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International Biodeterioration & Biodegradation 57 (2006) 63–68

www.elsevier.com/locate/ibiod

Veratryl alcohol degradation by a catechol-driven Fenton reactionas lignin oxidation by brown-rot fungi model

David Contrerasa, Juanita Freera,c, Jaime Rodrıguezb,c,�

aChemical Sciences Faculty, Renewable Resources Laboratory, Universidad de Concepcion, Casilla 160-C, Concepcion, ChilebForestry Sciences Faculty, Renewable Resources Laboratory, Universidad de Concepcion, Casilla 160-C, Concepcion, Chile

cBiotechnology Center, Renewable Resources Laboratory, Universidad de Concepcion, Casilla 160-C, Concepcion, Chile

Received 6 August 2005; received in revised form 24 October 2005; accepted 19 November 2005

Available online 4 January 2006

Abstract

Dihydroxybenzenes reduce Fe(III) to Fe(II), which react with H2O2 driving a Fenton reaction. This non-enzymic mechanism operates

in wood degradation by brown-rot fungi, which mainly degrade wood carbohydrates and, to a lesser extent, lignin. Consequently, less

attention has been focussed on lignin transformation by these organisms. In this work, the degradation of veratryl alcohol (VA), the

simplest lignin model compound, via a Fenton reaction driven by 1,2-dihydroxybenzene (catechol, CAT) was studied. Multivariate

analysis performed in order to determine the relationship between pH and concentrations of CAT, FeCl3 and H2O2 showed that the

highest VA degradation, 1mol base, was obtained at the CAT:FeCl3:H2O2 ratio of 0.375:0.375:5.0 at pH 3.4. Under these reaction

conditions, VA degradation and mineralisation were, respectively, 3.8 and almost 40 times greater than for a Fe(II)-Fenton reaction.

r 2005 Elsevier Ltd. All rights reserved.

Keywords: Veratryl alcohol; Fenton reaction; Catechol; Brown-rot fungi

1. Introduction

In brown-rot fungi, there are different strategies fordegradation of wood. As opposed to employment ofhydrolytic enzymes to initiate the degradation process,they have a mechanism for reducing Fe(III) that promotesa Fenton reaction (Green and Highley, 1997). Fentonchemistry involves the reaction of Fe(II) with H2O2, inacidic medium, producing a �OH radical as follows:

FeðIIÞ þH2O2! FeðIIIÞ þHO� þOH�. (1)

Two systems that reduce Fe(III) to Fe(II) have beenproposed for wood degradation by brown-rot fungi(Oviedo et al., 2003a) based on dihydroxybenzenes (DHBs)and glycopeptides, with the DHBs-based systems havingreceived most study (Kerem and Hammel, 1999; Paszc-zynski et al., 1999). DHBs reduce Fe(III) to Fe(II) and O2

e front matter r 2005 Elsevier Ltd. All rights reserved.

iod.2005.11.003

ing author. Biotechnology Center, Renewable Resources

niversidad de Concepcion, Casilla 160-C, Concepcion,

6 41 204 601; fax: +56 41 247 517.

ess: jrodrig@udec.cl (J. Rodrıguez).

to H2O2, driving a Fenton reaction (Jensen et al., 2001).Besides these, the participation of cellobiose dehydrogen-ase, which also reduces Fe(III) to Fe(II) and described forseveral white-rot fungi, has been proposed for Coniophera

puteana (Hyde and Wood, 1997) but, as this is not a typicalbrown-rot fungus, it can only be considered an isolatedevent.The enhancement and induction of the Fenton reaction

by catecholate compounds has been studied in diversebiological systems. Pro-oxidant properties by Fe(III)reducing phenolic compounds were found in soybeannodules (Moran et al., 1997). In addition, 2,3-dihydrox-ybenzoic acid and 3,4-dihydroxybenzoic acid have beenused to mimic the fungal mechanism (Goodell et al., 1997;Rodriguez et al., 1999, 2001; Oviedo et al., 2003b).In previous studies (Rodriguez et al., 1999, 2004), the

oxidation of a pulp mill effluent, EDTA and the non-phenolic lignin model veratryl alcohol (3,4-dimethoxyben-zyl alcohol, VA) by different DHB-driven Fenton reactionswere discussed. The maximum VA degradation rate wasobserved using catechol (CAT). Other workers haveconducted research on the degradation of pollutants and

