Environ. Sci. Technol. 1994, 28, 408-418

Effects of N03-, CI-, F-, sol-, and C032- on Pb2+ Immobilization by Hydroxyapatite

Qi Ying Ma,' Terry J. Logan, and Samuel J. Traina

Department of Agronomy, The Ohio State University, Columbus, Ohio 43210

James A. Ryan

RREL, U.S. EPA, Cincinnati, Ohio 45268

Remediation of Pb-contaminated wastes has received considerable attention recently. We have previously shown that hydroxyapatite [Ca5(P04hOH] can reduce Pb2+ concentrations below the EPA action level (72.4 nmol L-1) and, thus, has the potential for in situ Pb2+ immobil­ization against leaching. This research investigated the effects of N03-, CI-, F-, 8042-, and C032- on hydroxyap­atite-Pb2+ interactions. 80lutions containing initial Pb2+ concentrations of 24.1-482 fimol L -1 were reduced to below 72.4 nmol L-1 after reaction with hydroxyapatite, except in the presence of high levels of C032- and Pb2+. Concen­trations of CI-, F-, and 8042- decreased, whereas N03- and C032- concentrations were unchanged after reaction with hydroxyapatite. Hydroxypyromorphite [Pb5(P04hOH] precipitated after the reaction of hydroxyapatite with Pb2+ in the presence of N03-, 8042-, and C032-, while chloropy­romorphite [Pb5(P04hCl] and fluoropyromorphite [Pb5-(P04hF] formed in the presence ofCI- and F-, respectively. The ability of hydroxyapatite to rapidly remove Pb2+ from solution in the presence of high levels of N03-, CI-, F-, 804

2-, and C032- demonstrates its great potential for reducing the environmental impact ofPb2+ -contaminated wastes.

Introduction

Lead is a widespread constituent of the earth's crust. Its concentration in soil ranges from 2 to 200 mg kg-1 and averages 16 mg kg-1 (1). Due to its long-term and extensive use, Pb2+ is a major contaminant of solid wastes and soils, especially in landfills. Existing and abandoned disposal sites are a potential source of groundwater and surface water pollution. According to The Conservation Foun­dation (2), Pb2+ levels in 10 % of the nation's rural wells exceeded the EPA's drinking water standard of 241 nmol L-1. Concerns over water pollution from heavy metal­contaminated landfills have generated tremendous interest in developing technologies that can cleanup these con­taminated waste sites. Much of the attention in recent years has focused on Pb2+ among all the heavy metals due to its potential hazard to the environment and human health.

Phosphate forms insoluble lead orthophosphates after reaction with Pb, and both aqueous P and hydroxyapatite (HA) [Ca5(P04>sOH] have been used to treat Pb2+­contaminated wastes and water (3-7). Among all inorganic P sources, apatites are the most economical to use because of their ready availability and low cost. Hydroxyapatite has been studied as a cation exchanger to remove heavy metals from wastewater (5-7). We have recently shown

• Address correspondence to this author at her present address: Soil and Water Science Department, Institute of Food and Agri­cultural Sciences, University of Florida, Gainesville, FL 32611-0510.

408 Environ. Sci. Techno!., Vol. 28, No.3, 1994

that HA can effectively attenuate aqueous Pb2+, ex­changeable Pb2+, and Pb2+ in contaminated soil material (8). Hydroxyapatite reduced initial dissolved Pb2+ con­centrations of 24.1-2410 fimol L-1 to below 96.5 nmol L-1 after 0.5 h. Aqueous Pb2+ in Pb-contaminated soil material was reduced from 11.0 to 0.17 fimol L-1 after reacting with HA for 5 h. 80lution pH, initial aqueous Pb2+ concentra­tions, and especially aqueous P concentrations are impor­tant factors in the Pb2+ immobilization process (8). Hy­droxypyromorphite (HP) precipitation and HA dissolution were the main mechanisms for removal of aqueous Pb.

Before HA can be successfully used as a Pb-immobilizing material, three factors need to be considered. Hydroxy­apatite has to be able to immobilize Pb2+ in the presence of interfering cations, anions, and dissolved organic matter; the reaction products have to be stable in the contaminated environment; and the reaction should be rapid. Numerous studies have investigated the effects of ions on the properties of HA, but little information is available on HP (9--11). There are three types of substitutions that can occur in HA or HP structures. The cations Pb2+, Ba2+,

Zn2+, Fe3+, and Mg2+ can substitute for Ca2+, while the oxyanions AS043-, V043-, C032-, and 8042- can replace structural P043-. Additionally, anions such as F- and CI­can exchange with OH- (12). Thus, when HA reacts with Pb2+ in the presence of CI- or F-, the following minerals can form: chlorapatite (CA) [Ca5(P04)3Cl], fluorapatite (F A) [Ca5(P04hF], chloropyromorphite (CP) [Pb5(P04h­Cl], and fluoropyromorphite (FP) [Pb5(P04)3F]. In the case ofC032- and 8042-, incomplete replacement ofP043-by C032- or 8042- can result in the formation of Ca5-(P04,C03hOH, Ca5(P04,804hOH, Pb5(P04,C03hOH, or Pb5(P04,804hOH. In the present study, HA was reacted with Pb2+ in the presence of N03-, CI-, F-, 8042-, or C032-,

Thus, both substitution and dissolution/precipitation reactions involving these anions were possible.

Hydroxyapatite is noted for its isomorphous substitu­tion. Both CI- and F- can substitute for OH- in apatite. We found in previous work that N03- had no effect on Pb-HA interaction and did not interact with HA, and thus it was included as a control in this experiment (8). Fluorapatites are the most abundant mineral apatites and have received extensive study. They have larger crystallite size and are thermaly and chemicaly more stable than HA (12). No significant effects on the crystallinity of apatites have been observed as a consequence of CI- incorporation; however, the large unit cell volume of CA suggests that CI­contributes to the instability of apatite (12). Among all apatite substitutions, that by C032- is considered the most common. This is attested to by the abundance of carbonate apatite [(Ca,Na,Mg)s(P04,C03,804,F)aF] in nature (13) . Carbonated apatite has smaller crystallite size, higher solubility, and lower thermal stability than HA (12).

0013-936X/94/0928-0408$04.50/0 © 1994 American Chemical Society

Table 1. Initial Pb2+ and Anion (NOa-, CI-, F-, 8042-, and

COa2-) Concentrations at Different Anion/Pb2+ Molar Ratios

initial Pb2+ eon en anion conen (f.Lmol L-l)

(f.Lmol L-l) 2 4 8 12

24.1 48.2 96.4 193 289 121 241 482 964 1450 241 482 964 1930 2890 482 964 1930 3860 5780

Substitution of S042- for P04a- is generally much less important than COa2- substitution, and such substitution does not occur to a significant extent (12). One example of S042--substituted apatite is wikeite [Ca5(P04,S04,­Si04h(F,OH)] (13). Sulfate-substituted apatite has lower chemical stability than HA and has similar crystal size.

