Desorption of Pentachlorophenolfrom Soils Using Mixed SolventsA M I D P . K H O D A D O U S T ,M A K R A M T . S U I D A N , *G E O R G E A . S O R I A L , A N DD I O N Y S I O S D . D I O N Y S I O U

Department of Civil and Environmental Engineering,University of Cincinnati, Ohio 45221-0071

R I C H A R D C . B R E N N E R

National Risk Management Research Laboratory,U.S. Environmental Protection Agency,Cincinnati, Ohio 45268

Desorption of pentachlorophenol (PCP) from contaminatedsoils in mixed solvents of water and ethanol wasinvestigated using desorption isotherm experiments. Thefollowing cosolvent volume fractions of ethanol in the mixedsolvent were considered: 0, 0.3, 0.56, 0.79, 0.95, and 1.0.Three fractions of a synthetic soil (Edison soil) withapproximately 1% organic matter were the main soilsused in this study in addition to K-10 montmorillonite clayand Ottawa sand. The effect of soil organic matter andsoil surface area on desorption in mixed solvents wasevaluated. Analysis of desorption data revealed that PCPdesorption increased with PCP solubility in mixed solvent upto 0.79, 0.95, and 0.56 fraction ethanol for Edison soil,K-10 montmorillonite, and Ottawa sand, respectively. Lowerdesorption of PCP from Edison soil in solvents with morethan 0.79 fraction ethanol resulted from interactions betweensolvent and soil organic matter. For Edison soil, highestPCP desorption in all mixed solvents was obtained for thesoil fraction with the smallest surface area. Desorptionof PCP in mixed solvents containing more than 0.79 fractionethanol was lower for soils with organic matter than forother soils.

IntroductionPentachlorophenol (PCP) is a wood preserving compoundoften found in contaminated soils at wood preserving sitesin the U.S. (1). Solvent washing of contaminated soils maybe a viable soil cleanup technology (2, 3). In remediation ofcontaminated soils, the selection of solvent for either in-situor ex-situ washing of soils with solvent depends on severalfactors that influence the effectiveness of the solvent incontaminant removal. The solubility of the organic pollutantin the solvent and the miscibility of the solvent with waterare important factors. The fraction of the water-miscibleorganic liquid, the cosolvent, in an aqueous solutiondetermines the solubility of the compound in that solution.Higher cosolvent fractions in the solution increase thesolubility of the hydrophobic organic compounds (HOCs) inlog-linear fashion (4-6).

The sorption of HOCs onto soil from an aqueous solutionhas been primarily attributed to sorption of HOCs to soil

organic matter (7-10). Sorption, which is related to theconcentration of the HOC in the liquid and the organic mattercontent of the soil, depends on the solubility of the HOC inthe liquid. Equilibrium sorption studies have shown that thesorption of HOCs decreases with the solubility of thecontaminant in the liquid in log-linear fashion (11-13). Leeet al. (14) showed that the equilibrium sorption of ionizableor dissociating HOCs also decreased with liquid solubility,similar to neutral HOCs.

Rao et al. (12) suggested that HOC sorption onto soils andsolubility in solvents are determined by hydrophobic (solvo-phobic) interactions, where the hydrophobic surface ofsorbent is in contact with a hydrophobic sorbate (solute). Anincrease in the fraction of organic cosolvent leads to anexponential decrease in sorption with a decreasing carbon-aceous surface area for the sorbate, resulting in highermobility of the HOC in the absence of hindrance by diffusionthrough the organic matter.

In sorption or physiosorption of HOCs onto naturalsorbents, negligible activation energy is required for phys-iosorption of the HOC to an adsorptive surface or partitioninginto an organic phase. In contrast, desorption of HOCs fromthe once-sorbed phase or surface generally requires activationto overcome sorptive interactions that may have developedbetween sorbate and sorbent after the initial sorption (15).Assessing different types of geosorbents and their inorganicand organic sorptive components during sorption-desorp-tion of HOCs, Luthy et al. (16) suggested fast and slowdesorption from amorphous (soft) and dense (hard) organicmatter, respectively. Reversible sorption occurs when majorsorbate-sorbent interactions have been overcome. In additionto physiosorption, chemisorption of substituted phenoliccompounds such as PCP may occur due to interactionsbetween the substituted groups or the phenolate anion andthe soil surface or soil organic matter. In the absence ofspecific sorbate-sorbent and sorbent-solvent interactionsother than physiosorption, desorption of HOCs from soilsinto aqueous solutions, therefore, is expected to increasewith increasing liquid solubility of the HOC. A higher fractionof cosolvent in the mixed solvent is expected to enhance thedesorption of HOCs.

