Journal of Environmental Management 163 (2015) 204e213

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Journal of Environmental Management
journal homepage: www.elsevier.com/locate/jenvman

Research article

Evaluation of a horizontal permeable reactive barrier for preventing
upward diffusion of volatile organic compounds through theunsaturated zone
Mojtaba G. Mahmoodlu a, d, *, S. Majid Hassanizadeh a, Niels Hartog a, b, Amir Raoof a,
Martinus Th. van Genuchten a, caUtrecht University, Department of Earth Sciences, The Netherlands
KWR Watercycle Research Institute, Nieuwegein, The Netherlands
Federal University of Rio de Janeiro, Department of Mechanical Engineering, BrazildDepartment of Watershed and Rangeland Management, Gonbad Kavous University, Iranbca r t i c l e i n f o

a b s t r a c t

Article history:
Received 5 April 2015
Received in revised form
11 August 2015
Accepted 19 August 2015
Available online xxx

Permeable reactive barriers are commonly used to treat contaminant plumes in the saturated zone.
However, no known applications of horizontal permeable reactive barriers (HPRBs) exist for oxidizing
volatile organic compounds (VOCs) in the unsaturated zone. In this study, laboratory column experiments were carried out to investigate the ability of a HPRB containing solid potassium permanganate, tooxidize the vapors of trichloroethylene (TCE), toluene, and ethanol migrating upward from a contaminated saturated zone. Results revealed that an increase in initial water saturation and HPRB thickness
strongly affected the removal efﬁciency of the HPRB. Installing the HPRB relatively close to the watertable was more effective due to the high background water content and enhanced diffusion of protons
and/or hydroxides away from the HPRB. Inserting the HPRB far above the water table caused rapidchanges in pH within the HPRB, leading to lower oxidation rates. The pH effects were included in areactive transport model, which successfully simulated the TCE and toluene experimental observations.
Simulations for ethanol were not affected by pH due to condensation of water during ethanol oxidation,
which caused some dilution in the HRPB.
© 2015 Elsevier Ltd. All rights reserved.

Keywords:
Horizontal permeable reactive barrier
Potassium permanganate
VOC vapors
Unsaturated zone
Diffusion

1. Introduction
A number of remediation techniques (e.g., soil vapor extraction
and bioventing) are currently available for treating an unsaturated
zone contaminated with volatile organic compounds (VOCs) (Strooand Ward, 2010). Although these techniques have been demonstrated to be effective for treating the unsaturated zone, they sufferfrom several drawbacks such as their required long-term operation,
while in some cases incomplete transformation of the contaminants can generate toxic compounds or environmentally unacceptable forms (Hinchee, 1993; Cho et al., 2002; Stroo and Ward,2010). During the past few decades, in-situ chemical oxidation(ISCO) has been established as an alternative technology to treat* Corresponding author. Utrecht University, Department of Earth Sciences, The
Netherlands.
E-mail address: m.g.mahmnoodlu@gmail.com (M.G. Mahmoodlu).
http://dx.doi.org/10.1016/j.jenvman.2015.08.025
0301-4797/© 2015 Elsevier Ltd. All rights reserved.

dissolved VOCs in the saturated zone (Stroo and Ward, 2010;
Tsitonaki et al., 2010; Yuan et al., 2013). However, the literatureshows only a few ISCO studies on using dissolved permanganate orother oxidants to treat the unsaturated zone contaminated with
VOCs (Siegrist et al., 1998; Hesemann and Hildebrandt, 2009; Cronk
et al., 2010).
Vertical permeable reactive barriers (VPRBs) are a wellestablished technique and common conﬁguration for remediationof contaminants in the saturated zone (Yeh et al., 2010; Gibert et al.,2013). However, VPRBs are not suitable for the unsaturated zone
because of generally limited horizontal ﬂow in the unsaturated
zone. Horizontal permeable reactive barriers (HPRBs), on the other
hand, can be employed in the unsaturated zone to prevent or limit
the vertical leaching of contaminants (Siegrist et al., 1998; Mackova
et al., 2006; Leal et al., 2013) or the upward migration of contaminant vapors.
The potential of dry solid potassium permanganate granules foroxidizing various VOCs has been reported in the literature

M.G. Mahmoodlu et al. / Journal of Environmental Management 163 (2015) 204e213(Mahmoodlu et al., 2013, 2014b). The oxidation of TCE, toluene, andethanol is described by the following overall reaction equations:

C2 HCl3ðgÞ þ 2KMnO4ðsÞ /2Kþ þ 2MnO2ðsÞ þ 3Cl þ 2CO2ðgÞ þ Hþ(1)
C6 H5 CH3ðgÞ þ 12KMnO4ðsÞ þ 2H2 O/12Kþ þ 12OH
þ 12MnO2ðsÞ þ 7CO2ðgÞ

(2)

