Journal of Environmental Polymer Degradation, Vol. 5, No. 2, 1997

Review of the Kinetics of Alkaline Degradation of in View of Its Relevance for Safety Assessment of Repositories

L. R. Van Loon l'z and M. A. Glaus 1

The degradation of cellulose (a substantial component of low- and intermediate-level radioactive waste) under alkaline conditions occurs via two main processes: a peeling-off reaction and a base- catalyzed cleavage of glycosidic bonds (hydrolysis). Both processes show pseudo-first-order ki- netics. At ambient temperature, the peeling-off process is the dominant degradation mechanism, resulting in the formation of mainly isosaccharinic . The degradation depends strongly on the degree of polymerization (DP) and on the number of reducing end groups present in cellulose. Beyond pH 12.5, the OH- concentration has only a minor effect on the degradation' rate. It was estimated that under repository conditions (alkaline environment, pH 13.3-12.5) about 10% of the cellulosic materials (average DP = 1000-2000) will degrade in the first stage (up to 105 years) by the peeling-off reaction and will cause an ingrowth of isosaccharinic acid in the interstitial pore water. In the second stage (105-106 years), alkaline hydrolysis will control the further degradation of the cellulose. The potential role of microorganisms in the degradation of cellulose under alkaline conditions could not be evaluated. Proper assessment of the effect of cellulose degradation on the mobilization of radionuclides basically requires knowing the concentration of isosaccharinic acid in the pore water. This concentration, however, depends on several factors such as the stability of ISA under alkaline conditions, sorption of ISA on cement, formation of sparingly soluble ISA-salts, etc. A discussion of all the relevant processes involved, however, is far beyond the scope of the presented overview.

KEY WORDS: Cellulose; alkaline degradation; peeling off; isosaccharinic acid; kinetics.

INTRODUCTION known in the literature that cellulose is unstable under alkaline conditions and will degrade to water-soluble, Low- and intermediate-level radioactive waste low molecular weight compounds [3]. The type of deg- (L/ILW) contains substantial amounts of cellulosic ma- radation products formed depends strongly on the com- terials. In Switzerland, about 50 % of the organic waste position of the solution in contact with the cellulose. In planned to be emplaced in the repository for low and the presence of Ca 2+, a primary cation in cement pore intermediate radioactive waste (L/ILW-repository) is water, isosaccharinic acid (ISA) will be the main deg- cellulosic [1]. The use of large amounts of cement for radation product [4, 5]. Isosaccharinic acid (Fig. I) is constructing the repository causes an alkaline environ- assumed to form--in analogy with (Fig. ment (pH of the cement pore water remains above 12.5 D--stable complexes with tri- and tetravalent radio- for periods of the order of 105 years [2]). It is well- nuclides such as Am 3+ and Pu 4+ [6-8] and might lower their sorption on the cement phase [9]. This would lead Paul Scherrer Institute, Laboratory for Waste Management, CH-5232 to an enhanced release of such radionuclides to the geo- Villigen PSI, Switzerland. and biosphere. 2To whom correspondence should be addressed. When trying to assess the potential effect of the

97 1064-7546/97/0400-0097512.50/0 1997 Plenum Publishing Corporation 98 Van Loon and Glaus

COOH COOH dation product. The ISA concentration based on this I I COOH H--C--OH H--C--OH assumption depends on the cellulose content of the re- i i HOH2C--C--OHI HO--C--HI ~H2 pository. High cellulose contents lead to ISA concentra- (~H2 H--C--OH H~C--OH tions high enough (up to 0.1 M) to complex virtually all I I of the radionuclides present so that no sorption occurs. H--C--OH H--C--OH H--C--OH I I I Such an assumption is very conservative. A more real- CH20H CH20H CH20H istic picture, however, can be obtained when informa- isosaccharinic acid gluconicacid metasaccharinic acid tion on the following questions is available and taken Fig. 1. Chemicalstructures of isosaccharinic acid, gluconic acid, and into account: metasaccharinic acid. Is the degradation of cellulose such a fast process that it is reasonable to assume an instantaneous presence of cellulose in a repository, it is very important degradation? and to have an idea of the concentration range of ISA in the Will cellulose be degraded completely to ISA? cement pore water, because the concentration of a li- In the following, we try to answer these questions based gand, together with the stability of the complex formed, on information available in the literature. We discuss is one of the key parameters governing the ultimate ef- the mechanisms and kinetics of the alkaline degradation fect of complexation on sorption. The concentration of of cellulose in detail w.r.t, their consequences on the ISA in the cement pore water depends on several fac- long-term degradation of cellulose in a cementitious re- tors, such as the pository. amount of cellulose emplaced in the cement (cel- lulose loading), CELLULOSE degradation kinetics of cellulose, stability of ISA formed, Cellulose is the most abundant organic material on sorption of ISA on the cement phase, and earth. It is the main component of plant sources, serving formation of sparingly soluble ISA-salts with, as the structural material by which plants, trees, and e.g., . grasses sustain the strength to stay upright. It occurs in The first two factors are directly related to the degra- almost-pure form in cotton fibers (90%) and, to a lesser dation of cellulose, whereas the others have to do with extent, in flax (80%), jute (60-70%), and (40- the properties of the degradation product itself or with 50%). Cellulose is a linear macromolecule composed of the interaction between the degradation product and re- up to 10,000 (1,4)-/~-D-glucopyranose units (Fig. 2), and pository components and, consequently, are not directly only the configuration of the C1 position is different from related to the degradation of cellulose. In this overview, that of amylose that is made of (1,4)-c~-D-glucopyranose we focus only on the factors that are directly related to [10]. the degradation process of cellulose. The cellulose molecule has a nonreducing end and Bradbury and Sarott [9] assumed, in their overview a reducing one. The reducing end is a latent aldehyde of sorption of radionuclides on cement, that the degra- and, like an aldehydo function, responds to both reduc- dation of cellulose occurs instantaneously and com- tion and oxidation processes. As will be seen later, the pletely and that isosaccharinic acid is the main degra- reducing end group plays a key role in the alkaline deg-

