Pyromorphite Formation from Goethite Adsorbed Lead

Exchangeable lead was determined using a method modified from Tessier et al. (15) and Rapiz et al. (16). An aliquot of goethite suspension from the ad...
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Environ. Sci. Technol. 1997, 31, 2673-2678

Pyromorphite Formation from Goethite Adsorbed Lead PENGCHU ZHANG,* JAMES A. RYAN, AND L. TARAS BRYNDZIA National Risk Management Research Laboratory, U.S. Environmental Protection Agency, 5995 Center Hill Avenue, Cincinnati, Ohio 45224

Conversion of labile soil Pb species into stable Pb minerals is a crucial process in the development of an in situ soil Pb remediation strategy. A complete understanding of the reaction with specific soil Pb species is therefore of fundamental importance. A synthetic goethite, R-Fe(OH)3, with surfaces saturated with Pb2+ was used to represent adsorbed Pb, one of the primary soil Pb species. Hydroxyapatite (Ca5(PO4)3OH) and NaH2PO4 were the source of solid and soluble phosphate, respectively. When stoichiometric concentrations of soluble phosphate were added directly to the suspension of Pb-adsorbed goethite, the thermodynamically stable lead phosphate mineral chloropyromorphite (Pb5(PO4)3Cl) was rapidly precipitated. By contrast, when the same goethite suspension was reacted in a dialysis system in the presence of hydroxyapatite, the formation of chloropyromorphite is slow and appears to be controlled by the rate of dissolution of the hydroxyapatite. Chloropyromorphite minerals produced in these experiments vary in morphology in accordance with the reaction conditions in which the mineral formed. However, the extractability of the goethite-adsorbed Pb by MgCl2 was dramatically reduced because of the reaction with added phosphates. This supports the hypothesis that the bioavailability of adsorbed soil Pb could potentially be reduced to insignificant levels in Pb-contaminated soil, under normal soil pH conditions, by amendment with a phosphate source.

Introduction It has been suggested that the bioavailability of soil Pb can be reduced by amending Pb-contaminated soils with solid phosphate minerals, such as apatite or phosphate rock, and producing the stable Pb(II) phosphate mineral, pyromorphite (1-3). To properly evaluate this hypothesis, the reactivity and reaction kinetics of major soil Pb species with phosphate minerals need to be investigated. The major species of interest are adsorbed Pb, Pb-bearing minerals, and Pb-organic complexes because they represent the main sources of and sinks for soil Pb and are therefore the primary factors that determine the availability of free Pb2+ ion in soil solutions. The hydrous metal oxides of Fe, Al, and Mn control, to a great extent, the concentrations and transport of many trace metals in soils and natural waters through the mechanisms of sorption and coprecipitation (4-6). Hydrous oxides of Fe and Mn can scavenge heavy metals and, thus, are believed to play an important role in the retention of heavy metals in polluted soils (7). However, the dissolution of the oxides under reducing conditions may weaken the heavy metal bond * Corresponding author e-mail: [email protected]; telephone: 513-569-7869; fax: 513-569-7879.

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 1997 American Chemical Society

and thereby promote solubilization of metal ions (8). The potential always exist that the oxides sorbed with heavy metals become a pollutant source as environmental conditions such as the pH, redox, and concentrations of metals change. Goethite (R-Fe(OH)3) is usually the dominant hydrous iron oxide mineral in soils. Interactions between goethite and Pb have been extensively investigated because they represent a common soil constituent and a typical pollutant and, therefore, are ideal candidates with which to establish a model that describes the relationship between soil matrices and pollutants for monitoring purposes and also to predict transport of metals in soils and natural waters, a necessity to develop and evaluate strategies for soil remediation. There are two phenomena observed in the present investigation of goethite-Pb interaction: (a) the adsorption/desorption of Pb is pH dependent and (b) the sorbed Pb is equilibrated with free Pb2+ ion in the solution. Any changes in pH in the systems results in a new equilibrium amount of sorbed Pb. The second phenomenon is the reversibility of the proportionality between the sorbed Pb and Pb2+ in solution, which can be partial (9) or full (10). Goethite-adsorbed Pb is, therefore, a potential source of bioavailable Pb in soils. The sorbed Pb is expected to be released into solution when pH is low, e.g., the ingested soil in the gastrointestinal liquid in an empty stomach with its acid pH or when rainfall and irrigation waters come in contact with Pb-contaminated soils. The importance of adsorbed Pb in both bioavailability and transportation makes it one of the target Pb species in soil Pb immobilization. This paper illustrates the results of formation of pyromorphite from reactions of phosphate(s) with goethite-adsorbed Pb. The behavior of adsorbed Pb in the presence of phosphate, as described in this paper, demonstrates the potential to form pyromorphites, which are important in the development of in-situ Pb immobilization technology.

