Journal of Contaminant Hydrology 164 (2014) 193–208

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Journal of Contaminant Hydrology

j ourna l homepage: www.e lsev ie r .com/ locate / jconhyd

Oxidation of trichloroethylene, toluene, and ethanol vapors bya partially saturated permeable reactive barrier

Mojtaba G. Mahmoodlu a,⁎, S.Majid Hassanizadeh a, Niels Hartog a,b, Amir Raoof a

a Utrecht University, Department of Earth Sciences, The Netherlandsb KWR Watercycle Research Institute, Nieuwegein, The Netherlands

a r t i c l e i n f o

⁎ Corresponding author. Tel.: +31 302535024; fax:E-mail addresses: m.gharehmahmoodlu@uu.nl,

m.g.mahmnoodlu@gmail.com (M.G. Mahmoodlu).

http://dx.doi.org/10.1016/j.jconhyd.2014.05.0130169-7722/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

Article history:Received 27 February 2014Received in revised form 13 May 2014Accepted 26 May 2014Available online 17 June 2014

The mitigation of volatile organic compound (VOC) vapors in the unsaturated zone largelyrelies on the active removal of vapor by ventilation. In this study we considered an alternativemethod involving the use of solid potassium permanganate to create a horizontal permeablereactive barrier for oxidizing VOC vapors. Column experiments were carried out to investigatethe oxidation of trichloroethylene (TCE), toluene, and ethanol vapors using a partiallysaturated mixture of potassium permanganate and sand grains. Results showed a significantremoval of VOC vapors due to the oxidation. We found that water saturation has a major effecton the removal capacity of the permeable reactive layer. We observed a high removalefficiency and reactivity of potassium permanganate for all target compounds at the highestwater saturation (Sw = 0.6). A change in pH within the reactive layer reduced oxidation rateof VOCs. The use of carbonate minerals increased the reactivity of potassium permanganateduring the oxidation of TCE vapor by buffering the pH. Reactive transport of VOC vaporsdiffusing through the permeable reactive layer was modeled, including the pH effect on theoxidation rates. The model accurately described the observed breakthrough curve of TCE andtoluene vapors in the headspace of the column. However, miscibility of ethanol in water incombination with produced water during oxidation made the modeling results less accuratefor ethanol. A linear relationship was found between total oxidized mass of VOC vapors perunit volume of permeable reactive layer and initial water saturation. This behavior indicatesthat pH changes control the overall reactivity and longevity of the permeable reactive layerduring oxidation of VOCs. The results suggest that field application of a horizontal permeablereactive barrier can be a viable technology against upward migration of VOC vapors throughthe unsaturated zone.

© 2014 Elsevier B.V. All rights reserved.

Keywords:VOC vaporsDiffusionWater saturationUnsaturated zonePermeable reactive barrierSolid potassium permanganate

1. Introduction

During the past few decades, the migration of volatileorganic compounds (VOCs) by diffusion from contaminatedsoil or groundwater into overlying buildings has receivedconsiderable attention (McHugh et al., 2013; Provoost et al.,2009). Human health risks of VOCs are typically dominated

+31 30 2534900.

by the extent of exposure through inhalation of indoor air.The health risks from VOC vapor inhalation are much greaterthan those from drinking comparably contaminated water.Hence, methods to diminish or prevent vapor intrusion intobuildings are of great public interest.

A number of techniques exist for treating unsaturatedzone contaminated with VOCs. Despite their successful use,they all suffer from several shortcomings. For example, soilvapor extraction requires long-term operation and does notconvert the contaminants to less toxic compounds (Cho et al.,2002). Bioventing is an in-situ bioremediation technology

194 M.G. Mahmoodlu et al. / Journal of Contaminant Hydrology 164 (2014) 193–208

that degrades VOCs. However, the performance of this methodcan be affected by soil permeability and water contentrestrictions. Moreover, the method is not effective foraerobic biodegradation of many chlorinated hydrocarbons(Hinchee, 1993; USEPA, 1995). In-situ chemical oxidation(ISCO) of VOCs has been well developed as a remediationtechnology of dissolved VOCs in groundwater (Heiderscheidtet al., 2008; Li and Schwartz, 2004; Tsitonaki et al., 2010; Yuanet al., 2013). However, only a few studies have applied ISCO tothe unsaturated zone using permanganate (Hesemann andHildebrandt, 2009) or other oxidants (Cronk et al., 2010). Inthese studies, the oxidantwasmostly introduced as an aqueoussolution, thus effectively saturating the unsaturated zone. Bycontrast, our recent study showed that dry solid potassiumpermanganate granules were able to oxidize TCE, toluene, andethanol (target compound) vapors, according to the followingoverall reaction equations (Mahmoodlu et al., 2013):

C2HCl3 gð Þ þ 2KMnO4 sð Þ→2Kþ þ 2MnO2 sð Þ þ 3Cl− þ 2CO2 gð Þ þ Hþ

ð1Þ

C6H5CH3 gð Þ þ 12KMnO4 sð Þ þ 2H2O→12Kþ þ 12OH− þ 12MnO2 sð Þ þ 7CO2 gð Þ

ð2Þ

C2H5OH gð Þ þ 4KMnO4 sð Þ→4Kþ þ 4OH− þ 4MnO2 sð Þ þ 2CO2 gð Þ þ H2O:

