1988).

Sample treatment: Solution exposure and removal

Single crystals were immersed in scintillation vials containing about 4 mL of solution and shaken. Near the end of their exposure time, the solution was decanted, and each crystal was removed from the vial with stainless steel tweezers pinched on a comer that was not to be used for surface analysis. The sample was held vertically; and at exactly the time that exposure should end, a stream of high purity N2 was used to sweep all remaining solution from the crystal. At the same time, the comer of a Kimwipe was held to the last droplet that was clinging to the lowest crystal edge in such a way that the tissue did not touch the crystal.

This method of mechanically removing the droplet was adopted to minimize contamination and to avoid evaporation that could result in precipitation of the material remaining in solution on the surface. To prove the effectiveness of this method, we exposed a calcite sample ( Call ) to 10m4 M Cd( ClO,)z - 6H2O and analyzed it immediately after the drying procedure described above. For this particular test, we chose ClO; as the only ligand because of its known lack of ten- dency to pair with metal ions that would result in surface complexing. So, if Cl was observed at the calcite surface, it could remain only as a result of evaporation, proving that the solution had not been completely removed. Cl, has good sensitivity in XI’S spectra (Table 1 ), so even small amounts of surface ClO; would be detectable. Analysis immediately after solution exposure showed typical peaks for the components of calcite (O,,, CIJ, and Ca,), and adsorbed Cdw was observed (Fig. I), but no trace of Cl* was detected on the blown- dry surface. Because of its success, this method of solution removal and drying was adopted in preference to rinsing with distilled MilliQ water or with ethanol, the traditional method used for cleaning and drying samples for use in ultrahigh vacuum systems. Separate experiments show that water rinsing removes a substantial proportion of the adsorbed Cd, alcohols always leave a slight residue of contamination ( STIPP and HOCHELLA, 199 I )

Instruments

X-ray photoelectron spectroscopy

For this study, samples were analysed in a VG ES CALAB Mk II. Vacuum was typically between 10e9 and IO-” mbar (1.0 mbar = 0.75 torr). Nonmonochromatic Mg K., X-rays (hv = 1256.6 eV) were used as the excitation source at a power of 240 W. A mono- chromatic X-ray source was not suitable because it caused unstable sample charging (sometimes causing peak broadening) on the carbonate samples. An analyzer pass energy of 50 eV with a step size of 1 eV was used for survey scans; for the regional (narrow) scans, a 20 eV analyzer pass energy with a step size of 0.1 eV was used. Spectra were analyzed with the aid of Shirley background-subtraction and least-squares Gaussian-Lore&an curve-fitting, both performed with VG 5000/5250 computer software.

The 3 mm diameter, analyzer-entrance aperture, with a sampling diameter of 1 mm, was used for many samples in order to achieve good energy resolution while maintaining a reasonable count rate. In several instances, duplicate samples were analyzed with a 1 mm diameter, analyzer-entrance aperture (sampling diameter 0.33 mm). Although these latter scans had poor signal-to-noise ratios, the im- proved resolution allowed discrimination of peaks that were closely spaced. In some cases, especially where intensity ratios were the chief aim of the analyses, the 6 X 15 mm aperture with a sampling area of 2 X 5 mm was used in order to increase the signal-to-noise ratio.

In all cases, binding energy measurements from the regional scans were reproducible to within 0.1 eV. The intensity of small peaks representing surface species did not decrease during long periods (days to months) of exposure to ultrahigh vacuum ( 1O-‘o mbar). However,


P!; 360- cas, : +;: 140

342 346 360 364 368 362 347.7 361.2 406.9 412.7

700- 01,

622 626 630 634 638 202 206 210 214 218 631.9 Binding Energy (eV)

120

5 80 u n t * 40

280 284 288 1 292 290.1

Binding Energy (eV)

FIG. 1. Medium resolution regional scans for components of calcite with adsorbed Cd*’ on cleaved calcite sample ( Call ) exposed to 10e4M Cd(C104)2. 6Hz0 for I min. The surface was blown dry with N2. Cadmium surface coverage is calculated to be about a monolayer. Spectra for Ca,,, Clr, and O,, are typical of calcite. Cadmium is present at the surface, but Cl is not. (All scans have been shifted to absolute binding energy.)

we found again that the relative intensity of peaks representing surface hydration species decreased during extended exposure to X-rays (STIPP and HOCHELLA, 1991), while the composition of the near- surface remained unchanged during analysis. In order to minimize X-ray-induced modification for all samples in this study, we selected the lowest X-ray power, the highest detector settings, and the shortest analysis times possible to maintain reasonable signal-to-noise ratios. In order to allow us to make direct comparisons between samples, we exposed each sample to X-rays of the same intensity for the same amount of time.