Materials and Methods Goethite and Hydroxyapatite. The goethite used in this study was prepared as described by Atkinson (11), and its structure was confirmed by X-ray diffraction analysis. After aging for 24 h, the goethite suspension was placed in a dialysis tube suspended in double deionized water. The water was changed daily until the conductivity of the dialysis water equaled that of deionized water. The suspension was refrigerated until use. Synthetic hydroxyapatite was obtained from Bio-Rad laboratories (Bio-Gel HTP). The reported surface area was 45 m2 g-1 (12), with a molar Ca/P ratio of 1.625, which is close to the stoichiometric ratio of 1.667 in an ideal hydroxyapatite crystal. The XRD pattern for the material exhibited characteristic peaks of hydroxyapatite, consistent with the results of Xu and Schwartz (13). Pb Adsorption and Extraction. To prepare Pb-adsorbed goethite, suspensions of 20 g L-1 of goethite were prepared in a 0.1 M NaNO3 and 0.015 M Pb(NO3)2 solution at five different pH values: 3.0, 4.0, 5.0, 6.0, and 7.0 ((0.2). The pH of the solution was adjusted and maintained by addition of 0.1 M HNO3 or 0.1 M NaOH. The suspensions were shaken continuously; every 24 h an aliquot (3 mL) was taken, centrifuged at 13 000 rpm for 30 min, and filtered through a 0.2 µm membrane; and the Pb concentration of the filtrate was then determined. The goethite-adsorbed Pb was determined by the difference between added Pb and that remaining in solution. Results from our experiment and those reported in the literature (14) indicated that adsorption equilibrium was reached after 24 h. After equilibrium, the Pb-adsorbed goethite suspensions were centrifuged to remove

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excess solution and stored as wet paste in a refrigerator until needed. This resulted in five lead-adsorbed goethites with Pb concentrations ranging from 0.1 to 0.67 mmol g-1. Exchangeable lead was determined using a method modified from Tessier et al. (15) and Rapiz et al. (16). An aliquot of goethite suspension from the adsorption experiment was mixed with an equal volume of 1.0 M MgCl2 solution previously adjusted to pH 7, shaken for 30 min, centrifuged, filtered, and analyzed. Exchangeable Pb was the difference of Pb in the desorption filtrate and the Pb in the liquid phase of the suspension prior to desorption. Dialysis Experiments. The Pb-adsorbed goethite paste was resuspended in 0.10 M NaNO3 and 0.01 M NaCl solution at a solid concentration of 20 g L-1. A 20.0 mL sample was placed into a dialysis tube (Spectra/Por 7). The tube had a molecular weight cutoff of 1000, allowing free passage of inorganic ions. The tubing was suspended on a parallel bar rotator in 500 mL of 0.10 M NaNO3 and 0.01 M NaCl solution in a polyethene container. A dialysis tube containing 2.0 g of hydroxyapatite was also placed in the same 500 mL polyethylene container. The experimental control contained only Pb-adsorbed goethite in a container with 500 mL 0.10 M NaNO3 and 0.01 M NaCl. The pH of the dialysis solution was monitored weekly. Liquid samples were taken weekly using a syringe and filtered through a 0.2 µm membrane for soluble Pb and Ca analysis. To keep a constant dialysis volume, an aliquot of NaNO3 and NaCl solution was added to the container to replace the volume taken during sampling. Preliminary tests indicated that at least 48 h was needed for Pb to reach equilibrium in the dialysis system, consistent with the results of Berggren (17). After 49 days of dialysis, a portion of the dialyzed Pbadsorbed goethite suspension was analyzed for exchangeable Pb by MgCl2 extraction, and the remainder was filtered and air-dried. The remaining solids were analyzed on an X-ray diffractometer (XRD) and a scanning electron microscope coupled with an analytical energy dispersive X-ray spectrometer (SEM/EDX). A precipitate was found on the membrane containing Pb-adsorbed goethite that had been dialyzed with apatite. These membranes were rinsed, dried, and analyzed using XRD and SEM/EDX. Batch Study. A stock goethite suspension was prepared at a solid concentration of 20.0 g L-1 and Pb concentration of 0.015 M Pb(NO3)2. Twenty-five (25.0) mL of the suspension was added in each of four beakers containing 500.0 mL of 0.1 M NaNO3 and 0.01 M NaCl solution. The total Pb in a suspension was 3.75 × 10-4 mol and 0.5 g of goethite. The pH of the suspensions was adjusted with 0.10 M HNO3 or 0.10 M NaOH to a pH of 4.00, 5.00, 6.00, or 7.00 ((0.10). The pH was maintained with an automatic titrator for 3 h, after which time the suspension was stirred with a magnetic bar for 18 h before sampling and adding phosphate. Based on the molar ratio of P/Pb ) 3/5 of chloropyromorphite (Pb5(PO4)3Cl), 2.50 × 10-4 mol of phosphate was added into each of the four suspensions. This amount of phosphate was slightly higher than that needed (2.25 × 10-4) to transfer all Pb (in solution and surface bound) into chloropyromorphite. Phosphate was introduced by pumping 10.0 mL of 0.025 M NaH2PO4, prepared in a 0.10 M NaNO3 and 0.01 M NaCl solution, into the suspension within 30 min. Simultaneously, 3.75 × 10-4 mol of CaCl2 solution was added into the suspension to simulate a solution of dissolved apatite. After addition of phosphate, pH in the suspension dropped approximately 0.5 to 1.0 unit, and a 0.10 M NaOH solution was used to adjust the pH to its level prior to phosphate addition. The phosphate-amended suspensions containing goethite were stirred for 12 h prior to sampling. The sample was centrifuged for 30 min at 13 000 rpm and filtered through a 0.2 µm membrane, and the filtrate was analyzed for P, Ca, and Pb on an inductively coupled plasma spectrometer (ICP,

