Estimation of transport parameters of phenolic compounds and inorganic contaminants through composite landfill liners using one-dimensional mass transport model Gamze Varank ⇑ , Ahmet Demir, Kaan Yetilmezsoy, M. Sinan Bilgili, Selin Top, Elif Sekman Yildiz Technical University, Faculty of Civil Engineering, Department of Environmental Engineering, 34220 Davutpasa, Esenler, Istanbul, Turkey article info Article history: Received 21 August 2010 Accepted 8 June 2011 Available online 13 July 2011 Keywords: Leachate Contaminant transport Geomembrane Phenolic compounds Inorganic contaminants 1D model abstract One-dimensional (1D) advection–dispersion transport modeling was conducted as a conceptual approach for the estimation of the transport parameters of fourteen different phenolic compounds (phenol, 2-CP, 2- MP, 3-MP, 4-MP, 2-NP, 4-NP, 2,4-DNP, 2,4-DCP, 2,6-DCP, 2,4,5-TCP, 2,4,6-TCP, 2,3,4,6-TeCP, PCP) and three different inorganic contaminants (Cu, Zn, Fe) migrating downward through the several liner sys- tems. Four identical pilot-scale landfill reactors (0.25 m 3 ) with different composite liners (R1: 0.10 + 0.10 m of compacted clay liner (CCL), L e = 0.20 m, k e =1 10 8 m/s, R2: 0.002-m-thick damaged high-density polyethylene (HDPE) geomembrane overlying 0.10 + 0.10 m of CCL, L e = 0.20 m, k e =1 10 8 m/s, R3: 0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick bentonite layer encapsulated between 0.10 + 0.10 m CCL, L e = 0.22 m, k e =1 10 8 m/s, R4: 0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick zeolite layer encapsulated between 0.10 + 0.10 m CCL, L e = 0.22 m, k e = 4.24 10 7 m/s) were simultaneously run for a period of about 540 days to investigate the nature of diffusive and advective transport of the selected organic and inorganic contaminants. The results of 1D transport model showed that the highest molecular diffusion coefficients, ranging from 4.77 10 10 to 10.67 10 10 m 2 /s, were estimated for phenol (R4), 2-MP (R1), 2,4-DNP (R2), 2,4-DCP (R1), 2,6-DCP (R2), 2,4,5-TCP (R2) and 2,3,4,6-TeCP (R1). For all reactors, dispersion coefficients of Cu, ranging from 3.47 10 6 m 2 /s to 5.37 10 2 m 2 /s, was determined to be higher than others obtained for Zn and Fe. Average molecular diffusion coefficients of phenolic compounds were estimated to be about 5.64 10 10 m 2 /s, 5.37 10 10 m 2 /s, 2.69 10 10 m 2 /s and 3.29 10 10 m 2 /s for R1, R2, R3 and R4 systems, respectively. The findings of this study clearly indicated that about 35–50% of transport of phenolic compounds to the groundwater is believed to be prevented with the use of zeolite and bentonite materials in landfill liner systems. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Leachates from municipal solid waste (MSW) landfills and var- ious discarded products contain a wide mixture of chemical pollu- tants and constitute a potential risk to the quality of receiving water bodies, such as surface water or groundwater (Paxéus, 2000; Christensen et al., 2001; Baun et al., 2004; Oman and Junestedt, 2008). A number of chemicals from MSW leachates are released during the lifetime of the landfill and result in emission of various volatile organic compounds (VOCs) and toxic inorganic pollutants. For this reason, MSW landfills have been identified as one of the major threats to groundwater resources (US EPA, 1984). Therefore, the impact of landfill leachate on the surface and groundwater has given rise to a number of studies in recent years (Fatta et al., 1999; Looser et al., 1999; Abu-Rukah and Al-Kofahi, 2001; Saarela, 2003; Longe and Enekwechi, 2007; Rowe, 2005). Pollutants in MSW landfill leachate can be divided into four groups: dissolved organic matter; inorganic macro components; heavy metals; and xenobiotic organic compounds (Kjeldsen et al., 2002). Researchers have reported from full-scale landfills, test cells, and laboratory studies that average heavy metal concentra- tions of landfill leachate are fairly low and heavy metals in landfill leachate at present are not a major concern (Revans et al., 1999; Kjeldsen and Christophersen, 2001). However, the issue of xenobiotic organic compounds (XOCs) in landfill leachates have been addressed in a number of studies (Christensen et al., 2001; Kjeldsen et al., 2002; Baun et al., 2004; Slack et al., 2005; Oman and Junestedt, 2008; Bejerg et al., 2009). The XOCs include a variety of aromatic hydrocarbons, phenols, chlorinated aliphatics, pesticides, and plastizers. Among them, 0956-053X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2011.06.005 ⇑ Corresponding author. Tel.: +90 212 383 5377; fax: +90 212 383 5358. E-mail addresses: [email protected](G. Varank), [email protected](A. Demir), [email protected](K. Yetilmezsoy), [email protected](M.S. Bilgili), [email protected](S. Top), [email protected](E. Sekman). Waste Management 31 (2011) 2263–2274 Contents lists available at ScienceDirect Waste Management journal homepage: www.elsevier.com/locate/wasman
Estimation of transport parameters of phenolic compounds and inorganiccontaminants through composite landfill liners using one-dimensional masstransport model
Gamze Varank ⇑, Ahmet Demir, Kaan Yetilmezsoy, M. Sinan Bilgili, Selin Top, Elif SekmanYildiz Technical University, Faculty of Civil Engineering, Department of Environmental Engineering, 34220 Davutpasa, Esenler, Istanbul, Turkey
a r t i c l e i n f o
Article history:Received 21 August 2010Accepted 8 June 2011Available online 13 July 2011
Keywords:LeachateContaminant transportGeomembranePhenolic compoundsInorganic contaminants1D model
0956-053X/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.wasman.2011.06.005
One-dimensional (1D) advection–dispersion transport modeling was conducted as a conceptual approachfor the estimation of the transport parameters of fourteen different phenolic compounds (phenol, 2-CP, 2-MP, 3-MP, 4-MP, 2-NP, 4-NP, 2,4-DNP, 2,4-DCP, 2,6-DCP, 2,4,5-TCP, 2,4,6-TCP, 2,3,4,6-TeCP, PCP) andthree different inorganic contaminants (Cu, Zn, Fe) migrating downward through the several liner sys-tems. Four identical pilot-scale landfill reactors (0.25 m3) with different composite liners (R1:0.10 + 0.10 m of compacted clay liner (CCL), Le = 0.20 m, ke = 1 � 10�8 m/s, R2: 0.002-m-thick damagedhigh-density polyethylene (HDPE) geomembrane overlying 0.10 + 0.10 m of CCL, Le = 0.20 m,ke = 1 � 10�8 m/s, R3: 0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick bentonitelayer encapsulated between 0.10 + 0.10 m CCL, Le = 0.22 m, ke = 1 � 10�8 m/s, R4: 0.002-m-thick damagedHDPE geomembrane overlying a 0.02-m-thick zeolite layer encapsulated between 0.10 + 0.10 m CCL,Le = 0.22 m, ke = 4.24 � 10�7 m/s) were simultaneously run for a period of about 540 days to investigatethe nature of diffusive and advective transport of the selected organic and inorganic contaminants. Theresults of 1D transport model showed that the highest molecular diffusion coefficients, ranging from4.77 � 10�10 to 10.67 � 10�10 m2/s, were estimated for phenol (R4), 2-MP (R1), 2,4-DNP (R2), 2,4-DCP(R1), 2,6-DCP (R2), 2,4,5-TCP (R2) and 2,3,4,6-TeCP (R1). For all reactors, dispersion coefficients of Cu,ranging from 3.47 � 10�6 m2/s to 5.37 � 10�2 m2/s, was determined to be higher than others obtainedfor Zn and Fe. Average molecular diffusion coefficients of phenolic compounds were estimated to beabout 5.64 � 10�10 m2/s, 5.37 � 10�10 m2/s, 2.69 � 10�10 m2/s and 3.29 � 10�10 m2/s for R1, R2, R3 andR4 systems, respectively. The findings of this study clearly indicated that about 35–50% of transport ofphenolic compounds to the groundwater is believed to be prevented with the use of zeolite and bentonitematerials in landfill liner systems.
