Alkali/urea pretreatment of rice straw at low temperature for enhanced biological hydrogen production

Optimization of biogas production from straw wastes by different pretreatments: Progress, challenges, and prospects

Lignocellulosic biomass (LCB) presents a promising feedstock for carbon management due to enormous potential for achieving carbon neutrality and delivering substantial environmental and economic benefit. Bioenergy derived from LCB accounts for about 10.3 % of the global total energy supply. The generation of bioenergy through anaerobic digestion (AD) in combination with carbon capture and storage, particularly for methane production, provides a cost-effective solution to mitigate greenhouse gas emissions, while concurrently facilitating bioenergy production and the recovery of high-value products during LCB conversion. However, the inherent recalcitrant polymer crystal structure of lignocellulose impedes the accessibility of anaerobic bacteria, necessitating lignocellulosic residue pretreatment before AD or microbial chain elongation. This paper seeks to explore recent advances in pretreatment methods for LCB biogas production, including pulsed electric field (PEF), electron beam irradiation (EBI), freezing-thawing pretreatment, microaerobic pretreatment, and nanomaterials-based pretreatment, and provide a comprehensive overview of the performance, benefits, and drawbacks of the traditional and improved treatment methods. In particular, physical-chemical pretreatment emerges as a flexible and effective option for methane production from straw wastes. The burgeoning field of nanomaterials has provoked progress in the development of artificial enzyme mimetics and enzyme immobilization techniques, compensating for the intrinsic defect of natural enzyme. However, various complex factors, such as economic effectiveness, environmental impact, and operational feasibility, influence the implementation of LCB pretreatment processes. Techno-economic analysis (TEA), life cycle assessment (LCA), and artificial intelligence technologies provide efficient means for evaluating and selecting pretreatment methods. This paper addresses current issues and development priorities for the achievement of the appropriate and sustainable utilization of LCB in light of evolving economic and environmentally friendly social development demands, thereby providing theoretical basis and technical guidance for improving LCB biogas production of AD systems.

Effect of deep eutectic solvent pretreatment on biohydrogen production from corncob: pretreatment temperature and duration

Deep eutectic solvent pretreatment with different temperatures and durations was applied to corncob to increase hydrogen yield via photo-fermentation. The correlation of composition, enzymatic hydrolysis, and hydrogen production in pretreated corncobs, as well as energy conversion was evaluated. Deep eutectic solvent pretreatment effectively dissolved lignin, retained cellulose, and enhanced both enzymatic hydrolysis and hydrogen production. The maximum cumulative hydrogen yield obtained under a pretreatment condition of 50°C and 12 h was 677.45 mL; this was 2.72 times higher than that of untreated corncob, and the corresponding lignin removal and enzymatic reduction of sugar concentration were 79.15% and 49.83 g/L, respectively; the highest energy conversion efficiency was 12.08%. The hydrogen production delay period was shortened, and the maximum shortening time was 18.9 h. Moreover, the cellulose content in pretreated corncob was positively correlated with both reducing sugar concentration and hydrogen yield and had the strongest influence on hydrogen production.

Low-temperature torrefaction assisted with solid-state KOH/urea pretreatment for accelerated methane production in wheat straw anaerobic digestion

Low-temperature torrefaction assisted with solid-state KOH/urea applied onto wheat straw was proposed to break down the lignocellulosic material to enhance biomethane production in anaerobic digestion (AD). The optimization of key parameters applying the Box-Behnken design and response surface methodology showed that an addition of 0.1 g/g_straw KOH/urea at 180 °C while torrefying for 30 min was the optimal condition for producing biomethane. Results indicate that co-applying KOH and urea in torrefaction synergistically enhanced the biodegradability of straw by effectively removing lignin and largely retaining cellulose, giving rise to a 41 % increase in the cumulative methane production compared to untreated straw (213 mL/g-volatile solids (VS_raw)) from batch AD. Additionally, the nitrogen- and potassium-rich digestates helped to improve soil fertility, thus achieving a zero-waste discharge. This study demonstrated the feasibility of using solid-state KOH/urea assisted low-temperature torrefaction as an effective pretreatment method to promote methane production during AD.