Hydroxypyromorphite, like HA, shows a remarkable tendency to form solid solutions with respect to anions by the substitution of F- or Cl- for OH-. The stability sequence for the pyromorphite series is as follows: Pb5-(P04hCl> Pb5(P04)aOH > Pb5(P04hF (14). In contrast, the stability sequence for apatite is as follows: Ca5(P04>aF > Ca5(P04hCl > Ca5(P04hOH (15). Such differences in stability are probably related to the crystal-chemical characteristics of these two groups of compounds, which suggest that the results obtained by studying apatite may not apply to pyromorphite. Isomorphous substitution has been suggested as the process for the formation of CP and FP from the reaction of aqueous Pb2+ with HA in the presence of Cl- and F- (5-7). However, HA dissolution followed by CP or FP precipitation may also occur, as we have previously observed for HP (8). There are few reports on the substitution of S042- or C032- for P043- in pyromorphite. Such substitution is considerably less in the pyromorphite series of related minerals than the apatite series (16). In the pyromorphite series, Cl- is the major halide ion, as compared to F- and OH- in the apatite series (16).

This research was conducted (1) to determine the effects of NOa-, Cl-, F-, S042-, and C032- on Pb2+ immobilization by HA; (2) to examine the reaction products; and (3) to investigate the mechanisms of those reactions.

Experimental Section

Experimental Procedures. Different concentrations of Pb2+ were reacted with HA in the presence of various levels of N03-, Cl-, F-, S042-, or C032- to test the effects of these anions on Pb2+ immobilization by HA. Solutions of PbCh and PbF2 were used as Pb2+ sources for the Cl­and F- treatments, and Pb(N03h solution was used for the N03-, S042-, and C032-treatments. Sodium salts were used to supply N03-, Cl-, F-, S042-, and C032-. A sample of 0.1 g of HA (Bio-Rad) [see Ma et al. (ref 8) for a description of these materials] was reacted with 200 mL of solution containing 24.1,121,241, and 482 t-tmolofPb2+ L-l as Pb(N03h, PbCI2, and PbF2, respectively. At each Pb2+ level, four different anion concentrations were used; 2, 4, 8, and 12 times the respective initial Pb2+ molar concentrations (Table 1). All solutions were adjusted to pH 6 with dilute HN03, HCI, HF, or NaOH. The suspensions were shaken for 2 h and then filtered with 0.2-,um Nucleopore polycarbonate membrane filters. The filtrates were analyzed for total P043-, Pb2+, Ca2+, N03-,

Cl-, F-, S042- and C032-, and solution pH. The solid phases

were analyzed by X-ray diffraction (XRD) and scanning electronic microscopy (SEM). Measured total POl-, Ca2+, Pb2+, N03-, Cl-, F-, S042- and C032- concentrations and solution pH were used as inputs to the MINTEQA2 equilibrium speciation model (17). Due to the variation reported for the equilibrium constantK, 1/20 log K around the saturation index = 0 was used as the accepted uncertainty (18). Equilibrium constants of 10-143.3,10-106.2, and 10-44.35 were added to the MINTEQA2 database for FP [Pb5(P04hF], CA [Ca5(P04hCl], and glauberite [Na2-Ca(S04)2], respectively (12, 17, 19, 20).

We hypothesize that Pb2+ was precipitated as CP or FP when reacted with HA in the presence of Cl- and F-, and thus HA simply acted as the P source. To elucidate such a mechanism, aqueous P was reacted with Pb(N03)2 in the presence of Cl- and F- at controlled pH. A total of 0.08 g of NaH2P04 was mixed with 0.07 g of NaCI or 0.05 g of NaF in 50 mL of distilled deionized water. Solution pH was adjusted to 3, 5, 7, and 9 with a Mettler DL 70 autotitrator set in the pH-stat mode using dilute HCl, HF, or NaOH. Then 50 mL of 19.3 mmol L-1 Pb(N03h solution was added slowly to the above solution while the pH was maintained at a desired level and the solutions were stirred for 10 min. The suspensions set overnight before being washed and oven dried at 110 DC. The samples were then analyzed by XRD.

To further demonstrate that CP and FP were formed mainly through precipitation/dissolution instead of sub­stitution, both CI- and F-of the same initial concentrations as PbCl2 or NaCI and PbF2 or NaF were reacted with 0.1 g of HA. The initial CI- and F- concentrations were 48.2, 96.4, 193, and 289 ,umol L-1, respectively. The filtrates were analyzed for total CI-, F-, P043-, Pb2+, and pH at the end of 2 h of shaking.

Analytical Methods. A Perkin-Elmer 3030B atomic absorption spectrophotometer was used to analyze total Ca2+ concentrations and Pb2+ concentrations >4.8 ,umol L -1; a Varian SpectrAA-20 atomic absorption spectrometer equipped with a graphite furnace atomizer was used to measure Pb2+ concentrations <4.8 ,umol L-1. Total NOa-concentrations were determined with a Lachat Quickchem autoanalyzer (Method 10-107-04-1-A). Total dissolved P043-was measured colorimetrically with a Beckman DU-6 spectrophotometer as ascorbic reduced phosphomolybdate (21). The concentrations of F- and Cl- were analyzed by ion chromatography on a Dionex ion chromatograph 2000i. A Dionex Fast-Sep Anion-1 column was used, and the retention times for F- and Cl- were 1.12 and 2.09 min, respectively. The flow rate was 2.1 mL/min. Solution pH was measured with an Orion/Ross combination elec­trode and an Orion EA 920 pH meter. All experimental treatments in this study were prepared in triplicate and were conducted in acid-washed (0.1 M HCl) polycarbonate labware.

All XRD analyses were conducted with a Philips X-ray diffractometer (Philips Electronic Instrumentation Co., Mahweh, NJ) using Cu K-a radiation at 35 kV and 20 mAo Measurements were made using a step-scanning technique with a fixed time of 4 s/0.05° 28. A total of 601 data points were obtained from 15 to 450 28. All XRD analyses were performed using back-filled, randomly oriented mounts. Selected samples were observed with a JEOL JSM-A20 scanning electronic microscope (SEM; JEOL, USA Inc., Peabody, MA) using a Tracor Northern (5500) X-ray analyzer equipped with a 5502 upgrade. The samples were

Environ. Sci. Technol., Vol. 28, No.3, 1994 409

Table 2. ~nitial and Final Pb2+ Concentrations after Reaction with Hydroxyapatite in the Presence of Different -Anion/Pb2+ Molar RatIOs

initial Pb2+ anion/Pb2+ (fJmol L-l) molar ratios NOa- CI-

24.1 2 0.82:i: 0.34 0.68:i: 0.43 4 0.19 :i: 0.34 14.8:i: 7.58 8 10.1:i: 17.4 31.3:i: 13.8

12 1.64:i: 2.85 3.04:i: 3.52 121 2 1O.3:i: 0.72 2.80:i: 2.70

4 12.2:i: 3.96 6.66:i: 4.87 8 0.00:i: 0.00 30.9:i: 13.8

12 0.82:i: 2.85 3.04:i: 3.52 241 2 12.8:i: 10.6 14.8:i: 7.58

4 4.68:i: 2.51 11.4:i: 3.14 8 16.3:i: 23.7 0.82 :i: 0.72

12 0.00:i: 0.00 2.80:i: 0.68 482 2 13.2:i: 3.57 1.88:i: 0.53

4 6.18:i: 5.36 0.53 :i: 0.48 8 7.05:i: 5.50 15.3:i: 0.24

12 3.72:i: 3.48 7.38:i: 5.16

mounted on a stainless steel stub using double-stick tape and then coated with Au and Pd.