� 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Leachates from municipal solid waste (MSW) landfills and var-ious discarded products contain a wide mixture of chemical pollu-tants and constitute a potential risk to the quality of receivingwater bodies, such as surface water or groundwater (Paxéus,2000; Christensen et al., 2001; Baun et al., 2004; Oman andJunestedt, 2008). A number of chemicals from MSW leachates arereleased during the lifetime of the landfill and result in emissionof various volatile organic compounds (VOCs) and toxic inorganicpollutants. For this reason, MSW landfills have been identified asone of the major threats to groundwater resources (US EPA,1984). Therefore, the impact of landfill leachate on the surface
and groundwater has given rise to a number of studies in recentyears (Fatta et al., 1999; Looser et al., 1999; Abu-Rukah andAl-Kofahi, 2001; Saarela, 2003; Longe and Enekwechi, 2007; Rowe,2005).
Pollutants in MSW landfill leachate can be divided into fourgroups: dissolved organic matter; inorganic macro components;heavy metals; and xenobiotic organic compounds (Kjeldsen et al.,2002). Researchers have reported from full-scale landfills, testcells, and laboratory studies that average heavy metal concentra-tions of landfill leachate are fairly low and heavy metals in landfillleachate at present are not a major concern (Revans et al., 1999;Kjeldsen and Christophersen, 2001). However, the issue ofxenobiotic organic compounds (XOCs) in landfill leachates havebeen addressed in a number of studies (Christensen et al., 2001;Kjeldsen et al., 2002; Baun et al., 2004; Slack et al., 2005; Omanand Junestedt, 2008; Bejerg et al., 2009).
The XOCs include a variety of aromatic hydrocarbons, phenols,chlorinated aliphatics, pesticides, and plastizers. Among them,
2264 G. Varank et al. / Waste Management 31 (2011) 2263–2274
phenol is the precursor to the synthesis of many organic com-pounds and is of high concern because of potential toxicity(Boopathy, 1997). Phenol and substituted phenols are commontransformation products of several pesticides. Many substitutedphenols, including chlorophenols, nitrophenols, and cresols, havebeen designated as priority pollutants by the US EnvironmentalProtection Agency (Boyd et al., 1983).
The liner system is one of the most important elements of amodern engineered landfill. There are two pathways for contami-nant transport through composite liners: advection and diffusionof inorganic and organic solutes through defects in the geomem-brane and subsequently through the soil liner; and diffusion of or-ganic solutes through the intact geomembrane and subsequentlythrough the soil liner (Edil, 2003; Rowe, 2005). Due to its highstrength, impermeability, and resistance to chemicals, the highdensity polyethylene (HDPE) geomembranes are the most widelyused components of a modern liner system in solid waste landfills.However, many studies have shown that geomembranes are essen-tially impervious to diffusion of inorganic contaminants but organ-ic compounds can readily penetrate through geomembranes in ashort period of time (Sangam and Rowe, 2001; Joo et al., 2005).
The other significant component of a modern liner system is soilliner generally comprising clay material. Early concerns regardingcontaminant transport through clay liners focused on advectivetransport (e.g. contaminants migrating along with the flow ofwater through the clay) but recently researchers have concludedthat diffusive transport (contaminant migration driven by the dif-ference in concentration between the upper and lower sides of theliner) is often the dominant mode of contaminant transportthrough well-built liner systems (Kim et al., 2001; Foose et al.,2002; Kalbe et al., 2002; Edil, 2003) including compacted clay lin-ers (Toupiol et al., 2002; Willingham et al., 2004; Bezza andGhomari, 2008), geosynthetic clay liners (Malusis and Shackelford,2004; Rowe et al., 2005) and, composite liners (Foose et al., 2002;Kalbe et al., 2002; Edil, 2003).
Although several studies on emission and impact of variousVOCs and toxic inorganic pollutants have been published moreand more in recent years (Rowe et al., 2000; Lake and Rowe,2004; El-Zein and Rowe, 2008; McWatters and Rowe, 2009), thereare almost no systematic papers in the literature specifically de-voted to a study of the estimation of transport parameters of var-ious phenolic compounds through composite landfill liners usingone-dimensional mass transport model. Therefore, clarification ofthe place of the present subject in the scheme of MSW landfillscan be considered as a particular field of investigation for theleachate management. For this reason, the present study aims atfulfilling the gap in this field by particularly focusing upon thetransport mechanisms of various phenolic compounds. In thisstudy, fourteen different phenolic compounds (phenol, 2-chloro-phenol (2-CP), 2-metilphenol (2-MP), 3-metilphenol (3-MP),4-metilphenol (4-MP), 2-nitrophenol (2-NP), 4-nitrophenol(4-NP), 2,4-dinitrophenol (2,4-DNP), 2,4-dichloropenol (2,4-DCP),2,6-dichloropenol (2,6-DCP), 2,4,5-trichlorophenol (2,4,5-TCP),2,4,6-trichlorophenol (2,4,6-TCP), 2,3,4,6-tetrachlorophenol (2,3,4,6-TeCP), pentachlorophenol (PCP)) and also three different inor-ganic contaminants (Cu, Zn, Fe) were selected to represent organicand inorganic compounds of the leachate constituents.
Considering the above-mentioned facts, the specific objectivesof this study were (1) to determine the transport parameters(molecular diffusion and dispersion coefficients) of the selectedinorganic and organic compounds migrating downward throughthe clay and composite liners; (2) to assess the applicability ofsimple diffusion models for the preliminary comparison of theperformance of different liner systems; and (3) to evaluate theimportance of organic and inorganic contaminant transport bymeans of landfill leachate management and groundwater quality.
2. Materials and methods
2.1. Reactor setup and operation
Four identical pilot-scale landfill reactors (R1, R2, R3 and R4)were simultaneously run for a period of about 540 days to investi-gate the nature of diffusive transport of the selected organic andadvective transport of inorganic contaminants. All parts of thereactors were made of HDPE pressurized pipes with a wall thick-ness of 0.005 m. The diameter (DR), height (HR), effective volume(VE) and total volume (VT) of the reactors were 0.40 m, 2.5 m,0.20 m3 and 0.25 m3, respectively.
The reactors were comprised from two parts with heights of2.0 m and 0.50 m to install alternative liners. The bottom of theupper part of the reactors consisted of a 0.15-m gravel drainagewith a perforated pipe (0.025 m diameter) inserted to collect andto discharge the generated leachate. The leachate collection wasperformed by opening the discharge valve on a daily-basis at thebeginning of the experiment, and at 1 or 2 week intervals for thefollowing periods. The discharged leachate from each reactor wasstored to use for recirculation. Distilled water was placed at thelower part of the four reactors simulating groundwater, and dis-tilled water samples were obtained from the valves existing atthe bottom. Landfill gas was collected via the perforated pipes,which were located in the center of each reactor (0.04 m diameterand 1.70 m height). Temperature probes were also located at1.20 m depths from the top of the waste to measure temperaturevariation in each landfill reactor. A detailed schematic of the exper-imental set-up is depicted in Fig. 1.