The Preparation Processes and Influencing Factors of Biofuel Production from Kitchen Waste

Kitchen waste is an important component of domestic waste, and it is both harmful and rich in resources. Approximately 1.3 billion tons of kitchen waste are produced every year worldwide. Kitchen waste is high in moisture, is readily decayed, and has an unpleasant smell. Environmental pollution can be caused if this waste is treated improperly. Conventional treatments of kitchen waste (e.g., landfilling, incineration and pulverization discharge) cause environmental, economic, and social problems. Therefore, the development of a harmless and resource-based treatment technology is urgently needed. Profits can be generated from kitchen waste by converting it into biofuels. This review intends to highlight the latest technological progress in the preparation of gaseous fuels, such as biogas, biohythane and biohydrogen, and liquid fuels, such as biodiesel, bioethanol, biobutanol and bio-oil, from kitchen waste. Additionally, the pretreatment methods, preparation processes, influencing factors and improvement strategies of biofuel production from kitchen waste are summarized. Problems that are encountered in the preparation of biofuels from kitchen waste are discussed to provide a reference for its use in energy utilization. Optimizing the preparation process of biofuels, increasing the efficiency and service life of catalysts for reaction, reasonably treating and utilizing the by-products and reaction residues to eliminate secondary pollution, improving the yield of biofuels, and reducing the cost of biofuels, are the future directions in the biofuel conversion of kitchen waste.

Improved Biohythane Production from Rice Straw in an Integrated Anaerobic Bioreactor under Thermophilic Conditions

This study evaluated the feasibility of continuous biohythane production from rice straw (RS) using an integrated anaerobic bioreactor (IABR) at thermophilic conditions. NaOH/Urea solution was employed as a pretreatment method to enhance and improve biohythane production. Results showed that the maximum specific biohythane yield was 612.5 mL/g VS, including 104.1 mL/g VS for H₂ and 508.4 mL/g VS for CH₄, which was 31.3% higher than the control RS operation stage. The maximum total chemical oxygen demand (COD) removal stabilized at about 86.8%. COD distribution results indicated that 2% of the total COD (in the feed) was converted into H₂, 85.4% was converted to CH₄, and 12.6% was retained in the effluent. Furthermore, carbon distribution analysis demonstrated that H₂ production only diverted a small part of carbon, and most of the carbon flowed to the CH₄ fermentation process. Upon further energy conversion analysis, the maximum value was 166.7%, 31.7 times and 12.8% higher than a single H₂ and CH₄ production process. This study provides a new perspective on lignocellulose-to-biofuel recovery.

Valorization of agricultural wastes for biofuel applications

Continuous environmental degradation, volatility in the oil market, and unimpressive functioning of fossil-based (FB) fuels in compression ignition engines have expanded the tempo of the search for alternative fuels. Due to the astronomical rise in global population, improved agricultural, commercial, and manufacturing activities, enhanced farming and other food production and utilization ventures, agricultural waste generation, renewable fuel consumption, and emission of toxic gases. The need for cost-effective, readily available, and environmentally benign agricultural waste to biofuels has never been more crucial. Biofuels are renewable, biodegradable, low-cost, and eco-friendly fuels that are produced by microorganisms from waste lignocellulosic biomass. Conversion of agricultural wastes to biofuel does not exacerbate food security, contributes to waste management, prevents environmental degradation, and ensures energy security. This study reviews the conversion of agricultural wastes into biofuels with special emphasis on bioethanol, biohydrogen, biobutanol, biomethane, biomethanol, and biodiesel for various applications. It is safe to conclude that wastes generated from agricultural activities and processes are useful and can be harnessed to meet the affordable and accessible global renewable energy target. The result of this investigation will improve the body of knowledge and provide novel strategies and pathways for the utilization of agricultural wastes. Going forward, more collaborative and interdisciplinary studies are required to evolve state-of-the-art, ecofriendly, and cost-effective conversion pathways for agricultural wastes to promote the utilization of the generated renewable fuels. More human, financial, and infrastructural investments are desirable to motivate the conversion of agricultural waste into biofuels to ensure environmental sanitation and sustainability, promote renewable fuel utilization, and avert the raging implosion of our planet.

Effect of Ammonia Fiber Expansion Combined with NaOH Pretreatment on the Resource Efficiency of Herbaceous and Woody Lignocellulosic Biomass

The most essential issue facing the world today is the provision of energy and sustainable consumption of natural resources. Pretreatment is an essential step to produce biofuels from lignocellulosic biomass. In this study, ammonia fiber explosion (AFEX) combined with NaOH (A-NaOH) pretreatment effects on the characteristics of Pennisetum sinese (herbaceous), oak (hardwood), and camphor wood (softwood) were assessed using enzymatic efficiency analysis, thereby identifying the composition properties of subsequent bio-H₂ production. The results show that the lignin removal (84.2%, 59.7%, and 36.7%, respectively) at 5%A-NaOH conditions and enzymatic efficiency (36.2%, 9.7%, and 6.5%, respectively) of Pennisetum sinese (P. sinese), oak, and camphor wood were significantly increased under 4% A-NaOH conditions. Further A-NaOH pretreatment significantly promoted dark fermentation bio-H₂ production (152.3, 99.1, and 76.9 mL/g TS, respectively) and volatile acid production (4660.2, 3720.2, and 3496.2 mg/L, respectively) of P. sinese, oak, and camphor wood. These findings show that A-NaOH pretreatment is an effective means of utilization of lignocellulose resources.

