566 research outputs found
Reactive transport of pollutants in porous media
No full textAccidental or deliberate crude oil spills have been, and still continue to be, a significant source of environmental pollution, and pose a serious environmental problem, due to the possibility of air, water and soil contamination. Chlorinated volatile organic compounds (VOCs), such as 1,1-dichloroethylene (DCE) and aromatic hydrocarbons, BTEX (benzene, toluene, ethyl benzene and xylene) constitute a significant fraction of hazardous air and water pollution. Human beings are exposed to elevated levels of a wide spectrum of VOCs, many of which have been found to be toxic and potentially carcinogenic. Removal of these organic contaminants from water and wastewater has been achieved using several treatment technologies, such as advanced oxidation processes, air stripping, reverse osmosis, ultrafiltration and adsorption.
Adsorption processes can be successfully used when contaminants are not amenable to fast biological degradation. Permeable Reactive Barriers (PRB) are one of the most promising passive treatment technologies, due to their effectiveness regarding various contaminants, and their low cost compared to other in situ technologies. Typical PRB configuration consists in a permeable treatment zone placed vertically to the flow path of groundwater, which contains reactive material that immobilises or decomposes the contaminants by adsorption as the groundwater flows through it.
PRBs are installed as permanent, semi-permanent, or replaceable units. A wide variety of pollutants are degraded, precipitated, sorbed or exchanged in the reactive zone, including chlorinated solvents, heavy metals, radionuclides and other organic and inorganic species.
Conventional permeable reactive barriers for the decontamination of water are based on systems which most widely use Granular Activated Carbon (GAC).
GAC has been shown to be only slightly effective in treating water containing very soluble compounds, such as oxygenated organics, or low molecular weight compounds, such as DCE and vinyl chloride (VC). However, their use for the removal of organic contaminants in water and wastewater applications can be complicated by the presence of dissolved natural organic matter in the water stream being treated, which can decrease the removal efficiency of GAC. When activated carbon is saturated, it has to be regenerated or renewed, which is rather an expensive operation. The adsorbed molecules are then released and still have to be destroyed by thermal treatments. Moreover, this additional treatment also degrades the activated carbon adsorption properties in the long term [1]. Zero-valent iron (ZVI), which directly degrades several contaminants, appears to be ineffective too, both on irreducible compounds such as DCE and chlorobenzenes as well as on hydrocarbons. Furthermore, when ZVI is used, it causes a reduction in the permeability of the barrier due to encrustations or precipitation of minerals which derive from the reactions between the ions of the oxidised metal and the substances contained in the groundwater [1-2]. Therefore, when operating with a barrier based on metallic iron alone, the chemical reduction reaction of the reducible compounds can require from 1 to 2 days. In this case, it is only the thickness of the iron which can ensure the time necessary for completing the reactions and large quantities are required to guarantee the complete decontamination of the groundwater.
Recently, high-silica zeolites were shown to be more effective than activated carbon or ZVI in removing certain organics from water [3-4].
The selection of zeolites from among the large variety of adsorbent materials is based on their stability and efficiency properties.
To date, the adsorption mechanisms of zeolites in gas phase systems have been widely investigated. On the contrary, studies and applications on organic pollutants adsorption in microporous zeolitic materials from aqueous media have been relatively scarce. Adsorption from gas phase systems can significantly differ from that observed from the corresponding aqueous solutions, due to the highly polar nature of water molecules. In literature, it has been reported that water plays a very important role in the diffusion of hydrocarbons in the zeolite pore system. In particular, large amounts of co-adsorbed water molecules block the migration of host molecules such as alkanes and olefins, thus reducing the adsorption capacity of zeolites, especially at low adsorbate concentrations. As a consequence, water acts as a screen between the cationic sites of the zeolite and the hydrocarbon molecules (screening effect) and reduces both the sorption volume (steric effect) and the aperture of the zeolite windows (blocking effect). On the contrary, small amounts of co-adsorbed water lower the extent of specific adsorption without significant blocking effects. However, as mentioned above, this research on hydrocarbon adsorption has also mainly been focused on single components from air matrices, whereas there are few studies involving aqueous dilute solutions. Nonetheless, in most environmental applications, these pollutants are present as very dilute aqueous solution mixtures.