The sorption of HOCs onto soils from aqueous and mixedsolvents has been studied by numerous investigators, whilethe desorption of HOCs from soils into mixed solvents hasnot been studied as extensively. This research examined thedesorption of PCP, a dissociating HOC and a U.S. EPA prioritypollutant, from soils into mixtures of water and ethanol.Ethanol was selected as cosolvent in water-ethanol mixturesdue to its high PCP solubility, its complete miscibility withwater, and its low cost. Desorption of PCP in water-ethanolmixed solvents was investigated using soils with varied soilorganic matter content and soil surface area (Table 1).

* Corresponding author phone: (513)556-3695; fax: (513)556-2955;e-mail: makram.suidan@uc.edu.

TABLE 1. Desorption of PCP from Soils Using Water-EthanolMixtures: Experimental Matrix

soil surface area

with soil organic matter without soil organic matter

Edison soil montmorillonite Ottawa sand

20 × 40 100 × 140 >200 large surface small surfaceU.S. U.S. U.S. area areamesh mesh mesh

Environ. Sci. Technol. 1999, 33, 4483-4491

10.1021/es980759z CCC: $18.00 1999 American Chemical Society VOL. 33, NO. 24, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 4483Published on Web 11/09/1999

Experimental SectionChemicals. The chemicals (solutes and solvents) used in thisstudy are listed in Table 2. MilliQ grade deionized water (DI)with a resistivity greater than 18 MΩ was produced insidethe laboratory by processing tap water through a Milliporepurification system (Millipore Corporation, Bedford, MA).The various water-ethanol mixtures were prepared fromthis DI water and the 95% ethanol solution.

Soils. Uncontaminated Edison soil, containing 31% sand,6% no. 9 gravel, 28% silt, 20% top soil, and 15% clay, wasobtained from the U.S. EPA Synthetic Soil Blending SystemProduction Center in Edison, NJ. After sieving the Edisonsoil, the 20 × 40, 100 × 140, and >200 U.S. mesh fractionsof soil were selected for this study. The soil fractions, exceptthe >200 U.S. mesh fraction, were washed with water anddried in an oven at 105 °C prior to spiking with PCP. Othersoils included K-10 montmorillonite clay and 20 × 30 U.S.mesh Ottawa sand. Some properties of the soils are listed inTable 3.

BET Surface Area and Pore Size Analysis for Soils. TheBET (19) surface area, pore size, and pore size distributionfor different soils were measured with an Accelerated SurfaceArea and Porosimetry system, ASAP 2010 (MicromeriticsInstrument Corporation, Norcross, GA), using nitrogenadsorption/desorption isotherms.

PCP Spike. Acetone was used as a wetting agent for spikingthe soil with PCP. A solution of PCP in acetone was addedto a dry soil fraction. This soil-acetone-PCP slurry (500 gof soil-100 mL of acetone) was placed inside a cylindricaltumbler with baffles and allowed to mix for 3 weeks at 16rpm. The tumbler was a 304 stainless steel cylinder 12.7 cmlong with a 10.1-cm ID. The tumbling procedure providedfor slow deposition of the PCP onto the soil, while the acetonevolatilized through 4-µm stainless steel frits placed insideportholes on the sides of the tumbler.

PCP Analysis. Liquid samples were filtered through Magna0.45-µm nylon membranes (Micron Separations Inc., West-boro, MA) and acidified to pH 2 with concentrated hydro-chloric acid. The samples were then extracted with tolueneto transfer the PCP from the aqueous phase to the organicphase for gas chromatograph (GC) analysis. An internalstandard calibration was performed using 2,4,6-tribromo-phenol. Internal standard was added to toluene beforeextraction. The toluene extracts were injected into the GCfor PCP analysis. Extraction efficiency of PCP into toluenewas greater than 98%. PCP analysis determined the totalconcentration of PCP species. Since PCP has a pKa of 4.7,when liquid samples were acidified to pH 2, all the PCP speciespresent in the sample were in the neutral (undissociated)form. Therefore, the reported PCP values include the totalPCP extracted from the soil (pentachlorophenol and penta-chlorophenolate, C6Cl5OH and C6Cl5O-).

PCP concentrations in liquid samples were analyzed witha J&W 30-m long, 0.53-mm i.d. DB-5 fused silica capillarycolumn (J&W Scientific, Folsom, California) using a 5890

Series II HP (Hewlett-Packard, Palo Alto, CA) GC with anelectron capture detector (ECD). Helium was the carrier gas,and a 95% argon-5% methane gas mixture was the makeupgas.

PCP Solubility. An equal volume of 100 mL of differentwater-ethanol mixtures was placed inside 160 mL glasshypovials. Excess PCP was added to each bottle at a PCP/ethanol mixing ratio of 0.63 g/g (50 g of PCP/78.9 g of neatethanol or 50 g of PCP/100 mL of ethanol). This PCP/ethanolratio of 0.63 was approximately 21% in excess of 0.52, thePCP/ethanol ratio reported by Bevenue and Beckman (20)for the solubility of PCP in neat ethanol at 30 °C (52 g ofPCP/100 g of ethanol). After shaking the bottles on a rotatingshaker at 18 rpm for 7 days at 24 °C, an excess of solid PCPremained in each bottle. The concentration of PCP in liquidwas determined with the GC.

