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Journal of Environmental Management 140 (2014) 111e119

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Journal of Environmental Management

journal homepage: www.elsevier .com/locate/ jenvman

Permeable Adsorptive Barrier (PAB) for the remediationof groundwater simultaneously contaminated by some chlorinatedorganic compounds

A. Erto a,*, I. Bortone b, A. Di Nardo b, M. Di Natale b, D. Musmarra b

aDipartimento di Ingegneria Chimica, dei Materiali e della Produzione Industriale, Università degli Studi di Napoli Federico II, P.le Tecchio 80,80125 Napoli, ItalybDipartimento di Ingegneria Civile, Design, Edilizia e Ambiente, Seconda Università degli Studi di Napoli, Via Roma 29, 81031 Aversa (CE), Italy

a r t i c l e i n f o

Article history:Received 2 August 2012Received in revised form10 March 2014Accepted 17 March 2014Available online 18 April 2014

Keywords:Permeable Reactive Barrier (PRB)AdsorptionActivated carbonGroundwater contaminationRemediation

* Corresponding author. Tel.: þ39 (0)81 7682236; fE-mail address: aleserto@unina.it (A. Erto).

http://dx.doi.org/10.1016/j.jenvman.2014.03.0120301-4797/� 2014 Elsevier Ltd. All rights reserved.

a b s t r a c t

In this paper, a Permeable Reactive Barrier (PRB) made with activated carbon, namely a PermeableAdsorptive Barrier (PAB), is put forward as an effective technique for the remediation of aquiferssimultaneously contaminated by some chlorinated organic compounds.

A design procedure, based on a computer code and including different routines, is presented as a toolto accurately describe mass transport within the aquifer and adsorption/desorption phenomena occur-ring inside the barrier.

The remediation of a contaminated aquifer near a solid waste landfill in the district of Napoli (Italy),where Tetrachloroethylene (PCE) and Trichloroethylene (TCE) are simultaneously present, is consideredas a case study. A complete hydrological and geotechnical site characterization, as well as a number ofdedicated adsorption laboratory tests for the determination of activated carbon PCE/TCE adsorptioncapacity in binary systems, are carried out to support the barrier design.

By means of a series of numerical simulations it is possible to determine the optimal barrier location,orientation and dimensions. PABs appear to be an effective remediation tool for the in-situ treatment ofan aquifer contaminated by PCE and TCE simultaneously, as the concentration of both compoundsflowing out of the barrier is everywhere lower than the regulatory limits on groundwater quality.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

The remediation of contaminated groundwater is of the utmostimportance since billions of people all over the world use it fordrinking and irrigation purposes. In particular, Tetrachloroethylene(PCE) and Trichloroethylene (TCE) are important industrial solventsand, due to their widespread use and subsequent disposal, they arecommonly found at hazardous waste disposal sites, especially inurban industrial areas (ATSDR,1997a,b). In these areas, PCE and TCEare often found mixed together in groundwater, the TCE origi-nating, in part, from the reductive de-chlorination of PCE (Vogeland Mc Carty, 1985). These compounds are persistent in natureand produce toxic and carcinogenic intermediates, too. The Italianregulatory limits for groundwater quality have recently been set at1.1 and 1.5 mg L�1 for PCE and TCE, respectively.

ax: þ39 (0)81 5936936.

Consequently, there is a pressingneed todefineeffectivemethodsfor the simultaneous removal of these compounds from contami-nated groundwater. Permeable Reactive Barriers (PRBs) appear to bea promising alternative to conventional remediation methods, suchas pump-and-treat (Rivett et al., 2006), for the in situ treatment ofpolluted groundwater. This technology consists in introducing wallsof reactive media in the subsurface in order to intercept thecontaminated water plume. If the building material is more perme-able than the surrounding aquifer materials, the groundwater flowpasses through the barrier under natural hydraulic gradient.

PRBs can be efficiently adopted as a versatile technology for theremoval of both heavy metals (Park et al., 2002; Di Natale et al.,2008; Jun et al., 2009; Han et al., 2011) and organic compounds(Lorbeer et al., 2002; Ake et al., 2003; Erto et al., 2011a).

