Environ. Sci. Technol. 2005, 39, 175-180
Electrochemical Study of 2,3-Dihydroxybenzoic Acid and Its Interaction with Cu(II) and H2O2 in Aqueous Solutions: Implications for Wood Decay R A N L I U , †,‡ B A R R Y G O O D E L L , § JODY JELLISON,⊥ AND A R I A A M I R B A H M A N * ,† Departments of Civil and Environmental Engineering, Wood Science and Technology, and Biological Sciences, University of Maine, Orono, Maine 04469
The electrochemical behavior of 2,3-dihydroxybenzoic acid (2,3-DHBA) and the electron-transfer characteristics between Cu(II) and 2,3-DHBA were studied in aqueous solutions using cyclic voltammetry (CV). The overall electrochemical oxidation process of 2,3-DHBA by Cu(II) may be classified as a chemical reaction involving oneelectron oxidation of 2,3-DHBA to its semiquinone radical in solution, followed by an electron-transfer reaction involving the oxidation of the semiquinone radical to a quinone at the electrode surface. In the presence of H2O2, oxidation of 2,3-DHBA by Cu(II) is enhanced due to the regeneration of Cu(II) by H2O2 oxidizing Cu(I). The redox cycling between Cu(I)/Cu(II) and H2O2 also produces hydroxyl radicals (OH•). Even though the presence of OH• may not be detected at the surface of a glassy carbon electrode, production of electroactive dissolved oxygen (O2) suggests the presence of OH•. The production of O2 is dependent on Cu(II):H2O2 concentration ratio. At the electrode surface and when the initial Cu(II):H2O2 is less than 1, O2 is produced, suggesting that H2O2 may act as a scavenger for OH•; at initial Cu(II):H2O2 > 1, the production of O2 is not favored, and OH• will be involved in the oxidation of Cu(I) and the organic ligand. The reaction mechanisms proposed in this study indicate that OH• production by chelator-mediated Fenton reactions is favorable under conditions found in the wood cell wall.
Introduction Brown rot decay is the most common and destructive type of decay in Northern Hemisphere forests and structural wood products. It causes billions of dollars in loss each year in the United States and the rest of the world (1). To prevent wood decay in an environmentally appropriate manner, a systematic understanding of the mechanisms of wood biodegradation by brown rot fungi is needed. The mechanisms of wood degradation by brown rot fungi have been studied for decades. It is believed that the brown * Corresponding author phone: (207)581-1277; fax: (207)581-3888; e-mail:
[email protected]. † Department of Civil and Environmental Engineering. ‡ Present address: Department of Civil and Environmental Engineering, Carnegie Mellon University, Pittsburgh, PA. § Department of Wood Science and Technology. ⊥ Department of Biological Sciences. 10.1021/es049714q CCC: $30.25 Published on Web 11/26/2004
2005 American Chemical Society
rot decay process is initially a nonenzymatic process and that it involves a catalytic system that produces hydroxyl radicals capable of attacking wood components (2-4). Low molecular weight compounds (chelators) produced by brown rot fungi such as Gloeophyllum trabeum (Gt chelators) have been proposed to mediate the redox cycling of Fe(II)/Fe(III), which reacts with H2O2 to produce highly reactive hydroxyl radicals. A study using the model chelator, 2,3-dihydroxybenzoic acid (2,3-DHBA), showed that this ligand not only has a high affinity for binding Fe(III) but also will reduce Fe(III) to Fe(II) (4). Recent research (5-7) has confirmed that three compounds isolated from the brown rot fungus G. trabeum (4,5-dimethoxycatechol and 2,5-dimethoxyhydroquinone) have the ability to reduce Fe(III) to Fe(II) and to initiate an extracellular Fenton reaction in brown rot wood degradation processes. Recently, quinones were also shown to be the dominant redox-active moieties of the natural organic matter, which is an end product of wood biodegradation (8). Cyclic voltammetry (CV) is a widely used technique for acquiring qualitative information about electrochemical reactions. In particular, this technique offers rapid measurement of the redox potentials of electroactive species and convenient evaluation of the effect of environmental parameters upon redox processes (9). Cyclic voltammetry has been used to characterize the oxidation state and the type of binding between hydroquinones and metals ions in dimethyl sulfoxide (DMSO) solutions (10, 11). Copper is known to be involved in biological electrontransfer processes. It is an essential micronutrient for fungal growth and functions as a metal activator of several fungal enzymes (e.g., oxidases and in the synthesis of pigments) (12). Copper, like iron, can undergo Fenton reactions with H2O2 to produce hydroxyl radicals. The role of copper complexes and peroxide-producing systems in lignin depolymerization has been previously examined (13). Copper may also play a role in white rot biodegradation of lignocellulose, and even though models have been proposed using iron as the model transition metal, copper may work in these mechanisms equally well for the brown rot fungi (4). Models for the degradation of lignocellulose employing copper as a transition metal have also been proposed (13). Copper was studied in this work because it also forms complexes with the chelators isolated from wood decay fungi, and it could be used in voltammetry studies more readily than iron due to its higher solubility. In this paper, the reaction of Cu(I)/Cu(II) and 2,3-DHBA (a model ligand for Gt chelators) in the absence and presence of H2O2 is studied using CV. The data provide information on redox mechanisms and help to improve our understanding of reaction mechanisms related to a catalytic system proposed to be involved in wood decay processes.