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FIGURE 1. Averaged Pb concentration in the solution of the systems in which Pb-adsorbed goethite was dialyzed with and without hydroxyapatite. Thermal Jarrel Ash). To determine the exchangeable Pb from goethite surfaces in the suspension, 10.0 mL of the suspension and 10.0 mL of 1.0 M MgCl2 solution were shaken for 30 min, centrifuged, filtered, and analyzed for P, Ca and Pb by ICP. Following sampling after the first addition of phosphate, the goethite suspensions were amended with a second addition of 2.50 × 10-4 mol of NaH2PO4. The same procedures for stirring, sampling, and extraction were followed as for the first addition of phosphate. At the end of the experiment, the suspension was filtered through a 0.45 µm membrane, and the solids were collected and air-dried for XRD and SEM/ EDX analysis. A portion of the solid was stored as wet paste in a refrigerator for further analysis. Analytical Methods. An atomic absorption spectrophotometer (AA, Perker-Elemer Zm4000) equipped with a flame head (FAA) and graphite furnace (GFAA) was used to analyze Pb in the dialysis solutions. The detection limit for GFAA was 0.5 µg L-1 with Mg and P as the modifiers. An inductively coupled plasma spectrometer was employed to analyze P, Ca, and Pb concentrations in the batch experiment solutions and Ca and P concentrations in dialysis solutions. The detection limit for the three elements was 2 µg L-1. Solution and suspension pH were measured with an Accumet combination electrode coupled with a pH meter. The identity of all crystalline materials was confirmed with an X-ray diffractometer (Scintag, XDS 2000; Cu KR radiation, a potential of 30 KV filament current of 20 mA). Solids were air-dried, crushed, and mounted on a glass sample holder for XRD analysis or coated with carbon on double-stick tape for SEM/EDX analysis. Step-scanning at 0.04°2θ/s was employed to obtain the XRD patterns. The dialysis tube membrane was mounted with double-sided tape on sample holders and coated with carbon, or Au, under vacuum for SEM/EDX analysis. The SEM/EDX measurements were conducted with a JEOL scanning electron microscope (JSM 5300). The distribution of Pb(II), PO4, Ca, CO2, NO3, and Cl species in the solutions were modeled using the computer code EQ3/6 (Version 7.0), which was developed for geochemical modeling of aqueous systems (18).

