unsaturated transport processes in undisturbed heterogeneous porous media: ii. co-contaminants

Unsaturated Transport Processes in Undisturbed Heterogeneous Porous Media:II. Co-Contaminants

P. M. Jardine,* G. K. Jacobs, and J. D. O'Dell

ABSTRACTDepartment of Energy facilities involved in defense-related activities

have generated huge quantities of low-level radioactive mixed wasteduring the past several decades. The waste is composed of organicallycomplexed contaminants, also known as co-contaminants, which aretypically disposed in shallow land burial sites. The objective of thisstudy was to provide an improved understanding of the geochemicalprocesses controlling co-contaminant transport in heterogeneous, un-saturated subsurface media. Large undisturbed columns were isolatedfrom a proposed waste site consisting of fractured saprolitic shale,and the steady-state Unsaturated transport of Co(II)EDTA2~,Co(IH)EDTA , and SrEDTA2 was investigated at -10 cm pressurehead. Subsurface Fe and Al sources effectively dissociated the Sr-EDTA2 - co-contaminant and Sr was transported as a reactive, un-complexed species. The EDTA readily complexed with Fe and Al,resulting in significant solid-phase modification of the porous mediavia chelate-enhanced dissolution and redox alterations. Displacementof Co(II)EDTA2 through the subsurface media was characterized bya MnO2-mediated oxidation of the co-contaminant with subsequentformation of Co(III)EDTA~. The latter co-contaminant was an ex-tremely stable complex that was transported through the subsurfaceas a single, reactive entity and exhibited an overall retardation thatwas similar to the uncomplexed contaminant Co2+. Modeling resultsusing equilibrium and nonequilibrium formulations of the convective-dispersive equation suggested that a large portion of the transportedCo(III)EDTA~ was controlled by time-dependent sorption reactionswith the solid phase. Although the solid-phase retention ofCo(III)EDTA- and Co2* were similar, the sorption kinetics of theformer were more sluggish relative to Co2+ and contaminant transportwas accelerated in the presence of EDTA.

LOW-LEVEL RADIOACTIVE WASTE previously gener-ated at U.S. Department of Energy facilities within

the Weapons Complex is typically composed of in-organic fission byproducts mixed with various chelat-ing agents and organic acids. Much of the mixed wasteresulted from decontamination of nuclear equipmentand hot cells. A wide variety of reagents had beenused in decontamination efforts, with chelating agentsgenerally preferred since they formed stable, water-soluble complexes with a wide variety of metals andradionuclides (Ayres, 1971). The most commonly usedchelating agents were the aminopolycarboxylates NTA,EDTA, and DTPA (Means and Alexander, 1981; Tosteand Lechner-Fish, 1989; Riley and Zachara, 1992).

Organically complexed contaminants, referred to asco-contaminants, which comprise low-level radioac-tive waste, were commonly disposed in shallow landburial sites via pits and trenches. The presence of the

P.M. Jardine and O.K. Jacobs, Environmental Sciences Division,Oak Ridge National Lab., P.O. Box 2008, Oak Ridge, TN 37831-6038; and J.D. O'Dell, Dep. of Plant and Soil Science, Univ. ofTennessee, Knoxville, TN 37901-1071. Joint contribution fromOak Ridge National Lab. and the Univ. of Tennessee. This re-search was funded by the Subsurface Science Program of theEcological Research Division, Office of Health and Environmen-tal Research, U.S. Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems. Publi-cation no. 4068. Received 8 July 1992. * Corresponding author.

Published in Soil Sci. Soc. Am. J. 57:954-962 (1993).

complexing agent is believed to alter the geochemicalbehavior of the disposed contaminant in subsurfacemedia. Field observations by Means et al. (1978) sug-gested that EDTA complexation of 60Co enhanced thesubsurface transport of the contaminant from disposaltrenches at the Oak Ridge National Laboratory, OakRidge, TN. Ion exchange, gel filtration chromatog-raphy, and gas chromatography were used to dem-onstrate the persistence of the EDTA-60Co complexin the trench disposal sites. Using laboratory batchtechniques, Swanson (1981, 1982, 1983) also showedthat the adsorption of Eu, Ni, and Co on Hanford,Savannah River, and Oak Ridge soils was greatly re-duced in the presence of EDTA and DTPA. However,Weiss and Columbo (1980), investigating the effectof EDTA on Co and Am adsorption by soil constitu-ents, found that, although EDTA generally reducedcontaminant-solid interactions, co-contaminant asso-ciations in the presence of montmorillonite did noteffect Co adsorption. Likewise, field studies of Kirk-ham and Jones (1982) showed little, if any, movementof 60Co-EDTA complexes during a 3-yr period inHanford soils using water fluxes typical of naturalconditions. Conflicting observations on the mobilityof contaminants in the presence of organic ligands ismost likely the result of complicated geochemical andbiological reactions during transport. The mobility ofcomplex co-contaminant mixtures in subsurface mediamay be complicated by competitive sorption and co-sorption, aqueous and surface complexation, solid-phasemodification, and biological degradation.

