Harald Brelid

Adsorption of Arabinoxylan on Cellulosic Surfaces: Influence of Degree of Substitution and Substitution Pattern on Adsorption Characteristics

This study presents results that show that the fine structure of arabinoxylan affects its interaction with cellulosic surfaces, an important understanding when designing and evaluating properties of xylan-cellulose-based materials. Arabinoxylan samples, with well-defined structures, were prepared from a wheat flour arabinoxylan with targeted enzymatic hydrolysis. Turbidity measurements and analyses using NMR diffusometry showed that the solubility and the hydrodynamic properties of arabinoxylan are determined not only by the degree of substitution but also by the substitution pattern. On the basis of results obtained from adsorption experiments on microcrystalline cellulose particles and on cellulosic model surfaces investigated with quartz crystal microbalance with dissipation monitoring, it was also found that arabinoxylan adsorbs irreversibly on cellulosic surfaces and that the adsorption characteristics, as well as the properties of the adsorbed layer, are controlled by the fine structure of the xylan molecule.

CP/MAS C-13 NMR study of pulp hornification using nanocrystalline cellulose as a model system

The hornification process of paper pulp was investigated using solid-state (13)C NMR spectroscopy. Nanocrystalline cellulose was used to serve as a model system of the crystalline parts of the fibrils in pulp fibers. Characterization of the nanocrystalline cellulose dimensions was carried out using scanning electron microscopy. The samples were treated by drying and wetting cycles prior to NMR analysis where the hornification phenomenon was recorded by spectral changes of the cellulose C-4 carbon signals. An increase of the crystalline signal and a decrease of the signals corresponding to the accessible amorphous domains were found for both paper pulp and nanocrystalline cellulose. These spectral changes grew stronger with repeating drying and wetting cycles. The results show that cellulose co-crystallization contribute to hornification. Another conclusion is that the surfaces of higher hydrophobicity in cellulose fibrils have an increased preference for aggregation.

Spatial distribution of metal ions in spruce wood by synchrotron radiation microbeam X-ray fluorescence analysis

Summary The possibility of using synchrotron radiation microbeam X-ray fluorescence (μ -XRF) for the determination of the morphological distribution of inorganic elements in wood has been investigated. A number of samples were analyzed and some of the results are presented in this paper. The new application of the method showed good results and it was concluded that the technique is useful for specific in situ metal ion analysis of wood. One of its special advantages is that it is a non-destructive method, which may allow analysis of the same sample before and after a chemical treatment. This study shows the natural distribution of a selection of metal ions in wood. Some differences in the distribution and amount of certain metals could also be observed in a sample that had been subjected to treatment with an EDTA-solution.

Acid hydrolysis of cellulosic fibres: Comparison of bleached kraft pulp, dissolving pulps and cotton textile cellulose

The behaviour of different cellulosic fibres during acid hydrolysis has been investigated and the levelling-off degree of polymerisation (LODP) has been determined. The study included a bleached kraft pulp (both never-dried and once-dried) and two dissolving pulps (once-dried). Additionally, cotton cellulose from new cotton sheets and sheets discarded after long-time use was studied. Experimental results from the investigation, together with results found in literature, imply that ultrastructural differences between different fibres affect their susceptibility towards acid hydrolysis. Drying of a bleached kraft pulp was found to enhance the rate of acid hydrolysis and also result in a decrease in LODP. This implies that the susceptibility of cellulosic fibres towards acid hydrolysis is affected by drying-induced stresses in the cellulose chains. In cotton cellulose, it was found that use and laundering gave a substantial loss in the degree of polymerisation (DP), but that the LODP was only marginally affected.

Degradation/dissolution of softwood hemicellulose during alkaline cooking at different temperatures and alkali concentrations

The behaviour of hemicellulose was studied during alkaline cooking of softwood meal (Pinus sylvestris). Alkali concentrations and temperatures were chosen to cover a broad range, relevant for hemicellulose dissolution/degradation during soda pulping (0.25-1.25 mol OH/kg liquor and 25168°C). Some cooks were performed after pre-treatment with sodium borohydride. To assure accurate data suitable for kinetic evaluation, the temperature profile was measured and the alkali concentration was kept virtually constant during the cooks. During alkaline cooking, there was extensive degradation/ dissolution of hemicellulose. The behaviour of the major hemicellulose, (galacto)glucomannan and (arabinoglucurono)xylan, differed from each other during delignification. A significant part of the glucomannan rapidly dissolved or degraded. The removal rate of glucomannan was strongly dependent on temperature and even at temperatures below 100°C a substantial part of the glucomannan was removed. No obvious influence of the alkali concentration could be observed. Approximately 25% of glucomannan was more resistant to degradation/dissolution even after prolonged delignification (above 90% of lignin removal). The loss of xylan is dependent on both alkali concentration and temperature. The removal of xylan was practically linearly proportional to the delignification. The amount of the removed carbohydrates due to dissolution in an alkaline solution and to primary peeling was quantified. Approximately 15% of the glucomannan and approximately 5% of the xylan was dissolved in an alkaline solution. Approximately 45% of the native glucomannan was degraded due to primary peeling at a cooking temperature of 168°C. Primary peeling of xylan was of minor importance. Pre-treatment with sodium borohydride reduced the aldehyde end-groups in the carbohydrates and primary peeling was prevented. The yield was increased, mainly due to higher retention of glucomannan. The increased yield of glucomannan may act as a physical barrier as delignification was retarded.

Impact of dissolved sodium salts on kraft cooking reactions.

