Environ. Sci. Technol. 1994, 28, 1219-1228

Effects of Aqueous AI, Cd, Cu, Fe(II), NI, and Zn on Pb Immobilization by Hydroxyapatite

01 Ylng Ma'

Department of Soil and Water SCience, University of Florida, Gainesville, Florida 32611

Samuel J. Traina and Terry J. Logan

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

James. A. Ryan

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

The effects of aqueous AI, Cd, Cu, Fe(lI), Ni, or Zn on Pb immobilization by hydroxyapatite [CalO(P04)s(OHh] were studied. Lead was removed mainly via hydroxyapatite dissolution and hydroxypyromorphite [PblO(P04)S(OHh] precipitation in the presence of these metals with a Pb removal efficiency of 37-100%. These metals inhibited Pb immobilization by hydroxyapatite in the order: Al > Cu > Fe (II) > Cd > Zn > Ni and Cu > Fe (II) > Cd > Zn > Al > Ni at high and low initial Pb concentrations, respectively. The inhibition was probably through the precipitation of amorphous to poorly crystalline metal phosphates, decreasing the amount of dissolved P available for precipitation with dissolved Pb ions. Hydroxyapatite was effective in removing these added metals, especially at low concentrations. Hydroxyapatite selectively re­moved Pb from solution in the presence of aqueous AI, Cd, Cu, Fe(II), Ni, or Zn. The results support our earlier finding that hydroxyapatite has the potential to be used for in situ immobilization of Pb in Pb contaminated soils and wastes.

Introduction

Concerns over contamination of groundwater and surface water by heavy metals from previously abandoned disposal sites and some currently operating sites have generated programs to remediate contaminated soils (1). The presence of heavy metals in the effluent streams from chemical and metal plating industries has been a major concern to communities and municipalities. Heavy metals are toxic to humans and aquatic life. The ubiquitous nature of heavy metals, their toxicity even in trace quantities, their tendency to bioaccumulate in the food chain, and the stricter environmental regulations related to heavy metal discharges make it necessary to develop schemes for the removal of heavy metals from both wastewaters and landfillieachates (2). Likewise, there is growing interest in in situ reduction ofPb bioavailability in soils contaminated by leaded gasoline, lead based paints, and lead batteries disposal. Commonly encountered metals of concern include Cu, Ni, Cr(ll), Cd, Pb, Hg(II), Zn, and Ag(I) (3). Lead was chosen for study due to its extent of contamination oflandfills, wastewaters and soils.

Hydroxyapatite (HA) [CalO(P04)S(OH)2] has a high removal capacity for divalent heavy metal ions and has been used for wastewater treatment (4-6). We have recently shown that HA can effectively attenuate aqueous Pb, resin exchangeable Pb, and Pb in contaminated soil material (7). Hydroxyapatite can reduce initial dissolved

0013-936X/94/0928-1219$04.50/0 © 1994 American Chemical Society

Table 1. Saturation Indices of Selected Solids after Reaction of Hydroxyapatite with 482 ~mol of Pb L-I Solutions, in Presence of Different Metal/Pb Molar Ratios

saturation indices

metals minerals 1 3 5 7

Al Ai(OH)s (am") -0.32 -0.41 0.46 0.75 AlPO.·2H2O(va") 2.3 2.7 3.4 3.4 Ca5(PO.)gOH -5.3 -8.1 -9.0 -10 Pb5(PO.)gOH -7.2 1.6 3.3 2.9

Cu CUS(P04)2 -2.5 -3.3 -2.1 -0.55 Ca5(PO.)sOH -8.2 -14 -14 -12 Pb5(PO.)sOH -2.6 0.80 2.8 5.4

Fe (I!) Fe2(PO.)3.8H2O(vi") -5.8 -5.8 -5.9 -5.9 Ca5(P04)sOH -11 -15 -17 -18 Pb5(P04)gOH -7.7 -3.8 -3.3 -3.2

Ni Nis(PO.l2 -1.9 -0.92 -0.91 -0.77 Ca5(PO.laOH 1.1 0.19 -0.95 -1.4 Pb5(P04laOH -9.7 -3.2 -2.4 -2.2

Zn ZnS(P04)2 -4.5 -4.7 -5.2 -6.0 Ca5(PO.)sOH -2.2 -6.0 -8.3 -11 Pb5(PO.)aOH -0.29 -1.4 -0.58 -0.57

• am, va, and vi denote amorphous, variscite, and vivianite, respectively.

