Efficient removal of residual lignin from eucalyptus pulp via high-concentration chlorine dioxide treatment and its effect on the properties of residual solids

A clean high-concentration chlorine dioxide bleaching process at room temperature facilitating the rapid degradation of lignin and its kinetic

Elemental Chlorine-Free (ECF) bleaching is a globally prevalent pulping technology that primarily utilizes chlorine dioxide (ClO2) as a bleaching agent. However, the chlorination side reactions during ECF bleaching lead to the formation of chlorinated adsorbable organic halides (AOX), which are difficult to degrade and pose a potential threat to the ecological environment. This study presents a novel high-concentration ClO2 pulping bleaching technology at room temperature, which effectively suppresses the chlorination side reactions through the strong oxidizing properties of high-concentration ClO2, thereby achieving both bleaching and a reduction in AOX generation. The results indicate that high concentrations of ClO2 efficiently removed residual lignin from the pulp, promoted the cleavage of lignin ether bonds, and had no adverse impact on the strength or crystalline structure of the cellulose. The amount of AOX was effectively controlled to 0.41 kg/t pulp. Kinetic analysis demonstrated that the degradation of lignin in this bleaching system follows first-order reaction kinetics, with an activation energy of 1310.78 J/mol. Approximately 95 % recovery of the ClO2 solution was achieved through a four-step reabsorption method. This technology advances the development of pulping and bleaching techniques and provides theoretical guidance for the innovation of cleaner pulping and papermaking technologies.

Optimizing lignin demethylation using a novel proton- based ionic liquid: 1, 2-propanediamine/glycolic acid catalyst

Demethylation modification of lignin is an effective strategy to overcome the barrier to its high-value conversion. The purpose of this study focuses on the new proton-based ionic liquid (PIL) 1, 2-propanediamine/glycolic acid (PD/GA) as a catalyst and solvent to achieve the targeted oxidation of lignin. The PD/GA solvents have higher selectivity and efficiency. Optimal phenolic hydroxyl (PH)-increment was achieved, demonstrating enhanced demethylating effect on lignin by modulating the acid-base molar ratio, reaction temperature, and reaction time. Compared to ethanolamine/acetic acid (CE/AC) treatment, the PD/GA treatment at molar ratio 1.25, temperature 60 °C, and 3 h increased the PH-content from 37.74 to 59.91 %. Additionally, the lignin treated with PD/GA exhibited excellent recyclability, featuring a larger Brunauer-Emmett-Teller surface area (1.45 m2.g-1), total pore volume (9.51*10-3 cm3.g-1), and mesoporous size (26.15 nm). The treated lignin yielded maximum ultraviolet resistance and antioxidant activity. These results present new avenues for the development of green and efficient lignin demethylation methods.

Effect of Substituents on Molecular Reactivity during Lignin Oxidation by Chlorine Dioxide: A Density Functional Theory Study

Lignin is a polymer with a complex structure. It is widely present in lignocellulosic biomass, and it has a variety of functional group substituents and linkage forms. Especially during the oxidation reaction, the positioning effect of the different substituents of the benzene ring leads to differences in lignin reactivity. The position of the benzene ring branched chain with respect to methoxy is important. The study of the effect of benzene substituents on the oxidation reaction's activity is still an unfinished task. In this study, density functional theory (DFT) and the m062x/6-311+g (d) basis set were used. Differences in the processes of phenolic oxygen intermediates formed by phenolic lignin structures (with different substituents) with chlorine dioxide during the chlorine dioxide reaction were investigated. Six phenolic lignin model species with different structures were selected. Bond energies, electrostatic potentials, atomic charges, Fukui functions and double descriptors of lignin model substances and reaction energy barriers are compared. The effects of benzene ring branched chains and methoxy on the mechanism of chlorine dioxide oxidation of lignin were revealed systematically. The results showed that the substituents with shorter branched chains and strong electron-absorbing ability were more stable. Lignin is not easily susceptible to the effects of chlorine dioxide. The substituents with longer branched chains have a significant effect on the flow of electron clouds. The results demonstrate that chlorine dioxide can affect the electron arrangement around the molecule, which directly affects the electrophilic activity of the molecule. The electron-absorbing effect of methoxy leads to a low dissociation energy of the phenolic hydroxyl group. Electrophilic reagents are more likely to attack this reaction site. In addition, the stabilizing effect of methoxy on the molecular structure of lignin was also found.

