Oxidation of Cellulose and Carboxylic Acids by

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In this paper, the kinetics of hypochlorous acid oxidation of carbohydrates and three low-molecular-weight carboxylic ... HOCl can react easily with many pulp compo- ... with an A/DED sequence (Table I). ..... It is well known that ... equivalents of carbon dioxide (CO2), three .... actions are believed to undergo an analogous.

Oxidation of Cellulose and Carboxylic Acids by Hypochlorous Acid: Kinetics and Mechanisms Z. ZHOU, A.-S. JÄÄSKELÄINEN and T. VUORINEN

Hypochlorous acid (HOCl) is formed in chlorine dioxide bleaching, and it may react with lignin, hexenuronic acid, carbohydrates and dissolved organic compounds as a strong oxidant. In this paper, the kinetics of hypochlorous acid oxidation of carbohydrates and three low-molecular-weight carboxylic acids were investigated. Treatment of fully-bleached eucalyptus kraft pulp with HOCl under acidic conditions showed that pulp carbohydrates are oxidized, especially at high temperatures. In addition, three carboxylic acids (formic, oxalic and glycolic acids) reacted quickly with HOCl. All of these reactions followed first order reaction kinetics with respect to HOCl. Oxalic acid reacted most easily with HOCl, followed by formic and glycolic acids. The activation energy of HOCl oxidation was estimated to be 55±2 kJ/mol for cellulose, whereas it was 61±1 and 88±2 kJ/mol for formic and glycolic acids, respectively. Hypochlorous acid (HOCl) is formed in chlorine dioxide bleaching, and it may react with lignin, hexenuronic acid, carbohydrates and dissolved organic compounds as a strong oxidant. In this paper, the kinetics of hypochlorous acid oxidation of carbohydrates and three low-molecular-weight carboxylic acids were investigated. Treatment of fully-bleached eucalyptus kraft pulp with HOCl under acidic conditions showed that pulp carbohydrates are oxidized, especially at high temperatures. In addition, three carboxylic acids (formic, oxalic and glycolic acids) reacted quickly with HOCl. All of these reactions followed first order reaction kinetics with respect to HOCl. Oxalic acid reacted most easily with HOCl, followed by formic and glycolic acids. The activation energy of HOCl oxidation was estimated to be 55±2 kJ/mol for cellulose, whereas it was 61±1 and 88±2 kJ/mol for formic and glycolic acids, respectively

INTRODUCTION The chemistry of chlorine dioxide bleaching is rather complicated, because several inorganic chlorine compounds are formed [1-3], such as chlorate (ClO3⎯), chlorite (ClO2⎯), hypochlorous acid (HOCl) and elemental chlorine (Cl2). These intermediates may react with pulp components and dissolved molecules and in that way influence the overall bleaching efficiency and the final pulp quality. Among these chlorine-containing compounds hypochlorous acid (HOCl) is a very strong oxidant, and it has been proved to react with many Z. Zhou, A.-S. Jääskeläinen and T. Vuorinen Dept. Forest Products Technology Helsinki University of Technology Otakaari 7, P.O.Box 3320 Espoo 02015 Finland (e-mail: [email protected])

organic structures [4]. One equivalent of HOCl is formed for every two equivalents of ClO2 consumed [1,4-5], and it therefore plays an important role in chlorine dioxide bleaching. HOCl can react easily with many pulp components, especially with lignin and hexenuronic acid (HexA) [5,6]. In the final bleaching stages, where the contents of lignin and HexA are very low, HOCl may also attack cellulose (and/or hemicelluloses) causing oxidation and degradation [7,8]. The oxidation of cotton cellulose by hypochlorite was studied by a few researchers in the 1960’s [9-11]. Most of those studies were performed within a pH range of 5-10, and some others within a higher pH range of 10-12. The extent of oxidation was found to be pH dependent. According to these studies, the oxidation of cellulose by hypochlorite was most effective around pH 7. This correlated quite well to an earlier study, where the maximum rate of oxy-

cellulose formation was reported to occur at neutral pH [12]. In that work, it was observed that the consumption of chlorine reached a maximum at pH 2 and 7, while clear minimum values were obtained at pH 1 and 4-5 [12]. Beside the oxidation of cellulose by hypochlorite, the oxidation of cellulose by aqueous chlorine solutions at pH 1-7 has been studied. Under these conditions, chlorine is in equilibrium with HOCl (known also as chlorine hydrolysis) shown by equilibrium (1) K1 Cl2 + 2 H2O