2.2. Composite liner systems
Four alternatives with different composite liners (0.10 + 0.10 mof compacted clay liner (CCL) for R1, 0.002-m-thick damaged high-density polyethylene (HDPE) geomembrane overlying 0.10 + 0.10m of CCL for R2, 0.002-m-thick damaged HDPE geomembraneoverlying a 0.02-m-thick bentonite layer encapsulated between0.10 + 0.10 m CCL for R3, and 0.002-m-thick damaged HDPE geo-membrane overlying a 0.02-m-thick zeolite layer encapsulatedbetween 0.10 + 0.10 m CCL for R4) were employed for the reactorliners. The clay material and geomembrane used in this study wereobtained from Komurcuoda Sanitary Landfill in (41�804100N,29�2202100E) in Istanbul, Turkey. The soil and geomembrane werevirgin materials and not in contact with waste in the KomurcuodaSanitary Landfill before sampling and use in the test cells.
According to the Turkish Solid Waste Control Regulation, thehydraulic conductivity of the compacted liner materials (i.e. clay,etc.) must be lower than or equal to 10�8 m/s for sanitary landfilllining. Therefore, prior to running the reactor systems, the hydrau-lic conductivities of both clay and bentonite were experimentallyadjusted to be about k 6 10�8 m/s. The detailed experimental pro-cedure and physical and chemical properties of the used materialscan be found in the work of Varank (2010). Since the porosity andthe effective grain diameter of zeolite used in R4 were experimen-tally determined, the hydraulic conductivity of zeolite was com-puted as the average of values obtained from the well-knownKozeny-Carman (d10 < 0.003 m) and Slitcher (d10 = 1 � 10�5–5 �10�3 m) equations, respectively (Odong, 2007):
k ¼ gm
� �� ð8:3� 10�3Þ n3
ð1� nÞ2
" #ðd10Þ2 ð1Þ
k ¼ gm
� �� ð1� 10�2ÞðnÞ3:287ðd10Þ2 ð2Þ
where g is the acceleration due to gravity (9.81 m/s2), m is thekinematic viscosity (1.01 � 10�6 m2/s for a water temperature of
Watertightness was ensured at the edges of the geomembranespecimens used in test cells R2 to R4
Fig. 1. A detailed schematic of the experimental set-up.
G. Varank et al. / Waste Management 31 (2011) 2263–2274 2265
20 �C), n is the porosity of zeolite used in this study (nzeolite = 0.493),d10 (or de) is the effective grain diameter of zeolite(d10 = 1.5 � 10�5 m). In this study, we also defined an equivalenthydraulic conductivity (ke) for the underlying composite liners ineach reactor system using the arithmetic average method proposedby Golan and Whitson (1986):
ke ¼k1L1 þ k2L2 þ � � � þ knLn
L1 þ L2 þ � � � þ Ln¼Pn
i¼1kiLiPni¼1Li
ð3Þ
In this study, a 0.008-m-diameter hole (a = 5 � 10�5 m2, N = 1)acting as the representative of a damaged section was manuallydrilled through the geomembrane specimens by using a rotary dril-ling machine to simulate the advective–dispersive transport of theselected inorganic contaminants (Cu, Zn, Fe) migrating downwardthrough the several liners into the model groundwater system. Inorder to ensure watertightness at the edges of the geomembranespecimens, the 0.002-m-thick HDPE geomembranes used in testcells R2–R4 were cut in slightly larger than the reactor diameter(DR = 0.40 m), and thoroughly compressed without remaining anyspace below. Considering the watertightness at the edges, it canbe noted that the only important point for the advective–dispersive transport of the selected inorganic contaminants wasthe manually drilled hole in the geomembrane specimens testedin the present work.
2.3. Source of municipal solid wastes
The disposed municipal solid wastes in the landfill reactorswere obtained from the Odayeri Sanitary Landfill (41�140800N,28�5101600E) in Istanbul, Turkey. The average composition of solidwaste samples collected from Odayeri Sanitary Landfill was 44%organic, 8% paper, 6% glass, 6% metals, 5% plastic, 54% textile, 9%nylon, 8% diaper, and 9% ash and others (Demir et al., 2004). Thereactors were filled with approximately 150 kg of fresh MSW.
2.4. Transport of organic and inorganic contaminants and formulation
Flow and solute transport through composite liners and under-lying geologic layers are three-dimensional (3D) processes (Foose,2010). The conceptual approach used herein is to analyze a 1Dsystem that approximates the characteristics of the 3D system, assimilarly conducted by Foose (2010). In this study, the followingassumptions are made for the estimation of transport parametersof the selected inorganic (Cu, Zn, Fe) and organic compounds(phenolic compounds) through an attenuation layer or layerswhich overlies an aquifer with horizontal flow (Foose, 2010): (1)porous medium is isotropic and saturated, (2) composite linerand underlying geologic layers are horizontal and homogeneous,(3) solute transport in the underlying aquifer occurs only viaadvection, (3) flow in the aquifer occurs in one-dimensional
2266 G. Varank et al. / Waste Management 31 (2011) 2263–2274
motion to the layers and is uniform and steady-state, (4) solutetransport in the underlying aquifer occurs only via advection, andDarcy’s law is valid, (5) no contaminant decay occurs in the com-posite liner and underlying geologic layers or aquifer.
In the 1D system (e.g. the x direction), transport by advection–dispersion and hydrodynamic dispersion (Dx) can be expressed asfollows (Ogata and Banks, 1961):
Transport by advection ¼�vxnCdA ð4Þ
Transport by diffusion ¼nDx@C@x
� �dA ð5Þ
Hydrodynamic dispersion ¼Dx ¼ ax �vx þ Dm ð6Þ
where �vx is the average linear velocity [L/T], n is the porosity (con-stant for unit of volume), C is the concentration of solute [M/L3], dAis the elemental cross-sectional area of the conceptual cubic controlvolume [L2], Dx is the hydrodynamic dispersion coefficient [L2/T], ax
is the longitudinal dispersivity [L], and Dm is the molecular diffusioncoefficient [L2/T]. Katsumi et al. (2001) have reported that advectiveand dispersive transport must be considered for clay liners due totheir low hydraulic conductivity. Considering the simultaneous ef-fect of advective and dispersive transport, the diffusive flux (Fx) ina 1D system is computed using the following equation (Ogata andBanks, 1961):
Fx ¼ vxnC � nDx@C@x
� �ð7Þ
In the analytical solution of 3D system, total amount of soluteentering the conceptual cubic element and the difference inamount entering and leaving the element are expressed, respec-tively, as follows (Ogata and Banks, 1961):
Fxdydzþ Fydxdzþ Fzdxdy ð8Þ@Fx
@xþ @Fy
@yþ @Fz
@z
� �dxdydz ð9Þ
For a non-reactive solute, the difference between flux in and outcan be considered to be equal to the amount accumulated withinthe element. Therefore, based on the predefined equations, the rateof mass change @C
@t
� �in the element is computed by using the advec-
tion–dispersion equation as follows (Ogata and Banks, 1961):
@C@t¼ Dx
@2C@x2
!þ Dy
@2C@y2
!þ Dz
@2C@z2
!" #
� �vx@C@x
� �þ �vy
@C@y
� �þ �vz
@C@z
� �� ð10Þ
Based on the general assumptions, the solution of Eq. (10) for1D transport system at an elapsed time, t, can be obtained by Ogataand Banks (1961) and Katsumi et al. (2001):
Cðx ¼ L; tÞC0
¼ 12
erfc1� TR
2ffiffiffiffiffiffiffiffiffiffiffiffiTR=PL
p !
