soil-water partitioning and desorption hysteresis of volatile organic compounds from a louisiana...

SOIL-WATER PARTITIONING AND DESORPTION HYSTERESIS OFVOLATILE ORGANIC COMPOUNDS FROM A LOUISIANA

SUPERFUND SITE SOIL

R. R. KOMMALAPATI1, K. T. VALSARAJ2∗ and W. D. CONSTANT3

1 Department of Civil Engineering, Prairie View A and M University, Prairie View, Texas, USA; 2

Gordon A and Mary Cain Department of Chemical Engineering, Louisiana State University, BatonRouge, Louisiana, USA; 3 Department of Civil and Environmental Engineering, Louisiana State

University, Baton Rouge, Louisiana, USA(∗ author for correspondence, e-mail: [email protected])

(Received 25 February 2000; accepted 3 February 2001)

Abstract. The adsorption and desorption of three volatile organic compounds (1,2- dichloroethane,1,1,2- trichloroethane and 1,1,2,2-tetrachloroethane) from a previously uncontaminated clayey soilsample from a Superfund site in North Baton Rouge, Louisiana was studied. In the linear rangeof the adsorption isotherm, the partition constants were not affected by the presence of the co-solutes. The adsorption isotherms over a wide concentration range on the soil followed the nonlinearFreundlich isotherm. The desorption of the compounds showed significant hysteresis at all concen-trations studied. Approximately 20 to 70% of the adsorbed mass of organic compounds resisted thedesorption even after five months of successive desorption steps. The desorption of four compounds(1,2-dichloroethane, 1,1,2-trichloroethane, 1,4-dichlorobenzene and hexachlorobutadiene) from acontaminated soil sample from the same site was also studied. The aqueous concentration declined asthe successive desorption steps progressed. For hexachlorobutediene the desorption can be visualizedas occurring in two stages. The first stage involved a ‘loosely bound’ or ‘reversible’ fraction and thesecond stage involved a ‘tightly bound’ or ‘resistant’ fraction.

Keywords: desorption, hysteresis, volatile organic compounds

1. Introduction

Petro Processors, Inc. is a Superfund site located in North Baton Rouge, Louisiana.This waste site which was operated in the mid sixties and seventies covers approx-imately 50 ha. It was used to dispose petrochemical and related wastes composedmainly of chlorinated solvents. Under a Consent Decree agreed to by the variousindustrial parties, federal, state and local governmental agencies in the U.S. DistrictCourt, Middle District of Louisiana, the remedy was to cover the site using a claycap and install a system of wells to hydraulically contain and recover free phaseliquids for above ground treatment. This is usually known as the pump-and-treat (Pand T) method (NPC, 1995). The industry uses a groundwater geochemical model(MODFLOW� ) to predict the movement of the groundwater plume for both satur-ated and unsaturated conditions at the site. An important parameter needed in thismodel is the soil-water partition constant. In using the MODFLOW� algorithm,

Environmental Monitoring and Assessment 73: 275–290, 2002.© 2002 Kluwer Academic Publishers. Printed in the Netherlands.

276 R.R. KOMMALAPATI ET AL.

the site is divided into numerous grids and the value of partition constants ineach grid determined from a knowledge of the soil properties, viz., organic mattercontent, mineral fraction and surface area (NPC, 1995). Since the site geology iscomplex and heterogeneous, adsorption will vary from point to point along thesite. As per the Supplemental Remedial Action Plan (SRAP) approved for the siteby the U.S. EPA, for modeling the groundwater plume using MODFLOW� , onlya few of the priority pollutants present at the site were to be selected for monit-oring; these were called ‘harbinger compounds’. The selection was based on theexisting groundwater quality data, water quality standards and factors affecting themobility in groundwater, for example, light chlorinated compounds arrive first atthe monitoring wells when the plume of contamination migrates (NPC, 1995). Thecompounds chosen were vinyl chloride, 1,2-dichloroethane, 1,1,2-trichloroethane,1,1,2,2-tetrachloroethane and 1,4-dichlorobenzene.