Low-energy electron di@raction

For this study, we used a VG model 640-reverse view LEED system mounted on an ultrahigh vacuum chamber. Vacuum was typically in the range of 10m9 to IO-“’ mbar. In order to prevent charging of the insulating calcite surface, an electron beam energy of greater than 80 eV was necessary. Beam energies higher than about 130 eV led to rapid electron-induced damage of the surface, and the diffraction pattern was lost after only a few tens of seconds.

Auger electron spectroscopy

AES was performed using the VG ES CALAB Mk II system. Be- cause carbonate surfaces are easily modified by electron-stimulated desorption resulting in the loss of CO*, the electron beam accelerating voltage, beam current density, and exposure times were all minimized.

The beam voltage was set at 3 KeV, and the beam current was main- tained in the nA range. Large analyzer pass energies and slit sizes were used to maximize counts at the expense of energy resolution. In addition, the beam was defocused and rastered over the largest area compatible with the type of analysis required. Charging was minimized by placing the sample at 45” with respect to the beam. Multiple scans (usually five) were made from 25 to 600 eV kinetic energy: the data were then smoothed and differentiated.

RESULTS AND DISCUSSION

Our study of the behaviour of Cd*+ at the calcite surface took three different directions. First, we studied pure otavite powder in order to obtain reference information (absolute binding energies for each element) to parallel data already available for pure calcite, the other endmember of the solid- solution series ( STIPP and HOCHELLA, 199 1) . A second di- rection of study was to examine otavite overlayem that had precipitated to a depth of at least 90 di on the cleavage surface of Iceland spar. Finally, we observed metal adsorption of a monolayer or less resulting from short exposures to dilute and slightly acidic solutions, where precipitation of either otavite or Cd-CaCOs solid-solution was highly unlikely. In this work, we speak of a monolayer of Cd as if the metal had

Cd*+ solid-state diffusion in calcite 1947

occupied only the cationic (Ca) sites in the surface atomic structure. LEED, XPS, and radiotracer evidence (presented later) demonstrates that this assumption is probably valid. On the calcite { 10 1) cleavage face, a monolayer is 3 8, deep, and on a planar surface, there are 7.5 X lo-” moles of Ca atoms/cm’ if all metal sites are filled.

Pure Otavite Powder (CdCOs )

Several samples of pure, clean otavite powder were analyzed by XPS after surface decoration with a very small amount of evaporated gold. The absolute binding energies of Cdsd, Cls, and O,, were determined from the A&/shift (relative to 84.0 eV) for each sample; the mean values are presented in Table 3 along with some previously determined values.

The spectra for the components of otavite (not shown) exhibited assymetric shoulders on the high binding energy side of the main peaks. At higher photoelectron energy resolution, separate low-intensity peaks appeared at positions slightly shifted to the higher binding energy side of the main peaks for each element. STIPP and HWHELLA ( 199 1 f observed simiiar peaks for freshly cleaved, calcite surfaces that had been exposed to water either as liquid or as vapour; these were interpreted to represent the hydrolysis products S - Ca0I-I and S * CO3H (Fig. 2a ) . By analogy with that work,

Table 3: Absolute binding energies for the elements of otavite.

Absolute

binding

energy (ev)

Method Reference

289.5 f 0.1 gold this work 283.7 f 0.1 Osb gold Sommer (1975)

406.2 f 0.1 gold rbis work

411.9 f 0.1 gold this work

531.6 f 0.1 gold this work

532.1 f 0.1 Oqc gold Sommer (1975)

a Values originally reported in Sommer (1975) were determined by comparison with the biding energy of Awf at 83.0 eV, but his valuea, as shown above, have been adjusted to Audf at 84.0 eV, in order to be consistent with OIV work.