þ expðPLÞerfc1þ TR
2ffiffiffiffiffiffiffiffiffiffiffiffiTR=PL
p !( )
ð11Þ
where L is thickness of the clay liner and x is the vertical downwardcoordinate with origin at the surface of the liner, C is the concentra-tion of solute at time of t (herein described as the average concen-tration in distilled water), and C0 is the initial value of C at time of t(herein described as the average concentration in leachate). Theparameter TR is the dimensionless time factor and PL is the Pecletnumber representing the relative magnitudes of advective anddispersive transport. In Eq. (11), TR and PL are computed as follows(Katsumi et al., 2001):
TR ¼mSx � tR � L ¼
mSx � tð1þ qdKp=nÞ � L ð12Þ
PL ¼mSx � L
Dð13Þ
where mSx is the seepage velocity (in the x direction), qd is the drydensity of the clay, Kp is the clay-solute partition coefficient, andD is the dispersion coefficient for the solute. The term ð1þ qdKp
=nÞ in Eq. (12) is called the retardation factor, R. When substitutingEqs. (12) and (13) into Eq. (11), the final form of this equation can beobtained as follows:
Cðx ¼ L; tÞC0
¼ 12
erfcR�L�mSx �t
R
� �ffiffiffiffiffiffiffiffiffiffiffiffi
4�D�tR
� �q0B@
1CAþ exp
mSx � LD
� �erfc
R�LþmSx �tR
� �ffiffiffiffiffiffiffiffiffiffiffiffi
4�D�tR
� �q0B@
1CA
8><>:
9>=>;ð14Þ
In a parametric study conducted by Katsumi et al. (2001), three dif-ferent values of retardation factor (R = 1, R = 2 and R = 5), two differ-ent values of hydraulic conductivity (k = 10�9 m/s and k = 10�9 m/s), and three different values of dispersion coefficient (D = 1 �10�9 m2/s, D = 2 � 10�10 m2/s and D = 4 � 10�11 m2/s) were per-formed to investigate the migration of inorganic and organic chem-icals in compacted clay liners. The authors concluded that thedispersion coefficient and retardation factor affected the perfor-mance of liners when the hydraulic conductivity of the clay linerwas low (10�9 m/s), whereas the parameters had much smallereffect when the hydraulic conductivity was higher (10�8 m/s).Therefore, in the conceptual approach used herein, we selectedretardation factors as R = 1 (for organics) and R = 2 (for inorganics),which are consistent with the values assumed by Katsumi et al.(2001) for the performance-based design of the landfills. Katsumiet al. (2001) have also reported that for organic compounds, thecontribution of leakage through the geomembrane defects is negli-gible because molecular diffusion through the geomembrane is farmore significant. As stated by Katsumi et al. (2001), for organic con-taminants tested in this study, diffusion can be ignored through thegeomembrane because the geomembrane is significantly thinnerthan the clay liner (Lg = 0.0015–0.002 mm� 0.20 m). Moreover,for organic chemicals, advection can be considered to be zero be-cause the geomembrane limits leakage to very small quantities(Katsumi et al., 2001). As a result, in this study ðmSx ffi 0Þ, Eq. (14)can be simplified for organic compounds as follows (Katsumiet al., 2001):
Cðx ¼ L; tÞC0
¼ 12
erfcLffiffiffiffiffiffiffiffiffiffiffiffi4�D�t
R
� �q0B@
1CAþ expð0Þerfc
Lffiffiffiffiffiffiffiffiffiffiffiffi4�D�t
R
� �q0B@
1CA
8><>:
9>=>; ð15Þ
Cðx ¼ L; tÞC0
¼ erfcLffiffiffiffiffiffiffiffiffiffiffiffi4�D�t
R
� �q0B@
1CA ð16Þ
Since the second term with exponential function can be ne-glected in most cases, Eq. (14) is simplified for inorganic com-pounds as follows (Ogata and Banks, 1961):
Cðx ¼ L; tÞC0
¼ 12
erfcR�L�mSx �t
R
� �ffiffiffiffiffiffiffiffiffiffiffiffi
4�D�tR
� �q0B@
1CA
8><>:
9>=>; ð17Þ
As seen in Eq. (17), leakage through the geomembrane defects isthe primary transport mechanism of inorganic contaminantsthrough composite liners. Katsumi et al. (2001) have reported thatthis flow and transport process is three-dimensional (3D), whichmakes it difficult to simulate. Therefore, the 3D system can be ana-lyzed readily if it is approximated as an equivalent 1D system hav-ing a leakage rate (Q) through an area (Ae) (Katsumi et al., 2001;
Table 1Parametric values used in 1D advection–dispersion model for the estimation of thetransport parameters.
Constituent Unit Value
Head of liquid on the top of the geomembrane (hw) m 0.025–0.050Thickness of the damaged HDPE geomembrane
(Lg)m 0.002
Thickness of the composite liner (Lc, Lb, Lz) Clay (Lc, m) 0.20Bentonite(Lb, m)
0.02
Zeolite (Lz,m)
0.02
Equivalent thickness of the composite liner (Le) For R1 andR2 (m)
0.20
For R3 andR4 (m)
0.22
The quality of the intimate contact between thegeomembrane and its underlying clay liner (Cf)
Equivalent hydraulic conductivity (ke) For R1, R2,R3 (m/s)
61 � 10�8
For R4 (m/s) 4.24 � 10�7
Number of defects (N) – 1Area of single defect in geomembrane (a) m2 5 � 10�5
Surface area of the reactor (AR) m2 0.126
G. Varank et al. / Waste Management 31 (2011) 2263–2274 2267
Foose, 2010). In this study, the leakage rate of inorganic contami-nants is computed for a circular defect in a composite liner usingthe following equation (hw < 3 m, and defect diameter a 6 5 �10�4 m2) (Giroud, 1997; Erickson and Thiel, 2002; Foose, 2010):
Q ¼ Cf 1þ 0:1hw
L
� �0:95 !
ðaÞ0:1ðhwÞ0:9ðkÞ0:74ðNÞ ð18Þ
where Q is the leakage rate per unit area (m3/m2/s), Cf is the relatedto the quality of the intimate contact between the geomembraneand its underlying clay liner, hw is the head of liquid on the top ofthe geomembrane (m), L is the thickness of the composite liner(m), a is the area of single defect in geomembrane (m2), k is thehydraulic conductivity (herein defined as equivalent hydraulic con-ductivity, ke) of the underlying composite liner (m/s), and N is thenumber of defects having area a.
Eq. (18) contains the factor Cf which accounts for the degree ofintimate contact between the geomembrane and its underlyingclay liner. Empirical studies indicated that compacted clay linerinstallations with good construction quality assurance achieve thatwould be considered a ‘‘good’’ liner contact rating, resulting in a Cf
value of about 0.21. On the other hand, without good constructionquality and quality assurance, the hydraulic contact between ageomembrane and a compacted clay liner might be ‘‘poor’’, result-ing in a Cf value of about 1.15. Using the results published in theliterature, Thiel et al. (2001) recommend a conservative value ofCf = 0.01 for contact between the bentonite component of the geo-membrane supported geosynthetic clay liner and an adjacent geo-membrane. Moreover, it is reported that a value of Cf = 0.05 forfabric-encased geosynthetic clay liners to represent ‘‘excellent’’contact conditions (Erickson et al., 2002).
