Reactive transport of pollutants in porous media

Abstract

Accidental 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.