field assessment of lead immobilization in a contaminated soil after phosphate application

The Science of the Total Environment 305(2003) 117–127

0048-9697/03/$ - see front matter� 2002 Elsevier Science B.V. All rights reserved.PII: S0048-9697Ž02.00469-2

Field assessment of lead immobilization in a contaminated soilafter phosphate application

Ricardo Melamed , Xinde Cao , Ming Chen , Lena Q. Ma *a b b b,

Center for Mineral Technology, Ministry of Science and Technology, Av. Ipe 900, Ilha da Cidade Universitaria, Rio de Janeiro,a ˆ21941-590, Brazil

Soil and Water Science Department, University of Florida, Gainesville, FL 32611-0290, USAb

Received 28 May 2002; accepted 15 September 2002

Abstract

A pilot-scale field demonstration was conducted at a Pb-contaminated site to assess the effectiveness of Pbimmobilization using P amendments. The test site was contaminated by past battery recycling activities, with averagesoil Pb concentration of 1.16%. Phosphate amendments were applied at a 4.0 molar ratio of PyPb with threetreatments: T1, 100% P from H PO ; T2, 50% from H POq50% from Ca(H PO ) ; and T3, 50% from H POq3 4 3 4 2 4 2 3 4

5% phosphate rock. Soil samples were collected and characterized 220 days after P application. Surface soil pH wasreduced from 6.45 to 5.05 in T1, to 5.22 in T2, and to 5.71 in T3. Phosphate treatments effectively transformed upto 60% of total soil Pb from the non-residual fraction(sum of water soluble and exchangeable, carbonate, Fe–Mnoxide, and organic fractions) to the residual fraction relative to the control. In addition, P treatments reduced ToxicityCharacteristic Leaching Procedure(TCLP) Pb from 82 mg l to below EPA’s regulatory level of 5 mg l in they1 y1

surface soil. Scanning electron microscopy-energy dispersive X-ray elemental analysis and X-ray diffraction analysisindicated formation of insoluble chloropyromorphitewPb (PO ) Clx mineral in the P-treated soils. Although H PO is5 4 3 3 4

necessary to dissolve meta-stable Pb in soil for further lead immobilization, it should be used with caution due to itspotential secondary contamination. A mixture of H PO and Ca(H PO ) or phosphate rock was effective in3 4 2 4 2

immobilizing Pb with minimum adverse impacts associated with pH reduction.� 2002 Elsevier Science B.V. All rights reserved.

Keywords: Lead immobilization; Phosphate amendment; Contaminated soil; Field demonstration

1. Introduction

Accumulation of heavy metals in soils and theirtransport through the soil matrix are potentialthreats to human health, especially to children’shealth by ingestion of Pb-contaminated soil(Het-tiarachchi et al., 2000; Yang et al., 2001). In this

*Corresponding author. Tel.:q1-352-392-1951; fax:q1-352-392-3902.

E-mail address: [email protected](L.Q. Ma).

regard, metal bioavailability and mobility are twomajor concerns. Increasing awareness of the hazardthat toxic elements can cause to the environmentand to humans makes it necessary to remediatemetal contaminated sites. Among the remediationtechnologies available for contaminated sites, insitu immobilization techniques are of particularinterest because they are relatively more cost-effective compared to conventional techniques, e.g.

118 R. Melamed et al. / The Science of the Total Environment 305 (2003) 117–127

excavation, and off-site disposal(Rabinowitz,1993; Yang et al., 2001).Application of phosphate amendments to soils

has been identified as a potentially effective insitu remediation technology(Cotter-Howells,1996; Hettiarachchi et al., 2000; Ma et al., 1994;Ryan et al., 2001). These amendments are availa-ble in various forms, environmental-friendly, andsimple to use. Phosphate has been shown toeffectively immobilize Pb from aqueous solutionsas well as various contaminated soils(Ma et al.,1993, 1994; Ma and Rao, 1997), to reduce plantPb uptake(Laperche et al., 1997), and to mitigateacid mine drainage by coating the pyrite surfacewith FePO which hinders sulfide oxidation, reduc-4

ing the transport of heavy metals(Evangelou,1996).The mechanisms involved in P-induced Pb

immobilization include ionic exchange and chem-ical precipitation. Several studies suggest that apotential retention mechanism of hydroxyapatitefor Pb (Suzuki et al., 1984) is ionic exchange withCa. However, Ma et al.(1993) demonstrated thatPb reacts with hydroxyapatitewCa (PO ) OHx in5 4 3

