Environ. Sci. Technol. 1994, 28, 1219-1228
Effects of Aqueous AI, Cd, Cu, Fe(II), Ni, and Zn on Pb Immobilization by Hydroxyapatite QI Ylng Ma'
Department of Soil and Water Science, University of Florida, Gainesville, Florida 3261 1 Samuel J. Tralna and Terry J. Logan
Department of Agronomy, The Ohio State University, Columbus, Ohio 43210 James. A. Ryan RREL, U S . EPA, Cincinnati, Ohio 45268
Table 1. Saturation Indices of Selected Solids after The effects of aqueous Al, Cd, Cu, Fe(II), Ni, or Zn on P b Reaction of Hydroxyapatite with 482 pmol of Pb L-1 immobilizationby hydroxyapatite [Calo(P04)6(OH)2]were Solutions, in Presence of Different Metal/Pb Molar Ratios studied. Lead was removed mainly via hydroxyapatite saturation indices dissolution and hydroxypyromorphite [ P ~ I O ( P ~ & ( O H ) ~ I precipitation in the presence of these metals with a P b 1 3 metals minerals 5 7 removal efficiency of 37-100%. These metals inhibited -0.41 0.46 0.15 A1 Al(OH)3 (ama) -0.32 P b immobilization by hydroxyapatite in the order: A1 > AlPO4.2HzO(vaa) 2.3 2.1 3.4 3.4 Cu > Fe(I1) > Cd > Zn > Ni and Cu > Fe(I1) > Cd > Zn -8.1 -9.0 -10 CadPOdsOH -5.3 3.3 2.9 -7.2 1.6 Pbs(POr)sOH > A1 > Ni at high and low initial P b concentrations, cu Cus(PO4)z -2.5 -3.3 -2.1 -0.55 respectively. The inhibition was probably through the -14 -14 -12 Caa(PO4)aOH -8.2 precipitation of amorphous to poorly crystalline metal 0.80 2.8 5.4 PbdP04)aOH -2.6 phosphates, decreasing the amount of dissolved P available Fe(I1) Fez(P04)3.8H20(via) -5.8 -5.8 -5.9 -5.9 -11 -15 -17 -18 Caa(PO4)aOH for precipitation with dissolved P b ions. Hydroxyapatite -3.3 -3.2 -7.7 -3.8 Pbs(P04)sOH was effective in removing these added metals, especially Ni Nia(P04)2 -1.9 -0.92 -0.91 -0.77 at low concentrations. Hydroxyapatite selectively re1.1 0.19 -0.95 -1.4 Caa(PO4hOH moved P b from solution in the presence of aqueous Al, -2.4 -2.2 -9.7 -3.2 Pbs(POdaOH Cd, Cu, Fe(II), Ni, or Zn. The results support our earlier -4.7 -5.2 -6.0 Zn Zns(PO4)z -4.5 -8.3 -11 -2.2 -6.0 CadPOhOH finding that hydroxyapatite has the potential to be used -0.58 -0.57 Pbs(POr)sOH -0.29 -1.4 for in situ immobilization of P b in P b contaminated soils and wastes. Oam, va, and vi denote amorphous, variscite, and vivianite, respectively.