As part of the ongoing program to support the modeling work, we studiedthe soil/water partitioning of the ‘harbinger’ compounds using soils from the site(Kommalapati et al, 2000). Our initial work involved the determination of lin-ear adsorption isotherm constants for four of the five harbinger compounds (1,2-dichloroethane, 1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane and 1,4-dichloro-benzene) on uncontaminated site soils varying in clay and organic matter content.Correlations were developed for the partition constants (Valsaraj et al., 1999). Thispaper extends the above database to include the effects of co-contaminants on thepartitioning of the compounds. We also studied the desorption of the above testcompounds from both laboratory spiked soils and aged (field) contaminated soilsfrom the site. However, for aged soils, 1,1,2,2-tetracholoroethane was present atvery low concentrations and was not considered, but we included the desorptionbehavior of one additional compound from the site soil, viz. hexachlorobutadiene,which is the most prevalent high molecular weight organic compound presentat high concentrations throughout the site. Hexachlorobutadiene is highly hydro-phobic and has very low mobility in the groundwater. Hence it was not includedamong the ‘harbinger’ compounds in the SRAP.

2. Experimental Section

2.1. MATERIALS

2.1.1. SoilsSix uncontaminated soil borings were obtained from the site with soils represent-ative of silty clay, recent silty sand, Pleistocene clay and Pleistocene sand. Thelocations and sampling depths were described in our earlier work (Valsaraj et al.,1999). For the experiments reported here only the Pleistocene clay soil was used.The large lumps of soil were broken up and air dried for three days, before beingground separately and sieved through a 2 mm sieve (US Standard No: 10) and

SOIL-WATER PARTITIONING AND DESORPTION HYSTERESIS 277

TABLE I

Properties of the site soil used in the experiments

(A) Uncontaminated PPI Site Soila:

Location 147–162 feet from the surface

(Boring No: BB 2853–1)

Type of soil Pleistocene clay

Percent sand 22.2

Percent clay 49.4

Percent silt 28.4

Percent organic carbon 0.25

(B) Contaminated PPI Site Soil:

Location 45 to 50 feet and 60–65 feet from the surface

(Well No: W 1129–1)

Type of soil silty sand

Percent organic carbon < 0.02

Hexachlorobutadiene (HCBD) 1414 ± 1 µg g−1

1,4-dichlorobenzene (DCB) < 1 µg g−1

1,1,2-trichloroethane (TCA) < 3 µg g−1

1,2-dichloroethane (DCA) < 3 µg g−1

a Two other types of soils used in some experiments are sandy and silty clayey soilsfor which the properties were given in our earlier paper (Valsaraj et al., 1999). Thelocation of the soil collection is also shown in detail in Valsaraj et al, 1999.

stored for experiments. The soil was analyzed for some of the priority pollutants asper EPA method 8240 to insure that it was free of the contaminants.

Contaminated soil was also obtained from the site in sealed plastic containers.The soil was selected from a region where the contaminants are present at traceconcentrations and not in the form of a separate non-aqueous phase liquid (NAPL).500 grams of this soil were homogenized and used for the desorption experiments.A fraction of the soil was used to determine the texture, size distribution and or-ganic matter content. The soil was determined to be silty, sandy type. Triplicate soilsamples were analyzed for priority pollutants, particularly the test contaminants;these concentrations served as the initial soil contamination before desorption wasinitiated. The remaining soil was stored in sealed containers with no head space tobe used for subsequent experiments.

The properties of both the clayey (uncontaminated) and the contaminated soilsare given in Table I.