m CH20H O r CH20H CH2 OH o H H H

H H HIgH I/H I I I I I I H OH OH H OH n-2 nonreducing end reducing end Fig. 2. Chemical structure of cellulose. Review of the Kinetics of Alkaline Degradation of Cellulose 99

the cellulose chain, resulting in soluble degradation O-% products such as isosaccharinic acid. The peeling-off re- action is controlled by two competing reactions: a pro- gressive shortening of the cellulose molecule in which glucose units are progressively eliminated from the cel- crystalline region amorphous region crystalline region lulose molecule (starting at the reducing end group) and Fig. 3. Microfibrillic structure of cellulose [12]. (e) Reducing end a stopping reaction in which the reducing end group is group; ( nonreducing end group. converted to an alkalistable end group while it is still attached to the cellulose chain. The stopping reaction can be subdivided into a chemical and a physical stop- radation of cellulose. The cellulose molecular chains are ping reaction. The former is the transformation of a re- ordered into strands as cellulose microfibrils through in- ducing end group into a stable metasaccharinic acid (Fig. ter- and intramolecular hydrogen bonding [11]. These l) end group. The latter implies that a reducing end microfibrils have crystalline and amorphous regions group reaches the crystalline region of the cellulose and (Fig. 3). The alkaline degradation takes place in the is no longer accessible to alkali [12]. The physical stop- amorphous regions of cellulose [12, 13]. ping reaction is not an abrupt process since there is a gradual transition from the amorphous to the crystalline region rather than a distinct interface. A schematic pre- CEMENT PORE WATER sentation of the peeling-off reaction is given in Fig. 4.

The chemistry of cement pore water has been dis- General Reaction Kinetics cussed in detail by Bemer [2] and Neall [14]. The com- The kinetics of the peeling-off reaction of cellulose position is determined to a large extent by the compo- have been studied in detail by Haas et al. [12]. They sition of the cement and the water flow through the performed kinetic studies on hydrocellulose (prepared repository. The evolution of the pore water chemistry from cotton) with a degree of polymerization (DP) of can be grossly divided into three stages. In the first stage, 166 at different temperatures (ranging from 65 to 132 oc) the dissolution of NaOH and KOH--present in cement in 1.25 M NaOH solutions. The liquid/solid ratio in their as "impurities"--causes a (Na,K)OH cement pore water studies was so high (100/1) that the NaOH concentra- saturated with respect to Ca(OH) 2. The pH of this initial tion remained practically constant during the whole re- pore water lies around 13.4 (the concentration of OH- action time. is 0.3 M) and the Ca concentration is about 2 mM. In For the progressive chain degradation, at a constant the second stage, when all NaOH and KOH has been concentration of OH-, the following equation can be leached out, the composition of the pore water is deter- mined by the dissolution of Ca(OH)2. The pH of the pore water falls to 12.5 and the Ca concentration lies around 20 mM. In the third stage--when all the Ca(OH)2 has dissolved--the pH falls further and hydrated Ca-Si G-G-G-G-G-G-G-G-G-G r phases (CSH-gels) start dissolving. Peeling off OH Based on assumptions made for the water flow and 1 the permeability and porosity of the cement, the pH of G-G-G-G-G-G-Gr + nGe the cement pore water was calculated to remain above 12.5 for periods of the order of 105 years [2]. Chemical stopping

DEGRADATION OF CELLULOSE UNDER G-G-G-G-G-G-Gms a ALKALINE CONDITIONS Physical stopping G-G-G-G-G-G-Gr, c Peeling-Off Reaction Fig. 4. Schematicpresentation of the peeling-offreaction. G is a glu- cose monomeric unit (glucopyranose), Gm~ais a stable metasacchar- General inic acid end group, Ge is a glucose unit eliminated, and n is the number of glucose units eliminated. Gr is a reducing end group in the The peeling-off reaction is an endwise degradation amorphous region of the cellulose fiber and G~,c is a reducing end process by which a reducing end group is split off from group in the crystalline region of the cellulose fiber. 100 Van Loon and Glaus written: The decrease in reactive reducing end groups is caused by the two stopping reactions and can be com- d(Q) -- k I (Gr) (1) bined as dt d(G~) with (Ge) the mole fraction of glucose units eliminated, - k2 " (Gr) + kcr'(Gr) = kt " (Gr) (6) dt (Gr) the mole fraction of reducing end groups available for the reaction, and kt the pseudo-first-order rate con- where kt (kt = k2 q- kcr) is the total rate constant for stant. The mole fraction of a component is defined here chain termination. as the number of moles of that component divided by At t = 0, (G0 = (GOo, the initial reducing end- the number of moles of all components in the system. group content. Integration of Eq. (6) gives For the system with cellulose described here, the num- (GO = (G,)o e -k''' (7) ber of moles of all components equals the initial degree of polymerization (DP0). As an example, the mole frac- Substitution of Eq. (7) in Eqs. (1) and (3) gives tion of Q is defined as dCa~) - kl " (G~)o " e -k''t (8) Ge dt (Gr - (2) OPo d(ISA...... ~) _ kl " (G~)o " e -kt't (9) Assuming that the glucose units are eliminated as iso- dt saccharinic acid (no fragmentation reaction), 3 Eq. (1) At t = 0, (Go) and (ISA) = 0 and Eqs. (8) and (9), after can also be written as integration, give d(ISA) = k, (Gr) (3) k~ .t) (10) dt (Ge) = ~ (Gr)o " (1 - e -k' where (ISA) is the mole fraction of isosaccharinic acid formed. (ISA) = ~k~ (Gr)o (1 - e -k' .t) (11) For the chemical stopping reaction, the following rate equation can be written: Substitution of Eq (7) in Eqs. (4) and (5) gives d(MSA) d(MSA) - k2 (G~) (4) - kl " (Gr)o e -kt'' (12) dt dt