ARTICLE IN PRESSD. Contreras et al. / International Biodeterioration & Biodegradation 57 (2006) 63–6864

xenobiotics (including pentachlorophenol and azo dyes)using the DHB-driven Fenton system as well, as throughthe use of catecholate compounds produced by the brown-rot fungi themselves (Goodell et al., 1997, 2004). The rateof chemiluminescence emission of the DHB-driven Fentonreaction was slower than in a Fe(II) Fenton reaction,suggesting that different mechanisms are involved indifferent systems. The enhanced activated species produc-tion and oxidation of substrates can be understood eitherby the Fe(III) cyclic reduction by CAT or the hydroxylatedintermediary compounds (Chen and Pignatello, 1997, 1999;Kang et al., 2002), or as production of activated speciesdifferent from those involved in a Fenton reaction(Hamilton et al., 1966; Tamagaki et al., 1989) and speciesderived from a Fe(III) peroxo complex (Ensing et al., 2003)stabilised by CAT as chelant (Eq. (2))

½FeðIIIÞCAT�þþH2O2! ½FeðIIIÞðCATÞðOOHÞ� þHþ.

(2)

Despite brown-rot fungi degrades primarily the woodcarbohydrate fraction, the objective of this work was toevaluate the reactivity of a CAT-driven Fenton reactiontowards lignin to better understand its oxidation, using VAas a simple lignin model compound. CAT, FeCl3 and H2O2

concentrations and pH for VA degradation by a CAT-driven Fenton reaction were investigated in order todetermine the optimum ratio between them for VAdegradation.

2. Materials and methods

2.1. Materials

In all experiments and analyses nanopure water (NPW) was used.

Unless otherwise stated, all reagents were p.a. grade. CAT (Sigma); 30%

(w/w) H2O2, sodium formate and formic acid (Fluka); 96% VA (Aldrich);

hydrochloride acid, ferric chloride and ferrous sulphate (Merck); and

HPLC-grade acetonitrile (J.T. Baker) were employed. Fe(II) and Fe(III)

solutions in HCl 0.01M were prepared immediately before each

experiment. H2O2 concentration was determined by titration with a

KMnO4 secondary standard solution.

2.2. Multivariate experiments

For multivariate analysis, response surface methodology (RSM) was

used as given by Barros et al. (2001). This model is based on a central

composite circumscribed design consisting of a factorial design and star

points. The data were analysed using the Modde 7.0 software. The

variable values were coded and normalised in unitary values: �1 is defined

as the lowest value of a variable and +1 as its highest value. From these

extreme values of variables, the central point (coded 0) was set and assayed

in triplicate. The star points were distributed at a distance of n1/2 from the

central point, where n is the number of variables.

The influence of the four following variables was studied: FeCl3concentration (X1), CAT concentration (X2), pH (X3) and H2O2

concentration (X4). The response was VA degradation (Y%), expressed

as a percentage of the initial amount. The initial concentration of VA

(400mM), temperature (25 1C) and reaction time (120min) were kept as

constant parameters. Eight star points distributed at a distance of two

from the central point were carried out.

A second-order function that described the behaviour of the system was

determined by a matricial algorithm. The optimised values of the analysed

variables were obtained from the maximum values of this function using

the ‘‘optimiser tool’’ from the Modde 7.0 software. The statistical

validation was performed by an ANOVA test with 95% confidence

level.

The different concentrations of CAT, FeCl3 and H2O2, and pH values,

are given in Table 1. The pH of the reactions was adjusted with 3.7% HCl.

All experiments were performed as indicated in Section 2.3, and the VA

concentration was determined at reaction time 150min as described in

Section 2.4. The pH values were monitored along the reaction. From the

beginning until the end of the runs the pH valued did not change more

than 0.2 units.

2.3. Time-course measurement

To assess the progress of VA degradation, the concentration of VA was

measured at 15-min-intervals over 150min. This procedure was performed

at central (0 codified experiments in Table 1) conditions to determine when

the reaction reached equilibrium. A time-course kinetic was also

performed under the conditions shown in Table 2 to check the optimised

VA degradation values obtained from the multivariate analyses and make

comparison with those of a Fe(II) Fenton reaction.

