LEAD REMOVAL FROM SOILS VIA BENCH-SCALE SOIL

WASHING TECHNIQUES

By Steven R. Clinel and Brian E. Reed,2 Associate Members, ASCE

ABSTRACT: The retention and release of lead from eight different soils collected from the eastern UnitedStates were investigated. After characterization using well-established test procedures, the study soils wereartificially contaminated with lead. Upon contamination, the lead-retention capacity of the study soils wasstatistically correlated with their physical and chemical properties. Lead capacity of the eight study soils wasbest correlated with the parameters of soil pH and the organic soil fraction. The efficiencies of six washingsolutions in removing lead from the contaminated soils were then investigated via batch washing experiments.Unlike conventional field-scale soil washing, all particle size fractions were washed and recovered in thesebench-scale experiments. Hel and EDTA obtained the best Pb-removal efficiencies. Removals tended to beindependent of soil type and washing solution concentration. In addition to the physical/chemical propertiesof the washing solutions and of the study soils, the effects of the initial lead contamination level and washingsolution concentration upon lead removals were examined. Lead-retention and release kinetic studies wereperformed to confirm equilibration between the aqueous and solid phases. Kinetics of both lead retention andrelease were very rapid for most study soils.

INTRODUCTION

Heavy metals such as lead, mercury, zinc, nickel, chro­mium, and cadmium are often found in soils at high concen­trations as a result of past environmental-disposal practices.Unlike many organic pollutants that can be eliminated orreduced by chemical oxidation techniques or microbial activ­ity, heavy metals will not degrade. Soil washing has recentlybecome a popular ex-situ technique for remediating sites con­taminated with organics and heavy metals. In conventionalsoil washing processes, excavated soil is vigorously mixed witha solution(s) (typically water) that separates the contaminantsfrom the large particle size fractions. The recyclable wash­water is then treated to remove the colloidal particles andthe original contaminants.

This research focused on lead contamination, since lead isone of the most common contaminants at Superfund sitesacross the nation, particularly at sites where past industrialactivities include battery breaking and recycling, oil refining,paint manufacturing, metal molding and plating, and smelting(USEPA: Superfund 1991).

Several retention mechanisms can be operative within asoil system. Cation exchange and specific adsorption are twomechanisms controlling metal adsorption. (The data pre­sented here are most applicable to soils where these retentionmechanisms are predominant.) Heavy metals can also be re­tained by mechanisms other than sorption (e.g., solid-statediffusion and precipitation reactions) especially when leadexists as PbCO" PbS04 , or as an organic lead form. Theknowledge of soil characteristics is often beneficial in pre­dicting the lead-retention mechanisms that might be opera­tive. Heavy-metal retention has been found to generally in­crease with increases in soil pH, cation-exchange capacity(CEC), organic content, clay content, and the metal oxidecontent of a soil. In addition, the strength of metal retentiongenerally increases as the initial concentration of the contam-

'Res. Assoc.. Envir. Sci. Div .. Oak Ridge Nat. Lab .. P.O. Box 200X.Oak Ridge. TN 37831-6036.

'Asst. Prof.. Dept. of Civ. & Envir. Engrg.. West Virginia Univ ..P.O. Box 6101, Morgantown. WV 26506-6101.

Note. Discussion open until March I. 1996. To extend the closingdate one month. a written request must be filed with the ASCE Managerof Journals. The manuscript for this paper was submitted for review andpossible publication on December 16. 1993. This paper is part of theJournal oj Environmental Engineering. Vol. 121. No. 10. October. 1995.©ASCE. ISSN 0733-9372/95/0010-0700-0705/$2.00 + $.25 per page.Paper No. 7531.

700/ JOURNAL OF ENVIRONMENTAL ENGINEERING / OCTOBER 1995

inant decreases. (The average binding strength of a soil de­creases once the high-energy binding sites are selectively filledfirst. )

MATERIALS AND METHODS

All experimental work was performed at the West VirginiaUniversity Department of Civil and Environmental Engi­neering. A majority of the experiments were conducted intriplicate. Solutions were prepared, using reagent-gradechemicals, in nitric acid rinsed volumetric flasks then storedin polypropylene containers. All samples for metal(s) analysiswere filtered through a 0.45-fJ.,m membrane filter and acidi­fied. Lead concentrations were determined using a PerkinElmer 2380 atomic-absorption spectrophotometer (AA)equipped with a flow spoiler (air-acetylene flame, A = 217nm). Percent recoveries (spiked samples) were performed onthe AA for approximately one in every eight samples to en­sure that significant matrix interferences were not present.