Results Dialysis Experiments. The average soluble Pb level in the dialysis solutions without hydroxyapatite reached a concentration of 10-4-10-5 mol L-1 and exhibited little change during the 49 days of dialysis (Figure 1). Modeling by EQ3/6 failed to provide evidence for the formation of any solid under these conditions, and no solids were detected in the filtrate. By contrast, addition of hydroxyapatite to the dialysis system

a

FIGURE 2. Photomicrograph of precipitate on the dialysis tube membrane (a) and its elemental spectrum determined by X-ray energy dispersive analysis (b). The spectrum is similar to that obtained from synthetic chloropyromorphite.

FIGURE 3. Effect of hydroxyapatite on exchangeable Pb in the Pbadsorbed dialysis system. dramatically lowered the Pb concentration in the dialysis solution (Figure 1). After 7 days of dialysis, Pb concentrations in the solutions were reduced by 2 orders of magnitude relative to the corresponding goethite suspension without hydroxyapatite. After 21 days of dialysis, the Pb concentration in solutions with hydroxyapatite reached a concentration of 10-7-10-8 mol L-1, which was 3 orders of magnitude lower than the Pb concentration in the corresponding dialysis solutions without hydroxyapatite. During the dialysis period, a gray-white precipitate gradually accumulated on the dialysis tube. The gray-white precipitate only formed on the tubes containing the goethite suspension in the presence of hydroxyapatite and not on the tubes containing only hydroxyapatite or in the bulk solution. The precipitate was tightly bound to the internal wall of the tubing and could not be removed even with vigorous ultrasonification. The SEM image of this precipitate indicates the presence of well-developed crystals with hexagonal morphology (Figure 2a). The chemical composition of the precipitate obtained by EDX (Figure 2b) indicates that the molar ratio of P-Pb-Cl of the solid is identical to that in chloropyromorphite synthesized from Pb(NO3)2, NaCl, and NaH2PO4 solutions. The XRD pattern for the precipitated material confirmed that it was indeed chloropyromorphite. After 49 days of dialysis, the goethite suspensions were analyzed for exchangeable Pb by extraction with 1.0 M MgCl2 solution (Figure 3). The percent exchangeable Pb of the total Pb adsorbed on goethite in the dialysis systems without hydroxyapatite was approximately 52%, regardless of the total

amount of adsorbed Pb on the goethite. In contrast, the percent exchangeable Pb on lead-adsorbed goethite that was dialyzed in the presence of hydroxyapatite ranged from 20% to 40% and was positively related to the total amount of Pb adsorbed on the goethite. This reduction in exchangeable Pb might be attributed to either (a) removal of exchangeable Pb from the surface and precipitation with the phosphate and/or (b) formation of a lead phosphate solid at the goethite surface. The first possibility is well-established by precipitation of Pb from solution and the formation of chloropyromorphite on the walls of dialysis tubes. Although a new solid phase was not detected on the goethite surface by SEM and/ or XRD, the formation of a phosphate-Pb solid on the surface cannot be ruled out because the EDX spectra (not shown) indicated the chemical composition of the goethite surface includes P and Cl in addition to Pb and Fe, indicating the possible existence of chloropyromorphite and/or other lead phosphates on the goethite surface. The soluble Ca concentration in the dialysis systems that contained hydroxyapatite (Figure 4a) was continuously increased during the dialyzing period. As the only source of Ca was through dissolution of hydroxyapatite and the amount of Ca in solution had not reached steady state at 49 days of dialysis, it can be assumed that the dissolution of hydroxyapatite was still occurring. In contrast, the soluble P level was constant during the 49 days of dialysis at 10-6-10-7 mol L-1 (Figure 4b), indicating that there was no dissolution of hydroxyapatite or that the P was removed from solution as the hydroxyapatite dissolved. Since it has been shown that chloropyromorphite was formed, it can be assumed that the amount of P in solution was governed by the formation of chloropyromorphite. Thus, this difference between expected P from dissolution of hydroxyapatite and P observed in solution (Figure 4b) allows an estimate of the amount of chloropyromorphite which can be produced. Batch Study. The Pb adsorbed on goethite surfaces was calculated as the difference between the total added Pb and measured soluble Pb. This was calculated to be 0.16, 0.22, 0.40, and 0.68 mmol/g at pH 4.02, 5.00, 5.96, and 7.04, respectively. The soluble Pb concentrations are inversely related to pH as adsorption increased with increasing solution pH (Figure 5a). Following the addition of 2.5 × 10-4 mol of P, which is the amount of phosphate needed to convert the added Pb into chloropyromorphite, the soluble Pb concentrations decreased to 1.98 × 10-5 and 3 × 10-8 mol L-1 (Figure 5a) at pH 4 and pH 5, respectively. A second addition of 2.5 × 10-4 mol of P to the suspensions caused further reduction in soluble Pb concentrations such that only the system at pH 4.02 exhibited a value above the detection limit (Figure 5a).