Improving anaerobic digestion of corn straw by using solid-state urea pretreatment

The resistant structure and high carbon/nitrogen ratio (C/N) of cellulosic substrate are the barriers during their anaerobic digestion (AD). Solid-state urea pretreatment was developed in this study to pretreat corn straw and adjust C/N ratio simultaneously for the downstream AD of corn straw. Results showed solid-state urea pretreatment was efficient in lignin removal and achieved the highest lignin reduction of 7.06% with C/N ratio = 15 during pretreatment. Scanning electron microscopy demonstrated the destruction of the dense structure by pretreatment, which benefited to the hydrolysis of corn straw. The cumulative methane yields of the pretreated corn straw ranged from 234.07 to 250.03 mL/g VS, which were obviously higher than that of the untreated corn straw. The maximum methane yield of 250.03 mL/g VS was achieved with C/N = 15 during pretreatment, which was 23.91% higher than that of the untreated group. In addition, AD digestates from the pretreated groups had 9.62% higher nutrients than that from the untreated group. The solid-state urea pretreatment can destroy the dense structure of corn straw and regulate the C/N ratio during AD, thus benefit the methane production and fertilizer use of the digestate, which is a potential choice during the AD of cellulosic substrate.

Effects of enzymatic hydrolysis and alkalization pretreatment on biohydrogen production by chlorella photosynthesis

The effects of alkalization pretreatment and enzymolysis on biohydrogen production with Chlorella vulgaris microalgae biomass by photosynthesis were studied, the alkalization pretreatment enzymolysis was to alkalize biomass raw materials before enzymolysis, the biohydrogen production kinetics equation of microalgae biomass was put forward by comparing the biohydrogen process of enzymatic hydrolysis with that of alkaline pretreatment enzymatic hydrolysis. The experimental results show: the optimum initial substrate concentration for biohydrogen production by enzymatic hydrolysis and alkaline pretreatment was 24 g/L, the maximum biohydrogen was 132.1 mL and 294.5 mL, the maximum specific biohydrogen production was 22.0 mL/g and 49.1 mL/g, and the maximum biohydrogen content was 43.9% and 56.8%. The effect of biohydrogen production by enzymatic hydrolysis after alkaline pretreatment of microalgae biomass is obviously better than that by direct enzymatic hydrolysis, which provides scientific reference and development of high efficiency and low cost biohydrogen production technology by photosynthesis of microalgae biomass.

A review on biological recycling in agricultural waste-based biohydrogen production: Recent developments

Hydrogen has become a research highlight by virtue of its clean energy production technology and high energy content. The technology of biohydrogen production from biological waste via fermentation has lower costs, provides environment-friendly methods regarding energy balance, and creates a pathway for sustainable utilization of massive agricultural waste. However, biohydrogen production is generally limited by lower productivity. Many studies have been conducted aimed at improving biohydrogen production efficiency. Hence, this review is intended to describe improving routes for biohydrogen production from agricultural waste and highlights recent advances in these approaches. In addition, the critical factors affecting biohydrogen production, including the pretreatment method, substrate resource, fermentation conditions, and bioreactor design, were also comprehensively discussed along with challenges and future prospects.

Advances in pretreatment of lignocellulosic biomass for bioenergy production: Challenges and perspectives

As a clean and renewable energy, bioenergy is one of the most promising alternatives to fossil fuels. Lignocellulose possesses great potential for bioenergy production, but the recalcitrant and heterogeneous structure limits its application. Pretreatment technology offers an effective solution to fractionate the main components of the lignocellulose and uncover the available cellulose. The obtained feedstock can be applied to bioconversion into energy, e.g., bioethanol, biogas, biohydrogen, etc. Here, the current state of lignocellulose pretreatment technologies was comprehensively reviewed, the advances in bioenergy production from pretreated lignocellulose was described, with particular attention to key challenges involved. Several new strategies for overcoming pretreatment barriers to realize highly efficient lignocellulose bioconversion were highlighted. The insights given in this review will facilitate further development on lignocellulosic bioenergy production, towards addressing the global energy crisis and climate change related to the use of fossil fuels.