Materials and Methods For the CV experiments, a conventional three-electrode electrochemical cell was used (Metrohm 746 VA trace analyzer and a Metrohm 747 VA stand), equipped with a glassy carbon electrode (surface area 0.126 cm2) as working electrode, a platinum auxiliary electrode, and an Ag/AgCl reference electrode. All CV experiments were conducted under a laminar flow hood. The solutions were prepared by adding the appropriate amounts of CuCl2, 2,3-DHBA, and/or H2O2 stock solutions to a 0.5 M KNO3 electrolyte solution. Aliquots of 0.05 M HCl or 0.05 M NaOH were added to obtain the desired pH. Prior to taking each voltammetric scan, 20 mL of sample solution VOL. 39, NO. 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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was placed in a Teflon vessel on the VA stand, and ultrapure Ar was passed through the solution for 15 min to remove oxygen. All measurements were performed at room temperature. Between each scan, the working electrode was polished for 10 s with small circular movements with 0.5 µm of alumina (Al2O3) on a polishing pad, rinsed with deionized water, immersed in diluted HCl for 10 s, rinsed again with deionized water, and dried with a filter paper. Different concentrations of the electrolyte KNO3 (0.01, 0.05, 0.1, 0.5, 1.5, and 3 M) were used to test for the iR drop (solution resistance) effect. The cyclic voltammograms obtained at different concentrations of KNO3 remained the same, indicating that iR drop is insignificant under our experimental conditions. The chemicals 2,3-dihydroxybenzoic acid (99%), CuCl2 (anhydrous, 99%), H2O2 (30% w/w), NaOH (99%), and HCl (analytical grade) were all purchased from Sigma-Aldrich.
Results and Discussion Electrochemistry of 2,3-DHBA. The electrochemical behavior of 2,3-DHBA was studied in aqueous solution at various pH values. Figure 1 shows the CV scans of 2,3-DHBA at four different pH values. The anodic peaks A and B in each voltammogram represent the oxidation of 2,3-DHBA into the “semiquinone” and the “quinone” forms of the ligand, respectively (10, 11). The oxidation to the quinone form, which occurs at the surface of the glassy carbon electrode, is irreversible since no corresponding reduction peak to the oxidation peak B appears. The cathodic peak C in each voltammogram may represent the reduction of the semiquinone form back to 2,3-DHBA at the surface of electrode. The potential of peak A shifts to less positive values as the pH increases from 790 mV at pH 2 to 400 mV at pH 12 (Figure 1). This shift in the oxidation potential indicates that oxidation of 2,3-DHBA becomes more favorable with increasing pH. The potential shift may be attributed to the speciation of 2,3-DHBA in aqueous solution as a function of pH and the higher electron density of the molecule at a higher pH. At pH 2, the fully protonated 2,3-DHBA is the dominant species, whereas at pH of 4.4 and 7, the diprotonated species, and at pH 12, the monoprotonated species are dominant. The large oxidation-reduction peak potential separation between peaks A and C in the voltammograms in Figure 1 indicates that the electro-oxidation of 2,3-DHBA ligands is kinetically slow (14). The height of peak C is much smaller than that of peak A for all voltammograms. This quasireversible process may be attributed to the redox kinetics behavior between 2,3-DHBA and its semiquinone species (10). The further oxidation of semiquinone to quinone species is an “EC” mechanism (15), which is indicated by the oxidation peak B with no corresponding reduction peak. Oxidation from the semiquinone radical to its di-radical form is an electron-transfer process at the surface of the electrode, known as the “E” mechanism, which is followed by a chemical process from the di-radical form to the quinone in solution (the “C” mechanism). No reduction peak appears even at very fast scan rates (5.0 V/s) indicating that the di-radical analogue is very short-lived and is converted into quinone very quickly. The overall redox reaction scheme of 2,3,-DHBA at the surface of the electrode may be shown as
The CV study of 2,3-DHBA shows that the electrochemical oxidation of 2,3-DHBA to its semiquinone form becomes 176
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FIGURE 1. CV scans for 5 mM 2,3-DHBA in 0.5 M KNO3 at a scan rate of 0.5 V s-1, (a) pH 2; (b) pH 4.4; (c) pH 7; (d) pH 12.
