A bionic system with Fenton reaction and bacteria as a model for bioprocessing lignocellulosic biomass

Effect of heterogeneous fenton-like pretreatment on semi-permeable membrane-covered co-composting: Humification and microbial community succession

This study focused on the impacts of heterogeneous Fenton-like pretreatment on the humification and bacterial community during co-composting of wheat straw with cattle dung covered with a semi-permeable membrane. In this study, FeOCl and low concentration of H2O2 were used for pretreatment and composting, which lasted for 39 days. The results showed that the pretreatment promoted the humification process, with degree of polymerization and percentage of humic acid increasing by 53.2 % and 7.3 %, respectively. Furthermore, the diversity and structure of bacterial communities were altered by pretreatment. During the thermophilic phase, pretreatment considerably promoted the metabolism of carbohydrate. According to redundancy analysis, C/N, moisture and organic matter were the key environmental factors that dominated the microbial community. In summary, heterogeneous Fenton-like pretreatment provided a novel idea for improving the humic acid content and maturity of the compost pile.

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.

High-purity alkaline lignin extraction from Saccharum ravannae and optimization of lignin recovery through response surface methodology

Saccharum ravannae, known as "Ekra" in the Northeast region of India, is an elephant grass species that abundantly grows in the natural habitat of Assam. This study aims to utilize this wild grass species and extract alkaline lignin of high purity through KOH-mediated alkaline hydrothermal pretreatment using the Oil bath process. Lignin recovery was optimized using RSM (response surface methodology) combined with a central composite model. Three process parameters, namely KOH concentration (1-3 %), reaction time (50-200 min), and solid loading (5-15 %), varied to optimize the combined effect of these parameters. RSM predicted a maximum lignin recovery of 15.38 g/100 g of raw biomass at optimum conditions (2.4 % KOH, 6.41 % solid loading, 176.57 min). Three experimental runs were performed at optimum conditions, and 15.81 ± 0.32 g/100 g lignin recovery was obtained, thus verifying the predicted result. Maximum 93.7 % purity of extracted lignin was achieved in a different operating condition (3 % KOH, 10 % solid loading, 125 min). The commercial and extracted alkaline lignin with maximum purity was characterized by Nuclear Magnetic Resonance (NMR), Fourier Transform Infrared Spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS). The extracted lignin shows higher phenolic content and more functional groups than commercial lignin and can be used for future applications.

Strengthening Fenton-like reactions to improve lignocellulosic degradation performance by increasing lignocellulolytic enzyme core microbes during rice straw composting

This study aimed to explore the effect of Fenton-like reactions on lignocellulosic degradation performance and identify their driving factors during composting. Rice straw was pretreated by inoculating Aspergillus fumigatus Z1 and then adding Fe (II), which resulted in Fenton-like reactions. The treatment groups included CK (control), Fe (addition of Fe (II)), Z1 (inoculation of A. fumigatus Z1), and Fe + Z1 (inoculation of A. fumigatus Z1 and addition of Fe (II)). The results suggested that Fenton-like reactions can produce lignocellulolytic enzymes and degrade lignocellulose, due to the variation in microbial community composition and diversity. In addition, functional modular microbes were identified by network analysis, which can produce endoglucanase and xylanase. Regarding ligninase, bacteria were more favorable for producing manganese peroxidase, and fungi were more favorable for producing laccase. Additionally, reducing sugars, organic matter, total nitrogen and amino acids were key microhabitat factors of functional modular bacteria, while organic matter, reducing sugars, amino acids and C/N were key microhabitat factors of functional modular fungi, thereby promoting the degradation of lignocellulose. This study provides technical support for lignocellulosic degradation based on Fenton-like reactions.

How can vanillin improve the performance of lignocellulosic biomass conversion in an immobilized laccase microreactor system?