ð3Þ

VOC vapor oxidation presented in our earlier study occurredthrough the exposure of VOC vapor to excess amounts of solidpotassium permanganate. As a result, any potential long-termeffects on the reactivity of potassiumpermanganate through theaccumulation of reaction products, such as manganese dioxide(MnO2) and pH changes, could not be assessed. Also, theexperiments were performed at low moisture conditions withonly ambient air providing initial humidity. While the studyconfirmed the potential of using permanganate for permeablereactive barriers in the unsaturated zone, it remained unclearhow water content and chemical evolution would affectthe reactivity of a permeable reactive barrier. In this studywe therefore performed a series of column experiments to(1) evaluate the ability of solid potassium permanganate asa horizontal permeable reactive layer to oxidize the vaporof three VOCs under various degrees of water saturation,(2) investigate the impact of the accumulations of by-productson the long-term reactivity of potassium permanganate, and(3) numerically simulate the migration and oxidation processof each target compound diffusing through the permeablereactive layer.

2. Materials and methods

2.1. Materials

The contaminants (target compounds) used in this studywere pure TCE, toluene, and ethanol (from Sigma-Aldrich,Merck, and ACROS, respectively). Solid potassium perman-ganate of 99% purity was obtained from Sigma-Aldrich andwell mixed with sand to create a permeable reactive layer.The sand used in this study originated from a river bed inPapendrecht (Filcom Company, The Netherlands) and was

sieved to retain sizes of 0.5–1 mm. The porosity of sand wasestimated to be approximately 0.35. Since the mean size ofpotassium permanganate grains was almost equal to the sandmean grain size, we assumed the same porosity for potassiumpermanganate.

Two additional TCE experimentswere conducted to test theeffect of pH buffering by adding sodium bicarbonate (NaHCO3)(≥99.0%, Merck) and calcium carbonate (CaCO3) (≥99.0%,Merck). Deionized (DI) water was used to adjust the requiredinitial water saturation in the permeable reactive layer, and toinvestigate the effect of adding water on the reactivity ofpermeable reactive layer.

A glass cylinder of 5.0 cm length and 4.0 cm internaldiameter, capped by a steel stainless lid, was used to constructthe experimental columns. The columns were divided into twoparts bymeans of a glass filter (P0, ϕ = 0.3, Robu & Schott, ISO4793), which was fused to the inner wall of the columns.Horizontal permeable reactive layers consisting of a combina-tion of solid potassium permanganate, sand, and DI water,were placed on top of the glass filters, through which VOCvapor could diffuse from below (Fig. 1).

2.2. Sampling and measurements

During the experiments, gas samples of 1.5 ml wereperiodically taken from the headspace of the reactive and controlcolumns using a 2.5 ml gas-tight syringe (SGEAnalytical Science,Australia). To eliminate the effect of a pressure drop due tosampling, the same volume of air (1.5 ml) was simultaneouslyinjected in the upper part of the column through a separatevalve. The gas sample subsequently was injected into a 10-mltransparent glass vialwhichwas cappedwith amagnetic cap andhard septum (Magnetic Bitemall; red lacquered, 8 mm centerhole; Pharma-Fix-Septa, silicone blue/PTFE gray; Grace Alltech).Sampling vials were immediately placed into the tray of a gaschromatograph (GC). Gas samples of 2.0 ml were taken by anautosampler using the headspace syringe of the GC from eachvial. Samples were next injected into the GC. The GC (Agilent6850) was equipped with a flame ionization detector (FID).Separation was done on an Agilent HP-1 capillary column(stationery phase: 100% dimethylpolysiloxane, length: 30 m, ID:0.32 mm, film thickness: 0.25 μm). A temperature programmedrun was used to analyze the samples. VOC concentrations weredetermined using a headspace method as employed in previousstudies (e.g. Almeida and Boas, 2004; Przyjazny and Kokosa,2002; Sieg et al., 2008; Snow, 2002). The limits of quantification(LOQ) were calculated by using a signal-to-noise ratio of 10:1(Kubinec et al., 2005).

To measure the total organic carbon (TOC) content, the sand(D0 = 0.5–1 mm) was grained before the experiment andsieved to a particle size fraction of b250 μm. Measurementswere next carried out using Fisons Instruments analyzer (NA1500NCS)with a cycle timeof 180 s and a source temperature of190 °C.

2.3. Experimental procedure

Mixtures of potassium permanganate grains (20 g) andsand (10 g) with different water saturation (0.0, 0.2, 0.4, and0.6) were used to create the permeable reactive layers. First,potassium permanganate and the sand grains were placed

Fig. 1. Schematic view of the column experiments and the main processes during the migration of VOC vapor through a permeable reactive layer.

