Environ. Sci. Technol. 2001, 35, 3798-3803
Formation of Chloropyromorphite in a Lead-Contaminated Soil Amended with Hydroxyapatite JAMES A. RYAN* National Risk Management Research Laboratory, US EPA, 5995 Center Hill Avenue, Cincinnati, Ohio 45224 PENGCHU ZHANG Department of Geochemistry, Sandia National Laboratories, MS0750, Albuquerque, New Mexico 87185 DEAN HESTERBERG Department of Soil Science, North Carolina State University, Box 7619, Raleigh, North Carolina 27695 JEFF CHOU AND DALE E. SAYERS Department of Physics, North Carolina State University, Raleigh, North Carolina 27695
Conversion of soil Pb to pyromorphite [Pb5(PO4)3Cl] was evaluated by reacting a Pb contaminated soil collected adjacent to a historical smelter with hydroxyapatite [Ca5(PO4)3OH]. In a dialysis experiment where the soil and hydroxyapatite solids were placed in separate dialysis bags suspended in 0.01 M NaNO3 solution a crystalline precipitate, identified as chloropyromorphite, formed on the dialysis membrane containing the soil. The aqueous composition of the solution indicated that dissolution of solid-phase soil Pb was the rate-limiting step for pyromorphite formation. Addition of hydroxyapatite to the soil caused a decrease in each of the first four fractions of sequential extractable Pb and a 35% increase in the recalcitrant extraction residue. After a 240-d incubation at field-moisture content there was a further increase in the recalcitrant extraction residue fraction of the hydroxyapatite-amended soil to 45% of the total soil Pb. The increase in the extraction residue fraction in the hydroxyapatite amended 0-d incubated soil as compared to the control soil illustrates that the chemical extraction procedure itself caused changes in extractability. Thus, the chemical extraction procedure cannot easily be utilized to confirm changes occurring in amended soils. The further increase after the 240-d incubation implies that the reaction also occurs in the soil during incubation. Extended X-ray absorption fine structure (EXAFS) spectroscopy indicated that after the 240-d incubation the hydroxyapatite treatment caused a change in the average, local molecular bonding environment of soil Pb. Lowtemperature EXAFS spectra (chi data and radial structure functions - RSFs) showed a high degree of similarity between the chemical extraction residue and synthetic pyromorphite, providing additional evidence that the change of soil Pb to pyromorphite is possible by simple amendments of hydroxyapatite to soil.
Introduction It has been hypothesized that other more soluble soil Pb species can be converted into less soluble lead orthophos3798
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phates (pyromorphites) by adding phosphate. Thus, providing a potential technique for in-situ immobilization of soil Pb as an alternative to current remediation technologies for contaminated soils. A detailed thermodynamic basis for the reaction of Pb(II) and phosphates in aqueous solutions has been established by Nriagu et al. (1-4). Because pyromorphites, [Pb5(PO4)3(Cl, OH, F..)], are the most stable Pb phosphate minerals found under normal environmental conditions encountered in nature, thermodynamics predict that other solid phase species would be converted to pyromorphite by a dissolution-precipitation mechanism. Within the family of pyromorphite minerals, chloropyromorphite [Pb5(PO4)3Cl] is several orders of magnitude less soluble than hydroxyl-, bromo-, and fluoro-pyromorphites (3). Due to the ubiquity of chloride in nature, it is expected that chloropyromorphite is the dominant environmental form of pyromorphite. Pyromorphite has been reported in Pbbearing mineral deposits, mine-waste contaminated soils (5), garden soil (6), and soils contaminated with Pb ore (galena) adjacent to a phosphoric acid plant (7). Pyromorphite formation was observed in a highly contaminated soil (34% Pb content) amended with hydroxyapatite (8). Recent research demonstrated that phosphate addition in the form of either phosphate salt or a relatively stable rock phosphate reduced aqueous Pb concentrations to low levels in synthetic Pb solutions or Pb-contaminated soil solutions (9, 10) due to the rapid and exclusive formation of pyromorphites. Several Pb phosphates [e.g., Pb3(PO4)2 and Pb4O(PO4)2], whose solubilities are similar to hydroxypyromorphite [Pb5(PO4)3OH] at pH 4-7, were not identified in the PbH2O-P2O5 systems (2, 9, 11, 12). Furthermore, the concentration of cations and anions commonly found in soil solutions did not significantly affect formation of pyromorphite or its composition (11, 12). Thus, phosphate addition to Pb contaminated soil should result in the formation of pyromorphite. However, direct evidence for the formation of pyromorphite in Pb contaminated soils upon phosphate addition is limited. Hodson et al. (13) evaluated metal immobilization by a 2% bonemeal addition in three contaminated soils (Pb contents 9882, 