Research paper

Advection and retardation of non-polar contaminants in compacted claybarrier material with organoclay amendment

Sadra Javadi a, Mohammad Ghavami a, Qian Zhao a,⁎, Bate Bate b,c

a Department of Civil and Environmental Engineering, University of Louisville, USAb Institute of Geotechnical Engineering, College of Civil Engineering and Architecture, Zhejiang University, Hangzhou, Chinac Department of Civil, Architectural, and Environmental Engineering, Missouri University of Science and Technology, USA

a b s t r a c ta r t i c l e i n f o

Article history:Received 28 March 2016Received in revised form 21 October 2016Accepted 27 October 2016Available online 12 November 2016

Compacted clay liners (CCLs) are widely used as hydraulic barriers in landfills, underground storage tanks, ver-tical cutoff walls and surface impoundments. Most commonly, the breakthrough of contaminant flows in CCLstakes a long time due to the reduced advection rate; however, the attenuation of non-polar fluid or organic con-taminants in CCLs is relatively low because of the non-reactive nature ofmost CCLmaterials. Surfactant-modifiedbentonites are promising barrier amendments as they are coated with organic surfactant and are capable ofuptaking the non-polar species from the aqueous phase. In this study, laboratory tests were carried out to eval-uate the swelling, permeability and contaminant retention of compacted silty clay amended with an organoclay(hexadecyltrimethylammonium (HDTMA) modified bentonite) against both gasoline and organic solution. Theswelling properties and the hydraulic conductivities of compacted soils with varying liquids were evaluatedand the transport of naphthalene, a representative of polycyclic aromatic hydrocarbons (PAHs) and a possiblecomponent of NAPLs, in organobentonite-amended silty clay was examined through batch sorption and columntests. The results indicated that the addition of 10% HDTMA bentonite to compacted silty clay slightly increasedthe permeability of themixture to water. However, higher swelling tendency and lower permeability to gasolinewere also observed.With 5% of HDTMA bentonite amendment, the compacted silty clay soil had amuch strongerretardation capacity for naphthalene.

© 2016 Elsevier B.V. All rights reserved.

Keywords:OrganobentoniteCompacted clay linersAmendmentReactive geo-materialClay barrierHydraulic conductivitySwellingRetardation

1. Introduction

Organic pollutants such as petroleum-related products and non-aqueous-phase liquids (NAPLs) pose serious risks to the public healthand ecosystems (Fels, 1999; Pawełczyk et al., 2016). Polycyclic aromatichydrocarbons (PAHs) and fuels are common organic pollutants in thesubsurface due to inappropriatewaste disposal practices, gas station op-erations and industrial activities (Samanta et al., 2002). Most common-ly, geosynthetics and engineered earthen materials are used in linerbarriers to reduce advection and themass transport of the contaminantflow. The low hydraulic conductivities, reliable performance and lowcost of compacted clay liners (CCLs) (Boynton and Daniel, 1985; Roweet al., 2004) make CCLs ideal in applications for engineered barriers.Typically, the advection rate in CCLs is so low that the molecular diffu-sion governs the contaminant migration (Shackelford, 2014). However,previous field applications raised several concerns regarding the

chemical compatibility between CCL soils and certain contaminants:(1) pure phase organic compounds or petroleum-related products cancause the soil to shrink and crack with an increase in hydraulic conduc-tivity (Broderick and Daniel, 1990; Fernandez and Quigley, 1985;Bowders and Daniel, 1987; Hamdi and Srasra, 2013; Lake and Rowe,2005), and (2) the mass flux of contaminants can be significant even ifthe flow rate is low due to the non-interactive nature of conventionalclay and clay-type materials used in CCLs (Brown and Burris, 1996;Bright et al., 2000; Francisca and Glatstein, 2010). This is of significantconcern where low concentration of organic pollutants in the ground-water can cause problems in the long term (Ma et al., 2015). Therefore,improvement of the barrier performance of CCLs against certain con-taminant flows still merits further examination.

For CCLs with non-interactive components against the contami-nants, mass transport is controlled by advection-dispersion becauseno chemical or biochemical reactions reduce the contaminant concen-tration in the pore volumes (Shackelford, 1994; Lu et al., 2010;Francisca and Glatstein, 2010; Du et al., 2009). Recently, a “new” con-cept of an active material amended low permeability-reactive barrier,in which the soil matrix or soil amendments in the barrier are capable

Applied Clay Science 142 (2017) 30–39

⁎ Corresponding author at: WS Speed 119, 2301 S 3rd St, Louisville, KY 40292, USA.E-mail addresses: sadra.javadi@louisville.edu (S. Javadi), m.ghavami@louisville.edu

(M. Ghavami), qzgeotech@gmail.com (Q. Zhao), batebate@zju.edu.cn (B. Bate).

http://dx.doi.org/10.1016/j.clay.2016.10.0410169-1317/© 2016 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

