Diffusion of lignin macromolecules within the fibre walls of kraft pulp. Part I: Determination of the diffusion coefficient under alkaline conditions

The intrinsic rate of diffusion of soluble lignin from fibre walls to bulk liquor has never been determined previously because of experimental difficulties; for example, the diffusion rate determined in a stirred cell is affected by the mechanical action of stirring. In our work, the intrinsic rate of diffusion of lignin macromolecules from the fibre walls of a softwood kraft pulp was determined under alkaline conditions using a displacement cell which eliminated external heat and mass transfer resistances and pulp fibre disturbances. The effects of such experimental conditions as pulp bed height and liquid flow rate were studied. The diffusion rate can be described by a diffusion model for a hollow cylinder with a very wide range of diffusion coefficients. The diffusion rate increased with increasing pH. Our results provide a new understanding of the lignin diffusion process in fibre walls, which is affected by the size of lignin molecules and the pores, and by the electrostatic interactions between intrafibre pore walls and lignin.     

木素接枝改性物及其应用研究进展

本文简要介绍了木素接技改性的研究概况,重点阐述了这种兼具天然高分子和合成高分子二者优点的改性木素产品的应用情况,其广泛用于絮凝剂、分散剂、土质稳定剂、驱油剂、表面活性剂和钻井液处理剂等,均取得良好的效果.     

Molar mass-dependent profiles of functional groups and carbohydrates in kraft lignin

Technical lignins are complex, irregular, polyphenolic compounds obtained in large quantities as by-products of the pulp and paper industries or according to current biorefinery setups. The availability of kraft lignin is increasing due to larger scale retrieval from process liquors, which opens new possibilities for further refining or new applications of such lignins. In the present study, sequential ultrafiltration of kraft lignin was performed to fractionate the lignin and to elucidate molar mass-dependent changes in lignin structure. Two industrial black liquors and three precipitated lignins were fractionated, and their functional groups were determined, providing molar mass-dependent profiles. Interrelations between structural parameters and functional groups, the molecular weight ranges, and the different lignin sources are discussed. This will help to establish structure-property-application relationships (SPARs) for technical lignins which are required for any future large-scale application.     

Fractional and structural characterization of ball-milled and enzyme lignins from wheat straw

Ball-milled and enzyme lignins were extracted with 90 and 50% dioxane–water from 6 days ball-milled wheat straw, and subsequently cellulase-treated straw residues, respectively. The crude lignin preparations were purified using a two-step precipitation method instead of the traditional ether precipitation procedure, and fractionated into pure milled lignin (PML), pure enzyme lignin (PEL), hemicellulose-rich milled lignin (HRML), lignin-rich enzyme lignin (LREL), and solubilized lignin during enzyme treatment (SLET) fractions. The five lignin fraction were studied using spectroscopic and degradative tecyhniques. The PML and PEL fractions showed very low content of associated polysaccharides (2.36–2.86%). The PML is mainly composed of β-O-4 ether bonds in the lignin structural units. The less common β-5 and β-β carbon–carbon linkages are also present. The results obtained also indicated that the lignins in wheat straw cell walls appeared to be very closely associated to p-coumaric and ferulic acid, and glucuronic acid or 4-O-methylglucuronic acid. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1633–1641, 1998     

Transformations of (14C-lignin) cell walls of wheat by rumen microorganisms

The lower halves of apical internodes of wheat harvested at the flowering stage were labelled with [U-¹⁴C] phenylalanine (phe) or with [O¹⁴CH₃] sinapic acid (sin). Cell wall residues (CWR) and saponified residues (SR) were incubated in a fermenter simulating the rumen for 7 days with rumen fluid or without microorganisms (controls). PheCWR was labelled in all lignin units (measured as aldehydes from nitrobenzene oxidation), in phenolic acids and slightly in proteins. Labelling of pheSR was more lignin-specific. SinCWR and sinSR were specifically labelled in syringyl units of lignin. The fermentation of CWR resulted in phenylpropane-derived unit losses in the following decreasing order: ferulic acid>p-coumaric acid>syringaldehyde>vanillin>p-hydroxybenzaldehyde. If allowance is made for slight losses in controls, 61, 52, 61 and 63% of the phenylpropanes of pheCWR, sinCWR, pheSR and sinSR, respectively, were transformed into an acid-precipitable fraction, an acid-soluble fraction and ¹⁴CO_2. The comparison of pheCWR and sinCWR degradation showed that syringyl units were solubilised into acid-precipitable molecules to a greater extent than the other lignin units; demethylation of the syringyl units of lignins was also evident from the different productions of ¹⁴CO₂. Alkali-resistant lignins of SR were mainly transformed into acid-precipitable molecules and were weakly degraded. Lignin solubilisation and degradation seem to be governed by different mechanisms which depend on both cell wall structure and rumen microflora.     

