Aquifer washing by micellar solutions: 2. DNAPL recovery mechanisms for an optimized alcohol–surfactant–solvent solution

Ž .Journal of Contaminant Hydrology 30 1998 1–31

Aquifer washing by micellar solutions: 2. DNAPLrecovery mechanisms for an optimized

alcohol–surfactant–solvent solution

Richard Martel a,b,), Rene Lefebvre a, Pierre J. Gelinas b´ á INRS-Georessources, 2535, boul. Laurier, C.P. 7500, Sainte-Foy, Que. GIV 4C7 Canada´

b GREGI, Department of Geological Engineering, LaÕal UniÕersity, Quebec, Que. G1K 7P4 Canada´

Received 6 June 1996; revised 19 February 1997; accepted 19 February 1997

Abstract

A large sand column experiment is used to illustrate the principles of complex organicŽ . Ž .contaminants DNAPL recovery by a chemical solution containing an alcohol n-butanol , a

Ž . Ž .surfactant Hostapur SAS , and two solvents d-limonene and toluene . The washing solution ispushed by viscous polymer solutions to keep the displacement stable. The main NAPL recovery

Ž . Ž .mechanisms identified are: 1 immiscible displacement by oil saturation increase oil swelling ,Ž .oil viscosity reduction, interfacial tension lowering, and relative permeability increase; 2

miscible NAPL displacement by solubilization. Most of the NAPL was recovered in a Winsor,Ž .type II system ahead of the washing solution. The 0.8 pore volume PV of alcohol–surfactant–

solvent solution injected recovered more than 89% of the initial residual DNAPL saturationŽ .0.195 . Winsor system types were determined by visual observation of phases and confirmed byelectrical resistivity measurements of phases and water content measurements in the oleic phase.Viscosity and density lowering of the oleic phase was made using solvents and alcohol transferfrom the washing solution.

Small sand column tests are performed to check different rinsing strategies used to minimizewashing solution residual ingredients which can be trapped in sediments. An alcoholrsurfactantrinsing solution without solvent, injected behind the washing solution, minimizes solvent trappingin sediments. More than five pore volumes of polymer solution and water must be injected afterthe rinsing solution to decrease alcohol and SAS concentrations in sediments to an acceptablelevel. To obtain reasonable trapped surfactant concentrations in sediments, the displacement front

) Corresponding author. Fax: q1 418 654-2615; E-mail: [email protected]

0169-7722r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0169-7722 97 00030-2

( )R. Martel et al.rJournal of Contaminant Hydrology 30 1998 1–312

between the rinsing solution and the subsequent the following polymer solution has to be stable.q 1998 Elsevier Science B.V.

Keywords: Surfactant; Alcohol; Solvent; d-limonene; n-butanol; Hostapur SAS

1. Introduction

1.1. NAPL contamination remediation

Ž .Nonaqueous phase liquids NAPLs cannot be completely recovered from contami-Žnated aquifers by conventional pump-and-treat technology Mackay and Cherry, 1989;

.Haley et al., 1991 . Capillary forces retain NAPLs as immobile drops in porous orfractured media. The NAPLs trapped at residual saturation, which occupy between 5 and

Ž .40% of the pore volume Schwille, 1984 , have low water solubility. Decades arerequired for their dissolution by water flowing through zones where they are present. Alimited number of technologies is available for the restoration of sites contaminated byimmiscible liquids. Most of the time, NAPL recovery from sediments requires theirexcavation and off-site treatment. However, in certain cases, excavation is not feasible

Ž .or cannot be done economically, for example when the NAPL source: 1 is located atŽ . Ž .depth in the saturated zone; 2 is located under buildings; or 3 contaminates a large

volume of sediments. Biotechnologies are commonly used for NAPL remediation butŽ .they are not efficient for the restoration of sediments which: 1 are highly contami-

Ž .nated; or 2 contain chlorinated solvents. Other technologies such as air sparging orbio-venting work well for volatile compounds but cannot be applied when mainlylong-chain hydrocarbons are present in sediments. Surfactant solutions can enhanceremediation by increasing NAPL mobility and solubility to improve pump-and-treat

Ž .performance EPA, 1995 .

1.2. Research program

This study is part of a four-year research project aiming to develop surfactantsolutions to restore contaminated soils by in-situ dissolution of DNAPLs such as those

Ž . Ž .present at the Mercier site Quebec MEF, 1994 . The development of this in-situ´Ž . Ž .technology requires work at many scales Fig. 1 : 1 firstly, at the vial scale for the

selection of washing solutions by phase diagram construction, then for the evaluation ofwashing solutions compatibility with aquifer material, and finally for the measurement

Ž . Ž . Ž .of physical properties Martel et al., 1993, 1998b ; 2 in small sand columns 130 g forŽ . Ž .washing solution optimization Martel and Gelinas, 1996; Martel et al., 1998b ; 3 in´

Ž .large sand columns 65 kg to investigate, in detail, DNAPL recovery and transportŽ . Ž .mechanisms this paper ; 4 in 2-D sand boxes to reproduce permeability contrasts and

evaluate the performance of polymer solutions used to improve front stability of theŽ . Ž .injected washing solutions Martel et al., 1997a ; 5 in etched glass micromodels to

Ž . Ž .simulate NAPL recovery in fractured bedrock Tittley, 1994 ; and finally 6 in the fieldŽ .for an evaluation of the technology at a pilot scale Martel et al., 1998c .

( )R. Martel et al.rJournal of Contaminant Hydrology 30 1998 1–31 3

Fig. 1. Scales of study in the surfactant solution research project.