Soil Characterization and Contamination Procedure

Each study soil was artificially contaminated with lead[Pb(NO,hl at three different concentrations: 10, 100, and1,000 mg/L Pb. Sodium nitrate (NaNO) was used to adjustthe solutions to an ionic strength of 0.04 M to simulate thatof ground water. One hundred fifty grams of air-dried, ASTMNo. 10 sieved, soil was placed in a 2-L Nalgene containerwith each Pb-NaNO, solution at a 10: I liquid/soil (LIS) massratio. Chemical equilibrium software suggested that lead spe­ciation is predominately as Pb(OHb,) at pHs above 5.0 forthe 1,000 mg/L Pb contamination level. Changes in initialmetal concentrations can also lead to shifts in the dominantretention mechanisms that occur. At high initial heavy-metalconcentrations, for example, a large percentage of the metalspresent will precipitate, increasing the significance of surfaceprecipitation as a metal retention mechanism. Thus, the 1,000mg/L lead solution was pH adjusted to 4.8 with nitric acidbefore being mixed with the soil in order to simulate leadsorption rather than precipitation. The samples were placedon mechanical shakers for 21 days. The samples were thenremoved from the shakers. and the slurry pHs were taken.The slurries were then left undisturbed for 24 h to allow thecolloidal clay particles to settle. A small aliquot was filteredand preserved for lead analysis, and the remaining superna­tant was decanted off of the soil mass, which was then allowedto air dry.

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Using the 10:1 LIS ratio, the 10, 100, and 1,000 mg/L lead­contaminating solutions would increase the soil lead concen­trations by 100, 1,000, and 10,000 mg/kg, respectively, if allthe lead was retained by the study soils. Two methods wereused to determine the final contamination levels. The firstmethod was based upon mass-balance principles, accordingto

Soil Pb = {[Pbo'Yo] - [Pbf'Yf ]} -7- [W,] (1)

where Pbo = lead concentration of the stock contaminatingsolution. Pbf and Yf represent the lead concentration andvolume, respectively, of the supernatant solution decantedfrom the soil. The Vo variable represents the volume of thestock lead solution added to the soil mass, W,. As a check,the contaminated soils were digested in triplicate for totallead using SW 486 Method 3050 (USEPA: Test 1986).

Batch Washing Procedure

After the spiked soils were air-dried and allowed to ageapproximately 2-3 months, the contaminated soils were thencrushed to pass through an ASTM No. 10 sieve. Aliquots ofthe spiked soils were washed (in triplicate) with the six so­lutions listed in Table 1. (The tap water washes provided abaseline to determine if lead was held tightly by the soils).In the wash experiments, 25 mL of washing solution wereadded to a 60 mL bottle that contained the 1.0 ± 0.02 galiquot of contaminated soil. The samples were placed onmechanical shakers. After 24 h, the pH of each sample slurrywas measured and recorded. Approximately 10 mL of eachsample was then 0.45-l--lm-filtered and acidified for lead anal­ysis. It was assumed that the lead mass determined for thefiltrates represented the lead released from the contaminatedsoils. Soil washing efficiencies were computed by dividing thelead quantities found in the filtrates by the initial lead masspresent on each sample before the wash. The Pb value ob­tained from the average of both the mass balance approach[( 1)] and the EPA Method 3050 digestions served as the initialsoil Pb concentration for a majority (75%) of the spiked sam­ples. On average, the mass balance approach yielded greater(2-15%) initial Pb soil concentrations than did the soil diges­tions. [If only one of these values was used as the initial Pbconcentration, it was due to the fact that the experimentaldata suggested that the value was more representative of thelead quantity actually present on the spiked soils. The actualinitial lead values used in the computations can be foundelsewhere (Cline 1993).]

Retention and Release Kinetics

For the retention kinetics studies, 50 g of soil were mixedwith each contaminating solution (10: 1 LIS mass ratio) andplaced on a stirrer. (Again, the ionic strength was also ad­justed uniformly with NaN03 .) At predetermined times be­tween 15 min and 21 days, the pH of each slurry reactor wasmeasured, and a lO-mL aliquot was withdrawn and preservedfor lead analysis.

A 25:1 LIS ratio was employed in the lead release kinetics.

TABLE 1. Washing Solutions Investigated

Washing solutions Concentrations(1) (2)

Tap water (H,O) Not applicableHydrochloric acid (HCI) 0.1 Nand 1.0 NEDTA (Na,C,,,H,"OHN,'2H,O) 0.01 M and 0.1 MAcetic acid (CH,COOH) 0.1 Nand 1.0 NCalcium chloride (CaCI,) 0.1 M and 1.0 M

Note: All prepared solutions were reagent grade.

For each contaminated soil, 1.0 ± 0.02 g samples were placedinto seven 60 mL Nalgene bottles containing the appropriatewashing solution and were immediately placed on the me­chanical shaker. One sample was removed from the shakerat each of the following time periods: 15 min, 30 min, 1 h, 4h, 8 h, 1 day, and 7 days. The slurry pH of each sample wasmeasured, and a lO-mL aliquot was taken and preserved forlead analysis. The lead values obtained from the filtrates wereplotted against the time that the samples were taken.