C2 H5 OHðgÞ þ 4KMnO4ðsÞ /4Kþ þ 4OH þ 4MnO2ðsÞ þ 2CO2ðgÞ
þ H2 O(3)
Our recent study of VOC vapor diffusion through a partiallysaturated permeable reactive barrier showed that water saturation has a considerable effect on the removal capacity of a HPRB
(Mahmoodlu et al., 2014b). The study also found that changes in pH
values generated during the VOC oxidations according to theirstoichiometric reactions (Eqs. (1)e(3)) are responsible for reducing
the oxidation rates of the VOCs. Although that study showed thatthe prevailing chemical processes and water saturation strongly
affected the reactivity and longevity of a HPRB, it remained unclear
how HPRB thickness, its location relative to the water table, andwater availability in general would affect the oxidation process, andconsequently the reactivity and longevity of the HPRB in the unsaturated zone. For these reasons we performed a series of VOC
column transport experiments under unsaturated conditions.
Speciﬁc objectives of our study were to (1) evaluate the ability of a
HPRB to oxidize the vapors of three VOCs in the unsaturated zone,(2) evaluate the impact of HPRB location relative to the water tableon the long-term reactivity of the barrier, (3) investigate the effectof HPRB thickness on the oxidation process, and (4) numericallysimulate the reactive transport of VOC vapors diffusing through theunsaturated zone containing a HPRB.
2. Materials and methods
2.1. Experimental setup
Fig. 1 shows a schematic of the experimental setup used in ourstudy. A glass cylinder of 30 cm length and 5.0 cm internal diameter
was used to construct the experimental columns. Five inlet andoutlet ports and three sampling ports were installed along thecolumn. A peristaltic pump (Ismatec, Switzerland) was employed totransfer deionized (DI) water and dissolved VOC from stock solutions into the lower part of the column. A 10-L aluminum balloon(Tesseraux Company, Germany) was attached to the stock solution
container to prevent changes in air pressure when liquid was
pumped into the column. The balloon was ﬁlled with argon gas.
The column contained four domains. The lower part was thesaturated zone contaminated with a particular dissolved VOC. The
second domain comprised the unsaturated zone between the watertable and the top of the sand column. The third domain was the
HPRB within the unsaturated zone consisting of a mixture of potassium permanganate and sand. The fourth domain was theheadspace above the unsaturated zone. For the control experiments we used the same setup, but without any potassium permanganate in HPRB.
2.2. Contaminants and porous medium
The target contaminants used in this study were TCE, toluene,
and ethanol (SigmaeAldrich, Merck, and ACROS, respectively).

205

Stock solutions of aqueous-phase TCE, ethanol, and toluene wereindividually prepared in 10-L glass vessels (Fisherbrand) by dissolving the chemicals in degassed-deionized water (Table 1). The
vessels containing the stock solutions were continuously mixedusing magnetic stirrers (IKA, Germany) to maintain homogeneous
solutions.
The sand used for the column experiments originated from ariver bed in Papendrecht (Filtersand, Filcom, The Netherlands) andwas sieved to restrict the particle size between 0.5 and 1.0 mm.
Selected properties of the sand are given in Table 1. Before its use
we ﬁrst soaked the sand overnight in pure hydrochloric acid (ACS
reagent, 37%, SigmaeAldrich) to remove iron oxide coatings. We
then rinsed the sand with DI water to increase the pH to the natural
background value. The acid-washed sand was subsequently heated
to 800  C for 4 h to remove soil organic matter. Mercury chloride(SigmaeAldrich) was used for both the stock solution and the sandto inactivate microorganisms, thus eliminating possible biodegradation processes during the experiments. We injected 3.0 ml of the
2 g l1 mercury chloride solution into the stock solutions of thetarget compounds. The sand was further soaked ﬁrst overnight in asolution of 10 mg l1 of mercury chloride. We subsequently dried
the sand in a fume hood.
Solid potassium permanganate of 99% purity (SigmaeAldrich)
was used as the oxidant in the HPRBs. Since the mean size of thepotassium permanganate grains was almost equal to the mean
grain size of sand, we assumed that the physical properties of thepotassium permanganate-sand mixture were the same as for thesand itself.
2.3. Column preparation
The columns were packed with sand from the bottom up to aheight of 26 cm. This created two parts: sand and a 4-cm thickheadspace in the top of the column. Before packing the columns,
ﬁberglass was inserted into the inlet and outlet ports to prevent any
sand from entering the tubes. The sand columns were next ﬂushed
using CO2 gas for 4 h to replace air in the soil pore spaces with CO2
gas. We saturated only 6.0 cm of the columns via the bottom port
using a syringe. To control and monitor the water table, the sandcolumns were connected to a water column using two tubes. The
water column, also ﬁlled with DI water up to 6.0 cm, was connected
in turn to a waste container using a tube at a level that allowed us tomaintain a desired constant water table in the sand columns. DI
water was subsequently injected into the saturated part of the sandcolumn using a peristaltic pump to mimic horizontal groundwaterﬂow (Fig. 1).
A mixture of dry potassium permanganate grains and dry sand(10 g of each) was prepared for use in the HPRBs. In one case, themixture was emplaced dry and in a second case mixed with DI
water to reach a relative water saturation of 0.2. The mixture wasplaced ﬁrst into a small plastic container and shaken for 10 min toensure homogeneous mixing of sand and potassium permanganate. This mixture was next wetted with DI water to the desired
water saturation. To promote uniformity of moisture, the wetted
mixture was shaken further for 15 min, after which the material
was placed at the desired elevation in the unsaturated zone. This
created a HPRB of about 1.0 cm thick. The mixture was placed atelevations of either 7.0 cm or 12.0 cm above the water table. Thiswas done to investigate the effect of HPRB elevation relative to thewater table on VOC concentrations in the headspace. In one
experiment with TCE (100 mg l1) we also used a dry HPRB located12.0 cm above the water table. After placing the HPRBs in the unsaturated zone, the sand columns were immediately capped with avapor-tight stainless steel lid.
Two additional experiments were conducted with TCE in order206