Results

Effect of NOa- on Pb2+ Immobilization by HA. Dissolved Pb2+ concentrations were below 17.0 nmol L-1 after HA reacted with 24.1-482 !lmol L -1 Pb2+ in the presence of different levels of N03- (Table 2). Nitrate had little effect on final Pb2+ concentrations as previously shown (8). The XRD data showed that HP was present together with excess HA (Figure IB). The peak intensities of HP increased with an increase in initial Pb2+ concen­trations, indicating more formation of HP at higher Pb2+ concentrations, and they did not change with varying N03-concentrations (data not shown). Typical SEM micro­graphs of HA and HP are presented in Figure 2. Clearly, the morphology of HA (flake shaped) was different from that of HP (needle shaped).

Phosphate concentrations and solution pH decreased, while Ca2+ concentrations increased with an increase in initial Pb2+ concentrations, and they did not change with an increase in a N03-jPb2+ molar ratio (Table 3). Nitrate concentrations did not change after HA reacted with Pb­(N03)2 (data not shown), suggesting that N03- was not present in solid phase. Saturation indices of the filtrates calculated from MINTEQA2 indicated that the solution was in equilibrium with respect to HA and undersaturated with respect to HP (Table 4).

~ ::l

8

i Q) CI:

final Pb2+ con en (nmol L -1 )

1400

1200

1000

800

600

400

200

F- 8042-

6.42 :i: 4.05 2.41:i: 1.83 3.23:i: 1.93 7.14:i: 5.74 0.77 :i: 0.82 22.3:i: 9.89 6.52:i: 1.64 6.56:i: 5.98 2.27 :i: 1.83 20.6:i: 15.8 3.86:i: 1.88 32.6:i: 22.3 4.20:i: 1.79 6.66 :i: 3.14 7.87:i: 2.56 7.77 :i: 4.20

15.6:i: 6.42 24.4:1: 11.1 17.7:i: 8.98 16.2:i: 6.56 14.3:i: 5.94 11.6 ± 4.87 35.8:i: 12.7 9.32:i: 9.12

29.1 ± 14.1 1.93 :i: 0.39 47.3:i: 1.06 12.9:i: 9.22 44.4:i: 8.35 1.88:i: 1.06 60.8:i: 8.40 0.43:i: 0.63

A: hydroxyapatite (Ca1O(PO.)6(OH)2)

H: hydroxypyromorphite (Pb,0(PO.)6(OH)2)

C: hydroxypyromorphite (Pb,0(PO.)6CI2)

F: fluoropyromorphite (Pb,0(PO.).F2

)

A A

$: glauberite (wNa2ca(SO.i2i A A

A A A H Na

2Co

3 A

s H A

020

COa2-

8.21 :i: 5.41 10.8:i: 6.81 16.8:i: 5.21 26.5:i: 25.9

57.4 ± 24.0 35.9:i: 17.6

169:i: 40.1 121 ± 39.0

96.0:i: 9.75 127:i: 23.1 46.0:i: 7.53

134:i: 8.16

87.4 ± 12.1 93.6:i: 21.0

277:i: 53.6 343:i: 10.7

A A

45 50

Effects of Cl- and F- on Pb2+ Immobilization by HA. Chloropyromorphite and FP were formed after aqueous P043- reacted with Pb2+ in the presence of CI­and F- at pH 3, 5, 7, and 9, respectively (Figure 3). The peak intensities of both CP and FP were highest at pH 3, second highest at pH 7, and lowest at pH 5 and 9. A weakly crystallized CP was formed at pH 9 with broad peaks. The morphology of CP was different from those of both HA and HP and appeared as irregularly shaped spheres adhering to HA surface (Figure 4). The shape of the original HA crystal could still be seen. The morphology ofFP resembled that of HP, but with fewer needle-shaped crystals. Loose particles of HA were observed together with CP or FP.

Figure 1. Reaction products of hydroxyapatite with 482 !lmol L-l Pb2+ in the presence of 5.78 mmol L-l of N03-, CI-, F-, 804

2-, or C032-. XRD pattern of hydroxyapatite after reaction with water (A); XRD patterns of hydroxyapatite after reaction with Pb2+in the presence of N03- (6); CI- (C); F- (D); 804

2- (E); and C032- (F).

Significantly more CI- and F- were removed by HA from PbCl2 and PbF2 solutions than from NaCI and NaF solutions, respectively (Figure 5A,B). This difference

410 Environ. ScI. rechnol., Vol. 28, No.3, 1994

became larger at higher initial Cl- and F- concentrations. The ranges of CI- and F- concentration reduction by HA from PbCb and PbF2 were 12.1-83.5 and 43.2-362 !lmol L-1, and those from NaCI and NaF were 12.1-13.9 and 38.9-57.4 !lmol L -1, respectively (Figure 5A,B). Assuming that all of the CI- and F- from NaCI and NaF were incorporated as CA and FA and that an equivalent amount of HA was dissolved as a result, then at least 1.26 and

Table 3. Final PO,3- and Ca2+ Concentrations and Solution pH after HA Reacted with Pb2+ at Different Anion/Pb2+ Molar Ratios (N03-, CI-, F-, SO,2-, and C032-)

anion/Ph molar ratios

initial Pb2+ P043- (I'mol L-l) Ca2+ <lLmol L-l) solution pH (I'mol L-l) 2 4 8 12 2 4 8 12 2 4 8 12

N03-

0 68.8 (0.2)0 147.3 (0.94) 6.75 (0.02)b 24.1 62.7 (1.35) 64.3 (1.87) 65.1 (0.47) 66.0 (1.16) 185.1 (4.74) 192.6 (0.35) 193.0 (2.26) 185.4 (1.09) 6.65 (0.03) 6.65 (0.10) 6.70 (0.05) 6.69 (0.03)

