Sustainable treatment of waters polluted with fuel-based pollutant mixtures by high silica zeolites.

Introduction Chlorinated volatile organic compounds, such as 1,1-dichloroethylene and aromatic hydrocarbons (BTX: benzene, toluene, and xylene) constitute a significant fraction of fuel-based hazardous air and water polluants [1]. The decontamination of groundwater is one of the most difficult and expensive environmental problem due to the difficulty associated with pollutants removal from water systems and to the serious health problems they can pose if allowed to enter the environment. Permeable Reactive Barriers (PRB) based on zeolites are one of the most promising passive treatment technologies for the remediation of polluted ground waters, due to their effectiveness regarding various contaminants, and their low cost compared to other in situ technologies [2,3]. In this work the efficiency of hydrophobic high silica zeolite materials (Y, MOR, and ZSM-5) for the removal of organic contaminant was tested, and the host–guest interactions occurring during adsorption processes and the process selectivity were carefully studied. In particular, evidences of 1,2-dichloroethane (DCE), methyl tert-butyl-ether (MTBE) and toluene (TOL) adsorption from dilute solutions into organophilic zeolites will be presented. A combined diffractometric (PWXRD), thermogravimetric (TGA), gas chromatographic (GC) and infrared (IR) study was used to: 1) investigate the adsorptive properties of hydrophobic synthetic zeolites; 2) compare the adsorption data for a mixture of these contaminants with concentrations in the ppb and ppm range; 3) characterise the zeolite structure after contaminants adsorption; 4) localise the organic species in the zeolite channel systems; 5) highlight the interactions between organic molecules and framework oxygen atoms. Experimental Adsorbents were hydrophobic commercial zeolites purchased with a very high silicon to aluminium ratio (SAR) and in their protonated (MOR and Y) or ammonium (ZSM-5) forms. In all cases, Na2O content was lower than 0.1% wt. Kinetics and adsorption isotherm batch data were obtained via Headspace Solid Phase Microextraction-GC. PXRD patterns were collected before and after adsorption on a Bruker D8 Advance diffractometer equipped with SOL-X detector. Thermal analyses (TG and DTA) were performed in air up to 900°C at 10°C/min. Infrared spectra were collected on a Thermo Electron Corporation FT Nicolet 5700 Spectrometer with 4 cm-1 resolution using special cells connected to high vacuum lines allowing in-situ adsorption/desorption experiments. Results and Discussion The effective adsorption in highly siliceous zeolites of single mono-aromatic molecules, volatile organic compounds [4,5] and MTBE [6,7] when dissolved in the aqueous matrix has been demonstrated and the position of these pollutants in the zeolites framework has been localized exactly by diffractometric techniques. Experimental results prove that the adsorption kinetics of all components is fast and indicate that competition exists between the organic compounds at low co-solute concentrations [4-6]. The reversibility, type and strength of the host-guest interactions (namely, H-bonding and/or van der Waals type) were defined by FTIR spectroscopy [7]. Rietveld analysis of PXRD data elucidated the vicinity of organic molecules and zeolite oxygens atoms [4-6]. The embedding of DCE, MTBE, TOL induced a combined effect of widening/contraction of all channel systems, which was highlighted by the variations of O-O distance of channel systems, and this favours the adsorption of the organics [4-6]. The competitive adsorption for a mixture of these contaminants induces diffusion of the molecules through the zeolite channel systems thus causing a ridistribution of DCE, MTBE, TOL species highlighted by Rietveld refinements (see Figure below). Location of MTBE-TOL, DCE-TOL, and MTBE-DEC mixtures into ZSM-5. Conclusions The very favorable adsorption kinetics along with the effective and highly irreversible adsorption of DCE, MTBE and TOL molecules into zeolite pores make these cheap and environmental friendly materials applicable for the treatment of water contaminated with fuel-based pollutants. This type of information is crucial to design and optimize the PRB technology for water remediation based on high silica zeolites. Acknowledgements The authors wish to thank Research Center for Non-Conventional Energy, Istituto ENI Donegani − Environmental Technologies (Novara, Italy) for their financial support. References [1] R. Vignola, R. Bagatin, A. De Folly D’Auris, E. Previde Massara, D. Ghisletti, R. Millini, R. Sisto, Chem. Eng. J. 178 (2011) 210–216. [2] R. Vignola, R. Bagatin, A. De Folly D’Auris, C. Flego, M. Nalli, D. Ghisletti, R. Millini, R. Sisto, Chem. Eng. J. 178 (2011) 204–209. [3] A.R. Gavaskar, B.C. Kim, S.H. Rosansky, S.K. Ong, E.G. Marchand, Environ. Prog. 14 (1995) 33–40. [4] A. Martucci, L. Pasti, N. Marchetti, A. Cavazzini, F. Dondi, A. Alberti, Micropor. Mesopor. Mater. 148 (2012) 174-183. [5] L. Pasti, A. Martucci, M. Nassi, A. Cavazzini, A. Alberti, R. Bagatin, Micropor. Mesopor. Mater. 160 (2012) 182–193 [6] R. Arletti, A. Martucci, A. Alberti, L. Pasti, M. Nassi, R. Bagatin, J. Solid State Chem. 194 (2012) 135-142. [7] I. Braschi, G. Gatti, C. Bisio, G. Berlier, V. Sacchetto, M. 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