Environ. Sci. Techno/. 1995, 29, 1118-1126
Lead- 1 .on from Aqueous Solutions and Contaminated Soils Using Phosphate Rocks m m
QI Y I N G M A * Soil and Water Science Department, University of Florida, Gainesville, Florida 3261 1-0290
TERRY J . LOGAN AND SAMUEL J. TRAINA School of Natural Resources, The Ohio State University, 2021 Coffey Road, Columbus, Ohio 43210
This research investigated the effectiveness of phosphate rocks in immobilizing Pb from aqueous solutions and contaminated soils. Different amounts of phosphate rocks were reacted with aqueous solutions containing Pb or with Pb-contaminated soils. The results showthat phosphate rocks were effective in immobilizing Pb from aqueous solutions with a minimum Pb removal of 38.8-100%. The reaction time had less effect on Pb immobilization than the quantity of phosphate rocks used. Selected phosphate rocks reduced water-soluble Pb from a contaminated soil by 56.8-100%. The primary mechanism of Pb immobilization was via dissolution of phosphate rocks and precipitation of a fluoropyromorphite-like mineral. Different methods of mixing phosphate rocks and soil and incubation time had little effect on Pb immobilization. Our results strongly demonstrate that phosphate rocks may provide a cost-effective way to remediate Pb-contaminated water, soils, and wastes.
1118 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 4,1995
Introduction Lead is awidespread constituent of the earth's crust. It has always been present in soils, ranging in total content from 2 to 200 pglg and averaging 16pglg (1). It is toxic to both humans and animals, especially to young children. As a result of worldwide accumulation, lead presents a more serious environmental and health hazard than do any other elements (2). Sources of Pb contamination in soils can be classified into three broad categories: agricultural activities such as application of insecticide PbHAs04, industrial activities such as mining and smelting, and urban activities such as use of lead gasoline and lead paint. Since 1910, approximately 11.4 million tons of Pb have been used in the United States in the forms of paint (43%)and gasoline additives (57%) (3). Much research has been conducted on remediation of Pb-contaminated sites by employing chemical, physical, or biological treatments, and significant progress has been achieved ( 4 ) . More recently, increasing attention has been given to in situ remediation of metalcontaminated soils (5). Significant theoretical and experimental evidence supports the hypothesis that lead phosphates are the most insoluble and stable forms of Pb in soils, and they can form rapidly in the presence of adequate Pb and phosphate. Among d the Pb-P minerals, chloropyromorphite has the lowest solubility, thus it is most stable under favorable environmental conditions (6). Formation of chloropyromorphite [Pblo(PO4)6Cl2]in soils contaminated through mining activities was reported (7). Cotter-Howells and Thornton imply that many Pb-rich grains were composed of chloropyromorphite, identified using scanning electron microscopy (SEMI,although the mode of formation of this mineral is unclear. Weathering of galena (PbS)to insoluble chloropyromorphite in soils contaminated by galena was also demonstrated (8). Ruby et al. hypothesize that formation of chloropyromorphite contributes to the low Pb bioavailibiilty in these contaminated soils. In a study of municipal landfill leachate, chloropyromorphite was indicated to be present in the system (9). However, actual identification of the mineral was not performed, instead an equilibrium speciation model 'MINTEQ' was used in their study. In laboratory batch experiments, it was reported that Pb solubility in soils decreased with an increase in P concentrations and that Pb solubility was governed by chloropyromorphite (10). In addition, trisodium phosphate has been used to precipitate Pb in wastes from TV picture tube manufacturing at Westinghouse for many years ( 1 1 ) . Furthermore, we have shown that hydroxyapatite [Calo(PO&(OH)2]is effective in immobilizing aqueous Pb in the presence of anions (Nos-, C1-, F--,S042-, or COS2-)or cations (A3+, Cd2+,Cu2+,Fez+,Ni2+,or Zn2+)(12,13). The primary mechanism of Pb immobilization in these systems is through hydroxyapatite dissolution and hydroxypyromorphite precipitation or through chloropyromorphite and fluoropyromorphite precipitation in the presence of C1and F-, respectively ( 1 3 ) . Moreover, we have also proven that not only hydroxyapatite but also phosphate rocks, whose composition is close to Cal,~(P04)6F2 with substantial COS2- substitution in the structure, are effective in immobilizing aqueous Pb (14). We have also demonstrated that hydroxyapatite is effectivein immobilizing Pb not only
