Bonemeal Additions as aRemediation Treatment for MetalContaminated SoilM A R K E . H O D S O N * A N DEÄ V A V A L S A M I - J O N E S

Department of Mineralogy, The Natural History Museum,Cromwell Road, London, SW7 5BD England

J A N E T D . C O T T E R - H O W E L L S

Department of Plant and Soil Science, University of Aberdeen,Cruickshank Building, St. Machar Drive,Aberdeen, AB24 3UU Scotland

The ability of bonemeal additions (finely ground, poorlycrystalline apatite, [Ca10(PO4)6OH2]) to immobilize pollutantmetals in soils and reduce metal bioavailability throughthe formation of metal phosphates has been evaluated.Leaching column experiments were carried out oncontaminated soils with pH varying between 2.7 and 7.1.Monitoring of leachates over a three month period indicatedthat bonemeal additions resulted in the immobilization ofmetals and an increase in the pH of the column leachate,the soil pore water and the soils themselves. Analyticalscanning electron microscopy of the bonemeal treated soilat the end of the experiment revealed that Pb and Znwere associated with phosphorus. X-ray diffraction identifiedseveral newly formed phases in the bonemeal treatedsoil at the end of the experiments that had peaks of similarintensity and positions as reference Pb and Ca-Znphosphates. Batch experiments and subsequent extractionof metals from controls and bonemeal amended soilsusing 0.01 M CaCl2 and DTPA indicated that bonemealadditions reduced the availability of the metals in the soils.Bonemeal amendments appear to have potential as aremediation treatment for metal contaminated soils.

IntroductionRemediation of metal contaminated land is an importantcurrent environmental issue. The majority of remediationstrategies focus on either civil engineering methods (e.g.excavation, disposal and encapsulation) or process basedtechnologies such as soil washing. Further development ofphytoextraction technologies is required before large-scaleremediation using this technique can be implemented (1).An alternative, chemical/mineralogical remediation methodinvolving metal phosphate formation has been suggested (2,3). Many metal phosphates (e.g. Pb, Zn, Cd) are highlyinsoluble (4). If pollutant metals in contaminated soils couldbe converted into phosphates, then the metals would beimmobilized in situ and their bioavailability would bereduced.

The majority of experiments examining metal phosphateformation have concentrated on Pb, which can form pyro-

morphite (Pb5(PO4)3Cl) (2, 3, 5). Experiments have beencarried out using highly soluble forms of phosphate, e.g. K2-HPO4 (3). Pyromorphite formed but the highly soluble natureof this P source means that an eutrophication risk would beassociated with such a treatment. Laboratory experimentsusing rock apatite have also been successful (6), but rockapatite is highly insoluble (4) and so might not release Psufficiently rapidly to remediate contaminated soil on anacceptable time scale. Experiments using finely powderedsynthetic hydroxyapatite (which has a solubility intermediatebetween K2HPO4 and rock apatite) in soils and solutions haveresulted in the formation of pyromorphite (2, 5, 7), but theuse of synthetic hydroxyapatite on a field scale is economicallyunfavorable. It was recently suggested that poorly crystallineapatite, e.g. bone apatite (in the form of bonemeal-finelycrushed bone), might represent a low-cost, readily availablephosphate source that could be used to remediate metalcontaminated land without causing excessive P runoff (7).Preliminary experiments (8) using a moderately contami-nated soil indicated that bonemeal treatments could be auseful remediation method. The current experiments werecarried out to determine whether bonemeal additions couldbe a suitable remediation treatment for soils heavily con-taminated with metals and in the acidic to neutral pH range.

MaterialsSoil samples were taken from the historic mining sites ofParys Mountain (PM), Leadhills (LH) and Wanlockhead (WH)(9) in the United Kingdom (UK) and were chosen to give arange of metal contamination and pH values (Table 1). Thesoil was sieved to e2 mm which also homogenized the soil.

The bonemeal used was obtained from a commercialsupplier. It was sieved to produce a 90-500 µm fraction foruse in the experiments because this size fraction was foundto be the most effective of those previously tested (90-500µm, 500-2000 µm and 2000-3300 µm) (8).

Bonemeal is steam sterilized at temperatures above 100°C which is standard procedure for termination of microbialactivity. However, in the UK and further afield, there isconcern over the safety of skeletal tissue of cattle. In a previousstudy (8) we showed that incineration of bonemeal at 400 °Chad no effect on the results of bonemeal treatments to metalcontaminated soils.

