Evaluation for the Removal E ﬃ ciency of VOCs and Heavy Metals by Zeolites-Based Materials in the Wastewater: A Case Study in the Tito Scalo Industrial Area

: The current study was designed to demonstrate the e ﬃ ciency of selected zeolites in the immobilization of heavy metals and volatile organic compounds from water in the industrial area of Tito Scalo (Basilicata Region in Southern Italy). The e ﬃ ciency of zeolite materials has been evaluated by analyzing real water samples, by a multi-technique approach. Gas chromatography (GC) and inductively coupled plasma optical emission spectrometry (ICP-OES) were selected for the detection of volatile organic compounds (VOCs) and heavy metals respectively, and then by thermal analysis (TG, DTA) and X-ray powder di ﬀ raction (XRD) to verify the presence of contaminants in the structural channels of the adsorbents. ZSM-5 zeolite (MFI topology) was suitable for volatile organic compounds, showing removal e ﬃ ciencies > 87%. 13X (FAU topology) was more selective for in situ abatements of heavy metals, with e ﬃ ciencies up to 100%. After VOCs and heavy metals removal, structure reﬁnements of loaded zeolites highlighted variations of both lattice parameters and extraframework content conﬁrming the pollutants immobilization in the framework microporosities. The occurrence of these species was also conﬁrmed by DTA curves showing di ﬀ erent phenomena explained on the basis of the nature and number of extraframework species hosted in the zeolite micropores.


Introduction
Contamination of surface and groundwater is considered a global emergency due to the very fast development of industries, the badly conceived urbanization, as well as the agricultural practices. Being frequently a recipient for these contaminants, the aquatic ecosystem is seriously deteriorated and sometimes hopelessly compromised by the occurrence of natural and human-induced chemicals. A wide range of hazardous pollutants is released from nonpoint broadly distributed sources, including bioaccumulative and toxic chemicals (i.e., heavy metals such as lead, mercury, and chromium) and persistent and mobile organic compounds (PMOCs) (i.e., volatile organic compounds (VOCs), methyl-tertbutylether (MTBE), short-chain perfluoroalkyl acids (PFAAs), and tris (2-chloroethyl) phosphate (TCEP)) [1][2][3][4]. Their presence at trace and ultra-trace levels might cause the development of sub-lethal toxic effects in water and groundwater systems [5] due to the difficulty associated with pollutants removal from water systems, the variety of source sizes and shapes, the pollutants fast mobility, water solubility, and sorption tendency. Spatial and temporal variations in water chemistry are mandatory to protect and enhance the status of aquatic ecosystems, as well as to bring applicable solutions to the subject.
In the present work, we undertook a study of the industrial area of Tito Scalo which is designated as "Siti di Interesse Nazionale (SIN)" (contaminated Sites of National Interest) and, according to Italian law, its management is entrusted to the national authorities [6]. This contaminated site is characterized by the presence of groundwater containing chlorinated solvents (trichlorethylene, 1,2-dichloroethylene, 1,1-dichloroethylene, and vinyl chloride), metals (As, Fe, and Mn), and sulfates in concentrations above the reference threshold (CSC). In order to control both heavy metals and VOCs in this site, we have selected zeolites with different silicon vs. aluminum molar ratio (SAR) as a function of pollutants composition and nature. Zeolites represent a benchmark in the area of porous materials, due to their efficiency as ion exchanger (i.e., heavy-ion removal) and organic adsorbent media. The three-dimensional networks containing well-defined channels and cavities imparts high porosity and a large surface area to these materials whose selectivity and activity towards guest pollutant species can be modulated by acting on their structure and chemical features. Additionally, rapid sorption kinetics, no salt and humic substances interference, high stability in water during prolonged periods of time and against aggressive chemicals make them a viable alternative to other reactive media. The ability of zeolites with a low Si/Al ratio to remove heavy metal ions, such as Cd 2+ , Pb 2+ , Ni 2+ , Cu 2+ , and Zn 2+ by ionic exchange has been largely demonstrated to produce drinking water [7][8][9][10][11]. Besides, zeolites characterized by SAR values higher than 5 mol/mol are more hydrophobic materials which are widely used in adsorption-related applications [12][13][14][15][16].
This study, investigated at pilot scale and under real environmental conditions, provides a rapid and low-cost solution for the decontamination of the Tito Scalo polluted streams by the identification of efficient selective adsorbent media able to immobilize the contaminating species. In particular, ZSM-5 zeolite (MFI topology) turned out to be suitable for chlorinated solvent molecules, while 13X (FAU topology) was more selective for in situ abatements of heavy metals. The efficiency of zeolite materials has been evaluated during the water treatment by analyzing real water samples, by gas chromatography (GC) and inductively coupled plasma optical emission spectrometry (ICP-OES) for VOCs and heavy metals respectively, and then verifying the presence of contaminants in the structural channels of the adsorbents by thermal analysis (TG, DTA) and X-ray powder diffraction (XRD).