2.4. Chemical analysis

VA was quantified by reverse-phase HPLC, using a Lichrocart 125-4/

Lichrospher 100 RP-18 (particle diameter 5 mm) column in a HPLC pump

(Knauer 64) with a constant flow of 1mLmin�1. The mobile phase was

acetonitrile:methanol:formic acid (100:900:1). A 20-mL loop was used.

Detection was performed with a UV-VIS detector (Knauer Variable

Wavelength Monitor) at 277 nm with a digital integrator (PE-Nelson

Model 1022). Total organic carbon (TOC) was determined using a total

carbon analyser (TOC-VCPN, Shimadzu).

2.5. Speciation calculus

All the speciation calculus at different pH and CAT concentrations

were performed with the software ‘‘CHEAQS pro V. 2004.1’’ written by

Wilko Verweij (http://home.tiscali.nl/cheaqs).

3. Results and discussion

3.1. Multivariate analysis

From the results of the factorial planning experiments(Table 1) a polynomial response to VA degradation wasobtained (Eq. 3), which considers the relative importanceand interactions between the different variables. Thecoefficients were normalised according to codified variablevalues

Y% ¼ 78ð�8:8Þ þ 9:5X1ð�4:1Þ þ 15X2ð�4:1Þ

� 19X3ð�4:1Þ þ 6:8X4ð�4:1Þ � 11X21ð�3:9Þ

� 7:3X22ð�3:9Þ � 11X2

3ð�3:9Þ

þ 12X1X2ð�4:9Þ þ 19X1X3ð�4:9Þ, ð3Þ

where Y is the VA degradation, X1 the FeCl3 concentra-tion, X2 the CAT concentration, X3 the pH, and X4 theH2O2 concentration.For a better understanding of the importance of each

variable, their coefficients in the polynomial were plotted in

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Table 2

Optimised values of variables obtained from factorial analysis

Variable Relation Value

VA 1 400a

Fe(III) (X1) 0.375 150a

CAT (X2) 0.375 150a

pH (X3) – 3.4

H2O2 (X4) 5 2000a

aUnits: 10�6 molar.

-30

-20

-10

0

10

20

30

Fe-pH pH CAT Fe-CAT pH-pH Fe(III)-Fe(III) Fe(III) CAT-CAT H2O2

Coe

ffici

ent v

alue

Fig. 1. Coefficients from the response surface placed in increasing order

from the left, with error bars showing 95% confidence levels.

Table 1

Calculated and experimental results of VA degradation by the CAT-driven Fenton reaction

Fe(III) (X1)a CAT (X2)