MethodsLeaching Columns. Two hundred grams of either soil(controls) or a 1:50 bonemeal:soil mix, with the bonemealevenly distributed throughout the soil, were packed into 250mL polypropylene columns which were made from invertedbottles of 60 mm diameter, with their bases cut off (see ref8 for experimental details). The soil was added to the columnsin 50 g portions and was lightly compressed betweenadditions. The final density of soil in the columns wasapproximately 1 g cm-3. A quartz wool plug at the base ofthe columns acted as a 6 µm filter for the leachate. The uppersurfaces of the columns were covered with quartz wool, andthe wool was irrigated twice daily by manual sprinkling (usinga pipet) of a dilute ionic solution similar in composition tonatural rain (12) (pH ) 4.4, 1.94 mg NO3

- L-1, 0.49 mg NH4+

L-1, 1.87 mg Na+ L-1, 0.25 mg Mg2+ L-1, 0.29 mg Ca2+ L-1, 3.41mg Cl- L-1, 2.65 mg SO4

2- L-1). The wool ensured that the“rainfall” was distributed evenly over the entire surface ofthe soil column. The rate of column irrigation was equivalentto rainfall of 0.9 m yr-1. The columns remained well drainedand undersaturated throughout the experiments. The col-

* Corresponding author phone: +44 (0) 118 931 8911; fax: +44(0)118 931 6660; e-mail: m.e.hodson@reading.ac.uk. Current ad-dress: Department of Soil Science, The University of Reading,Whiteknights, P.O. Box 233, Reading, Berkshire, UK, RC6 6DW.

Environ. Sci. Technol. 2000, 34, 3501-3507

10.1021/es990972a CCC: $19.00 2000 American Chemical Society VOL. 34, NO. 16, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 3501Published on Web 07/15/2000

umns were kept in the dark in an air-conditioned laboratoryat a temperature of 18-24 °C.

Small, c. 10 mL, amounts of fresh column leachate werecollected approximately every 500 h. The different treatmentsdid not influence the leach rate from the columns. Theleachate was collected in polypropylene sample bottlescontaining 0.25 mL of reagent grade nitric acid. The im-mediate acidification of samples was carried out in order toprevent metal phases precipitating from solution prior toanalysis. From previous trial experiments, in which fresh,unacidified leachate was filtered through 0.2 µm filters whichwere then examined using a scanning electron microscope(SEM), we were confident that no particulate matter waswashed out of the columns during leaching. Acidifiedsolutions were filtered through 0.2 µm filters as a precau-tionary measure and analyzed using inductively coupledplasma-atomic emission spectroscopy (ICP-AES).

At the end of the experiment (2064 h of leaching),unacidified leachate was collected for pH measurement.Column material was centrifuged to extract soil pore waterfor pH and metal analysis. The soils were air-dried and theirpH measured (11).

Batch Experiments 1: Predicted Metal Availability. Twochemical extractions (0.01 M CaCl2 solution and pH 7diethylenetriaminepentaacetic acid solution, DTPA) that arewidely reported in the scientific literature as being proxiesfor bioavailability of metals in soils to plants (13-16) wereused to assess the relative availability of metals in the treatedand untreated soils.

The extractions were not carried out on the soil residuesleft at the end of the leaching column experiments aspotentially available metals had already been leached fromthem, particularly from those soils which were not treatedwith bonemeal (see below). Instead 6 g of fresh contaminatedsoil or 1:50 bonemeal:soil mixtures were mixed with 120 mLof artificial rain solution and, after vigorous stirring, wereleft at room temperature to react for 7 days. The choice ofthe ratio of soil mixture to solution was arbitrary. After 7days the mixtures were air-dried so that any highly mobilemetals which may have gone into solution during the batchexperiments were repartitioned into the solid phase. Thisprevented the extractions being biased toward the more fixedmetal fractions. Chemical extractions using 0.01 M CaCl2

and pH 7 DTPA were carried out on subsamples of the solidresidues.

The extraction procedure using 0.01 M CaCl2 recom-mended by ref 14 and followed in this study is as follows. Tenmilliliters of 0.01 M CaCl2 solution were shaken with 1 g ofsample for an hour and then centrifuged for 15 min at 200rpm. The centrifugate was filtered through 0.2 µm filters andacidified with reagent HNO3 to a strength of 2.5% HNO3 priorto analysis by ICP-AES.