Gentle and effective pretreatment is necessary to produce clean lignocellulosic biomass-based fuels. Herein, inspired by the efficient lignin degradation in the foregut of termites, the microreactor system using immobilized laccase and recoverable vanillin was proposed. Firstly, the co-deposition coating of dopamine, hydrogen peroxide and copper sulfate was constructed for laccase immobilization and a high immobilization efficiency of 87.0% was obtained in 30 min. After storage for 10 days, 82.2% activity was maintained in the laccase-loaded microreactor, which is 210.0% higher than free laccase. In addition, 6% (w/w) vanillin can improve lignin degradation in the laccase-loaded microreactor without impairing laccase activity, leading to a 47.3% increment in cellulose accessibility. Finally, a high cellulose conversion rate of 88.1% can be achieved in 1 h with glucose productivity of 2.62 g L^-1 h^-1. These demonstrated that the appropriate addition of vanillin can synergize with immobilized laccase to enhance the conversion of lignocellulosic biomass.

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.

Understanding of bacterial lignin extracellular degradation mechanisms by Pseudomonas putida KT2440 via secretomic analysis

BACKGROUND

Bacterial lignin degradation is believed to be primarily achieved by a secreted enzyme system. Effects of such extracellular enzyme systems on lignin structural changes and degradation pathways are still not clearly understood, which remains as a bottleneck in the bacterial lignin bioconversion process.

RESULTS

This study investigated lignin degradation using an isolated secretome secreted by Pseudomonas putida KT2440 that grew on glucose as the only carbon source. Enzyme assays revealed that the secretome harbored oxidase and peroxidase/Mn²⁺-peroxidase capacity and reached the highest activity at 120 h of the fermentation time. The degradation rate of alkali lignin was found to be only 8.1% by oxidases, but increased to 14.5% with the activation of peroxidase/Mn²⁺-peroxidase. Gas chromatography-mass spectrometry (GC-MS) and two-dimensional ¹H-¹³C heteronuclear single-quantum coherence (HSQC) NMR analysis revealed that the oxidases exhibited strong C-C bond (β-β, β-5, and β-1) cleavage. The activation of peroxidases enhanced lignin degradation by stimulating C-O bond (β-O-4) cleavage, resulting in increased yields of aromatic monomers and dimers. Further mass spectrometry-based quantitative proteomics measurements comprehensively identified different groups of enzymes particularly oxidoreductases in P. putida secretome, including reductases, peroxidases, monooxygenases, dioxygenases, oxidases, and dehydrogenases, potentially contributed to the lignin degradation process.

CONCLUSIONS

Overall, we discovered that bacterial extracellular degradation of alkali lignin to vanillin, vanillic acid, and other lignin-derived aromatics involved a series of oxidative cleavage, catalyzed by active DyP-type peroxidase, multicopper oxidase, and other accessory enzymes. These results will guide further metabolic engineering design to improve the efficiency of lignin bioconversion.

Combined Biological and Chemical/Physicochemical Pretreatment Methods of Lignocellulosic Biomass for Bioethanol and Biomethane Energy Production—A Review

Lignocellulosic biomass is a low-cost and environmentally-friendly resource that can be used to produce biofuels such as bioethanol and biogas, which are the leading candidates for the partial substitution of fossil fuels. However, the main challenge of using lignocellulosic materials for biofuel production is the low accessibility to cellulose for hydrolysis of enzymes and microorganisms, which can be overcome by pretreatment. Biological and chemical pretreatments have their own disadvantages, which could be reduced by combining the two methods. In this article, we review biological–chemical combined pretreatment strategies for biogas and bioethanol production. The synergy of fungal/enzyme–NaOH pretreatment is the only biological–chemical combination studied for biogas production and has proven to be effective. The use of enzyme, which is relatively expensive, has the advantage of hydrolysis efficiency compared to fungi. Nonetheless, there is vast scope for research and development of other chemical–biological combinations for biogas production. With respect to ethanol production, fungal–organosolv combination is widely studied and can achieve a maximum of 82% theoretical yield. Order of pretreatment is also important, as fungi may reduce the accessibility of cellulose made available by prior chemical strategies and suppress lignin degradation. The biofuel yield of similarly pretreated biomass can vary depending on the downstream process. Therefore, new strategies, such as bioaugmentation and genetically engineered strains, could help to further intensify biofuel yields.

