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FAIR-CT95-0560
Wood Extracts in Pulp and Paper Manufacture, Technical and Environmental Implications and Biological Removal |
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Contract No: | FAIR-CT95-0560 |
| Date Prepared: | June 1999 | |
| Source: | Final Report Abstract |
Objectives
The project considered the various problems (pitch deposition and effluent toxicity) caused by wood extractives in the manufacturing of paper pulps from several types of wood, as well as the potential of biotechnology to solve or control some of these problems. With this objective two different paper manufacturing processes (softwood thermo-mechanical and eucalypt Kraft pulping) were investigated. The different studies can be outlined as follows:
Activities
Gas chromatography (GC) was used as a reference methodology to identify and quantify wood extractives. However, some adaptation of chromatographic conditions were necessary. These included the use of short-length high-temperature capillary-columns with thin films in GC (with flame-ionisation detection) to analyse different wood extractives including high molecular weight lipids, in a single chromatographic run. When a precise identification of individual compounds was required. GC coupled to mass spectrometry (MS) with medium-length capillary column was used with or without prior fractionation of the acetone extract by solid-phase extraction (SPE) using aminopropyl-phase columns. The SPE method was adapted to fractionate both eucalypt and pine extracts, and used for preliminary evaluation of the toxicity of pine fractions . However, for a precise identification of those compounds responsible for the acute toxicity found in softwood extractives, high-performance liquid chromatography (HPLC) analysis of pine extractives was optimised and individual peaks collected and used for Microtox tests. Finally, different problems inherent to evaluation of toxicity of wood extractives forming unstable colloidal suspensions were considered.
The chemical composition of pine extractives was well-documented when the project was initiated Confirmatory results were obtained by both GC-MS and HPLC using evaporative light-scattering detection. In addition, toxicity evaluations were carried out with different softwoods, the acute toxicity of which were much higher than that of eucalypt extractives. The extractive content of eucalypt wood (up to 1.5%) was lower than found in pine and includes around 20% lipophilic compounds. The analysis of pitch deposits from eucalypt pulps and mill equipment indicated their predominantly-lipophilic nature including some non-extractable compounds which were analysed by pyrolysis. The chemical characterisation of eucalypt wood focused on this type of compounds.
Results
Contrarily to the results reported for other hardwoods, a large predominance of free and esterified sitosterol was found in Eucalyptus globulus extractives (each amounting up to 30-35% of total chrornatographed lipophilic compounds), together with free and bound stigmastanol, fucosterol and cycloartenol, steroid ketones and hydrocarbons, free fatty-acids, triglycerides, waxes, tocopherols, etc. The chemical evolution of these compounds during eucalypt Kraft cooling and bleaching was investigated in depth. The results revealed that some of them, including the major free and esterified sitosterol, survived cooking and totally-chlorine free (TCF) bleaching and were major components of pitch deposits and pulp extracts.
Different physico-chemical treatments for pitch control were evaluated in mill or laboratory-scale experiments. These included pitch deposition control by two opposite strategies based on chemical additives: surfactants increasing colloidal pitch stability, and commercial products fixing pitch on fiber surface. Although some of the results obtained have been satisfactory it is not economically viable to use these products to increase pulp cleanliness.
The effect of chip pre-steaming and seasoning was also investigated. The former decreased extractive content and could be of interest as a pre-treatment before fungal treatments. Chip seasoning also produced a favourable effect on extractive content (sitosterol degradation being observed) and colloidal pitch level, but it appeared as less efficient than biological depitching with selected fungal strains.
Since triglycerides were known as extractive compounds causing pitch problems during mechanical pulping, the first enzymes investigated for pitch control in the present project were lipases. A large screening including more than 160 strains from culture collections or isolated from woody materials was carried out in agar-plates containing Tween-80. Some of the best strains degrading eucalypt wood extractives were also incorporated into the studies on enzyme production for pulp treatment. Up to ten fungi were then selected for physiological studies on lipolytic enzyme production (including effect of pH, calcium concentration, carbon and nitrogen sources, etc). Finally, Ophiostoma piceae and Fusarium oxysporum were used for scaling-up the production of liopolytic enzymes. The latter species appeared as a strong lipase producer, whereas Ophiostoma piceae produced both lipases and sterol esterases , which were isolated and characterised. The ability to hydrolyse sterol esters is of the greatest interest for the enzymatic control of pitch in eucalypt pulps.