In a specific design, liquid head build-up, hw, may vary from lessthan 0.025 m for cap applications, up to 0.30 m for regulated appli-cations (Erickson and Thiel, 2002). In a parametric study conductedby Akgun (1997), three different values of leachate head (hw =0.01 m, hw = 0.1 m and hw = 1 m) were subjected to several linertypes such as clay-only, geomembrane-only, geomembrane/claycomposite to calculate the expected leachate rate for the structure.Moreover, Giroud et al. (1997) calculated the rates of leachatemigration through the defect for different values of leachate heads(i.e. hw = 0.01 m, hw = 0.04 m, hw = 0.16 m, etc.) and geomembranedefect diameters (i.e. d = 0.0005 m, d = 0.001 m, d = 0.002 m,d = 0.005 m, d = 0.0113 m, etc.) in design examples presented inanother parametric study.
Considering both the above-mentioned facts and the scale ofthe studied system (DR = 0.40 m, VT = 0.25 m3), in this conceptualapproach, we conducted different values of leachate heads(hw = 0.0025–0.050 m) to be used in 1D advection–dispersion mod-el for the estimation of the transport parameters, as well as for thecalculation of the radius of the wetted area. Furthermore, the Cf va-lue was selected as 0.01 indicating a conservative contact rating, asconducted by Thiel et al. (2001). The length of the simulation timewas about t = 514.17 days for inorganic compounds. Finally, theleakage rate (Q) was computed in m3/s for a reactor surface areaof about AR = 0.126 m2.
It is apparent from the literature that various analytical solu-tions (Rowe, 1998; Touze-Foltz et al., 1999), empirical equations(Touze-Foltz and Giroud, 2003; Giroud and Touze-Foltz, 2005),experimental studies (Koerner and Koerner, 2002; Touze-Foltzet al., 2006) and numerical modelling works (Foose et al., 2001;Saidi et al., 2006) were successfully conducted to determine advec-tive flow rates through composite liners incorporating geosyn-thetic clay liners. In many of these studies, it is assumed thatthere is an interface between the soil layer surface and the geo-membrane, so that geomembrane is not actually in perfect contactwith the soil layer (Brown et al., 1987). In this case, the liquid that
flows through the geomembrane defect then flows laterally tosome distance in the interface prior to percolating through the soilcomponent. This results in a concept of a ‘‘wetted area’’ of the soilcomponent (Touze-Foltz and Giroud, 2003). Since the interfaceflow has a significant influence on the rate of flow through a com-posite liner, proper definition and quantification of contact condi-tions is important at the interface between the two components ofthe composite liner, the geomembrane and the soil layer (Touze-Foltz and Giroud, 2003; Rowe, 1998).
In this study, the radius of the corresponding wetted area (Rw)assuming conservative contact conditions (Cf = 0.01) adopted inthe determination of the flow rate (Q) in Eq. (18) is calculated usingthe following equation previously proposed by Giroud and Bona-parte (1989):
Rw ¼ffiffiffiffiffiCf
p
rðaÞ0:05ðhwÞ0:45ðkeÞ�0:13 ð19Þ
assuming a gradient equal to 1.Table 1 summarizes the selected parametric values used in 1D
advection–dispersion model for the estimation of the transportparameters. Based on the above-mentioned design considerationsand the continuity equation, the seepage velocity (mSx) in transportof inorganic constituents through geomembrane defects in com-posite liners can be obtained from Eqs. (3) and (18) as a functionof leachate head (hw), thickness of the composite liner (L) andthe equivalent hydraulic conductivity (ke):
mSx ¼ f ðhw; L; keÞ ¼ ð0:00372Þ 1þ 0:1hwL
� �0:95 !
ðkeÞ0:74 ð20Þ
When substituting the selected retardation factors and simula-tion periods (R = 1 and t = 535.63 days for organics and R = 2 andt = 514.17 days inorganics) into Eqs. (16) and (17), molecular diffu-sion (Dm) and dispersion (Dd) coefficients are specifically computedfor organic and inorganic compounds respectively, as follows:
2268 G. Varank et al. / Waste Management 31 (2011) 2263–2274
Dm ¼ f ðL;C;C0Þ ¼0:022L
erfc�1 CC0
� �0@
1A
2
ð21Þ
Dd ¼ f ðL; mSx;C;C0Þ ¼L�ð257:09ÞðmSxÞ
32:07
� �erfc�1 2C
C0
� �0@
1A
2
ð22Þ
2.5. Analytical procedure
All analyses were conducted according to the relevant methodsdescribed in the standard methods of APHA (2005). To determineheavy metal content of the landfill leachate, the digestion of leach-ate and solid waste samples were realized in a microwave furnace(ETHOS 1600). The quantity of the samples used for digestion was5 mL for leachate samples and 1 g for solid waste samples. Solidwaste samples were dried at 103 �C for 24 h and ground to passa 0.0015-m screen. Leachate and solid waste samples were put ina Teflon vessel, which is resistant to pressure, and digested with6 mL HNO3, 3 mL HCl, and 0.25 mL H2O2. After the digestion pro-cess, the samples were filtered and the volume of the sampleswas completed to 100 mL. Metal concentrations determined byatomic absorption spectrophotometer (Perkin–Elmer Simaa 6000Model).
To determine phenol and phenolic compounds in leachate,SPME method is used as conducted by Ribeiro et al. (2002).Eighty-five micrometre polyacrylate fiber (from SUPELCO), a SPMEfiber holder (from SUPELCO) and (25 � 10�5 m � 30 m � 0.25 lm)column are used for this method. The fiber was conditioned in theGC injector for 1 h at 250 �C. The vial capacity was 4 mL, handling2 mL of sample. The temperature and stirring velocity (750 rpm)were controlled during extraction. The pH of the samples were ad-justed (pH < 2) with H2SO4 by using pH meter (Jenway 3040 IonAnalyser) and a pH probe (HI1230, Hanna Instruments). Na2SO4
was used to saturate samples. GC/FID analyses were carried outusing a Varian 3900 Model GC/FID Gas Chromatograph with thehelium carrier gas at 10 mL/min. Injector and detector tempera-tures are 250 and 320 �C, respectively. The temperature programis increased to 280 �C at a heating rate of 6 �C/min and held at thistemperature for 5 min. Phenolic compounds are quantified by peakarea using external standard method. Quantification is achievedusing peak area calculations, and compound identification is partlycarried out using correlations between retention times. EPA 8040Aand EPA 8040B phenol calibration mixtures (from SUPELCO) con-taining eighteen phenols with an individual concentration of2000 lg/L in isopropyl alcohol was used to obtain fourteen phenolderivatives.
