Brown rot decay of wood is the most destructive and costly form of fungal deterioration of wood in-service (Green 2000). The initial fungal decay of wood cannot be exclusively induced by enzymes because these are not able to penetrate the cell wall. The diameters of cell wall micro-pores above the fibre saturation point (FSP) do not exceed 2 nm, while the hydrodymamic diameters of enzymes such as peroxidises, laccases, cellulases, or xylanases are not smaller than 4 nm (Flournoy et al. 1991). Brown rot fungi are the only species that are able to degrade cellulose and hemicelluloses without prior removal of lignin. It was demonstrated that white rot decay led to a much higher increase in the cell wall pore diameter than brown rot decay did at the same mass loss (ML) level (Flournoy et al. 1991, 1993). This specific decay pattern and the absence of exo-β-glucanases in several brown rot fungi (Highley and Dashek 1998) led to the conclusion that diffusion of low molecular weight (MWlow) agents might cause oxidative degradation of polysaccharides, resulting in strength loss prior to ML (Green et al. 1991; Winandy and Morrell 1993; Green and Highley 1997).
Fenton reagent (Fenton 1894) is known to play a key role in brown rot decay because it is a diffusible MWlow agent that can penetrate the cell wall matrix, even in a chelated form (Koenigs 1974; Kirk et al. 1991; Hyde and Wood 1997; Rättö et al. 1997; Hammel et al. 2002). It can thus reach the non-crystalline regions of cellulose and cleave the glucan chains, which results in tensile strength loss (Green et al. 1991; Kleman-Leyer et al. 1992; Green and Highley 1997). According to the mechanism described by Fenton (1894), a redox reaction proceeds between hydrogen peroxide and ferrous ions, resulting in highly oxidative hydroxyl radicals with a redox potential of +2180 mV for the one-electron reduction to water (Koppenol and Liebman 1984). Hydroxyl radicals can cleave glycosidic bonds of polysaccharides, generating carbonyl and carboxyl groups (Halliwell 1965; Gilbert et al. 1984; Kirk et al. 1991). Degradation of wood ultimately leads to mineralisation, i.e., production of carbon dioxide and water, as reported previously (Xie et al. 2010, 2012; Norbakhsh et al. 2014). Wood components and their degraded intermediates may facilitate the Fenton reaction, which is accompanied by ML and loss of tensile strength and production of CO2 (Xie et al. 2010, 2012). The last quoted studies revealed that the MWlow iron chelator 2,3-dihydroxybenzoic acid (DHBA) accelerated the decomposition of H2 O2 in the solution containing ferrous ions but reduced the ML and tensile strength loss. Moreover, the degree of mineralisation was reduced compared with the blank without DHBA. The Fenton reaction [Eq. (1)] preferentially proceeds under moderately acidic conditions generating highly oxidative hydroxyl radicals. At pH 4 (as in this study), the reaction proceeds faster than at lower pH because at this pH, ferrous irons exist mainly as Fe(OH)2, which reacts faster with H2 O2 than the hexa-aquoferrous ion does, which is predominant at pH below 3 (Wells and Salam 1965).
To enhance the effectivity of the Fenton reaction, ferric ions formed must be reduced to ferrous ions. Reduction of Fe(III) might be caused by peroxide [Eq. (1)] or peroxyl radicals [Eq. (2)] (Barb et al. 1951; Laat and Gallard 1999).
In addition, ferric ions can also be reduced by phenolic compounds whose standard potential E0 is below that of the Fe3+/Fe2+ pair (+0.77 V; Vanýsek 2005). These phenolic compounds can be free phenolic groups in lignin or some of its aromatic degradation products. As an example, the standard potential E0 of guaiacol amounts to +0.77 V (Chiavari et al. 1988). Additional electron releasing groups attached to the guaiacol ring such as methoxy or alkyl groups further reduce its E0. It has been recently shown that chemical modification with 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) and glutaraldehyde (GA) is one way to enhance the resistance of wood to decay fungi (Verma et al. 2008; Xiao et al. 2012). Three reasons are considered to explain the increased decay resistance of modified wood (Verma et al. 2008, 2009; Verma and Mai 2010): (a) Changes in the cell wall polymers (due to blocking of hydroxyl groups) might render these substrates unrecognisable for enzymes. (b) A smaller micro-pore size in the wood cell wall due to bulking or cross-linking of cell wall polymers might further reduce the accessibility of enzymes and of diffusible agents with MWlow compared with native wood (Hill et al. 2005). (c) Reduction of the cell wall moisture content (MC), which is generally equivalent to the FSP during fungal degradation, inhibits the diffusion process of the decay agent into the cell wall (Hill 2009).