Table 2. Saturation Indices of Selected Solids after Reaction of Aqueous Pb with Hydroxyapatite in Presence of Different Cd/Pb Molar Ratios

initialPb saturation indices (~mol L-I) minerals 1 3 5 7

482 Cds(PO.h -2.8 -3.4 -4.9 -5.7 Ca5(PO')30H -1.6 -6.1 -10 -13 Pb5(P04laOH -0.38 1.3 1.5 0.87

241 CdS(PO')2 -3.0 -2.4 -2.5 -2.8 Cas(PO.)sOH 0.69 -2.8 -4.6 -6.1 Pb5(PO')30H 0.42 0.44 0.12 0.42

121 Cd3(P04h -4:8 -2.4 -2.6 -3.1 Ca5(P04)sOH 3.3 -0.44 -2.9 -4.7 Pb5(P04)30H -0.53 -0.29 0.26 -1.9

24.1 Cd3(PO.l2 -7.5 -10 -5.9 -4.1 Cas(PO')30H 4.5 4.2 3.5 2.5 Pb5(PO')30H -3.7 -3.4 -2.8 -1.7

Pb of 24.1-2410 }.Lmol L-l to below 95 nmol L-l in 0.5 h, indicating that Pb immobilization by HA is rapid and effective. Aqueous Pb in Pb-contaminated soil material was reduced from 11.0 to 0.17 }.LmolL-l byHA. Wefound that hydroxypyromorphite (HP) [PblO(P04)s(OHh] is the final reaction product in most cases and is formed through HA dissolution followed by HP precipitation (7). We also found that HA can effectively immobilize aqueous Pb in the presence of varying levels of dissolved NOs, CI, F, S04, and COs, although F and COs lowered the effectiveness of HA in immobilizing Pb at high initial concentrations

Environ. Sci. Technol., Vol. 28, No.7, 1994 1219

0.30 35 300 10 100

0.10 10

1.0 0.1

0.01 ~ rt~~ 0.1 Initial Pb .. I!I".... ~151 0.01

m--~-.(p.., 0.01 ..... 1!3 concentration ~ ~~.ar" (!lmol L·1) (3 0.001 . ''''s.

0.001 Jft_-r E 0.001 ............ 'f]

3.0.0005 0.0001 Zn Cd -+- 482

c 0.0001 0 0 1 2 3 4 5 6 7 8 0 1 2 345 678 o 1 2 3 4 5 6 7 8 +== ~

"'0" 241 -c ~ 300 500 500 c 100 100 8 100 .0 10

_ 121 Cl. 10 10 Cti c IT: "E,T" 24.1

0.1 0.1 0.1

0.01 0.01 13"'-8-'-" s-·.....£l 0.01

0.001 0.001 ;'

0.001

0.0001 0.0001 Cu

0.0001 o 1 2 3 4 5 6 7 8 o 1 2 34567 8 0 1 2 3 4 5 6 7 8

Metal/Pb molar ratios Figure 1. Final concentrations of aqueous Pb after reaction of 24.1-482 I-Imol of Pb L-1 with HA in the presence of different concentrations of aqueous AI, Cd, Cu, Fe(II), NI, or Zn. The bars stand for standard deviations of triplicates.

100 • • • • 100 • • = ... • 100 • --to: • I ~.~.~. ... ........ .. , 80 80 80 "'I

60 60 60 Initial Pb concentration

c:: (j.lmoIL,1) 0 40 40 40 :;::::

:::J '0 Ni Zn Cd en '-£3-' 24.1 E 20 20 20 ,g 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 'C OJ ..... 121 > 0 E

100 ~ 100 ..... . R:::::!

~ 100 . " . • • .0 " \\ .. _ ... ~ 241

a.. ...... ':!2. 80 .... ~ 80 80

"~ 0 \

\

" ...... 482 ..... •.... "" \

60 ' .... 60 60 \

\

\ ,. ..., 40 40 ........ 40 .~ ...

Fe Cu AI 20 20 20

0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8

Metal/Pb molar ratios Figure 2. Percentage of aqueous Pb removed by HA in the presence of different concentrations of aqueous AI, Cd, Cu, Fe(II), NI, or Zn. The bars stand for standard deviations of triplicates.

1220 Environ. Sci. Technol., Vol. 28, No.7, 1994

1300 A Metal/Pb ratio = 1 1300 B Metal/Pb ratio = 7

1200 1200

1100 1100

1000 1000

900 900

tI) - 800 800 c ~

700 0 700 (,)

~ 600 :;: 600 co

Cii a:

500 500

400 400 Cu

300 300

200 200 Fe

100 100

AI 0 0

20 25 30 35 40 45 50 20 25 30 35 40 45 50

029 Figure 3. XRD patterns of hydroxyapatite (HA) after reaction with 482 ~mol of Pb L -1 In the presence of aqueous AI, Cd, Cu, Fe(II), NI, or Zn at metallPb molar ratio of 1 (A) and 7 (B). HP denotes hydroxypyromorphlte.

of dissolved Pb, F, and COa (8). Lead immobilization was again via a mechanism of dissolution and precipitation; HP formed after HA reacted with aqueous Pb in the presence of NOa, S04, or COa; and chloropyromorphite [PblO(P04)6CI2] and fluoropyromorphite [PblO(P04)sF2]

formed in the presence of CI and F, respectively. Clearly, HA has great potential to immobilize Pb in Pb­contaminated soils and wastes.