M.G. Mahmoodlu et al. / Journal of Environmental Management 163 (2015) 204e213

Fig. 1. Schematic of the experimental setup and sand columns used in this study.

to test the effects of HPRB thickness on VOC accumulations in theheadspace. For the ﬁrst experiment we mixed 40 g potassium
permanganate with 40 g sand, resulting in a reactive layer of 4.0 cm
thick. For the second experiment we used 80 g of each, so that thelayer thickness was 8.0 cm. We also carried out a series of control
experiments without potassium permanganate for all target compounds. All experiments were conducted in duplicate in a fumecabinet at room temperature (22 ± 1  C). We assumed that thetemperature and pressure inside the columns remained constant
and were equal to room temperature and atmospheric pressure,respectively.

3. VOC transport simulations
Numerical simulations of water ﬂow and contaminant transport
in the unsaturated zone are generally carried out using the Richards
equation for ﬂow and the advectionediffusion equation for transport. In our case no vertical ﬂow occurred since the top and bottom
boundary conditions were impermeable to ﬂow, while a water table was maintained at a ﬁxed level (6 cm) above the bottom
boundary. The pressure head distribution in the soil column hence
is in equilibrium with the water table, while water contents above
the saturated zone are deﬁned by the soil water retention curve. In
our study we used for this purpose the retention equation of van
Genuchten (1980):

2.4. VOC transport experiments
The VOC transport experiments started by switching the column
inﬂow from DI water to one of the stock solutions containing dissolved concentrations of 130 mg l1, 50 mg l1, and 8  103 mg l1
for TCE, toluene, and ethanol, respectively. We measured the concentration of dissolved VOC in the stock solutions and also at the
inlet and outlet ports during the experiments. Aqueous samples of
1.5 ml were taken periodically for this purpose using 2.5-ml gastight syringes.
Gas samples of 1.5 ml were taken systematically from theheadspace of both the reactive columns (containing HPRB) and thecontrol columns using gas-tight syringes. Simultaneously we
injected the same amount of air (1.5 ml) into the headspace via aseparate valve to avoid any air pressure drops due to sampling.
The extracted liquid and gas samples were injected into 10-ml
transparent glass vials, which were sealed with a hard septum
and magnetic cap. VOC concentrations were measured using a gas
chromatograph (GC). The GC (Agilent 6850) was equipped with aﬂame ionization detector and an Agilent HP-1 capillary column.
VOC concentrations of both the liquid and gas samples wereanalyzed using a headspace method with temperature programmed runs of the GC (Almeida and Boas, 2004; Sieg et al., 2008).

Swe ðhÞ ¼qw ðhÞ  qw
r
w ¼
qw
s  qr1þ1=n1 þ ðajhjÞn
1

h<0
h0

(4)

where 0  Swe  1.
The major chemical transport processes to be considered in ourstudy were diffusion in the gas and liquid phases, interfacial exchange of VOCs between the air and water phases, dissolution ofpotassium permanganate in the water phase of the HPRB, oxidationof VOCs by potassium permanganate, and the related consumptionof potassium permanganate during oxidation.
The governing equation for VOC diffusive transport in the gas
phase within the unsaturated zone, the HPRB, and the headspace isgiven by:
g
g vCA

g
v
g vC
qg De;A A
¼
q
vx
vt
vx

!
 rAdiss

ðA ¼ TCE; ethanol; tolueneÞ
(5)where qg is equal to 1.0 in the headspace. Hence, the rate of VOC
dissolution into water (in the headspace) is equal to zero.
Boundary conditions for Eq. (5) are a known constantM.G. Mahmoodlu et al. / Journal of Environmental Management 163 (2015) 204e213

207

Table 1
Experimental conditions and input parameters for VOC transport simulations.
Parameter

VOC

Value

References

e

0.35

e

r , (g m3)e1.34  106

e

3

e

0.05

e

qw
r , ()qw
s , ()

e

0.02

e

0.4

a, (m1) in Eq. 4

e

e

e

20

e

e

4.5

e

1.0

e

3.0

a

f, ()b bc

TOC, (g m)n, () in Eq. 4
ai0, (m1)s in Eq. (8) ()2 1DwH þ , (m s )2 1DwOH , (m s )w , (g m3)
CA0

HC;A , ()

1 1
koxid
s )
A , (M

DA , (m2 s1)
g

2 1
Dw
A , (m s )

1
kdiss
A , (m s )g in Eqs. (12) and (14) ()b in Eqs. (12) and (14) ()u in Eqs. (12) and (14) ()z in Eqs. 1 and 2, ()Barber (1995)ee