2.6. Statistical analysis
Each chemical analysis (concentration of phenolic compounds,heavy metal content, etc.) was performed in triplicate and repeatedat least three times to observe the reproducibility, and experimen-tal results were reported as the mean value of each parameter. Alldescribed statistics reported in this study were calculated usingthe statistical functions in spreadsheets of Microsoft Excel� 2007or DataFit
2005 Oakdale Engineering) used as open database connectivity(ODBC) data sources. An online calculator (Department of Electricaland Computer Engineering, College of Engineering, Illinois Univer-sity, Urbana-Champaign, USA) was used for calculating the compli-mentary error function (erfc) or the inverse complimentary errorfunction (erfc�1) obtained from the one-dimensional mass trans-port model. The erfc was calculated with an error of less than
1 � 10�7 by using Chebyshev’s approximation. The complimentaryerror function is expressed as follows:
erf ðxÞ ¼ 2ffiffiffiffipp
Z x
0e�t2
dt ð23Þ
erfcðxÞ ¼ 2ffiffiffiffipp
Z 1
xe�t2
dt ¼ 1� erf ðxÞ ð24Þ
3. Results and discussion
3.1. Transport of organic compounds and inorganic contaminants
In this study, leachate quality and groundwater contaminationwere regularly monitored by the means of organic (phenolic com-pounds) and inorganic (Cu, Zn, Fe) contaminants. Variations ofphenolic compounds and inorganic contaminants in leachate gen-erated in pilot-scale landfill reactors during the experimental per-iod (t = 535.63 days for organics and t = 514.17 days for inorganics)are summarized in Tables 2 and 3, respectively. Moreover, Tables 4and 5 present the variations of phenolic compounds and inorganiccontaminants in distilled water underlying the studied modellandfill site, respectively.
As seen from Tables 2–5, considering all phenolic compoundsobserved herein, transport ratio of organic contaminants from R1and R2 systems through clay and clay + geomembrane liners weredetermined to be about 34.52% and 35.05% on average, respec-tively. It can be concluded from these results that the dominantmechanism in organic contaminant (phenol and phenolic com-pounds) transport from leachate to groundwater is molecular dif-fusion and geomembrane layer is ineffective in organiccontaminant transport through composite liners. On the averagebasis, transport ratio of phenolic compounds from R3 and R4 sys-tems through composite liners consisting of additional bentoniteand zeolite materials were determined as 17.19% and 21.70%,respectively. These results indicated that using bentonite or zeoliteas a liner component reduced the transport ratio approximately35–50% compared to the transport ratios obtained from R1 andR2 systems.
Experimental results indicated that the total amounts of con-taminants transported through the compacted clay liner (R1) andcompacted clay + damaged HDPE geomembrane (R2) were approx-imately two times greater than others (R3 and R4) consisting ofcompacted clay + damaged HDPE geomembrane + bentonite (R3)and compacted clay + damaged HDPE geomembrane + zeolite(R4). Based on the experimental findings, it can be noted that con-ventional landfill liners may prevent groundwater contamination,but not thoroughly. Since geomembranes are thin enough the stea-dy-state conditions for organic contaminants are quickly reached,as similarly reported by Katsumi et al. (2001). In this case, organiccontaminants in landfill leachates constitute a serious threat togroundwater. Therefore, in order to assess the pollution of thegroundwater and to perform a preliminary comparison of the per-formance of different liner systems, a conceptual approach wasproposed in this study. A simplified 1D advection–dispersiontransport model was readily conducted to simulate leaching fromMSW landfills and the solute transport into the groundwater, aswell as to estimate the transport parameters of the studied organicand inorganic contaminants.
3.2. Determination of diffusion and dispersion coefficients
Based on present projection criteria and Eqs. (20)–(22)described herein, one-dimensional (1D) advection–dispersion cal-culations were conducted as a conceptual approach for the estima-tion of the transport parameters (i.e. molecular diffusion and
Table 3Variation of inorganic contaminants in leachate (C0) generated in pilot-scale landfill reactors with different composite liners (t = 514.17 days).
a R1 (0.10 + 0.10 m of CCL, Le = 0.20 m, ke 6 1 � 10–8 m/s).b R2 (0.002-m-thick damaged HDPE geomembrane, 0.10 + 0.10 m of CCL, Le = 0.20 m, ke 6 1 � 10�8 m/s).c R3 (0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick bentonite layer encapsulated between 0.10 + 0.10 m CCL, Le = 0.22 m, ke 6 1 � 10�8 m/s).d R4 (0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick zeolite layer encapsulated between 0.10 + 0.10 m CCL, Le = 0.22 m, ke = 4.24 � 10�7 m/s).
Table 2Variation of phenolic compounds in leachate (C0) generated in pilot-scale landfill reactors with different composite liners (t = 535.63 days).
a R1 (0.10 + 0.10 m of CCL, Le = 0.20 m, ke 6 1 � 10–8 m/s).b R2 (0.002-m-thick damaged HDPE geomembrane, 0.10 + 0.10 m of CCL, Le = 0.20 m, ke 6 1 � 10�8 m/s).c R3 (0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick bentonite layer encapsulated between 0.10 + 0.10 m CCL, Le = 0.22 m, ke 6 1 � 10�8 m/s).d R4 (0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick zeolite layer encapsulated between 0.10 + 0.10 m CCL, Le = 0.22 m, ke = 4.24 � 10�7 m/s).
Table 4Variation of phenolic compounds in distilled water (C) underlying the model landfill site (t = 535.63 days).
a R1 (0.10 + 0.10 m of CCL, Le = 0.20 m, ke 6 1 � 10–8 m/s).b R2 (0.002-m-thick damaged HDPE geomembrane, 0.10 + 0.10 m of CCL, Le = 0.20 m, ke 6 1 � 10�8 m/s).c R3 (0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick bentonite layer encapsulated between 0.10 + 0.10 m CCL, Le = 0.22 m, ke 6 1 � 10�8 m/s).d R4 (0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick zeolite layer encapsulated between 0.10 + 0.10 m CCL, Le = 0.22 m, ke = 4.24 � 10�7 m/s).
G. Varank et al. / Waste Management 31 (2011) 2263–2274 2269
dispersion coefficients) of fourteen different phenolic compounds(phenol, 2-CP, 2-MP, 3-MP, 4-MP, 2-NP, 4-NP, 2,4-DNP, 2,4-DCP,2,6-DCP, 2,4,5-TCP, 2,4,6-TCP, 2,3,4,6-TeCP, PCP) and three differ-ent inorganic contaminants (Cu, Zn, Fe) migrating downwardthrough the several liner systems.
As seen from Fig. 2, the highest molecular diffusion coefficientwas estimated to be 9.379 � 10�10 m2/s for 2-MP, 2,4-DCP and2,3,4,6-TeCP in R1 system (0.10 + 0.10 m of CCL, Le = 0.20 m,ke = 1 � 10�8 m/s). The lowest molecular diffusion coefficientwas determined as 0.77 � 10�10 m2/s for 2-CP. Based on the
Table 5Variation of inorganic contaminants in distilled water (C) underlying the model landfill site (t = 514.17 days).
a R1 (0.10 + 0.10 m of CCL, Le = 0.20 m, ke 6 1 � 10�8 m/s).b R2 (0.002-m-thick damaged HDPE geomembrane, 0.10 + 0.10 m of CCL, Le = 0.20 m, ke 6 1 � 10�8 m/s).c R3 (0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick bentonite layer encapsulated between 0.10 + 0.10 m CCL, Le = 0.22 m, ke 6 1 � 10�8 m/s).d R4 (0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick zeolite layer encapsulated between 0.10 + 0.10 m CCL, Le = 0.22 m, ke = 4.24 � 10�7 m/s).
2270 G. Varank et al. / Waste Management 31 (2011) 2263–2274
transport mechanisms observed in R1, the molecular diffusioncoefficient of phenol was found to be about 2.79 � 10�10 m2/s,which is consistent with the values obtained by Xie et al.(2009) in a study on the analysis of diffusion–adsorption equiva-lency of landfill liner systems for organic contaminants. The re-sults indicated that molecular diffusion coefficients of 2-MP,2,4-DCP, 2,3,4,6-TeCP, 3-MP, 4-MP and 2-NP were estimated tobe higher than the values obtained for other phenolic compoundsthrough clay liners. When considering all phenolic compoundsobserved herein, the average molecular diffusion coefficient ofthe organic compounds was estimated to be about 5.465 �10�10 m2/s for the R1 system.