RESULTS AND COMMENT

Selected properties of the test soils are presented in Table2. After the 21-day contamination period, the pH values wereslightly lower than at the start of the contact time. Final slurrypH values for the 10, 100, and 1,000 mg/L lead contaminationsaveraged 6.1, 5.9, and 4.8, respectively for the eight studysoils. During the lead-contamination step, nearly all the leadwas retained by the study soils at the lower two contaminationlevels. The data are presented in Table 3. It is believed thatthe low percentage of lead retention for soil 3 at the lowcontamination level is due to experimental error. It appearsthat the lead capacity of all the study soils was exceeded atthe 1,000 mg/L contamination level, with the possible excep­tion of soil 7, which possessed a higher initial pH than didthe other study soils. Multiple linear regression analyses wereperformed to determine which pair(s) of soil parameters hadthe greatest influence on the retention capacity of the studysoils (Kennedy and Neville 1986). The parameter pairs ofpH/volatile solids and pH/CEC significantly affected lead re­tention, yielding ,2 values of 0.97 and 0.95, respectively (ex= 0.01; n = 8; and k = 2). Since parameter pairs (theindependent variables) are usually not completely indepen­dent of each other, t-tests were also performed for each in­dependent variable to ensure that each significantly influ­enced lead retention. The calculated t-values for each parameterpair were significant (ex = 0.01; D.F. = 5). These statisticalresults are quite similar to that observed by others (LaBauveet al. 1988).

The results of the batch soil washing experiments are sum­marized in Table 4. The tap water washes resulted in anaverage removal efficiency that was less than 3%, indicatingthat the sorbed lead could not be readily removed by rinsingalone even though the soils were artificially contaminated inthe laboratory. From the remaining washing solutions, EDTAand HCl achieved the greatest average lead-removal efficien­cies (92% and 89%, respectively) followed by CH3COOH(45%) and CaCl2 (36%). Good reproducibility among tripli­cate samples was obtained. Removal efficiencies also variedwith the initial lead contamination level. The average lead­removal efficiency from all test soils at the 1,000 mg/L con­tamination level was more than 10% greater than that of the10 mg/L contaminations, likely due to selective lead uptakeby the high-energy binding sites of the study soils. Hence,the strength of metal retention appears to be greatest for thelowest lead-contamination level.

The average standard deviations listed in Table 4 generallydecreased with increasing initial lead concentrations. Such atrend is likely caused by the greater influence of the soils'indigenous lead content at the 10 mglL lead contaminationlevel than at the larger contamination levels. The large stan­dard deviations for the 10 mg/L contaminations may also bedue to the decrease in AA sensitivity at low lead concentra­tions or the relatively small 1.0 ± 0.02 g soil aliquots usedin the batch wash experiments. Although good agreementexists among the results of most of the triplicate washes, it ispossible that the small aliquots were not always representativeof the lead concentration of the artificially contaminated soils.

JOURNAL OF ENVIRONMENTAL ENGINEERING / OCTOBER 1995/ 701

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TABLE 2. Selected Properties of Test Solis

Soil Indigenous CEC Total volatile % Silt FeMo AID MnMO

number Sample location Soil pH" Pb (mg/kg)b (meq/l00 g)" solids (wt%)d and clay' (mg/kg)' (mg/kg)9 (mg/kg)'(1) (2) (3) (4) (5) (6) (7) (8) (9) (10)

I Monongahela Co., W.Va. 6.2 ± 0.20 6.3 ± 1.3 6.4 ± 0.60 1.60 ± 0.10 1.7 1,350 ± 80 600 ± 50 522 Marion Co., W.Va. 7.0 ± 0.10 19 ± 1.5 20.5 ± 2.0 4.37 ± 0.20 37.2 5,000 ± 200 2,110 ± 120 515 ± 103 Marion Co., W.Va. 6.1 ± 0.10 20.3 ± 0.3 23.5 ± 1.5 4.54 ± 0.10 28.8 4,200 ± 375 2,475 ± 170 580 ± 505 Erie Co., N.Y. 5.6 ± 0.03 22.8 ± 0.3 25.6 ± 0.60 5.53 ± 0.20 30.5 2,600 ± 70 850 ± 90 18 ± 56 Erie Co., N.Y. 5.7 ± 0.01 14.8 ± 0.8 15.4 ± 0.30 2.45 ± 0.10 41.6 1,800 ± 430 970 ± 10 80 ± 177 Erie Co., N.Y. 8.0 ± 0.10 21 ± 0.5 20.5 ± 3.20 4.05 ± 0.20 24.7 3,650 ± 300 1,680 ± 160 725 ± 608 Erie Co., N.Y. 5.5 ± 0.10 20.4 ± 1.4 7.6 ± 0.40 1.46 ± 0.04 3.5 1,100 ± 50 1,230 ± 100 80 ± 209 Erie Co., N.Y. 6.6 ± 0.02 25.1 ± 1.5 17.9 ± 2.9 3.00 ± 0.02 14.2 3,100 + 230 2,750 + 260 350 + 10

