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6th Asian Regional Conference on Geosynthetics - Geosynthetics for
Infrastructure Development, 8-11 November 2016, New Delhi, India
626
ROLE OF DIFFUSION ON THE TRANSPORT
OF VOLATILE ORGANIC CONTAMINANTS
THROUGH GEOMEMBRANES IN COMPOSITE
LINER SYSTEMS
R.K. ANJANA AND D.N. ARNEPALLI
Department of Civil Engineering, Indian Institute of Technology Madras
ABSTRACT
Geomembranes are popularly employed as part of composite liners in engineered landfi lls to
control contaminant migration from the landfi ll. Volatile organic compounds (VOCs) present in
the leachate can migrate through geomembranes by advection through defects or by diffusion.
Among these processes, diffusion is an important contaminant transport phenomenon since
diffusive transport may result in contaminant migration through intact geomembranes over
relatively short times. In view of this, the objective of the present study is to provide an overview
of the research carried out on diffusion of VOCs through geomembranes. The manuscript
discusses the infl uence of diffusion on the contaminant transport process, for an otherwise well-
designed barrier system, on the long-term performance of the barrier system. The study also
elaborates on the mechanism by which diffusion occurs through a geomembrane in response to
a concentration gradient. Recent research on alternative methods to minimise VOC diffusion
through geomembranes have been highlighted.
1. INTRODUCTION
In recent years, there have been many advances in the use of geosynthetics as contaminant barriers. The
predominant liner system until the 1980s was compacted clay liners (CCLs). However, there are few
drawbacks to CCLs, which have paved the way for the increased usage of geosynthetic liners in landfi ll
engineering (Touze-Foltz 2012; Anjana and Arnepalli 2015). Geomembranes are often used as barriers to
restrict the migration of contaminants present in leachate and landfi ll gas from modern sanitary landfi lls,
leachate ponds, and containment systems for fuel storage (Rowe et al. 2004, Koerner 2005; McWatters
and Rowe, 2010). Geomembranes are also commonly found as part of liner systems to minimize reservoir
seepage. Geomembranes are used in such applications because of their high service life, resistance to
stress cracking and leakage of contaminants, etc. Geomembranes are made of common polymers like
high density polyethylene (HDPE), linear low density polyethylene (LLDPE), polyvinyl chloride (PVC)
(Park and Nibras 1993; Nefso and Burns 2007; Rowe 2005; Koerner 2005).
The primary contaminant transport mechanisms relevant to typical landfi ll applications are advection
(leakage) and diffusion. Advection is a physical process whereby contaminants introduced into a
groundwater fl ow system migrate in solution or in suspension along with the movement of leachate or
groundwater. It is governed by Darcy’s Law, with the Darcy fl ux, va, given by :
v2=ki [1]
627
Role of Diffusion on the Transport of Volatile Organic Contaminants
Through Geomembranes in Composite Liner Systems
where k is the hydraulic conductivity (coeffi cient of permeability) and i is the hydraulic gradient, which
is often controlled by the height of leachate mounding on the landfi ll liner. The advective contaminant
transport through a geomembrane occurs through the fl aws in the geomembrane. The fl aws in the
geomembrane arise due to manufacturing defects, improper handling during transportation, damage from
indentation on placement of the gravel drainage layer and stress cracking upon placement of waste (Rowe
2005). Majority of the fl aws can be identifi ed with the help of non-invasive leak detection surveys. Also,
these fl aws can be averted by practising stringent quality control and quality assurance measures during
the manufacture of the geomembranes, by providing geotextile protection layers over the geomembrane
surfaces in the fi eld or by using the geomembrane in conjunction with geosynthetic clay liner. Assessment
of advective transport of contaminants has been the subject of thorough and exhaustive research (Bouazza
and Vangpaisal 2006; Bouazza, et al. 2008; Lake 2000; Saidi et al. 2008; Rowe 1998, 2005; Take et al.
2007; Brachman and Gudina 2008; Stark and Choi 2005).
Gross defects in the geomembranes and their effects on contaminant transport are fairly obvious. However,
the fl ow of contaminants due to the subtler process of diffusion needs to be given due importance since
its contribution to the overall transport of the contaminants is signifi cant even over relatively small times.