a pH (X3) H2O2 (X4)a Experimental resultsb Calculated resultsb

100 (�1) 55 (�1) 3 (�1) 1000 (�1) 75 68

200 (+1) 55 (�1) 3 (�1) 1000 (�1) 77c 15

100 (�1) 155 (+1) 3 (�1) 1000 (�1) 75 74

200 (+1) 155 (+1) 3 (�1) 1000 (�1) 72 74

100 (�1) 55 (�1) 5 (+1) 1000 (�1) 0 �9.4

200 (+1) 55 (�1) 5 (+1) 1000 (�1) 30 21

100 (�1) 155 (+1) 5 (+1) 1000 (�1) 5.6 �2.9

200 (+1) 155 (+1) 5 (+1) 1000 (�1) 69 80

100 (�1) 55 (�1) 3 (�1) 2000 (+1) 88 82

200 (+1) 55 (�1) 3 (�1) 2000 (+1) 26 30

100 (�1) 155 (+1) 3 (�1) 2000 (+1) 90 88

200 (+1) 155 (+1) 3 (�1) 2000 (+1) 91 89

100 (�1) 55 (�1) 5 (+1) 2000 (+1) 3 4.8

200 (+1) 55 (�1) 5 (+1) 2000 (+1) 23 36

100 (�1) 155 (+1) 5 (+1) 2000 (+1) 9.7 11

200 (+1) 155 (+1) 5 (+1) 2000 (+1) 89 95

50 (�2) 105 (0) 4 (0) 1500 (0) 1.9 17

250 (+2) 105 (0) 4 (0) 1500 (0) 59 48

150 (0) 5 (�2) 4 (0) 1500 (0) 3.2 14

150 (0) 205 (+2) 4 (0) 1500 (0) 80 80

150 (0) 105 (0) 2 (�2) 1500 (0) 64 67

150 (0) 105 (0) 6 (+2) 1500 (0) 0 �4.2

150 (0) 105 (0) 4 (0) 500 (�2) 50 64

150 (0) 105 (0) 4 (0) 2500 (+2) 95 93

150 (0) 105 (0) 4 (0) 1500 (0) 86 79

150 (0) 105 (0) 4 (0) 1500 (0) 83 79

150 (0) 105 (0) 4 (0) 1500 (0) 86 79

aUnits: 10�6 molar.bPercentage of initial amount.cExclude value.

D. Contreras et al. / International Biodeterioration & Biodegradation 57 (2006) 63–68 65

decreasing order (Fig. 1). Considering the first-order andthe interaction coefficient of each variable in Eq. (3) andFig. 1, it can be seen that, the main effects are due to theFe(III)–pH (X1X3) interaction and pH (X3), in accordancewith the importance of the Fe(III) solubility (whichdepends of the pH) in Fenton reaction. The effect ofCAT concentration (X2) is higher than those of Fe(III) (X1)and H2O2 (X4) concentration.The positive coefficient of Fe(III) (X1), CAT (X2) and

H2O2 (X4) implies that there is a positive influence on VAdegradation with an increase in these variable values.The negative second-order coefficient of Fe(III) ðX2

1Þ, pHðX2

3Þ and CAT ðX22Þ indicates a maximum region described

by a parabola. The second-order coefficient of H2O2 iszero, indicating the absence of a maximum region and alinear behaviour described by a positive slope linearfunction.The negative first-order coefficient for pH implies a

negative effect of this parameter, hence, at higher pHvalues there would be less VA degradation. This iscorrelated with the decreasing solubility of Fe(III) as pHincreases.The coefficient of Fe(III)–CAT (X1X2) shows a positive

interaction between these variables, being more importantthan the effect of Fe(III) (X1) and H2O2 (X4) bythemselves.

ARTICLE IN PRESS

Fig. 2. Response surfaces for the VA degradation: plot A as a function of

pH and CAT; plot B as a function of pH and Fe(III); and plot C as a

function of CAT and Fe(III) concentration. The experimental concentra-

tions assayed are inside the rectangle area formed by the dotted lines.

Optimised values chosen are showed by a black point.

D. Contreras et al. / International Biodeterioration & Biodegradation 57 (2006) 63–6866

In Fig. 2, the contour plots of the polynomial responseare shown. In these graphs two variables are plotted, whilemaintaining the other two variables constant at optimisedvalues (Table 2). The contour plots are predicted by thepolynomial response (Eq. 3), where the experimentalassayed area is inside the rectangle formed by the dottedlines. In this area the validity of the model is guaranteed(confidence 95%), but outside this it must be consideredwith caution. The regions of maximum VA degradation areshown by a 100% layer, and values chosen for eachvariable are shown by black points in their respective area(Fig. 2).

In plot A of Fig. 3, which shows VA degradation as afunction of CAT (X2) and pH (X3), a maximum region canbe observed with CAT concentrations ranging from 140 to173 mM and pH from 3.0 to 3.5. VA degradation as afunction of Fe(III) concentration and pH is shown in plotB. There is an elliptical maximum with the Fe(III)concentration varying from 141 to 239 mM and the pHranging from 3.0 to 5.0. In plot C, the CAT–Fe(III)interaction is shown, in which maximum degradation is at139–214 mM for CAT and 147–226 mM for Fe(III). Theregion of maximum VA degradation is an elliptical areasimilar to that described for the Fe(III) and pH interaction.The CAT–Fe(III) interaction coefficient from the poly-nomial and the optimised values for CAT and Fe(III)concentration (Table 2) underline the importance of the 1:1ratio of CAT:Fe(III), since this monoligand complex[Fe(III)CAT]þ is the main species that interacts withH2O2 (Hamilton et al., 1966; Tamagaki et al., 1989).Similar results have been found for the Fe(III):ligand, witheither EDTA or CDTA, where the importance lies on a 1:1ratio in the Fenton system reactivity (Engelmann et al.,2003). Qian et al. (2002) have also found that at DHB:ironratios 41:1, the excess chelator will inhibit iron reduction,as the hexadentate ligand is formed to coordinate iron andlimit its reactivity.

