Incorporation of Pb at the Calcite (104)–Water Interface

Jul 9, 2014 - Lead (Pb) is a common environmental pollutant, and its transport in surface waters and groundwater is controlled in part by sorption and...
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Incorporation of Pb at the Calcite (104)−Water Interface Erika Callagon,† Paul Fenter,*,‡ Kathryn L. Nagy,† and Neil C. Sturchio† †

Department of Earth and Environmental Sciences, University of Illinois at Chicago, Chicago, Illinois 60607, United States Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, Illinois 60439, United States

‡

S Supporting Information *

ABSTRACT: Lead (Pb) is a common environmental pollutant, and its transport in surface waters and groundwater is controlled in part by sorption and precipitation reactions at mineral surfaces. Using in situ specular and resonant anomalous X-ray reflectivity measurements, we investigated the interaction of the calcite (104) surface with a dilute Pb- and EDTA-bearing solution that is slightly undersaturated with respect to calcite. The X-ray results reveal Pb coherently substituting for Ca in the near-surface layers of strained calcite with Pb/(Pb + Ca) atom fractions as high as 0.28 in the outermost layer. The larger ionic radius of Pb2+ relative to Ca2+ is accommodated in calcite by vertical displacements of Pb relative to the Ca site. In situ atomic force microscopy images obtained during the reaction suggest that Pb incorporation below the surface occurs after initial dissolution followed by regrowth of a strained epitaxial Pb-rich calcite solid-solution at the calcite (104)−water interface. This process could produce a widespread host phase for Pb in groundwater aquifers and soil pore fluids.

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INTRODUCTION The mobility and bioavailability of dissolved metals in the environment are controlled largely by interactions with mineral surfaces through processes that include the formation of surface complexes, incorporation of ions, and the accretion of mineral overgrowths. Reaction mechanisms affecting metal ions at mineral−water interfaces often remain incompletely understood, despite multiple efforts using a variety of experimental and modeling approaches. Metal−mineral surface reactions can be especially difficult to discern when dissolution and growth of the substrate in response to small changes in saturation state are significant over the time scale of investigation. Calcite, a nearly ubiquitous mineral with relatively fast dissolution and growth rates in natural waters, has great environmental significance because of its ability to buffer pH and host a variety of trace metals. The incorporation of divalent metals in calcite has been studied extensively.1−9 Much of the early research (i.e., before 1994) was based on the batch powder-reaction approach, or ex situ observations of calcite surfaces after reaction,10 from which it is inherently difficult to discern the uptake mechanism. Sorption is often considered to be driven by an assumed pH-dependent surface charge.11,12 Nevertheless, the observation of fast and slow desorption kinetics2,4,13,14 (associated with reversible and irreversible reactions, respectively) suggests that the sorption mechanism is not well described as simple adsorption. The advent of in situ methods for observing metal−mineral interactions (i.e., atomic force microscopy (AFM) and synchrotron X-ray methods) has led to major progress in understanding uptake processes of Pb2+ and other divalent metals (Co2+, Zn2+, Ba2+, Cu2+, and Cd2+)6,8,15−19 and reveals that these metals can substitute into the Ca site at the calcite surface. The interaction of Pb with calcite is particularly interesting because Pb2+ is larger than Ca2+ and therefore incompatible with the calcite structure (but compatible with the © 2014 American Chemical Society

orthorhombic aragonite structure). The complex uptake behavior of Pb at the calcite surface is indicated by reports of multiple sorption/incorporation mechanisms. For example, the immobilization of Pb by precipitation on calcite (at 1−100 μM Pb) has been observed in some cases8,18,20 to be caused by the formation of specific phases (e.g., cerussite (orthorhombic PbCO3)8 and hydrocerussite (trigonal Pb3(CO3)2(OH)2)).20 The ability of Pb to coherently adsorb/partition in the calcite structure is not as well understood. Previous studies using synchrotron X-ray methods established that Pb can incorporate into the octahedral Ca site in calcite (in the presence of EDTA), but these studies did not resolve the spatial site distribution of Pb relative to the calcite (104) surface plane.16,21,22 Recent radiotracer and X-ray absorption spectroscopy (XAS) studies (without added EDTA) indicated that Pb2+ occurs as a reversibly bound adsorbed surface complex on calcite in a nonoctahedral (trigonal pyramidal or square pyramidal) coordination geometry,20,23 that diffusion into the lattice is not significant,7 and that the extent of Pb sorption on calcite varies with pH.24,25 Nevertheless, in situ, high-resolution measurements that are able to directly distinguish the full distribution of Pb relative to the calcite (104) surface have not yet been reported. It is therefore not apparent to what extent previous observations reflect distinct processes versus multiple mechanisms operating simultaneously under given sets of conditions. To understand better the apparent complexity of Pb−calcite interactions, we used a suite of complementary in situ probes (synchrotron X-ray reflectivity (XR), resonant anomalous X-ray reflectivity (RAXR), and AFM) to elucidate the distribution of Received: Revised: Accepted: Published: 9263