0013-936X95/0929-1118$09.00/0
@ 1995 American Chemical Society
TABLE 1
Specific Surface Areas (SSA) and Chemical Analysis of Phosphate Rock Samples and Their Abbreviations cu hg/kiJ)
mining companies
states
abb
(m2 9-11
Ca (Oh)
P (Yo)
Agri-Chemical Corp. Agrico Chemical Co. C. F. Chemical, Inc. Occidental Chemical Corp. Nu-Gulf Industries, Inc. IMC Corp. NU-West Industries, Inc. calcined rock raw ore wash plant product Monsanto J. R. Simplot Co. phosphate acid plant pipeline slurry Texasgolf Inc. Comico Fertilizers
FL FL FL FL FL FL
AC ACC CF NG IMC
15.8 7.48 6.57 18.2 12.2 13.5
29.7 27.2 33.5 33.7 32.7 34.6
13.3 15.0 15.3 15.6 15.2 16.5
ID ID ID ID
NC NR NW MO
4.88 11.0 5.33 6.48
32.7 24.5 30.1 33.2
15.9 11.8 14.4 16.2
118 104 151 205
37 1 892 362 820
55.5 58.1 217 114
ID UT NC MT
SP
4.95 7.24 18.7 4.22
31.1 33.8 33.5 29.7
14.7 15.5 14.3 14.8
137 0.00 49.2 7.22
496 428 132 313
21.5
SSA
oc
ss TG cc
from aqueous solutions but also from Pb-contaminated soil (14). Thus, we can conclude, based on the above information, that the interaction of Pb and P, possibly forming hydroxypyromorphite, chloropyromorphite, or fluoropyromorphite, is an important buffer mechanism controlling the migration and fixation of Pb in water, soils, and wastes, thus reducing Pb solubility as well as bioavailibiilty. Finely ground phosphate rocks have been used as phosphate fertilizers for manyyears, especially in acid soils (15). Fluorapatite, the principal constituent of igneous phosphate rocks and a member of space group p63lm with a = 9.367 A, c = 6.844 A,and z = 2 occurs in massive and well-formed crystals (16). On the other hand, hydroxyapatite occurs only rarely in nature. The structure of hydroxyapatite is similar to that of fluorapatite, for OHcan occupy the sites of F- on the 6-fold axis. However, hydroxyapatite (logKD = 14.46) is much more soluble than fluorapatite (logK" = -0.21) (17). The apatitesin phosphate rocks are poorly crystallized, and their compositions differ considerably from those of pure apatites (18). Their chemical reactivity and thermal stability vary widely as a result, depending on the degree of isomorphic substitution of carbonate for phosphate in the fluorapatite crystal lattice. In general, the solubility of phosphate rocks increases with an increase in carbonate substitution (19, 20). The current research is a continuation of our previous studies(12-14). This study evaluatedthe feasibility of using phosphate rocks to attenuate Pb from aqueous solutions and contaminated soil. The objectives of this study were (a) to investigate the effectiveness of various phosphate rocks in reducing Pb concentrations from aqueous solutions and contaminated soils using both batch and column experiments; (b) to evaluate the properties of phosphate rocks that influence their efficiency of Pb immobilization; and finally (c) to examine the effects of different methods of mixing phosphate rocks and soil and incubation time on the efficiency of Pb immobilization.
Materials and Methods Lead Immobilization from Aqueous Solutions Using Phosphate Rocks. Fourteen phosphate rock samples from five states were collected for the current study. The samples were ground to fine powder using a disk mill, and their specific surface areas (4.22-18.7 m2 g-l), as measured by
Cd hg/kg) 8.15 7.45 5.26 6.36 2.45 7.82
Cr
hsncs) 48.2 58.0 63.3 57.6 56.9 71.2
0.00 0.00 0.00 0.00 0.00 0.00
0.00 0.00 0.00
Ni
Pb
hence)
(mance)
16.3 26.5 9.73 0.00 2.68 0.00
0.00 0.00 7.61 0.00 0.00 26.6
110 180 100 50.9 105 6.46 4.88 8.05
4.53
0.00 15.9
0.00 0.00 0.00 0.00 49.0
Zn (manta) 47.2 82.4 29.4 54.1 33.6 79.4 910 1350 1030 1610 1360 34.9 326 176