1.3. Proposed injection sequence

The injection sequence proposed for field application is derived from the petroleum-Ž .enhanced recovery literature Lake, 1989 but adapted to the special needs of environ-

mental remediation. Such a sequence was applied at a pilot scale for DNAPL recoveryŽ . Ž .Martel et al., 1998c Fig. 2 . The first fluid injected was a polymer solution as apreflush step to limit preferential flow of the washing solution in the more permeablelayers. This step also avoids preferential adsorption of washing solution ingredients onto

Ž . Ž .sediments Martel et al., 1998b . Then, the washing solution alcohol–surfactant–solventis injected to recover the trapped NAPL. The rinse cycle starts with the injection of analcohol–surfactant solution to recover the solvent used in the washing solution. Thealcohol–surfactant solution contains xanthan gum to maintain a stable front at thewashing solution interface. The rinse cycle continues with many pore volumes ofxanthan gum solution followed by many pore volumes of water. Water injection isnecessary before inoculation of the sediments. Xanthan gum must also be eliminatedbecause it is a preferential carbon source for bacteria and delays biodegradation of

Ž .washing solution ingredients Roy et al., 1995 . Since the objective of the experiment isto document recovery mechanisms with surfactant solution flooding, there was neither a


Fig. 2. Schematic representation of the proposed injection sequence in the field.

preflush step with a polymer solution nor an inoculation of the sediments in the columnfor the biodegradation of ingredients at the end of the test.

1.4. ObjectiÕes

This paper presents laboratory results obtained from a DNAPL recovery test in alarge sand column with an optimal alcohol–surfactant–solvent solution. The aim of thistest was not to completely recover the DNAPL, but rather to document DNAPLrecovery mechanisms. Different rinsing strategies were also studied in small sandcolumns to minimize residual ingredients from the washing solution left in sedimentsafter the test. Field injection strategies based on these results are also discussed. The

Ž .objective of this paper is: 1 to illustrate the principles of complex contaminantŽ .recovery by micellar solution; and 2 to discuss field injection strategies.

2. Background

2.1. Oil trapping and recoÕery in porous media

Three forces act on NAPL droplets present in a porous medium: capillary forces,Ž .gravitational or buoyancy forces, and viscous forces Bradner and Slotboom, 1975 .

Ž .Capillary forces P are responsible for NAPL trapping as they resist their mobilization.c

They are defined by the Young–Laplace equation:2s cos uow wr

P s 1Ž .c rŽ .where s is the NAPL–aqueous phase interfacial tension Nrm , u is the water-solidow wr

Ž . Ž .contact angle in the presence of NAPL wettability , and r is the radius metres of theŽ .pore containing the interface. Buoyancy forces D P are hydrostatic in nature. TheB


Ž .hydrostatic pressure drop through a NAPL droplet is a function of the height h metresŽ .of the droplet and is given by Eq. 2 :

D P sD r gh 2Ž .B

Ž y3 .where D r is the NAPL–aqueous phase density difference kg m and g is theŽ y2 .gravitational acceleration m s . Viscous forces are created by the flow of the

displacing fluid in the porous media. When a displacing fluid flows past a droplet of aŽ . Ž . Ž .given length for horizontal flow or a given height for vertical flow L metres , theŽ .pressure drop D P in the displacing fluid is obtained from Darcy’s Law:V

VLmD P s 3Ž .V k

Ž y1 .where V is the darcy velocity or the specific discharge m s , m is the dynamicŽ y1 .viscosity of the displacing fluid mPa s , and k is the intrinsic permeability of the

Ž 2 .porous media m .Capillary forces can be reduced by lowering the interfacial tension or by changing

Žwettability. They can also be offset by increasing viscous or buoyancy forces Morrow,.1979 . For NAPL recovery, water flooding, without interfacial tension reduction, leaves

Žresidual NAPL as various size ganglia Larson et al., 1980; Chatzis et al., 1983;.Wardlaw and McKellar, 1985 . Considering the practical limits of water flooding,

Ž . Žsurfactant solutions can mobilize NAPL mainly by 1 decreasing capillary forces lower. Ž . Žinterfacial tension, modify wettability , but also by 2 increasing viscous forces higher

. Ž . Žviscosity of the displacing fluid and, by 3 increasing buoyancy forces larger density.contrast between aqueous and NAPL phases . Surfactant solution can also increase the

‘‘solubility’’ of NAPL in water by micellar solubilization andror encapsulation inŽ .microemulsions Baran et al., 1994 .

2.2. PreÕious studies

ŽMost of the previous laboratory studies on surfactants have used large volume up to. Ž .19 pore volumes and low concentration surfactant solutions 0.1–4% for NAPL

Žrecovery Texas Research Institute Inc., 1977, 1979, 1985; Ellis et al., 1984, 1985; Nashet al., 1987; Hurtig et al., 1988; Tuck et al., 1988; Gannon et al., 1989; McDermott etal., 1989; Vigon and Rubin, 1989; Abdul et al., 1990; Ducreux et al., 1990; Clarke et al.,

.1991; Fountain et al., 1991; Kan et al., 1992; Pennell et al., 1992; Peters et al., 1992 .ŽSome field tests with these solutions are also reported Nash, 1988; Sale et al., 1989;

.Abdul et al., 1992 . In this study, a small volume of a concentrated alcohol–surfactant–solvent solution was used to recover DNAPL. Previous works show that the combina-tions of alcohols and surfactant to dissolve light oils and solvents are more effective than

Ž .the use of either of them separately Desnoyers et al., 1983a; Martel et al., 1993 .Complete diesel dissolution in a Winsor type I system was achieved in sand columnwith a concentrated alcohol–surfactant solution. Unfortunately, these solutions cannot

Ždissolve significant quantities of heavy and viscous Mercier DNAPL Martel et al.,.1993 . However, if a light organic solvent is added to the alcohol–surfactant mixture,

Mercier DNAPL can be dissolved, confirming similar findings with bitumen recovery inŽ .tar sands Desnoyers et al., 1983b; Sarbar et al., 1984 .


The washing solutions were selected through pseudo-ternary phase diagrams andŽ .optimized by sand column experiments Martel et al., 1998b . The principle of the

method is to create water-in-NAPL or NAPL-in-water microemulsions in Winsor type IIŽ . Ž .and type I systems Winsor, 1954 . No Winsor type III system middle phase , where

Ž y3 y4 y1.interfacial tension is ultra-low 10 –10 mN m is expected, since solvents areŽused to dissolve DNAPL. NAPL recovery is a function of displacement type miscible

.or immiscible but depends also, in a porous medium, on volumetric sweep efficiencyŽ .areal and vertical . In sand column experiments, sweep efficiency is a function of thestability of the displaced front which depends on fluids densities and viscosities, and onporous media heterogeneities. The effect of front stability on injected slug dilution willbe discussed in this paper.