For the DTPA extractions the DTPA solution was preparedas described in ref 15, comprising 5 mM DTPA, 10 mM CaCl2

and 0.1 M triethanolamine in a deionized water matrix withthe pH of the solution adjusted to 7.3 by addition of HCl.Twenty five milliliters of the DTPA solution and 5 g of sample

were shaken at 120 cycles per minute for 2 h and then filteredthrough Whatman No. 42 filter paper. The 5:1 solution-to-soil ratio was used rather than a 2:1 ratio as suggested in theinitial publications detailing the DTPA test (16) due to thehigh metal content of the soils (15). The filtrate was acidifiedwith reagent HNO3 to a strength of 2.5% HNO3 prior toanalysis by ICP-AES.

Batch Experiments 2: pH Effects. To investigate the effectthat an increase in the pH of the soil could have on metalrelease from the soils in the absence of bonemeal three setsof experiments were run for each soil (PM, LH and WH). Sixg of soil were shaken with 30 mL of artificial rain; 6 g of a 1:50mix of bone meal and soil were shaken with 30 mL of artificialrain; and 6 g of soil were shaken with 30 mL of sodiumhydroxide solution of suitable strength to cause the same pHchange as was observed in the bonemeal treated soils.Experiments were run for 24 h, after which suspensions werecentrifuged at 1000g and filtered through 0.2 µm filters. ThepH of the supernatant was measured, and then the solutionswere acidified using reagent nitric acid to a strength of 2.5%nitric acid prior to analysis by ICP-AES.

Scanning Electron Microscopy (SEM). At the end of theleaching column experiment, dry soil and bonemeal:soilmixture was mounted in epoxy resin, polished to give a crosssection through grains and carbon coated prior to analysisusing a Hitachi S-2500 probe operating at 15 kV and 1 nA(spot size 1 µm) with an Oxford Link energy-dispersive X-ray(EDX) analysis system. X-ray mapping was used to identifyareas in which Pb, Zn and P occurred together, andquantitative spot analyses were performed at these points.

X-ray Diffraction (XRD). XRD analysis was carried outon a bonemeal-rich fraction and also the <2 µm fraction ofthe soils at the end of the leaching column experiments.SEM analysis indicated that Pb and Zn bearing phosphatesmight occur associated with bonemeal. Studies by otherworkers (e.g. ref 17) suggested that Pb-phosphates might bepresent as submicron sized particles. Thus the separations,of the bonemeal-rich and <2 µm fractions were carried outto increase the likelihood of detecting newly formed metalphosphates using XRD. Bonemeal particles were separatedfrom the bulk of the soil by floating them off the soil in water.The material, which still contained fine grained material fromthe soil as well as bonemeal, was dried and ground to a sizeof <20 µm. The <2 µm fraction of the soil was collected bysedimentation. Both bonemeal-rich and <2 µm separateswere mounted on silicon wafer stubs as random powders.Analyses were carried out using Cu KR1 X-rays generated byan Enraf-Nonius PSD 120 operating at 45 kV and 45 mA witha curved position sensitive detector and a fixed-beamsample-detector geometry (5°) (18).

Results and DiscussionQuality Control. All experiments and digestions were carriedout in triplicate. ICP-AES analyses were carried out withappropriate quality control (matrix-matched standards ex-ceeding the range of elemental concentrations in unknowns,blanks and a monitoring standard of known composition

TABLE 1. Metal Concentrations (µg Metal g-1 Dry Soil)a and pHb in the e2 mm Fraction of Soils and the 90-500 µmBonemeala

sample texture pH (H2O) pH (CaCl2) Zn Pb Cd Cu Ni

Parys Mountain (PM) silty clay loam 2.71 2.53 4872 15043 16 2317 2Leadhills (LH) silty clay loam 4.71 4.48 213 9882 0 83 50Wanlockhead (WH) silty clay loam 7.1 7.1 14282 136260 56 1668 25bonemeal 89 <0.2 <0.02 5 <0.05

a Digested using 70% m/m nitric acid (10). Digestion of a well characterized in house mine waste standard supplied by Imperial College GeochemistryGroup (sample HRM31, Derbyshire minewaste) indicated that the analyses accurately reflected bulk metal concentration of the samples. b Measuredfollowing the method given in ref 11.

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that was analyzed after every fifth sample). On the basis ofreplicate analyses of in house reference materials, accuracyand precision of the analyses were both e ( 5%.

Leaching Columns: Leachate, Pore Water and Soil pH.The bonemeal additions generally resulted in a significantincrease in leachate, pore water and soil pH (Table 2). Thisis consistent with the consumption of protons duringdissolution of Ca-phosphate as suggested in ref 2.

The increase in pH could cause the precipitation of metaloxides and hydroxides. It is, therefore, possible that thereduction in metal release from the soils treated withbonemeal (see below) was due to the pH rise associated withthe dissolution of bonemeal.