Nz-BET adsorption with a Micromeritics Flowsorb 2300 surface area analyzer are listed in Table 1. The samples were also digested using HF-HC1/HN03 dissolution in a Parr Bomb (21). The digests were analyzed by inductively coupled plasma (ICP) (Table 1). The most abundant elements in addition to Ca and P include Al, Fe, K, Na, Mg, S, and Si. Samples from Nu-West (NC, NR, and NW), Monsanto (MO), and the J. R. Simplot Company (SP) contained significantly higher Cd, Cu, Ni, Cr, and Zn than the other phosphate rock samples. X-ray diffraction (XRD) analysis of these samples indicates that they are mainly fluorapatite [Ca10(P04)6F21 or carbonated fluorapatite [Calo(PO4,C03)6F21, and the second most abundant mineral was quartz (Si02). Trace amounts of dolomite [CaMg(C03)zl, calcite (CaC03),or gypsum (CaSO4*H20) were also present in some samples. Samples of phosphate rocks (0.1 or 0.2 g) were reacted with 200 mL of 4.82 or 48.2 pmol Pb L-' as Pb(N03)~.The phosphate rocklPb ratios ranged from 500 to 50 glg. Deionized distilled water of 200 mL was used as a control. The suspensions were shaken continuously from 2 h to 2 d. Lead Immobilizationfrom a Contaminated SoilUsing Phosphate Rocks. In this study, column experiments were used to evaluate the feasibility of using phosphate rocks to immobilize Pb from a contaminated soil and to evaluate the effects of different methods of mixing phosphate rocks and soil and incubation time on Pb immobilization efficiency. The two most effective Florida phosphate rocks in immobilizing Pb [C. F. Chemical (CF) and Occidental Chemical (OC)]were chosen. The soil (Burch)was collected from an old apple orchard in Washington State that has been contaminated by many years of applications of PbHAsO4 insecticide and had a total Pb content of 2560 mglkg via the same digestion method as phosphate rocks. A total of 10 g of Burch soil was mixed with 0, 1, 2, or 4 g of phosphate rocks. After uniform mixing, the soilphosphate rock mixtures were placed in 60-mL plastic syringe cylinders containing a layer of m e x glass fibers in the bottom to prevent solid loss during leaching. HPLCgrade water was added to bring the solidlsolution ratio to 2:l. Each column was covered with a polyurethane foam plug to reduce water vapor loss yet allow adequate air VOL. 29, NO. 4,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
1
1119
exchange and was incubated at room temperature for 1,7, and 16 d. After 1-dincubation, 60 mL of deionized distilled water was leached through each mixture for 24 h using a Zero-Max extractor Model 24-01. The samples were then returned for continued incubation and extracted again at the end of 7 and 16 d. Thus, all samples were extracted three times, which simulates continuous leaching in soils by rain. In the following three column studies, the same quantity of phosphate rocks and Burch soil were used via the same extraction method as above. The only difference among these experiments were the methods of mixing phosphate rocks and Burch soil. In one study, 0, 1, 2, or 4 g of phosphate rocks was mixed with 30 g of Ottawa reagentgrade sand and placed at the bottom of the syringes, and 10 g of Burch soil was placed on the top of the mixtures of phosphate rocks and sand. The sand was used as a supporting matrix for phosphate rocks. Thus, phosphate rocks and the contaminated soilwere placed in the column without any physical mixing, and only the soil leachates had physical contact with phosphate rocks. In a different study, 10 g of Burch soil was first incubated with water at field capacity for 2 d, and then the collected soil leachate was passed through the column packed with the mixtures of 0,1,2,or 4 g of phosphate rocks and 30 g of reagent grade sand. In another study designed to investigate the effect of incubation time on Pb immobilhtion from a contaminated soil by phosphate rocks, Burch soil was mixed with 0, 1, 2, and 4 g of phosphate rocks and incubated for 2, 7 , 14,28,or 56 d. At the end of eachincubation time, a different set of samples was extracted, which differed from the sequential leaching mentioned in the first column experiment that leached the same set of samples. Analytical Method. The suspensions in all studies were filtered through 0.2-pm Nucleopore polycarbonate membrane filters. Perkin-Elmer 3030B and 4100ZL atomic absorption spectrophotometers were used to analyze total metal concentrations. Total dissolved P was measured colorimetrically with a Beckman DU-6 spectrophotometer (22). Solution pH was measured with an OrionIRoss combination electrode and Orion EA 920 pH meter. All experimental treatments in this study were prepared in triplicate and were conducted in acid-washed (2% "03) polycarbonate labware. Selected samples were observed with a JEOL JSM-A20 scanning electronic microscope (SEM; JEOL, USA Inc., Peabody, MA) using a Tracor Northern (5500) analyzer equipped with 5502 upgrade. The samples were mounted on stainless steel stubs using double-stick tape and then coated with Au and Pd.