HOCl + H3O++Cl-

(1)

At pH 3-5 HOCl is the dominant species in the solution, while at or below pH 2.5 elemental chlorine is the primary species [13]. The influence of pH was investigated as well on the oxidation rate of low-molecular-weight

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carbohydrates as cellulose models [14,15], in addition to cellulose oxidation by aqueous chlorine [16]. It was concluded that chlorine oxidation might take place via both ionic and radical reactions, and that the final oxidized products were dependent on the pH values [17]. In addition to the pulp, some low-molecular-weight components may also contribute to the consumption of HOCl. In modern pulp mills which produce bleached chemical pulps, increased closure of the bleach plant water system has led to the increased concentrations of a variety of dissolved organic substances [18]. These low molecular weight (Mw) compounds originate from degraded pulp components, and exist in process waters. For instance oxalic, formic and glycolic acids have been commonly identified in some bleaching effluents [19,21]. Therefore, it is of interest to find out how these low Mw carbohydrate compounds interact with HOCl. In this work, the reactivity of cellulose (and/or hemicelluloses) and three low-molecular-weight carboxylic acids towards HOCl was examined. The kinetics and mechanisms are discussed in detail. The knowledge would help to add these reactions to the computational simulation of chlorine dioxide bleaching of pulp [22].

EXPERIMENTAL Cellulose pulp A mill-cooked, 2-stage O2-delignified eucalyptus (Eucalyptus urograndis) kraft pulp was bleached in laboratory with an A/DED sequence (Table I). The fully bleached pulp had a kappa number of 0.9, a brightness of 88.8 ISO %, and a viscosity of 890 mL/g. The lignin and HexA contents in the bleached pulp were 0.5 % and 4 mmol/kg, respectively, as determined by UV resonance Raman (UVRR) spectroscopy from handsheets [23]. The content of hemicelluloses was determined by gas chromatography (GC) after total acid hydrolysis [24]. The pulp contained about 12% of xylans after A/DED bleaching treatment, and this figure decreased to be 7% after alkaline extraction applied to partly remove the xylans. Chemicals Sodium hypochlorite (2.5 % NaOCl, Fluka Chemika, Switzerland) was commercially purchased. Acetate (pH 5, 1 M) and phosphate (pH 3, 0.2 M) buffers were prepared in the laboratory from acetic acid (100 %, Bang & Bonsomer Oy, Finland), sodium acetate (99.0 %, Fluka Chemika, Switzerland), phosphoric acid (85 %, VWR International Oy, Finland) and sodium phosphate (98.0 % Na3PO4·12H2O, J.T.Baker Chemicals, Holland).

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TABLE I A/DED BLEACHING CONDITIONS AT PULP CONSISTENCY OF 10% A

D0

Temperature (˚C)

95

95

70

75

Time (min)

120

5

60

180

pH

E1

D1

3.5*

2.6**

10.6**

5.1**

10.5 (before A)