TABLE II

Properties of various contaminants present in the PPI site soil

Property DCA TCA TetCA DCB HCBD

Molecular weight 98.9 133.4 167.8 147 260.7

Vapor pressure (mm Hg) 87 30 6.5 0.4 0.15

Aqueous solubility (mg L−1) 8,300 4,400 3,000 65 3.2

log Kow 1.48 2.18 2.56 3.39 4.90

2.1.2. Chemicals and GlasswareThe test chemicals used for spiking the uncontaminated soil in the laboratory werethe three ‘harbinger’ compounds used in the groundwater model for the site, viz.,1,2-dichloroethane, 1,1,2-trichloroethane, 1,1,2,2-tetrachloroethane. 1,4-dichloro-benzene was used in our earlier study (Valsaraj et al., 1999), and the results wereused here for comparisons. All chemicals were obtained from Aldrich ChemicalCompany (Milwaukee, WI) and were used as supplied (> 98% purity). An addi-tional chemical, namely, hexachlorobutadiene was used for the desorption studyfrom contaminated soil for the reasons stated earlier. The properties of the testchemicals are given in Table II.

All soil-water linear adsorption isotherm constants were determined using 125mL Trace Clean� amber bottles purchased from VWR Scientific (Sugar Land, TX)certified to be free of organics. For the desorption study centrifuge tubes (50 mL)made of Teflon� (Fisher Scientific, St Louis, MO) were used as batch reactors.EPA certified clean vials of 40 mL capacity from VWR Scientific (Sugar Land,TX) were used for the collection and storage of aqueous solutions before analysis.Sodium azide was used as the preservative and biocide in the vials for experimentswith contaminated field soils.

2.1.3. MethodologyAdsorption from a Multicomponent Mixture and a Single Component System:

ASTM Standard Method D 5285–92 was used for the determination of soil-waterpartition coefficients (ASTM, 1993). The method used was the same as that re-ported in our previous work (Valsaraj et al., 1999). All the experiments were doneat least in triplicate. One set of experiments was conducted with DCA alone inwater as the test contaminant at a concentration of 2.5 mg L−1 and another set wasconducted with all four compounds each spiked at 2.5 mg L−1 in water. Know-ing the initial and final concentrations of the test organic in the aqueous phaseafter equilibration, total weight of the solution and soil, the soil-water partitioncoefficient Ksw was calculated as described previously (Valsaraj et al., 1999).

Desorption from Laboratory-Spiked Soil: About five grams of uncontaminatedsoil were added to 50 mL centrifuge tubes and water spiked to the desired con-


centration with the test contaminants (1,2-dichloroethane, 1,1,2-trichloroethane,and 1,1,2,2-tetrachloroethane) was added. Special care was taken to remove airbubbles. The bottles were equilibrated for the desired time (24–48 hr) and theaqueous solution was separated after centrifuging. This constituted the adsorptionstep. The contaminated soil was then subjected to a sequential desorption. To thecentrifuge tubes containing the contaminated soil (from the adsorption step), de-ionized water was added to fill the tubes to the top. The bottles were equilibratedagain for the desired time (48 hr to as much as 2 weeks) and supernatant separatedafter the centrifugation. The supernatant was collected in clear EPA certified vialsand analyzed for the test contaminants. The centrifuge tubes were refilled withdeionized water and the experiment continued until the aqueous concentrationswere below the detection limit (5 mg L−1).

Desorption from Field Contaminated Soils: About five grams of contaminatedsoil homogenized and stored as mentioned earlier was measured and transferredinto a 50 mL pre-weighed Teflon centrifuge tube. The exact weight of each tubewith soil was noted. Two milliliters of sodium azide stock solution (40 g L−1) wasadded to each of the centrifuge tubes to give a final concentration of about 2 g L−1

in the tube to suppress biological activity during the equilibration. The centrifugetubes were filled to the top so as to have a minimum head space and sealed tightly.The tubes were then agitated on a mechanical shaker for a week. The tubes wereremoved from the shaker and centrifuged at 12,000 rpm for 20 min. The super-natant was separated into EPA certified clear vials. The weight of centrifuge tubesafter separating the supernatant was noted again. A measured amount of water wasadded to the clear vials to fill them to the top. The supernatant was analyzed forthe test contaminants. The centrifuge tubes were filled again with 2 mL of sodiumazide solution, followed by water and the experiment repeated. This procedure wascontinued until the aqueous concentrations of the test contaminants were below thedetection limit or the concentrations did not change significantly.

