TOWARDS SUSTAINABLE MATERIALS FOR CO₂ REMEDIATION: FROM FUNCTIONAL CARBON-HYDROXYAPATITE COMPOSITES TO WASTE-DERIVED RESOURCES

Sustainable development represents one of the most urgent challenges nowadays, involving a complex and multidimensional interplay of environmental, economic, and social factors. The transition to a circular economy is increasingly regarded as a key pathway to enhance environmental sustainability by replacing the linear “take-make-dispose” model with approaches focused on recycling, reusing, and reducing waste. The Ph.D. project, in collaboration with a national research center, Ricerca sul Sistema Energetico (RSE S.p.A.), explores sustainable chemistry applied to both materials and processes for energy and environmental applications. Two complementary research directions were investigated: CO2 conversion through bio-electrochemical and electrochemical routes, and development of sustainable functional materials from biomass and wastes. These research directions converge on a common theme, namely the investigation of carbon-based materials and hydroxyapatite, including their combination into innovative composite materials for environmental applications. The focus of the research was firstly directed toward the study of CO2 bioelectromethanogenesis (BEM), a process in which microbial metabolism works in synergy with electrochemical systems to reduce carbon dioxide into methane (CH4). Ternary composite cathodes based on biomass-derived carbon, copper nanoparticles, and hydroxyapatite were designed and characterized. After morphological and structural characterization, HAP-containing biocathodes were proven to promote a more CH4 productive BEM process than HAP-free materials, owing to their ability to increase surface area, adsorb bicarbonate species, buffer the local pH, and promote microbial adhesion. Despite the remarkable CH4 production, this composite suffers from low stability due to the lack of homogeneous distribution of hydroxyapatite on the carbon scaffold. To overcome this issue, during the period abroad in Lille (France), granted by Erasmus+ Traineeship in collaboration with Dr. Vitaly Ordomsky from CNRS-Université de Lille, a one-pot strategy was employed to synthesize a carbon nitride (CN) as a conductive scaffold, combined with HAP and copper nanostructures (HAP_Cu@CN). Advanced characterization techniques, including X-ray diffraction, TEM, and XPS, confirmed the presence of CN material and the successful integration of the components, particularly HAP, completely embedded with carbon nitride scaffolds. HAP_Cu@CN, tested in CO2 electroreduction test (CO2ER), showed selectivity, expressed as Faradic Efficiency (FE), of 60% at -1.0 V vs RHE in the formation of formate, HCOO-, with H2 from hydrogen evolution reaction (HER) identified as the other detected product, demonstrating that HAP addresses the reaction pathway, by modifying the local reaction environment and stabilizing intermediates through non-covalent interactions, without significantly hindering charge transfer. Building on the sustainable processes investigated for CO2 emission mitigation in the first part, the second part of the thesis addresses material sustainability by optimizing individual components of the composite materials (specifically carbon scaffold and hydroxyapatite) with waste- and biomass-derived alternatives and greener production pathways. Biochar was produced from sugarcane biomass through two pyrolysis temperatures, while hydroxyapatite was extracted from waste-to-energy plant ashes. From a morphological point of view, by XRPD and Raman spectroscopies, the sugarcane-derived biochar exhibited high surface area and graphitic features, thanks to the high content of lignocellulosic component. The surface area obtained, after pyrolytic treatment, increased by 10-times of order of magnitude in respect to the biochar used for bioelectromethanogenesis, reaching 200 and 100 m2 g-1, for the two chosen pyrolysis temperatures. As for graphitic properties, both materials demonstrated enhanced order and disordered bands, which are indicative of the presence of graphitization of the materials and consequently their conductive properties. With these preliminary results, sugarcane-derived biochar emerges as a promising component for efficient electrocatalysts, combining high specific surface area with improved electrical conductivity. The simultaneous enhancement of porosity and graphitic character suggests its suitability for electrochemical applications where both active surface availability and charge transport are critical factors. On the other hand, ash-derived HAP samples showed tuneable crystallinity, Ca/P ratios, and surface acid-base properties depending on extraction conditions. Particularly, four HAPs were extracted, changing mixing techniques, temperature, and maturation time. Since the sources were inorganic ashes, which could contain many impurities, the composition of HAP samples included many hetero atoms, with a marked difference in Ca/P ratio for two of the HAP samples (HAP_3 and HAP_4). However, all four samples demonstrated typical features of synthetic HAP in terms of specific surface area, structure, and surface moieties, analysed through N2 adsorption-desorption isotherms, XRPD, FT-IR, and Raman spectroscopies, respectively. Furthermore, a deep surface studies were carried out through isothermal liquid-solid titration and volumetric microcalorimetry, analysing the number of acid and basic sites (and relative ratio A/B, amphoteric index) and their relative strength, respectively. Further going into details, two samples (HAP_1 and HAP_2) showed higher crystallinity, lower specific surface area and a more amphoteric nature (even though it is always shifted towards basicity) with an A/B ratio higher than 0.30. On the other hand, more amorphous HAPs (HAP_3 and HAP_4) displayed higher densities of amphoteric, predominantly basic sites (A/B < 0,30), which are relevant for catalytic applications. These results demonstrate that waste ashes can be effectively valorised into functional inorganic materials with specific properties by tailoring extraction parameters. Overall, this work shows how sustainable chemistry can be addressed through complementary approaches, including the use of green materials, rational structural design, and the optimization of catalytic processes.