where (MSA) is the mole fraction of stable metasac- and charinic acid end groups formed and k2 the pseudo-first- d(Gt) order rate constant for the chemical stopping reaction. - kcr" (Gr)o " e -k''t (13) Although no chemical reaction is involved in the dt physical stopping process, a similar rate equation can be At t = 0, (MSA) and (Gt) = 0 and integration of Eqs. written for this physical stopping reaction: (12) and (13) gives d(Gt) - kcr (G~) (5) k2 .t) dt (MSA) = ~ (G~)o " (1 - e -k' (14)

where (Gt) is the mole fraction of reducing end groups and not available for reaction and kcr is the formal rate con- stant of termination caused by inaccessibility. kcr e-kt" t ) (Gt) = ~t (G~)0 " (1 - (15)

The maximum amount of (ISA) at t = oo can be cal- 31n the peeling-off reaction, the reducing end group is split off from the cellulose chain, resulting in the formation of an intermediate culated from Eq. (11): product Ge. The intermediate product reacts further via two reaction kl pathways to give the ultimate degradation products. Isosaccharinic (ISA)max = ~ (Gr)o (16) acid is formed via an internal rearrangement of the intermediate product (benzilic acid type of rearrangement). Via a fragmentation reaction the intermediate molecule is split into smaller molecules such The maximum amount of cellulose degraded equals the as lactic acid, formic acid, and acetic acid [3-5, 15, 16]. maximum amount of glucose units peeled off and can Review of the Kinetics of Alkaline Degradation of Cellulose 101

be written as 0.012 , ~ ,

k 1 (cellulose degraded)max = (Ge)ma x = kt (Gr)~ (17) "O"OO 0.01 0.008 o'} From Eqs. (16) and (17) it can be deduced that (ISA)max o "o 0.006 and (cellulose degraded)max depend on the initial mole t'- ...... _o fraction of reducing end groups in the cellulose, (G00, 0.004 and on the ratio of the rate constants of the propagation DP = 5000 reaction and the stopping reactions. It. 0.002 ..... DP = 10000 s The initial mole fraction of reducing end groups is 0 i I I I I I I defined as 0 50 100 150 200 250 300 350 400

moles of reducing end groups Time (days) (Gr) o = (18) moles of glucose units in the chain Fig. 5. Degradation of cellulose as a function of time at [OH-] = 0.3 M and 25~ for different degrees of polymerization (DP), The The mole fraction of reducing end groups depends on solid lines were calculated using Eq. (10), with k, = 3.65 10 2 h-~ the average amount of reducing end groups in a cellu- and kt = 6.91 10 -4 h -~. lose molecule and on the degree of polymerization. Both these parameters have an effect on the amount of cel- lulose that will be degraded. In theory, one molecule of Values of these constants at lower temperatures can cellulose, containing n glucose units (degree of poly- be estimated by applying the Arrhenius equation: merization DP = n), has one reducing end group. In k = A e -ea/n" r (20) reality, not all cellulose molecules will have reducing end groups because the reducing end groups may have or, in the linear form, been transformed to nonreducing end groups during the Ea pulping process [17-20]. Consequently, the mole frac- logk = logA - 0.434 "-- (21) tion of reducing end groups in cellulose with DP = n is R.T with 1 0 < (Gr)0 < - (19) n A = Arrhenius parameter (h- l) k = rate constant (h -1) The presence of one reducing end group per cellulose molecule will result in the maximum value of the con- centration of reducing end groups (Gr)o for a cellulose Table I. Overview of the Rate Constants for Cellulose Degradation molecule with DP = n and, consequently, results in the (Peeling-OffReaction) at Different Temperatures in 1.25 M NaOH upper limit of the maximum amount of degradable cel- [121 lulose. The amount of isosaccharinic acid formed and also T k~ k2 kcr k, (~ (h-') (h i) (h-') (h-') the amount of cellulose degraded by the peeling off re- action depend strongly on the degree of polymerization 2Y 0.0393 0.000014 0.00073h 0.00074 of cellulose. The larger the cellulose molecule, the lower 65 4 0.0041 0.057 0.061 the mole fraction of reducing end groups [as can be seen 78 17.8 0.031 0.263 0.29 from Eq. (18)] and the lower the extent of degradation 87 46.3 0.097 0.64 0.74 100 147 0.50 1.60 2.10 (Fig. 5). 132 1550 6.98 13.0 19.98

Effect of Temperature on the Peeling-Off Reaction "Extrapolated by the Arrhenius equation (21): Kinetics logkI = 16.21 - 5249 1/T "" logkl (25~ = -1.41 -4- 0.35 The rate constants kl, k2, kcr, and k t were deter- logk2 = 17.12 - 6548 1/T ~ logk2 (25~ = -4.86 + 0.46 mined by Haas et al. [12] at different temperatures be- logkt = 13.86 - 5062 lIT ~ logkl (25~ = -3.13 + 0.44 tween 65 and 132~ and an OH- concentration of 1.25 bThe value of kcr at 25 ~ was calculated from the extrapolated values M. The authors used hydrocellulose with DP = 166. kt and k2 by using the equation kcr = kt - k2 because kcr is not a The values are summarized in Table I. measured value but calculated from k t and k2. 102 Van Loon and Glaus