Hydroxyapatite is well-known for its isomorphous substitution as expressed by the formula: N 1O(R04)s y 2 where N = Ca2+, Pb2+, Na+, K+, Sr2+, Mn2+, Zn2+, Cd2+, Mg2+, Fe2+, and AP+; R04 = P043-, COa2-, HP042-, P20 74-,

As043-, V04a-, S042-, and Si044-; and Y = OH-, F-, Cl--:-, H20, and Br-. These substitutions affect the crystallinity, morphology and lattice parameters of the apatites and, as a consequence, alter their stability (9). Naturally occurring apatites can be divided into two major groups: (1) the apatite series in which Ca is the dominant cation and (2) the pyromorphite series in which the dominant cation is Pb. Lead does not substitute to a large extent for Ca in the apatites series, and Ca does not substitute in large amounts for Pb in the pyromorphite series (10). Whereas, many studies have focused on the interactions of HA with heavy metals such as Cd, Cu, Ni, and Zn (6, 11-13), there is little published data on the effects of these metals on Pb immobilization by HA. Our previous study showed that, in the absence of other metals, HA dissolution and HP precipitation were the main mechanisms for Pb immobilization by HA (7). These chemical reactions can

be described as (14)

CalO(P04)S(OHMc) + 14H+ . -. dISsolution

10Ca2+ + 6H2P04- + 2H20 log kO, 28.92 (1)

10Pb2+ + 6H2P04- + 2H20 -precipitation

PblO(P04)S(OH)2(C) + 14H+ log kO, -8.28 (2)

The proclivity of "apatite-like" solids to incorporate a number of different metal ions into crystal lattice sites suggests that the immobilization of Pb by HA could be strongly affected by the presence of other dissolved metal ions. Such effects could result from alterations of reaction 1 and/or reaction 2. Clearly, before HA can be readily adopted for the treatment of Pb-contamimanted soils, sediments, and natural waters, an assessment of the effects of other metal ions on Pb immobilization by HA is warranted. The present study examines the effects of dissolved metal ions (M) on Pb/HA interactions, where M represents AI, Cd, Cu, Fe(II), Ni, or Zn. These metals were chosen for study since they are likely to be present in large aqueous concentrations in many Pb-contaminated soils and sediments.

Experimental Section

Experimental Procedures. Different concentrations of Pb were reacted with HA in the presence of varying

Environ. Sci. Technol., Vol. 28, No.7, 1994 1221

7.0

6.5 ....... ....-~-e---a

6.0

5.5

5.0

4.5

7.0

6.5

6.0

5.5

5.0

4.5

6.0

5.5

5.0

4.5

Initial Pb concentration

(Ilmol L·1)

Ni I 4.0 --I-..,..-...,.-...,.-...,.--r--r--r---f 0..

Zn 4. 0 -t-,...--.,...-.,...-.,...-.,...-.,...-~ --&- 24.1 4.0 -t--r--r--r--r--r--r--.---4

Cd c:: o

:;::; o 1 2 3 4 5 6 7 8 o 1 2 3 456 7 8 o 1 2 3 4 5 6 7 8

::::l '"6 CI)

7.0 Fe 6.5

6.0

5.5

5.0

4.5

7.0 Cu

6.5

6.0

5.5

5.0

4.5

7.0 AI 6.5

6.0

5.5

5.0

4.5

-.- 121

-I!r- 241

-+- 482

4. 0 -+-,....-,--+::;:::$=i~-l 4.0 -t--r--r--l---r---r----r-...--I 4.0 -t-,...--.,...-.,...-.,..-.,..-...--.---l o 1 2 3 4 5 6 7 8 o 1 2 3 456 7 8

Metal/Pb molar ratios

o 1 2 3 4 5 6 7 8

Figure 4. Final solution pH values after reaction of HA with 24.1-482 ~mol of aqueous Pb L -1, In the presence of different concentrations of aqueous AI, Cd, Cu, Fe(II), Ni, or Zn. The bars stand for standard deviations of triplicates.

