Ozone dioxane delignification from the cell walls of Japanese cypress (Chamaecyparis obtusa Endl.)

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1 J Mater Cycles Waste Manag (2006) 8: Springer 2006 DOI /s ORIGINAL ARTICLE Shinso Yokota Kazuya Iizuka Futoshi Ishiguri Zensaku Abe Nobuo Yoshizawa Ozone dioxane delignification from the cell walls of Japanese cypress (Chamaecyparis obtusa Endl.) Received: July 27, 2005 / Accepted: March 22, 2006 Abstract Delignification from the cell walls with a combination of ozone oxidation and dioxane water extraction using thin sections of a softwood, Japanese cypress (Chamaecyparis obtusa Endl.), was studied to determine its suitability for the production of recyclable cellulose-based materials from wood waste. The visible-light absorption spectra of treated wood sections revealed that delignification from the cell walls with ozone increased with increasing ozonization time. Ozone delignification proceeded from the lumen side toward the middle lamella within the secondary wall of a cell, and it proceeded faster in early wood than in late wood within an annual ring. Mild ozonization for 10 30min was sufficient for the removal of lignin from the cell walls when sections were extracted with dioxane after ozonization. The results obtained here demonstrate that microspectrometry coupled with the Wiesner reaction is useful for the quantitative analysis of lignin in cell walls. Key words Ozonization Dioxane Lignin Cell wall Microspectrophotometry Introduction In recent years, the treatment of very large quantities of industrial wood and agricultural waste, such as that from mushroom culture, has become a serious problem in Japan. This waste from wood is now being applied to biomass utilization in many countries. However, using wood as a fuel, for example, would not help reduce atmospheric carbon dioxide levels, even if large quantities of energy could be obtained from biomass. Better and more environmentally friendly applications of wood waste would be to use it as a recyclable biomass resource. S. Yokota K. Iizuka F. Ishiguri Z. Abe N. Yoshizawa (*) Forest Products, Department of Forest Science, Faculty of Agriculture, Utsunomiya University, Utsunomiya, Tochigi , Japan Tel ; Fax nobuoy@cc.utsunomiya-u.ac.jp The present study, therefore, focused on using wood waste such as wood meal and planer scraps with the objective of producing base materials without lignin for saccharification with enzymes and subsequent bio-ethanol production and for developing agricultural materials. Ozone has the potential to delignify and brighten wood without producing harmful compounds. In addition, ozone is attracting interest for its potential as a replacement for chlorine and its derivatives in pulp and paper manufacturing because it minimizes the formation of organic halogenated compounds. 1 Other reports also revealed that ozone oxidation can be used to enhance the susceptibility of wood to enzymatic hydrolysis. 2 4 Ozone strongly oxidizes lignin and carbohydrates, resulting in decreased fiber strength from the decreasing cell wall thickness caused by excessive ozonization. 1,5 Microscopic observations revealed that the permeability of ozone into cell walls is different among wood fibers in morphologically different regions within a growth ring. 6 However, it is difficult to visually estimate the characteristics of residual lignin in cell walls, and the process of delignification with the progress of ozonization has not yet been investigated. Dioxane is well known as a good solvent for lignin, and it can be easily recovered from the reaction mixtures. Dioxane delignification, therefore, has been extensively applied to pulping, bleaching of pulp, and the isolation of the lignin carbohydrate complex. 7 9 In this study, we observed the progress of lignin removal from the cell walls in wood using a coupled method of ozone dioxane treatment and scanning electron microscopy (SEM). The degree of delignification from the cell walls was examined by visible-light (VL) microspectrometry coupled with the Wiesner reaction. Materials and methods Samples Transverse sections of 50 µm in thickness were sliced from small wood blocks ( cm) of Japanese cypress