Diffusion starts to become the dominant transport process at very low k values in the range of 2 to
3x10-10 m/s (Shackelford 1990). For example, in an intact geomembrane with no fl aws, contaminants
such as VOCs can migrate substantially through the non-porous geomembrane by molecular diffusion.
Transport of VOCs through the landfi ll cover and basal liner systems contributes to the surrounding
atmospheric and groundwater contamination. Thus, the diffusive transport through geomembranes and
their ability to retard the release of contaminants should be subjected to extensive studies. Contaminant
transport of volatile organic compounds (VOCs) through the composite liner system has been the focus
of past research (Park and Nibras 1993; Rowe et al. 1995a; Park et al. 1996a, 1996b; Xiao et al. 1997;
Aminabhavi and Naik 1999; Sangam and Rowe 2001; Edil 2003; Joo et al. 2004, 2005; McWatters and
Rowe 2007, 2009, 2010, 2014; Jones et al. 2011; Saheli et al. 2011; Touze-Foltz et al. 2011). These studies
show that although HDPE geomembranes are resistant to diffusion of ionic contaminants, they fail to
prevent VOC migration. Volatile organic compounds can readily diffuse through HDPE geomembranes,
although the permeation coeffi cient will vary depending on factors such as the crystallinity of the
geomembrane, temperature and in some cases, the chemical composition and concentrations in the
contaminant source (Rogers 1985; Sangam and Rowe 2001).
2. CONTAMINANTS OF INTEREST
Volatile organic compounds (VOCs) that are carcinogenic, mutagenic, and/or teratogenic are found
in residential and industrial wastes (e.g., cleaners, paints, paint thinners, fi nger nail polish remover,
etc.) that are commonly disposed in municipal solid waste (MSW) landfi lls. These VOCs end up in
leachate (chemical fl uid that forms as water percolates through the waste) as well as the landfi ll gas
(emanating from the waste mass due to biodegradation). VOCs can pose signifi cant health risks even in
low concentrations. About thirty organic landfi ll gas constituents are classifi ed as hazardous air pollutants
(HAP) by the US Environmental Protection Agency.
The most common VOCs found in landfi ll gas and leachate are aromatic hydrocarbons (benzene, toluene,
ethyl benzene and xylenes) and halogenated hydrocarbons (trichloroethylene, tetrachloroethylene,
dichloromethane, 1,1,1 trichloroethane) and phenol (Klett et al. 2005; Kjeldsen et al. 2010). They
volatilize quickly, but resist degradation in the environment. Studies have shown that transport of VOCs
through landfi ll liners is more critical than transport of inorganic compounds (e.g., toxic heavy metals
and ionic contaminants) even though VOCs found in leachate are of lower concentrations (Rowe 1998;
Park and Nibras 1993; Park et al. 1996; Sangam and Rowe 2001; Edil 2003). In engineered landfi lls, the
contaminants of interest are present in dilute aqueous phase or vapour phase (Rowe et al. 2002).
628 R.K. Anjana, et al.
3. PROCESS OF DIFFUSION THROUGH GEOMEMBRANE
Diffusion is the movement of molecules or ions from a region of higher chemical potential to a region of
lower chemical potential until the concentration attains equilibrium (Quigley et al. 1987; Shackelford and
Daniel 1991a). The process of diffusive mass transfer across an interface (geomembrane) is the result of a
chemical potential driving force. This driving force is usually expressed in terms of concentrations of the
contaminant. The rate of diffusion in gases is about 5 cm/min; in liquids, the rate of diffusion is about 0.05
cm/min; in solids, the rate of diffusion is as low as 0.00001 cm/min (Cussler 2009). The rate of diffusion
process can be expedited by agitation.
According to Rogers (1985), the diffusive transport of contaminants through a homogenous geomembrane,
in the absence of defects, is supposed to occur by the following process: seperation of the penetrant on
the inner surface layer (adsorption), migration to the outer surface layer under a concentration gradient
(molecular diffusion), and seperation from the outer surface layer (desorption). This diffusive migration of
the contaminants can be imagined as a sequence of unit diffusion jumps during which the penetrant passes
over a potential barrier. These jumps are considered to occur because of “cooperative rearrangement”
of the penetrant molecule and the surrounding polymer chain segment. To allow this rearrangement of
molecules, certain van der Waals and other bonds/interactions need to broken. This process requires a
localisation of energy to be available to allow a diffusive jump of the penetrant molecule in the polymer
structure. Hence, the diffusive motion depends on the energy availability and the relative mobilities of
the penetrant molecules and polymer chains. The energy required to create this rearrangement depends on
the size and shape of the penetrant molecule. As the molecule size and shape increases, energy required
to break the interactions increases. This gives a plausible explanation for the variation of diffusion
coeffi cients Dg for different contaminants.