In-situ generation of H2O2 by zero valent iron to control depolymerization of lignocellulose in composting niche

Lignocellulosic degradation is a bottleneck of bioconversion during the composting process. In-situ generation of H₂O₂ in the composting system was an ideal method for efficiently promoting lignocellulase degradation, and zero valent iron (ZVI) was concerned because it can generate H₂O₂ by reducing dissolved oxygen. This study focused on the effects of ZVI treatment on lignocellulose degradation, microbial communities, and carbohydrate-active enzymes (CAZymes) genes during composting. Its results indicated that ZVI increased H₂O₂ content during composting, accompanied by the formation of •OH. The degradation rates of lignin, cellulose and hemicellulose in ZVI group (20.77%, 30.35% and 44.7%) were significantly higher than in CK group (17.01%, 26.12% and 38.5%). Metagenomic analysis showed that ZVI induced microbial growth that favored lignocellulose degradation, which increased the abundance of Actinobacteria and Firmicutes but reduced Proteobacteria. At the genus level, the abundance of Thermomonospora, Streptomyces, and Bacillus significantly increased. In addition, glycoside hydrolases and auxiliary activities were important CAZymes families of lignocellulose degradation, and their abundance was higher in the ZVI group. Redundancy analysis showed that the increased H₂O₂ and •OH content was a critical factor in improving lignocellulose degradation. Overall, H₂O₂ as a co-substrate enhanced the enzymatic efficiency, •OH unspecifically attacked lignocellulose, and the increase in functional microbial abundance was the main reason for promoting lignocellulose degradation in composting.

Effect of Fenton pretreatment and bacterial inoculation on cellulose-degrading genes and fungal communities during rice straw composting

The aims of this article were to study the effect of Fenton pretreatment and bacterial inoculation on cellulose-degrading genes and fungal communities during rice straw composting. The rice straw was pretreated by Fenton reactions and functional bacterial agents were then inoculated during the cooling phase of composting. Three treatment groups were carried out, the control (CK), Fenton pretreatment (FeW) and Fenton pretreatment and bacterial inoculation (FeWI). The results indicated that Fenton pretreatment and bacterial inoculation changed the fungal communities composition and increased fungal diversity, leading to changes in the cellulose-degrading genes. In addition, a network analysis showed that in the FeWI treatment, the fungi from modules 1, 5 and 8 were core hosts of the cellulose-degrading genes driving the cellulosic degradation. Moreover, Fenton pretreatment and bacterial inoculation changed the core module fungal communities and strengthened the correlation between the core fungi and the cellulose-degrading genes, thereby promoting cellulosic degradation. Based on redundancy and structural equation model analyses, the NH₄⁺-N, TOC, pH and Shannon index were important factors influencing the variations in the cellulose-degrading genes. This study provides a foundation for cellulosic degradation during cellulosic waste composting.

Humification process and mechanisms investigated by Fenton-like reaction and laccase functional expression during composting

This study aims to explore the impacts of the Fenton-like reaction on hydrogen peroxide, hydroxyl radicals, humic substance (HS) formation, laccase activity and microbial communities during composting to optimize composting performances. The results indicated that the activity of laccase in the presence of the Fenton-like reaction (HC) (35.92 U/g) was significantly higher than that in the control (CP) (29.56 U/g). The content of HS in HC (151.91 g/kg) was higher than that in CP (131.73 g/kg), and amides, quinones, aliphatic compounds and aromatic compounds were promoted to form HS in HC by 2D-FTIR-COS analysis. Proteobacteria contributed most greatly to AA1 at phylum level, Pseudomonas and Sphingomonas abundances increased in HC. Redundancy analysis indicated that there was a strong positive correlation among the Fenton-like reaction, laccase and HS. Conclusively, the Fenton-like reaction improved the activity of laccase, promoted the formation of HS and enhanced the quality of compost.

