Degradation of acrylic copolymers by Fenton’s reagent

Carsten Maia,*, Andrzej Majcherczykb, Wiebke Schormannb, Aloys Huttermannb

aInstitute of Wood Biology and Wood Technology, Busgenweg 4, 37077 Gottingen, GermanybInstitute of Forest Botany, University of Gottingen, Busgenweg 2, 37077 Gottingen, Germany

Received 17 May 2001; received in revised form 25 July 2001; accepted 4 August 2001

Abstract

The degradation of different copolymers of acrylamide and acrylic acid by Fenton’s reagent was studied. The polymers testedwere either homopolymers or copolymers containing lignin sulfonate, guaiacol or 3,4-dihydroxylbenzoic acid, respectively. Acryl-amide copolymers (PAAm) were degraded faster than polymers of acrylic acid (PAA). Among the PAAm, the copolymers of ligninsulfonate and guaiacol were degraded at a significantly higher rate than the corresponding homopolymer, whereas among the PAA,

the rate of degradation was highest with copolymers of guaiacol and 3,4-dihydroxylbenzoic acid. The decrease of H2O2, i.e. the rateof hydroxyl radical production in the presence of a certain polymer, did not correlate with the rate of its degradation. It was con-cluded that the incorporation of lignin and certain phenolic compounds into an acrylic chain may accelerate the decay of these

polymers by wood decaying fungi, which reportedly produce hydroxyl radicals extra-cellularly, and through the use of advancedoxidation systems applied in sewage cleaning. # 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Fenton’s reagent; Lignin copolymer; Acrylic; Degradation

1. Introduction

Water soluble, poly(acrylic acid) and polyacrylamideare highly resistant to biodegradation. One method ofenhancing the degradability of such non-hydrolyzablemacromolecules entails grafting components which arereadily degradable into the main polymer backbone.Several attempts have been made in which some natu-rally occurring polymers of plant or microbial origin,such as lignin [1–13], starch [14], cellulose [15], andpoly(hydroxylbutyric acid) [16] were introduced into asynthetic polymer structure. The naturally occurringfractions of the resulting products have shown appre-ciable biodegradibility.The main property of macromolecules which compli-

cates biodegradation by microorganisms is their highmolecular weight. It prevents the passage of the poly-mers through the plasma membrane of microbial cellsso that a preceding extra-cellular reduction to smallpieces prior to intra-cellular mineralization is necessary.This extra-cellular degradation step can be brought

about by extra-cellular enzymes or chemically, e.g. byreduced oxygen species, such as hydroxyl radicals.In the present study, the decomposition of acrylic

copolymers which contain lignin sulfonate or lignin-likemonomeric phenolics by Fenton’s reagent was investi-gated. The hydroxyl radical possesses an extremely highredox potential for the one-electron reduction to water[17]:

HO� þHþ þ e� ! H2O E0 ¼ þ2180 mV

Any oxidant with a higher redox potential wouldreact with water forming hydroxyl radicals. For thisreason, the hydroxyl radical is the strongest oxidant inbiological systems. It has been suggested that hydroxylradicals are involved in the degradation of both lignin(by white-rot fungi) and cellulose (by brown-rot fungi).The degradation of 14C-labeled lignin by UV/H2O2,Fenton’s reagent, photosensitized rivoflavin as well asby UV- and g-irradiation has been previously described[18]. Both 14C-methoxy labeled lignin and ring- andside-chain lignins were extensively degraded by bothphotosensitized rivoflavin and UV/H2O2. A rapid,nearly complete degradation was observed withFenton’s reagent.

0141-3910/01/$ - see front matter # 2001 Elsevier Science Ltd. All rights reserved.

PI I : S0141-3910(01 )00209-9

Polymer Degradation and Stability 75 (2002) 107–112

www.elsevier.com/locate/polydegstab

* Corresponding author. Tel.: +49-551-393484; fax: +49-551-

392705.

