Effect of Triton X-100 on the Rate of Trichloroethene Desorption from

Environ. Sci. Techno/. 1995, 29, 1069-1080

Effect of Triton X=lOO on the Rate of Trichloroethene Desorption from Soil to Water JAMES J . DEITSCH A N D J A M E S A. SMITH* Department of Civil Engineering and Applied Mechanics, University of Virginia, Charlottesville, Virginia 22903-2442 ~

~~~

~~~~

~

Continuous-flow stirred tank reactor (CFSTR) experiments and batch sorption experiments were conducted to determine the effect of Triton X-100, soil organic carbon content, and soil/contaminant contact time on the rate of trichloroethene (TCE) desorption from t w o aquifer soils to water. Soil A and soil B have organic carbon contents of 24 and 1.36%, respectively. TCE desorption from soil A was strongly kinetic, and the rate of desorption decreased when the soil/contaminant contact time was increased from 1 t o 4 wk. TCE desorption from soil B could be approximated by assuming instantaneous equilibrium between the soil and aqueous phases. Aqueous solutions of Triton X-100 at concentrations of 30, 300, and 3000 mg/L increased the rate of TCE desorption from soil A relative to an aqueous solution with 0 mg/L Triton X-100 following both 1- and 4-wk soil/ contaminant contact times. The increased rates of TCE desorption observed for the 30 and 300 mg/L Triton X-100 solutions are caused by an increase in the mass-transfer coefficient. The increased rates of TCE desorption observed for the 3000 mg/L Triton X-100 solutions are caused by the combined effects of an increased concentration gradient (resulting from a reduction in the equilibrium sorption coefficient) and an increased mass-transfer coefficient.

0013-936X/95/0929-1069$09.00/0

0 1995 American Chemical Society

Introduction Pump-and-treat is a common method for the remediation of contaminated aquifers. Implementation of this remediation technique usually involves groundwater extraction fromwithdrawalwells screened in the contaminated aquifer and treatment at land surface. The treated water is then returned to the subsurfaceby injection wells or is discharged to a nearby river. Unfortunately, pump-and-treat systems rarely cause permanent remediation of aquifers, and several reasons have been suggested to explain the ineffectiveness of this technology ( 1 ) . First, aquifers may be contaminated by nonaqueous-phase organic liquids, and remediation is limited by the rate of dissolution of the organic liquid into the surrounding groundwater. Second, following years of continued subsurface contamination, organic pollutants are transported into low-permeability lenses in aquifers. During remediation, removal of these contaminants is limited by the low advection velocities in these formations. Third, groundwater remediation may be limited by the slow desorption rate of contaminants from soil to water. This latter problem is the focus of this paper. The persistence of nonionic organic contaminants in the environment is well documented (2-5). Sorbed concentrations of 1,2-dibromoethane were detected in Connecticut agricultural field soils 20 yr after application of the soil fumigant was discontinued (2, 3). Aqueous concentrations of trichloroethene (TCE) up to 1 mg/L have been detected in groundwater at Picatinny Arsenal, New Jersey, more than 6 yr after the source of contamination was removed (5). Several researchers have reported strong nonequilibrium conditions in the field (5- 7), and the extent of nonequilibrium appears to be correlated to the residence time of the contaminant in the soil (2-9). The observed persistence and nonequilibrium conditions observed for these contaminants in the environment indicate that the commonlyused assumption of rapid and reversible sorption is not always valid. The cause of nonequilibrium sorption has been the subject of much research (9-22). Intraparticle pore diffusion has been proposed as the cause of kinetic sorption/ desorption (10-14):however, this mechanism should affect both sorbing (e.g.,nonionic organic solutes)and nonsorbing (e.g., Br-, tritiated water) solutes because it is a physical transport limitation (15-18). Kinetic sorption has been observed for sorbing solutes, but not for nonsorbing solutes in laboratory tests (15-18). Intra-organic matter diffusion has also been proposed as the cause of kinetic sorption/ desorption ( 1 , 15-23) and is consistent with research showing that partition into soil organic matter is believed to be the dominant sorption mechanism for relatively nonpolar organic contaminants in aqueous systems (2426'). Soil organic matter is composed primarilyof complex, macromolecular humic substances (20) with molecular weights ranging from 300 to 200 000 (27). The humic substances create a three-dimensional structure perforated withvoids (15-18,23). Near the soil/organic matter/water interface, the organic matter arrangement may be flexible and able to change in response to variations in pH, ionic strength, and temperature (I, 22, 23, 28). Given our knowledge of the mechanism of nonionic solute sorption and the structure of soil organic matter, it is reasonable to

VOL. 29, NO. 4, 1995 /ENVIRONMENTAL SCIENCE & TECHNOLOGY 1069

believe that rate-limited sorption/desorption is at least partly a result of diffusive resistances encountered by the solute within the soil organic matter (15-23). The rate of mass transfer of an organic contaminant from soil to water is often quantitatively expressed as dS-- -a[S

dt

-

where S is the sorbed solute concentration (MIM),a is the mass-transfer rate coefficient (l/Tl, KD is the equilibrium sorption distribution coefficient (L3/M), and C is the aqueous phase solute concentration (M/L3). Equation 1 indicates that the rate of solute desorption from soil is a function of the mass-transfer rate coefficient, a, and the concentration gradient between the organic matter and aqueous phases. Several researchers have proposed the use of surface-active agents (surfactants) to increase the rate of nonionic solute desorption from soil to water in contaminated aquifers (I,23,29). At concentrations greater than critical micelle concentration, surfactants such as Triton X-100 have been shown to increase the apparent aqueous solubility of nonionic organic contaminants (3033) and reduce the equilibrium sorption distribution coefficient, KO (32,341. By reducing KD,the concentration gradient in eq 1 and the rate of solute desorption from soil will increase. In this regard, the use of surfactants shows promise in conjunction with pump-and-treat remediation for the remediation of contaminated aquifers. Presently, there are at least two important limitations to the surfactant remediation technology described above. First, to achieve a measurable reduction in KD,surfactant concentrations must be significantly greater than critical micelle concentration, particularly if the soil-water system is contaminatedwith chlorinated solvents or gasoline-range hydrocarbons with aqueous solubilities greater than a few hundred milligramsper liter. Therefore, the material costs of using surfactants at concentrations above critical micelle concentration may offset the benefits of increased desorption rates and shorter remediation times. Second, during the operation of pump-and-treat systems, aqueous solute concentrations are typically low, and the concentration gradient between the sorbed and aqueous phases may be close to a maximum. Thus, any reduction in KD would have a minimal effect on the concentration gradient. The primary objective of the research presented in this paper is to investigate the effect of the surfactant Triton X-100 on the mass-transfer rate coefficient of trichloroethene (TCE) at surfactant concentrations above and below critical micelle concentration. Triton X-100 is a nonionic heterogeneous octylphenol ethoxylate surfactant with the following molecular structure: CH3

