Microwave irradiation--A green and efficient way to pretreat biomass

Enhanced production of bioethanol through supercritical carbon dioxide-mediated pretreatment and saccharification of dewaxed bagasse

The pretreatment and saccharification of dewaxed bagasse (DWB) has been investigated under various reaction conditions ranging 2000 to 3200 psi, at 70 ± 1 °C in supercritical carbon dioxide (SCC). This has been in attempt to transform the DWB into fermentable sugar and bioethanol in high yields. The effect of SCC mediated pretreatment and enzymatic hydrolysis on structural and morphological alterations in DWB has been ascertained through diverse analytical methods. The sugar has been released through cellulase (40 FPU/mL) mediated enzymatic hydrolysis of pretreated DWB in sodium acetate buffer (pH 4.7) within 1 h at SCC 2800 psi, 70 ± 1 °C. The released sugar was subsequently fermented in the presence of yeast (Saccharomyces crevices, 135 CFU) at 28 ± 1 °C over 72 h to afford the bioethanol. The SCC mediated process conducted in acetic acid:water media (1:1) at 2800 psi, 70 ± 1 °C over 6 h has afforded the pretreated DWB with maximum yield towards the production of fermentable sugar and bioethanol. The production of fermentable sugar and bioethanol has been electrochemically estimated through cyclic voltammetry (CV) and square wave voltammetry (SWV) over glassy carbon electrode in KOH (0.1 M). The electrochemical methods were found selective and in close agreement for estimation of the yields (%) of fermentable sugars and bioethanol. The yield (%) of fermentable sugar estimated from CV and SWV were 80.10 ± 5.34 and 79.00 ± 5.09 respectively. Whereas the yield (%) of bioethanol estimated from CV and SWV were 81.30 ± 2.78% and 78.6 ± 1.25% respectively. Present investigation delivers a SCC mediated green and sustainable method of pretreatment of DWB to afford the enhanced saccharification, to produce bioethanol in high yields.

Enhancing saccharification of bamboo shoot shells by rapid one-pot pretreatment of hydrated deep eutectic solvent

In this work, a rapid one-pot hydrated deep eutectic solvent (DES) pretreatment was proposed to facilitate the conversion of carbohydrates from lignocellulosic biomass to monosaccharides. Specifically, the pure and hydrated DES based on benzyl triethylammonium chloride (BTEAC), formic acid (FA) and water was used to pretreat bamboo shoot shells (BSS) by microwave heating. The pretreated solid residues were enzymatically saccharified to produce fermentable sugars, and the hydrolyzed carbohydrates and lignin remained in the hydrolyzate. The results showed that the yield of monosaccharides from the hydrated DES hydrolyzate (193.7-228.4 g/kg) was significantly higher than that (45.9-66.1 g/kg) of pure DES. The 30% hydrated DES pretreatment achieved the best glucose yield (89.03%) and a total monosaccharides yield of 555.4 g/kg, which corresponded to a conversion ratio of carbohydrates to monosaccharides of 87.0%. The proposed process is a robust method for the efficiently convert carbohydrates from BSS into monosaccharides.

Different pre-treatments and kinetic models for bioethanol production from lignocellulosic biomass: A review

Lignocellulosic biomass is the generally explored substrate to produce bioethanol for environmental sustainability due to its availability in abundance. However, the complex network of cellulose-hemicellulose-lignin present in it makes its hydrolysis as a challenging task. To boost the effectiveness of conversion, biomass is pre-treated before enzymatic hydrolysis to alter or destroy its original composition. Enzymes like Cellulases are widely used for breaking down cellulose into fermentable sugars. Enzymatic hydrolysis is a complex process involving many influencing factors such as pH, temperature, substrate concentration. This review presents major four pre-treatment methods used for hydrolysing different substrates under varied reaction conditions along with their mechanism and limitations. A relative comparison of data analysis for most widely studied 10 kinetic models is briefly explained in terms of substrates used to get the brief insight about hydrolysis rates. The summary of pre-treatment methods and hydrolysis rates including cellulase enzyme kinetics will be the value addition for upcoming researchers for optimising the hydrolysis process.

Pretreatment and composting technology of agricultural organic waste for sustainable agricultural development

With the continuous development of agriculture, Agricultural organic waste (AOW) has become the most abundant renewable energy on earth, and it is a hot spot of research in recent years to realize the recycling of AOW to achieve sustainable development of agricultural production. However, lignocellulose, which is difficult to degrade in AOW, greenhouse gas emissions, and pile pathogenic fungi and insect eggs are the biggest obstacles to its return to land use. In response to the above problems researchers promote organic waste recycling by pretreating AOW, controlling composting conditions and adding other substances to achieve green return of AOW to the field and promote the development of agricultural production. This review summarizes the ways of organic waste treatment, factors affecting composting and problems in composting by researchers in recent years, with a view to providing research ideas for future related studies.

Recent advances toward structural incorporation for stabilizing heavy metal contaminants: A critical review

Heavy metal pollution has resulted in serious environmental damage and raised significant public health concerns. One potential solution in terminal waste treatment is to structurally incorporate and immobilize heavy metals in some robust frameworks. Yet extant research offers a limited perspective on how metal incorporation behavior and stabilization mechanisms can effectively manage heavy metal-laden waste. This review sets forth detailed research on the feasibility of treatment strategies to incorporate heavy metals into structural frameworks; this paper also compares common methods and advanced characterization techniques for identifying metal stabilization mechanisms. Furthermore, this review analyses the typical hosting structures for heavy metal contaminants and metal incorporation behavior, highlighting the importance of structural features on metal speciation and immobilization efficiency. Lastly, this paper systematically summarizes key factors (i.e., intrinsic properties and external conditions) affecting metal incorporation behavior. Drawing on these impactful findings, the paper discusses future directions in the design of waste forms that efficiently, effectively treat heavy metal contaminants. By examining tailored composition-structure-property relationships in metal immobilization strategies, this review reveals possible solutions for crucial challenges in waste treatment and enhances the development of structural incorporation strategies for heavy metal immobilization in environmental applications.