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of uptaking, immobilizing and/or degrading the pollutants activelywhile maintaining a satisfactory hydraulic performance (Wagner et al.,1994; Crocker et al., 1995; Lo and Yang, 2001), was proposed (Wileset al., 2005; Lo et al., 1997; Bartelt-Hunt et al., 2005; Lu et al., 2015).The reduced release rate of the contaminant mass due to contami-nant/soil reactions is commonly referred to as retardation and a retarda-tion factor (R) (Freez and Cherry, 1979) is typically used to quantify thepace of the slow release. It is important to note that the flow rate doesimpact the apparent or measured retardation factor significantly.Shackelford (1994) demonstrated that for fast flow rate scenarios, theretardation factor could be estimated from the distribution coefficient(or partitioning coefficient, Kd). However, the application of the sameequation tends to yield an overestimated retardation factor for lowflow rate scenarios (Peclet number b 1) (Shackelford, 1994). Nonethe-less, the mass flux through the porous media is significantly reduced ifa reactive soil amendment is present and the retarded transport of con-taminants occurs (Gullick and Weber, 2001; Varank et al., 2011a,2011b).

Many different types of natural or engineered materials have beensuggested as clay liner amendments to both enhance the reactivity ofthe liners and provide low hydraulic conductivity (LaGrega et al.,1994; Bartelt-Hunt et al., 2005). Most commonly, fly ash, zeolite,organo-zeolite, coated sand and organoclays have been suggested aspotential reactive amendments to enhance the effectiveness of CCLs(Lo and Liljestrand, 1996; Prasad et al., 2012; Varank et al., 2011a,2011b; Younus and Sreedeep, 2012). Among the suggested additives,organoclays have been proven to be effective sorbents for organic pol-lutants (Benson et al., 2015; De Paiva et al., 2008; Gates et al., 2004;Soule and Burns, 2001). Organoclays are aluminosilicates/organic sur-factant hybrids that have high organophilic phase to uptake nonpolaror low-polarity species from aqueous phase. When in contact with or-ganic liquids, organoclays exhibit low conductivity, high swelling ten-dency and high plasticity. The organic surfactants in organoclaysprovide partitioning media to the organic molecules in the interlayerspaces (Lee et al., 2012; Zhao and Burns, 2012). Consequently,organoclays can be used as a secondary compound liner barrier in in-dustrial activities such as petroleum sewage refinement and the treat-ment of waste water due to their ability to bind with organiccompounds (De Paiva et al., 2008; Lo and Yang, 2001; Seung andTiwari, 2012).

Previous studies show that organoclay amendments may improvethe efficiency of the clay barrier when facing organic liquids or petro-leum products (Lo and Yang, 2001; Moon et al., 2007; Smith et al.,2003; Yang and Lo, 2004). The permeability of organoclays for gasolinewas demonstrated to decrease two to four orders of magnitude com-pared to that for water (Smith et al., 2003; Moon et al., 2007). Althoughorganoclays may have higher permeability for water, their impact onthe overall conductivities of compacted soils might be negligible dueto low dosage (Yang and Lo, 2004; Ghavami et al., 2016). Additionally,the beneficial attenuation capability of non-polar species in the soilmay be greatly enhanced by the organoclay additives (Seliem et al.,2011; Liu et al., 2014; Li and Denham, 2000). This is because engineeredorganoclays, especially engineered organophilic clays, are extremely ef-fective in terms of sorbing low or non-polar organics, chlorinated or-ganics, polycyclic hydrocarbons and pesticides from aqueous phase(Boyd et al., 1988; Jaynes and Boyd, 1990, 1991; Larsen et al., 1992;Montgomery et al., 1991; Owabor et al., 2010; Qu et al., 2008; Smithet al., 1990; Katsumi et al., 2008; Seung and Tiwari, 2012; Zhu et al.,2015). The sorption of organic contaminants onto organo-carbon richclays (organophilic clays), whose distribution coefficients can be hun-dreds of thousands of times higher than those of non-sorptive soils, istypically through partitioning (Redding et al., 2002; Nzengung, 1996;Shu et al., 2010). In addition, kinetic studies of hydrophobic organicma-terials (e.g. naphthalene and diuron) sorption onto organoclays revealthat sorption happened very fast and sorption equilibrium was oftenreached when the flow rate was low (Nzengung et al., 1997). Extensive

laboratory column tests were conducted to assess the barrier perfor-mance of conventional CCLs against contaminants and to derive trans-port parameters (Acar and Haider, 1990; Crooks and Quigley, 1984).However, relatively few studies explored the applicability of usingorganoclays as CCL amendment forwaste containment andpollutant at-tenuation (Lo andMak, 1998). This is due to the difficulty of controllingand comparing soil components/properties and the long duration ofbreakthrough tests in compacted soils.