E a = activation energy (J mol-i) Effect of Base Concentration on the Peeling-Off R = universal gas constant (J mol -I K-i) Reaction Kinetics T = absolute temperature (K) The concentration of OH- in the cement pore water Plotting log k versus the reciprocal absolute tempera- in the initial stage of cement degradation is about 0.3 M tures yields a straight line with slope 0.434 EJR and and the Ca level is about 2 mM. To evaluate the deg- intercept log A with the Y-axis. With these equations, radation of cellulose under such repository conditions, values for kl, k2, and k t at 25~ were calculated (see the effect of pH (or OH- concentration) and the effect Table I). The errors on the estimated values were cal- of Ca 2+ on the peeling-off reaction have to be discussed. culated from the errors on the slope and the intercept of The rate constants discussed in the previous section were the regression line [21], the Student t value for (n - 1) derived for reaction conditions of 1.25 M NaOH in the = 4 degrees of freedom, and the 95 % confidence level. absence of Ca. No further systematic studies on the ef- Figure 6 illustrates the Arrhenius equation for the dif- fect of [OH-] and [Ca 2§ on the rate constants (kl, k2, ferent reactions of the peeling off process. It is clearly kcr, and kt) have been found in the literature for cellu- shown that the relationship between the logarithm of the lose. Although no experimental data on the effect of rate constants and the reciprocal of the absolute temper- OH- and Ca 2§ are available for cellulose, some data ature is a linear one according to Eq. (21). The activa- exist for other polymeric materials. Studies on amylose tion energy (Ea) of the different reactions can be calcu- [22] and/3-(1-3)-glucans [23] showed that the hydroxyl lated from the slope of the lines. The propagation ion does not participate in the rate-determining reaction reaction has an activation energy of 101 kJ mo1-1 and step and that anionic species are involved in the peeling- the activation energy of the overall stopping reaction is off and chemical stopping reaction for these polysac- 97 kJ mol -~. charides. It was further shown that the alkaline degra- The extent of cellulose degradation by the peeling- dation of amylose with ot-(1-4)-glycosidic bonds (at off reaction depends on the ratio of the rate constants k~/ temperatures < 120~ proceeds by the same reaction kt [Eq. 17] and, consequently, on the reaction temper- mechanisms as the alkaline degradation of cellulose with ature. At lower temperatures, the degree of degradation /~-(l-4)-glycosidic bonds [22], i.e., is smaller than at higher temperatures. This can be ex- a chain propagation reaction (peeling off reac- plained properly by the slightly higher activation energy tion), resulting in the formation of isosaccharinic of the propagation reaction w.r.t, the overall stopping acid; and reaction. a chemical stopping reaction by which the re- ducing end group is transformed into an alkali- stable metasaccharinic acid end group. A physical stopping reaction does not occur because amylose does not have a microfibrillar structure such as 2 - cellulose. Conclusions drawn from the study on amy- lose [22] are applicable to a large extent to cellulose. 0 The rate constant for the peeling-off reaction (k0 increased with the concentration of OH- until 0.3 M. ,r {:~ -2 Beyond 0.3 M, k~ remained constant (see Fig. 7). The 0 0 k 1 k 1 rate constant for the chemical stopping reaction (k2) was negligibly small for a concentration of OH- below 0.1 M and could not be quantified. Beyond 0.3 M, k2 in- -6 t t t I i creased until the concentration of OH- reached a value 2.2 2.4 2.6 2.8 3 3.2 3.4 of 1.5 M. Beyond 1.5 M, k2 remained constant. It was also observed that up to 0.1 M, amylose was completely 1/T (x 1000 ~ degraded and that, for 0.1 M < [OH-] < 1.5 M, the Fig. 6. Arrhenius plot for the peeling-off reaction [chain propagation extent of degradation decreased. Beyond 1.5 M, the ex- (k0, chemical (k2) and overall (k0 stopping reaction] of cellulose in tent of degradation remained constant [22]. 1.25 M NaOH. Open symbols represent experimental data [ 12]; filled In analogy to amylose, it is assumed that the reac- symbols are extrapolated values. The error bars on the extrapolated values represent the 95 % confidence level estimated from the error on tivity of the reducing end group in cellulose is propor- the slope and the intercept of the regression line. tional to its degree of ionization: Review of the Kinetics of Alkaline Degradation of Cellulose 103

320 i i i , 2 d(Q) 0 0 0 - k~ (Gr) t (29) 280 dt 240 kl o t 11,5 where k~ is the "conditional" OH-dependent rate con- 200 2:: stant. "-" 160 Equation (29) is very similar to Eq. (1). The for- 120 k2 ..,.~ malism developed by Haas et al. [12] for the peeling- 80 0.5 off reaction was a special case of the more general for- malism presented by Eq. (27) because their studies were 40 performed at a high and constant concentration of OH-. 0 I I I k 0 0 1 2 3 4 For the stopping reaction, a further deprotonation of the reducing end group is required so that a second [OH] (M) deprotonation step has to be introduced. The rate equa- Fig. 7. Dependence of rate constants for the peeling-off reaction (k0 tion for the chemical stopping reaction can be written as and chemical stopping reaction (k2) of amylose on the hydroxyl con- centration at 100~ [22]. d(MSA) - k~ (G~-) (30) dt

where k~ is the OH-independent first-order rate constant cellulose - Gr + OH- ,~ cellulose - G7 + H20 for the chemical stopping reaction and (G2-) the mole fraction of twofold deprotonated reducing end groups. (22) Since at low temperatures the chemical stopping re- The deprotonation constant is defined as action plays a minor role in the overall stopping reaction of the alkaline degradation of cellulose (see Table I, k 2 (G/) K, - (23) << kcr), the dependence of k2 on the hydroxyl concen- (Gr) [OH- ] tration is not discussed further here. where (G/-) is the mole fraction of deprotonated reduc- The effect of [OH-] on the physical stopping re- ing end groups and (GO is the mole fraction of proton- action was not studied by Lai and Sarkanen [22] be- ated reducing end groups. cause, as already mentioned, such a stopping process From the mass balance, does not occur in amylose. For cellulose, however, the following rate equation--based on Eq. (5)--can be writ- (G,) t = (Gr) + (Gr) (24) ten assuming that the reaction rate is proportional to the and Eq. (23), it can be shown that mole fraction of deprotonated end groups:

