washing of cadmium(ii) from a contaminated soil column

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Washing of Cadmium(II) from a Contaminated SoilColumnAllen P. Davis a , Dilip Matange a & Mohammad Shokouhian aa Environmental Engineering Program, Department of Civil Engineering, University ofMaryland, College Park, MD 20742Published online: 22 Sep 2010.

To cite this article: Allen P. Davis , Dilip Matange & Mohammad Shokouhian (1998) Washing of Cadmium(II) from aContaminated Soil Column, Journal of Soil Contamination, 7:3, 371-393

To link to this article: http://dx.doi.org/10.1080/10588339891334276

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Journal of Soil Contamination, 7(3):371–393 (1998)

INTRODUCTION

HERE are numerous documented instances of contamination of surface andsubsurface soils by heavy metals. Cadmium is a metal of major environmen-

tal concern due to its high toxicity and relatively high mobility in the terrestrialenvironment (Boekhold et al., 1991). At the present time, there are limited tech-nologies available for treating metal-contaminated soils. Thus, development ofnew and effective techniques for soil treatment is essential.

Washing of Cadmium(II)from a Contaminated Soil Column

Allen P. Davis*, Dilip Matange, andMohammad Shokouhian

Environmental Engineering Program;Department of Civil Engineering; University ofMaryland; College Park, MD 20742

The washing of cadmium (from CdO(s)) froma soil column employing either an acidsolution or EDTA (a strong metal chelator)was examined. For Cd(II) levels of 50 to1000 mg/kg, the fraction removed wasessentially independent of the initial Cd(II)concentration. The most efficient washingof cadmium was achieved using an acidwash solution at pH 2.5. Lower Cd(II) re-movals were found at lower pH, apparentlydue to inhibition of CdO(s) dissolution byconstituents released from the soil underhighly acidic conditions. EDTA wash solu-tions were employed at EDTA:cadmiummolar ratios ranging from 1:1 to 10:1. Up to90% removal of total Cd(II) was achievedat the 10:1 ratio after the passage of thefirst 50 PV of wash solution. Although higherchelate levels enhanced Cd(II) removal,the utilization efficiency of EDTA for cad-mium decreased.

KEY WORDS: EDTA, acid solution, CdO(s), Cd(II).

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One successful technology for removing heavy metals from contaminated soilis soil washing. In the soil washing process, the soil is excavated and the soilparticles are vigorously mixed with a wash solution. Metal-rich soil particles areremoved, other metals affiliated with the soil are solubilized, and the wash solutionand clean soil are separated. Generally either a strong mineral acid or a strongmetal chelator is used to chemically enhance the metal removal. For example,several experiments were carried out by Esposito et al. (1989) on soil artificiallycontaminated with either 20 or 1000 mg Cd(II)/kg soil, as well as other heavymetals. Addition of the chelating agent ethylenediaminetetraacetic acid (EDTA)was found to increase the efficiency of metal removal. A 3:1 molar ratio of EDTAto total metal contaminant present in the soil produced optimum metal removal.Peters and Shem (1992) also found EDTA to be efficient in the washing of leadfrom an artificially contaminated soil. Tuin and Tels (1990b) found that 75 to 81%of the Cd in contaminated soils could be removed via washing with 0.1 N HCl. Ofsix metals washed from the soil, cadmium was the easiest removed.

A number of soil parameters must be considered in order to evaluate theeffectiveness of a soil washing process. Soils with a greater percentage (>50%) ofsand and gravel respond better to soil washing due to the difficulties of removingcontaminants from chemically active clay material (Van Benschoten et al., 1994).Other factors that affect metal extraction efficiency from soils include the metalcontamination history of the soil, the liquid-soil extraction ratio, initial metalconcentrations, and extraction kinetics (Tuin and Tels, 1990b; Van Benschoten etal., 1994).