Sample Analysis: Aqueous samples were analyzed using EPA Method 601 forpurgeable halocarbons on HP 5890 series II gas chromatograph equipped with apurge and trap unit and an electrochemical detector. Samples were diluted if neces-sary (for concentrations above the method range) before analysis. The method de-tection limit was 5 mg L−1 for all the test contaminants. Soil samples were analyzedusing EPA Standard Method 8240. The method detection limits for soil sampleanalyses were set at 3 mg kg−1 for all compounds except hexachlorobutadiene forwhich the detection limit was 100 mg kg−1. All sample analyses were conducted bya commercial laboratory (Gulf Coast Analytical Laboratories, Inc., Baton Rouge,Louisiana).


TABLE III

Effect of co-contaminants on the soil-water linear adsorption constant for1,2-dichloroethane on selected soils

Soil type foc DCA alone DCA with co-solutes ANOVA-P

Ksw(L kg−1) Ksw(L kg−1)

sandy 0.0011 0.09 ± 0.09 0.05 ± 0.01 0.0001

clayey 0.0025 0.41 ± 0.22 0.22 ± 0.10 0.0559

silty clayey 0.0113 0.71 ± 0.42 0.29 ± 0.08 0.1267

3. Results and Discussion

3.1. EFFECTS OF CONTAMINANT MIXTURES ON PARTITION CONSTANTS

In our previous work (Valsaraj et al., 1999), we reported the linear partition con-stant, Ksw for each of the four harbinger compounds determined on different soiltypes. These values were measured from water spiked with the four ‘harbinger’compounds (1,2-dichloroethane, 1,1,2-trichloroethane, 1,1,2,2-tetrachloroethaneand 1,4-dichlorobenzene) together, each at an initial spike concentration of 2.5 mgL−1. Thus, there was no evaluation of the effect of co-contaminants or co-soluteson the sorption of an individual compound. Recognizing that the waste at the sitecontains several other compounds, besides the ‘harbingers’, it was important toascertain what effect co-contaminants have on the sorption process. Hence, weconducted one study where we compared the measurements made earlier for onecompound (1,2-dichloroethane) from a mixture of all four compounds with thatmade using only 1,2-dichloroethane as the contaminant in the spiked water. Theresults of this trial along with the statistical evaluation are given in Table III. Atfirst glance, it appears that the partition coefficient for 1,2-dichloroethane from thesingle component system is higher than that from the multicomponent mixture.However, the standard deviations associated with the measurements are high, andthe ANOVA statistical analysis indicated that there was no statistically significantdifference between the two means. Note that these measurements were made at lowinitial aqueous spike concentrations (2.5 mg L−1 in water) and it is questionableas to whether it can be directly extrapolated to higher aqueous concentrations andhigher soil phase concentrations. Indeed this specific conclusion is not new and canbe anticipated from the currently available literature (Weber and DiGiano, 1996).


TABLE IV

Freundlich isotherm constants for contamin-ants on Pleistocene Clayey soil

Compound KF n r2

DCA 2.5 × 10−6 0.647 0.89

TCA 7.9 × 10−5 0.782 0.93

TetCA 2.3 × 10−4 0.858 0.97

Note: Freundlich isotherm gives the soil con-centration W = KF Cw

1/n, where KF and nare constants determined from fitting the ad-sorption data. r2 is the correlation coefficientfor the linear fit.