9.31  109e

5.3  109TCE
TolueneEthanolTCE

13050
8.0  103
4.3  101

Toluene2.8  101Ethanol

2.4  104TCE

8.0  101

Toluene2.5  104Ethanol

6.5  104TCE

7.9  106

Toluene

7.6  106Ethanol

1.1  105TCE

9.1  1010Toluene9.4  1010Ethanol

1.2  109TCE
Toluene8.0  105
1.3  105EthanolTCE

1.0  104
0.1

Toluene1.0  104

TCE

2.0

e

Toluene

3.0

e

TCE

1.25

e

Toluene

14.0

e

TCE

1.0

e

Toluene12.0eDane and Topp (2002)
Dane and Topp (2002)

Fan and Scow (1993) and Stroo and Ward (2010)
Fan and Scow (1993) and Lide (2008)
ITRC (2011) and Lide (2008)
Kao et al. (2008) and Mahmoodlu et al. (2014)
Mahmoodlu et al. (2014)
Mahmoodlu et al. (2014)
Estivill et al. (2007)
Hers et al. (2000)
Green and Perry (2007)
Fogler (2006) and Lewis et al. (2009)
Hers et al. (2000) and Freitas (2009)
Green and Perry (2007) and Freitas (2009)d

MeasuredeeaPorosity of both sand and HPRB.
Bulk density of sand.
Total organic carbon in dry sample.
d
A series of batch experiments was performed to calculate the dissolution rate coefﬁcient of VOCs.
bcconcentration of the target compound at the water table boundarygði:e:; CAg ðx ¼ 0; tÞ ¼ CA0
Þ and a no-ﬂux condition at the top boundaryof the headspace. We further assumed continuity of ﬂuxes acrossthe interface between water and air. Since CAw was constant for all
target compounds in the saturated zone during the experimentsw ), C g was taken to be equal to C w multiplied by the(equal to CA0
A0A0
corresponding Henry's constant. Initial concentrations of the target
compounds were zero throughout the modeling domain, i.e.,g
CA ðx; t ¼ 0Þ ¼ 0.
The diffusion rate of VOC vapor is lower in soil than in free air
because of pore space tortuosity and the presence of water.

Literature data indicate that diffusion coefﬁcients of VOC vapors in
air are to 3 to 4 orders of magnitude greater than those in the waterphase (Wiedemeier et al., 1999; Partridge et al., 2002; Tillman andWeaver, 2005). The effective gas diffusion coefﬁcient of VOC vapor
as a function of air content in the unsaturated zone and HPRB was
expressed as follows (Millington and Quirk, 1961; Partridge et al.,
2002; Yao et al., 2013):
10

Dge;A ¼

qg 3 g
D
f2 A

(6)

208

M.G. Mahmoodlu et al. / Journal of Environmental Management 163 (2015) 204e213

Nomenclature
Molecular diffusion coefﬁcient of OH in the water phase
Henry's constant of VOC
Porosity of both sand and HPRB
Dissolution rate of VOC into waterAB
Sw [e]Swe [e]VOC target compound
Potassium permanganate
Water relative saturation
Effective water saturation

2 1
Dw
OH [L T ]HC,A [e]f [e]qw [e]
qg [e]

Volumetric water content

rAoxid [NL3T1]

Volumetric air content

Dissolution rate constant of VOC

qw
r [e]

kdiss
[LT1]A

Residual water content

Oxidation rate coefﬁcient in the water phase

qw
s [e]

Saturated water content
VOC concentration in the air phase[NL3T1]koxid
Aai [L1]ai0 [L1]

CAg [NL3]
CAw [NL3]w [NL3]
CA0
CBw [NL3]w
CB;max[NL3]
3
Cþ
]
H [NL
3
C
]
OH [NLDgA [L2T1]
Dge;A [L2T1]
2 1
Dw
A [L T ]2 1Dwe;A [L T ]
2 1
[L
T ]
Dw
Hþ
a
b

rAdiss [NL3T1]

Speciﬁc airewater interfacial area
Speciﬁc surface area corresponding to solid phase

VOC concentration in the water phase
Initial concentration of VOC in the water phase
Concentration of permanganate in the water phase
Maximum concentration of permanganate in the water phase

n [e]
h [L]
t [T]

van Genuchten parameter in Eq. (4)van Genuchten parameter in Eq. (4)Soil water pressure head
Time

Molar concentration of proton in water phase
Molar concentration of hydroxide ions
Molecular diffusion coefﬁcient of VOC in free air

X [L]
s [e]
g [e]

Distance above the water table
Fitting parameter in Eq. (8)Fitting parameter in Eqs. (12) and (14)Effective diffusion coefﬁcient of VOC in airu [e]Fitting parameter in Eqs. (12) and (14)Molecular diffusion coefﬁcient of VOC in the water phase
Effective diffusion coefﬁcient in the water phase

b [e]
z [e]

Fitting parameter in Eqs. (12) and (14)
Number of mole of aproton or bhydroxide in Eqs. (1)e(3)Molecular diffusion coefﬁcient of Hþ in the water phase

Produced during oxidation of TCE.
Produced during oxidation of toluene.

