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Pyromorphite Growth on Lead-Sulfide Surfaces A N D R E W G . S T A C K , †,‡ R O L F E R N I , § N I G E L D . B R O W N I N G , §,| A N D W I L L I A M H . C A S E Y * ,†,‡ Department of Land, Air and Water Resources, University of California, Davis, California 95616, Department of Geology, University of California, Davis, California 95616, Department of Chemical Engineering and Materials Science, University of California, One Shields Avenue, Davis, California 95616, and National Center for Electron Microscopy, Lawrence Berkeley National Laboratory, Berkeley, California 94720
Electrochemical Scanning Tunneling Microscopy (ECSTM) and electron microscopies have been used to follow the nucleation and growth of ∼10-15 nm pyromorphite (Pb5(PO4)3Cl,OH) particles on a galena (PbS) substrate under oxidative conditions. The particle sizes and crystal morphologies are found to be strongly affected by solution and oxidation potential, and in the earliest stages the particles are generally sufficiently small to be mobilized in a soil. It is clear that the particles grow epitaxially under these conditions, based on observations of the particles’ adherence to the surface during imaging, their preferred crystallographic orientation, their growth along surface features on the galena, and commensurate atomic structures. Through cyclic voltammetry, we show that the presence of phosphate also partially passivates the surface of the galena to oxidation. We propose two possibilities for the mechanism of passivation, one is that pyromorphite nucleation inhibits the retreat of steps, and the second is that adsorbed phosphate stabilizes a lead-terminated surface structure by coordinating lead and slowing its dissolution.
Introduction Colloids and molecular clusters form commonly in natural environments (1, 2), but the local conditions that facilitate their formation and the mechanisms of growth are not well understood. Of special importance are colloids formed from toxic soil contaminants such as lead or radionuclides, which can be mobilized in a soil or groundwater by erosional or other processes (3-5). The transport of these colloids inhibits remedial strategies based on the sequestration of toxic elements as immobile solids. One promising strategy for the treatment of lead contamination is to add a source of phosphate that immobilizes the lead by precipitating the relatively insoluble mineral pyromorphite (Pb5(PO4)3Cl,OH) (6-11). Pyromorphite has been found to grow epitaxially on hydroxyapatite under acidic conditions in the presence of aqueous lead (12), although homogeneous nucleation can * Corresponding author phone: (530)752-3211; fax: (530)752-1552; e-mail:
[email protected]. † Department of Land, Air and Water Resources, University of California. ‡ Department of Geology, University of California. § Department of Chemical Engineering and Materials Science, University of California. | Lawrence Berkeley National Laboratory. 10.1021/es049487s CCC: $27.50 Published on Web 09/29/2004
2004 American Chemical Society
also occur (13, 14). Galena is the dominant ore of lead and is a common constituent of lead-contaminated sites, so understanding the extent to which pyromorphite grows epitaxially, and adheres to the underlying substrate, is of utmost importance. In lead-sulfide and phosphate-containing field sites, lead-phosphate reaction rims have been observed on galena grains (10) as well as lead-phosphate solids present as free particles (15). Here, using a variety of microscopies and cyclic voltammetry, we follow the nucleation and growth of pyromorphite and explore the conditions affecting their growth during the oxidative dissolution of galena in the presence of aqueous phosphate.