’ Sommer’~ Cl, value is in the range of “adventitious” carbon (Stipp and liocbella, 1991) and probably does not represent carbon from the carbonate group.

’ On wide resolution scans, it is not possible to distinguish the slightly shifted peak that represents oxygen orbed from CdOH on the hydrated surface (hydrolysis species explained later in the text) from the main oxygen peak representing near-surface CdC03. Determination of 01, absolute binding energy using the composite peak results in a value a few te&bs of 1 eV higher thsn that found from the peak representing only near-surface oxygen in CdCOJ.

the small, higher binding energy peaks observed for the elements of pure otavite powder (Fig. 2b) have been assumed to represent 5’. CdOH and S- COaH and will be referred to hereafter as hydration shoulders.

Otavite Overgrowth Precipitated on Calcite

Several, freshly cleaved, calcite samples were immersed in a solution that initially was saturated with both CdCO3 and pure CO,. Slow degassing over several months led to precipitation of a smooth, cloudy overgrowth on the originally shiny, cleavage surfaces. Otavite is significantly less soluble than calcite (compare pKslOtavjte) = 12.1 (STIPP et al., 1992) with pKsgcalfiteI = 8.43 SLUMMER and BUSENBER~, 1982)). Be- cause of this difference and because of the initial lack of Ca in the solution, it was a safe assumption that only otavite would precipitate as CO2 evolved and pH increased. Disso- lution of calcite would then be prevented by a continu~ly thickening rind of CdCO3, and subsequent precipitation, of a Cd-CaCOs solid-solution would be unlikely. PAPADOPOU- LOS and ROWELL ( 1988) studied the behaviour of calcite in contact with Cd solutions and found calcite growth to be inhibited following formation of a CdCOs coating. Exposure times in the solution varied as shown in Table 4. Each sample was analyzed immediately after drying, except sample 0t04, which was stored in ultrahigh vacuum for one month before analysis.

Our assumptions concerning the precipitation of a nearly pure, otavite overlayer were verified by surface analysis. Sev- eral separate samples exposed to precipitation for a week ( OtOl ) were examined with XPS and LEED. The chemical composition revealed by XPS showed almost pure CdCO3 to at least a depth of about 30 A. LEED confirmed that the overlayer was crystalline, and the diffraction pattern was identical to that expected from the lattice parameters of otavite at the 95% confidence level. Thus, based on the chemical and structural information, the overlayers were confirmed to be otavite, presumably growing epitaxially over calcite. The LEED difI?action maxima for samples OtOf were more dif- fuse than those observed for freshly cleaved Iceland spar and resembled maxima of recently precipitated calcite ( STIPP and HOCHELLA, 1991). This could mean either that the precipitated overlayers had more adventitious carbon con~ination than the pure calcite surfaces, or that there is some dis- tortion in the atomic structure of the overlayer, possibly caused by minor Ca substitution. Unfortunately, the otavite repeat distances in the { 101) plane are within 2% of those for calcite and, in addition, are within the uncertainty margin of the LEED technique, making direct comparison of lattice parameters for samples with composition varying along the CdCO,-CaCO3 join impossible.

Samples OtOl and OtO2 were exposed to solution that was slightly supersaturated with CdC03 for the duration of sample exposure (conditions and results are presented in Ta- ble 4). We see evidence that otavite precipitation was the dominant surface reaction. Analyses performed immediately after the samples were removed from solution show Ca, peaks that were harely above background (Fig. 2), even when the sample was positioned to maximize analysis depth. The ratio of Cd to Ca increased from 0 on the original calcite to about 65 after two months. The fwhm (full width at half


(a) calcite (b) otavite overlayer

60

600

C 700

C 0 0 It II :: i 400 t

s contamination

400

200

290.1 406.5 412.3 Binding Energy (eV) Binding Energy (eV)

FIG. 2. (a) Spectra from the { 10 I } cleavage surface of Iceland spar exposed to distilled, deionized water at equilibrium with air. The main peaks represent CaC03 in the near-surface. The smaller peaks (at higher binding energy) are “hydration shoulders” that represent the hydrolysis species on the surface. (b) Spectra from an otavite layer grown over { 10 I} cleavage surfaces of Iceland spar (sample OtOl ). The intensity of the Ca, peak is very weak compared to the pure calcite sample. Hydration shoulders are again present on the higher binding energy side of the main O,, and CdW peaks. (All scans have been shified to absolute binding energy.)

maximum) for all peaks of sample 0202 were similar to values found for pure otavite powder. This evidence, together with the LEED results, proves that the solutions were pre- cipitating nearly pure otavite and that solid-solution precipitation involving Ca was highly unlikely for at least a depth of 90 A into the surface.

