Ensemble machine learning prediction of anaerobic co-digestion of manure and thermally pretreated harvest residues

This study aimed to clarify the statistical accuracy assessment approaches used in recent biogas prediction studies using state-of-the-art ensemble machine learning approach according to 10-fold cross-validation in 100 repetitions. Three thermally pretreated harvest residue types (maize stover, sunflower stalk and soybean straw) and manure were anaerobically co-digested, measuring biogas and methane yield alongside eight thermal preprocessing and biomass covariates. These were the inputs to an ensemble machine learning approach for biogas and methane yield prediction, employing three feature selection approaches. The Support Vector Machine prediction with the Recursive Feature Elimination resulted in the highest prediction accuracy, achieving the coefficient of determination of 0.820 and 0.823 for biogas and methane yield prediction, respectively. This study demonstrated an extreme dependency of prediction accuracy to input dataset properties, which could only be mitigated with ensemble machine learning and strongly suggested that the split-sample approach, often used in previous studies, should be avoided.

Electron transfer and microbial mechanism of synergistic degradation of lignocellulose by hydrochar and aerobic fermentation

In response to the problem of asynchronous fermentation between lignocellulose and perishable materials in compost, the combined technology of low-temperature hydrochar and compost has been studied. Hydrochar was prepared through low-temperature hydrothermal reactions and applied to aerobic fermentation. The response relationship between lignocellulose content, electron transfer capability, and microbes was explored. The results showed that a pore structure with oxygen-containing functional groups was formed in hydrochar, promoting electron transfer during composting. With the rapid increase in composting temperature, the lignocellulose content decreased by 64.36 mg/g. Oceanobacillus, Cerasibacillus, Marinimicrobium, and Gracilibacillus promoted the degradation of lignocellulose and the carbon/nitrogen cycle during aerobic fermentation, and there was a significant response relationship between electron transfer capability and functional microbes. The combined application of hydrochar and aerobic fermentation accelerated the degradation of lignocellulose. This study provides technical support for the treatment of heterogeneous organic waste.

Pretreatment of Jerusalem artichoke stalk using hydroxylammonium ionic liquids and their influences on 2,3-butanediol fermentation by Bacillus subtilis

Pretreatment of lignocellulose is a vital step for biological production of bio-chemicals and bio-fuels. In this work, the pretreatment of Jerusalem artichoke stalk (JAS) by hydroxylammonium ionic liquids was evaluated based on pretreatment efficiency including polysaccharide recovery and enzymatic digestibility, and influence of ionic liquids on 2,3-butanediol fermentation using Bacillus subtilis. The results showed ethanolammonium acetate (EOAA) was efficient in JAS pretreatment, and maximum cell density was increased 25% when EOAA concentration was not greater than 0.3 mol/L in medium, while the total concentration of acetoin and 2,3-butanediol was 15% greater than the control at 0.1 mol/L EOAA. After the pretreatment under optimized conditions of 170 °C for 5-h and liquid-solid ratio of 18, about 87% cellulose and 75% hemicellulose were recovered, and glucose yield of 64% and xylose of 66% were obtained after 24-h hydrolysis of JAS residue by cellulase (15 FPU/g) with solid loading of 10 wt%.

Self-generated peroxyacetic acid in phosphoric acid plus hydrogen peroxide pretreatment mediated lignocellulose deconstruction and delignification

BACKGROUND

Peroxyacetic acid involved chemical pretreatment is effective in lignocellulose deconstruction and oxidation. However, these peroxyacetic acid are usually artificially added. Our previous work has shown that the newly developed PHP pretreatment (phosphoric acid plus hydrogen peroxide) is promising in lignocellulose biomass fractionation through an aggressive oxidation process, while the information about the synergistic effect between H3PO4 and H2O2 is quite lack, especially whether some strong oxidant intermediates is existed. In this work, we reported the PHP pretreatment system could self-generate peroxyacetic acid oxidant, which mediated the overall lignocellulose deconstruction, and hemicellulose/lignin degradation.