Sampling
The SIN is located about 4 km from the village of Tito (Basilicata, Italy) ( Figure 1) within an area characterized by intense industrial activities. The samples of polluted groundwater were collected in piezometric wells located in three different areas inside the SIN (Figure 1). Before the sampling activity, each piezometric well was purged. The low-flow sampling of groundwater was carried out in dynamic mode by means of an electro-submersible pump. The samples were collected using 1 L glass bottles with pressure caps ensuring that there was no gap between the sample and the cap. The samples were fixed using hydrochloric acid and stored in a refrigerator at 4 • C until the analysis. 50 mL of groundwater was collected in plastic bottles for metals determination. The samples were analyzed within ten days after collection. References about thermodynamic studies pertaining to selected VOCs (e.g., t-amyl methyl ether, ethyl t-butyl ether, di-isopropyl ether, n-hexane, fluorobenzene) are available in the literature [17][18][19][20][21][22][23][24][25][26][27].

Adsorption Experiments
The adsorption of the contaminants was investigated using the batch method. Adsorption experiments were carried out in 20 mL crimp top reaction glass flasks sealed with PTFE septa (Supelco, PA, USA). In the case of VOCs adsorption experiments, the flasks were filled in order to have the minimum headspace. For VOCs, a solid/solution ratio of 4:1 (mg mL −1 ) was employed, whereas for heavy metals the solid/solution ratio was 1:1 (mg mL −1 ). After 24 h equilibration at a temperature of 25.2 ± 0.5 °C under stirring, the zeolites were separated from the solution by filtration using 25 mm syringe filters with PVDF membrane 0.45 μm (Agilent Technologies, Santa Clara, CA, USA). The initial concentrations (C0) and the concentrations at equilibrium (Ce) of the contaminants in the water samples were determined (by ICP-OES for metals and HS-GC-MS for VOCs) and the adsorbed quantities were calculated with the following equation: where q (mg g −1 ) is the contaminant uptake (mg g −1 ), C0 is the initial contaminant concentration in the solution (mg L −1 ), Ce the contaminant concentration in the solution at equilibrium (mg L −1 ), V is the solution volume (L) and m is the mass of adsorbent (g). In the present work, the adsorption isotherm

Sampling
The SIN is located about 4 km from the village of Tito (Basilicata, I characterized by intense industrial activities. The samples of polluted g piezometric wells located in three different areas inside the SIN (Fi activity, each piezometric well was purged. The low-flow sampling of in dynamic mode by means of an electro-submersible pump. The sam

Adsorption Experiments
The adsorption of the contaminants was investigated using the batch method. Adsorption experiments were carried out in 20 mL crimp top reaction glass flasks sealed with PTFE septa (Supelco, PA, USA). In the case of VOCs adsorption experiments, the flasks were filled in order to have the minimum headspace. For VOCs, a solid/solution ratio of 4:1 (mg mL −1 ) was employed, whereas for heavy metals the solid/solution ratio was 1:1 (mg mL −1 ). After 24 h equilibration at a temperature of 25.2 ± 0.5 • C under stirring, the zeolites were separated from the solution by filtration using 25 mm syringe filters with PVDF membrane 0.45 µm (Agilent Technologies, Santa Clara, CA, USA). The initial concentrations (C 0 ) and the concentrations at equilibrium (C e ) of the contaminants in the water samples were determined (by ICP-OES for metals and HS-GC-MS for VOCs) and the adsorbed quantities were calculated with the following equation: where q (mg g −1 ) is the contaminant uptake (mg g −1 ), C 0 is the initial contaminant concentration in the solution (mg L −1 ), C e the contaminant concentration in the solution at equilibrium (mg L −1 ), V is the solution volume (L) and m is the mass of adsorbent (g). In the present work, the adsorption isotherm was not estimated, since the real samples contained a variety of different compounds that could be characterized by different adsorption properties. In addition, competitive phenomena could also modify the single-component adsorption. Therefore, to investigate the capability of zeolite in the removal of a complex mixture of contaminants, we evaluated the adsorbed quantity with respect to the total VOC concentration (see Section 2.5). For the adsorption of VOCs onto zeolites, different isotherm models have been employed [30][31][32][33][34][35][36].