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 landfill leachates (2). Likewise, there is growing interest in in situ reduction of P b bioavailability in soils contaminated by leaded gasoline,lead based paints, and lead batteries disposal. Commonly encountered metals of concern include Cu, Ni, Cr(II), Cd, Pb, Hg(II), Zn, and Ag(1) (3). Lead was chosen for study due to its extent of contamination of landfills, wastewaters and soils. Hydroxyapatite (HA) [Calo(P04)6(OH)21has 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 P b in contaminated soil material (7). Hydroxyapatite can reduce initial dissolved 0013-936X/94/0928-1219$04.50/0
0 1994 American Chemical Society
Table 2. Saturation Indices of Selected Solids after Reaction of Aqueous Pb with Hydroxyapatite in Presence of Different Cd/Pb Molar Ratios initial P b (pmol L-1)
minerals
1
-2.8 -1.6 -0.38 -3.0 0.69 0.42 -4:8 3.3 -0.53 -7.5 4.5 -3.7
saturation indices 3 5 -3.4 -6.1 1.3 -2.4 -2.8 0.44 -2.4 -0.44 -0.29 -10
4.2 -3.4
-4.9 -10 1.5 -2.5 -4.6 0.12 -2.6 -2.9 0.26 -5.9 3.5 -2.8
7 -5.7 -13 0.87 -2.8 -6.1 0.42 -3.1 -4.7 -1.9 -4.1 2.5 -1.7
P b of 24.1-2410 pmol L-l to below 95 nmol L-l in 0.5 h, indicating that P b immobilization by HA is rapid and effective. Aqueous P b in Pb-contaminated soil material was reduced from 11.0 to 0.17 pmol L-l by HA. We found that hydroxypyromorphite (HP) [Pblo(P04)6(OH)21is the final reaction product in most cases and is formed through HA dissolution followed by H P precipitation (7).We also found that HA can effectively immobilize aqueous P b in the presence of varying levels of dissolved NO3,C1, F, SO4, and CO3, although F and COS lowered the effectiveness of HA in immobilizing P b at high initial concentrations Environ. Scl. Technol., Vol. 28, No. 7, 1994 1219
0.30 ,
300 , 100
0.10
10 1.o
0.01
0.1
7
i
Initial Pb concentration
0.01
h
ji
(pmol L-1)
0.001
-
0.0001
.-0
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 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
0.0001 0*0°' 0 1 2 3 4 5 6 7 8
Metal/Pb molar ratios
-I
Figure 1. Final concentrations of aqueous Pb alter reaction of 24.1-482pmol of Pb L-I with HA in the presence of different concentrations of aqueous AI, Cd, Cu, Fe(II), Ni, or f n . The bars stand for standard deviatlons of trlplicates.
loo]
.j
100- y -%.*
80-
80-
60-
60-
*,\N.
Y initial Pb concentration
40-
40
I
I
,
I
I
20
0 1 2 3 4 5 6 7 8
401FE 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
20 0 1 2 3 4 5 6 7
0 1 2 3 4 5 6 7 8
Metal/Pb molar ratios Flgure 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. Technoi., Vol. 28, No. 7, 1994
MetaVPb ratio = 7
700
4
I
700
4
400 -
300-
300 -
1
100-
.
AI
0 20
25
30
35
40
45
20
! O
30
35
40
45
20
Flgure 3. XRD patterns of hydroxyapatlte (HA) after reaction with 482 Hmol of Pb L-I at metai/Pb molar ratio of 1 (A) and 7 (B). HP denotes hydroxypyromorphlte.
of dissolved Pb, F, and COS(8). Lead immobilization was again via a mechanism of dissolution and precipitation; H P formed after HA reacted with aqueous P b in the presence of NO3, SO4, or COB;and chloropyromorphite [Pblo(PO4)&12] and fluoropyromorphite [Pbl0(P04)6F21 formed in the presence of C1 and F, respectively. Clearly, HA has great potential to immobilize P b in Pbcontaminated soils and wastes. Hydroxyapatite is well-known for its isomorphous substitution as expressed by the formula: Nlo(ROr)6Yz where N = Ca2+,Pb2+,Na+, K+, Sr2+,Mn2+,Zn2+,Cd2+, Mg2+,Fez+,and A13+;R04 = P04%,co32-,HP0d2, P207&, As04%, V0d3-, S042-,and sio46; and Y = OH-, F-, C1-, HzO, 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 P b in the pyromorphite series (10). Whereas, many studies have focused on the interactions of HA with there heavy metals such as Cd, Cu, Ni, and Zn (6,11-13), is little published data on the effects of these metals on P b immobilization by HA. Our previous study showed that, in the absence of other metals, HA dissolution and H P precipitation were the main mechanisms for P b immobilization by HA (7). These chemical reactions can
25
In the presence of aqueous AI, Cd, Cu, Fe(II),Ni, or Zn
be described as (14) Cal0(PO4),(OH),(c) + 14H+
+
10Ca2+ 6H2P0,
-
dissolution
+ 2H20
+ 2H20 precipitation Pblo(P04)6(OH)2(C)+ 14H'
log k", 28.92 (1)
10Pb2++ 6H2P0,
log k", -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 P b 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 P b immobilization by HA is warranted. The present study examines the effects of dissolved metal ions (M) on PWHA interactions, where M represents Al, 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 P b were reacted with HA in the presence of varying Envlron. Scl. Technol., Vol. 28, No. 7, 1994 1221
70.1-
7.0
6.5
6.5
6.0
6.0
5*51
5.5
5.0
5.0
4.51
1 Ni 54.0, r
0
Initial Pb concentration
4.5-
I
I
I
,
,
,
I
1
2
3
4
5
6
7
4.5
1 Zn 4.0, ,
I
I
I
,
I
I
0
2
3
4
5
6
7
8
1
(pmol L") -8- 24.1
4.0 8
0
6.5
6.5
6.0
6.0
5.5
5.5
5.0
5.0
4.5
4.5
1-0.4
4.0 0
1
2
3
4
5
6
7
8
I
1
I
2
I'
3
I
4
I
5
I
6
I
7
I
8
-c
121
-A-
241
I
0 1 2 3 4 5 6 7 8
MetaVPb molar ratios Final solution pH values after reaction of HA with 24.1-482 pmol of aqueous Pb L-I, in the presence of different concentrations of aqueous Ai, Cd, Cu, Fe(II),Ni, or Zn. The bars stand for standard deviations of triplicates. Flgure 4.