Methods Electrochemical Scanning Tunneling Microscopy. Electrochemical scanning tunneling microscopy (EC-STM) was performed using a Digital Instruments MultiMode system with the Electrochemical STM package and Pico STM tips covered in Apiezon wax (Molecular Imaging). The typical EC-STM setup is a four electrode system where the potential of the sample (or working electrode) relative to the surrounding solution is measured/controlled against a reference electrode by a potentiostat and the current is sensed via a counter electrode. The potential of the STM tip (for tunneling) is determined relative to that of the sample. For an explanation of EC-STM, please see ref 16. In our experiments, freshly cleaved galena (PbS, Ward’s Scientific) was used as the working electrode and an 0.5 mm diameter platinum wire as the counter electrode in a custom fluid cell. In this cell, the surface area of the working electrode exposed to solution is ∼1 cm2, and the solution volume is ∼0.2 mL. As is common in EC-STM experiments, the cell was too small to fit a standard reference electrode, and an oxidized, 0.5 mm diameter silver wire was used as a quasireference electrode. This was calibrated against a saturated calomel electrode (SCE) in the same solution used in the experiments and has a potential of approximately +0.045 VSCE (i.e., +0.045 V vs the SCE). Thus, potentials listed here can be converted to VSCE by adding +0.045 V to the potentials versus the silver wire (VAg wire). Similarly, to convert to the normal hydrogen electrode scale (VNHE), one must add +0.286 V. Due to the tendency of the tip to come in contact with particles and crash, the scan rate was limited to 5 Hz maximum. Unfortunately, using this slower scan rate increases the effect of scanner drift which introduces some distortion in the images. All solutions used were in 0.01 M NaCl background electrolyte. Phosphate solutions were made using NaH2PO4. To limit oxygen contamination, all solutions were sparged with argon for at least 30 min prior to use, and argon was flowed over the cell during experiments. Scanning Electron Microscopy. Scanning electron microscopy (SEM) samples were oxidized using the same electrodes as in the EC-STM cell but controlled with a PAR model 263A potentiostat. Samples were oxidized just prior to imaging. SEM was performed at the W. R. Wiley Environmental Molecular Science Laboratory using a LEO 982 field emission scanning electron microscope. Scanning transmission electron microscopy (STEM) samples were separated from the galena substrate by sonication in heated methanol for 30 min and then allowed to dry in air and transferred to a copper grid. STEM was performed at the National Center for Electron Microscopy using a Tecnai F20 STEM/TEM microscope. Cyclic Voltammetry. Cyclic voltammetry (CV) was performed using the same potentiostat, reference, and counter VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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electrodes as described above. Galena samples ∼1 cm3 in size were cleaved just prior to each experiment. Electrical contact to the sample was made by tying a platinum wire around the sample and then lowering the sample onto the solution. Special care was taken not to allow the platinum wire to become exposed to the solution as this would make for inaccurate measurements. Solution volumes were 10 mL. Argon was flowed over the cell during the experiments. The scan rate was set at 10 mV/s, with a 15 s equilibration time prior to starting the experiment. All experiments used the open-circuit potential (OCP; i.e., the potential of the system with zero, or very small, current flowing through the cell) as the initial and final potentials.
Results and Discussion Particle Composition and Habit. We observe the growth of extraordinarily small pyromorphite particles in situ using EC-STM. Figure 1 shows a consecutive sequence of EC-STM images of the same area of a galena surface, under a potential of +50 mVAg wire, and after the addition of several drops (∼75 µL) of 0.001 M sodium-phosphate solution to the cell. Each image took approximately 100 s to collect, and all four images took 7 min to collect (due to drift, portions of each image were cut away). The images were taken over the same area of a galena surface. The first image shows monolayer steps of 3 Å on the galena surface and an etch pit that is two monolayers deep (6 Å). As a consequence of the potential, the surface oxidatively dissolves and releases aqueous Pb(II) ions to solution (hence the roughened surface and the etch pit) (17). The next three images show clearly that particles grow on the surface that are approximately 10-15 nm in width but ∼1 nm in height. The particles appear to grow rapidly once nucleated, from the first image to the second in the sequence the particles are transformed from hardly observable to well-formed. Figure 2 shows two consecutive images, one 60 × 60 nm and