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Chemical Engineering Journal 178 (2011) 204– 209
Contents lists available at SciVerse ScienceDirect
Chemical Engineering Journal
jo u r n al hom epage: www.elsev ier .com/ locate /ce j
eolites in a permeable reactive barrier (PRB): One year of field experience in aefinery groundwater—Part 1: The performances
odolfo Vignolaa, Roberto Bagatina,∗, Alessandra De Folly D’Aurisb, Cristina Flegoc, Massimo Nalli c,anila Ghisletti c, Roberto Millini c, Raffaello Sistod
Department of Environmental Technologies, eni S.p.A., Istituto eni Donegani, Via G. Fauser 4, I-28100 Novara, ItalyDepartment of Chemistry, La Sapienza Università di Roma, Piazzale A. Moro 5, I-00185 Roma, Italyeni S.p.A., Refining & Marketing Division, San Donato Milanese Research Center, Physical Chemistry Department, Via F. Maritano 26, I-20097 S. Donato Milanese, ItalyStudies & Researches Department, Renewable Energy & Environment Technologies, eni S.p.A. Piazzale E. Mattei 1, I-00144 Roma, Italy
r t i c l e i n f o
rticle history:eceived 3 August 2011eceived in revised form 14 October 2011ccepted 18 October 2011
eywords:eoliteSM-5ordeniteater treatment
dsorption properties
a b s t r a c t
This paper describes the performances of zeolites utilized for one year as adsorbents in a permeablereactive barrier (PRB) located under a coastal refinery. The average organic contamination was 5 mg/L oftotal petroleum hydrocarbons, whose toxic constituents are aromatic (BTEX) and polynuclear aromatichydrocarbons (PAHs), and 5 mg/L of MTBE, specific gasoline additive, while the average concentrationsof more representative inorganic ions were: Na+, 8537 mg/L; Cl−, 10,700 mg/L. The target of the entireprocess was: � hydrocarbons, 350 �g/L; MTBE, 10 �g/L; and BTEX, 1, 15, 50 and 10 �g/L, respectively.Two working tests of six months each were performed. In the first phase two zeolite filters, constituted byZSM-5 (120 kg) and mordenite (150 kg), were used to treat water with a flow (Q) kept fixed at 4 m3/d. Inthe second phase next each filter a new one of same material has been added to have a total of four filters,two of ZSM-5 (120 kg each) followed by two of mordenite (150 kg each), while the flow was doubled. In
both tests the concentration of the organic contaminants in the outlet water remained constantly underthe limits. Between the two phases, the test was interrupted for six months for removing by backwashingthe inorganic and biological deposits formed on the filters. The performances of zeolites were evaluated onthe basis of the composition of both water sampled at the outlet of zeolite filters (by gas-chromatographicanalysis, GC–MS) and contaminants adsorbed in the zeolite channels (by thermogravimetric analysis, TGAand GC–MS).. Introduction.
A permeable reactive barrier (PRB) consists essentially of aermeable diaphragm of a reactive material, placed in situ trans-ersely to the flow of a polluted plume, capable of degrading oremoving contaminants, transported by groundwater during itsatural movement. This technology replaces, where the conditionsre favourable, the conventional “pump and treat” technology,n which the plume is extracted from groundwater by pumpingnd then sent to a dedicated water treatment. The configurationsormally utilized in PRB technology are “funnel and gate” andontinuous barrier. In the first case the water is conveyed by anmpermeable diaphragm (funnel) to the permeable reactive zone
gate), while in the second one the reactive material is distributedcross the width of the contaminated groundwater plume. A vari-nt of the “funnel and gate” consists of the “drain and gate”, where∗ Corresponding author. Tel.: +39 0321447310; fax: +39 0321447506.E-mail address: [email protected] (R. Bagatin).
385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.cej.2011.10.050
© 2011 Elsevier B.V. All rights reserved.
a drain captures the portion of groundwater containing the plume,including the conveyance, and facilitates the test area, thus allow-ing a reduction of the extent of the barrier itself.
