particle-size dependent sorption and desorption of pesticides within a water−soil−nonionic...

Particle-Size Dependent Sorptionand Desorption of Pesticides withina Water-Soil-Nonionic SurfactantSystemP E N G W A N G A N D A R T U R O A . K E L L E R *

Bren School of Environmental Science and Management,University of California, Santa Barbara, 93106

Received October 30, 2007. Revised manuscript receivedFebruary 17, 2008. Accepted February 20, 2008.

Although nonionic surfactants have been considered insurfactant-aided soil washing systems, there is little informationon the particle-size dependence of these processes, and thismay have significant implications for the design of these systems.In this study, Triton-100 (TX) was selected to study its effecton the sorption and desorption of two pesticides (Atrazine andDiuron) from different primary soil size fractions (clay, silt,and sand fractions) under equilibrium sorption and sequentialdesorption. Soil properties, TX sorption, and pesticide sorptionand desorption all exhibited significant particle-size dependence.The cation exchange capacity (CEC) of the bulk soils and thesoil fractions determined TX sorption capacity, which inturn determined the desorption efficiency. Desorption ofpesticideoutof theclayfraction is the limitingfactor inasurfactant-aided washing system. The solubilization efficiency of theindividual surfactant micelles decreased as the amount ofsurfactant added to the systems increased. Thus, instead ofattempting to wash the bulk soil, a better strategy might be toeither (1) use only the amount of surfactant that is sufficientto clean the coarse fraction, then separate the fine fraction, anddispose or treat it separately; or (2) to separate the coarsefractions mechanically and then treat the coarse and fine fractionsseparately. These results may be applicable to many otherhydrophobic organic compounds such as polyaromatichydrocarbons (PAHs) and polychlorinated biphenyls (PCBs)strongly sorbed onto soils and sediments.

1. IntroductionPesticide spills and accidents involving pesticide handlingtake place each year on farms and pesticide formulating andmanufacturing plants resulting in high pesticide concentra-tion at these sites (1). There are many other situations wheresoils or sediments are contaminated with hydrophobicorganic compounds (HOCs) that are strongly sorbed andnot bioavailable for natural or enhanced biodegradation. Theability of surfactants to enhance the water solubility of HOCsprovides a potential means of remediating pesticide-contaminated soils and sediments by surfactant-aided soilwashing (2, 3).

Loss of anionic surfactants (e.g., linear alkylbenzenesulfonate (LAS) and sodium dodecyl sulfonate (SDS)) bycomplexation with divalent cations in soils (e.g., Ca2+, Mg2+)can be so significant that the use of anionic surfactants for

remediating contaminated soils rich in divalent cations istypically ineffective (4, 5). Cationic surfactants, due to theirpositive charge, tend to significantly sorb onto the soilparticles via cation exchange (6, 7), resulting in significantsurfactant loss. Thus, the use of nonionic surfactants for soilwashing has received considerable research attention (e.g.,refs 1, 2, 8–10,). However, nonionic surfactants can also sorbonto soil to some extent (2, 8–10). Surfactant loss via sorptiononto soils results in reduced solubilization of HOCs and,more importantly, the sorbed surfactants can serve as aneffective partitioning medium for the HOCs (3, 6, 7, 11) whichfurther complicates the sorption and desorption behavior ofthe HOC within these systems. Thus, understanding sur-factant sorption is critical in understanding the mechanismby which HOCs can be removed within surfactant-aided soilwashing systems.

Most soil washing processes remove contaminants fromsoils either by dissolving them in a washing solution and/orconcentrating them into a smaller amount of soil throughparticle size separation (12, 13). By separating the fine fromthe coarser particles, the soil washing process effectivelyconcentrates the contaminants into a smaller amount of soilthat can be further treated or disposed of. Therefore,understanding soil particle-size dependent surfactant sorp-tion behavior and its effect on HOC partitioning among thesize fractions is highly relevant to soil washing.