The objectives of this study are to quantify the free-swelling behav-ior of organobentonite in varying liquids and its permeability to varyingliquids and to evaluate the NAPL attenuation capacity as additives in asilty clay. Hexadecyltrimethylammonium (HDTMA+) surfactant modi-fied bentonite was chosen as a representative CCL amendment withengineered high-organic carbon content and organophilicity. The barri-er performance of silty clay/HDTMA bentonite against gasoline (repre-sentative of petroleum products) and PAH (common components inNAPLs) transport was assessed. Permeability tests on saturated speci-mens of compacted silty clay with HDTMA bentonite amendmentwere conducted in the flexible wall permeameter. Free swelling testsof the silty clay/HDTMA bentonite in water and gasoline were per-formed. Batch sorption tests and low-flow rate column tests were car-ried out to quantify the retardation coefficient and diffusion coefficientof naphthalene transport in compacted silty clay/HDTMA bentonite soil.

2. Materials

Three different types of soils such as (silty clay, Ca-bentonite andHDTMA modified bentonite) were used in this study. The natural low-plasticity silty clay soil (Nugent Sand Company) contained 1% gravel,12% sand, and 87%fineparticles (ASTMD422). Ca-bentonite soil had ap-proximately 85% calcium montmorillonite (American Colloid Compa-ny). HDTMA bentonite was synthesized from the calcium bentonite byexchanging Ca2+ cation with hexadecyltrimethylammonium cation(HDTMA+), following the method described in a previous study(Lorenzetti et al., 2005). The synthesized HDTMA bentonite had a totalorganic carbon of 21.44%, according to a carbon analyzer. The basalspacing of the base bentonite and HDTMA bentonite was determinedby X-ray diffraction (XRD) analysis. X-ray diffraction patterns were re-corded between 2° and 20° (2θ) using CuKa radiation (n = 1.5406 Å)at a scanning speed of 2°/min. The basal spacing of the base bentonitewas observed to increase from 15.06 Å to 19.44 Å after intercalation ofHDTMA cations (Fig. 1). Methylene blue absorption technique wasused to determine the specific surface area of each studied soil(Santamarina et al., 2002). The SEM photo of Ca-bentonite andHDTMA bentonite was recorded using a FEI Nova NanoSEM 600 witha working distance of 5–6 mm (Fig. 2). Ca-bentonite was observed tohave large aggregates and curvy edges, and after the surfactant interca-lation, the particles showed less foliated structure with rough edges.

Fig. 1. XRD results of Ca-bentonite and HDTMA bentonite.

31S. Javadi et al. / Applied Clay Science 142 (2017) 30–39

Atterberg limits and the specific gravity of soils were determined inaccordance with ASTM D4318 and ASTM D854, respectively. The soilproperties are summarized in Table 1. Naphthalene was selected asthe representative organic contaminant because of its simple structure(Chen et al., 2005) and frequent presence in leachate/wastewater ofthe landfills and industrial facilities (Wolfe et al., 1986). Naphthalenewas purchased from Fisher Scientific Co. in crystal form. The solubilityof naphthalene inwater is as low as 31.6mg/L at 25 °C. To prevent naph-thalene dissolution/precipitation reaction in soils, it was diluted in themethanol and water matrix to the designed concentration. The proper-ties of permeant liquids and contaminant flows are summarized inTable 2.

3. Methods

3.1. Free swelling test

The one-dimensional oedometer was used to measure the freeswelling index, following ASTM D4546. Each oven-dried soil samplewas moistened to a water content of 13–14% and then placed in a ringwith a diameter of 66 mm and height of 20 mm, respectively. The soilspecimens from the fixed ring were placed in the oedometer apparatusand subjected to 1 kPa overburden pressure. When the height of thespecimens reached equilibrium, water or other liquids were added tothe oedometer cell and the change of height of the specimens weremeasured by a vertical deformation gauge. The measurement of free-swelling potential for each specimen was completed in 24 h.

3.2. Compaction test

Synthesized HDTMA bentonite, calcium bentonite, and silty clay soilwere oven dried for 24 h. The standard compaction test was performedon three soil admixtures of: 100% silty clay; 90% silty clay + 10% Ca-

bentonite; and 90% silty clay+ 10% HDTMA bentonite (by weight), fol-lowing the ASTM standard D698, Method A. The relationship betweenwater content and dry density of each compacted specimenwas obtain-ed from sufficient numbers of repeated tests. Themaximumdry densityand the optimum water content of each admixture were determined(Fig. 3).

3.3. Hydraulic conductivity test

The flexible wall permeameter (Trautwein Soil Testing Equipment)was used to evaluate the hydraulic conductivity of samples accordingto ASTM D5084. The falling head, rising tail water elevation method(D5084- Method C) was employed to measure hydraulic conductivity.Two bladder accumulators were used at inflow and outflow points tostore the organicfluids. According to the compaction curve, each soil ad-mixture was compacted at the optimum water content. After the com-paction process, each soil specimen was trimmed to a diameter of50±0.2mmand a height of 100±0.2mm. The specimenswere placedin flexible wall permeameter with a cell pressure of 13.8 kPa andflushed with de-aired water for approximately 4 days, followed by ap-plied back pressure. The back and cell pressure simultaneously in-creased up to 192 and 206 kPa, respectively, and a B-value N0.95 wasreached. Then the hydraulic conductivity of each specimen was mea-sured when the flow reached a steady state under a hydraulic gradientof 2.5. Three hydraulic conductivity measurements were recorded andthe mean values were obtained.