The passive character (no energy input is required), high effi-ciency and relatively low operating and maintenance cost in thelong-term are the principal advantages of this technique. Otherbenefits include the absence of mechanical devices (such aspumps), focused cleanup only on the contaminated area,

A. Erto et al. / Journal of Environmental Management 140 (2014) 111e119112

conservation of clean water and continued productive use of thesite almost immediately after installation (Gavaskar,1999; Lee et al.,2009).

A variety of reactive materials can be used to build PRBs forchlorinated compounds remediation, mainly represented by zero-valent iron (D’Andrea et al., 2005; Higgins and Olson, 2009; Junet al., 2009; Moon et al., 2005) in which organic pollutants aredegraded through a series of reduction reactions. These reactionsare greatly unselective and may frequently lead to solid precipita-tion deriving from inorganic reductions, often co-present inpolluted groundwater, and salt precipitation. In addition, thesereactions are usually very slow at high water flow-rates; hence,barriers should be very wide in order to extend the contaminantresidence time in the system (Moon et al., 2005). The occurrence ofpreferential flows through the barrier and pore clogging due tosolid precipitation can lead to the inactivation of a part of thebarrier (Mackenzie et al., 1999; Li et al., 2006; Lee et al., 2009;Higgins and Olson, 2009).

A solution to overcome these drawbacks is using sorbent ma-terials, especially activated carbons, instead of reactive media (Akeet al., 2003; Di Natale et al., 2008; Erto et al., 2011a). Adsorption is arecognized technology for the removal of pollutants fromwater andmany different adsorbent materials can be used, including byproducts and waste materials (Akhtar et al., 2006; Di Natale et al.,2009; Balsamo et al., 2012; Molino et al., 2013; Leone et al.,2014). However, the use of activated carbon for the removal ofchlorinated compounds appears as very promising in reactivebarriers because the pollutant can be immobilized into the barrier,avoiding any precipitation phenomena (Lorbeer et al., 2002; Ertoet al., 2010b). In this sense, the Permeable Adsorptive Barrier(PAB) can be considered as a particular case of PRB made ofadsorptive material.

Despite investigations on the removal of chlorinated com-pounds with different reactive materials (Lorbeer et al., 2002;D’Andrea et al., 2005; Moon et al., 2005), very little is knownabout design and optimization of PRBs for the remediation ofgroundwater simultaneously contaminated by more than onepollutant.

In this work, for the first time an activated carbon PABwas testedfor the in-situ remediationof anaquiferwithmultiple contamination(i.e. PCE and TCE). This study is collocated in awider research activityin which thermodynamic studies on single-compound and multi-component adsorption of organic pollutants are coupled with theirpractical application to the remediation of polluted groundwaterwith PAB (Erto et al., 2009, 2010a,b, 2011a,b, 2012). In particular, in arecent work (Erto et al., 2011a), the design and optimization of a PABwas carried out dealing with a single-compound contamination (i.e.PCE)while in the presentwork the designprocedurewas specificallystructured to take into account the co-presence of PCE and TCE. Tothis end, the physical and chemical interactions among pollutedwater, aquifer and barrier, including contaminant transport andcapture by the barrier, were described by an ad hoc computer code.The barrier design was supported by dedicated laboratory tests onbinary adsorption of PCE and TCE on a selected activated carbon, toconsider thepossible interactionsbetween the twopollutants duringadsorption. Finally, the overall procedure was applied to the reme-diation of a contaminated aquifer near a solid waste landfill in thedistrict of Napoli (Italy) and themain barrier properties (i.e. location,orientation, dimensions) to capture the pollutant plume were eval-uated by optimal iterative calculation.

2. Experimental analysis of PCE/TCE binary adsorption

The design of adsorption systems mainly depends on sorbentadsorption capacity in equilibriumconditions and their performances

are likely to be influenced by the multicomponent competitive in-teractions of the compounds simultaneously present in solution (Do,1998; Erto et al., 2011b, 2012). Hence, the assessment of the adsorp-tion capacity in the co-presence of different solutes and the definitionof appropriate theoreticalmodels are necessary for a proper design ofa cost-effective remediation system.