Inductively Coupled Plasma Optical Emission Spectrometry
The concentration of As, Fe, and Mn in the water samples, before and after the contact with zeolite 13X, was determined by ICP-OES (Perkin-Elmer Optima 3100 XL, Inc. 940 Winter Street Waltham, MA 02451 USA) (axial view) equipped with a solid-state charge-coupled device detector (CCD), a peristaltic pump and a low-flow GemCone nebulizer coupled to a cyclonic spray chamber. Analytical lines 188.98 nm, 259.94 nm, and 293.30 nm were selected for the quantitative determination of As, Fe, and Mn respectively. The ICP-OES measuring parameters were: plasma flow = 15 L min −1 , auxiliary flow = 0.5 L min −1 , nebuliser flow = 0.65 L min −1 , RF power = 1350 W.

Gas Chromatography
The concentration of VOCs in the aqueous solution before and after the contact with zeolite ZSM-5 was determined by Headspace Gas Chromatography coupled to Mass Spectrometry (HS-GC-MS). The analyses were carried out with an Agilent GC-MS system (Santa Clara, CA, USA) consisting of a GC 6850 Series II Network coupled to a Pal G6500-CTC injector and a Mass Selective Detector 5973 Network.
The injected solutions consisted of 10 mL of sample solutions containing 1.8 g of NaCl and 10 µL of 500 mg L −1 of fluorobenzene in methanol as an internal standard. An HP-VOC GC column (L = 30 m, I.D. = 0.2 mm, df = 1.12 µm film thickness, Agilent, Santa Clara, CA, USA) was used. The carrier gas was high purity helium with a constant flow rate of 1.0 mL min −1 . The oven temperature gradient started at 35 • C for 5 min, then ramped to 70 • C at 5 • C min −1 with a hold time of 0 min, finally it ramped 230 • C at 20 • C min −1 with a hold time of 1 min. All samples were injected in split mode (10:1).
The total VOCs concentration in each sample was determined according to the procedure proposed by the Italian Institute for Environmental Protection and Research (ISPRA) [37] by considering the area of the chromatographic peaks in a specific retention window, from 4.6 to 17.2 min which was the retention times for 2-methylpentane and decane respectively. The sum of the areas of the peaks within the selected retention window was expressed as a concentration of n-hexane through external calibration to represent the total VOC concentration. The areas of the peaks of ethers (MTBE, ETBE, TAME, and DIPE) were subtracted from the total, if present.

Quality Assurance
The limit of detection (LOD) for the analytes (VOCs, As, Fe and Mn), calculated from the blank value plus three times the standard deviation of the 10 replicates of blank solutions [38], were 58.94 µg L −1 for As, 1.36 µg L −1 for Fe, 2.58 µg L −1 for Mn, and 0.17 mg L −1 for VOCs.
The procedural quality control for the analysis of the metals was verified with recovery experiments by adding a known quantity of a standard solution containing the three metals. The recovery obtained was 92 ± 11% for As, 98 ± 5% for Fe, and 96 ± 9% for Mn. For VOCs, quality control was carried out by determining the recovery of fluorobenzene which was added to each sample before analysis; the recovery obtained was 103 ± 13.6%.
Reagent blanks were analyzed to account for possible contamination during sample preparation; the reagent blanks were subtracted from the value of the concentration of the target analytes present in the samples.