The work developed in the present thesis is part of a wider project whose purpose is to study the interaction and mobility of groundwater pollutants adsorbed in zeolite pores, in order to improve the efficiency of permeable reactive barriers. This project involves Ferrara and Bologna Universities with the financial support of the ENI and the scientific support of Dr. Roberto Bagatin of the research centre of Novara. Several techniques were employed such as X ray diffraction, gas chromatography, IR spectroscopy, thermal analyses, as well as computational studies.
In this thesis, combined diffractometric, thermogravimetric and gas chromatographic techniques were employed to study the adsorption process in order to: 1) investigate the adsorptive properties of these hydrophobic synthetic zeolites; 2) characterise their structure after the adsorption of selected contaminants (1-2 dichloroethane, tert-butyl methyl ether and toluene); 3) localise the organic species in the zeolite channel system; 4) probe the interactions between organic molecules and framework oxygen atoms; 5) compare the adsorption data for a mixture of these contaminants with concentrations in the ppb and ppm range; 6) characterise the kinetic of the adsorption processes.
In particular, the thermodynamic and kinetic of the adsorption processes of contaminants on hydrophobic zeolites were obtained by using complementary, batch, linear and non-linear chromatography and thermogravimetry techniques. Batch and non-linear chromatography were mainly used to measure the adsorption isotherms for the compounds of interest. The adsorption isotherm is useful in representing the capacity of a zeolite to adsorb organics from waste, and in providing description of the functional dependence of capacity on the concentration of pollutants. Experimental determination of the isotherm allows to evaluate the feasibility of adsorption for treatment, to select a zeolite, and to estimate adsorbent dosage requirements. Moreover, it is possible to evaluate the adsorption energy distribution of the process from isotherm parameters. Batch and linear chromatography, instead, were employed to investigate the kinetic of the adsorption. Kinetics deals with changes in chemical properties in time and is especially concerned with the rate of changes and plays a fundamental role in determining the proper time contact for the removal of pollutant components from wastewater. In addition, an original theoretical model able to give information regarding the kinetic and the thermodynamic constants of systems in which both reactions and adsorption processes occur simultaneously was developed. To investigate the adsorption mechanism, diffraction techniques were employed to localize the organics adsorbed into the zeolite structure. The information gathered by this latter investigation – in cooperation with the Earth Science Department UNIFE - allows to define the interactions between organic molecules and zeolite framework. Finally, adsorption on mesoporous materials was investigated. It is well known that water is contaminated by different classes of substances, and zeolites are mainly suitable for molecules with dimensions comparable to that of their pores. However, many compounds belonging to the class of emergent contaminants have large molecular dimensions, and in such cases mesoporous materials can be more efficient than zeolites. To accomplish this task MCM-41 and HMS were synthesized and characterised – this work was carried out at the ‘Institut Charles Gerhardt (ICG), Matériaux Avancés pour la Catalyse et la Santé (MACS)’ at Montpellier (France) with the supervision of Prof. Francesco di Renzo and Dr. Anne Galarneau – and then the adsorption of acid perfluorooctanoic onto these mesoporous materials was performed
Structural modifications induced by adsorption of pharmaceuticals from water on Y organophilic zeolite
No full textDomestic and commercial wastewaters contain a variety of organic wastewater contaminants such as pharmaceuticals
and personal care products. These compounds undergo incomplete removal in wastewater treatment plants
and they are found in the surface waters receiving the effluents of these plants. In the present work the removal of
several drugs (which differ in chemical properties and molecular dimensions) from water by Y organophilic zeolite
(SiO2/Al2O3 ratio equal to 200) was investigated. All selected drugs (erythromycine, carbamazepine, levofloxacin,
hydrochlorothiazide, ketoprofene, diclofenac) are ubiquitous contaminants in the sewage waters, nor effectively
removed by conventional activated sludge treatment and membrane bioreactors (MBRs). This study has a dual
purpose: i) to measure the sorption capacity of hydrophobic commercial zeolite material weighed against drugs
dissolved in water and to quantify aspects of their removal efficiency for potential use in wastewater and groundwater
remediation, and ii) to understand the zeolite structure features for adsorption of drugs in aqueous solutions.