In this study, the simultaneous contamination by PCE and TCEhas been considered. To this aim, specific laboratory tests wereperformed, as reported in the following.

2.1. Materials and methods

Aquacarb 207EA� is a commercially available non impregnatedgranular activated carbon, produced by Sutcliffe Carbon startingfrom a bituminous coal. This material has a narrow particle sizedistributionwith an average diameter of 1.2 mm, a BET surface areaof 950 m2 g�1 and a micropore volume of 0.249 cm3 g�1 mainlycentred in the pore size region of 8e18�A (Erto et al., 2013). Dry bulkdensity (rb) is about 500 kg m�3, porosity (nb) is 0.4 m3 m�3 andhydraulic conductivity is about 0.001 m s�1.

A commercial mineral water, whose composition can be repre-sentative of groundwater, was used for sample preparation. It had apH ¼ 8 and an ionic strength of 0.0046 M; a complete list of itschemical properties is reported in Erto et al. (2010a).

In all experimental runs, the aqueous solutions used in theadsorption tests were prepared by adding PCE and TCE (SigmaAldrich, 99.5%) directly to mineral water.

2.2. Laboratory test procedure

Batch adsorption tests were conducted at T ¼ 10 �C in a PIDcontrolled thermostatic oven, using 200 ml amber stained,headspace-free glass vessels of mineral water.

After equilibration, PCE and TCE concentrations in solutionweremeasured with a gas chromatograph (Agilent, GC 6890) equippedwith an electron capture detector (ECD) and a Purge & Trap system(Tekmar LSC-2000). Analytical methods comply with the EPAmethod 5030B.

The experimental run accuracy was checked by allowing for amaximum error of 8% in PCE or TCE mass balance. In order toconfirm the accuracy, reliability, and reproducibility of the collecteddata, all batch tests were performed in triplicate and average valuesonly are reported. PCE and TCE carbon adsorption capacity (ui,i ¼ 1,2) was determined by single material balance as:

ui ¼C0i � C*

im

$V (1)

where C*i is the PCE or TCE equilibrium concentration, C0

i is the PCEor TCE initial concentration,m is the activated carbon mass and V isthe solution volume.

Further details on the experimental procedure and the analyt-ical methods are reported in Erto et al. (2010a).

2.3. Experimental results

For the binary system indicated, the results of PCE and TCEadsorption tests onto Aquacarb 207EA� at the temperature of 10 �Care reported in Fig. 1.

Experimental data reveal that PCE adsorption capacity is higherthan that of TCE for all the investigated equilibrium concentrations.Moreover, it is interesting to report that a comparison with single-compound data previously available (Erto et al., 2009, 2010a,b)shows that PCE adsorption capacity depends very little on thepresence of TCE; on the contrary, TCE adsorption is proportional to

Fig. 1. PCE and TCE adsorption capacity in binary system. T ¼ 10 �C, pH ¼ 7 � 3.

A. Erto et al. / Journal of Environmental Management 140 (2014) 111e119 113

the amount of PCE present. These results are consistent with thosereported in previously available studies on the samemixture and insimilar experimental conditions (Erto et al., 2011b, 2012). For lowconcentration values of the two compounds, such as those usuallyfound in polluted groundwater, the multi-component Langmuirmodel satisfactorily interprets the experimental data. For a binarysystem, the model can be written as (Erto et al., 2011b):

u1 ¼u1;MAXK1C1

1þK1C1þK2C2u2 ¼u2;MAX

K2C21þK1C1þK2C2

(2)

where u1 and u2 are the adsorption capacity of PCE and TCErespectively, u1,MAX and u2,MAX are the correspondent maximumvalue and C1, C2 and K1, K2 are equilibrium concentrations andadsorption constants, respectively.