more favorable as the pH increases. This indicates that the reducing ability of 2,3-DHBA is greater at high pH. This may provide an explanation to the question of why transition metal reduction via Gt chelator is more favored at pH 4.0 or greater as compared to low pH environments (16). Oxalate produced by brown rot fungi has been proposed to control the pH of the fungal environment by creating a pH gradient between the immediate fungal environment (pH≈2.5) and the wood cell wall (pH≈5.5-6.0) (4, 17). The redox reaction between a transition metal and 2,3-DHBA (or Gt chelator) leading to the initiation of the chelator-mediated Fenton reaction and production of hydroxyl radicals is more favorable in the wood cell wall, where the pH is relatively higher. Electrochemistry of Cu(II) and 2,3-DHBA Interaction. Electrochemical analysis of the interaction between Cu(II) and 2,3-DHBA was conducted with different ratios of Cu(II) to 2,3-DHBA at pH 4.4. Figure 2 illustrates the comparison between a Cu(II):2,3-DHBA solution (10:1) and a solution of 2,3-DHBA only.
TABLE 1. Cyclic Voltammetry Data for Various Cu(II):2,3-DHBA Ratios in 0.5 M KNO3 Solutions at pH 4.4 peak B
peak E
Cu:DHBA ratiosa
potential (mV)
current (µA)
DHBA only 1:10 1:5 1:2 1:1 2:1 5:1 10:1 Cu only
1340 1370 1380 1400 1400 1410 1420 1450
67.74 77.84 81.98 86.73 89.49 95.93 105.25 111.33
a
peak F
peak G
potential (mV)
current (µA)
potential (mV)
current (µA)
potential (mV)
current (µA)
250 250 270 250 280 300
11.2 12.94 18.99 23.95 73.03 99.80
90 160 150 170 160
-28.28 -33.00 -51.44 -77.90 -59.28
-256 -160 -110 -80 -90
-32.00 -41.95 -71.35 -124.3 -86.71
[2,3-DHBA] ) 0.5 mM and [Cu(II)] varies from 0 to 5 mM.
radicals), followed by an electron-transfer reaction involving the product at the surface of the electrode represented by the E step (i.e., anodic oxidation of the semiquinone radicals to quinone). This reaction is shown in eq 2.
FIGURE 2. CV scans of Cu(II) and 2,3-DHBA at pH 4.4 in 0.5 M KNO3 at a scan rate of 0.5 V s-1. Solid line corresponds to 5 mM Cu with 0.5 mM 2,3-DHBA, and dashed line corresponds to 0.5 mM 2,3-DHBA only.