RESULTS

The PHP pretreatment profile on wheat straw and corn stalk were investigated. The pathways/mechanisms of peroxyacetic acid mediated-PHP pretreatment were elucidated through tracing the structural changes of each component. Results showed that hemicellulose was almost completely solubilized and removed, corresponding to about 87.0% cellulose recovery with high digestibility. Rather high degrees of delignification of 83.5% and 90.0% were achieved for wheat straw and corn stalk, respectively, with the aid of peroxyacetic acid oxidation. A clearly positive correlation was found between the concentration of peroxyacetic acid and the extent of lignocellulose deconstruction. Peroxyacetic acid was mainly self-generated through H2O2 oxidation of acetic acid that was produced from hemicellulose deacetylation and lignin degradation. The self-generated peroxyacetic acid then further contributed to lignocellulose deconstruction and delignification.

CONCLUSIONS

The synergistic effect of H3PO4 and H2O2 in the PHP solvent system could efficiently deconstruct wheat straw and corn stalk lignocellulose through an oxidation-mediated process. The main function of H3PO4 was to deconstruct biomass recalcitrance and degrade hemicellulose through acid hydrolysis, while the function of H2O2 was to facilitate the formation of peroxyacetic acid. Peroxyacetic acid with stronger oxidation ability was generated through the reaction between H2O2 and acetic acid, which was released from xylan and lignin oxidation/degradation. This work elucidated the generation and function of peroxyacetic acid in the PHP pretreatment system, and also provide useful information to tailor peroxide-involved pretreatment routes, especially at acidic conditions.

Structure-property-performance relationships of lactic acid-based deep eutectic solvents with different hydrogen bond acceptors for corn stover pretreatment

Herein, ten types of lactic acid-based deep eutectic solvents (DESs) with differently structured hydrogen bond acceptors (HBAs) were used for corn stover pretreatment. Among the tested DESs, those composed of HBAs with short alkyl chain were more effective to remove lignin and xylan, resulting in higher enzymatic digestion of the pretreated solids than their counterparts with long alky chain. Also, functional groups of HBAs demonstrated significant effects on biomass deconstruction. In order to interpret the different pretreatment performance of the tested DESs, Kamlet-Taft solvent polarity parameters of the tested DESs were correlated to their lignocellulose pretreatment performance. It was found that hydrogen bond acidity (Kamlet-Taft α parameter) had strong positive relationships with pretreatment efficacy of the studied DESs. These findings not only clarified the structure-property-performance relationships of the DESs, but also provided novel insights into design and selection of DESs for lignocellulose pretreatment.

Understanding the dissolution of softwood lignin in ionic liquid and water mixed solvents

Lignin is the most abundant heterogeneous aromatic polymer on earth to produce a large number of value-added chemicals. Besides, the separation of lignin from the lignocellulosic biomass is essential for cellulosic biofuel production. For the first time, we report a cosolvent-based approach to understand the dissolution of lignin with 61 guaiacyl subunits at the molecular level. Atomistic molecular dynamics simulations of the lignin were performed in 0%, 20%, 50%, 80%, and 100% 1-Ethyl-3-Methylimidazolium Acetate (EmimOAc) systems. The lignin structure was significantly destabilized in both 50%, and 80% EmimOAc cosolvents, and pure EmimOAc systems leading to the breakdown of intrachain hydrogen bonds. Lignin-OAc and lignin-water hydrogen bonds were formed with increasing EmimOAc concentration, signifying the dissolution process. The OAc anions mostly solvated the alkyl chains and hydroxy groups of lignin. Besides, the imidazolium head of Emim cations contributed to solvation of methoxy groups and hydroxy groups, whereas ethyl tail interacted with the benzene ring of guaiacyl subunits. Effective dissolution was obtained in both the 50% and 80% EmimOAc cosolvent systems. Overall, our study presents a molecular view of the lignin dissolution focusing on the role of both cation and anion, which will help to design efficient cosolvent-based methods for lignin dissolution.