Desorption Isotherms. Similar to sorption, reverse sorp-tion or desorption of HOCs from soils into solution may bedescribed by a linear partition equation

where qe (mg/kg) is the concentration of sorbate (solute)remaining on the soil at equilibrium, Ce (mg/L) is theequilibrium concentration of solute in the liquid, and Kp

(L/kg) is the sorption constant (sorption or partition coef-ficient). A nonlinear equation for describing sorption is theFreundlich (21) equation

where the Freundlich constants Kf and 1/n are empiricalconstants that may be related to the capacity or affinity ofthe sorbent for the sorbate and the intensity of sorption,respectively.

In this study, soil spiked with PCP was mixed with cleansolvent to determine the removal of the sorbate from the soilby the liquid (release of sorbate by the sorbent into thesolvent). In desorption isotherm experiments, the followingrange for the soil:solvent mass to volume contact ratio (g:mL) was used: 1:1 to 1:200. Different masses of PCP-spikedsoil were placed inside 160 mL glass bottles; solvent wasadded to each bottle using various soil:solvent contact ratios.The bottles were then capped and placed inside a rotatingshaker at 18 rpm. Desorption isotherm experiments wereconducted at 24 °C. After 3 weeks of shaking, the PCPconcentration in the liquid phase (Ce) was measured withthe GC. The values for qe, the concentration of PCP remainingon the soil after desorption (PCP sorbed onto the soil), werecalculated from the measured PCP concentrations in theliquid phase by mass balance: qe ) qi - Ce(V/M), where qi

is the initial PCP concentration in soil (mg/kg), V is the volumeof solution (L), and M is the soil mass (kg).

ResultsPCP Solubility. A log-linear solubility model proposed byRubino and Yalkowsky (5) for an exponential increase insolubility of HOCs in mixtures of water and water-misciblecosolvent is

where fc is the cosolvent volume fraction in the mixed solvent,σ is the cosolvency power of the solvent for the solute, andSm, Sc, and Sw are solubilities in mixed solvent, pure cosolvent,and water, respectively. The solubility of PCP in severalwater-ethanol mixtures is shown in Figure 1. A cosolvencypower (σ) of 4.75 was calculated from the measured solubili-

TABLE 2. Chemicals

chemical specification source

PCP 99% Aldrich Chemicals,Milwaukee, WI

2,4,6-tribromo-phenol

98% Aldrich Chemicals,Milwaukee, WI

ethanol 190 proof,USP grade

Midwest Grain Products,Weston, MO

ethanol 200 proof,USP grade

Midwest Grain Products,Weston, MO

acetone optima grade Fisher Chemicals,Fairlawn, NJ

qe ) KpCe (1)

qe ) KfCe(1/n) (2)

log Sm ) log Sw + σfc (3)

log(Sc/Sw) ) σ (4)

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ties of PCP in water and neat ethanol (eq 4). Linear regressionof the experimental solubility data determined a σ value of4.70 (R 2 ) 0.98), a 1% deviation from the σ given by thelog-linear solubility model (eq 3).

Deviation from the log-linear model has been observedfor HOC solubility in mixed solvent systems. Using water-ethanol mixed solvents and several organic solutes withdifferent structures, Rubino and Yalkowsky (6) observed apositive deviation from the log-linear solubility predictionfor the 0.6 volume fraction of ethanol in mixed solvent,attributing this deviation to cosolvent-water interactions.Similar deviations from log-linear solubility in water-ethanolmixed solvents were observed for several polycyclic aromaticcompounds (4, 22, 23).

Linear Analysis of Desorption Isotherm Data for EdisonSoil. Sorption constants were obtained from linear regressionof PCP desorption data. The linear regression of data resultsin an y-intercept for the line which is presented by

where qe (mg/kg) and Ce (mg/L) are the same as in eq 1, Km

(L/kg) is the linear sorption constant for the mixed solvent(similar to Kp), and I (mg/kg) represents the amount of PCPthat did not desorb from the soil.