Sample Ot03 was exposed to the solution for 10 months and was analyzed immediately after drying. Ca,, peaks were clearly visible and the Cd/Ca ratio had fallen to about 3 (Table 4). Thefwhm for all peaks were considerably larger than for pure otavite or for the previous two samples, sug- gesting more than a single environment for each constituent. This evidence suggests that a Ca-CdC03 solid-solution may have been formed during the sample’s long exposure to the solution.

If metal sorption from solution continues throughout the entire exposure time as suggested by typical two-stage uptake/ time curves (particularly the Cd*+ data of DAVIS et al., 1987,

and MCBRIDE), 1980, then our evidence indicates that the surface of 0t03 has been depleted with respect to Cd or en- riched with respect to Ca, and hints at the possibility of solid- state diffusion. On the other hand, PLUMMER and BUSENBERG ( 1992) have observed rapid uptake of Sr2+ by aragonite followed by release back to solution and, subsequently, very slow uptake. If the Cd-calcite system was behaving in this way, then decrease of the surface Cd/Ca ratio and the evidence of solid-solution would suggest loss of Cd2+ to the solution.

Sample 0t04 tests these hypotheses. After exposure to solution for two months, the near-surface Cd/Ca ratio was expected to be the same as for sample 0t02 (about 65 ) . How- ever, before analysis, sample 0204 was stored in ultrahigh vacuum for an additional month. During this time, loss by desorption in solution was impossible. Recrystallization aided by ambient humidity was also precluded by the extreme conditions of the vacuum chamber. When finally analyzed, Ca2,

Cd*+ solid-state diffision in calcite 1949

Rble rl: Conditionn and renultr from various otavite o=k’-.

srmplr mImba SampIe birtoq

Approxinmtt C~+J,, Cd/CapepL uudylir Pe8k intcauity depth b clear? ratio

oto2 2 month0 in rolution, 90 A no 65 analyzed immediate.Iy after drying

otos 10 montba in eolution, 90 A yts 3.0 8Miysed immediately aa drying

oto4 2 month8 in sokaion, 30 A ye8 2.8 then 1 month In vacuum

a Solution is initially saturated with CdC!Ost,) and pure CO?; Ca WM not present. Slow degas&g over rtvtd months lead to predpitation of an otavitc laya growth over the uldte deavage rurfaca.

b Difkrent depth are achieved by &an&g the angle of the sample with nrpcet to the uwlyser column.

peaks were clearly visible, the Cd/Ca ratio were once more very low, and the&&n of the peaks were much wider than would be expected for pure otavite or pure calcite. Again, the evidence indicates the development of a C!a-cdcO~ solid- solution, but this time it developed in the absence of a solution phase. This is the f%st of three independent pieces of evidence that demonstrates that solid-state diffusion of Cd’+ in calcite

proceeds relatively quickly, with a’rate on the order of tens of AngsWms in only a few weeks.

Absolute binding energies were determined for Ca,, Cd%, Olr, and C,, in the otavite overlayer samples, and the results are presented in Table 5. For comparison, data for pure calcite, pure otavite, and a calcite sample exposed briefly to a Cd solution ( Call ) are also included. The binding energies

Table 5: Abaoiute binding energies (eV) for the ekments present in otavite overgrowthr on ctldtt. AU mmph rtftrtmtd with An+

pure okvik powder 289.5 f 0.1 495.2 l 0.1 531.6 f 0.1

oto2 2 mo. in solution, 289.7J:O.l 347.7f0.2 405.SfO.l 531.7fO.l analyzed immediately

oto4a 2 mo. in solution 289.9 rf: 0.2 347.7 f 0.2 406.6 f 0.2 631.8 f 0.2 thenlmo.invaeuum

