Chemical biowaste upgrading: carbon dots, biopolymers and actives

The herein reported thesis finds its roots in the concepts of sustainability and circular economy and has as purpose the valorisation of agro- and fishery-waste for the production of added-value products. In particular, different types of waste biomass were used as starting material for the synthesis of carbon dots (CDs) used as photocatalysts, for the obtainment of biopolymers, namely collagen and chitin, and for the extraction of valuable compounds using supercritical CO2. The investigation started studying the dependence of structure and photocatalytic behaviour of CDs on the carbon source. Six different types of carbon nanoparticles were synthesized and compared from the morphological and optical point of view revealing a strong correlation of their properties with the starting material and the synthetic method. The citric acid-derived CDs were then employed to photoactivate the atom transfer radical polymerization of a methacrylate leading to the obtainment of the polymer in high conversions (89%) and narrow dispersity (1.4). The study on CDs proceeded then with the employment of more complex starting material, namely fish scales. Naturally nitrogen doped carbon dots were successfully synthesized starting from bass scales, fully characterized and employed as photocatalyst for the reduction of methyl viologen. The bass-CDs were compared with classic citric acid-derived CDs resulting in higher initial photoreduction rate (7.5·10−8 M·s−1 vs 4.9·10−8 M·s−1). The fish-derived nanoparticles were then applied as photocatalyst for the continuous flow degradation of azo dyes highlighting a quantitative degradation of five model dyes in only 2 min. This study was conducted during a research period abroad (September 2021-April 2022) in the laboratories of the Université de Liège under the supervision of Prof. Jean-Christophe Monbaliu. The upgrading of fishery waste was then developed more during the second part of this thesis where the possibility to extract biopolymers from fish scales or crab carapaces was explored. After a review study on the production of UV-shields using biopolymers, an investigation on the preparation of UV-blocking films from mullet scales-derived gelatin and bass-CDs was conducted. The obtained materials were fully characterized from the morphological, mechanical and optical point of view; by adding just 5% of CDs the films blocked almost 70% of the UV radiation with negligible change in opacity and in transparency. A further study on the possibility to obtain chitin from crab shells using a one-step protocol using ionic liquids was then conducted. All characterization data confirmed that in the conditions tested, ammonium formate prepared in situ seems to be a promising candidate for one-pot chitin pulping process, allowing quantitative isolation, high purity and a high degree of acetylation (DA > 90%). To have a complete overview on the possibility to exploit waste biomass for the production of added-value products, an investigation on the valorisation of agro-waste was then conducted. In this frame, a supercritical CO2 extraction method was developed to obtain natural preservatives from hops and jimsonweed. scCO2 extracts were compared to ethanolic ones: the supercritical fluid led to the selective extraction of volatile compounds resulting in a better recovery for preservatives. The scCO2 technique was then applied to obtain fatty acids for cosmetic formulations from fruit pomace, resulting again more selective when compared to classical organic solvents.