March July 2, July 9, July 9,

26, 2014 2014 2014 2014

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λ = 0.885 Å. The X-ray reflectivity, R(Q), is the fraction of the incident beam flux that is reflected from the surface into the detector as a function of momentum transfer, Q, which in turn is related to the scattering angle of reflection, 2θ, by the relation Q = (4π/λ)sin(2θ/2). In brief, the specular reflectivity, R(Q), has a shape characterized as a crystal truncation rod (CTR), which is a weak rod of reflected intensity oriented perpendicular to the surface and which connects subsequent Bragg peaks in reciprocal space. Resonant anomalous X-ray reflectivity spectra (i.e., R(Qi, E)) were acquired by measuring the XR signal as a function of photon energy, E, near the Pb L3 edge at selected, but fixed, vertical momentum transfers, Qi. Thirteen RAXR spectra were measured at momentum transfer values 0.21 Å−1 ≤ Qi ≤ 4.45 Å−1. The energy-dependent scattering factors (i.e., the anomalous dispersion terms) for Pb near its L3 absorption edge were obtained by measuring the transmission X-ray absorption near-edge structure of the Pb2+ species for a 0.1 m solution of Pb(NO3)2 dissolved in water revealing a nominal edge energy of 13.046 keV. The derived real and imaginary components to the scattering factors (i.e., f ′(E) and f ″(E)) are shown in Figure S1 (Supporting Information). The calcite unit cell is defined by a vertical layer spacing, d104 = 3.035 Å, and a surface unit cell area of AUC = 20.198 Å2 (Figure 1, inset). Calcite (104) surfaces were prepared by cleaving optical-quality calcite with a razor blade and transferring the freshly cleaved surface immediately (within seconds) to ∼10 mL of the reaction solution in a polypropylene centrifuge tube. The sample was reacted with the solution for ∼24 h and then mounted in the sample cell for X-ray characterization. A duplicate sample measurement with the same reaction solution was performed, which yielded similar results. The XR measurements were performed using a thin-film sample cell29 with a beam cross-section of 0.05 mm × 0.5 mm. This cell geometry maintains a capillary film of solution (only a few micrometers thickness) across the sample surface, and this minimizes background intensity caused by X-ray scattering

Pb near the calcite−water interface and the mechanism of its incorporation. These measurements were performed in the presence of EDTA and may mimic natural conditions where trace metal ions (as well as Ca) may be strongly complexed by dissolved organic matter. Interfacial electron density profiles along the surface normal direction were obtained by XR, and Pb-specific profiles were obtained using RAXR26 for a sample that reacted with a Pb-bearing solution for ∼24 h. Complementary real-time observations by in situ AFM provided high-resolution topographic images of the reacting interface and give insight into the mechanism of Pb incorporation during transient changes in saturation state.

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MATERIALS AND METHODS Solution Chemistry. The reaction solution, with [Pb]total = 91 μm, was prepared as described by Sturchio et al.16 Equilibrium solubility-speciation calculations (by PHREEQC27 with the MINTEQ.v428 database) show a slight undersaturation of calcite (saturation index = −0.07 = log(IAP/ Ksp)) and greater undersaturations of aragonite and Pb phases (Tables 1 and S1, Supporting Information). Saturation of Pb solid phases in the bulk solution was inhibited by the presence of EDTA, which forms aqueous complexes with Pb. X-ray Reflectivity and Resonant Anomalous X-ray Reflectivity. The experimental XR measurements in the present study are similar to those described previously29−31 and were performed at sector 6-ID of the Advanced Photon Source. The X-ray data were obtained using a CCD area detector, which enables fast data acquisition to minimize X-ray beam exposure (the stability of the sample during the measurements is documented in Figure S2, Supporting Information). Specular XR measurements used a photon energy of 14.0 keV, corresponding to an X-ray wavelength of Table 1. Initial Speciation of the Reaction Solution with Composition Adopted from Sturchio et al.16a species

conc (mol kg−1)