Leaching Columns: Metal Content of Leachates andPore Water. Metal concentrations in the leachates and porewaters from bonemeal treated soils were generally signifi-cantly less than, or more rarely the same as, those from theuntreated soils (e.g. Table 3). The only exception to this wasthe greater Cu concentration in the leachate from thebonemeal treated WH soils. Reductions in metal releasebetween the untreated and treated soils were greatest for thePM soil and least for the WH soil. Phosphorus release fromthe bonemeal treated columns was minimal (Table 3) butdid occur indicating that bonemeal dissolution was takingplace and that at least some of the P released was notconverted to metal phosphates. The reduction in metalrelease from the bonemeal treated columns could have beendue to any one or a combination of 1) precipitation of metal

phases in response to the pH rise, 2) adsorption of metalsonto bonemeal particles, and 3) precipitation of metalphosphates.

Batch Experiments 1: Predicted Metal Bioavailability.For the majority of samples bonemeal additions eitherreduced, or had no effect on, metal availability as estimatedby CaCl2 and DTPA extractions (Table 4). For PM and LH theCaCl2 extractions predicted reductions in the availability ofmore metals in the bonemeal treated soils than the DTPAextractions. Generally the DTPA extracted more metal fromthe soils than the CaCl2 extractions in accordance with currentthinking that CaCl2 displaces ions from exchange sites,whereas DTPA can also chelate ions and dissolve someamorphous oxyhydroxides (13). Thus it would appear thatthe bonemeal treatments generally reduced the concentrationof metal ions held on exchange sites and reduced theconcentration of some chelatable metals. However, somemetals remained in a chelatable form and the concentrationof these metals was greater than the concentration of metalsheld on exchange sites.

The WH Cu extraction results (an increase in CaCl2

extractable Cu but a decrease in DTPA extractable Cu in thebonemeal treated soils) suggest that the bonemeal additionsreduced the concentration of chelatable metal, while theadditions increased or had no effect on the concentration ofmetals held on exchange sites. The higher concentrations ofthe DTPA extractable Cu compared to the CaCl2 extractableCu mean that any changes in the concentrations of metals

TABLE 2. pHa of Leachate, Pore Water and Soil from Leaching Columns after 2064 h of Leachingb

Parys Mountain,soil only (PMU)

Parys Mountain,soil + bonemeal (PMT)

Leadhills,soil only (LHU)

Leadhills,soil + bonemeal (LHT)

Wanlock-head,soil only (WHU)

Wanlock-Head,soil + bonemeal (WHT)

leachate 2.50 ( 0.03 2.47 ( 0.09 3.76 ( 0.62 5.93 ( 1.64 6.40 ( 0.33c 8.10 ( 0.09c

pore water 2.69 ( 0.05c 4.25 ( 0.09c 4.74 ( 0.33c 6.89 ( 0.07c 4.88 ( 1.53c 7.43( 0.09c

soil (H2O) 2.52 ( 0.05 3.23 ( 0.48 4.02 ( 0.21c 5.85 ( 0.33c 6.97 ( 0.12c 7.31 ( 0.14c

soil (CaCl2) 2.35 ( 0.10c 3.24 ( 0.48c 4.06 ( 0.02c 5.76 ( 0.21c 6.98 ( 0.14c 7.31 ( 0.10c

a Measured after the method of ref 11. bValues are means ( standard deviation from triplicate experiments. cIndicates a significant differencebetween the treated and untreated soils at the 95% level.

TABLE 3. Metal and P Content of Leachates from Soil Columns (mg Metal L-1)a

time from start ofexperiment (h) metal PMU PMT LHU LHT WHU WHT

576 Zn 173.3 ( 15.5b 12.0 ( 4.2b 0.5 ( 0.2 NA 75.4 ( 19.0 27.1 ( 46.71056 159.9 ( 23.7b 13.5 ( 4.1b <0.02 <0.02 75.8 ( 16.3 92.7 ( 10.21560 99.6 ( 14.9b 9.1 ( 0.0b 0.2 ( 0.1 <0.02 60.6 ( 6.7b 41.72 ( 7.9b

2064 58.7 ( 21.4b 7.7 ( 4.9b 0.1 ( 0.0 <0.02 69.42 ( 11.6b 30.0 ( 19.3b

576 Pb 2.1 ( 0.2b 0.6 ( 0.2b 20.3 ( 2.2 NA 0.4 ( 0.2 0.4 ( 0.21056 1.4 ( 0.5b <0.2b 18.1 ( 3.1b <0.2b <0.2 <0.21560 1.8 ( 1.0 2.1 ( 0.0 15.4 ( 3.3b <0.2b 0.3 ( 0.0 0.3 ( 0.02064 2.1 ( 0.9 2.1 ( 3.2 14.4 ( 3.4b 1.9 ( 2.2b <0.2 <0.2576 Cd 0.6 ( 0.0b 0.1 ( 0.0b <0.02 NA 2.0 ( 0.3b 1.0 ( 0.2b