3.2. EFFECT OF CONTAMINANT CONCENTRATION ON LINEAR ADSORPTION

ISOTHERM

The soil-water partition constants are usually determined at concentrations in theaqueous phase well below the saturation solubility of compounds. For the caseswe reported earlier (Valsaraj et al., 1999), we assumed linear adsorption isotherms.Although reasonable at low aqueous and soil phase concentrations, this assumptionis not necessarily valid over very large concentration ranges. As the aqueous con-centration of a contaminant approach its saturation solubility in water, significantdeviations from linearity are observed for adsorption on surfaces (Adamson andGast, 1997). The reason is that the high concentration of adsorbed solute makesthe surface more hydrophobic and hence the uptake of the hydrophobic solute isincreasingly favored. We observed similar behavior for DCA, TCA and TetCA onPleistocene clayey soil. Adsorption was linear at low concentrations, but increasedmarkedly as aqueous concentrations approached their solubility limit. In Figure 1we present the adsorption isotherm for DCA. Its saturation aqueous solubility is 8.3× 106 µg L−1 and had an adsorption constant of 0.41 ± 0.22 L kg−1 which wasmeasured at equilibrium aqueous concentrations < 103 µg L−1. The linear isothermwhen extrapolated to a high concentration under-predicted the actual soil phaseconcentration by an order of magnitude. Notice that the largest concentration stud-ied presently was 3.0 × 105 µg L−1, which is 27 times smaller than the saturationsolubility but more than two orders of magnitude larger than the earlier study. Abetter predictor over the entire concentration range is the well-known Freundlichisotherm which is shown in Figure 1. Notice that both axes are logarithmic andhence a straight line is to be expected for the Freundlich isotherm. Isotherm con-stants, KF and n and the associated r2 for the linear fit are determined and presentedin Table IV. Similar approach was applied to the other two compounds (1,1,2-trichloroethane and 1,1,2,2-tetrachloroethane) to obtain the Freundlich isothermconstants and are enumerated in Table IV. Note that KF generally increased as the


Figure 1. Experimental data and Freundlich isotherm fit for the adsorption of 1,2-dichloroethane onPPI Pleistocene clayey soil.

octanol-water partition constant (hydrophobicity) of the compound increased. Itwas also obvious that as the compound became more hydrophobic, n value tendsto one, indicating that the linear isotherm is more applicable for TetCA, a morehydrophobic chemical than DCA. It was also noteworthy that in all cases n <1 indicating that the isotherms are concave. The observations presented here areimportant since the pore water contaminant concentrations at the PPI site are likelynear saturation solubility in view of the high contaminant concentrations in the soil.

3.3. ADSORPTION AND DESORPTION FROM LABORATORY-SPIKED SOILS

In order to further understand the behavior for the four compounds as to the ad-sorption and desorption pathways, a series of experiments was performed wherebythe adsorption step was followed by successive desorption using distilled water.Results of adsorption and desorption studies with laboratory spiked soils for 1,4-dichlorobenzene were presented earlier (Valsaraj et al., 1999). The adsorption anddesorption behavior of three other compounds (1,2-dichloroethane, 1,1,2-trichlor-ethane and 1,1,2,2-tetrachloroethane) on Pleistocene clay were studied here. No-tice first of all that in all of these experiments, uncontaminated soil (Pleistoceneclay) from the site was first contaminated in the laboratory during the adsorp-tion step and subjected to sequential desorption by replacing the aqueous phase


Figure 2. Adsorption and desorption isotherms for 1,2-dichloroethane on Pleistocene clayey soil.Different initial spike concentrations and therefore different starting soil phase concentrations areshown. The straight line shown is the Freundlich isotherm fit to adsorption data.

with distilled water at predetermined intervals as discussed in the experimentalsection. Five separate runs each with different initial aqueous contaminant con-centrations in the spike solution (1, 15, 90, 100 and 500 mg L−1) were made.Obviously the initial soil concentrations increased with increasing spike concen-trations. Figures 2 to 4 are plots of the adsorption and desorption for the three com-pounds, 1,2-dichloroethane, 1,1,2-trichlorethane and 1,1,2,2-tetrachloroethane, re-spectively. The horizontal axis represents the equilibrium aqueous concentration(µg L−1) and the vertical axis represents the equilibrium soil phase concentration(µg g−1). The solid lines shown in the plot are the adsorption isotherms obtainedin each case using Freundlich isotherm parameters from Table IV. As can be seenfrom the plot, the adsorption of all three compounds can be best represented by theFreundlich isotherm.