K, [OH-] d(Gt) K~ [OH-] (G~-) = "(GOt (25) - k'r" (Gr)t (31) 1 + K1 [OH-] dt 1 + K 1 [OH-] The rate equation for the propagation reaction can be with written as K, [OH-] d(Ge) k. = k.p . (32) dt = k[ (Gr) (26) 1 + K 1 [OH-] Equation (31) is similar to Eq. (27) for the peeling-off where k[ is the OH-independent first-order rate constant reaction. This similarity can be justified as follows. for the peeling-off reaction. Combining Eqs. (25) and Since the physical stopping reaction occurs when a re- (26) results in ducing end group reaches the crystalline region of the d(Ge) K, [OH-] cellulose fiber, its rate will depend strongly on the rate = k~ (GOt (27) dt 1 + K, [OH-] of the peeling-off reaction. The faster the peeling-off re- action, the faster a reducing end group will reach the With crystalline region. Consequently, the rates of both re- K, [OH-] actions have to be strongly correlated. The ratio kl/kcr kl = k~ (28) 1 + K~ [OH-] is constant and equals kt/kcr. From the considerations made above, the maxi- Eq. (27) can be written as mum amount of cellulose that will be degraded can be 104 Van Loon and Glaus written as ence in rate constant between 1.25 and 0.3 M OH- is kl very small because Kl of the reducing end group is = 30 (cellulose degraded)max = (Ge)max = ~ (Gr)0 (33) or the deprotonation constant pKa = 12.5 [22, 23, 26]. Beyond pH 13, the reducing end groups are almost com- where kl and k t are the intrinsic (i.e., OH-independent) pletely deprotonated. rate constants. Because of the assumption that the rate of stopping is proportional to the rate of degradation, Effect of Ca2+ on the Peeling-Off Reaction Kinetics the extent of cellulose degradation is independent of the Ca 2+ has a significant effect on the peeling-off re- OH- concentration. This was experimentally confirmed action [4, 5, 24, 27]. Ca 2§ seems to catalyze the ben- by Machell and Richards [24], who did not see an effect zilic acid type of rearrangement, leading to the forma- of the alkali concentration on the extent of cellulose tion of ISA. Also, the chemical stopping reaction is degradation in the range 0.125 M < [OH-] < 1.25 M. catalyzed. The overall effect of the presence of Ca 2+ is For a concentration of OH- higher than 2 M, a signifi- a lower degree of degradation and the formation of rel- cant effect of [OH-] on the extent of degradation was atively more ISA at the expense of side products. No observed. At these concentrations, however, the struc- systematic kinetics studies on the alkaline degradation ture of cellulose is changed (transition of crystalline to of cellulose in presence of Ca 2 § have been performed. amorphous cellulose), resulting in a higher accessability of the cellulose to OH- [25]. The reaction rate, however, depends on the con- Base-Catalyzed Cleavage of Glycosidic Bonds centration of OH-. The values of the first-order rate (Alkaline Hydrolysis) constants for cement pore water conditions (i.e., [OH-] = 0.3 M) can be calculated by applying the following General Mechanisms and Reaction Kinetics equation: Besides the peeling-off reaction, the base-catalyzed K 1 [OH-] cleavage of glycosidic bonds is another important pro- ki.o.3 = kl " (34) 1 + K~. [OH-] cess in the alkaline degradation of cellulose that needs to be discussed in detail. The kinetics of alkaline hy- with drolysis have been studied by Lai and Sarkanen [28], Lai [29], and Franzon and Samuelson [30]. ki,0. 3 = rate constant for [OH-] = 0.3 M kl = intrinsic rate constant Alkaline hydrolysis of cellulose could not be stud- ied separately from the peeling-off reaction because hy- [OH-] = concentration of OH (M) K~ = deprotonation constant for the deprotonation drolysis produces new reducing end groups, initiating a of CrOH of the reducing end group (K I = peeling off reaction. 30) As illustrated in Fig. 8, the degradation is initiated by cleavage of a glycosidic bond. The newly formed The values of k~ were calculated by applying Eq. (34) reducing end groups give rise to a chain degradation for [OH-] = 1.25 M and the values of ki,l.25 at 25~ as summarized in Table I. The values of the different rate constants for 0.3 M G-G-G-G-G-G-G-G-G-G OH- and 25~ are summarized in Table II. The differ- I OH" (slow)

Table II. Overview of the Rate Constants for Cellulose G-G-G-G-G r + G-G-G-G-G Degradation by the Peeling-Off Reaction for 25~ and Different OH- Concentrations OH" (fast) [OH-] k, k2 kc~ kt (M) (h-1) (h-') (h-1). (h-') G-G-Gmsa + XnGe + G-G-G-G

1.25 3.93E-2 1.40E-5 7.31E-4 7.45E-4 Fig. 8. Schematic overview of the alkaline degradation of cellulose 0.3 b 3.65E-2 1.30E-5 6.78E-4 6.91E-4 by the base-catalyzed cleavage of glycosidic bonds and the peeling- off reaction. Gr = reducing end group, G.... = alkali-stable meta- a kcr = k, - k2. saccharinic acid end group, Ge = glucose units eliminated as ISA or bCalculated by Eq. (34). other products; and x, = the number of peeled-off glucose units. Review of the Kinetics of Alkaline Degradation of Cellulose 105