Using models developed in this paper, the mechanisms of surfactant-enhanced desorption are identified. If a surfactant, such as Triton X-100, can increase a at concentrations less than the critical micelle concentration, material costs for surfactant remediation will decrease and preexisting large concentration gradients will not negate the benefits of surfactant addition to the aquifer. 1070. ENVIRONMENTAL SCIENCE & TECHNOLOGY I VOL. 29, NO. 4,1995

Experitnental Section Soil. The two soil types used in this study were collected at Picatinny Arsenal, New Jersey. Because of improper disposal of TCE at the field site between 1950 and 1985,the unconfined sand-and-gravel aquifer has been contaminated. Presently, the groundwater contamination is being remediated with a pump-and-treat system. A detailed description of the field site, with information about the site hydrogeology and groundwater contamination, is given by Imbrigiotta et al. (35). Soil Ais a composite sample collected from a peat layer near the center of the groundwater plume at depths of 6-12 ft. Because of its high organic carbon content, this layer is believed to be a significant source of TCE to the groundwater because of slow desorption from the soil organic matter to the subsurface water. Soil B is a composite sample collected from several locations in the plume of contamination from depths of 5-40 ft. This sample is typical of soil from the unsaturated zone and shallowgroundwater zone at the field site. Both soils were air dried for 24 h and then heated at 105 “C for 24 h. They were then passed through a No. 10 sieve (2-mm openings) to ensure that subsequent soil subsamples would be relatively homogeneous. All soil samples were then “conditioned” as described in the next section prior to kinetic and equilibrium sorption experiments and quantification of soil organic carbon contents. Amethanol-soil extraction as described by Sawhney et al. (36) was performed to determine the residual TCE concentrations on the soils. The residual concentrations were below detection limits. The organic carbon contents of soils Aand B were quantified by Huffman Laboratories (Golden,CO) and determined to be 24 and 1.4%,respectively. CFSTR Methodology. A continuous-flow stirred tank reactor (CFSTR) apparatus was used to study the effect of Triton X-100 on the kinetic desorption of TCE from soil to water and to study the sorption kinetics of Triton X-100 to soil A. Individual CFSTRs consisted of a cylindrical glass reactor (1.0 cm i.d. x 15 cm) with Teflon end fittings and stainless steel screens. The fittingswere secured and formed air-tight seals at the ends of the glass reactor. Omnifit twoway valves were connected to the inflow and outnow fittings of the CFSTR with stainless steel tubing. All the surfaces inside the CFSTRs were either glass or Teflon. Each CFSTR contained either 5 g of soil A or 7.5 g of soil B. Deionized, organic-free water was added to the soil and mixed until the soil was completely wetted (Le., all the air trapped in the soil was released) and the reactor was completely filled. To condition the soil, the CFSTRs were fastened to a rotary shaker and shaken continuously at 150 rpm at ambient room temperature. Distilled water containing 200 mg/L sodium azide was pumped through each CFSTR at 12 mL/h for 3 d. A Manostat cassette drive pump was used in all CFSTR experiments. The sodium azide solution eliminates microbiological activity in the CFSTRs under aerobic conditions, thus preventing biodegradation of TCE ( 4 ) . The 3-d flow (conditioning) period was necessary for two reasons. First, airbubbles in the CFSTRs were removed, thus limiting the potential for volatilization losses after the injection of TCE. Second, small amounts of soil initially exited the CFSTRs during the conditioning period. After the third day, no additional soil escaped from the CFSTRs. Thus, the mass of the soil in the CFSTRs remained constant and was determined by subtracting the mass of the soil

TABLE 1

Summary of CFSTR TCE Desorption Experiments equilibration influent surfactant soil time (weeks) concn ( m g W A A A A A A A A B B a

1 1 1

1 4 4 4 4 1 1

0 (4) 30 (2) 300 (3) 3000 (2) 0 (2) 30 (2) 300 (2) 3000 (2) 0 (3) 30 (3)

experiment name 1-wk 0 mg/L CFSTR 1-wk 30 mg/L CFSTR 1-wk 300 mg/L CFSTR I-wk 3000 mg/L CFSTR 4-wk 0 mg/L CFSTR 4-wk 30 mg/L CFSTR 4-wk 300 mg/L CFSTR 4-wk 3000 mg/L CFSTR I-wk 0 mg/L CFSTR I-wk 30 mg/L CFSTR

Number of reactors shown in parentheses.

which exited the CFSTR during the conditioning period from the initial mass of soil in the CFSTR. The volume of water in the CFSTR was then calculated based on the mass of the soil in the CFSTR and the difference in the mass between the conditioned and the empty CFSTRs. After the 3-d flow period, the pumping was discontinued, and the valves controlling flow into and out of the reactor were closed. The CFSTRs used in the TCE desorption experiments and the Triton X-100 sorption experiments were prepared in this manner. The CFSTRs used in the TCE desorption experiments were contaminated with [l4C1TCE. A 500-pCi sample of [l4C1TCE (specific activity equal to 6.2 mcilmmol) was obtained from Sigma Chemical Co. and mixed with nonradioactive TCE to yield a net volume of 6 mL of neat liquid. The resultant chemical and radiochemical purity of the radioisotope was greater than 98%. Aspecificvolume of [14C]TCE was injected into each CFSTR such that equilibrium aqueous TCE concentrations of approximately 250 mg/L resulted. Following the injection of TCE, the CFSTRs were continuously shaken at 150 rpm for a 1- or 4-wk equilibration period and then sampled. The CFSTRs were separated into groups, which used either water or water with a specific concentration of Triton X-100 as its inilow reservoir. Each reservoir also contained 200 mg/L sodium azide. The solutions were then pumped into the CFSTRs with a peristaltic pump at constant flow rates (approximately 2 mLlh) until the sampling was completed. The flow rates were measured throughout the course of the sampling period and were constant for individual CFSTRs. The variation of flow rates among CFSTRs was less than 5%. The small variations did not affect the desorption rates. Effluent from each CFSTR was collected periodically in 7-mL scintillation vials filled with 5 mL of Beckman Ready Safe scintillation cocktail. The mass of effluent captured (approximately0.5 mL) was quantified gravimetrically. The radioactivity of the samples was measured with a Packard Tri-Carb 19OOCA liquid-scintillation analyzer, and the corresponding TCE concentrations were calculated with a standard curve relating concentration to counts per minute (cpm). Quench was approximatelyconstant for all samples, therefore counts per minute were not converted to disintegrations per minute (dpm). The sampling periods lasted between 7 and 10 d. The TCE desorption experiments and nomenclature are outlined in Table 1. For example, the experiment studying the effect of a 30 mg/L Triton X-100 solution on the rate of TCE desorption after a 1-wk