10 543, and 136 260 mg kg-1) with a column experiment. They illustrated that the addition of bonemeal was an effective means of reducing metal leachate and extractable metals from the soil but were unable to confirm the formation of pyromorphite. Laperche et al. (8) identified pyromorphite in the products obtained from the reaction of the Pb-enriched fraction (82 g Pb/kg) of a highly contaminated soil (Pb content ) 3-4% w/w) mixed with a synthetic hydroxyapatite. Cerrusite (PbCO3) was the dominant soil Pb mineral and the major source of soil Pb. However, a contaminated soil with a single Pb species at such high concentrations is not often found. The diversity of species distribution and relatively low Pb content (e.g., less than 10 000 mg kg-1) makes species identification and quantification difficult. For development and evaluation of remediation technologies, it is necessary to illustrate the conversion of the target pollutant to intermediate and final product(s). Therefore, the acceptability of immobilizing Pb in contaminated soils by amendment with phosphates depends on direct confirmation of pyromorphite formation to supplement other inferences collected from soil and solution experiments. The focus of this research was to identify the products of soil Pb and apatite reaction in a contaminated soil containing * Corresponding author phone: (513)569-7653; fax: (513)569-7879; e-mail:
[email protected]. 10.1021/es010634l CCC: $20.00
2001 American Chemical Society Published on Web 08/17/2001
TABLE 1. Total Elemental Concentrations and Suspension pH of the Soilc weight % SOMa
Si
Al
Fe
Ca
K
Mg
P
2.1
25 .8
6.88
3.50
1.55
2.60
0.85
0.14
Pb
Mn
Ba
Zn
Cu
Ni
Cd
pHb
8106
673
751
2432
1255
19.4
158
6.78
mg/kg
a
Soil organic matter. b Measured in 1:2 soil:water suspensions after 24 h. c Except Pb, the data listed in this table were provided by Dr. L. Ma, University of Florida.
moderate levels of Pb (Pbtotal = 8000 mg kg-1). Untreated and hydroxyapatite treated soils were analyzed by a sequential extraction technique and X-ray absorption spectroscopy (XAS). The XAS studies provided direct evidence of soil Pb forms which could not be obtained by X-ray diffraction (XRD) techniques at these Pb concentrations. A dialysis system allowing physical separation of Pb and P sources provided information on the plausibility of the reaction, and it allowed determination of whether the reaction was kinetically controlled by dissolution of apatite or soil Pb or by Pb and P diffusion in the solution.
Materials and Methods Soil and Minerals. The soil used in this study was collected from the area of a historical lead smelting plant in Montana. Before any measurements were conducted, the bulk soil was air-dried, passed through a 2 mm sieve, and homogenized. The contents of major and trace elements and selected soil properties are listed in Table 1. The major identified mineral forms of soil Pb contained in the sample were cerrusite and litharge. Synthetic hydroxyapatite [Ca5(PO4)3OH] was obtained from Bio-Rad laboratories (Bio-Gel). The Ca/P molar ratio of 1.625 was close to the ideal ratio of 1.667, and XRD analysis showed characteristic peaks for hydroxyapatite, both from this study and the literature (14). Chloropyromorphite [Pb5(PO4)3Cl] was synthesized as a reference material. Fortyone (41.0) grams (0.3 mole) of NaH2PO4‚H2O was dissolved in 1.0 L of 0.01 M NaCl solution, and 1.0 L of 0.5 M Pb(NO3)2 solution was slowly added into the phosphate solution while vigorously stirring. The precipitate was aged for 2 days, filtered through a 0.45 µm membrane, washed with deionized water, and air-dried. The resulting precipitate had a molar ratio of Pb/P ) 1.664 which is close to the theoretical ratio of 1.667 for pyromorphite. The SEM/EDX image and spectra showed the precipitate was of irregular morophology and mainly comprised the elements Pb, P, and Cl. The XRD analyses indicate that the precipitate was crystalline chloropyromorphite. Incubation Experiment. In an incubation experiment, 1.0 kg of air-dried, sieved ( 10 K) for the chloropyromorphite sample during data collection, because of poorer thermal contact between the mineral and the cryogenic sample holder. (The soil residue sample consisted of one
TABLE 2. Fitting Results of Low-Temperature Pb LIII-EXAFS Spectra for Untreated Soil and Soil Treated with Hydroxyapatite and Pyromorphite Standardsa sample Pb-contaminated soil-no treatment hydroxyapatite treated soil incubated for 0 d hydroxyapatite treated soil incubated for 240 d hydroxypyromorphite chloropyromorphite sequential extraction residue
shell
bond type
relative coord no.b
radial distance (Å)
Debye-Waller factorc (Å2)
1st 2nd 1st 2nd 1st 2nd 3rd 1st 1st 2nd 3rd 1st 2nd 3rd
Pb-O Pb-O Pb-O Pb-O Pb-O Pb-O Pb-O Pb-O Pb-O Pb-O Pb-P Pb-O Pb-O Pb-O
1.1 1.8 1.1 1.4 1.2 1.4 1.8 3.5 2.4 1.4 1.7 2.9 0.9 0.7
2.29 2.51 2.27 2.46 2.28 2.88 3.17 2.33 2.42 2.66 3.18 2.39 2.85 3.33