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FIGURE 4. Effect of dialysis time on soluble Ca (a) and phosphate (b) concentrations.

FIGURE 5. Effect of added phosphate on soluble Pb (a) in goethite suspensions and exchangeable Pb (b) from goethite surfaces.

FIGURE 6. Recovery of phosphate (a) and Ca (b) from the goethite suspension after 2.5 × 10-4 and 5.0 × 10-4 mol of phosphate added, respectively. Approximately 60-70% of the adsorbed Pb was extractable by 1.0 M MgCl2 (Figure 5b). Addition of 2.5 × 10-4 mol of P to the suspension decreased the extractable Pb such that 1-6% of the adsorbed Pb was extractable, and the extractable amount was positively related to the amount of Pb adsorbed on the goethite surface (Figure 5b). The second addition of 2.5 × 10-4 mol of P reduced the 1.0 M MgCl2 extractable Pb to 1-2% of adsorbed Pb, resulting in no significant difference among the samples. Only 0.3-2.8% of the first 2.5 × 10-4 mol of P added to the suspension was recovered in the solution phases (Figure 6a). However, essentially all (90-100%) of the second addition of 2.5 × 10-4 mol of P was recovered in the solution phase (Figure 6a). To create a similar chemical composition as that of dissolved chloroapatite (Ca5(PO4)3Cl), calcium chloride (CaCl2) was added simultaneously with NaH2PO4 to the goethite

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suspension. As demonstrated, more than 95% of added Ca stayed in the aqueous phase as soluble Ca (Figure 6b), confirming the model prediction by EQ3/6, in which no solids such as chloroapatite and hydroxyapatite should be produced under these conditions. Further, XRD analysis also failed to detect the presence of these solids. Additionally, soluble Ca concentration was unaffected by the second addition of 2.5 × 10-4 mol of P, suggesting that there was no consumption of Ca by the added P to form a calcium phosphate mineral. The solids collected from the reacted suspensions by filtration with a 0.45 µm membrane were analyzed by SEM/ EDX. The SEM analysis failed to yield visible crystals of chloropyromorphite. However, the EDX spectrum indicated a combined chemical composition of chloropyromorphite and goethite. The XRD patterns from the reacted goethite surfaces contained the characteristic peaks of chloropyro-

FIGURE 7. XRD patterns for Pb-adsorbed goethites and a synthetic chloropyromorphite. morphite and goethite (Figure 7). No evidence for the formation of other P-Pb products was observed.

Discussion Although the exact distribution of Pb species in the dialysis system could not be quantitatively determined, the information obtained from the dialysis solutions helps to understand the fate of adsorbed Pb in soil solutions. From simulation of the dialysis solution by EQ3/6, chloropyromorphite is the only stable mineral that the solution could generate. Other lead phosphate, lead-iron phosphate, iron phosphate, and lead carbonate minerals would not precipitate from the low soluble P and Pb concentrations in the systems tested. Examination of the dialysis system (apatite surface, goethite surface, their respective dialysis tubes, and the bulk solution) allows for determination of where the reaction is occurring and what is controlling it. Examination of apatite surfaces illustrated that no Pb had adsorbed or precipitated onto the apatite surface and that the reactions only occurred in solution near the Pb-adsorbed goethite surface, suggesting that desorption of Pb from the goethite surface was controlling the precipitation of pyromorphite in this system. As reported, the direct contact of hydroxyapatite and resin adsorbed-Pb resulted in a formation of pyromorphite at the resin surface (1). Although the rate of P release to solution through hydroxyapatite dissolution did not seem to be the rate-limiting factor for the overall formation process of pyromorphite in the adsorbed-Pb-hydroxyapatite system, the effect of excess amount of hydroxyapatite used in these experiments on the dissolution rate should be considered. Soluble P remained relatively constant and at a low concentration during the 49 days of dialysis, indicating either no dissolution of hydroxyapatite or a continuous removal of P from the solution. As Ca concentration in the solution was continuously increasing during the 49 days, it is reasonable to conclude that hydroxyapatite was being dissolved and the P was being removed from solution. This removal of P from solution could have been from adsorption by goethite or precipitation of a phosphate mineral. Phosphate adsorption on goethite has been studied intensively, and it has been suggested that the adsorbed phosphate forms both binuclear bridging Fe-OP(O)2O-Fe and monodenate Fe-OP(O)3 surface complexes on goethite surfaces (19). These surface complexes are strong, and phosphate could not be desorbed even by washing at low pH (20). In this study, adsorption of phosphate on goethite surface may not be ruled out, but would be limited. At a low pH (e.g., pH 4), where low Pb adsorption occurs, there may be adsorption sites available on goethite surfaces. However, the precipitation of pyromorphite is a very rapid process (seconds), whereas adsorption of phosphate onto goethite surfaces is relatively slow.