Thermal Analyses
Thermogravimetric (TG) and differential thermal analysis (DTA) measurements of both loaded and bare zeolite samples were performed in constant air flux conditions using an STA 409 PC LUXX ® -Netzch (Gerätebau, Germany) operating at 10 • C/min heating rate, from room temperature (RT) to 900 • C. The tests were conducted using sample amounts of approximately 65 mg.

X-ray Powder Diffraction Data Collection and Refinement Strategy
X-ray diffraction patterns on powders of ZSM-5 and 13X zeolites before and after contact with the waters of the reference wells were carried out on a Bruker D8 Advance diffractometer (Cu Kα1,2 radiation) equipped with a Sol-X detector. Diffraction data were collected at RT, in 10-120 • 2θ ranges, with a counting time of 20 s each 0.02 • 2θ. The structural refinements were run through a full profile Rietveld analysis using the GSAS [39] software and the EXPGUI graphical interface [40]. Unit-cell and structural parameters were determined starting from the monoclinic P2 1 /n (for ZSM-5) and the cubic Fd-3m (for 13X) space groups, respectively. The Bragg peak profiles were modeled by a Pseudo-Voigt function (Thompson-Cox-Hastings) with the peak cut-off set to 0.001% of the peak maximum by means of three Gaussian terms (i.e., GU, GV, and GW, respectively), and the two Lorentzian broadening coefficients (i.e., LX, and LY, respectively) plus an asymmetry contribution. The 2θ zero shift, scale factor, and unit-cell parameters were refined. The instrumental background was empirically fitted using a Chebyshev polynomial of the first type with 18 (ZSM-5) and 20 (13X) variable coefficients data set. Soft restraints were initially imposed on tetrahedral bond distances (1.625 Å, tolerance (σ) value of 0.04 Å) and then completely removed in the final cycles. Finally, atomic coordinates, site occupancy, and isotropic atomic displacement parameters were refined. Tables 2 and 3 report the details of the data collections and Rietveld refinements. The crystal size determination was achieved by the integral breadth based calculation of volume-weighted mean crystallize sizes assuming intermediate crystallite size broadening modeled by a Voigt function.

Adsorption
The total VOCs concentration in each sample is reported in the second column (C 0 ) of Table 4. Table 4 also reports the contaminants uptake (q) and removal efficiency obtained by using zeolite ZSM-5 as adsorbent material. Analogously, Table 4 shows the initial contaminants concentration (C 0 ), contaminants uptake (q), and removal efficiency obtained from adsorption on zeolite 13X for Fe and Mn metal ions.
Volatile organic compounds, and among them chlorinated substances, are commonly found in groundwaters due to their vast use as solvents [41]. VOCs were present in all the samples analyzed in a concentration range from 0.75 to 163 mg L −1 , with an important contribution of 1,2-dichloroethylene and trichloroethylene which were identified in all the samples. Zeolite ZSM-5 has been selected in this work due to its high uptake towards VOCs [42][43][44][45][46][47][48]; in fact, it showed high removal efficiencies (>87%) for this class of contaminants present in the groundwater samples from Tito Scalo, with the adsorbed quantity (q) increasing with the increase of initial contaminant concentration in the water sample.
For concerned metals, As was absent in all the water samples, Mn was detected in all samples except one (PM28) in a concentration range between 400 and 1750 µg L −1 . Fe also was present in all samples except one (PM1), but in lower concentrations compared to Mn, from about 1.5 to 57 µg L −1 . In groundwaters, under an anaerobic environment. their presence can be caused by reducing conditions that promote the stability of Fe and Mn as soluble divalent ions [49]. For both metals, zeolite 13X proved to be an efficient adsorbent, with efficiencies up to 100%. Lower removal efficiencies (53% and 68% for Fe and Mn respectively) were obtained only for those samples showing a higher initial metal content, this decrease in the removal efficiency may be due to the saturation of the adsorbent material towards these metals.