E-mail address: cmai@gwdg.de (C. Mai).

Koenigs [19] suggested that the initial attack on crys-talline cellulose in wood by brown-rot fungi proceedsthrough an H2O2/Fe

2+ system. The breakdown of cel-lulose molecules in vitro at a biological pH was firstdescribed by Halliwell [20]. The degradation of water-soluble polymers such as PAA, poly(methacrylic acid)and poly(vinylpyrrolidone) by UV/H2O2 was recentlyreported [21].

2. Materials and methods

2.1. Copolymerization of acrylic and lignin sulfonate/phenolic compounds

The co-polymerization of acrylamide (AAm) andacrylic acid (AA) onto lignin sulfonate (LS) was per-formed as has already been described [12]. A solution of160 mg lignosulfonate and 1 g (14.07 mmol) of acryl-amide or 1.051 g (13.88 mmol) of acrylic acid in 8 ml ofwater was adjusted to pH 4.5 (acrylamide) or 4 (acrylicacid) with 1 M NaOH or 0.1 M H2SO4. Subsequently,50 ml of a laccase solution (12 units, final concentration)was added and the reaction was started by the additionof 20 ml (0.15 mmol) of t-butylhydroperoxide (t-BHP,70%). The reaction mixture was incubated for 24 h at30 �C, diluted with 50 ml of distilled water, dialyzed for48 h and thereafter reduced to about 40 ml using arotary evaporator. The remaining solution was freeze-dried to recover the lignin-polyacrylic copolymer.Acrylamide and acrylic acid copolymers of guaiacol

and 3,4-dihydroxylbenzoic acid were produced by thelaccase/Fe2+/t-butylhydroperoxide initiation system[22]. The synthesis procedure was identical to that of thelignin copolymers, but lignin sulfonate was replaced by97 mg (0.78 mmol) of guaiacol and 120 mg (0.78 mmol)of 3,4-dihydroxylbenzoic acid. Additionally, 0.7 mg(1.79 mmol) of (NH4)2Fe(SO4)2. 6 H2O were added to thereaction mixture as a solution in 100 ml of water.

2.2. Molecular weight determination

The average molecular weight (M� w) of the copolymerswas analyzed by gel permeation chromatography (GPC)[12]. The high-performance liquid chromatography sys-tem was HP 1090 (Hewlett-Packard, Waldbronn, Ger-many) with an automatic injector, a diode arraydetector, and a Pascal Work Station with Hewlett-Packard GPC-Software. The column set consisted oftwo 7.8�300 mm styrene-divinylbenzene 16 mm TSK-columns, G6000 PWxl and G5000 PWxl (Toso-Haas,Stuttgard, Germany), one 8�300 mm hydro-xylethylmethacrylate column, HEMA BIO 1000 (10mm), and a HEMA BIO 1000 guard column (PolymerStandard Service PPS, Mainz).

The eluent, 0.15 M NaCl solution in highly purifiedwater (Water-Purification-System ’’Milli-Q’’, Millipor,Eschborn, Germany), was filtered through a 4.5 mmcellulose acetate filter prior to analysis. The analysis wasrun in an isocratic mode with a flow-rate of 1 ml min�1

at a constant temperature of 40 �C. The sample con-centration was 5 mg ml�1; an injection volume of 20 mlwas generally used. Benzene sulfonic acid was used asan internal standard at a concentration of 0.1 mg ml�1.The system was calibrated with 10 poly(acrylic acid-Na-salt) standards (molecular weight at peak maximum,M� p, 1 100 000, 467 300, 193 000, 130 500, 62 900, 35000, 16 000, 7500, 4000, 855). The elution of the copo-lymers was detected by absorption at 220 and 280 nmwith a 4 nm band width; poly(acrylic acid-Na-salt)standards were detected at 220 nm.