Identifying driving factors of humic acid formation during rice straw composting based on Fenton pretreatment with bacterial inoculation

The aims of this study were to identify the driving factors of humic acid (HA) during rice straw composting based on Fenton pretreatment with bacterial inoculation. Rice straw was pretreated by Fenton reactions and then inoculated during composting, which was set up CK (control), FeW (Fenton pretreatment) and FeWI (Fenton pretreatment + functional bacterial agents). Results indicated that Fenton pretreatment and inoculation of functional bacteria increased the concentration of HA components, which was due to that bacterial composition was changed and bacterial diversity was decreased. Moreover, Fenton pretreatment and inoculation of functional bacteria increased the bacterial amounts of shikimic acid metabolism genes and the correlation between HA components and shikimic acid metabolism genes. Therefore, the functional bacteria were core driving factors, and NH₄^--N, pH, cellulose and bacterial diversity as key environmental factors to promote the formation of HA components.

Hydrogen peroxide plus ascorbic acid enhanced organic matter deconstructions and composting performances via changing microbial communities

This work aims to investigate the influence of hydrogen peroxide (H₂O₂) and ascorbic acid (ASCA) on the physicochemical characteristics, organic matter (OM) deconstructions, humification degree and succession of bacterial communities for co-composting of bagasse pith and dairy manure. The results indicated that H₂O₂ and ASCA accelerated the degradation of lignocellulose, improved the transformation of dissolved organic matter (DOM), and enhanced the content of humic substance (HS) and the degree of its aromatization. The bacterial communities were significantly changed in the presence of additives, in which the relative abundances of Firmicutes and Actinobacteria significantly increased. Redundancy analysis (RDA) indicated that the degradation of OM and lignocellulose more influenced the bacterial community compositions. Conclusively, adding H₂O₂ and ASCA accelerated lignocellulose degradation efficiency, and improved the composting process, which provided an optimized method to dispose of lignocellulose wastes and livestock manure.

Fiber degradation and carbohydrate production by combined biological and chemical/physicochemical pretreatment methods of lignocellulosic biomass - A review

Sustainable biorefinery concepts based on lignocellulosic biomass are gaining worldwide research interest because of their inexpensiveness and abundance. The recalcitrance of lignocellulosic biomass poses a major hindrance to enhance biofuel production. Therefore, a pretreatment step is critical to prepare the substrates for the downstream process. Combining pretreatment steps help to lower the severity of the drawbacks of a single pretreatment step. This paper systematically reviews the combined biological and chemical/physicochemical pretreatment based on fiber degradation and sugar yield. An energy-efficient biological pretreatment method combined with a chemical pretreatment that accelerates the pretreatment times has been seen to be efficient for fiber degradation and sugar yields. However, fungal species, culture conditions, biomass type, the severity of chemical pretreatment and the order of sequential pretreatment influences the relative component contents and sugar yield. Even the same biomass from different sources undergoing similar pretreatment conditions could result in a varying amount of digestibility.