,
1
Results and Discussion Lead Immobilization from Aqueous Solutions Using PhosphateRocks. The effectiveness of various phosphate rocks in reducing aqueous Pb concentrations in the current study varied considerably. When the initial Pb concentrations were 4.82 pmol L-1, all the phosphate rocks were effective in reducing Pb concentrations to below 72.4 nmol L-' (the current EPA action level for Pb) after 2 h of reaction (Table 2), and 98.5-99.9% of the added Pb was removed by phosphate rocks. However, when the initial Pb concentrations were increased to 48.2 ymol L-l, the effectiveness of the phosphate rocks decreased, with only the CF sample effectively reducing Pb concentrations to tolerable levels (Table 2). Nevertheless,the percentage of Pb removed 1120 =ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4, 7995
v)
.-e 0
U
m
2100
1800
1500
Y t
3
1200
8
9
J=
m
900
Q)
a
600
300
20
25
30
35
20
45 0
I
25
28
FIGURE 1. XRD patterns of 0.1 g of phosphate rocks before and after reaction with 200 mL of 48.2 pmol
ranged from 38.8 to 100.0%, indicating that all of the phosphate rocks could reduce Pb concentrationseffectively. The current results are in agreementwith our previous study in which we have shown that phosphate rocks from North Carolina, Florida, and Idaho reduced aqueous Pb concentrationsfrom4.82-24.1 to below0.012-3.79pmolL-1,with a minimum reduction of 84% (14). In the absence of added Pb(N03),the final Pb concentrations in the blanks (phosphate rocks with deionized distilled water only) were low, ranging from 0.00 to 6.02 nmol L-l and Pb concentrations in 71% of the samples were below 1 nmol L-l in the current study. This indicates that the phosphate rock samples contained no significant soluble Pb, and thus the addition of natural phosphate rocks to Pb-contaminated soils or natural water would impose no Pb hazard to the environment. Generally speaking,the effectivenessof phosphate rocks in removing Pb was increased significantly by increasing the amount of phosphate rocks added at the same initial Pb concentrations. A smaller increase in Pb removal was obtained by increasing the reaction time from 2 to 6 h; however, Pb removal efficiencyimproved greatly when the reaction time increased to 1 or 2 d (Table 2). For example, the percentage of Pb removed by sample CC increased from 38.8 to 45.9%when reaction time increased from 2 to 4 h; it increased to 61.6% when the amount of CC added was doubled (Table 2). In another example, the percentage of Pb removed by sample NC increased from 58.1 to 91.7% when reaction time increased from 2 h to 1 d, and it increased to 97.5% when the amount of NC added was
45
4
L-'
Pb for different times
doubled (Table 2). Although only selected samples were tested for different reaction times, we can still conclude that both reaction time and the amount of phosphate rock added were important for Pb removal by phosphate rocks. In an earlier study (13), we have shown that Pb immobilization by hydroxyapatite in the presence of Fdissolution was mainly through fluorapatite [Ca10(P04)6F21 and fluoropyromorphite [PbdP04)6F21precipitation:
10Pb"
+ 6HzPO;
f
2FPb,,(PO,),F,(c)
+ 12Hf
(2)
Similarly, we propose that dissolution of phosphate rocks and precipitation of carbonated fluoropyromorphite-like mineral is the primary mechanism for Pb removal by phosphate rocks, which can be expressed by similar equations as above. Assuming equal presence of P04,and C032-, F- and OH- as an example:
VOL. 29, NO. 4,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
1121
,'
c
L. 6'
.-.
.
-
.ii
FIGURE 2. SEM micrographs of CF phosphate rock before and alter reaction with 2W mL of 48.2pmol L Pb.
'
10PbZi
+ 3H,PO; + 3CO;- + F- + OH-
prrcipi,a,ion
Pb,o(P0,)3(C0,)3FOH(c)+ 6Ht (4) According to the above hypothesis, the higher the solubilityofapatites. themoreeffectivethey arein reducing Pb concentrations since the main function of apatites, in this case, is to supply P to precipitate Pb in aqueous solutions. We have shown (14) that the more soluble the hydroxyapatite, the more effective it is in reducing Pb concentrations. Thus, hydroxyapatite should be more effective than phosphate rocks in immobilizingPb as shown in the following example. The effectiveness of apatites in removing Pb can be measured based on the quantity of Pb removed perunitofapatitesused. The followingtwocases represent the highest Pb removal capacity ofhydroxyapatite and phosphate rocks among all the studies we conducted so far. In one of our earlier studies (141, 0.2 g of hydroxyapatite (Bio-Rad)attenuated Pb concentrations in 50 mL of solution from 500 mg L-' to 4.46 pg Le', which convened to a Pb removal capacity of 0.125 g of Pb/g of hydroxyapatite. In the current study, the most effective phosphate rock CF reduced Pb concentrations of 200 mL from 10 mg L-' to 1.32 pg L-', which was equivalent to a Pb removal capacity of 0.02 g of Pb/g of phosphate rock (Table 2). Apparently, hydroxyapatite is about six times more effective in reducing Pb concentrations than 1122. ENVIRONMENTAL SCIENCE & TECHNOLOGY/ VOL. 29. NO. 4.1995
FIGURE 3. SEM micrographs of OC phosphate rock before and after reaction with 200 mL of 48.2 pmol 1-' Pb.