3.1 Governing Laws
Diffusion can be described by means of a mathematical model based on a fundamental law. There are two
choices for such a model (Cussler 2009). First choice for the model of diffusion involves a mass transfer
coeffi cient k, which is a simple approximate method.
Contaminant fl ux = k (Concentration difference) [2]
The diffusive fl ux in the above relationship (Eq. 2) can be defi ned as the amount of contaminant/penetrant
passing through the geomembrane of unit area normal to the direction of fl ow during unit time, and the
proportionality constant k is the mass transfer coeffi cient. The diffusive fl ux can be reduced by increasing
the distance between the regions where the initial concentration gradient exists. This is incorporated in
the model and equation 2 has been modifi ed to obtain the following mathematical relation.
Contaminant flux, J = D Concentration difference
Distance [3]
The proportionality coeffi cient D is known as the diffusion coeffi cient. Fick’s law of diffusion, which
involves a diffusion coeffi cient D, is the fundamental model of diffusion. According to Cussler (2009), the
mass transfer model is said to have lumped parameters, where the dependent variable (i.e., concentration
of the contaminant) is a function of time alone. The variation of the concentration with position is
ignored. However, the Fick’s diffusion model has distributed parameters, for the dependent variable is
a function of all the independent variables (position and time). The choice between the two models is
based on the application rather than the accuracy and precision of the model. Fick’s diffusion model is
more appropriate for applications concerning diffusive mass fl ux in liner systems, since the variation
of concentration with position and time is obtained. The variation of the concentration of the penetrant
with position is important since the thickness of the liner systems will decide the magnitude of diffusive
fl ux of the contaminants. The variation of concentration with time will decide the contaminating lifespan
629
Role of Diffusion on the Transport of Volatile Organic Contaminants
Through Geomembranes in Composite Liner Systems
of the landfi ll. The contaminating lifespan of the landfi ll is the period of time during which the landfi ll
contains contaminants which could have an unacceptable impact if released to the environment (Rowe
et al. 2004).
Diffusion of vapour or aqueous contaminants for dilute solutions through geomembranes occurs in
three steps: adsorption, diffusion and desorption (Haxo and Lahey 1988; Park and Nibras 1993; Prasad
et al. 1994; Sangam and Rowe 2001; Pierson and Barroso 2002). Initially, the contaminant partitions
between the source medium and adjacent surface of the geomembrane. Then, the compound diffuses
through the geomembrane driven by chemical potential. Finally, the compound partitions between the
outer geomembrane surface and the receiving medium (Sangam and Rowe 2001). In the fi rst step of the
process, it is assumed that a portion of the contaminant molecules are sorbed onto the geomembrane such
that, this portion is effectively immobilised within the polymer matrix (Rogers 1985). The distribution
of the contaminant between the geomembrane and ambient phases for high penetrant concentrations
is described by Nernst distribution function, which relates the concentration in the geomembrane (i.e.,
sorbed concentration cg) with the concentration in the fl uid (i.e., ambient penetrant concentration cf).
cg = K cf
[4]
where K is a function of temperature and may be a function of cg. For low contaminant concentrations like
those encountered in modern landfi ll leachate, Sgf is the Henry’s law solubility coeffi cient (partitioning
coeffi cient) which is independent of pressure and concentration (Rogers 1985; Naylor 1989). When
Henry’s law is obeyed, the equilibrium achieved between the ambient penetrant and polymer phase can
be expressed as
cg = Sgf cf [5]
where Sgf is known as the partitioning coeffi cient and is a constant for the given molecule, fl uid,
geomembrane and temperature of interest. The value of Sgf implies a preference for the geomembrane
or the contaminant solution. In case of hydrophobic organic contaminants (i.e. those with low solubility
in water) which can readily dissolve in the geomembrane, the value of Sgf is greater than one (higher the
hydrophobicity of the contaminant, greater the Sgf). Thus Sgf for ethylbenzene is greater than for benzene
which is greater than for dichloromethane (Rowe 2007). Conversely, hydrophilic contaminants (i.e. those
highly soluble in water, like ionic salts such as NaCl) do not readily dissolve in polymeric geomembrane
and have a value of Sgf which is less than unity since, at equilibrium, most of the substance will be
dissolved in the water rather than the GM.