The data from the desorption isotherms for the 100 × 140U.S. mesh soil with an initial PCP loading of 99 mg/kg arepresented in Figure 2. The data from Figure 2 indicate thatsolvents containing 0.56 and 0.79 fraction ethanol achievedthe highest PCP removals from the 100 × 140 U.S. mesh soil.Low PCP recoveries were obtained with water and neatethanol. The solvents containing 0.3 and 0.95 fraction ethanolproduced PCP recoveries between those yielded by waterand neat ethanol and those given by the solvents containing0.56 and 0.79 fraction ethanol. The PCP recovery levelsachieved with the 0.56 and 0.79 ethanol fractions in solventwere comparable to each other. Similar trends were observedfor the 20 × 40 and >200 U.S. mesh soil fractions. At highvolumes of mixed solvent (high soil:solvent contact ratio),

low concentrations of PCP were measured in the liquid,resulting in less precision in the measured sorbed mass ofPCP remaining on the soil (calculated from mass balance),reflected by the scatter of desorption data in Figure 2 for lowPCP concentrations.

The Km values determined from the slopes of the lines inFigure 2 are plotted in Figure 3a against the fraction of ethanolin mixed solvent in log-linear (semilog) fashion. The datapresented in Figure 3a show exponential behavior for thedesorption of PCP from soil with increasing fraction of ethanolin solvent from 0 to 0.56 fraction ethanol. PCP desorptiondecreased for soils in contact with solvents containing morethan 0.79 fraction ethanol. Smaller values of Km (lowersorption) were indicative of greater PCP desorption from thesoil. The results for 100 × 140 U.S. mesh soil indicate low Km

values for the 0.56 and 0.79 ethanol fractions, intermediateKm values for the 0.3 and 0.95 ethanol fractions, and high Km

values for water and neat ethanol. The linear sorptionconstants from Figure 3a show that the desorption of PCPfrom the three fractions of Edison soil followed in similarfashion with respect to the fraction of ethanol in mixedsolvent, where desorption from three fractions of Edison soiloccurred in descending order for soil particle size (U.S.mesh): 20 × 40, 100 × 140, >200.

The values for I in Table 5 show that between 5.4 and 23.3mg/kg PCP did not desorb from the 100 × 140 U.S. mesh soilfraction. Higher values of I were obtained for all three fractionsof Edison soil using neat ethanol (23-34 mg/kg) and for 20× 40 and >200 U.S. mesh soil fractions using water (23-30mg/kg).

Nonlinear Analysis of Desorption Data for Edison Soil.The PCP desorption data were analyzed using the Freundlichequation. The solid lines in Figure 4 represent the fit of thedesorption data to the Freundlich equation for the 100 × 140U.S. mesh Edison soil. Figure 5a shows the Freundlichconstant 1/n determined from the slopes of the solid linesin Figure 4. The 1/n values from Figure 5a indicate that thelowest sorption intensity of PCP for the 20 × 40 U.S. meshsoil was obtained with the 0.56 fraction ethanol, while the0.79 fraction ethanol gave the lowest sorption intensity ofPCP for the 100 × 140 and >200 U.S. mesh fractions. Thedata from Figure 5a indicate that for most mixed solvents,the desorption of PCP was greatest from the 20 × 40 U.S.mesh soil, the fraction with the largest particle size. The valuesfor Freundlich constant Kf determined from the data in Figure5a are listed in Table 6; higher Kf values were obtained forwater and neat ethanol, and the lowest value was obtainedfor the mixed solvent containing 0.79 fraction ethanol.

Analysis of Desorption Data for Other Soils. The PCPdesorption isotherm data for the K-10 montmorillonite clayand 20 × 30 U.S. mesh Ottawa sand show an increase indesorption of PCP from soil with increasing fraction of ethanolin mixed solvent. The linear sorption constants are presentedin Figure 3b. The data for the >200 U.S. mesh Edison soil areincluded for comparison. The desorption data presented inFigure 3b demonstrate a rapid increase in desorption of PCPfrom K-10 montmorillonite for solvents containing from 0.3to 0.56 fraction ethanol, then a more sluggish increase in

TABLE 3. Properties of Soils

sourcePCP spike

(mg/kg) pHborganic matterc

(mass %)BET specific

surface aread (m2/g)specific pore

volumed (cm3/g)

20 × 40 U.S. mesh Edisona U.S. EPA 99 8.23 1.2 7.9 0.0182100 × 140 U.S. mesh Edison U.S. EPA 99 8.40 0.7 8.3 0.0217>200 U.S. mesh Edison U.S. EPA 99 8.31 1.1 17.4 0.0348K-10 montmorillonite Aldrich 84 3.26 <0.1 221.6 0.295620 × 30 U.S. mesh Ottawa sand Fisher 33 8.59 <0.1 0.07

a U.S. standard sieves. b ASTM method D2974-87 (17). c ASTM method D4972-89 (18). d See Experimental Section.