equilibration period is referred to as the 1-wk 30 mg/L CFSTR. The surfactant sorption experiments used Triton X-100 solutions made from 250 pCi of [3HlTritonX-100 (specific activity equal to 2.37 mCilmg) mixed with nonradioactive Triton X-100. A 3000 mg/L f3H1TritonX-100 solution was pumped at 2 mL/h through two CFSTRs prepared with soil A. The effluent was sampled and analyzed in the manner described previously. The assumption of a completelymixed reactor was tested by conducting a Br- tracer experiment. Three CFSTRs were prepared with soil B and mixed with a solution of 800 mg/L Br-. A n 800 mg/L Br- solution was pumped through the three CFSTRs for 3 d at 12 mLlh. After the flow period, the three CFSTRs were equilibrated for 4 wk to allow the Brto diffuse into the intraparticle pore spaces of the soil. After the equilibration, deionized water was pumped through each CFSTR, and effluent samples were collected. The concentrations of Br- were measured using an Orion Model 94-35 Br- electrode and an Orion Model 90-01 single junction reference electrode. The ionic strength of the samples was adjusted by a 5 M sodium nitrate solution. The readings were measured using an Accumet Model 950 pHlion meter. The accuracy of the TCE and TritonX-100experimental methodologies was tested by conducting blank CFSTR experiments to determine whether losses of TCE and Triton X-100 occurred during the experimental procedure. Four CFSTRs were prepared without soil. Measured volumes of P4C1TCEwere injected into two of the CFSTRs and allowed to equilibrate for 1 wk. A 3000 mg/L [3HlTritonX-100 solution was pumped through the other two CFSTRs without soil. The TCE and Triton X-100 blank CFSTRs were then sampled according to the procedure described previously. The pH and ionic strength of the CFSTR environments were measured periodically during the experiments. The pH and ionic strength were approximately constant throughout the duration of the particular experiments. The approximate values of pH and ionic strength were 6.5 and 0.0064, respectively. Batch Methodology. The equilibrium distribution of Triton X-100 between water and soil A was quantified by a conventional batch equilibration method (37). Different masses of [3HlTritonX-100, 0.5 g of soil A, and 15 mL of distilled water were combined in 15-mL (nominalvolume) glass centrifuge tubes with Teflon-lined caps. The centrifuge tubes were then shaken at 20 "C in the dark for 48 h. After shaking, the tubes were centrifuged at 2000g for 45 min to separate the water and soil phases; 0.5 mL of the aqueous supernatant in each tube was transferred to 5 mL of scintillation cocktail, and the sample was analyzed with a Packard 19OOCA liquid scintillation analyzer. The measured radioactivity was related to aqueous concentration by a standard curve. The sorbed concentration of the solute was then calculated by difference. Batch sorption experiments were also performed to quanufy the equilibrium distribution of TCE between soil A and solutions with equilibrium aqueous Triton X-100 concentrations of 0,30,300,900,and 3000 mglL. For these experiments, 0.5 g of soil A, 15 mL of distilled water, a specified mass of nonlabeled Triton X-100, and [14C]TCE were combined in 15-mL glass centrifuge tubes. These tubes were then processed identically to the abovedescribed batch equilibration procedure, except the supernatant was analyzed for TCE instead of Triton X-100. VOL. 29, NO. 4,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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For each sorption isotherm, three additional centrifuge tubes were handled similarly to the above-described tubes for quality assurance. Two of these tubes contained water, radiolabeled solute, and no soil. These tubes were used to quantify solute losses caused by processes other than sorption to the soil (volatilization,biodegradation, etc.). In almost all cases, solute recovery from these tubes was greater than 95%. If solute recovery was less than 90%, the isotherm experiment was repeated. The third tube contained water, soil, and no radiolabeled solute. This tube was used to quantify the background radiation and to identify if the glassware, soil, or water was contaminated with radioactivity. In all cases, the measured radiation in these tubes was less than or equal to background levels (less than 30 dpm).

where SKand SEare the concentrations of TCE sorbed in kinetic and equilibrium regions (mglkg),respectively, Cis the aqueous concentration ofTCE (mg/L),Fis the fraction of the sorbent which is kinetically limited, and KDis the lineat. partition coefficient of TCE (Llkg). The equaticn governing the kinetic desorption of TCE is (5)

where a is a constant mass-transfer coefficient. The governing equation for the equilibrium-controlledregions is obtained by differentiating eq 4b with respect to time, resulting in dSE dt

-= (1 - F)K

Yodel Development The governing equation for the conservation of mass for the aqueous phase in the CFSTR, assuming a completely mixed system, is

r,V,

+ V,- ddtC = Q(Cin- C )

where r, is a reaction term (mg L-’ h-l), V, is the volume of water in the reactor (L), Q is the flow rate through the CFSTR (Llh),Ci, is the inflow solute concentration (mg/L), C is the aqueous phase solute concentration throughout the reactor (mg/L),and dCldt is the time rate of change of the solute concentration. Equation 2 provides the framework for the four CFSTR models developed for this study. The Br- CFSTRs and the TCE and Triton X-100 blank CFSTRswere modeled assumingthat no reactions (sorption, biodegradation, etc.) occurred during the respective experiments. Therefore, eq 2 was modified by setting r, = 0. The analytical solution to the resulting equation is

