Advanced Nanocomposites for Photocatalytic Degradation of Organic Pollutants in Water

Water plays a vital role in sustain life on earth. However, the quality of the water resources is seriously affected by the pollution caused by industrial, municipal and agricultural activities. To remediate the toxic organic and inorganic contaminants in water, several physical, chemical and biological water treatment methods are employed. However, current water treatment technologies present limited performance, materials of high cost, and need highly energy consuming infrastructure. In this scenario, the utilization of advanced materials and innovative methods could be helpful for reducing the cost and enhancing the performance of the wastewater treatment. One of most promising method is the Advanced Oxidation Processes (AOPs). The AOPs use semiconductor materials that are activated by the UV or Visible light irradiation that are able to mineralize organic water pollutants. Currently, commercial available photocatalysts are mostly used in slurry or powder form, but this requires post-treatment processes in order to avoid secondary pollution, causing the increase of the overall treatment cost. To face up such limitation and to resolve problems related to the post-treatment process, the immobilization of the photocatalysts on solid compact matrices is a feasible solution. In particular, the use of polymeric matrices as host solid materials may result in a final system that combines the photocatalytic properties of the semiconductor component with the mechanical properties, usability and cost effectiveness of the polymers.

In this study are developed polymeric nanocomposites for their utilization in AOPs. The study is focused on the (i) investigation of the chemical and physical characteristics of the prepared samples and (ii) evaluation of their photocatalytic performance on the degradation of organic pollutants in water.

This study was possible thanks to the collaboration of the Smart Materials group of Istituto Italiano di Tecnologia (IIT) in Genova, the research group of Interdisciplinary Laboratory for Advanced Materials Physics (I-Lamp) at Università Cattolica del Sacro Cuore (UCSC) in Brescia and the Radiation Laboratory of the University of Notre Dame (ND, Indiana, USA).

The promising results are summarized and illustrated in this thesis, which has been structured in five chapters:

The Chapter 1 is dedicated to the water pollutants, starting from the causes and reaching to the remediation processes. In this chapter, the description of the Advanced Oxidative Processes (AOPs) and their utilization as an innovative solution for water decontamination are presented. Specifically, are discussed the characteristic of this process and the different types of semiconductors which are used in this process concluding on the utilization of polymeric nanocomposites in the wastewater treatment through AOPs.

Chapter 2 presents the development of a porous nanocomposite mat composed of cellulose acetate (CA) and tin dioxide (SnO₂) for the photocatalytic degradation of organic pollutants. The nanocomposite is prepared by the combination of the electrospinning technique and the in-Situ thermal activation of tin precursor directly in the fibrous mats. The final nanocomposites are used for the photocatalytic degradation of an anionic dye (Methylene Blue (MB)) and a cationic one (Methyl Orange (MO)) in water under UV irradiation. The mats are able to photodegrade both types of dyes with an efficiency of more than 90%. After the photodegradation process the by-products are studied with the Liquid Chromatography-Mass Spectrometry (LC-MS) which indicates the de-methylation of the organic structure for both dyes and a hydroxylation process in the MO azo-dye as the main degradation roots.

The characteristic and photocatalytic activity of the CA polymer fibers combined with cerium nanoparticles (CeNPs) and platinum (Pt) nanostructures (CA/CeNPs/Pt) are discussed in the Chapter 3. The mats are prepared by the combination of the supersonic beam deposition (SCBD) of Pt nanoclusters directly on the surface of electrospun CA fibrous mats, which are pre-loaded with a cerium salt precursor and transformed into CeNPs directly in the solid mats by a simple thermal treatment. The photocatalytic performance of the final composites is evaluated by the degradation of the MB under visible light irradiation, and is proved that the mats reach a degradation efficiency of 70%. As demonstrated the presence of Pt nanoclusters enhances the photocatalytic activity of the nanocomposites, while the detection of the reactive oxygen species in solution showed that holes and •OH radicals are the main species involved in the photodegradation process.

In the Chapter 4 a novel system composed by a Bipolar Membrane (BMP) and the combination of cadmium sulfide (CdS) and palladium (Pd) nanostructures are used for the degradation of phenolic compounds in water. The combination of CdS and Pd in the different sides of the membrane improved the photocatalytic activity of the system, that resulted from the simultaneous oxidation of 4-chlorophenol and the reduction of 4-nitrophenol under visible light irradiation. The choice of these compounds allows to obtain a mechanistic insight into the sequential electron transfer between segregated phases embedded in the membrane.

The Chapter 5 summarizes the results obtained throughout this study and outlines possible future developments and related applications.