Fig. 3 shows that the highest molecular diffusion coefficient wasestimated to be 10.671 � 10�10 m2/s for 2,6-DCP and 2,4,5-TCP inR2 system (0.002-m-thick damaged HDPE geomembrane overlying0.10 + 0.10 m of CCL, Le = 0.20 m, ke = 1 � 10�8 m/s). As similarlyobtained in R1, the lowest molecular diffusion coefficient wasdetermined to be 1.04 � 10�10 m2/s for 2-CP. In this system, themolecular diffusion coefficient of phenol was estimated as 2.67� 10�10 m2/s, which is in line with the values obtained by Xieet al. (2009) for compacted clay liners (CCL). The rates of diffusivetransport for the phenolic compounds such as 2,6-DCP, 2,4,5-TCP,2,3,4,6-TeCP, 4-NP and 2,4-DCP, were found to be higher than oth-ers in R2 system. Considering all phenolic compounds in R2 sys-tem, the average molecular diffusion coefficient of the organiccompounds was estimated as 5.37 � 10�10 m2/s.
Phenolic compounds in the R1 system
Phen
ol
2-C
P
2-M
P
3-M
P
4-M
P
2-N
P
4-N
P
2,4-
DN
P
2,4-
DC
P
2,6-
DC
P
2,4,
5-TC
P
2,4,
6-TC
P
2,3,
4,6-
TeC
P
PCP
Mol
ecul
ar d
iffus
ion
coef
ficie
nts
(x 1
0-10 m
2 /s)
0
2
4
6
8
10
Fig. 2. Estimated molecular diffusion coefficients (t = 535.63 days, Le = 0.20 m,R = 1, ke 6 1 � 10�8 m/s) for phenolic compounds in the R1 system (0.10 + 0.10 mCCL).
The highest molecular diffusion coefficient was estimated to be4.77 � 10�10 m2/s for 2,4-DNP in R3 system (0.002-m-thick dam-aged HDPE geomembrane overlying a 0.02-m-thick bentonite layerencapsulated between 0.10 + 0.10 m CCL, Le = 0.22 m, ke = 1� 10�8 m/s) (Fig. 4). As similarly found in both R1 and R2, the low-est molecular diffusion coefficient was determined to be0.95 � 10�10 m2/s for 2-CP. On the basis of the transport mecha-nisms observed in R3, the molecular diffusion coefficient of phenolwas determined to be about 2.72 � 10�10 m2/s, which is compara-ble with the values previously reported by Xie et al. (2009). The re-sults clearly demonstrated that the molecular diffusion coefficientsestimated in R3 (with bentonite addition) were found to be lowerthan those obtained in R1 and R2. Moreover, in R3 system, the ratesof diffusive transport for 2,4-DNP, 4-NP, 2-NP, 2-MP and 2,4,5-TCPwere found to be higher than the molecular diffusion coefficientsestimated for other phenolic compounds. For the R3 system theaverage molecular diffusion coefficient of the phenolic compoundswas estimated to be about 2.69 � 10�10 m2/s.
The estimated molecular diffusion coefficients for phenoliccompounds in R4 system (0.002-m-thick damaged HDPE geomem-brane overlying a 0.02-m-thick zeolite layer encapsulated between0.10 + 0.10 m CCL, Le = 0.22 m, ke = 4.24 � 10�7 m/s) is depicted inFig. 5. In R4, the highest molecular diffusion coefficients were esti-mated to be 5.824 � 10�10 m2/s and 5.04 � 10�10 m2/s and for2,3,4,6-TeCP and phenol, respectively. On the other hand, the low-est molecular diffusion coefficients were determined to be about
Phenolic compounds in the R2 system
Phen
ol
2-C
P
2-M
P
3-M
P
4-M
P
2-N
P
4-N
P
2,4-
DN
P
2,4-
DC
P
2,6-
DC
P
2,4,
5-TC
P
2,4,
6-TC
P
2,3,
4,6-
TeC
P
PCP
Mol
ecul
ar d
iffus
ion
coef
ficie
nts
(x 1
0-10 m
2 /s)
0
2
4
6
8
10
12
Fig. 3. Estimated molecular diffusion coefficients (t = 535.63 days, Le = 0.20 m,R = 1, ke 6 1 � 10�8 m/s) for phenolic compounds in the R2 system (0.002-m-thickdamaged HDPE geomembrane overlying a 0.10 + 0.10 m CCL).
Phenolic compounds in the R3 system
Phen
ol
2-C
P
2-M
P
3-M
P
4-M
P
2-N
P
4-N
P
2,4-
DN
P
2,4-
DC
P
2,6-
DC
P
2,4,
5-TC
P
2,4,
6-TC
P
2,3,
4,6-
TeC
P
PCP
Mol
ecul
ar d
iffus
ion
coef
ficie
nts
(x 1
0-10 m
2 /s)
0
1
2
3
4
5
6
Fig. 4. Estimated molecular diffusion coefficients (t = 535.63 days, Le = 0.22 m,R = 1, ke 6 1 � 10�8 m/s) for phenolic compounds estimated in the R3 system(0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick bentonitelayer encapsulated between 0.10 + 0.10 m CCL).
Phenolic compounds in the R4 system
Phen
ol
2-C
P
2-M
P
3-M
P
4-M
P
2-N
P
4-N
P
2,4-
DN
P
2,4-
DC
P
2,6-
DC
P
2,4,
5-TC
P
2,4,
6-TC
P
2,3,
4,6-
TeC
P
PCP
Mol
ecul
ar d
iffus
ion
coef
ficie
nts
(x 1
0-10 m
2 /s)
0
1
2
3
4
5
6
7
Fig. 5. Estimated molecular diffusion coefficients (t = 535.63 days, Le = 0.22 m,R = 1, ke = 4.24 � 10�7 m/s) for phenolic compounds in the R4 system (0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thick zeolite layer encap-sulated between 0.10 + 0.10 m CCL).
G. Varank et al. / Waste Management 31 (2011) 2263–2274 2271
1.12 � 10�10 m2/s and 1.21 � 10�10 m2/s for 4-MP and 2-CP,respectively. As similarly observed for R1, R2 and R3, the estimatedmolecular diffusion coefficient of phenol in R4 is also within therange of diffusion coefficients of phenols (ranging from 1.1 �10�12 m2/s to 4 � 10�7 m2/s) obtained by Xie et al. (2009). The re-sults indicated that the molecular diffusion coefficients estimatedin R4 (with zeolite addition) were mostly found to be lower thanthose obtained in R1 (only CCL) and R2 (damaged HDPE geomem-brane overlying CCL), but higher than those in R3 (damaged HDPEgeomembrane overlying bentonite layer encapsulated betweenCCL). In R4 system damaged HDPE geomembrane overlying zeolitelayer encapsulated between CCL), the rates of diffusive transportfor 2,3,4,6-TeCP, phenol, 2,4,5-TCP, 2,4,6-TCP and PCP were foundto be higher than the molecular diffusion coefficients of otherphenolic compounds. When considering all phenolic compoundsobserved herein, the average molecular diffusion coefficient ofthe organic compounds was estimated to be about 3.29 �10�10 m2/s for the R4 system. These results clearly indicated thatabout 35–50% of transport of phenolic compounds to the ground-water can be significantly prevented by choosing a suitable mate-rial, such as zeolite and bentonite, in composite liner systems.