Kinetics and adsorption isotherm batch data were obtained via HPLC-DAD. Breakthrough curves were obtained
by using 0.2x2 cm SS column and a standard HPLC equipment. Powder diffraction patterns were measured on a
Bruker D8 Advance Diffractometer equipped with Sol-X detector. Thermogravimetric (TG) and differential thermal
analyses (DTA) measurements were performed in air up to 900C using a STA 409 PC LUXX® - Netzch
at 10C/min heating. After adsorption, the X-ray diffraction patterns of untreated and exhausted samples show
relevant differences both in the intensity and position of the diffraction peaks, indicating that the zeolite crystal
structure was markedly modified by the pharmaceuticals adsorption experiment. Rietveld refinement demonstrated
that the adsorption of the tested pharmaceuticals in Y zeolite induces strong unit cell parameter variations as well
as remarkable distortion of the framework, thus confirming the adsorption of the drugs inside the channel system.
The results of this study indicate that Y zeolite is an efficient materials for the removal of selected contaminants in
wastewater remediation rate
Evidences of competitive adsorption of hydrocarbons into an hydrophobic ZSM-5
No full textVolatile organic compounds and hydrocarbons constitute a significant fraction of the hazardous air and water pollution. Research on hydrocarbon removal has been mainly focussed on single components from air matrix, whereas the studies involving aqueous dilute solutions are few. However, in most environmental applications, these pollutants are present in the form of mixtures in very dilute aqueous solution. In this study, we report on evidence of 1,2-dichloroethane (DCE), methyl tert-butyl-ether (MTBE) and toluene (TOL) adsorption into ZSM-5 zeolite.
A combined diffractometric and gas chromatographic study was used to: 1) investigate the adsorptive properties of ZSM-5; 2) characterise its structure after the adsorption of selected contaminants; 3) localise the organic species in the zeolite pores; 4) compare the adsorption data for a mixture of these contaminants with concentrations in the ppb and ppm range.
The ZSM-5 sample is a commercial adsorbents, (SiO2/Al2O3 =280), purchased by Zeolyst International. Kinetics and adsorption isotherm batch data were obtained via Headspace Solid Phase Microextraction -Gas Chromatography. XRD powder patterns were collected on ZSM-5 before and after adsorption on a Bruker D8 Advance diffractometer. Thermal analyses (TG and DTA) were performed in air up to 900°C at 10°C/min. The experimental results prove that the kinetic of adsorption of all components is fast and indicates that competition exists between the organic compounds at low cosolute concentrations.
Rietveld refinements indicate that the relevant incorporation of DCE, MTBE and TOL in the ZSM-5 causes significant increase of all unit cell parameters in comparison to those of the untreated material as well as strong distortions of the channel systems where the contaminants are hosted. The very favorable adsorption kinetics along with the effective and highly irreversible adsorption of DCE, MTBE and TOL molecules into zeolite ZSM-5 pores make this cheap and environmental friendly material a tool with interesting applications for the removal of hydrocarbons from wastewater
Dynamic Chromatography: a Stochastic Approach
No full textDuring the chromatographic separation process, analyte reactions are often observed leading to band broadening and/or elution of peak clusters. For many different chemical compounds the reaction can be reduced to a simple isomerisation kinetic scheme where elution is the result of adsorption–desorption on the surface stationary phase coupled with a flipping two-level reaction system. In this paper, the chromatographic peak shape for a reacting analyte is calculated in frequency domain when the reaction follows a simple reversible first order scheme. Both reaction and dynamic chromatographic systems have been considered. The derived solutions are expressed in closed form in the Fourier domain. Several limit solutions obtained under conditions of very slow and moderately fast kinetics are exploited. The effects of both kinetics rate constants and retention time on the chromatographic peak shape are singled out
MULTIANALYTICAL GC-MS METHOD OF WATER SOLUBLE COMPOUNDS IN ATMOSPHERIC AEROSOL
No full textWater-soluble organic compounds (WSOCs) are an important group of chemical tracers that may give relevant information on the relative strengths of primary emission sources and secondary photochemical processes on air quality. In fact, they can be primarily emitted into the atmosphere by a multiplicity of sources -- including power plants, vehicular circulation, meat cooking operations and biomass burning -- or secondarily produced by photochemical atmosphere reactions from both biogenic and anthropogenic precursors.