C = Cin

+ (C,, - C,,)

exp

(3)

where C, is the initial solute concentration (mg/L) in the reactor and t is the time (h). Equation 3 quantifies the effluent solute concentration from the Br- CFSTRs and the TCE and Triton X-100 blank CFSTRs as a function of time. The TCE desorption and Triton X-100 sorption experiments were simulated by incorporating sorption/desorption terms into the CFSTR modeling framework. Three models were used to simulate the experimental data: a two-region sorption/desorption model, an equilibrium sorption/desorption model, and a time-varying mass-transfer coefficient sorptionldesorption model. The models used to simulate TCE desorption from the soil to the water in the CFSTRs are developed in this section. The models used to simulate Triton X-100 sorption to the soil in the CFSTRs are not developed in this paper. However, the models are similar except that the effect of the nonlinear sorption of Triton X-100 to the soil is incorporated. The two-region model (38-40) assumes that the sorbent is composed of kinetic and equilibrium sorption regions. The equilibrium relation defining the distribution of solute in the kinetic and equilibrium regions, respectively are

8-dCt + (1 - F)C- d t

H D

(6)

KDis a function of time because the Triton X-100 solutions can reduce the distribution coefficient of TCE. dKD/dt accounts for the rate of change of KD as the aqueous concentration of TritonX-100 increases from 0 mg/L to the inflow concentration. The total sorbed concentration of TCE in the sorbent is expressed as

s = SK + s,

(7)

therefore

-=-+-=-ct[SK-FKDQ+(l-F)K8-+ dS ~ S K~ S E dt dt dt

C dt

Equation 8 is modified by multiplying by the mass of soil in the CFSTR, Ms (kg),and dividing by the volume of water in the CFSTR, V, (L). It is then substituted into eq 2 for r,, yielding

Equations 5 , 6 , and 9 were solved simultaneously using a fourth-order Runge-Kutta numerical method programmed by the authors. An appropriate time interval was selected to minimize numerical errors. The two-region model was fit to the experimental data by adjusting the two unknown parameters, F and a. The criterion for best fit was to minimize the sum of the squared residuals between the numerical solution and the experimental data. The equilibrium model assumes that the sorbent is composed of only equilibrium sorption regions (i.e.,F = 0 ) . Therefore, eq 9 reduces to

When dKD/dt = 0 (Le., the influent solution is water or the influent Triton X-100 solution does not alter KD),eq 10 can be solved analytically, yielding 1072

ENVIRONMENTAL SCIENCE &TECHNOLOGY / VOL. 29, NO. 4, 1995

~~

%

-

o 30 mg/L TRITON X-100

€ 4000

A

L

2

0 mg/L TRITON X-100

300 mg/L TRITON X-100

o 900 mg/L TRITON X-100

3000

3,000mg/L TRITON X-100

fim 1000 a: 0 v)

0 0

100

50

150

200

250

300

AQUEOUS TCE CONCENTRATION (mg/L) FIGURE 1. Results of batch sorption experiments that quantify the effect of 0,30,300,900, and 3000 m g h Triton X-100 solutions on the sorption of trichloroethene to soil A. TABLE 2

Organic Carbon Mormalized TCE Distribution Coefficients, KDc,Determined from Batch Sorption Experiments aqueous Triton X-100 concn (mg/L)

hC, organic carbon normalized distributioncoeff (Ukg)

0 30 300 900 3000

73.1 73.1 73.1 20.9 13.5

P-z 750 0

$c 600 z W

9 450 8z 300 0 W

$s I5Oo 1

0

2 3 4 AQUEOUS REACTOR VOLUMES

X

6

LABORATORY DATA

- ANALYTICAL SOLUTION

If dKDldt is not zero, eq 10 can be solved numerically.The sorbed concentration was calculated using the equilibrium isotherm relation. The time-varying mass-transfer coefficient kinetic sorptionldesorption model (multi-region model) assumes that the entire sorbent is a kinetic sorption medium (Le., F = 1). Therefore, eq 9 reduces to -4

0

where a is now a time-varying mass-transfer coefficient. The sorbed phase concentration of TCE is governed by eq 5 with SKreplaced by S and F = 1. The modified version of eqs 5 and 12 were solved simultaneously using a fourthorder Runge-Kutta numerical method. The multi-region model was fit to the experimental data by adjusting the magnitude of a throughout the simulation. As with the two-region model, the sum of the squared residuals was

5

1

2 3 4 5 AQUEOUS REACTOR VOLUMES

6

7

FIGURE 2. Results from the bromide ion tracer and trichloroethena blank continuous-flow stirred tank reactor (CFSTR) experiments fit with the analytical solutions of the systems' governing equation.

minimized. The multi-region model was used to obtain a numerical simulationwhich accuratelyfit the experimental data throughout the entire concentration profile. Thus, the multi-region model served as a curve-fitting tool to obtain the best numerical fit. VOL. 29, NO. 4,1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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range of aqueous concentrations that include the critical micelle concentration of Triton X- 100 (approximately 130 mg/L) (31) are well characterized by a Freundlich sorption isotherm model. The Freundlich isotherm model is commonly expressed as

3000

S = KFC

(13)

where S has units of mglkg. The Freundlich coefficient, KF,is 495.0 and n is 0.430. The correlation coefficient is 0.945 based on 14 data points. Figure 1presents isotherm data quantifying the sorption of TCE to soil A from aqueous solutions containing 0, 30, 300,900, and 3000 mglL equilibrium aqueous concentrations of Triton X-100. The sorption isotherms are highly linear and are described by the following linear model:

s = foCKocC 10 15 AQUEOUS REACTOR VOLUMES

5

0

20

FIGURE 3. Results from the continuous-flow stirred tank reactor (CFSTR) experiment studying the sorption of Triton X-100 to soil A. The data are fit with equilibrium and two-region models.