Investigating the adsorption of Cd on soil from so-lutions containing various organic ligands, Elliott andDenneny (1982) showed that reduced Cd retention fol-lowed EDTA > NTA > oxalate ~ acetate. They notedthat the ability of the organic complexants to influenceCd adsorption decreased with decreasing pH and hy-pothesized the preferential binding of ligands to met-als (Fe and Al) released by acid-catalyzed dissolutionof soil components. Huang et al. (1988) also sug-gested that Zn(II) removal by SiO2, A12O3, and somezeolites in the presence of EDTA was significantlyreduced in alkaline pH regimes and slightly enhancedin the acid pH range. These authors presented evi-dence of specific interactions of EDTA with surfacehydroxyl groups since mineral zeta potentials wereshifted further into the acid pH range following EDTAadditions. Chang et al. (1983) also inferred specificadsorption of EDTA on hematite as evidenced by ashift in the mineral isoelectric point to lower pH val-ues. The bound EDTA was inferred to occupy twolattice ferric irons. Further studies by Chang and Ma-tijevic (1983) suggested that the lattice bonds betweenthe surface EDTA complexed Fe(III) ion and O wereAbbreviations: EDTA, ethylenediaminetetraacetate; NTA, nitri-lotriacetate; DTPA, diethylenetriaminepentaacetate; DDI, doubledeionized; ICPAES, inductively coupled plasma atomic emissionspectrophotometry; UV/VIS, ultraviolet/visible; 1C, ion chroma-tography; TOC, total organic carbon; TIC, total inorganic carbon;CEC, cation-exchange capacity; CD, equilibrium convective—dis-persive; CDK, nonequilibrium convective-dispersive.

954

JARDINE ET AL.: TRANSPORT PROCESSES IN UNDISTURBED HETEROGENEOUS POROUS MEDIA: II. 955

sufficiently weakened to allow release of theFe(III)EDTA- complex into solution. Sorption of Niand Co-EDTA complexes on soil from various DOEfacilities was also noted by Swanson (1983). This au-thor further noted solid-phase modification of thesesoils in the presence of Na-EDTA and -NTA salts.

In general, the interaction of co-contaminant mix-tures with subsurface media is usually performed inbatch reactors using homogenized samples. Batch sys-tems are invaluable for deciphering complex sorptionmechanisms in a rapid period of time; however, batchexperiments are closed systems and reaction productsand byproducts are not removed from the system dur-ing the course of the reaction. The interaction of re-action products or byproducts with the original co-contaminant mixture may complicate or provide fic-titious experimental results relative to natural subsur-face environments. Since co-contaminant leakage fromwaste burial sites involves predominately unsaturatedsubsurface processes, studies are needed that addressthe complex geochemical reactions of co-contaminantmixtures during unsaturated transport in heteroge-neous subsurface media. Accordingly, the objectiveof this study was to provide an improved understand-ing of the geochemical processes controlling co-con-taminant transport in heterogeneous, unsaturatedsubsurface media.

MATERIALS AND METHODSLarge, undisturbed soil columns (=14-cm diam. by =40-

cm length) were isolated from the C horizon of a proposedsolid waste storage area (Melton Branch Watershed) on theOak Ridge Reservation in eastern Tennessee using techniquesdescribed by Jardine et al. (1993). The subsurface material isprimarily fractured saprolite derived from weathered limey shaleand is coated with amorphous Fe and Mn oxides. Frequently,the layered fractured saprolite is interbedded with clay lensesweathered from limestone. The latter bedding structure is pref-erentially weathered within the porous media and carbonatesare essentially absent to depths of =3 m. The clay lense ma-terial experiences a much more rapid rate of mineral dissolu-tion than the surrounding saprolite, which is evident from itslower pH and higher dissolved Si and Al concentrations. Amore detailed description of the subsurface material can befound in Jardine et al. (1993).

The undisturbed subsurface columns were prepared for un-saturated transport studies as described by Jardine et al. (1993)and were maintained under a steady-state pressure head of—10 cm. This condition closely mimics the natural flow char-acteristics of the soil during most storm events, when unsat-urated flow occurs through mesopores and micropores (Wilsonet al., 1992, 1993).

Displacement ExperimentsThe co-contaminants used in this study were SrEDTA2-,

Co(II)EDTA2-, and a mixture of Co(II)EDTA2- andCo(III)EDTA-, which will be designated as Co(II/III)EDTA.These co-contaminants were utilized because decontaminationefforts of low-level radioactive waste commonly involves theuse of chelating agents, such as EDTA, that complex fissionbyproducts such as 90Sr and 60Co. The use of Co(II)- andCo(III)-EDTA complexes was intended to simulate co-con-taminant sources from anaerobic and aerobic burial trenches,respectively [in the presence of EDTA, Co(II) is slowly oxi-dized by atmospheric O2 to Co(III) during a period of severalyears]. Each co-contaminant influent solution was 0.02 M withrespect to the dissolved metal-EDTA complex and 0.002 M

with respect to CaBr2. The ionic strength (/) of each solutionwas adjusted to / = 0.15 using CaCl2 and checked by analysisand solution speciation with GEOCHEM (Sposito and Matti-god, 1980; Parker et al., 1987). The SrEDTA2- andCo(II)EDTA2~ solution complexes were prepared by slowlydissolving analytical grade Ca2EDTA salt into 0.02 M SrCl2or CoCl2, respectively. The solutions were allowed to equili-brate for 24 h prior to use to assure formation of the contam-inant-chelate complex (Hering and Morel, 1990).

Preparation of the Co(II/III)EDTA solution initially involvedcrystallizing a mixed Co(II)K2 EDTA and Co(III)K EDTA saltusing the following procedure. A 0.56 equimolar solution ofreagent-grade CoCl2 and H4EDTA was prepared in a 3.4 Mpotassium acetate aqueous solution. The solution was heatedto a near boil and 50 mL of 3% H2O2 per 100 mL of solutionwas gradually added. The solution was cooled to 298 K and=500 mL of reagent-grade ethyl alcohol per 150 mL of so-lution was added. This mixture was further cooled at 277 Kovernight. The supernatant was subsequently decanted, and theprecipitate was redissolved in a minimal amount of DDI waterand filtered through a 0.2-jun Nuclepore filter (Nuclepore,Costar Corp., Cambridge, MA). The process of salt purifica-tion involved three more chilled reprecipitations and dissolu-tions using ethyl alcohol and DDI water, respectively. Afterthe final reprecipitation, the salt was dried at 343 K and crushedinto a granular form. Aqueous solutions of the purified saltwere prepared and the mass quantity of Co^I^EDTA-1 andCo(II)EDTA2- were determined using an ion chromatographequipped with chemically suppressed conductivity detectioncoupled with spectral array detection (D.L. Taylor, Oak RidgeNational Laboratory, 1992, unpublished data). This techniqueaccurately quantified the purified salt as consisting of 53%Co(II)K2 EDTA and 47% Co(III)K EDTA.