Therefore, phosphate is rapidly consumed by forming pyromorphite instead of being adsorbed on goethite surfaces. At a high pH, the goethite surface sites were fully occupied with adsorbed Pb, and the surface was altered by polymeric adsorbed Pb (21). Therefore, the added phosphate anions could not directly contact the goethite surface to form a surface complex. Rather, phosphate interacts with the adsorbed Pb to precipitate chloropyromorphite or a pyromorphite-like solid at the goethite surface. Subtracting the measured soluble P from the predicted P released from apatite dissolution, the phosphate precipitated by Pb was between 1.8 and 1.6 × 10-5 mol. Accordingly, the immobilized Pb would be from 2.0 × 10-5 to 3.0 × 10-5 mol or 20-70 ((10)% of the adsorbed Pb, depending upon the total Pb adsorbed on goethite surfaces. Together with reduction in soluble Pb and exchangeable Pb as shown in Figures 1 and 3, respectively, as well as the formation of chloropyromorphite in solution, the phosphate was precipitated by Pb released from the goethite surface to form chloropyromorphite. However, the amount of phosphate released to solution by hydroxyapatite dissolution was not sufficient to convert all goethite adsorbed Pb into stoichiometric chloropyromorphite. Therefore, exchangeable Pb could still be extracted even after the 49 days of dialysis. In addition to the reduction in exchangeable Pb, it was noted in Figure 3 that the fraction of exchangeable Pb increased with the Pb adsorbed on goethite surface when the system contained hydroxyapatite. This was attributed to the fact that the relatively larger fraction of adsorbed Pb was precipitated and removed from the goethite surfaces on which the adsorbed Pb was relatively low, e.g., approximately 60% of adsorbed Pb was transformed into pyromorphite from the goethite with a Pb loading of 1 × 10-5 mol Pb g-1; however, only about 20% of total adsorbed Pb was precipitation from the goethite with a Pb loading of 6.5 × 10-5 mol Pb g-1. In the batch study, the first 2.5 × 10-4 mol of phosphate was not recovered from the aqueous phase in any of the goethite suspensions, whereas from 90 to 100% of the second 2.5 × 10-4 mol of phosphate was recovered from the aqueous phase of each sample. As the amount of each phosphate addition was stoichiometrically equivalent to that necessary to convert all the Pb to pyromorphite and the extractable Pb also disappeared, it is reasonable to assume that reaction has occurred and all Pb and P are in the insoluble chloropyromorphite mineral phase. Further, the recovery of the second addition of phosphate would imply that no surface Pb was available for further reaction. In addition to the formation of chloropyromorphite, it is also possible that iron phosphate may have precipitated on the goethite surface. Small crystallites of the mineral tincticite (Fe6(PO4)4(OH)6‚7H2O) were found on the top layer of goethite using transmission electron microscopy (22). A detailed study conducted by Martin et al. (23) on the goethite surface precipitation of phosphate revealed the formation of iron phosphate phases as crystallites on the goethite surface when equilibrated with a 10-3 M phosphate solution. However, it should be noted that the new phase was formed in 90 days and at 60 °C. There was no reported observation of precipitation in less than 18 days in the goethite/phosphate system (23). In the suspensions used in this experiment, the concentration of phosphate was low (within 10-4-10-5 M), and the contact time between goethite-adsorbed Pb and phosphate was relatively short (12 h); therefore, it was unlikely that a significant amount of added phosphate was consumed by forming iron phosphate crystallites. Furthermore, the formation of a polymeric layer of adsorbed Pb on the goethite surface (21) may prevent phosphate anions from reaching the reaction sites on the goethite surface. The competition for the phosphate anion between the formation of solid chloropyromorphite and/or iron phosphates kinetically favors chloropyromorphite, thus preventing the consumption of