2.3. Incubation of the co-polymers with Fenton’sreagents

The studies on the degradation of acrylic co-polymerswere performed with regard to the catalytic degradationof cellulose via the H2O2/Fe

2+ systems reported byHalliwell [20]. In 6.7 ml of sodium acetate buffer (0.2 M)with a pH of 4.2, 75 mg amounts of the co-polymerisatewere dissolved at a time and 6.7 ml of a hydrogen per-oxide solution (1 vol.%) was added. The Fenton reac-tion was started by the addition of 0.34 ml of a 10 mMsolution of FeSO4

.7 H2O in water; the concentration ofoxygen in the solution can be estimated to be 3.2 10�4

mol l�1 (at 25 �C) [23]. The reaction solutions were keptin a dark water bath at 30 �C. At intervals noted inResults and discussion, 1 ml of the solution was extrac-ted to determine the average molecular weight (M� w) ofthe polymers. To stop the reaction, 0.1 ml of a solutionwas added which contained 5 mg of the iron chelatordesferrioxamine and 5 mg of hydroquinone in 1 ml ofwater.

2.4. Determination of hydrogen peroxide

The concentration of hydrogen peroxide was deter-mined iodometrically. 1 ml of the peroxide-containingsolution was dissolved in 50 ml of water, 1 g potassiumiodide was added and the solution was acidified with 1ml of hydrochloric acid (2 M). The solution was kept inthe dark for 30 min. Subsequently, the brown reactionmixture was titrated with 0.02 M thiosulfate solution toa faint yellow color and after the addition of 1 ml starchsolution (0.2 wt.%) the titration was continued until theblue color of the starch–I2 complex disappeared.

2.5. Chemicals

The calcium lignin sulfonate (Wafex CAM) was acommercial product from Holmen (Sweden). The hemi-

108 C. Mai et al. / Polymer Degradation and Stability 75 (2002) 107–112

cellulose content of Wafex CAM amounted to 3%. Allother chemicals used in this study were obtained fromMerck (Darmstadt, Germany) and were of analyticalpurity.

3. Results and discussion

Different copolymers of acrylamide and acrylic acidwhich contained lignin sulfonate, guaiacol and 3,4-dihydroxylbenzoic acid were synthesized in the presenceof peroxides, a phenoloxidase (laccase) and in the caseof guaiacol and 3,4-dihydroxylbenzoic acid of an ironsalt. The contents of the incorporated phenolic com-pounds in the acrylamide copolymers amounted to10.5% (lignin sulfonate) [12], 5.6% (guaiacol) and 2.2%(3,4-dihydroxylbenzoic acid) [22]. The composition ofthe copolymers of acrylic acid could not be determinedby elemental analysis due to the absence of nitrogen inboth components, but it can be expected that the pro-portion of incorporated phenolics is similar to that ofthe acrylamide copolymers. The HPLC chromatogramsof the acrylamide copolymers (Fig. 2, left column) wererecorded at two different wavelength: at 220 nm bothcomponents are detected, whereas at 280 nm only theincorporated phenolics show an absorption. A compar-ison of the pattern at the two wavelengths shows thatthe phenols are equally distributed over the wholepolymer range and that they can also be found in thehigh molecular weight fraction. Since lignin sulfonate isa phenylpropanoid polymer with an average molecularweight (M� w) of 35 800 g mol

�1 [24], it can be expectedthat lignin forms the centre of the molecules and thatseveral acrylic side chains are attached (grafted) to it. Incase of the simple phenolics it is not clear if they arelocated at the end of an acrylic chain or if they areincorporated into the main chain.As shown in Fig. 1A, all acrylamide (AAm) polymers