Cdl 1 min in sohation of 290.0f0.1 347.TfO.l 405.9f0.2 631.9fO.l lo-’ M Cd-pexehbmtt, ~~~~~~y

pure e&it3 (I&d apar)

290.1 f 0.1 347.7 f 0.1 631.9 f 0.1

a Average from only two samples.

b Detded by Stipp and Eochella (1991).


of Ot02 and CM34 are not significantly different from each other. However, the shifts of binding energy as the surface concentration of Cd decreases from pure otavite powder to pure calcite show an interesting trend. Sample OtU2, which is nearly pure otavite at the surface, shows binding energies for C,, and Or, that are bracketed by those of otavite powder and calcite but that are close to energies expected for pure otavite. Sample OtO4, which had a considerably higher surface con~~tmtion of Ca, shows binding energies that are closer to those of pure calcite. The change in atomic binding energies, as the ratio of Cd/Ca in the surface changes, reflects a modification of atomic environment. This could be explained by a difference in bond distance between carbonate groups and each of the two different metal atoms, as would be expected during development of a solid-solution,

Uptake by Cakite Powder

Aged calcite powder ( C'uPl ) was exposed to a gently stirring solution at a concentration of about 1 g of solid per litre of solution. We calculated the concentration of a solution that would be undersaturated with respect to both CaCOs and CdCO:, using the equilibrium model and constants discussed previously. It contained 10-6.4 M Cd-perchlorate and lo-* M KOH; it was at equilibrium with 100% CO*, was undersaturated with respect to CdCOs , and initially contained no Ca2+. Its slight acidity (5.2 < pH < 5.9) would allow adsorption of Cd’+ while inhibiting precipitation. During the time that the powder was exposed to the solution, it is certain that some calcite dissolved and it is possible that some Ca-CdCOs might have precipitated, but after 55 h, 75% of the cadmium had been taken up from solution. Using an average surface area of about 0.2 m’/g (BET) and assuming that all surfaces were ( 10 1 f faces, the Cd2+ surface coverage could be calculated to be about one-quarter of a monolayer. Based on XPS relative signal intensities (Table 1) and our experience with many other spectra from samples having monolayer Cd coverage, if all of the Cd were to be present at the surface, it should be possible to see weak but distin- guishable peaks in the Cdsd region. The powder was analyzed

as soon as possible (<3 h) after filtering and vacuum drying. XPS analysis showed peaks for CLs, Olsr and Ca2, that were typical for calcite powder that had been exposed to water, but the Cdsd region had no intensity above background. Cad- mium had been taken up from solution, but by the time the powder was analyzed by XPS, the metal was no longer detectable at the surface. Thefwhm for all peaks from CaPl were wider than those observed for pure calcite powder, indicating more than one major structural environment for each of the elements of calcite. This could result from Cd substitution in the calcite crystal lattice.

Uptake by Calcite Cleavage Surfaces

A cleaved crystal of Iceland spar (C&Y1 ) was exposed for 1 min to a solution of 10m4 M Cd perchlorate. -y-scintillation data from similar crystals and solutions were used with estimated surface areas (determined as described in the experimental section) to calculate that about one monolayer of Cd*+ would have been taken up by the surface. Steps and microfractures could not be taken into account in the surface area estimation, but they would increase the estimated surface area, making true surface concentration lower. However, analysis by XPS about 2 h after exposure showed clear peaks for Cd3d, and the peaks for Caz,, C,, and Oi, were all typical for those of calcite exposed to water. The spectra for sample C&l are shown in Fig. 1, and Cd intensity ratios are presented in Table 6.

LEED analyses of samples with identical history to CaXi , and from other similar samples that had been exposed instead for 20 min to a solution of the same composition, all showed clear, regular, diffraction patterns. Their maxima were slightly more diffise, but their spacings were not statistically different from patterns derived from pure, fresh calcite. This evidence proves that the near-surface retains its crystallinity, even after surface hydration and after uptake of the equivalent of a monolayer or more of Cd*+.