Table 6Estimated dispersion coefficients and radius of the wetted area (t = 514.17 days, Le = 0.22damaged HDPE geomembrane overlying a 0.10 + 0.10 m CCL).
Dispersion coefficients and the corresponding values of theradius of the wetted area (t = 514.17 days, Le = 0.22 m, R = 2,ke = 1 � 10�8 m/s for R2 and R3, and ke = 4.24 � 10�7 m/s for R4)of the inorganic compounds (Cu, Zn ve Fe) estimated for the R2,R3 and R4 systems (including 0.002-m-thick damaged HDPE geo-membrane) are given in Tables 6–8, respectively. For all inorganiccontaminants, dispersion coefficients estimated for the R3 system(ranging from 2.62 � 10�7 m2/s to 8.36 � 10�4 m2/s) were foundto be lower than those obtained in R2 (ranging from3.61 � 10�7 m2/s to 8.16 � 10�3 m2/s) and R4 (ranging from9.09 � 10�5 m2/s to 5.37 � 10�2 m2/s) systems. The results indi-cated that Cu had the highest dispersion (ranging from 3.47 �10�6 m2/s to 5.37 � 10�2 m2/s) among all inorganic contaminants.On the other hand, the lowest dispersion coefficients (ranging from2.62 � 10�7 m2/s to 2.10 � 10�2 m2/s) were estimated for Fe in allreactor systems. It is noted that the effect of the various geochem-ical processes on the transfer of contaminants might be differentdue to differences in mobility and physio-chemical behavior ofeach inorganic contaminant, as similarly reported by Ladha(1999). The results obviously indicated that the transport of inor-ganic compounds to the groundwater can be noticeably decreased
m, R = 2) for inorganic contaminants (Cu, Zn ve Fe) in the R2 system (0.002-m-thick
Table 7Estimated dispersion coefficients and radius of the wetted area (t = 514.17 days, Le = 0.22 m, R = 2) for inorganic contaminants (Cu, Zn ve Fe) in the R3 system (0.002-m-thickdamaged HDPE geomembrane overlying a 0.02-m-thick bentonite layer encapsulated between 0.10 + 0.10 m CCL).
Inorganic contaminant Cu Zn Fe Parametric values
Average (C/C0) 0.241 0.034 0.005 Cf a (m2) ke (m/s)
Table 8Estimated dispersion coefficients and radius of the wetted area (t = 514.17 days, Le = 0.22 m, R = 2) for inorganic contaminants (Cu, Zn ve Fe) in the R4 system (0.002-m-thickdamaged HDPE geomembrane overlying a 0.02-m-thick zeolite layer encapsulated between 0.10 + 0.10 m CCL).
Inorganic contaminant Cu Zn Fe Parametric values
Average (C/C0) 0.078 0.042 0.012 Cf a (m2) ke (m/s)
2272 G. Varank et al. / Waste Management 31 (2011) 2263–2274
with the particular use of bentonite material in composite linersystems. As seen from Tables 6–8, the radius of the wetted areavalues ranged from 0.025 m to 0.098 m for the R2 and R3 systems(ke 6 1 � 10�8 m/s) and from 0.016 m to 0.060 m for the R4 system(ke = 4.24 � 10�7 m/s) in this conceptual approach (hw = 0.025–0.050 m).
Finally, it is noted that the conceptual approach described here-in has several limitations. The molecular diffusion coefficients re-ported herein are determined in each cell for the mineral liner asa whole and that no analysis is performed to determine indepen-dently the diffusion coefficients for the various layers. As the mate-rials are homogeneous in R1, the diffusion coefficients in this casecould be extrapolable to other configurations with different soilthicknesses, however, there is a limitation of the study for cellsR3–R4. In order to extrapolate the results to landfill sites wheredifferent proportions of zeolite or bentonite can be used, there isa need to determine the diffusion coefficients independently forthe various layers. Moreover, the equations presented in this paperare limited to constant retardation factors (R = 1 for organics andR = 2 for inorganics) of organic and inorganic contaminants testedin various lining systems. Thus, to increase the precision of the pro-posed advection–dispersion transport model, some additionalsorption measurements are needed for determination of the retar-dation factors of the various contaminants in the soils of the study.
4. Conclusions
Transport mechanisms relevant to the performance of fourdifferent liner systems were investigated for various phenolic
compounds and inorganic contaminants based on a simplified 1Dadvection–dispersion transport model. In this study, the highestmolecular diffusion coefficients were estimated to be 9.38 �10�10 m2/s for 2-MP, 2,4-DCP and 2,3,4,6-TeCP in R1 (0.10 +0.10 m of CCL, Le = 0.20 m, ke = 1 � 10�8 m/s), and 10.67 � 10�10
m2/s for 2,6-DCP and 2,4,5-TCP in R2 (0.002-m-thick damagedHDPE geomembrane, 0.10 + 0.10 m of CCL, Le = 0.20 m, ke = 1 �10�8 m/s). For R3 (0.002-m-thick damaged HDPE geomembraneoverlying a 0.02-m-thick bentonite layer encapsulated between0.10 + 0.10 m CCL, Le = 0.22 m, ke = 1 � 10�8 m/s) and R4 (0.002-m-thick damaged HDPE geomembrane overlying a 0.02-m-thickzeolite layer encapsulated between 0.10 + 0.10 m CCL, Le = 0.22 m,ke = 4.24 � 10�7 m/s) systems, the highest molecular diffusioncoefficients were estimated to be about 4.775 � 10�10 m2/s for2,4-DNP, and 5.82 � 10�10 m2/s and 5.04 � 10�10 m2/s and for2,3,4,6-TeCP and phenol, respectively. On the average basis, themolecular diffusion coefficients of phenolic compounds were pre-dicted to be about 5.64 � 10�10 m2/s, 5.37 � 10�10 m2/s, 2.69 �10�10 m2/s and 3.29 � 10�10 m2/s for R1, R2, R3 and R4 systems,respectively.
The results showed that Cu had the highest dispersion coeffi-cients (ranging from 3.47 � 10�6 m2/s to 5.37 � 10�2 m2/s) amongall inorganic contaminants. In all reactors, the lowest moleculardiffusion coefficients, ranging from 0.77 � 10�10 m2/s to 1.210 �10�10 m2/s, and the lowest dispersion coefficients, ranging from2.62 � 10�7 m2/s to 2.10 � 10�2 m2/s, were estimated for 2-CPand Fe, respectively. The findings of this study clearly demon-strated that about 35–50% of transport of phenolic compounds tothe groundwater can be prevented with the particular use of ben-tonite and zeolite materials in landfill liner systems.
G. Varank et al. / Waste Management 31 (2011) 2263–2274 2273
Although predictions of contaminant transport described hereinwere limited by the assumptions and some boundary conditions,the simplified 1D transport model was readily used for the preli-minary comparison of the performance of different mineral linersystems. However, it can be concluded that the investigation ofthe potential effects of different operating conditions (e.g. head ofliquid on the top of the geomembrane, thickness of the compositeliner, area of single defect in geomembrane, the quality of the inti-mate contact between the geomembrane and its underlying clayliner, retardation factor, number of geomembrane defects, etc.)seems to be worthy of testing by future studies to develop a rele-vant database for MSW landfill leachate management, as well asto evaluate the impact of organic (phenolic compounds) and inor-ganic (Cu, Zn, Fe) contaminant transport on groundwater quality.
Acknowledgements
This research has been supported by The Scientific and Techno-logical Research Council of Turkey (TUBITAK–CAYDAG) (ProjectNumber: 105Y334) Ankara, Turkey.
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