-

4.0

0.9

0.78

-

-

-

ClO2 charge, %

-

1.16

-

1.55

NaOH charge, % Brightness, ISO %

-

-

0.96 75.5

0.33 88.8

Kappa number H2SO4 charge, %

* Initial pH; ** Final pH

The chemicals used for iodometric titration were prepared in the laboratory, including 1M potassium iodide (VWR International Oy, Finland), 1 M sulphuric acid (VWR International Oy, Finland) and 0.01 N thiosulphate (Na2S2O3·5 H2O, Oy FF-Chemicals Ab, Finland). HOCl treatment of cellulose 20 g (O.D.) of the fully bleached pulp was mixed with 1800 mL of distilled water in a lab disintegrator (model F-1, Fendo Oy, Finland), at the lowest revolution for 15 seconds. The reaction was carried out in a temperature-controlled jacketed glass reactor (Laborexin Oy, Finland) equipped with a mechanical stirrer. The pulp suspension was mixed with 40 mL of 1 M buffer solutions (phosphate and acetate buffers for pH 3 and pH 5, respectively). The reactor was filled with distilled water to end up at a final buffer concentration of 0.02 M and 1% pulp consistency. After stabilization of the temperature, 15 mL of 2.5 % NaOCl solution (0.2 g/L active chlorine, 1 % charge based on O.D. pulp weight) was added. Aliquot portions of 5 mL were taken out for titration every 5 to 10 minutes and the concentration of active chlorine was determined by iodometric titration as described in the standard method SCAN-C 29:72. The total reaction time was 60 min. The reaction was stopped by filtering the pulp in a Büchner funnel and washing it with 4 L of distilled water. The data presented are averaged from 4-6 repetitions. HOCl treatment of carboxylic acids The oxidation of oxalic, formic and glycolic acids was carried out in a temperature-controlled jacketed glass reactor (Laborexin Oy, Finland) using a mechanical stirrer. The experiments were carried out at pH 5 which was obtained by using an acetate buffer. Aqueous solutions of oxalic and glycolic acid were prepared by dissolving 252 mg (0.002 moles) oxalic acid · 2 H2O and 1.54 g (0.02 moles) glycolic acid in 85 mL of distilled water. The solution of formic acid was prepared by diluting 40 mL of 1 M formic acid (0.04 moles) with 45 mL of distilled water. The reactor was filled

with 1900 mL of 0.02 M phosphate buffer and 15 mL of 2.5% NaOCl solution. After the temperature was stabilized the organic acid solution was added, giving a total volume of 2 L. The reaction temperature was 16 °C for oxalic acid; 25, 35 and 50 °C for formic acid; 40, 60 and 80 °C for glycolic acid.

RESULTS AND DISCUSSIONS In HOCl oxidation, pH plays an important role. The composition of the HOCl solution varies with pH as described by equilibria (1) and (2) [13].

K2 H3O+ + ClO-

HOCl + H2O

(2) In acidic aqueous solutions, HOCl is in equilibrium with chlorine according to Equation (1). Increasing the pH will result in the dissociation of HOCl and the formation of hypochlorite ion (OCl-), as illustrated by equilibrium (2). The equilibrium constants for (1) and (2) are K1 = 3·10-4 M2 and K2 = 3.2·10-8 M at 25°C [25]. Thus, HOCl is the predominant species between the pH values chosen for this study (pH 3 and 5), while hypochlorite ion dominates in the alkaline range and free chlorine molecules are present at pH 3 and below, depending on the chloride ion concentration [13,26]. At lower pH elemental chlorine (Cl2), or at higher pH hypochlorite ions (OCl-), have to be taken into account as well.

HOCl oxidation of fully bleached pulp After bleaching with the A/DED sequence, the relative contents of residual lignin and HexA, which are both known to react with HOCl, were very low (0.5% and 4 mmol/kg, respectively). Thus their contribution to HOCl consumption during cellulose oxidation can be neglected. Reference trials with only buffer

JOURNAL OF PULP AND PAPER SCIENCE: VOL. 34 NO. 4 OCTOBER/NOVEMBER/DECEMBER 2008

d [HOCl] = −k1 [HOCl][Cellulose] dt

and HOCl showed that the active chlorine concentration remained constant in both buffer solutions even at the highest reaction temperature of 80 °C. During HOCl treatment of the fullybleached pulp, the concentration of active chlorine decreased constantly as a function of time at both pH 3 and 5 (Figure 1). The consumed HOCl was many times higher than the residual HexA and lignin contents in the pulp. This fact indicates the consumption of HOCl by pulp carbohydrates. At 80 ˚C, about 68 % of the HOCl was consumed in one hour, while only 10 % and 25 % was consumed at 50 and 65 ˚C, respectively. The first order kinetic equation for the consumption of HOCl by cellulose (and/or hemicelluloses) is described as Eq. C. In these experiments the cellulose was in large excess to HOCl, so its concentration was considered constant throughout the experiments.