Derived high reducing sugar and lignin colloid particles from corn stover

Lignocellulosic biomass is considered as the largest potential candidate to develop alternative energy, such as biofuel, biomaterial. However, the efficient conversion of cellulose and practical utilization of lignin are great challenges for sustainable biorefinery. In this study, high reducing sugar yield and different size of lignin colloid particles (LCPs) were obtained via tetrahydrofuran-water (THF-H2O) pretreatment of corn stover (CS). THF-H2O as a co-solvent, could efficiently dissolve lignin and retain cellulose. After the pretreatment, 640.87 mg/g of reducing sugar was produced, that was 6.66-fold higher than that of the untreated CS. Meanwhile, the pretreatment liquor could form spherical LCPs with different sizes ranged from 202 to 732 nm through self-assembly. We studied the optimal pretreatment condition to simultaneously realize the high reducing sugar yield (588.4 mg/g) and excellent LCPs preparation with average size of 243 nm was under TH22 (THF-H2O pretreatment at 120 °C for 2 h). To further explore the formation of LCPs with different sizes. We studied the lignin structure changes of various conditions, concluded the size of LCPs was related to the lignin concentration and syringyl/guaiacyl (S/G) ratio. As the increase of the lignin concentration and S/G, the sizes of LCPs were increased. G-type lignin was easier to dissolve in the mild pretreatment supernatant, contributing to form smaller LCPs with a good dispersibility. In the severe condition, both of S and G-type lignin were dissolved due to the lignin depolymerization, formed the larger sphere particles. This work provides a novel perspective for the technical design of lignocellulosic biomass conversion.

Improving enzymatic hydrolysis of mechanically refined poplar branches with assistance of hydrothermal and Fenton pretreatment

The combination of different pretreatment methods can effectively overcome recalcitrance of lignocellulosic biomass to ensure its highly efficient conversion into bio-based products. In this study, the combined pretreatments of chemical methods (hydrothermal treatment and Fenton treatment) with mechanical refining were used to improve the enzymatic hydrolysis efficiency of poplar branches. The results indicated that hydrothermal pretreatment and Fenton pretreatment can effectively improve the enzymatic hydrolysis of poplar substrates, e.g., the maximum glucose conversion yield and glucose concentration reached 92.4% and 20.8 g/L, respectively. The pre-hydrolysates contained some valuable components such as monosaccharides, oligosaccharides, acetic acid, furfural, and hydroxymethylfurfural. The main characteristics (specific surface area, water retention value, fines content, and surface lignin concentration) of poplar substrates were obviously changed by the combined pretreatment, which benefit the enzymatic hydrolysis.

Lignin induced iron reduction by novel sp., Tolumonas lignolytic BRL6-1

Lignin is the second most abundant carbon polymer on earth and despite having more fuel value than cellulose, it currently is considered a waste byproduct in many industrial lignocellulose applications. Valorization of lignin relies on effective and green methods of de-lignification, with a growing interest in the use of microbes. Here we investigate the physiology and molecular response of the novel facultative anaerobic bacterium, Tolumonas lignolytica BRL6-1, to lignin under anoxic conditions. Physiological and biochemical changes were compared between cells grown anaerobically in either lignin-amended or unamended conditions. In the presence of lignin, BRL6-1 accumulates higher biomass and has a shorter lag phase compared to unamended conditions, and 14% of the proteins determined to be significantly higher in abundance by log2 fold-change of 2 or greater were related to Fe(II) transport in late logarithmic phase. Ferrozine assays of the supernatant confirmed that Fe(III) was bound to lignin and reduced to Fe(II) only in the presence of BRL6-1, suggesting redox activity by the cells. LC-MS/MS analysis of the secretome showed an extra band at 20 kDa in lignin-amended conditions. Protein sequencing of this band identified a protein of unknown function with homology to enzymes in the radical SAM superfamily. Expression of this protein in lignin-amended conditions suggests its role in radical formation. From our findings, we suggest that BRL6-1 is using a protein in the radical SAM superfamily to interact with the Fe(III) bound to lignin and reducing it to Fe(II) for cellular use, increasing BRL6-1 yield under lignin-amended conditions. This interaction potentially generates organic free radicals and causes a radical cascade which could modify and depolymerize lignin. Further research should clarify the extent to which this mechanism is similar to previously described aerobic chelator-mediated Fenton chemistry or radical producing lignolytic enzymes, such as lignin peroxidases, but under anoxic conditions.