Co-fermentation of residual algal biomass and glucose under the influence of Fe3O4 nanoparticles to enhance biohydrogen production under dark mode

The present study reports Fe₃O₄ nanoparticles (Fe₃O₄ NPs) induced enhanced hydrogen production via co-fermentation of glucose and residual algal biomass (cyanobacteria Lyngbya limnetica). A significant enhancement of dark fermentative H₂ production has been noticed under the influence of co-fermentation of glucose and residual algal biomass using Fe₃O₄ NPs as catalyst. Further, using the optimized ratio of glucose to residual algal biomass (10:4), ∼ 37.14 % higher cumulative H₂ has been recorded in presence of 7.5 mg/L Fe₃O₄ NPs as compared to control at 37 °C. In addition, under the optimum conditions [glucose to residual algal biomass ratio (10:4)] presence of 7.5 mg/L Fe₃O₄ NPs produces ∼ 937 mL/L cumulative H₂ in 168 h at pH 7.5 and at temperature 40 °C. Clostridum butyrium, employed for the dark fermentation yielded ∼ 7.7 g/L dry biomass in 168 h whereas acetate (9.0 g/L) and butyrate (6.2 g/L) have been recorded as the dominating metabolites.

Microwave-Assisted Urea-Based-Hydrotropic Pretreatment of Rice Straw: Experimental Data and Mechanistic Kinetic Models

The three major lignocellulose components can be transformed into various biomass-derived platform fuels, chemicals, and materials upon pretreatment and chemical upgrading. Lignocellulose pretreatment is an important step to obtain an eco-friendly, economical, and effective biomass utilization process. The combination of microwave heating and hydrotropic pretreatment is considered as a green method of lignocellulose pretreatment. Experimental data and two mechanistic kinetic models of microwave-assisted pretreatment of rice straw are presented. Here, the use of urea solution as the hydrotropic agent was examined to facilitate the degradation of three major lignocellulose components. The first kinetic model assumes that the soluble lignin does not undergo condensation, while the second one assumes that part of the soluble lignin condenses to a solid product. The mechanistic models were validated with a series of experimental data obtained from microwave-assisted hydrotropic pretreatment of rice straw. The results show that both models could generally describe the experimental data well. However, based on the evaluation of the results of the kinetic models, it turned out that the rate of lignin condensation was relatively slow compared to the rate of lignin degradation to soluble lignin (the value of k c is relatively small compared to the value of k l1). Hence, the kinetic model with exclusion of lignin condensation is suggested more since it is mathematically simpler. The proposed mechanistic model can also predict the cellulose and hemicellulose dissolution and thereby can be used as a process optimization tool. The microwave-assisted urea-based hydrotropic pretreatment conducted at a solid-liquid ratio of 1:35, a urea concentration of 36.8%, a reaction temperature of 90 °C, and a pretreatment duration of 73.6 min is predicted to give a solid residue with low lignin content and high cellulose content which resulted in a cellulose to lignin ratio of 5.53. Cellulosic biomass characterization revealed that microwave-assisted hydrotropic pretreatment was able to produce higher crystallinity and thermally stable cellulosic biomass.

Enhanced enzymatic hydrolysis and hydrogen production of sugarcane bagasse pretreated by peroxyformic acid

Pretreatment plays a key role in biofuel production from lignocellulosic biomass. In this study, the main factors of peroxyformic acid (PA) pretreatment were optimized in the light of enzymolysis efficiency and composition analysis of pretreated sugarcane bagasse (SCB). Lignin was significantly removed (59.0%) and a complete saccharification level (103.6%) was obtained for the pretreated SCB with slight cellulose loss (9.2%) under the optimized pretreatment conditions. The effects of PA pretreatment on the structural characteristics of SCB were also studied and the digestibility of pretreated SCB was also evaluated by dark fermentative hydrogen production with an enriched anaerobic cellulolytic microbial consortium MC1. The hydrogen production increased by 195.5% (based on initial SCB) and the abundance of dominant hemicellulose-degradation genus Thermoanaerobacterium increased from 23.8% to 40.2% due to the remaining and accessible hemicellulose in PA pretreated SCB.

A Review of Key Technologies and Trends in the Development of Integrated Heating and Power Systems in Agriculture

Petroleum agriculture, characterized by mechanization and chemistry, is developing rapidly in China. However, petroleum agriculture has not only brought food safety problems, but also caused great obstacles to the sustainable development of society. In view of the disadvantages of oil agriculture, we provide an upgrading plan for energy systems in agriculture. This work can help reduce carbon emissions and improve food security. We introduce the most advanced technologies in Chinese agricultural development and the technical scope includes new agricultural energy power generation, agricultural energy use and the safe operation of agricultural energy systems. We describe the detailed data of agricultural bioenvironmental and energy engineering to clarify the level of agricultural energy efficiency in China. The overall conclusion of this paper is that the deep integration of agriculture and energy internet has become the development trend of agricultural energy systems.