Three calcite cleavage samples, C&Z?, C&3, and C&4, exposed to solutions containing 10 -4 M Cd f radiolahelled io9Cd; 5 X 10 -3 M HCl; solutions open to CO2 in air) for 1,

Table 6: Intensity ratios for calcite cleavage samples exposed for various times to Cd solutions (lo-’ M Cd(C10&6&0; ro9Cd, and 5 x 10v3 M HC1; COx at equilibrium with atmosphere; no initial Ca concentration). All sampler were tilted to maximize surface semitivity. Information depth is about 30.k.

7

Sample Solution equivalent approximate time XPS

number exposure monolayer8 lapse before Intensity Ratios

time (min) Cd taken up” XPS anaiyds Cd/O CdfCa

CaXl CaX2 cax3 cox4

1 1 2 hours- 0.11 0.16 1 1 18 hours 0.04* 0.06*

10 2.8 28 hours 0.036 0.046

100 4 48 hours 0.01* 0.026

4 Calculated using ~-scintillation data from solution and from crystals.

b These ratios are calenlated using estimated peak intensities. The cadmium pealrs (see Fig. 3b for an example) were so dithcult to see that precise measurement of intensity was impossible.

Cd2+ solid-state diffusion in calcite 1951

10, and 100 min, respectively, were analyzed between 1 and 2 days after exposure (see Table 6). During the time after solution exposure but before analysis, the dry cleavage fragments were exposed to air but were kept under a glass cover to prevent excessive contamination by adventitious carbon. Scintillation counting of both the solutions and the crystals indicated Cd uptake equivalent to about 1,2.8, and 4 monolayers, respectively. C’ax2 had been exposed to solution for exactly the same amount of time as CaXl with the same amount of Cd taken up by the surface, so we expected a similar intensity for the CdJd peak. CaX3 and CaX4 were exposed longer to the acidic solution. We expected some dissolution of CaCOs and perhaps some precipitation of C+Cd( ,_&03. Nevertheless, if the Cd remained in the near- surface, relative CdJd peak intensity would be expected to increase with surface coverage. The XPS spectra showed that all calcite component peaks were typical of a surface exposed to water. For all three samples, the amount of surficial adventitious C was insignificant; it could not have caused high attenuation of photoelectrons from the sample surface. However, the cadmium peaks for all three samples were scarcely above background. Samples with higher cadmium uptake but analyzed after a longer storage time had lower Cd3d peak intensity ratios (Cd/O and Cd/Ca) (Table 6 and Fig. 3). This, combined with the results from the calcite powder, provides the second, separate piece of evidence for fast diffusion of Cd into the bulk of the calcite crystal.

Specijic su$ace features analyzed by AES

Rather than diffusion through the solid state as such, it is possible that microfractures in contact with the humidity of laboratory air could have served as a conduit for movement of adsorbed Cd away from the calcite surface. If this were to be true, one would expect a concentration of Cd at and near

(a) 140

I Sample CaXl

03 Sample CaX4

C 320- 0 u - n i 280.

=p 1’1 I” , I 240 -

Ia * I1. 0 I III II 400 404 1 408 414 416

405.9 412.7 Cdsd Binding Energy (eV)

FIG. 3. Cdu regional scans for calcite samples exposed to 10e4 M Cd. (a) Call took up the equivalent of one monolayer and was analyzed immediately. (b) CuX4 took up the equivalent of four monolayers but was analyzed after storage for 2 days in air, covered by a glass dish. (Scans have been shifted to absolute binding energy.)

the site of microfractures and surface defects. Evidence from scanning force microscope (SFM) images at nanometer scale proves a scarcity of microfractures on fresh and solution- exposed cleavage surfaces of Iceland spar (STIPP and EG- GLESTON, 1992 ) . In order to see if concentrations of Cd were high& at surface defect sites, we took advantage of the small analysis area of AES. Several calcite cleavage flakes that had been dipped for 100 min in 10d4 M Cd solution (“‘Cd; 5 X 10e3 HCI; same conditions as CaX4) were analyzed 4 h later. In scanning mode, we focused on various surface features and used only short analysis times in order to minimize surface damage. Figure 4 shows the types of surface features examined and some AES scans. Figure 4a is typical of all sites on all of the crystals. In all scans, the components of calcite (C, 0, and Ca) are present, and Cl from the HCl used to acidify the radiotracer solution is visible. However, only one site, a bruised area resulting from the scoring necessary to induce cleavage, showed a trace of Cd (Fig. 4b). Still, the Cd intensity is many times smaller than that expected based on the calculated surface coverage (about four monolayers) and AES relative elemental intensities (Table I ). This evidence clearly shows that cadmium was no longer present at the surface, nor was it concentrated on surface defects. If it migrated into the crystal by way of surface imperfections, it did so very quickly and almost completely.