3. Procedures for the large sand column experiment

3.1. Sand column filling

Ž .The column consists of a glass cylinder filled with sand Fig. 3 . Its properties arelisted in Tables 1 and 2. The upper end is sealed by a perforated Teflon cap which is

Ž .covered by a glass plate 1 cm thick for the observation of fluid circulation. The lowerend is sealed by a perforated stainless steel cap which is connected to a conic reservoir.The seal between the glass cylinder and the lower cap is provided by a Viton gasket. A

Ž .stainless steel screen 51 mm inside each cap prevents the loss of fines. To preventradial segregation of sand particles, dry sand was placed in the column in thin 1 cm

Ž .layers 800 g . The link between layers was obtained by the scarification of the upper 0.5cm of each layer before the addition of the next layer. Column compaction was achieved

Ž . Ž .by dropping a weight 1 kg from a controlled height 50 cm , 12 times for each layer. Adry density of 1721 kg my3 was obtained with this technique, which is sufficient to

Ž y3 . Ž .prevent preferential liquid channelling over 1700 kg m Ripple et al., 1973 . Intersti-tial air is eliminated from the sand column by circulating 18 pore volumes of carbondioxide. The water-soluble carbon dioxide is eliminated when the column is fullysaturated with deaerated, distilled, and demineralized water. The sand used for thecolumn experiments is the Mercier sand sieved to remove clay and silt size particleswhich represent 5% of the original material. Removing these particles prevents problemsrelated to fine materials. Physical characteristics of the sand are described in Table 2.

3.2. Residual DNAPL saturation in the column

Mercier DNAPL collected in the field in 1992 was used for the lab experimentsperformed in 1994. The DNAPL contains thousands of organic compounds and up to

Ž .16% of chlorinated solvents. At lab temperature 258C , Mercier DNAPL is moreŽ . Ž .viscous and denser than water Table 3 . To reach residual oil saturation S , DNAPLor

was injected by gravity into the sand column using a Teflon tube connected to a Teflonfunnel and pushed with distilled and deaerated water at high velocity with the help of aperistaltic pump. Initially, 2277.24 g of Mercier DNAPL was introduced upward under a


Fig. 3. Large sand column for washing experiment.

gradient of 0.40–0.45 and pushed with 34 l of water at 3.58 cm miny1. During waterinjection, no DNAPL was recovered in the effluent but DNAPL in the sand was near theupper end. An additional 519.65 g of DNAPL was introduced downward in the column

Ž .and pushed with 14 l of water. A small quantity of DNAPL 3.9 g was recovered at thebottom effluent. A total of 2793 g of Mercier DNAPL remains trapped in the column

Ž .and gives a residual DNAPL saturation S of 0.195 which is double that observed inorŽ .other small sand column experiments Martel et al., 1998b . Consequently, the pore

volume available for water circulation decreases from 13 658 to 10 998 cm3 and theŽ .water-filled porosity of the column changes from 0.3634 to 0.2926 Table 4 . The

presence of residual DNAPL in the column decreases the hydraulic conductivity of thecolumn by 70%.

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Table 1Characteristics of sand column

aColumn characteristics Length Diameter Area Volume Mass of sand Sand dry density Pore volume Porosity K2 3 y3 3 y4 y1Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .cm cm cm cm g kg m cm 10 m s

Large column 99.5 21.93 377.72 37583 64680 1721 13658.7 0.3634 2.31Small columns 13.80"0.04 3.735 10.96 151.21"0.46 257"12 1700"34 52.26"1.93 0.3457"0.0127 1.8"1.5

a Hydraulic conductivity.

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Table 2Characteristics of sand and solids

a bSand characteristics Grain sizes d Specific surface area Organic carbon Carbonates Quartz Feldspar Muscovite Chlorite1502 y1Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž . Ž .mm mm m g % % % % % %

Ž .Mercier sand without fines large column 0.063–4 0.6 0.006 0.57 62 34 4 0 0Ž .Mercier sand with fines small column -0.002–5 0.5 0.014 0.64 32 29 8 21 10

a Mean grain diameter.b Ž .Walky black method Agriculture Canada, 1977 .

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Table 3Physical properties of fluids injected in the sand column

a a bInjection Solution type Mass of fluid Pore Density Viscosity Darcy velocity Mobility Density Displacementy3 y1 y1Ž . Ž . Ž . Ž .sequence injected g volume kg m mPas m d ratio contrast front stability

y 1Ž .5 Polymer solution 390mgl 17540 1.31 997.05 7.0 0.39 M)1 D r g sin a s0 Unstabley1Ž .4 Polymer solution 600mgl 27920 2.09 997.05 19.8 0.39 M)1 D r g sin a -0 Unstable

Ž .3 Rinsing solution 3536.5 0.27 992.75 24.7 0.39 M-1 D r g sin a -0 Conditionally stable type II2 Washing solution 10149 0.80 946.20 7.8 0.38 M-1 D r g sin a )0 Stable1 Water 997.05 0.89

Mercier DNAPL 1048.18 15.64

a Ž .Viscosity m at 258C and at a shear rate of 1 per second for solutions with polymers.b Ž .Flow rate per area of the column m per day .M is viscosity ratio of the fluid ahead to the fluid injected.

Ž y3 .D r is density difference between the fluid ahead and the fluid injected kgm .Ž 2 y1.g is gravitational constant 9.8 m s .

Ž .a is dip angle of the sand column in radian for downward injection a s1.5p .