A linear kinetic model was used to simulate the dissolution rateof VOC vapors into the water phase (Schwarzenbach et al., 2003;
Yoshii et al., 2012) as follows:

rAdiss ¼ kdiss
A ai

CAg
 CAw
HC;A

!
(7)

The dissolution rate of vapor into the water phase depends on
the speciﬁc airewater interfacial area (Kim et al., 2001; Hoeg et al.,
2004; Cho et al., 2002). Also, the speciﬁc airewater interfacial area
is known to depend on water saturation as well as on the capillarypressure (Hassanizadeh and Gray, 1993; Raoof and Hassanizadeh,2013). For the purpose of this study we assumed that the speciﬁc
surface area depends only on relative water saturation accordingto:ai ¼ ai0 ð1  Sw Þs

(8)

Eq. (8) is consistent with both theoretical (Bradford and Leij,
1997; Oostrom et al., 2001) and experimental (Kim et al., 1999)
analyses of airewater interfacial areas as a function of relative
saturation. The data of Kim et al. (1999) for a coarse-textured sand,very similar to the one used in this study, indicate values ofapproximately 3.0 for the exponent s in Eq. (8), which we used alsoin our study.
VOC transport within the water phase, both in the unsaturated
zone and the HPRB, is assumed to be governed by:qw

a [L1]Oxidation rate of VOC



vCAw
vCAw
v
qw Dw
¼
þ rAdiss  rAoxid
e;A
vx
vt
vx

(9)

The effective VOC diffusion coefﬁcients in the water phase areassumed to be given by a similar equation as for air (Millington and
Quirk, 1961; Partridge et al., 2002; Yao et al., 2013):
10Dw
eff ;A ¼

qw 3 wDA
f2(10)

Boundary conditions for Eq. (9) are a constant concentration ofw Þ andeach VOC at the water table boundary ði:e:; CAw ðx ¼ 0; tÞ ¼ CA;0a no-ﬂux condition at the solideheadspace interface. We assumed

that the VOCs initially were absent in the water phase in the unsaturated zone, i.e., CAw ðx; t ¼ 0Þ ¼ 0.
Several studies have shown that the oxidation rate of VOCs in
the aqueous phase can be described using a second-order reaction
rate equation involving both the target compound and potassium
permanganate (Huang et al., 1999; Kao et al., 2008; Mahmoodlu
et al., 2014a). For unsaturated conditions, the oxidation rate of
VOC may be expressed asw w w
r oxid ¼ koxid
A CA CB qðB ¼ potassium permanganateÞ

(11)

Assuming that the dissolution rate of potassium permanganatein the water phase is much larger than the oxidation rate of dissolved potassium permanganate, mass transfer across the water/
solid interface was considered not to be limiting. We hencew
assumed CBw to be equal to CB;max
within the HPRB, and zero
elsewhere.
A preliminary analysis of the experiments revealed that theoxidation rate decreased with time and eventually became zero.
We considered two hypotheses for the decrease in reactivity of the
HPRB: (1) depletion of potassium permanganate in the water phaseand (2) a dependency of the oxidation rate on pH. The experiments
involved only a limited amount of potassium permanganate, which
could become depleted due to oxidation of the VOCs. Nevertheless,our experimental data showed that the availability of potassium
permanganate was not a limiting factor during the experiments
since a signiﬁcant amount of potassium permanganate remained
unconsumed at the end of each experiment. We hence considered
the second hypothesis that the VOC oxidation rates depended upon
pH. Our earlier study suggested that a decrease in pH during the
TCE experiments, and an increase in pH during the toluene andethanol experiments, as expressed by their stoichiometric reactions
in Eqs. (1)e(3), were responsible for the decrease in oxidation rate
(Mahmoodlu et al., 2014a). This effect was simulated by allowing
koxid
to depend on pH as described below. For TCE we used theAfollowing equations:
bTCE
koxid
TCE ¼ gTCE ðpH  uTCE Þ

forpH < 4(12)

with pH ¼ log CHþ . Variations in pH were modeled using the
diffusion equation:

M.G. Mahmoodlu et al. / Journal of Environmental Management 163 (2015) 204e213



vCHþ
v
vCHþ
oxid
qw Dw
¼
þ zTCE rTCE
þ
e;H
vx
vt
vx

(13)

in which gTCE, uTCE, and bTCE are ﬁtting parameters obtained as partof the simulation results. The parameter zTCE is equal to unity according to Eq. (1).
Eq. (12) was used to calculate the reaction rate constant for pH
values less than 4. The reaction rate constant for TCE is known to be
independent of pH in the range of 4e8 (Yan and Schwartz, 1999;
Waldemer and Tratnyek, 2006). For pH values larger than 4 weused a known value of the reaction rate constant as given in Table 1.
Similar equations were employed for the reaction rate coefﬁcientsof toluene and ethanol, which both decrease with increasing pH:

koxid¼ gA ðuA  pHÞbA
A209

as for TCE, Eq. (14) was used to calculate the reaction rate constantfor pH values larger than 12 only. For pH values less than 12 weused a known value of the reaction rate constant (Table 1).
Boundary conditions for Eqs. (13) and (15) are constant protonand hydroxide ion concentrations at the water table, i.e.,CHþ ðx ¼ 0; tÞ ¼ CHþ 0 and COH ðx ¼ 0; tÞ ¼ COH 0 , and zero ﬂuxes at
the upper headspace boundary. Since water that was injected into
the column had a pH of 7, CHþ 0 ¼ COH 0 ¼ 107 M, where M denotes molarity. These concentrations were also assumed to be theinitial conditions in the unsaturated zone.
The above set of Eqs. (5)e(15) were solved fully coupled using
COMSOL Multiphysics (Müller et al., 2013).