Electrogenerated alkaline hydrogen peroxide for rice straw pretreatment to enhance enzymatic hydrolysis

In this work, alkaline hydrogen peroxide (AHP) solution with 1 wt% H₂O₂ was electrogenerated by oxygen reduction with a current efficiency of 75.2% in a home-made gas diffusion electrode-based electrochemical cell and used for rice straw pretreatment (0.1 g H₂O₂/g rice straw, 10% (w/v) biomass loading, 55 °C, 2 h). Results showed that the AHP pretreatment removed 97.56% of the initial lignin, 85.75% of the initial hemicellulose, and only 0.56% of the initial cellulose, and the specific surface area and porosity of the AHP pretreated rice straw (AHP-RS) were greatly increased. Saccharification results showed that after 48 h of enzymatic hydrolysis AHP-RS achieved a 3.2-fold increase in reducing sugar concentration compared to the untreated rice straw (5.81 and 1.81 g L^-1), highlighting the potential use of this AHP solution for lignocellulose pretreatment.

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

BACKGROUND

The recalcitrance of lignocellulosic biomass offers a series of challenges for biochemical processing into biofuels and bio-products. For the first time, we address these challenges with a biomimetic system via a mild yet rapid Fenton reaction and lignocellulose-degrading bacterial strain Cupriavidus basilensis B-8 (here after B-8) to pretreat the rice straw (RS) by mimicking the natural fungal invasion process. Here, we also elaborated the mechanism through conducting a systematic study of physicochemical changes before and after pretreatment.

RESULTS

After synergistic Fenton and B-8 pretreatment, the reducing sugar yield was increased by 15.6-56.6% over Fenton pretreatment alone and 2.7-5.2 times over untreated RS (98 mg g^-1). Morphological analysis revealed that pretreatment changed the surface morphology of the RS, and the increase in roughness and hydrophilic sites enhanced lignocellulose bioavailability. Chemical components analyses showed that B-8 removed part of the lignin and hemicellulose which caused the cellulose content to increase. In addition, the important chemical modifications also occurred in lignin, 2D NMR analysis of the lignin in residues indicated that the Fenton pretreatment caused partial depolymerization of lignin mainly by cleaving the β-O-4 linkages and by demethoxylation to remove the syringyl (S) and guaiacyl (G) units. B-8 could depolymerize amount of the G units by cleaving the β-5 linkages that interconnect the lignin subunits.

CONCLUSIONS

A biomimetic system with a biochemical Fenton reaction and lignocellulose-degrading bacteria was confirmed to be able for the pretreatment of RS to enhance enzymatic hydrolysis under mild conditions. The high digestibility was attributed to the destruction of the lignin structure, partial hydrolysis of the hemicellulose and partial surface oxidation of the cellulose. The mechanism of synergistic Fenton and B-8 pretreatment was also explored to understand the change in the RS and the bacterial effects on enzymatic hydrolysis. Furthermore, this biomimetic system offers new insights into the pretreatment of lignocellulosic biomass.

Synergy of lignocelluloses pretreatment by sodium carbonate and bacterium to enhance enzymatic hydrolysis of rice straw

We studied a new strategy for pretreatment of rice straw (RS) to enhance enzymatic hydrolysis under mild condition. This approach uses the synergy of sodium carbonate (Na₂CO₃) and the bacterial strain Cupriavidus basilensis B-8 (hereafter B-8). After synergistic Na₂CO₃ and B-8 pretreatment (SNBP), the reducing sugar yield varied from 335.3mg/g to 799.6mg/g under different conditions. This increased by 13-31% over Na₂CO₃ pretreatment (284.2-719.2mg/g) and 3.42-8.15times over the untreated RS (98mg/g). Moreover, the composition of RS was changed significantly through decreases in lignin and hemicellulose. We confirmed this change by compositional analysis and physicochemical characterization of the structure of RS before and after pretreatment. We also elaborated a mechanism for SNBP to better explain RS changes and bacterial effects on enzymatic hydrolysis.

Combination of biological pretreatment with NaOH/Urea pretreatment at cold temperature to enhance enzymatic hydrolysis of rice straw

A stepwise pretreatment of combination of bacterial treatment with NaOH/Urea (NU) treatment was conducted to enhance enzymatic hydrolysis of rice straw (RS). The results showed that the composition of RS changed significantly, the lignin and hemicellulose decreased while cellulose increased. The biological treatment with a bacterium Sphingobacterium sp. LD-1 achieved mild conditions for the sequential NU treatment, reducing the concentration of the NU solution from 7%/12% to 4%/6% and increasing the temperature from -20°C to -10°C. The saccharification of rice straw co-treated with bacterium Sphingobacterium sp. LD-1 and 4%/6% NU at -10°C resulted in 1.396-fold and 1.372-fold increase of reducing sugar and glucose yield respectively than that of sole NU treatment.

