The isolation of nanocellulose from different agricultural residues is becoming an important research field due to its versatile applications. This work collects different production processes, including conditioning steps, pretreatments, bleaching processes and finally purification for the production of nanocellulose in its main types of morphologies: cellulose nanofiber (CNF) and cellulose nanocrystal (CNC). This review highlights the importance of agricultural wastes in the production of nanocellulose in order to reduce environmental impact, use of fossil resources, guarantee sustainable economic growth and close the circle of resource use. Finally, the possible applications of the nanocellulose obtained as a new source of raw material in various industrial fields are discussed.

Lignin, the most abundant aromatic polymer in nature, plays a critical role in lignocellulosic biomasses by providing structural support. However, its presence complicates the industrial exploitation of these materials for biofuels, paper production and other high-value compounds. Annually, the industrial extraction of lignin reaches an estimated 225 million tons, yet only a fraction is recovered for reuse, with most incinerated as low-value fuel. The growing interest in lignin potential has sparked research into sustainable recovery methods from lignocellulosic agro-industrial wastes. This review examines the chemical, physical and physicochemical processes for isolating lignin, focusing on innovative, sustainable technologies that align with the principles of a circular economy. Key challenges include lignin structural complexity and heterogeneity, which hinder its efficient extraction and application. Nonetheless, its properties such as high thermal stability, biodegradability and abundant carbon content place lignin as a promising material for diverse industrial applications, including chemical synthesis and energy generation. A structured analysis of advancements in lignin extraction, characterization and valorization offers insights into transforming this undervalued by-product into a vital resource, reducing reliance on non-renewable materials while addressing environmental sustainability.

Olive tree leaves, an abundant agricultural by-product without enough industrial market outlets, are presented in this study as a relevant resource of available carbohydrates to be chemically treated for monomeric sugar production. Characterization of two main granulometric fractions is the starting point for testing the specific effect and the relevance of three main factors (time, temperature, and sulfuric acid concentration) on diluted acid hydrolysis with respect to oligosaccharides, simple sugars, and fermentation inhibitory compounds production. The selected conditions (100 ${}^{\circ }$ C, 90 min, and 6% w/w H ${}_2$ SO ${}_4$ ) to perform the small scale hydrolytic process, considering response surface methodology (2 ${}^3$ factorial design with center points), implied production of acetic acid and hydroxymethylfurfural in concentrations not exceeding 1.10 kg m ${}^{-3}$ and 0.25 kg m ${}^{-3}$ , respectively. Thus, these experimental conditions were the reference framework to evaluate the effect of a meaningful scaling stage in a hydrolysis reactor, considering kinetic parameters based on hydrolysis rates and d-glucose and d-xylose generation.

Hemicellulosic biomass from olive-tree pruning (OTPB) was used as a raw material in order to produce a hemicellulosic hydrolysate to be fermented with the non-traditional yeast Candida guilliermondii FTI 20037 to obtain ethanol and xylitol. The main objectives of this research were to study the most relevant kinetic parameters involved in the bioconversion process and the correlation between stirred-tank bioreactor and agitated Erlenmeyer flask fermentation. In a first scale-up (using Erlenmeyer flasks) incubated on a rotary shaker at 200 rpm, fermentation assays were performed to determine the most convenient process conditions and the adaptation of the microorganism to the concentrated OTPB and added nutrients culture medium. The best conditions (2.5 kg m${}^{-3}$ of initial yeast cells, pH of 5.5 and 30 ${}^{°}$C) were set in a bench bioreactor. A comparative study on ethanol and xylitol production was conducted in two scale scenarios, obtaining different results. In the bioreactor, 100% of D-glucose and partially D-xylose were consumed to produce an ethanol yield of 0.28 kg kg${}^{-1}$ and an ethanol volumetric productivity of 0.84 kg dm${}^{-3}$ h${}^{-1}$ as well as a yield and volumetric productivity in xylitol of 0.37 kg kg${}^{-1}$ and 0.26 kg dm${}^{-3}$ h${}^{-1}$, respectively. The kinetic results allowed increasing the action scale and obtaining more real results than the previous steps to enable mini-plant and industrial scaling.

The present work describes a protocol of chemical activation, with acid catalyst, of olive endocarps to obtain acid insoluble lignin-rich materials with high capacities for the adsorption of furfural present in aqueous media. During biomass activation, factors such as acid concentration, reaction time and temperature, solid/liquid ratio and the presence of water extractives strongly affected both the surface characteristics of the treated bioadsorbents and their capacities for furfural retention (percentage increase close to 600% with respect to the crude biomass). Once a treated solid with good adsorbent properties was obtained, the optimal conditions for adsorption were found: stirring speed 80 rpm, temperature 303 K and adsorbent load 7.5 g solid/50 cm³. Kinetic study indicated the pseudo-second order model provided the best fit of the experimental data. At 303 K, the equilibrium adsorption capacities values ranged from 2.27 mg g^-1 to 29.29 mg g^-1, for initial furfural concentrations between 0.49 g dm^-3 and 12.88 g dm^-3. Freundlich model presented the best isotherm (R² = 0.996 and SE = 4.7%) providing K_F and n values of 0.115 (mg g^-1) (mg dm^-3)^-n and 0.610, respectively. Since physical interactions predominate in the adsorption of furfural on chemically activated olive endocarps, the furfural removal process could have occurred reversibly on the heterogeneous surface of the bioadsorbents.