Table 4Measured physical parameters for the sand column

Measured at water Measured at residual Measured at final DNAPLsaturation DNALPL saturation saturation after washing

Water saturated porosity 0.3634 0.2926 0.35583Ž .Water-filled pore volume cm 13658.7 10998.7 13372.7

y1 y4 y5Ž .Hydraulic conductivity m s 2.31=10 7.14=10 —3Ž .Volume of DNAPL in the sand cm 0 2660 286

S 0 0.1947 0.0209or

3.3. Sand column washing

The test recovery of residual DNAPL saturation in the large sand column wasŽ .performed at lab temperature 25"38C with previously optimized washing solution

Ž .Martel et al., 1998b . The different solutions were injected downward at an averagevelocity of 1.09"0.19 m per day. Downward washing was selected because the columnwas designed for vertical fluid circulation and because the displacement front is stable

Ž .when a lighter and more viscous washing solution is displacing water Lake, 1989 .DNAPL concentrations in soils and liquids were determined by spectrophotometryŽ .HACK DRr200 at 550 nm. Dichloromethane was used as a solvent for massdetermination of DNAPL. With this technique, DNAPL mass balance in the sandcolumn is determined with a precision of "6%. The analytical method is described in

Ž .Martel et al. 1998b . All components of the washing solution were analyzed in therecovered fluid and in the sand after the washing experiment. Solvents and alcoholcontents were determined by a temperature programmable gas chromatograph and a

Ž .flame ionization detector GCrFID . Surfactant concentrations were evaluated by aŽ .colorimetric technique described in Zhen Cao 1978 .

During DNAPL recovery, the mobile aqueous fluid volume present in the columnchanges because of the removal of residual oil. Thus, there is no constant reference fluidphase pore volume during the experiment. We arbitrarily use as a reference, pore

Ž . Žvolume V , the mobile aqueous phase saturated pore volume at the end of the test i.e.o. Ž .at the final small residual DNAPL saturation Table 4 . The rationale for this choice of

reference is that the mobile pore volume is modified as soon as the washing solution isinjected by the creation of an oil bank ahead of the washing solution. The mobile porevolume left behind the oil bank is very close to the final volume. This way, we alsoaccount for both the aqueous and oil phases produced in the effluent, which are mobile.

Ž .V is the volume of liquid aqueous and NAPL recovered in the column effluent.Ž .Residual DNAPL saturation in the column was partly removed with 0.80 PV VrVo

Ž . Žof washing solution. The solution contains an alcohol n-butanol , a surfactant Hostapur. Ž .SAS , and two solvents toluene and d-limonene in respective mass concentrations of

Ž .9.21%, 9.21%, 13.16% and 13.16%. Sodium orthosilicate 0.3% was added to theŽ .solution to minimize surfactant retention Lake, 1989 and to decrease washing solution

Žefficiency in order to simulate the lower field groundwater temperature Martel et al.,.1997a . The washing solution was followed by a 0.27 PV of rinsing solution containingŽ . Ž .alcohol n-butanol and surfactant Hostapur SAS . The rinsing solution contains 20% of


Ž . y1n-BuOHrSAS mass ratio of 1.0 and 620 mg l of xanthan gum to increase itsviscosity to 24.7 mPa sy1. Finally, 3.4 PV of polymer solution were injected with an

y1 Ž .initial concentration of 600 mg l of xanthan gum 2.09 PV and a final concentrationy1 Ž .of 390 mg l 1.31 PV to rinse the n-BuOHrSAS solution. The effluents were

recovered in 54 jars for DNAPL, d-limonene, toluene, n-BuOH, and SAS determina-tions. After the experiment, six sand samples were collected at different depths in thecolumn to determine residual DNAPL and ingredients concentrations in the sand.

4. Results

4.1. Effluents description

4.1.1. Effluents concentration and mass balancePolymer solution behavior was not studied since a satisfactory analytical method was

not available. Concentrations at breakthrough of the washing solution ingredients areŽ . Ž .presented in Fig. 4 a and Fig. 4 b . Ingredients concentrations were measured in shaken

Ž . Žsamples mixed aqueous and oil phase . n-BuOH and SAS emerge simultaneously Fig.Ž ..4 a and show no retardation or chromatographic effect. A long tailing is observed

Ž .however, with both ingredients. Mass balance of ingredients Table 5 was done toevaluate mass loss or analytical problems and to verify the accuracy of the test. Massbalance compares the mass of ingredients in recovered fluids and in sediments at the endof the test to the mass of ingredients injected in the column. Mass balance of SASŽ . Ž .117% and n-BuOH 109% show that the test was done well and that analytical resultsare reliable. The masses of SAS and n-BuOH are overestimated mainly because theDNAPL interferes with the chemical analysis method. SAS and n-BuOH did not reachinjection concentrations during the rinsing sequence. Dilution of the rinsing solution bythe polymer solution is obvious as shown by the tailing effect. Surfactant retention insand is discussed further later.

Analytical problems with toluene and d-limonene are indicated by the mass balancefor these components. The sum of the analyzed masses in the effluent and the columnsand represent respectively 65.7 and 70.3% of the toluene and d-limonene masses

Ž .initially injected Table 5 . This mass loss cannot be explained by biodegradationŽ .because bacteria cannot survive during the washing process Roy et al., 1995 . Nor can

it be explained by solvent adsorption on sand because an error of 2400% on toluene and940% on d-limonene analysis would have to be attributed. It also cannot be explainedtotally by solvent volatilization because it was minimized during the test, and chemicalanalyses were performed a short period of time after the test. n-BuOH which is asvolatile as toluene or d-limonene did not show signs of volatilization during the test.Solvent loss cannot be explained by fluid loss because mass balance of fluids confirm

Ž .the agreement between the injected and recovered fluids Table 5 . We believe that theanalytical results give a systematic underestimation of toluene and d-limonene concen-trations in fluids and that no mass is actually lost. This phenomena is frequently

Ž . Ž .Fig. 4. a Relative concentration of n-butanol and SAS in the column effluent. b Relative concentration oftoluene and d-limonene in the column effluent.