The solid-state physicochemical properties and biogas production of the anaerobic digestion of corn straw pretreated by microwave irradiation

The effect of different temperatures used in microwave pretreatment on enhancing methane production of corn straw was comparatively studied in this paper through the analysis of the physicochemical properties of the pretreated materials and the methane yield during anaerobic digestion. Analytic methods such as scanning electron microscopy, Fourier transform infrared spectroscopy, and X-ray diffraction were performed to detect the surface chemistry of the pretreated corn straw. The results indicated that microwave pretreatment could effectively disrupt the lignocellulosic structure to release cellulose, hemicellulose, and related derivatives and make them available for the process of anaerobic digestion. The outcome of the methanogenic assay demonstrated that methane production could be significantly improved by 73.08% concerning the variation of the temperatures in microwave pretreatment. This study provides technical support for pretreatment methods of lignocellulose materials and deems that microwave pretreatment boosts methane yield efficiently during the process of anaerobic digestion of lignocellulosic materials.

Development of co-culture yeast fermentation for efficient production of biobutanol from rice straw: A useful insight in valorization of agro industrial residues

Escalating environmental concerns and petroleum demands leads into the present study. In this investigation delignification of rice straw was optimized by NaOH and H2SO4 pretreatment using L16 Taguchi orthogonal array. NaOH pretreatment revealed higher delignification as compared to H₂SO₄ and; further subjected to separate enzymatic hydrolysis and co-fermentation (SHCF) using RSM as the SHCF demonstrated a maximum glucose and xylose yield of 575 and 205 mg/g. Further, butanol concentration of 4.32 g/L was achieved from 20 g/L of sugar loadings by co-culture of Saccharomyces cerevisiae and Pichia sp. at 72 h of incubation time which was 79.25% higher as compared to monocultures of Pichia sp. Scale-up experiments with higher sugar loadings (90 g/L) demonstrated a butanol concentration of 13.3 g/L. The release of amino acids in co-culture and monoculture systems demonstrated that the addition of S. cerevisiae promoted the butanol synthesis pathway which led to higher butanol concentration.

Evaluation of a novel pretreatment of NaOH/Urea at outdoor cold-winter conditions for enhanced enzymatic conversion and hythane production from rice straw

A novel pretreatment using NaOH/Urea (NU) solution at outdoor cold-winter conditions was developed to enhance the enzymatic saccharification and hythane production from rice straw (RS). Results revealed that the reducing sugar conversion of RS reached 90.02% after NU pretreatment at outdoor freezing temperature. Chemical composition analysis showed that the lignin removal was up to 62.74% with cellulose and hemicellulose loss of 0.56% and 18.87% after 3%-6% NU pretreatment at 100% solid loading for 3 months. Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis confirmed that the surface of pretreated RS exposed more cellulose and hemicellulose due to the disruption of resistant structure of lignocellulose. Subsequently, the enzymatic hydrolysate of pretreated RS was used as substrate to produce hythane by two-stage fermentation with the yield of 225.1 mL H₂/g sugar and 112.8 mL CH₄/g sugar. The energy conversion efficiency of hythane fermentation attained 10.4%, which was 22.8% and 190.5% higher than that for single H₂ and CH₄ fermentation. These results demonstrated that NU pretreatment at outdoor cold-winter conditions was practically and feasible way for improved hythane recovery from lignocellulosic biomass.

A novel integrated process to convert cellulose and hemicellulose in rice straw to biobutanol

A novel integrated process was established in this study to produce butanol from rice straw. In the first pretreatment, an alternative NaOH/Urea preatment operated at -12 ^oC efficiently removed 10.9 g lignin and preserved 91.54% cellulose and hemicellulose in 100 g rice straw. Subsequently, crude cellulase produced from Trichoderma viride was used to convert pretreated rice straw to mono-sugars for fermentation. The yields of glucose, xylose and arabiose obtained from 100 g rice straw were 31 g, 13.4 g and 0.48 g, respectively, resulting in a 69.45% saccharification efficiency of crude enzyme. Finally, to alleviate the carbon catabolite repression (CCR) and enhance butanol production, the coculture system of Clostridium beijerinckii and Saccharomyces cerevisiae was applied. Compared to monoculture of C. beijerinckii F-6, more sugars were consumed, especially the reduction rate of xylose reached to 81.87%, 32.99% higher than that in monoculture system. With more substrate facilitied into metabolism, the butanol concentration reached to 10.62 g/L corresponding to 0.28 g/g substrate, 115.38% higher than that in monoculture system. Overall, this integrated process was a low-energy consumption and efficient method for butanol production from rice straw.