HCO3− Na+ CO32− Cl− Pb(EDTA)2− NaCO3− NaHCO3 H2CO3 Ca2+ Ca(EDTA)2− CaCO3 CaHCO3+ Pb(CO3)22− PbCO3 CaOH+ PbH(EDTA) H(EDTA)3− CaH(EDTA)− PbOH+ PbHCO3+ Pb(OH)2 Pb2+ PbCl+ a

9.04 9.00 3.50 1.82 9.09 3.86 3.73 3.05 1.83 1.82 4.44 2.04 1.87 1.75 1.62 1.68 1.51 6.40 4.33 1.73 7.38 3.82 1.65

× × × × × × × × × × × × × × × × × × × × × × ×

10−3 10−3 10−4 10−4 10−5 10−5 10−5 10−5 10−5 10−5 10−6 10−6 10−9 10−9 10−9 10−10 10−10 10−11 10−11 10−11 10−12 10−12 10−14

Figure 1. XR of the calcite−water interface as a function of momentum transfer, Q, from a calcite surface equilibrated in a calcite saturation solution (CSS, black circles) and the Pb/EDTA solution (blue squares). The lines are calculations of the reflectivity signal for optimized structural models: black line, the calcite−water interface as described previously;34 red and blue lines, initial and final fits to the XR data for the calcite surface in contact with the Pb/EDTA solution. A schematic of the calcite (104) structure is shown, inset, with the vertical calcite layer spacing, d104, indicated.

The measured pH of this solution was 8.80. 9264

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electron density for the Ca site through the incorporation of Pb atoms, and interfacial water structure including an adsorbed layer and a layered near-surface water profile (initial fit in Figure 1, with a quality of fit of χ2 = 6.60, and R = 0.050). The complex structure factors from this XR MD structural fit were then used in the MI analysis of the RAXR spectra,35 which reveals the Q-dependent variation of the coherent amplitude, A, and phase, ϕ, of the Pb distribution (Figure 2). The Fourier transform of the Pb structure factor, A(Q)exp[iϕ(Q)], directly reveals the Pb atom distribution (MI density profile, Figure 3). This MI profile has discrete peaks in the Pb density, at and below the calcite surface, approximately coinciding with the Ca location along with a small contribution from an adsorbed layer above the surface. This picture is fully consistent with the variation of the coherent amplitude, having a total Pb coverage of θ ≈ AQ→0 ∼ 0.4 ML, a shape that is similar to a CTR form factor with a peak at Q ≈ 2 Å−1 indicating a layer spacing of 2π/ (2 Å−1) ≈ 3 Å, and a coherent phase that shows an average Pb distribution height of ≈ [ϕ/Q]Q→0 ∼ −2 Å (i.e., on average below the surface).35 This Pb-specific model was then incorporated into the analysis of the XR data to obtain a revised total electron density profile, followed by iterating the RAXR and XR analyses to find a structure that is fully consistent with both the XR (final fit in Figure 1 and Table S2, Supporting Information, with quality of fits χ2 = 6.56, and R = 0.055) and RAXR data (Figure S3, Supporting Information, Figure 2, and Table S3, Supporting Information, χ2 = 0.66, and R = 0.011). The derived total and Pb-specific interfacial electron density profiles (Figure δ) reveal that Pb is coherently incorporated into the calcite lattice in discrete layers distributed primarily