1056 0.4 ( 0.0b <0.02b <0.02 <0.02 1.6 ( 0.1b 0.3 ( 0.1b

1560 0.4 ( 0.0b <0.02b <0.02 <0.02 <0.02b 0.3 ( 0.1b

2064 0.3 ( 0.1b <0.02b <0.02 <0.02 1.3 ( 0.1b 0.1 ( 0.1b

576 Cu 33.0 ( 1.2b 0.3 ( 0.1b <0.02 NA 0.1 ( 0.1b 3.5 ( 0.9b

1056 28.7 ( 4.0b 0.1 ( 0.0b <0.02 <0.02 0.1 ( 0.1b 0.8 ( 0.7b

1560 26.5 ( 2.5b 0.4 ( 0.0b < 0.02 0.1 ( 0.0 0.1 ( 0.1b 0.4 ( 0.1b

2064 23.47 ( 3.3b 0.6 ( 0.4b <0.02 <0.02 <0.02 0.1 ( 0.1576 Ni 0.2 ( 0.0b 0.1 ( 0.1b <0.05 NA 0.1 ( 0.0 0.1 ( 0.1

1056 0.2 ( 0.1b <0.05b <0.05 <0.05 <0.05 <0.051560 0.3 ( 0.0b 0.1 ( 0.0b <0.05 <0.05 <0.05 <0.052064 0.1 ( 0.0b <0.05b <0.05 <0.05 <0.05 <0.05576 P <0.5b 1.8 ( 0.4b <0.5 NA <0.5 0.5 ( 0.7

1056 <0.5 0.3 ( 0.5 <0.5 <0.5 <0.5 <0.51560 <0.5 <0.5 <0.5 <0.5 <0.5 0.7 ( 0.72064 <0.5 8.2 ( 10.8 <0.5 <0.5 <0.5 <0.5

a Values are means ( standard deviation from triplicate experiments. bIndicates statistically significant differences between untreated andtreated columns at the 95% level. NA ) not available.

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held on exchange sites were negligible compared to theconcentration of chelatable metal in the soil.

A possible explanation for the Cu CaCl2 results is thetendency of Cu to form soluble organo-Cu complexes atnear neutral pH conditions (19) and the fact that WH had thehighest pH of the three test soils. The pH was increased bybonemeal additions; the bonemeal treated WH soils in boththe leaching column and batch experiments were the onlysoils in those sets of experiments to have pH > 7. However,it seems unlikely that the increase in CaCl2 extractable Cuwas due to the formation of soluble organo-Cu complexessince DTPA extractable Cu was higher in the untreated WHsoil than in the bonemeal treated soil.

It is possible that the WH DTPA results are not reliablefor any of the metals considered. Table 4 shows that c. 24µmoles of Zn, Pb, Cu and Cd were extracted from both thetreated and untreated WH soil. The chelating capacity ofDTPA in a 2:1 DTPA:soil is saturated by chelation of 10 µmolesof metal (16). In the 5:1 DTPA:soil mix used here chelationsaturation would occur after chelation of 25 µmoles of metalso the chelating capacity of the DTPA solution used to extractthe WH soils could have been at or close to saturation. If thiswere the case some potentially chelatable metals in the soilmay not have been extracted. Therefore the reason for thedifference in the availability of metals in the WH soils asassessed by the two different extraction procedures is unclear.

The differences in the results obtained using the twoextraction procedures serve to highlight the inherent dif-ficulties in determining bioavailability by chemical extractiontechniques. Field trials, whereby bioavailability is assessedby monitoring metal uptake by plants from untreated andbonemeal treated at a former mining site, are, therefore,now being carried out.

Batch Experiments 2: pH Effects on Metal Immobiliza-tion. Significant pH rises and metal immobilization wereobserved in all the bonemeal treated soils (Table 5). Whensoil pH was raised to a statistically identical level in freshsoils by NaOH addition, metal release from the PM and LHsoils was either the same as (Zn, Cd) or significantly morethan (Pb, Cu) from the bonemeal treated soils (Table 5). Therewere no significant differences in metal immobilization forthe bonemeal and NaOH treated WH soils. While experi-mental conditions differed between the batch experimentsand the leaching column experiments reported above, theresults are a good indication that, at least for the PM and LHsoils, metal immobilization in the bonemeal treated leachingcolumns was not due solely to the generally observed rise inpH.