Simultaneous microwave-assisted phosphotungstic acid catalysis for rapid improvements on the accessibility and reactivity of Kraft-based dissolving pulp

Improving the cellulose accessibility and reactivity in an efficient and convenient way has become the focused issue in the field of dissolving pulp manufacturing. We herein demonstrate a simple yet efficient strategy, namely a simultaneous microwave (MW)-assisted phosphotungstic acid (PTA) catalysis (MW-PTA_sim). The MW-PTA_sim treatment was efficient to improve Fock reactivity from 49.1 % to 85.8 % and decrease viscosity from 561 to 360 mL/g within 10 min, which was superior to the single MW treatment and the sequential MW-PTA_seq treatment. Besides, the MW-PTA_sim treated fiber had rougher and more fibrillated surfaces with an enhanced fiber accessibility, showing increased specific surface area (SSA) from 1.43 to 6.31 m²/g, mean pore diameter (MPD) from 6.92 to 11.20 nm and water retention value (WRV) from 101 % to 172 %. These positive enhancements are mainly due to a synergy that MW-enhanced rotation of PTA mediums was served as "spinning cutters" to attack the fibers, plus MW-accelerated PTA transfer and catalytic hydrolysis further improved the fiber accessibility. Moreover, PTA also demonstrates a high reusability and chemical stability. This process offers an effective and sustainable alternative for manufacturing a premium dissolving pulp.

Challenges and perspectives of green-like lignocellulose pretreatments selectable for low-cost biofuels and high-value bioproduction

Lignocellulose represents the most abundant carbon-capturing substance that is convertible for biofuels and bioproduction. Although biomass pretreatments have been broadly applied to reduce lignocellulose recalcitrance for enhanced enzymatic saccharification, they mostly require strong conditions with potential secondary waste release. By classifying all major types of pretreatments that have been recently conducted with different sources of lignocellulose substrates, this study sorted out their distinct roles for wall polymer extraction and destruction, leading to the optimal pretreatments evaluated for cost-effective biomass enzymatic saccharification to maximize biofuel production. Notably, all undigestible lignocellulose residues are also aimed for effective conversion into value-added bioproduction. Meanwhile, desired pretreatments were proposed for the generation of highly-valuable nanomaterials such as cellulose nanocrystals, lignin nanoparticles, functional wood, carbon dots, porous and graphitic nanocarbons. Therefore, this article has proposed a novel strategy that integrates cost-effective and green-like pretreatments with desirable lignocellulose substrates for a full lignocellulose utilization with zero-biomass-waste liberation.

Advances in physicochemical pretreatment strategies for lignocellulose biomass and their effectiveness in bioconversion for biofuel production

The inherent recalcitrance of lignocellulosic biomass is a significant barrier to efficient lignocellulosic biorefinery owing to its complex structure and the presence of inhibitory components, primarily lignin. Efficient biomass pretreatment strategies are crucial for fragmentation of lignocellulosic biocomponents, increasing the surface area and solubility of cellulose fibers, and removing or extracting lignin. Conventional pretreatment methods have several disadvantages, such as high operational costs, equipment corrosion, and the generation of toxic byproducts and effluents. In recent years, many emerging single-step, multi-step, and/or combined physicochemical pretreatment regimes have been developed, which are simpler in operation, more economical, and environmentally friendly. Furthermore, many of these combined physicochemical methods improve biomass bioaccessibility and effectively fractionate ∼96 % of lignocellulosic biocomponents into cellulose, hemicellulose, and lignin, thereby allowing for highly efficient lignocellulose bioconversion. This review critically discusses the emerging physicochemical pretreatment methods for efficient lignocellulose bioconversion for biofuel production to address the global energy crisis.

Pretreatment of sugarcane bagasse with acid catalyzed ethylene glycol-water to improve the cellulose enzymatic conversion

In this work, HCl catalyzed ethylene glycol-water pretreatment (HCl/EG-H₂O) of sugarcane bagasse (SCB) was explored with response surface methodology (RSM) and single factor analysis, which aim to investigate the influence of pretreatment variable on pretreated solid cellulose enzymatic conversion. The result showed that HCl/EG-H₂O pretreatment could selectively extract ∼89.9 % xylan and ∼61.2 % lignin in SCB, meanwhile maintain a relatively high cellulose retention (∼86.8 %). Pretreatment of SCB at 120 °C for 60 min with 1.00 % HCl and 90 % EG obtained the pretreated solid having maximum cellulose enzymatic conversion of 88.7 % under 10 FPU/g enzyme dosage, this enhancement of cellulose enzymatic conversion mainly attributed to structure change of SCB in pretreatment. The adding of enzymatic additives into the hydrolysis process could not only improve hydrolysis efficiency but also lower the enzyme dosage. Besides, the linear relationship between substrate characteristic parameters (such cellulose content, lignin removal rate etc.) and cellulose conversion were observed.

Recent Advancements and Challenges in Lignin Valorization: Green Routes towards Sustainable Bioproducts

The aromatic hetero-polymer lignin is industrially processed in the paper/pulp and lignocellulose biorefinery, acting as a major energy source. It has been proven to be a natural resource for useful bioproducts; however, its depolymerization and conversion into high-value-added chemicals is the major challenge due to the complicated structure and heterogeneity. Conversely, the various pre-treatments techniques and valorization strategies offers a potential solution for developing a biomass-based biorefinery. Thus, the current review focus on the new isolation techniques for lignin, various pre-treatment approaches and biocatalytic methods for the synthesis of sustainable value-added products. Meanwhile, the challenges and prospective for the green synthesis of various biomolecules via utilizing the complicated hetero-polymer lignin are also discussed.