A low pH value (optimum c. pH 3.0) in the Fenton-likereaction has been correlated with the amount of[Fe(III)–OOH]+2 complex formation (Safarzadeh-Amiriet al., 1996), but in the CAT-driven Fenton reaction theoptimum pH for VA degradation is higher (c. pH 3.4). Therelative concentrations of the different Fe(III) speciesat pH 1–5 and the predicted VA degradation is plotted(Fig. 3A). The predicted degradation curve shows amaximum region around pH 3.4. This is correlated withthe pH range where [Fe(III)CAT]+ species prevails over[Fe(OH)]+2 and [Fe(OH)2]

+ complexes. The speciation infunction of CAT concentration (Fig. 3B) shows that the[Fe(III)CAT]+ concentration is higher than the otherFe(III) species starting from 150mM CAT concentration(at pH 3.4), but the highest VA degradation is obtained near150mM CAT, decreasing at higher concentrations. Thismeans that the prevalence of the [Fe(III)CAT]+ species isnecessary to obtain the best VA degradation. However, theCAT:Fe(III) ratio must be around 1:1, since at higher ratios,CAT could compete with the VA as a substrate.

ARTICLE IN PRESS

0

20

40

60

80

100

pH

%

Fe+3

Fe(OH)+2

Fe(OH)2+

Fe(CAT)+

Predicted VADegradation

1 2 3 4 5

(A)

0

20

40

60

80

100

05 0 100 150 200 250 300

CAT( M)

%

Fe+3

Fe(OH)+2

Fe(OH)2+

Fe(CAT)+

Predicted VA Degradation

(B)

Fig. 3. The pH dependence of Fe(III) speciation and predicted VA

degradation with pH (plot A), and CAT concentration dependence of

Fe(III) speciation (black lines) and predicted VA degradation (grey line)

with pH (plot B). In both plots, VA degradation was calculated using the

polynomial response.

D. Contreras et al. / International Biodeterioration & Biodegradation 57 (2006) 63–68 67

3.2. Degradation of VA at optimised conditions

The best conditions predicted for VA degradation by theresponse surfaces are shown in Table 2. Under theseconditions, a time-course degradation was carried out andcompared with a Fe(II) Fenton reaction. The CAT-drivenFenton reaction reached VA degradation and TOCremoval levels of 95% and 38%, respectively, at a reactiontime of 1 h. While, the neat Fe(II) Fenton reaction yielded25% VA degradation and o1% TOC removal at the sametime and concentration.

In preliminary experiments under similar conditions asdescribed above, VA aromatic ring hydroxylation pro-ducts, demethoxylated products and benzyl alcohol groupoxidation products to aldehydes and carboxylic acids werefound (determined by gas chromatography with a massspectrometer detector). No significant difference betweenthe VA degradation products by the treatment with a

CAT/Fe(III)/H2O2 system or a Fe(II) Fenton reactioncould be observed.

4. Conclusions

The multivariate analysis shows a statistically validatedresponse surface, which allowed to find the best reactionconditions for VA degradation. This experimental condi-tion yielded 3.8 times greater VA degradation and almost40 times greater mineralisation than a Fe(II) Fentonreaction. The polynomial response shows an importantdependence between CAT–Fe(III) concentration andFe(III) concentration and pH.The enhanced degradation of VA by a CAT-driven

Fenton reaction can be associated with the directparticipation of the [Fe(III)CAT]+ species and the cyclicreduction of Fe(III) to Fe(II) by CAT and their reactionproducts.Further research is being carried out in our laboratory to

verify if the same conditions for maximum lignin degrada-tion are valid for cellulose degradation by the CAT-drivenFenton reaction. These results could contribute to under-stand the different abilities of brown-rot fungi to degradedifferent components of wood.

Acknowledgements

Authors are grateful to Dr. Sofıa Valenzuela for hercritical revision and comments of the manuscript. Thefinancial support for this work was provided by Fondecyt(Grant No. 1010840) and Conicyt (Ph.D. Fellowship 2003to David Contreras).

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