A working hypothesis is that chemical modification of wood may inhibit the Fenton reaction. It was found for acetylated wood that increasing weight percentage gains (WPGs) linearly decreased ML caused by brown rot fungi (Hill et al. 2005, Hill 2009). It was, however, not yet directly demonstrated that chemical modification inhibits the decay of wood through MWlow agents. The aim of the present work was to study the effect of two types of chemical modification – DMDHEU and GA – on degradation by the Fenton reaction. The mode of action for fungal protection by DMDHEU is attributed mainly to cell wall bulking, while that of GA relies mainly on both cell wall bulking and cross-linking of cell wall polymers (Verma et al. 2009; Verma and Mai 2010; Xiao et al. 2012). The expectation is that this study may contribute additional knowledge to these complex reaction mechanisms.
Materials and methods
Wood veneers, measuring 100 mm×15 mm×80 μm (L×R×T), were cut from the radial surface of a Scots pine (Pinus sylvestris L.) sapwood block using disposable microtome blades (Reichert-Jung, Nussloch, Germany) as previously described (Xie et al. 2010). GA and DMDHEU are commercial ready-to-use solutions received from BASF AG (Ludwigshafen, Germany). Magnesium chloride hexahydrate (MgCl2·6H2 O) was used as a catalyst for both agents. Hydrogen peroxide (35 wt%) and iron(III) sulfate hydrate [Fe2(SO4)3·H2 O] were purchased from Sigma-Aldrich Chemie GmbH (Munich, Germany). Sodium thiosulfate, potassium iodide, and starch were purchased from AppliChem GmbH (Darmstadt, Germany).
The veneers were vacuum (100 mbar) impregnated (30 min) with aqueous solutions of DMDHEU (MgCl2·6H2 O) or GA (MgCl2·6H2 O) in concentrations of 0.4 (0.023 mol l-1), 1.2 (0.07 mol l-1), and 2.0 (0.12 mol l-1) mol l-1 (concentration in parentheses refers to the catalyst). After impregnation, the excess solution was blotted off the veneers with filter paper. The veneers were air-dried for 2 h and cured at 120°C for 12 h. The mass of each veneer was determined after drying. Prior to treatment with Fenton reagents, the veneers were leached with warm (40°C) deionized water for 4 h (with twice water change), in order to remove the un-reacted chemical.
Modified and unmodified veneers were cut in the middle: one half was subjected to Fenton treatment and the other half was stored in 0.1 mol l-1 acetate buffer (pH 4.0) as untreated controls. Fenton treatment was run in 100-ml serum bottles accommodating 50 ml treating solution [50 mmol l-1 H2 O2 and 0.2 mmol l-1 Fe2(SO4)3·H2 O] and three halves of veneer (ca. 1.5 mg ml-1). Wood veneers were firstly dispersed in the buffer solution; iron solution was added afterwards; H2 O2 solution was added last to initiate the Fenton reaction. The serum bottle was immediately closed with a rubber capsule. The capsule was tightened to the bottle with an aluminium cover to prevent produced gas in the treating solution from escaping. The serum bottle was then gently shaken in a dark water bath at room temperature (ca. 20°C) during the treating time. Fifteen serum bottles were prepared for each modification. Three of them were respectively withdrawn after 0.5, 2, 6, 24, and 48 h for determining gas production and H2 O2 concentration and measuring wet tensile strength of the veneers. The thickness of the veneers was determined by a dial gauge micrometre for tensile strength calculation. The air-dried veneers were scanned and their surface lightness was determined using the software Photoshop 7.0 (Adobe Systems GmbH, Munich, Germany). After tensile testing, the fragments were oven dried and weighed to assess ML due to the exposure.
The concentration of H2 O2 in the treating solution was measured iodometrically as described previously (Mai et al. 2002; Xie et al. 2010).
The nitrogen contents of DMDHEU-treated and untreated veneers were determined in an elemental analyser LECO CHN 2000-Analyzer (LECO Instrumente GmbH, Mönchengladbach, Germany). The proportion of DMDHEU in treated veneers was calculated as described previously (Xie et al. 2005).