In addition, it is worth noticing that, based on these hypotheses,the adsorption constants of both compounds (ui,MAx and Ki) shouldbe derived individually from single-component adsorption tests, asthey represent intrinsic properties of the sorbate-sorbent couple.For the investigated compounds, adsorption data on single-compounds are available from previous works (Erto et al., 2009,2010a) and the correspondent Langmuir model parameters arereported in Table 1.

For data reported in Fig. 1, the multicomponent Langmuir modelprovides a satisfactory interpretation, as the coefficient of deter-mination (R2) results to be 0.92 and 0.79, for TCE and PCErespectively.

3. Permeable Adsorptive Barrier design

A suitable PAB design procedure starts from a thoroughknowledge of the aquifer properties, its contamination state, and acomplete hydrological and geotechnical characterization of the

Table 1Langmuir parameters for PCE and TCE single-compound adsorption onto Aquacarb207 EA� at T ¼ 10 �C and pH 7 � 3.

Langmuir Parametersu ¼ umax

KC1þKC

Value R2 P-test F-test

Mean Std error T-test

PCE umax (mg g�1) 913.9 121.9 14.60 0.9976 <0.0001 1489.8DG (kJ mol�1) �23.27 0.372 32.73K (l mol�1) 19,830 3140 11.23

TCE umax (mg g�1) 229.2 9.142 18.61 0.9926 <0.0001 2016.7DG (kJ mol�1) �25.25 1.934 43.75K (l mol�1) 46,017 4127 10.06

entire site (Gavaskar, 1999; Erto et al., 2011a). Based on this infor-mation, the localization of the barrier can be proficiently chosen.The complete design mainly consists in the definition of distancefrom the pollutant plume (E), orientation with respect to the North(b), length (L), height (H), thickness (W) and barrier adsorbingmaterial.

3.1. PAB design procedure

A preliminary choice of design parameters (i.e. E, b, L, H and W)can be made by setting the minimum size of the contaminateddomain as the one that includes the pollutant plume in its initialposition.

The barrier has to be perpendicular to groundwater flow di-rection, in order to optimize pollutant capture and to reduce theintervention on the site (Craig et al., 2006). Furthermore, it needs tobe slightly larger than the cross sectional area of the contaminatedgroundwater in order to capture the contaminants in both verticaland horizontal directions.

Once location and orientation have been chosen, the PABthickness should be designed so as to provide sufficient residencetime for the contaminants within the treatment zone to be effi-ciently removed (i.e. for adsorption process to take place). Barrierwidth can be chosen by considering the following inequality:

Wub

> ðkcaÞ�1 (3)

where kc represents the mass transfer coefficient for adsorptionreactions, a represents the external specific surface area of theadsorbent particles, that can be determined by a preliminaryadsorbent characterization. Finally, ub is the average groundwaterflow velocity through the barrier, which must be previouslydetermined by a complete hydro-geological characterization of thesite.

After, it is necessary to check whether the thickness allows for athorough pollutant capture during thewhole lifetime of the barrier.To this aim, the pollutant concentration downstream the barrier(CWi) has to be everywhere lower than the fixed limit value (Climit),usually represented by regulatory limits specific for groundwater.Eventually, as the pollutant concentration at barrier inlet may varyduring the barrier working period, the occurrence of desorptionphenomena within the barrier must also be taken into consider-ation. Adsorption continues until the pollutant concentration at thebarrier inlet remains lower than the equilibrium values corre-sponding to the amount of pollutant adsorbed on solid, but if thepollutant inlet concentration decreases, the pollutant adsorbedmay be desorbed from the barrier solid, giving rise to a contami-nated plume at the exit of the barrier itself. When desorption oc-curs, a thicker barrier ensures a lower release of the pollutantadsorbed, avoiding critical outbound concentrations. Therefore, thebarrier must be designed to both retain intense concentrationpeaks and for long-term performances, also considering theoccurrence of any desorption phenomena. Finally, the minimumdimensions of the barrier have to be identified for it to be cost-effective and to this purpose an iterative procedure, very time-consuming, must be adopted in order to optimize the barrierdimensions.