solution, forming stable pyromorphite-type miner-als wPb (PO )3X; XsF, Cl, Br and OHx, suggest-5 4

ing dissolution of hydroxyapatite followed byprecipitation of pyromorphite as the primary mech-anism. As such, the solubility of P amendmentsdictates their effectiveness in Pb immobilization.While chemical precipitation of metals depends

on the solubility products of the solids formed,metal sorption involves adsorption, surface precip-itation or co-precipitation, and intra-particle diffu-sion. Melamed et al.(2000) reported that whenphosphate rock is used to immobilize Pb at itsnatural pH 8.7, soluble P concentration is low,resulting in a relatively slow Pb immobilization.However, at pH 3.7, phosphate rock dissolves andPb immobilization is instantaneous, forming apyromorphite-type mineral. Although much knowl-edge about the mechanisms involved in immobi-lization of Pb using P amendments has beenacquired, implementation of this technology in thefield has been limited. Thus, a field demonstrationof this technology, at a site heavily contaminatedwith Pb from battery recycling wastes, wasundertaken.

The main objective of this pilot-scale fieldexperiment was to evaluate the effectiveness of Pamendments on in situ Pb immobilization. Thespecial tasks were(1) to identify formation of lesssoluble pyromorphite-like minerals;(2) to deter-mine Pb distribution after phosphate treatments;(3) to assess Pb leaching characteristics. Thispaper reports field data from a contaminated sitewhich was determined on day 220 after P appli-cation in February 2000. Results from this exper-iment provide the evidence on the effectiveness ofusing P amendments to immobilize Pb in contam-inated soil at field scale, narrowing the gapbetween field and laboratory experiments.

2. Materials and methods

2.1. Site description

The contaminated site is located northwest ofJacksonville, Florida. It is a nearly-level area ofapproximately 4100 m(71.4=57.3 m) with sur-2

face runoff in a west to southwesterly direction. Itwas probably exposed to Pb contamination due toits use as battery recycling and as a salvage yardwith discharge of waste oil during the 1940s.Based on previous reports, levels of total Pb andTCLP-Pb are elevated but restricted mostly to thesurface horizon(Cao et al., 2001).The site is dominated by disturbed urban soils

lacking structure. The soil in areas that have notbeen filled or otherwise disturbed is classified asa Spodosol(sandy siliceous thermic ultic alaquod).Selected chemical and physical properties of com-posite soil samples from the site are given in Table1. The soil is neutral, and heavily contaminatedwith Pb. Mineralogical characterization by XRDrevealed the presence of CaCO and PbCO(cerus-3 3

site) in the coarse non-magnetic soil fractions()1 mm), which is related to the slightly alkalineconditions of the site(Cao et al., 2001).

2.2. Experimental plot establishment

In our preliminary laboratory experiments thisfield soils were subject to Pb immobilizationoptimization by testing P sources and applicationrates using both batch and column experiments

119R. Melamed et al. / The Science of the Total Environment 305 (2003) 117–127

Table 1Selected physical and chemical properties of the soila

PHb CECc OMd Sand Silt Clay PeT PbT CuT ZnTcmol kgy1 ...................................%................................... ................................g kg ................................y1

Soil 6.95"0.19 5.75"0.85 3.91"0.90 87.7"1.37 9.0"1.58 3.35"0.54 0.89"0.16 11.6"1.57 2.64"0.11 1.95"0.83

Data represent an average of twelve replicates with a standard deviation; pH was determined with a 1:1 ratio of soilywater;a b

Cation exchange capacity; Organic matter; Total concentration.c d e

(Cao et al., 2001). Sources of P included solubleKH PO , NH H PO , H PO and less soluble2 4 4 2 4 3 4

Ca(H PO ) , as well as phosphatic clay and phos-2 4 2

phate rock. Results of these initial screening testsrevealed that a mixture of Ca(H PO ) qCaClq2 4 2 2