Effect of Fenton pretreatment combined with bacteria inoculation on humic substances formation during lignocellulosic biomass composting derived from rice straw

The goal of this work was to explore the effect of Fenton pretreatment combined with bacteria inoculation on the formation of humic substances (HS) during rice straw composting. In this study, the compound bacterial agents were inoculated after Fenton pretreatment during rice straw composting. The results suggested that the coupling effects of Fenton pretreatment and bacteria inoculation promoted the humification process, which might be the reason of organic fractions degradation and transformation. In addition, the bacterial communities structure and diversity were changed by Fenton pretreatment and inoculation. Key microbial genera linking to the transformation of organic fractions were determined by network analysis. Redundancy analysis and structural equation model analysis indicated that Fenton pretreatment, inoculation, amino acid, soluble sugar and beta-diversity as the key factors affecting organic fractions transformation during composting. Therefore, the combined application Fenton pretreatment with bacteria inoculation provided a new method to promote the HS amount.

Improved lignocellulose degradation efficiency based on Fenton pretreatment during rice straw composting

This study aims to explore the effect of Fenton pretreatment on organic fractions, enzymes activities and microbial communities during composting. In this study, rice straw was chosen to be composted after pretreatment. The results indicated that Fenton pretreatment significantly increased the degradation of organic matter and coarse fiber contents, which might be the reason that Fenton pretreatment enhanced lignocellulose-degrading enzymes activities during composting, including CMCase, FPase, xylanase, manganese peroxidase, lignin peroxidase and laccase. Additionally, Fenton pretreatment reshaped bacteria community. The key enzymes and environmental factors, which affected organic fractions degradation were identified by redundancy analysis. Furthermore, structural equation modeling and variation partitioning analysis further revealed possible mechanisms of organic fractions degradation in different treatments during composting. In summary, the combined application Fenton pretreatment and composting improved lignocellulose degradation efficiency, which provided for an effective and environment-friendly way to manage lignocellulose wastes.

Response of Cupriavidus basilensis B-8 to CuO nanoparticles enhances Cr(VI) reduction

CuO nanoparticles (NPs) released into aqueous environments induce metal toxicity, which generally exerts negative effects on various organisms and leads to great challenge for wastewater biotreatment. In this study, a promotion effect of CuO NPs on biological process was first found. Cr(VI) reduction by Cupriavidus basilensis B-8 (hereafter B-8) was enhanced in the presence of CuO NPs. The efficiency of Cr(VI) bioreduction was much higher with B-8 and CuO NPs (approximately 100%) than with B-8 (approximately 37.6%) and CuO NPs (39.9-44.7%) alone, indicating a stimulatory effect of CuO NPs on Cr(VI) reduction by B-8. Our material analyses revealed different response mechanisms of B-8 to Cr(VI), with and without CuO NPs. The addition of CuO NPs influenced the interaction of Cr(VI) with the N-, P-, S-, and C-related functional groups of B-8. Transcriptomic analysis indicated that multiple mechanisms, including Cr(VI) uptake and reactive oxygen species detoxification, were induced by Cr(VI). Many genes involved in various metabolic processes were significantly upregulated by the addition of CuO NPs. To a certain extent, the pressure of DNA repairment by B-8 induced by Cr(VI) was also alleviated by the presence of CuO NPs. They contributed to facilitate B-8 growth and enhance Cr(VI) reduction, even with 50 mg/L Cr(VI). This study not only elaborated the mechanisms of bacterial Cr(VI) reduction when enhanced by CuO NPs, but also provided a novel perspective for wastewater biotreatment.