Molecular diffusion is the second step of the process. As geomembranes are relatively thin, steady-
state diffusion through the geomembranze can be established quickly (Shackelford 1991), such that the
mass fl ux of the contaminant in dilute concentrations can be expressed in accordance with Fick’s fi rst
law. Systems with higher concentrations show behaviour which deviates from that observed with low
concentrations or gases. According to Fick (1855), the process of diffusion can be considered analogous
to heat conduction and electrical conduction. Fick’s laws was developed with the help of Fourier’s work
(Fourier 1822). Fick’s fi rst law is expressed as
J = -Dgcg
x [6]
where J is the diffusive fl ux, x is the space coordinate normal to the geomembrane surface and Dg is the
diffusion coeffi cient. Dg is specifi c to the geomembrane and the contaminant of interest (McWatters and
Rowe 2007). As diffusion proceeds, unsteady state/transient diffusion is established. According to Crank
(1975), the rate of change of contaminant concentration at any point or plane within the geomembrane
(x>0) is given by
630 R.K. Anjana, et al.
c
t= Dg 2cg
x2 [7]
which must be solved for the appropriate initial and boundary conditions. This is the fundamental
differential equation for unidirectional diffusion (i.e. concentration gradient is along x-axis) in an isotropic
medium. Here, Dg is independent of position, time and concentration. The above equation is commonly
known as Fick’s second law.
The fi nal step of the contaminant diffusion process involves desorption of the penetrant from the
geomembrane to the outside fl uid. This can also be described using Henry’s law.
c̵g = S̵gf cf [8]
where S’gf is the partitioning coeffi cient between the geomembrane and the outside fl uid. Sgf and S’gf may
be assumed to be the same for dilute aqueous solutions. This is especially true if the source and receptor
fl uid is same (Sangam 2001). Both the partitioning coeffi cients are similar and can be obtained by batch
sorption tests.
Since the primary interest is in the concentrations of contaminant in water (not in the geomembrane) it
is convenient to express the diffusion equations in terms of the concentration in adjacent solutions cf.
Substituting equation 5 in equation 6, the diffusive fl ux from the source side of the geomembrane to the
receptor side of the geomembrane is given by
J = -Dgcg
x=-DgSgf cf
x = -Pgcf
x [9]
where Pg is known as permeation coeffi cient of the geomembrane (permeability in polymer literature) and
the permeance Pg is given by
Pg= DgSgf [10]
The concentration profi les across the geomembranes are depicted in Figure 1. Assume C0 and Cl as
the contaminant concentrations on either side of the geomembrane respectively (C0>Cl). Note that the
concentration profi le is independent of the diffusion coeffi cient. If the contaminant is more soluble in the
membrane than in the adjacent solution (hydrophobic contaminants like ethylbenzene), the concentration
profi le has sudden discontinuities at surface as shown in Fig. 1(a). If the contaminant is more soluble
in the adjacent solution (hydrophilic contaminant like NaCl), the concentration profi le is similar to the
profi le in Fig. 1(b). Fig. 1(c) shows the drop in chemical potential across the geomembrane, where 0 and
l are the chemical potentials on either side of the geomembrane. This potential, which drops smoothly
with concentration, is the driving force responsible for diffusion.
Fig. 1: Concentration profi les across geomembranes (Modifi ed from Cussler 2009)
631
Role of Diffusion on the Transport of Volatile Organic Contaminants
Through Geomembranes in Composite Liner Systems
3.2 Methods to Assess Diffusion
There are a number of methodologies (Shackelford 1991; Rowe 1998) that can be used to deduce the
partitioning, diffusion and permeation coeffi cients: weight gain method, pouch method, steady state
method (double compartment diffusion), time lag method and transient method. The methods have been
discussed in detail in the following section.
In weight gain method, increase in mass of geomembrane immersed in contaminant fl uid of interest from
initial value mo until mass of geomembrane becomes constant at mn[(mt-mo)/(mn-mo) versus is plotted].
The Sgf and Dg values are obtained from the following equations.