The in situ process of soil washing, in which the washing solution is in somemanner forced through the in-place soil matrix, is known as soil flushing. Effi-ciency of metal removal via soil flushing is less controllable than with excavationand soil washing due to the variable in situ soil characteristics. The characteristicsof the washing fluid that is applied to the contaminated soil are chemicallymodified through interactions with the soil. Thus, the efficiency of a soil flushingsystem varies as the flushing continues. Davis and Singh (1995) examined theflushing of a soil column contaminated specifically with zinc(II) solids. IncreasedZn(II) removal efficiency was found at lower pH and with increasing concentra-tions of EDTA. Also, the form of the Zn(II) contamination played an important rolein the overall washing efficiency.

In the present study, several parameters that affect the efficiency of soil flushingfor Cd(II) removal are investigated. The soil is artificially contaminated employingCdO(s) to fix the Cd(II) loading. A specific cadmium phase is used because it islikely that highly contaminated soils contain metals present as distinct geochemicalphases in addition to sorbed species, as noted in several cases for lead-contami-nated soils (Hessling et al., 1990; Van Benschoten et al., 1997). Tuin and Tels(1990a) noted the majority of cadmium in the “exchangeable” fraction based onsequential extraction on actual contaminated soils, although the metal was distrib-uted among all fractions. Nevertheless, subsequent sorption phenomena are impor-

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tant to the success of the flushing process since metals that become dissolved fromthe metal solids still will interact with the soil surface. Specific parameters inves-tigated include effects of initial cadmium loading of the soil, pH of the washsolution, and addition of a strong chelating agent (EDTA) at various concentra-tions.

MATERIALS AND METHODS

The methodology used for this work is similar to the one that was used by Davisand Singh (1995) for the washing of zinc(II)-contaminated soil. The soil for thisstudy was taken at a depth of two feet from an open grass field at the Universityof Maryland, College Park, MD. Some of the physical and chemical properties ofthe soil were determined by the Department of Agronomy, University of Maryland,indicating a sandy loam soil fractionated as 74% sand, 16% silt, and 10% clay. Thesoil pH was 6.1 and the cation exchange capacity (CEC) was 3.41 meq/100 g. Thesoil was air-dried and sieved through a 1.18-mm opening sieve. Batches of 500 gof the soil were dry-spiked with powdered cadmium oxide (Fisher Sci.) to producecontamination ranging from 50 to 1000 mg Cd/kg soil. The contaminated soil wasshaken overnight in a plastic bottle to disperse the cadmium solids throughout thesoil.

The cadmium content of the unspiked soil was determined using a strong acidextraction, following the methodology presented by Berrow and Stein (1983). Onegram of soil was mixed with 10 mL of 6 M HCl in a 100-mL Pyrex beaker andheated to complete dryness on a steam bath. This process was repeated four times.The residue was extracted with 0.06 M HCl, filtered, and the filtrate was analyzedfor cadmium by graphite furnace atomic absorption spectrophotometry. The totalcadmium in the original soil was calculated as 0.2 mg/kg, and thus is negligible ascompared to the 50 to 1000 mg/kg loading of the contaminated soil.

In each washing experiment, 11.9 g of contaminated soil was packed in aPlexiglass column (1.9 cm internal diameter, 5 cm length), producing a 3-cm soilcolumn with a bulk density of 1.4 g/cm3. Aluminum foil-covered rubber stoppersand a small amount of glass wool were used to seal both ends of the column. Washsolutions of various concentrations of acid or EDTA were used to remove cad-mium from the soil, all at a fixed ionic strength of 5 × 10–2 M NaNO3 (E.M.Science). Water used in all experiments was deionized using a Hydro-Servicereverse osmosis/ion exchange apparatus (Model LPRO–20). The effects of changein pH of the wash solution from 6 to 2, and change in concentration of EDTA(tetrasodium, Fisher; 1.6 × 10–5 to 1.1 × 10–3 M, equivalent to total solutionEDTA:total soil Cd(II) molar ratios from 1:1 to 10:1) at pH 6 were studied.