Although HOC sorption onto different soil size fractionshas received considerable attention in the literature (14) therehas been little work on understanding the effect of addingsurfactants to these pesticide-soil systems (3, 15, 16). To ourknowledge, the sorption and desorption of HOC onto andfrom primary soil size fractions (i.e., clay, silt, and sand sizefractions) in the presence of surfactants has not been reportedelsewhere.

Equilibrium HOC sorption experiments within soil–wa-ter–surfactant systems have been conducted by numerousresearchers (e.g., refs 9, 10, and 17,), but there have beenrelatively few desorption experiments reported (1, 8, 18).Although the results of equilibrium sorption experimentscan shed valuable insights into the desorption behavior ofHOC within surfactant-aided soil washing systems, there arestill gaps in understanding the parameters that control HOCdesorption from the different size fractions. Also, it isuncertain whether the results of equilibrium sorptionexperiments can be used for predicting HOC desorption ina soil washing system.

The nonionic surfactant Triton-100 (TX) was consideredin this study since it has been intensively studied withinsurfactant-aided soil washing systems (e.g., refs 1, 3, 8, 10,16–19,) in the past few years. This project focused on exploringthe effect of TX on the sorption and desorption of twopesticides (Atrazine and Diuron), as an example of a moregeneral application for removing HOCs from contaminatedsoils by ex situ surfactant-aided soil washing. More specif-ically, the objectives of the study were (1) to investigateequilibrium soil particle-size dependent surfactant sorptionand pesticide partitioning behavior and (2) to examineparticle-size dependent pesticide desorption behavior undersequential desorption.

2. Materials and Methods2.1. Chemicals. Atrazine (2-chloro-4-ethylamino-6-isopro-pylamino-1,3,5-triazine) was purchased from Supelco Inc.(Bellefonte, PA) with a reported purity >97%, and Diuron(3-(3,4-dichlorofenyl)-1,1-dimethylurea) was purchased fromChemService Inc. (West Chestnut, PA) with a reported purity

* Corresponding author phone: 01-805-453-1822; fax: 01-805-456-3807; E-mail: [email protected].

Environ. Sci. Technol. 2008, 42, 3381–3387

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Published on Web 04/02/2008

>99%. Triton-100 (t-octylphenoxypolyethoxyethanol) waspurchased from Sigma-Aldrich (St. Louis, MO). The chemicalswere used as received. Selected physicochemical propertiesof these compounds can be found in the SupportingInformation.

2.2. Bulk Soils and Soil Particle Size Fractions. Foursoils and one sediment (denoted as Ag#1, Ag#2, Ag#3, Clayey,and Sediment) were collected from Santa Barbara, California.The water dispersible clay (<2 µm), silt (2∼50 µm), and sand(>50 µm) fractions were separated using a low energy method,which involved using only water as dispersant, gentle mixing,repeated wet sedimentation, dialysis desalination, and freeze-drying. The details of the particle size separation can be foundin the Supporting Information. The organic carbon (OC),cation exchange capacity (CEC), and pH of the soils weremeasured using the standard methods described by Carter(20). BET surface area (SA) measurement was conducted ona TriStar 3000 gas adsorption analyzer (Micromeritics Inc.,Norcross, GA) using N2.

2.3. Equilibrium Sorption of Pesticide and TX. Thesorption experiments were conducted in duplicate by thebatch equilibration technique. The TX sorption isotherm wasdetermined in the absence of the pesticides. Deionized (DI)water was used for preparing all aqueous solutions. Pesticidepartitioning was determined in the absence and presence ofTX. Pesticide concentrations used were 15.00 mg/L forAtrazine and 15.95 mg/L for Diuron. Initial TX concentrationspanned over a large range (0∼20 g/L), below and above itscritical micelle concentration (CMC ) 0.12 g/L). A 0.01 MCaCl2 background electrolyte was used to minimize changein ionic strength, and 0.02% NaN3 was used as microbialgrowth inhibitor in all cases. Aliquots of 2.00 g of a bulk soil,0.30 g clay fraction, 0.70 g silt fraction, or 1.00 g sand fractionwere treated with 10 mL of solution containing pesticide andsurfactant with varying concentrations in 15 mL glasscentrifuge tubes. The amount used for each particle sizefraction was based on its average weight fraction within thebulk soil.