The co-contaminant influent solutions, which all containednonreactive Br, were independently introduced into the col-umns as pulse additions. The pH and the redox electrode po-tential, Eh, of each influent solution is presented in Table 1.The carrier solution that followed the pulse was 0.05 M CaQ2(/ = 0.15) with a pH of 5.6 and Eh of 554 mV. Methodsdescribing influent additions to the columns were provided inJardine et al. (1993) and the physical parameters pertinent toeach column displacement study are listed in Table 1. The fateand transformation of the co-contaminant was thoroughly mon-itored over time, with each experiment requiring numerousmonths of steady-state leaching because of the use of realisticunsaturated conditions.

Analysis of Column EffluentEffluent from the various column displacement experiments

was analyzed for a complete array of inorganic and organicconstituents to establish mass and charge balance in the aqueousphase. Such extensive analyses are essential for decipheringcomplex geochemical reactions during co-contaminant trans-port. The major and trace elements were determined by IC-PAES and Br~ and SO2- were determined by 1C. Detectionof Br~ in the effluent of the SrEDTA displacement experimentinvolved converting the metal-EDTA complex into Co(III)EDTAby adding excess CoCl2 and oxidizing the solution with knownquantities of H2O2. This procedure eliminated the original in-terfering metal chelate, and Br was accurately quantified usingstandard 1C principles for anion detection.

Total EDTA was quantified by two different methods: UV/VIS spectrophotometric analysis and TOC analysis. Directmeasurement of effluent EDTA from the SrEDTA displace-ment experiment was possible using the method described byBhattacharyya and Kundu (1971). In general this method con-verts the effluent metal-EDTA complex into Fe(III)EDTA,which is quantified via spectrophotometric absorbance scans.The concentration of EDTA determined using the procedureof Bhattacharyya and Kundu (1971) agreed extremely well

956 SOIL SCI. SOC. AM. J., VOL. 57, JULY-AUGUST 1993

Table 1. Physical and chemical characteristics for column displacement experiments.Influent

Column

Midslope 1Midslope 2

Influent type

Co(II)EDTACo(n/ffl)EDTA

SrEDTA

PH

4.405.024.80

Redoxpotential

mV325401566

Pressureheadcm-10-10-10

Watercontent

cm3 cm-3

0.4190.4150.415

Bulkdensityg cm-3

1.511.561.56

Lengthcm40.040.040.0

Radiuscm7.08.58.5

Waterflux

cm h"1

0.07290.1710.193

Tracerpulse

durationh

27.118.317.8

Peclettnumber

6.42 ± 0.404.86 ± 0.295.30 ± 0.49

t Best fit to Br effluent using equilibrium convective-dispersive model; error estimates are 95% confidence intervals.

with TOC measurements using a Model 700 Total Carbon Ana-lyzer (OI Corp., College Station, TX). Since the indigenousorganic matter in these soils was low (<0.2% on a mass basis)and the effluent organic C was typically <1 mg C L-1 priorto the injection of co-contaminants, TOC analysis, coupledwith spectral analysis, served as a good measure of the totalEDTA eluted from the column studies. Effluent EDTA fromthe Co(II)EDTA2- and Co(II/III)EDTA displacement experi-ments could not be quantified using the method of Bhattacha-ryya and Kundu (1971) since the metal-EDTA complexesformed in these systems were much too stable for replacementby Fe(III). Nevertheless, direct spectrophotometric determi-nation of the effluent EDTA from these systems was possibleusing absorbance scans from 300 to 700 nm (see below), andthe quantified EDTA again agreed very well with TOC mea-surements.

Effluent from the various column displacement experimentswas also analyzed for TIC using the Model 700 Total CarbonAnalyzer. Values of TIC were generally <2 mg C L"1. Ef-fluent pH and Eh were measured using standard techniques.Measurements of Eh were performed with a Pt electrode anda Ag-AgCl reference electrode. Redox potential readings werereported relative to the normal H electrode.

Modeling the Displacement ExperimentsThe transport of co-contaminants through the subsurface me-

dia was complicated by sorption and co-sorption, redox reac-tions, and solid-phase modification. A rigorous theoreticaltreatment of the data requires the use of a multispecies-mul-ticomponent hydrogeochemical transport model (Yeh and Tri-pathi, 1990, 1991). For the purposes of our study, co-contaminant transport was modeled with various formulationsof the convective-dispersive equation, which considered bothequilibrium and nonequilibrium processes (see Jardine et al.,1993). The transport of metal-EDTA complexes were treatedas single species, and co-sorption reactions were lumped intoa retardation factor, R. For modeling purposes, metal-EDTAtransformations and redox reactions were ignored because ourintent at the time was not to simulate the geochemical mech-anisms operative during transport, but rather to model the lumpedretardation and nonequilibrium behavior of the co-contaminantin the subsurface media.