phosphate rocks on a unit solid weight basis as expected from differences in theirsolubilities. The lower Pb removal efficiency can be attributed to lower solubility and purity of phosphate rocks than hydroxyapatite. Six samples that had the highest Pb removal capacity were selected for XRD analysis. All these samples were treated with 48.2 pmol L-' Pb and reacted with 0.1 g of phosphate rocks for different times. Assuming that all the Pb removedbyphosphate rock formed fluoropyromorphite, then the maximum fluoropyromorphite in the samples would have ranged from 2.55 to 2.19% on a solid weight basis, exceeding the hypothetical XRD detection limit of 1 wt. %. On the other hand, if all the Pb removed by phosphate rocks containing significant amounts of carbonate formed hydrocerussite lPb3(C03)2(OH)~l, the maximum amount of hydrocerussite formed would be about 3.25wt. %, Subsequent to reaction with dissolved Pb, XRD analysis indicated the formation of carbonated fluoropyromorphite in samples OC, CF, and S P hydrocerussite in sample NC; and both fluoropyromorphite and hydrocerussite in sampleSS. However, no Pb-containingmineral was detected in sample MO (Figure 1). In all instances, only one XRD peak could be identified as fluoropyromorphite-like mineral. The results again support our earlier hypothesis that phosphate rock dissolution and fluoropyromorphite precipitation were the primary mechanisms for Pb removal. However, hydro-
45 -8- 0 . 1 g P R + 2 h
8*4
8.2
40
35
30
25
20
7.2
15 0.0
9.6 19.3 28.9 38.6 48.2
,C 0.0
I
I
1
I
9.6 19.3 28.9 38.6 4E
Initial Pb concentrations (pmol L'' ) FIGURE 4. Changes in Ca and P concentrations and filtrate pH after reaction of CF phosphate rock (PR) with aqueous Pb.
cerussite formation indicates that other Pb minerals can also form when conditions are favorable (Figure 1). The source of C03*- may have been from dissolution of phosphate rock as described in eq 3. Given enough time, hydrocerussite will likely convert to fluoropyromorphite since the latter is more stable thermodynamically. Although about equal amounts of dissolved Pb were removed by samples OC and MO, the absence of any XRD-detectable Pb-minerals in the sample MO suggested that other mechanisms, such as the formation of poorly crystalline or noncrystalline solids, cation substitution, or adsorption, may have played important roles in Pb immobilization by these phosphate rock materials. The presence of fluoropyromorphiteswas also supported by SEM micrographs (Figures 2and 3) in addition to XRD data (Figure 1). Needle-shaped minerals were observed in samples CF and OC. On the other hand, no differences were observed by SEM between the untreated samples and those treated with Pb in samples SS, SP, and TG (data not shown). The results again indicate that mechanisms other than precipitation may also be important in the reactions of Pb with phosphate rocks. In general, the final Ca concentrations increased (with some exceptions),and the final P concentrations and filtrate pH decreased with an increase in initial Pb concentrations, reaction time, or the amount of phosphate rock added (data not shown). A typical example of changes in Ca and P concentrations and filtrate pH for sample CF is given in Figure 4. It is probable that, as more Pb was precipitated as fluoropyromorphite, more P was consumed and more H+ was released to solution resulting in lower P concentrations and filtrate pH (eq 4). Furthermore, lower pH and P concentrations caused more dissolution of phosphate rock, thus releasing more Ca into solution. Clearly, dissolution
of phosphate rocks and subsequent precipitation of Pbphosphate was a dominant mechanism in most of these systems. In contrast, the dissolved Ca concentrations of samples CC, IMC, and MO decreased with an increase in either reaction time or the amount of phosphate rock added, possibly due to either co-precipitation of Ca with fluoropyromorphite or precipitation as calcite (data not shown). Generallyspeaking,higher finalCa and P concentrations or lower solution pH in the blank treatments (phosphate rocks with deionized water only) indicate higher solubility of a phosphate rock. For a given phosphate rock, higher specific surface areas increase its dissolution rate. The relationshipsbetween Pb removal efficiency (asdetermined by percent Pb removed by phosphate rocks) and solubilities of phosphate rocks (as indicated by final concentrations of dissolved Ca and P, the solution pH in the blanktreatments, and the specific surface areas of phosphate rocks) were examined with least-square linear regression analyses. All four variables were correlated to Pb removal