Oxygen-delignified Kraft Pulp Research Articles

Reevaluation of the manganese requirement for the biobleaching of kraft pulp by white rot fungi

Manganese dependent peroxidase (MnP) is the main enzyme implicated in the biobleaching of kraft pulps by white rot fungi. The goal of this study was to evaluate the Mn requirement for biobleaching of eucalyptus oxygen delignified kraft pulp (OKP) by various white rot fungi: Trametes versicolor, Phanerochaete sordida, Phlebia radiata, Stereum hirsutum and Bjerkandera sp. strain BOS55. All of the strains tested produced MnP and provided extensive bleaching of OKP when 33 μM Mn was included in the medium. Bjerkandera sp. strain BOS55 was the only strain that also displayed MnP production and biobleaching activity of EDTA-extracted OKP in the complete absence of Mn. However, MnP and biobleaching activity in the absence of Mn was dependent on the presence of organic acids in the medium. The fact the biobleaching was correlated to MnP activity irrespective of whether Mn was present or absent suggests that there may be roles for MnP in Bjerkandera under Mn-deficient conditions. Although manganese-independent peroxidase (MIP) and lignin peroxidase (LiP) were also detected, the titres were much smaller in comparison with those of MnP, so their relative role in biobleaching can be predicted to have a minor importance in comparison with MnP. Only in the case of Bjerkandera, was the expression of LiP stimulated in the presence of oxalate but final brightness was not substantially affected.

A spectroscopic method for determining lignin content of softwood and hardwood kraft pulps

A rapid method for determining the kappa number of unbleached and oxygen-delignified kraft pulps in the range 3–35 is presented. This novel method was based on the multivariate analysis of VIS spectral data on pulp samples. The calculated models and the test results indicated that partial least squares (PLS) and principal component regression (PCR) models yielded similar results, PLS being slightly more accurate. It was also found that for practical purposes a separate model for each wood feedstock and delignification process is needed.

Role of Organic Acids in the Manganese-Independent Biobleaching System of Bjerkandera sp. Strain BOS55

Bjerkandera sp. strain BOS55 is a white rot fungus that can bleach EDTA-extracted eucalyptus oxygen-delignified kraft pulp (OKP) without any requirement for manganese. Under manganese-free conditions, additions of simple physiological organic acids (e.g., glycolate, glyoxylate, oxalate, and others) at 1 to 5 mM stimulated brightness gains and pulp delignification two- to threefold compared to results for control cultures not receiving acids. The role of the organic acids in improving the manganese-independent biobleaching was shown not to be due to pH-buffering effects. Instead, the stimulation was attributed to enhanced production of manganese peroxidase (MnP) and lignin peroxidase (LiP) as well as increased physiological concentrations of veratryl alcohol and oxalate. These factors contributed to greatly improved production of superoxide anion radicals, which may have accounted for the more extensive biobleaching. Optimum biobleaching corresponded most to the production of MnP. These results suggest that MnP from Bjerkandera is purposefully produced in the absence of manganese and can possibly function independently of manganese in OKP delignification. LiP probably also contributed to OKP delignification when it was present.

Biobleaching of oxygen delignified kraft pulp by several white rot fungal strains

Twenty-five white rot fungal strains were tested for their ability to bleach Eucalyptus globulus oxygen delignified kraft pulp (OKP). Under nitrogen-limited culture conditions, eight outstanding biobleaching strains were identified that increased the brightness of OKP by more than 10 ISO units compared to pulp incubated in sterile control medium. The highest brightness gain of approximately 13 ISO units was obtained with Bjerkandera sp. strain BOS55, providing a high final brightness of 82% ISO. This strain also caused the greatest level of delignification, decreasing the kappa number of OKP by 29%. When the white rot fungal strains were tested in nitrogen-sufficient medium, the extracellular activities of laccase and peroxidases increased in many strains; nonetheless, the pulp handsheets were either destroyed or brightness gains were lower than those obtained under nitrogen-limitation. The titer of ligninolytic enzymes was not found to be indicative of biobleaching potential. However, the best biobleaching strains were generally characterized by a predominance of manganese dependent peroxidase (MnP) activity compared to other ligninolytic enzymes and by a high decolorizing activity towards the polyanthraquinone ligninolytic indicator dye, Poly R-478.

Xylanase treatment for the peroxide bleaching of oxygen delignified kraft pulps derived from three softwood species

Three commercial enzymes were used to evaluate the use of xylanase treatment for enhancing the peroxide bleaching of three oxygen delignified kraft pulps derived from the softwoods red cedar, Douglas fir, and western hemlock. Two bleaching sequences were examined, where the enzyme stage was placed before or between the two peroxide stages. The brightness gain achieved was dependent on the enzyme preparation, the origin of the pulp and the position of the enzyme stage in the bleaching sequence. In general, xylanase was found to enhance the pulp brightness achieved with two peroxide stages by 1–1.5% ISO. It could directly brighten oxygen delignified pulps before and after the first peroxide stage. Direct brightening of the oxygen delignified pulps, before peroxide bleaching, appeared to be correlated with the amount of UV-absorbing material solubilized during the enzyme stage.