RESULTS AND DISCUSSIONDisplacement of the co-contaminants Co(II)EDTA2~,

Co(II/III)EDTA, and SrEDTA2- in undisturbed soil col-umns obtained from the midslope region of the MeltonBranch field site was investigated at —10 cm steady-state pressure head. Two columns were used in the stud-ies (Table 1) and were obtained within 2 m of each other.Both columns exhibited similar physical properties (bulkdensity, water content, and Peclet number); however,the average water flux of Column 1 was significantlylower than that for Column 2 at the same steady-statepressure head (Table 1). The latter circumstance resultedbecause Column 1 was primarily composed of weathered

saprolite, whereas Column 2 had a small amount of claylense material interbedded with the saprolite (see above).The highly weathered clay lense regimes create an en-vironment that is conducive to more rapid water andsolute transport.

Cobalt(II)EDTA2- Displacement StudiesThe unsaturated displacement of Co(II)EDTA2- through

the undisturbed soil column revealed simultaneousbreakthrough of Co and EDTA as a function of time(Fig. la). Similar Co and EDTA effluent concentrationsduring the entire displacement experiment suggested thatthe metal and the chelate were transported as a singleentity. Spectral analysis of the effluent via absorbancescans confirmed this contention; however, the metal-chelate complex was determined to be Co(IH)EDTA~.The Co(III)EDTA~ spectrum is quite unique and highlysensitive and exhibits two Amax at 381 and 535 nm (datanot shown, see Flaschka and Barnard, 1976). Indepen-dent measurements of Co and EDTA concentrations viaICPAES and TOC analysis, respectively (Fig. la), wereessentially identical to concentrations of Co(III)EDTA~measured directly by absorbance scans. These resultssuggest that the effluent Co and EDTA were entirelycomplexed as Co(III)EDTA-. An ion chromatographymethod that utilized electrochemical and spectral arraydetection in series also confirmed that Co(III)EDTA~was the complex present in the effluent and that noCo(II)EDTA2- remained (D..L. Taylor, Oak Ridge Na-tional Laboratory, 1992, unpublished data). Since ef-fluent C could be attributed almost exclusively to EDTA,the biodegradation of the EDTA during transport wasinsignificant.

The breakthrough of Co(III)EDTA- was significantlydelayed relative to nonreactive Br~, indicating the for-mer co-contaminant was reactive with the solid phase(Fig. la). Using batch techniques, Huang and Lin (1981)and Girvin et al. (1991) showed significant adsorptionof Co(III)EDTA- on -y-Al-,O3 and suggested that Hbonding and electrostatic adsorption mechanisms wereoperative. Electrostatic adsorption mechanisms appearmore likely in our study since the retained co-contami-nant was >87% reversible in a CaCl2 carrier solution.Peak breakthrough of Co(III)EDTA~ was more rapid thanuncomplexed Co2+ in the same subsurface media (Fig.la; Jardine et al., 1993, Fig. 4) suggesting chelate en-hanced mobility of the co-contaminant.

The presence of Co(III)EDTA- in the effluent sug-gested in situ oxidation of the influent Co(II)EDTA2~during transport. Since the influent Co(II)EDTA2~ in theabsence of subsurface material was stable during the du-ration of the displacement experiment (-5000 h), oxi-


862Time (h)

1724 2586 3448

5 -

x.oE

,

tf

o 0.4Eug 0.3 |-u

I °-2" 0.1

0.0

560

J, 540.cujc 52°o

£ 500

480

460

i "

*?"\oOO «

'? * Co

(A)Melton Mldslope 1-10cm pressure head

Co(ll)EDTA Influent

(B)

°a Mna

°D

(C)Eh

pH

6.25

6 9Pore Volume

12

5.75

5.50

Fig. 1. Observed solute effluent concentrations, redox potential(Eh), and pH for the Co(II)EDTA2- displacement study at—10 cm pressure head on a Melton midslope soil column.

dation via atmospheric O2 seemed unlikely. A moreprobable scenario of oxidation catalyzed by soil mineralsbecame evident on inspection of the effluent Mn gen-erated during displacement of the co-contaminantCo(II)EDTA2- (Fig. Ib). A significant pulse of Mn wasdisplaced through the column and was delayed relativeto the effluent Co(III)EDTA- in a similar manner as thedivalent cations Sr2+ and Co2+ (see Jardine et al., 1993).The elevated concentrations of Mn were not a result ofdesorption via electrostatic cation-exchange processesbecause previous displacement studies on this soil usinguncomplexed Co2* and Sr2+ showed a constant concen-tration of Mn in the effluent. Although electrostatic ex-change may have controlled the breakthrough time of theMn, its higher concentrations during Co(II)EDTA2~ dis-placement was most likely a result of the accelerateddissolution of amorphous MnO2 that is prevalent in thesesoils. The mineral MnO2 serves as an excellent oxidantand probably catalyzed the oxidation of Co(II)EDTA2~to Cp(III)EDTA- during transport (Bartlett, 1986). Batchstudies confirmed the oxidation of Co(II)EDTA2~ to

Co(III)EDTA- in the presence of crushed MnO2 concre-tions obtained from soil near the field site (unpublished,1992). During this redox reaction, MnO2 would be re-duced to Mn2+, which would then be transported throughthe subsurface media as a reactive divalent cation exhib-iting a breakthrough profile similar to Sr2+ and Co2+ (seeJardine et al., 1993). As discussed by Bartlett (1986),organic chelating agents encourage reverse dismutationof Mn(IV) and production of Mn(III). This latter Mnspecies is one of the most powerful and reactive oxidiz-ing agents likely to be encountered in soil. Because ofthis, Mn(III) is quite unstable and reduces to Mn(II) inthe presence of organic chelates.