3.4. Batch sorption test

Batch sorption tests were carried out to quantify the naphthalenesorption onto the HDTMA bentonite and silty clay. The stock solutionsconsisted of 70% deionizedwater (DI) and 30%methanol with naphtha-lene concentrations of 10, 20, 40, 60, 80, 100, 140, 180, and 200 μg/mL.

Fig. 2. SEM image of (a) Ca-bentonite and (b) HDTMA bentonite.

Table 1Soil properties.

Properties Silty clay Calcium bentonite HDTMA bentonite

Organic surfactant – – HDTMA+

Liquid limit (%) 26 88.3 74.9Plasticity index (%) 4.5 37.3 16.3Fine content % (b#200 sieve) 87 85.5 100Specific gravity 2.62 2.58 1.75Specific surface area (m2/g) 33.5 276.28 162.21Total organic carbon (%) b1 2.58 21.44

Table 2Physicochemical properties of permeant liquids.

Liquids Type Polarity

Density(kg/L) at20 °C

Dynamicviscosity(cP)

Water Deaired 10.2 1 1Gasoline Unleaded #87 b3 0.76 0.55Naphthalene/methanol Crystalline/reagent

grade5.1 0.791 0.56

32 S. Javadi et al. / Applied Clay Science 142 (2017) 30–39

The proportion of deionized water and methanol in the total volume ofthe stock solutionwas kept constant. Prior to the experiments, 10mL ofstock solutionwith varying naphthalene concentrationswas added intothe conical centrifuge tubes (CORINGCo.). After adding 0.2 g of sorbentsto the tubes, they were agitated for 24 h. Next, sample tubes were cen-trifuged at 1500 rpm for 30 min and 8 mL aliquot of the supernatant ineach tubewas filtered through 0.2 μm syringe filters (Acrodisc Co.). Thefiltered sampleswere then extractedwith hexane at a ratio of 10:1 (v/v)to assess the concentration of naphthalene in an aqueous phase. The ex-tracted naphthalene/hexane solutions were tested in the gas chroma-tography equipped with flame ionization detector (GC-FID, Clarus 480,Perkin Elmer Co.). The concentration of the absorbed naphthalene bysoil was measured as:

S ¼ Vm C0−Cð ÞMs

ð1Þ

where S (μg/g) is the concentration of naphthalene sorbed by HDTMAbentonite, C0 (μg/mL) and C (μg/mL) are the initial and equilibrium con-centrations of naphthalene in an aqueous phase, respectively. Vm (mL)is the volume of the batch reactor, and MS (g) is the total weight ofthe HDTMA bentonite sorbent. S was plotted with C and the slope ofthe linear fit was determined as the partitioning coefficient, Kd.

A series of non-equilibrium sorption studies for naphthalene in twosoil samples was performed to quantify the sorption kinetics. A set of15 mL glass conical centrifuge tubes (CORING Co.) with Teflon septawere used to conduct the kinetic studies. Vials were filled with a super-natant consisting of 70% DI and 30% methanol with a naphthalene con-centration of 200 μg/mL. Then, 0.2 g of HDTMA bentonite was added toeach vial, and all vials were shakenwith a shaker. After each preset timeperiod (15 s to 48 h), the supernatant was filtered and extracted beforetested in the GC.

3.5. Column test

The retardation factor (Rd) of naphthalene transport in compactedHDTMA bentonite/silty clay was determined by a column test. The soilspecimen was prepared from 5% of HDTMA bentonite and 95% of siltyclay, which was moistened with an optimum water content of (13–14%), and compacted in a standard compaction mold resulting in abulk dry density of 1.78 to 1.85 g/cm3 (Table 3). The trimmed soil sam-ple was 50 ± 0.2 mm in diameter and 68 ± 0.2 mm in length and waswrapped with Teflon tape/latex membrane and placed in the flexiblewall permeameter. The experimental setup is illustrated in Fig. 4. Thesoil column was first saturated and flushed with de-aired water (ap-proximately two pore volumes) until a steady state was reached. Theobtained steady flow ratewas approximately 1mL/h. Then a bladder ac-cumulatorwas connected to the soil to feed 1 pore volume of pulse-typecontaminant solution, which consisted of 70% DI and 30% methanolwith a naphthalene concentration of 130 μg/mL. The inflow wasswitched back to de-airedwater after 1 pore volume of naphthalene so-lution was injected. An additional 13 mL of de-aired water was fed tothe soil for 12 pore volume (approximately 30more days). The effluentwas extracted and tested for naphthalene concentration twice a day, fol-lowing the same technique of the sorption test. For this low-flow ratecolumn test, the retardation factorwas determined by thefirst-momentequation (Valocchi, 1985).