another 100 × 100 nm, of a different area of the surface during the same experiment showing that the size and distribution of the particles are relatively homogeneous within the area imaged but have a tendency to grow on step edges. This tendency indicates clearly that the PbS surface affects nucleation and growth. The particle denoted by the arrow was moved by the tip during the scan, as is apparent from subsequent scans, indicating that the adhesion forces can be overcome. In the experiment shown in Figures 1 and 2, we incrementally added small amounts of phosphate solution while imaging a galena surface that was being oxidized. This was continued until precipitation was observed. In Figure 3, we started with a 100 µM solution of NaH2PO4 in the fluid cell and stepped the potential of the galena surface in a positive (oxidizing) direction until precipitation was observed (Figure 3 was taken at +25 mVAg wire sample potential). The image shown in Figure 3 is a scan collected from bottom-to-top, i.e., the bottom is earlier in time and shows the precipitation of ∼50 nm particulates within the single image. The image took approximately 100 s to be collected. A galena surface is observed in the lower portion of the image, with abundant steps and showing a morphology that is characteristic of an oxidized galena surface (17, 18), but becomes completely covered with particles during the course of the image. The image quality is degraded somewhat during the initial precipitation, as is evident in the middle portion of the image (Figure 3). To verify that the particles are indeed lead phosphate, we conducted a series of experiments where oxidation was allowed to proceed for much greater times (15 min to 1 h) and varied the oxidation potential and phosphate concentration. We then imaged the results using electron microscopies ex situ. For a 15 min oxidation, at a sample potential of +100 mVAg wire (Figure 4a), we observed large, 5530
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FIGURE 1. EC-STM images from galena surface in 0.01 M NaCl after the addition of several drops of 0.001 M NaH2PO4 solution. Sample potential was +50 mVAg wire (oxidizing conditions) and the tip potential was -50 mVAg wire, scale-bar is 15 nm, and z-scale is 3 nm. Particle sizes are 10-15 nm in breadth and about a nanometer in height. The last image in the sequence may exhibit some tipdoubling artifacts because of a particle adhered to the tip. Also, some distortion is present due to scanner drift. well-formed hexagonal crystals that are pyramidally capped and are consistent with the pyromorphite crystal habit (19). Figure 4b shows a scanning transmission electron microscope (STEM) image of a particle created under the same conditions but approximately 100 nm in length; it has the same morphology as the particles shown in Figure 4a. Selectedarea chemical analysis of these crystallites show that they
FIGURE 3. An EC-STM image of galena taken in 100 µM NaH2PO4 immediately after the sample potential was increased to +25 mVAg wire from 0 mVAg wire. Tip potential was -75 mVAg wire. The image size is 1 µm and was taken by scanning from bottom to top. The image took approximately 100 s to collect. Precipitation of ∼50 nm particles is evident during the course of the scan, which coat the surface. The z-scale is 10 nm.
FIGURE 2. Sequential EC-STM images of galena in the presence of phosphate under the same conditions as Figure 1 and show several particles on the surface of the galena. The top image is 60 × 60 nm. The bottom image, taken immediately after, is 100 × 100 nm. The black box indicates the same approximate geographical area as the top image, but there is distortion due to drift. The particle marked by the arrow represents either a single particle that was moved by the tip during the course of the second image or several particles that had aggregated together, possibly after becoming dislodged by tip. contain phosphorus and lead, consistent with the composition of pyromorphite. Additionally, since the chloride endmember of pyromorphite is less soluble than the hydroxide end-member when the background electrolyte is NaCl (20), it is reasonable to conclude that this is the chloride endmember of pyromorphite. Pyromorphite crystallite size and habit depend on the chemical conditions but are clearly influenced by the underlying galena surface structure under these conditions. At highly oxidizing potentials (+100 mVAg wire) and relatively high phosphate concentrations (0.001 M), the crystallites grow to ∼1-2 µm in length and are well formed (Figure 4a). When the surface oxidized at less-severe potentials (+35 mVAg wire) at the same concentration of phosphate (0.001 M), we observe small, oriented aggregates of crystals 1 µm in length or less (Figure 4c). The particles show a morphology similar to those observed in the EC-STM experiments of Figures 1 and 2 (especially the last image in the sequence of Figure 1). Figure 4d shows particles oxidized at +100 mVAg wire and a phosphate concentration of 100 µM. Here the particles still have an obvious hexagonal crystal habit but are 500 nm in size. Figure 4e shows a sample that had been oxidized for an hour at an oxidation potential of +35 mVAg wire. This sample is almost covered with pyromorphite crystallites in some areas, but in other areas the sample is still relatively free of particles. Since
