Environ. Sci. Technol. 2009, 43, 711–717
X-ray Absorption Near Edge Structure Study of Lead Sorption on Phosphate-Treated Kaolinite ROBERT W. TAYLOR,† WILLIAM F. BLEAM,‡ T H I L I N I D . R A N A T U N G A , * ,† CRISTIAN P. SCHULTHESS,¶ ZACHARY N. SENWO,† AND DON RUFUS A. RANATUNGA§ Department of Natural Resources and Environmental Sciences, P.O. Box 1208, Alabama A&M University, Normal, Alabama 35762; Department of Soil Science, University of Wisconsin—Madison, 1525 Observatory Drive, Madison, Wisconsin 53706; Department of Plant Science, University of Connecticut, 1376 Storrs Road, U-4067, Storrs, Connecticut 06269; and Department of Chemistry, Oakwood University, 7000 Adventist Boulevard, Huntsville, Alabama 35896
Received July 20, 2008. Revised manuscript received November 14, 2008. Accepted November 19, 2008.
Presorbed phosphate significantly increases Pb sorption on the phyllosilicate kaolinite in the pH range from 4 to 8. The sorbed Pb-to-P molar ratios over this pH range stray little from the molar ratio found in the mineral pyromorphite, suggesting sorbed phosphate reacts with soluble Pb to form a surface precipitate similar to pyromorphite. X-ray absorption near edge structure (XANES) studies at the Pb L3-edge support this interpretation. In particular, the fine structure of first-derivative Pb L3-edge XANESspectraofPbspeciessorbedtophosphate-treatedkaolinite samples covering a pH range extending from 4 to 10 match the fine structure of the spectrum of pyromorphite. Although the productisstructurallyandcompositionallysimilartopyromorphite, the ion activity product in the pH range 4-6 was undersaturated with respect to the solubility product of pyromorphite.
Introduction Environmental chemists have long recognized the potential benefit of using phosphate to reduce biological availability of Pb in soil (1-3). The desired outcome is the precipitation of an insoluble Pb phosphate mineral known as pyromorphite, Pb5(PO4)3(OH,Cl). This precipitation reaction would yield pyromorphite particles as a separate phase and result in a soil solution saturated with respect to this phase. Another reaction is possible where much of the reactive phosphate remains sorbed to mineral surfaces and a product with structure similar to pyromorphite forms at the mineral-water interface. Phosphate sorbed to mineral surfaces tends to enhance metal sorption from solution. The enhanced pH-dependent sorption of Pb occurs in the acidic pH range where sorbed phosphates levels are highest (4-6). Similar behavior is * Corresponding author phone: 256-372-4225; fax: 256-372-5429; e-mail:
[email protected]. † Alabama A&M University. ‡ University of Wisconsin—Madison. ¶ University of Connecticut. § Oakwood University. 10.1021/es8020183 CCC: $40.75
Published on Web 01/09/2009
2009 American Chemical Society
observed with Cd, known to form an insoluble phosphate of the apatite mineral class (7, 8). There is also evidence that adsorbed phosphate will desorb as it reacts with high levels of soluble Pb to form pyromorphite crystals as a separate phase (5). The formation of surface precipitates by Pb and other metals at oxide mineral surfaces makes it very likely phosphate sorbed at the surface of kaolinite will also react with Pb to form a surface precipitate. Surface precipitates are difficult to identify by conventional X-ray diffraction or electron microscopy given their inherently small particle size. Weesner and Bleam (4) used Pb L3-edge extended X-ray absorption fine structure (EXAFS) to identify surface precipitates with a structure resembling pyromorphite at the surface of goethite and boehmite. Rapid Pb-207 isotope exchange kinetics demonstrated the ultrafine particle size of these pyromorphite surface precipitates (4). Ler and Stanforth (9) inferred the formation of a pyromorphite surface precipitate from Pb-to-P mole ratios adsorbed to the surface of goethite but lacked structural data to confirm the product. Yoon et al. (10) used EXAFS spectroscopy and magnetic susceptibility to demonstrate ultrafine rare-earth phosphate surface precipitates form when lanthanide cations adsorb to phosphated-boehmite. Kaolinite is usually the most abundant phyllosilicate mineral in highly weathered soils such as ultisols and oxisols and is one of the primary sinks for phosphate in these soils. Hence, it may play an important role in immobilization of Pb (6). Previous studies indicate that phosphates may be strongly sorbed onto kaolinitic clays via an inner-sphere mechanism and also by formation of Al-P surface precipitates (11, 12). The kaolinite surface differs significantly from the aluminum and iron oxide minerals examined in earlier studies making it impossible to predict whether surface precipitation of pyromorphite occurs at the kaolinite-water interface and, if so, under what conditions. Low EXAFS intensity (13) at the Pb L3-edge combined with Pb surface excesses on the order of 1 µmol m-2 makes any Pb EXAFS study challenging even when the oxide adsorbent has a surface area in excess of 100 m2 g-1. The specific surface area of kaolinite, which cannot be synthesized, is a full order of magnitude lower than the goethite, boehmite, and smectite adsorbents used previously (14, 15). This study measures the effect of phosphates on Pb sorption in the pH range 4-10, relying on Pb L3-edge X-ray absorption near edge structure (XANES) spectra and surface excess mole ratios to determine the nature of the product that forms at the kaolinite surface.