ln[HOCl]t = −k1 [Cellulose]t + ln[HOCl]0 (4) Integration of Eq. C gives Eq. D, which shows a linear correlation of ln[HOCl] against time in accordance with the experimental results (Figure 2). The slopes of the regression lines in Figure 2 Consumption of HOCl (mM) by a fully bleached eucalyptus pulp (1% consistency) at pH 3 and pH 5, at 50, 65 and 80 ˚C. Reaction rates are shown in M-1s-1.2 represent the reaction rate constants k1[Cellulose], as shown beside the corresponding temperatures. The oxidation rates were of the same magnitude at pH 3 and pH 5. Previous studies on cellulose oxidation have mainly discussed the products formed during the oxidation and their impact on pulp quality. The reaction rate and rate constants were rarely mentioned.

pH = 3

3,0

50 C 

2,5

[HOCl] (mM)

[HOCl] (mM)

3,0

65 C 

2,0

80 C 

1,5

pH = 5

50 C  65 C 

2,5 2,0

80 C 

1,5

1,0 0,5

Based on these kinetic data, the activation energy of cellulose oxidation with HOCl can be calculated according to the Arrhenius Equation. The activation energy of cellulose oxidation by HOCl was calculated to be 55±2 kJ/mol [Figure 3]. This value is close to the previously reported activation energy for starch oxidation by hypochlorite 66±6 kJ/mol at pH 5.5 [25]. The difference in the physical state of starch and cellulose certainly results in dissimilar reaction kinetics, although the oxidation mechanisms would not likely be distinguished from each other. In order to specify the effect of hemicellulose content on the reaction rate of HOCl oxidation, the pulp after alkaline extraction was treated in the same way as the original fully bleached pulp. The alkaline extraction (NaOH 18%) reduced the amount of hemicelluloses in the pulp from 12% to 7%. The pulp from the same bleaching batch was used throughout this study. Under the same experimental conditions, the two pulps, however, did not show a

(3)

1,0

0

10

20

30

40

50

0,5

60

Reaction time (min)

0

10

20

30

40

50

60

Reaction time (min)

Figure 1 Consumption of HOCl by a bleached kraft pulp (1% consistency), at pH 3 and pH 5.

pH = 3 1,0

65 °C

0,8

(9.2 0.80)10-5

0,8

0,6

0,2

(1.7 0.05)10-4

0

15

30

45

50 °C (3.0 0.30)10-5

65 °C (8.0 0.51)10-5

0,6 0,4

80 °C 0,4

ln[HOCl]

ln[HOCl]

1,0

pH = 5

50 °C (3.7 0.30)10-5

80 °C (2.5 0.32)10-4

0,2

60

0,0

0

Reaction time ( min)

15

30

45

60

Reaction time (min)

Figure 2 Consumption of HOCl (mM) by a fully bleached eucalyptus pulp (1% consistency) at pH 3 and pH 5, at 50, 65 and 80 ˚C. Reaction rates are shown in M-1s-1. JOURNAL OF PULP AND PAPER SCIENCE: VOL. 34 NO. 4 OCTOBER/NOVEMBER/DECEMBER 2008

3

3,0

[HOCl] (mM)

lnk

0,0 -0,5 -1,0

2,5

-1,5 2,0

-2,0 -2,5 0,00287

0,00294

0,00301

1,5

0,00308

-1 1/T (K )

Figure 3 Effect of temperature on the oxidation rate (s-1) of cellulose by HOCl at a consistency of 1%, at both pH 3 and pH 5.

1,2

25 °C

ln[HOCl]

0,9

0,3

6

9

40 °C

60 °C

0,6

a

45

(1.1±0.05)10-2

0,3

80 °C (5.8±0.33)10-2

0,0 30

(2.0±0.03)10-3

0,9

-0,3 15

15

1,2

(2.3±0.11)10-2

0

12

Figure 4 Consumption of HOCl (mM) by oxalic acid at pH 5 and 16 °C. Initial oxalic acid: 1mM.

50 °C

0,0

3

(3.3±0.11)10-3

35 °C (8.3±0.25)10-3

0,6

0

Reation time (min)

ln[HOCl]

0,00280

60

0

Reaction time (min)

15

30

b

45

60

Reaction time (min)

Figure 5 Consumption of HOCl by formic acid (a) and glycolic acid (b) during HOCl oxidation at pH 5. Initial concentrations: [HOCl]0 = 3 mM, [FA]0 = 20 mM and [GA]0 = 10 mM. Reaction rates in M-1s-1.