The interesting aspects of this work are the desorption curves shown in Figures2 to 4. Let us look at Figure 2, which describes the adsorption and desorptionbehavior of 1,2-dichloroethane and same discussion is applied to the other twocompounds, 1,1,2-trichloroethane and 1,1,2,2-tetrachloroethane (Figures 3 and 4).It is evident that the aqueous concentrations during the initial few desorption stepsremained high, however, the aqueous concentrations were rapidly reduced to ator below detection limits particularly for samples with low initial concentrations.


Figure 3. Adsorption and desorption isotherms for 1,2,2-trichloroethane on Pleistocene clayey soil.Different initial spike concentrations and therefore different starting soil phase concentrations areshown. The straight line shown is the Freundlich isotherm fit to adsorption data.

For samples with higher initial aqueous concentrations the desorption continuedover a larger number of desorption steps. If adsorption was 100% reversible, thethermodynamic pathway taken for both adsorption and desorption of a compoundwill be identical and we would see a desorption curve identical to the adsorptionisotherm. Evidently, the desorption pathway is quite different from the adsorptionpathway. This deviation between adsorption and desorption pathways is not new,and has been observed by several authors previously, for example, Kan et al. (1997;1994). The desorption is composed of two stages, one in which the contaminantis desorbed rather quickly (the first few desorption steps) and a second stage inwhich the contaminant is desorption resistant, as indicated by the low aqueousconcentrations during the later desorption stages. The aqueous concentrations didnot change appreciably when the equilibrium time was increased from 2–3 days to10–15 days indicating that the observed resistance to the desorption was not dueto simple kinetic or diffusional limitations. The possibility of transport limitationsthrough organic matter or the microporosity of solids as the mechanism of the res-istant desorption is also questionable. For example, Kan et al. (1997) reported thata surrogate material (sodium dodecyl benzene sulfonate coated anatase) which isnon-porous and homogeneous had a similar desorption resistant fraction. However,this fraction was recoverable by extraction with methylene chloride at high temper-


Figure 4. Adsorption and desorption isotherms for 1,2,2,2-tetrachloroethane on Pleistocene clayeysoil. Different initial spike concentrations and therefore different starting soil phase concentrationsare shown. The straight line shown is the Freundlich isotherm fit to adsorption data.

atures. Thermodynamic considerations dictate that the minimum requirement foradsorption and desorption to follow different paths is a structural rearrangementin the solid phase subsequent to adsorption, and hence the desorption occurs froma different molecular environment than that to which it was adsorbed (Adamsonand Gast, 1997). Portions of the adsorbed material are perhaps occluded by con-formational changes of organic matter around the organic molecule and hence thedesorption is not favored. A number of related hypotheses to explain the desorptionresistance in terms of the behavior of organic matter in the soil have been advanced(Pignatello and Xing, 1996).

The percent desorption resistant fractions for each of the three chemicals atdifferent initial soil concentration ranges were determined to range from 20 to 70%for the cases reported in Figures 2–4. This is within the range of values reportedby several other authors. For example, Kan et al. (1997) reported that 50% ofa polychlorinated biphenyl congener was ‘irreversibly sorbed’ to Lula sediment.Similarly, Carroll et al. (1994) reported that more than 45% of the adsorbed massresisted desorption from contaminated Hudson River sediments.


Figure 5. Aqueous phase concentrations of DCA, TCA and DCB during successive desorption fromcontaminated soil from the PPI site.