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into a small plastic container and shaken for 10 min in orderto obtain a homogeneous mixture. This mixture subsequentlywas wetted with DI water required to obtain the desiredwater saturation. In order to obtain homogeneity, the wettedmixture was shaken for 15 min. The mixture was next placedon the glass filter of each column described above. Thiscreated a permeable reactive layer of about 1.0 cm thick witha surface area 13.2 cm2. The columns were immediatelycapped with a vapor-tight stainless steel lid. Finally, 2.5 ml ofthe pure phase of a particular VOC was introduced via thelower level valve into the bottom of the column (Fig. 1). Toprevent the pressure from increasing due to the injection ofpure phase of the target compounds, the same volume of air(2.5 ml) was withdrawn from the lower part of the columnbefore injection. Preliminary observations of the ethanoloxidation during high-water saturation experiments showedthat the ethanol pool was depleted. Hence, for the ethanolexperiments when the pool was about depleted, we injectedan additional volume 2.5 ml of pure ethanol into the bottomof the column.

To prevent any photodecomposition of potassium per-manganate, all columns were wrapped in aluminum foil. Aseries of control experiments with 30 g sand, no potassiumpermanganate, and identical water saturations was carriedout for all target compounds. All experiments were conduct-ed in a fume cabinet at room temperature (22 ± 1 °C) induplicate. We assumed that the temperature and pressureinside the columns remained constant and were equal toroom temperature and atmospheric pressure, respectively.

Two separate additional experiments were conducted forTCE, in which 1.0 g of dry basic salts was added to thepermeable reactive layer, in order to buffer proton produc-tion. For one experiment, we used sodium bicarbonate and inthe other experiment calcium carbonate.

For the TCE experiments, we further tested the effect ofadding water during the experiments on the reactivity ofthe permeable reactive layer. The water was added to thepermeable reactive layer in two different ways. In one case, wemade an instantaneous injection of 1.0 ml DI water to thepermeable reactive layer using a syringe (SGE AnalyticalScience, Australia). In the other experiment, the same amountof DI water was injected continuously at the rate of approxi-mately 2.1 × 10−3 ml min−1, using a syringe pump (KDSModel 100 Series).

3. Processes and equations

Fig. 1 shows a schematic of experimental column setup.The columns contain four domains. The first domain consistsof a pool of VOC liquid. The second domain comprises the airspace above the liquid pool and below the permeablereactive layer. The third domain is the permeable reactivelayer consisting of potassium permanganate and sand withits water and air phases. The fourth domain is the headspaceabove the permeable reactive layer.

For our simulations we assumed that the composition ofthe liquid pool and the air space above the liquid pool did notchange with time. This assumption was certainly valid forTCE and toluene since the liquid did not deplete and theconcentration in the air space above it quickly reached itsequilibrium value (corresponding to its vapor pressure)

because of rapid diffusion. For ethanol, this assumption wasonly weakly valid since its partial pressure changed withtime due to full miscibility of ethanol with water in thecolumn. The water evaporated from the permeable reactivelayer, diffused down and dissolved in the ethanol pool, andvice versa. This resulted in a lower ethanol fraction within theliquid phase (b100%), and thus decreasing its partial pressureover time. Assuming equilibrium between water in thepermeable reactive layer and the ethanol pool, we calculatedthe mole fraction of ethanol in the mixture (ethanol andwater). Based on the calculated mole faction of ethanol in thenon-ideal mixture (Kuhn et al., 2009), the partial pressure ofethanol was estimated to be approximately 80% of its vaporpressure.

Diffusion in the headspace above the permeable reactivelayer was thought to be fast enough to consider this domainas a well-mixed zone (i.e., with no vertical concentrationgradients). However, to take into account the storage of VOCmass within the headspace, and also for the purpose ofspecifying well-defined boundary conditions, we chose toinclude this domain in our modeling.

The main processes taking place in the permeable reactivelayer are: (1) upward diffusion of VOC vapors in the airphase, (2) dissolution of VOC vapor into the water phase,(3) dissolution of solid potassium permanganate into thewater phase, and (4) oxidation of dissolved VOC by dissolvedpermanganate in water (Fig. 1).

We assumed that there was no change in air pressureand temperature in the column. We hence disregarded theadvection term in the transport equations. Based on the verysmall amount of TOC (0.05%) in sand and its oxidation in thepresence of potassium permanganate (Mumford et al., 2005)in the reactive experiment, we also assumed the adsorptionof VOC to the sand grains to be negligible. Hence, thegoverning transport equation for a target compound in thegas phase of permeable reactive layer and headspace,assuming a one-dimensional transport, can be written as:

θg∂Cg

A

∂t ¼ θgDgeff ;A

∂2CgA

∂x2−rdiss

A ; A ¼ TCE; toluene; ethanol ð4Þ

where θg denotes the air content, A denotes the targetcompound, CAg is the concentration in air [NL−3], Deff,A

g is theeffective gas diffusion coefficient [L2T−1], and rdissA is the rateof dissolution in water. The latter term is absent in theheadspace, and θg is hence equal to 1.0.