0.0017 0.0068 0.0050 0.0090 0.0095 0.0079 0.010 0.013 0.017 0.014 0.011 0.016 0.016 0.0026
a Estimated standard errors on fitting paramaters: coordination numbers, ( 15%; radial distance, ( 0.02 Å for first shell,( 0.05 for higher shells; Debye-Waller factor, ( 0.005 Å2. b Coordination numbers are not corrected to a standard of known coordination number. c The Debye-Waller factor is the root-mean square deviation in absorber-scatterer distance and thus provides a measure of the degree of disorder in the average local coordination environment around atoms of the element of interest (Pb in this case). This disorder is a result of thermal disorder - vibration of atoms around an average position due to thermal motion, decreases with decreasing temperature; and site disorder - how uniform the average position (coordination distances) of atoms are in a given coordination shell.
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thicker layer of material sandwiched between two pieces of Kapton tape, while the chloropyromorphite sample consisted of multiple thin layers of material separated by tape folded onto itself). The frequencies of oscillations in the EXAFS spectra in Figure 5 show a high degree of similarity between the two samples, even out to a wavevector of 18 Å-1. These results provide strong evidence that the soil extraction residue was dominated by chloropyromorphite. The unique peak at 4.4 Å-1 in the chloropyromorphite spectrum is likely a multiple scattering peak (28). Overall, the EXAFS data were consistent with the sequential extraction data indicating that incubation of contaminated soil with hydroxyapatite for 240-d did alter the chemical speciation of soil Pb. The fact that the extraction residue contained chloropyromorphite provided supporting evidence for the formation of chloropyromorphite during the incubation; however, we cannot rule out the possibility that chloropyromorphite formed as a result of the chemical extraction itself.
Discussion The EXAFS spectra for the 0-d incubated soil treated with hydroxyapatite and the control soil imply that they are not different. In contrast, the sequential extraction data indicates the forms of soil Pb contained in the two samples are different. The high ratio of water to solid, relatively low pH, and continuous agitation in the extraction process created a environment favorable for both soil Pb and apatite P release into the extraction solution. Reported data (9, 15, 24, 25, 29) illustrate that the precipitation of pyromorphite is rapid when both Pb and P are in the solution. Thus, pyromorphite formation during sequential extraction offers a plausible explanation for the differences observed by the two measurement techniques. If so, then the use of sequential extractions to determine alterations of the Pb minerals present in amended samples are inappropriate, and other techniques that are able to quantify solid Pb species without extraction are required. The hypothesis that various fractions of soil Pb were transformed into chloropyromorphite and remained in the residual Pb fraction seems apparent from the results obtained from the dialysis experiment (Figures 1 and 2), the sequential extraction experiment (Figure 3), and the EXAFS spectra for the 240-d incubated soil treated with hydroxyapatite and the extracted soil residue (Figures 4 and 5). But at this time it is difficult to quantify the rate and extent of the transformation in more than very general terms. A comparison of the sequential extraction for 0-d and 240-d incubation of the hydroxyapatite treated soil illustrated that changes in soil Pb occurred during the incubation. If one accepts the difference in the residual fraction of the sequential extraction as the amount converted to pyromorphite, then at least 10% (difference between hydroxyapatite amended samples at 240-d and 0-d) and maybe as much as 45% (difference between hydroxyapatite amended samples at 240-d and control at 0-d) of the soil Pb had been altered during the incubation. The EXAFS spectra for the 240-d incubated soil treated with hydroxyapatite indicated that this treatment induced changes in the average, local molecular bonding of soil Pb. The extracted soil residue had an RSF and fitting results that were similar to chloropyromorphite indicating that chloropyromorphite may have formed during incubation, but its formation during the sequential extraction to obtain the residue cannot be ruled out. The confirmation of chloropyromorphite formation in the dialysis experiment supports the hypothesis that formation occurred during the incubation.
Acknowledgments This research was supported in part by an appointment to the Postdoctorate 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 US. Department of Energy and 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 12, 2001. Revised manuscript received July 11, 2001. Accepted July 11, 2001. ES010634L
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