efficiency to some extent. Lead removal efficiency of 48.2 pmol L-l initial Pb treated with 0.1 g of phosphate rock had better correlation with final Ca and P concentrations and filtrate pH in the blank treatments ( r = 0.30, -0.28, and -0.60, respectively) than did those of 4.82 pmol L-l initial Pb ( r = -0.03, -0.12, and -0.32, respectively). The results thus suggest that solubility of phosphate rocks is more critical for Pb removal at higher initial Pb concentrations as shown by its better correlation. Specific surface area is important only at lower initial Pb concentrations (r = -0.39 at initial Pb concentration of 4.82 pmol L-' compared to r (0.09 at initial Pb concentrations of 48.2pmol L-l). However, in all instances, the absolute values of the correlation coefficients were low. Among all four variables, pH seemed to be the VOL. 29, NO. 4, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY
1123
TABLE 3
lead Concentrations in Leeches of Contaminated Soif before and after Reaction with Phoshate Rocks (PR) by Mixing or Not Mixing with Soil final Pb concentrations (nmol/L) PR mixed with soil in columns
PR not mixed with soil in columns
PR
PR added (9)
(1 d)'
(7 dIa
(16 d)'
leachates with soil (0 d)'
leachates alone (2 d)'
CF
0 1 2 4 0 1 2 4
1250 f 278
493 f 128 147 f 22.7 59.1 f 13.1 7.48 f 1.70 526 f 163 113 f 41.1 115 f 10.9 20.2 f 8.29
219 f 86.8 77.8 f 19 37.3 f 7.52 11.O f 4.7 405 f 35.4 175 f 75.1 100 f 10.9 20.2 f 6.35
202+96.8 0.94 f 0.96 2.55 f 2.27 8.28 f 6.63 184 f 80.2 9.28 f 13.0 5.47 f 4.97 7.38 f 12.8
56.9+1.98 17.0+5.90 0.00 f 0.51 1.37 f 2.29 56.9 f 1.98 79.1 f 23.2 0.00 f 7.16 0.00 f 0.00
oc
a
339 f 236 47.8 i 8.00 1436 f 47.7 410 f 32.3 201 f 17.2 58.7 f 8.25
Incubation times.
1800
1
Incubation time (d)
-
1
0 2.8
V."
2
3
,
4
I
I
I
0
1
2
3
I
I
I
I
0
1
2
3
4
1
I
I
I
I
I
I
0
1
2
3
4
Phsophate rock added (9) FIGURE 5. Changes in elemental concentrations of Burch soil after incubation (mixing) with CF phosphate rocks for different times
most important to Pb attenuation, followed by P, Ca, and specific surface area. In summary, phosphate rocks from five states were effective in immobilizing aqueous Pb, with Pb removal ranging from 38.8 to 100%. The main mechanism of Pb immobiljzationby phosphate rocks was through carbonated fluorapatite dissolution and precipitation of fluoropyromorphite or hydrocerussite as shown by XRD and SEM 1124
ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4, 1995
data. However, adsorption and substitution may also have occurred. It seems that specific surface area and initial solubility of phosphate rocks are correlated to Pb removal efficiency only to some extent. Whereas, the parameters that influence the effectiveness of phosphate rocks in removing Pb from solution were not clear from this experiment, it is apparent that all the materials evaluated showed some potential for use in remediating Pb-
250
,
1
---1800
-
0
1
2
3
4
n
1
lncubation
I
I
I
0
1
2
3
0
1
2
3
4
Phsophate rock added (9) FIGURE 6. Changes in elemental concentrations of.Burch soil after incubation (mixing) with OC phosphate rocks for different times
contaminated materials. Lead Immobilization from a Contaminated Soil Using Phosphate Rocks. Samples OC and CF, among the most effective Florida phosphate rocks in immobilizing Pb from aqueous solutions, were chosen for the following column studies. In the study where phosphate rocks were mixed with Burch soil and extracted sequentially,both phosphate rocks (OC and CF) were effective in reducing Pb concentrations in the leachates, with Pb reduction ranging from 56.8 to 98.5% (Table 3). Longer incubation did not have a significant effect on Pb removal by phosphate rocks, but increased phosphate rock addition sigmficantly increased leachate Pb reduction. For example, at 1-d incubation, Pb concentration was reduced from 410 to 58.7 nmol-' when the amount of OC phosphate rock was increased from 1 to 4 g (Table 3). As discussed previously, we proposed that phosphate rock dissolution and fluoropyromorphite precipitationwere the main mechanisms of Pb reduction. At all incubation times, final P and Ca concentrations increased final Pb concentrations decreased; and suspension pH stayed constant, with an increase in the amount of phosphate rock added, which generally agreed with our previous observations in the aqueous Pb experiment. At all phosphate rock levels, final P, Ca, and Pb decreased at longer