"McLean (1982)."Method 3050 (USEPA: Test 1986).'Method 9081 (USEPA: Test 1986)."No. 209D (Standard 1985).'In accordance with ASTM 422, the value represents the wt% of sample passing the No. 200 ASTM sieve.'McKeague and Day (1966).'Mehra and Jackson (1960).

FIG. 1. HCI Wash Results for 10, 100, and 1,000 mg/L Pb Contam­inations

HCI wash solutions, respectively. The average standard de­viations obtained for the analysis of the HCl washes (Table4), were lower than those of the other washing solutions,indicating that removal efficiencies may have been somewhatindependent of soil type, particularly for the 100 and 1,000mg/L lead contaminations. This phenomenon was most likelydue to dissolution of the soil structure resulting from the harshlow pH values the soils were subjected to. Lead release couldhave been partly due to selectivity of the soil surface groupsfor H + over the bound Pb2 + . From an electrostatic viewpoint,negatively charged surface sites have a greater affinity for tri­and divalent ions than monovalent ions. However, H + ionsare attracted more strongly than any other cation. Regardless

No. 1 No.2 No.3 No.5 No.6 No.7 No.8 No_ 9

No. 1 No.2 No.3 No.5 No.6 No.7 NO.8 No.9

-

VV

DO.1N HCI !Z31.0N HCI ~

17VV

IIvVV

HCI Washes (25: 1 L/S Ratio) lDO.1N Hel lZ::al.0N Hel

17 17 17 17 BV V V

~ V ~/...j

HCI Washes (25: 1 L/S Ratio)

BII

Study Soil No.

Study Soil No.

Study Soil No.

I/­

I/-

17

17

V

~V

vV

vV

V

17

1,000 mg/L Pb

10 mg/L Pb [:;

No. 1 No.2 No . .:5 No.5 No.6 No.7 No.8 No.9

100 mg/L Pb HCI Washes (25: 1 L/S Ratio)DO.IN Hel ~'.ON Hel

7 ~ /' ~7 17

/' /' /' ~ r:;V V V-

V /

/ /'V / /V V /

>- 120uc

'" 100'0;,::W 80

"0 60>0E 40"cr.0 20Cl.

a

<a)

>- 120uc

'" 100'0;,::

W 80

"0 60>0E 40"cr.0 20Cl.

a

(b)

>- 120uc

" 100'0;,::W 80

"0 60>0E 40"cr.0 20Cl.

0

<c)

TABLE 4. Summary of Soil Washing Efficiencies

Note: 21-day equilibration period; 10: I LIS mass ratio employed."%Pb removed from solution = (Pb'in."lPb'nili«') X 100."Slurry pH value after equilibration period.'Not applicable.

Hydrochloric Acid Washes

Removal efficiencies for the HCI washes are presented inFig. I for the 10, 100, and 1,000 mg/L lead contaminations.After the 24 h wash time, the 25: 1 LIS slurry pH values rangedfrom 1.1 to 1.8 and from 0.1 to 0.9 for the 0.1 N and 1.0 N

"Average lead-removal efficiencies (% mass) from soils 1, 2, 3, 5, 6,7,8, and 9... ± Values" represent one standard deviation.

"Average removal efficiencies (% mass) of the 10, !OO, and 1,000mg/L lead contaminations.

''Average lead-removal efficiencies (% mass) from soils 1,2,3,5,6,and 7.

10 mg/L Pb 100 mg/L Pb 1,000 mg/L PbContamination Contamination Contamination

Soil % Pb % Pb % Pbnumber removed" pHb removed" pHb removed" pHb

(1) (2) (3) (4) (5) (6) (7)

I 89.1 6.0 99.7 6.0 23.3 4.62 98.2 6.6 99.9 6.4 6.4 4.63 54.4 5.4 99.4 5.3 66.6 4.35 96.2 5.1 99.4 5.0 83.1 4.56 94.9 4.9 97.2 4.8 42.1 4.27 98.7 7.2 99.8 7.2 98.1 6.48 78.3 - '" 87.7 - , 20.3 - '"9 93.6 6.1 99.8 58 54.5 48

TABLE 3. Percent of Initial Adsorbate Lead Removed from So­lution

10 mg/L Pb 100 mg/L PbWashing contamina- contamina- 1,000 mg/L Pb Averagesolution tion tion contamination efficiencyb