The washing solutions were introduced from the bottom of the column tosaturate the soil at a flow rate of 3 mL/min (velocity = 1.06 cm/min). Approxi-mately 5.5 h were required to pump 1 L (250 pore volumes) of extraction solution

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through the soil column. A fraction collector (ISCO Retriever II) using 15 and 30mL vials was employed to capture the column effluent samples, which wereacidified with HNO3 (Fisher). Dilutions up to 500 times were made and sampleswere analyzed for cadmium using a Varian Tectron Model AA–5 atomic absorp-tion spectrophotometer (AAS), flame configuration. A detection limit of approxi-mately 0.1 mg/L Cd(II) was obtained with a linear range up to 3 mg/L. All labmaterials and glassware were acid-washed to ensure complete removal of anyadsorbed metals.

Batch experiments were also carried out to evaluate the Cd(II) adsorptioncharacteristics of the soil with respect to pH variation. Solutions of three differentconcentrations of Cd(NO3)2 (6 to 30 mg/L as Cd) were prepared (I = 5 × 10–2 MNaNO3). Twenty 125 mL Nalgene bottles were each filled with 100 mL of theprepared solution. To 19 bottles, 1 g of soil was added. The initial pH of the bottleswas adjusted over the range of 1 to 8. The bottles were placed overnight in a shaker,after which the final pH of each sample was measured. The samples were filteredusing 0.2 µm membrane (Gelman) filters (disregarding the initial filtrate to mini-mize possible effects from filter sorption), and analyzed for dissolved Cd(II) usingAAS. The amount adsorbed was calculated from the difference between the samplewithout soil and the final dissolved Cd(II) concentrations. Experiments were alsocompleted with the addition of 10–3 M Fe(NO3)3 (at low pH only) and 10–3 MCa(NO3)2 to the Cd(II) solution to evaluate effects of these metals on Cd(II)adsorption characteristics.

Finally, several experiments were completed to investigate preliminarily thedissolution kinetics of the CdO(s) that was used as the contaminant source. Solu-tions of 5 × 10–2 M NaNO3 (300 mL) were prepared and adjusted to pH 2.0, 2.5,and 3.0 using HNO3. One gram of CdO(s) was added to these solutions; subse-quently ten samples were taken at one minute interval. The samples were rapidlyfiltered (0.2 µm) and analyzed for dissolved Cd(II) using AAS. Dissolution experi-ments at these pH values were also completed individually in the presence ofCa(NO3)2, Fe(NO3)3, Al(NO3)3, and NaH2PO4, each at 10–3 M.

RESULTS AND DISCUSSION

Effect of Cadmium Loading on Washing Efficiency

Figure 1A shows the cumulative Cd(II) removal efficiency of a wash solution atpH 6 for initial cadmium loadings of 50 to 1000 mg Cd(II)/kg. Only 11 to 22% ofthe total cadmium in the soil column is removed after the passage of 250 porevolumes (PV) of the wash fluid. Approximately one half of the removal occurswithin the passage of the first 50 PV. The removal efficiency is greatest for 100 mgCd(II)/kg and lowest for 300 mg Cd(II)/kg, although the difference is not large.The fraction Cd(II) removed by the wash was independent of the initial cadmium

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FIG

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A

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Loa

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s, p

H 6

. I

=5

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w R

ate

= 3

mL/

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. D

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1000

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FIG

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Cd(

II) C

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loading. Accordingly, Figure 1B demonstrates that there is an approximate linearrelationship between the effluent cadmium concentrations that were measured atboth 50 and 200 PV, and the initial Cd(II) loading. Thus, because removal effi-ciency is essentially constant at all cadmium levels, soils with higher initial metalcontamination have correspondingly higher residual Cd(II) concentrations aftersimilar washing durations. Tuin and Tels (1990b) found greater percent Cu, Ni, Pb,and Zn extractions at higher metal loadings from artificially polluted soils. The lowpercent extractions at low loading were attributed to metals affiliated with high-strength sorption sites.