H PO displayed high immobilization efficiency.3 4

The results also demonstrated that application ofphosphate rock or phosphatic clay alone was effec-tive for Pb immobilization only at the low concen-tration range of Pb due to the limitation of thehigh P application rate required.Phosphoric acid was the most effective in reduc-

ing Pb extracted using Toxicity CharacteristicLeaching Procedure(TCLP) to below regulatorylevels, which was attributed to its effectiveness inenhancing the solubility of meta-stable cerussite(PbCO), the main form of Pb at the site, and3

allowing the precipitation of pyromorphite, a geo-chemically more stable phase(Cao et al., 2001).The drawback of adding H PO may take a higher3 4

risk of potential eutrophication due to its highmobility. The use of soluble H PO in combination3 4

with Ca(H PO ) or with phosphate rock was thus2 4 2

a rational step to take. H PO addition would3 4

fulfill the need to solubilize cerussite, precipitatingthe readily available Pb, while the phosphate rockwould supply a slow continuous source of phos-phate ions, while minimizing the decrease in soilpH. Further studies, focusing on P applicationrates, indicated that among various molar ratio ofPyPb tested, 4.0 was sufficient to achieve the goalsfor this site(Cao et al., 2001).The experimental plots were established in the

highly contaminated zone of the site. Each plotwas an approximately area of 4 m , which were2

circled by high-density polyethylene geomembraneliner of 2.5 mm in thickness to prevent floodingout of or into the plots. The plots were separatedby 1.5 m in distance from each other to avoid

possible inter-plot contamination. Phosphateamendments were applied to three plots at a molarratio of 4.0 PyPb with three treatments. The totalamount of P added was calculated for the surfacesoil of 0–20 cm depth. To pre-acidify the soil, halfof the amount of P was initially applied to thethree plots on 17 Febuary 2000 as a CaClq2H PO mixture in 25 l of water and sprayed3 4

uniformly in each area. The plots were then cov-ered with a plastic sheet to maintain moisturecontents in the surface layers and to preventleaching from rainfall. The addition of equivalentamount of CaCl was to provide adequate Cl2

needed for the formation of least soluble chloro-pyromorphitewPb (PO ) Clx. On 27 March 2000,5 4 3

40 days after the first application, the second halfof the P amendments was applied as H PO in T1,3 4

Ca(H PO ) in T2 and 5% phosphate rock in T3,2 4 2

and mixed to a depth of 20 cm by a shovel. Pleasenote, 5% phosphate rock was mixed with soil inT3 instead of 50% of the remaining P. Additionalphosphate was added as phosphate rock to com-pensate for its low solubility with a PyPb molarratio of 4.5 in T3 treatment. The plot without Paddition was set as the control(T0). After thesecond P application, the plots were exposed toair.

2.3. Sampling procedure

Three composite soil samples were collectedfrom each of the plots 220 days after first Papplication, using a soil probe of 2-cm diameterat 6 depths(0–10, 10–20, 20–30, 30–40, 40–60and 60–80 cm). After air-dried, soils were passedthrough a 2-mm sieve. Subsoil samples weredigested with HNOyH O hot block digestion3 2 2

procedure(USEPA Method 3050a).


2.4. Soil Pb fractionation and TCLP test

Sequential extraction was performed on soilsamples using the methodology of Tessier et al.(1999). The extractions were carried out in 40 mlcentrifuge tubes with 1 g of soil. The procedureseparated the lead into five operationally-definedfractions: water soluble and exchangeable(WE),carbonate bound(CB), Fe–Mn oxides bound(FM), organic matter(OM) and residual(RS). Areference soil material SRM 2710(NIST, Gaith-ersburg, MD) was used to compare Pb recoverybased on sequential extraction with certified val-ues. Lead recovery was satisfactory with103"10%. The Toxicity Characteristics LeachingProcedure(TCLP) was used to evaluate the effi-ciency of P amendments on lead toxicity(USEPA,1992).

2.5. Chemical analysis

Lead was analyzed by an atomic absorptionspectrophotometer equipped with a graphite fur-nace(Perkin Elmer SAMMA 6000, Norwalk, CT).Other elements were analyzed with a multi-channelinductively coupled plasma spectrophotometer(Thermo Jarrel Ash ICAP 61-E, Franklin, MA).Total P was measured colorimetrically with aShimadzu 160 U spectrometer using the molybdateascorbic acid method(Olsen and Sommers, 1982).Soil pH was measured with a 1:1 ratio of solid toDI water. Soil organic matter was determined usingthe Walkley–Black procedure(Nelson and Som-mers, 1982). Cation exchange capacity(CEC) wasdetermined using the method of Rhoades(1982).

2.6. X-ray diffraction (XRD) analysis

The clay fraction(-2 mm) of selected soilsamples were separated by centrifugation and wereanalyzed using XRD. Samples were scanned from2 to 608 2u with CuKa radiation on a computer-controlled diffractometer (Philips ElectronicInstruments, Inc., Mahwah, NJ) equipped withstepping motor and graphite crystal mono-chromator.