The catalyzed oxidation of Co(II)EDTA2~ toCo(III)EDTA- in the presence of MnO2 was accom-panied by an increase in the effluent Eh (Fig. Ic). Theincreasing values of Eh reached a maximum at residencetimes equivalent to the peak breakthrough concentrationof Co(III)EDTA- and suggested that the soil solutionbecame more highly oxidized because of the redox re-action. Although pH measurements were somewhat in-frequent, effluent pH remained relatively constant duringthe displacement experiment (Fig. Ic). Effluent Si con-centrations were constant and Al concentrations werenegligible during the Co(II)EDTA2- displacement ex-periment (Fig. la), suggesting that enhanced phyllosili-cate dissolution was not occurring during the transportand transformation of Co(II)EDTA2- in these soils. Arelatively small pulse of Fe was generated during thedisplacement experiment and exhibited a similar break-through profile as the Co(III)EDTA- complex (Fig. laand Ib). As will become evident below, the enhancedFe transport was most likely a result of transformationof excess influent CaEDTA2- to FeEDTA" via solid-phase dissolution. The excess CaEDTA2- in the influentresulted from slightly less than equimolar concentrationsof added CoCl2 when preparing the Co(II)EDTA2- com-plex. Nevertheless, enhanced Fe-oxide dissolution wasnot significant during the transport of Co(III)EDTA~ inthese soils.

Cobalt(II/III)EDTA Displacement StudiesThe unsaturated displacement of a mixed influent so-

lution of Co(II)EDTA2- and Co(III)EDTA- (53 and 47%,respectively) through an undisturbed soil column wasinvestigated to simulate co-contaminant transport processesresulting from a leaky aerobic trench. As was noted forthe Co(H)EDTA2- displacement experiment, simulta-neous breakthrough of equimolar Co and EDTA wasevident during transport (Fig. 2a). This observation sug-gested that the metal and the chelate were mobilizedthrough the column as a single entity, and spectral analy-sis of the effluent confirmed that the metal-chelate com-plex was entirely Co(III)EDTA-. The lack of effluentCo(II)EDTA2~ coupled with a significant pulse of Mndisplaced through the column (Fig. 2b) suggested MnO2mediated oxidation of the influent Co(H)EDTA2- duringtransport. Considering the molar ratio of influentCo(II)EDTA2- to effluent Mn for both the Co(II)EDTA2-and Co(II/III)EDTA displacement experiments (notshown), >85% of the effluent Mn in the latter displace-ment experiment can be explained by the presence ofCo(II)EDTA2-. In other words, the presence of influent


290

Time (h)

580 87010

1160 1450

EDTA

\ 0.6oEug 0.4

0.2 1-

0.0

640

620

600

580

(A)

Melton Midslope 2-10cm pressure head

Co(ll/lll)EDTA Influent

(B)

i Mn

(C)

4.2

4.0

12 153.8

Pore Volume

Fig. 2. Observed solute effluent concentrations, redox potential(Eh), and pH for the Co(II/III)EDTA displacement study at—10 cm pressure head on a Melton midslope soil column.

Co(HI)EDTA- did not significantly enhance the redox-catalyzed dissolution of MnO2.

Oxidation of influent Co(II)EDTA2- during the Co(II/III)EDTA displacement experiment was accompanied bya small increase in the effluent Eh (Fig. 2c). This re-sponse is similar to that observed for the Co(II)EDTA2-displacement experiments; however, the magnitude ofthe Eh increase was not as marked. The effluent pHremained at a relatively constant value of 4.1 during thedisplacement experiment (Fig. 2c). The lower effluentpH reflects the presence of clay lense material within thecolumn that is interbedded with fractured saprolite. Theclay lense material experiences a much more rapid rateof mineral dissolution than the surrounding saprolite andresults in effluent Al concentrations of =0.5 mM andpH values of =4. The high concentration of dissolvedAl causes a decrease in the effective CEC of the solidphase since divalent cations of interest, such as Ca, Mn,Sr, and Co, do not compete well with Al for negativelycharged solid surface sites. The lower effective CEC ofthe bulk soil column accounts for the more rapid break-

through of Mn in the midslope Column 2, Co(II/III)EDTAexperiment, vs. the midslope Column 1, Co(II)EDTAexperiment (Fig. 1 and 2, Table 1).

Effluent Si and Al concentrations were constant (Aldata not shown; however, its average concentration was0.4 mM) and Fe concentrations were negligible duringthe Co(II/III)EDTA displacement experiment (Fig. 2a).This suggested that enhanced dissolution of the phyllos-ilicates and Fe oxides was not occurring during the trans-port of Co(II)EDTA2- and Co(IH)EDTA- in these soils.

The breakthrough of Co(III)EDTA- was significantlydelayed relative to nonreactive Br~, indicating the for-mer co-contaminant was reactive with the solid phase(Fig. 2a). These results are similar to co-contaminantbreakthrough curves observed in the Co(II)EDTA2~ dis-placement experiments (Fig. la). Again, electrostatic ad-sorption mechanisms are believed to account for theretarded Co(III)EDTA~ during transport since the co-contaminant adsorption was >95% reversible in a CaCl2carrier solution.

Strontium EDTA2- Displacement StudiesLow-level radioactive waste is also composed of 90Sr,