Table 5Mass balance on fluids and ingredients in the large sand column test

Fluids DNAPL SAS n-BuOH Toluene d-LimoneneŽ . Ž . Ž . Ž . Ž . Ž .l g g g g g

Injected 59.898 2793.0 1288.00 1288.00 1335.00 1335.00aRecovered in effluent 60.862 2498.1 1363.53 1404.00 857.75 891.36

Analysed in sand — 175.8 149.43 n.d. 20.00 47.16Deviation q0.964 y119.1 q224.96 q116.00 y457.25 y396.48

Ž .Mass balance ratio % 101.6 95.7 117.5 109.0 65.7 70.3Ž .recoveredrinjected

a Ž . Ž . Ž .Recovered effluent 62.817 l plus delay 0.428 l minus recovered DNAPL 2.383 l .

observed when the calibration curve of solvents is made with water, whereas samples ofŽeffluent contain other organic compounds Yvon Couture 1996, personal communica-

. Ž Ž ..tion . The corrected solvent concentrations Fig. 4 b are adjusted to give a massbalance of 100%. The conclusions regarding solvent behavior are based on these

Žcorrected concentrations. Toluene and d-limonene have very similar behavior Fig.Ž ..4 b . No retardation is observed, but mass loss occurs during washing through the

transfer of solvents into the trapped DNAPL phase. This is apparent when comparing theŽ .respective widths in terms of PV of the solvents relative to SAS or n-BuOH. The first

peak at 1.1 PV corresponds to solvents present in the injected washing solution, whereasthe second peak at 1.8 PV is related to the solvents recovered by the solvent freeŽ .n-BuOHrSAS rinsing solution.

Ž .For this washing experiment, mass balance for the DNAPL is very good Table 5 .The sum of the analysis for the effluent and sediments totals 95.7% of the DNAPL

Ž .initially put in the column. The estimated maximum residual DNAPL saturation SorŽ Ž .. Ž .recovery varies from 89.4% Fig. 7 a based on DNAPL determination in the effluent

Ž .to 93.7% based on DNAPL determined in the sand . The 89.4% recovery is consideredin this paper because the effluent is studied in detail.

4.1.2. Injected and produced fluids concentrations and physical propertiesAs the ingredients of the washing solution transfer in the oil phase, density and

Ž Ž . Ž ..viscosity change Fig. 5 a and Fig. 5 b . Density measurements were made with aŽ .picnometer and viscosity with a viscometer Cannon-Fenske . DNAPL density is

Ž .reduced at the beginning because a partitioning tracer test not discussed here usingy1 Ž1100 mg l of iso-butanol in water was performed before the flooding experiment Fig.

Ž .. Ž y3 .5 a . Oil phase density increases owing to SAS density of 1050 kg m transfer at theoil–water interface. At 0.95 PV, the DNAPL becomes a LNAPL because the solventsand n-butanol have partitioned into the oil phase. A lighter oil phase increases upwardbuoyancy forces which is unfavorable in a downward flooding experiment. At 1.2 PV,DNAPL dissolved in the washing solution increases its density. The aqueous phasedensity remains close to water density as most of the ingredients of the washing solutiontransfer into the oil phase.

Oil phase viscosity decreases from its original value to reach the washing solutionŽ Ž ..viscosity at 1.2 PV Fig. 5 b . Further decreases in oil viscosity are related to water


Ž . Ž .Fig. 5. a Injected fluid density and produced phase density in the column effluent. b Injected fluid viscosityand produced oil phase viscosity in the column effluent.


Ž Ž ..transfer into the oil phase Fig. 6 e . A lower oil phase viscosity decreases the viscousforces required for its mobilization and makes the recovery easier. In order to maintain astable front between the preflush, the washing and the rinsing solutions, the viscosity ofthese injected fluids increases. The following rinsing sequence with polymer solutionshave unstable displacement fronts. The first polymer solution which has a viscosity of19.8 mPa sy1 should have been more viscous to prevent fingering in the rinsing solution.The low viscosity polymer solution is injected at the end of the process to bring theporous media close to its water saturation. Front stability is discussed later in this paper.

4.2. DNAPL recoÕery mechanisms

4.2.1. Produced fluid phase behaÕiorŽ Ž .. Ž .DNAPL was recovered in different phases Fig. 6 a : 1 as a separate DNAPL

Ž .phase in the oil bank ahead of the washing solution; 2 dissolved in a water-in-oilŽ Žmicroemulsion phase denser than the water phase Winsor type II system Winsor,

.. Ž .1954 in the core of the oil bank; 3 dissolved in a water-in-oil microemulsion phaseŽ . Ž .lighter than the water phase Winsor type II system in the tail of the oil bank; 4

Ž .dissolved in a oil-in-water microemulsion phase Winsor type I system within theŽ .washing solution slug; 5 dissolved in an oil-in-water microemulsion phase and in an

Ž .oil phase lighter than the microemulsion Winsor type I system within the rinsingsolution. In this oil recovery experiment, phase conditions go from a Winsor type IIsystem to a Winsor type I without the observation of a Winsor type III system where amiddle-phase microemulsion in equilibrium with an aqueous and an oleic phase is

Ž y1 .observed. Since IFT are kept high over 0.1 mN m with an alcohol–surfactant–solventsolution, it is normal not to observe a type III system. Most of the DNAPL wasrecovered under Winsor type II conditions. Winsor type I conditions can recover littleDNAPL since optimal washing conditions did not persist because the injected washingsolution slug was very small.

Ž Ž ..Initially, the minor oil phase recovered is pure DNAPL Fig. 6 a . SurfactantŽ .transfers at the oil–water interface and decreases interfacial tension IFT . This IFT drop

increases NAPL mobilization and the oil proportion in the effluent. Because oilsaturation is increased and, therefore, so is its relative permeability, an oil bank iscreated. The proportion of oil phase in the effluent is maintained between 0.8 and 1.0

Ž Ž ..PV Fig. 6 b and is considerably increased between 1.0 and 1.3 PV owing to solventŽ Ž ..and alcohol transfer from the washing solution to the oil phase Fig. 6 c . Even if the

Ž Ž ..oil phase proportion is kept high Fig. 6 b , DNAPL recovery in water-in-oil mi-croemulsion is slowing down as less and less DNAPL is available in the porous mediaŽ Ž .. Ž . ŽFig. 6 a . At 1.2 PV, the oil phase water-in-oil microemulsion phase disappears Fig.Ž .. Ž6 b and the DNAPL is recovered in the aqueous phase oil-in-water microemulsion

. Ž .phase . A very small oil phase LNAPL is also produced between 1.9 and 2.25 PV.

Ž . Ž .Fig. 6. a Type of phase environments produced in the column effluent, b Mass fraction of phases producedŽ .in the column effluent. c Concentrations of ingredients of the washing solution in the aqueous and oil phases.

Ž . Ž .d Water content in the oil and microemulsion phases produced. e Electrical resistivity of phases producedŽ .in the column effluent. f Partition coefficient of ingredients in phases produced in the column effluent.