Boundary conditions for Eq. (4) are a constant concentra-tion of the target compound (CAg (t, x = 0) = CA0g ) at thelower boundary of the permeable reactive layer and no-fluxat the top of the headspace. The simulation further impliescontinuity fluxes across the interface between the permeablereactive layer and the headspace. The value of CA0g for TCE andtoluene was taken equal to the vapor pressure of the targetcompounds. However, CA0g for ethanol was taken to be 80%of its vapor pressure. Initial concentrations of the targetcompound were equal to zero throughout the domain, i.e., CAg

(x, t = 0) = 0.Several literature studies show that for VOCs with high

vapor pressures, vapor diffusion dominates the migration ofVOCs through unsaturated soil (Berscheid et al., 2010; Shenet al., 2014; USEPA, 1993). The rate of vapor diffusion is

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obviously slower in soil than in free air. The effective gasdiffusion is influenced by the pore space tortuosity, which itselfdepends on porosity and the volumetric air content (Raoof andHassanizadeh, 2013; USEPA, 1993). The effective gas diffusioncoefficient of VOC vapor in the permeable reactive layer wasexpressed as follows (Millington and Quirk, 1961; Pennell etal., 2009; Yao et al., 2013):

Dgeff ;A

¼ DgAθg

103

ϕ2 ð5Þ

where DAg is the molecular diffusion coefficient of the target

compound in free air [L2T−1] and ϕ denotes the porosity of theporous medium.

The dissolution of the VOC compounds into water wasmodeled as a linear kinetic process expressed as (Yoshii et al.,2012):

rdiss

A ¼ kdAaiCgA

HAC

−CwA

!ð6Þ

where kAd is the dissolution rate constant [LT−1], ai is the

specific air–water interfacial area, HCA denotes Henry's constant,

and CAw is the concentration of the target compound in soilwater of the permeable reactive layer [NL−3].

The dependence of the dissolution rate on the specificair–water interfacial area, ai, has been reported by variousresearchers (Cho et al., 2005; Costanza and Brusseau, 2000;Hoeg et al., 2004; Kim et al., 2001). The specific air–waterinterfacial area is known to depend on water saturation aswell as on capillary pressure (Hassanizadeh and Gray, 1993;Joekar-Niasar et al., 2010; Raoof et al., 2013). However, forthe purpose of this study we assumed that ai depends onlyon the water saturation, Sw, according to the followingequation (Zhang et al., 2012):

ai ¼ ai0Sw 1−Sw� �α ð7Þ

where α is a fitting parameter and ai0 is the specific interfacialarea corresponding to residual saturation (Zhang et al., 2012).

Regarding the spread of target compounds in soil water, weassumed one-dimensional diffusive transport. The governingequation for the aqueous phase, which pertains to only thepermeable reactive layer, is hence:

θw∂Cw

A

∂t ¼ θwDw

eff ;A∂2Cw

A

∂x2þ rdissA −roxidA ð8Þ

where θw is the water content, rAoxid,w is the oxidation rate andDeff,Aw is the effective diffusion coefficient in water.Boundary conditions for Eq. (8) are a zero flux at both the

bottom and top of the permeable reactive layer. We furtherassumed that the target compounds are initially absent inwater, i.e., CAw (x, t = 0) = 0.

The effective diffusion coefficient of the target compoundassumed to be given by a similar equation as for air (Pennellet al., 2009; Yao et al., 2013):

Dweff ;A ¼ Dw

Aθw

103

ϕ2 ð9Þ

where DAw denotes the molecular diffusion coefficient of the

target compound in water [L2T−1].Dissolved permanganate is known to be able to oxidize a

variety of VOCs in water (Kao et al., 2008; Mahmoodlu et al.,2014; Waldemer and Tratnyek, 2006). Mahmoodlu et al.(2014) proposed a second-order equation for the oxidationrate of VOCs in the aqueous phase, given as a function of thetarget compound and potassium permanganate concentra-tions. For unsaturated conditions, the oxidation rate can bewritten as:

roxidA ¼ kACwA C

wMnO−

4θw ð10Þ

where kA denotes the reaction rate coefficient in the aqueousphase [N−1T−1] and Cw

MnO−4is the concentration of dissolved

permanganate in water [NL−3].Potassium permanganate commonly dissolves quickly in

soil water up to its solubility of 64 g l−1 at 20 °C (USEPA,1999). Since we used an excess amount of potassiumpermanganate, the consumption of potassium permanganatedue to oxidizing VOCs was disregarded. We hence equatedthe maximum concentration of permanganate in water equalto the solubility of potassium permanganate, Cw

MnO−4 ;max.

Preliminary analysis of the experimental results showedthat oxidation rate ceased after a certain period of time. Ourresults suggested that this was due to a decrease in the pHduring the TCE experiment, and an increase in the pH duringthe toluene and ethanol experiments, as expressed by theirstoichiometric reaction equations (Eqs. (1) to (3)). In order tomodel this effect, we allowed for a dependency of kA on pH,according to the following equations:

kTCE ¼ κTCE pH−ωTCEð ÞβTCE ð11Þ

∂Hþ

∂t ¼ ζTCEroxid

TCE ð12Þ

in which κTCE denotes the reaction rate constant in water[N−1T−1] as determinedwith batch experiments (Mahmoodluet al., 2014), ωTCE and βTCE are fitting parameters obtained aspart of the simulation results, and H+ denotes the protonconcentration [NL-3]. The parameter ζTCE in Eq. (12) is thenumber of moles of protons produced during oxidation of TCE.According to Eq. (1), its value is equal to unity.