Desorption stimulated by the X-ray beam

The loss of adsorbed material caused by X-ray exposure during XPS analysis has already been discussed, and the steps we took to reduce the effects have been described (see section on experimental details). In some cases, however, the phe- nomenon can be used to provide useful information. Four cleavage fragment samples, CaXS to Cax8, were exposed for 1 min each to 10m4 M cadmium solution (“‘Cd; 5 X 1 0m3 M HCl; COZ at equilibrium with air). The fragments themselves were scintillation-counted before and then immediately after exposure to solution. Surface uptake of Cd for all samples was calculated to be on the the order of a monolayer. Two samples were analyzed by XPS within an hour; two were stored for 24 h before analysis (one was kept in air with about 95% humidity; the other, in the ultrahigh vacuum chamber). After XPS analysis, the samples were again scintillation-counted, and Cd lost from the surface as a result of X-ray stimulated desorption was determined from the difference in counts before and after beam exposure. The results are shown in Table 7.

The concentration of near-surface cadmium (compare ratios Cd/O and Cd/Ca) was significantly higher in the samples that were analyzed immediately after exposure to solution. However, the scintillation-counting data showed that these fresh samples lost an average of about 30% of their total Cd due to X-ray stimulated desorption during analysis. On the other hand, samples stored for one day before analysis lost an average of only about 12% of their Cd in the X-ray beam. These results demonstrate that the amount of Cd that was available at the surface for X-ray stimulated desorption decreased substantially with time after uptake because it had moved away from the surface. This is the third of the independent sources of evidence for Cd diffusion into the bulk of the calcite crystal.


8rea of no apparent defects bruised area where cleavage was induced by repeated gentle scratching with a razor blade corner cleavage steps

edge large fracture scratch made with a pin mierotiaetures induced by scratch

(a) 60

dJEJ 20 dE

-20

-60

-100

(b) 2000

-P dNE o

d

200 300 400

Kinetic Energy (eV)

FIG. 4. Diagram of a calcite crystal showing types of surface locations that were tested by AES for Cd enrichment. The samples were exposed to lo-’ M Cd for 100 min and analyzed about 4 h later. Caieulations from radiotracer data showed that the ~uiv~ent of four mono~ye~ had been taken up by the crystal. (a) A spectrum @picaI for all sites of all samples. Components of calcite are clear, but Cd is not visible. (b) The only AES scan (out of more than thirty) that shows a trace of Cd was collected at site b.

SUMMARY AND CONCLUSIONS

The absolute binding energies for the elements of otavite were determined using XPS and the gold referencing technique on synthetic otavite powder. They are: 298.5 f 0.15 for Cls, 405.2 and 4 11.9 + 0.15 for Cd+ and Cd+ respectively, and 531.6 4 0.15 for O,,.

Many of the experiments of this study used single crystals of cleaved Iceland spar that had been exposed to aqueous solutions. The most eff%tive way to dry these samples for XPS / LEED analysis was to blow a stream of high-purity NZ across their surface. Samples treated in this way showed no evidence of evaporation ofresidual solution. Thus, there was no need to subject them to dissolution during a rinse with distilled water or to adventitious carbon contamination during a wash with alcohol.

LEED analyses of calcite surfaces that had been exposed to Cd uptake from solution for 1 or 20 min, or to otavite

precipitation for a week, prove that the top few monolayers maintain their crystal structure. Lattice parameters were not signifi~ntly different from those of calcite or otavite, respectively, at the 95% confidence level.

