Adsorption properties of particles for environmental applications

The contaminants of emerging concerns (CECs) are defined by the United States Geological Survey (USGS) as: “any synthetic or naturally occurring chemical or any microorganism that is not commonly monitored in the environment but has the potential to enter the environment and cause known or suspected adverse ecological and/or human health effects.” In other words, CECs are any substances that can be suspected to cause harm. Basically, they are substances used in human activities every day for all kinds of purposes. Because they are so numerous, ubiquitous and chemically different, they are frequently divided into categories that describe their purpose, use or other characteristics. Some common categories are: pharmaceuticals, personal care products, agricultural runoff (pesticides, pathogens and fertilizers), fuel-based-compounds, chlorinated solvents, flame retardants (perfluorinatedalkyl compounds), plasticizers, dyes and endocrine disruptors. CECs are continuously entering water sources throughout the world because of their widespread use. Conventional wastewater and recycled water treatments are only partially effective in their removal or degradation, so they are discharged into the environment with treated wastewater effluent, recycled water and wastewater plant sludge. Other sources of CECs include industrial wastewater effluents, untreated wastewater from manufacturing facilities, landfill leachates, effluents from poultry farms and animal feeding facilities where veterinary drugs may be used. Effects of CECs on human and ecosystem health are largely unknown and relatively little is known about the ways they travel through the environment or how they may be transformed or degraded during their residence time in the environment. Some studies have shown that even very low exposure to certain CECs can have impacts on biological systems. CECs are generally present at very low concentration in natural waters. Therefore, it is important on one hand to develop analytical methods able to detect these molecules in accurate way to better evaluate their impact on the environment, and on the other hand to enhance the efficiency of water remediation technologies in order to decrease their potential adverse effects on biota. Several methods have been reported to be suitable for both of these purposes; sorption based technologies were demonstrated to be efficient and economical methods both for the enrichment of trace pollutants and for their removal from water. Among the large number of inorganic adsorbents, zeolites were proven to be efficient in removing organic compounds from environmental matrixes. Recently, mesoporous silica materials have also received increasing interest because their properties (high surface area, high pore volume, controlled pore size) make them promising as adsorbents in contaminant removal processes. Moreover, due to the thermal and chemical stability of these siliceous adsorbents, they can be considered environmentally compatible. The aim of the present work is to study the adsorption properties of microporous (Beta, ZSM-5, Y and ferrierite zeolites) and mesoporous (MCM-41 and HMS) siliceous materials towards various organic compounds in aqueous solutions. This investigation has the dual purpose of evaluating the application of the selected adsorbents both in remediation technologies of natural waters and as medium for solid phase extraction systems of pre-concentration. For what concerns the studied contaminants, four compounds (ketoprofen, hydrochlorothiazide, atenolol, erythromycin) have been selected as members of CECs. These four drugs belong to different therapeutic classes and have been detected in natural waters of several countries because they are not efficiently removed by conventional wastewaters treatments plants. Moreover they are characterized by different physico-chemical properties as molecular dimensions, acid/base behaviour, hydrophobicity, ecc. Perfluorooctanoic acid (PFOA) was selected because, due to its high water-solubility, it has been frequently found in aquatic environment; moreover it is extremely persistent and toxic. The selected organic pollutants were toluene and methyl-tert-butyl ether (MTBE): they were chosen as representative of BTEX (benzene, toluene, ethylbenzene, xylene) and fuel oxygenate compounds, respectively. Because of its high water solubility and hence high bioavailability, MTBE belongs to the class of emerging contaminants too. In this thesis, chromatographic, thermogravimetric and diffractometric techniques were employed to study the adsorption process in order to: 1) investigate the adsorptive properties of the siliceous materials; 2) characterise their structures after the adsorption of the selected contaminants; 3) localise the organic species in the zeolites channel systems; 4) probe the interactions between organic molecules and framework oxygen atoms; 5) characterize the kinetic of the adsorption process. In particular, the thermodynamics and kinetics of the adsorption process of contaminants on hydrophobic zeolites were studied by using complementary techniques: chromatography and thermogravimetry. Chromatography was mainly used to measure the adsorption isotherms of the studied compounds. The adsorption isotherm is useful for representing the capacity of a zeolite for adsorbing organics from water, and in providing description of the functional dependence of capacity on the concentration of pollutants. Experimental determination of the isotherm allows for evaluating the feasibility of adsorption for treatment, in selecting a zeolite and in estimating adsorbent dosage requirements. Moreover, from isotherm parameters it is possible to evaluate the adsorption energy distribution of the process. The abovementioned techniques were also employed to investigate the kinetics of the adsorption. Kinetics deals with changes in chemical properties in time and is concerned especially with rates of changes: hence it plays a fundamental role in determine the proper contact time for the removal of pollutant components from wastewater. To investigate the adsorption mechanism, diffraction techniques were employed to localize the organic adsorbed into the zeolite structure. The information gathered by this last investigation – in cooperation with the Earth Science Department UNIFE - allow to define the interactions between organic molecules and zeolite framework. The experimental data revealed that the amount of the organic pollutant embedded inside the framework was influenced by the lattice structure, the hydrophobicity (SiO2/Al2O3 ratio: SAR) and the thermal treatments of the adsorbent. For mesoporous silica materials, an important role is also played by the procedure of template removal (thermal treatment or solvent extraction). Both hydrophobic and electrostatic interactions were demonstrated to contribute to the adsorption process: in fact, hydrophobicity and dissociation constant of the solute strongly affect the adsorption. In many cases the selected molecule was proved to be adsorbed inside the framework of the siliceous materials. Generally, it was also proved that the adsorption process was very fast in all the studied zeolites towards several classes of pollutants. Also the adsorption kinetic of PFOA on mesoporous silica materials was satisfying if compared with literature data dealing with other types of adsorbents. To investigate the possible competition of natural organic matter towards contaminants adsorption, the effect of two lignin-derived phenolic monomers (caffeic acid and parahydroxybenzaldheyde) with molecular dimensions comparable to those of the pores of the adsorbent material on the adsorption properties of zeolites was considered. This last part of the work of thesis is a fraction 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, Bologna and Piemonte Orientale Universities with the financial and scientific support of ENI Research Center of San Donato Milanese. The results revealed that zeolites are selective adsorbents for organic pollutants. In fact, it was demonstrated that toluene is preferentially and almost exclusively adsorbed from mixtures of toluene and humic acid monomers in aqueous solutions. In conclusion, favourable adsorption kinetics along with the effective and selective adsorption of contaminants into zeolites and mesoporous siliceous materials make these cheap and environmentally-friendly materials a tool with interesting applications for the removal or enrichment of organic pollutants from contaminated waters.