The anodic peaks A and B and the cathodic peak C represent redox cycling of 2,3-DHBA at the electrode surface, as described above. The anodic peak E may be assigned to the oxidation of Cu(I) to Cu(II), as established using a solution of only Cu(II) (18). Although no Cu(I) was added to the solution, Cu(I) may be generated at the surface of the electrode when the voltammetric scan is initiated at 0 V. Peak F may be assigned to the reduction of Cu(II) to Cu(I), and Peak G may be assigned to the reduction Cu(I) to Cu(0). The peak potentials and currents for B, E, F, and G at different Cu(II):2,3-DHBA ratios are shown in Table 1. The presence of Cu(II) increases the anodic current of the semiquinone/quinone couple (peak B; Table 1). This could be due to 2,3-DHBA being oxidized to its quinone form by two consecutive one-electron-transfer steps at the surface of the electrode, and at the same time, Cu(II) oxidizing 2,3DHBA into its semiquinone radical in the bulk solution (19) that leads to an overall higher semiquinone/quinone oxidation peak. The semiquinone radical generated in solution diffuses to the surface of the electrode where it is oxidized into the quinone form. The excess of the semiquinone radical in the presence of Cu(II) results in an increase in the current as compared to the oxidation of 2,3-DHBA at the electrode surface in the absence of Cu(II). The increase in the height of peak B with the higher Cu(II) concentration in solution indicates that formation of the semiquinone radical is favored at high Cu(II) concentrations. The overall process can be classified as a “CE” mechanism, with the C step representing the chemical reaction in the bulk solution (i.e., oxidation of 2,3-DHBA by Cu(II) and generation of the semiquinone
The initial step in the oxidation of 2,3-DHBA by Cu(II) involves the formation of an aqueous Cu(II)-DHBA complex followed by an intramolecular electron transfer within the Cu(II)-DHBA complex to produce Cu(I) and the DHBA radical. The semiquinone structure is then further oxidized at the surface of the working electrode following its diffusion to the electrode’s surface to form the quinone species. Alternatively, it may be possible, in analogy to the oxidation of 2,3-DHBA by Fe(III), that the Cu(II)-DHBA complex is subsequently oxidized by an additional Cu(II) to form the semiquinone intermediate with the concurrent reduction of only the second Cu(II) to Cu(I) (20). However, in the presence of an excess of 2,3-DHBA, involvement of a second Cu(II) in the oxidation of 2,3,-DHBA by Cu(II) is not likely due to low Cu(II) concentrations. Ferrozine assay results with iron and 2,3-DHBA have confirmed that an excess of 2,3-DHBA (Fe(III):DHBA ) 1:3) indeed inhibits Fe(III) reduction (20). This inhibitory effect of Fe(III) reduction may be due to the hexadentate coordination of 2,3-DHBA with Fe(III), which prevents Fe(III) from further reaction. However, Cu(II) does not form a hexadentate complex with 2,3-DHBA. Our observation suggests that the formation of semiquinone is not inhibited by an excess concentration of 2,3-DHBA with respect to Cu(II). As shown in Table 1, the current for peak B increases with Cu(II) concentration, even at a low (1:10) Cu(II):2,3-DHBA ratio. Electrochemistry of Cu(II) and H2O2 Interaction. Figure 3a shows the voltammograms for the interaction between Cu(II) and H2O2 at two different concentrations of H2O2 at pH 4.4. For comparison, CV scans of solutions with only Cu(II) and H2O2 are also shown. Solutions that contain only H2O2 do not exhibit any peaks within the voltammetric scan range used here. Peaks F and G are assigned to the reduction of Cu(II) to Cu(I) and Cu(I) to Cu(0), respectively. In the presence of H2O2, peak G, assigned to the reduction of Cu(I) to Cu(0) at the surface of the electrode, decreases due to the VOL. 39, NO. 1, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. CV scans of 0.5 mM 2,3-DHBA, 0.5 mM Cu(II) + 0.5 mM 2,3-DHBA, and 0.5 mM Cu + 0.5 mM 2,3-DHBA + 2.5 mM H2O2 at pH 4.4 in 0.5 M KNO3 at a scan rate of 0.5 V s-1.
FIGURE 3. (a) CV scans of 5 mM Cu(II), 5 mM Cu(II) + 1 mM H2O2; 5 mM Cu(II) + 10 mM H2O2; and 5 mM H2O2 only at pH 4.4 in 0.5 M KNO3 at a scan rate of 0.5 V s-1. (b) Variation of peak H in panel a with respect to Cu(II):H2O2 ratio for initial Cu(II) concentrations of 1 and 5 mM. oxidation of Cu(I) to Cu(II) by H2O2 and possibly by OH•, as shown by the following reactions:
H2O2 + Cu(I) + H+ f OH• + Cu(II) + H2O
(3)
evolution is observed in water when Fe:H2O2 e 1 while no O2 is observed when Fe:H2O2 > 1 (23).