Results Batch Results. The isotherm data quantifyingTriton X- 100 sorption to soil A from water (data not shown) over a wide

3 250

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2 200 f,

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10

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2 400

MODEL

TWO-REGION

300

z

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0

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z

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30

L

L

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EQUILIBRIUM MODEL

0

0

2

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where S is the equilibrium sorbed concentration of TCE (mg/kg), C is the equilibrium aqueous concentration of TCE (mg/L),foc is the fraction of the organic carbon in the sorbent, and KO,is the organic carbon normalized partition coefficient (Llkg). The calculated partition coefficients for the isotherm data in Figure 1 are given in Table 2. For the 0, 30, and 300 mglL Triton X-100 solutions, the partition coefficients are not statistically different ( p = 0.05) based

2 250

o0 mg/L TRITON TRITON X-100 x-100

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FIGURE 4. Results from continuous-flow stirred tank reector (CFSTR) experiment studying the effects of 0, 30, 300,and 3OOO m a Triton X-100 solutions on the rate of TCE desorption from soil A after a 1-wk equilibration. The data are fit with the equilibrium, two-region, and multi-region models.

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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 4 , 1 9 9 5

3 300

a,

E 250

v

z

200

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300 0 mg/L TRITON X-1 00

a, E 250 z 1 w


dI- 150 z

0 F 200

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W

0 100

0 0 w

0 0 50 w

z

50

0 10

20

30

40

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0

10

20

30

40

700 300 mg/L TRITON X-100

t ; 200 2 2 150 K

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Y


z 0 500

EQUILIBRIUM MODEL

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0

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0

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2

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0 0 100 w

50

w

P o

P o 0

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20

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40


0

10

20

30

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FIGURE 5. Results from continuous-flow stirred tank reactor (CFSTR) experiment studying the effects of 0, 30,300, and 3ooo mg/L Triton X-100 solutions on the rate of TCE desorption from soil A after a 4-wk equilibration. The data are fit with the equilibrium, two-region, and multi-region models.

on analysis with a student’s t-test. However, partition coefficientsfor the 900 and 3000 mg/L Triton X- 100solutions are sigmflcantlyless ( p= 0.05) than the partition coefficients for the 0, 30, and 300 mg/L Triton X-100 solutions. CFSTR Results. The averaged effluent concentrations obtained from triplicate Br- CFSTRs and duplicate TCE blank CFSTRs were plotted separately in Figure 2. (All subsequent references to CFSTR data are the averaged data obtained from duplicate, triplicate, or quadruplicate CFSTRs.) To facilitate comparison of CFSTRs with slightly different flow rates, the data obtained from CFSTRs were plotted versus aqueous reactor volumes. The number of aqueous reactor volumes is defined as the total volume of flow over some time, t, divided by the volume of water in the reactor, V,. The corresponding analytical solutions are also shown in Figure 2. The exponential correlation coefficients, r, between the experimental data and the analytical solutions were -0.9995 and -0.998 for the Brand TCE blank CFSTR experiments, respectively. Based on the strong correlation, it was concluded that the soilwater mixtures in the Br- CFSTRs were completely mixed. If the transport of the Br- was hindered by intraparticle or interparticle diffusion, the concentration profile would have deviated from the exponential curve. The initial concentrations of TCE in the TCE blank CFSTRs matched the expected concentration based on the mass of TCE originally added to the CFSTR. This fact coupled with the strong

correlation indicate that the mass of TCE was conserved in the CFSTR. The results obtained from the Triton X-100 blank CFSTRs (data not shown) indicated that the sorption of Triton X-100 to glass and Teflon as well as other loss mechanisms was negligible. The results obtained from the CFSTR experiment investigating Triton X-100 sorption to soil A are shown in Figure 3. The equilibrium model did not accurately predict the rate of Triton X-100 sorption to soil A, whereas the two-region model provided a good fit, indicatingthat Triton X- 100sorption is kinetically limited. The two-regionmodel numerical simulation was used to predict the aqueous concentration of Triton X-100 in the 1- and 4-wk 3000mg/L CFSTR experiments. The predicted aqueous Triton X-100 concentrations, in conjunction with the information obtained from the TCE/Triton X-100 batch sorption experiments, were used to simulate the reduction of the sorption distribution coefficient of TCE by the TritonX-100 in the 3000 mg/L CFSTRs. This was accomplished by varying the TCE distribution coefficient in the numerical model as the concentration of Triton X-100 increased from 0 to 3000 mgll in the 3000 mg/L CFSTRs. The data obtained from the experiments which investigated the effect of Triton X-100 on the rate of TCE desorption from soil A following exposure to TCE for 1and 4-wk equilibration periods are shown in Figures 4 and 5, respectively. For each experiment, the equilibrium VOL. 29, NO. 4,1995 /ENVIRONMENTAL SCIENCE &TECHNOLOGY

1076

250

0 mg/L TRITON X-100

7

300 n

2 J

E 200

Y

250

Y

z

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0

2 150

IZ

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2 150

MULTI-REGION

U

z 200 0 F

MULTI-REGION

z u

8 100 z

TWO-REGION

0 w 50

20 100 0

u1

2

P

0

50 0

0

10

20


0

10

20


FIGURE 6. Resulrs from continuous-flow stirred tank reactor (CFSTR)experiment studying tbe effects of 0 and 30 mgA Triton X-la0 solutions on the rete of ICE desorption from soil B fit with the equilibrium, two-region, and multi-region models.

model, the two-region model, and the multi-region model were fit to the data. The initial aqueous concentrations of TCE in all but one of the 1- and 4-wk CFSTRs were within 4% of the aqueous TCE concentrations predicted from the 48-h batch sorption experiments. (The 4-wk 300 mg/L CFSTR deviated from the expected initial concentration by 10%. The results from this CFSTR were consistent with the results obtained from the 4-wk 30 and 300 mg/L CFSTRs and therefore are included.) This provides strong evidence that the mass of TCE in the CFSTRs was conserved and that the distribution coefficients obtained from the 48-h batch experiments described in the Methods section were appropriate for the 1- and 4-wk equilibration times used in the CFSTR experiments. As evidenced by these data, the concentration of TCE in the reactors decreasedwith time in response to the i d o w of water with a 0 mg/L concentration of TCE. For all cases in Figures 4 and 5, the equilibrium model, which assumes an instantaneous equilibrium between the sorbed and aqueous concentrations of TCE, did not adequatelydescribe the experimental data. For the 3000 mg/L CFSTRs, the high concentrations predicted by the equilibrium model were a result of the reduction of the distribution coefficient by the 3000 mg/L Triton X-100 solutions. The two-region model, which accounts for kinetic sorption and desorption, provided a more accurate description of the data than the equilibrium model but consistently overpredicted the concentration of TCE in the CFSTR for aqueous reactor volumes greater than 10-20. The multi-region model, which accounts for kinetic sorption with a time-varying mass-transfer coefficient, provided the best description of the experimental CFSTR data in Figures 4 and 5. The effect of Triton X- 100 on the desorption of TCE from soil B after a 1-wk equilibration period was studied (Figure 6). The KO, of soil B was approximated from the soil A sorption data. The initial TCE concentrations in the soil B CFSTRs were slightly higher than predicted by the soil A &,value. Therefore,the KO,of soil B was approximated by using the initial concentration data from the soil B CFSTRs. This KO, value (KO,= 80 L/kg) was used in the model simulations. In contrast with the experiments studying the desorption of TCE from soil A (Figures 4 and 5), all 1078