The gas pressure in the enclosed serum bottle was determined at a defined time by means of a tensiometer, which is a pressure transducer equipped with a hypodermic needle (Thies Klima, Göttingen, Germany). The gas was sampled from the serum bottle and the composition of the gas (O2, CO2, N2, N2 O) was analysed as previously described (Xie et al. 2012).
Zero-span tensile strength was determined according to Xie et al. (2010, 2012). Tensile strength retention was calculated by comparing the tensile strength of veneers subjected to Fenton reagent with that of the unmodified veneer half stored solely in acetate buffer (control).
Results and discussion
Colour change and ML
Modification of wood caused an increase in WPG with increasing concentrations of DMDHEU or GA due to incorporation of chemicals in the wood structures (Figure 1). After leaching in water, the WPGs of treated wood were reduced due to removal of non-reacted chemicals and water-soluble wood extractives. Veneers treated with DMDHEU obtained higher WPGs than did those treated with GA at the same treating concentration. This can be explained by the greater molar weight of DMDHEU (178 g mol-1) compared with GA (100 g mol-1). In addition, major parts of GA might evaporate during oven drying, while DMDHEU readily undergoes auto-condensation, which prevents evaporation. Modification with DMDHEU or GA apparently did not change the colour of wood strips (not shown). Treatments with the Fenton solution for 48 h apparently darkened the unmodified wood strips due to formation of chromophoric groups as a result of lignin oxidation (Xie et al. 2012). The modified wood exhibited less colour change; the colour of modified wood with 2.0 mol l-1 chemicals was comparable with that of the unmodified veneer strips stored solely in buffer. The IR spectra of veneers treated with 2.0 mol l-1 DMDHEU or GA wood did not change due to Fenton treatment (not shown). At comparable concentration, the lightness of GA-modified wood was greater than that of DMDHEU-modified wood (Figure 2), which shows that GA modification might be more efficient than DMDHEU modification in preventing lignin oxidation.
The veneers exhibited minor ML in the initial 6 h of Fenton treatment (Figure 3a, b). In this stage, degradation of cell wall polymers is likely to occur, but the amounts of water-soluble compounds were obviously low. The ML of unmodified veneers after 48 h of exposure to Fenton reagent amounted to 18.0% and was somewhat lower than that described by Xie et al. (2012). This is attributed to the lower pH (3.0) and higher temperature (30°C) in the previous study. Under these conditions, the rate of the Fenton reaction is known to be higher (Pérez et al. 2002; Jung et al. 2009). Modification with DMDHEU and GA reduced the ML during the exposure to Fenton reagent; the increasing WPGs of the treated veneers further reduced the ML (Figure 3a,b). After 48 h of Fenton treatment, veneers modified with 2.0 mol l-1 chemicals exhibited only an ML of 4.0% (DMDHEU) and 2.4% (GA). The MLs of GA-modified veneers were slightly less than those of DMDHEU-modified wood compared at equal treatment concentrations.
Tensile strength of wood
As reported previously (Xie et al. 2005, 2007; Xiao et al. 2010), the process of chemical modification reduced wet tensile strength by up to 50%, especially at high chemical concentrations (see values at 0 h incubation time in Figure 3c,d). This tensile strength loss is attributed to hydrolysis of cell wall polysaccharides catalysed by the Lewis acid MgCl2·6 H2 O, cross-linking of cell wall polymers, and embedding of the resin molecules in the cell wall matrix. Veneers unmodified and modified with 0.4 mol-1 DMDHEU or GA, which exhibited a comparable strength prior to Fenton treatment, revealed a linear loss in tensile strength with the incubation time. At high chemical concentrations (1.2 and 2.0 mol l-1), the tensile strength of the veneers was hardly affected by Fenton treatment.
Change in nitrogen content of DMDHEU- modified veneers
The nitrogen content of the unmodified veneers was below the detection limit (0.05%). Modification with DMDHEU increased the N content as a function of the concentration of the modification chemical (Figure 4a). The N content in veneers treated with 0.4 mol l-1 DMDHEU was reduced considerably during Fenton treatment, which was accompanied with increased detection of gaseous nitrogen compounds (Figure 4b). This is a sign of degradation of the insoluble DMDHEU-cell wall complex, which leads to mineralised gaseous products. The N content in the treating solution was not determined, but it is assumed that the modified veneers release water-soluble degradation products prior to mineralisation.