The tubes with soil and the pesticide and/or surfactantsolution were then rotated at 60 rpm for 24 h to reach sorptionequilibrium in an end-over-end shaker at 22 ( 2 °C, andthen centrifuged at 5000g for 30 min at the same temperature.A number of researchers have used 24 h as a mixing periodfor studying the partitioning of HOCs within soil–water-–surfactant systems. For example, Li and Bowman (6),Hayworth and Burris (7), Sanchez-Camazano et al. (8), Shenget al. (11), Edwards et al. (17), Rodriguez-Cruz et al. (18), Koet al. (21), Zhu et al. (22), Park and Jaffe (23), Sun et al. (24),Kibbey and Hayes (25), and Abu-Zreig et al. (27), all foundthat the sorption equilibrium of the HOC and surfactant(cationic, anionic, or nonionic) within soil–water–surfactantsystems was reached within a 24 h mixing period. Additionalresults from this study are provided in the SupportingInformation, and they indicate that 24 h are more thansufficient to reach equilibrium sorption for both pesticideand surfactant.

Pesticide and surfactant concentrations in the supernatantwere analyzed using high performance liquid chromatog-raphy (HPLC). The sorption of pesticide and surfactant onthe centrifuge tubes was determined to be negligible, andthe amount of the pesticide and surfactant blank (with nosoils) did not show any significant change before and aftermixing. Thus, the amount of pesticide and surfactant sorbedwas calculated as the difference between the initial and finalmass in the aqueous phase.

2.4. Pesticide Desorption in the Presence of TX. Pesticidedesorption experiments were conducted using the sameexperimental procedure as for the sorption experimentsexcept that the desorption experiments consisted of one

TABLE 1. Measured Soil Properties

soil weight % OC (%)CEC

(cmol/kg)BET surfacearea (m2/g) pH

Ag #1 (sandyloam)

bulk 100% 1.51 6.20 3.5 7.3clay 5.3% 4.95 40.2 30.8 6.7silt 17.1% 1.82 13.0 4.5 8.5sand 77.6% 0.50 3.0 1.3 8.3Ag #2 (loam)bulk 100% 1.50 15.2 9.4 7.4clay 6.8% 4.36 59.0 53.1 7.2silt 47.6% 1.29 16.0 8.4 8.4sand 45.6% 0.15 6.0 1.4 8.3Ag #3 (loam)bulk 100% 1.52 15.4 14.1 7.6clay 13.8% 4.50 54.4 63.1 7.9silt 41.0% 1.13 19.0 8.0 8.2sand 45.2% 0.11 3.0 2.1 8.2clayey (sandy

clay loam)bulk 100% 1.37 15.7 13.1 7.1clay 14.0% 1.80 50.4 71.0 7.5silt 28.4% 0.66 18.0 7.0 6.9sand 57.6% 0.50 8.0 1.0 8.5sediment

(loamy sand)bulk 100% 1.12 5.4 2.0 8.1clay 2.7% 6.02 42.2 36.6 6.4silt 21.1% 1.28 13.0 4.5 8.6sand 76.2% 0.27 3.0 0.9 8.5

FIGURE 1. TX sorption isotherms onto (a) clay fractions and (b)silt fractions. Note: The x axis presents the equilibrium TXaqueous concentration.

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sorption step followed by five consecutive desorption steps.Only the bulk soils and their clay and silt size fractions wereused for the desorption experiments. During the sorptioncycle, aliquots of 2.00 g of a bulk soil, 0.30 g clay fraction, or0.70 g silt fraction were treated with 10 mL of a solutioncontaining only pesticide in 15 mL glass centrifuge tubes.The concentrations were 15.00 mg/L for Atrazine and 15.95mg/L for Diuron in all cases. During each desorption step,5 mL of the supernatant were replaced with 5 mL of surfactantsolution. The concentration of these 5 mL of surfactantsolution was adjusted so that the 10 mL of supernatant wouldbe at the desired surfactant concentration for each case. ThreeTX concentrations were employed: 1.00, 2.00, and 3.00 g/L.The blank experiments were conducted with water onlycontaining the corresponding background electrolytes. Nosignificant change in pH was observed before and after thesorption/desorption experiments.