Diffision of Cd ‘+ into the bulk of calcite crystals was demonstrated by three observations:

1)

2)

Samples whose surface uptake ranged from less than a monolayer to the equivalent of four monolayers provided clear peaks for Cd3d when analyzed immediately by XPS, but in analyses performed a day or two after exposure, Cdsd peaks were barely above background. Cadmium on samples analyzed immediately after exposure was more susceptible to X-ray stimulated desorption than cadmium on samples analyzed the next day, dem- onstrating that time had allowed transport of the cadmium away from the outermost solid Iayers, leaving it unavail- able for desorption by the X-ray beam.

Table 7: Intensity ratios for calcite cleavage samples exposed to ‘OgCd solutions and anaIyaed by XPS immediately or after one day of storage.

Sample” aaalysed Cd/O Cd/& Cd loot by surface*

CaX5 immediately 0.08 0.113 25% &X6 immediately 0.09 0.116 36% Cax? after 24 hours in air 0.06 0.086 14% CaX8 a&r 24 hours 0.06 0.092 10%

in ultra-high vacuum

’ All samples were exposed for 1 minute to 10e4 M Cd(C!J0&*6&0, l”“Cd, aud 5 x lo-’ M HCl; CO1 aC equilibrium wiith air.

* Determined from difkence in y-scintiition dats before and after exposure to x-ray beam.

Cd’+ solid-state diffusion in calcite 1953

3) A layer of otavite grown over the { 101 > cleavage surface of calcite and analyzed alter two months of precipitation showed a peak for Ca2, that was barely above background and O,, and C,, binding energies that were bracketed by those of calcite and otavite. A similar sample, after storage for one additional month in ultrahigh vacuum, showed a clear signal for Cazp, a lower intensity of Cdsd, and bond environments closer to those of calcite.

Structural evidence for solid-solution is provided by binding energies for the Ca-CdCO~ layers that fallbetween otavite and calcite, and by our inability to resolve the wide XPS peaks into separate component peaks. The otavite overgrowth sample stored in ultrahigh vacuum for a month verifies that Ca moved into the CdCOs overlayer (while Cd moved out), proving that a solid-solution can be formed by solid-state diffusion and that the rate of diffusion is on the order of tens of Angstroms in a few weeks.

Investigation of individual features on calcite cleavage surfaces by AES a few hours a&er uptake failed to reveal evidence of local Cd enrichment. The scarcity of microfractures in optical grade Iceland spar and the AES evidence strongly suggest, but do not prove, that the flux of Cd into and Ca out of the calcite crystal is not limited to migration along microfractures.

Direct, atomic-level data gained in these experiments sub- stantiates some surface process models elucidated from solution chemistry studies. The CdCOs surface is hydrated (to a depth of a monolayer or less) with species that appear to be SsCdOH and S-C03H, where S’s represents the solid surface. Solid-solution formation proceeds with time and exposure to a trace-metal solution. However, the concept of a “hydrated, disordered surface layer” with a depth of many monolayers (DAVIS et al., 1987) is not supported by our surface structural data unless the disordered layer quickly rc- crystallizes to the normal calcite structure upon drying. For- mation of a thin, surhcial, solid-solution precipitate may occur during initial contact with solution, but with time on the order of days to weeks, the trace metal diffuses into the solid, and no layer remains visible at the surface.

Solid-state diffusion is probably not limited to the Cd-calcite system. The results of the present study suggest that this process should be considered in mechanistic models for trace- metal uptake from solution by all carbonates. Large-scale hydrogeochemical models designed to simulate and predict concentration and transport of dissolved trace metals or ra- dioactive elements in carbonate terrains also may need to include some facility for accounting for the effects of solid- state diffusion. Finally, new work is necessary to determine if or how much of a role solid-state diffusion plays in uptake of other trace metals by calcite and by other rock-forming mineral groups.

Acknowledgments-We acknowledge the following with appreciation: Carrick Eggleston for his willingness for discussion and help with the LEED photos; Barge Nielsen for his unending encouragement and his assistance with LATEX, the tables and figures; and the Center for Materials Research at Stanford for allowing use oftheir surface science laboratory. Thanks also go to three anonymous reviewers. A portion of this work was supported by a grant from the American Chemical Society ( PRF #22892-AC&2 to MFH ) . SLS is grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC) for

the Post-Graduate Scholarship and for the L. B. Reynolds and the Nute Memorial Fellowships for personal funding during this work.

Editorial handling: G. Sposito

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