(peeling-off) process by progressively converting the Franzon and Samuelson [30] studied the alkaline deg- terminal units to isosaccharinic acid and other products radation of cellulose at 170~ in 1.25 M NaOH. They (see previous section). observed a change in DP of the cellulose and a weight A few mechanisms are known for the base-cata- loss. The overall degradation was found to be the result lyzed cleavage of glycosidic linkages. For cellulose, an of two processes: the cleavage of glycosidic bonds (re- internal nucleophilic substitution (SNi) was found to be suiting in a decrease in DP), followed by a peeling-off the most evident one [29]. The SNi mechanism for base- reaction starting from the newly created reducing end catalyzed cleavage of glycosidic bonds can be divided group (resulting in weight loss). The amount of cellu- into two processes [31]. In the first step, a deprotonation lose units peeled off after each chain break (Xn) was reaction occurs: shown to be constant (x,, = 65) and independent of the

K DP within a wide range. The rate-controlling step in the GlcOH + OH- ~ ~ H20 + GlcO- (35) overall degradation process is the cleavage reaction. The amount of cellulose left after degradation (Y) In the second step, the intermediate product (GlcO-) is was found to be related to the degradation time (t) by transformed: the following equation: k GlcO- - degradation products (P) (36) In (Y) = -k t (42) where GlcOH is the glycoside (cellulose in our case) with and GlcO- is the anionic intermediate. K is the equilib- Y = fraction of cellulose left rium constant between neutral and ionic glycosides (de- k = rate constant (h -t) protonation constant) and k is the specific rate constant t = time (h) in the conversion of anionic intermediates to degrada- tion products. The latter is the rate-determining step in Lai and Sarkanen [28] studied the alkaline degradation the overall reaction. The rate of alkaline degradation of of cellulose at different temperatures between 146 and glycosides can be expressed as 186~ in 1.25 M NaOH. They came to conclusions similar to those of Franzon and Samuelson [30] and d(P) - k (GlcO-) (37) showed that the rate of degradation conformed with the dt following equation: where (P) represents the mole fraction of glycosides re- In (Y) = --kob s x n t (43) acted after time t (or the mole fraction of degradation products formed) and (GlcO-) is the mole fraction of with ionized, intermediate glycosides at time t. Y = fraction of unreacted cellulose The "deprotonation" constant K is defined as t = time (h) kob s = rate constant (h-i) (GlcO-) x,, = average number of glucose units peeled off (x. K = {(GlcOH)o - (GlcO-) - (P)} [OH-] (38) = 65) where (GlcOH)o is the mole fraction of glycosides at t Equations (42) and (43) are very similar. Since the num- = 0. Combination of Eqs. (37) and (38) yields a rate ber of glucose units peeled off for each chain break (x,) equation, which is integrated to give is constant, it can be integrated in the rate constant as (GlcOH) o - (P) K. k [OH-] was done by Franzon and Samuelson [30]: In (GlcOH)o = -1 + K" [OH-] " t (39) kobs " xn = k (44) If Effect of Temperature on the Reaction Kinetics K. k" [OH-] kobs = (40) Lai and Sarkanen [28] determined the rate con- 1 + K. [OH-] stants for the degradation of cellulose in 1.25 M NaOH where kob~ is the pseudo-first-order rate constant of the at different temperatures between 146 and 185 ~ Table reaction, Eq. (39) becomes III summarizes their results. A rate constant for 25~ was calculated by applying the Arrhenius equation [see (GlcOH)o - (P) In = -kobs " t (41) Eq. (21)]. Figure 9 illustrates the temperature depen- (GlcOH)o dence of the rate constant. At a temperature of 25~ 106 Van Loon and Glaus

Table IIl. Rate Constants for Degradation of Cellulose in 1.25 M 20 ii,,i,,,,i,,,, I .... OH (from Ref. 28 with the Exception of the Data for 170~ from Ref. 30)

Temperature kobs x n 15 (~ (h-')

v 256 2.17" 10 -t~ r 146 4.00. 10 -3 o 165 23.0" 10 -3 10 I- e- 170 38.7" 10 -3 X .1r 185 129" 10 -3 09 ,.,O aExtrapolated by applying the Arrhenius equation (21): log(kobs x,) = 15.41 - 7461 I/T ~ log(kobs " x,) = --9.66 + 0.17.

0~ ~===l~hltlitlllllll the rate constant is about 7 orders of magnitude lower 0 0.5 1 1.5 2 than at 146~ The base-catalyzed cleavage of glyco- sidic bonds is very sensitive to temperature. [NaOH] (M) Effect of Base Concentration on the Reaction Kinetics Fig. 10. Dependence of k,,b,~ x, on the concentration of OH- at 185~ [28]. Lai and Sarkanen [28] found a linear dependence of kobs " x, on the concentration of OH- for 0 M < [OH] < 2 M. Figure 10 illustrates this dependence for Effect of Ca 2+ on the Reaction Kinetics the degradation of cellulose at 185~ Assuming that a similar linear relationship is valid for lower reaction No information is available on the possible effect temperatures, a rate constant for the degradation of cel- of Ca 2 § on the base-catalyzed hydrolysis of cellulose. lulose in 0.3 M OH- at 25~ can be calculated by ap- plying the following equation: RELEVANCE FOR PERFORMANCE kobs, l.25 X n k~ " Xn = 1.25 0.3 (45) ASSESSMENT

and results in a value of kob, " x, = 5.25 10 -11 h -1 In earlier safety assessment studies [1, 9], it was for 25~ assumed that cellulose was degraded instantaneously and completely to isosaccharinic acid, resulting in high con- centrations of ISA in the cement pore water. From the o -...% , kinetic data discussed in this work, it is clear that this is not the case. Depending on the DP of the cellulose --~-- measured values and on the number of reducing end groups, only a small "2

i fraction of cellulose will degrade almost instantaneously t'- -4 to isosaccharinic acid. The rest of the cellulose will de- .-.= grade slowly by alkaline hydrolysis. In this section, we x -6 calculate the degree of degradation of cellulose (under ,4o conditions like those of a L/ILW-repository) caused by v .go -8 the peeling-off reaction and base-catalyzed hydrolysis, based on the information described above. -10 The parameters needed for calculating the degra- I I dation of cellulose are the kinetic parameters of the peel- 2 2.5 3 3.5 ing-off reaction (k0, the overall stopping reaction (k,), 1/T (x 1000 ~ and the alkaline hydrolysis (kob,) and the mole fraction