Rd ¼

XC=C0ð ÞPVΔPV

XC=C0ð ÞΔPV

−0:5PV0 ð2Þ

Where Rd. is the dimensionless retardation factor, C0 (μg/mL) and C(μg/mL) are the initial and equilibrium concentrations of naphthalene inthe aqueous phase, respectively, PV (mL) is the corresponding pore vol-ume to each measured concentration, ΔPV (mL) is the differential porevolume between each sampling step, and PV0 (mL) is the initial injectedpore volume of the solution to the soil column.

4. Results and discussion

4.1. Free swelling

The results of the free swelling tests of the three soils: silty clay, siltyclay + 10% Ca-bentonite, and silty clay + 10% HDTMA bentonite inwater and gasoline are presented in Fig. 5. The free swelling of siltyclay soil in water increased significantly as 10% of bentonite wasadded (Fig. 5a). Previous studies indicated that this was due to the hy-dration of bentonite soil (Chapuis, 1990; Sun et al., 2009). When thesilty clay soil was mixed with 10% HDTMA bentonite, the free swellingof the water showed similar behavior as 100% silty soil. This was dueto the inhibited swelling tendency of HDTMA bentonite in water, asthe intercalated HDTMA surfactant tended to be hydrophobic and didnot hydrate in water (Benson et al., 2015; Sreedharan andSivapullaiah, 2014).

However, when a mixture of silty clay and HDTMA bentoniteinteracted with nonpolar organic liquids such as gasoline, it swelledmuch more than in water. A detailed comparison suggested that 10%of HDTMA bentonite additive may increase the free swelling tendencyof the mixture soil by approximately ten times in gasoline (Fig. 5b). Itwas assumed that the non-polar species could cause the interlayer ex-pansion of the HDTMA bentonite due to their interactionwith the inter-calated aliphatic carbon chains (Slade andGates, 2004). In contrast, siltyclay alone or mixed with bentonite had little or no volume change ingasoline. It can be concluded that when interacting with petroleumproducts, organoclay may be a superior material than natural clay insoil barriers. It should be noted that this study represented the case ofsoil swelling under no or low confining stress. While the hydraulic

Fig. 3. Compaction curves of three soil admixtures.

Table 3Lab soil column test parameters.

Column component 95% silty clay + 5% HDTMA bentonite

Diameter (cm) 5.1Height (cm) 6.8Gs 2.57Bulk unit weight (g/cm3) 2.02Porosity 0.3Pore volume (cm3) 42

33S. Javadi et al. / Applied Clay Science 142 (2017) 30–39

conductivities of thesemixtures can be affected by their swelling poten-tial, the overburden stress may reduce swelling potential.

4.2. Hydraulic conductivity

The hydraulic conductivity of silty clay, silty clay + 10% Ca-benton-ite, and silty clay + 10% HDTMA bentonite is presented in Table 4. Theconductivity of silty clay mixed with 10% HDMTA to gasoline was alsoinvestigated. When silty clay was mixed with 10% bentonite, the hy-draulic conductivity decreased approximately one order of magnitudecompared to hydraulic conductivity of silty clay due to increasing theplasticity index of the specimen from 4.4 to 10. It should be noticedthat increasing the plasticity index of the specimen can decrease the hy-draulic conductivitywhenwater is used as a permeant liquid (Benson etal., 1994; Lo and Yang, 2001). Also, the “plasticity index” of organoclaymay increase when the pore fluid is non-polar liquid (Soule andBurns, 2001). When HDTMA bentonite was added to silty clay soil, thehydraulic conductivity to water increased approximately 1 order ofmagnitude compared to 100% silty clay, whichwas consistent with pre-vious studies (Li et al., 1996; Lorenzetti et al., 2005; Moon et al., 2007).This phenomenon can be related to increasing the hydrophobicity as10% HDTMA was added to the specimen. It was expected that hydro-phobicity of the soil can lead to facilitated water conduction throughsoil pores.

The hydraulic conductivity of the silty clay-HDTMA bentonite mix-ture to gasoline is approximately 1.5 orders of magnitude lower than

the hydraulic conductivity to water which can be related to theorganophilicity of HDTMA when permeated with an organic liquid.The results were in agreement with previous studies (Lo and Yang,2001; Moon et al., 2007; Smith et al., 2003). Smith et al. (2003) investi-gated on intrinsic permeability of the two types of compactedorganoclays, BTEA-bentonite and HDTMA bentonite to water and un-leaded gasoline. The permeability of both types of organobentonitewas observed to decrease approximately two orders of magnitudewhen the permeant liquid was changed from water to unleaded gaso-line. Further studies by Yang and Lo (2004) showed that the conductiv-ity of compacted organoclay (BB-40) to gasoline (10−9 cm/s) was fourorders ofmagnitude lower than the conductivity of Na-bentonite to gas-oline (10−5 cm/s). The latest study byMoon et al. (2007) was conduct-ed on permeability of conventional clay andHDTMAmodified bentoniteto water and unleaded commercial gasoline. The overall results in thisstudy indicated that the HDMTA modified bentonite had a higher hy-draulic conductivity for water but much lower permeability when ex-posed to petroleum related products like unleaded gasoline.