Figure 2 shows the cumulative removal of Cd(II) from the soil column at pH 2.The removal fraction varies only from 0.14 to 0.35 after the passage of 250 PV ofthe wash fluid, with the greatest fractional removal at 100 mg Cd(II)/kg, and theleast at 1000 mg. In this case nearly all of the effective washing was completed by50 PV, with one-half to 80% of the removed cadmium occurring by this point. Withmost of the Cd(II) loadings, little removal occurred after 50 PV. The effluentcadmium concentration at 50 PV was approximately proportional to the initialCd(II) loading at pH 2, as was found at pH 6. By 200 PV, most of the removableCd(II) was gone and little Cd(II) was being removed from the column.

The reproducibility of cadmium washing was evaluated by repeating experi-ments for 1000 mg Cd(II)/kg soil loadings; essentially identical results wereobtained at this loading for washes at both pH 6 and pH 2 (Figures 1A and 2).

Effect of pH on Washing Efficiency

Figure 3 shows the cumulative removal of Cd(II) from 500 mg Cd(II)/kg soil overa pH range of 2 to 6. As expected, decreasing the pH below 6 continuouslyincreased the effectiveness of the wash. However, surprisingly, the maximumremoval (approximately 94%) occurred at pH 2.5; nearly all of this removal (92%)resulted before the first 50 PV of wash fluid was passed. Removal efficiencydecreased to 76% at pH 2.3, and was only 23% at pH 2.0.

The pH of the column effluent was monitored in selected cases. For the experi-ments with initial wash solution at pH 6, the effluent pH varied little and remainedbetween pH 5 and 6 (Figure 4). However, for influent at pH 2, the effluent pH wasinitially 2.8 to 3.7, increasing to 3.8 to 4.6 at 23 PV, due to proton bufferinginteractions with the soil. Thereafter, the pH gradually dropped to near 2 as the soilbuffer capacity was depleted.

Because the Cd(II) contamination in the soil results from CdO(s), the effective-ness of a wash fluid initially depends on its ability to dissolve the metal solid. Asthe wash solution contacts the CdO(s) within the soil matrix, dissolution occurs via:

CdO H Cd H Os( ) + → ++ +2 22← (1)

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Accordingly, decreasing the solution pH promotes the dissolution process, result-ing in greater concentrations of aqueous Cd(II). The solubility of CdO(s) in 5 ×10–2 M NaNO3 was calculated using the database and speciation program,MINTEQ2A. Results indicate that Cd+2 is the dominant species over the pH rangeof 2 to 10. For all practical purposes, CdO(s) is predicted to be completely solublefor all pH less than 8.

The proton-enhanced dissolution rate of slightly soluble oxide minerals iscontrolled by the concentration of protonated hydroxyl groups on the mineralsurface (Stumm and Wieland, 1990). This surface protonation produces the precur-sor species for the detachment of the metal ion from the surface as the rate limitingfinal step in the dissolution. In the acid-washing soil treatment process, protons areadsorbed by the CdO(s) at low pH, weakening the metal-oxygen lattice bonds, andthus increasing the cadmium dissolution rate. During the soil flushing, this processconveys the Cd(II) into the solution phase. The dissolution is rapid under acidicconditions, but very slow at higher pH.

Cd(II) Adsorption

Once the Cd(II) is released into solution through dissolution, adsorption of theCd(II) onto some constituents of the soil is likely, depending on the solution pH.This adsorption limits the total Cd(II) removal at higher pH. Figure 5 shows resultsof batch adsorption of three Cd(II) concentrations onto the soil at pH 1 to 8. As thepH of the suspension increases, the degree of adsorption increases. These resultsare in accordance with previous Cd(II)/soil adsorption experiments of others (i.e.,Garcia-Miragaya and Page, 1978; Kuo and Baker, 1980; Soon, 1981). Employinga surface complexation reaction to describe the adsorption, protons compete withCd2+ for adsorption sites on the soil (S-OH), thus resulting in less Cd(II) adsorption

at low pH.