2 141 (Chamaecyparis obtusa Endl.). These sections were immersed in distilled water and kept until being used in the experiment. Delignification with ozone dioxane Ozonization of sections was performed in a 100-ml reaction flask fitted to a rotary evaporator. Both faces of each section were clipped to a 40-mesh Teflon sheet (NRK, PFK-500; Tokyo, Japan) to prevent the sections from being damaged. One hundred samples were allowed to react for min with an ozone oxygen mixture (flow rate, 100 l/h) containing 3% ozone by weight at room temperature. Ozone was produced by passing pure oxygen through an ozonizer (BH-2; Chiyoda Seisakusho, Tokyo, Japan) at a flow rate of 250 ml/min, and its concentration was determined by the conventional iodometric titration method. After ozonization, the sections were extracted with 50ml of dioxane water solution (9/1, v/v) in a petri dish for min at room temperature and then subjected to SEM observation. Microspectrometry Residual lignin in the cell walls of sections treated with ozone was observed with an ordinary light microscope coupled with a spectrometer (UMSP 80; Carl Zeiss, Hamburg, Germany) after the Wiesner reaction. The Wiesner reaction was performed as follows: a few drops of 1% phloroglucinol ethanol solution were poured onto a section mounted on a glass slide, one drop of 3.5% HCl was added to the section, and the section was covered with a cover slip. Using these sections, the VL absorption spectra from nm were measured in 1-nm steps with a spectrometer (spot diameter, 0.5 nm; bandwidth, 5 nm) in the center of the area delignified with ozone. Measurements were repeated ten times at every step for the measured point using a Lambda scan program (Carl Zeiss). All measurements were performed within 10 min because the color gradually fades after the color reaction. The ten VL absorption spectra obtained from each measurement were averaged to give the mean spectrum. SEM observation After ozone treatment, dioxane treatment, or both, sections were dried by critical-point drying. They were mounted on specimen stubs by bond gum and coated with gold for 7min using an ion-sputter apparatus. The extent of delignification from the cell walls was observed by an SEM (JSM-5200; JEOL, Tokyo, Japan). Results and discussion Transverse sections treated with ozone for varying times were observed under an ordinary light microscope after applying the Wiesner color reaction to visually estimate the distribution of residual lignin in the cell walls. Transverse sections ozonized for 5 30min are shown in Fig. 1, which shows the progress of delignification with ozone. The extent of the deep-red color in the tissues of the areas exposed to ozone gradually decreased with increasing reaction time. The delignification proceeded in a concentric circle from the lumen toward the middle lamella within a secondary wall (Fig. 1a,b). Almost complete delignification from the secondary walls occurred after ozonization for 30 min in early wood. The ozone delignification in the secondary walls of tracheids proceeded faster in the early wood than in the late wood (Fig. 1b,c). In the middle lamella of cell corners, the disappearance of lignin-derived color was observed after ozonization for 30 min (Fig. 1c). Almost complete lignin removal from both the secondary walls and middle Fig. 1. Transverse sections stained with Wiesner reagent after a 5-min ozonization, b 10- min ozonization, and c 30-min ozonization. The lignin color reaction decreased from the lumen side of a cell toward the middle lamella; the lignin color reaction almost disappeared in the cell corners. Bars 50µm. Note: Deep color shows the presence of lignin in these photographs