Pretreatment of corn stover via sodium hydroxide-urea solutions to improve the glucose yield

Because of the abundance and renewability of lignocellulosic biomass, lignocellulose-derived biofuels and chemicals are promising alternatives to fossil resources. In this study, we developed a strategy for pretreating lignocellulose (corn stover) using a sodium hydroxide-urea solution (SUs) and evaluated changes in the efficiency and structure. The results showed that treatment with 6% NaOH/12% urea at 80 °C for 20 min gave a glucose yield of 0.54 g/g corn stover. Recycling of the NaOH/urea was also explored, and the average glucose yield over four pretreatment cycles was 0.44 g/g corn stover. The structural characteristics of corn stover were investigated by scanning electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy and gas chromatography-mass spectrometry. Compared with untreated corn stover, NaOH/urea-pretreated corn stover had more micropores, mesopores, and macropores, higher crystallinity, and a higher cellulose content. This pretreatment process is economical and efficient and has good application prospects for lignocellulose biorefinery.

Urea-pretreated corn stover: Physicochemical characteristics, delignification kinetics, and methane production

Solids loading is a key factor in aqueous or gaseous ammonia production from urea. Methane production from urea-pretreated corn stover, as well as the physicochemical characteristics and delignification kinetics of the corn stover, were investigated with four solids loading values (10%-70%) and five ratios of urea to corn stover (1:100-7:10) at 35 °C for 6 weeks. A 1:20 ratio of urea to corn stover was optimal for achieving high lignin removal with ≤50% solids loading, and 7:10 was optimal with 70% solids loading. Under the two optimal conditions, 85.56% and 82.35% of cellulose and 85.76% and 85.49% of hemicellulose were retained. The maximum lignin removal rates of 69.67% and 68.27% and methane production of 294.70 and 292.56 L/kg volatile solids (VS) were achieved, respectively. The delignification kinetics of the urea-pretreated corn stover conformed to three first-order reactions. Most of the lignin was degraded within the first 3 weeks.

Sustainability metrics of pretreatment processes in a waste derived lignocellulosic biomass biorefinery

Excessive utilization of fossil fuels has resulted in serious concerns about climate change. Integrating biorefinery technology to convert waste-derived-lignocellulosic biomass into biofuels and biopolymers has become an emerging topic toward our sustainable future. Pretreatment to fractionate the building block chemicals from the biomass is a crucial unit operation to ease the downstream processes in biorefinery. However, application of solvents and chemicals in the process can create many operational and environmental challenges in sensitive areas like highly populated cities. To shed light on how to determine a green biorefinery, this study presents the sustainability metrics of various pretreatment techniques and their operational risks during urbanization. The proposed green indexes include fractionation outputs, chemical recyclability, operational profile, and safety factors. In line with the design principles of lignin valorization, the issue of urban biomass and water-and-energy nexus are addressed to support future development and application of urban biorefinery for municipal waste management.

Enhanced butanol-hydrogen coproduction by Clostridium beijerinckii with biochar as cell's carrier

In this study, the effects of biochar on the fermentation performance of butanol-hydrogen coproduction by Clostridium beijerinckii F-6 were investigated. Results showed that the biochar with rich porous and graphitized structure can significantly promote the coproduction of butanol and hydrogen. The productivity of butanol and hydrogen reached 0.148 g/L/h and 0.299 mmol/L/h with biochar addition which were 20.23% and 48.76% higher than that in control without biochar addition, respectively. Moreover, the whole energy conversion efficiency calculated based on the heat value showed increment from 43.69% to 51.75% with biochar addition. Combined analysis of organic acids accumulation and oxidation-reduction potential fluctuation proved that biochar can regulate reducing power during fermentation and accelerate the conversion of acid phase to solvent phase. Scanning electron microscope images showed that biochar acted as carriers for cells absorption. Confirmation experiment further proved that biochar enhanced the butanol tolerant ability of Clostridium beijerinckii F-6.

State of the art and challenges of biohydrogen from microalgae

Biohydrogen from microalgae has attracted extensive attention owing to its promising features of abundance, renewable and self sustainability. Unlike other well-established biofuels like biodiesel and bioethanol, biohydrogen from microalgae is still in the preliminary stage of development. Criticisms in microalgal biohydrogen centered on its practicality and sustainability. Various laboratory- and pilot-scale microalgal systems have been developed, and some research initiatives have exhibited potential for commercial application. This work provides a review of the state of the art of biohydrogen from microalgae. Discussions include metabolic pathways of light-driven transformation and dark fermentation, reactor schemes and system designs encompassing reactor configurations and light manipulation. Challenges, knowledge gaps and the future directions in metabolic limitations, economic and energy assessments, and molecular engineering are also delineated. Current scientific and engineering challenges of microalgal biohydrogen need to be addressed for technology leapfrog or breakthrough.