Assessment of the Pretreatments and Bioconversion of Lignocellulosic Biomass Recovered from the Husk of the Cocoa Pod

The production of biofuels (biogas, ethanol, methanol, biodiesel, and solid fuels, etc.), beginning with cocoa pod husk (CPH), is a way for obtaining a final product from the use of the principal waste product of the cocoa industry. However, there are limitations to the bioconversion of the material due to its structural components (cellulose, hemicellulose, and lignin). Currently, CPH pretreatment methods are considered a good approach towards the improvement of both the degradation process and the production of biogas or ethanol. The present document aims to set out the different methods for pretreating lignocellulosic material, which are: physical (grinding and extrusion, among others); chemical (acids and alkaline); thermochemical (pyrolysis); ionic liquid (salts); and biological (microorganism) to improve biofuel production. The use of CPH as a substrate in bioconversion processes is a viable and promising option, despite the limitations of each pretreatment method.

Breakthrough in hydrolysis of waste biomass by physico-chemical pretreatment processes for efficient anaerobic digestion

Anaerobic digestion (AD) is the most comprehended process to stabilise the waste biomass efficiently and to obtain bioenergy. The AD starts with the hydrolysis process, where the major liability is the action of inhibitors during the hydrolysis process. The biomass pretreatment preceding anaerobic digestion is obligatory to improve feedstock biodegradability for enhanced biogas generation. It can be prevailed by the application of various pretreatment processes. This review explains the major inhibiting compounds and their formation during hydrolysis that affect the efficiency of anaerobic digestion and the benefits of the physico-chemical pretreatment (PCP) method for enhancing hydrolysis in the digestion of waste biomass. The synergistic effect of PCP on macromolecular release, liquefaction and biodegradability were presented. The feasibility of the pretreatment process was evaluated in terms of energy and cost assessment for pilot scale implementation. The outcome of this review reveals that the physico-chemical process is one of the best pretreatment methods to enhance anaerobic digestion by optimising various parameters and increasing the solubilization by about 90%. The thermochemical pretreatment at lower temperature (<100) increases the net energy yield. The solubilization of waste biomass in terms of macromolecular release and liquefaction cannot describe the pretreatment potential. The effectiveness of pretreatment was evaluated by the substrate pre-treatment followed by anaerobic digestibility of pretreated substrate.

Development of different pretreatments and related technologies for efficient biomass conversion of lignocellulose

Lignocellulose, a kind of biological resource widely existing in nature, which can be transformed into value-added biochemical products through saccharification, fermentation or chemical catalysis. Pretreatments are the necessary step to increase the accessibility and digestibility of lignocellulose. This paper comprehensively reviewed different pretreatment progress of lignocellulose in recent year, including mechanical/thermal, biological, inorganic solvent, organic solvent and unconventional physical-chemical pretreatments, focusing on quantifying the influence of pretreatments on subsequent biomass conversion. In addition, related pretreatment techniques such as genetic engineering, reactor configurations, downstream process and visualization technology of pretreatment were discussed. Finally, this review presented the challenge of lignocellulose pretreatment in the future.

Advancement of biorefinery-derived platform chemicals from macroalgae: a perspective for bioethanol and lactic acid

The extensive growth of energy and plastic demand has raised concerns over the depletion of fossil fuels. Moreover, the environmental conundrums worldwide integrated with global warming and improper plastic waste management have led to the development of sustainable and environmentally friendly biofuel (bioethanol) and biopolymer (lactic acid, LA) derived from biomass for fossil fuels replacement and biodegradable plastic production, respectively. However, the high production cost of bioethanol and LA had limited its industrial-scale production. This paper has comprehensively reviewed the potential and development of third-generation feedstock for bioethanol and LA production, including significant technological barriers to be overcome for potential commercialization purposes. Then, an insight into the state-of-the-art hydrolysis and fermentation technologies using macroalgae as feedstock is also deliberated in detail. Lastly, the sustainability aspect and perspective of macroalgae biomass are evaluated economically and environmentally using a developed cascading system associated with techno-economic analysis and life cycle assessment, which represent the highlights of this review paper. Furthermore, this review provides a conceivable picture of macroalgae-based bioethanol and lactic acid biorefinery and future research directions that can be served as an important guideline for scientists, policymakers, and industrial players.

Graphical abstract

Anaerobic Digestion of Lignocellulosic Waste Materials

Nowadays, the climate mitigation policies of EU promote the energy production based on renewable resources. Anaerobic digestion (AD) constitutes a biochemical process that can convert lignocellulosic materials into biogas, used for chemical products isolation or energy production, in the form of electricity, heat or fuels. Such practices are accompanied by several economic, environmental and climatic benefits. The method of AD is an effective method of utilization of several different low-value and negative-cost highly available materials of residual character, such as the lignocellulosic wastes coming from forest, agricultural or marine biomass utilization processes, in order to convert them into directly usable energy. Lignin depolymerization remains a great challenge for the establishment of a full scale process for AD of lignin waste. This review analyzes the method of anaerobic digestion (biomethanation), summarizes the technology and standards involved, the progress achieved so far on the depolymerization/pre-treatment methods of lignocellulosic bio-wastes and the respective residual byproducts coming from industrial processes, aiming to their conversion into energy and the current attempts concerning the utilization of the produced biogas. Substrates’ mechanical, physical, thermal, chemical, and biological pretreatments or a combination of those before biogas production enhance the hydrolysis stage efficiency and, therefore, biogas generation. AD systems are immensely expanding globally, especially in Europe, meeting the high demands of humans for clean energy.