tested were appreciably degraded by the Fentonreagent; however, the variance of kinetic of degradationdisplayed a dependence on the incorporated phenoliccompound. Thus, the decrease of the average molecularweight (M� w) of the poly(lignin sulfonate-AAm) and thepoly(guaiacol-AAm) proceeded with a far higher ratethan that of the polyacrylamide homopolymer (PAAm)and that of poly(3,4-dihydroxyl benzoic acid-AAm).Within 24 h the M� w of poly(lignin sulfonate-AAm)decreased to 2.3% and the M� w of poly(guaiacol-AAm)to 5.8% of the initial value. In contrast, the corre-sponding M� w values of PAAm and poly(3,4-dihydroxylbenzoic acid-AAm) amounted to 52.6 and 46.7%,respectively. These distinct differences in the alterationofM� w among the AAm (co-)polymers are also visible inthe HPLC chromatograms at the onset and after 24 h ofincubation with Fenton’s reagents (Fig. 2). The two bigpeaks which appear at retention times of about 33 and

38 min come from desferrioxamine and hydroquinone,respectively; these compounds were added to stop thecatalytic cycle. The iron chelator desferrioxamine wasable to prevent the iron-catalyzed formation of thehydoxy radical from H2O2 and the superoxide radical(O2

�.) [25]; it also inhibited initiation through a Fe2+/t-butylhydroperoxide initiation system [26].In the case of the phenolic copolymers, the incubation

with Fenton’s reagent brought about a complete deco-loration of the solution after several days of incubation.The degradation of the acrylic acid (AA) copolymer

proceeded with a clearly lower rate than the (co)poly-mer species of AAm (Fig. 1B). Here the initial M� w ofacrylic acid homopolymer (PAA) was reduced by 32.8%within 5 days compared to 86.6% in the case of PAAm;the decrease of M� w within 24 h amounted to only10.2%. A similar extent of degradation was observedwith poly(lignin sulfonate-AA) which showed a decreaseofM� w of 31.2% within 5 days and of 3.2% within 24 h.Compared to the corresponding poly(lignin sulfonate-AAm) in which a 97.7% reduction occurred, the differ-ences in the degree of degradation are very drastic.The highest rate of degradation by Fenton’s reagents

among the AA polymers after 5 days was observed with

Fig. 1. Time course of the alteration of the average molecular weight

(M� w) of acrylamide (A) and acrylic acid (B) homo- and co-polymers,

respectively, caused by Fenton’s reagent.

C. Mai et al. / Polymer Degradation and Stability 75 (2002) 107–112 109

Fig. 2. HPLC chromatograms of different acrylamide copolymers at the onset and after incubation with Fenton’s reagent over 24 h (high absorp-

tion: 220 nm, low absorption 280 nm; left: before the incubation, right: after incubation. A: homopolymer, B: poly(guaiacol-AAm), C: poly(lignin

sulfonate-AAm), D: poly(3,4-dihydroxybenzoic acid-AAm).

Fig. 3. Residual amount of hydrogen peroxide in the reaction medium (% of initial concentration) after 5 days of incubation of the copolymerizates

with Fenton’s reagent (A: acrylamide, B: acrylic acid).

110 C. Mai et al. / Polymer Degradation and Stability 75 (2002) 107–112

poly(guaiacol-AA) and poly(3,4-dihydroxyl benzoicacid-AAm) whose M� w was reduced by 55.5 and 46.9%,respectively. After 24 h, the decrease of M� w was 25.6%[poly(guaiacol-AA)] and 32.4% [poly(3,4-dihydroxylbenzoic acid-AAm)], respectively.In comparison to the AAm polymers, the degree of

decolorization was much lower than it was when theAA polymers were tested. Obviously, the rate of theFenton reaction was slower, i.e. fewer hydroxyl radicalswere produced in the PAA reaction mixture. To estab-lish whether the variable behavior was influenced by therate of hydroxyl radical production, we determined theamount of hydrogen peroxide remaining in the reactionmixture after 5 days of incubation (Fig. 3). No sig-nificant differences were observed between the corre-sponding AAm and AA (co)polymers and nocorrelation was established between the amount ofhydrogen peroxide remaining and the rate of degrada-tion. Apparently, the higher rate of PAAm degradationis not a consequence of a higher rate of hydroxyl radicalproduction. A comparison among the AAm (co)poly-mers revealed that a greater decrease of H2O2 occured inthe presence of those polymer species which were degra-ded at a low rate [homopolymer, poly(3,4-dihydroxylbenzoic acid-AAm)].