3.2. PAB modelling equations

The transport and fate of contaminants in an aquifer in presenceof a PAB, i.e. a system formed by solids (soil and barrier material)and liquid of different properties, can be described by advectionedispersion processes coupled with adsorption phenomena

Table 2Case study: aquifer characteristic, PAB properties and numerical model parameters.

Aquifer characteristicPolluted area total extent, A 1.1 km2

Aquifer bed height, H 10 mPiezometric gradient, J 0.01 m m�1

Porosity, ns 0.25Dry soil bulk density, rs 1500 kg m�3

Hydraulic conductivity, Ks 5$10�5 m s�1

Longitudinal dispersivity, aL 1 mTransverse dispersivity, aT 0.01 mPCE Molecular diffusion coefficient, D*

d1 1.02$10�9 m2 s�1

TCE Molecular diffusion coefficient, D*d2 1.04$10�9 m2 s�1

Numerical model parametersHorizontal space step, Dx 6 mTransversal space step, Dy 6 mTime step, Dt 60 days

PAB propertiesReactive media Activated carbonDry bulk density, rb 520 kg m�3

Porosity, nb 0.4Hydraulic conductivity, Kb 0.01 m s�1

Horizontal space step, Dxb Dx/100Transversal space step, Dyb DyBarrier width, W 3 mBarrier length, L 900 mDistance from contaminant plume, E 6 mBarrier distance from domain boundary, X 1266 mTransversal domain extension, Y 996

A. Erto et al. / Journal of Environmental Management 140 (2014) 111e119114

involving the media of the barrier. The basic equations of ground-water motion and dynamics under these working hypotheses arereported in the following.

The design procedure is here reported for the remediation of asite contaminated by the simultaneous presence of two pollutantsand under the hypothesis of constant pollutant concentrationprofiles throughout the height of the aquifer (i.e. along z-dimension).

For such a system, the mass transport for the two contaminantscan be described by two partial derivative differential equations(Legend for the compounds: 1 ¼ PCE, 2 ¼ TCE, used throughout thepaper):

vC1vt

þ rbnb

vu1

vtþ u!VC1

nb� VðDh1VC1Þ ¼ 0 (4)

vC2vt

þ rbnb

vu2

vtþ u!VC2

nb� VðDh2VC2Þ ¼ 0 (5)

in which C1 and C2 represent contaminant concentrations in thefluid, u! the unit flux vector, u1 and u2 the contaminant concen-trations on the solid, rb and nb the dry adsorbing material bulkdensity and porosity, respectively.

The resolution of Eqs. (4) and (5) requires estimating thediffusional mass transport due to hydrodynamic dispersion withinthe porous media. The hydrodynamic dispersion coefficients Dh1and Dh2 can be both expressed as:

Dh1 ¼ Dþ D*d1 (6)

Dh2 ¼ Dþ D*d2 (7)

in which D is the tensor of mechanical dispersion and D*d1 and D*

d2are the coefficients of molecular diffusion (scalars). The moleculardiffusion effect is generally secondary and negligible as comparedto the mechanical dispersion effect, and only becomes importantwhen groundwater velocity is very low (Bortone et al., 2013).

The unit flux vector can be determined by Darcy equation,starting from the knowledge of Ks, i.e. the hydraulic conductivity ofthe soil.

The resolution of Eqs. (4) and (5) must be coupled also withexperimental relationships that correlate the adsorption capacity ofeach pollutant with its concentration in the fluid phase, i.e. anadsorption isotherm such as the one represented by Eq. (2).Moreover, an equation representing the flow of matter (pollutant)per unit volume near the surface of the sorbent is necessary, and itcan be represented by the product of a transport coefficient, kci, aspecific area, a, and a driving force. Consequently, the second termon the left hand side of Eqs. (4) and (5) can be written as:

rbnb

vu1

vt¼ kc1a

hC1 � C*

1ðu1;C2Þi

(8)

rbnb

vu2

vt¼ kc2a

hC2 � C*

2ðu2;C1Þi

(9)

In Eqs. (8) and (9), C*1 and C*

2 are equilibrium concentrationsderiving from the adsorption isotherms of contaminants onadsorbent material, in binary system.