Transcriptome analysis of Zymomonas mobilis ZM4 reveals mechanisms of tolerance and detoxification of phenolic aldehyde inhibitors from lignocellulose pretreatment

BACKGROUND

Phenolic aldehydes generated from lignocellulose pretreatment exhibited severe toxic inhibitions on microbial growth and fermentation. Numerous tolerance studies against furfural, 5-hydroxymethyl-2-furaldehyde (HMF), acetate, and ethanol were reported, but studies on inhibition of phenolic aldehyde inhibitors are rare. For ethanologenic strains, Zymomonas mobilis ZM4 is high in ethanol productivity and genetic manipulation feasibility, but sensitive to phenolic aldehyde inhibitors. Molecular mechanisms of tolerance for Z. mobilis toward phenolic aldehydes are not known.

RESULTS

We took the first insight into genomic response of Z. mobilis ZM4 to the phenolic aldehyde inhibitors derived from lignocellulose pretreatment. The results suggest that the toxicity to cells is caused by the functional group of phenolic aldehyde, similar to furfural and HMF, rather than aromatic groups or phenolic hydroxyl groups. Transcriptome response against 4-hydroxybenzaldehyde, syringaldehyde, and vanillin, representing phenolic groups H, S, and G, respectively, was investigated. The atlas of the important genes responsible for significantly enhanced and repressed genes at the genomic level was illustrated. 272 genes with twofold greater expressions than non-treated controls and 36 gene clusters in response to challenges of these phenolic aldehydes were identified. Several reductases encoded by ZMO1116, ZMO1696, and ZMO1885 were found to play the key roles in reducing phenolic aldehydes into the corresponding phenolic alcohols. Reduction of phenolic aldehydes by overexpression of ZMO1116, ZMO1696, and ZMO1885 in Z. mobilis ZM4 resulted in the increased inhibitor conversion and ethanol productivity, especially for 4-hydroxybenzaldehyde and vanillin. Several transporter genes such as ZMO0282, ZMO0283, ZMO0798, ZMO0799, and ZMO0800 was also displayed significantly increased expressions against the phenolic aldehydes.

CONCLUSIONS

The genes encoding reductases are with potentials on phenolic aldehydes-tolerant genes contributing to the reduction of phenolic aldehydes into the corresponding phenolic alcohols forms for Z. mobilis ZM4. Overexpression of the key genes improved the conversion ratio and ethanol productivity of 4-hydroxybenzaldehyde and vanillin with high toxicity. New knowledge obtained from this research aids understanding the mechanisms of bacterial tolerance and the development of the next-generation biocatalysts for advanced biofuels production.

Effect of fungal and phosphoric acid pretreatment on ethanol production from oil palm empty fruit bunches (OPEFB)

Oil palm empty fruit bunches (OPEFB), a lignocellulosic residue of palm oil industries was examined for ethanol production. Milled OPEFB exposed to simultaneous saccharification and fermentation (SSF) with enzymes and Saccharomyces cerevisiae resulted just in 14.5% ethanol yield compared to the theoretical yield. Therefore, chemical pretreatment with phosphoric acid, a biological pretreatment with white-rot fungus Pleurotus floridanus, and their combination were carried out on OPEFB prior to the SSF. Pretreatment with phosphoric acid, combination of both methods and just fungal pretreatment improved the digestibility of OPEFB by 24.0, 16.5 and 4.5 times, respectively. During the SSF, phosphoric acid pretreatment, combination of fungal and phosphoric acid pretreatment and just fungal pretreatment resulted in the highest 89.4%, 62.8% and 27.9% of the theoretical ethanol yield, respectively. However, the recovery of the OPEFB after the fungal pretreatment was 98.7%, which was higher than after phosphoric acid pretreatment (36.5%) and combined pretreatment (45.2%).