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phosphate by reactions other than chloropyromorphite crystallization until there is no available Pb left in the system. However, when there was excess phosphate in the suspension, as after the second addition of 2.5 × 10-4 mol of phosphate, iron phosphate crystallites could form, but the goethite surface might already have been covered by the newly formed chloropyromorphite after the first addition of phosphate, and there would be no goethite surface sites left for phosphate adsorption and reaction. The recovery of the second addition of phosphate from solution would, however, seem to refute formation of an iron phosphate phase on the goethite surface. Impurities such as Mn, Pb, Mg, and Ba in natural goethite may also have interacted with solution phosphate to generate the mineral griphite, an hydroxy manganese-iron phosphate mineral (23), and this may lead to a concern that Pb substituted for Mn in graphite precipitated on the goethite surfaces. Chloropyromorphite was the only mineral identified by XRD, and although other phases may have co-precipitated with chloropyromorphite, their presence was not detected by X-ray diffraction (23). The other minerals are not likely to have formed, however, because the formation of chloropyromorphite is kinetically more favorable. Results from the exchangeable Pb study illustrate a change of Pb form. These observations lead to the conclusion that the formation of pyromorphite from adsorbed Pb is the dominant process when aqueous phosphate is present in near-stoichiometric proportions. One of the mechanism(s) for the precipitation of chloropyromorphite in this system is speculated to involve a solution reaction occurring between the desorbed Pb and aqueous phosphate. To maintain equilibrium between surface adsorbed Pb and soluble Pb in the bulk solution, the adsorbed Pb is released to the solution from the goethite surface by desorption when the soluble Pb is removed by forming chloropyromorphite with soluble phosphate. The newly formed chloropyromorphite may precipitate on the goethite surface and/or remain suspended in the solution. However, a recent study by Xia et al. (24) suggested that the adsorbed phosphate on an oxide surface creates adsorption sites for Pb. Binding of Pb2+ ion directly to the adsorbed phosphate on the surface was observed using X-ray absorption spectroscopy (21). Such a surface reaction may also have occurred in our experiments. The added phosphate becomes partially adsorbed on the goethite surface, and soluble Pb becomes associated with goethite through a bridging of adsorbed phosphate. Similarly, one may expect that the adsorbed Pb on the goethite surface may bind with soluble phosphate through the bridging mechanism discussed previously. To fully explore the reaction mechanisms, however, further studies with more sophisticated surface analytical techniques are needed. Published work (1-3) as well as our own experiments illustrate that precipitation of chloropyromorphite in both saturated/supersaturated solutions is rapid. These conditions result in the formation of solids that appear as acicular crystals or amorphous solids when viewed by SEM. XRD analysis of the solids have confirmed they are the mineral chloropyromorphite. In the dialysis system, the rate and amount of Pb released to solution is controlled by Pb desorption from the solid goethite surface, and the rate and amount of P released to solution is controlled by apatite dissolution. The formation of chloropyromorphite starts when the solution reaches saturation and does not allow the solution to become supersaturated, thereby providing a favorable environment for nucleation and crystal growth. The well-crystallized chloropyromorphite obtained from the dialysis system should be less reactive in terms of dissolution due to its stoichiometric composition, smaller specific surface area, highly ordered crystal structure, and exceedingly low Ksp (10-84.5) and has therefore the potential to reduce the bioavailability of Pb2+

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in aqueous solutions such as soil solutions and gastrointestinal fluids.

Acknowledgments The authors gratefully thanks Drs. K. Brackett, P. Clark, and M. Schock for their assistance in SEM/EDX and XRD analysis. This research was supported in part by an appointment to the Postgraduate Research Participation Program at the National Risk Management Research Laboratory administrated by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and the U.S. Environmental Protection Agency. Although the research in this paper has been undertaken by the U.S. Environmental Protection Agency, it does not necessarily reflect the views of the Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.

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Received for review February 3, 1997. Revised manuscript received May 19, 1997. Accepted May 29, 1997.X ES970087X X

Abstract published in Advance ACS Abstracts, July 15, 1997.