-3

-2

-4

lnk

lnk

-3 -4 -5

-5 -6

-6

-7

0,00308

0,00315

0,00322

a

0,00329

0,00336

1/T (K-1)

0,00272

0,00280

b

0,00288

0,00296

1/T (K-1)

0,00304

Figure 6 Arrhenius plot: oxidation of formic acid (a) and glycolic acid (b) by HOCl.

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JOURNAL OF PULP AND PAPER SCIENCE: VOL. 34 NO. 4 OCTOBER/NOVEMBER/DECEMBER 2008

clear difference in the oxidation. Therefore, the kinetics of this pulp oxidation by HOCl can be regarded as representative of cellulose oxidation by HOCl.

HOCl treatment of carboxylic acids In order to investigate the reactivity of dissolved materials with HOCl, three organic acids, namely oxalic, glycolic and formic acids, were investigated. It is well known that more than three hundred low molecular weight organic compounds have been indentified in effluents from the production of bleached kraft pulp [27-29]. The compounds identified can be classified into three main groups according to their chemical properties: acids, phenolic compounds and neutral compounds [29]. The three acids selected in this study are by-products of some bleaching reactions, and account for a large portion of the alkanoic acids identified in kraft pulp bleaching effluents. For the three carboxylic acids the HOCl treatment trials were conducted at pH 5. Oxalic acid was the most reactive of the organic acids studied. To be able to monitor the changes in the concentration of HOCl (Figure 4), the experiments were conducted at very low initial concentrations. Oxalic acid was rapidly oxidized by HOCl even below room temperature. During the reaction the concentration of residual HOCl decreased by the initial concentration of oxalic acid, indicating a stoichiometric reaction of HOCl with oxalic acid. The probable reaction mechanism is shown in Figure 7. This reaction was too fast to be recorded by the method applied at higher temperatures, so that no further attempt was made to determine the activation energy of the oxidation. However, with the present data, the first order reaction rate can be calculated via the zero time integration method, where the reaction rate can be expressed as:

d [HOCl ] = k [HOCl ]0 • [OA ]0 dt .

Therefore, at 16 °C:

k=

∆[HOCl] ∆t[HOCl]0 ⎡⎢⎣OA ⎤⎥⎦ 0

= (1.1±0.02) M-

1 -1 s .

The k value can be generalized to be (2.1 ±0.03) M-1s-1 at 25 °C. The reactivities of formic and glycolic acid were much lower than that of oxalic acid. Therefore a large excess of these acids was used relative to HOCl to enhance its disappearance, and to simplify the calculation of rate constants (the organic acid concentration remained almost constant throughout the reac-