The key component in PRB technology is constituted by the reac-tive material, which should be selected on the basis of the natureof the contaminants to be removed and on the hydro-geologicalconditions of the site. Zero-valent iron (ZVI) is the most commonmaterial used in PRBs, followed by granulated activated carbon(GAC). Other examples of reactive materials include microorgan-isms, natural zeolites, peat, phosphates, limestone and amorphousferric oxide [1].
Depending on the nature of the material, the elimination of thecontaminants occurs by different processes such as degradation,adsorption or precipitation. In any case, the effectiveness of thematerials depends on their physical–chemical properties as well ason the nature of the compounds to be removed.
Particularly important is the problem related to the decon-tamination of groundwater from hydrocarbons, chlorinatedhydrocarbons and oxygenates (e.g. MethylTertButylEther, MTBE).For such kind of compounds the most widely used reactive
neering Journal 178 (2011) 204– 209 205
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Table 1Main characteristics of zeolites utilized.
Characteristics ZSM-5 Mordenite
SiO2/Al2O3 [mol/mol] 2100 230Na2O [wt%] < 0.01 < 0.05
R. Vignola et al. / Chemical Engi
aterials seem to be ineffective. For instance, ZVI, used in PRBechnology since the years 1990s, directly degrades several con-aminants, but appears to be ineffective on irreducible compoundsuch as 1,2-dichloroethane (1,2-DCA), chlorobenzenes as well asydrocarbons [2,3]. On the other hand, the use of GAC was encour-ged by economics and by the possibility of removing a wide rangef contaminants, such as chlorinated solvents, hydrocarbons andetal ions. However, GAC presents several disadvantages due to
he physical–chemical characteristics of its surface: pore plugging,nteractions with humic substances and inorganic ions, alwaysresent in groundwater, adverse effect of pH on the adsorption ofrganics [4]. Additionally, GAC has been shown to be slightly effec-ive in treatment of water containing very soluble compounds, suchs oxygenated organics, or low molecular weight compounds, suchs vinyl chloride (VC) [5].
Removal of MTBE is challenging because this compound is par-icularly recalcitrant and different processing systems based onhemical, physical–chemical and biological features proved to becarcely effective, particularly when considering the restrictiveevels required for this contaminant, such as 350 �g/L in water,
hether treated as a hydrocarbon, or still worse, the 20–40 �g/Lor its olfactory threshold.
Surfactant modified zeolites (SMZs) have been described asighly effective for the treatment of hydrocarbon-containingastewater, although evidencing progressive surfactant release
with negative impact both on economics and environmental pro-ection) [6,7].
Encouraging results have been reported also for siliceoushydrophobic) zeolites [8].
This paper illustrates the design of the industrial demo-scaleRB in “drain and gate” configuration, according to the prescrip-ions of the environmental plan of safety of the industrial site. Theow of water drained from an existing trench, or part of it, instead ofeing removed by “pump and treat” processes outside the ground,rossed a gate formed by specific reactive zeolites, thus creating
“drain and gate” PRB configuration. The system was placed justelow the ground level, but higher than the groundwater table, soater flew due to a siphon structure whose operation is ensured
y the passive hydraulic upstream – downstream gradient. Dur-ng the design phase the sources of pressure losses have beeneduced to a minimum and devices have been used to minimizehe possible formation of gas that could put at risk the continu-ty needed for passive hydraulic operation. As drain of the PRBn existing trench was employed, while the gate (reactive ele-ent) was performed using two pairs of filters, each containing
n adsorbent material selected according to its specificity towardsifferent classes of contaminants present: a first zeolite suited forhe adsorption of aliphatic and light aromatic components and aecond zeolite useful towards larger hydrocarbons and MTBE. Theour filters can be operated in series or in parallel and are sized toreat a range of 4 to 8 m3/day (depending on configuration). Thedvantages of zeolite application concern with the organic con-aminant selectivity, the rapid kinetics, the lack of interferencesith salt and humic substances (also at tens g/L) and, mainly, the
ffective capacity in the removal of almost all the organic com-ounds present in groundwater of petrochemical and petrol sites.owever, such prerogatives were demonstrated only in labora-
ory works, where the experimental conditions and especially theuration are limited and under constant control [2,8,9]. The effec-iveness of the zeolites is ensured by their regular channels able toelectively adsorb organic molecules with dimension and polarityomparable with those of the contaminants to be removed [10].