This paper describes the development of a GC-MS procedure for the simultaneous analysis of several WSOCs with a wide range of water solubility, including carboxylic acids and sugars. The response surface methodology (RSM) including central composite design (CCD) was applied to optimize solvent extraction: the factors considered were the solvent type (characterized by p’ parameter ) and volume (10-20 ml).
On the basis of RSM, the optimum extraction solvent was a mixture of metane:dichlorometane (90:10) using a volume of 10 ml. To validate the optimized conditions, a comparative study was performed towards the aqueous extraction, as reference solvent for water-soluble components, for blank filters spiked with standard solutions of WSOCs, and real PM samples.
The optimized procedure provides the low detection limits (≤2 ngm-3) and the good reproducibility (RSD%≤13%) required by environmental monitoring of chemical markers of atmospheric processes
Adsorption of 1,2-Dichloroethane onto hydrophobic zeolites
No full textChlorinated volatile organic compounds (VOCs), such as dichloromethane (DCM) and 1,2-dichloroethane (DCE) constitute an important environmental class of pollutants due to their high toxicity, inertness and widespread application in industry. These compounds can entry in the environment from effluent discharge of industries that use or produce DCE, air emissions and leachates from waste disposal sites. Despite DCE being the most abundant chlorinated groundwater pollutant on Earth, an efficient reductive in situ detoxification technology for this compound is not known. For instance, zero-valent iron (ZVI), which directly degrades several contaminants appears to be ineffective on irreducible compounds such as DCE, chlorobenzenes, as well as hydrocarbons. Recently, high-silica zeolites were shown to be effective in removing these types of compounds from water. In this study we focused on hydrophobic zeolites (ZSM-5, Y and Mordenite (MOR)), since they are the most promising zeolites support for VOC combustion, and many studies in literature deals with the adsorption properties of these zeolites from gas phase mixture. However, it has also been found that in gas phase water play an important role in regulating the interaction between organic compounds and zeolite. To date, studies and applications on organic pollutant adsorption on microporous zeolitic materials from aqueous media have been relatively scarce.
In this work DCE adsorption from aqueous solutions onto hydrophobic zeolites was studied by using different combined techniques (gas chromatographic and thermogravimetric) to investigate the mechanisms of adsorption
Adsorption and Location of methylterbutylether and toluene nonto hydrofhobic ZSM-5 zeolite: a Diffractometric, Thermogravimetric and Gas Chromatographic study.
No full textChlorinated volatile organic compounds (VOCs), such as 1,1-dichloroethylene (DCE) and aromatic hydrocarbons, BTX (benzene, toluene, and xylene) constitute a significant fraction of the hazardous air and water pollution. Among this category of compounds, Methyl Tertiary Butyl Ether (MTBE) and toluene (TOL) are of special relevance since both are toxic and commonly found in natural water.
Recently, high-silica zeolites were shown to be more effective in removing certain organics from water than activated carbon. Particularly, hydrophobic ZSM-5 (MFI-type framework topology) turned out that ZSM5 is the most promising zeolite support for VOC combustion,and many studies in literature deals with the adsorption properties of this zeolite from gas phase mixture. However, it has also been found that in gas phase water play an important role in regulating the interaction between organic compounds and zeolite.
In this work, MTBE and TOL adsorption onto an organophilic zeolite ZSM5 was studied by using different combined techniques (diffractometric, thermogravimetric and gas chromatographic) to investigate the mechanisms of adsorption: 1) the adsorptive kinetics and thermodynamics properties of hydrophobic synthetic ZSM5; 2) characterise its structure after the adsorption of MTBE and TOL from aqueous solutions; 3) localise the organic species and water in the ZSM5 channel system
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