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 29, NO. 4,1995

three models provided reasonably accurate descriptions of the experimental data. The multi-region model most accurately fit the CFSTR data. Unlike the previous experiments, however, this model fit was achieved using a constant mass-transfer coefficient. The changes in the sorbed concentrations of TCE were calculated during the two- and multi-region model numerical simulations by using the appropriate version of eq 8. The sorbed concentrations were used to calculate the percentage of TCE removed from the soil and then plotted versus aqueous reactor volumes. The percentages of TCE removed from soil A for the 1- and 4-wk equilibration periods, as well as the percentages of TCE removed from soil B are shown in Figure 7. The primary conclusion derived from the model analysis was that Triton X-100, at all three concentrations and for both equilibration times, increased the rate at which TCE was removed from soil A compared to the rate of TCE removal from soil A in the corresponding 0 mg/L CFSTRs. In contrast, Triton X-100 did not increase the rate of TCE desorption from soil B. The calculated percentages of TCE removed from soilA after 40 aqueous reactor volumes are presented in Table 3. The data in Figure 7 and Table 3 also indicate that the equilibration period affects the rate of TCE desorption. For example, in contrast with the 65.2%of TCE removed after 40 aqueous reactor volumes from the 1-wk 0 mglL CFSTR, only 53.7%of the TCE was removed from the 4-wk 0 mg/L CFSTR after 40 aqueous reactor volumes. Similar observations can be made for the 30, 300, and 3000 mg/L CFSTR experiments (Table 3). Analysis of the data in Figure 7 also indicates that the organic carbon content of the soil affects the rate of TCE desorption. For example, approximately99.9%of the TCE was removed from soil B &, = 0.014) in the 0 mg/L CFSTR after 25 aqueous reactor volumes, whereas approximately 50% of the TCE was removed from soil A = 0.24) in the I-wk 0 mg/L CFSTR after 25 aqueous reactor volumes. Comparison of the data from the 30 mg/L CFSTRs for soils A and B result in similar conclusions. The time-varying mass-transfer coefficients necessary to fit the multi-region model to the 1- and 4-wk CFSTR data using soil A are plotted versus aqueous reactor volumes in

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Figure 8. Similar trends were observed for both experiments. The sequence of the calculated a values for the TritonX-100concentration.sused in the 1- and4-wkCFSTR

experiments is 300 > 30 > 3000 > 0 mg/L. Comparatively, the a values calculated in the 1-wk CFSTRs were larger than those in the 4-wk CFSTRs. VOL. 29. NO. 4, 1995 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

1077

TABLE 3

Calculated Percentages (Based on Multi=Region Model) of TCE Removed from Soil A after 40 Aqueous Reactor Volumes for 1- and 4-wk Continuous4low Stirred Tank Reactors CFSTR

TCE removed (%)

1-wk 0 mg/L 1-wk 30 mg/L 1-wk 300 mg/L 1-wk 3000 mg/L 4-wk 0 mg/L 4-wk 30 mg/L 4-wk 300 mg/L 4-wk 3000 mg/L

65.0 75.5 79.6 78.8 51.4

59.7 61.8 59.3

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FIGURE 8. Time-varying mass-transfer coefficients, a,used to fit the data obtained from the experiments studying the effect of 0,30, 300,and 3ooomg/LTriton X-lo0 solutionson the rate of TCE desorption from soil A after 1- and 4-wk equilibrations.

Discussion A KO,value of 73.1 L/kg was determined fiom the TCE/

Triton X-100 batch sorption experiments for the sorption of TCE from water, which is consistent with values found in the literature (5,41,42). Based on the statistical analysis of the sorption data presented in the Results section, the 900 and 3000 mg/L Triton X-100 solutions reduced the sorption of TCE to soil A, whereas the 30 and 300 mg/L solutions had no measurable effect. These results are consistent with previous research on the effects of surfactants on the sorption of relatively water-soluble organic solutes such as tetrachloromethane (32, 34). 1078 1 ENVIRONMENTAL SCIENCE & TECHNOLOGY I VOL. 29, NO. 4,1995