levels of Al, Cd, Cu, Fe(II), Ni, or Zn to test the effects of these metals on Pb immobilization by HA. One-tenth gram of HA (Bio-Rad)was reacted with 200 mL of solutions containing 24.1, 121, 241, and 482 pmol of P b L-1 as Pb(NO&. The HA/Pb molar ratiosranged from 41.2 to 2.06. At each Pb level, four different metal concentrations were used: 1,3,5, and 7 times the respective P b concentration on a molar basis as nitrate salt; Fe(I1) was added in the form of Fe(C104)~ (the nitrate salt could not be obtained). All solutions were adjusted to pH 6 with dilute "03 or NaOH, except Fe(C104)2and Al(N03)3which were adjusted to pH 4.71 and 4.62, respectively, to prevent precipitation of iron(I1) and aluminum hydroxides. The suspensions were shaken for 2 h and then filtered through 0.2-pm Nucleopore polycarbonate membrane filters. The filtrates were analyzed for totalp, Pb, Ca, Cd, Cu, Ni, Zn, Al, 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 PO4was 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 HC1) 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/O.O5O 20. A total of 601 data points were obtained from 15 to 4 5 O 28. 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 MINTEQAZ chemical equilibrium speciation program (16)was used to calculate the equilibrium distributions and activities of aqueous species using total dissolved Pb, Ca, Al, Cd, Cu, Fe(II),Ni, Zn, NO3, and PO4 concentrations and solution pH as the initial inputs. Thermodynamic stabilities of each solid were established by comparing the appropriate ion activity products (IAP)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
h
I
30-1
,
I
80
I
30-1
.
70,
initial Pb
concentration (pmoi c')
'1 0 .+
0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
m
Cd 0 1 2 3 4 5 6 7 8
7
500
1
400
80 60 40
100
20 0
I
I
I
I
I
I
I
0 1 2 3 4 5 6 7 8
0 0I 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 reactlon of 24.1-482pmol of aqueous Pb L-l, 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.
Results
in immobilizing P b in the presence of the added metals was in the order: A1 C Cu C Fe(I1) < Cd < Zn C Ni, at high initial P b concentrations. In more dilute P b solutions, the order was Cu < Fe(I1) < Cd C Zn C A1 C Ni.
Added metals reduced the effectiveness of P b immobilization by HA as shown by higher final P b concentrations compared to those of the controls (samples in which P b was the only added metal ion). This was especially evident at higher initial P b concentrations and higher M/Pb molar ratios (Figure 1). At initial P b concentrations of 24.1 pmolL-l, the final P b 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 P b immobilization by HA. However, at initial P b concentrations of 121-482 pmol L-1, final P b concentrations were much higher, ranging from 10.8 nmol L-l to 306 pmol L-l, and increased with an increase in both initial P b levels and M/Pb molar ratios. Nevertheless, HA reduced P b concentrations significantly in spite of the presence of high levels of the added metals. Nickel had little effect on P b immobilization by HA (Figure 2). Al, Cd, and Zn caused decreases in P b immobilization by HA only at the greatest initial P b concentrations and at M/Pb ratios greater than 1. Maximum inhibition was approximately 64, 22, and 6 % , respectively. Copper and Fe(I1) exhibited the greatest inhibition on P b immobilization (Figure 2). This occurred at all but the lowest P b level and at most M/Pb ratios greater than 1. Maximum inhibition by Cu and Fe(I1) was 63 and 37 % , respectively. The effectiveness of HA
Hydroxypyromorphite was detected by XRD after HA reaction with 482 pmol of P b L-l, except at a M/Pb molar ratio of 7 in the presence of Fe(I1) or Al, and HA was present after the reaction in all treatments (Figure 3). The H P peak intensity decreased in the order: Ni > Zn > Cd > Cu > Fe(I1) > Al. Similar but less intense H P patterns were observed for initial P b concentrations of 241 and 121 pmol L-l, and no H P peaks were observed with initial P b concentrations of 24.1 pmol L-l (data not shown). If all of the P b removed by HA, at an initial Pb concentration of 24.1 pmol L-I, were precipitated as HP, then there should have been at least 6 % H P (by weight) present as a reaction product. In the presence of Fe(I1) and Al, there should have been about 15 and 9% H P present, respectively, at a M/Pb molar ratio of 7 and initial P b concentrations of 482 pmol L-I (based on the amount of dissolved P b removed from solution and an estimate of the amount of HA that dissolved). Since XRD can normally detect crystalline solids present at concentrations 1 1 wt % of the sample matrix, the absence of H P peaks in some of the reaction products suggeststhat a mechanism other than that described in reaction 2 was responsible for a t least some of the P b attenuation in this study. Whereas, coprecipitation of P b with aluminum/iron [Fe(II) or Fe(II1)l hydroxides and/or adsorption by Al, Fe hydroxides could have caused the lack of visible H P peaks in Figure
is an indication of degree of under- or supersaturation with respect to a solid phase.