(1 ) (2) (3) (4) (5)

Tap water" 4.2 ± 3.0 < 1.0 3.4 ± 3.0 2.90.1 N HCl" 88.7 ± 12.3 84.6 ± 5.7 81.0 ± 7.2 84.61.0 N HCI" 96.0 ± l7.0 90.7 ± 5.5 90.9 ± 3.8 92.50.01 M EDTA" 95.3 ± 20.2 95.3 ± 9.4 90.0 ± 4.3 93.5(1.1 0 M EDTA" 84.1 ± 11.3 95.9 ± 14.3 93.7 ± 6.8 91.20.1 N Acetic

acid'" 14.8 ± 12.1 26.3 ± 13.4 54.2 ± 12.7 31.81.0 N Acetic

acid'" 44.3 ± 12.7 55.7 ± 6.9 72.0 ± 6.2 57.30.1 M CaCV 10.3 ± 6.1 16.6 ± 14.3 47.4 ± 19.5 24.81.0 M CaCL" 36.0 ± 27.8 43.0 ± 26.1 62.7 ± 17.9 47.2

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i:;' 120cQ> 100~W 80

i:;' 120c

"'v 100-=W 80

10 mg/L Pb EDTA Washes (25: 1 L/S Ratio)

00.01 1ft EDTA~O.lM EDTA

100 mg/L Pb EDTA Washes (25: 1 L!S Ratio)

00.01 u EDTA &:s::30.1M EDTA

No.9

No.9

No.8

No. B

No.7

No.7

Study Soil No.

No. ~ No.5 No.6

No.:3 No. 5 No. 6

No.2

No.2

NO.1

No.1

60

40

20

o

60

40

20

o

o>oE"0::

.00..

o>aE"0::

.00..

(a)

of the release mechanism, the HCI removal efficiencies couldnot be correlated with the physical and chemical character­istics of the soils. The greatest variation in lead removals fromthe HCI washes occurred for the 10 mg/L contaminations andagain is likely due to the influence of the indigenous leadcontent (Table 2), which is large relative to the quantity oflead added at this contamination level. The larger than 100%removal efficiency obtained for soil 3 at the 10 mg/L Pb level[see Fig. 1(a)] is believed to be a result of an incorrect estimateof the soil lead contamination present before being washed.Both the mass-balance method [see (1)] and the soil-digestionapproach seem to underestimate the initial quantity of leadon the soil, perhaps due to analytical interferences caused bycertain constituents present in the soil itself. Finally, leadremovals were slightly dependent on the acid concentration.The average washing efficiencies obtained for the eight soilsat 10, 100, and 1,000 mg/L lead-contamination levels were7%,6%, and 10% greater for the 1.0 N HCl washes than forthe 0.1 N washes, respectively.

EDTA Washes(b)

Study Soil No.

Calcium Chloride Washes

FIG. 2. EDTA Wash Results for 10, 100, and 1,000 mg/L Pb Con­taminations

The addition of a secondary exchange cation, such as CaZ + ,

can promote the removal of heavy metals held via cationexchange. CaClz, if added in sufficient quantity, can driveexchangeable lead from the soil surface sites. Since heavymetals are generally retained by soils via other mechanismsin addition to cation exchange, CaClz solutions should not be

aqueous system in the pH range of 3-6, where Hz[EDTAJ2­is often the dominant form in which the metal ions that arepresent generally form 1:1 stoichiometric complexes (Bell1977). Although the slurry pH values obtained after washesfrom both EDTA concentrations were in the range in whichlead is theoretically bound to the EDTA ligand, the lowerlead removals obtained with the high EDTA concentrationsmay still be related to the pHs of the soil slurries. EDTAbegins to protonate significantly at pH values less than 4.Protonation reduces EDTA solubility thereby reducing theeffectiveness of the ligand to chelate metals. NazHzEDTAhas a solubility of 10.8 g/100 mL at 22°C, while the fullyprotonated H4EDTA species only has a solubility of 0.20g/100 mL (PribiI1972). The average pH of the treated samplesat the 10 mg/L Pb level was 4.73 ::t: 0.3 and 4.46 ::t: 0.2 forthe 0.01 M and 0.1 M washes, respectively. Realizing thatthe washing experiments were not true aqueous systems, pro­tonation may have begun at a pH between these two values,so that the number of donor atoms available for chelation onthe ligand was reduced for the 0.10 M washes. Ragahavan etal. (1989) discussed that the presence of large concentrationsof ions not participating in chelation (such as the large quan­tity of Na+ ions present in the EDTA solutions) can alsolower the stability of metal-chelant complexes.

SL.dy Soil No.