Review on upgrading organic waste to value-added carbon materials for energy and environmental applications

Value-added materials such as biochar and activated carbon that are produced using thermo-chemical conversion of organic waste have gained an emerging interest for the application in the fields of energy and environment because of their low cost and unique physico-chemical properties. Organic waste-derived materials have multifunctional abilities in the field of environment for capturing greenhouse gases and remediation of contaminated soil and water as well as in the field of energy storage and conversion. This review critically evaluates and discusses the current thermo-chemical approaches for upgrading organic waste to value-added carbon materials, performance enhancement of these materials via activation and/or surface modification, and recent research findings related to energy and environmental applications. Moreover, this review provides detailed guidelines for preparing high-performance organic waste-derived materials and insights for their potential applications. Key challenges associated with the sustainable management of organic waste for ecological and socio-economic benefits and potential solutions are also discussed.

Insight into the recent advances of microwave pretreatment technologies for the conversion of lignocellulosic biomass into sustainable biofuel

The utilization of renewable lignocellulosic biomasses for bioenergy synthesis is believed to facilitate competitive commercialization and realize affordable clean energy sources in the future. Among the pathways for biomass pretreatment methods that enhance the efficiency of the whole biofuel production process, the combined microwave irradiation and physicochemical approach is found to provide many economic and environmental benefits. Several studies on microwave-based pretreatment technologies for biomass conversion have been conducted in recent years. Although some reviews are available, most did not comprehensively analyze microwave-physicochemical pretreatment techniques for biomass conversion. The study of these techniques is crucial for sustainable biofuel generation. Therefore, the biomass pretreatment process that combines the physicochemical method with microwave-assisted irradiation is reviewed in this paper. The effects of this pretreatment process on lignocellulosic structure and the ratio of achieved components were also discussed in detail. Pretreatment processes for biomass conversion were substantially affected by temperature, irradiation time, initial feedstock components, catalyst loading, and microwave power. Consequently, neoteric technologies utilizing high efficiency-based green and sustainable solutions should receive further focus. In addition, methodologies for quantifying and evaluating effects and relevant trade-offs should be develop to facilitate the take-off of the biofuel industry with clean and sustainable goals.

Recent advances in green solvents for lignocellulosic biomass pretreatment: Potential of choline chloride (ChCl) based solvents

Biomass wastes exhibit a great potential to be used as a source of non-depleting renewable energy and synthesis of value-added products. The key to the valorization of excess lignocellulosic biomass wastes in the world lies on the pretreatment process to recalcitrant barrier of the lignocellulosic material for the access to useful substrates. A wide range of pretreatment techniques are available and advances in this field is continuously happening, in search for cheap, effective, and environmentally friendly methods. This review starts with an introduction to conventional approaches and green solvents for pretreatment of lignocellulosic biomass. Subsequently, the mechanism of actions along with the advantages and disadvantages of pretreatment techniques were reviewed. The roles of choline chloride (ChCl) in green solvents and their potential applications were also comprehensively reviewed. The collection of ideas in this review serve as an insight for future works or interest on biomass-to-energy conversion using green solvents.

Pretreatment of lignocellulosic biomass: A review on recent advances

Depleting fossil reserves and growing energy needs have raised the demand for an alternative and clean energy source. The use of ubiquitously available lignocellulosic biomass for developing economic and eco-friendly large scale biorefinery applications has provided the much-needed impetus in this regard. The pretreatment process is a vital step for biomass transformation into added value products such as sugars, biofuels, etc. Different pretreatment approaches are employed to overcome the recalcitrance of lignocellulosic biomass and expedite its disintegration into individual components- cellulose, hemicellulose, and lignin. The conventional pretreatment methods lack sustainability and practicability for industrial scale up. The review encompasses the recent advances in selective physical and chemical pretreatment approaches such as milling, extrusion, microwave, ammonia fibre explosion, eutectic solvents etc. The study will allow a deeper understanding of these pretreatment processes and increase their scope as sustainable technologies for developing modern biorefineries.

Optimization of biogas yield from lignocellulosic materials with different pretreatment methods: a review

Population increase and industrialization has resulted in high energy demand and consumptions, and presently, fossil fuels are the major source of staple energy, supplying 80% of the entire consumption. This has contributed immensely to the greenhouse gas emission and leading to global warming, and as a result of this, there is a tremendous urgency to investigate and improve fresh and renewable energy sources worldwide. One of such renewable energy sources is biogas that is generated by anaerobic fermentation that uses different wastes such as agricultural residues, animal manure, and other organic wastes. During anaerobic digestion, hydrolysis of substrates is regarded as the most crucial stage in the process of biogas generation. However, this process is not always efficient because of the domineering stableness of substrates to enzymatic or bacteria assaults, but substrates' pretreatment before biogas production will enhance biogas production. The principal objective of pretreatments is to ease the accessibility of the enzymes to the lignin, cellulose, and hemicellulose which leads to degradation of the substrates. Hence, the use of pretreatment for catalysis of lignocellulose substrates is beneficial for the production of cost-efficient and eco-friendly process. In this review, we discussed different pretreatment technologies of hydrolysis and their restrictions. The review has shown that different pretreatments have varying effects on lignin, cellulose, and hemicellulose degradation and biogas yield of different substrate and the choice of pretreatment technique will devolve on the intending final products of the process.