Sustainable fabrication of hybrid silver-copper nanocomposites (Ag-CuO NCs) using Ocimum americanum L. as an effective regime against antibacterial, anticancer, photocatalytic dye degradation and microalgae toxicity

In this study, a sustainable fabrication of hybrid silver-copper oxide nanocomposites (Ag-CuO NCs) was accomplished utilizing Ocimum americanum L. by one pot green chemistry method. The multifarious biological and environmental applications of the green fabricated Ag-CuO NCs were evaluated through their antibacterial, anticancer, dye degradation, and microalgae growth inhibition activities. The morphological features of the surface functionalized hybrid Ag-CuO NCs were confirmed by FE-SEM and HR-TEM techniques. The surface plasmon resonance λ_max peak appeared at 441.56 nm. The average hydrodynamic size distribution of synthesized nanocomposite was 69.80 nm. Zeta potential analysis of Ag-CuO NCs confirmed its remarkable stability at -21.5 mV. XRD and XPS techniques validated the crystalline structure and electron binding affinity of NCs, respectively. The Ag-CuO NCs demonstrated excellent inhibitory activity against Vibrio cholerae (19.93 ± 0.29 mm) at 100 μg/mL. Anticancer efficacy of Ag-CuO NCs was investigated against the A549 lung cancer cell line, and Ag-CuO NCs exhibited outstanding antiproliferative activity with a low IC₅₀ of 2.8 ± 0.05 μg/mL. Furthermore, staining and comet assays substantiated that the Ag-CuO NCs hindered the progression of the A549 cells and induced apoptosis as a result of cell cycle arrest at the G₀/G₁ phase. Concerning the environmental applications, the Ag-CuO NCs displayed efficient photocatalytic activity against eosin yellow degradation up to 80.94% under sunlight irradiation. Microalgae can be used as an early bio-indicator/prediction of environmental contaminants and toxic substances. The treatment of the Ag-CuO NCs on the growth of marine microalgae Tetraselmis suecica demonstrated the dose and time-dependent growth reduction and variations in the chlorophyll content. Therefore, the efficient multifunctional properties of hybrid Ag-CuO NCs could be exploited as a regime against infective diseases and cancer. Further, the findings of our investigation witness the remarkable scope and potency of Ag-CuO NCs for environmental applications.

Pretreatment of moso bamboo with p-toluenesulfonic acid for the recovery and depolymerization of hemicellulose

Bamboo and its mechanical processing residues have broad prospects for high value-added utilization. In this research, p-toluenesulfonic acid was used for the pretreatment of bamboo to investigate the effects of extraction and depolymerization of hemicellulose. The response and behavior of changes of cell-wall chemical components were investigated after different solvent concentration, time, and temperature pretreatment. Results indicated that the maximum extraction yield of hemicellulose was 95.16 % with 5 % p-toluenesulfonic acid at 140 °C for 30 min. The depolymerized components of hemicellulose in the filtrate were mainly xylose and xylooligosaccharide, with xylobiose accounting for 30.77 %. The extraction of xylose from the filtrate reached a maximum of 90.16 % with 5 % p-toluenesulfonic acid at 150 °C for 30 min pretreatment. This research provided a potential strategy for the industrial production of xylose and xylooligosaccharide from bamboo and for the future conversion and utilization.

High-performance arabinoglucuronoxylan-based biosurfactants for oily sludge separation

Arabinoglucuronoxylan (AGX), an important carbohydrate polymer in lignocellulosic biomass, contains many functional groups. It has excellent amphiphilic modification properties and has potential application in the separation of oily sludge. In this study, a simple strategy for producing high-performance AGX-based biosurfactants was studied and the raw AGX was extracted from bamboo by hydrothermal treatment. AGX-based biosurfactants with amphiphilic structure were produced by AGX and dodecanal succinic anhydride (DDSA) using rapid homogeneous esterification reactions in deep eutectic solvents (DES). This resulted in a significant reduction in the surface tension of the water from 72.32 to 29.76 mN·m^-1. These parameters are similar to those achieved using sodium dodecyl sulfate (SDS), a widely employed synthetic surfactant. Other physicochemical properties, including foamability, emulsification activity, stability, solubilization, and detergency were significantly improved compared to the unmodified AGX, demonstrate that AGX-based biosurfactants are promising detergents for oily sludge remediation and oil recovery. The results provide a new pathway for high value utilization of arabinoglucuronoxylan.

Lignin Degradation via Chlorine Dioxide at Room Temperature: Chemical Groups and Structural Characterization

Lignin degradation is an effective means of achieving the high-value application of lignin, but degradation usually requires the use of high temperatures and harsh reaction-conditions. This study describes a green, mild approach for the degradation of lignin, in which chlorine dioxide (ClO₂) was used for the oxidative degradation of lignin (IL) in an acidic aqueous suspension at room temperature. The optimal process conditions were: 30 mL of ClO₂ solution (2.5 mg·L^-1), pH 4.5 and 3 h. The FT-IR, NMR (¹H NMR, 2D-HSQC and ³¹P NMR), XPS and GPC analyses indicated that lignin could be degraded by ClO₂ relatively well at room temperature, to form quinones and muconic acids. Additionally, DIL was reduced to substances with a high phenolic-hydroxyl (OH) content (RDIL) under the presence of NaBH₄, which further confirmed the composition of DIL and which can be applied to the development of lignin-based phenolic resins, providing a reference for the further modification as well as the utilization of DIL.