incubation time, which was partially due to the nature of sequential extraction. Since the same samples were extracted multiple times, the soluble salts were steadily displaced from the soil columns. In any event, the highest levels of phosphate rock addition reduced the concentrations of dissolved Pb to levels below the current EPA drinking water limit of 72.4 nmol L-' (Table 3). In another study where Burch soil was placed on top of the mixture of phosphate rock and Ottawa sand, both phosphate rocks (CF and OC) were effective in reducing Pb concentrations (Table 3). Lead concentrations were reduced from 202 and 184 nmol L-' to below 10 nmol L-l. The results suggest that there was no effect by separating the contaminated soil from phosphate rocks. Both Ca and P concentrations and solution pH increased when phosphate rocks were increased from 0 to 1 gll0 g of soil. However, when phosphate rock was increased to 2 gll0 g of soil, P concentrations were decreased, indicating more precipitation of P with Pb. It was unexpected to find that aqueous Pb was much lower after the untreated contaminated soil reacted with water for 2 d before the extraction (Table 3). The Pb concentrations in the blanks were 202 and 183 nmol L-l in previous experiment (leachates with soil) as compared to 57 nmol L-l in this experiment (leachates alone). VOL. 29, NO. 4,1995 / ENVIRONMENTAL SCIENCE &TECHNOLOGY
1125
Incubating soil with water at field capacity reduced Pb concentrations significantly. It is possible that during the incubation, more soluble Pb was either readsorbed by soil particles or transformed to a less soluble form. Both phosphate rocks (CF and OC) were effectivein reducing Pb concentrations (Table 3). Lead concentrations were reduced from 57 nmol L-' to below2 nmol L-' at the addition of 2 2 g of phosphate rockdl0 g of soil. In a different study to examine the effect of incubation time on Pb immobilization by phosphate rocks, both phosphate rocks (CF and OC) were effective in reducing aqueous Pb concentrations in the contaminated soil, regardless of their incubation times, with higher amounts of phosphate rock resulting in greater effectiveness in reducing Pb concentrations. For example, after 1-d incubation, Pb concentrations were reduced from 1503 to 277, 143, or 33 nmol L-l after reaction with 1, 2, or 4 g of CF phosphate rock/lO g of Burch soil, respectively (Figure 5). Similarly, after 56-d incubation, Pb concentrations were reduced from 324 to 245, 106, or 77 nmol L-' (Figure 5). Similar results were obtained with OC phosphate rock (Figure 6). Both Ca and P concentrations increased with an increase in the amount of phosphate rocks reacted with the soil for all incubation times, indicating greater dissolution at higher quantity of phosphate rocks within limited reaction time (Figures 5 and 6). No trend was observed regarding pH changes with the quantity of phosphate rocks added to the soil, but overall there was little change in pH values. As observed previously, Pb concentrations decreased with an increase in incubation time in soils treated with water (blank treatment) (Figures5 and 6). For example, Pb concentrations in the blank decreased from 1958 nmol L-' after 1-d incubation to 356 nmol L-' after 7-d incubation (Figure 5). And then, Pb concentrations in the blank increased from 141 nmol L-l after 28-d incubation to 320 nmol L-I after 56-d incubation (Figure 5). The changes in Pb concentrations in the blank can be attributed to both chemical and biological reactions in the incubated soil. Initially, soluble Pb was converted to less soluble Pb by either adsorption or precipitation. Since the soil was incubated at field capacity, the soil could have been reduced enough to convert S042- to S2-. The S2- formed may then have precipitated as PbS. On the other hand, longer incubation time may have resulted in dissolution of organic matter, which would then solubilize Pb by forming organic Pb complexes. The exact mechanism of Pb concentration change in the untreated soil needs further investigation. The changes in Ca concentrations followed similar trends as those of Pb concentrations. Calcium concentrations decreased up to 28-d incubation and then increased from 28 to 56-d incubation (Figures 5 and 6). Similar reactions as Pb could have occurred to Ca. Again, without further investigation,no conclusion can be made regarding changes in metal concentrations. Similar changes in Ca concentrations in a flooded Crowley silt loam were observed by Gilmour and Gale (23). However, no clear explanation was given in their discussion. Both phosphate rocks were effective in reducing Pb from Burch soil, with a greater amount of phosphate rocks providing greater reduction. Incubation time and different methods of mixing phosphate rocks and soil had little effect on the efficiency of Pb immobilization by phosphate rocks.