4. Results and discussion

forpH > 12(14)
4.1. Experimental results

while pH variations are controlled by the transport equation:



vCOH
v
vCOH
q w Dw
¼

e;OH
vx
vt
vx
þ zA rAoxidðA ¼ toluene; ethanolÞ

(15)

in which zA is the number of moles of hydroxide ions involved in
the stoichiometric reaction (ztoluene ¼ 12 andzethanol ¼ 4). Similarly

Fig. 2. Comparison of measured and simulated breakthrough curves for the vapor ofthree target compounds in the headspace. The partially saturated HPRB (Sw ¼ 0.2) was
placed 12.0 cm above the water table. Results are given for both the control experiments and the reactive experiments when a reactive HPRB.

4.1.1. VOC concentrations in the headspace
Fig. 2 shows the normalized observed VOC concentrations (C/C0)
as a function of time for the control and reactive experiments (thelatter containing a HPRB), where C is the observed VOC concentration in the headspace and C0 the maximum VOC vapor concenw Þ, which was constanttration above the water table ðC0 ¼ HC;A CA0
during the experiments. Results are for the case when the HPRB
was placed 12 cm above the water table.
A comparison of the reactive (containing the HPRB) and control
experiments shows that the HPRB was very efﬁcient in oxidizing
the VOC vapors during unsaturated conditions. This is reﬂected by avery slow rise of VOC concentrations in the headspace of thereactive columns (Fig. 2). The experimental results also show thatthe HPRB in the sand column was far more effective than that in ourearlier setup (Mahmoodlu et al., 2014b) which involved the diffusion of VOC vapors only through a HPRB. This is due to the fact that
VOC concentrations in the gas phase reaching the HPRB are now
much lower as compared to those in our earlier studies.
The data in Fig. 2 further show a very slow increase in theethanol concentration of the headspace. HPRB was found to be
more effective for ethanol than for the other two compounds. The
reactivity of HPRB for ethanol lasted much longer than for TCE andtoluene. We believe that the production of water during theoxidation of ethanol did somewhat temper the concentration riseof produced hydroxide ions, consequently causing a slower increase in pH of the HPRB. An increase in the residence time ofethanol in the water phase due to its dissolution and hydrophilic
nature may have further increased the exposure time to dissolved
potassium permanganate and thus allowing more removal of
ethanol mass from the water phase.

4.1.2. HPRB elevation effect on the headspace VOC concentrations
Fig. 3 shows headspace VOC concentrations when the HPRB wasplaced 7.0 cm above the water table, rather than 12 cm. A comparison of Figs. 2 and 3 reveals that the HPRB at 7.0 cm was far more
effective than when the same HPRB was located 12 cm above thewater table. Observed VOC concentration in the headspace reachedmaximum values far below the maximum possible concentration.
This is because of the high background water content and possible
diffusion of protons and/or hydroxides (especially downwards) toneutralize the effect of pH on the oxidation rate, thus leading to ahigher removal efﬁciency and reactivity of HPRB for all VOCs. Fig. 3
further shows that the increase in the concentration of toluene in
the headspace was much higher than that of TCE and ethanol. Thisis likely due to the reaction rate constant in the water phase, which
is much smaller for toluene than for TCE and ethanol.

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Fig. 3. Comparison of measured and simulated breakthrough curves for the VOC vapor in the headspace. A partially saturated (Sw ¼ 0.2) The HPRB was placed 7.0 cm above thewater table. The vertical axes of the lower plots were enlarged for better clarity.

4.1.3. Initial water saturation effect of HPRB on headspace TCE
concentrations
Fig. 4 depicts the effect of initial water saturation on the reactivity of the HPRBs when the barrier was placed 12 cm above thewater table. The partially saturated HPRB is clearly far more effective in delaying TCE transport in than the dry barrier. Three factors
combine to cause the slower TCE transport. First, TCE oxidation
occurs only in the water phase, with the oxidation capacity
increasing at higher water saturations. Also, VOC partitions fromthe gas phase in the water phase, thus causing an apparent retardation of VOC diffusion (particularly ethanol) through the HPRB
and hence increasing its residence time in the partially saturated
sand (Mahmoodlu et al., 2014b). Finally, the effective diffusion coefﬁcient in the gas phase decreases at lower air contents (and hence
higher water contents) as described with Eq. (6). Consequently, thehigher the water content, the slower the gaseous diffusion process
through the HPRB.
4.1.4. HPRB thickness effect on VOC headspace concentrations
Fig. 5 shows the effect of HPRB thickness on headspace TCE
concentrations as a function of time. As indicated earlier, two moreexperiments were conducted with thicker HPRB (4.0 cm and8.0 cm) at a relative water saturation of 0.2. Results show that athicker HPRB containing more potassium permanganate causes
more effective oxidation of the VOC vapors. This is evident from theFig. 4. Concentrations of TCE vapors in the headspace of the control and HPRB experiments at two different initial water saturation values. The HPRB was placed
12.0 cm above the water table.