levels of AI, Cd, Cu, Fe(II), Ni, or Zn to test the effects of these metals on Pb immobilization by HA. One-tenth gram ofHA (Bio-Rad) was reacted with 200 mL of solutions containing 24.1, 121, 241, and 482 ~mol of Pb L -1 as Pb­(N03h The HA/Pb molar ratios ranged from 41.2 to 2.06. At each Pb level, four different metal concentrations were used: 1,3,5, and 7 times the respective Pb concentration on a molar basis as nitrate salt; Fe (II) was added in the form of Fe(CI04h (the nitrate salt could not be obtained). All solutions were adjusted to pH 6 with dilute HNOg or NaOH, except Fe(CI04)2 and AI(NOg)g which were adjusted to pH 4.71 and 4.62, respectively, to prevent precipitation of iron(II) and aluminum hydroxides. The suspensions were shaken for 2 h and then filtered through 0.2-~m Nucleopore polycarbonate membrane filters. The filtrates were analyzed for total P, Pb, Ca, Cd, Cu, Ni, Zn, AI, Fe(II), and pH. The solid phases were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM).

Analytical Methods

Perkin-Elmer 3030B and 4100ZL atomic absorption spectrophotometers were used to analyze total metal concentrations. Total dissolved P04 was measured color­imetrically with a Beckman DU-6 spectrophotometer (15). 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 HCI) polycarbonate labware.

1222 Environ. Sci. Technol .• Vol. 28. No.7, 1994

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 mA. Measurements were made using a step-scanning technique with a fixed time of 4 s/0.05° 20. A total of 601 data points were obtained from 15 to 45° 20. All XRD analyses were performed using back-filled, randomly oriented mounts. Photo micrographs were collected with a JEOL JSM-820 scanning electron microscope (SEM; JEOL, USA Inc., Peabody, MA). The samples were mounted on a stainless steel stub and then coated with Au and Pd for observation.

Chemical Speciation. The MINTEQA2 chemical equilibrium speciation program (16) was used to calculate the equilibrium distributions and activities of aqueous species using total dissolved Pb, Ca, AI, Cd, Cu, Fe(ll), Ni, Zn, N03, and P04 concentrations and solution pH as the initial inputs. Thermodynamic stabilities of each solid were established by comparing the appropriate ion activity products (lAP) with the corresponding formation constant (KO). The logarithmic ratio of these terms (saturation index) were used to establish the stability order for precipitation or dissolution of solids:

saturation index = log(IAP/KO) (3)

If the saturation index (SI) for a particular mineral is negative, the system is undersaturated with respect to that mineral. On the other hand, if the SI is positive, the system is supersaturated. Equilibrium condition is achieved if the SI equals zero. The absolute value of SI

80 80 70 70 70 60 ~+-......... t--~ ... 60 60 50 ""'1

,..~

50 50 40

40 40 30 Initial Pb

30 30 concentration .... 20 ~ 20 20 (Ilmol L'1) "0

10 10 Cd E Ni 10 Zn ..:;

0 24.1 c 0 0 ··B·· 0

0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 :;:::; ~ .... c -+- 121 Q) u c 80 100 500 8 .EI a.. 70

~.

···-Ek .. ~.

..~ .. 241 I _._Err

400 '<I' ......... E1- ...... ,,"'iJ 80 0 60 a.. ~ 50 60 300 --+- 482 c u::: 40

30 40 200

20 20 100

10 Cu 0 0 0 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8 0 1 2 3 4 5 6 7 8

Metal/Pb molar ratios Figure 5. Final dissolved P concentrations after reaction of 24.1-482 Jtmol of aqueous Pb L -1, with HA In the presence of different concentrations of aqueous AI, Cd, Cu, Fe(II), Ni, or Zn. The bars stand for standard deviations of triplicates.

is an indication of degree of under- or supersaturation with respect to a solid phase.

Results

Added metals reduced the effectiveness of Pb im­mobilization by HA as shown by higher final Pb concen­trations compared to those of the controls (samples in which Pb was the only added metal ion). This was especially evident at higher initial Pb concentrations and higher M/Pb molar ratios (Figure 1). At initial Pb concentrations of 24.1 Jtmol L -1, the final Pb concentrations were below 15.0 nmol L-l and did not change with M/Pb molar ratios, indicating that the metals added at these levels had little inhibiting effect on Pb immobilization by HA. However, at initial Pb concentrations of 121-482 Jtmol L-l, final Pb concentrations were much higher, ranging from 10.8 nmol L-l to 306 Jtmol L-l, and increased with an increase in both initial Pb levels and M/Pb molar ratios. Nevertheless, HA reduced Pb concentrations significantly in spite of the presence of high levels of the added metals.