which is frequently complexed by chelating agents priorto shallow land burial. The unsaturated displacement ofSrEDTA2- through the undisturbed soil column revealeda much more delayed breakthrough of Sr relative to EDTA(Fig. 3a and 3b). This result suggested that the metaland the chelate were not transported as a single entity.In fact, the breakthrough of Sr in this soil resembled thatof a reactive divalent cation (Jardine et al., 1993). Anincreased flux of numerous soil cations during the dis-placement experiment was evident, with the cations Feand Al exhibiting concentration profiles similar to thatof EDTA (Fig. 3a). Spectral analysis of the effluent viaabsorbance scans suggested that Fe was entirely com-plexed by EDTA in the form Fe(III)EDTA-. TheFe(IH)EDTA" spectrum is quite unique and highly sen-sitive, with a An,.̂ at 256 nm (data not shown, see Flaschkaand Barnard, 1976). Although the Fe was 100% com-plexed as Fe(III)EDTA~, a significant portion of the to-tal effluent EDTA was present in another form. Althoughabsorbance scans did not reveal the presence of any othermetal-EDTA complexes, the spectral analysis used herecan only quantify those complexes that absorb a signif-icant portion of ultraviolet or visible light. Since a sig-nificant pulse of Al was displaced through the column,in an initially similar manner as the EDTA, one mightanticipate the presence of an effluent A1(III)EDTA-complex (Fig. 3a). Since A1(III)EDTA- does not exhibita characteristic absorbance spectrum, it cannot be quan-tified via spectral analysis. A technique for directly mea-suring A1(III)EDTA- using ion chromatography iscurrently being developed (D.L. Taylor, Oak Ridge Na-tional Laboratory, 1992, unpublished data). Therefore,cationic and anionic components of the column effluentwere speciated using GEOCHEM (Sposito and Matti-god, 1980; Parker et al., 1987) to provide a relativeindication of what cations were associated with the EDTA.

Speciation of the effluent at numerous times duringthe displacement experiment indicated that 95 to 100%of the total EDTA was associated with Fe and Al ofvarying proportions (Fig. 4). The speciation results alsosuggested that Sr existed entirely as Sr2+, which agrees


T!m« (h)

64410

XoE

2 2

> 600

= 580

560

..

•KEDTA

Melton Midslope 2-10cm pressure headSrEDTA Influent

Al .

*o V

(B)

SICo

••" '». «. „««

Mn' I .

(c)

Eho 0° o o

2 t

4.3 a.

4.2 ~

6 9Pore Volume

12 154.0

Fig. 3. Observed solute effluent concentrations, redox potential(Eh), and pH for the SrEDTA2- displacement study at -10cm pressure head on a Melton midslope soil column.

with the observation that Sr is not transported as a co-contaminant (Fig. 3b). Speciation results further indi-cated that a small percentage (0-5%) of the effluent EDTAwas associated with indigenous Co derived from the soil(Fig. 4). This agreed with the observed displacement ofa small amount of Co in the soil column that exhibiteda breakthrough profile similar to that of EDTA (Fig. 3aand 3b, note scales of concentration axis). TheCo(HI)EDTA- present in this displacement experimentcould not be quantified since concentrations were belowthe detection limits of the spectrophotometer.

The results above suggest that, during the displace-ment of SrEDTA2" through the subsurface material, Feand Al sources effectively dissociated the co-contami-nant and Sr was transported through the column as areactive divalent cation. The chelate EDTA readily be-came associated with solid- and solution-phase Fe, Al,and Co and was also transported as a reactive component(Fig. 3a and 3b). Since solution-phase Fe3+ was ex-tremely low (undetectable), the Fe(HI)EDTA- complexmost likely formed via chelate-induced Fe-oxide disso-

100

so

E 60

40

20

o observed FeEDTA• observed remaining EDTA

—— predicted Fe(lll)EDTA- - - predicted AI(III)EDTA

predicted Co(l l l )EDTA

6 8Pore Volume

10 12 14

Fig. 4. Observed and predicted distribution of effluent metal-EDTA complexes as a function of pore volume for theSrEDTA2- displacement study. Predicted curves weredetermined by speciating individual effluent samples fornumerous times using GEOCHEM (Sposito and Mattigod,1980; Parker et al., 1987).

lution. Chang et al. (1983) and Chang and Matijevic(1983) showed specific adsorption of EDTA on hematiteand suggested that the lattice bonds between the surfaceEDTA-complexed Fe(III) ion and O were sufficientlyweakened to allow release of the entire Fe(III)EDTA~complex into solution. The A1(III)EDTA~ complexprobably developed from several sources because sig-nificant Al is present within the soil phyllosilicates, iscoprecipitated with Fe oxides, and exists on the ex-change complex of the solid phase. The Fe oxides thatare associated with these soils contain 20 to 35% Alimpurity (Arnseth and Turner, 1988). Therefore, be-cause Fe(III)EDTA~ presumably develops from chelate-induced Fe-oxide dissolution, it is highly probable thatA1(III)EDTA- complexes develop in a similar mannerbecause the Fe oxides are heavily coprecipitated with Al.Chelate-enhanced dissolution of soil phyllosilicates is mostlikely not occurring because Si concentrations are rela-tively constant during the displacement experiment (Fig.3b). The possibility of Si reprecipitating within the col-umn prior to elution, however, cannot be dismissed andincongruent dissolution of the octahedral layers of thephyllosilicates is also possible. A more probable sourceof readily available Al is on the exchange sites of thesoil or dissolved in the soil solution. This Al source maybe hydrolyzed and polymerized (Hodges and Zelazny,1983; Jardine et al., 1985) yet is readily available forpotential interactions with EDTA. The small quantity ofCo(IH)EDTA- that forms during transport is most likelyderived from coprecipitated Co impurities within Fe andMn oxides.

The presence of readily available Al sources relativeto Fe may explain the varying proportions of effluentFe(III)EDTA- and A1(III)EDTA- during the course ofthe displacement experiment (Fig. 4). The initial columneluent had a higher percentage of EDTA associated withAl relative to Fe. The proportion of Fe(HI)EDTA- mon-itonically increased during the displacement experimentto consume -90% of the total eluted EDTA (Fig. 4)even though solution Al concentrations were almost sixtimes larger than Fe concentrations at long times (Fig.3a). It is apparent that some of the A1(HI)EDTA- is


transported through the system more rapidly than theFe(IH)EDTA" because of the higher proportion of theformer in the effluent at early times. Thus it may bereasonable to suspect that some complexed Al developsfrom readily available sources (i.e., dissolved in solu-tion) that do not require time-dependent solid-phasemodification reactions to control their presence in solu-tion. Evidence is presented below that suggests the trans-port of the majority of Fe(III)EDTA~ is kineticallycontrolled, presumably a result of time-dependent dis-sociation reactions of the chelated structural cations. Thisscenario is the most likely explanation of whyFe(III)EDTA~ complexes dominate the effluent at longertimes (Fig. 4).