S OH Cd S OCd H– –+ → ++ + +2 2← (2)

These adsorption sites may be on any of the various organic and inorganic constitu-ents of the soil. As demonstrated in Figure 5, even at pH 1, the soil exhibits acapacity to sorb a limited amount of cadmium. Thus, even though dissolution ofthe CdO(s) may be complete under acidic washing conditions, sorptive interactionsmay prevent the complete removal of the cadmium from the soil column duringflushing operations.

Continued addition of wash fluid to the column reduces the concentration ofCd(II) in the soil water, possibly allowing some desorption of Cd(II) from the soil.However, a portion of the cadmium is likely to be very difficult to remove; thisremaining cadmium is affiliated with the strongest sorption sites of the soil matrix.

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FIG

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adm

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Ads

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d(II)

Con

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oil.

I =

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Effects of 10–3 M Ca2+ and Fe(III) on the Cd(II) adsorption characteristics arealso presented in Figure 5. The effect of Fe(III) was limited to pH < 2.5 due tosolubility concerns. There was an obvious decrease in the amount of cadmiumadsorbed onto the soil when these metals were present, the effect of Ca(II) beingmore than that of Fe(III). The inhibition of Cd(II) adsorption caused by Ca(II) andFe(III) should be due to competition between these metals for the soil surface sitesat high metal loadings:

S OH Ca S OCa H– –+ → ++ + +2← (3a)

S OH Fe S OFe H– –+ → ++ + +3 2← (3b)

These results are consistent with those of other workers, i.e., Cavallaro andMcBride (1978) and Tiller et al. (1979), which have shown that the ability of a soilto retain trace metals is reduced by other metals present in the matrix.

CdO(s) Dissolution

As discussed earlier, the dissolution of CdO(s) (Eq. 1) predicts greater solubility atlower pH. Experimental results shown in Figure 6 demonstrate that the CdO(s) isessentially completely dissolved within 10 minutes upon contact with water at pH2. Experiments were also performed to study the dissolution characteristics ofcadmium oxide in the presence of 10–3 M calcium, ferric, aluminum, and phosphatesalts. Figure 6 demonstrates that CdO(s) dissolution is reduced substantially withthe addition of these species, exemplified at pH 2. The minimum dissolution(25%), when 10–3 M ferric nitrate was added, was a factor of four lower than thatwithout Fe(III). The other compounds produced only about 30% dissolution. It isexpected that these species inhibit the surface protonation of the CdO(s), possiblythrough a competitive adsorption process. Similar inhibitory effects were noted atpH 2.5 and 3.

Based on these results, it is suggested that the unexpected decrease in cadmiumwash efficiency that is found at pH 2 results from effects of various ions leachedfrom the soil, interfering with the CdO(s) dissolution. Figure 7 presents the cumu-lative calcium and iron leached from the uncontaminated soil with the addition ofwash solution at pH 2 and 6. At pH 2 a large amount of calcium is immediatelyreleased from the soil. Dissolved iron is noted at later times. The levels at pH 6 aremuch less. Both of these metals, as well as other species leached from the soilunder the acidic conditions likely result in the inhibited cadmium washing effi-ciency at pH 2.

Effect of EDTA

The effect of employing a strong chelating agent, EDTA, for removal of cadmiumfrom the soil was also evaluated. Evaluation of the aqueous speciation predictedfrom adding 10–1 M CdO(s) in a solution of 3 × 10–1 M EDTA shows that the

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FIG

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E 6

Dis

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dO in

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Pre

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alci

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, A

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(III)

, an

d P

hosp

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Sal

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pH 2

. I

= 5

× 10

–2 M

NaN

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FIG

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Vol

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dominant cadmium species is the Cd-EDTA2- complex over the entire 2 to 8 pHrange. There is no CdO(s) precipitate in the system; equilibrium calculationsindicate that all CdO(s) will be dissolved.