2.5. HPLC Analysis. A Shimadzu HPLC system (Shi-madzu, Nakagyo-ku, Kyoto, Japan) was equipped with twoLC-10AT VP pumps, a Sil-10AF autosampler, a DGU-14Adegasser, and a SPD-M10AVP diode array detector. AShimadzu Premier C18 5 µ reverse phase column was usedwith a length of 250 mm and an inner diameter of 4.6 mm.The analyses were performed at a constant flow rate of 1.0mL/min. The solvent concentration gradients were designedso that all the homologues of the same surfactant elutedunder the same peak. The UV detector monitored theabsorbance at 222 nm for Atrazine, 247 nm for Diuron, and225 nm for TX. Some samples were diluted as needed. Thecalibration was conducted daily and R2 was greater than 0.98in all cases. The retention time of Atrazine, Diuron, and TXwas 5.3, 5.6, and 8.0 min, respectively.

3. Results and Discussion3.1. Characterization of the Bulk Soils and Their SizeFractions. The measured properties of the bulk soils andtheir size fractions are presented in Table 1. Generallyspeaking, the soil properties were highly particle-size de-pendent. As the size of the fractions decreased, their OC, SA,and CEC increased. The clay fractions had consistently higher

OC, SA, and CEC than the bulk soils and the silt and sandfractions. The correlation coefficient between OC and CECwas 0.83 and that between OC and SA was 0.72 (c.f. SupportingInformation).

3.2. TX Sorption Isotherms. For all soils and soil fractions,TX sorption first increased with increasing aqueous surfactantconcentrations until a saturation sorption capacity wasreached, which is consistent with other researchers (1, 3, 16).Figure 1 presents the TX sorption isotherms onto the clayand silt fractions as examples. The average of duplicatemeasurements was used in preparing these graphs. Thestandard errors were smaller than 10% of the averages; errorbars are all less than the symbol size and thus are notpresented. The TX saturation sorption capacities, definedhere as the average of the plateau points in the TX sorptionisotherms, can be found in Table 2.

The analysis indicated that the sorption capacity for TXin different soil fractions was highly related to CEC with acorrelation coefficient of 0.90, whereas the correlationcoefficient between OC and TX sorption capacities was low(0.43), indicating that soil organic matter (SOM) was not thedominant phase for TX sorption, which is consistent withother studies focused on bulk soil behavior (1, 27). However,the strong correlation between TX sorption capacity and CEChas not been reported before. Unlike cationic surfactantsorption, which takes place via cation exchange reaction (6, 7),the sorption of nonionic surfactant is less clear. Edward etal. (17) proposed a three-stage sorption model for TX sorptiononto sediment, which states that at high bulk solutionsurfactant concentrations, a patchy bilayer of the sorbed TXis formed, with the base layer consisting of the “head-on”sorbed TX held on the hydrophilic (i.e., charged) patches ofthe sediment surface. The high correlation between TXsorption capacities and CEC in this study seems to supportthis hypothesis. Since CEC and surface area (SA) are stronglycorrelated, the sorption capacity of TX is also highly relatedto SA, with a correlation coefficient of 0.93. The correlationcoefficient between the TX sorption capacity for bulk soilsand their clay content was 0.83.

TABLE 2. TX sorption Capacities in Different Soils and Soil Fractions, Amount of Pesticide Presorbed before the DesorptionExperiments, And Break-Even Concentrationsa

soils Diuron Atrazine

TX sorption capacity (mg/kg) presorbed (mg/kg) break-even conc. (g/L) presorbed (mg/kg) break-even conc. (g/L)

bulk

Ag#1 4,100 ( 340 27.4 ( 1.2 3.1 ( 0.4 12.8 ( 0.9 5.2 ( 0.5Ag#2 28,000 ( 940 28.5 ( 1.9 9.3 ( 0.5 14.0 ( 0.8 10.7 ( 0.6Ag#3 28,000 ( 1640 33.7 ( 1.2 9.0 ( 0.7 15.0 ( 0.9 11.0 ( 0.8clayey 31,000 ( 2800 24.6 ( 1.0 9.3 ( 0.5 13.8 ( 0.7 10.5 ( 0.5sediment 3,700 ( 310 18.3 ( 0.9 3.0 ( 0.4 10.9 ( 0.5 5.9 ( 0.4

clay


silt


sand


a Note: data were presented as average ( standard error of the duplicate measurements.