Similar relationships were employed for the reaction ratecoefficients of toluene and ethanol as follows:

kA ¼ κA ωA−pHð ÞβA ; A ¼ toluene; ethanol ð13Þ

∂OH−

∂t ¼ ζAroxidA ð14Þ

where κA denotes the reaction rate constant in water [N−1T−1]as determined previously (Mahmoodlu et al., 2014),ωA and βA

are fitting parameters, OH- denotes the molar concentration ofhydroxide ions, and ζA is the number of moles of hydroxideions consistent with the stoichiometric reaction (ζtoluene = 12and ζethanol = 4)).

The above set of coupled equations was solved simulta-neously using COMSOL Multiphysics.

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4. Results and discussion

4.1. Oxidation process and water saturation effect

Fig. 2 depicts normalized concentrations (C/C0) of thetarget compounds in the control experiments as a function oftime for different initial water saturations. Here, C denotesthe observed concentration of VOC vapor in the headspaceand C0 is the maximum observed concentration of VOC vaporin the lower part of the column above the liquid pool. Thelatter was found to correspond to the vapor pressure ofthe particular VOC. The results in Fig. 2 indicate that theconcentration of the target compounds in the headspaceincreased gradually with time up to the maximum observedconcentrations. Results of the control experiments showedthat an increase in water saturation retarded vapor migrationthrough the partially saturated sand layer (Fig. 2). A likelyexplanation is the effect of water saturation on the effectivediffusion coefficient of the VOCs. Literature shows that vapordiffusion is very sensitive to water saturation, with morerapid vapor movement when the medium is drier. Tillmanand Weaver (2005) found that with a 10% increase in watersaturation, the effective vapor diffusion coefficient of TCEdecreased by three orders of magnitude. Another explanationfor the effect of water saturation is the partitioning of VOCdue to its dissolution in water. This caused a retardation ofthe VOCs (particularly ethanol) and hence longer residencetimes in the partially saturated sand during the controlexperiments.

A comparison of the control and reactive experimentsrevealed that the permeable reactive layer was very effectivein oxidizing VOC vapors. This can be clearly seen in Fig. 3,where the build-up of VOC concentrations in the headspace ismuch slower for the columns with a permeable reactive layerthan in the control columns. The permeable reactive layer isfar more effective at higher water saturation degrees. Thisis because oxidation occurs in the water phase with theoxidation capacity being higher at higher water saturations.Moreover, the larger volume ofwater provides a large reservoirfor the dissolving vapors.

4.2. Reactivity of potassium permanganate

Our experimental results revealed that the reactivity ofthe permeable reactive layer decreased during the course ofthe experiments. We found that water saturation has a strongeffect on the reactivity of potassium permanganate. Thepermeable reactive layer at the highest water saturation (i.e.,Sw = 0.6) was found to be more reactive. As shown by Eqs. (1)to (3), the accumulation of by-products, particularly whenlimited amounts of water are present, may explain the decreasein reactivity of the permeable reactive layer. Two main by-products might have affected the reactivity of potassiumpermanganate: protons or OH− ions and manganese dioxide(Eqs. (1) to (3)).Highly acidic or basic conditionswould decreasethe oxidation rate. Furthermore, under normal conditions,manganese dioxide would precipitate on the potassium per-manganate grains and reduce their reactive surface.

The accumulation of by-products in the water phase couldnot be monitored during the experiment. However, weexpected that the accumulation of protons in the TCE

oxidation experiment (Eq. (1)), would result in highly acidicconditions for the closed system. Produced protons wouldreact directly with permanganate and generate permanganicacid (Forsey, 2004; Housecroft and Sharpe, 2005) as follows:

MnO−4 þHþ→HMnO4 ð15Þ

This reaction occurs under very acidic condition (Housecroftand Sharpe, 2005). Sincemonitoring of thepH in thewater phasewas not possible during our experiments, the pH values wereestimated based on stoichiometric reactions. Our calculations forthe TCE oxidation experiments showed that the pH could havedecreased to about 1.25. The literature shows that manganesedioxide is increasingly soluble under very acidic conditions (Kaoet al., 2008). Therefore, the formation of manganese dioxideprecipitates and subsequent coating of the reactive surface ofpermanganate is not a likely reason for reductions in the TCEoxidation rate. We hence conclude that the reactivity ofpotassium permanganate during the oxidation of TCE de-creased because of the acidic conditions created in the waterphase during the TCE column experiments.

To control the inhibitory effect of acidity on the reactivityof potassium permanganate during the oxidation of TCEvapor, two separate experiments were performed using twobasic salts under the highest water saturation (Sw = 0.6). Inone experiment, we added sodium bicarbonate to the initialpermeable reactive layer. In another experiment, calciumcarbonate was employed. Their dissolution in water and thepH buffering capacity were triggered by the proton produc-tion due to TCE oxidation. In line with the hypothesis that pHexerted the main control on the oxidation rates, the resultssuggest that the addition of both carbonates positivelyaffected the reactivity of the permeable reactive layer in theTCE oxidation experiments (Fig. 4). However, the overalleffect was still relatively small.