FIGURE 1. Solubility of PCP as a function of fraction of ethanol inwater-ethanol mixed solvents.

qe ) KmCe + I (5)

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PCP desorption from 0.56 to 0.95 fraction ethanol, and finallya decrease in desorption with neat ethanol. The data fromFigure 3b indicate an increase in desorption of PCP fromOttawa sand for an increase from 0 to 0.56 fraction ethanolin mixed solvent. For K-10 montmorillonite, desorption ofPCP decreased with neat ethanol, while for Ottawa sand,PCP desorption did not change significantly from 0.56 to 1.0fraction ethanol in mixed solvent. Sorption constants forwater were the greatest. The PCP sorption constant for thevarious soils ranged from 0.02 to 0.2 L/kg for 0.56 fractionethanol in mixed solvent.

The values of the Freundlich constant 1/n determinedfrom the desorption data for other soils are presented inFigure 5b. The data for the >200 U.S. mesh soil are shownfor comparison. These data indicate that for Ottawa sand,the highest desorption intensity was given by the mixedsolvent containing 0.56 fraction ethanol, while for K-10montmorillonite, the desorption intensity decreased withincreasing fraction of ethanol in mixed solvent.

Solution pH. The pH of mixed solvent was measuredbefore and after the experiments at several soil:solvent ratiosfor solvents used in the desorption isotherm experiments.The data presented in Figure 6 (parts a and b) show the pH

of the mixed solvents in contact with clean and PCP-spikedEdison soil, respectively. The pH data for the clean soilindicate that the soil pH (Table 3) affected the solvent pHafter soil-solvent contact for 0, 0.3, 0.56, and 0.79 ethanolfractions in mixed solvent. With a higher fraction of ethanolin mixed solvent, the solvent pH was less dependent on thesoil pH (lower solution pH values). A similar trend wasobserved for the pH data for spiked soils presented in Figure6b; the slightly higher pH values were possibly effected bycontact between soil and acetone during the spiking of soilwith PCP. The solution pH values for K-10 montmorillonitelisted in Table 4 show that the pH of solution was less than4.7 for all mixed solvents.

DiscussionLinear and Freundlich Sorption Constants. The linearsorption constants for Edison soil shown in Figure 3a indicatean increase in desorption from all three fractions of Edisonsoil with increasing fraction of ethanol for fc from 0 to 0.79followed by a decrease in desorption from all three fractionsof Edison soil for fc g 0.95. The Freundlich constants 1/n forEdison soil shown in Figure 5a indicate an increase indesorption for fc from 0 to 0.3 fraction ethanol before a

FIGURE 2. Desorption of PCP from 100 × 140 U.S. mesh Edison soil: linear isotherms.

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decrease at 0.56 fraction ethanol. A comparison of the linearand nonlinear sorption constants from Figure 3a and 5ashows that, for linear isotherms, highest desorption occurredat 0.79 fraction ethanol for all three soil fractions, while fornonlinear isotherms, highest desorption occurred at 0.56fraction ethanol for 20 × 40 U.S. mesh soil and at 0.79 fractionethanol for 100 × 140 and >200 U.S. mesh fractions. The

linear sorption constants show a similar trend for desorptionfrom the three soil fractions in descending order of soilparticle size: 20 × 40, 100 × 140, >200 U.S. mesh, whereasthe nonlinear sorption constants do not indicate a similartrend for soil particle size although showing that highestdesorption occurred for the 20 × 40 U.S. mesh fraction. Thelinear and nonlinear desorption parameters for Edison soil,listed in Tables 5 and 6, respectively, show higher regressioncoefficients in regression of linear isotherm desorption data.

For K-10 montmorillonite, the linear sorption constantsfrom Figure 3b show an increase in desorption with increasingfraction of ethanol from 0 to 0.95 fraction ethanol in mixedsolvent, whereas the Freundlich sorption constants fromFigure 5b do not show an increase in desorption withincreasing fraction of ethanol. Similar results were obtainedfor Ottawa sand. The linear and Freundlich sorption constantsfor Edison soil shown in Figures 3b and 5b, respectively, aregreater than the linear and Freundlich sorption constantsfor K-10 montmorillonite and Ottawa sand. In Figure 3b, forfc g 0.59, sorption constants are higher for Edison soil thanfor K-10 montmorillonite and Ottawa, indicating lowerdesorption of PCP from Edison soil than from the other soils.Lower desorption of PCP from Edison soil may be attributedto the organic matter present in the soil, since K-10montmorillonite and Ottawa sand had negligible organicmatter. The nonlinear constants in Figure 5b indicate that,for soils with very low organic matter content, the Freundlichconstant 1/n does not show a trend in the desorption of PCPwith respect to the fraction of ethanol in mixed solvent. ForK-10 montmorillonite and Ottawa sand, soils with very loworganic matter content, the data in Table 3 show a differenceof 3 orders of magnitude in BET specific surface area. Thelinear and Freundlich sorption constants presented in Figures3b and 5b, respectively, do not indicate a significant differencein desorption of PCP from K-10 montmorillonite and Ottawasand, showing that desorption was not affected by thedifference in soil surface area for soils with very low organicmatter.