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3.3. EquilibriumPesticideSorptionwithinSoil–Water-TXSystems. Figure 2presents the relationship between equi-librium aqueous Diuron concentrations and aqueous TXconcentrations for the various soil fractions (Figure 2a-c) oftwo bulk soils (Ag#1 and Ag#3 in Figure 2d). In all cases, thestandard errors were within 15% of the averages of theduplicates, so the error bars are presented only for Ag#1 andAg#3 for clarity. At first the aqueous pesticide concentration(i.e., the sum of water-dissolved concentration plus theconcentration present in the micelles) decreased withincreasing aqueous surfactant concentration before the CMCwas reached, due to sorption of surfactant onto the soilparticles and subsequent partitioning of the pesticide intothe sorbed surfactant. After the CMC was reached, theaqueous pesticide concentration increased with increasingaqueous surfactant concentration due to the presence ofsurfactant micelles. Atrazine showed similar behavior toDiuron, although less pronounced due to lower partitioninginto both sorbed TX and TX micelles (Supporting Informa-tion), given its lower hydrophobicity.

From Figure 2a-c, it can be seen that the partitioningbehavior of the pesticide to the sorbed surfactant and to themicelles is quite different for the different soil particles withinthe same size group, based on their CEC. This effect can alsobe most clearly seen in Figure 2d for the different size fractionsof the same soils (Ag#1 and Ag#3).

For these equilibrium partitioning systems, we define abreak-even concentration as the aqueous equilibrium sur-factant concentration at which the aqueous equilibriumpesticide concentration in the presence of the surfactant isequal to that in the absence of the surfactant. As an example,the determination of the break-even concentration for thesand fraction of Ag#1 is depicted in Figure 2c. The break-even concentrations were all determined experimentally. Inorder to clearly compare the slopes of the rising limbs of

Diuron aqueous concentrations curves across the differentsize fractions, the scale of the x-axes in Figure 2 have beenconstrained to TX concentrations smaller than 3.0 g/L.However, the experiments were conducted for aqueous TXconcentrations up 20 g/L, to determine the break-evenconcentrations. Figures for the entire range of TX concentra-tions for Diuron equilibrium sorption with the clay and siltfractions are presented in the Supporting Information. Themeasured break-even concentrations of the bulk soils andtheir size fractions are presented in Table 2. Thus, if sorption/desorption equilibrium is assumed, the break-even concen-tration can be used to predict the pesticide desorptionbehavior within soil washing systems. Within a surfactant-aided soil washing system, an enhanced pesticide desorptionwill not occur until the surfactant break-even concentrationis reached in the aqueous phase. As can be seen, the sandfractions have the lowest break-even concentrations and theclay fractions have the highest break-even concentrations,by a factor of 5-6. Thus, desorption of the pesticide associatedwith the clay fractions will require significantly more sur-factant than the other fractions.

As shown in Figure 2d, above the CMC of TX, at any givenequilibrium TX aqueous concentration, the equilibriumDiuron aqueous concentrations of the sand fractions aregreater than those of the clay and silt fractions. Thus, in asoil washing system where active mixing occurs, pesticidemolecules may transfer from the coarser to the finer fractions,possibly via micelles as an intermediate, resulting in ad-ditional pesticide accumulation on the clay fractions. Al-though this study was not designed to explicitly test thistransfer mechanism, there is some evidence in the literatureto support it (16).