121 57.8 (0.79) 56.3 (0.97) 57.6 (1.03) 60.2 (0.64) 399.2 (6.60) 395.0 (3.81) 390.1 (7.62) 379.2 (6.60) 6.48 (0.05) 6.52 (0.03) 6.54 (0.01) 6.48 (0.03) 241 53.6 (1.12) 55.3 (0.24) 57.6 (0.69) 56.4 (1.11) 690.3 (3.81) 682.0 (8.76) 686.1 (9.00) 674.5 (16.2) 6.36 (0.03) 6.44 (0.04) 6.35 (0.03) 6.43 (0.01) 482 53.6 (1.03) 53.1 (0.64) 55.9 (0.64) 58.4 (2.50) 1277 (15.8) 1274 (30.1) 1273 (9.00) 1275 (15.2) 6.23 (0.01) 6.28 (0.10) 6.22 (0.02) 6.25 (0.03)

CI-0 70.6 (2.52) 131 (5.74) 6.76 (0.10)

24.1 62.6 (0.64) 68.6 (2.10) 72.0 (0.00) 64.81 (1.48) 159 (3.24) 177 (2.74) 191 (0.00) 168 (5.49) 6.81 (0.08) 6.79 (0.03) 6.80 (0.06) 6.70 (0.09) 121 54.0 (2.10) 57.0 (0.00) 53.2 (1.68) 52.8 (1.68) 351 (2.74) 355 (0.00) 348 (0.50) 352 (1.25) 6.70 (0.11) 6.66 (0.08) 6.57 (0.09) 6.55 (0.02) 241 48.0 (5.06) 48.6 (1.68) 46.8 (3.80) 47.2 (2.10) 583 (6.49) 588 (5.74) 577 (1.25) 578 (1.75) 6.57 (0.05) 6.46 (0.01) 6.49 (0.09) 6.42 (0.05) 482 38.4 (2.52) 38.4 (0.00) 36.4 (0.64) 38.0 (1.68) 1088 (6.99) 1074 (3.49) 1069 (3.49) 1058 (2.74) 6.50 (0.03) 6.39 (0.06) 6.31 (0.02) 6.37 (0.10)

F-0 66.8 (2.10) 104 (2.99) 6.65 (0.03)

24.1 51.6 (1.68) 51.4 (1.06) 53.8 (1.68) 56.8 (0.42) 80.1 (0.50) 49.2 (2.99) 32.7 (1.00) 27.4 (0.75) 6.39 (0.05) 6.54 (0.09) 6.37 (0.04) 6.18 (0.05) 121 34.2 (1.68) 38.2 (1.06) 38.2 (0.42) 42.6 (0.84) 174 (3.99) 112 (3.49) 86.0 (4.24) 80.0 (3.74) 6.23 (0.02) 6.21 (0.05) 6.16 (0.01) 6.18 (0.05) 241 27.4 (2.10) 28.4 (0.42) 30.2 (0.84) 30.6 (1.26) 355 (5.49) 304 (5.49) 280 (4.49) 264 (11.0) 6.04 (0.04) 5.82 (0.11) 5.85 (0.10) 5.91 (0.08) 482 17.9 (0.42) 21.0 (2.32) 17.9 (2.10) 17.7 (2.94) 820 (3.99) 755 (19.7) 344 (55.4) 202 (54.1) 5.84 (0.04) 5.69 (0.04) 5.39 (0.07) 5.50 (0.16)

8042-

0 68.5 (1.58) 125 (8.28) 6.75 (0.05) 24.1 62.4 (0.94) 64.7 (1.03) 67.4 (0.70) 70.6 (1.45) 169 (5.75) 166 (6.93) 184 (7.00) 184 (11.1) 6.71 (0.00) 6.86 (0.06) 6.77 (0.09) 6.77 (0.06)

121 59.4 (0.95) 61.2 (1.58) 64.9 (1.37) 70.5 (1.69) 395 (28.8) 373 (6.18) 307 (5.77) 274 (1.18) 6.56 (0.03) 6.55 (0.03) 6.62 (0.05) 6.65 (0.07) m 241 57.0 (1.47) 62.1 (1.37) 71.8 (1.47) 72.1 (1.21) 422 (9.53) 300 (6.85) 319 (13.8) 275 (17.6) 6.47 (0.03) 6.60 (0.07) 6.66 (0.08) 6.69 (0.07) ;:, :5. 482 56.6 (1.21) 55.8 (1.58) 71.7 (0.47) 75.1 (0.69) 401 (34.5) 491 (32.3) 348 (33.4) 311 (23.7) 6.50 (0.04) 6.48 (0.04) 6.61 (0.04) 6.71 (0.01) a ? C032-en ~ 0 66.2 (5.49) 112 (4.49) 6.95 (0.03) -t 24.1 51.6 (2.63) 21.2 (2.82) 53.5 (3.70) 58.8 (2.82) 71.4 (8.73) 56.1 (13.0) 19.5 (8.72) 9.90 (6.81) 7.25 (0.01) 7.34 (0.11) 7.96 (0.33) 8.85 (0.10) (J) 0 121 10.9 (0.43) 23.6 (2.07) 57.5 (1.76) 59.1 (2.01) 39.2 (4.54) 10.5 (0.63) 6.57 (4.15) 18.0 (18.9) 7.77 (0.02) 9.23 (0.31) 10.1 (0.05) 10.4 (0.02) :::T ;:, 241 5.79 (0.81) 47.9 (1.38) 65.5 (3.10) 64.6 (0.31) 20.7 (1.00) 5.41 (2.50) 3.41 (7.30) 10.6 (1.80) 8.65 (0.53) 9.70 (0.17) 10.3 (0.02) 10.5 (0.02) ~ 482 0.77 (0.27) 42.0 (1.10) 60.3 (1.41) 60.0 (5.34) 11.1 (0.38) 8.48 (4.33) 14.7 (3.32) 10.7 (3.62) 9.32 (0.27) 10.2 (0.04) 10.5 (0.01) 10.6 (0.02) < fl o Values in parentheses are standard deviations of triplicate measurements. b Geometric means and standard deviations of triplicate measurements. !\)

!1> z ~

!" ~

<0 <0 .... .... .... ....

Figure 2. SEM micrographs of hydroxyapatite after reaction with water (A) and after reaction with 482 p.mol L-l of Pb(N03h in the presence of 5.78 mmol L-l of N03- (8).

3.92% of CA and FA, respectively, should have been present on a total solid mass basis at the end of the reactions, exceeding the theoretical XRD detection limit of 1 %. But neither CA and FA were detected by XRD (data not shown), indicating that Cl- and F- were not

present mainly as CA and FA. More F- was removed from solution than Cl- at the same initial concentrations. The final PO,3- concentrations and pH were lower in the presence of PbCl, and PbF, than NaCI and NaF (Figure 5C,D). In addition, lower PO,3- concentrations and pH were observed in the presence of PbF, than PbCl,. There were small differences in POl-concentrations and solution pH between NaCI and NaF treatments. Phosphate concentrations increased with an increase in initial F­concentrations, and they did not change with increasing Cl- concentrations (Figure 5C). Solution pH decreased with increasing concentrations of either Cl- or F- (Figure 5D).