Leaching Columns: Adsorption Effects on Metal Im-mobilization. Lower concentrations of Zn, Pb, Cd and Cu(PM soil), Pb (LH soil) and Cd (WH soil) were extracted bythe CaCl2 from the bonemeal treated soils than from theuntreated soils (Table 4). This indicates that metal retentiondue to bonemeal additions was not due to adsorption ofmetals onto either soil or bonemeal particles as this extractionis thought to displace ions off exchange sites (13).

Leaching Columns: SEM and XRD Results - Evidencefor Metal Phosphate Formation. Lead, Zn and P were onlyobserved occurring together within, or on the edge of, reactedbonemeal particles. Typical energy dispersive X-ray spectra(EDX) are shown in Figure 1. Concentrations of Pb and Znassociated with P were greatest in the PMT soils and leastin the WHT soils. Cadmium and Cu were always belowdetection levels. Analyses had low analytical totals due tothe instability of the phases under the electron beam (phasessustained beam damage after ca. 10 s of analysis).

TABLE 4. CaCl2 and DTPA Extractable Metals (µg of Metal Per g of Dry Solid) in the Soils and 1:50 Bonemeal:Soil Mixtures fromBatch Experiment 1a

CaCl2 PMU PMT LHU LHT WHU WHT

Zn 32.0 ( 4.8b 16.1 ( 3.6b 0.6 ( 0.2 1.4 ( 1.9 38.3 ( 0.3 26.8 ( 3.1Pb 112.6 ( 4.7b 7.0 ( 1.4b 621.6 ( 1.4b 39.6 ( 1.2b 6.6 ( 0.2 9.0 ( 4.0Cd 0.2 ( 0.0b <0.2b <0.2 <0.2 8.6 ( 0.2b 4.2 ( 0.2b

Cu 22.9 ( 1.7b 1.3 ( 0.3b <0.2 0.1 ( 0.2 0.6 ( 0.0b 2.6 ( 0.7b

DTPA PMU PMT LHU LHT WHU WHT

Zn 27.5 ( 5.2b 7.0 ( 1.0b 0.5 ( 0.1 1.0 ( 0.3 821.0 ( 20.0 745.3 ( 68.2Pb 659.5 ( 11.7 713.8 ( 80.8 3153.0 ( 564.8 2363.1 ( 104.4 1907.6 ( 64.8 2163.2 ( 29.3Cd <0.1 <0.1 <0.1 <0.1 20.1 ( 0.4 13.5 ( 0.4Cu 17.0 ( 1.7b 8.1 ( 0.7b 0.9 ( 0.7 2.0 ( 0.1 79.3 ( 0.3b 55.5 ( 2.4b

a Values are means ( standard deviation of extractions of material from triplicate experiments. bIndicates statistically significant differencesbetween untreated and treated soil at the 95% level.

TABLE 5. pH and Metal Concentration in Solutions (mg Metal L-1) from Experiments Designed To Separate pH and BonemealEffects on Metal Immobilizationa

pH Zn Pb Cd Ni Cu

PMU 2.92 ( 0.03 8.4 ( 0.3 4.1 ( 0.1 0.1 ( 0.0 0.1 ( 0.0 5.0 ( 1.2PMT 5.25 ( 0.34b 0.3 ( 0.3b <0.2b,c <0.02b <0.05b 0.04 ( 0.0b,c

PM + 0.06 M 5.38 ( 0.18b 0.3 ( 0.2b 0.5 ( 0.2b <0.02b <0.05b 0.2 ( 0.0b

NaOHLHU 4.41 ( 0.03 0.1 ( 0.0 15.7 ( 0.4 <0.02 <0.05 <0.02c

LHT 5.87 ( 0.02b 0.1 ( 0.0 <0.2b,c <0.02 <0.05 <0.02c

LH + 0.006 M 5.81 ( 0.08b 0.1 ( 0.1 0.3 ( 0.1b <0.02 <0.05 0.1 ( 0.0NaOHWHU 6.65 ( 0.05 14.7 ( 1.4 0.3 ( 0.0 0.4 ( 0.0 0.1 ( 0.0 0.3 ( 0.2WHT 7.44 ( 0.03b 3.9 ( 1.2b <0.2b 0.2 ( 0.0b <0.05b <0.02b

WH + 0.0006 M 7.44 ( 0.05b 1.9 ( 0.3b <0.2b 0.2 ( 0.0b <0.05b <0.02b

NaOHa Values are means ( standard deviation of triplicate experiments. bSignificantly different from untreated soil at the 95% level. cSignificantly

different from the NaOH treated soil at the 95% level.