Experimental Methods Mineral Samples. The kaolinite used for this study was kaolinite KGa-1 (Warren County, GA) obtained from the Clay Minerals Society reference clay collection. Surface area (N2 /BET) of the clay is 10.05 ( 0.02 m2 g-1 (16). The National Museum of Natural History supplied the minerals used as references: Pyromorphite (no. 147776, Merkur Mine, Ems, Germany), Senegalite (no. 137180, Kouroudiako Iron Deposit, Falema River, East Senegal), Variscite (no. 87484-5, Lucin, UT). These samples were ground to pass a 270-mesh sieve. The kaolinite exchange complex was saturated with Na+ by suspending the clay in 1 M NaCl solution, and washing repeatedly with distilled, deionized water until the supernatant tested negative for chloride using the silver nitrate test (17). Sorption reactions were carried out under nitrogen in 0.02 M NaNO3 electrolyte solution with intermittent shaking on a wrist action shaker at room temperature (22.3 ( 1 °C). All reagents were purged with nitrogen gas prior to sorption VOL. 43, NO. 3, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Phosphate and Lead Sorption by Kaolinite KGa-1 as Influenced by pH final pH 4.47 5.92 7.53 10.10 4.45 6.31 7.86 9.65 3.85 5.81 7.73 9.88 a
sorbed phosphate, µmol m-2
sorbed lead, µmol m-2
sorbed Pb-to-P molar ratio
0.495a 0.790a 1.086a 1.112a 0.263b 0.239b 0.171b 0.028b 0.544c 0.564c 0.549c 0.364c
0.977c 1.103c 1.118c 1.156c b
1.80 1.95 2.04 3.18 c
No phosphate added. No lead added. Phosphate added first and allowed to reach sorption equilibrium before addition of lead.
FIGURE 1. The pH dependent phosphate sorption by kaolinite KGa-1 in the absence and presence of Pb, open and filled squares respectively. Each data point is the average of three replicates and the scale prohibits plotting error bars representing the 95% confidence interval. experiments. Clay suspensions in the reaction tubes were also purged with nitrogen gas. All sorption experiments described below were carried out in securely capped and sealed tubes under nitrogen environment to exclude CO2 from the system. Phosphate Sorption Envelope. The kaolinite KGa-1 (0.5 g) was suspended in polyethylene centrifuge tubes containing 3 mg L-1 P (9.7 × 10-5 M PO43- in the form of KH2PO4) in 0.02 M NaNO3 (30 mL). The pH value of each suspension was immediately adjusted with 5 M HNO3 or 5 M NaOH, and the sorption reactions were carried out under a nitrogen environment by agitating on a wrist-action shaker for 24 h since preliminary experiments indicated that the system reached equilibrium within 24 h. Finally, the suspension solutions were centrifuged and the supernatant solutions were carefully decanted and separated for chemical analysis. Control experiments were carried out under same conditions in centrifuge tubes without addition of clay to observe formation of any solid phases at a particular pH. Lead Sorption Edge. The kaolinite KGa-1 suspensions were prepared as described above using 30 mL of 0.02 M NaNO3 containing 39 mg L-1 Pb (1.89 × 10-4 M) in the form of Pb(NO3)2. The Pb sorption interval was 24 h (which allows system to reach equilibrium): Pb(NO3)2 addition and pH adjustment at t ) 0 h followed by continuous suspension agitation on a wrist-action shaker, and centrifugation at t ) 24 h, and carefully decanting the supernatant for chemical 712
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FIGURE 2. The pH dependent Pb sorption by kaolinite KGa-1 in the absence and presence of phosphate, open and filled squares respectively. Each data point is the average of three replicates and the scale prohibits plotting error bars representing the 95% confidence interval. analysis. Control experiments were carried out under same conditions in centrifuge tubes without addition of clay to observe formation of any solid phases at a particular pH. Combined Phosphate–Lead Sorption Envelope. The kaolinite KGa-1 suspensions for phosphate adsorption were prepared as described above. Following an interval of 24 h, during which the suspensions were continuously agitated, Pb(NO3)2 stock solution