In another experiment, dissolved Pb2+ concentrations ranged from 0.53 to 31.1 and from 0.77 to 60.8 nmol L - l after HA reacted with aqueous Pb'+ in the presence of different levels ofCl- and F-, respectively (Table 2). The final Pb2+ concentrations changed randomly with an increase in initial Pb2+ concentrations in the presence of Cl-; however, they increased with an increase in initial Pb2+ concentrations in the presence of F-, especially at higher initial Pb'+ concentrations (241 and 4821'mol L - l) and F- concentrations (Table 2). The XRD data show that CP and FP were formed after reacting HA with PbCI, and PbF, solutions, respectively (Figure 1 C,D). The peak intensities of CP and FP increased with an increase in initial Pb2+ concentrations but they did not change with a change in either Cl- or F- concentrations (data not shown). Neither CA nor FA was detected by XRD.

As in the NOa-system, PO,,3- concentrations and solution pH decreased and Ca2+ concentrations increased with increasing initial Pb2+ concentrations after HA reacted with Pb'+ in the presence ofCl- or F- (Table 3). However, Ca2+ concentrations increased with an increase in initial Pb2+ concentrations at lower F-/pb'+ molar ratios (2 and 4), and they decreased with an increase in initial Pb'+ concentrations at higher F-/Pb2+ molar ratios (8 and 12). Phosphate concentrations increased, while both Ca'+ and solution pH decreased with an increase in F-/Pb2+ molar ratios, and they stayed constant with an increase in Cl-I Pb' + molar ratios (Table 3). At each initial Pb2+ level, lower P043- and Ca2+ concentration and solution pH were

Table 4. Saturation Index of 482 ~mol L - I Pb2+ Solutions after Reaction with Hydroxyapatite in the Presence of Different Anion/Pb%? Molar Ratios

minerals, formula anion/ Pb2:+ molar ratios equilibrium anions and abbrev 2 4 8 12 constant (log K)(I ±1/ 20 log k

saturation indices NO,- C..,(PO.bOH (HA) 1.01 1.23 0.84 1.03 -58. )9 (19) ±2.91

Pb,(PO.bOH <HP) -5.37 -1>.88 - 7.02 -8.26 - 76.79 (19) ±3.84 CI- C..,(PO.hOH (HA) 1.10 0.30 -0.37 -0.02 -58.19 (19) ±2.91

Ca,(PO.bCI (CA) - 4.17 - 4.48 - 4.83 - 4.40 -53.08 (13) ±2.65 Pb, (PO.>sOH <HP) 2.15 -0.95 5.91 4.68 - 76.79 (19) ±3.84 Pb,(PO.hCI (CP) - 9.30 - 12.9 -1>.36 - 7.67 - 83.70 (10) ±4.19

F- C..,(PO.bOH (HA) - 4.80 -5.91 - 9.89 - lOA - 58.19 (19) ±2.91 C..,(PO.bF (FA) 1.80 1.28 -2.24 - 2.69 -58.86 (19) ±2.94 Pb, (PO.),OH <HP) -8.52 -8046 - 10.9 - 9.63 - 76.79 (19) ±3.84 Pb, (PO.bF (FP) - 8.48 - 7.83 - 9.83 - 8.50 - 71.63 (19) ±3.58 CaF2: 1.85 2.67 2.88 2.70 - 10.41 (19) ±0.52

80.2:- Ca, (PO.>,OH (HA) 0.52 0.57 0.67 0.84 -58. 19 (19) ±2.91 Pb,(PO.bOH (HP) - 7.08 -3.86 - 1.85 -8045 -76.79 (19) ±3.84 Na,Ca(SO.>, 26.2 28.4 29.5 30.3 44.35 (20) ±2.22

CO,'- Ca,(PO.bOH (HA) 0.97 7.55 9.14 8.38 - 58.19 (19) ± 2.91 Pb,(PO.bOH (HP) -3.50 -0.36 0.26 -0.44 - 76.79 (19) ±3.84

(I Number in parentheses refers to the reference.

.,2 Environ. ScI. Technol.. Vol. 28, No. 3, 1994

"' C ::> 0 u

'" ." ro Q; a:

1000

800

600

400

200 pH=3 A

o~~~~~~~~ 20 25 30 35 40 45 50

'400

'200 E

'000

800

600

400

200 pH = 3

25 30 35 40 45 50

0 29

Figure 3. XRD patterns of the reaction products of aqueous P with Pb(N03h in the presence of CI- at different pHs: (A) pH = 3, (8) pH = 5, (e) pH = 7. and (D) pH = 9; and in the presence of F- at different pHs: (E) pH = 3. (F) pH = 5. (G) pH = 7. and (H) pH = 9.

Figure 4. SEM micrographs of hydroxyapatite after reaction with 482 JJ.mol L -I of PbCI2 in the presence of 5.78 mmol L - 1 of CI- (A) and after reaction with 482 JJ.mol L- 'of PbF2 in the presence of 5.78 mmol L- ' of F- (8).

obtained in the presence of F- than with Cl-. Both Cl- and F- concentrations decreased after HA

reacted with aqueous Pb2+ , with higher initial anion

concentrations resulting in larger concentration decreases (Figure 6A,B). However, more F- was consumed than Cl­under the same conditions. The saturation indices in the Cl- system showed that the solutions were in near­equilibrium with HA, supersaturated with respect to HP, and undersaturated with respect to CA and CP (Table 4) . Those in the F- system showed that the solutions were in equilibrium with FA, supersaturated with respect to fluorite (CaF, ), and undersaturated with respect to HA, HP, and FP. Neither FA nor fluorite was detected by XRD.

Effects of 80.'- and C03' - on Pb'+ Immobilization by HA. Dissolved Pb'+ concentrations ranged from 0.43 to 32.6 and from 8.21 to 343 nmol L - I after HA reacted with aqueous Pb(N03l2 in the presence of different levels of SO,'- and C03' - , respectively (Table 2). Sulfate had little effect on final Pb' + concentrations; however, final Pb2+ concentrations increased in the presence of C032- ,

indicating that C03'- reduced the effectiveness of HA in immobilizing Pb' +. The XRD data show that HP was formed after HA reacted with Pb(N03l2 in the presence of SO,'- or C03'- (Figure lE,F). In addition to HP, glauberite [Na,Ca(SO,l2j was formed in the presence of SO,'- (Figure IE). Varying concentrations of SO,'- had no effect on the XRD patterns of HP (data not shown); however, higher COa2- concentrations resulted in smaller HP peaks. Trace amounts of HP could be seen in the SO,'- treatments, but no needle-shaped HP was observed in HA samples treated with C03'- (Figure 7 A,B).