2.7. Scanning electron microscopy and energydispersive X-ray analysis

Selected soil clay samples analyzed by XRDwere further examined using a scanning electronmicroscope (SEM, JSM-6400yTN500, JEOL,USA), equipped with energy dispersive X-rayelemental spectrometry(EDX). Air-dried soil daysamples were mounted on carbon stubs and thencarbon coated.

3. Results and discussion

3.1. Lead concentrations in soil profiles

Due to the extreme heterogeneity of the contam-inated site, it is difficult to compare the temporalchanges in soil Pb concentrations for differenttreatments. This was apparent since Pb concentra-tions in all four experimental plots were different.Furthermore, Pb concentrations varied significantlyeven within the same experimental plot dependingon the precise location of soil sampling. Total Pbconcentrations in the surface soil(0–10 cm) were10 907, 5965, 15 919 and 3762 mg kg for T0,y1

T1, T2 and T3, respectively(Fig. 1a). In general,Pb concentrations were the greatest in the subsur-face at a depth of 10–20 cm in all soil profiles,reaching concentrations of 3.1, 1.7, 2.2 and 1.2%in T0, T1, T2 and T3, respectively(Fig. 1a).Elevated Pb concentrations in the subsurface ofthe control soil may suggest downward migrationof Pb in the soil profile. It is possible that afterapproximately 60 years, lead in the soil wasleached to a depth of 10–20 cm. A five-year studyof land application of municipal sludge to a forestsoil has shown that most metal movement seemsto be limited to the upper 5 cm of soil; however,repeated applications in the following yearincreased metal leaching to the underlying soil(Harris and Urie, 1986). In most contaminatedsoils, metals do not appear to leach downward insignificant quantities in the short run, because oftheir strong interactions with the soil. However, in


Fig. 1. Distributions of Pb concentration(a), P concentration(b), and pH(c), in the profile soils(0–80 cm) taken 220 days afterfirst P application. T0, the control; T1, H PO alone; T2, H POqCa(H PO ), and T3, H POqphosphate rock.3 4 3 4 2 4 3 4

the long run, metals can leach downwards in asoil due to their complexation with solubilizedorganic matter especially in an alkaline environ-ment where organic matter is more soluble(Mar-

schner and Wilczynski, 1991). This may be trueat the demonstration site where soil organic matter(3.91%) and pH(6.95) are much higher than thoseof typical Florida soils(Chen et al., 1999). More


than 2000 mg Pbykg was detected in 30–40 cmdeep soils of T1 and T2 plots.Therefore, this demonstration site was confront-

ed with both spatial and vertical Pb contamination.It could also be seen from Fig. 1a that a highheterogeneity for Pb distribution occurred at thissite, which may limit the effective evaluation of Ptreatments. However, this limitation could be over-come by using the same molar ratio of 4.0 PyPbduring P application.

3.2. P concentrations in soil profiles

Since the treatment plots were exposed to air,soil P enrichment and leaching may be an environ-mental concern for in situ Pb immobilization usingP amendments. Soluble P sources(e.g. H PO ,3 4

KH PO ) pose a high risk of enhanced P eutroph-2 4

ication, while less soluble P sources(e.g. apatite,phosphate rock) pose less. Hettiarachchi et al.(2001) reported that Bray-1 extracted much moreP in P-treated soils using triple super phosphatethan in soils using phosphate rock. Therefore, it isimportant to evaluate P distribution when phos-phate was used to immobilize Pb in a field testfrom the viewpoint of potential secondary contam-ination of P and P utilization efficiency.As expected, most P applied was concentrated

in the surface soil(0–10 cm; Fig. 1b). The Pretained at the surface(0–10 cm) was 45.1, 54.3and 73.5% of the total added P for T1, T2 andT3, respectively. Downward migration of P wasobserved in this soil with low buffer capacity,which should be favorable for Pb immobilizationoccurrence in the subsurface soil. Generally, Pconcentrations declined sharply with depth, withless P moving down the soil profile in T3 than inT1 and T2. Phosphorus retained in the wholeprofile (0–80 cm) accounted for 86.3%, 88.5%and 94.2% of the total P applied for T1, T2 andT3, respectively. The fact that less P moved downthe soil profile and less P was lost from T3 thanthe other two treatments suggested that T3 posedthe least eutrophication risk among the three treat-ments and it provided more P to react with Pb inthe soil.