It was anticipated that the gasoline hydrocarbons entered the inter-lamellar space of HDTMAbentonite due to the hydrophobic characteris-tic of organoclay, and wet the external surface of HDTMA bentonitemore efficiently thanwater. Previous studies indicated that organoclaysmay swell in non-polar liquids like bentonite swells in water (Benson etal., 2015). Consequently, the swelling of HDTMA modified clay and theincreased drag force on the gasoline are expected to be themain reasonsfor the decreased conductivity of gasoline in organoclay-amended silty

Fig. 4. Schematic column test setup.

34 S. Javadi et al. / Applied Clay Science 142 (2017) 30–39

clay soil. It was not investigated in this study, but it was demonstratedthat organic-modified soils tend to have lower conductivities for non-polar or low polarity fluids (Li et al., 1996; Benson et al., 2015).

The obtained hydraulic conductivity data of tested soils was com-pared with that of two additional soils (Bate, 2010; Ghavami et al.,2016). The HDTMA bentonite from Bate (2010) was an organoclaymodified from Na-bentonite with a total organic carbon content of

15.1%. The PM199 from Ghavami et al. (2016) was a commercialorganoclay and was combined with the same silty clay in the study.

The fluid conductivities of organobentonite amended soils, especial-ly compacted soils were summarized (Table 4). Three studies werecompared showing that organobentonite additives had limited impacton the overall hydraulic conductivities of compacted soil mixtures(even if the organobentonite fraction was as high as 50%). However,

Fig. 5. Free swelling of soils in (a) water and (b) gasoline.

Table 4Average fluid conductivities of compacted soils with varying permeants.

Materials Soil preparationVoidratio Permeant liquid

Liquidlimit

Plasticityindex

Fluidconductivity(m/s)

Intrinsicpermeability(m2) Source

Silty clay (100%) Compacted(ρd = 1.85 g/cm3)

0.42 Water 26 4.4 5.04 × 10−9 5.16 × 10−16 This study

Silty clay (90%) + Ca-bentonite(10%)

Compacted(ρd = 1.82 g/cm3)

0.43 Water 32 10 7.70 × 10−10 7.78 × 10−17

Silty clay (90%) + HDTMAbentonite (10%)

Compacted(ρd = 1.78 g/cm3)

0.42 Water 30 6.6 1.57 × 10−8 1.61 × 10−15

0.42 Gasoline – – 9.20 × 10−10 6.7 × 10−17

HDTMA bentonite Isotroically consolidated(ρd = 0.567 g/cm3)

2.03 Water 219 89 6.82 × 10−9 6.49 × 10−16 Bate (2010)

Silty clay (50%) + PM199organobentonite (50%)

Compacted(ρd = 1.43 g/cm3)

0.55 Water 31 6.2 1.4 × 10−8 1.5 × 10−15 Ghavami et al.(2016)

Compacted(ρd = 1.42 g/cm3)

0.54 Methanol – – 5.7 × 10−9 4.2 × 10−16

Compacted(ρd = 1.43 g/cm3)

0.55 Naphthalene/methanolsolution

– – 6.5 × 10−9 5.3× 10−16

35S. Javadi et al. / Applied Clay Science 142 (2017) 30–39

the conductivities of themethanol, gasoline and naphthalene/methanolsolution in silty clay/organobentonite mixtures were slightly decreasedwhen compared with 100% silty clay. Additionally, the conductivities ofall compacted soils with varying fluids were low, due to theirreduced pore voids and increased dry densities. This suggests thatorganobentonite amendments can be used in the compacted soils tolower the advection of contaminant flow. It is not examined in thisstudy, but it is also expected that bentonite/organobentonite amend-ments can be used to further reduce the advection rate in CCLmaterials.

4.3. Batch sorption test

The equilibrium isotherms for naphthalene sorption onto HDTMAbentonite and silty claywere also determined (Fig. 6a). Linear isothermswere observed for the range of naphthalene concentration for bothsorption tests. The distribution coefficients of Kd for silty clay andHDTMA bentonite were 4.53 mL/g and 1355.5 mL/g, respectively.HDTMA bentonite had a large organic carbon content that led to around300 times greater Kd compared to silty clay.

