The valorization of lignocellulosic biomasses derived from olive groves offers advantages related to sustainability, waste management, and process economy. This work explores the chemical activation of fragmented olive endocarps as a strategy to produce a novel bioadsorbent for furfural removal from aqueous solutions. Using Response Surface Methodology (RSM), robust mathematical models were developed to predict both bioadsorbent yield and furfural adsorption percentage under varying activation conditions. The optimal process involved an initial chemical attack with 88.4% H2SO4 at 30 °C for 21.79 hours followed by autoclaving at 121 °C for 1 hour. This resulted in a material with a 752% increase in adsorption capacity compared to the raw material. FTIR analysis revealed significant structural changes, including degradation of hemicellulose and cellulose and enrichment in lignin. Moreover, chemical activation increased the specific surface area and pore volume, yielding a mesoporous adsorbent. Adsorption kinetics followed a pseudo-second-order model (R2 = 0.999-1.000, SE < 5%) while equilibrium data at 303 K fitted the Freundlich isotherm (R2 = 0.999, SE = 4.7%). The desorption of furfural from activated olive endocarps using water was enhanced by temperature, with 70.1% of the aldehyde recovered at 40 °C after six sequential bioadsorbent regeneration stages.

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 pruning (OTP) is one of the most abundant sources of biomass waste in the Mediterranean basin. This is especially relevant in southern Spain where olive oil production represents a large part of the economy. Olive tree prunings are mostly either burned or are spread in olive orchards as an organic amendment, or used for heat generation on a domestic scale. However, the lignocellulosic composition of OTP makes it a potential source of biopolymers, thus providing an excellent economic alternative for the olive oil sector. In this work, pretreated OTP fibers were subjected to an optimized alkaline treatment followed by a single-step bleaching reaction with H2O2. Afterwards, the cellulose pulp was transformed chemically to obtain cellulose acetate. Noncellulosic components were removed effectively from OTP, thus obtaining a pulp highly purified in cellulose with 71% crystallinity and 355 degrees C maximum degradation temperature. Nevertheless, a very large amount of cellulose (ca. 50%) was eliminated throughout the process, especially during acid pretreatment, which was responsible for 38% solubilization. A similar level of acetylation and degree of substitution was obtained by using acetylation times in the range of 1 to 6 h. No large differences were observed in the infrared spectra and X-ray diffractograms of the synthesized acetates. However, their thermal stability varied significantly with reaction time, evolving from a multistep degradation pattern to a single and sharp peak between 300 and 400 degrees C with increasing time. Thermogravimetric curves revealed that at least 5 h (preferably 6 h) were needed to obtain cellulose acetate from OTP with adequate thermal stability for further processing.

A two-step chemical process was carried out on olive pruning residues according to an optimised sequence that led to the isolation of natural fibre with a high cellulose content. Reaction time, temperature and HNO 3 concentration in the acid hydrolysis stage were optimised by means of the Response Surface Methodology to achieve the highest removal of hemicellulose and lignin and the highest crystallinity index, minimising cellulose hydrolysis. Subsequent hydrolysis with NaOH allowed to obtain a pulp enriched in cellulose (83.28 wt.%). Analysis revealed that the cellulose isolated had a high crystallinity index (70.06%) and thermal stability ( T max = 357°C). The cellulose obtained was finally used for the manufacture of polymer biocomposites and to evaluate its viability as a filler for polymeric materials. The selected polymer matrix used was polylactic acid (PLA) and the amount of filler was 5 and 15% by weight, respectively. In general, the fibres did not improve the mechanical properties of PLA, and maintained unchanged its melting temperature. Microscopic analysis revealed that PLA/fibre adhesion was stronger for treated fibres. Contradictorily, the composites with untreated fibres presented slightly higher thermal stability. Water uptake increased with the concentration of fibres, being higher in those materials with untreated fibre.

Research studies for cellulose recovery from lignocellulosic materials are essential in order to propose sustainable alternatives to harness residual biomasses, solving problems caused by their abundance and inadequate use. In this study, olive-tree pruning biomass has been subjected to different pretreatments with different organosolvents (acetone, ethanol, and γ-valerolactone) with microwave radiation assistance. The effect of operating parameters has been studied, considering specific ranges of variables values according to each experimental design but, in any case, located in the ranges of 33–67% (chemical compound concentration), 130–170 °C (temperature), 5–30 min (reaction time), and 1/20–1/5 (solid/liquid ratio, s/L). Based on the R2 and R2adj values (mostly above 0.97), the experimental data were adequately adjusted to four selected response variables: post-solids cellulose and lignin content apart from removal percentages of both structural components. The optimization process resulted in post-treatment solids with meaningful cellulose yields (higher than 84.7%) and reduced lignin content (lower than 4.2%). The best results were obtained using 66.5% acetone (155 °C, 8.4 min and s/L = 1/19), involving greater material deconstruction, a high percentage of delignification (96.7%), not very significant cellulose loss (29.4%), and a post-treatment solid consisting almost exclusively of cellulose (≈99%).

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 KF 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.