Solution pH. In octanol-water distribution of PCP,Westall et al. (24) and Jafvert et al. (25) observed appreciablehydrophobicity of dissociated PCP. Schellenburgh et al. (26)evidenced that the sorption of pentachlorophenolate ontonatural sorbents was predominantly a partitioning processbetween the aqueous and organic phases. Jafvert (27) showedthat the sorption of pentachlorophenolate to natural sorbentswas predominantly due to hydrophobic interactions betweensolute and sorbent, modified by electrostatic interactionsdue to the changes in soil-solution pH. Miller and Faust (28)observed that PCP sorption onto an organo-clay from asolution containing 0.15 fraction of ethanol was greatest (closeto 100%) when solution pH was between 3 and 4.

In water-methanol mixtures, Lee et al. (29) indicated thatthe pKa of PCP increased with increasing fraction of methanolin mixed solvent from 4.7 in water to 8.6 in neat methanol,an alkaline shift in the organic acid dissociation constant ina solvent with low dielectric constant. Although conditionalpKa values for PCP in water-ethanol mixtures were notdetermined in this study, the conditional pKa values of PCPare expected to increase with higher fraction of ethanol inmixed solvent, similar to water-methanol mixtures. HigherpKa values result in lower dissociation of PCP at highercosolvent fractions. The pH data from Figure 6 indicate thatdesorption of PCP in water occurred primarily as dissociatedPCP, pentachlorophenolate. Due to the expected increase inpKa for PCP in mixed solvents with increasing fraction ofethanol, the solution pH values from Figure 6 indicate thatPCP desorbed from Edison soil as both pentachlorothan-phenolate and neutral PCP in mixed solvents with fc g 0.3,with more neutral PCP at higher fc. The pH of mixed solventsin contact with K-10 montmorillonite and Ottawa sand was

FIGURE 3. Linear sorption constant Km in desorption of PCP fromseveral soils.

TABLE 4. Solution pH in Extraction of PCP from K-10Montmorillonite

soil:solvent ratio (g:mL)

volume fraction ofethanol in solvent, fc 1:10 1:100 clean solvent

0 3.61 4.24 5.740.3 3.71 4.22 5.840.56 3.89 4.56 6.210.79 3.56 4.24 6.430.95 3.08 3.84 7.04

TABLE 5. Linear Analysis for Desorption of PCP from 100 ×140 U.S. Mesh Edison Soil

volume fractionof ethanol in

mixed solvent, fc

linear sorptionconstant, Km (L/kg)

nonreversiblesorption I(mg/kg)

regressioncoeff (%)

0 3.9 10.22 80.20.3 0.43 6.1 95.70.56 0.13 5.4 53.90.79 0.08 12.9 62.10.95 0.28 10.6 81.21 1.43 23.3 79.9

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less than 4.3 and 8.9, respectively, indicating that thedesorption of PCP from K-10 montmorillonite occurredprimarily in the form of neutral PCP, whereas PCP desorbedfrom Ottawa sand primarily in the form of dissociated PCP.

Sorption Constant Km. Rao et al. (12) and Fu and Luthy(13) derived expressions for predicting the effect of a misciblecosolvent on sorption of HOCs by soil from solvent-watermixtures by relating the sorption constant and the organiccarbon dependency of sorption to the log-linear solubilityrelationship to obtain an equation of the form

where Km and Kw are sorption (partition) constants in mixedsolvent and water, respectively, and R is an empirical constantwhich accounts for solvent-sorbent interactions. When R )1, the sorption of HOCs decreases log-linearly with thesolubility of the solute in mixed solvent, independent of anysorbent-solvent interactions.

The slopes (Rσ) of the log-linear plots presented in Figure3 were calculated from fc ) 0.0 (water) to fc ) 0.56 (50%ethanol by mass) for all soils. The R values listed in Table 7,which were determined from eq 4 using a σ value of 4.747,range from 0.56 to 0.68 for Edison soil and from 0.56 to 0.85for all soils. The deviation of R from 1 indicates that, although

desorption of PCP from soil into mixtures of water-ethanolwas primarily solubility limited, sorbent-solvent interactionsalso affected the desorption process. The value of R obtainedfor K-10 montmorillonite, 0.85, was closest to 1.0. Since theorganic matter content of K-10 montmorillonite was verylow, most of the PCP was adsorbed on the surface of the clay.Since the pH for K-10 montmorillonite was 3.26, the majorityof the sorbed PCP on the surface of soil was undissociatedPCP. In the absence of significant sorbent-solvent interactionsaffecting the desorption process, the desorption of neutralPCP increased with fraction of ethanol in mixed solvent.