An early study has shown that the optimum concentration of EDTA required toremove heavy metals from a contaminated soil was a 3:1 EDTA:total metal molarratio in a batch system (Esposito et al., 1989). In the present study, theEDTA:cadmium ratio ranged from 1:1 to 10:1 based on total Cd(II) in the soilcolumn and total EDTA in 1 L of wash solution.

Figure 8 demonstrates that the Cd(II) removed using 1:1 EDTA at three differentcadmium loadings continuously increases with wash volume; a plateau after thepassage of a critical amount of wash solution, as seen in the earlier experiments,does not occur. The total cadmium removed after the passage of 250 PV is 40 to50% of the initial cadmium added. Continued application of this solution wouldlikely result in greater removal of Cd(II). The effluent Cd(II) concentration is againproportional to the initial Cd(II) loading.

The promotion of metal oxide dissolution by an organic ligand is also a two-stepmechanism. The process follows the same general form as proton-promoted dis-solution: a rapid, reversible sorption of reactive chemical species from solutiononto the surface, in this case, the EDTA, followed by the detachment of the metalfrom the surface of the crystalline lattice, rapid regeneration of the surface and re-equilibrium of the reactive surface species (Morel and Hering 1993). Thus, in thesoil washing, the EDTA that is supplied first complexes with the CdO(s) surface.The resulting Cd-EDTA complex becomes detached and is released into the soilwater solution.

When the EDTA concentration was increased to a 3:1 mole ratio, the amount oftotal cadmium removed increased, while again the fraction of Cd(II) removed wasindependent of the initial Cd(II) loading (Figure 9). The maximum removal is ashigh as 70% when the initial cadmium concentration is 50 mg Cd(II)/kg soil. Theremoval efficiency varies little for loadings of 100 to 1000 mg Cd(II)/kg, between45 and 53%. At 50 PV, 20 to 30% removal is found in all cases.

By increasing the EDTA concentration to 10:1 the total removal efficiency ofwash fluid varied from 47 to 83% for different cadmium loadings (Figure 10). Inthese cases, the major portion of the cadmium removal was accomplished beforethe passage of 50 PV of wash solution. This is in contrast to the 1:1 and 3:1 EDTAwashes, where it was seen that there is a gradual increase in removal of cadmiumthroughout the entire wash cycle.

Elliott et al. (1989) demonstrated that there was an increase in lead recoveryfrom Pb-polluted soils from washing with increasing amounts of EDTA above thestoichiometric requirement, but the lead release with each incremental increase inEDTA concentration diminished as complete recovery was approached. In con-trast, experimental results of Peters and Shem (1992) have shown little effect inincreasing the EDTA concentration in the wash fluid on the removal efficiency oflead (nitrate) from soil (although their method of metal contamination was differ-

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FIG

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E 8

Fra

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d(II)

Rem

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fro

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onta

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Soi

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a F

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ED

TA

:Tot

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d =

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pH

6,

I =

5 ×

10–2

M N

aNO

3, F

low

Rat

e =

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FIG

UR

E 9

Fra

ctio

n of

Cd(

II) R

emov

ed f

rom

Con

tam

inat

ed S

oil a

s a

Fun

ctio

n of

Was

h V

olum

e. E

DT

A:T

otal

Cd

= 3:

1, p

H 6

, I

= 5

× 10

–2 M

NaN

O3,

Flo

w R

ate

= 3

mL/

min

.

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FIG

UR

E 1

0

Fra

ctio

n C

d(II)

Rem

oved

fro

m C

onta

min

ated

Soi

l as

a F

unct

ion

of W

ash

Vol

ume.

ED

TA

:Tot

al C

d =

10:1

, pH

6,

I =

5 ×

10–2

M N

aNO

3,F

low

Rat

e =

3 m

L/m

in.