No.2 No.3 No 5 No.6 No. 7 No.8 No.'NO.1

1.000 mg/L Pb EDTA Washes (25: 1 LiS Ratio) -00,01 ... EDTA ~o, 1M (OTA -

r-r-

r- ----

.-o«J

'" 120uc

" 100;gW 800 60>0E 40"0::

.0 200..

A chelate is a ligand that contains two or more electron­donor groups so that more than one bond is formed betweenthe metal ion and the ligand. Chelants such as EDTA canreadily form soluble complexes with Pbz+, reducing the quan­tity of metals retained by soil particles and thereby increasingheavy-metal mobility. Thus, chelating agents are well suitedfor removing metals bound by soils. EDTA forms 1: 1 molarratio complexes with several metal ions. Lead aqueometalcomplexes (e.g., Pbz+, Pb(OH)-, etc.) do not exist in thepresence of the EDTA ligand at molar ratios greater thanI: I. In this study, the EDTA:Pb molar ratio was always greaterthan 1: 1. Note also that common soil constituents (e.g., Caz+ ,

Na+, Mg, AI, and Fe) may compete with heavy metals forthe chelating agents, so that excess chelate quantities are oftenneeded to ensure complete contaminant removals.

EDTA was highly effective in removing lead from the con­taminated study soils. Only small differences were observedin removal efficiencies of the 0.01 or 0.10 M EDTA washes.Soil washing efficiencies for EDTA are presented in Fig. 2for the 10, 100, and 1,000 mg/L lead-contamination levels. Itdoes not appear that the average removal efficiencies (Table4) for the 0.01 M and 0.10 M EDTA washes were significantlydifferent for the 100 and 1,000 mg/L Pb contaminations. Othershave also found lead desorption to be independent of EDTAconcentration (Peters and Shen 1992). Like the HCI washes,the greatest variation in Pb removals from the eight studysoils was observed in the low Pb-contamination levels. Fur­thermore, the standard deviations obtained for the 100 and1,000 mg/L Pb contamination levels were relatively low whencompared to those obtained for other wash solutions listedin Table 4, suggesting that the EDTA removals were alsoindependent of soil type.

While lead removals by EDTA are largely independent ofEDTA concentration, it is interesting that the removals ob­tained from several of the 0.01 M washes were slightly greaterthan the removals from the 0.10 M washes at the 10 and 100mg/L contamination levels (see Fig. 2). (This occurred forseven of the eight soils at the 10 mg/L Pb level and for fiveof the eight soils at the 100 mg/L Pb level.) This phenomenonwas most likely not due to the existence of large matrix in­terferences during AA analysis since the average percent re­coveries of 103 ::t: 3% and 101 ::t: 3% were obtained for the0.01 M and 0.10 M EDTA samples, respectively.

The final slurry pH values of the EDTA samples were inthe range of 4.0-5.4 for the 0.01 M washes and between pH4.3 and 4.8 for the 0.10 M washes. The EDTA ligand isgenerally most effective in chelating heavy metals from an

JOURNAL OF ENVIRONMENTAL ENGINEERING! OCTOBER 1995! 703

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60

70

FIG. 4. Lead-Retention Kinetics for Soil No. 7

7

3730

5 6

Days

Days

21

INITIAL Pb SOLUTION CONC... 10 mg/L Pb• 100 mg/L Ph• 1.000 mg/L Pb

147

2 3 4

Time Elapsed

Hours

RETENTION KINETICSSOIL No.710: 1 LIquid/Soil Mass RatioIonic Strength: 0.04 M (NaNO 3)

"ass Balance Value

3060 Dile.Uoh Value

1l'- t ?,." IRELEASE KINETICS , \SOIL No.7

(1000 mg/L Pb Conlamin~tion)

2:5: 1 LiqUid/Soil Mass RatiO.

~ . O.IN HCl . O.OIM EDTA . O.IM Cact20 1.0N HCl ~ 0.10M EDTA 0 1.0M CaClz

a02468

Hours

O~' ~ '6 8/

2000

Time Elapsed

50

40

30

20

10

liD ,------7'/''----------------~

12000

bii 10000..-:"-onE 8000

6000

4000

expected to achieve complete removal of heavy metals. Inaddition, the presence of a high chloride concentration in thesoil pore water can lead to the formation of soluble lead­chloride complexes. According to chemical speciation soft­ware, lead exists as various soluble chloride species below thepoint where the precipitation of the hydroxyl species begins(pH 5.5). The PbCl + complex is significant at a chlorideconcentration of 10- 1 M (3,500 mg/L Cl-). In this research,the chloride concentrations of the 0.10 M and 1.0 M CaCl2

wash solutions are 7,000 mg/L and 70,000 mg/L Cl- , respec­tively.