Globally, huge amounts of cotton and sunflower stalks are generated annually. These wastes are being underutilized since they are mostly burned in the fields. So, in this work, we proposed a three-step method consisting of acid pre-treatment, alkaline hydrolysis, and bleaching for the extraction of cellulose pulps. These pulps were characterized to assess their morpho-structural and thermal properties. The design of experiments and response surface methodology were used for the optimization of the acid pre-treatment in order to achieve maximum removal of non-cellulosic compounds and obtain pulps enriched in cellulose. For cotton stalks, optimal conditions were identified as a reaction time of 190 min, a reaction temperature of 96.2 °C, and an acid (nitric acid) concentration of 6.3%. For sunflower stalks, the optimized time, temperature, and acid concentration were 130 min, 73.8 °C, and 8.7%, respectively. The pulps obtained after bleaching contained more than 90% cellulose. However, special care must be taken during the process, especially in the acid pre-treatment, as it causes the solubilization of a great amount of material. The characterization revealed that the extraction process led to cellulose pulps with around 69–70% crystallinity and thermal stability in the range of 340–350 °C, ready to be used for their conversion into derivatives for industrial applications.

The overall purpose of this research work was to apply a microwave-assisted digestion process combined with ethanol organosolv as pretreatment for pistachio shells (PS) and cherry tree pruning (CTP) biomasses for cellulose production. This process would reduce the technical and environmental disadvantages of the accumulation of these agricultural wastes. A central composite design based on the Response Surface Methodology was applied to check the effect of reaction time, temperature, and concentration of ethanol acid solution. Adequate models have been obtained for the contents of cellulose and lignin of the treated solids as well as for solubilized cellulose percentage. Best conditions implied 67 ^∘ C, involving the production of a cellulose-enriched material (81.1 ^3 for CTP and PS, resulting in enzymatic hydrolysis yields at 12 h of 97.2

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.

Debaryomyces hansenii has been employed to study, initially, the influence of the oxygen availability on D-xylose to xylitol fermentation, as this parameter is considered as one of the most critical variables for this bio alcohol accumulation. Apart from the air supplied in the fermentation process through the stirring vortex (0.0 v/v/min), additional aeration rates (0.1-2.0 v/v/min) effects were discussed. Furthermore, a change in the fermentative medium composition as well as a comparative analysis of D. hansenii behavior with respect to fermentation of D-glucose and D-xylose mixtures solutions, with the aim of producing both xylitol and ethanol bioproducts, were performed. For these purposes, specific growth rates, biomass productivities, specific substrate-uptake rates, overall biomass yields, specific xylitol formation rates and overall xylitol yields values have been calculated, applying a differential method to the kinetic data. Aeration influence was clearly evinced since a faster D-xylose metabolism, for aeration values close to 1.0 v/v/min, was noted. This yeast exhibited a sequential substrate consumption, firstly D-glucose and then D-xylose. The maximum xylitol yield (0.32 kg kg- 1) was obtained for 0.5 v/v/min airflow, remarking a significant reduction of this parameter for both above and below the quoted air supply value.

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.

Furfural presence in fermentation culture media provokes an inhibition effect in yeast growth. In fact, is a key toxin in lignocellulosic hydrolyzates. This research work attempts to analyze the use of olive endocarp (OE) as adsorbent to remove furfural from aqueous solutions. The adsorption experimental sets studied the effect of particle size, agitation speed, adsorbent load, temperature, and initial furfural concentration. As results, higher adsorption percentages were observed when fragmented OE was smaller than 1.2 mm, agitation speed in the range 80-250 rpm, and the adsorbent load was 90 g OE/300 cm3 of furfural solution (1 g dm-3). The equilibrium adsorption capacities, qe, values were varied from 0.270 to 4.750 mg g-1 for initial furfural concentrations of 0.78 g dm-3 to 5.89 g dm-3. The adsorption data fit to pseudo-second order kinetic model showed good fit values (R2 >= 0.997, SE <= 5%). Dubinin-Radushkevich (DR) model was considered as the best fit (R2 = 0.999, SE = 2.95%) for the different studied isotherms. An adsorption free energy value of 896 J mol-1 indicated a physical adsorption mechanism. Remarkable increases in furfural adsorption percentages have been achieved submitting natural olive endocarps to acid treatments.

Processes efficiency for second-generation ethanol production depends mainly on the type of lignocellulosic raw material. Therefore, the optimization (considering a central composite design) for each step involved in olive-tree pruning biomass valorization was studied: (1) alkaline pretreatment of the original feedstock, (2) diluted acid hydrolysis of pretreated solids and (3) fermentation of the hemicellulosic hydrolyzates for ethanol production by Scheffersomyces stipitis. The recommended alkaline pretreatment conditions were 30 min, 90 degrees C and 0.5% w/v NaOH, with losses of 88.3% of acetyl groups from starting biomass, but only 6.9% of D-xylose. Comparing both, in natura and previously treated acid hydrolyzates at the most effective conditions (2.0% w/v H2SO4 and 60 min) revealed more inhibitory effect for non-treated liquor, with 4.8, 2.1 and 1.6 times higher concentrations of acetic acid, furans and phenolic compounds, respectively. A significant improvement in ethanol production was observed in treated hemicellulose liquor (20.4 g dm(-3), Y-P/S = 0.20 g g(-1) and Q(p) = 0.21 g dm(-3)h(-1)). In contrast, the yeast could not satisfactorily ferment the reference hydrolyzate. Biomass pretreatment with alkali previously to dilute acid hydrolysis was a suitable strategy for olive-tree pruning biomass biotransformation, substantially decreasing the hydrolyzate toxicity, without requiring an additional detoxification step.