For soil A, the equilibrium model was unable to adequately describe the data obtained fromthe 1-and 4-wk CFSTRs, whereas the two- and multi-region kinetic models provided superior description of the data. The inability of the equilibrium model to describe the soil A CFSTR data provides strong evidence that the desorption of TCE from soilA is kinetically limited. However, the equilibrium model and the two- and multi-region kinetic models were able to fit the soil B CFSTR data reasonably well, indicating that the desorption of TCE from soil B was not kinetically limited for the CFSTR flow rates studied. The rate of TCE desorption from soil A was increased by all three Triton X-100 concentrations in the 1-and 4-wk CFSTRs using soil A. These results support the conclusions ofAronstein et al. (43)that surfactants, even at concentration less than critical micelle concentration, can increase the rate of solute desorption, thus increasing the bioavailability of the solutes. The increased rate of TCE desorption from soilA may have been caused by two mechanisms: the Triton X-100 may have increased the concentration gradient between the sorbed and aqueous phases and/or increased the mass-transfer coefficient, a. The results from the TCE/ Triton X- 100 batch sorption experiments and the Triton X-100 sorption kinetics experiment enable the cause of the enhanced desorption to be determined. According to the TCE/Triton X- 100 batch sorption experimental results, the concentration gradient was not increased in the 30 and 300 mg/L CFSTRs. The increased rate of TCE desorption in these CFSTRs was a result of an increased mass-transfer coefficient (a).Analysis of Figure 8 confirms that a was larger in the 30- and 300 mg/L CFSTRs than in the 0 mg/L CFSTRs. The a values were approximately 3 times larger in the 30 and 300 mg/L CFSTRs than in the 0 mg/L CFSTR after 40 aqueous reactorvolumes. Analysis of the TCElTriton X-100 batch data also indicates that the increased rate of TCE desorption observed in the 3000 mg/L CFSTRs may have been the result of an increased concentration gradient and/or an increased masstransfer coefficient. By incorporating the effect of the reduced distribution coefficient into the multi-region model, the effects of the two mechanisms were differentiated. Analysis of Figure 8 indicates that the enhanced desorption of TCE in the 1- and 4-wk 3000 mg/L CFSTRs was caused primarily by the increased concentration gradient. Although a was increased in the 3000 mg/L CFSTRs, the effects were secondary compared to the increased concentration gradient. However, the values of a were still approximately twice as high as the values of a observed in the 0 mg/L CFSTRs after 40 aqueous reactor volumes. The rate-limited sorption/desorption of nonionic organic compounds has been proposed to be a result of diffusive resistances encountered within the soil organic matter. At the time soil is exposed to contamination, a solute may diffuse into the soil organic matter along accessible “paths”. In response to environmental changes over extended periods of time, the alkyl and aryl functional groups of the soil humic and fulvic acids may rearrange, thus blocking old diffusive paths and creating new ones. For example, Murphy et al. (28)have noted that, depending on factors such as pH and ionic strength, structural orientation of humic substances may change from a coiled configuration to a more open configuration. When desorption is induced, some fraction of the sorbed solute may have a less tortuous path to travel for desorption into the

bulk solution while another fraction of the sorbed solute may have a more tortuous path to travel for desorption. This latter fraction of sorbate would cause observations of kinetic desorption effects, particularly if the rate of solute uptake of all the sorbed solute was relatively rapid. In theory, the diffusive resistances encountered by this fraction of sorbate could be reduced by altering the configuration of the soil organic matter to provide a less tortuous desorption path for the sorbate. Presumably, the addition of Triton X- 100 to the CFSTRs increased the rate of TCE desorption by reducing the tortuosity of the soil organic matter. TritonX-100may have caused the tortuosity to change by one or both of the following two mechanisms. First, the addition of a surfactant to water reduces the surface tension of the water until critical micelle concentration is reached ( 4 4 ) . Triton X-100 may have reduced the interfacial tension between the water and the soil organic matter, allowing the water to wet the hydrophobic regions of the soil organic matter. Therefore, the water content was increased, and the tortuosity of the soil organic matter was reduced. Second, the sorption of Triton X- 100 to the soil organic matter may have caused the organic matrix to expand and reduce the tortuosity. This is consistent with the observation that the sorption of nonionic surfactants is controlled by the soil organic matter (33,45).Citing increased rates of herbicide desorption from natural soil during miscible-displacement column experiments with increasing volume fractions of methanol, Nkedi-Kizza et al. (46)similarly hypothesized that methanol may cause “swelling” of the soil organic matter and thereby increased the rate of solute diffusion out of the soil organic matter and into the bulk solution. Evidence from this study suggests that there may be an optimal Triton X-100 concentration to enhance the masstransfer coefficient. At some threshold concentration level, it appears that additional surfactant starts to inhibit the mass transfer of solute from the soil organic matter into the water (Le., the 3000 mglL a values are lower than the 30 and 300 mglL a values). One possible explanation for this phenomena is that increased sorption of Triton X-100 to the soil organic matter may block more diffusive paths than are opened. The slower desorption rates observed in the4-wk CFSTRs compared to the 1-wkCFSTRs are consistent with previous findings that the extent of nonequilibrium sorption increases with contaminant equilibration time (2-9,21). The slower desorption rates in the 4-wk CFSTRs were presumably caused by greater configurational changes to the soil organic matter during the 4-wk equilibration period than during the 1-wk equilibration period. Although environmental factors such as pH, ionic strength, and temperature were constant during the 4-wk equilibration,the mechanical agitation from the rotaryshaker may have caused structural changes in the soil organic matter. These changes may parallel changes that occur in river sediments during repeated settling and resuspension between the river bottom and the water column. In contrast with the 1- and 4-wk CFSTRs using soil A = 0.24), Triton X-100 did not enhance the rate of TCE desorption from soil B = 0.014) because the desorption of TCE from soil B occurred at a rate approximated by an equilibrium sorption/desorption process (Figure 6). This result is consistent with results from previous research showing that the rate of sorbate desorption to water and extracting solvents from low organic carbon soil is rapid if

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the equilibration period is short (Le., days) (2, 36, 47). However, kinetic desorption during solvent extraction has been observed for low organic carbon soils that have been exposed to contamination for long time periods (Le.,years) (2-5, 36‘). Preliminary results of ongoing sod Column studies being conducted in the authors’ laboratory have shown that Triton X-100 increases the rate of TCE desorption from long-term field contaminated soils with low organic carbon contents. The dependence of desorption rates on the organic carbon content of the soil along with the lack of kinetic effects observed in the Br- CFSTRs that could be attributed to intraparticle diffusion indicate that diffusion through the soil organic matter is probably the primary cause of kinetic sorption and desorption observed in these studies. Carroll et al. (20) have also observed rates of desorption to increase with decreasing organic carbon content. Conclusions. The slow, kinetic desorption of organic contaminants from long-term contaminated aquifer soil to water limits the efficiency of pump-and-treatremediation systems. The use of surfactants to increase the rate of contaminant desorption from soil to water appears to be a viable remediation technology. Triton X-100 increased the rate of TCE desorption from soil A at concentrations above and below critical micelle concentration. The data obtained from the batch sorption experiments and the CFSTR experiments indicated that the increased TCE desorption rate in the 30 and 300 mglL CFSTRs was caused by an increase in the mass-transfer coefficient. In contrast, an increased concentration gradient appeared to be the primary cause of enhanced desorption in the 3000 mg/L CFSTRs. Evidence suggests that there is an optimal Triton X- 100 concentration for increasing the mass-transfer coefficient. The ability to increase the mass-transfer coefficient with an aqueous concentration of TritonX-100 below critical micelle concentration will reduce the material costs of injecting surfactant solutions into an aquifer. The rate of TCE desorption was also affected by the organic carbon content of the soil and the equilibration period. The desorption of TCE was slower from soil A = 0.24) than from soil B = 0.014). The correlation between soil organic carbon contents and TCE desorption rates supports the hypothesis that the kinetic desorption of contaminants from soil to water is caused by the ratelimited diffusion of the contaminants through the soil organic matter into the aqueous phase. Increased equilibration periods (times of exposure) decreased the rate of TCE desorption from soil to water in both the presence and the absence of Triton X-100.