Ž .Fig. 6 continued .


Ž .Fig. 6 continued .


Finally, the effluent is 100% aqueous phase to the end of the test, since no significantquantity of washing ingredients are in the fluid of the column.

Visual description of phases was confirmed by water content measurements in oil andŽ Ž ..microemulsion phases Fig. 6 d and resistivity measurements of aqueous and oil

Ž Ž ..phases Fig. 6 e . The water content was measured by the ASTM D 1744-83 methodŽ . Ž .Karl Fischer method ASTM, 1990 . The resistance was measured with an ohmmeterŽ .Beckmantech 310 or Micronta 22-212 . The mobilized DNAPL, as pure phase or in a

Ž . Ž Ž ..water-in-DNAPL microemulsion, has a low water content less than 5% Fig. 6 d andŽ . Ž Ž ..high resistivity over 2000 ohms Fig. 6 e . At 1.0 PV the washing solution front

breaks through. The resistivity drop in the oil phase is related to increasing water contentin the oil phase. Water content in the oil phase contributes to a change in its density andthe water-in-DNAPL microemulsion phase becomes lighter than the aqueous phase.Between 1.0 and 1.2 PV, the proportion of water in the microemulsion phase continues

Ž Ž .. Žto increase and reaches 50% Fig. 6 d . At this point, the water-in-DNAPL oil. Ž .external microemulsion becomes a DNAPL-in-water water external microemulsion.

At 1.5 PV, the water content of the microemulsion is higher than the water content ofŽ .the washing solution 60% because of the dilution created by the rinsing solution.

Between 1.2 and 1.5 PV, the decreasing DNAPL concentrations in the microemulsionŽ . Ž Ž ..phase aqueous phase decreases resistivity Fig. 6 e . At 1.8 PV, the rinsing solution

breaks through and the electrical resistivity of the aqueous phase begins to increase.Ž .Partition coefficient K is the concentration ratio of an organic compound in the oleicdw

Ž Ž ..phase to the aqueous phase Fig. 6 f . K indicates that n-butanol is hydrophile,dw

solvents are lipophile and SAS is intermediate. At 1.0 PV, owing to SAS breakthroughs, K of solvents is lowered by one order of magnitude for toluene and threedw

order of magnitude for d-limonene.

4.2.2. DNAPL recoÕeryŽ .Fig. 7 a shows the cumulative DNAPL recovery at the column effluent. The first

34% of the recovered DNAPL was mobilized, the following 35% was emulsified in aDNAPL-in-water emulsion which was followed by 16% of emulsified LNAPL. The last4% was dissolved in a stable DNAPL-in-water microemulsion. The emulsified LNAPLand DNAPL separate in two phases in less than 1 h. More than 70% of the initialDNAPL was recovered ahead of the washing solution in an oil bank. Downward

Žcirculation of only 0.80 PV of washing solution decreases S from 0.195 to 0.020 Fig.orŽ ..7 b . Desaturation is radical before the arrival of the washing solution and is slowed

down during the circulation of the solution.ŽDNAPL breaks through at 0.6 PV, at which point, the washing solution appears Fig.

.8 . In fact, most of the DNAPL is mobilized in a oil bank ahead of the washing solution.At 0.8 PV maximum oil production is observed with a DNAPL mass fraction in theeffluent of 0.55. At 1.2 PV, the washing solution is at optimal concentration and therecovery slows down rapidly thereafter, because the washing solution slug is too smalland becomes diluted by the rinsing solution behind it. Additional DNAPL solubilization

Ž .is observed during the rinsing operation between 1.8 and 1.9 PV because solventrecovery makes the rinsing solution close to the initial washing solution compositionconcentration.


Ž . Ž .Fig. 7. a Cumulative DNAPL recovery in the column effluent. b Residual DNAPL saturation in the column.


Fig. 8. Mass fraction of DNAPL produced and relative concentration of ingredients in the column effluent.

Based on these experimental results, DNAPL recovery can be predicted for the casewhere more PV of washing solution would be injected in the column. The additionalrecovery should be in Winsor type I system which is observed when the washingsolution is at optimal concentration. A first-order relationship, derived by Martel and

Ž .Gelinas 1996 , could be used to predict NAPL recovery in this system. At 1.2 PV, 0.1´PV of washing solution can dissolve 21% of the DNAPL still in place in the columnŽ . Ž .Fig. 9 . The mass of DNAPL removed M after ‘‘i’’ tenths of PV injected is described

Ž .by Eq. 4 where M is the mass of DNAPL in the column at 1.2 PV. Using thiso

relationship, we would predict that 2.5 PV of solution would be needed to dissolve morethan 99.5% of the initial residual DNAPL saturation. For comparison purposes, a goodagreement is observed between two Mercier DNAPL recovery experiments in horizontal

Ž .small sand columns at 88C S s0.1 and the predicted recovery curve for the largeor

sand column.

MsM 1yey0 .236 i 4Ž . Ž .o

4.3. Residual products recoÕery

4.3.1. Front stability and ingredients trapping in sedimentsŽ .For vertical sand column experiments, the front of the displacing injected fluid is

Ž .stable when the fluid’s mobility ratio viscosity ratio is smaller than 1 and when the


Fig. 9. Predicted and observed DNAPL recovery in the sand column.

Ž . Ž .gravity term D r g sin a is larger than 0 Lake, 1989 . Downward injection ofwashing solution in a water saturated sand column gives a stable displacement frontŽ .Table 3 . When the rinsing solution is injected behind the washing solution, thedisplacement front has conditional stability. In a type II conditional stability, a favorable

Ž .mobility ratio can stabilize an unfavorable gravity term if the Darcy velocity V isŽ . Ž . Ž .greater than the critical Darcy velocity V defined in Eq. 5 Chouke et al., 1959 . Inc

Ž y1 .this experiment, since the Darcy velocity 0.38 m d used is close to the criticalŽ y1 .discharge 0.50 m d , the displacement front is at the stability limit.