Sgf = ȡGM
cFF
൬m
mo
- 1൰
[11]
In this equation, ρGM is the density of the geomembrane and CFF is the fi nal equilibrium contaminant fl uid
concentration.
Dg= 0.049 tGM
2
t1/2
[12]
where tGM denotes the thickness of the geomembrane and t1/2 is the time required to get (mt-mo)/(mn-mo) =
0.5. Weight gain method is faster than alternative tests but each contaminant must be examined separately.
In case of VOCs, this method is not suitable since errors due to mass loss can occur while weighing.
Partitioning coeffi cient is obtained by means of either batch sorption tests or column sorption tests. Batch
sorption tests are simpler and are carried out in short period of time. They may not represent realistic fi eld
scenarios when obtaining sorption characteristics of compacted clay liners or composite liner systems.
However, to obtain sorption properties of geomembranes alone, batch sorption tests are effective. For
batch sorption tests, geomembrane samples are immersed/suspended in cells fi lled with a contaminant of
interest. Contaminant concentrations are measured at regular intervals until equilibrium is reached. Mass
loss in the cells is monitored over time to obtain Sgf. In case of compacted clay liners and composite liners,
column sorption experiments are effective since they simulate the geomaterial-contaminant interaction.
The only limitation of column sorption experiments is that they are time-intensive and cumbersome.
In pouch method, geomembrane pouch fi lled with the contaminant of interest is placed in a liquid of
known composition and concentration. As diffusion occurs, weight change of the geomembrane is
monitored. The main concern in this method is that the pouch should not have any defects i.e. advective
transport should not be present. This method assumes that the diffusion in the geomembrane is slow in
comparison to the diffusion in the fl uid.
Steady state method involves diffusion of contaminant from one side (source) of the geomembrane to
the other side (receptor). The geomembrane is secured between compartments: a source compartment
containing the solution of interest (e.g., an actual or synthetic leachate) and a collection compartment from
which samples are withdrawn for chemical analysis of specifi ed chemical species. The concentration of the
chemical species of interest is higher in the source reservoir than it is in the collection reservoir (in most
cases, it is zero); therefore, a concentration gradient is established across the sample. The concentration
gradient established across the sample is linear. Change in source and receptor side concentration is
monitored with time. Sgf is obtained from Henry’s law and Dg is inferred from breakthrough curves. The
time required to establish steady-state conditions with adsorbing solutes will be longer than that for non-
adsorbing solutes. A disadvantage of the method is that the mass of the chemical species diffusing from the
source reservoir must be continuously replenished while the mass of the chemical species diffusing into
the collection reservoir is continuously removed in order to maintain a constant concentration gradient
across the sample and, therefore, to establish and maintain steady-state conditions. Also, the time required
632 R.K. Anjana, et al.
to establish steady-state conditions can vary, depending on the type of polymer in the geomembrane. For
example, PVC geomembranes reach equilibrium sooner than HDPE geomembranes.
Time lag method monitors mass movement through geomembrane with time where c0=constant, cl=0 (Fig.
1). Cumulative mass through geomembrane F versus time is plotted. To obtain the time lag , extrapolate
steady state value to F=0. The diffusion coeffi cient Dg is obtained by
Dg= tGM
2
6ɒ [13]
Slope of the steady state line gives the permeability coeffi cient Pg. The partitioning coeffi cient Sgf is
obtained by
Sgf= Pg
Dg [14]
The technique that is suitable for actual situations uses a two-compartment diffusion cell, where
contaminant diffusion is monitored from the source to the receptor. When equilibrium is reached, the
value of Sgf can be deduced from mass balance considerations. The diffusion coeffi cient can then be
calculated knowing the value of Sgf by fi tting the variation of the source and receptor concentration with
time using computer software that models the boundary conditions, phase change and transport through
the geomembrane (Rowe and Booker 2005).