the particles in Figure 4e are well-formed compared to the aggregates of particles created under the same conditions but a smaller amount of time (Figure 4c), it is apparent that particle morphology matures with continued oxidation. It could be that the particles of Figure 4c represent several nuclei observed in the act of coalescence into single particles. The more well-formed particles of Figure 4e may indicate that the aggregates of particles have completely grown together. This dependence on reaction conditions indicates that particle size and habit are strongly affected by solution and reaction conditions. Smaller particles might be expected to form in environmental settings with sharp gradients in Gibbs Free Energy, causing a change in the solubility quotient of pyromorphite. Such environments can be found near redox fronts, where the solubility changes sharply with redox state of the anion or cation. Examples are redox fronts that are found commonly where there is bacterial activity (see e.g., ref 21) or downstream from a waste pile or contaminated site after a rain event (see e.g., ref 22). The types of particles shown in Figure 4 were not observed in control experiments. During the EC-STM experiments that served as a control (i.e., without phosphate), we did not observe any precipitation on surfaces under the same conditions. Similarly, we did not observe conspicuous precipitation on surfaces at the open circuit potential in the presence of phosphate, although some dissolution of galena occurs at the OCP, as is expected (17). However, ex situ SEM images of samples oxidized in the absence of phosphate reveal particles that are likely to be elemental sulfur (Figure 4f), as has been previously observed (23). The particles in Figure 4f cannot be pyromorphite as they do not contain any detectable phosphorus, they are morphologically distinct from the pyromorphite particles, and they sublimate underneath the electron beam as do particles of elemental sulfur (see ref 23 for a detailed discussion of their properties). In contrast, pyromorphite particles are quite stable under the electron beam. Furthermore, the particles shown in Figures 1-3 (the EC-STM experiments) are unlikely to be elemental sulfur because they are observed only when phosphate is present. There is a strong preferred crystallographic orientation of the pyromorphite particles relative to the galena substrate VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Ex situ images of oxidized galena surfaces. The scale is as marked. (a) SEM image of galena reacted at +100 mVAg wire for 15 min in 0.001 M NaH2PO4 (0.001 M NaCl). The large (1-2 µm) hexagonal particles are probably pyromorphite, as determined by energy-dispersive X-ray spectroscopy and crystal habit. Most pyromorphite crystals have a distinct orientation relative to the surface normal. (b) Z-contrast STEM image of a particle 136 nm in length (apex to base) from an experiment at the same conditions as in part (a). Electron energy loss spectroscopy also shows a composition similar to pyromorphite. (c) SEM image of galena reacted at 0.001 M phosphate solution at +35 mVAg wire, reacted for 15 min. These particles have a smaller size and are less well crystallized than those of (a). (d) SEM image of galena reacted in 100 µM phosphate at +100 mVAg wire. These particles are smaller than those of (a) but are well formed. (e) SEM image of galena reacted for 1 h in the presence of 0.001 M phosphate solution at +35 mVAg wire. Particles containing phosphorus cover the surface in some areas, but other areas are almost bare. (f) SEM image of galena reacted in the absence of phosphate at +100 mV for 15 min. Spectra of the surface show no phosphorus. It is likely the very small particles and the lighter gray material are elemental sulfur. The smaller particles (spots) in (a) and (d) are an artifact of drying because the samples were not washed of background electrolyte prior to evacuating the SEM chamber. Part (c) has been exposed to DI water prior to imaging and shows no smaller particles. that indicates that the particles are not growing by homogeneous nucleation (i.e., precipitation from solution), at least under these conditions. There are two primary lines of evidence indicating heterogeneous nucleation: first, the crystallites are aligned roughly parallel to the dominant step directions (Figures 2 and 4a). Second, the apices of the crystals in Figure 4 are oriented at a particular angle to the surface normal, approximately 30°-45°. Since the angle is particular it implies a structural relationship between the pyromorphite nuclei and the galena surface. To assess this structural relationship, we compared the crystal lattice of pyromorphite and galena and found a plane of Pb(II) atoms in the pyromorphite structure commensurate with the galena {001} surface. This plane is illustrated in Figure 5a; four lead atoms of pyromorphite unit cell lie in the plane and form a parallelogram with sides 4.204 and 4.237 Å in length and internal angles of 86.1 and 94.0°. The corresponding lead 5532