D r gsin aV s 5Ž .c m mws rs

yk kws rs

where k and k are respectively the relative permeability to washing and rinsingws rsŽ 2 .solution m and m and m are the viscosities of both fluids. When the polymerws rs

solution pushes the rinsing solution, the displacement front is unstable because ofunfavorable viscosity ratio and gravity term. Viscous and gravity fingers develop as wellas dispersion at the back of the rinsing solution slug and contribute to its dilution. Thiscan explain the tailing effect observed with SAS and n-BuOH in the column effluentŽ Ž .. Ž .Fig. 4 a and also SAS trapping in sediments Fig. 10 . As observed, near the top andat the base of the column, SAS was adsorbed on sand and trapped in pores because aconcentrated SAS solution is highly viscous without n-butanol. Viscous fingering at thefront of the second polymer solution was not monitored. Even downward washing


Fig. 10. Residual ingredients in the column sediments after the washing experiment.

solvents and NAPL are retained more at the top than at the base of the column. Solventtransfer in the NAPL phase favored an upward movement of the NAPL phase bybuoyancy forces. In fact, solvent transfer in the NAPL phase decreases the NAPLdensity and viscosity and results in an increase in its saturation and relative permeability.The next section will show that a different rinsing sequence could actually better removewashing solution ingredients.

4.3.2. Rinsing residual productsTo evaluate the proper procedures required for the removal of residual ingredients left

behind in the column at the end of the washing process, different rinse cycle tests wereŽ . Ž .performed in small sand columns at field temperature 88C Fig. 11 . The sand which

contains 5% fines of clay and silt sizes was confined by 1 cm of Mercier sand withoutŽ .fine particles at both ends i.e. the sand used in the washing experiment . Ville Mercier

sand with fines is described in Table 2. This sand has a higher specific surface area and

Fig. 11. Experimental set-up for rinsing strategy evaluation in the small sand column.


organic carbon content and has a lower hydraulic conductivity and porosity than theMercier sand without fine particles. The procedures for filling, compaction, and water

Ž .saturation were the same as the ones described in Martel and Gelinas 1996 . All´solutions were injected horizontally with a peristaltic pump at a mean fluid velocityvarying from 0.56 to 0.70 cm miny1.

Injection of 1.6 PV of the washing solution used in the washing experiment wasŽ y1 .preceded by xanthan solution 200 mg l and rinsed by different volumes of n-

Ž . Ž y1 .BuOHrSAS solution 15 or 20% in water or in polymer solution 500 mg l . TheŽ y1 .rinse cycle was completed with xanthan solution 500 mg l or a combination of

Ž y1 .xanthan solution 500 mg l and water. After the rinse cycle, toluene, d-limonene, andn-butanol concentrations were directly determined in the soil by GCrFID. After asoxhlet extraction of SAS in soil samples with methanol, its concentration was measured

Ž .by colorimetry Zhen Cao, 1978 . One test was doubled to check the precision of theresults.

Ž . Ž .In Fig. 12 a and Fig. 12 b , each pair of points represents residual d-limonene andtoluene in a single sand column test. Rinsing the washing solution with water leaves

Ž y1 .important residual concentrations of toluene up to 3200 mg kg and d-limoneneŽ y1 . Ž Ž ..more than 1000 mg kg Fig. 12 a . Injection of an alcohol-surfactant solution afterthe washing solution significantly decreases the solvent concentrations. Increasing thevolume of the n-BuOHrSAS rinsing solution decreases the residual solvent concentra-tions. Residual solvent concentrations are much lower but still high when 0.3 to 1 PV ofn-BuOHrSAS 15% is injected in the sand column after the washing solution. Two testsusing residual DNAPL saturation initially present in the columns, showed no significant

Ž Ž ..effect in solvent trapping Fig. 12 a . Increasing the n-BuOHrSAS concentration to20% in the rinsing solution can decrease residual solvent concentrations below the

Ž y1 . Ž Ž ..detection limit 10 mg l Fig. 12 b . To achieve this, more than 0.6 PV of n-BuOHrSAS 20% rinsing solution must be injected after the washing solution. Theseresidual solvent concentrations can be eliminated if biodegradation is performed in situ

Ž .with acclimatized bacteria and with proper nutrients Roy et al., 1995 .Residual SAS concentrations left in the sediments are a function of the PV of

Ž Ž ..polymer solution injected after the rinsing solution Fig. 12 c . Residual concentrationsin SAS stabilize around 900 mg ly1 when 5 PV of 500 mg ly1 xanthan in water areinjected in the column after the n-BuOHrSAS rinsing solution. Further injection ofwater does not significantly decrease residual SAS concentrations in sediments. Residualconcentration in SAS lower than 1000 mg kgy1 are easily biodegraded under anaerobic

Ž .conditions with acclimated field bacteria Roy et al., 1995 . Residual n-butanol concen-trations left in the sediments are independent of the PV of polymer solution injectedafter the rinsing solution. Residual concentrations in n-butanol fluctuate betweendetection limit and 100 mg ly1, a level where it is easily biodegraded. Whatever thenumber of PV selected to recover residual SAS, n-butanol will remain at low concentra-tion.

The rinsing strategy in the field should be to inject 0.75 PV of n-BuOHrSAS 20%with 500 mg ly1 of xanthan followed by 5.0 PV of water with decreasing concentrations

Ž y1 y1.of xanthan 600 mg l to 0 mg l . Water is then injected at the end to decreasepolymer concentrations. This is required since high xanthan gum concentration in



sediments can delay biodegradation of washing solution ingredients because bacteriaŽwould first prefer polysaccharides to solvents and to hydrocarbon chains of SAS Roy et

.al., 1995 .

5. Discussion

In the field, DNAPL recovery is expected to be less efficient than in a sand column.In sand column experiments, the flooding is perpendicular to sediment layers and the

Žinstability of the displacement front is caused only by fingering viscous and gravity.fingers . In the field, the flooding is generally parallel to layers and a lower sweep

efficiency is created by the bypassing of contaminated areas by the washing solutioncaused by permeability heterogeneities. Fingering can be prevented by designing stabledisplacement conditions, whereas bypassing caused by heterogeneities can only bereduced. One could expect also that the DNAPL recovery rate is overestimated in thecolumn study by using freshly contaminated material while in field cases, the contami-nants had very long residence time to diffuse into dead-end pores or fractures. However,the Mercier sands and gravels are of glacial origin which have undergone very lowweathering. The volume of DNAPL diffused in the matrix would be insignificantrelative to the DNAPL in the intergranular pore space.