OH• + Cu(I) + H+ f Cu(II) + H2O
(4)
Electrochemistry of the Interaction between Cu(II) and 2,3-DHBA in the Presence of H2O2. Cyclic voltammetric measurements were made in aqueous solutions, containing constant concentrations of Cu(II) and/or 2,3-DHBA with varying concentrations of H2O2 at pH 4.4. Figure 4 illustrates the CV scans of 2,3-DHBA only, Cu(II) + 2,3-DHBA, and Cu(II) + 2,3-DHBA + H2O2 at pH 4.4. The anodic peak A was assigned to the oxidation of Cu(II)-2,3-DHBA complexes and free 2,3-DHBA into semiquinone as before. In the presence of H2O2, this oxidation reaction is more favorable since it occurs at a less positive potential than when H2O2 is absent (Figure 4). The anodic current of the semiquinone/quinone couple (peak B) also increases nearly 2-fold in the presence of H2O2. Peak H, which was assigned to the reduction of O2, appears only in the presence of H2O2 (Figure 4). For constant concentrations of Cu(II) and 2,3-DHBA, both the anodic peak B and cathodic peak H increase with an increase in H2O2 concentration as shown in Figure 5. In our preliminary experiments, we have verified that the peaks not identified in Figures 4 and 5 are not related to the reactions involving copper, DHBA and H2O2.
Peak F, however, increases with H2O2 concentration due to the increase in the concentration of Cu(II) brought about by the oxidation of Cu(I) to Cu(II) by H2O2 and OH• in solution. For a 1:2 ratio of Cu(II):H2O2, a new peak H appears. This peak may be assigned to the reduction of dioxygen (O2) at the surface of electrode, as established separately by conducting voltammetric scans of an electrolyte solution saturated with and without O2 (18). Peak H, however, is absent for Cu(II):H2O2 > 1 (Figure 3b). Therefore, we propose that when Cu(II):H2O2 < 1, the following reactions may be dominant instead of eq 3 (22):
H2O2 + OH• f HOO• + H2O T O2•- + H+ •-
Cu(II) + O2 •
f Cu(I) + O2
Cu(II) + HOO f Cu(I) + O2 + H
(5) (6)
+
(7)
A similar behavior has been proposed previously for reactions involving different concentrations of Fe and H2O2: dioxygen 178
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FIGURE 5. CV scans of 0.5 mM Cu(II) and 0.5 mM 2,3-DHBA with different concentrations of H2O2 at pH 4.4 in 0.5 M KNO3 at a scan rate of 0.5 V s-1.
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FIGURE 6. Redox cycling scheme of Cu(II) + 2,3-DHBA + H2O2 system.
The mechanism for the increase of peak B current in the presence of H2O2 (Figure 5) can be described as follows: At the surface of the electrode, 2,3-DHBA is oxidized to semiquinone and quinone forms. In solution, Cu(II) reacts with DHBA (eq 2), and the byproduct, Cu(I), reacts with H2O2 (eq 3). Reaction in eq 3 forms highly reactive OH•. The OH• can then attack Cu(I) (eq 4) or 2,3-DHBA as shown below:
OH• + DHBA + H+ f semiquinone radical + H2O
(8)
OH• + semiquinone radical + H+ f quinone + H2O (9) The presence of H2O2 generates more semiquinone radicals than are generated in the absence of H2O2, as observed by an increase in anodic peak B (Figure 4). This is primarily due to the fact that Cu(I) acts as a catalyst in the Fenton reaction to oxidize 2,3-DHBA (eqs 2 and 3). In the presence of H2O2, the OH• generated by the reaction of H2O2 and Cu(I) can either attack 2,3-DHBA to form the semiquinone radical species directly (eq 8) or attack H2O2 to form HOO• and O2•- (eq 5), which in turn can reduce Cu(II) to Cu(I) (eqs 6 and 7). This redox cycling continually generates the semiquinone radical species, which is indicated by the increase in peak B in the voltammograms (Figure 5). In the presence of excess H2O2, the overall process forms the redox cycling as shown in Figure 6. The electrochemical study of Cu(II) and 2,3-DHBA interaction shows that Cu(II) can oxidize 2,3-DHBA to its semiquinone form under acidic conditions, similar to the oxidation reaction reported for Fe(III) (4, 16). The CV study of Cu(II), 2,3-DHBA and H2O2 interactions shows that Cu(II) can also react with dihydroxy derivatives at low pH conditions to drive the Fenton reaction to generate OH•. In the