tion). The consumption of HOCl followed first order reaction kinetics in the O O OH O Cl oxidation of both forOA 2 CO2 + HCl + HOCl mic and glycolic acids -H2O O O OH O H (Figure 5). The reaction of formic acid O O OH O Cl with HOCl was studFA CO2 + HCl + HOCl -H O 2 H H ied at 25, 35 and 50 ˚C in order to calculate the activation energy. O OH O OH k_11 k_12 The reaction of glycol2 CO2 + HCl GA 1) H OH + 2 HOCl -H2O HOCl O OH ic acid with HOCl was H -2 HCl slower and therefore HOCl k_21 studied at slightly -H2O higher temperatures O O Cl O O OH HOCl k_22 HOCl (40, 60 and 80˚C) in a CO2 + HCl GA 2) H O H -CO -HCl reasonable duration 2 -H2O H H H -HCl H time. The reaction rates in terms of HOCl consumption are Figure 7 Reaction mechanisms suggested for HOCl oxidation of the shown in Figure 5. three carboxylic acids: oxalic (OA), formic (FA) and glycolic acids Applying the same (GA). method used for cellulose oxidation, the activation energies for foralso possibly affects the pH of the system due mic and glycolic acid oxidations were to the formation of hydrochloric acid. Both calculated to be 62±3 kJ/mol and 87±2 kJ/mol, consequences are undesirable for pulp producrespectively (see Figure 6). tion. The HOCl oxidation of the carboxylic Oxidation of carbohydrates by acids probably follows an analogous mechaHOCl nism, as shown in Figure 7. HOCl reacts formally by an electrophilic substitution of the The reactions of hypohalous acids in OH group with Cl+ with the elimination of waaqueous media have been studied extensively ter. The intermediate products rearrange interby Deborde and Gunten [26]. Furthermore, nally as indicated by the arrows to finally yield Fort and Denivelle have investigated the reactwo equivalents of carbon dioxide and one tions of alkyl hypochlorites in solvents [31]. equivalent of hydrogen chloride. The reaction However, the reaction kinetics were not been of formic acid proceeds analogously, the interextensively reported. The HOCl oxidation of mediate formed decomposing to carbon dioxorganic substances generally occurs in a stepide and hydrogen chloride. In the case of wise manner (Figure 8). At first, HOCl underglycolic acid there are two possible reaction goes electrophilic substitution with organic pathways, both consuming three equivalents of compounds, or addition to the C=O bond formHOCl and giving the same end products: two ing organic hypochlorite, and then the reaction equivalents of carbon dioxide (CO2), three proceeds via elimination of the organic hyequivalents of hydrogen chloride (HCl) and pochlorite. The active groups in the organic two equivalents of water. In one pathway the substances which are chlorinated by HOCl can hydroxyl group is oxidized via the aldehyde to be carboxylic, hydroxyl or aldehyde groups oxalic acid, which consumes two equivalents (Step 1 in Figure 8). These active groups can of HOCl, and the further converted as shown in influence the reaction rates of both steps in the Figure 7. The other option is that the carboxyl course of oxidation. group is attacked yielding oxalic acid and forIn Figure 8 the rank of reaction rates is mic acid, but the decomposition of the intermeproposed based on both experimental and steric diate would yield formaldehyde in addition to structural considerations. It is supported by carbon dioxide and hydrogen chloride. Formalmany other studies in the literature [32-35]. dehyde itself will be oxidized by HOCl to forThis proposal explains why two reaction routes mic acid, which is further converted as can proceed simultaneously in the oxidation of described above. glycolic acid by HOCl. The oxidation of these three carboxylic In the cellulose chain, the primary and acids by HOCl not only raises the risk of oversecondary hydroxyl groups react with HOCl consumption of the bleaching chemicals, but through both pathways 2b) and 2c) (Figure 9).

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O

1a)

+

R

O

K1a HOCl

OCl

OH O

1b) 1c)

H R

HOCl

H

K1c HOCl

R

+

O

R1

O

k2a R

Cl

O

H

R2

O

H

O

2a)

H HOCl

HOCl

H

+

H

CO2

HCl

R1

Rn

O OH

H2O

Step 1 Formation of organic hypochlorites

O

+

H

Cl

R1

OH

Rm

OCl

O

O

O

OCl

O

k2c

H

OH

R

HOCl

+

OH

H

H2O

OH

K1b

+

R

+

R

R1

+ HCl

Cl

+

H

R1

Cl k2c

O

O

k2b

O R1

+

O R2

H

+

HCl

HCl

R2

R2

R

2b)

R1

O

H

R2

O

Cl

R1

O

+ R2

H

R1

H

2c)

O

O

k2b

+

HCl

H

O

k2c

+

R1

Cl

HCl

R2

R2

Step 2 Cleavage of organic hypochlorites

Figure 8 Reaction of HOCl with carbohydrates in two steps. R, R1 and R2 refer to alkyl structures. Suggested decreasing order of equilibria constants and reaction rates: K1a> K1c >> K1b, k2c > k2a >> k2b.

Pathway 2c), which forms ketones or formaldehyde demands less energy than 2b), which cleaves the bond between C2 and C3 yielding only aldehydes. Furthermore, the primary hydroxyl is more reactive than the secondary, as is well known [36]. Therefore, in the treatment of cellulose with HOCl, only the primary hydroxyl groups were taken into account to estimate the concentration of hydroxyl groups in the reaction mixture. The reaction rates of HOCl oxidations in this study are summarized in Table II, as well as some activation energies. The structure characteristic of glycolic acid determines the slow rate in both oxidation steps, which can reflect the small value of the rate constant and the large value of the activation energy in Table II.