Fig. 5. Concentrations of TCE vapor in the headspace for three different HPRB thicknesses. The ratio of initial mass of KMnO4 to initial mass of sand was equal to 1.0 for all
experiments. The vertical axis of the lower plot was enlarged for better clarity.

data in Fig. 5, which show that the build-up of the headspaceconcentration of TCE is now much slower for the columns containing thicker HPRBs. The thicker HPRBs remained effective for amuch longer period of time at the same degree of water saturation.
An increase in the travel time of VOC vapors through a thicker
HPRB, and consequently more dissolution of VOC vapors into thewater phase will hence enhance the oxidation capacity. Assuming auniform water distribution within HPRB, the only change is the
thickness of the HPRB and consequently longer exposure to potassium permanganate during the upward diffusion of the VOCs.
Since the TCE reaction rate constant is relatively high, most of the
VOC mass is initially removed from the bottom part of the HPRB. As
soon as the bottom part of the HPRB becomes less active due tolocally increasing pH values, the upper part will become more
important for the dissolved VOC reactions. The pH values hence will
also not remain uniform within a thicker HPRB as the diffuse

M.G. Mahmoodlu et al. / Journal of Environmental Management 163 (2015) 204e213

211

reaction front within the HPRB slowly moves upward.
4.1.5. Reactivity of the HPRBs
We found that water saturation exerted a strong inﬂuence on
the reactivity of potassium permanganate within the HPRBs. Our
experiments showed that the reactivity of the HPRB is affected byboth location relative to the water table (Figs. 3 and 4) and initialwater saturation of the HPRB (Fig. 4). For the setup with the HPRB
close to the water table (Fig. 3), the high background water content
and the possible diffusion of protons or/and hydroxide away fromthe barrier, especially downwards toward the water table, may haveneutralized the effect of pH on the oxidation rate. This would leadto a higher reactivity and removal efﬁciency of HPRB for all threecontaminants.
Our data show that the reactivity of the HPRB diminished during
the course of the experiments, in particular for the setup with the
HPRB far from the water table (Fig. 2). As shown by Eqs. (1)e(3),there are two main reaction products: manganese dioxide andprotons or hydroxide ions. Manganese dioxide can precipitate on
potassium permanganate grains surfaces under normal conditions,
thereby reducing the reactive surface of the potassium permanganate grains. Moreover, highly basic or acidic conditions would
decrease the oxidation rate of VOCs during the course of experiments. For a partially-saturated HPRB, our calculations for the TCE
oxidation experiments showed that the pH was able to decrease tohighly acidic conditions (e.g., see also Huling and Pivetz, 2006; Kaoet al., 2008). This can have two effects: (1) an increase in the solubility of manganese dioxide and (2) a reduction in HPRB reactivity
during the TCE oxidation experiments.
We also expected highly basic conditions for the toluene
oxidation experiments due to the production of hydroxide ions as
indicated by Eq. (2). Data in the literature indicate that the oxidation rate of aromatic rings diminishes with increasing basicity of
the water phase, and vice versa (Lobachev et al., 1997; Forsey,2004). This causes a reduction in the reactivity of the HPRB during the oxidation of toluene.
The reactivity of the HPRB during the ethanol experiment was
relatively constant and never reached zero (Fig. 2). This resulted in aconcentration of only about 20% of the maximum concentration in
the headspace for a partially saturated HPRB placed far above thewater table. As shown by Eq. (3), production of water shouldtemper the rise of pH in the HPRB. The reactivity in this casedecreased slightly and did not change with time.
4.2. Numerical modeling results
We numerically solved Eq. (4)e(15) to determine VOC transport
from the saturated zone through the water and gas phases of theunsaturated zone and the HPRB into the headspace of the experimental setup. Since the VOC concentrations were considered to beconstant in the saturated zone (equal to the input concentration),
only transport in the unsaturated zone and the headspace weremodeled. Values of various input parameters used in the simulations are given in Table 1.
4.2.1. Soil water distribution
The experimental procedure allowed an equilibrium soil water
distribution to be reached in the sand column, with the water tableﬁxed at x ¼ 0, before starting the VOC transport experiments. Thisequilibrium distribution, shown in Fig. 6a, is given immediately by
the soil water retention curve as described with Eq. (4). We also
considered cases where the partially-saturated HPRB (Sw ¼ 0.2) was
placed either 12 cm above water table (Fig. 6b) or 7.0 cm above
water table (Fig. 6c). Since potassium permanganate formed half of
the HBRB mass, we did not expect a signiﬁcant change in water

Fig. 6. Soil water distributions as a function of height above the water table for threecases: a) a dry HPRB placed 12.0 cm above the water table, b) a HPRB with Sw ¼ 0.2
placed 12.0 cm above the water table, and c) a HPRB with Sw ¼ 0.2 placed 7.0 cm above
the water table.