Soil washing efficiencies for CaCl2 are presented in Fig. 3for 100 mglL lead-contamination level. (The 10 and 1,000mg/L Pb samples are similar.) Lead removals were clearlydependent on CaCI2 concentration. Lead-removal efficienciesfrom the 1.0 M washes were more than two times the removalsfrom the 0.1 M washes. The average standard deviations (Table4) are particularly large for the CaCl2 washes, suggesting thatthe efficiency of this washing agent was dependent on thephysical and chemical characteristics of the test soils. Lead­removal efficiencies for CaCl2 for soilS were highest. SoilShad the largest CEC value (Table 2) of all the test soils. Thus,it is probable that cation exchange was the dominant lead­release mechanism. Lead removals for soil 7 were poor at allthree lead-contamination levels, most likely due to the highsoil pH (8.0 ± 0.10). In addition, soil 7 was collected froman agricultural site that was most likely amended with limeand/or fertilizers, which may have contributed to the high soilpH and organic-matter content observed. Thus, if cation ex­change is the dominant removal mechanism for the CaCI2

washes, large lead removals from soil 7 would not be antic­ipated since the soil was probably already saturated with ex­cess calcium (i.e., small Ca2 + concentration gradient).

Retention and Release Kinetics FIG. 5. Lead-Release Kinetics for 1,000 mg/L Pb Contaminationsof Soil No.7

Study Soil No.

FIG. 3. CaCI2 Results for 100 mg/L Pb Contamination Level

A typical plot illustrating the kinetics of lead retention ispresented in Fig. 4 for one of the study soils. Retention wasrapid, with nearly all adsorption occurring within the first fewminutes. This provides further evidence that sorption was thedominant retention mechanism, since cation exchange andspecific adsorption reactions generally possess higher reactionrates than precipitation reactions, which involves the for­mation of intricate, three-dimensional lattice structures. Forseveral of the study soils, the 100 mg/L initial lead solutionshad faster retention kinetics than the 10 mg/L lead solutions.Lead from the 10 and 100 mg/L solutions was essentiallycompletely removed, but lead removal appears greater forthe 100 mg/L samples when computed on a percentage basis.This may be due to the insensitivity of the AA at metal con­centrations close to the minimum detection limit of the equip­ment. The slower kinetics for the 1,000 mg/L Pb samples waslikely due to the formation of Pb(OHh(s)' The maximum leadquantities retained by the soils during the kinetic testing cor­relate well with the percentages of lead removed from solutionduring artificial soil contamination (Table 3). Thus, the se-

lection of a 21-day retention time in the contaminating pro­cedure appears to provide a very good approximation of quasi­equilibrium conditions.

Typical Pb-release kinetic results are presented in Fig. 5for the 1,000 mglL Pb contamination of soil 7. The total lead­release quantities obtained at the end of the 7-day testingperiod were generally similar to the lead-release values ob­tained in the 24-h batch wash experiments. As observed byother researchers (Tuin and Tels 1990; Van Benschoten etal. 1991; Benjamin and Leckie 1981), lead release was foundto be initially rapid, with the majority of the lead removaloccurring within the first hour. The slopes of the release curvesleveled out after the rapid step. Thus, additional removals ofany significance would require a very large contact time. Theremovals obtained during the rapid step for the HCl andEDTA solutions were independent of the solution concen­tration, but a dependence of solution strength was observedduring the slower release step. As expected, lead-releasequantities from the calcium experiments were less than fromeither the HCl or the EDTA experiments. The kinetics ofCaClz release appear to be nearly as rapid as those of HCIand EDTA. The rapid CaCl2 kinetics should be expectedsince cation-exchange reactions are rapid and often limitedby mass transport through the liquid film.

Erratic variations in Pb release with time may have resultedfrom the experimental protocol employed: Unlike the reten­tion kinetics experiments, the aliquot samples for lead anal­ysis were not taken from a large slurry reactor. Instead, sev­eral25-mL batch samples (25:1liquid:soil ratio) were placedon the mechanical shaker and removed at various time in­tervals.

CaC1 2 Washes (25:1 LiS Ratio)

00.01 M CoCI2 ~O.IM CoCI2

>- 120 100 mg/L Pbuc

" 100:~

w 80""6 60>0

E 40"a:.D 20D-

o

7041 JOURNAL OF ENVIRONMENTAL ENGINEERING 1OCTOBER 1995

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CONCLUSIONS AND RECOMMENDATIONS

The lead capacity of the eight study soils was best correlatedwith the parameter pairs of soil pH and organic content andsoil pH and CEC. Kinetics of both lead retention and releasewere rapid for most study soils. HCl and EDTA obtained thebest Pb-removal efficiencies in this study. These removalstended to be independent of soil type and washing solutionconcentration. It is suspected that dissolution of some of thesoil components controlled lead removal in the HCl washesand that chelation was the dominant lead-release mechanismfor the EDTA washes. Lead removal for CaCh was by ionexchange with Ca2 + and/or complexation with the chloridespecies. Although removals by CaCl2 were the lowest of allthe washing solutions investigated, the use of CaCl2 to re­mediate heavy-metal contaminated soils will not destroy thesoil structure, as often occurs with strong acids.