In sorption isotherm experiments using a Webster siltyclay loam with 3% organic carbon content, Lee et al. (29)found that the sorption of PCP onto soil from water-methanol mixed solvents was linear. Similar to the resultsfrom this study presented in Figure 3a, they observed a log-linear decrease in Km with increasing fraction of methanol,reaching a minimum at 0.9 volume fraction methanol beforeincreasing in neat methanol. The Rσ values for Edison soillisted in Table 6 are in the range of Rσ values reported by Leeet al. (14, 29) of 3.88 and 2.56 for sorption of neutral andanionic species of PCP, respectively.

In sorption of several HOCs onto a soil with 1.94% organiccarbon content using water-methanol and water-acetonemixtures, Fu and Luthy (13) obtained R values between 0.44

FIGURE 4. Desorption of PCP from 100 × 140 U.S. mesh Edison soil: Freundlich isotherms.

log Km ) log Kw - Rσfc (6)

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and 0.57, attributing this deviation to the hydrophobicity ofthe solute and sorbent-solvent interaction. They observedthat the more hydrophobic solute effected a smaller deviationfrom the relationship between log-linear sorption and log-linear solubility (R ) 1).

Nonreversible Sorption. The amount of sorbate remain-ing on the sorbent after the desorption or extraction processhas been called the irreversible or nonreversible sorption (Iin eq 4). Desorption is influenced by the interactionsoccurring between solute and sorbent during the previoussorption process. In spiking of all soils with PCP, the durationof the sorption process was 3 weeks. For Edison soils, thespiking process involved primarily the sorption of PCP fromthe acetone phase into soil organic matter. During contactbetween acetone and soil organic matter for several weeks,PCP had the opportunity to penetrate the soil organic mattermatrix. In this study, for all mixed solvents, a fraction of PCPwas nonextractable from PCP spiked soils. The nonextractablefraction of sorbed PCP, not available to solvent, was retained

in the soil organic matter by sorptive forces other thanphysical or hydrophobic forces.

Issacson and Frink (30) reported nonreversible sorptionof phenol, 2-chlorophenol, and 2,4-dichlorophenol on soils,observing hysteresis in the desorption of solute between thesorption and desorption experiments using aqueous solu-tions. Their desorption data were obtained under nonequi-librium conditions using the lowest flow rates. They attributedthe nonreversible sorption to the previous sorption process(prior to desorption) based on specific sorbate-sorbentinteractions. In batch desorption experiments using a soilwith high organic matter content (20-30%), Warith et al.(31) obtained a 20-90% irreversible sorption of PCP on soilin contact with water. Among other chlorophenols sorbedon several soils with synthetic and natural organic matter,Lagas (32) found a fraction of the sorbed PCP to benonextractable with hexane at pH 9.3. Banerji and Wei (33)observed 27 to 52% nonreversible sorption during recoveryof PCP from several soils loaded with 500 mg/kg PCP using

FIGURE 5. Freundlich constant 1/n in desorption of PCP from severalsoils.

TABLE 6. Nonlinear Analysis for Desorption of PCP from 100× 140 U.S. Mesh Edison Soil

volume fraction ofethanol in mixed

solvent, fc

Freundlichconstant, 1/n

Freundlichconstant, Kf

regressioncoeff (%)

0 0.46 3.37 87.80.3 0.34 2.23 66.80.56 0.49 1.48 70.00.79 0.06 3.08 24.30.95 0.28 2.59 46.51 0.39 3.39 68.6

FIGURE 6. Solution pH in extraction of 20 × 40 U.S. mesh Edisonsoil.

TABLE 7. Sorbent-Solvent Interaction Parameters

rσ r

(a) Edison Soil20 × 40a 3.25 0.68100 × 140a 2.67 0.56>200a 3.12 0.66

(b) Other SoilsK-10 montmorillonite 4.05 0.8520 × 30 U.S. mesh Ottawa sand 3.09 0.65

a U.S. mesh size.

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successive extractions with water and 2-propanol. In contrast,in sorption-desorption isotherm experiments using aqueoussolutions, Bellin et al. (34) observed that sorption of PCP inseveral alkaline soils with organic carbon contents from 0.12to 0.95% was completely reversible.

Sorbent-Solvent and Solute-Sorbent Interactions. Forsoils with very low organic matter, a sharp decrease indesorption did not occur for richer ethanol solutions (fc g0.95). For K-10 montmorillonite, desorption decreased slightlyin neat ethanol, while for Ottawa sand, no significant decreasein desorption occurred for fc > 0.56. For Edison soil, the datapresented in Figure 3a show an exponential increase in PCPdesorption with increasing solubility of PCP in mixed solventswith higher fractions of ethanol up to 0.79 ethanol fractionand an exponential decrease in desorption of PCP in solventswith 0.95 and 1.0 fraction ethanol.