3 142 lamella was found with ozonization for more than 30 min (data not shown). The progression of delignification was apparent as the duration of contact with ozone gas increased. A transverse section of a fiber from chemithermo mechanical pulp min after ozonization revealed that lignin removal with ozone first occurred at the outermost layer and extended inward with an increase in the ozone penetration into the cell wall. 10 In the present experiment, the secondary walls were ozonized much more rapidly than the middle lamella of the cell corners. It is clear that the progress of ozone delignification was topochemically heterogeneous in the morphologically different regions of tissues. This might result from differences in the content of lignin originally existing in the tissues. There might also be a difference in the longitudinal and horizontal permeability of ozone through the cell wall. VL absorption spectra after the Wiesner reaction are shown in Fig. 2. The spectra in the intact tissues exhibited an absorption maximum at around 570 nm, indicating the presence of lignin. All absorbancies at 570nm in the secondary walls of tracheids decreased with increasing ozonization time. After ozonization for 10 min, a great decrease in absorbance was found in the inner part of the secondary walls, whereas the outer part showed only a slight decrease, this fact being in agreement with the microscopic observation shown in Fig. 1b. In the middle lamella, the complete disappearance of the absorption maximum occurred after ozonization for more than 30 min. Yoshizawa et al. 6 also reported that, in the middle lamella, a relatively Fig. 2. Visible-light absorption spectra of the secondary walls in tracheids stained with Wiesner reagent after ozonization. Numerals indicate the ozonization time slow progress of delignification with ozone was observed at first, in comparison with delignification in the secondary walls of tracheids; this tendency increased with the increase in the thickness of the section. In all spectra obtained in this experiment, it is noteworthy that maximum absorption was consistently found at around 570 nm, irrespective of the apparent decrease in absorbance. The Wiesner reagent reacts with coniferyl aldehyde and synapyl aldehyde units in lignins and gives a bright red color. 11,12 It has also been reported that the Wiesner reaction becomes progressively stronger with the increase in the Klason-lignin content. 12 These findings suggest that the data obtained with microspectrometry coupled with the Wiesner reaction indicate a decrease in the lignin content with an increase in the ozonization time. This fact was confirmed by Yoshizawa et al., 6 who demonstrated that UV absorbance at 279 nm, indicating the presence of guaiacyl lignin, decreased with an increase in the ozonization time. It is likely that the degradation of guaiacyl aromatic rings progresses with ozone oxidation. 10 In these experiments, the process of delignification of the cell walls by ozone oxidation was qualitatively observed. However, it will be necessary to confirm whether lignin degradation actually occurs in a morphologically different region. The results of ozone treatment, dioxane treatment, or both are shown in Fig. 3. Figure 3a shows a transverse section extracted with dioxane for 120 min after 5-min ozonization. In this section, no lignin removal from the middle lamella of cell corners was found, although aqueous dioxane is well known to be a good solvent for lignin. 7,9 As shown in Fig. 3b, however, apparent lignin removal from the cell corners was observed in the section after ozonization treatment for 10 min and a subsequent dioxane extraction for 10 min, without reference to the remaining lignin color reaction, as shown in Fig. 1b. This fact suggests that certain modification of the lignin structure by ozone oxidation occurred and the modified lignin became susceptible to dioxane extraction, resulting in lignin dissolution. Similar results were found in a section with ozonization alone for 30 min, in which lignin removal occurred from the cell corners without dioxane extraction (Fig. 3c). Figure 3d shows complete lignin removal from both the middle lamella between neighboring cells and the cell corners in a section extracted with dioxane for 30 min after ozonization for 30 min. However, excessive exposure to ozone for more than 60min caused thinning of the cell walls, as shown in Fig. 3e, in which the thinning proceeded from both the inner and outer walls of a single cell. As a result, the absence of the primary wall and inner layer of the secondary walls was observed. The results obtained here indicate that mild ozonization for min is sufficient for lignin removal from the middle lamella of the cell corners. Extraction with dioxane after ozonization was also effective for removing lignin. It has been shown that the ozone attack upon ligninrelated model compounds is initiated at several sites and proceeds through a number of lignin degradation mechanisms. 10,16 18 Gierer 19 explained the general course of ozonolysis resulting in the cleavage of the original aromatic

143 Fig. 3a e. Scanning electron microscopy photographs of transverse sections extracted with dioxane after ozonization. a Dioxane extraction for 120 min after 5-min ozonization.