from the solution phase. It also limits further reaction progress because of the small solution volume (typically ≤ 1 μL) that is in direct contact with the sample surface. Atomic Force Microscopy. In situ phase images32 were obtained with an Asylum Research MFP-3D stand alone AFM operated in AC mode, using pyramidal probes mounted on Si3N4 cantilevers (nominal resonant frequency, 19 kHz; nominal spring constant, 0.1 N/m; Bruker). The scan rate was set at 1−1.5 Hz, which produced 5 μm × 5 μm images in approximately 5 min. A cleaved calcite crystal was mounted using marine epoxy on the base of the fluid cell that was immediately filled by calcite-saturated solution. A rubber membrane holding the probe was placed on top to seal the cell, and the AFM operating parameters were optimized for fluid imaging. The cell was then flushed with 7 mL of deionized water (DIW) to remove any fine particles remaining on the cleavage surface. After optimizing the imaging conditions (over the course of ∼2 h of contact with DIW), 10 mL of the Pb/ EDTA reaction solution was flushed through the cell within a few seconds to ensure full solution exchange in the ∼2 mL cell volume so that the initial reaction process of the calcite surface could be imaged. Several experiments were conducted, each yielding a series of images of the in situ variation of topography of the calcite surface obtained over 3−5 h during reaction. Images were captured less frequently (every 15−30 min) toward the end of each experiment to minimize tip-induced perturbations. Growth and dissolution rates were estimated by measuring lateral step motions with time using Gwyddion,33 an image analysis freeware.

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RESULTS AND DISCUSSION Structural Analysis of Pb Incorporation from XR and RAXR Data. The XR data for a calcite (104) surface in contact with the Pb/EDTA solution are shown in Figure 1 and compared to those obtained in contact with a Pb-free calcitesaturated solution.34 The data were measured to Qmax = 7 Å−1, corresponding to a vertical spatial resolution of 0.45 Å. The data for the Pb-bearing calcite surface exhibit the same qualitative features observed previously for the Pb-free calcite surface with the characteristic CTR shape between the substrate Bragg reflections (observed at Q = 2.07, 4.14, and 6.21 Å−1). The primary difference between these two data sets is the systematic enhancement of the reflectivity signal between the Bragg peaks for the Pb-bearing calcite surface, without the appearance of intensity oscillations that would be indicative of a well-defined film, consistent with previous observations.16 This qualitative insight is reinforced by RAXR measurements (Figure S3, Supporting Information). These results show a modest (∼20%) modulation in the RAXR signal as a function of energy for all momentum transfers measured, providing direct evidence for a Pb sorption structure with crystallographically controlled sites at the calcite surface. The spectral shape shows only a weak variation as a function of Q, suggesting that the average height of the Pb distribution is close to the calcite surface plane.35 The total and element-specific density profiles were obtained by an iterative analysis that combined both model-independent (MI) and model-dependent (MD) approaches. Briefly, a MD fit to the XR data, obtained based on the understanding of the calcite−water structure,34 suggested an increase in electron density at the Ca site. The model includes displacements of the top six Ca and CO3 layers (with displacement parameters defined in Figure S4, Supporting Information), variable

Figure 2. Variation of the coherent (a) amplitude, A, and (b) phase, ϕ, of the Pb-specific structure factor (black circles) obtained from the MI analysis of the RAXR data (shown in Figure S3, Supporting Information). The lines correspond to the optimized Pb distribution described in the text. Also shown, in (c), is the variation of ϕ/Q, whose value in the limit of Q → 0 corresponds to the average height of the Pb distribution with respect to the calcite surface plane. The solid squares are comparable results from a previous X-ray standing wave measurement,16 and the open square indicates the coherent amplitude scaled to the coverage observed in this sample. 9265