Fig. 9. Arrhenius plot for the alkaline hydrolysis of cellulose in 1.25 of reducing end groups (Gr)o. As discussed earlier, the M OH [28, 30]. mole fraction of reducing end groups depends on the Review of the Kinetics of Alkaline Degradation of Cellulose 107

Table IV. Average Degree of Polymerization (DP) of Native ] "~) ...... I ...... I ...... I ...... ~ ...... F ...... I ...... I ...... I ...... I ...... Cellulose of Different Origins (from Ref. 32) r 1 further cleavage followed by peeling off- Material Average DP "~ initial peeling off "~ 0.8 reaction Cotton fiber 14,120 Cotton fiber 5,000 ,~.-~ 0.6 Cotton fiber 10,800 Cotton fiber 5,200 o.4 Cotton fiber (seed hairs) 9,950 ~ 0.2 Cotton fiber 3,250 U. Ramie fiber 6,500 L, p,,,,,I ...... I ...... I ...... I ...... t ...... I ...... I Ramie fiber 4,600 10 .3 10 "1 101 10 a 10 s 0 r Bast fiber 9,550 Bast fiber 8,000 Time, (years) Bast fiber 9,900 Fig. 11. Overall degradation of cellulose with DP = 1000 and having Valonia 26,500 one initial reducing end group per cellulose molecule, at 25~ and Acetobacter xylenum 5 ~700 0.3 M OH- as a function of time. The solid line shows the remaining Acanthamoeba castellani 2,000-6,000 cellulose calculated by Eq. (46) using (Gr)o = 0.001, k= = 3.65 Gymnosperm wood 8,450/7,800 10 -2 h -L, kt = 6.91 " 10 -4 h -I, and k,,b~ ' x = 5.25 10 -It h -I. Angiosperm wood 8,200/9,350

follows:

number of reducing end groups and on the the degree of (1 - (kl/kt) " (Gr)o " (1 - e-k"t)) (cellulose left) = polymerization of the cellulose. The molecular weight ekobs x, l (or degree of polymerization) of cellulose and the (46) amount of reducing end groups depend strongly on the origin of cellulose and on the pulping process used. Figure 11 illustrates the overall degradation of cellu- Consequently, it is difficult to predict the extent of cel- lose. In this scenario, we have a fast ingrowth of ISA lulose degradation without making some assumptions. in the cement pore water from the beginning on, reach- For the average number of reducing end groups, we take ing its maximum value after about 2 years. A further the conservative value of one per cellulose molecule. increase in ISA is expected to occur very slowly when The molecular weight of native cellulose has been stud- the transport of ISA out of the repository is ignored. ied intensively. A summary overview is given by Mo- Proper assessment of the degradation of cellulose rohoshi [32] and presented here in Table IV. Similar on the fate of radionuclides in a repository requires a values were also published by Goring and Timell [33]. more detailed discussion of the concentration of ISA in High-temperature pulping processes (T > 150~ par- the cement pore water of a repository. This concentra- tially degrade native cellulose by the hydrolysis reac- tion depends on several factors and processes such as tion, decreasing its average DP. The average DP of cel- the cellulose loading, the degradation kinetics, the sol- lulose in paper and cotton lies around 1000-2000. A ubility of sparingly soluble Ca-salts of ISA, the sorption value of 1000 is taken as a representative value for cel- of ISA on cement, the stability of ISA under alkaline lulose present in radioactive waste. The kinetic con- conditions, and the water flow through the repository. stants summarised in Table II for [OH-] = 0.3 M were Such a discussion is far beyond the scope of this paper used and it was assumed that the pH of the cement pore and, therefore, will be discussed elsewhere [34, 35]. water stayed constant for 106 years. The degradation reaction starts with the fast peel- ing-off reaction and stops when all reducing end groups SUMMARY have been transformed to stable end groups or reducing end groups no longer accessible to OH- (after about 2 Two processes are involved in the alkaline degra- years). The further reactions are then the very slow dation of cellulose: the fast peeling-off reaction and the cleavage of glycosidic bonds followed by a fast peeling- slow base-catalyzed cleavage of glycosidic bonds (al- off reaction starting from the newly created reducing end kaline hydrolysis). groups. The overall degradation of cellulose for this case Based on the kinetic data available for these two can be expressed by combining Eqs. (10) and (43) as processes, the full degradation of cellulose under near- 108 Van Loon and Glaus