3.4. DESORPTION FROM FIELD CONTAMINATED SOIL

Long term persistence in soils of intrinsically biodegradable compounds even whenother environmental factors are not limiting microbial growth, supports the hy-pothesis advanced by Pignatello and Xing (1996) and Hatzinger and Alexander(1995) that the field contaminated (aged) soil, where contact times are months toyears, can be enriched in the slow desorption fraction owing to partial dissipationor degradation of more labile fractions before collection. In this spirit, we conduc-ted an experiment using field, contaminated soil from the PPI site. The soil wascollected and processed as described in the experimental section. Subsequently, itwas analyzed for the contaminants of interest. The soil was chosen such that therewere only trace levels of contaminants and no free non-aqueous phase liquid waspresent. Table I shows the results thus obtained from the commercial laboratory thatconducted the analysis. Except for HCBD which had an initial soil concentrationof 1414 ± 1 µg g−1, 1,2-dichloroethane (DCA) had a concentration < 3 µg g−1

and 1,4-dichlorobenzene (DCB) was < 1 µg g−1. This was because the laboratoryhad set high detection limits considering the very high concentration of HCBD inthe soil. This meant that except for HCBD the residual soil phase concentrationafter each desorption step for the other three compounds could not be accuratelyascertained.


Figure 6. Aqueous phase concentration of HCBD during successive desorption from contaminatedsoil from the PPI site.

Figure 5 shows the supernatant aqueous phase concentration of DCA, TCAand DCB after each desorption step. TetCA concentrations were very low (belowdetection limits) and thus TetCA results were not included in the discussion. Theaqueous concentrations of these compounds were between 30 and 35 µg L−1 dur-ing the initial desorption step, but were reduced to less that 5 µg L−1 after fivedesorption steps for DCB and fluctuated between 5 and 10 µg L−1 for DCA. Theseaqueous concentrations were well below the saturation aqueous solubility of thecompounds. The reported concentration at each step is the mean of four replicatesamples. After about six desorption steps (35 days) the aqueous concentration ofDCB was reduced to below detection limit value in one or more of the samplesand could not be detected in any of the samples after 15 desorption steps (105days). The aqueous concentration of DCA and TCA also declined sharply. Evenafter 20 desorption steps (140 days) a constant release of these two compounds atabout 10 µg L−1 was observed. As pointed out earlier, the soil phase concentrationscould not be estimated after each step and hence the desorption isotherms were notavailable.

The case with HCBD was different. HCBD is one of the most prevalent com-pounds present at high concentrations throughout the site. It was found at a highsoil concentration, 1414 ± 1 µg g−1 initially in the soil. The aqueous concentrationafter each desorption step is plotted in Figure 6. The initial concentration in water


in equilibrium with the soil was 2120 ± 114 µg L−1, but the concentration did notdecline as rapidly as was observed for the other three compounds. The aqueousconcentration declined to about 1200 µg L−1 after 20 desorption steps (140 days).For this compound the aqueous concentration after each step was used to determinealso the mass remaining on the soil through a mass balance. Knowing the initialsoil phase concentration and the concentration of HCBD in each desorption stepone can estimate HCBD concentration remaining on the soil by simply subtractingthe mass removed from the initial soil concentration. Thus a desorption isothermrelating the aqueous concentration to the equilibrium soil concentration was de-termined, and is shown in Figure 7. Each point on the curve (Figures 5 and 6) is themean of four replicate experiments. If sorption was entirely reversible, an isothermcan be obtained by connecting the origin to the first desorption point (dark brokenline). The slope of this e line (= 667 L kg−1) represents a partition constant Ksw

rev

if the partitioning was entirely reversible. It is quite clear that the adsorption anddesorption pathways are quite different, and that a considerable hysteresis in thedesorption exists. A linear fit to the points obtained from the sequential desorption(solid line) will give a slope equivalent to a partition coefficient for the looselybound or readily desorbing fraction which we will represent as Ksw