pecific zeolites were identified for efficient removal of MTBE, 1,2-CA and VC. Particularly, ZSM-5 zeolite turned out suitable forliphatic, halogen-aliphatic and mono-aromatic molecules, suchs BTEX and halogen–benzene derivatives, while mordenite wasBinder [wt%] Clay (20) Alumina (20)Pellet size (diameter × length) [mm] 1.5 × 3 1.5 × 3
found more appropriate for molecules with two or more aromaticrings, eventually halogen- and alkyl-substituted, and ethers suchas MTBE. Furthermore, these microporous adsorbents are proposedfor the decontamination of groundwater with the use of permeablereactive barriers. To assure the high permeability necessary for thefunctioning of the PRB, zeolites were not used in form of powder,but in form of extrudates.
Filters containing the two hydrophobic zeolites, ZSM-5 andmordenite with high SiO2/Al2O3 molar ratio, placed in succession,constituted an innovative adsorption system suitable for the in situabatement of contaminants in groundwater. Its effectiveness (interms of removal of the contaminants below the target concentra-tions imposed by the Italian legislation or local authorities) wasdemonstrated during the whole period of treatment of groundwa-ter, located close to a coastal refinery. The field treatment, withthe duration of one year, was divided into two operational phaseswith different water flow: the first one to verify the material per-formance and the second one to verify the system limits. Afterthe tests, a detailed physico-chemical characterization of the usedmaterials was carried out in order to determine the effective pres-ence of contaminant molecules within the zeolite pores and toverify the modifications, if any, induced by the prolonged perma-nence of the zeolites in groundwater [11].
2. Experimental
2.1. Samples
Zeolites were supplied by Tosoh Corp. as extruded cylinders; thecharacteristics are reported in Table 1.
In both cases zeolites with high silica content, properly selectedfor their high hydrophobicity, were used. Note the use of differ-ent binders for the two zeolites: clay for ZSM-5 and alumina formordenite. In both cases, the binder is set at 20 wt%.
Zeolite samples to be characterized have been collected afterPhase B from top and bottom of each filter.
2.2. Sizing of “drain and gate”
Figs. 1 and 2 show the schematic representations (in section andin plan, respectively) of the experimental “drain and gate” PRB withthe system of reactors (gate) fed by groundwater received from thedrain. As outlined in Fig. 1, the flow of water is ensured by the siphonoperation, rather than falling piezometrically. The presence of thesiphon does not change the “passive” behaviour of the hydraulicsystem. Siphon operation allows building the room that housesthe gate near the surface, rather than submerged in groundwa-ter, with obvious advantages for both construction and operationand does not affect the integrity of the high density polyethy-lene sheet, which ensures the tightness of the drainage trench.The siphon arrangement results in the liberation of gases dissolvedin water within the reactors, which are placed in depression. Thisphenomenon is controlled by a vacuum chamber equipped with a
drain that provides both the emptying regular of the formed gasand the initial priming of the siphon. The piezometric fall to thewater downstream is obtained by implementing a well for the col-lection of the output in which the discharge is obtained through a
206 R. Vignola et al. / Chemical Engineering Journal 178 (2011) 204– 209
al PRB
stotcadndm
2
tuQ
coTcCNcr
d
fi
Fg
Fig. 1. Schematic of the experiment
uction pipe and the hydraulic level remains lower than that of therench, thanks to a pump which raises water to treatment systemf the refinery. The difference between water level of the well andhat of the trench, as low as few centimeters, is sufficient to over-ome the pressure loss due to several meters of reactive material,nd drives the water flow through the siphon. This solution wasesigned to maintain the passive properties of PRB and, simulta-eously, to meet the prescription to prevent the dispersion in theownstream aquifer. The reactive system has been sized for sixonths of operation at flow rate of 8.4 m3/day.