Bioconversion of lignin into bioplastics by Pandoraea sp. B-6: molecular mechanism

Lignin is a byproduct in the pulp and paper industry and is considered as a promising alternative for the provision of energy and chemicals. Currently, the efficient valorization of lignin is a challenge owing to its polymeric structure complexity. Here, we present a platform for bio-converting Kraft lignin (KL), to polyhydroxyalkanoate (PHA) by Pandoraea sp. B-6 (hereafter B-6). Depolymerization of KL by B-6 was first confirmed, and > 40% KL was degraded by B-6 in the initial 4 days. Characterization of PHA showed that up to 24.7% of PHA accumulated in B-6 grown in 6-g/L KL mineral medium. The composition, structure, and thermal properties of the produced PHA were analyzed, revealing that 3-hydroxybutyrate was the only monomer and that PHA was comparable with the commercially available bioplastics. Moreover, the genomic analysis illustrated three core enzymatic systems for lignin depolymerization including laccases, peroxidases, and Fenton-reaction enzymes; five catabolic pathways for LDAC degradation and a gene cluster consisting of bktB, phaR, phaB, phaA, and phaC genes involved in PHA biosynthesis. Accordingly, a basic model for the process from lignin depolymerization to PHA production was constructed. Our findings provide a comprehensive perspective for lignin valorization and bio-material production from waste.

Laccase pretreatment of wheat straw: effects of the physicochemical characteristics and the kinetics of enzymatic hydrolysis

BACKGROUND

Wheat straw, the most abundant lignocellulosic biomass in China, is rich in cellulose that can be hydrolyzed and then converted into biofuels, such as bioethanol and biohydrogen. However, the accessibility of cellulose and the enzyme activity are greatly reduced in the presence of lignin. This significantly increases the enzyme cost in the saccharification, which hampers industrial production of lignocellulosic biofuels. In this study, a laccase treatment system mediated by 1-hydroxybenzotriazole was employed to modify and degrade lignin to enhance subsequent enzymatic saccharification of wheat straw. A kinetic model considering enzyme adsorption on lignin was proposed to estimate the saccharification performance.

RESULTS

Fourier transform infrared spectroscopy (FTIR) analyses showed that the peak intensity of lignin structure characteristics significantly changed after laccase-mediated system (LMS) treatment. 2D-nuclear magnetic resonance (NMR) analyses indicated that the aromatic ether bonds were cleaved and that guaiacyl (G) was oxidized after LMS treatment. X-ray diffraction (XRD) analyses suggested that the crystallinity of lignocellulose increased due to the partial degradation of lignin. As a result, the nonproductive adsorption of the enzyme on lignin was reduced by 28%, while the reducing sugar yield increased by 26%. A semi-empirical kinetic model was used to estimate the reducing sugar yield, the initial hydrolysis rate (K _M ) and deactivation rate coefficient (α) of LMS-pretreated wheat straw were 0.157 (h^-1) and 0.214 (h^-1), respectively. The model showed high accuracy (predicting error < 10%) for describing the behavior of laccase-treated wheat straw hydrolysis when the solid loading is < 5%.

CONCLUSIONS

The adsorption ability of the enzyme to lignin was reduced after LMS pretreatment. Physicochemical analyses showed that the chemical groups of lignin and lignocellulose were changed, with the crystallinity of the lignocellulose increasing after LMS treatment. A semi-empirical kinetic model was proposed to estimate the reducing sugar yield, which showed high accuracy for predicting the hydrolysis performance of laccase-treated wheat straw.

Use of bacteria for improving the lignocellulose biorefinery process: importance of pre-erosion

BACKGROUND

Biological pretreatment is an important alternative strategy for biorefining lignocellulose and has attracted increasing attention in recent years. However, current designs for this pretreatment mainly focus on using various white rot fungi, overlooking the bacteria. To the best of our knowledge, for the first time, we evaluated the potential contribution of bacteria to lignocellulose pretreatment, with and without a physicochemical process, based on the bacterial strain Pandoraea sp. B-6 (hereafter B-6) that was isolated from erosive bamboo slips. Moreover, the mechanism of the improvement of reducing sugar yield by bacteria was elucidated via analyses of the physicochemical changes of corn stover (CS) before and after pretreatment.