Structural and Thermal Characterization
Structural and thermal characterization of the adsorbents at the end of the adsorption test confirmed the incorporation of heavy metals and VOC S inside the microporosities of selected zeolites. A careful examination of both positions and intensities of the X-ray diffraction peaks showed strong differences with respect to those of the as-synthetized materials, especially at low 2θ angles. Figure 2 compares the XRD patterns of the fresh zeolites with those of the samples after their permanence in polluted groundwater collected in piezometric wells located in different areas inside the SIN. Volatile organic compounds, and among them chlorinated substances, are commonly found in groundwaters due to their vast use as solvents [41]. VOCs were present in all the samples analyzed in a concentration range from 0.75 to 163 mg L −1 , with an important contribution of 1,2dichloroethylene and trichloroethylene which were identified in all the samples. Zeolite ZSM-5 has been selected in this work due to its high uptake towards VOCs [42][43][44][45][46][47][48]; in fact, it showed high removal efficiencies (>87%) for this class of contaminants present in the groundwater samples from Tito Scalo, with the adsorbed quantity (q) increasing with the increase of initial contaminant concentration in the water sample.
For concerned metals, As was absent in all the water samples, Mn was detected in all samples except one (PM28) in a concentration range between 400 and 1750 μg L −1 . Fe also was present in all samples except one (PM1), but in lower concentrations compared to Mn, from about 1.5 to 57 μg L −1 . In groundwaters, under an anaerobic environment. their presence can be caused by reducing conditions that promote the stability of Fe and Mn as soluble divalent ions [49]. For both metals, zeolite 13X proved to be an efficient adsorbent, with efficiencies up to 100%. Lower removal efficiencies (53% and 68% for Fe and Mn respectively) were obtained only for those samples showing a higher initial metal content, this decrease in the removal efficiency may be due to the saturation of the adsorbent material towards these metals.