Figure 9: Reaction scheme of hydroxyl groups in cellulose with HOCl. Rx (x=m, n, 1 and 2) refers to alkane structures.

In most cases the rate-limiting step is the formation of organic hypochlorites, and supporting conclusions have been reported by other authors [31].

CONCLUSIONS

HOCl can oxidize pulp cellulose when the contents of residual lignin and hexenuronic acid groups are very low, which may result in the degradation of the cellulose. The Arrhenius activation energy for cellulose oxidation by HOCl was determined to be in the range of 55±2 kJ/mol. In addition, the three major carboxylic acids dissolved from pulp carbohydrates can also be oxidized by HOCl, and the oxidation reactions are believed to undergo an analogous mechanism, where the final products are water and carbon dioxide. Of the three compounds, oxalic acid was most reactive towards HOCl, followed by formic acid and glycolic acid. The activation energies for HOCl oxidation were calculated to be 62±3 kJ/mol and 87±2 kJ/mol, for formic and glycolic acids, respectively.

Cellulose

*(3.4±0.08)•10-2

Oxalic acid

(2.1±0.03)

Activation energy (Ea, kJ/mol) 55±2 _

Formic acid

(3.3±0.13) •10

61±1

Glycolic acid

**(4.7±0.05)•10-5

88±2

-3

* based on the content of primary hydroxyl groups in pure cellulose **extrapolated via the Arrhenius equation 6

ACKNOWLEGEMENT The authors would thank the Finnish Funding Agency for Technology and Innovation (TEKES) and a consortium of Finnish companies (Andritz, Jaakko Pöyry, Kemira Chemicals, Metsä-Botnia, Stora Enso) for the financial support. Ms. Jääskeläinen A.-S. acknowledges the financial support from the Academy of Finland.

REFERENCES 1.

2.

3.

4.

TABLE II REACTIONS WITH HOCL IN AQUEOUS SOLUTIONS AT 25 °C Rate constants (k, M-1s-1)

All these reactions undergo the same mechanisms, consisting in both formation and cleavage of organic hypochlorite. This study contributes to the knowledge of side reactions during chlorine dioxide bleaching of pulp, providing essential information to simulate the whole bleaching chemistry in the D-stage under real conditions.

5.

KOLAR, J.J., LINDGREN, B.O. and PETTERSSON, B., “Chemical Reactions in Chlorine Dioxide Stages of Pulp Bleaching”, Wood Sci. Techn., 17:117-128 (1983). DENCE, C.W. and REEVE, D.W., “Pulp Bleaching: Principles and Practice”, p263-266, TAPPI Press, Atlanta (1996). JONCOURT, M.J., FROMENT, P., LACHENAL, D. and CHIRAT, C., “Reduction of AOX Formation during Chlorine Dioxide Bleaching”, TAPPI J., 83(1):144-148 (2000). FOLKES, L.K., CANDEIES, L.P. and WARDMAN, P., “Kinetics and Mechanisms of Hypochlorous Acid Reactions”, Arch. Biochem. Biophys., 323(1):120-126 (1995). VUORINEN, T., JÄÄSKELÄINEN, A-S., ADORJAN, I., LEHTIMAA, T., TOIKKA, K. and ZHOU, Z.,“Fundamentals and Characteristics of Modern Hardwood Pulp Bleaching”, ABTCP-PI 2005: 38th Pulp and Paper International Congress and Exhibition, Electronic publication (CD-ROM), Sao Paulo, Brazil, October 17–20, (2005).

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6.

7.

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REFERENCE: Zhou, Z., Jääskeläinen, A.-S. and Vuorinen, T., Oxidation of Cellulose and Carboxylic Acids by Hypochlorous Acid: Kinetics and Mechanisms, Journal of Pulp and Paper Science, 34(4): October/November/December 2008. Paper offered as a contribution to the Journal of Pulp and Paper Science. Not to be reproduced without permission from the Pulp and Paper Technical Association of Canada. Manuscript received May 12, 2006; revised manuscript approved for publication by the Review Panel November 2, 2006. KEYWORDS: TPLEASE PROVIDE

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