3.3. pH in soil profiles

Application of P amendments significantlyimpacted the pH of this sandy soil with relativelylow buffer capacity and CEC(5.75 cmolykg)(Table 1 and Fig. 1c). Among the three treatments,T1 promoted the greatest decrease in soil pH atthe surface, and T3 the least. The surface soil pHdecreased from 6.45(T0), to 5.71 (T3), to 5.22(T2) and to 5.05(T1), with -1.5 pH unit reduc-tion. For typical Florida soils, an average pH of5.04 is expected(Chen et al., 1999). The leastreduction of pH in T3 plot was consistent with theresults of Hettiarachchi et al.(2001), wherebyH PO reduced soil pH from 7.0 to 5.2, while3 4

phosphate rock had little effect even at the highlevel of P (2500 mgPykg) on five metal-contam-inated soils and mine wastes. Furthermore, P-induced pH reduction is mostly limited to the top30 cm of soil, an indication of limited movementof P in the soil profile. At subsurface depths downto 30 cm, soil pH increased with depth(Fig. 1c).However, soil pH decreased with depth from 30to 80 cm in all treatments, including the controlsoil, ranging from 4.9 to 5.36, which is a typicalpH range for Florida soils.Reduction of soil pH was expected with addition

of H PO . It has been reported that inducing soil3 4

acidic conditions will promote the solubility of Pbcompounds, leading to effective Pb immobilizationvia formation of Pb pyromorphite(Yang et al.,2001). A field scale study suggested that adequatetriple super phosphate(32 gykg) should be appliedin order to reduce soil pH to levels that allowedeffective reduction of bioavailable Pb(Brown etal., 1999). Dissolution of the initial Pb phase hasbeen reported to be the limited factor in theformation of pyromorphite at pH values between5 and 8 and conversion of PbO to pyromorphitewas found to be most rapid at pH 5(Laperche etal., 1996). However, care should be exercisedwhen applying H PO due to enhanced mobility3 4

of P and other heavy metals(Cao et al., 2001).

3.4. Formation of pyromorphite-like mineral

Formation of a chloropyromorphitewPb (PO ) Clx mineral in the lead contaminated5 4 3


Fig. 2. X-ray diffraction patterns of the surface soils(0–10 cm) in the control and P-treated plots on 220 days after first P application.T0, the control; T1, H PO alone; T2, H POqCa(H PO ), and T3, H POqphosphate rock. CS, Cerussite; ClP,3 4 3 4 2 4 3 4

Chloropyromorphite.

soil after P application was confirmed by XRDdata(Fig. 2). The chloropyromorphite mineral wasobserved after 220 days of P application for T1and T2 treatments, but not in T3 treatment, asindicated by no peak at 2.96 and 2.86 A(Fig. 2).˚However, soil samples taken at later dates(330and 480 days later) showed formation of chloro-pyromorphite in soil samples taken from T3(Caoet al., 2001). Also, SEM element maps document-ed a Pb–P association in both surface(0–10 cm;Fig. 3a) and subsurface(30–40 cm; Fig. 3b)samples of T3. The EDS analysis showed that thePb particles contained significant amount of Ca, P,Pb, Fe, Cl, Cu and Zn(data not shown), indicatingthe occurrence of chloropyomorphite. Similarly,SEM-EDS analysis of soils from T1 and T2showed association of Pb with P.Coupled with formation of chloropyromorphite,

H PO -induced dissolution of cerrusite(PbCO),3 4 3

the main form of Pb at the site(Fig. 2), was alsoevident as indicated by the disappearance of thepeaks at 3.56 A for soil samples taken from T1,˚

T2 and T3(Fig. 2). Our data are consistent withthe hypothesis that P-induced Pb immobilizationwas mainly through a dissolution-precipitationmechanism(Ma et al., 1993).

3.5. Fractionation of lead in soil profiles

Chemical fractionation was carried out to under-stand lead transformations in the soil profileinduced by P-treatment. Chemical fractionation hasbeen extensively used to assess diagenetic pro-cesses, mobility and bioavailability of heavy met-als in soils. Although the geochemical phases insequential extraction schemes are operationallydefined by the reagents used, it is of generalconsensus that the water soluble and exchangeable,carbonate bound, Fe–Mn oxides and organicbound phases(together as non-residual) are morebioavailable than the residual phase(Ma and Rao,1997).Lead fractionation from the control soil at six

different depths indicated that Pb was primarily


Fig. 3.