The effect of the accumulated by-products on the reactivity ofthe permeable reactive layer should be reduced by addingwater.Therefore, in two sets of complementary experiments, we addedwater to both the reactive and control TCE experiment (Fig. 5). Inthe first set of experiments, we expected to observe the lowestreactivity of the permeable reactive layer. In this case, 1.0 ml ofwater was injected instantaneously into the permeable reactivelayer using a syringe. Results showed that the concentration ofTCE vapor in the headspace initially decreased rapidly, but thenincreased gradually up to a maximum (Fig. 5a). Results for theTCE control experiment showed only a minor reduction in theTCE vapor concentration of the headspace due to the dissolutionprocess. In the second set of experiments, after reaching steady-state conditions, water was injected continuously (at the rate ofaround 2.1 × 10−3 ml min−1) into the permeable reactive layerusing a syringe pump. As shown in Fig. 5b, the concentration ofTCE vapor in the headspace decreased gradually until the pumpwas turned off. The increase in the reactivity of the permeablereactive layer due to added water is attributed to a reduction inthe proton concentrations. This allows the reaction to continueuntil protons accumulated again to previous levels.

Similar to the TCE oxidation experiment, the reactivity ofthe permeable reactive layer towards toluene and ethanoldecreased during the experiments. However, in contrast to theTCE oxidation experiment, and in accordancewith Eqs. (2) and

Fig. 2. Effect of water saturation on VOC vapors diffusing through the partially saturated sand.

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(3), the produced hydroxide ion increased the pH in the waterphase (Mahmoodlu et al., 2014).

Oxidation rate of aromatic rings have been shown todecrease with increasing basicity of the water phase (Forsey,2004; Lobachev et al., 1997; Rudakov and Loachev, 1994). Wehence expected the oxidation rate of toluene to decrease duringthe toluene experiments (as basicity increased). As shown byEq. (2), the oxidation of 1.0 mol toluene by potassiumpermanganate produces 12.0 mol hydroxide ion and consumes2.0 mol water. We hence can expect a rapid rise in pH andconsequently a decrease in the toluene oxidation rate bypotassium permanganate in the permeable reactive layer.

Literature studies also show a dependency of the ethanoloxidation rate on proton ion (Sen Gupta et al., 1989). Theoxidation rate of ethanol hence should increase with acidity andconversely. The stoichiometric reaction of ethanol shows thatthe oxidation of 1.0 mol ethanol by potassium permanganate

produces 4.0 mol hydroxide ion (Eq. (3)). However, the reactionalso produces 1 mol of water, which should temper the con-centration of produced hydroxide ions and consequently aslower increase in pH of the permeable reactive layer. Thus, aslow increase in pH may have caused the lower reactivity ofpotassium permanganate during the oxidation of ethanol.

4.3. Simulation results

Simulations showed that the concentration of the targetcompounds below the permeable reactive layer reachedequilibrium immediately. Hence, we simulated only thepermeable reactive layer and the headspace domains usingEqs. (4) and (8). All input parameters are given in Table 1.Simulation results together with experimental data areshown in Figs. 6 to 8. While there is good agreement betweenthe simulation results and the experimental data for TCE and

Fig. 3. Concentration of VOC vapors diffusing through the permeable reactive layer for two different water saturations.

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Fig. 4. Effect of NaHCO3 and CaCO3 on the reactivity of the permeable reactive layer during TCE oxidation.

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toluene, a discrepancy occurs for the ethanol experiments,which increases at the higher water saturations (Fig. 8). Alikely reason is that the assumed concentration of ethanolvapor in the air space below the permeable reactive layer(CA0g ) was too low. In the simulations, we set CA0g equal to 80%of the ethanol vapor pressure (see Section 3) to account

Fig. 5. Effect of adding more water to the reactive layer on the reactivity of the permethe upper valve, b) continuous injection of 1.0 ml DI water (at the rate of aboutreplications of a reactive experiment including dry permeable reactive layer.

for the dissolution of water vapor in the ethanol pool. Thiseffect was not present initially but became significant onlyafter a long time. This may have resulted in the discrepancybetween experimental data and simulation results.

As explained in Sections 3 and 4.2, we expected thechange in pH to be the main reason for the decrease in the

able reactive layer during TCE oxidation. a) Injecting 1.0 ml DI water through2.1 × 10−3 ml min−1) by a syringe pump. Reactive 1 and 2 represent two

Table 1Experimental conditions and modeling parameters of column experiments.

Parameter VOC Value References

Porosity of the permeable reactive layer, ϕ, (−) – 3.5 × 10−1 –

Water content, θw, (−) – 7.0 × 10−2,1.4 × 10−1 and 2.1 × 10−1 –

Volume of the headspace (m3) – 20.7 × 10−6 –

Volume of the permeable reactive layer (m3) – 13.2 × 10−6 –

Solubility of KMnO4, CwMnO−

4 ;max,(mol m−3) at 20 °C – 4 × 102 USEPA (1999)Specific interfacial area corresponding to residualsaturation, ai0, (m−1)

– 4.0 Zhang et al. (2012)

Fitting parameter α in Eq. (7) (−) 6.0 –

Henry's constant of VOCs, HC, (−) TCE 4.3 × 10−1 Fan and Scow (1993)Toluene 2.8 × 10−1 Fan and Scow (1993)Ethanol 2.4 × 10−4 ITRC (2011)