Enhanced co-production of biohydrogen and algal lipids from agricultural biomass residues in long-term operation

This study evaluates a two-stage process by dark fermentation and microalgal cultivation for hydrogen and lipid production from different agricultural biomass residues, such as corn cob, corn stalk, rice straw and wheat straw. Results indicate that corn stalk was the best fermentation feedstock for hydrogen production and the highest accumulative hydrogen volume of 762.3 mL L^-1 was obtained in batch mode. Dark fermentative effluent from corn stalk was rich in acetate and butyrate, and was favorable to lipid production. The maximum algal biomass and total lipid content reached 1461.1 mg L^-1 and 35.2%, respectively. Meanwhile, the energy conversion efficiency in two-stage cultivation significantly increased from 5.78% to 16.96%. The system was stable and effective in long-term operation, and the average hydrogen production rate of 811.1 mL L^-1 d^-1 and lipid concentration of 588.5 mg L^-1 were achieved. This study provides a promising process for enhancing energy production from agricultural wastes.

Rice husk derived double network hydrogel as efficient adsorbent for Pb(II), Cu(II) and Cd(II) removal in individual and multicomponent systems

In this study, lignin extracted from rice husk was used to synthesis double network hydrogel adsorbent, named RH-CTS/PAM gel. RH-CTS/PAM gel exhibited macroporous structure and high buried water content, which gave rise to the exceptional adsorption performance. As results, in individual systems, the equilibrium time of Pb(II), Cu(II) and Cd(II) with initial concentration of 200 mg/L could be reached within 10 min, with the theoretical maximum adsorption capacity of 374.32, 196.68 and 268.98 mg/g, respectively. The adsorption rate and capacity of Pb(II), Cu(II) and Cd(II) in multicomponent systems were lower than that of individual systems. However, in a few cases of ternary system, higher adsorption rate and capacity was observed compare to binary systems. Adsorption mechanism indicated that both oxygen-containing and nitrogen-containing functional groups played a dominant role during the adsorption process, and mainly through chemical interaction along with a small amount of physical interaction.

High-solid pretreatment of rice straw at cold temperature using NaOH/Urea for enhanced enzymatic conversion and hydrogen production

A high-solid loading pretreatment using NaOH/Urea solution at -12 °C was proposed to pretreat rice straw (RS) for enhanced saccharify and hydrogen production. Results shown NaOH/Urea pretreatment exhibited excellent pretreatment performance at solid loading ranged from 10% to 100% (w/v) with an average reducing sugar conversion of 80.22% was obtained which was 31.89% higher than that untreated RS. Upon fermentation of 100% solid loading pretreated hydrolysate, the H₂ yield of 72.5 mL/g-pretreated RS was calculated based on substrate consumption, which enabled 49.5% higher reducing sugar transfer to H₂ through material balance. FTIR and XRD analysis further demonstrated that the cold NaOH/Urea pretreatment at 100% (w/v) could effectively disrupt the lignin structure and decrease the cellulose crystallinity. The present study suggested a high solid loading pretreatment with NaOH/Urea at cold temperature could be a valuable alternative for better techno-economic of the lignocelluloses - to - sugars - to H₂ routes.

A novel clean production approach to utilize crop waste residues as co-diet for mealworm (Tenebrio molitor) biomass production with biochar as byproduct for heavy metal removal

Proper management of waste crop residues has been an environmental concern for years. Yellow mealworms (larvae of Tenebrio molitor Linnaeus, 1758) are major insect protein source. In comparison with normal feed wheat bran (WB), we tested five common lignocellulose-rich crop residues as feedstock to rear mealworms, including wheat straw (WS), rice straw (RS), rice bran (RB), rice husk (RH), and corn straw (CS). We then used egested frass for the production of biochar in order to achieve clean production. Except for WS and RH, the crop residues supported mealworms' life activity and growth with consumption of the residues by 90% or higher and degraded lignin, hemicellulose and cellulose over 32 day period. The sequence of degradability of the feedstocks is RS > RB > CS > WS > RH. Egested frass was converted to biochar which was tested for metal removal including Pb(II), Cd(II), Cu(II), Zn(II), and Cr(VI). Biochar via pyrolysis at 600 °C from RS fed frass (FRSBC) showed the best adsorption performance. The adsorption isotherm fits the Langmuir model, and kinetic analysis fits the Pseudo-Second Order Reaction. The heavy metal adsorption process was well-described using the Intra-Particle Diffusion model. Complexation, cation exchange, precipitation, reduction, deposition, and chelation dominated the adsorption of the metals onto FRSBC. The results indicated that crop residues (WS, RS, RB, and CS) can be utilized as supplementary feedstock along with biochar generated from egested frass to rear mealworms and achieve clean production while generating high-quality bioadsorbent for environment remediation and soil conditioning.