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atoms on the galena surface are separated by 4.197 Å and form 90° angles to each other (Figure 5b). The four atoms in this plane of the pyromorphite unit cell repeat in two directions, with cell-to-cell distances as shown (Figure 5a,b). This matches the most probable orientation of pyromorphite on galena surfaces since the degree of mismatch is small enough to be accommodated by relaxation (or strain) in the structure (24, 25). Figure 5c,d shows an idealized pyromorphite particle growing on a galena surface as might occur after nucleation. The c-axis of the pyromorphite makes an angle of 36° relative to the surface normal, which is consistent with the experimentally observed particle orientation in Figure 4a,d. Most of the pyromorphite particles adhere strongly enough to the galena surface to allow consecutive EC-STM images to be taken but some can be dislodged with repeated imaging (Figure 2, marked by an arrow). Because the lateral force induced by the STM tip is much less than what would be affected by grain-to-grain contact in a disturbed or weathering soil, we speculate that Pb5(PO4)3Cl particles can be physically dislodged and released to solution. If so, these particles could act as a vehicle for the transport of lead beyond the contaminated site. Alternatively, if conditions are such that particles grow faster than they are transported away from the galena surface, such as when phosphate and lead are in abundance, a reaction rim surrounding the galena grains will form as is observed here (Figures 3 and 4e) and in the field (10). Effect of Phosphate on Oxidation. We performed cyclic voltammetry experiments on the system in order to determine if the presence of phosphate can passivate the galena surface and prevent further oxidation. The electrochemistry of galena is quite complex due to the numerous (meta)stable surface and aqueous species of sulfur (17, 26, 27). Typically in CV experiments on galena, oxidation results in the formation of elemental sulfur or other metastable intermediates. A number of peaks have been observed from different types of surface sites (e.g., sites located on the edge of a step vs sites located within the plane of the surface). The addition of phosphate to this system complicates matters further because phosphoric-acid species give up their protons at a much less reducing potential than hydronium ion, creating an additional peak (28). Figure 6 shows the results of the CV study with and without phosphate. Parts a and b of Figure 6 each show four cycles of the potential, i.e., the experiment was started at the opencircuit-potential (typically between -60 and -130 mVAg wire sample potential) and ramped to -0.7 VAg wire, then to 0.3 VAg wire, and back to the OCP, in four cycles. During the CV experiments, the results of Paul et al. (26) and Sivenas and Foulkes (27) were first reproduced. The end points of the voltammograms in Figure 6 were chosen such that so that Pb(II) reduction to lead metal does not occur, nor oxidation of the surface to such a degree that the surface is passivated by elemental sulfur, nor the formation of hydrogen gas from H2PO4-. In Figure 6a, at oxidizing potentials (positive), there is an anodic current peak (labeled A) whose height and width remain relatively constant, indicating an oxidation of a species whose concentration remains constant with each cycle. It is likely that this peak represents surface sulfur species located on step edges (17) or other active sites that are being oxidized to elemental sulfur. Under reducing conditions (Figure 6a) we observe a peak that is present as a brief shoulder during the first cycle (labeled B) but in subsequent cycles is replaced by a peak whose height grows with each cycle (labeled C). Peak B is most likely the reduction of oxidized sites on the surface of the galena (some oxidation does occur at the OCP). Since peak C grows with successive cycles, it indicates that the concentration of the species involved is increasing with each cycle. This species is most probably the reduction of