field conditions existing in a cementitious L/ILW-re- 2. U. Bemer (1990) A Thermodynamic Description of the Evolution pository ([OH-] = 0.3 M, 25~ was estimated to take of Pore Water Chemistry and Speciation During the Degradation of Cement, PSI-Bericht 62, Paul Scherrer Institute, 105-106 years. In the relatively short initial stage, about Villigen, Switzerland. (Also published as NAGRA Technical 1-10% (depending on the degree of polymerization of Report NTB 90-12, NAGRA, Wettingen, Switzerland) cellulose and on the average amount of reducing end 3. R. L. Whistler and J. N. BeMiller (1958)Adv. Carbohydr. Chem. Biochem. 13, 289-329. groups) of the cellulose will degrade by the peeling-off 4. M. J. Blears, G. Machell, and G. N. Richards (1957) Chem. Ind. reaction and yield mainly isosaccharinic acid. After this Aug. 24, 1150-1151. initial stage, the degradation will continue by both base- 5. G. Machell and G. N. Richards (1960) J. Chem. Soc. A2, 1932- 1939. catalyzed cleavage and the peeling-off reaction, the for- 6. D. T. Sawyer (1964) Chem. Rev. 64, 633-643. mer controlling the overall reaction rate. 7. A. D. Moreton (1993) Mater. Res. Soc. Syrup. Proc. 294, 753- For performance assessment purposes, it can be as- 758. 8. B. F. Greenfield, G. F. Holtom, M. H. Hurdus, N. O'Kelly, N. sumed that only a part of the cellulose (1-10%) will J. Pilkington, A. Rosevaer, M. W. Spindler, and S. J. Williams degrade in the first 104 years after repository closure in- (1995) Mater. Res. Soc. Syrup. Proc. 353, 1151-1158. stead of taking a complete and instantaneous degrada- 9. M. H. Bradbury and F. A. Sarott (1995) Sorption Databases for the Cementitious Near-Field of a L/ILW Repository for Perfor- tion. This can be justified by the kinetic data available mance Assessment, PS1-Bericht 95-06, Paul Scherrer Institute, in the literature. Villigen, Switzerland. (Also published as NAGRA Technical Report NTB 93-08, NAGRA, Wettingen, Switzerland.) The temperature has a large effect on the degrada- 10. K. Okamura (1991) in D. N.-S. Hon and N. Shiraishi (Eds.), tion rate of cellulose. The extent of degradation, how- Wood and Cellulosic Chemistry, Marcel Dekker, New York and ever, is affected much less because both the propagation Basel, pp. 89-112. 11. H. Kr~ssig (1985) in J. F. Kennedy, G. O. Phillips, D. J. Wed- reaction and the stopping reaction are equally sensitive lock, and P. A. Williams (Eds.), Cellulose and Its Derivatives: to temperature and the ratio of the two reaction rates Chemistry, Biochemistry and Applications, Marcel Dekker, New determines the extent of degradation. Raising the tem- York, Chichester, Brisbane, pp. 3-25. 12. D. W. Haas, B. F. Hrutfiord, and K. V. Sarkanen (1967) J. perature from 25 to 100~ will increase the extent of Appl. Polym. Sci. 11,587-600. degradation by 30%. This is important because in the 13. M. Lewin (1985) in J. F. Kennedy, G. O. Phillips, D. J. Wed- initial stage of cement hydration, the temperature is lock, and P. A. Williams (Eds.), Cellulose and Its Derivatives: Chemistry, Biochemistry and Applications, Marcel Dekker, New higher. The effect of temperature on cellulose degrada- York, Chichester, Brisbane, pp. 27-35. tion is limited and hence does not need to be taken into 14. F. Neall (1994) Modelling of the Near-Field Chemistry of the account. SMA Repository at the Wellenberg Site, PS1-Bericht 94-18, Paul Scherrer Institute, Villigen, Switzerland. Because of uncertainties in the kinetic data, addi- 15. E. Sjrstrrm (1977) TAPPI60, 151-154. tional studies on the kinetics of cellulose degradation are 16. Y. Z. Lai (1991) in D. N.-S. Hon and N. Shiraishi (Eds.), Wood necessary. Especially, the effect of Ca 2 § on the peeling- and Cellulosic Chemistry, Marcel Dekker, New York and Basel, pp. 455-523. off and stopping reaction and the effect of [OH-] need 17. A. R. Procter and R. H. Wiekenkamp (1969) Carbohydr. Res. to be further investigated. 10, 459-462. 18. A. R. Procter and H. M. Apelt (1969) TAPPI 52, 1518-1522. 19. J. R. G. Bryce (1980) in J. P. Casey (Ed.), Pulp and Paper, Chemistry and Chemical Technology, Vol. 1, 3rd ed., John Wiley ACKNOWLEDGMENTS & Sons, New York, pp. 429-436. 20. V. L. Chiang and K. V. Sarkanen (1984) J. Wood Chem. Tech- nol. 4, 1-18, The authors would like to thank Dr. E. Wieland, 21. J. C. Miller and J. N. Miller (1988) Statistics for Analytical Dr. M. Bradbury, and Dr. J. Pearson Jr. (all from PSI) Chemistry, Ellis Horwood, Chichester. for giving useful comments to improve the manuscript. 22. Y. Z. Lai and K. V. Sarkanen (1969) J. Polym. Sci. C 28, 15- 26. This work was partly financed by the Swiss National 23. R. A. Young, K. V. Sarkanen, P. G. Johnson, and G. G. Allan Cooperative for the Disposal of Radioactive Waste (1972) Carbohydr. Res. 21, 111-122. (NAGRA). 24. G. Machell and G. N. 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30. O. Franzon and O. Samuelson (1957) Svensk Papperstidning 23, 34. L. R. Van Loon and M. A. Glaus (1997) in preparation. 872-877. 35. M. H. Bradbury and L. R. Van Loon (1997) Cementitious Near- 31. Y. Z. Lai (1972) Carbohydr. Res. 24, 57-65. Field Sorption Databases for Performance Assessment of a 32. N. Morohoshi (1991) in D. N.-S. Hon and N. Shiraishi (Eds.), L/ILW Repository in a Palfris Marl Host Rock, PSI-Bericht (in Wood and Cellulosic Chemistry, Marcel Dekker, New York and preparation), Paul Scherrer Institute, Villigen, Switzerland. (Also Basel, pp. 331-392. published as NAGRA Technical Report NTB 96-04, NAGRA, 33. D. A. I. Goring and T. E. Timell (1962) TAPPI45, 454-460. Wettingen, Switzerland.)