Enzymatic Synthesis of Lignin‐Based Concrete Dispersing Agents

Lignin is the most abundant aromatic biopolymer, functioning as an integral component of woody materials. In its unmodified form it shows limited water solubility and is relatively unreactive, so biotechnological lignin valorisation for high‐performance applications is greatly underexploited. Lignin can be obtained from the pulp and paper industry as a by‐product. To expand its application, a new synthesis route to new dispersing agents for use as concrete additives was developed. The route is based on lignin functionalisation by enzymatic transformation. Screening of lignin‐modifying systems resulted in functionalised lignin polymers with improved solubility in aqueous systems. Through grafting of sulfanilic acid or p‐aminobenzoic acid by fungal laccases, lignin became soluble in water at pH≤4 or pH≤7, respectively. Products were analysed and evaluated in miniaturised application tests in cement paste and mortar. Their dispersing properties match the performance criteria of commercially available lignosulfonates. The study provides examples of new perspectives for the use of lignin.     

Temperature dependence of X-ray diffractograms of amorphous lignins and polystyrenes

Temperature dependencies of X-ray diffractograms of lignins and polystyrene derivatives were investigated. Bulky side groups, such as methoxyl functionalities were found to expand intermolecular distance, while hydroxyl groups reduce its mean value. Intermolecular distance expands discontinuously at the temperature corresponding to the glass transition temperature.     

The Simultaneous Repression of CCR and CAD, Two Enzymes of the Lignin Biosynthetic Pathway, Results in Sterility and Dwarfism in Arabidopsis thaliana

Cinnamoyl CoA reductase(CCR)and cinnamyl alcohol dehydrogenase(CAD)catalyze the last steps of monolignol biosynthesis.In Arabidopsis,one CCR gene(CCR1,At1g15950)and two CAD genes(CAD C At3g19450 and CAD D At4g34230)are involved in this pathway.A triple cad c cad d ccr1 mutant,named ccc,was obtained.This mutant displays a severe dwarf phenotype and male sterility.The lignin content in ccc mature stems is reduced to 50% of the wild-type level.In addition,stem lignin structure is severely affected,as shown by the dramatic enrichment in resistant inter-unit bonds and incorporation into the polymer of monolignol precursors such as coniferaldehyde,sinapaldehyde,and ferulic acid.Male sterility is due to the lack of lignification in the anther endothecium,which causes the failure of anther dehiscence and of pollen release.The ccc hypolignified stems accumulate higher amounts of flavonol glycosides,sinapoyl malate and feruloyl malate,which suggests a redirection of the phenolic pathway.Therefore,the absence of CAD and CCR,key enzymes of the monolignol pathway,has more severe consequences on the phenotype than the individual absence of each of them.Induction of another CCR(CCR2,At1g80820)and another CAD(CAD1,At4g39330)does not compensate the absence of the main CCR and CAD activities.This lack of CCR and CAD activities not only impacts lignification,but also severely affects the development of the plants.These consequences must be carefully considered when trying to reduce the lignin content of plants in order to facilitate the lignocellulose-to-bioethanol conversion process.     

Characterization of Residual Lignins Isolated from Unbleached and Semibleached Softwood Kraft Pulps

Abstract

Residual lignins in an unbleached and a semibleached softwood (Pinus taeda L.) kraft pulp were isolated by enzymatic hydrolysis of polysaccharides in the pulps. After purification, the residual lignins were characterized. A dissolved lignin was also isolated from the alkaline extraction spent liquor by acidification and characterized.

Results of the characterization indicate that extensive degradation of residual lignin in kraft pulp occurred during the first two stages of bleaching. The results also strongly support the previous finding that stable covalent linkages between residual lignins and carbohydrates in pulp any be the most probable cause for the residual lignins to resist delignification during kraft pulping and prebleaching.