.3. Experimental conditions
Two operational phases, six months long each, were conductedo better evaluate the effective performance of the materialssed. The phases were characterized by different water flow (Q);
= 4 m3/day in the first one and Q = 8.4 m3/day in the second phase.Contamination – during the six months of treatment, the con-
entrations of inlet organic contaminants were almost constantlyf 5 mg/L for total petroleum hydrocarbons and 5 mg/L for MTBE.he proximity of the sea was well evidenced by a constant con-entration, always very high, of inorganic ions: Na+: 8537 mg/L;l−: 10,700 mg/L; Ca2+: 630 mg/L; SO4
2−: 26 mg/L; Mg 2+: 36 mg/L;O3
−: 10 mg/L; Fe3+: 2.6 mg/L. Output target levels: total hydro-arbons, 350 �g/L; MTBE, 10 �g/L; BTEX, 1, 15, 50 and 10 �g/L,espectively.
Adsorption system – the dimensions of treatment filters wereifferent for the two phases.
Phase A: 120 kg ZSM-5 filter (Z) followed by 150 kg mordenitelter (M) in a Z + M succession.
ig. 2. Representation of a spatial arrangement of the elements of the drain andate.
, operating siphon system reactive.
Phase B: new filters of 120 kg ZSM-5 (Z′) and 150 kg mordenite(M′) were added so each previous filter is followed by a new one ina succession Z + Z′ + M + M′. Groundwater went through filters frombottom to top (up flow).
Estimated working life of zeolites – ∼six months atQ = 8.4 m3/days.
Volatile and semi-volatile compounds were determinated by GCanalysis according to EPA methods 5021 and 3510, respectively.
Water samples to be analyzed have been collected from the inletof Z filter and at the outlets of filters Z and M (during Phase A) orfilters Z′ and M′ (during Phase B).
2.4. Instrumental analysis
The thermogravimetric analysis (TG) was carried out on groundsamples using a thermobalance mod. Hi Res 2950 of TA Instru-ments connected to a MKS system for feeding gas, both controlledby computer. The thermobalance is equipped with a quartz furnaceoperating at atmospheric pressure that can reach a temperature of1000 ◦C, with holes to allow the flushing of gas to get inside a con-trolled atmosphere; at the centre of it, is suspended a platinumcrucible containing the sample. The tests were conducted usingsample amounts of approximately 15 mg.
Air was used as the references gas at a flow of 90 sccm – initialstage: stabilization for 30 min at 30 ◦C – heating ramp of 10 ◦C/minfrom 30 ◦C to 900 ◦C.
Analysis of the vapour phase was performed by gas chro-matography coupled with mass spectrometry (HS-GC–MS), usinga GC HP-6890 Plus system, equipped with column SPB-1 (l = 15 m,ID = 0.25 mm, film thickness = 0.25 �m), on-line with a MS detec-tor Agilent-5973. GC analysis conditions were: isothermal at 60 ◦Cfor 1 min, heating 5 ◦C/min to 310 ◦C, final isotherm at 310 ◦C for5 min. The vapour phase was obtained by heating at 55 and 130 ◦Cfor 30 min in the headspace HS 7694E on-line with the GC–MSapparatus. Closed containers of 10 mL volume were utilized; theywere filled with approx. 1 g of pellets as they are in the air. The MSdetector worked at 230 ◦C, mass range m/z = 33–400, positive ions.