Microwave irradiation coupled with zero-valent iron that enhances the composite geopolymerization of chromite ore processing residue and its mechanisms

In this work, microwave (MW) irradiation was employed to enhance the zero-valent iron (ZVI)-dominated de-contamination of chromite ore processing residue (COPR). A coupling system and the traditional two-step procedure were both conducted to evaluate the effects of MW irradiation on the reduction and the incorporation of COPR into the composite materials-based geopolymers. The factors including the ratios of liquid to solid, the mass ratios of ZVI to COPR, and the acid dosage had some obvious influence on the reduction of COPR in the MW system. The compressive strengths of 31.54 and 41.56 MPa were determined from the two-step procedure and the coupling system at the COPR dosage of 10% (mass ratio), respectively. The employment of MW irradiation not only strengthened the formation of the geopolymer matrices but also improved the chemical stabilization of Cr species in the solidified blocks. The coupled process was more conducive to incorporating the treated COPR into the geopolymer-based crystalline microstructures compared with the subsequent usage of ZVI reduction and MW irradiation.

Advances in Pretreatment of Straw Biomass for Sugar Production

Straw biomass is an inexpensive, sustainable, and abundant renewable feedstock for the production of valuable chemicals and biofuels, which can surmount the main drawbacks such as greenhouse gas emission and environmental pollution, aroused from the consumption of fossil fuels. It is rich in organic content but is not sufficient for extensive applications because of its natural recalcitrance. Therefore, suitable pretreatment is a prerequisite for the efficient production of fermentable sugars by enzymatic hydrolysis. Here, we provide an overview of various pretreatment methods to effectively separate the major components such as hemicellulose, cellulose, and lignin and enhance the accessibility and susceptibility of every single component. This review outlines the diverse approaches (e.g., chemical, physical, biological, and combined treatments) for the excellent conversion of straw biomass to fermentable sugars, summarizes the benefits and drawbacks of each pretreatment method, and proposes some investigation prospects for the future pretreatments.

Ideal conditions of microwave-assisted acid pretreatment of sugarcane straw allow fermentative butyric acid production without detoxification step

Sugarcane straw (SCS) was pretreated with dilute sulfuric acid assisted by microwave to magnify fermentable sugars and to minimize the concentration of inhibitors in the hydrolysates. The optimum conditions for maximum recovery of sugars were 162 °C and 0.6% (w/v) H₂SO₄. The low level of inhibitors, such as acetate (2.9 g/L) and total phenolics (1.4 g/L), in the SCS slurry from the pretreatment stage allowed the enzymatic hydrolysis and fermentation steps to occur without detoxification. Besides consuming the total sugar content (31.0 g/L), Clostridium beijerinckii Br21 was able to use acetate from the SCS hydrolysate, to give butyric acid at high conversion factor (0.49 g of butyric acid /g of sugar). The optimized pretreatment conditions spared acid, time, and the detoxification stage, making bio-butyric acid production from SCS extremely attractive.

Persea Americana Agro-Industrial Waste Biorefinery for Sustainable High-Value-Added Products

Significant problems have arisen in recent years, such as global warming and hunger. These complications are related to the depletion and exploitation of natural resources, as well as environmental pollution. In this context, bioprocesses and biorefinery can be used to manage agro-industrial wastes for obtaining high-value-added products. A large number of by-products are composed of lignin and cellulose, having the potential to be exploited sustainably for chemical and biological conversion. The biorefinery of agro-industrial wastes has applications in many fields, such as pharmaceuticals, medicine, material engineering, and environmental remediation. A comprehensive approach has been developed toward the agro-industrial management of avocado (Persea americana) biomass waste, which can be transformed into high-value-added products to mitigate global warming, save non-renewable energy, and contribute to health and science. Therefore, this work presents a comprehensive review on avocado fruit waste biorefinery and its possible applications as biofuel, as drugs, as bioplastics, in the environmental field, and in emerging nanotechnological opportunities for economic and scientific growth.

Thermal Decomposition and Combustion of Microwave Pre-Treated Biomass Pellets

The objective of the study was to investigate a more effective use of commercially available biomass pellets (wheat straw, wood, peat) using microwave pretreatment to improve heat production. Pellets were pretreated using the originally designed microwave torrefaction device. The effects of microwave (mw) pretreatment were quantified, providing measurements of the weight loss and elemental composition of pellets and estimating the effect of mw pretreatment on their porosity, surface area and calorific values at pretreatment temperatures of T = 448–553 K. Obtained results show that the highest structural variations and elemental composition during mw pretreatment were obtained for wheat straw pellets, with an increase in reactivity, a decreasing in the duration of the thermal decomposition by about 40% and an increase in the yield of combustible volatiles. Increased reactivity of pretreated pellets enhanced the ignition and burnout of volatiles, decreasing the duration of the burnout of pretreated wheat straw, wood and peat pellets by 40%, 24% and 9%, respectively, and increasing the peak and average values of the flame temperature, heat output, and produced heat energy by 40–50%, with a correlating increase of combustion efficiency and the mass fraction of carbon-neutral CO2 emission. Thus, the applicability of microwave pretreatment for the control and improvement of heat production was confirmed.

Biodegradation of creosote-treated wood by two novel constructed microbial consortia for the enhancement of methane production

Lignocellulose biodegradation is limited because of its recalcitrant structure particularly when polluted by toxic and carcinogenic compounds such as creosote oil (CRO). As far as we know, this might be the first report that explores the biodegradation of creosote treated wood (CTW) to serve biomethane production. Two novel CTW-degrading microbial consortia, designated as CTW-1 and CTW-2, were screened and constructed to enhance methane production from CRO-treated pine sawdust. After 12 days of biological pretreatment by CTW-1 and CTW-2, a significant reduction in lignocellulosic content of CTW was recorded; estimated as 49 and 43%, respectively. More than 64 and 91% of cumulative biogas and methane yields were obtained from biodegraded CTW over control. Ecotoxicity of treated and untreated CTW was compared by Microtox test. The biodegraded CTW hydrolysates showed a toxicity decrease of more than 80%, suggesting the promising role of constructed microbial consortia for biofuel production and bioremediation.