The breakthrough curve for EDTA was essentially anupside-down, mirror image of the effluent Eh distribu-tion during the SrEDTA2- displacement experiment (Fig.3a and 3c). During the ascending and descending limbsof the Fe(III)EDTA- and A1(III)EDTA- breakthrough,Eh values rapidly decreased then increased, respectively,with Fe- and Al-EDTA peak effluent concentrations co-inciding exactly with the Eh minimum. The observedchange in Eh was most likely due to the formation ofFe(III)EDTA~ during transport. Laboratory measure-ments showed this species to have a significantly lowerredox potential than the SrEDTA2 ~ and the carrier so-lution CaCl2. Thus, significant concentrations ofFe(III)EDTA- resulted in a more highly reduced envi-ronment (Fig. 3a and 3c). This scenario may explain thepulse of Mn that was displaced through the column (Fig.3b). Since the formation of Fe(III)EDTA- created a morereduced environment during transport, soil MnO2 wasmost likely reduced, causing the subsequent transport ofMn2+ through the subsurface media. The chelate-in-duced dissolution of Mn coprecipitated with Fe oxidesprobably did not significantly affect the total effluent Mnflux. Arnseth and Turner (1988) have shown that the Feoxides in this soil have a Mn/Fe molar ratio of 0.02 to0.05, which is clearly insufficient to account for the ef-fluent Mn/Fe molar ratio of 0.28.

Modeling the Co-Contaminant DisplacementExperiments

Contaminant and co-contaminant effluent concentra-tions from the various displacement experiments weremodeled with the CD and CDK equations (Jardine et al.,1993). As discussed above, this modeling exercise is notentirely rigorous since co-contaminant transport wascomplicated by sorption and cosorption, redox reactions,and solid-phase modification. The approach is justifiedby our intentions to model the lumped retardation andkinetic behavior of the co-contaminants in subsurfacemedia, rather than the specific geochemical mechanismsoperative during transport. The use of the one-site equi-librium or two-site nonequilibrium model for describingco-contaminant transport is justified by the absence ofphysical nonequilibrium in the soil system. Effluent Br~was well described by the equilibrium CD equation (Ta-ble 1), which suggests that conditions of nonequilibriumwithin the soil are a function of kinetically controlledchemical processes rather than physical transportprocesses.

Observed Co(III)EDTA- effluent concentrations fromboth the Co(H)EDTA2- and Co(H/III)EDTA displace-

0.04

I 0.03Oi_

C0)oo 0.02oTJIDO

"S 0.01ac.

0.00

0.07

c °'06

o

g 0.05•*-c\ 0.04oo-o 0.03oo

I 0.02o:

0.01

0.00

Melton Midslope 2— 10cm pressure head

(A)

o obsurved Co(l l l)EOTA—— kinetic fit- - - equilibrium tit

nn n ~ ~

10 15

Pore Volume

Melton Midslope 2-10cm pressure head

o observed Sr- - - - - equilibrium fit Sr

o observed EDTA- - - equilibrium tit EOTA- kinetic fit EDTA

(B)

15 20

Pore Volume

Fig. 5. (A) Observed Co(III)EDTA- effluent for theCo(II)EDTA2~ displacement experiment, (B) and observedSr and EDTA effluent for the SrEDTA2- displacementexperiment, with model-fitted curves determined using theequilibrium and nonequilibrium (kinetic) convective-dispersive model and the physical parameters of Table 1.

ment experiments were not adequately described by theCD model with optimization of R (Fig. 5a with Co(H/III)EDTA data not shown, Table 2). In both cases, theequilibrium model-fitted curves had a more delayedbreakthrough and significantly higher peak concentrationthan the observed Co(III)EDTA- data. The modelingresults appear to indicate a loss of mass during transport,yet Co(III)EDTA- was at least 87% reversible duringthis displacement experiment. Presumably the observeddata would tail at low concentration for times much longerthan 15 pore volumes (Fig. 5a). Application of the no-nequilibrium or kinetic CDK model to the observedCo(III)EDTA~ data of both displacement experimentsresulted in a good description of co-contaminant trans-port (Fig. 5a, Table 2). Model-fitted parameters R, F(fraction of equilibrium sites), and a (first-order ratecoefficient) had good 95% confidence intervals on theestimated values and the autocorrelation matrix for op-timized parameters revealed no adverse collinearity ef-fects. Model-fitted F and a. values suggested a largeproportion of the transported Co(HI)EDTA- was con-trolled by time-dependent cosorption reactions with thesolid phase.

Nonequilibrium model-fitted R values forCo(III)EDTA- transport agreed reasonably well with Rvalues obtained from batch adsorption isotherms on soilobtained near the columns using methods similar to those


Table 2. Model-fitted parameters and statistics from the application of the equilibrium and nonequilibrium convective—dispersiveequation to observed co-contaminant and contaminant breakthrough curves.