The sorption kinetic of naphthalene on HDTMA bentonite is shownin Fig. 6b. The kinetic study was conducted to estimate the time toreach naphthalene sorption equilibrium and to compare the sorption

rate versus flow and diffusion rate in the HDTMA amended soil. The ki-netic study confirmed that sorption of naphthalene onto HDTMA ben-tonite happened quickly, as approximately 83% of naphthalene wasabsorbed within the first 20 s. The rapid sorption of naphthalene wasfollowed by a slow sorption stage that was completed in 12 h. In addi-tion, the kinetic parameters were estimated by fitting the data withone-site transfer model (OSMTM, Nzengung et al., 1997):

Ct ¼ Ce þ C0−Ce½ & exp −εC0

Ce

! "t

# $ð3Þ

where Ct is the solution concentration at time (t) (μg/mL), Ce is the so-lution concentration at equilibrium (μg/mL), C0 is the initial solutionconcentration (μg/mL), t is the time (min), and ε is themass transfer co-efficient (1/min). The governing mass transfer coefficient (ε) was opti-mized based on the constant distribution coefficient (KP) measuredfrom batch sorption study. The kinetic parameters are summarized inTable 5. This agrees with some previous studies (Nzengung, 1993) andindicates that equilibrium sorption governed naphthalene transport inthe low flow-rate column. Additionally, the desorption of naphthalenewas assumed to happen fast as well (Nzengung, 1993) and the one-di-mensional solute transport model is valid (Freez and Cherry, 1979).

4.4. Naphthalene breakthrough and retardation

The performed column test was conducted under low flow rate con-ditions to represent themass transport in lowpermeability earthen bar-riers. Assuming the steady state flow and equilibrium sorption, thetransport of pollutions within soil media can be described by a one-di-mensional advection-dispersion-retardation equation (Freez andCherry, 1979).

R∂Cr

∂t¼ D

∂2Cr

∂x2

!−υ

∂Cr

∂x

! "ð4Þ

Where D is the hydrodynamic dispersion coefficient, v is the seepagevelocity, x is the microscopic distance in the direction of transport, t isthe time, and Cr is the concentration of solute in the pore water of thesoil. The average seepage velocity over the tested time was recordedto monitor the availability of steady-state condition during the test(Fig. 7). Also, the average hydraulic conductivity of specimen duringthe breakthrough test wasmeasured around 8.84× 10−9 m/s followingASTM D5084, method B.

Fig. 7. Seepage velocity monitoring during retardation experiment.

Table 5Sorption kinetics and equilibrium sorption parameters.

Sorbate-sorbent KP (Kd) (mL/g) R2 ε (1/min) Equilibrium time (h)

Naphthalene-HDTMA bentonite 1355.50 0.98 0.45 12Naphthalene-silty clay 4.53 0.95 N.A. N.A.

Fig. 6. (a) Sorption isotherms of naphthalene sorption onto HDTMA bentonite and siltyclay soil. (b) Sorption kinetic of naphthalene onto HDTMA bentonite.

36 S. Javadi et al. / Applied Clay Science 142 (2017) 30–39

The naphthalene breakthrough curve for silty clay columnwith 5% ofHDTMA bentonite soil is illustrated in Fig. 8. The naphthalene solutionpassing through the silty clay/HDTMA bentonite column showed thatthe concentration peaked after 142 h, which was approximately 9times lower than the initial concentration. The decrease of effluent con-centration is because of the high sorption capacity of HDTMA bentonite.The retardation factor in a pulse-type injection was interpreted from afirst moment analysis (Valocchi, 1985) as the advection rate was lowin the column (Shackelford and Redmond, 1995). With Eq. (2), the

retardation factor of the naphthalene solution passing through thecompacted silty clay/HDTMA bentonite soil column was determinedas 6.98 (the initial concentration, C0, was 130 μg/mL, and the initialpore volume of introduced pollution, PV0, was 1). This number wascompared with the retardation coefficient derived from the distributioncoefficient (Freez and Cherry, 1979):

R ¼ 1þ ρbn:Kd ¼ 1þ 2:02

0:3: 0:05' 1355:5þ 0:95' 4:5376ð Þ≅486: ð5Þ

Table 6Summary of retardation factor and diffusion coefficient for low& high flow rate studies.

Study Contaminants Soil matrixVelocity(cm/s)

Retardationfactor

Diffusioncoefficient(cm2/s)

High flow rateLiu et al. (1991) Naphthalene Lincoln - sand 2 × 10−3 5 N.A.

Eustis - sand 12Thierrin et al.(1995)

Benzene Sandy aquifer 2.5 × 10−2 1.02 N.A.Toluene 1.04p-Xylene 1.12Naphthalene 1.32

Owabor et al.(2010)

Naphthalene Sandy soil 2 × 10−3 25.77 N.A.

Larsen et al. (1992) Naphthalene Aquifer material N.A. 1.61 N.A.

Low flow rateLo et al. (1997) Benzene, toluene, o-xylene, ethyl-benzene, phenol, 2-chlorophenol,

2,4-dichlorophenol, and 2,4,6-trichlorophenolBB-40 4.93 × 10−6 N.A. N.A.