Material utilization of green waste: a review on potential valorization methods

Considering global developments like climate change and the depletion of fossil resources, the use of new and sustainable feedstocks such as lignocellulosic biomass becomes inevitable. Green waste comprises heterogeneous lignocellulosic biomass with low lignin content, which does not stem from agricultural processes or purposeful cultivation and therefore mainly arises in urban areas. So far, the majority of green waste is being composted or serves as feedstock for energy production. Here, the hitherto untapped potential of green waste for material utilization instead of conventional recycling is reviewed. Green waste is a promising starting material for the direct extraction of valuable compounds, the chemical and fermentative conversion into basic chemicals as well as the manufacturing of functional materials like electrodes for electro-biotechnological applications through carbonization. This review serves as a solid foundation for further work on the valorization of green waste.

Preparation and Swelling Behaviors of High-Strength Hemicellulose-g-Polydopamine Composite Hydrogels

Hemicellulose-based composite hydrogels were successfully prepared by adding polydopamine (PDA) microspheres as reinforcing agents. The effects of PDA microsphere size, dosage, and nitrogen content in hydrogel on the mechanical and rheological properties was studied. The compressive strength of hydrogel was increased from 0.11 to 0.30 MPa. The storage modulus G’ was increased from 7.9 to 22.0 KPa. The gaps in the hemicellulose network are filled with PDA microspheres. There is also chemical cross-linking between them. These gaps increased the density of the hydrogel network structure. It also has good water retention and pH sensitivity. The maximum cumulative release rate of methylene blue was 62.82%. The results showed that the release behavior of hydrogel was pH-responsive, which was beneficial to realizing targeted and controlling drug release.

Circular economy in olive oil production - Olive mill solid waste to ethanol and heavy metal sorbent using microwave pretreatment

Olive mill solid waste (OMSW) is an abundant agricultural waste without viable solution. The effects of OMSW different pretreatments (microwave or autoclave), different additives (water, formic, or sulfuric acid), and utilization of different saccharification enzymes (Cellic® CTec2 or Accellerase® 1500) were tested on saccharification process and sugar release, and on fermentation inhibitors formation and ethanol production. Microwave treatment with formic acid resulted in highest saccharification rates (90% of cellulose fraction) and fermentation yields (15.9 g/L ethanol), although loss of sugars and fermentation inhibitors production was notable. Microwave with water treatment resulted in less saccharification and ethanol (9.6 g/L). To facilitate economical process and to extract maximum value, solid remnants after saccharification were tested as heavy metal sorbent. Microwave with water resulted in the best sorbent, followed by microwave with formic acid. Addition of sulfuric acid, to either microwave or autoclave, resulted in very poor saccharification and absorbance capacity. Therefore, combination of ethanol and sorbent production from OMSW are suggested.

Enhancing the Enzymatic Saccharification of Grain Stillage by Combining Microwave-Assisted Hydrothermal Irradiation and Fungal Pretreatment

Grain stillage from the liquor industry was pretreated by using microwave-assisted hydrothermal pretreatment, fungal pretreatments, and their combination to enable efficient enzymatic hydrolysis for sugar production. The microwave-assisted hydrothermal (MH) pretreatment was optimized by using a response surface methodology, and the respective maximum reducing sugar yield and saccharification efficiency of 17.59 g/100 g and 33.85%, respectively, were achieved under the pretreatment conditions of microwave power = 120 W, solid-to-liquid ratio = 1:15 (g·mL^-1), and time = 3.5 min. The fungal pretreatment with Phanerochaete chrysosporium digestion (PC) achieved the maximum ligninolytic enzyme activities in 6 days with 10% inoculum size at which the reducing sugar yield and saccharification efficiency reached 19.74 g/100 g and 36.29%, respectively. To further improve the pretreatment efficiency, MH and PC pretreatments were combined, but the sequence of MH and PC mattered on the saccharification efficiency. The MH + PC pretreatment (the MH prior to the PC) was better than PC + MH (the PC prior to the MH) in terms of saccharification efficiency. Overall, the MH + PC pretreatment achieved superior reducing sugar yield and saccharification efficiency (25.51 g/100 g and 66.28%, respectively) over all other studied pretreatment methods. The variations of chemical compositions and structure features of the raw and pretreated grain stillage were characterized by using scanning electron microscopy and Fourier transform infrared spectroscopy. The results reveal that both MH and PC pretreatments mainly functioned on delignification and decreasing cellulose crystallinity, thus enhancing the enzymatic saccharification of the pretreated grain stillage. The combined MH and PC pretreatment could be a promising method to enable cost-efficient grain stillage utilization for downstream applications such as biofuels.

Recent trends on seaweed fractionation for liquid biofuels production

Concerns about fossil fuels depletion has led to seek for new sources of energy. The use of marine biomass (seaweed) to produce biofuels presents widely recognized advantages over terrestrial biomasses such as higher production ratio, higher photosynthetic efficiency or carbon-neutral emissions. In here, interesting seaweed sources as a whole or as a residue from seaweed processing industries for biofuel production were identified and their diverse composition and availability compiled. In addition, the pretreatments used for seaweed fractionation were thoroughly revised as this step is pivotal in a seaweed biorefinery for integral biomass valorization and for enabling biomass-to-biofuel economic feasibility processes. Traditional and emerging technologies were revised, with particular emphasis on green technologies, relating pretreatment not only with the type of biomass but also with the final target product(s) and yields. Current hurdles of marine biomass-to-biofuel processes were pinpointed and discussed and future perspectives on the development of these processes given.