At the saturation surfactant sorption the aqueous Diuronconcentrations decreased to the same level for the clay andsilt fractions of each of the two soils (Ag#1 and Ag#3) (Figure

FIGURE 2. Diuron sorption in the presence of TX with (a) clay fractions; (b) silt fractions; (c) sand fractions; (d) all size fractions ofAg#1 and Ag#3. Note: (1) The error bars shown for soils Ag #1 and Ag #3 are representative of the experimental error for all soilsconsidered; (2) x and y axes are equilibrium TX and Diuron concentrations respectively.


2d). However, beyond the saturation surfactant sorption, withincreasing micelle concentrations, the aqueous pesticideconcentrations in equilibrium with the silt fractions werealways higher than the ones with the clay fractions, suggestinghigher affinity of the pesticide onto the clay fractions underthe same aqueous equilibrium surfactant concentration.From a soil washing perspective, it indicates greater difficultyassociated with desorbing pesticide sorbed onto clay fractionsthan silt fractions.

Since the interaction between TX monomers and eitherpesticide was found to be negligible (c.f. Supporting Infor-mation), a dimensionless pesticide partitioning coefficientfor this system can be defined as follows:

R )Pestoc +Pestss

Pestw +Pestmc(1)

Where R is the ratio of the mass of sorbed pesticide in solidphase to the mass of pesticide in aqueous solution (mg/mg);Pestoc, Pestss, Pestw, and Pestmc are the mass of the pesticideassociated with original OC, sorbed surfactant, water, andsurfactant micelles, respectively (all in mg).

Based on a mass balance

Pestt )Pestoc +Pestss +Pestw +Pestmc (2)

where Pestt is the total mass of pesticide added to soil–wa-ter–surfactant system (mg). Rearranging

Pestoc +Pestss )Pestt - (Pestmc +Pestw) (3)

At equilibriumKmc ) 1000 × (Kmc, dl/ C pest, w) is a constant,where Kmc is a dimensional pesticide partitioning coefficientinto the TX micelles (L/g) as reported in the SupportingInformation, Kmc,dl is a dimensionless pesticide partitioningcoefficient into the TX micelles (mg/mg), Cpest,w is the aqueouspesticide concentration (mg/L), and 1000 is a unit conversionfactor. Also since Pestw ) Cpest,wVw, where Vw is the volumeof water (L), then Pestw ) 1000 × (Kmc, dl/Kmc) Vw. Pestmc )MmcKmc,dl, where Mmc is the mass of the TX molecules presentas micelles (mg). Thus, the following expression can bederived:

R )Pestt - (MmcKmc,dl + 1000 × (Kmc,dl ⁄ Kmc)Vw)

MmcKmc,dl + 1000 × (Kmc,dl ⁄ Kmc)Vw(4)

Since R, Mmc, and Kmc can be measured and Pestt and Vw areknown for the current experiments, the above equation canbe solved for Kmc,dl. The relationship between Kmc,dl and theequilibrium aqueous TX concentration of Diuron, in thepresence of Ag#1 clay and silt fractions, is presented in Figure3. The x-axis in Figure 3 starts from an aqueous TXconcentration 0.12 g/L, which is the CMC of TX. As can beseen, Kmc,dl decreases as the TX micelle concentration

increases, since the capacity of the micelles to store pesticidemolecules decreases with increasing loading. Also, the Kmc,dl

of the clay fraction is always smaller than the Kmc,dl of thefraction, reflecting the stronger partitioning of the pesticideto the clay particles even in the presence of TX micelles.

In sum, based on the results of the equilibrium sorptionexperiments the largest amount of surfactant is needed todesorb the pesticide associated with the clay fractions. Thus,it is the clay fractions that may determine the amount ofsurfactant needed for a surfactant-aided soil washingapplication.

3.4. Pesticide Desorption in the Presence of TX bySequential Washing. Figure 4 presents Diuron desorptionisotherms from the clay fractions in the absence (Figure 4a)and presence (Figure 4b) of surfactant. In the absence ofsurfactant, pesticide desorption proceeds monotonically(Figure 4a). Adding TX (Figure 4b) results in an abruptincrease in the amount of pesticide sorbed onto the soilparticles during the first and sometimes even the seconddesorption steps, followed by a decrease in sorbed pesticidethereafter. The initial increase is caused by the sorption ofthe surfactant onto to the soil (Figure 4c), which is followedby partitioning of more dissolved pesticide onto the sorbedsurfactant phase. Once the aqueous TX concentration exceeds

FIGURE 3. Kmc,dl as a function of equilibrium aqueous TXconcentration for Ag #1 clay and silt particles.