As to initial conditions, contaminant liquid concentration in allthe area is previously determined throughout the entire flowdomain and the initial contaminant concentration on sorbent ma-terial is assumed to be zero:

The boundary conditions are stated as follows:

C1 ¼ 0; C2 ¼ 0 x ¼ 0cyct

C1 ¼ 0; C2 ¼ 0 y ¼ 0cxct

C1 ¼ 0; C2 ¼ 0 y ¼ Y cxct

vC1vt

þ u!VC1nb

� VðDh1VC1Þ ¼ 0 x ¼ X cyct

vC2vt

þ u!VC2nb

� VðDh2VC2Þ ¼ 0 x ¼ X cyct

(10)

assuming a reference frame coinciding with the boundary of thedomain, X is the distance between the barrier and the origin, and Yis the size of the domain in y direction. A complete description ofthe set of equations for the modelling of an aquifer in the presenceof a PAB was previously reported in Erto et al. (2011a).

Many commercial computer codes can be used to support PABdesign, but no specific dedicated software has been developed yet.In this work, two computer codes were coupled in order to properlydesign the barrier, taking into account both the pollutant transportwithin the whole aquifer and the adsorption/desorption phenom-ena taking place within the barrier. The first code is a commercial3D model flow, PMWIN, that uses a block-centred finite differencescheme for the saturated zone and that includes a groundwaterflow model (MODFLOW) and a pollutant transport model (MT3D).In particular, this code allows solving the numerical integration ofEqs. (4)e(5) with the boundary conditions (10) while the secondcode, developed by the authors, ADSORP-CODE, describes theadsorption phenomena involving the pollutants when they passthrough the barrier. In particular, ADSORP-CODE solves the Eqs. (2),(8) and (9) inside the barrier with their initial and boundaryconditions.

For each barrier dimension chosen, the calculation tool allowsfor the numerical determination of the evolution of the pollutantplume over the calculation domain in time. The definition ofoptimal PAB size, i.e. minimizing its dimensions, can be obtained byiterative computer simulations (Bortone et al., 2013).

Finally, for a useful graphical presentation of data, a GIS(Geographic Information System) application, named AMBSIT�, has

A. Erto et al. / Journal of Environmental Management 140 (2014) 111e119 115

been specifically developed (by CIRIAM, Seconda Università diNapoli).

4. Case study

In this work, a contaminated aquifer near a solid waste landfillin Giugliano in Campania, a town in the metropolitan area North ofNapoli (Italy), was chosen as a case study for the application of the

Fig. 2. PCE (A) and TCE (B) iso-concentration lines for the case stud

PAB design procedure described above. Site characterization showsthe presence of a large number of pollutants, both inorganic andorganic, and in particular of PCE and TCE. The remediation of thisaquifer from the simultaneous presence of these two contaminantswas carried out by solving the groundwater contaminant transportand adsorption equations. In a previous work (Erto et al., 2011a),the same site was selected to test the design of a similar PAB for theremediation from PCE only, so that the extension to a binary system

y considered. The position of the PAB has been also reported.

Fig. 3. PCE outlet concentration at the barrier point S as function of thickness (E ¼ 6 m,L ¼ 900 m and m ¼ 90�). ( ) CIN1 e ( ) CW1 ¼ 1 m e ( ) CW1 ¼ 2 m e ( )CW1 ¼ 3 m e ( ) CW1 ¼ 4 m.

Fig. 4. TCE outlet concentration at the barrier point P as function of thickness (E ¼ 6 m,L ¼ 900 m and m ¼ 90�). ( ) CIN2 e ( ) CW2 ¼ 1 m e ( ) CW2 ¼ 2 m e ( )CW2 ¼ 3 m e ( ) CW2 ¼ 4 m.

A. Erto et al. / Journal of Environmental Management 140 (2014) 111e119116

(i.e. with PCE and TCE) is expected to provide further new in-dications on PAB applicability to polluted groundwaterremediation.