Structural and Thermal Characterization
Structural and thermal characterization of the adsorbents at the end of the adsorption test confirmed the incorporation of heavy metals and VOCS inside the microporosities of selected zeolites. A careful examination of both positions and intensities of the X-ray diffraction peaks showed strong differences with respect to those of the as-synthetized materials, especially at low 2θ angles. Figure 2 compares the XRD patterns of the fresh zeolites with those of the samples after their permanence in polluted groundwater collected in piezometric wells located in different areas inside the SIN.  In 13X, the cubic Fd-3m symmetry was maintained but the refined lattice parameters and unit-cell volume (V) was remarkably decreased (Table 2). A sensible variation of diffraction profile width was noticed in loaded samples, confirmed by the full width at half maximum (FWHM) values as a function of 2theta ( • ) as reported in Figure 3a. In 13X, the cubic Fd-3m symmetry was maintained but the refined lattice parameters and unitcell volume (V) was remarkably decreased (Table 2). A sensible variation of diffraction profile width was noticed in loaded samples, confirmed by the full width at half maximum (FWHM) values as a function of 2theta (°) as reported in Figure 3a. The average grain sizes of the 13X samples decreased from 120 Å in the fresh sample to about 500 Å. No noticeable distortion of the framework was highlighted and the channel's geometry remained about circular as attested by ellipticity, ε (ε = largest/shortest oxygen-oxygen bond distances), and the Crystallographic Free Area, CFA (calculated assuming an oxygen ionic radius of 1.35 Å), parameters, reported in Table 5. After heavy metal-exchange, similar wt. % losses during TG analysis (25-29 wt. %) were observed in the 13X sample: ~3 wt.% losses in the 25-100 °C range due to physically adsorbed water, 22~26 wt.% losses in the 100-900 °C range [8]. These differences can be explained on the basis of the nature and number of metal exchangeable cations hosted in the zeolite micropores (being Si/Al ratio constant) and, consequently, to the nominal charge (Z) and radius (r) of the charge-compensating cations. These two latter factors are normally combined in the ratio Z/r which gives a measure of the ionic potential of exchangeable extraframework (EF) cations.
The DTA curves showed different phenomena occurring with respect to the unloaded samples ( Figure 4. The broad exothermic and not well-defined signal observed above 200 °C, with a maximum at ~500 °C, has been usually related to the loss of the hydroxyl groups that compose the zeolite structure. At higher temperature, another exothermic peak was taken as a reference of zeolites thermal stability and was attributed to the collapse of the zeolite structure and/or to the The average grain sizes of the 13X samples decreased from 120 Å in the fresh sample to about 500 Å. No noticeable distortion of the framework was highlighted and the channel's geometry remained about circular as attested by ellipticity, ε (ε = largest/shortest oxygen-oxygen bond distances), and the Crystallographic Free Area, CFA (calculated assuming an oxygen ionic radius of 1.35 Å), parameters, reported in Table 5. After heavy metal-exchange, similar wt% losses during TG analysis (25-29 wt%) were observed in the 13X sample:~3 wt% losses in the 25-100 • C range due to physically adsorbed water, 22~26 wt% losses in the 100-900 • C range [8]. These differences can be explained on the basis of the nature and number of metal exchangeable cations hosted in the zeolite micropores (being Si/Al ratio constant) and, consequently, to the nominal charge (Z) and radius (r) of the charge-compensating cations. These two latter factors are normally combined in the ratio Z/r which gives a measure of the ionic potential of exchangeable extraframework (EF) cations.
The DTA curves showed different phenomena occurring with respect to the unloaded samples ( Figure 4). The broad exothermic and not well-defined signal observed above 200 • C, with a maximum at~500 • C, has been usually related to the loss of the hydroxyl groups that compose the zeolite structure. At higher temperature, another exothermic peak was taken as a reference of zeolites thermal stability and was attributed to the collapse of the zeolite structure and/or to the recrystallization into a new phase [50][51][52][53]. These results suggested that the 13X thermal stability increases with increasing ionic radii of the ions in the channel-void system and, consequently, the framework distortion by electrostatic interactions, decreases with the increase in ionic radius. recrystallization into a new phase [50][51][52][53]. These results suggested that the 13X thermal stability increases with increasing ionic radii of the ions in the channel-void system and, consequently, the framework distortion by electrostatic interactions, decreases with the increase in ionic radius. For ZSM-5 samples after VOCs adsorption, a comparison of the XRD patterns before and after their permanence in polluted groundwater highlighted relevant changes in the peak intensities in the low 2θ region but slight modifications of their position. Moreover, the absence of reflection peaks which are forbidden in the monoclinic P21/n symmetry revealed that this symmetry was maintained. The unit cell parameters were not remarkably modified (Tables 2 and 3) but a noticeable distortion of the framework was highlighted (Table 5) by the Rietveld crystal structure refinement, clearly indicating a remarkable change in the ellipticity and crystallographic free area of both straight and sinusoidal channels (Table 5). No noticeable variations in the FWHM values as a function of 2θ (°) were detected and the average grain sizes were ~100 Å before and after VOCs adsorption.
TG analyses showed a first weight loss associated with the elimination of species (water and/or VOCs) weakly bonded to the surface (25-100 °C, weight loss 2.0%), and a second one (100-900 °C, weight loss ~1 wt.%). In all cases, the last weight loss was related in DTA analyses to two exothermic reactions ascribed to VOCs decomposition ( Figure 4). Similar results were already observed in the same ZSM-5 sample after adsorption of BTEX and/or VOCs from water [42][43][44][45][46][47][48].
The occurrence of both structural modifications coupled with information on the phase's thermal behavior strongly suggests that both selected zeolites are efficient in the pollutants removal from water.
These results suggested that filters containing the ZSM-5 and 13X, placed in succession, can constitute an efficient adsorption system suitable for the in situ abatement of contaminants in groundwater, such as in the Permeable Reactive Barriers system (PRB). This water remediation technology is often preferred with respect to thermal desorption, groundwater pump-and-treat systems, biodegradation, chemical oxidation, and reduction because of the significant technical and economic advantages, as well as low impacts on the environment [54][55][56][57][58][59][60][61][62][63][64][65][66][67]. With respect to other remediation technologies, the PRB option represents the optimum solution in longevity and cost and does not suffer from back diffusion and rebound issues [68][69][70][71]. It has been reported that the PRB system could be installed efficiently with about 60% cheaper cost than that of the equivalent Pump and Treat system [72][73][74][75][76][77].
On the basis of this pilot-scale test, a multi-layered PRB could be constructed by combining different reactive materials (13X and ZSM-5). These reactive adsorbent media may successfully remove a wide variety of contaminants, including heavy metals, chlorinated solvents, and aromatic hydrocarbons. For ZSM-5 samples after VOCs adsorption, a comparison of the XRD patterns before and after their permanence in polluted groundwater highlighted relevant changes in the peak intensities in the low 2θ region but slight modifications of their position. Moreover, the absence of reflection peaks which are forbidden in the monoclinic P2 1 /n symmetry revealed that this symmetry was maintained. The unit cell parameters were not remarkably modified (Tables 2 and 3) but a noticeable distortion of the framework was highlighted (Table 5) by the Rietveld crystal structure refinement, clearly indicating a remarkable change in the ellipticity and crystallographic free area of both straight and sinusoidal channels (Table 5). No noticeable variations in the FWHM values as a function of 2θ ( • ) were detected and the average grain sizes were~100 Å before and after VOCs adsorption.
TG analyses showed a first weight loss associated with the elimination of species (water and/or VOCs) weakly bonded to the surface (25-100 • C, weight loss 2.0%), and a second one (100-900 • C, weight loss~1 wt%). In all cases, the last weight loss was related in DTA analyses to two exothermic reactions ascribed to VOCs decomposition ( Figure 4). Similar results were already observed in the same ZSM-5 sample after adsorption of BTEX and/or VOCs from water [42][43][44][45][46][47][48].
The occurrence of both structural modifications coupled with information on the phase's thermal behavior strongly suggests that both selected zeolites are efficient in the pollutants removal from water.
These results suggested that filters containing the ZSM-5 and 13X, placed in succession, can constitute an efficient adsorption system suitable for the in situ abatement of contaminants in groundwater, such as in the Permeable Reactive Barriers system (PRB). This water remediation technology is often preferred with respect to thermal desorption, groundwater pump-and-treat systems, biodegradation, chemical oxidation, and reduction because of the significant technical and economic advantages, as well as low impacts on the environment [54][55][56][57][58][59][60][61][62][63][64][65][66]. With respect to other remediation technologies, the PRB option represents the optimum solution in longevity and cost and does not suffer from back diffusion and rebound issues [67][68][69][70]. It has been reported that the PRB system could be installed efficiently with about 60% cheaper cost than that of the equivalent Pump and Treat system [71][72][73][74][75].
On the basis of this pilot-scale test, a multi-layered PRB could be constructed by combining different reactive materials (13X and ZSM-5). These reactive adsorbent media may successfully remove a wide variety of contaminants, including heavy metals, chlorinated solvents, and aromatic hydrocarbons.