A similar degradation process of DMDHEU-modified veneers due to weathering has been shown previously, where the ML of DMDHEU occurred faster than the ML of veneer (Xie et al. 2005). N loss is consistent with ML and tensile strength reduction. Veneers modified with 2.0 mol l-1 DMDHEU exhibited a slight N reduction within the first 2 h of exposure, which might be attributable to leaching of unfixed DMDHEU rather than to degradation by Fenton reagent.
Despite higher N contents, veneers with higher WPGs of DMDHEU produced minor amounts of nitrogen gas after exposure to Fenton reagent for 6 h, after which gas production did not increase. It is therefore assumed that the enhanced initial nitrogen gas production is due to mineralisation of leached, unfixed DMDHEU rather than the mineralisation of the DMDHEU-containing cell wall complex. In total, the amount of DMDHEU released from the veneers is relatively small compared with the WPGs of wood.
Decay of hydrogen peroxide
The concentration of H2 O2 in the solution containing untreated veneers decreased linearly from 50 to 30 mmol l-1 in the course of the incubation (Figure 5). Solutions containing veneers modified with 0.4 mol l-1 DMDHEU or GA revealed a slightly lower decomposition rate of H2 O2 than did those with untreated veneers. The decomposition rate further reduced in the presence of veneers of increasing WPG. At the highest chemical concentration (2.0 mol l-1), H2 O2 concentration decreased by only 6.5% (DMDHEU) and 8.5% (GA). This shows the inhibiting effect of the treatments with respect to the reducing decay of H2 O2, which results in the formation of hydroxyl radicals.
Under the conditions of this study, decomposition of H2 O2 proceeds mainly via the Fenton reaction. To enhance the turnover of the reaction, ferric ions must be re-reduced to ferrous ions. Phenolic constituents of wood might function as redox partners, such as lignin (with its free phenolic groups), its degradation products, or phenolic extractives (Xie et al. 2010). Chemical modification might block phenolic hydroxyl groups and thus inhibit the reduction of ferric ions; this would result in a reduced decomposition rate of H2 O2 via the Fenton reaction. However, other modes of action are possible (see below).
Gas production by the Fenton system
Production of CO2 due to the Fenton reaction was highest in the solution containing untreated veneers; the amount increased exponentially with the incubation time (Figure 5a, b). The increasing rate of CO2 can be explained by the increasing volume of pores in the cell wall, which leads to exposure of more cell wall material and/or by the enhanced reduction of ferric ions by fragments of cell wall polymers, particularly lignin, which drive the Fenton reaction. Fenton reagent in acetate buffer at pH 3.0 (without wood or chelators) produces CO2 by oxidative degradation of acetate (Xie et al. 2012). In the present study, however, which applies acetate buffer at pH 4.0, CO2 was not produced from buffer constituents during the incubation (not shown). This indicates that the Fenton reaction hardly proceeds in buffer alone because reduction of ferric ions does not occur. If it is assumed that this reduction initially proceeds in the wood with the participation of wood constituents, particularly lignin, the CO2 produced should derive mainly from mineralisation of wood and not of buffer. In a later phase, however, these degradation products might also promote the formation of hydroxyl radials outside the veneers that may also mineralise acetate buffer.
The total mass of unmodified veneer strips was initially 75 mg, and 18% was lost after 48 h of Fenton treatment (Figure 3a). As the carbon content of Scots pine sapwood amounts to 47.4% (Xie et al. 2005), the Fenton system should produce 10.6 mmol l-1 CO2, if the ML would be completely due to mineralisation. However, approximately 8.0 mmol l-1 was actually produced (Figure 5c), indicating that at least 2% of decomposed cell wall fragments remained in the solution. Modification of the veneers with DMDHEU and GA caused a decrease in the production rate and total amount of CO2 during the treatment with Fenton reagent (Figure 5c, d).
Veneers modified with 2.0 mol l-1 DMDHEU and GA revealed MLs of 4.0% and 2.4%, respectively, after 48 h of exposure to Fenton reagent (Figure 3a, b). If these MLs would derive exclusively from leached DMDHEU and GA, which were completely mineralised, they would produce 1.7 and 1.8 mmol l-1 CO2, respectively. As the actual amount of CO2 produced was 3.0 mmol l-1, some mineralisation of the modified veneers must have occurred.