Nickel had little effect on Pb immobilization by HA (Figure 2). AI, Cd, and Zn caused decreases in Pb immobilization by HA only at the greatest initial Pb concentrations and at M/Pb ratios greater than 1. Maxi­mum inhibition was approximately 64, 22, and 6 % , respectively. Copper and Fe(1I) exhibited the greatest inhibition on Pb immobilization (Figure 2). This occurred at all but the lowest Pb level and at most M/Pb ratios greater than 1. Maximum inhibition by Cu and Fe (II) was 63 and 37 %, respectively. The effectiveness of HA

in immobilizing Pb in the presence of the added metals was in the order: Al < Cu < Fe(II) < Cd < Zn < Ni, at high initial Pb concentrations. In more dilute Pb solutions, the order was Cu < Fe(II) < Cd < Zn < Al < Ni.

Hydroxypyromorphite was detected by XRD after HA reaction with 482 Jtmol of Pb L -1, except at a M/Pb molar ratio of 7 in the presence of Fe(II) or AI, and HA was present after the reaction in all treatments (Figure 3). The HP peak intensity decreased in the order: Ni > Zn > Cd > Cu > Fe(II) > AI. Similar but less intense HP patterns were observed for initial Pb concentrations of 241 and 121 Jtmol L-l, and no HP peaks were observed with initial Pb concentrations of 24.1 Jtmol L-l (data not shown). If all of the Pb removed by HA, at an initial Pb concentration of 24.1 Jtmol L-l, were precipitated as HP, then there should have been at least 6 % HP (by weight) present as a reaction product. In the presence of Fe(II) and AI, there should have been about 15 and 9% HP present, respectively, at a M/Pb molar ratio of7 and initial Pb concentrations of 482 Jtmol L-l (based on the amount of dissolved Pb removed from solution and an estimate of the amount of HA that dissolved). Since XRD can normally detect crystalline solids present at concentrations ~1 wt % of the sample matrix, the absence of HP peaks in some of the reaction products suggests that a mechanism other than that described in reaction 2 was responsible for at least some of the Pb attenuation in this study. Whereas, coprecipitation of Pb with aluminum/iron [Fe (II) or Fe (III) ] hydroxides and! or adsorption by AI, Fe hydroxides could have caused the lack of visible HP peaks in Figure

Environ. Sci. Technol., Vol. 28, No.7, 1994 1223

1.9.,--------, 1.9.--------, Zn 1.7

1.9.,---------, 1.7 Ni 1.5

1.3 ....... ..--i--+_--1.1

0.9 .... 0.7 ~ (5 0.5 i---l....--e.....-.-.-.. E 0.3 g ._ .. e-- .. _El~··8

1.5 1.3

1.1

0.9 0.7 0.5 0.3

1.7 Cd 1.5 1.3

1 .1 0.9 0.7 0.5 0.3

Initial Pb concentration

(Ilmol L·1)

c o :;::;

0.1 +--,---,--.--r-,--,..-r-! 0.1 +--'-T"'"""'I,,-r--,---r--4 -+- 482

e:? o 2 3 4 5 6 7 8 o 1 2 3 4 5 6 7 8 o 1 2 3 4'5 6 7 8 -c

Q) C,.) c o C,.)

1.9.,...-----------. Fe

1.9.--------,

ctS ()

ctl c i.L

1.7

1.5

1.3

1.1

0.9 0.7 0.5

Cu 1.7

1.5

1.3

1.1

0.9 0.7 0.5 0.3

4.6 4.1

3.6 3.1

2.6 2.1 1.6 1.1

0.6

-.- 121

.. 13 .. 24.1

O. 1 +-,---,--,.--,--r-r--,--! O. 1 -<f"'-¥-""",;;r---.-.,.-..---r---4 012345678 012345678

Metal/Pb molar ratios

o 1 2 3 4 5 6 7 8

Figure 6. Final Ca concentrations after reaction of 24.1-482 /-tmol of aqueous Pb L-l, with HA in the presence of different concentrations of aqueous AI, Cd, Cu, Fe(II), Ni, and Zn. The bars stand for standard deviations of triplicates.

3h, no diffraction patterns of any Fe or Al solids were detected.

Solution pH and P concentrations decreased, while Ca concentrations increased with increasing Pb concentra­tions (Figures 4-6). Generally speaking, solution pH decreased in the order: Ni > Zn > Cd> Al > eu > Fe(II) except at initial Pb concentrations of24.1 /-tmol L -1. Total dissolved P concentrations decreased in the order: Ni > Zn > Cd > Fe(II), while changes in P concentrations in the Al and Cu systems varied with initial Pb concentrations (Figure 5). At an initial Pb concentration of 482 /-tmol L -1, dissolved P concentrations decreased in the order Al > Ni > Zn > Cd> Fe(II) > Cu, and at an initial Pb concentration of 121 ~mol L-1, the order was Ni > Al > Zn, Cu > Cd > Fe(II). Dissolved Ca concentrations decreased in the order Cu > Cd > Al > Fe (II) > Zn > Ni at an initial Ph concentration of 24.1 ~mol L -1, and the order was Al > Cd, Fe (II) , Zn > Cu, Ni at initial Pb concentrations of 121-482 ,umol L-1. In general, Ca concentrations were the lowest and highest in the presence of Ni and AI, respectively, excluding treatment of initial Pb concentration of 482 ~mol L-1 (Figure 6).