4 143 Fig. 3a e. Scanning electron microscopy photographs of transverse sections extracted with dioxane after ozonization. a Dioxane extraction for 120 min after 5-min ozonization. b Dioxane extraction for 10 min after 10-min ozonization. The arrowheads show lignin removal from the cell corners. c No dioxane extraction after 30-min ozonization. The arrowheads show lignin removal from the cell corners. d Dioxane extraction for 30 min after 30-min ozonization. e Dioxane extraction for 120 min after 60-min ozonization. The thinning of cell walls from both the inside and outside was observed. Bars 10 µm. or olefinic double bond and the formation of carbonylcontaining fragmentation products. The results obtained here suggest that a mild oxidation by ozone and subsequent extraction with dioxane can be used successfully for lignin removal from the cell walls to produce cellulose-based materials for various uses of wood waste. However, the effects of ozonization on lignin removal depend on the size distribution of the wood samples. Further research will be necessary to find the optimal conditions for ozonization by varying the material size, because severe ozonization causes the degradation of carbohydrates within cell walls. Conclusions In the present study, mild ozonization lasting from 10 to 30 min was sufficient for lignin removal from the morphologically different regions in wood tissues under the studied conditions of ozone delignification. Extraction with dioxane after ozonization was very effective in dissolving the ozoneoxidized lignin. Ozone delignification occurred faster in the secondary walls of tracheids than in the middle lamellae of cell corners and was faster in early wood than in late wood. The results obtained here demonstrate that microspectrometry coupled with the Wiesner reaction is useful for the quantitative analysis of lignin in the cell walls. References 1. Roncero MB, Queral MA, Colom JF, Vidal T (2003) Why acid ph increases the selectivity of the ozone bleaching processes. Ozone Sci Eng 25: Neely WC (1984) Factors affecting the pretreatment of biomass with gaseous ozone. Biotechnol Bioeng 26: Hayashi N, Shimizu K, Hosoya H (1989) Pretreatment of ozone for increasing the enzymatic susceptibility of autohydrolyzed softwoods. Mokuzai Gakkaishi 35: Kabeya H, Kubo T, Hosokawa J (1992) Combined chemical and physical pretreatments for enzymatic hydrolysis of thermomechanical pulp. Mokuzai Gakkaishi 38: Chirat C, Lachenal D (1994) Effect of ozone on pulp components: application to bleaching of kraft pulps. Holzforschung 48:

5 Yoshizawa N, Sagiya S, Yokota S, Idei T (1996) Microspectrometrical analysis of ozone delignification from wood cell walls. Holzforschung 50: Aoyama M, Sakakibara A (1979) Hydrolysis of lignin with dioxane-water (XVII). Isolation of a new lignol from hardwood lignin. Mokuzai Gakkaishi 25: Yashiro M, Tanaka S, Sugai Y (1982) The heat of dissolution and the solubility of dioxane lignin in some organic solvents. Bull Niigata Univ For 15: Azuma J, Takahashi N, Isaka M, Koshijima T (1985) Lignin-carbohydrate complexes extracted with aqueous dioxane from beech wood. Mokuzai Gakkaishi 31: Kojima Y, Yoon SL, Kayama T (1988) Delignification from the cell wall of wood fibers with ozone. Mokuzai Gakkaishi 34: Srivastava LM (1966) Histochemical studies on lignin. TAPPI 49: Sarkanen KV, Ludwig CH (1971) Definition and nomenclature. In: Sarkanen KV, Ludwig CH (eds) Lignins. Wiley InterScience, New York, pp Fergus BJ, Goring DAI (1970) The distribution of lignin in birch wood as determined by ultraviolet microscopy. Holzforschung 24: Musha Y, Goring DAI (1975) Distribution of syringyl and guaiacyl moieties in hardwoods as indicated by ultraviolet microscopy. Wood Sci Technol 9: Fujii T, Shimizu K, Yamaguchi A (1987) Enzymatic saccharification on ultrathin sections and ultraviolet spectra of Japanese hardwoods and softwoods. Mokuzai Gakkaishi 24: Balousek PJ, McDonough TJ, Mckelvey RD, Johnson DC (1981) The effects of ozone upon a lignin model containing the β-aryl ether linkage. Svensk Paper 84:R49 R Kojima Y, Miura K, Kayama T (1978) Oxidative degradation of softwood lignin model compounds with ozone. Res Bull Coll Exp For Hokkaido Univ 35: Eriksson T, Gierer J (1985) Studies on the ozonation of structural elements in residual kraft lignins. J Wood Chem Technol 5: Gierer J (1990) Basic principles of bleaching. I. Cationic and radical processes. Holzforschung 44:

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