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 ∘ 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.

The application of an effective removal treatment to minimize inhibitory compounds, present after hydrolysis treatment, might mean a great advance on renewable biofuel generation strategy since growth-inhibiting substances, such as phenolic compounds and furans, are one of the major problems in the development of fermentative step. Detoxification of olive-tree pruning hydrolyzates using NaBH4, as reducing agent, has had a positive effect on fermentative parameters using Pichia stipitis CBS 6054. Oxalic acid treatment conditions have been optimized (150 ∘C and 75 mM of oxalic acid in order to obtain the maximum fermentable sugar recovery without an excessive inhibitory compounds production) considering the combined severity factor, CSF, (CSF = 1.56 ± 0.05). Hydrolyzate characterization at optimal condition showed concentrations (in g/L) of 16.64 D-glucose, 15.05 D-xylose, 5.16 L-arabinose, 2.87 acetic acid, 3.18 phenolic compounds, and 0.96 furans. Sodium borohydride conditioning process has been studied using a response surface methodology. The use of NaBH4 (0.03 mol/L during 30 min at pH= 6.0) has been demonstrated to be a successful conditioning method for almost entirely furans elimination as well as 40% of phenolic compounds reduction. As a result, it has been possible to ferment sugary liquors which did not ferment without such treatment, obtaining a maximum ethanol yield of 27% (3.8 g/L) in treated hydrolyzates under optimum conditions.

In the current study, the potential application of baker's yeast as biological agent for the detoxification of rice straw hemicellulosic hydrolyzate containing high initial D-xylose content has been evaluated with the goal of improving ethanol production by Scheffersomyces stipitis. As required, various biodetoxification conditions in terms of treatment time, cell density and pH were assessed by measuring ethanol yield (YP/S) and ethanol volumetric productivity (QP) when the treated hydrolyzate was fermented by S. stipitis. Our results showed that baker's yeast is able to reduce the toxicity of hydrolyzate with only 6 h biotreatment. Interestingly, the maximum ethanol production from biotreated hydrolyzate was not correlated with the complete removal of furan and phenolic compounds, but when acetic acid was reduced from the medium. Under selected biotreatment conditions (5.0 g dm(-3) baker's yeast concentration at pH 3.0 for 6 h), the fermentative performance of S. stipitis was noticeably favored in bench top bioreactor, i.e., fermentable sugars were completely consumed with production of 23.0 g dm(-3) ethanol after 44 h (YP/S = 0.24 g g(-1) and QP = 0.52 g dm(-3) h(-1)). Based on our results, baker's yeast may be considered a promising detoxification method for application in biorefineries, especially because its failure to consume D-xylose, which is the major sugar in these media, besides it is recognized as safe (GRAS status) and largely available commercially. Certainly, this bioprocess could be an important step toward processing lignocellulosic biomass for the development of second-generation ethanol production.

In this work the effect of spent diatomaceous earth (SDE), as pore forming or silica carrier, on the properties of clay ceramic bricks was investigated. Mixtures of clay and SDE waste were, compacted, dried and fired at 950 degrees C during 6 h. The technological properties of waste-clay bricks were analysed to evaluate the influence of the waste incorporation. The SDE (1-5 wt-%) addition in the clay matrix causes a reduction between 6 and 12% of the bulk density compared with the control samples (only clay) which implies good insulating characteristics. In addition, the waste incorporation increases the water absorption within the range 4-31% and decreases the compressive strength between 12 and 26%. However, the incorporation of the waste is beneficial as no remarkable changes in the technological properties are observed. These results indicate that the waste has a predominant pore forming effect over a pore filling effect.

After acid hydrolysis with \(\hbox {H}_2\hbox {SO}_4\) from olive pruning, subsequent fermentations were carried out in order to compare two non-traditional yeasts: Candida guilliermondii and Pichia stipitis. During the fermentations, sugar uptake as well as ethanol and xylitol production were determined. However, both yeast employed for the biotransformations showed different behaviours; C. guilliermondii produced ethanol from D-glucose and xylitol from D-xylose but, in contrast, P. stipitis only was able to produce ethanol from hexoses and pentoses although, due to the inhibitors amount (acetic acid and polyphenols mainly), it required a detoxification step. To solve this problem, activated charcoal treatment as well as a vacuum evaporation process (concentration ratio 2.7) were performed as physical detoxification methods with positive results. The maximum ethanol and xylitol yields (\(Y_{P/S}\)) (calculated on consumed sugars) obtained with C. guilliermondii were 0.38 and 0.31 kg \(\hbox {kg}^{-1}\) respectively; while P. stipitis was able to produce 0.33 kg of ethanol per kg of fermentable sugar.