3. Results and discussion
3.1. Contaminant evolution and bed performances
The removal of contaminants by zeolite materials has been eval-uated during the water treatment by analyzing water samples atthe outlet of treatment filters by GC analysis. At the end of oneyear of field treatment, the effective contaminant removal was
R. Vignola et al. / Chemical Engineering Journal 178 (2011) 204– 209 207
Fig. 3. Evolution and treatment with zeolite-based system (ZSM-5 and mordenitein succession) of the most significant contaminants present in the groundwater oft(b
qnt
ocasiB
sswsfslZC
qt
Fz
Fig. 5. Benzene contamination and zeolite-PRB treatment: inlet ( ) and outletZSM-5 ( ).
he refinery: MTBE + BTEX inlet (−�−), outlet ZSM-5 (–�–) and outlet mordenite···�···); MTBE inlet ( ), outlet ZSM-5 ( ) and outlet mordenite ( );enzene inlet ( ) and outlet ZSM-5 ( ).
uantified by the presence of contaminants in the structural chan-els adsorbents by thermogravimetric analysis and the nature ofhe contaminants determined by HS-GC–MS.
The overall GC results of the zeolite-PRB treatment along thene-year field trial are summarized in Fig. 3, where time courseontamination and its abatement are reported. The first phase ofbout six months (Phase A), with Q = 4 m3/day, was followed by aecond phase (Phase B) in which the water flow was Q = 8 m3/day,n order to assess the maximum efficiency of the treatment system.etween the two phases there was a break of about six months.
During Phase A, the concentration at the exit of PRB was con-tantly kept below the target levels. At the beginning of Phase B,tarted after the stop of six months, the efficiency of the systemas fully restored after proper backwashing. The overall results
how that the channels of zeolite adsorbents are not affected byouling. As far as the MTBE removal concerns, mordenite has con-istently maintained the concentration of MTBE below the targetevel, confirming its specific adsorption of MTBE. On the other hand,SM-5 has reduced MTBE below 10 �g/L only for the first 100 days.omplete data are shown in Fig. 4.
As far as benzene concerns, Fig. 5 shows how the abatement isuite complete just after the first zeolite bed, being the concentra-ion at the exit in any case well below detection limit (0.1 �g/L).
ig. 4. Trend of the presence of MTBE in groundwater and effects of treatment witheolite-PRB: inlet ( ), outlet ZSM-5 ( ) and outlet mordenite ( ).
Fig. 6. Time course and treatmente of GROs: inlet ( ), outlet ZSM-5 ( )and outlet mordenite ( ).
So the specific adsorbent capacity of ZSM-5 towards hydrocarbonswas confirmed.
The graphics in Figs. 6 and 7 describe the evolution of gasolinerange organics (GROs), hydrocarbons from C6 to C12, and dieselrange organics (DROs), hydrocarbons from C13 to C28, before adafter the zeolite beds.
Fig. 7. Time course and treatment of DROs: inlet ( ), outlet ZSM-5 ( ) andoutlet mordenite ( ).