In the following, the main hydrological and geological siteproperties are reported, together with a detailed description of thestate of contamination specifically referred to the presence of PCEand TCE. Furthermore, the numerical results of the simulations arereported in terms of both contaminant concentrations and barrierdimensions. Finally, an interesting comparison with the resultsreported in the cited previous work is presented.

4.1. Site characterization

The groundwater aquifer is located at a depth of 35e40 m fromthe land surface and confined by an aquitard (50 m). An imper-meable layer is present at the bottom of the aquifer; hence a hor-izontal movement of polluted water can be predicted. Thegroundwater flux lines are East-West oriented under a piezometricgradient of 0.01 (Erto et al., 2011a).

The soil composition of Giugliano landfill area can be approxi-mated to a single mineral (Neapolitan yellow tuff) whose hydraulicconductivity is 5*10�5 m s�1. Specific tests (not reported) showedthat the adsorption capacity of this material for organic compoundscan be considered as negligible; hence, the initial solid concentra-tion can realistically be assumed to be zero throughout the entireflow domain (as previously stated).

The aquifer characteristics, the main properties of the adsorbingmaterial chosen for the barrier and the numerical parameters usedin the numerical simulations were summarized in Table 2:

In Fig. 2, a graphical representation of the site contamination,the map of the area, the pollutant iso-concentrations and the po-sition of PAB were reported. PCE and TCE are simultaneously pre-sent in the same aquifer, even if their contamination was reportedseparately for the sake of graph readability.

As can be observed, PCE concentration values change in the areawith peaks more than 20 times higher than the Italian regulatorylimits for groundwater quality, set at 1.1 mg L�1 (Fig. 2A). Similarly,TCE concentration values change in the area with peaks more than6 times higher than the Italian regulatory limits, set at 1.5 mg L�1

(Fig. 2B).In Fig. 2 the final position and size of the PABwere also reported.

Points S and P, indicated on the barrier, represent the most criticalpoints, i.e. where the highest concentration of PCE and TCE,respectively, are reached during the run time.

For the sake of simplicity, the total volume of pollutedgroundwater was assumed to be constant during the monitoringtime of the aquifer and the dissolution of further amounts ofpollutant or the dilution of its concentration due to rainfall wasneglected.

4.2. Numerical results

In this section, the numerical results of PAB design procedurewere reported. Several iterations based on different geometricalparameters of the barrier were necessary to determine the optimalposition and dimensions of the barrier.

In particular, the length and the height of the barrier mainlydepend on the extent of the contaminant plume, whose size isknown in advance. The PAB results to be a continuous trench 10 mhigh (H), 900 m long (L), put on North direction (b), at 6 m far fromthe boundary pollution (E) and penetrating the aquifer at full-depthdown to the aquitard.

Once the minimum dimensions of H, E, b, and L have beendetermined, the iterative procedure allows minimizing PAB thick-ness (W), through a trial-and-error procedure. It is worth observing

that the higher the thickness, the lower the barrier outflowpollutant concentrations e CW1 and CW2 e for PCE and TCE,respectively. Differently from the single-compound case, in a binarycontamination system the optimal thickness of the barrier corre-sponds to the higher of the two values that allows complying withthe regulatory limits simultaneously for PCE and TCE.

Fig. 3 reports PCE outlet concentration (CW1), as a function of therun time, at the most critical barrier point S for this compound (asindicated in Fig. 2), for different thickness values of PAB fromW ¼ 1 m to W ¼ 4 m. The PCE inlet concentration (CIN1) in corre-spondence of the same point was reported as well, together withthe PCE regulatory limit (Clim1), for a fair comparison with PCEoutlet concentration. In a similar way, Fig. 4 reports the results forTCE in correspondence of the barrier point P, with the TCE inletconcentration and TCE regulatory limit (CIN2, Clim2, respectively).