presence of 2,3-DHBA, the Fenton reaction between Cu(I) and H2O2 is enhanced due to the continual generation of Cu(I), not only from the reaction between Cu(II) with HOO• and O2•but also from the reduction of Cu(II) by 2,3-DHBA. Thus, more OH• is produced than expected from the Fenton reaction alone. This is consistent with studies of OH• activity by Qian et al. (24), in which OH• is produced by the reaction involving Fe(II) and H2O2 and where radical production or activity occurs over a longer time span in the presence of 2,3-DHBA than in its absence. Fe(III) may undergo the same reaction mechanisms with 2,3-DHBA and H2O2 as Cu (II) (Figure 6). Specifically, in the fungal decay environment O2 may react with the dimethoxyhydroquinone radical to produce H2O2 and dimethoxybenzoquinone, with this redox cycle forming H2O2 from O2 (6). This may provide a possible source of H2O2 in the wood cell wall. Thus, the production of hydroxyl radicals could occur in the wood cell wall with available Fenton reagents, H2O2, and Fe(II) (from reduction of Fe(III) by the chelator).
From this study it can be concluded that: (i) 2,3-DHBA is oxidized at the surface of the glassy carbon electrode to its semiquinone and quinone forms via two consecutive one-electron-transfer steps. Oxidation of 2,3DHBA is facilitated as pH increases, which suggests that the redox reaction between the metal (Cu(II) or Fe(III)) and 2,3DHBA may be more favorable in the less acidic wood cell wall (pH 4-6) rather than in the highly acidic environment immediately adjacent to the fungal hyphae. The presence of a reducing environment next to the fungal hyphae also contributes to the inhibition of oxidation of the organic chelator (25). This would, therefore, prevent the generation of OH• radicals in close proximity to the fungus where damage to fungal membranes could occur but permit production of OH• radicals within the wood cell wall where oxidation of wood cell wall components would proceed as part of the decay process. (ii) In the presence of Cu(II), the overall electrochemical oxidation process of 2,3-DHBA may be classified as a CE mechanism. The increase of anodic current of the semiquinone/quinone couple with Cu(II) indicates that 2,3-DHBA is oxidized by Cu(II) in solution to generate the DHBA semiquinone radical (the C step), followed by further oxidation of the semiquinone to the quinone at the electrode surface (the E step). Our results indicate that 2,3-DHBA is oxidized by Cu(II) under acidic conditions. (iii) In the presence of H2O2, Cu(II) is regenerated via the Fenton reaction between Cu(I) and H2O2 under the experimental conditions, with OH• also formed. In the presence of excess H2O2, the OH• produced from the oxidation of Cu(I) in solution is scavenged by H2O2 producing HOO• and O2•-. Further reaction of Cu(II) with these two radicals generates O2. This could provide a source for H2O2 in the wood cell wall via the reaction between O2 and dimethoxyhydroquinone. (iv) The redox cycle involving Cu(II), 2,3-DHBA and excess H2O2 leads to a continuous regeneration of Cu(II) and production of OH•, which accelerates the formation of DHBA semiquinone radicals. This study shows evidence that a chelator-mediated Fenton reaction is favored within the wood cell wall, where the free radicals perturb the wood structure and initiate the degradation process. Also, reaction mechanisms for the interaction of Cu(II), 2,3-DHBA, and H2O2 have been proposed that provide at least partial explanations for the H2O2 cycle and the mechanism for the nonenzymatic, chelator-mediated Fenton reactions in brown rot wood decay processes.
Acknowledgments Funding for this work was provided by a grant from the University of Maine Wood Utilization Research Funds. This is Main Agricultural and Forest Experiment Station publication number 2771. The authors are grateful to Mr. Yuhui Qian for his help with the experiments.
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Received for review February 23, 2004. Revised manuscript received October 11, 2004. Accepted October 18, 2004. ES049714Q