was added to the suspension to yield a Pb concentration of 39 mg L-1 (1.89 × 10-4 M). The combined phosphate–lead sorption interval was 48 h: KH2PO4 addition and initial pH adjustment at t ) 0 h followed by continuous suspension agitation on a wrist-action shaker, Pb(NO3)2 addition and a second pH adjustment at t ) 24 h followed by continuous suspension agitation on a wrist-action shaker, and centrifugation at t ) 48 h to collect supernatant solution samples. Chemical Analysis and Solution Speciation. Determination of dissolved Pb in the supernatant solutions was carried out by inductively coupled plasma-optical emission spectroscopy (ICP-OES). The Murphy and Riley (18) colorimetric method was used for chemical analysis of dissolved phosphate. Sorption was calculated as the difference between initial and final concentrations of dissolved Pb and phosphate. Simulation of ion speciation in the equilibrium solutions was performed using MINTEQA2, Version 1.5 (Allison Geoscience Consultants, Inc.). The distribution of Pb and phosphate species in 0.02 M NaNO3 solution was calculated based on total soluble Pb, total soluble phosphate at fixed equilibrium pH values at 23 °C. X-ray Absorption Data Collection. The Pb L3-edge X-ray absorption near edge structure (XANES) spectra were collected at the National Synchrotron Light Source (Brookhaven National Laboratory) on beamline X10C. The number of scans was scaled according to the Pb content of the sample to achieve a uniform signal-to-noise ratio over kaolinite samples. The number of scans ranged from 13 for samples with the highest Pb contents to as many as 40 scans for the sample with the lowest Pb content. Each data scan required about 16 min and ranged from 13 005 to 13 155 eV. Powdered samples were mounted in plexiglass sample holders, tamped, and sealed with Kapton tape. Fluorescent intensity was measured using a 13-element solid-state detector with simultaneous transmission scanning of Pb foil for calibration using an argon-filled gas ionization chamber. The Pb L3edge XANES spectrum of the reference mineral pyromorphite (National Museum of Natural History mineral sample no. 147776) has been published previously (4). X-ray Absorption Data Analysis. Analysis of all XAS data relied on the Athena public-domain application, a compo-
FIGURE 3. A. The Pb L3-edge stacked fluorescent XANES spectra of Pb sorbed by kaolinite KGa-1 at about pH 4. Actual pH values, total sorbed phosphate and total sorbed lead appear in Table 1. Open and filled circles indicate the absence and presence of cosorbed phosphate respectively. The data points represent the merging of numerous scans while the smooth lines plotted in each spectrum represent the three-point smoothing of the data points. B. The Pb L3-edge fluorescent postedge XANES spectra between 13070 and 13110 eV of Pb sorbed by kaolinite KGa-1 at about pH 4. Actual pH values, total sorbed phosphate and total sorbed lead appear in Table 1. Open and filled circles indicate the absence and presence of cosorbed phosphate, respectively. The data points represent the merging of numerous scans while the smooth lines plotted in each spectrum represent the three-point smoothing of the data points. nent of the IFEFFIT XAS Analysis package (19). Athena performs background correction, normalization, and data smoothing. Data smoothing employs the three-point smoothing algorithm of Athena. The XANES spectrum of pyromorphite (4) did not require data smoothing to discern fine structure.
Results and Discussion Sorption Envelopes. Results from the sorption envelope studies appear in Table 1 and Figures 1 and 2. These results clearly indicate an enhancement of Pb sorption on the surface in the presence of phosphate. Based on speciation simulations using MINTEQA2, the system is undersaturated with
respect to potential Pb solid phases, namely Pb(OH)2 and hydroxylpyromorphite, in the pH range between 4 and 6. Thus, the loss of Pb and phosphate from solution in the pH range from 4 to 6 occurs strictly by sorption to the kaolinite surface. We estimate