des,1 and is 166L kg−1 in the present case. Extrapolating this backward to the Y axis (light brokenline) suggests that approximately 1,204 ± 13 µg g−1 of the HCBD is tightly bound(or desorption resistant) to the soil and may desorb only very slowly (months toyears). Thermodynamics dictates that the desorption curve cannot, in reality, ex-tend to a finite value on the y-axis, and that at some concentration near the y-axis,a sharp decrease in soil concentration should be observed. An attempt was made toobtain the desorption constant for the tightly bound fraction, by drawing a line fromthe origin to intersect the extrapolated line for the loosely bound fraction as shownin Figure 7. The slope of this line is obtained by taking the difference betweenthe initial soil aqueous concentration (1414 µg g−1) and the estimated remainingsoil concentration (desorption resistant) from the extrapolated line (1204 µg g−1)and dividing it by the aqueous concentration of 20 µg L−1 as discussed below.The slope thus obtained is Ksw

des,2 = (1414 - 1204)/20 = 10.5 L g−1 = 10,500 Lkg−1. Kan et al. (1998) reported that the aqueous concentration of dichloroben-zene, p,p′-DDT and polychlorinated biphenyls and a number of other compoundsin equilibrium with the maximum ‘irreversibly’ sorbed compartment concentrationon three different sediments was in the range of 1 to 20 µg L−1. Hence we chose thevalue of 20 µg L−1 as the aqueous concentration for the calculation of the slope.This indicates a 160-fold decrease in HCBD concentration relative to its aqueoussolubility. Thus an estimated ratio of Ksw

des,2/Kswdes,1 for HCBD on PPI site soil

is 63. This is only meant to conceptualize the magnitudes of the two compartmentsinto which the HCBD partitions within the soil. Gess and Pavlostathis (1997) re-ported that the ratio for HCBD on an estuarine sediment from Bayou d’Inde inLouisiana is 33. A number of investigators have also shown that the ratio of the


Figure 7. Desorption isotherm for HCBD from the contaminated PPI site soil sample.

partition constants varied from 1 to 20 (Pignatello and Xing, 1996) for a variety ofother compounds on contaminated soils.

The observed hysteresis in sorption process on soil has profound implicationsfor the remediation of contaminated soils. There are several important consequencesof the above findings. Firstly, it is clear that satisfactory removal of residual soil-sorbed HCBD or other compounds (i.e., the fraction left after the free phase re-moval) by conventional pump-and-treat is difficult and may require an inordinateand unacceptable time frame. In other words, much of the contaminants are ‘irre-versibly’ bound to the soil. If recovery is the only remedy, other enhanced pumpand treat schemes should be considered. Secondly, since most of the contamin-ants are bound to the soil and leaches only slowly, and since microbes capable ofconsuming these specific contaminants are known, monitored natural attenuationor enhanced bioremediation are better options for site remediation. This would, ofcourse, require a continuous monitoring of the plume so that no off-site migrationof the contaminants occur during the site cleanup. Thirdly, since the contaminantsare predominantly remains bound to soil particles, its movement off-site with thegroundwater is likely not significant.


Acknowledgments

This work was supported by a grant through the LSU Hazardous Waste ResearchCenter and sponsored by the U.S. District Court, Middle District of Louisiana. Wealso thank the personnel at NPC Services, Inc. for collecting and analyzing samplesfrom the PPI site.

References

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Carroll, K. M., Harkness, M. R., Bracco, A. A., Balcareel, R. B.: 1994, Application of a per-meant/polymer diffusional model to the desorption of polychlorinated biphenyls from HudsonRiver sediments, Environ. Sci. Technol. 28, 253–258.

Gess, P. and Pavlostathis, S. G.: 1997, Desorption of chlorinated organic compounds from acontaminated estuarine sediment, Environ. Toxicol. Chem. 16, 1598–1605.

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soil-water partitioning and desorption hysteresis of volatile organic compounds from a louisiana...

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