Needle·shaped HP crystals were observed on HA surfaces after reaction with Pb in the presence of Ni, Cd, or Zn, while little to no HP precipitates were visible on HA surfaces in the presence of Cu, Fe(II), or Al (Figures 7-9). Different shaped precipitates were observed in the presence of Cu, but the solids formed were not detected by XRD (Figure 8B). Larger and longer crystals were observed in the presence of Fe(II) and AI, but again no Fe or Al solids were detected by XRD.

1224 Environ. SCi. Techno!., Vol. 28, No.7, 1994

MINTEQA2 data generally indicated that solutions containing Ni were undersaturated, those containing Zn or Cd were in near equilibrium, and those containing Cu were supersaturated with respect to HP at initial Pb concentration of 482 ~mol L -1 (Tables 1 and 2). The degree of undersaturation with respect to HA increased in the order Ni < Al < Zn < Cd < Cu < Fe(II) , which was similar to the order of the effectiveness of HA in immobilizing Pb in the presence of these metals. This trend was also observed for initial Pb concentrations of 24.1-241 ~mol L-1 (data not shown). Saturation indices ofHP and Cd3-

(P04h increased, while those of HA decreased as initial Pb concentrations increased in the presence of Cd (Table 2). Similar trends also apply to AI, Cu, Fe, Ni, or Zn systems (data not shown).

Hydroxyapatite was effective not only in removing Pb in the presence of AI, Cd, Cu, Fe(II), Ni or Zn but also in reducing the concentrations of these metals themselves. All metal concentrations decreased after reaction with HA and the reduction varied from metal to metal and with the initial metal ion concentrations. As was observed for Pb, final dissolved AI, Cd, Cu, Fe(Il), Ni, and Zn concentrations increased with increased initial metal concentrations (increase in either initial Pb concentrations or M/Pb molar ratios). In general, the effectiveness of HA in removing these metals was in the order Al > Zn > Fe(II) > Cd > Cu > Ni; whereas, the order of the effectiveness of those metals in inhibiting Pb immobilization by HA was Al > Cu > Fe(II) > Cd > Zn > Ni. Thus, the amount of metals removed was not related to its effectiveness in inhibiting Pb immobilization by HA. In addition, the amount of Zn

Figure 7. SEM mlcrographs of hydroxyapatite after reaction with 482 jJmoI of Pb L - 1 at a M/Pb molar ratio of 7 In the presence of NI (A) and Cd (8).

and Fe(II) removed from solutions increased with an increase in their final concentrations; those of Cd, Ni, and Cu decreased and those of Al removed reached a plateau (Figure 10). The results suggest that HA was capable of removing additional Fe(II) and Zn, while its capacity in removing Cd, Ni, CUI and AI was reduced with an increase in their concentrations (likely because of simultaneous increases in dissolved Pb).

The reaction of HA and dissolved Pb with Cd produced a pale yellow solid, alight blue precipitate was present in those samples reacted with Cu, and a pinkish brown solid was evident in the presence of Fe(II). No color changes (from the white color of HA) were observed for those samples reacted with AI, Ni or Zn.

Upon reaction with HA, dissolved Pb was preferentially removed from the equimolar binary metal solutions (M/ Pb molar ratio of 1). Similarly, Suzuki et al. reported that HA removed more Pb than Cuat the same initial conditions (17). However, Takeuchi and Arai found that HA was most effective in removing Cu, followed by Pb and Cd (6). Additionally, Miyake etal. concluded that the effectiveness of synthetic carbonate apatites in removing heavy metals was in the order: Pb > Hg(II) > Cd > Zn (18). These differences may have been caused by the difference between HA and synthetic carbonate apatite and by the presence ofPb with the other metals in our study. Miyake et a1. reported that the maximum amounts of Zn and Cd removed by synthetic carbonate apatites were 502 and

Flgure 8. SEM micrographs of hydroxyapatite after reaction with 482 jJmoI of Pb L - 1 at a M/Pb molar ratio of 7 In the presence of Zn (A) and Cu (8).

603 mmol mol-' apatite compared to 2452 and 490 mmol mol- ' HA of Zn and Cd, respectively, in our study (18). They reported that the structure of HA was destroyed by an increase in Zn and Cd uptake, which was not found in our experiment, even though more Zn was removed by HA in our study than in their study. The increase in M/Pb molar ratio reduced the peak intensity of HP, but HP can be seen even at the highest Zn/Pb molar ratio (Figure 3).