Fig. 3. Elemental maps of a surface(0–10 cm) soil clay sample(a) and a subsurface soil clay samples at depth of 30–40 cm(b) collected from the T2 plot treated with H POq3 4

Ca(H PO ), using scanning electron microscopy coupled with2 4

energy dispersive X-ray analysis.

associated with the carbonate fraction in the lead-contaminated soil(50–70%, Fig. 4), which isconsistent with the XRD data(Fig. 2). The pres-ence of cerussite may be attributed to the fact thatthe soil had relatively high pH(6.95) (Table 1).All P treatments were able to modify the partition-ing of Pb from the non-residualypotentially avail-able fraction to the residualyless-available fraction(Fig. 4). In this respect, the major transformationwas a decrease of the carbonate-bound Pb(up to40%), while the residual fraction increased signif-icantly (up to 60%) relative to the control, at thesurface soil. This was consistent with the designedstrategy of dissolving cerussite with phosphoricacid, and the precipitation of Pb as chloropyro-morphite(Fig. 2). Previous investigations indicat-ed a significant reduction of soil Pb inexchangeable fraction and increase in residualfraction upon P addition(Berti and Cunningham,1997; Ryan et al., 2001). The increase of Pb inthe residual fraction result from formation of chlo-ropyromorphite. Ma and Rao(1997) reported that99.9% of Pb pyromorphite was associated with theresidual fraction. All the non-residual extractantsare ineffective to dissolve Pb from pyromorphite.Fe–Mn oxide bound Pb also decreased(;10%)

at the soil surface after P applications. Thisdecrease of Pb in the Fe–Mn oxide bound fractionmay be attributed to the dissolution of the oxidesand release of sorbed Pb, as a consequence of lowsoil pH (Hayes and Katz, 1996) caused by phos-phoric acid additions, the precipitation of P withsoluble Pb, andyor to chloropyromorphite forma-tion from Pb adsorbed on oxide surfaces(Zhanget al., 1997).The efficiency of P treatments to immobilize Pb

generally decreased with depth, reflected by theprogressive decrease of the residual fraction. How-ever, this fraction was still enhanced up to 20%,at a soil depth of 30–40 cm, compared to thecontrol (Fig. 4). The total Pb in the soil profilewas highest at a depth of 10–20 cm in all plots


Fig. 4. Lead fractionation in the soil profiles(0–80 cm) collected 220 days after first P application. T0, the control; T1, H PO3 4

alone; T2, H POqCa(H PO ), and T3, H POqphosphate rock. WE, water-soluble and exchangeable; CB, carbonate; FM, Fe–3 4 2 4 3 4

Mn oxide; OM, organic matter; RS, residual.

(Fig. 1a). Thus, the consumption of P in thesurface layer(0–10 cm) may have decreasedefficiency of Pb immobilization at 10–20 cm. Infact, levels of P were much higher at the surfacethan in the 10–20 cm layer(Fig. 1b).The proportion of exchangeable Pb and Fe–Mn

oxide bound Pb increased with soil depth(Fig. 4).The increase in exchangeable Pb was from approx-imately 5% at the surface to 20% at 60–80 cm inall plots. Fe–Mn oxide bound Pb increased fromapproximately 15% at the surface to 30% at 60–80 cm in T1, while this fraction actually increasedby approximately 5% in T0. In general, thesechanges could result from decreased pH in thetreated plots(Fig. 1c), promoting Pb leaching andP attenuation. However, the complexity of thesystem does not allow definite conclusions fromthe field data.Although metals associated with Fe–Mn oxides

are generally considered as bioavailable, weemphasize that this fraction plays a significant rolein the attenuation of metal leaching(Hayes andKatz, 1996). As the metal adsorption phenomenabecome important at lower depths, the system pHbecomes critical. The adsorption of metals onoxides decreases as the system pH is decreased,producing a sigmoidal function(Sposito, 1984).

