Venezia, Virginia (2022) Multifunctional hybrid materials from biowaste valorization. [Tesi di dottorato]

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Abstract

The waste to wealth concept is a challenging research field that promotes a future sustainable lifestyle valorising waste into a wide range of valuable compounds and products. This approach is particularly promising for bio-wastes (BWs) because of their large abundance as well as remarkable chemical and biological richness. As a result, industry and academic researchers are working to increase the economic and environmental value of BWs by developing strategies for recycling and converting them into added-value compounds and materials. Among BWs, Humic acids (HA) are the alkali-soluble fraction of natural organic matter that survives the biological and chemical degradation of both vegetal and animal biomasses. Furthermore, they are the most abundant bioproduct of biorefinery processes. According to a circular-chemistry approach, they are viewed as a promising and cost-effective source for high-value products and novel materials because of their intriguing properties. Indeed, phenolic, and carboxylic groups in HA are primarily responsible for improved plant growth and nutrition, flame retardancy features due to char formation in combustion processes, and even antiviral and inflammatory activity. Furthermore, their amphiphilic nature determines self-assembly in aqueous environment building up supramolecular structures which can act as metal chelating agents and can interact with organic contaminants. Moreover, quinone moieties confer redox (antioxidant and/or pro-oxidant) behavior to these bioavailable mixtures thanks to their ability to generate, stabilize, or scavenge Reactive Oxygen Species (•OH, •O2-, •OOH), also known as ROS. Despite the substantial potential offered by bioavailable HA materials, their full technological exploitation is strongly limited by their segregation leakage and/or degradation phenomena in aqueous environment. Accordingly, despite their large abundance, they are mainly considered as waste and only a small amount is employed for low value processes including soil amending. Furthermore, their presence in water environment raises big issues to decontamination processes, since they contribute to fouling phenomena and improve pollutant persistence preventing efficient removal. In this context, the PhD project aims at defining different synthetic approaches to turn this environmental issue into a technological tool, through the design of multifunctional materials based on HA biomolecules. The project has been developed with the support of the company Verde Vita s.r.l. based in SASSARI, where the composting plant is located. This firm provides for quality compost certified by “Consorzio Italiano Compostatori” as HA source. The PhD thesis has been focused on two main parts: • Design of hybrid nanostructured HA-based materials. • Design of HA-based polymeric materials. The molecular combination of these heterogeneous moieties with an inorganic matrix (e.g., SiO2, TiO2, ZnO) is an effective method for limiting HA aqueous degradation phenomena, thus improving physicochemical stability, mechanical properties, and even enhancing intrinsic HA features in the final hybrid nanostructured HA-based materials. This approach has been combined with electrospinning technology to produce bioactive and sustainable nanocomposite films for active packaging applications made of electrospun biodegradable and bioderived polymers and hybrid nanoparticles. Furthermore, HA have been explored as functional additives for polymeric materials. In particular, proper synthesis strategies have been designed to use these biomolecules as functional biowaste flame retardant for epoxy-based systems or as an additive for gelatine hydrogels to obtain hybrid HA-3D network with tunable rheological features and significant biological activity, including antimicrobial and antioxidant efficacy, for a broader range of biotechnological applications. From a scientific point of view, the thesis project has contributed to clarify the chemistry of HA with a special focus on their interaction with either organic or inorganic components as well as on the physico-chemical features of hybrid HA-based hybrid materials. From a technological point of view, the research activity has provided viable routes for HA valorization, by developing different materials with intriguing features and concrete application perspective in food-packaging, biomedical as well as automotive fields. Furthermore, developed synthesis strategies could be easily extended to BWs valorisation other than HA, thus significantly contributing to the challenging mission of giving waste a new value.

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