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ent). Figure 11 demonstrates that higher EDTA levels remove the cadmium faster,and to a greater extent. Similar results were observed for other Cd(II) contamina-tion levels. However, as the EDTA concentration in the wash solution increases,the ∆Cd(II) released/∆EDTA added decreases (Table 1). The addition of 1:1EDTA caused a significant increase in the amount of Cd(II) removed over thatwithout EDTA. As Cd forms a 1:1 complex with EDTA, from 24 to 41% of theadded EDTA is being utilized for its intended purpose. By increasing EDTA to 3:1,in some cases the amount of Cd(II) removed is decreased somewhat. At 10:1, theoverall amount of Cd(II) removed is generally increased significantly, but the∆Cd(II)/∆EDTA is only a few percent. This ratio is low because other cations suchas Fe(III), Ca(II), and Al(III) compete with Cd(II) for complexation with EDTA athigh chelate levels. Also, not all EDTA reacts with the metals, but may passthrough uncomplexed, and some of the EDTA itself may become adsorbed onto thesoil (Elliott et al., 1989; Peters and Shem, 1992). Therefore, based on EDTAutilization efficiency, lower EDTA levels are optimum. However high chelateconcentrations remove more Cd(II) in fewer pore volumes. A balance betweenthese two variables must be maintained in soil flushing systems.

SUMMARY

The results obtained in this study show that washing a soil column contaminatedwith Cd(II) (via CdO(s), 50 to 1000 mg/kg soil) with solution of pH 6 producedCd(II) removals of only 22 to 35% after the passage of 250 PV of wash solution.Approximately half of the Cd(II) removal occurred within the first 50 PV of washsolution. Relative (percent) removal of cadmium from the soil matrix was indepen-dent of the initial cadmium loading, both at low pH and with the addition of EDTA.Thus, washing of soils with higher initial cadmium produced soils with higherresidual cadmium.

A maximum Cd(II) removal of 94% was achieved from a wash at pH 2.5. Whenthe pH was lowered or increased, there was a substantial decrease in the removalof cadmium. The reduction in removal efficiency of Cd(II) below pH 2.5 is likelyto be due to leaching of other metals from the soil itself (i.e., Ca(II), Fe(III), Al(III))that inhibit the dissolution of the CdO(s). Experimentally it was found that thedissolution rate of CdO(s) decreased significantly in the presence of Ca(II), Fe(III),Al(III), and H2PO4

–.The presence of a strong cadmium complexing agent, EDTA, increased the

dissolution of cadmium from the soil matrix, thereby increasing the removalefficiency of the wash fluid. The Cd(II) removal increased with an increase inconcentration of the EDTA. The maximum cadmium removal with EDTA (85%)occurred with a 10:1 total (molar) EDTA:Cd(II) ratio, the highest value examined.High EDTA levels also produced more rapid Cd removal. However, the efficiency

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FIG

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E 1

1

Fra

ctio

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Cd(

II) R

emov

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rom

Con

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oil a

s a

Fun

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Was

h V

olum

e at

tot

al C

adm

ium

(II)

= 3

00 m

g/kg

Soi

l, pH

6.

Flo

wR

ate

= 3

mL/

min

, I

= 5

× 10

–2 M

NaN

O3.

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of EDTA utilization for Cd removal decreases with increasing EDTA:Cd(II) ratios.Thus, a balance must be made between the amount of EDTA added (and wasted)and the speed and extent to which the Cd(II) is removed from the soil.

REFERENCES

Berrow, M. L. and Stein, W. M. 1983. Extraction of metals from soils and sewage sludges byrefluxing with aqua regia. Analyst 108(1283), 277–285.

Boekhold, A. E., van der Zee, S. E. A. T. M., and de Haan, F. A. M. 1991. Spatial patterns of cadmiumcontents related to soil heterogeneity. Water, Air, and Soil Pollution 57–58(Part II), 479–488.