2 ineering Journal 178 (2011) 204– 209
a4oecwo
DgoZs5gla(atpim
3
3
oTbietwtIcfitlto
wileiocm
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zhtge(
0
200
400
600
800
1000
Tem
pera
ture
(°C
)
94
96
98
100
Mod
ified
Wei
ght (
%)
14013012011010090807060504030Time (min)
Fig. 8. TG traces of ZSM-5 samples: fresh ( ), filter Z TOP (
tuted cyclopentanes;iii) aromatic hydrocarbons: benzene, ethylbenzene, cumene,
xylenes and other mono-, bi- and tri-substituted aromatic rings,
0
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30 40 50 60 70 80 90 10 0 110 120 13 0 140
08 R. Vignola et al. / Chemical Eng
During plant operation, GROs inlet concentrations ranged from minimum of 1300 to a maximum of 10,000 �g/L (mean value900 �g/L) and DROs between a minimum of �g 20 to a maximumf 280 �g/L (mean value 124 �g/L). The action of the two differ-nt zeolites is clear: ZSM-5 is specific for hydrocarbons, since itan reduce of about 90% the total petroleum hydrocarbons (TPHs),hile mordenite maintains their concentration well below the limit
f 350 �g/L.Examining separately the process of adsorption of GROs and
ROs in succession on the two zeolites, it can also be highlighted thereater relative efficacy of the ZSM-5 on GROs and of the mordeniten DROs. The average removal efficiency of GROs is in fact 96% onSM-5 and 78% of the residual fraction on mordenite. For DROs theituation is reversed: the average removal efficiency is 41% on ZSM-
and 71% of the residual fraction on mordenite. This behaviour is inood agreement with the expected performances from the two zeo-ites, being ZSM-5 ideally suited for adsorption of small moleculesnd mordenite more suitable for adsorption of larger hydrocarbonsand MTBE). At the end of the one-year field trial, zeolite materi-ls were sampled from filters and examined in order to verify thereatment effects on their physical–chemical characteristics, and inarticular the properties that could modify their adsorption capac-
ty, in order to have indication on the longevity of the adsorbentaterials.
.2. Characterization of the adsorbed organic compounds
.2.1. Thermogravimetric analysisThermogravimetry was employed for determining the amount
f contaminants adsorbed in zeolite samples at the end of the tests.he interpretation of the analytical data was however affectedy the continuous and significant weight loss during the initial
sotherm at 30 ◦C. Assuming that this weight loss is associated to thelimination of adsorbed water and that the amount of organic con-aminants released at low temperature is negligible (being trappedithin the zeolite pores), the initial weight of each sample was
aken as measured at the end of the 40 min long isotherm at 30 ◦C.t was also reasonably assumed that the adsorbed molecules wereompletely released at the end of the analysis (900 ◦C) and that thenal weight corresponds to that of the dry adsorbent (neglectinghe weight loss associated to the decomposition of inorganic phasesikely deposited during the tests). Finally the overall weight loss inhe region 30–900 ◦C was corrected by the weight loss determinedn the fresh adsorbent measured after the same thermal treatment.
The TG curves of ZSM-5 and mordenite samples in comparisonith fresh materials are shown in Figs. 8 and 9. Since water was
mposed to flow from the bottom, two different samples were col-ected at the top and at the bottom of each column, in order tovaluate the homogeneity of the adsorption. The results, reportedn Table 2, confirm that (with the exception of the second columnf ZSM-5) the adsorption mainly occurred at the bottom of theolumns and that adsorption in ZSM-5 prevailed with respect toordenite.It should also be noted the possibility that the diffusion of con-
aminants into the channels may, in the field test, be influenced byhe concentration and composition of the complex mix of contam-nation line.
.2.2. Analysis of the organic moleculesInformation on the nature of organic species trapped within the
eolite pores were obtained by HS-GC–MS. The adsorbents wereeated at 55 and 130 ◦C (i.e. the temperatures corresponding to
he main weight losses in TG analysis) for 30 min and the evolvedas analyzed by GC–MS. The analysis carried out at 55 ◦C weressentially featureless, with only traces of aromatic compoundsC2-benzenes) detected in the ZSM-5 (used in two stages), both), filter Z bottom ( ), filter Z′ TOP ( ), filter Z′ bottom( ).
top and bottom samples. The weight loss at 55 ◦C is therefore likelydue to water release that cannot be detected by GC–MS analysis.More significant were the results obtained in the analysis of thegas produced after heating at 130 ◦C. As shown in Table 3, the rela-tive amount of organic species released at high temperature werefound to be basically dependent on the position of the sample alonggroundwater flow (ZSM-5 > mordenite and bottom > top).