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Pb incorporation than did the earlier XSW results. The equivalence of both sets of results can be seen by plotting the coherent amplitude and phase values measured by the in situ XSW measurements (at Q = 2π/d104 = 2.07 Å−1) alongside the values measured here by RAXR (solid squares, Figure 2). The XSW-measured coherent phase is fully consistent with the present results, because of the presence of displacements that are oriented in both the positive and negative directions. The XSW coherent amplitude (∼0.04) is substantially lower than the present measurement at this condition (∼0.4), but that is mostly due to the differences of the Pb coverage in the two experiments being compared (0.08 ML by XSW, and ∼0.4 ML by RAXR). The XSW result, when scaled to accommodate the higher Pb coverage of the present measurements, is similar to that in the present measurement near Q = 2 Å−1 (open square, Figure 2a). These RAXR results suggest that significant displacements of Pb with respect to the Ca lattice sites are needed to accommodate the differences in size and preferred coordination geometry of Pb versus Ca (with ionic radii of 1.18 Å vs 1.00 Å, respectively, and a coordination number of 9 for Pb and Ca in orthorhombic carbonates vs octahedral coordination in calcite). Observing the Pb Incorporation Process by AFM. AFM measurements were performed to constrain the mechanism by which Pb incorporates below the calcite surface. These results were obtained on a separate set of calcite samples using the same initial solutions as the X-ray experiments. The starting surface (after flushing the AFM cell with DIW) revealed abundant etch pits and 3 Å high steps corresponding to elementary steps on the calcite surface (Figure 4a), features likely resulting from the initial cleavage and reaction with water. Upon introduction of the Pb/EDTA solution, the surface dissolved further as marked by the straightening of steps, increasing size of existing etch pits, and formation of new etch pits (Figure 4a, b). This transient dissolution continued for about 20 min, producing rhombohedral-shaped topography that is typical of calcite (red and yellow arrows, Figure 4b). The calcite surface then began to accrete through step advance. As growth continued, etch pits were filled, and steps developed a scalloped appearance. Steps eventually coalesced and partially covered wide terraces. This growth behavior persisted until the

Figure 3. Derived interfacial structure of the calcite surface after equilibration with Pb/EDTA solution in the form of the electron density profile, ρ(z), along the surface normal direction. The total interfacial density profile from XR (black line, corresponding to the structure in Table S2, Supporting Information) is shown along with the derived Pb-specific profiles from the initial MI result (red dashed line, corresponding to the fit to the data in Figure S3a, Supporting Information) and the fully optimized MD result (thick blue line) from the fit shown in Figure S3b, Supporting Information (second RAXR analysis), corresponding to the structures in Table S3, Supporting Information.

within and below the calcite surface plane, as expected from the preliminary analysis (above). Each Pb layer is centered near the Ca site with decreasing occupations four layers into the calcite crystal (and with minimal contribution from adsorption on top of the calcite surface). However, the Pb locations are systematically displaced by ∼0.2 Å from both the observed and ideal Ca positions, indicating strain caused by the relatively large size of Pb. This result is apparently distinct from the conclusions of previous X-ray standing wave (XSW) measurements of this system16 in which Pb was observed to be located in the Ca site (with an average displacement of only −0.03 Å). The present XR/RAXR results reveal a more detailed picture of

Figure 4. In situ AFM phase images of a calcite surface (a) after reaction with DIW for ∼2.5 h and before the introduction of Pb, and (b) 19; (c) 63; (d) 83; (e) 123; and (f) 166 min after injecting the Pb/EDTA solution. Individual layer numbers are labeled starting with “0” as the deepest layer in any one image. Dissolution is evident in (b), marked by the disappearance of small terraces and coalescence of etch pits. The prominent monolayerdeep pit in the upper part of the image (marked with “0”) eventually filled in and was covered by new layers with scalloped edges, presumably originating from the obtuse steps (yellow arrows; red arrows indicate acute steps). The mechanism of incorporation at depth is demonstrated at the location of the yellow dot, where one layer dissolved and three layers subsequently grew. 9266