As can be observed, the PCE inlet concentration is expected to beover the regulatory limit for almost 60 years in the calculationdomain, while the TCE plume is higher than its limit for a shortertime. Furthermore, the great potentiality of the barrier for an

A. Erto et al. / Journal of Environmental Management 140 (2014) 111e119 117

effective remediation of the site is highlighted and, as expected, anincrease in barrier thickness always results in a reduction of theoutlet concentration, for both compounds. In particular, a 3 mbarrier, also inclusive of a safety coefficient, provides the best re-sults for PCE remediation, while for TCE a 2 m barrier would belargely sufficient. It is worth underlining again that the same site issimultaneously contaminated by PCE and TCE. Hence, the

Fig. 5. PCE iso-concentration into the aquife

minimum optimal barrier thickness allowing for the simultaneouscontrol of PCE and TCE concentrations is 3 m and is effective evenwhen inlet PCE and TCE concentration decreases and desorption ofpreviously captured PCE and/or TCE may occur.

It is very interesting to observe that a 3 mwide barrier appearedto be optimal for the treatment of the aquifer when only PCEcontamination is considered, as reported in a previous work (Erto

r as a function of the barrier run time.

A. Erto et al. / Journal of Environmental Management 140 (2014) 111e119118

et al., 2011a). Therefore, the same barrier is able to protect theaquifer also when a binary contamination is present, confirmingthe high potentialities of PAB for multiple pollution remediation.This result is really remarkable as it provides new insights on thepossibility to perform an in-situ remediation of groundwater alsowhen the contamination is even more complex, as in most realcases. Simultaneously, it highlights the great potentiality of

Fig. 6. TCE iso-concentration into the aquife

activated carbons for such applications, thanks to their good cap-ture properties and general non-selectivity for a wide number oforganic and inorganic compounds.

Once the optimal barrier parameters were defined and checkedfor both compounds in their respective most critical points, thenumerical simulations were used to calculate PCE and TCE con-centrations all over the domain investigated as a function of run

r as a function of the barrier run time.

A. Erto et al. / Journal of Environmental Management 140 (2014) 111e119 119

time in the form of contour plots, as reported in Figs. 5e6, for PCEand TCE, respectively.

Figs. 5e6 confirm the effectiveness of the designed barrier allover the polluted aquifer and over a large run time. Indeed, thedesign tool includes all the hydrological properties of the aquifer-barrier system, resulting in a slow motion of the plume towardsthe barrier. In these conditions, as the whole pollutant plumepasses through the barrier no overflow is observed at the barrierand the whole plume is captured at any time. As a final verification,in fact, downstream the barrier, PCE and TCE concentrations arealways below their respective regulatory limits. Furthermore, evenif the barrier is continuously loaded by the pollutants it is able toface the possible occurrence of desorption phenomena all over thedomain. At the end of the observation time, the barrier is notsaturated yet, hence the residual adsorption capacity can beconsidered as a further resource for aquifer protection against newcontaminations.

5. Conclusion

In this work, a Permeable Reactive Barrier built with an acti-vated carbon, namely Permeable Adsorptive Barrier (PAB), wasadopted for the remediation of a contaminated site in whichdifferent organic compounds are simultaneously present. A com-plete procedure for an accurate PAB design was presented. Startingfrom a thorough site hydrological and geotechnical characteriza-tion, the design tool includes a set of equations describing the masstransport within the aquifer and the adsorption/desorption phe-nomena inside the barrier. The procedure was applied to acontaminated aquifer near a solid waste landfill in the district ofNapoli (Italy), in which Tetrachloroethylene (PCE) and Trichloro-ethylene (TCE) are simultaneously present with concentrations farover their respective regulatory limits.

A set of dedicated laboratory adsorption tests was carried out tosupport the numerical calculations, in order to determine activatedadsorption capacity in a PCE/TCE binary system. Numerical resultslead to the optimization of barrier location, orientation and di-mensions, assuring that the concentration of PCE and TCE flowingout of the barrier is always lower than the stated regulatory con-centration limits for groundwater quality. In conclusion, PAB can beconsidered as a valid remediation technology for the in-situ treat-ment of an aquifer simultaneously contaminated by both pollutants.Moreover, the same barrier designed to capture PCE is also able tosimultaneously capture TCE, providing new insights on PAB appli-cations tomultiple contaminated groundwater both for remediationand for protection from new accidental discharge of pollutants.

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