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Acknowledgments The authors thank Thomas Imbrigiotta of the U.S. Geological Survey for technical and logistical support throughout this investigation. This research has been supported by the Office of Exploratory Research ofthe US. Environmental Protection Agency.

Notation BrC Cin CO

CFSTR

bromide ion aqueous phase solute concentration (mg/L) inflow solute concentration (mglL) initial concentration of solute in the CFSTR (mg/L) continuous-flow stirred tank reactor

VOL. 29, NO. 4,1995 I ENVIRONMENTAL SCIENCE &TECHNOLOGY

1079

fraction of the organic carbon in the sorbent fraction of sorbent that is kinetically limited equilibrium sorption distribution coefficient (Llkg) Freundlich sorption coefficient parameter organic carbon normalized distribution coefficient (Llkg) mass of soil inside the CFSTR (g) Freundlichsorption coefficient fitting parameter flow rate through the CFSTR (Llh) reaction term (mg L-’ h-l) total sorbed solute concentration (mglkg) concentration of TCE sorbed in the equilibrium regions (mglkg) concentration of TCE sorbed in the kinetic regions (mglkg) trichloroethene volume of water in CFSTR (L) mass-transfer coefficient (Llh)

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(18) Brusseau, M. L.; Rao, P. S. C. Environ. Sci. Technol. 1991, 25, 1501- 1506. (19) Pignatello, J. J. In Reactions and movement oforganic chemicals in soils; SSSA Special Publication 22; Sawhney, B. L., Brown, K., Eds.; Soil Science of America: Madison, WI, 1989; pp 45-80. (20) Carroll, K. M.; Harkness, M. R.; Bracco, A. A,; Balcarcel, R. R. Environ. Sci. Technol. 1994, 28, 253-258. (21) Pignatello, I. J.; Ferrandino, F. J.; Huang, L. Q. Enuiron. Sci. Technol. 1993, 27, 1563-1571. (22) Kan, A. T.; Fu, G.; Tomson, M. B. Environ. Sci. Technol. 1994, 28, 859-867. (23) Deitsch, J. J.; Smith, J. A. Water-Resour. Invest. Rep. (U.S. Geol. Sun/.) 1994, NO. 93-4015. (24) Karickhoff, S. W.; Brown, D. S.; Scott, T. A. Water Res. 1979,13, 241-248. (25) Chiou, C. T.; Peters, L. J.; Freed, V. H. Science 1979, 206, 831832. (26) Smith, J. A.; Witkowski, P. J.; Chiou, C. T. Rev. Environ. Contam. Toxicol. 1988, 103, 127-151. (27) Beckett, R.; Jue, Z.; Giddings, J. C. Environ. Sci. Technol. 1987, 21, 289-295. (28) Murphy, E. M.; Zachara, J. M.; Smith, S. C.; Phillips,J. L.; Wietsma, T. W. Environ. Sci. Technol. 1994, 28, 1291-1299. (29) Edwards, D. A.; Liu, Z.; Luthy, R. G. J. Environ. Eng. (N.Y.) 1994, 120, 5. (30) Kile, D. E.; Chiou, C. T. Enuiron. Sci. Technol. 1989,23,832-838. (31) Kile, D. E.; Chiou, C. T. Environ. Sci. Technol. 1990,24,205-208. (32) Smith, J. A.; Tuck, D. M.; Jaff6P. R.; Mueller, R. T. Org. Substances Sediments Water 1991, 1 , 201-230. (33) Edwards, D. A.; Luthy, R. G.; Liu, Z. Enuiron. Sci. Technol. 1991, 25, 127-133. (34) Vigon, B. W.; Rubin, A. J. 1. Water Pollut. Control Fed. 1989, 61, 1233-1240. (35) Imbrigiotta, T. E.; Martin, M. Water-Resour.Invest. Rep. (U.S. Ge01. Sun/.) 1989, NO.88-4220, 343-350. (36) Sawhney, B. L.; Pignatello, J. 7.; Steinberg, S. M. J. Environ. Qual. 1988, 17, 149-152. (37) Smith J. A.; Jaff6,P. R.; Chiou, C. T. Environ. Sci. Technol. 1990, 24, 1167-1172. (38) Cameron, D. A.; Klute, A. WaterResour. Res. 1977, 13, 183-188. (39) Rao, P. S. C.; Davidson, J. M.; Jessup, R. E.; Selim, H. h4. Soil Sci. SOC.Am. J. 1979, 43, 22-28. (40) De Camargo, 0.A.; Biggar, J. W.; Nielsen, D. R. SoilSci. SOC.Am. J. 1979, 43, 884-890. (41) Peterson, M. S.; Lion, L. W.; Schoemaker, C. A. Environ. Sci. Technol. 1988,22, 571-578. (421 Chiou, C. T.; Porter, P. E.; Schmedding, D. W. Environ. Sci. Technol. 1983, 17, 227-231. (43) Aronstein, B. N.; Calvillo, Y. M.; Alexander, M. Environ. Sci. Technol. 1991,25, 1728-1731. (44) Fountain, J. C.; Klimek, A.; Beikirch, M. G.; Middleton, T. M. J. Hazard. Mater. 1991, 28, 295-311. (45) Urano, K.; Saito, M.; Murata, C. Chemosphere 1984, 13, 293300. (46) Nkedi-Kizza, P.; Brusseau, M. L.; Rao, P. S. C.; Hornsby, A. G. Environ. Sci. Technol. 1989, 23, 814-820. (47) Pignatello, J. J. Environ. Toxicol. Chem. 1990, 9, 1107-1115.

Received for review August 16, 1994. Revised manuscript received December 13, 1994. Accepted December 27, 1994.@

ES940526H @

Abstract published in Advance ACS Abstracts, February 1, 1995.

Effect of Triton X-100 on the Rate of Trichloroethene Desorption from

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