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experiment was terminated. The yellow dot in Figure 4 shows a nominally fixed location on the surface where the removal of one layer was followed by growth of three layers during ∼3 h of reaction. These phase images do not show any significant changes of contrast upon growth of the Pb-containing layer (presumably because Pb is incorporated as an impurity), as may be expected if the new layer had a significantly different structure or composition. The dissolution/regrowth process can be further illustrated by considering the evolution of the two types of steps on the calcite surface that are characterized by their geometry and reactivity: acute steps and obtuse steps.36,37 The obtuse steps both retreated and grew more rapidly than the acute steps, as seen in the changing shapes of etch pits with time (Figure 4b− f; obtuse and acute steps are labeled with yellow and red arrows, respectively). This observation, coupled with the RAXR results, suggests that Pb (ionic radius 1.18 Å) incorporates at the obtuse steps during growth, consistent with previous findings that cations larger than Ca (ionic radius 1.00 Å) preferentially incorporate in these sites during calcite growth,38 with the exception of Zn.17,38,39 The mean step-retreat rate of obtuse steps during dissolution (∼0.2 nm s−1) was estimated from two images obtained 19 min apart. The step-advance rate (∼0.2 nm s−1), from seven sequential images over 3 h, is comparable to the observed stepretreat rate. These rates are much lower than those reported during far-from-equilibrium dissolution experiments (1.6−3.2 nm s−1),40 and during growth (∼10 and ∼4 nm s−1, at the obtuse step and acute steps, respectively).41 A decrease in growth rate with time, particularly in the first ∼3 h of the reaction, is consistent with a reduced stability of the compositionally modified calcite associated with the buildup of strain in the compositionally modified surface42 (Figure S5, Supporting Information). Discussion. The present results provide new insights into the structure and mechanism of Pb incorporation into calcite. This is informed by considering the evolution of the solution speciation as a function of the mass of calcite dissolved, calculated using PHREEQC (Figure 5).27 These calculations did not allow for precipitation of calcite or aragonite at saturation and thus give insight into the potential solution speciation in the case where rapid dissolution could cause transient oversaturation, consistent with AFM observations. The calculations show that the solutions were initially undersaturated with calcite. Our observations suggest that the solutions became supersaturated with respect to the Pb-rich calcite layer upon dissolution of ∼10−5 mol L−1 (∼10−8 moles Ca in the ∼1 mL volume of the AFM cell, explaining the transition from dissolution to growth during the first 20 min of reaction time). These calculations (which did not account for solid-solution effects43) assume equilibrium throughout the entire cell volume. However, this might not have been the case for the entire duration of the experiment, and a considerably higher saturation may have been present at the region closer to the calcite surface. The speciation calculations show that the dominant Pb species (other than the negatively charged Pb/ EDTA chelate) was PbCO30, consistent with previous results suggesting that the amount of sorbed Pb correlates strongly with the availability of this species.24 The observed vertical displacements of Pb with respect to the Ca site (as high as 0.28 Å; Table S2, Supporting Information) are consistent with the value expected by the coherent incorporation of the larger Pb ion in the calcite lattice (∼0.2

Figure 5. Calculated (a) Ca (solid lines) and Pb (dashed lines) speciation, and (b) pH and saturation indices (SI) of calcite and cerussite (total Pb = 9.09 × 10−5 m; total EDTA = 1.09 × 10−4 m; total Ca = 9.66 × 10−5 m) of the experimental solution as a function of calcite dissolved. Initial CO2 is the atmospheric value (3.8 × 10−4 atm) and was allowed to vary during the reaction progress.

Å). From a crystal chemistry perspective, Pb is expected to be incorporated at the obtuse calcite steps more readily than the acute steps due to the more favorable adsorption geometry (Figure 6). Therefore, the observation of four Pb-bearing layers below the calcite surface can be best explained by incorporation of Pb in the growing PbxCa1−xCO3 layer during step-advance, both in and near pre-existing etch pits as well as those produced by the initial dissolution of the surface. The observed decrease in the Pb concentration with depth (Figure 3) apparently reflects the decreasing fraction of subsurface layers exposed by etch pits when the Pb-containing solutions were placed in contact with the surface. Furthermore, the fractional Pb concentration within the calcite lattice is relatively high (∼28% within the top calcite layer) with little or no Pb adsorbed on top of the calcite surface. The decrease in the observed growth rate during the first ∼100 min of growth (Figure S5, Supporting Information) and the displacements of Pb and Ca relative to Ca bulk positions (Tables S2 and S3, Supporting Information) in the calcite lattice suggest that solution transport and/or lattice strain may control the rate of step-advance and Pb incorporation. We saw no evidence (from

Figure 6. Schematic of the Pb incorporation site at a step, showing preferred incorporation at the obtuse step (right) during growth due to the relatively flexible coordination geometry with two coplanar CO3 groups (allowing for vertical displacement to accommodate the larger Pb ion), rather than the acute step (left) that requires coordination between two noncoplanar CO3 groups (the potential complexation of Pb with any solvated species is not shown). 9267

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to the calcite saturation state that can be driven by daily/ seasonal variations in calcite solubility (e.g., as driven by changes in temperature or pCO2).