Environ. Sci. Technol., Vol. 28, No. 7, 1994 1223
1.9
I
1.3 Initial Pb concentration (pmol L')
.-wU m
1+ 0 1 2 3 4 5 6 7 8
0 1 2 3 4 5 6 7 8
482
0 1 2 3 4'5 6 7 8 )
! 241
1.9 1.74
cu
-e- 121
'
0 1 2 3 4 5 6 7 0
9 1 2 3 4 5 6 7 8
24.1
0 1 2 3 4 5 6 7 0
MetaVPb molar ratios Figure 8. Final Ca concentrations after reaction of 24.1-482 @molof aqueous Pb L-', with HA in the presence of different concentratlons of aqueous AI, Cd, Cu, FeUI), Ni, and Zn. The bars stand for standard devlations of triplicates.
3b, no diffraction patterns of any Fe or A1 solids were detected. Solution pH and P concentrations decreased, while Ca concentrations increased with increasing P b concentrations (Figures 4-6). Generally speaking, solution pH decreased in the order: Ni > Zn > Cd > A1 > Cu > Fe(I1) except at initial P b concentrations of 24.1 pmol L-l. 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 P b concentrations (Figure 5 ) . At an initial P b concentration of 482 pmol L-l, dissolved P concentrations decreased in the order AI > Ni > Zn > Cd > Fe(1I) > Cu, and at an initial P b concentration of 121 pmol L-1, the order was Ni > A1 > Zn, Cu > Cd > Fe(I1). Dissolved Ca concentrations decreased in the order Cu > Cd > A1 > Fe(I1) > Zn > Ni at an initial P b concentration of 24.1 pmol L-I, and the order was A1 > Cd, Fe(II),Zn > Cu, Ni at initialPb concentrations of 121-482 pmol L-1. In general, Ca concentrations were the lowest and highest in the presence of Ni and Al, respectively, excludingtreatment of initial P b concentration of 482 pmol L-l (Figure 6). Needle-shaped HP crystals were observed on HA surfaces after reaction with P b in the presence of Ni, Cd, or Zn, while little to no H P precipitates were visible on HA surfaces in the presence of Cu, Fe(II), or A1 (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(I1)and Al, but again no Fe or A1 solids were detected by XRD. 1224 Environ. Scl. Technol., Vol. 28, No. 7, 1994
MINTEQAP 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 P b concentration of 482 pmol L-l (Tables 1 and 2). The degree of undersaturation with respect to HA increased in the order Ni < A1 < Zn < Cd < Cu < Fe(II), which was similar to the order of the effectiveness of HA in immobilizing P b in the presence of these metals. This trend was also observed for initial P b concentrations of 24.1-241 pmol L-' (data not shown). Saturation indices of H P and Cds(P04)~ increased, while those of HA decreased as initial P b 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 P b in the presence of Al, 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 dissolvedA1,Cd, Cu, Fe(II), Ni, and Zn concentrations increased with increased initial metal concentrations (increase in either initial P b concentrations or M/Pb molar ratios). In general, the effectiveness of HA in removing these metals was in the order AI > Zn > Fe(I1) > Cd > Cu > Ni; whereas, the order of the effectivenessof those metals in inhibiting P b immobilization by HA was A1 > Cu > Fe(I1) > Cd > Zn > Ni. Thus, the amount of metals removed was not related to its effectiveness in inhibiting P b immobilization by HA. In addition, the amount of Zn
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