1126 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY I VOL. 29, NO. 4,1995
The primarymechanism of Pb immobilization by phosphate rocks was through dissolution of phosphate rocks and precipitation of a fluoropyromorphite-like mineral. This study also demonstrates the diversity in applications of phosphate rocks in immobilizing Pb from contaminated water, soils, and wastes. The mechanism in changes of metal concentrations with incubation time in untreated soil awaits additional study.
Acknowledyments This research was sponsored in part by the Florida Institute of Phosphate Research (Contract 92-01-101R). Approved for publication as Florida Agricultural Experiment Station Journal Series R-04304. Special thanks are extended to Mr. Shaoqin Yao and Mrs. Pranitha Gaddam for partidy conducting the experiment.
Literature Cited (1) Stubbs, R. L. In Lead in the environment; Applied Science Publishers Ltd: Zoological Society of London, Regent's Park, London, 1973; pp 1-7. (2) Jaworski, J. F. In Lead, mercury, cadmium and arsenic in the environment; Hutchinson, T. C., Meema, K. M., Eds.; John Wiley & Sons: New York, 1987; pp 3-16. (3) Mielke, H. W.; Heneghan, J. B. Chem. Speciation Bioavailability 1991, 3, 129-134. (4) Holden, T. How to select hazardous waste treatment technologies for soils and sludges: alternative, innovative, and emerging technologies; Noyes Data Corporation: Park Ridge, NJ, 1989; Vol. 163. (5) Czupyma, G.; Levy, R. D.; MacLean, A. I.; Gold, H. In situ immobilization of hea y-metabcontaminated soils; Noyes Data Corporation: Park Ridge, NJ, 1989; Vol. 173. (6) Nriagu, J. 0. Geochim. Cosmochirrr. Acta 1973, 37, 367-377. (7) Cotter-Howells, J.; Thornton, I. Environ. Geochem. Health 1991, 13, 127-135. (8) Ruby, M. V.; Davis,A.; Nicholson, A. Environ. Sci. Technol. 1994, 28, 646-654. (9) Gounaris, V.; Anderson, P. R.; Holsen, T. M. Environ. Sci. Technol. 1993, 27, 1381-1387. (10) Christensen, T. H.; Nielsen, B. G. In H e a y Metals in the Environment; CEP Consultants Ltd: Edinburgh, U.K., 1987. (11) Lewicke, C. K. Environ. Sci. Technol. 1972, 6, 321-322. (12) Ma, Q. Y.; Traina, S. J.; Logan, T. J.; Ryan, J. A. Environ. Sci. Technol. 1994, 28, 1219-1228. (13) Ma, Q. Y.; Logan, T. J.; Traina, S. J.; Ryan, J. A. Environ. Sci. Technol. 1994,28, 408-418. (14) Ma, Q. Y.; Traina, S. J.; Logan, T. J.; Ryan, J. A. Environ. Sci. Technol. 1993, 27, 1803-1810. (15) Chien, S. H.; Friesen, D. K. In Future directions for agricultural phosphorus research;Sikora,F. J., Ed.; TVA Muscle Shoals, 1992; Bulletin Y-224; pp 47-52. (16) McClellan, G. H. 1. Geol. SOC. London 1980, 137, 675-681. (17) Lindsay, W. L. Chemical equilibria in soils; John Wiley & Sons, Inc.: New York, 1979. (18) Smith, J. P.; Lehr, J. R. 1. Agric. Food Chem. 1966, 14, 342-349. (19) Chien, S. H. Soil Sci. 1977, 123, 117-121. (20) Jahnke, R. A. Am. 1. Sci. 1984, 284, 58--78. (21) Bemas, B. Anal. Chem. 1968, 40, 1682-1686. (22) Olsen, S. R.; Sommers, L. E. In Methods of soil analysis. Part 2. Chemical and microbiological properties, 2nd ed.; Page, A. L., Miller, R. H., Keeney, D. R., Eds.; Soil Science Society of America: Madison, WI, 1982; Part 2; pp 403-446. (23) Gilmour, J. T.; Gale, P. M. In Formation, Chemistry & Biology of Wetland Soils; Hook, D. D., Ed.; Croom Heln Limited: London, 1987; pp 279-292.
Received f o r review September 6, 1994. Revised manuscript received November 29, 1994. Accepted December 12, 1994.@
ES9405585 @
Abstract published in Advance ACS Abstracts, January 15, 1995.