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Although it is possible that any newly formed metal-phosphate phase formed by the substitution of metal ionsfor calcium ions in the bonemeal by an exchange reaction,it has been suggested elsewhere (7) that the formation ofmixed metal phosphates is unlikely to be due to substitutionbecause ionic substitution is a very slow process at ambienttemperatures and pressures. In addition, the rise in pH ofthe bonemeal treated soils and the presence of P in theleachates from the bonemeal treated soils indicate thatbonemeal dissolution was occurring. If this was the case ionicsubstitution of metal ions for Ca2+ would be hindered by thedissolution of the bonemeal.

It seems likely, therefore, that metal phosphates wouldhave formed by precipitation after calcium phosphatedissolution. Hydroxylated metal phosphate may form by

where M ) a metal ion.The presence of a small Cl peak in the X-ray spectra (Figure

1) is interpreted as partial conversion of hydroxyl phosphatephases to chloro-phosphate phases, which, for Pb at least,are more stable (4).

Alternatively metal phosphates with no associated hy-droxyl groups may have formed

or a mixed metal-calcium phosphate may have formed

However, it was not possible to resolve the grains of bonemealand any grains of newly formed metal-bearing phases at theedge of the bonemeal particles by SEM. The area over whichthe EDX spot analysis were carried out may, therefore, haveincluded both newly formed metal-bearing phases andsurrounding bonemeal so that both these components wouldhave been included in the spectra shown in Figure 1. Thusthe Ca detected may be present in either the bonemeal and/or a metal-Ca phosphate. It is thought that the Al and Si seenin the PMT analysis is due to minute grains of a silicatemineral being included in the analysis. The resolutionproblems mean that it is not possible to say whether theoccurrences of Pb, Zn, Fe, Al and P indicate submicron crystalsof newly formed metal-phosphate or other phases whichnucleated on, or in, the porous bonemeal.

X-ray diffraction patterns of the bonemeal-rich fraction(Figure 2) and the <2 µm fractions (not shown) obtained atthe end of the leaching column experiments indicated thepresence of many minerals in the soil. Jarosite, goethite,plagioclase, haematite, muscovite, quartz, anatase, cha-mosite, cerrusite, kaolinite, calcite and dolomite were identi-fied leaving several unassigned peaks which were not presentin XRD patterns obtained from the untreated soils.

The unassigned peaks on the WH trace were markedlydissimilar to those reported for crystalline metal bearingphosphates (20). However, the unidentified peaks on thepatterns obtained from the PM and LH bonemeal-richseparates were similar, but not identical, to the publishedpeak positions and intensities of documented crystalline Pb-and Zn-bearing phosphates (PDF 20-248, 251 and 258) (20).Small differences in peak positions relative to published data(20) could be due to the phosphates in the bonemeal treatedsoil being impure (i.e. containing more than one type of

FIGURE 1. Typical EDX spectra for occurrences of Zn, Pb, Ca andP in treated soils: a) PMT, b) LHT, c) WHT.

FIGURE 2. X-ray diffraction traces obtained from bone meal rich separates from the treated soils: a) PMT, b) LHT, c) WHT. Peaks whichindicate the presence of metal phosphates are indicated on the PMT and LHT traces.

10M2+(aq) + 6H2PO4-(aq) + 2H2O S

M10(PO4)6(OH)2(s) + 14H+(aq) (1a)

XM2+(aq) + YH2PO4-(aq) S

MX(PO4)Y(s) + 2YH+(aq) (1b)

XM2+(aq) + ZCa2+(aq) + YH2PO4-(aq) S

MXCaZ(PO4)Y(s) + 2YH+(aq) (1c)

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metal ion) or poorly crystalline (so that peak broadeningoccurs making the precise position of the peak hard todetermine). Other workers (21-23) have reported shifts inthe peak positions of pyromorphite due to substitution of Cafor Pb in the crystal lattice. Unidentified peaks with d-valuesof 0.296, 0.215, and 0.208 nm on the PM trace suggest thepresence of a zinc phosphate, R CaZn2(PO4)2, phase PDF20-248 (20). On the LH trace unidentified peaks with d-valuesof 0.401 and 0.371 nm suggest the presence of a zincphosphate, γ CaZn2(PO4)2, phase PDF 20-251 (20), whileunidentified peaks with d-values of 0.343, 0.328 and 0.298nm suggest the presence of a lead phosphate, Pb5(PO4)3OH,phase PDF 24-586 (20).