Phosphate concentrations and solution pH increased and Ca2+ concentrations decreased with an increase in either SO,'-/Pb2+ or C03' -/Pb2+ molar ratios (Table 3). Phosphate concentrations decreased with increasing initial Pb'+ concentrations at low SO,'-/Pb2+ molar ratios (2 and 4) and increased at high SO,2-/Pb2+ molar ratios (8 and 12) (Table 3). Calcium concentrations increased, while solution pH decreased with increasing initial Pb2+ con­centrations in the presence of SO,'-. Phosphate concen­trations increased with an increase in initial Pb2+ con-

Environ. ScI. Technol., Vol. 28, No. 3, 1994 413

~

~ "0 E .5 c: .2 ca E OJ () c: 8 (3 .E c: 0

1:5 ::J

al a:

~ "0 E .5 c: .2 ca E OJ () c: 0 ()

cO

0" a. Cii c: iI:

100

A 80

60

40

20

180 340 500 660 820 980

Initial CI" concentration (j.lmol L")

90

80 ---£ ---- iII

70

60

50 \ \

40

~~ 30

20 -----C -----.,

10 20 180 340 500 660 820 980

~ 400,-------------______ -. "0 §. c: 300 o ~

1200 8 U. .5 100

8

20 180 340 500 660 820 980

Initial F' concentration (j.lmol L")

6.9,--------------......,

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C.6.5i~ § '~.

i 6.3 'iIt'~--ID-_

~ 6.1 iI:

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5.9 o

20 180 340 500 660 820 980

Initial cr or F' concentration (j.lmol L") Initial cr or F' concentration (j.lmol L")

-e-- NaCI

~ NaF

Figure 5. Concentration reduction of CI- and F- by HA from PbCI2 and PbF2 compared with those from NaCI and NaF; associated POl- concentrations and pH changes after HA reacted with PbCI2, PbF2, NaCI, and NaF.

500~----------~ 800 A cr B F' C SO 2,

4 ~ '0

800 AnionlPb2+

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molar ratios

If) c: o .~

al 300 '0

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600

600

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400

/ /

/ 200 // 200

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o 04T~~~~~~~~ o 1 00 200 300 400 500 0 100 200 300 400 500 100 200 300 400 500

Initial Pb2+ concentration (j.lmol L")

Figure 6. Concentration reduction of CI- (A), F- (8), and 8042- (C) by hydroxyapatite after reaction with Pb2+ at different anlon/Pb2+ molar ratios.

centrations except at COa2-/Pb2+2+molar ratio of2, at which they decreased (Table 3). Solution pH increased and Ca2+ concentrations decreased with an increase in COa2-jPb2+ molar ratios. At each initial Pb2+ level, P04a- concen­tration was lower in the presence ofCOa2-than with S042-. Sulfate concentrations decreased after HA reacted with Pb2+, and the decrease was proportional to the initial S042-concentrations, while COa2' concentrations did not change after the reaction (Figure 6C). The saturation indices in the S042- system showed that the solutions were in

414 Environ. Sci. Technol., Vol. 28, No.3, 1994

equilibrium with HA, undersaturated with respect to HP, and supersaturated with respect to glauberite (Table 4). The saturation indices in the COa2- system showed that the solutions were supersaturated with HA and were in equilibrium with HP (Table 4).

Discussion

Effect ofN03- on Pb2+ Immobilization by HA. We have demonstrated that Pb2+ removal by HA is mainly

Figure 7. SEM micrographs of hydroxyapatite after reaction with 482 ~mol L-' of Pb(N03h In the presence of 5.78 mmol L-' of SO.2- (A) and with 482 JJ.mol L- ' of Pb(N03h in the presence of 5.78 mmol L- ' of CO,2-.

through HA dissolution followed by HP precipitation in the presence of NO,- (8). However, Pb'+ adsorption by HA or Pb2+ substitution for Ca'+ on HA or HP cannot be ruled out at the present as alternate mechanisms. We also hypothesize that NO,- did not participate in the HA­Pb2+ interaction and, thus, has no effect on Pb2+ immo­bilization by HA. The following equations describe the reaction of HA with Pb(NO,), (8):

Ca,(PO,),OH(c) + 7H+ -diuolution

5Ca'+ + 3H,PO,- + H,O (1)

5Pb'+ + 3H PO - + H 0 2 4 2 ... precIpItatIon

Pb,(PO,),OH(c) + 7H+ (2)

Nitrate was not included in the above reactions, since no solid containing NO,- was formed. According to the above equations, the function of HA is to supply P for the formation of HP.

X-ray diffraction data indicate that HP was the product when HA reacted with aqueous Pb'+ in the presence of NO,- (Figure 1B). When Pb(NO,)z reacts with aqueous P, Pb,(NO,)(PO,) may form (22). However, this mineral was not detected by XRD in the study. In addition, SEM micrographs show that HP was a separate phase formed near the HA surface (Figure 2B). It appears t hat most of the precipitation occurred near the HA surface where the solubility constant of a Pb- P mineral was probably

exceeded due to locally high P concentrations. The fact that PO,3- concentrations and solution pH

decreased, while Ca2+ concentrations increased, with an increase in initial Pb2+ concentrations further supports our HA dissolution/ HP precipitation hypothesis (Table 3). As discussed above, the function of HA was to supply P for the formation of HP. Since the precipitation of the lead phosphates is rapid (8), the limiting step in the above reactions was most likely the dissolution ofHA (eqs 1 and 2) . Assuming these arguments are true, then we would expect P043- concentrations and solution pH to decrease at higher Pb2+ levels as more PO,3- reacts with Pb'+' At the same time, Ca2+ concentrations will increase with an increase in initial Pb2+ concentrations as more HA dissolves. On the other hand, if the dominant mechanism is adsorption or cation substitution, then initial Pb'+ levels would have little effect on PO,3- concentrations and solution pH.

Dissolved Pb'+ concentrations in the presence of dif­ferent levels of NO,- were low and all below the EPA action level of 72.4 nmol L-l (Table 2), supporting our hypothesis that NO,- had no effect on Pb'+ immobilization by HA. The fact that NO,- concentration was not changed after HA reacted with Pb(NO,)z (data not shown) together with constant P043- and Ca2+ concentrations and solution pH in the presence of varying NO,- concentrations also supports the hypothesis that NO,- did not participate in Pb'+-HA reaction (Table 3). Undersaturation ofPb(NO,)z solution with respect to HP after reaction with HA, however, is inconsistent with the detection ofHP by XRD (Table 4 and Figure 1).