FIGURE 4. Diuron desorption isotherm from the clay fractions inthe presence of (a) water only and (b) TX (2.00 g/L), and (c) TXsorption during the desorption cycles. Note: the y axes of (a)and (b) are the Diuron sorbed concentration while that of (c) isthe sorbed TX concentration.


its CMC, micelles are present in the aqueous phase. Thepesticide molecules partition into the micelles in the aqueousphase.

A desorption efficiency coefficient, E, can be defined asfollows:

E )(1-Ds)

(1-Dw)(5)

where Ds and Dw are the fractions of pesticide remainingsorbed after a given number of desorption steps in thepresence (Ds) and absence (Dw) of surfactant relative to theinitial amount of pesticide presorbed. In this study weconsidered five desorption steps. An E>1 indicates enhancedpesticide desorption, while E < 1 represents an inhibitedpesticide desorption. If at the end of five desorption stepsthe amount of pesticide remaining sorbed is greater than theinitial amount of pesticide presorbed, a negative E value isgenerated, in which case, instead of desorption, an overallenhanced sorption occurs. The measured Ds, Dw, and E arepresented in Table 3.

From the results of the equilibrium sorption experiments,the correlation coefficients between the break-even con-centrations and TX sorption capacities are 0.89 and 0.81 forAtrazine and Diuron, respectively, while those between thebreak-even concentrations and the amount of pesticidepresorbed in the absence of TX are 0.69 and 0.55 for Atrazineand Diuron. On the other hand, the correlation coefficientsbetween TX sorption capacity, at TX concentrations of 1.0,2.0, and 3.0 g/L, and Ds are 0.83, 0.89, and 0.80 for Diuron,and 0.78, 0.90, and 0.88 for Atrazine. Therefore, the resultsof both equilibrium sorption and sequential washing indicatethat the amount of TX sorbed determines the overall pesticidedesorbability. Since TX sorption capacity is determined byCEC, this soil property controls surfactant-enhanced pesticidedesorption. Also, these results indicate that to determine theamount of surfactant to be used in a surfactant-aided soilwashing system, desorption of pesticide sorbed onto sorbedsurfactant is a more significant factor than the amount of thepesticide presorbed in the absence of surfactant.

These findings establish the usefulness of characterizingequilibrium sorption for predicting pesticide desorptionbehavior within a soil washing processes. In fact, thecorrelation coefficient between the break-even concentra-tions and E is -0.91, -0.88, -0.87 for TX concentrations of

1.0, 2.0 and 3.0 g/L for Diuron, and -0.84, -0.85, and -0.87for Atrazine for the same TX concentrations. The highlynegative correlation between the two suggests that themeasured break-even concentrations from the pesticideequilibrium sorption experiments serve to predict well therelative desorbability of an HOC in surfactant-aided soilwashing systems using a particular surfactant.

The results point to the difficulty in desorbing Atrazinefrom the clay fractions of the Ag#2, Ag#3, and clayey soils.This finding is consistent with the results of the equilibriumsorption experiments which showed the clay fractions ofAg#2, Ag#3, and clayey soils had the highest break-evenconcentrations among all the size fractions. This reflects thefact that the clay fractions of these soils have the highest CECand thus highest TX sorption capacities. An additional factoris the lower affinity of Atrazine for TX micelles, relative to themore hydrophobic (higher Kow) Diuron.

The clay fractions showed a statistically lower E than thebulk soils and silt fractions for either pesticide using TX (Table3), indicating that pesticide desorption from the clay fractionsusing TX is more difficult than from the bulk soils and otherfractions. It has been reported that pesticide desorption fromclay fractions in the absence of surfactant exhibited higherdesorption hysteresis than the bulk soils and other soilfractions (14). Presumably, the difficulty associated withdesorbing pesticide out of the clay fractions is attributableto the higher structural hysteresis of the clay fractions, whichis related to the difficulty of pesticides sorbed insidemicropores diffusing out of the pore space (14). Thus,desorption of the pesticide from the smallest particles willrequire the largest amount of surfactant, which is consistentwith the break-even concentrations determined by equilib-rium sorption experiments.