Analyses of volatile or semivolatile organic environmental pollutants usually begin with concentrating
the analytes of interest through liquid-liquid extraction, purge-and-trap, headspace, or various other
techniques. Aqueous phase concentrations of contaminants through the geomembrane are established by
Purge & Trap Gas Chromatography (P&T GC). Vapour phase concentrations through the geomembrane
are established based on direct vapour phase VOC concentrations (Solid phase microextraction) and
vapour concentrations inferred from aqueous phase VOC concentrations (P&T GC). Solid phase
microextraction, or SPME, an adsorption/desorption technique developed at the University of Waterloo
(Ontario, Canada), is a novel technique which eliminates the need for solvents or complicated apparatus
for concentrating volatile or nonvolatile compounds in liquid samples or headspace. VOCs are extracted
from the sample medium and concentrated onto the thin fi lm coating of a thin fused silica fi ber in the
SPME syringe. After concentration of the sample, the fi ber is injected into the GC and analytes desorb
off the fi ber (Belardi and Pawliszyn 1989). Polymer coatings are selected specifi cally for the analytes
being identifi ed. A 75 m (fi lm thickness) Carboxen-polydimethylsiloxane (PDMS) is typically used for
BTEX contaminants showing good effi ciency and accuracy at detection limits at the ng/L (ppt) level. The
sensitivity of the SPME method using PDMS coating for these compounds is very high.
4. DIFFUSIVE TRANSPORT OF VOCS THROUGH GEOMEMBRANES
A number of studies regarding the use of geomembranes focused on the diffusion of volatile organic
compounds (VOCs) for virgin HDPE geomembranes (Park and Nibras 1993; Prasad et al. 1994; Müller
et al. 1998; Sangam and Rowe 2001; Touze-Foltz et al. 2011) virgin PVC, LLDPE with and without a co-
extruded ethylene vinyl-alcohol (EVOH) inner core geomembranes (McWatters and Rowe 2008, 2010),
EVOH fi lms (McWatters and Rowe 2014) fl uorinated HDPE geomembranes (Sangam and Rowe 2005)
and aged HDPE geomembranes (Islam and Rowe 2008; 2009).
August and Tatzky (1984) found that strongly polar penetrant molecules have permeation rates in a
HDPE geomembrane in the range 7x10-7 m3/m2/d for methanol to 9.4x10-6 m3/m2/d for trichloroethylene.
Diffusion coeffi cients of VOCs in virgin HDPE geomembranes range from 0.37 to 22.8x10-13 m2/s for
benzene and dichloromethane respectively with partition coeffi cients ranging from 1.8 to 189. The
633
Role of Diffusion on the Transport of Volatile Organic Contaminants
Through Geomembranes in Composite Liner Systems
resulting permeation coeffi cient lies between 1 and 70x10-12 m2/s. In PVC geomembranes, permeation
coeffi cients of VOCs are in the following order ethylbenzene>xylenes>toluene>benzene (McWatters
and Rowe 2007). In HDPE geomembranes, permeation coeffi cients of VOCs follow toluene>benze
ne>ethylbenzene>xylene (Sangam and Rowe 2001; Sangam 2001; Rowe et al. 2004). In coextruded
geomembranes, permeation of various contaminants shows little variation (McWatters and Rowe 2010;
Eun 2014). Coextruded geomembranes perform better than EVOH thin fi lms in attenuating VOC diffusion
since they combine the benefi cial effects of multiple polymers. Toluene is less permeable than TCE
through EVOH fi lm (McWatters and Rowe 2014). Critical appraisal of literature showed the resistance of
geomembranes to VOCs diffusion as: coextruded GM (EVOH inner core)>Coextruded GM (polyamide
inner core)>HDPE>LLDPE>PVC.
The signifi cance of diffusion on the contaminant transport through liners can be described with the aid
of breakthrough curves (BTCs), representing the temporal variation in the concentration of a given
chemical contaminant at the receptor end of the geomembrane (Shackelford 1988; Shackelford 2014).
BTCs can be obtained for a geomembrane with a particular thickness by (i) establishing steady-state
seepage conditions, (ii) continuously introducing a contaminant at the source side of the diffusion cell
containing a known chemical species at a concentration co, and (iii) monitoring the concentration of the
same chemical species at the receptor side of the diffusion cell as a function of time (Shackelford 1995;
Shackelford and Redmond 1995). Because the source concentration, co, is constant, the BTCs typically
are presented in the form of dimensionless relative concentration, ct/co, versus elapsed time. The time
required for the penetrant to migrate from the source side to the receptor side of the cell is referred to
as the “breakthrough time” or the “transit time” (Shackelford 2014). Usually, these BTCs are used to
estimate Sgf, Dg and Pg for the geomembrane and the contaminant of interest.