Finally, the good lead removals obtained by the washingsolutions may be due to the approach used to contaminatethe soils. The large ionic radii of the cationic lead speciesused may have prevented the lead from entering into thestructural lattice of the soil particles. Such a phenomenonwould cause most of the lead to be present on exterior surfacesites. Thus, lead removals by EDTA, or other complexingagents, may not have been as great if the contaminated soilswere contaminated with different lead sources. Additionalstudies (Cline et al. 1993) have addressed this point by per­forming wash experiments using soils contaminated with leadin the presence of either excess sulfate or carbonate sources,which typically form lead precipitants that require a very ag­gressive washing solution to remove them from a soil system.

ACKNOWLEDGMENTS

This paper was written by a contractor of the U.S. government. OakRidge National Laboratory is managed by Lockheed Martin EnergySystems, Inc., under contract DE-AC05-840R21400 with the U.S. De­partment of Energy. Accordingly, the U.S. government retains a non­exclusive. royalty-free license to publish or reproduce the published formof the contribution, or allows others to do so, for U.S. governmentpurposes. The research presented in this paper was conducted underUSEPA Contract No. 5009-92. The writer would also like to express hisappreciation for the efforts of his laboratory research assistants at WestVirginia University.

APPENDIX. REFERENCES

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Bell, C. F. (1977). Principles and applications of metal chelation. OxfordChemistry Ser., Oxford Univ. Press, Oxford, England.

Benjamin, M. M., and Leckie, J. O. (1981). "Multiple-site adsorptionof Cd, Cu, Zn, and Pb on amorphous iron oxyhydroxide." J. Colloidand Interface Sci., 79(1), 209-221.

Cline, S. R. (1993). "Efficiencies of soil washing/flushing solutions toremediate lead contaminated soils," MS thesis, Dept. of Civ. andEnvir. Engrg., West Virginia Univ., Morgantown, W.Va.

Kennedy, J. B., and Neville, A. M. (1986). Basic statistical methods forengineers and scientists. Harper and Row Publishers, New York, N.Y.

LaBauve, J. M., Kotuby-Amarcher, J., and Gambrell, R. P. (1988)."The effect of soil properties and a synthetic municipal landfill leachateon the retention of Cd, Ni, Pb, and Zn in soil and sediment materials."1. Water Pollution Control Federation, 60(3), 379-385.

McKeague, J. A., and Day, J. H. (1966). "Dithionite and oxalate ex­tractable Fe and Al as aids in differentiating various classes of soils."Can. J. Soil Sci., Vol. 46, 13-22.

McLean, E. O. (1982). "Soil pH and lime requirements." Methods ofsoil analysis part 2; chemical and microbiological properties; agronomyNo.9, 2nd Ed., Am. Soc. of Agronomy, Inc., Madison, Wis., 199­209.

Mehra, O. P., and Jackson, M. L. (1960). "Iron oxide removal fromsoils and clays by a dithionite citrate system buffered with soliumbicarbonate." Clays and clay minerals; Proc., 7th Nat. Cont, mono­graph No.5. Earth Sci. Ser., 317-327.

Peters, R. W., and Shem, L. (1992). "Adsorption/desorption charac­teristics of lead on various types of soil." Envir. Progress, 11(3),234­240.

Raghavan, R., Coles, E., and Dietz, D. (1989). "Cleaning excavatedsoil using extraction agents: a state-of-the-art review." EPA/600/2­89/034, Prepared by Foster Wheeler Envirosponse, Inc., for the USEPA,Risk Reduction Laboratory.

Standard methods for the examination of water and wastewater. (1985).16th Ed., Am. Public Health Assoc.

Superfund engineering issue: treatment oflead-contaminated soils. (1991).EPA540/2-9I/009, U.S. EPA Ofc. of Solid Waste and Emergency Re­medial Response, Washington, D.C.

Test methods for evaluating solid waste: physical chemical methods. (1986).SW-846 , 3rd Ed., U.S. EPA Ofc. of Solid Waste and EmergencyResponse, Washington, D.C.

Tuin, B. J., and Tels, M. (1990). "Extraction kinetics of six heavy metalsfrom contaminated clay soils." Envir. Technol., Vol. 11, 541-554.

Van Benschoten, J. E., Reed, B. E., Matsumoto, M. R., and McGarvey,P. J. (1991). "Effects of iron oxides on soil washing a metal contam­inated sandy soil." Water Envir. and Technol., 66(2),168-174.

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