The PCP desorption in ethanol solutions with fc g 0.95was not solubility limited (solute-solvent interaction) butwas influenced by sorbent-solvent interactions. The ethanol-rich solvents (fc g 0.95) effected a lower desorption of PCPfrom soil despite greater solubilities of PCP in the solvents.The pH data from Figure 6 show that, for fc ) 0.95, the pHvalues for the solution before and after contact with Edisonsoil were similar. Although no pH reading was taken in neatethanol, the pH data for fc ) 0.95 indicate that soil pH wouldnot effect a significant sorbent-solvent interaction for solu-tions with fc g 0.95.

For K-10 montmorillonite, the decrease in PCP desorptionwith neat ethanol may be attributed to sorbent-solventinteractions. Although the soil surface area and soil porevolume of K-10 montmorillonite are approximately 1 orderof magnitude greater than those of >200 U.S. mesh Edisonsoil (Table 3), the desorption data do not indicate the relativedecrease in desorption from K-10 montmorillonite in mixedsolvent with fc g 0.95. The desorption data from Figure 3bsuggest that sorbent-solvent interactions occurring for Edisonsoil were due to soil organic matter rather than the soil surfacearea.

The soil property data for Edison soil in Table 3 show thatthe specific surface area and pore volume for the smallestsoil fraction (>200 U.S. mesh) were approximately twice asgreat as the values for the largest soil fraction (20 × 40 U.S.mesh). The specific surface area and pore volume dataindicate that there were more sorption-desorption sitesavailable in the >200 U.S. mesh fraction than in the 20 × 40U.S. mesh fraction. For an equal initial concentration ofsorbed PCP in different fractions of Edison soil, the desorptionconstants from Figure 3a indicate lower desorption withincreasing number of sorption-desorption sites.

Lower desorption of PCP from Edison soil in solvents withfc g 0.95 was primarily due to the effect of hydrophobicsolvents on soil organic matter, a sorbent-solvent interaction.Freeman and Cheung (35) suggested that desorption of HOCsfrom soils and sediments in solvent was affected by theswelling of the three-dimensional matrix of the nonpolarpolymeric organic matter into a gel when organic matterabsorbed the solvent. They extracted less HOC from the gel-like organic matter with methanol than with hexane ordichloromethane, attributing higher desorption to the greaterswelling of the gel. They suggested maximum swelling (andHOC desorption) depended on the respective polarities ofthe solvent and the gel.

Solute preference for the solvent, the soil organic matter,or the soil mineral surface depends on competing attractiveforces. Major attractive sorption forces other than hydro-phobic interaction include hydrogen bonding, van der Waals-London forces, and ion exchange. Boyd (36) suggestedhydrogen-bonding between the hydroxyl group of monochlo-

rophenol and the soil surface groups, extending this capabilityto higher chlorophenols. Choi and Aomine (37) reportedadsorption of PCP in the dissociated form onto severalalkaline clays and adsorption of neutral PCP to humus. Nose(38) suggested that the electrophilic chlorines on the aromaticring of PCP induced a partial positive charge on the hydroxylgroup of PCP, leading to adsorption of PCP in the neutralform onto soil surfaces with negative charge. The incorpora-tion of chlorophenols into soil organic matter throughoxidative coupling of the phenolic hydroxyl group im-mobilizes chlorophenols in soil. Bollag and Liu (39) showedthe copolymerization of PCP into polymeric quinoid andphenolic oligomers using a model humic component. Usingradiolabeled 14C-PCP in soil, Weiss et al. (40) recovered 28.6%of the initial radioactivity in soil in the form of unextractablesoil residues containing humic and fulvic acids. Besidesincorporation into soil organic matter, the sorbed HOCs canbecome physically trapped inside the organic carbon matrixby clustering, formation of hydrogen and covalent bondsinside the organic matter matrix (41).

ConclusionsMixtures of water and ethanol ranging from 0.59 to 0.95ethanol fraction in mixed solvent were effective in desorbingPCP from synthetic and natural soils. For all soils, linearanalysis of desorption data showed a log-linear increase indesorption with increasing fraction of ethanol in mixedsolvent. The desorption of PCP increased exponentially upto 0.79, 0.95, and 0.56 ethanol fractions in mixed solvent forEdison soil, K-10 montmorillonite, and Ottawa sand, re-spectively. In concentrated ethanol solutions, desorption ofPCP from Edison soil was affected by sorbent-solventinteractions due to the effect of hydrophobic solvent on soilorganic matter. For Edison soil, desorption increased withincreasing soil particle size.

AcknowledgmentsThe funding for this research was provided in part by theNational Risk Management Research Laboratory of the U.S.Environmental Protection Agency in Cincinnati, OH. Thesoil porosity measurements were performed by GenovevaBuelna in the Department of Chemical Engineering at theUniversity of Cincinnati.

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Received for review July 24, 1998. Revised manuscript re-ceived July 20, 1999. Accepted October 1, 1999.

ES980759Z

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