FIGURE 5. Epitaxial growth model. (a) Cross-section through Pb5(PO4)3Cl structure (only lead atoms are shown) showing a plane of four lead atoms with a very similar orientation to the lead atoms in galena. This group of four atoms reproduces periodically in the same plane, with distances shown. (b) The {001} galena surface, (only lead atoms shown). (c) A model of a pyromorphite particle growing on a galena surface, looking down the c-axis of the pyromorphite. The galena surface is inclined 36° toward the bottom-right of the diagram. Lead atoms are gray, sulfur are yellow, chlorine are green, and phosphate molecules are shown as blue tetrahedra. Overlapping lead atoms between the pyromorphite and galena structures are shown in red, with their size exaggerated for clarity. The same atoms are highlighted in parts (a) and (b) as well. (d) Side view of the same orientation. the oxidized sulfur formed in the oxidative portion of the cycle (peak A). Since peak C is not observed until after the sample has been oxidized, it must arise from the reduction of a oxidation product, probably reduction of elemental sulfur to hydrogen sulfide. Figure 6b shows a voltammogram taken with phosphate present. The large peak on the reducing side is removed (peak C), but the shoulder remains (peak E) and grows a small amount in each successive cycle. Also, the peak on the oxidation side is much reduced in size (peak D). It is likely that the shoulder that is observed in the presence of phosphate (peak D) is actually a separate reaction from the one observed in absence of phosphate (peak A) but is obscured by the larger peak when phosphate is absent. One possible reason peaks A and C disappear (or diminish) in the presence of phosphate is that the oxidatively induced retreat of steps is inhibited by pyromorphite particles and oxidation only occurs on terraces. An alternate possibility is that phosphate adsorbs to lead atoms on the surface and stabilizes the surface structure. Higgins and Hamers (17) found that during oxidation of a galena step on a surface, the removal of sulfur atoms from step edges probably limits the overall reaction rate; i.e., lead atoms dissolve very quickly. Here, it is likely that phosphate binds to lead atoms on the step edge
and could slow their dissolution and the rate of retreat of the step, thus limiting the availability of sulfur to react. If this were true, it would also explain why the pyromorphite grows more commonly on steps: an adsorbed phosphate would provide an ideal nucleation site for pyromorphite growth. The interpretation that phosphate stabilizes a leadterminated surface structure is qualitatively consistent with our observations during EC-STM imaging of galena surfaces in the presence of phosphate. In the absence of phosphate, we often observed that STM image quality would degrade over time at oxidizing potentials (probably due to the surface becoming covered in nonconducting elemental sulfur). However, in the presence of phosphate we found that image quality often remained fair during oxidation, which may indicate that the surface is more stable in the presence of phosphate.
Acknowledgments We thank the AE, Janet Hering, and three anonymous reviewers for considerably improving this manuscript with perceptive comments and suggestions. Support for this research was from the U.S. DOE (DE-FG-03-02ER15325) to W.H.C. A portion of this research was performed at the W. VOL. 38, NO. 21, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Cyclic voltammetry data, scan rate ) 10 mV/s, reducing conditions are plotted to the right, cathodic currents are positive. (a) Voltammogram taken in 0.01 M NaCl, begun at the OCP, scanning to -0.75 VAg wire, scanned to 0.3 VAg wire, scanned for a total four cycles, and ending at the OCP. The peak labeled A probably indicates an oxidation of sulfur sites locate on the galena surface at a stepedge or reactive site. The oxidation probably results in elemental sulfur or possibly H2S. The shoulder, peak B, is likely the reduction of some oxidized sulfur sites on the surface. Peak C is probably the reduction of the elemental sulfur or H2S (created from peak A) to hydrogen sulfide. (b) Voltammogram taken in the presence of 0.001 M NaH2PO4, the same sample as in part (a). Peak D is an oxidation reaction of sulfur sites on the surface but not necessarily the same reaction producing peak A. Peak E is probably the same reaction as peak B. Peak C is missing, and peak A is either much reduced in size or missing as well, indicating that phosphate is protecting or passivating the surface somewhat. R. Wiley Environmental Molecular Sciences Laboratory located at Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated for the DOE by Battelle Memorial Institute under Contract DEAC0676RLO1830. Another portion of this work was supported by the Director, Office of Science, of the U.S. Department of Energy under Contract No. DE-AC03-76SF00098. The authors acknowledge the support of the staff and facilities at the National Center for Electron Microscopy.
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Received for review April 2, 2004. Revised manuscript received August 18, 2004. Accepted August 19, 2004. ES049487S