The formation of Pb hydroxyapatite and non-hydroxylatedZn bearing phosphate has been observed before when Pband Zn solutions were reacted with apatite (e.g. (7) for Pband Valsami-Jones, unpubl. for Zn).

Unidentified peaks on the XRD traces of the <2 µmseparates obtained from the bonemeal treated soils occupiedpositions which most likely indicated the presence of metalphosphates of the form (A3-xBx(PO4)2 with a strong possibilityof Zn making up some of A and Fe making up some of B. Thepotential metal phosphate peaks were absent on the <2 µmfractions obtained from the untreated soils.

The low concentration of Cl detected by EDX analysisusing the SEM makes it unlikely that any chloro-metal-phosphate compounds would be detected by XRD even infractions rich in the newly formed phosphates.

The dissolution of bonemeal in parallel with both theformation of hydroxylated Pb phosphate (Reaction 1a) andhydroxyl-free Zn bearing phosphates (Reaction 1b, 1c) willresult in the net removal of protons from solution. Ther-modynamics predicts that the least soluble species of eachmetal should form and thus in a mix of, for example, Pb andZn ions it would be expected that both Pb-hydroxypyro-morphite (a hydroxylated metal-phosphate) and a hydroxyl-free Zn phosphate would form. Thus the tentative XRDidentification of both hydroxylated and hydroxyl-free metalbearing phosphates in the bone meal treated soils isconsistent with both the increase in the pH of the soils andleachate as the bonemeal dissolved and also thermodynam-ics.

In previous experiments (24) limited dissolution ofhydroxylapatite at near neutral pH and the stability of otherPb bearing phases has been found to limit pyromorphiteformation. This could explain the lack of identifiable metalphosphate peaks on the WHT XRD traces and the lowconcentrations of Pb and Zn associated with P observed byEDX analysis using the SEM.

Suitability of Bonemeal Additions as a ContaminatedLand Remediation Treatment. This study has shown thatbonemeal additions to soils contaminated with Pb, Zn, Cdand Cu result in pH increases and a reduction in metal releasefrom soils. The pH increase appears to be due to thedissolution of the bonemeal. Although not fully characterized,the retention of the metals appears to be partially due to thepH increase and, at least for acidic soils (pH < 7), partiallydue to metal phosphate formation. Metal retention is leastmarked in neutral (pH 7) soil in which no XRD evidence formetal phosphate formation was obtained (though EDXanalysis indicates that Pb, Zn and P are associated). It seemslikely that at neutral pH, as in the WH soil, metal phosphateformation was slowed so that there was insufficient timeduring the leaching period for significant metal phosphateformation.

Assessment of metal availability by chemical extractionindicates that bonemeal additions either reduced or had noeffect on metal availability in soils. In the acidic soils(pH < 7) formation of highly insoluble Pb and Zn phosphatesappears to have reduced metal availability. Again, results are

less promising for the neutral (pH 7) soil though somereduction in metal availability was observed. It seems likelythat over time more significant reductions in metal availabilitywould also have occurred in all the soils as bonemealdissolution and metal phosphate formation continued.

In conclusion, although the mechanism of metal im-mobilization is not yet fully understood, the results from thecurrent study indicate that bonemeal additions have thepotential to be an effective means of remediating acidic(pH < 7) metal contaminated soil and may also have thepotential for remediating neutral soils.

AcknowledgmentsThe BOC Foundation and the Environment Agency jointlyfunded this project. David Jenkins (University of Wales,Bangor) provided assistance at Parys Mountain. Advice wasprovided by Andrew Bannister, Barnsley Metropolitan Coun-cil; Paul Charlesworth and Stan Redfearn, The BOC Founda-tion; Matthew Collins, Newcastle University; William Dubbinand Alan Warren, Natural History Museum; Ian Hall, Sun-derland City Council; Theresa Kearney, Environment Agency;Anthony Kemp, Bristol University; and Iain Thornton,Imperial College, London University. Meryl Batchelder,Gordon Cressey and Paul Schofield (all at NHM) are thankedfor carrying out the XRD analysis. John Hall (Animal Feeds)Ltd. provided the bonemeal.

Supporting Information AvailableTables 1 (mineralogy of soils used in leaching columnexperiments), 2 (metal concentrations in pore waters of soils),and 3 (typical quantitative EDX analyses of occurrences ofmetal and phosphorus in the bonemeal treated soils). Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

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Received for review August 18, 1999. Revised manuscriptreceived March 21, 2000. Accepted June 2, 2000.

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