Recent advances in the pretreatment of microalgal and lignocellulosic biomass: A comprehensive review

Microalgal and lignocellulosic biomass is the most sumptuous renewable bioresource raw material existing on earth. Recently, the bioconversion of biomass into biofuels have received significant attention replacing fossil fuels. Pretreatment of biomass is a critical process in the conversion due to the nature and structure of the biomass cell wall that is complex. Although green technologies for biofuel production are advancing, the productivity and yield from these techniques are low. Over the past years, various pretreatment techniques have been developed and successfully employed to improve the technology. This paper presents an in-depth review of the recent advancement of pretreatment methods focusing on microalgal and lignocellulosic biomass. The technological approaches involving physical, chemical, biological and other latest pretreatment methods are reviewed.

Multi-Step Exploitation of Raw Arundo donax L. for the Selective Synthesis of Second-Generation Sugars by Chemical and Biological Route

Lignocellulosic biomass represents one of the most important feedstocks for future biorefineries, being a precursor of valuable bio-products, obtainable through both chemical and biological conversion routes. Lignocellulosic biomass has a complex matrix, which requires the careful development of multi-step approaches for its complete exploitation to value-added compounds. Based on this perspective, the present work focuses on the valorization of hemicellulose and cellulose fractionsof giant reed (Arundo donax L.) to give second-generation sugars, minimizing the formation of reaction by-products. The conversion of hemicellulose to xylose was undertaken in the presence of the heterogeneous acid catalyst Amberlyst-70 under microwave irradiation. The effect of the main reaction parameters, such as temperature, reaction time, catalyst, and biomass loadings on sugars yield was studied, developing a high gravity approach. Under the optimised reaction conditions (17 wt% Arundo donax L. loading, 160 °C, Amberlyst-70/Arundo donax L. weight ratio 0.2 wt/wt), the xylose yield was 96.3 mol%. In the second step, the cellulose-rich solid residue was exploited through the chemical or enzymatic route, obtaining glucose yields of 32.5 and 56.2 mol%, respectively. This work proves the efficiency of this innovative combination of chemical and biological catalytic approaches, for the selective conversion of hemicellulose and cellulose fractions of Arundo donax L. to versatile platform products.

Kinetic analysis of microwave-enhanced cellulose dissolution in ionic solvents

Cellulose dissolution in mixtures of the ionic liquid 1-ethyl-3-methylimidazolium acetate with dimethylsulfoxide, [C₂C₁Im][OAc] + DMSO, have been kinetically compared using conventional heating and microwave heating in a single-mode cavity with a semiconductor generator. Microwaves led to enhancements in the dissolution rate between 21 and 57% under different conditions of temperature and concentration of ionic liquid. Rate enhancement by microwaves prominently occurred at temperatures above 60 °C. Based on an Arrhenius plot and wide-band dielectric measurements we advance the hypothesis that the faster dissolution is caused by ionic motion induced by microwaves in the timescale of formation and breaking of hydrogen bonds between cellulose chains and acetate anions.

Unassisted solar lignin valorisation using a compartmented photo-electro-biochemical cell

Lignin is a major component of lignocellulosic biomass. Although it is highly recalcitrant to break down, it is a very abundant natural source of valuable aromatic carbons. Thus, the effective valorisation of lignin is crucial for realising a sustainable biorefinery chain. Here, we report a compartmented photo-electro-biochemical system for unassisted, selective, and stable lignin valorisation, in which a TiO₂ photocatalyst, an atomically dispersed Co-based electrocatalyst, and a biocatalyst (lignin peroxidase isozyme H8, horseradish peroxidase) are integrated, such that each system is separated using Nafion and cellulose membranes. This cell design enables lignin valorisation upon irradiation with sunlight without the need for any additional bias or sacrificial agent and allows the protection of the biocatalyst from enzyme-damaging elements, such as reactive radicals, gas bubbles, and light. The photo-electro-biochemical system is able to catalyse lignin depolymerisation with a 98.7% selectivity and polymerisation with a 73.3% yield using coniferyl alcohol, a lignin monomer.

From waste to food: Optimising the breakdown of oil palm waste to provide substrate for insects farmed as animal feed

Waste biomass from the palm oil industry is currently burned as a means of disposal and solutions are required to reduce the environmental impact. Whilst some waste biomass can be recycled to provide green energy such as biogas, this investigation aimed to optimise experimental conditions for recycling palm waste into substrate for insects, farmed as a sustainable high-protein animal feed. NMR spectroscopy and LC-HRMS were used to analyse the composition of palm empty fruit bunches (EFB) under experimental conditions optimised to produce nutritious substrate rather than biogas. Statistical pattern recognition techniques were used to investigate differences in composition for various combinations of pre-processing and anaerobic digestion (AD) methods. Pre-processing methods included steaming, pressure cooking, composting, microwaving, and breaking down the EFB using ionic liquids. AD conditions which were modified in combination with pre-processing methods were ratios of EFB:digestate and pH. Results show that the selection of pre-processing method affects the breakdown of the palm waste and subsequently the substrate composition and biogas production. Although large-scale insect feeding trials will be required to determine nutritional content, we found that conditions can be optimised to recycle palm waste for the production of substrate for insect rearing. Pre-processing EFB using ionic liquid before AD at pH6 with a 2:1 digestate:EFB ratio were found to be the best combination of experimental conditions.