saturation of the HBRB. This allowed us to assume steady-state
saturation distributions during the calculations. We thus ignored
any possible water redistribution that may have taken place after
initiating the VOC experiments (possibly important only for thecase where the HPRB is located 12 cm above the water table).
4.2.2. Distribution of VOC concentrations for the HPRB placed 12 cm
above the water table
Fig. 2 shows calculated VOC concentration breakthrough curves
and our experimental data for the partially-saturated HPRB
(Sw ¼ 0.2) as well as the control experiments. We obtained
reasonably good agreement between simulation results and the
VOC data for all experiments. As explained earlier, we expectedchanges in the pH to be the main reason for the decrease in the VOC
oxidation rates during the experiments. Indeed, preliminary simulations based on our two alternative hypotheses (e.g., assuming
manganese dioxide and consequent coating of the reactive surfaceof potassium permanganate, or depletion of potassium permanganate mass and consequently reduction of its concentration in thewater phase) could not describe the experimental data. The modelin this case accurately described the observed breakthrough curvesof TCE and toluene vapors in the headspace. Modeling results

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conﬁrmed the controlling effect of pH on the oxidation rate of TCE
and toluene by potassium permanganate (Fig. 2). While theoxidation rate was negligible at relatively long times, with theconcentrations of TCE and toluene in the headspace reaching thecorresponding concentrations of the control experiments, theoxidation rate for the ethanol experiment decreased only slightly
and remained constant during the experiment due to produced
water (Fig. 2).
4.2.3. Distribution of VOC concentrations for the HPRB placed
7.0 cm above the water table
Fig. 3 compares the simulated VOC concentrations in theheadspace with our experimental data when the partiallysaturated HPRB (Sw ¼ 0.2) was placed 7.0 cm above the water table, while again also including the control experiments. The lower
plots in this ﬁgure provide close-ups of the reactive transport datain the upper plots. The plots again show very good agreement between the simulation results and the experimental data for all
target compounds.
As discussed earlier, we did not expect the pH to have a major
effect on the oxidation rate of the target compounds when the
HPRB was placed 7.0 cm above the water table. Modeling results
conﬁrmed our assumption of a negligible change in pH within the
HPRB due to possible diffusion of protons or/and hydroxides awayfrom the barrier, especially downwards into the higher water
saturation area, during oxidation of all VOCs.
5. Conclusions
This study evaluated the performance of partially-saturated
HPRBs consisting of solid potassium permanganate, sand and water, for oxidizing TCE, toluene, and ethanol vapors under unsaturated conditions. A comparison of the control and reactive
experiments revealed that the HPRB was very effective in oxidizing
VOC vapors.
The reactivity of the HPRB for ethanol lasted much longer thanfor TCE and toluene. We believe that this is due to the production ofwater during the oxidation process of ethanol, which would temper
the concentration of produced hydroxide ions and, consequently,
would lead to a higher and constant reactivity of the HPRB. Moreover, an increase in the residence time of ethanol within the waterphase due to its dissolution and hydrophilic nature may increase
the exposure time to dissolved potassium permanganate, thus
causing more mass transfer of ethanol from the air phase to thewater phase and consequently more mass removal of ethanol.
Our results show that an increase in initial water saturation of
the HPRB can be very effective for oxidizing VOC vapors. We further
found that the location of the HPRB relative to the water table, andconsequently its background water saturation, had a strong effecton the removal capacity and longevity of the HPRB. This because
the high background water content and diffusion of protons or/andhydroxide (to the higher water saturation area) neutralized the pH
effect on the oxidation rate. However, the higher water content
near the water table may increase the dissolution of potassium
permanganate into water and its subsequent diffusion away fromthe HPRB. This in turn would shorten the longevity of HPRB for
oxidizing VOC vapors in the unsaturated zone.
The build-up of VOC concentrations in the headspace was muchslower for the columns with a thicker HPRB. The increased exposure time to more potassium permanganate within the thicker
HPRB, and also the non-uniform change in pH during upward
diffusion of the VOCs, clearly did lead to higher efﬁciency andlongevity of the HPRB.
A reactive transport model that included diffusion of VOCs in thegas and water phases, dissolution of VOCs from gas into the soil

water, and oxidation of dissolved VOCs by dissolved potassium
permanganate could satisfactorily simulate the experimental data.
For the TCE and toluene experiments with the HPRB placed 12.0 cm
above the water table, we needed to include the effect of pHchanges on the oxidation rate in order accurately describe the data.
However, the simulations for ethanol vapor transport were conducted without the effect of pH since the production of water due
to the oxidation process prevented major change in pH. Simulationsof all cases when the HPRB was placed 7.0 cm above the water table
did not require us to account for any changes in pH. This could be
explained by diffusion of protons or/and hydroxide ions away fromthe HPRB, especially downwards to the higher water saturationarea.
We conclude that the laboratory results obtained in this study
are very promising for controlling upward VOC diffusion. Theysuggest that HPRBs may provide a realistic option for preventing
VOC vapors to reach the land surface in actual ﬁeld situations.
Acknowledgments
The authors would like thank to Emilio Rosales Villanueva(University of Vigo, Spain) for his thoughtful review and providing
critical comments. The comments by two anonymous referees
further helped to improve the manuscript. This work was supported by the Ministry of Science, Research and Technology of Iran.
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