Two-Stage Pretreatment to Improve Saccharification of Oat Straw and Jerusalem Artichoke Biomass

Pretreatment is a necessary step when lignocellulosic biomass is to be converted to simple sugars; however single-stage pretreatment is often insufficient to guarantee full availability of polymeric sugars from raw material to hydrolyzing enzymes. In this work, the two-stage pretreatment with use of acid (H2SO4, HNO3) and alkali (NaOH) was applied in order to increase the susceptibility of Jerusalem artichoke stalks (JAS) and oat straw (OS) biomass on the enzymatic attack. The effect of the concentration of reagents (2% and 5% w/v) and the order of acid and alkali sequence on the composition of remaining solids and the efficiency of enzymatic hydrolysis was evaluated. It was found that after combined pretreatment process, due to the removal of hemicellulose and lignin, the content of cellulose in pretreated biomass increased to a large extent, reaching almost 90% d.m. and 95% d.m., in the case of JAS and OS, respectively. The enzymatic hydrolysis of solids remaining after pretreatment resulted in the formation of up to 45 g/L of glucose, for both JAS and OS. The highest glucose yield was achieved after pretreatment with 5% nitric acid followed by NaOH, and 90.6% and 97.6% of efficiency were obtained, respectively for JAS and OS.

Enzymatic hydrolysis of brewer's spent grain to obtain fermentable sugars

Lignocellulosic biomass is a feedstock with the potential to be converted into value-added bioproducts. The use of enzymatic hydrolysis allows the cleavage of lignocellulose into their monomeric units, but there are some drawbacks that make its use in industrial biocatalysis unfeasible. In the present study, we describe the hydrolysis of brewer's spent grain (BSG) with an enzymatic cocktail produced by Aspergillus niger CECT 2700 and its comparison with commercial enzymes. In addition, it was determined whether pretreating the BSG (non-pressurized alkaline hydrolysis or treatment with cholinium glycinate ionic liquid) is necessary. Results show that both pretreatments enhanced xylose release (10.55 ± 0.07 g/L and 8.14 ± 0.13 g/L respectively), meanwhile the hydrolysis of raw BSG with the enzymatic cocktail produced solutions containing high levels of glucose (18.45 ± 1.66 g/L) and xylose (6.38 ± 0.26 g/L).

Deciphering the Fenton-reaction-aid lignocellulose degradation pattern by Phanerochaete chrysosporium with ferroferric oxide nanomaterials: Enzyme secretion, straw humification and structural alteration

Nowadays, Nano-biotechnology is emerging to be one of the most promising tools in environmental remediation. In this study, the degradation efficiency of lignocellulose by white-rot fungi was improved by addition of Fe₃O₄ nanomaterials (NMs) in solid-state fermentation. The highly-ordered cellulose crystalline was demonstrated to be broken down through infrared spectroscopy (FT-IR) and crystallinity index analysis. The decay of fluorescence intensity presented a lower degree of aromatic polycondensation and less conjugated chromophores in lignocellulose. Mechanistic analysis showed that NMs participated in the Fenton reaction and affected lignocellulose biodegradation process by regulating enzyme secretion. Specifically, the time variation curves of hydroxyl radicals and Fe²⁺ were discussed to illustrate the degradation pattern. The NMs remained stable after the fermentation and were possible to be recycled for the next cycle. All the results support that the synergism of Fe₃O₄ NMs and white-rot fungi would be a promising research direction in lignocellulose treatment.

Earthworms, Rice Straw, and Plant Interactions Change the Organic Connections in Soil and Promote the Decontamination of Cadmium in Soil

The joint effects of earthworms and crop straw on toxic metal speciation are not clear, and very limited information is available regarding the effects of their interaction on Cd mobility in Cd contaminated soil or in remediation processes involving plants. This study evaluated their impacts on Cd mobile form changes in soil and their effects on Cd uptake by plants. Treatments included both planted and unplanted-Cd-contaminated soil with or without rice straw and/or earthworms. The results revealed that earthworms, rice straw, and plant interactions change the Cd mobile forms in soil. The order of Cd concentration of different chemical forms was as follows: exchangeable > residual > bound to Fe-Mn oxide > bound to organic matter for earthworms, and exchangeable > bound to organic matter > residual > bound to Fe-Mn oxide for rice straw treatment, with a recovery rate of 96 ± 3%. The accumulation of Cd in plants increased in the presence of earthworms and decreased in the presence of rice straw. FT-IR spectra indicated that the degradation of rice straw increases C⁻O, C⁻O⁻H, C⁻H, and O⁻H functional groups which could complex with Cd ions. These findings highlighted that earthworms' activities and crop straw can modify soil properties and structure and promote the remediation of heavy metal. This study suggests that the ecological context of remediation instead of being limiting on soil-earthworms-plant interaction, should integrate the natural resources forsaken which can provide a positive influence on both plant health and the remediation of heavy metal in contaminated soil.