Nanostructured materials for water purification: synthesis, insights and performance evaluation

Membrane filtration and Advanced Oxidative Processes (AOPs) are among the most efficient and cost-effective methods employed in water purification. A system to integrate the two methods using photoactive colloidal particles was studied in this thesis, with the final purpose of overcoming membrane fouling, one of the main issues occurring in filtration processes. The production of nanostructured TiO2 microparticles through a simple and extremely rapid synthesis and an easy method to assemble a multifunctional coating, integrating inorganic particles on filtration membranes, were targeted as the most promising solutions from the technological and environmental point of view. The control of microwave-assisted heating applied to hydrothermal treatments, a relatively recent synthetic method, allowed the production of nanostructured mesoporous spherical TiO2 particles, bringing the synthesis to the minute scale, extremely rapid compared with conventional heating, and achieving products otherwise difficult to obtain without the help of surfactants or templating agent. The as-synthesised particles showed photoactivity under visible light, with rate of specific reactions (selective de-ethylation) 4 times higher compared with commercial photocatalysts. Furthermore, the particles were modified to extend the limited intrinsic absorbance of TiO2 in the visible light, with promising results given by formation of stoichiometric defects (in particular oxygen vacancies) through annealing under vacuum. This treatment allowed the achievement of comparable or even higher performance in photocatalytic degradation of rhodamine B with respect to commercial TiO2 photocatalysts, including Aeroxide P25, with degradation rate towards organic molecules (rhodamine B) of even 60-70% after 1 hours, compared to the 25% of P25. The production of a multifunctional coating for water treatment by integration of colloidal and nanometric TiO2 particles has been also studied. A simple technique to integrate TiO2 nanoparticles onto different substrate, in particular filtration membranes, was developed by simple electrostatic interactions involving the use of polyelectrolytes, water-soluble charged polymer forming organised layers when assembled in a macromolecular structure defined as polyelectrolyte multilayers (PEMs). Electrostatic assembly was applied as an environmentally friendly technique to anchor nanoparticles (P25) on different surfaces, transferring their properties to these. In particular, the application of TiO2 particles conferred hydrophilic and superhydrophilic to a relatively hydrophobic surface (Mylar) by controlling the multilayer assembly conditions, in particular the ionic strength of the polyelectrolyte solutions. The achievement of superhydrophilic behaviour on the treated surfaces, with contact angles below 15° on Mylar surfaces, and the possibility of removing fouled active layer from a membrane replacing it with a newly generated one can be both implemented as potential antifouling strategies in water treatment