The Fenton reaction also produced molecular oxygen, which derives from decayed H2 O2 (Figure 5e,f). In the presence of unmodified veneers, 20 mmol l-1 H2 O2 was decomposed in the solution within 48 h (Figure 5a, b). Complete conversion of H2 O2 would produce 10 mmol l-1 O2. Actually, only 2.3 mmol l-1 O2 was obtained; the rest was converted to CO2 via oxidative decay of wood and acetate/acetic acid. Modification of veneers with 0.4 mmol l-1 DMDHEU did not reduce the production of O2 during incubation with Fenton reagent as compared with the unmodified veneers. Increasing WPGs of DMDHEU, however, decreased O2 production (Figure 5e). This is consistent with less decomposition of H2 O2 and lower production of CO2.
The Fenton system containing GA-modified veneers exhibited a similar trend in production of O2 to that containing DMDHEU-modified veneers, but the former produced slightly less amounts of O2 and CO2 compared at the same treatment concentrations. Accordingly, modified wood is less susceptible to decay by the Fenton reaction.
Mode of action of decay protection
Hill et al. (2005) demonstrated that acetylated wood suffers less ML due to brown rot decay as a function of increasing WPG through acetylation and attributed this to micro-pore blocking, which prevents the penetration of the decay agent into the cell wall (Hill et al. 2005) and/or to reduced cell wall MC (Hill 2009). A direct effect of chemical modification on the resistance of wood to isolated decay agent has not yet been shown. Reduced mineralisation of the veneers reveals that modified veneers with increasing WPGs became increasingly resistant to oxidative degradation by hydroxyl radicals produced by Fenton reagent. Less tensile strength loss indirectly indicates less degradation of polysaccharides, while lower degrees of discoloration indicate lower oxidative decay of lignin. There are two interpretations for these findings: (a) Fenton reagent is unable to penetrate into the cell wall matrix or (b) chemical modification prevents the reduction of the ferric form of iron to the ferrous form. This reduction might be caused by wood constituents such as phenolic compounds of lignin or extractives or by phenolic compounds produced by the fungus (Goodell et al. 1997; Kerem et al. 1999; Xu and Goodell 2001; Qian et al. 2002). In previous studies, however, the addition of metal-binding, phenolic compound with MWlow DHBA to Fenton reagent did not enhance strength loss and mineralisation of unmodified micro-veneers (Xie et al. 2010, 2012) and was therefore not considered in this study.
It is assumed that ferrous ions are chelated by two acetate molecules, and ferric ions, by three acetate molecules; these complexes are much smaller than extracellular enzymes responsible for cell wall degradation. Increased cell wall bulking or cross-linking due to the modifications might reduce the diameters of cell wall micro-pores and the availability of water as a transport medium, thereby inhibiting penetration by iron complexes (mechanism a; Papadopoulos and Hill 2002; Xiao et al. 2012). Reduced penetration of iron complexes might also disable reduction of ferric irons by free phenolic groups of lignin. Despite lower WPGs, GA-modified veneers were somewhat more resistant to degradation by Fenton reagent than DMDHEU-modified ones were. This might be attributed to a stronger degree of cross-linking by GA and smaller pore diameters resulting thereof.
Both DMDHEU and GA might react with phenolic hydroxyl groups in lignin and thus prevent reduction of ferric ions via free phenolic groups (mechanism b). At this juncture, it cannot be elucidated which mechanism is predominant. DMDHEU mainly undergoes auto-condensation and GA also undergoes electrophilic aromatic substitution of lignin or condensation with hydroxyl groups of polysaccharides. Still, for both chemicals, reaction with phenolic hydroxyl groups in lignin is possible.
Modification of wood with DMDHEU and GA can efficiently inhibit the degradation by Fenton reagent, i.e., the formation of hydroxyl radicals, as evidenced by the reduced ML and strength reduction, and less mineralisation of wood. Compared at the same treating-concentration levels, GA causes somewhat higher resistance to degradation by Fenton reagent than DMDHEU does. Thus, the working hypothesis can be confirmed and the high resistance of GA- and DMDHEU-modified wood to brown rot decay can be attributed to higher resistance of the modified wood to the Fenton reaction.
We acknowledge the support from the Fundamental Research Funds for the Central Universities (no. DL12DB02) and the Program for New Century Excellent Talents in University of Ministry of Education of China (no. NCET-11-0608). Yanjun Xie thanks the postdoctoral grant from German Academic Exchange Service (DAAD).
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