Effects of the Preferential Oxidation of Phenolic Lignin Using Chlorine Dioxide on Pulp Bleaching Efficiency

Chlorine dioxide is widely used for pulp bleaching because of its high delignification selectivity. However, efficient and clean chlorine dioxide bleaching is limited by the complexity of the lignin structure. Herein, the oxidation reactions of phenolic (vanillyl alcohol) and non-phenolic (veratryl alcohol) lignin model species were modulated using chlorine dioxide. The effects of chlorine dioxide concentration, reaction temperature, and reaction time on the consumption rate of the model species were also investigated. The optimal consumption rate for the phenolic species was obtained at a chlorine dioxide concentration of 30 mmol·L-1, a reaction temperature of 40 °C, and a reaction time of 10 min, resulting in the consumption of 96.3% of vanillyl alcohol. Its consumption remained essentially unchanged compared with that of traditional chlorine dioxide oxidation. However, the consumption rate of veratryl alcohol was significantly reduced from 78.0% to 17.3%. Additionally, the production of chlorobenzene via the chlorine dioxide oxidation of veratryl alcohol was inhibited. The structural changes in lignin before and after different treatments were analyzed. The overall structure of lignin remained stable during the optimization of the chlorine dioxide oxidation treatment. The signal intensities of several phenolic units were reduced. The effects of the selective oxidation of lignin by chlorine dioxide on the pulp properties were analyzed. Pulp viscosity significantly increased owing to the preferential oxidation of phenolic lignin by chlorine dioxide. The pollution load of bleached effluent was considerably reduced at similar pulp brightness levels. This study provides a new approach to chlorine dioxide bleaching. An efficient and clean bleaching process of the pulp was developed.

Enhancement of separation selectivity of hemicellulose from bamboo using freeze-thaw-assisted p-toluenesulfonic acid treatment at low acid concentration and high temperature

The cellulose-rich residual solids are obtained with p-toluenesulfonic acid (p-TsOH) treatment. However, better fractionation of hemicellulose and separation is difficult to obtain during treatment. This study aims at investigating the separation selectivity of bamboo hemicellulose using freeze-thaw-assisted p-TsOH (F/p-TsOH) treatment. The desired separation effect was achieved at freezing temperature -40 °C, freezing time 20 h, p-TsOH concentration 3.0 %, treatment temperature 130 °C and time 80 min. 93.26 % hemicellulose separation was found, which was 32.88 % higher than that of conventional p-TsOH treatment. Furthermore, the separation yield of lignin decreased significantly from 69.29 % to 13.98 %. The distinct lignin characteristic absorption peaks were found, while that of hemicellulose was difficult to observe. The fiber crystallinity index increased from 50.42 to 56.55 %. Furthermore, greater selectivity for hemicellulose separation was achieved. The results provide a new research thinking for efficient fractionation of lignocellulosic biomass by organic acid treatment.

Superior separation of hemicellulose-derived sugars from eucalyptus with tropic acid pretreatment

Organic acid pretreatments can efficiently separate biomass-based hemicellulose and selectively produce hemicellulose-derived sugars. In this study, hemicellulose is separation as xylose, oligosaccharides in the tropic acid-catalyzed hydrothermal pretreatment of eucalyptus. The maximum yield of hemicellulose-derived sugars (85.78 %) with 71.25 % xylose selectivity (based on the total xylose in raw material) was achieved in the hydrolysate under optimal conditions (5 % TA, 160 ℃, 80 min). The yield of hemicellulose-derived sugar and the separation yield of hemicellulose increased by 11.06 % and 11.45 % compared with glycolic acid pretreatment in the similar severity factor. The separation yield of cellulose and lignin was decreased by 4.23 % and 0.98 %, respectively. This resulted in residual solids with higher biological stability (higher fiber crystallinity index, higher thermal stability, and higher lignin content). Therefore, higher hemicellulose separation selectivity and rich hemicellulose-derived sugars were obtained using TA pretreatment. The work would bring up a new method for biomass refining.