Conclusions
The current study was designed to test the efficiency of 13X and ZSM-5 adsorbent media in the heavy metals and volatile organic compounds removal of polluted water in the industrial area of Tito Scalo (Basilicata Region in Southern Italy). ZSM-5 zeolite was shown to be suitable for volatile organic compounds (in particular 1,2-dichloroethylene and trichloroethylene), showing high removal efficiencies (>87%). 13X was more selective for in situ abatements of heavy metals with efficiencies up to 100%.
The effective incorporation of VOCs and heavy metal species was confirmed by XRD structure refinements by the Rietveld method (i.e., the variation of both lattice parameters and extraframework content, channels geometry). After heavy metal-exchange on 13X, DTA curves showed different phenomena which can be explained on the basis of the nature and number of metal exchangeable cations hosted in the zeolite micropores (being Si/Al ratio constant) and, consequently, to the nominal charge (Z) and radius (r) of the charge-compensating cations. The exothermic peak occurring at high temperature was taken as a reference of zeolites thermal stability and it suggested that the 13X thermal stability increases with increasing ionic radii of the ions in the channel-void system. Consequently, the framework distortion by electrostatic interactions decreases with the increase in ionic radius.
For ZSM-5 samples after VOCs adsorption, the VOCs decomposition was attested in DTA analyses by two exothermic reactions. In this case, the occurrence of both structural modifications coupled with information on the phase's thermal behavior strongly suggested that ZSM-5 was efficient in the pollutants removal from water.
These results suggested that filters containing the ZSM-5 and 13X, placed in succession, can constitute an efficient adsorption system suitable for the in situ abatement of contaminants in groundwater, such as in the Permeable Reactive Barriers system (PRB). These reactive adsorbent media can successfully remove a wide variety of contaminants, including heavy metals, chlorinated solvents, and aromatic hydrocarbons.