Li et al. (2002) Chromate HDTMA-modified illite 1 × 10−4 N.A. N.A.Shackelford andDaniel (1991)

Cl− Kaolinite & Lufkin clay N.A. N.A. 4.4 to 6 × 10−6

Br− 4.8 to6.5 × 10−6

Shackelford (1995) Cl− Kaolin 7 × 10−7 1.4 3.75 × 10–6Na+ 2.4 5.8 × 10−6

Lo (2003) Total organic carbon of landfill leachate Mixture of BB-40, bentonite, anddecomposed volcanic rock

N.A. 8.92 2.3 × 10−6

Shackelford andRedmond (1995)

Cl− Kaolin 7 × 10−7 2.336 2.84 × 10−6

Na+ 9.53 8.78 × 10−6

Rodriguez-Cruz etal. (2007)

Linuron Modified sandy loam withODTMA-montmorillonite

N.A. 69.5 N.A.Atrazine 9Metalaxyl 10

This study Naphthalene HDTMA-bentonite 3.4 × 10−5 6.98 ~9 × 10−6

Fig. 8. Breakthrough curve of naphthalene solution in 95% silty clay + 5% HDTMA bentonite.

37S. Javadi et al. / Applied Clay Science 142 (2017) 30–39

ρb represents the bulk density (g/cm3), n is the porosity, andKd is theweighted distribution coefficient (mL/g). A significant difference existsbetween the two numbers. The difference is explained in that Freezeand Cherry's studymay be applied for low flow rate when the diffusionis the dominant transport mechanism (Shackelford, 1995). Instead, theretardation factor should be determined from the breakthrough curve:a. for an infinite, constant contaminant source, the retardation factor isthe value of pore volumewhich corresponds to relative effluent concen-tration of 0.5; and b. when the contaminant source is a finite, pulse-sig-nal type, the first moment equation should be applied (Shackelford,1993). Although Shackelford (1995) did not involve organoclay amend-ments in the tested soils, the observation from this study suggests thatthe same conclusions can be drawn on the naphthalene-HDTMA ben-tonite amended soil combination.

Additionally, the results suggest that the retardation of naphtha-lene transport in HDTMA bentonite amended (only 5% by weight)silty clay could lead to a significantly reduced mass flux of naphtha-lene, which could be even more pronounced as the amount oforganobentonite was increased. The contaminant source had amass flux of 0.113 mg/cm2 × day, whereas the retardation yieldedan average mass flux of 0.004 mg/cm2 × day in the effluent, approx-imately 29 times lower. From a mass balance perspective, it was de-termined that 54% of total introducedmass of naphthalene to the soilcolumn was still uptaken by the small fraction of HDTMA bentonite(5%) after 33 days. Transport modeling (data not shown) for thisstudy also suggested a naphthalene diffusion coefficient in theorder of 10−6 cm2/s for the HDTMA bentonite amended silty clay,which is comparable with previous studies on soil barriers(Headley et al., 2001) (Table 6). This study suggests that a small frac-tion of HDTMA bentonite has a limited impact on the hydraulic con-ductivity and diffusion coefficient of amended soils. However, itobviously provides increased retention and retardation of naphtha-lene in a compacted soil matrix and may be applicable for otherorganic contaminants as well. Consequently, organobentoniteamendments may have promising applications in soil liners as theycan significantly reduce the mass flux of organic contaminantsthrough the soil barrier without imposing high advection and diffu-sion rates.

5. Conclusions

In this study, the barrier performance of a compacted silty clayand HDTMA bentonite mixture against gasoline and dissolved PAHwas assessed. Obtained results indicated that 10% of HDTMA benton-ite additives had little or no impact on the free swelling of silty clay inwater. However, silty clay with HDTMA bentonite showed a muchhigher swelling potential in gasoline due to the interaction betweenthe organophilic phase in the HDTMA bentonite and non-polar liq-uid. When an additional 10% HDTMA bentonite was added tocompacted silty clay, slight increase in hydraulic conductivity(water as the permeant fluid) was observed (5.04 × 10−9 m/s to1.57 × 10−8 m/s). However, HDTMA bentonite amended silty clayhad decreased permeability for gasoline (9.20 × 10−10 m/s, approx-imately 1 order of magnitude lower). The combined results with pre-vious studies suggest that a small fraction of organoclay additiveshave limited impact on the hydraulic conductivities of amendedcompacted silty clay (slightly increased), whereas they may enhancethe confinement of organic fluid or low-polarity fluids. For the or-ganic contaminant (naphthalene) mass transport in the HDTMAbentonite amended silty clay, the column test revealed that a smallfraction of HDTMA bentonite (~5%) can impose a retardation coeffi-cient of approximately 7 to the silty clay-dominated soil matrix.Although the breakthrough time of naphthalene through the low-flow rate system did not increase significantly, a slow release ofnaphthalene was observed (average of 0.004 mg/cm2 × dayand peak of 0.011 mg/cm2 × day compared to the source of

0.113 mg/cm2 × day). The overall results in this study suggest thatHDTMA bentonite or other organophilic clays hold the promise ofbeing the amendment material in a CCL barrier for organic and/orlow polarity fluids.

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