either AFM or XR/RAXR) for the presence of any distinct Pb phases associated with heterogeneous growth of precipitates (e.g., cerussite or hydrocerussite) on the calcite surface. This is consistent with PHREEQC calculations showing that cerussite remains undersaturated even after substantial calcite dissolution (Figure 5b). These observations all support the conclusion that Pb is coherently incorporated as an impurity in the growing calcite surface under these conditions. The picture of Pb incorporation into calcite as presented here is distinct from that inferred from X-ray absorption spectroscopy of calcite powders, where it was concluded that Pb was adsorbed on top of the calcite surface as an inner-sphere surface complex.7,20 Those studies differed with respect to the present work in that Pb was adsorbed from an EDTA-free solution that was initially saturated with respect to calcite. We observed in separate XR and RAXR measurements that freshly cleaved calcite surfaces in an EDTA-free calcite-saturated solution (with 10−7 < [Pb] < 10−5) did not show any evidence for significant Pb adsorption or incorporation on the calcite terraces (Figure S6, Supporting Information). Together, the aforementioned X-ray absorption studies and this observation imply that Pb adsorption in the absence of EDTA may occur only at under-coordinated sites on the surface (e.g., step or kink sites, whose surface site density on the calcite cleavage surface is below the detection limit for the XR and RAXR measurements).7,18 More generally, it suggests that the calcite surface itself does not have a significant surface charge that would drive adsorption of the Pb2+ ion. Because the substantial surface loading of Pb that we observe in EDTA-containing solutions is achieved primarily through incorporation at steps during growth, the evolution in the solution saturation state with respect to calcite is apparently a necessary condition for Pb uptake. We interpret our results to indicate that the presence of an initial excess of EDTA in the experimental solution drives calcite dissolution, by which Ca becomes complexed with metal-free EDTA and releases CO32− to solution, which then drives a Pb-rich calcite solid-solution phase to supersaturation. The present observations also indicate that solid-state diffusion of adsorbed metal ions into the calcite lattice is not required for their incorporation at depth (cf. Stipp et al.44). Instead, the results of this study indicate clearly that Pb uptake at the calcite surface occurs predominantly by solid-solution incorporation within a Pb-rich, distorted calcite structure. This may act as an important potential sink for Pb in carbonate-rich systems and can help to explain the fast and slow release of metals upon dissolution. Environmental Implications. The present results suggest that sorption of Pb to calcite from an initially undersaturated solution is enabled by the transient changes in the calcite saturation state as indicated by the observed surface dissolution and subsequent growth of a Pb-rich calcite layer. This is supported by the observations that the most common dissolved Pb species in these solutions aside from Pb/EDTA complexes is PbCO30, the same as that in non-EDTA solutions,18,24 and no significant Pb incorporation is observed from solutions that are initially saturated with calcite. The specific behavior observed in these experiments is likely facilitated by the presence of initial excess EDTA, which can chelate Ca ions from the calcite surface and may provide the driving force for the observed dissolution/growth process. As such, we expect that this behavior may be observed in natural systems in which Ca is strongly complexed by dissolved organic acids. Similar sorption behavior also may occur whenever there are transient changes

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ASSOCIATED CONTENT

S Supporting Information *

Details about the solution preparation and speciation, X-ray reflectivity and resonant anomalous reflectivity data and data analysis as well as growth rate estimates by atomic force microscopy. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (630) 252-7053; e-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding

This work was supported by the Geosciences Research Program of the Office of Basic Energy Sciences, U.S. Department of Energy (DOE), through contract number DEAC02-06CH11357 at Argonne National Laboratory and DEFG02-03ER15381 at UIC. The X-ray data were collected at the X-ray Operations and Research beamline 6-ID-B at the Advanced Photon Source (APS), Argonne National Laboratory. The submitted manuscript has been created by UChicago Argonne, LLC, Operator of Argonne National Laboratory (“Argonne”). Argonne, a U.S. Department of Energy Office of Science laboratory, is operated under contract number DEAC02-06CH11357. The U.S. Government retains for itself, and others acting on its behalf, a paid-up nonexclusive, irrevocable worldwide license in said article to reproduce, prepare derivative works, distribute copies to the public, and perform publicly and display publicly, by or on behalf of the Government. Notes

The authors declare no competing financial interest.

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REFERENCES

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dx.doi.org/10.1021/es5014888 | Environ. Sci. Technol. 2014, 48, 9263−9269