3.6. TCLP lead in soil profiles

Without P treatments(T0), TCLP-extractablePb concentrations in the surface soils(0–10 cm)far exceeded 5 mgyl critical level of hazardouswaste(USEPA, 1995). Similar to the distributionof total Pb, the highest concentration of TCLP-extractable Pb was observed at 10–20 cm. Evenat the 20–30 cm profile, TCLP-Pb still exceededthe critical level. This is possibly because most ofthe Pb was within the carbonate fraction(Fig. 4),which would readily dissolve in the acidic TCLP-solution(Berti and Cunningham, 1997). Phosphateamendment was effective in reducing the TCLPPb to below the critical level in the surface soilsamples(Fig. 5). These results are of great signif-icance with respect to the disposal of the soil,because they show that P amendments can amendthe soil to a material that would be considerednon-hazardous. Although P treatments did reduceTCLP-Pb within the subsurface soils(10–20 cm),TCLP-extractable Pb concentrations in T1 and T3were still higher(up to 60 mgyl) than the criticallevel of 5 mgyl except in T2. This may be due toless soluble P available for Pb to form adequateamount of pyromorphite(Fig. 1b).


Fig. 5. Lead concentrations in the soil profiles using Toxicity Characteristic Leaching Procedure 220 days after first P application.T0, the control; T1, H PO alone; T2, H POqCa(H PO ), and T3, H POqphosphate rock.3 4 3 4 2 4 3 4

4. Conclusions

The efficiency of in situ P-induced Pb immobi-lization technology is site specific and dependsprimarily on the nature and extent of the contam-ination and on the type of soil at the site. Thetype and rate of the P amendment, and the appro-priate application management to be utilizedrequire careful scrutiny, and as such, preliminarylaboratory studies to assess the mechanismsinvolved with P applications and ultimately theirefficiencies are of fundamental importance.The results of the field pilot-scale study, at this

particular site, indicate that P amendments wereefficient in transforming more bioavailable Pb(non-residual) into a less-bioavailable form(resid-ual). The P-induced formation of pyromorphite inthe field was evidenced by both XRD and SEM-EDX data. Although H PO is needed to catalyze3 4

the dissolution of meta-stable Pb, making it avail-able for further immobilization reactions, its useshould be taken with caution. Phosphoric aciddecreased soil pH, especially for low-bufferingsandy soils, and consequently may cause leachingof heavy metals. Thus, low pH and other heavymetals leaching may be potential drawbacks of itsindiscriminate utilization. On the other hand, amixture of H PO and calcium phosphate or rock3 4

phosphate had excellent efficiencies, and both

treatments had less impact on soil pH. A strategy,which could work better than the one used in thisstudy, would be to invert the sequences of Papplication, i.e. to add calcium phosphate andphosphate rock first and then apply the phosphoricacid, thus producing the dissolution of cerussiteand the more soluble P amendments at the sametime. Effective remediation technology entailsminimizing both leaching and bioavailability.

Acknowledgments

This project was supported in part by the FloridaInstitute for Phosphate Research(Contract 97-01-148R) and the Ministry of Science and Technologyof Brazil. The authors are very thankful to MrThomas Luongo for his help in sample analysis.

References

Berti WR, Cunningham SD. In-place inactivation of Pb in Pb-contaminated soils. Environ Sci Technol 1997;31:1359–1364.

Brown SL, Chaney R, Berti B. Field test of amendments toreduce the in situ availability of soil lead. In: Wenzel WW,editor. The 5th International Conference on the Biogeochem-istry of Trace ElementsVienna, Austria, 1999. p. 506–507.

Cao X, Ma LQ, Singh SP, Chen M, Harris WG, Kizza P. Fielddemonstration of metal imobilization in containated soilsusing phosphate amendments. Gainesville, FL: Florida Insti-tute of Phosphate Research, 2001.


Chen M, Ma LQ, Harris WG, Hornsby A. Background con-centrations of 15 trace metals in Florida soils. J EnvironQual 1999;28:1173–1181.

Cotter-Howells J. Lead phosphate formation in soils. EnvironPollut 1996;93:9–16.

Evangelou VP. Pyrite oxidation inhibition in coal waste byPO and H O pH-buffered pretreatment. Int J Surf Min4 2 2

Reclamation Environ 1996;10:135–142.Harris AR, Urie DH. Heavy metal storage in soils of an aspenforest fertilized with municipal sludge. In: Co DW, HenryCL, Nutter WL, editors. The forest alternative for treatmentand utilization of municipal and industrial wastes. Seattle,WA: University of Washington Press, 1986. p. 168–176.

Hayes KF, Katz LE. Application of X-ray absorption spectros-copy for surface complexation modeling of metal ion sorp-tion. In: Brady PV, editor. Physics and Chemistry of MineralSurfaces. CRC Press, 1996.