Horizontal sand column experiments are recommended to minimize unstable dis-placement fronts between fluids of different densities. However, in this configuration,

Ž .gravity may lead to the overriding or underriding of fluids Martel et al., 1997a .The washing solution slug must also be wide enough to completely recover the

DNAPL because a second injection is more expensive. The injected volume required toget the proper width must consider dispersion effects at the leading edge and back of thewashing slug. In fact, injecting a second slug is inefficient since it is diluted again at thefront and the back, and this shortens the washing solution slug length at optimalconcentrations able to solubilize NAPL.

To better understand the complex processes related to aquifer restoration by washingŽ . Ž .solutions numerical models should be used to: 1 interpret lab and field tests; 2 design

Ž .injectionrpumping patterns; 3 test different washingrrinsing sequences. The modelŽmust include fluid–solid interactions, and DNAPL recovery processes mobilization,

. Ž .emulsification, solubilization . Numerical models such as TOUGH2 Pruess, 1991 andŽ .UTCHEM Center for Petroleum and Geosystems Engineering, 1994 could be modified

Ž .to include these phenomena Brown et al., 1994; Falta and Brames, 1995 . Formodelling purposes, capillary properties, relative permeability, as well as interfacialtension must be determined in the lab. The phase behavior of the optimal solutioninjected must also be characterized.

Ž .Fig. 12. a Residual solvent concentrations in sediments of the small column as a function of the pore volumeŽ .of the rinsing solution n-BuOHrSAS 15% injected. b Residual solvent concentrations in sediments of the

Ž .small column as a function of the pore volume of the rinsing solution n-BuOHrSAS 20% injected. cResidual SAS concentrations in sediments of the small column as a function of the pore volume of waterinjected.


6. Conclusion

A large sand column experiment is used to illustrate the principles of complexcontaminant recovery by a surfactant solution. The main NAPL recovery mechanisms

Ž . Ž .identified are: 1 immiscible displacement by oil saturation increase oil swelling , oilviscosity reduction, interfacial tension lowering, and relative permeability increase; andŽ .2 miscible displacement by solubilization. Most of the NAPL was recovered in aWinsor type II system ahead of the washing solution. The 0.8 PV of alcohol–surfactant–solvent solution injected recovered more than 89% of the initial residual

Ž .DNAPL saturation 0.195 . Winsor system types were determined by visual observationof phases and confirmed by electrical resistivity measurements of phases and watercontent measurements in the oleic phase. Viscosity and density lowering of the oleicphase was made by solvents and alcohol transfer from the washing solution.

Small sand column tests are used to check different rinsing strategies used tominimize washing solution residual ingredients in sediments. An alcohol–surfactantsolution, without solvent, injected behind the washing solution, minimizes solventtrapping in sediments. Residual solvents concentrations in sediments are a function ofthe volume and concentration of the alcohol–surfactant rinsing solution. Residualalcohol–surfactant concentrations in sediments are a function of the volume of rinsingwater or xanthan in water solution injected. More than 5 PV of polymer solution andwater must be injected after the rinsing solution to decrease alcohol and SAS concentra-tions in sediments to acceptable levels. To obtain reasonable trapped surfactant concen-trations in sediments, the displacement front between the rinsing solution and thefollowing polymer solution has to be stable. Horizontal sand column experiments arerecommended to minimize the appearance of an unstable displacement front betweenfluids of different densities. However, in this configuration, gravity may lead tooverriding or underriding of fluids. To better understand the complex NAPL recoveryprocesses by washing solutions, numerical models should be used.

Acknowledgements

ŽThis project is funded by the DESRT Program Development and Demonstration of.Site Remediation Technologies of the Quebec Ministry of Environment and Fauna, and´

Environment Canada. Special thanks to the students who helped with the laboratoryŽ .work Marie-Josee Roy, Lucie Gauthier, and Nathalie Roy . We also wish to thank,´

research assistants: Catherine Blais, Pierrette Vaillancourt, and Rene Dufault. Special´thanks to Mme Linda Lecours and M. Yvon Couture from the laboratory division of theMEF who performed analyses on volatile organic compounds in soil samples.

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ŽRichard Martel is professor at the Institut National de la Recherche Scientifique INRS-Georessources,´. Ž .Quebec University . He holds a B.Sc.A. in Geological Engineering 1983 and a M.Sc. in Hydrogeology

Ž . Ž . Ž .1986 and a Ph.D. 1996 from Laval University Quebec . Between 1986 and 1996, he was an hydrogeolo-´gist for the Quebec Ministry of the Environment and Fauna where he was involved in aquifer characterizationand restoration. He completed a Ph.D. on the development of surfactant solutions to solubilize oil and DNAPLat residual saturation in contaminated aquifers.

Ž . Ž .Pierre J. Gelinas, graduated in Geological Engineering 1968 at Laval University Quebec . After´graduate studies in hydrogeology and geotechnical engineering, he obtained a Ph.D. in Civil EngineeringŽ .1974 at the University of Western Ontario. He has been involved in teaching and research in hydrogeology

Ž . Ž .and engineering geology at the University of Ottawa Ontario 1972–1976 and since 1976 at LavalŽ .University Quebec . He was Chairman of the Department of Geology at Laval for six years from 1981 to´

1987. Current research interests include aquifer characterization and restoration, acid mine drainage, andregional hydrogeology.

ŽRene Lefebvre is professor at the Institut National de la Recherche Scientifique INRS-Georessources,´ ´. ŽUniversite du Quebec . He received a B.Sc.A. in geological engineering and a Ph.D. in geology Hydrogeol-´ ´

. Ž . Ž .ogy from Laval University Quebec as well as a M.Sc. degree from the University of Calgary Alberta in´Ž .geology geochemistry . He worked in the petroleum industry in western Canada, California, Alaska and

Texas. His teaching and research activities are mainly related to multiphase flow processes.

Documents

Aquifer washing by micellar solutions: 2. DNAPL recovery mechanisms for an optimized alcohol–surfactant–solvent solution