Lytic polysaccharide monooxygenase - A new driving force for lignocellulosic biomass degradation

Lytic polysaccharide monooxygenases (LPMOs) can catalyze polysaccharides by oxidative cleavage of glycosidic bonds and have catalytic activity for cellulose, hemicellulose, chitin, starch and pectin, thus playing an important role in the biomass conversion of lignocellulose. The catalytic substrates of LPMOs are different and the specific catalytic mechanism has not been fully elucidated. Although there have been many studies related to LPMOs, few have actually been put into industrial biomass conversion, which poses a challenge for their expression, regulation and application. In this review, the origin, substrate specificity, structural features, and the relationship between structure and function of LPMOs are described. Additionally, the catalytic mechanism and electron donor of LPMOs and their heterologous expression and regulation are discussed. Finally, the synergistic degradation of biomass by LPMOs with other polysaccharide hydrolases is reviewed, and their current problems and future research directions are pointed out.

Deep eutectic solvent with Lewis acid for highly efficient biohydrogen production from corn straw

To boost saccharification and biohydrogen production efficiency from corn straw, Lewis acid enhanced deep eutectic solvent (DES) pretreatment using choline chloride/glycerol was developed. A notable enhancement of the enzymatic hydrolysis efficiency from 26.3 % to 87.0 % was acquired when corn straw was pretreated with aqueous DES at 100 °C for 5 h using 2.0 wt% AlCl₃. A maximum biohydrogen yield of 114.8 mL/g total solids (TS) was achieved in the sequential dark fermentation stage, which was 2.1 times higher than that of the raw feedstock (37.1 mL/g TS). The enhanced efficient conversion was ascribed to the effective removal of lignin and hemicellulose, which led to the bio-accessibility of the straw. This work provides new sights for the rational design of efficient AlCl₃-aided aqueous DES system toward biohydrogen production from lignocellulosic biomass.

High-efficiency separation of hemicellulose from bamboo by one-step freeze-thaw-assisted alkali treatment

The selectivity of alkali treatment (AT) for hemicellulose separation is reduced due to the alkali solubility of lignin. It was improved using freeze-thaw-assisted alkaline treatment (FT/AT). In this study, bamboo hemicellulose was separated via a one-step freeze-thaw-assisted alkali treatment (OFT/AT). The effects of freezing temperature, freezing time, alkali concentration, and treatment time on bamboo components were studied. The separation yield of hemicellulose was 73.26%, compared to 64.00% using conventional FT/AT. The separation of lignin and cellulose was inhibited as alkali concentration decreased from 7.0% to 5.0%. The extraction yield of hemicellulose increased from 46.35% to 56.12%. Structural analysis of extracted hemicellulose revealed the effective inhibition of the breakage of the xylose backbone and arabinose side chain of hemicellulose. This indicated that the molecular structure of extracted hemicellulose was relatively complete. It provides theoretical support for the efficient separation of hemicellulose by AT.

Application and prospect of organic acid pretreatment in lignocellulosic biomass separation: A review

As a clean and efficient method of lignocellulosic biomass separation, organic acid pretreatment has attracted extensive research. Hemicellulose or lignin is selectively isolated and the cellulose structure is preserved. Effective fractionation of lignocellulosic biomass is achieved. The separation characteristics of hemicellulose or lignin by different organic acids were summarized. The organic acids of hemicellulose were separated into hydrogen ionized, autocatalytic and α-hydroxy acids according to the separation mechanism. The separation of lignin depends on the dissolution mechanism and spatial effect of organic acids. In addition, the challenges and prospects of organic acid pretreatment were analyzed. The separation of hemicellulose and enzymatic hydrolysis of cellulose were significantly affected by the polycondensation of lignin, which is effectively inhibited by the addition of green additives such as ketones or alcohols. Lignin separation was improved by developing a deep eutectic solvent treatment based on organic acid pretreatment. This work provides support for efficient cleaning of carbohydrate polymers and lignin to promote global carbon neutrality.

Kinetics of Lignin Separation during the Atmospheric Fractionation of Bagasse with p-Toluenesulfonic Acid

As a green and efficient component separation technology, organic acid pretreatment has been widely studied in biomass refining. In particular, the efficient separation of lignin by p-toluenesulfonic acid (p-TsOH) pretreatment has been achieved. In this study, the mechanism of the atmospheric separation of bagasse lignin with p-TsOH was investigated. The separation kinetics of lignin was analyzed. A non-simple linear relationship was found between the separation yield of lignin and the concentration of p-TsOH, the temperature and the stirring speed. The shrinking nucleus model for the separation of lignin was established based on the introduction of mass transfer and diffusion factors. A general model of the total delignification rate was obtained. The results showed that the process of lignin separation occurred into two phases, i.e., a fast stage and a slow stage. The results provide a theoretical basis for the efficient separation of lignin by p-TsOH pretreatment.