Column

Midslope 1

Midslope 2

Influent type

Co(D)EDTA

Co(II/ffl)EDTA

SrEDTA

Effluenttracer

Co(ni)EDTA

Co(ffl)EDTA

AEDTAft

Sr

Modelt

CDCDKCD

CDKCD

CDKCD

CDK

K*

3.09 ± 0.18#6.76 ± 0.833.15 ± 0.105.14 ± 0.223.13 ± 0.295.89 ± 0.505.31 ± 0.09

§

f§

tt0.262 ± 0.052#

tt0.416 ± 0.028

tt0.197 ± 0.037

tt§

oilh-'tt

0.000840 ± 0.000079#tt

0.00273 ± 0.00016tt

0.00537 ± 0.00012tt§

r2

0.4860.9770.8230.9950.0540.9630.980

Sum ofsquares

0.002700.000120.004050.000110.008760.000340.00019

t CD = equilibrium convective-dispersive model; CDK = nonequilibrium convective-dispersive model.i R = retardation coefficient.§ F = fraction of equilibrium sites.f a = first-order rate coefficient for nonequilibrium sites.# Confidence interval = 95%.tt Parameter not applicable.it X symbolizes various metal cations such as Fe (observed) and Al (estimated via solution speciation, see Fig. 4).§§ Model not appropriate since equilibrium case is adequate.

specified by Jardine et al. (1993). Batch-derived R val-ues for Co(H)EDTA2- and Co(II/III)EDTA displacementexperiments were 6.25 and 6.48, respectively, and com-pared reasonably well with transport-derived R valuesshown in Table 2. These results suggest that the entiremass of saprolite and surrounding soil within the columnis chemically active during unsaturated co-contaminanttransport. More important is the observation that co-con-taminant R values for Co(III)EDTA~ were very similarto the R values acquired for the uncomplexed contami-nant Co2+ at the same ionic strength (Table 2; Jardineet al., 1993, Table 2). Similar R values implied that anequivalent amount of Co(HI)EDTA- and Co2* were re-tained by the soil even though their mechanisms of ad-sorption are quite distinct. The peak breakthrough ofCo(III)EDTA~ (Fig. la and 2a), however, was morerapid than uncomplexed Co2+ (Jardine et al., 1993, Fig.4), suggesting that the former co-contaminant exhibitssurface interactions that are more kinetically controlledrelative to Co2+. Thus the subsurface transport of Co2+

is accelerated in the presence of EDTA due to the prev-alence of chemical nonequilibrium during co-contami-nant transport.

Effluent from the SrEDTA2- displacement experimentwas also modeled with various equilibrium and none-quilibrium formulations of the CD equation. Since Srand EDTA were not transported as a single entity, theywere modeled separately assuming no interaction be-tween the two species. Although EDTA was transportedprimarily as Fe(III)EDTA- and A1(III)EDTA- (Fig. 3aand 4), we ignored the cation association and modeledthe EDTA species as a composite. As noted for theCo(II)EDTA2- and Co(II/IH)EDTA displacement exper-iments, observed EDTA during the SrEDTA2- displace-ment experiment was not adequately described by theCD model with optimization of R (Fig. 5b, Table 2).The equilibrium model-fitted curves had a more delayedbreakthrough and significantly higher peak concentrationthan the observed EDTA data. Application of the CDKmodel resulted in a good description of EDTA transport.Model-fitted R, F, and a values had good 95% confi-dence intervals and suggested a significant portion of thetransported Fe(IH)EDTA- and A1(III)EDTA- was con-trolled by time-dependent cosorption and solid-phasemodification reactions with the solid phase.

Observed Sr effluent concentrations during the Sr-EDTA2- displacement experiment were well describedby the CD model with optimization of R. This agreedwith the observations noted for Sr2+ transport through asimilar unsaturated soil column that showed the break-through of this contaminant was reasonably well de-scribed by the CD model (Jardine et al., 1993). In fact,estimated R values for Sr transport in this study agreedwell with independently measured R values of Sr2+ in-teractions in a similar soil (Table 2; Jardine et al., 1993,Table 3). Thus, the transport of Sr is unaffected by thepresence of EDTA and this contaminant is mobilizedthrough the subsurface as a reactive divalent cation.

CONCLUSIONSThis study has provided an improved understanding

of the geochemical processes controlling co-contaminanttransport in heterogeneous, unsaturated subsurface me-dia. Using large undisturbed soil columns, it was notedthat the mobility of Co(H)EDTA2-, Co(III)EDTA-,and SrEDTA2- through subsurface media was signif-icantly modified by sorption, surface complexation,and redox reactions. Subsurface Fe and Al sources ef-fectively dissociated the SrEDTA2- co-contaminant andSr was transported as a reactive, uncomplexed species.Observed Sr effluent concentrations were well de-scribed by the CD model and suggested that this con-taminant moved through the subsurface media as areactive divalent cation. Displacement of Co(II)EDTA2"through the subsurface media was characterized by aMnO2-mediated oxidation of the co-contaminant to formthe very stable Co(III)EDTA- species. ObservedCo(III)EDTA- effluent concentrations were modeledwell with the CDK model and suggested the mobilityof this co-contaminant was controlled by time-depen-dent sorption reactions with the solid phase. The oxi-dation product Co(III)EDTA~ exhibited an overallretardation that was similar to the uncomplexed con-taminant Co2+; however, the sorption kinetics ofCo(III)EDTA- were appreciably slower than Co2+. Theresults of these experiments indicated that Co2+ trans-port in subsurface media is accelerated in the presenceof the chelating agent EDTA, whereas the transport ofSr is unaltered in the presence of EDTA.


ACKNOWLEDGMENTSThe authors appreciate the efforts of Dr. Frank Wobber, the

contract officer for the Department of Energy who has sup-ported this work, and are grateful to Vickie Lewis, who typedthis manuscript. The authors are indebted to the efforts ofVirginia Harless, who was involved in many of the analyticalstrategies and the preparation of visuals. Gratitude is also ex-pressed to Dr. S.Y. Lee, of Oak Ridge National Laboratory,who contributed many helpful suggestions during this study.

unsaturated transport processes in undisturbed heterogeneous porous media: ii. co-contaminants

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