Bleaching of radiata pine Kraft pulp with the EnZone process

Oxygen delignified Kraft pulps from radiata pine were treated with xylanase in the presence of a quelating agent prior to their chemical bleaching with the sequences WZE OPP, E PWZP and E OPWZP. Enzymes tested included Cartazyme from Sandoz, Ecopulp from Alko-ICI, Irgazyme 40 from Ciba-Genencor and Pulpzyme HB from Novo Nordisk. A positive effect of these xylanases was observed with the sequence Xo-E PWZP, which yielded pulps with a final brightness of 85–86% ISO. Application of the sequence Xo-E OPWZP led to pulps with brightness within the target value of 90% ISO, although enzyme effect in this case was less evident. In turn, enzymes were ineffective in the sequence XQ-WZE OPP. These results emphasize the need to design a proper chemical sequence to optimize biobleaching. In most experiments, beatability of enzyme-treated pulps was slightly affected, requiring up to 15% more revolutions in the PFI mill than the controls to reach the same tensile index.

Formation of Chlorinated Phenols during Bleaching of Eucalypt Kraft Pulps

Oxygen-delignified kraft pulps from mature and young eucalypt woods were treated with chlorine (C) or chlorine dioxide (D) and subsequently with alkali reinforced with oxygen, (EO) C or (EO) D . The bleaching filtrates were examined for chlorinated phenols by the in situ acetylation-gas chromatography method. A total of 43 chlorinated phenols were identified in the C-stage and/or the (EO) C -stage filtrates, whereas only four were found in the corresponding D-stage filtrates. About half of the identified compounds were chlorinated benzaldehydes and acetophenones, and 18 chlorinated phenols have not been positively identified previously. The two chloro-3,4,5-trimethoxybenzaldehydes and 2,4,6-trichlororesorcinol identified in the filtrates probably derive from extractives originally present in the wood. There was no indication of differences in the nature of the chlorinated phenols obtained from the mature and young eucalypt samples

Characteristics of the Chlorinated Organic Substances in Filtrates from Bleaching of Oxygen-Delignified Eucalypt Kraft Pulp

Oxygen-delignified kraft pulps from mature and young eucalypt woods were bleached with sequences involving chlorine (C), chlorine dioxide (D) and alkali-oxygen (EO). The bleaching filtrates were analysed for chlorinated organic compounds by gas chromatography with the use of a mass selective detector. Chlorinated compounds found included a range of chlorinated phenols, neutral compounds, aliphatic acids and phenolic acids. A total of 41 chlorinated phenols were identified, mostly in the filtrates following chlorination {(EO)[C]} and only few in the (EO)[D] filtrates. 2-Chlorosyringaldehyde was the only significant chlorinated phenol obtained after D-prebleaching. There was no indication of differences in the types of chlorinated phenols obtained from the mature and young eucalypt samples, although there were higher levels of phenols in filtrates from the younger wood sample. The dominant chlorinated neutral compounds found in the filtrates were chloroform, chloroacetones and chlorodimethylsulfones. Most chloroform and chloroacetones were found in the C-stage filtrates, while chlorodimethylsulfones were found in both C and D-stage filtrates to the same extent. An abundant compound with probable molecular formula C5H3ClO3 was also in the neutral fraction. The chloroacetic acids were the only aliphatic acids detected and were found in all filtrates examined. A method for the analysis of chlorinated phenolic acids involving sequential acetylation and methylation of the extracts was devised. Three chlorinated phenolic acids were identified in the filtrates as their methyl ester acetates. The total amount of chlorinated compounds analysed was consistently higher in the young eucalypt sample than in the mature sample, and correlated well with the generic parameters AOX and EOX. High molar mass (HMM) solids prepared from the C(EO) bleaching filtrates by ultrafiltration had higher chlorine contents than those from the D(EO) bleaching filtrates. Apparent molecular mass distributions of the HMM solids determined by high performance size exclusion chromatography were dependent on the method of detection.

On the selectivity of ozone delignification of softwood kraft pulps

Ozonation of kraft pulp and oxygen-delignified kraft pulp in acetic acid/water mixtures has been studied at different pulp consistencies with and without an inhibitor to obtain information about the limitations of the selectivity of the ozone treatment. The experiments have been performed according to a full factorial design and data have been analysed by stepwise regression analysis. The results show that a better delignification selectivity can be achieved when ozone is applied in acetic acid media instead of conventional aqueous media. By combining conventional oxygen delignification and ozonation in acetic acid, pulps with equal strength properties and better optical properties compared to those of conventional O(DC)E prebleached pulps can be obtained. This study has also revealed that the reagent is utilized far more efficiently in ozonation at high pulp consistency (35%) than in ozonation at low consistency (1%), and that a similar selectivity can be achieved in both types of ozonation by choosing appropriate conditions. The greater selectivity of ozonation in acetic acid than in water is interpreted in terms of different stabilities of the oxidant in these reaction media. The relation between the formation of hydroxyl radicals and the extent and selectivity of delignification during ozonation has been investigated. The final bleaching of pulps prebleached with ozone in acetic acid medium has been studied. Two final bleaching sequences are presented, by which pulps with ISO brightness levels of 79% and 90% can be produced.

The Effects of Chlorination Conditions on the AOX and Chlorinated Phenol Content of Kraft Bleach Plant Wastewaters

A central composite experimental design was used to develop models of the yields of Adsorbable Organic Halide (AOX) and chlorophenolic compounds in wastewaters produced by bleaching oxygen delignified kraft pulps. The following five variables were used to control the bleaching conditions: total available chlorine, chlorine dioxide substitution, incoming kappa number, chlorination time and temperature. The total available chlorine and the chlorine dioxide substitution were the most important parameters in all the models developed. The yields (g.tonne−1) of AOX and total chlorophenolic compounds increased linearly with increasing total chlorine application. Elevating the level of chlorine dioxide substitution caused the yield of AOX to decrease linearly while the yield of total chlorophenols diminished in a non-linear manner. Increasing the level of chlorine dioxide substitution at a given total chlorine application is an effective means of reducing the emissions of both AOX and chlorinated phenols.