4.1 Effect of Ageing
Ageing and durability of the geomembranes play a critical role in deciding the transport of contaminants
through liner systems (Arnepalli and Rejoice 2013a; 2013b). Joo et al. (2004) investigated the effect of
ageing on the migration of organic compounds by conducting batch immersion tests on both virgin and
5-year aged HDPE geomembranes. Aged geomembrane was exhumed from a real landfi ll after 5 years.
No signifi cant changes in partitioning and diffusion coeffi cients were evident, although the diffusion
coeffi cients were slightly lower for aged geomembrane. However, recent laboratory studies (Sangam
2001; Rowe et al. 2008) have shown that the crystallinity of the HDPE geomembrane increases with
the increase of ageing duration. Studies were carried out to fi nd the effect of ageing on geomembranes
on diffusion process (Islam and Rowe 2008, 2009). Permeation coeffi cients were calculated for both
unaged and aged geomembranes and were found to be reduced by about 35-40% for aged geomembrane.
The increase in crystallinity was implicated for the reduction in diffusive transport through the aged
geomembrane. Therefore, ageing of geomembrane appears to reduce the diffusive migration of organic
contaminants through HDPE geomembranes (other things being equal).
Islam and Rowe (2009) provided better insight on the effects of ageing on the diffusion process. The
diffusive migration of VOCs through a geomembrane is considered to be a molecular activated process
and suffi cient activation energy must be concentrated within the zone of chain segments for a successful
diffusion step to occur (Michaels and Bixler 1961). The mobility of amorphous chain segments is reduced
due to the presence of crystalline zone in the geomembrane. Higher activation energy is required for
the diffusion to take place because of the reduction in the amorphous region, which eventually reduces
the chance of diffusive migration (Michaels and Bixler 1961; Rogers 1985; Naylor 1989). Diffusion
through a polymeric material is also expected to decrease because of the irregular shape of interconnected
amorphous regions surrounded by impenetrable crystallites (Michaels and Bixler 1961). The irregularities
in amorphous regions or tortuosity increases as the degree of polymer crystallinity increases (Michaels
634 R.K. Anjana, et al.
and Parker 1959). Thus higher crystallinity in the aged geomembrane lowered the permeation of
contaminants through HDPE geomembrane because the crystalline zones perform as an impermeable
barrier for the diffusion process and reduce the volume of amorphous material where partitioning can
take place (Crank and Park 1968; Rogers 1985; Naylor 1989). Efforts were made to model the effects of
ageing of geomembranes on contaminant transport and the long-term performance of landfi ll composite
liners (Rowe and Arnepalli 2008a). Analyses showed that the reduced diffusion coeffi cient due to ageing
had no practical signifi cance on the contaminant transport since the peak benzene impact on the underlying
aquifer was the same for both aged and unaged geomembranes.
4.2 Temperature and Concentration Dependence
Temperature plays a critical role in the diffusive transport of contaminants (Crank 1975). It is known that
the diffusive migration of the contaminants occurs as a sequence of unit diffusion jumps during which the
penetrant passes over a potential barrier. The diffusive motion depends on the energy availability and the
relative mobilities of the penetrant molecules and polymer chains. This polymer chain segmental motion
is primarily affected by temperature and the concentration of the penetrant/contaminant. According
to Rogers (1985), an increase in temperature provides energy for a general increase in segmental
motion. If the energy is suffi cient, the polymer may pass through structural transitions which further
affect the diffusion process. As far as the contaminant is concerned, as the temperature increases, the
amount of energy available for diffusion is increased. This means that the contaminant molecules will
diffuse faster at a higher temperature. Temperature is an important driver for diffusion of contaminants
through membranes and all three of the permeation parameters, partitioning, diffusion and permeation
coeffi cients are dependent on temperature (Crank and Park 1968). The temperature dependence of the
contaminant transport was modelled in Rowe and Arnepalli (2008b). Results from this study showed that
the contaminant impact was almost the same for temperatures of 10°C and 20°C. However, increased
ambient temperature (35°C) led to twice the contaminant impact on the underlying aquifer.