Effects of CI- and F- on Pb2+ Immobilization by HA. We hypothesize that both CP and FP are formed via the same mechanism of HP formation: dissolution and precipitation. However, an ion substitution mechanism has also been suggested (5-7) . Similar equations describing the reactions of HA with Pb(NO,)z apply to PbCl, and PbF, systems, and the only difference is that NO,- was not involved in the reaction while both CI- and F- were

5Pb2+ + 3H,PO; + Cl" Pb,(PO,),Cl(c) + 6H+ precipitation

(3)

5Pb'+ + 3H,PO; + F" Pb,(PO,),F(c) + 6H+ precipitation

(4)

The function of HA is to supply P for the formation of CP or FP as it does for the formation ofHP. The fact that CP and FP were formed by reacting aqueous PO,3- with PbCI, or PbF, solutions, respectively, strongly supports our hypothesis of HA dissolution and CP and FP pre­cipitation (Figure 3) . In addition, HA was reacted with PbCI" NaCI, PbF" and NaF solutions to further verify our dissolution/precipitation hypothesis. The fact that significantly more CI- and F- were removed by HA from PbCI, and PbF, solutions than from NaCI and NaF solutions was also in agreement with our dissolution! precipitation hypothesis, CI- and F- being likely removed by the precipitation of CP and FP, respectively (Figure 5) . The fact that neither CA nor FA was detected by XRD suggests that the removal of CI- and F- from NaCI and NaF solutions may be through surface adsorption by HA or formation of solid solutions. Suzuki and Ishigaki reported that HA removed more F' from PbF, than from NaF solutions by ion substitution (5). Alternatively, we

Environ. ScI. Technol., Vol. 28, No.3, 1994 415

believe that HA dissolution and FP precipitation explain the results better. Fluoropyromorphite may have formed in their study when HA reacted with PbF2 solutions (5); whereas, no mineral was formed when HA reacted with NaF solution, and thus, more F- was consumed in the former than in the latter process. In addition, in our study, more F- was removed from solution than CI- at the same initial concentrations, which supports the observation that HA more readily incorporates F- than CI- (Figure 5) (12). LeGeros reported that more F- substitutes for OH- in HA than Cl- even if F- concentration is lower than that of Cl­(12). Lower P043- concentrations and pH values in the presence ofPbC12 and PbF2 solutions than in the presence of NaCI and NaF solutions suggest that some P043- was precipitated as FP and CP according to eqs 3 and 4. Similarly, lower P043- concentrations and pH were ob­served in the presence of PbF 2 solution than in the presence of PbC12 solution, confirming our dissolution and precip­itation hypothesis since more F- was removed than Cl­(Figure 5).

As was observed in the N03- solutions, dissolved Pb2+ concentrations were all below the EPA action level of 72.4 nmol L -1 after HA reacted with Pb2+ in the presence of Cl­or F- (Table 2), indicating that neither Cl- nor F- had significant effects on Pb2+ immobilization by HA. Take­ichi and Arai found that more Pb2+ was removed from Pb(N03)2 than from PbCl2 solutions, and the difference was larger at lower pH (7). They suggest that the increased surface area of HA at lower pH resulted in greater Pb2+ adsorption.

In contrast to the N03- solutions, the presence of both CI- and F- changed the solution chemistry and reaction products during immobilization ofPb2+ by HA. Formation of fluorite (CaF2) may have contributed to lower Ca2+ concentrations at higher F- concentrations (Table 3). However, fluorite was not detected by XRD even though MINTEQA2 predicted that the solutions were supersat­urated with respect to fluorite (Table 4). The amount of fluorite formed during the reaction may have been below the XRD detection limit of 1 % in our study. However, fluorite was observed by Suzuki et al. (6). We hypothesize that the formation of fluorite at higher F- concentrations removed additional Ca2+ from solution, resulting in the dissolution of HA and in subsequent increases in P043-

concentrations (Table 3). Lower POl- and Ca2+ concen­trations and solution pH values in the presence of F- than CI- suggest that FP was more effective in inhibiting HA dissolution than CPo Undersaturation of the solutions with respect to HA in the presence of F- compared to the equilibrium of the solutions in the presence of CI- further supports the above hypothesis (Table 4). It is interesting to note that FP was the most soluble pyromorphite mineral, while FA is the least soluble apatite mineral. Formation of FA would have caused higher solution pH as F­substituted for OH- in HA structure. Decrease in solution pH with an increase in F- concentrations suggests that such a substitution may be limited (Table 3).

Changes in anion concentrations give additional support to our hypothesis. Reduction of CI- and F- concentrations with the formation ofCP and FP would be 4.82,24.1,48.2, and 96.4 /.tmol L-1 at initial Pb2+ concentrations of 24.1, 121,241, and 482 /.tmol L-1, respectively, if all of the Pb2+ was present as CP or FP. The decreases in Cl- concen­trations were 8.7, 53.0, 132, and 440 /.tmol L-1 at initial Pb2+ concentrations of 24.1, 121, 241, and 482 /.tmol L-1,

416 Environ. Sci. Technol.. Vol. 28, No.3, 1994

respectively, much higher than that required for the formation of CPo Chloride either substituted for OH- on HA or was adsorbed by HA in addition to being incor­porated into the CP structure. Lower solution pHs than point of zero charge for hydroxyapatite of 7,5 support the adsorption of Cl- and F- by hydroxyapatite (Table 3) (23). Similar trends were observed for F-, except that more F­was consumed than CI- under the same conditions (Figure 6). These results support the conclusion of LeGeros et al. (12) that HA favors the incorporation of F- more than it does CI- and also agree with our previous experiment (Figure 5). If OH- of the HA (0.1 g) was completely replaced by either CI- or F-, 995 /.tmol L-1 Cl- or F- would have been consumed. The maximum Cl- and F- removed from the system was 440 and 655 /.tmol L-1, respectively. Thus, not all the OH- of HA was replaced by either CI­or F-. Most of the Cl- and F- may have been adsorbed on the surface of HA, CP, or FP since neither CA nor FA was detected by XRD. Under saturation of the solutions with respect to both CP and FP, as in the case of HP, again suggests that both adsorption and cation substitution are possible mechanisms for Pb2+ removal in addition to dissolution/precipitation.