Although degradation of sitosterol and resin acids by fungal cultures was observed, no information on the enzymes involved is currently available. The two fungi mentioned above were grown in 2, 20 and 72-litre fermenters, the enzymes obtained from the latter being partially purified and used to treat eucalypt and pine pulps. The most reproducible result was a significant reduction of resin acid content in crude pine pulp after enzymatic treatment.
The studies on pine and eucalypt extractives biocontrol were based on a large number of strains which were used to treat wood as well as a source of crude enzymes to treat pulps. The screening of fungi for removal of eucalypt extractives under solid-state fermentation (SSF) conditions was carried out on a total of 73 strains belonging to 33 species of basidiomycetes, 18 of ascomycetes and 16 of deuteromycetes. Many of these were isolated and identified from fruiting bodies growing on eucalypt wood in forest or in chip piles (at pulp mills), and included some species typically associated to eucalypt trees (one of them reported for the first time in Spain). In the case of pine wood, more than 100 strains were screened, also corresponding to different taxonomic groups. Several strains of Ophiostoma piliferum commercialised by (as Cartapip™) for pitch control were included in screening studies.
The composition of the lipophilic extractives of wood treated with the different fungal species was analysed by HPLC and/or GC-MS. In the case of eucalypt wood, some of the fungi degraded efficiently all the lipophilic compounds responsible for pitch deposition. However, other species including the commercial Ophiostoma piliferum reduced the sterol ester content but increased the free sitosterol (probably due to sterol esterase activity).
Microscopic studies on wood samples treated with fungi revealed degradation of spherical deposits in rays containing wood extractives.
Four fungal strains (Bjerkandera adusta, Poria subvermispora, Phlebia radiata and Pleurotus pulmonarius) decreasing sterols and sterol esters, causing a harmful loss of wood weight and showing good capacity for wood colonisation were selected for laboratory Kraft cooking and TCF bleaching experiments.
After the fungal treatments of industrial chips, a significant decrease in sterol esters and/or free sterols was found in eucalypt pulp (40-71%), black liquor (44-76%) and wood (32-77%) with respect to the corresponding untreated controls. These figures were lower than those obtained during fungal treatment of smaller wood chips but represent a substantial reduction of problem extractives for eucalypt pulp manufacture.
Moreover, it was found that:
In the case of pine wood, special attention was dedicated to extractive detoxification in addition to degradation of problematic extractive fraction. Based on the results, the white-rot fungi Trametes versicolor and Bjerkandera sp. were selected for pine and spruce wood depitching and detoxification during manufacture of themomechanical pulp (TMP). The latter appeared as a versatile organism for extractive degradation in different types of woods including pine, beech, spruce, poplar and birch (although degradation rates were lower in the three latter cases). Both fungi degraded triglycerides but, in contrast with previous experiments with smaller chips, the removal of resin acids was very limited when industrials chips were used. Furthermore, the fungal treatment being highly effective in the removal of triglycerides from wood was also found to lead to biomechanical pulps nearly free from these troublesome compounds.
In general the fungal treatment did not produce a significant deterioration of the mechanical properties of pulps, both tear and tensile indices being increased after Trametes versicolor treatment. Finally, although a decrease of pulp brightness was observed after fungal treatment, the bleachability of the biomechanical pulps was excellent and the final brightness levels were similar to those of control pulps. In conclusion, some of the fungi presented good potential for pitch control during Kraft and TW pulping. The selection of most promising fungal strains from an industrial point of view was a compromise between the optimum in terms of extractive removal and the lowest detrimental effect on pulp properties.
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