A case of great practical interest is that in which the diffusion coeffi cient depends only on the concentration
of diffusing substance. Such a concentration-dependence exists in most systems, but often, e.g. in dilute
solutions, the dependence is slight and the diffusion coeffi cient can be assumed constant for practical
purposes. In other cases, however, such as the diffusion of vapours in high-polymer substances, the
concentration dependence is a very marked, characteristic feature (Crank 1975). For many penetrant-
polymer systems, Dg is not a constant but rather is a function of c. The concentration-dependence is
a refl ection of the plasticising action of sorbed penetrant and/or various mechanisms which localise
(immobilise) a portion of the sorbed penetrant (Rogers 1985). According to Cussler (2009), the diffusion
coeffi cients vary with concentration for some contaminants. Hence, the concentration of the contaminant
on the source side of the diffusion cell is of importance. Usually, the contaminant concentrations on the
source side refl ect realistic fi eld leachate concentrations.
5. MINIMISING VOC DIFFUSION
Recent research has focused on fi nding alternative geomembranes which resist VOC diffusion since
diffusion of VOCs through single-polymer geomembranes has been an issue. Sangam and Rowe (2005)
examined the effect of surface fl uorination of an HDPE geomembrane on diffusion of VOCs. The surface
fl uorination consisted of applying elemental fl uorine, which exchanged with hydrogen along polymer
chains at the surface of a polyolefi n substrate. The diffusion of VOCs could be reduced by a factor of
between about 2 and 5 by using a fl uorinated HDPE as an alternative to a conventional GM. Rowe et al.
(2010) tested the durability of such fl uorinated HDPE geomembranes in the Arctic. It was noted that the
durability was maintained beyond the design life of the geomembranes.
Coextrusion of polymers has also been found to useful in resisting VOC diffusion. Coextrusion involves
extruding two or more layers of dissimilar polymers into a single fi lm. The fi rst co-extruded multilayer
635
Role of Diffusion on the Transport of Volatile Organic Contaminants
Through Geomembranes in Composite Liner Systems
geomembranes had high density polyethylene outer layers and a lower density polyethylene as the
inner layer (Kolbasuk 1990). These multi-layered geomembranes have evolved to include co-extruded
geomembranes with polyamide (nylon) as the innermost layer. Polyamides have been shown to have a
lower permeability to organic solvents and gases than pure polyethylene resins (Yeh and Fan-Chiang
1996; Gonzalez-Nunez 2001; McWatters and Rowe 2010). Coextruded geomembranes with ethylene vinyl
alcohol layers (EVOH) are also being manufactured to take advantage of the properties of polyethylene
as a water barrier while potentially substantially reducing the diffusion of VOCs. McWatters and Rowe
(2010) reported improved resistance to benzene, toluene, ethylbenzene and xylene diffusion for two
coextruded geomembranes, a polyamide (nylon) geomembrane GMB and an ethylene vinyl–alcohol
(EVOH) geomembrane. Ethylene vinyl alcohol (EVOH) has the potential to be a much better diffusive
barrier to VOCs than PVC, LLDPE or HDPE as illustrated in McWatters and Rowe (2014).
6. CONCLUDING REMARKS
The role of diffusion on the transport of volatile organic contaminants through geomembranes is
reviewed and presented in this paper. The use of geomembranes is benefi cial in retarding contaminant
transport through advection but it hardly helps in preventing the diffusive transport of contaminants
like VOCs. The laws governing the diffusive transport of VOCs through geomembranes have been
discussed at length. The signifi cance and interpretation of a measured value of Dg depends upon the
conditions of the experimental method, and the specifi c range of concentration, temperature and other
variables. It is necessary to clearly defi ne the reference plane in terms of concentration and thickness of
the geomembrane since the ultimate aim of the studies revolve around the estimation of Dg in terms of
molecular properties of the contaminants. Since diffusion of VOCs occurs over short times, the need of
fi nding barriers that were capable of preventing the diffusive transport came to the fore. The primary
outcome of such studies implied that single polymer geomembranes like HDPE, LLDPE and PVC are
inadequate in preventing VOC diffusion and the need of newer types of geomembranes was emphasised.
Fluorination of HDPE geomembranes and multi polymer geomembranes were brought into play to satisfy
the barrier requirements. Coextruded EVOH geomembranes have shown great promise in preventing
VOC transport. It is to be noted that the partitioning Sgf, diffusion Dg and permeation Pg coeffi cients
obtained from the studies should be exercised with caution and should be made use of only as guides.
This is primarily because these coeffi cients show dependencies on concentrations, space (i.e., thickness),
time, temperature, polymer properties, etc.
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