Development of sustainable processes for obtaining lignin from sugarcane by-products and its valorisation

Abstract

The sugarcane industry generates several by-products, including straw (SCS) and bagasse (SCB). Their conversion into value-added products is of the utmost importance from a circular economy perspective. Lignin - one of the three main components of biomasses - has attracted considerable interest as a versatile ingredient for different applications. Therefore, this thesis aims to valorise, in a sustainable way, sugarcane residues (SCS and SCB), through the development of lignin-based products. In Chapter 1, it is presented a literature review focused on the valorisation of lignin obtained from sugarcane by-products. In Chapter 2, the detailed objectives of the project are presented. Chapter 3 presents different pre-treatments for lignin extraction, namely, alkaline, organosolv and deep eutectic solvents (DES). Furthermore, the combination of pretreatments was carried out following an integrated process approach: hydrothermal pretreatment followed by an alkaline pretreatment. The alkaline pretreatment of SCB proved to be the most effective among all pretreatments, producing lignin with distinct properties, such as purity (89%), colour (light brown; L*/a*/b*: 60.3/6.1/19.1), odour (woody sweet) and production cost (29 €/kg). Regarding sustainability, all processes achieved excellent scores ranging from 83 to 93%, with the alkaline extraction achieving the highest value. Chapter 4 describes the purification of the lignin obtained with lower purity. In this context, the alkaline liquor from the SCS was subjected to a two-stage membrane filtration step: first the liquor was filtered with a polyvinylidene fluoride microfiltration membrane (500-1000 kDa), followed by filtration with a 10 kDa polyethersulfone ultrafiltration membrane to concentrate the permeate flux. After acid precipitation at pH 2, the lignin purity increased from 61% to 84%. Chapter 5 details the characterisation of the lignins obtained by the pre-treatments in terms of physicochemical and structural properties. The results confirmed the presence of the main functional groups in the seven lignins, with some differences attributed to the biomass. The highest content of total hydroxyl phenolic units was obtained for the organosolv SCB and integrated process lignins. It was also concluded that guaiacyl (G) units were predominant in the SCB pretreatments, followed by syringyl (S) units. The organosolv lignin showed lower molecular weight, while the highest molecular weight distribution was observed for the SCS lignins with DES and alkaline pretreatments. In Chapter 6, the antioxidant and antibacterial activities of the SCB and SCS lignins from the different optimised pretreatments were evaluated, and the relationship between chemical structure and lignin functionality was elucidated. The total phenolic content and antioxidant activity were significantly higher for the alkaline, organosolv and integrated process SCS and SCB lignins. Regarding the antibacterial activity, the alkaline lignins showed activity against both representative bacteria evaluated (Escherichia coli and Staphylococcus aureus), with the SCB alkaline lignin showing the best performance. Chapter 7 explores the development of two products in the areas of cosmetics and materials. For the cosmetics field, lignin was tested as a natural UV blocker, antioxidant, and pigment in an blemish balm (BB) cream formulation. Remarkable results were achieved in terms of in vitro and in vivo SPF of 9.5 ± 2.9 and 9.6 ± 0.8, respectively. Furthermore, the lignin-based BB cream showed a broad-spectrum UV protection (critical wavelength 378 ± 0.5 nm). Lignin also showed antioxidant capacity after incorporation, being safe for topical applications. For materials, lignin was explored as a waterproofing agent in coatings for cardboard packaging. The results showed that the lignin-based coating was able to increase the water resistance of paper by up to three times, suggesting that this coating is a promising solution to improve the performance of paper-based packaging. The coating also demonstrated resistance to cracking by 90° folding. The results of this study corroborate the potential of sugarcane residues as a natural and sustainable source of new ingredients for the development of innovative lignin-based products.

Date of Award	2 Feb 2024
Original language	English
Awarding Institution	Universidade Católica Portuguesa
Supervisor	Patrícia Filipa Santos Costa (Supervisor), Maria Inês Ferreira da Mota (Co-Supervisor) & João Porfírio da Silva Burgal (Co-Supervisor)