Silica Based Materials for Groundwater Depollution

ln this Ph.D. study has been devoted to the preparation and physicochemical characterization of silica-based solids with variable porosity (from completely amorphous to long-range ordered materials) for potential application in groundwater depollution. ln this respect, materials with different pore dimensions and architecture, particle size and textural properties have been selected and tested as sorbents for hydrocarbons, with particular attention to toluene and n- hexane molecules chosen as model of aromatic and aliphatic molecules, respectively. Microporous solids such as zeolites have been selected as adsorbents and the effect of their physico-chemical properties on the adsorption capacity were studied. Particular attention has been paid to the use of two dealuminated high silica HSZ-Y and ZSM-5 zeolites (with a SiOz/AlzOs ratio of 200 and 280, respectively) with commercial origin, that were selected on the basis of their pore d imension, hyd rophobicity, a nd textural properties. Moreover, to overcome the limitations related to the use of microporous materials, the interest was also given to the use of mesoporous materials. ln this respect, the interest was then devoted to the use of mesoporous silicas with different porosity. ln particular, two types of mesoporous silicas with irregular porosity (i.e., FUMED and AMSs silicas) and two types of mesoporous silicas with ordered porosity (i.e., SBA-15 and MCM-41) were studied. The physico-chemical feature of these solids (FUMED, AMSs, HSZ-Y, ZSM-S, MCM-41 and 58A-L5) were investigated by means of different experimental approaches (i.e. Nz adsorption, FTIR, TGA and ss-NMR). The first part of this Ph.D. study was focused to the understanding of the competition of organic contaminants for a specific adsorbent. This important issue is relevant from the environmental point of view in that, under real conditions, pollutants are generally present in groundwater as complex mixtures. From FTIR analyses it was possible to observe that the two zeolites behave differently in the presence of the toluene/n-hexane mixture: indeed, the HSZ-Y zeolite preferentially retains toluene, whereas n-hexane is preferentially adsorbed on ZSM-5. Volumetric analyses were performed to study the toluene adsorption capacity of these solids. This collection of data suggested that porosity and confinement effects play a key-role in driving toluene adsorption capacity. Additionally, some of the porous materials considered during this Ph.D. study were tested also as adsorbents for the removal of more complex molecules such as N-(phosphonomethyl)glycine (glyphosate, an herbicide largely used in the last years) from aqueous media. For this, the adsorption properties of ordered mesoporous silicas (mainly SBA-15, also functionalized with organic species) for glyphosate removal from aqueous solution was studied. Kinetics and equilibrium adsorption isotherms of glyphosate on these solids were obtained from the adsorption measurements of glyphosate in contact with the selected solids in aqueous media. UV-vis spectroscopy was used to determinate the glyphosate concentrations present in the aqueous solutions after the batch adsorptions.