Effect of 8042- and COa2- on Pb2+ Immobilization by HA. We hypothesize that Pb2+ was also removed by HA in the presence of either S042- or C032- through HA dissolution and HP precipitation, with S042- or C032-

possibly substituting for P043- in HA or HP structures. The fact that HP was the only mineral detected by XRD suggests that HA dissolution and HP precipitation were the dominant mechanisms during Pb2+ immobilization by HA and that substitution of S042- or C032- for P043-

was limited (Figure lE,F). Dissolved Pb2+ concentrations were all below the EPA

action level of 72.4 nmol L -1 except in the presence of high levels ofC032- (Table 2). This was mainly caused by lower HA solubility resulting from higher pH due to the presence of C032-, thus leading to less formation of HP as indicated by smaller HP peaks by XRD (data not shown). In addition to the formation of HP, glauberite was present after HA reaction with Pb2+ in the presence ofS042- (Figure IE). Glauberite is not commonly found in soil environ­ments. The more common mineral gypsum [CaS04·2H20] was not detected (19). The initial C032- concentrations in our experiment ranged from 10-4·32 to 10-2.24 mol L-1, which is high for a normal soil environment with a CO2 concentration of 0.006-0.015 atm. The C032- concentra­tion is less than 10-6·67 mol L-1 in normal soils of pH 7 and can reach 10-2.67 mol L-l or higher in sodic (alkali) soils of arid regions of pH 9 with a CO2 level of 0.003 atm (19). At such low C032- concentrations in normal soils, the effect ofC032- on Pb2+ immobilization by HA would be minimal. Clusters of amorphous-like material on the surface of HA treated with C032- may be the reprecipitated HA as suggested by calculated saturation indices (Figure 7B and Table 4). Increases in solution pH may have caused the supersaturation ofHA and led to HA reprecipitation. Since there was no C032- loss from solution, the formation of C032--substituted minerals was unlikely (data not shown).

Decreases in Ca2+ concentrations resulting from the formation of glauberite drove HA dissolution and led to higher P043- concentrations at higher S042-/Pb2+ molar ratios (Figure 1 and Table 3). Higher P043-concentrations may also have been a result of S042- substitution for P043-, since S042- concentrations decreased after the reaction,

although formation of glauberite was probably the main process responsible for decreases in S042- concentrations. It is clear that Ca2+ concentrations decreased with an increase in C032- concentrations due to lower HA solubility. But, it is unclear why P043- concentrations increased with an increase in C032- concentrations unless there was substantial C032- substitution for P043-. However, COS2-concentrations did not change after HA reacted with Pb­(NOsh (data not shown). The results thus indicate that the substitution ofCOs2- for POlo. in HA or HP was limited.

Summary and Conclusions

Hydroxyapatite can effectively immobilize aqueous Pb2+ in the presence of NOs-, CI-, F-, S042-, and C032-. Lead concentrations were reduced from initial levels of 24.1-482 ,umol L-l to below the EPA Pb2+ action level of 72.4 nmol L-l, except at high concentrations ofPb2+ and C032-.

The main mechanisms of Pb2+ removal were through the dissolution of HA and the precipitation of HP, CP, or FP. Hydroxyapatite was transformed to CP and FP after reaction with aqueous Pb2+ in the presence of Cl- and F-, respectively, and to HP in the presence of N03-, S042-, or C032-. Hydroxyapatite dissolution followed by HP, CP, or FP precipitation was the main chemical reaction, but Pb2+ adsorption by HA and cation substitution of Pb2+ for Ca2+ on HA may have also occurred. Both CP and FP were formed by reacting PbCl2 and PbF2 solutions with aqueous P, which further supports our hypothesis of HA dissolution and CP or FP precipitation. In addition, more Cl- and F- were removed from PbCb and PbF2 than from NaCl and NaF solutions, indicating that Cl- and F- were precipitated as CP and FP in addition to substitution for OH- on HA or sorption by HA, CP, or FP. The precipitated HP, CP, and FP showed different morphologies than HA. Both HP and FP were needle shaped, and CP showed irregularly shaped spheres compared to sheet-shaped HA. Glauberite appeared to be rod shaped, while reprecipitated HA was in flake-shaped clusters.

The presence of varying levels of N03- and Cl- had no significant effect on solution chemistry, while F-, S042-,

and C032- did. Solution pH and P043- concentrations decreased, whereas Ca2+ concentrations increased with an increase in initial Pb2+ concentrations in the presence ofN03-, Cl-, and F-. Concentrations of CI-, F-, and S042-

decreased after HA reacted with Pb2+, and the decrease was proportional to the initial anion concentrations. The formation of CP and FP, the substitution for OH- of HA, and the adsorption by HA all contributed to the removal of Cl- and F-, while glauberite formation accounted for the decrease in S042- concentrations, although S042-

substitution for P043- cannot be ruled out. The solutions were in near-equilibrium with HA after HA reaction with Pb2+ in the presence of N03-, Cl-, and 8042-, were undersaturated in the presence of F-, and were supersat­urated in the presence of C032-. Undersaturation was probably caused by slow HA dissolution due to FP coating on HA surface, while supersaturation resulted from higher solution pH due to C032-. The solutions were supersat­urated with respect to fluorite and glauberite, with only glauberite being detected by XRD. The solutions were undersaturated with respect to most of the lead phosphates formed, even though they were detected by XRD, which may have resulted from Pb2+ adsorption by HA or solid solution formation.

Lead immobilization by HA in the presence of N03-,

Cl-, F-, S042-, or C032- was mainly through dissolution of HA and precipitation of HP, CP, or FP. However, Pb2+ adsorption by HA, HP, CP, or FP or Pb2+ substitution for Ca2+ on HA is also possible. In general, variations in anion concentrations (N03-, Cl-, and 8042-) had no effect on Pb2+ immobilization by HA, whereas F- and C032-decreased Pb2+ immobilization by HA slightly. The fact that HA can reduce Pb2+ concentrations from 24.1-482 ,umol L -1 to below 96.5 nmol L -1 in the presence of high levels of anions such as N03-, Cl-, F-, S042-, and C032-

together with the stable reaction products and rapid reaction strongly indicate that HA has a great potential to immobilize Pb2+ in Pb-contaminated soils and waste materials.

Acknowledgments

Funding was provided by the U.S. EPA (Contract CR-816843-01-0) through a cooperative agreement to The Ohio State University. However, the article has not been subjected to EP A review, thus it does not necessarily reflect the views of the EPA. Mention of trade names or commercial products does not constitute endorsement or recommendation for use. Salaries and research fund were also provided in part by state and federal funds appro­priated to OSU-OARDC. OARDC Journal Article No. 188-93. We wish to acknowledge Dr. J. M. Bigham, Agronomy Department, OSU, for his assistance in using XRD. We would also like to thank Xiaosong Zhang and 8haoqin Yao for partially conducting the experiments.

Abbreviations used: HA, hydroxyapatite [Ca5-(P04)sOH]; HP, hydroxypyromorphite [Pb5(P04)OH]; CA, chlorapatite [Ca5(P04>sCl]; FA, fluorapatite [Ca5(P04)sF]; CP, chloropyromorphite [Pb5(P04)sCl]; FP, fluoropyromorphite [Pb5(P04)sF]; XRD, X-ray diffrac­tion; SEM, scanning electronic microscope.

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Received for review May 17,1993. Revised manuscript received October 29, 1993. Accepted November 19, 1993.'"

'" Abstract published in Advance ACS Abstracts, January 1, 1994.