Abstract This study examined the effects of inactive ions – ions not reacting with wood – during kraft cooking, which thus far have received modest attention. Six different sodium salts were added to kraft cooking liquors at two different levels of alkalinities. Delignification as well as the formation and degradation of hexenuronic acid (HexA) were observed of Scots pine sapwood meal. The delignification rate was greatly affected by the presence of additional anions. Chloride ions had the greatest retarding effect, while the addition of polyacrylate ions had almost no effect. When carbonate, sulphate and lactate ions were added to the liquors, the delignification rates were in-between the series with chloride and polyacrylates ions. We suggest that the anions affect the solubility of lignin fragments in analogy to the Hofmeister effect observed in various macromolecular systems in the presence of dissolved salts. When the reactions involving HexA were examined, the opposite results were obtained. In that case, the highest reaction rates were in the presence of chloride ions, and the lowest rates were obtained when no extra ions were added, and the second lowest rates were obtained in the presence of polyacrylate ions. As for delignification, the cooking series containing carbonate, sulphate and lactate ions had a reaction rate in-between the series with the highest and lowest rates. The differences obtained with various inorganic ions can be qualitatively explained by the Donnan equilibrium theory and by the variation in activity coefficients of hydroxide ions. Findings imply that knowledge of kraft cooking is far from complete. The effects of inactive ions on reaction kinetics should also receive more attention.

Removal of metal ions from wood chips during acidic leaching 1: Comparison between Scandinavian softwood, birch and eucalyptus

Acidic leaching of Scandinavian softwood, birch and eucalyptus has been studied, for the purpose of removing metal ions from the wood. Equilibrium experiments with wood powders were conducted to study the influence of pH. An increased release of metal ions was found, as expected, when the pH was lowered. For one sample of eucalyptus, the release of calcium accelerated when the pH was lowered to below 3, which led to the conclusion that a salt of low solubility could be present. Further investigations showed a high concentration of oxalate in this sample, which is likely to be an explanation for the accelerated release. Industrial cut wood chips, from Scandinavian softwood, birch and eucalyptus, were leached at different temperatures to study their leaching behavior. Birch and softwood were shown to have the same general leaching pattern for calcium and potassium. Eucalyptus showed the same leaching trend for potassium, but it was harder to remove calcium from eucalyptus than from softwood and birch. In the case of eucalyptus chips, which had a relatively high content of chloride (480 mg/kg wood), extensive removal of chloride was noted.

Reaction kinetics of softwood kraft delignification –General considerations and experimental data

For a delignification reaction to occur, cooking chemicals must be transported to the reaction site. After fragmentation reactions in the lignin structure, lignin fragments need to be solubilised in order to diffuse out of the fibre wall and complete the complicated delignification reaction. Despite major research efforts, there are still several aspects of this complex process that are not fully understood. This study is part of a larger investigation aimed at increasing the knowledge of the reaction kinetics of a kraft cook. In the present paper, different aspects of reaction kinetics of delignification and approaches to the modelling of kraft delignification are discussed. In the experimental part, the effect of active chemical concentrations on the reaction kinetics of delignification of Scots pine (Pinus silvestris) was investigated at five temperature levels. Cooks with the addition of an industrial softwood black liquor and increased sodium ion concentration were also performed. To minimise the influence of mass transport on the reaction rates, wood meal was used as a substrate and all cooks were conducted at a high liquor-to-wood ratio (200: 1) to minimise the deviation from initial bulk concentrations. The rate of delignification in the kraft cook increased with increasing hydrosulphide ion and hydroxide ion concentrations. An increase in sodium ion concentration retarded the rate of delignification, while an increase in concentration of dissolved kraft lignin in the cooking liquor increased the delignification rate during the main part of the cook. These results qualitatively resemble many of the results found in the literature. Indications of the influence of covalent bonds between lignin and carbohydrates on the delignification rate are shown and discussed. Data obtained in this investigation cover a broad range of cooking conditions, and are suitable for modelling the reaction kinetics of delignification without the influence of mass transfer resistance.

Removal of metal ions from wood chips during acidic leaching 2: Modeling leaching of calcium ions from softwood chips

The purpose of this work was to model leaching of calcium ions from softwood chips under acidic conditions. The first step was to study the leaching of calcium ions from well-defined sawn samples of spruce wood. Leaching experiments with varying geometries under acidic conditions (pH 2.5 and 60°C) were conducted to determine the diffusion coefficients,giving a diffusion coefficient of 5.9 x 10-10 m2/s in the longitudinal direction and 0.41 x 10-10 m2/s across the grain (in the radial and tangential directions). Results show that leaching of calcium ions from spruce can be described with a model assuming that the wood piece is semi-homogenous, with different diffusion coefficients in different directions. A model describing leaching of industrially cut wood chips, taking into account cracks, rough surfaces and size distribution in the batch of wood chips, was developed using a model of a wood chip with an enlarged surface area. The diffusion coefficient and the chip model were then used to develop models of the leaching process that can be used to simulate and evaluate different leaching designs.

The impact of ionic strength on the molecular weight distribution (MWD) of lignin dissolved during softwood kraft cooking in a flow-through reactor

Abstract The molecular weight distribution (MWD) of dissolved lignin as a function of time during kraft cooking of Scots pine (Pinus silvestris L) has been investigated, while the influence of sodium ion concentration ([Na+]) on the MWD was in focus. The kraft cooking was performed in a small scale flow-through reactor and the [Na+] was controlled by the addition of either Na2CO3 or NaCl. Fractions of black liquors (BL) were collected at different cooking times and the lignin was separated from the BL by acidification. The MWD of the dissolved lignin was analyzed by GPC. Results show that the weight average molecular weight (Mw) of dissolved lignin increases gradually as function of cooking time. An increase of [Na+] in the cooking liquor leads to Mw decrement. Findings from cooks with constant and varying [Na+] imply that the retarding effect of an increased [Na+] on delignification is related to the decrease in lignin solubility at higher [Na+].