Discussion

The presence of HP in the XRD patterns (Figure 3), at M/Pb ratios of 1 and initial dissolved Pb concentrations of 4821'IDoi L -', indicates that the primary mechanism of Pb immobilization by HA can still be attributed to eqs 1 and 2 (7). Ma et al. showed previously that HP precipita­tion occurred when Pb solutions as dilute as 241'mol L-' were reacted with HA. Nevertheless, the partial to complete inhibition ofHP precipitation in the binary metal solutions with M/Pb ratios of 7 indicates that additional reactions must have occurred in these systems. The presence of foreign metals such as AI, Cd, Cu, Fe(ll), Ni, or Zn could have altered the reactions in eqs 1 and 2 in numerous ways. Even though the data in Figure 10 are presented in a "sorption isotherm format", this does not necessarily imply that adsorption reactions were primarily responsible for the retention of metal ions by HA. The appearance of precipitates in the SEM micrographs

Envi'on. Set. Technol., Vot 28. No. 7. 1994 1225

Figure 9. SEM micrographs of hydroxyapatite after reaction with 482 ~moI of Pb l -l at a M/Pb molar ratio of 7 In the presence of Fe(II) (A) and AI (8).

Table 3. Ionic Radii log K.p of Various Metal Phosphates

ions radii (CN = 6)" (A) log K .. ' M3(PO.h

Ca 1.08 10.2 Ph 1.26 -4.43 Cu 0.81 2.21 Cd 1.03 6.53 Zn 0.83 7.06 Ni 0.77 8.82 Al 0.61 0.50' Fe 0.86 9.20

" From ref 19, and CN denotes coordination number. b From ref 20, and M denotes AI, Cd, Cu, Fe, Ni, and Zn, respectively. C From ref 14.

(Figures 7-9) and the color changes observed when binary mixtures of Pb and Cd, Cu or Fe(lI) were reacted with HA clearly indicate that at least some retention of AI, Cd, Cu, Fe(lI), Ni, and Zn was due to the incorporation of these ions into new solid phases. Formation of surface rinds of these secondary precipitates (Figures 7-9) could have decreased the dissolution of HA. Additional evidence for this is found in the increased undersaturation of HA with increased initial concentrations ofM (Tables land 2). The generally concomitant decreases in dissolved P and XRD­detectable HP with increased initial concentrations of M (Figures 3 and 5) are also consistent with the formation of M/ P precipitates. The solubility products given for M3(PO,h (Table 3) indicate that the phosphate solubility

1228 Environ. Sci. Technol .. Vol. 28. No. 7. 1994

increases in the order CU3(PO,h < Cd3(PO,h < Zn3(PO,h < Ni3(PO,),. Whereas these solids were not detected by XRD nor did 81 values provide evidence for their presence (Tables 1 and 2), at initial Pb concentrations of 121-482 "mol L -I, dissolved P concentrations were the lowest in the Cu solutions, followed by the Cd, Zn, and Ni solutions. As noted earlier, this was also the same as the order of inhibition of Pb attenuation by these metals (Figure 2). It seems likely that the presence of Cu, Cd, Fe(lI), or Zn inhibited the formation of HP through the precipitation of some amorphous to poorly crysta\line, mixed metal phosphates. The inclusion of Ca into these solids would account for the decreases in dissolved Ca observed at initial Pb concentrations of ~241 "mol L-I and M/Pb ratios >1 (for Cd, Cu, Fe(lI), and Zn) (Figure 6). However, there is little correlation between ionic radii and their effective­ness in inhibiting Pb immobilization by hydroxyapatite (Table 3), suggesting that the effects of substitutions of these metals for Ca on HA is limited.

The preserice of M ions could also have inhibited the attenuation of dissolved Pb by HA through direct par­ticipation in the reaction described in eq 2. Thus, coprecipitation ofM and Pb with phosphate could explain the decrease in XRD-detectable HP at higher concentra­tions of M and the observed increases in final Pb concentrations. However, it is not apparent how the inclusion of foreign metals into eq 2 would result in final pH values that would differ for each metal ion, nor would such a reaction account for the changes in Ca solubility unless it was also included in any precipitates. It is reported that Cd is readily incorporated as substituents in HA; whereas, Zn, AI, and Fe(ll) suppress crystal growth with significant incorporation (13). The extent to which these elements can be accommodated by the HP structure is not known.