Reaction rate constant of VOCs in water, κA,(mol−1 s−1)

TCE 8.0 × 10−1 Mahmoodlu et al. (2014)Toluene 2.5 × 10−4

Ethanol 6.5 × 10−4

Molecular diffusion coefficient of VOCs in air,DAg, (m2 s−1)

TCE 7.9 × 10−6 Estivill et al. (2007)Toluene 7.6 × 10−6 Hers et al. (2000)Ethanol 1.1 × 10−5 Green and Perry (2007)

Molecular diffusion coefficient of VOCs in water,DAw, (m2 s−1)

TCE 9.1 × 10−10 Fogler (2006) and Lewis et al. (2009)Toluene 9.4 × 10−10 Hers et al. (2000)Ethanol 1.2 × 10−9 Green and Perry (2007)

Dissolution rate coefficient of VOC vapors in water,kAd,(m s−1)

TCE 8.0 × 10−5 aMeasuredToluene 1.3 × 10−5

Ethanol 1.0 × 10−4

Fitting parameter β in Eqs. (11) and (13) (−) TCE 2.0 –

Toluene 3.0 –

Ethanol 3.0 –

Fitting parameter ω in Eqs (11) and (13) (−) TCE 1.25 –

Toluene 14.0 –

Ethanol 14.0 –

Number of mole of protonb or hydroxidec in Eqs. (1)to (3), ζ,(−)

TCE 1.0 –

Toluene 12.0 –

Ethanol 4.0 –

Fitting parameter η in Eq. (16), (mol m−3) TCE 44.74 –

Toluene 13.21 –

Ethanol 196.57 –

a Additional batch experiments were performed to estimate the dissolution rate coefficient of VOCs.b Produced during oxidation of TCE.c Produced during oxidation of toluene and ethanol.

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oxidation rate of the target compounds during the experi-ments. Modeling results confirmed the controlling effect ofpH. The oxidation rate became zero at relatively long times,with the concentrations of all target compounds in theheadspace reaching the corresponding concentrations of thecontrol experiments (Figs. 6 to 8).

We used the simulations to estimate the total mass of VOCvapors entering the layer of both the reactive and controlexperiments at different water saturation degrees at the end ofeach experiment. We next estimated the total oxidized mass ofVOC as the total mass entering the layer at the end of a reactiveexperiment minus the mass entering the layer of the corre-sponding control experiment. The calculated total oxidizedmasswas normalized using the volume of permeable reactive layerand plotted in Fig. 9 as a function of its initial water saturation.This figure shows a linear increase in the total oxidized mass ofVOCs with an increase in initial water saturation. Although theresults are based on a 1D-homogeneous diffusion model, theysupport the assumption that the reaction rate is hamperedby thechange in pH of the water phase during the oxidation of VOCs.This linear relationship can be expressed by:

moxidA ¼ ηSw ð16Þ

wheremAoxid denotes the oxidizedmass of VOCs per unit volume

of permeable reactive layer [NL−3] and η is a fitting parametergiven in Table 1 for each VOC.

4.4. Longevity of the reactive permeable barrier

The current experiments were performed under veryidealized conditions. Field conditions generally involve manyenvironmental factors such as pH, temperature, soil organicmatter, soil type, heterogeneity, and soil moisture conditions,which are not considered here. Nevertheless, our resultsprovide an indication of the reactive capacity of a permeablereactive barrier. There are two major differences between afield situation and our experimental setup: 1) the thicknessof the permeable layer in the field can be at least 100 timeslarger, and 2) the concentration of contaminants in the gasphase reaching the layer will be much lower than in ourexperiments. These two factors are critical when determiningthe most effective application of potassium permanganate inthe field.

In order to obtain a rough estimate of the longevity of ahorizontal permeable reactive barrier consisting of potassiumpermanganate and sand under field conditions, we consid-ered a hypothetical field situation with a building located

Fig. 6. Comparison of measured and simulated breakthrough curves for TCE vapor in the headspace at various water saturations.

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Fig. 7. Comparison of measured and simulated breakthrough curves for toluene vapor in the headspace at various water saturations.

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Fig. 8. Comparison of measured and simulated breakthrough curves for ethanol vapor in the headspace at various water saturations.

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Fig. 9. Effect of initial water saturation (S0W) on total consumed mass of VOCs per volume of the permeable reactive layer (mVOCoxid). The mass ratio of potassium

paramagnet over the sand was equal to 2.0 and the thickness of the permeable reactive layer was equal to 0.01 m.

