The agriculture and agri-food sectors produce substantial amounts of plant-based waste. This waste presents an identifiable research opportunity to develop methods for effectively eliminating and managing it, to promote zero-waste and circular economies. All these wastes can be valorized using downstream processes in an integrated manner, which results in the conversion of waste into secondary raw materials. Specifically, plant-based food wastes and/or byproducts are recognized sources of bioactive chemicals, including dietary fibers that are beneficial as food additives or functional food ingredients that can meet the technological and functional requirements of health-promoting value-added products. Additionally, cellulosic ingredients can be utilized directly within nonfood industries, such as textiles, resulting in a reduction in the environmental impact of secondary raw materials as well as an increase in market acceptance compared to those currently on the market. An overview of novel concepts for effective reuse, recyclability, and maximal utilization of plant-based food wastes and/or byproducts from food-processing industries is presented, pointing potential opportunities for the extraction of value-added dietary fiber with potential applications in food and nonfood industries. Direct application of lignocellulosic agricultural by-products as fibers in the textile sector can support the development of new environmentally friendly, biobased, and biodegradable raw materials to meet the ever-growing needs of the industry. Innovative concepts and contemporary technologies are researched for the effective utilization of plant-based waste and by-products from the agricultural and agro-industrial sectors to extract fibers for diverse applications, including the fashion industry. Two major routes are identified to produce cellulose fibers: the extraction and purification of natural cellulose fibers and the extraction and purification of cellulose pulp that is further processed into manmade cellulosic fiber and several sub routes are mapped. Scalability of experimental results at the laboratory or pilot level is a major barrier, so it is critical to develop closed-loop processes, apply standardization protocols, and conduct life cycle assessments and techno-economic analyses to facilitate large-scale implementation. In the experimental section of the thesis, the production of cellulose pulp from peach (Prunus persica) fruit wastes generated during the processing of a Greek compote and juice production industry were investigated. A three-step chemical process is used, including alkaline treatment with NaOH, organic acid (acetic and formic) treatment, and hydrogen peroxide treatment, with the goal of cellulose extraction and purification. The samples obtained were evaluated based on their α-cellulose content and degree of polymerization following a fractional factorial design using different reagent levels. They were further characterized by XRD, FTIR, SEM, and TGA techniques. Color, lightness, and lignin continent were also studied. The results of the XRD and FTIR spectra confirmed the presence of cellulose, revealing a very good crystallization of about 57%. The SEM analysis demonstrated a strong morphological agreement between the final product and the commercial dissolving grade pulp, and the comparison of the samples' SEM images with those of the raw material confirms a good purification. The successful extraction of the peach pulp was followed by its blending with a-cellulose powder at a ratio of 25:75 and the addition of the organic solvent N-methylmorpholine N-oxide (NMMO) to prepare the spinning dope. Next, laboratory experiments using a wet spinning machine confirmed the suitability of the pulp to produce regenerated cellulosic fiber from airgap spinning of NMMO solution, establishing proof of concept that the agro-industrial peach waste can be upcycled to produce manmade cellulosic fibers (MMCFs). A lab-scale process was developed after implementing technical adjustments to improve both the process and the morphology of the fiber. The latter was confirmed via scanning electron microscopy. Measurements of chlorinated phenols, organotin compounds by gas chromatography-mass spectrometry confirmed their absence in the produced fibers, making them compatible with the EU regulation on the Registration, Evaluation, Authorization, and Restriction of Chemicals (REACH) and the OEKO-TEX® Standard 100 as to the specific parameters. pH values confirm the compatibility of the produced cellulose fibers with the skin and indicate the absence of finishing or other chemicals. The ability to recycle NMMO is the principal element that renders the entire wet spinning process ecologically sustainable and cost effective. The wastewaters that include the diluted NMMO solution from the coagulation and washing baths of the fiber spinning process, undertake a purification procedure to separate the aqueous NMMO from all other impurities. These include disinfection, filtration, flocculation, and ion exchange. For condensation of the purified NMMO solution the reverse osmosis (RO) membrane technology was researched. The 1.76 wt.% feed solution was energy efficiently condensed using RO up to 9.84 wt.%, which was the upper limit possible imposed by the osmotic phenomena at 50 bar transmembrane pressure. The purification efficacy of this series of methodologies was confirmed by 1H-NMR measurements, which delivered spectra that were comparable to those of the commercial aqueous NMMO solution.
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