Hettiarachchi GM, Pierzynski GM, Ransom MD. In situstabilization of soil sead using phosphorous and manganeseoxide. Environ Sci Technol 2000;34:4614–4619.

Hettiarachchi GM, Pierzynski GM, Ransom MD. In situstabilization of soil lead using phosphorus. J Environ Qual2001;30:1214–1221.

Laperche V, Logan TJ, Gaddam P, Traina S. Effect of apatiteamendments on plant uptake of lead from contaminated soil.Environ Sci Technol 1997;31:2745–2753.

Laperche V, Traina SJ, Gaddam P, Logan TJ. Chemicalmineralogical characterization of Pb in a contaminated soil:reactions with synthetic apatite. Environ Sci Technol1996;30:3321–3326.

Ma LQ, Rao GN. Effects of phosphate rock on sequentialchemical extraction of lead in contaminated soils. J EnvironQual 1997;26:788–794.

Ma QY, Traina SJ, Logan TJ. In situ lead immobilization byapatite. Environ Sci Tech 1993;27:1803–1810.

Ma QY, Traina SJ, Logan TJ, Ryan JA. Effects of aqueousAl, Cd, Cu, Fe(II), Ni and Zn on Pb immobilization byhydroxyapatite. Environ Sci Technol 1994;28:1219–1228.

Marschner B, Wilczynski AM. The effect of liming on quantityand chemical composition of soil organic matter in a pineforest in Berlin, Germany. Plant Soil 1991;137:229–236.

Melamed R, Self P, Smart R. Kinetics and spectroscopy of Pbimmobilization on rock phosphate at constant pH. In: Singh-al RK, Mehrotra AK, editors. Environmental Issues and

Management of Waste in Energy and Mineral Production.Calgary, Alberta, Canada: A.A. BalkemayRotterdamyBrook-field, 2000. p. 301–306.

Nelson DW, Sommers LE. In: Page AL, Miller RH, KeeneyDR, editors. Total Carbon, Organic Carbon, and OrganicMatter. In Methods of Soil Analysis, Part 2 chemical andmicrobiological properties, vol. 9. Madison, Wisconsin:ASA, 1982. p. 539–577.

Olsen SR, Sommers LE. Phosphorus. In: Page AL, Miller RH,Keene DR, editors. Methods of Soil Analysis, Part 2chemical and microbiological properties, vol. 9. Madison,Wisconsin: ASA, 1982. p. 403–427.

Rabinowitz MB. Modifying soil lead bioavailability by phos-phate addition. Bull Environ Contam Toxicol 1993;51:438–444.

Rhoades JD. Cation exchange capacity. In: Page AL, MillerRH, Keeney DR, editors. Methods of Soil Analysis, Part 2chemical and microbiological properties, vol. 9. Madison,Wisconsin USA: ASA, 1982. p. 149–157.

Ryan JA, Zhang P, Hesterberg D, Chou J, Sayers DE. Forma-tion of chloropyromorphite in a lead-contaminated soilamended with hydroxyapatite. Environ Sci Technol2001;35:3798–3803.

Sposito G. The surface chemistry of soils. New York: OxfordUniversity Press, 1984.

Suzuki T, Ishigaki K, Miyake M. Synthetic Hydroxyapatitesas Inorganic Cation Exchangers. J Chem Soc Farady Trans1984;80:3157–3165.

Tessier A, Campbell PGC, Bisson M. Sequential extractionprocedure for the speciations of particular trace metals. AnalChem 1999;51:844–851.

USEPA 1992. Toxicity characteristic leaching procedure(method 1311, rec.0). July 1992. In Test methods forevaluating solid waste, physicalychemical methods(i. SW-846, ed.). Office of Solid Waste, Washington, DC.

USEPA 1995, Test methods for evaluation of solid waste. Vol.IA, Laboratory mannaul physicalychemical methods, SW846, 3ed. 40CFR Part 403 and 503. US Gov. Print. Office,Washington, DC.

Yang J, Mosby DE, Casteel SW, Blancher RW. Lead immo-bilization using phosphoric acid in a smelter-contaminatedurban soil. Environ Sci Technol 2001;35:3553–3559.

Zhang P, Ryan JA, Bryndzia LT. Pyromophite formation fromgoethite adsorbed lead. Environ Sci Technol 1997;31:2673–2678.

field assessment of lead immobilization in a contaminated soil after phosphate application

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