As far as the nature of the organic species trapped within thepores of ZSM-5 and mordenite, it should be pointed out that thedata obtained by the HS-GC–MS analysis should considered onlyqualitative, essentially for two reasons: (i) the complexity of thepool of contaminants contained in the groundwater and (ii) therelatively low temperature (130 ◦C) does not necessarily assure thedesorption of the heavier molecules. To facilitate the discussion, thelarge number of organic species detected by HS-GC–MS analysis isgrouped in the following classes of compounds (Table 3):
i) oxygenates: ethers (with MTBE as main component), alco-hols (mainly methanol and tert-butanol) and ketones (in traceamounts);
ii) aliphatic hydrocarbons: mainly olefins (1-butene) and substi-
Time (min)
Fig. 9. TG traces of mordenite samples: fresh ( ), filter M TOP( ), filter M bottom ( ), filter M′ TOP ( ), filterM′ bottom ( ).
R. Vignola et al. / Chemical Engineering Journal 178 (2011) 204– 209 209
Table 2Amounts of contaminants adsorbed as determined by TG analysis. The adsorption capacities determined in laboratory tests are reported for comparison (data in wt%).
ZSM-5 Mordenite
Filter Z (Phase A + Ba) Filter Z′ (Phase Bb) Filter M (Phase A + Ba) Filter M′ (Phase Bb)
Top 3.77 1.86 2.38 1.52Bottom 4.32 1.82 3.37 2.07
Adsorption capacities in laboratory tests
MTBE 0.5 2.4Benzene 3.0 0.1
a Used for 1 year.b Used for 6 months.
Table 3Classes of compounds detected by HS-GC–MS in the samples (data in area% normalized to the total area of the peaks detected in each sample).
ZSM-5 Mordenite
Filter Z (Phase A + Ba) Filter Z′ (Phase Bb) Filter M (Phase A + Ba) Filter M′ (Phase Bb)
Bottom Top Bottom Top Bottom Top Bottom Top
Total relative area of the peaksc 100 69 38 15 10 6 9 2Alcohols 11 10 15 18 7 16 8 27MTBE 32 31 31 23 7 2 n.d.d 35Oxygenates (Total) 46 42 47 41 18 19 8 62Aliphatic HC’s 8 7 18 19 9 8 37 7Aromatic HC’s 33 22 19 15 71 23 49 8HC’s (total) 41 30 37 34 80 31 86 15CO2 14 20 n.d.d 7 2 50 6 23S8 1 9 16 18 n.d.d n.d.d n.d.d n.d.d
a Used for 1 year.
i
tdZwtiiwcla
4
msbobeees
[
b Used for 6 months.c Expressed as % of filter Z bottom.d Not detected.
indane, indene and, only in mordenite samples, unsubstitutedand substituted naphthalenes;
v) CO2 and molecular sulphur (S8, present only in ZSM-5 samples).
Neglecting the presence of CO2 (most likely corresponding tohat dissolved in groundwater) and molecular sulphur, the resultsemonstrate that most part of the contaminants are adsorbed bySM-5, namely the first zeolite encountered by the water flow,hile the role of mordenite is to remove the organic molecules
oo large for being adsorbed in the medium pores of ZSM-5 and,n general, to complete the removal of all the contaminants. Thiss clearly demonstrated by the data reported in Figs. 4, 6 and 7,
here it is evident that MTBE, GROs and DROs are mostly but notompletely removed in the first zeolite (ZSM-5). The second zeo-ite (mordenite) is necessary in this case for finishing the process,bsorbing most of the residual contaminants escaped from ZSM-5.
. Conclusions
The evidences here collected confirm that zeolites are able toaintain the contaminant concentration at the exit of PRB con-
istently below the target levels and that their channels are notlocked by organic deposits. It was also validated the specificityf ZSM-5, more suitable for the adsorption of light hydrocar-ons, and of mordenite, whose structural features make it more
ffective against heavy hydrocarbons and MTBE. However, thexperience has also evidenced that devices for the removal of for-ign bodies must be taken into account in the design of futureystems.[
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