Whereas it is clear that some if not alI of the retention of metal ions by HA was due to the precipitation of secondary solids, some adsorption of Pb and M ions by HA can not be ruled out with the present data. Indeed competition between Pb and M ions for limited adsorption sites may in part be responsible for the reversals in slope observed for the Cd, Ni, and Cu sorption isotherms (Figure 10). However, such competition could also occur for lattice sites in a precipitation reaction. Clearly, the appearance of secondary solids (Figure 7 to 9) indicates that precipi­tation reactions were dominant in the present study.

The apparent equilibrium of Al with amorphous AI­(OHla and the supersaturation with respect to variscite (Table 1) suggest that some form of Al solid formed in the HA samples reacted with Pb and AI, but as was previously noted no solids other than HA and HP could be detected by XRD in the samples containing an AI/Pb ratio of 1 (Figure 3). However, XRD-detectable HP was completely absent at an AI/Pb ratio of 7. If amorphous Al(OH)3 did form in these samples, it should have served as a significant adsorptive sink for Pb; yet Al was most effective in inhibiting attenuation of dissolved Pb when present at initial concentrations ~1.45 mmol L -I (Figure 1). Itseems most likely that, at the higher initial Al concentrations, precipitation of amorphous variscite occurred, excluding Pb from the solid phase.

The effectiveness ofPb immobilization by HA was tested in the presence of various concentrations of AI, Cd, Cu, Fe(ll), Ni, and Zn. Hydroxyapatite reduced initial Pb concentrations of 24.1 "mol L-I to below 15.0 nmol L-I in the presence of !r3.37 mmol L-Iof dissolved AI, Cd, Cu,

3500 500 ~ AI 1400 Fe(lI) • 450 6. Cd

3000-1200 400 6.

2500 • • 350 ~ • 1000 • • 2000 6. 300

800 • 6.. 250 • 1500 • e. Initial Pb

"0) ~ 600 • 200 concentration .:.::

(5 1000 ~ 400 1 150 • (j.lmol L-1) E • 100 .s 500

i 200

c 50 0

0 0 0 24.1 .. :J I I I I

5i 0 0.5 1.5 2 2.5 0 1 2 3 4 2 3 4 E • 121 E -g 160 450 6. 241 > 2500- • Zn Ni Cu 0 140 •• 400 E Q)

~ . • • 482 .... 2000- 120 350 1/1 • iii Q) 100 ~ .6.

300 ::E 1500- 6. 250

6.. 80 • 200 1000 • 60 6. 6. • fit>. P

150 • .. 40 100 500 • 20 50

0 0 . 0 2 3 0 2 3 4 2 3 4

Final metal concentrations (mmol L'1)

Figure 10. Relationships of metals [AI, Cd, Cu, Fe(II), Ni, and Zn] removed from solution with the final dissolved metal concentrations.

Fe(II), Ni, or Zn. Attenuation of aqueous Pb from solutions containing greater initial Pb concentrations occurred to a lesser extent. In essentially all cases, the observed attenuation of Pb is consistent with the dis­solution of HA and the subsequent precipitation of HP. When HA was reacted with higher total concentrations of Pb and M, there was a decrease in the attenuation of aqueous Pb, and in the cases of Fe(II) and AI, a disap­pearance of HP as a reaction product. Nevertheless, formation of secondary precipitates was still the primary mechanism for attenuation of aqueous Pb and M ions.

It is apparent that, when present at concentrations <169 /-Lmol L-l, dissolved AI, Cd, Cu, Fe(II), Ni, and Zn will not significantly reduce the ability ofHA to attenuate dissolved Pb. Formation of XRD-detectable HP at Pb concentra­tions of 482 /-Lmol L-l and M/Pb ratios of 1 suggests the potential of using HA for in situ stabilization of Pb in contaminated soils and sediments. Although the forma­tion of HP was reduced at Zn, Cd, and Cu concentrations of 3.37 mmol L-l and did not occur at all when Fe (II) and Al were added at these levels, adsorption of these ions by hydrous metal oxides (hydroxides), organic matter, and aluminosilicate clays will generally maintain much lower dissolved M concentrations. Thus, it seems likely that HA can be used for effective, in situ stabilization of Pb in Pb-contaminated soils and sediments.

Acknowledgments

Funding was provided by the U.S. EPA (Contract CR-816843-01-0) through a cooperative agreement with The Ohio State University (OSU). Salaries and research fund were also provided in part by state and federal funds

appropriated to OSU/ OARDC. This is OARDC Journal Article No. 189-93. We wish to acknowledge Dr. J. M. Bigham, Agronomy Department, OSU, for his assistance in using XRD. We would also like to thank Mr. Shaoqin Yao for laboratory assistance.

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Received for review July 14, 1993. Revised manuscript received March 15, 1994. Accepted March 22, 1994.111

III Abstract published in Advance ACS Abstracts, May 1, 1994.