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above VOC-contaminated groundwater (Fig. 10). We consid-ered a horizontal permeable reactive barrier with a thicknessof 1.0 m, consisting of solid potassium permanganate andsand (with the mass ratio of potassium permanganate tosand equal to 2.0), and at a typical water saturation of 0.40.We assumed the initial concentrations of TCE and toluene ingroundwater to be 1% value of their solubilities in water.However, for ethanol we considered groundwater with avolumetric ratio of 10% ethanol (e.g., as reported by Freitas(2009) for North America). The reactive barrier was assumedto be constructed 2.0 m above the groundwater table(Fig. 10). Assuming that VOC vapors are transported to thereactive layer by diffusion only, the continuous mass fluxesreaching the permeable reactive layer could be calculated.Then, using the correlation equation (i.e., Eq. (16)), we

Fig. 10. A conceptual model for preventing the vapor int

calculated the lifetime of the permeable reactive barrier. Allparameter values and results are given in Table 2. Calcula-tions showed that the permeable reactive layer would be ableto oxidize TCE, toluene, and ethanol vapors for a period of247, 227 and 785 days, respectively. Obviously, the longevityof a horizontal permeable reactive barrier is affected by manyenvironmental factors. Nevertheless, this rough estimate ofthe longevity shows that horizontal permeable reactivebarrier could be a viable option for preventing VOC vaporsfrom reaching indoor space. In fact, these estimates are basedon experimental results that suffered from water limitationsin a sealed column. This caused the build-up of reactionproducts that negatively affected the reactivity of thepermeable reactive barrier. The absence or prevention oflimitations in water availability during field applications

rusion by a horizontal permeable reactive barrier.

Table 2Longevity of a partially saturated permeable reactive barrier consisting of potassium permanganate grains and sand in the unsaturated zone.

VOC Cg (mol m−3)a Sw (−)b Flux (mol m−2 s−1) mAoxid (mol m−3)c Longevity (day)

TCE 4.1 × 10−2 0.4 8.4 × 10−7 17.9 247Toluene 1.4 × 10−2 0.4 2.7 × 10−7 5.3 227Ethanol 1.7 × 10−1 0.4 1.2 × 10−6 78.6 758

a Concentrations of TCE and toluene in the gas phase calculated based on 1% of their solubility in water. For the estimation of ethanol concentration in air, weassumed a solution of 10% ethanol and 90% water. The ratio of initial mass of potassium permanganate to initial mass of sand was equal to 2.0 and the thickness ofpermeable reactive barrier was equal to 1.0 m.

b Water saturation.c Oxidized mass of VOCs per unit volume of permeable reactive layer.

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could further enhance the effectiveness of a horizontal perme-able reactive layer tomitigate vapor intrusion risks. This could bea focus of future studies.

5. Conclusions

In this study we investigated the possibility of using solidpotassium permanganate in a partially water saturated hori-zontal permeable reactive barrier for oxidizing VOC vapors. Theresults of the control experiments revealed that the upwardmigration of VOC vapors was affected by the degree of watersaturation. In addition to increasing tortuosity, an increase inwater saturation retarded themigration of VOC vapors throughdissolution of VOCs (particularly ethanol) in the water phase ofthe layer. The results of the reactive experiments showed thatwater saturation had a strong effect on the removal capacity ofthe reactive layer. We observed a high removal efficiency andreactivity of the layer for all target compounds at the highestwater saturation (Sw = 0.6). The change in pH of the waterphase during the oxidation of VOCs was found to be the mainreason for a reduction in the oxidation rate of the permeablereactive layer.

The developed model for reactive vapor transport, whichincluded pH-dependent oxidation rates, was able to satisfac-torily simulate the experimental data for toluene and TCE.For ethanol we found increasing discrepancy between thesimulation results and experimental data with increasingwater contents. This was attributed to the fact that, due to thehigh solubility of ethanol in water, the vapor concentration ofthe air space between the ethanol pool and the reactive layervaried with time. We did not account for these variations.Instead, we assumed a constant ethanol vapor concentrationto correspond to 80% of the saturated vapor. To improve thesimulations for ethanol, the variation of the ethanol vaporconcentration with time at the inlet boundary should beaccounted for. This would require simulating additionalprocesses, such as the evaporation of ethanol from the liquidpool, evaporation of water from the permeable reactive layerand its partitioning into the ethanol pool, and estimatingthe equilibrium vapor concentration of the air phase belowthe permeable layer. We were not able to monitor theseprocesses in our current setup. Moreover, the effect of theaccumulated by-products and possible interactions with eachother on the oxidation rate of ethanol should be considered.

Simulation results revealed that the total oxidized mass ofethanol vapor was higher than TCE and toluene for identicalwater saturations, despite a larger oxidation rate constant forTCE than for ethanol and toluene (Mahmoodlu et al., 2014;Waldemer and Tratnyek, 2006). This is due to the fact that

TCE and toluene oxidation is more affected by a change in pHof the water phase during their oxidation than ethanol.

A rough estimate of the field scale longevity of a permeablereactive layer suggests that horizontal permeable reactivebarriers can provide a viable option for preventing VOC vaporsfrom reaching the land surface. However, the performance ofsuch a methodology can be affected by various environmentalfactors such as pH, soil organic matter, minerals, temperature,soil type, heterogeneity, and particularly the presence of soilwater.

Acknowledgments

The authors would like to thank Rien van Genuchten(Federal University of Rio de Janeiro, Brazil), Emilio RosalesVillanueva (University of Vigo, Spain), Mart Oostrom (PacificNorthwest National Laboratory, USA), Kótai László (HungarianAcademy of Sciences, Hungary) and Tom Bosma (UtrechtUniversity, The Netherlands) for providing critical commentsthroughout the course of this research. The comments by thethree anonymous referees further helped to improve themanuscript. This work was supported by the Ministry ofScience, Research and Technology of Iran.

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