The results showed consistently lower desorption ef-ficiency (E) for Atrazine than for Diuron (Table 3) with theclay fractions under the same conditions, due to its loweraffinity to TX micelles than Diuron. As a result, moresurfactant is required to desorb Atrazine molecules out ofthe sorbed phase than to desorb Diuron. Thus, one can expecta higher desorbability for more hydrophobic organic com-pounds than Diuron and Atrazine. Further work may resultin a generalizable relationship between E and Kow.

Thus, in view of the greater difficulty in desorbing pesticideout of the clay fractions, for a contaminated soil with highclay content, desorbing all the pesticide from all size fractions

TABLE 3. Percentage of Pesticide Remaining Sorbed in the Presence (DS) and Absence (DW) of Surfactant, and DesorptionEfficiency Coefficients (E) after Five Desorption Cycles with TX

Diuron Atrazine

TX conc. (g/l) 0.00 1.00 2.00 3.00 0.00 1.00 2.00 3.00

Dw Ds E Ds E Ds E Dw Ds E Ds E Ds E

bulk

Ag#1 35% 36% 0.98 20% 1.24 16% 1.29 41% 20% 1.34 18% 1.38 13% 1.46Ag#2 32% 86% 0.20 53% 0.69 40% 0.88 33% 44% 0.84 31% 1.04 23% 1.16Ag#3 42% 84% 0.27 60% 0.70 42% 1.00 35% 52% 0.73 30% 1.08 24% 1.16clayey 28% 82% 0.24 46% 0.74 40% 0.82 32% 49% 0.75 33% 0.98 23% 1.13sediment 29% 22% 1.10 16% 1.18 14% 1.20 38% 18% 1.33 15% 1.37 11% 1.43

clay

Ag#1 43% 53% 0.82 28% 1.25 27% 1.27 45% 68% 0.57 44% 1.00 40% 1.09Ag#2 41% 94% 0.09 77% 0.39 48% 0.88 37% 97% 0.05 86% 0.21 102% -0.0Ag#3 47% 80% 0.28 58% 0.56 45% 0.84 38% 98% 0.04 88% 0.2 99% 0.01clayey 39% 100% 0.01 84% 0.26 62% 0.62 35% 99% 0.01 92% 0.12 104% -0.1sediment 35% 54% 0.71 34% 1.01 32% 1.05 42% 69% 0.52 51% 0.84 32% 1.16

silt

Ag#1 46% 40% 1.12 30% 1.29 26% 1.37 28% 40% 0.84 32% 0.95 24% 1.06Ag#2 53% 63% 0.79 46% 1.15 38% 1.32 36% 30% 1.10 36% 1.00 26% 1.16Ag#3 57% 58% 0.98 41% 1.36 40% 1.39 49% 36% 1.27 51% 0.96 46% 1.07clayey 53% 77% 0.48 60% 0.85 50% 1.06 52% 40% 1.24 51% 1.01 43% 1.17sediment 56% 51% 1.12 38% 1.39 30% 1.60 45% 37% 1.14 33% 1.23 22% 1.43


might be less efficient. Instead of attempting to washthe entire bulk soil, a better strategy might be to either (1)use only the amount of surfactant that is sufficient to cleanthe coarse fraction, then separate the fine fraction, anddispose or treat it separately; or (2) to separate the coarsefractions mechanically and then treat the coarse and finefractions separately.

Supporting Information AvailableAdditional information on pesticide properties, soil sizeseparation, pesticide and TX sorption kinetics, pesticidesolubility enhancement by TX, calculation of sample cor-relation coefficient of two variables, and actual correlationbetween the measured parameters. This material is availablefree of charge via the Internet at http://pubs.acs.org.

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ES702732G


particle-size dependent sorption and desorption of pesticides within a water−soil−nonionic...

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