4. Conclusions

The results of acrylic polymer degradation by Fen-ton’s reagent show that hydroxyl radicals are able tosignificantly reduce the average molecular weight ofboth the homopolymers and the phenolic copolymers.This degradation corresponds to the ability of thehydroxyl radicals to cause an abstraction of hydrogen insp3-hybridized carbon atoms as previously described[27], subsequently inducing a cleavage of the polymerchain. An acceleration of the degradation as a con-sequence of lignin sulfonate grafting or of the incor-poration of guaiacol into acrylic chains could be due tothe fact that incorporated phenolic compounds reactmore easily with hydroxyl radicals than aliphatic chains.Possible reactions which can bring about the degrada-tion of the lignin macromolecule or simple phenoliccompounds include: (1) the hydroxylation of aromaticrings [28], (2) the demethoxylation of methoxylizedaromatic rings [29], (3) the decarboxylation of aromaticacids [30], or (4) aromatic ring opening [30]. Using lig-nin model compounds (cresol, 4-methoxyveratrole),Gierer et al. [30] have reported the occurrence of thesetypes of reactions; however, ring opening products wereonly produced in small amounts. Since the reactions inour studies were carried out in the presence of oxygen,the presence of hydroxyl radicals could additionally leadto the oxidation of both components of the polymers[31].

Our studies show that polymers of acrylamide andacrylic acid can be degraded in vitro by hydroxyl radi-cals produced via the Fenton reaction. This is especiallytrue for acrylamide copolymers of lignin sulfonate andguaiacol and acrylic acid copolymers of guaiacol. Thesecompounds were degraded at a significantly higher ratethan the corresponding homopolymer. This finding canbe explained by the fact that the incorporated phenolics(lignin, guaiacol) react faster with hydroxyl radicalsthan sp3-hybridized carbon bonds in polyacrylic chains.Hydroxyl radicals are the strongest oxidants in biolo-

gical systems and have been assumed to be involved inthe degradation of lignin by white-rot fungi [18,32] aswell as in the degradation of cellulose by brown-rotfungi [19]. Our data show that these fungi, which areable to produce hydroxyl radicals, have the potentialability of degrading acrylic polymers. In vivo studies onthe degradation of such polymers by wood decayingfungi are in progress.Water-soluble polyacrylates and polyacrylamides are

of particular interest with regard to their industrialapplication, e.g. in the treatment of industrial effluents,such as drilling muds, co-builders in washing powdersor dispersion agents. Some of these polymers are foundin municipal waste waters. The degradation of water-soluble polymers such as PAA, poly(methacrylic acid)and poly(vinylpyrrolidone) by the photo-oxidativedegradation in the presence of H2O2 was recentlydescribed. Advanced oxidation systems which producehighly reactive oxygen species are already being used,e.g. in the degradation of phenolic and polyphenolicbleaching effluents in the pulp and paper industry [33].In addition, the light-enhanced Fenton reaction is beingused for the treatment of industrial waste waters todegrade toxic aromatic and aliphatic compounds. Thegrafting of acrylic side chains onto lignin and theincorporation of acrylic polymers with phenolic com-pounds present possibilities of enhancing degradabilityin processes where such oxidative systems are used. Theuse of such polymers can thus lead to a reduction in energycosts and in the application of hazardous chemicals.

Acknowledgements

We are grateful to the ‘‘Chemische Fabrik Stock-hausen GmbH&Co.KG’’ for all the help and theirfinancial support. In addition, we would like to thankDr. F. Krause and D. Starr for their sound advice.

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