RESULTS

The digestibility of CS pretreated with B-6 was equivalent to that of untreated CS. The recalcitrant CS surface provided fewer mediators for contact with the extracellular enzymes of B-6. A pre-erosion strategy using a tetrahydrofuran-water co-solvent system was shown to destroy the recalcitrant CS surface. The optimal condition for pre-erosion showed a 6.5-fold increase in enzymatic digestibility compared with untreated CS. The pre-erosion of CS can expose more phenolic compounds that were chelated to oxidized Mn³⁺ and also provided mediators for combination with laccase, which was attributable to B-6 pretreatment. B-6 pretreatment following pre-erosion exhibited a sugar yield that was 91.2 mg/g greater than that of pre-erosion alone and 7.5-fold higher than that of untreated CS. This pre-erosion application was able to destroy the recalcitrant CS surface, thus leading to a rough and porous architecture that better facilitated the diffusion and transport of lignin derivatives. This enhanced the ability of laccase and manganese peroxidase secreted by B-6 to improve the efficiency of this biological pretreatment.

CONCLUSION

Bacteria were not found useful alone as a biological pretreatment, but they significantly improved enzymatic digestion after lignocellulose breakdown via other physicochemical methods. Nonetheless, phenyl or phenoxy radicals were used by laccase and manganese peroxidase in B-6 for lignin attack or lignin depolymerization. These particular mediators released from the recalcitrance network of lignocellulose openings are important for the efficacy of this bacterial pretreatment. Our findings thus offer a novel perspective on the effective design of biological pretreatment methods for lignocellulose.

Efficient pretreatment of lignocellulosic biomass with high recovery of solid lignin and fermentable sugars using Fenton reaction in a mixed solvent

BACKGROUND

Pretreatment of biomass to maximize the recovery of fermentable sugars as well as to minimize the amount of enzyme inhibitors formed during the pretreatment is a challenge in biofuel process. We develop a modified Fenton pretreatment in a mixed solvent (water/DMSO) to combine the advantages of organosolv and Fenton pretreatments. The hemicellulose and cellulose in corncob were effectively degraded into xylose, glucose, and soluble glucose oligomers in a few hours. This saccharide solution, separated from the solid lignin simply by filtration, can be directly applied to the subsequent enzymatic hydrolysis and ethanol fermentation.

RESULTS

After the pretreatment, 94% carbohydrates were recovered as soluble monosaccharide (xylose and glucose) and glucose oligomers in the filtrates, and 87% of solid lignin was recovered as the filter residue. The filtrates were directly applied to enzymatic hydrolysis, and 92% of raw corncob glucose was recovered. The hydrolysates containing the glucose and xylose from the enzymatic hydrolysis were directly applied to ethanol fermentation with ethanol yield equals 79% of theoretical yield. The pretreatment conditions (130 °C, 1.5 bar; 30 min to 4 h) are mild, and the pretreatment reagents (H₂O₂, FeCl₃, and solvent) had low impact to environment. Using ferrimagnetic Fe₃O₄ resulted in similar pretreatment efficiency and Fe₃O₄ could be removed by filtration.

CONCLUSIONS

A modified Fenton pretreatment of corncob in DMSO/water was developed. Up to 94% of the carbohydrate content of corncob was recovered as a saccharide solution simply by filtration. Such filtrate was directly applied to the subsequent enzymatic hydrolysis and where 92% of the corncob glucose content was obtained. The hydrolysate so obtained was directly applied to ethanol fermentation with good fermentability. The pretreatment method is simple, and the additives and solvents used have a low impact to the environment. This method provides the opportunity to substantially maximize the carbohydrate and solid lignin recovery of biomass with a comparatively green process, such that the efficiency of biorefinery as well as the bioethanol production process can be improved. The pretreatment is still relatively energy intensive and expensive, and further optimization of the process is required in large-scale operation.