Biomethane production through anaerobic co-digestion with Maize Cob Waste based on a biorefinery concept: A review

Maize Cob Waste (MCW) is available worldwide in high amounts, as maize is the most produced cereal in the world. MCW is generally left in the crop fields, but due to its low biodegradability it has a negligible impact in soil fertility. Moreover, MCW can be used as substrate to balance the C/N ratio during the Anaerobic co-Digestion (AcoD) with other biodegradable substrates, and is an excellent precursor for the production of Activated Carbons (ACs). In this context, a biorefinery is theoretically discussed in the present review, based on the idea that MCW, after proper pre-treatment is valorised as precursor of ACs and as co-substrate in AcoD for biomethane generation. This paper provides an overview on different scientific and technological aspects that can be involved in the development of the proposed biorefinery; the major topics considered in this work are the following ones: (i) the most suitable pre-treatments of MCW prior to AcoD; (ii) AcoD process with regard to the critical parameters resulting from MCW pre-treatments; (iii) production of ACs using MCW as precursor, with the aim to use these ACs in biogas conditioning (H₂S removal) and upgrading (biomethane production), and (iv) an overview on biogas upgrading technologies.

The effect of microwave drying pretreatment on dry torrefaction of agricultural biomasses

This paper examines the effect of microwave drying on biomass characteristics and subsequent dry pyrolysis and characteristics of produced biochar from rice straw, sugarcane bagasse, rice husk and cotton stalk compared to oven drying at 105 °C. Dried samples from both methods are torrefied at 250 and 300 °C with 30-minutes residence time. Drying time reached 60 times faster with microwave. The fast and violent microwave drying ruptured the biomasses' surface, releasing more volatiles and having lower crystallinity; these lowered the heating value, energy yield and elemental carbon compared to oven drying except for cotton stalk only due to its woody nature which reduced devolatilization. Sugarcane, rice husk and cotton stalk have the most promising values of elemental carbon, energy yield and heating value reaching that of the bituminous coal. Torrefied rice straw showed high crystallinity of 50.7% while sugarcane and rice husk were completely amorphous.

Pretreatment Processes of Biomass for Biorefineries: Current Status and Prospects

This article seeks to be a handy document for the academy and the industry to get quickly up to speed on the current status and prospects of biomass pretreatment for biorefineries. It is divided into two biomass sources: vegetal and animal. Vegetal biomass is the material produced by plants on land or in water (algae), consuming sunlight, CO2, water, and soil nutrients. This includes residues or main products from, for example, intensive grass crops, forestry, and industrial and agricultural activities. Animal biomass is the residual biomass generated from the production of food from animals (e.g., manure and whey). This review does not mean to include every technology in the area, but it does evaluate physical pretreatments, microwave-assisted extraction, and water treatments for vegetal biomass. A general review is given for animal biomass based in physical, chemical, and biological pretreatments.

Different effects of ozone and aqueous ammonia in a combined pretreatment method on rice straw and dairy manure fiber for enhancing biomethane production

Low digestibility of lignocellulosic feedstock is the most important limitation for biogas production. The synergistic effects of ozone and aqueous ammonia (OSAA) on different types of lignocelluloses including rice straw and dairy manure fiber were investigated. OSAA significantly increased biogas production of rice straw by 114.2%-172.8% when compared with using ozonation alone, while increased by 6.2%-8.8% with manure fiber. OSAA pretreatment increased biogas production of manure fiber by 55.3%-103.6% when compared with soaking aqueous ammonia (SAA) alone, while by 28.8%-39.9% with rice straw. The specific effects of pretreatment time on anaerobic digestion of manure fiber differed noticeably from those on rice straw. Ozonation time had a major function for pretreatment of manure fiber via the OSAA process, but SAA pretreatment time was more important than that for rice straw.

Sugarcane Bagasse Hydrolysis Enhancement by Microwave-Assisted Sulfolane Pretreatment

Sugarcane bagasse is the major by-product of the sugarcane industry and, due to its abundant availability, it has been extensively studied for lignocellulosic bioconversion in the production of bioethanol and other value-added commercial products. In the study presented herein, a combined pretreatment using sulfolane, TiO2 and alkali microwave irradiation (MW-A) was assessed for the dissolution of lignin prior to enzymatic saccharification of holocellulose. Total reducing sugars (TRS) and saccharinic acid yields were investigated. The increase in NaOH concentration up to 5% and in temperature from 120 °C to 140 °C were found to have a positive influence on both yields. While increasing the reaction time from 5 to 60 min only led to an increase in TRS yield <2%, a reaction time of 30 min almost doubled the saccharinic acids production. TRS yields and saccharinic acid production were approximately 5% and 33% higher when the sulfolane-TiO2 reaction medium was used, as compared to MW-A in water, reaching up to 64.8% and 15.24 g/L of saccharinic acids, respectively. The proposed MW-A pretreatment may hold promise for industrial applications, given the good TRS yields obtained, and the associated enzyme and time/energy savings. The use of sulfolane-TiO2 reaction medium is encouraged if saccharinic acids are to be recovered too.

Development of a Microwave Irradiation Probe for a Cylindrical Applicator

A microwave irradiation probe was newly developed for downsizing microwave applicators and the overall microwave heating apparatus. The key component of the proposed probe is a tapered section composed of polytetrafluoroethylene (PTFE) and alumina. Insertion of the tapered section between the input port and the applicator vessel realizes impedance matching to the microwave power source and reduces the reflected power from the applicator. The proposed microwave probe for a cylindrical applicator was designed using 3D electromagnetic simulations. The permittivity data of two liquid samples—ultrapure water and 2 M NaOH solution—were measured and taken into simulations. The conductivity of the NaOH solution was estimated from the measurement results. The measured reflection ratio of the fabricated applicator was in good accordance with the simulated one. The frequency ranges in which the measured reflection ratio was less than 10% were from 1.45 GHz to 2.7 GHz when using water and from 1.6 GHz to 2.7 GHz when using the NaOH solution as the sample. The heating rate of the applicator was roughly estimated as 63 to 69 K for a 5 min interval during the 2.45 GHz microwave irradiation at the input power of 100 W.