Cavallaro, N. and McBride, M. B. 1978. Copper and cadmium adsorption characteristics of selectedacid and calcareous soils. Soil Sci. Soc. Am. J. 42(4), 550–556.

Davis, A. P. and Singh, I. 1995. Washing of zinc(II) from a contaminated soil column, J. Environ.Eng. ASCE 121(2), 174–185.

Elliott, H. A., Linn, J. H., and Shields, G. A. 1989. Role of Fe in extractive decontamination of Pb-polluted soils. Haz. Waste Haz. Mater. 6(3), 223–229.

Esposito, P., Hessling, J., Locke, B. B., Taylor, M., Szabo, M., Thurmau, R., Rogers, C., Traver, R.,Barth, E. 1989. Results of treatment evaluations of a contaminated synthetic soil. JAPCA 39(3),294–304.

Garcia-Miragaya, Y. and Page, A. L. 1978. Sorption of trace quantities of cadmium by soil withdifferent chemical and mineralogical composition. Water, Air, and Soil Pollution 9(3), 289–299.

Hessling, J. L., Esposito, M. P., Traver, R. P., and Snow, R. H. 1990. Results of bench scale researchefforts to wash contaminated soils at battery-recycling facilities, in Metals Speciation, Separation,and Recovery, (J. Patterson and R. Passino, Eds.), Lewis Publication, Inc. Chelsea, MI, Vol. II,497–514.

Kuo, S. and Baker, A.S. 1980. Sorption of copper, zinc, cadmium by some acid soils. Soil Sci. Soc.Am. J. 44(5), 969–974.

Morel, F. M. M. and Hering, J. G. 1993. Principles and Applications of Aquatic Chemistry, Wiley,New York, NY.

TABLE IRatio of ∆∆∆∆∆Cd/∆∆∆∆∆EDTA, theIncrease in Cd Removed

per Unit Increase in EDTAAdded to Wash Solution, after 250 PV

∆∆∆∆∆Cd/∆∆∆∆∆EDTA (%)mg Cd(II)/kg soil

EDTA:Cd LoadingsCompared 300 500 1000

0 to 1:1 42 24 411:1 to 3:1 –3.5 2.9 –2.43:1 to 10:1 5.5 –0.7 4.2

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Peters, R. W. and Shem, L. 1992. Use of chelating agents for remediation of heavy metal contami-nated soil. In: Environmental Remediation: Removing Organic and Metal Ion Pollutants, (G. W.Vandegrift, D. T. Reed, and I. Tasker, Eds.), ACS Symposium Series 509, American ChemicalSociety, Washington, D.C. 70–84.

Soon, Y. K. 1981. Solubility and sorption of cadmium in soils amended with sewage sludge. J. SoilSci. 32(1), 85–95.

Stumm, W. and Wieland, J. J. 1990. Dissolution of oxide and silicate minerals: rates depend onsurface speciation. In: Aquatic Chemical Kinetics, (W. Stumm, Ed.), Wiley, New York, NY, 367–400.

Tiller, K. G., Nayyar, V. K., and Clayton, P. M. 1979. Specific and non-specific sorption of cadmiumby soil clays as influenced by zinc and calcium. Aust. J. Soil Res. 17(1), 17–28.

Tuin, B. J. W. and Tels, M. 1990a. Distribution of heavy metals in contaminated clay soils beforeand after extractive cleaning. Environ. Tech. 11, 935–948.

Tuin, B. J. W. and Tels, M. 1990b. Removing heavy metals from contaminated clay soils byextraction with hydrochloric acid, EDTA or hypochlorite solutions. Environ. Tech. 11, 1039–1052.

Van Benschoten, J. E., Reed, B. E., Matsumoto, M. R., and McGarvey, P. J. 1994. Metal removalby soil washing for an iron oxide coated sandy soil. Water Environ. Research 66(2), 168–174.

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washing of cadmium(ii) from a contaminated soil column

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