73 CH9700362

THE DEGRADATION OF : A PROBLEM FOR THE SAFETY OF A REPOSITORY?

L.R. Van Loon, M.A. Glaus, S. Stallone, A. Laube

In an alkaline cementitious environment, cellulose degrades rapidly via a peeling-off reaction. The main degradation product is isosaccharinic (ISA), a polyhydroxy type of ligand forming stable complexes with tri- and tetravalent radionuclides. ISA can have an adverse effect on the sorption of radionuclides to an extent which depends on its concentration in the pore water. The concentration of ISA is gov- erned by several factors such as cellulose loading, cement porosity, extent of cellulose degradation, etc. The sorption of ISA on cement, however, is the process which governs the concentration of ISA in the pore water. The ISA concentration in a repository with a cellulose loading of 5 % is calculated to be of the order of 10-4 M. At this level, the effect of cellulose degradation products on radionuclide sorption is neg- ligibly small.

1 INTRODUCTION 2 CHEMICAL AND PHYSICAL STRUCTURE OF CELLULOSE The sorption of radionuclides on repository compo- nents (e.g. cement) is an important process since it Cellulose is the most abundant organic material on controls the release of radionuclides from the reposi- earth. It is the main component of vegetation, serving tory [1]. A strong sorption of radionuclides is desirable as the structural material by which plants, trees, as since it will allow only a small release of radionuclides well as grasses have the strength to stay upright. to the geo- and biosphere. The strong sorption behav- Cellulose is a linear macromolecule composed of up iour of radionuclides, however, could possibly be de- to 10000 (1,4)-(3-D-glucopyranose monomeric units creased by several orders of magnitude by the pres- (Fig. 1) [12]. The number of monomers in the chain is ence of organic ligands. Ligands such as EDTA, NTA, called the degree of polymerisation (DP). A cellulose citric acid etc. are inherent components of radioactive molecule has a non-reducing end and a reducing one. waste since these complexing agents are used in nu- The reducing end is a latent aldehyde and, like an clear power stations for decontaminating purposes. aldehydo function, responds to both reduction and Other ligands might be formed by degrading organic oxidation processes; it plays the key role in the alka- polymers present in low and intermediate level radio- line degradation of cellulose. active waste [2,3]. Cellulose materials such as cotton, paper and form a substantial part (ca. 50 %) of CH2OH the organic waste [1]. The use of large amounts of cement for constructing a repository causes alkaline environments in which the pH of the pore solution will remain above 12.5 for periods of the order of 105 years [4]. It is well known from the literature that cellu- lose is unstable under alkaline conditions and will de- non-reducing end reducing end grade to water soluble, low molecular weight com- Fig. 1: Chemical structure of cellulose, n is the num- pounds by the peeling-off reaction [5]. The main deg- ber of monomers. radation product of cellulose is isosaccharinic acid (ISA), which is stable under alkaline conditions [5-9]. ISA enhances the of Pu(IV) [7,8] and has an adverse effect on the sorption of Eu(lll), Th(IV) and crystalline region amorphous region crystalline region Ni(ll) [6]. For instance, in a solution of 10-3 M ISA, the solubility of Pu(IV) at pH 12 increases by a factor of 20000 [10]. The sorption of Pu(IV) [8], Eu(lll), Th(IV) and Ni(ll) [6], however, is affected to only a minor extent. The observed effects are - by analogy with [11] - interpreted to be due to a strong complexation of these metals. • reducing endgroup o non-reducing endgroup

A full assessment of the effect of cellulose degrada- Fig. 2: Physical structure of cellulose [14]. tion on the sorption requires a detailed understanding of the mechanisms involved. The present study gives The cellulose molecular chains are ordered into an overview of the different processes involved and strands as cellulose microfibrils through intra- and describes how to quantify them and how to use them intermolecular hydrogen bonding [13]. These microfi- in a safety assessment. brils have crystalline and amorphous regions (Fig. 2); 74 alkaline degradation takes place in the amorphous acid type of rearrangement to isosaccharinic acid, the regions of cellulose [14,15]. main degradation product of alkaline degradation of cellulose. The different reactions involved (chain 3 DEGRADATION OF CELLULOSE: PEELING- propagation, chemical and physical stopping reaction) OFF PROCESS are all first order reactions [14]. The rate of alkaline degradation of cellulose decreases continuously until The degradation of cellulose under alkaline conditions all reducing end-groups have been transformed to occurs via a fast "peeling-off" process and a slow end-groups no longer available for the peeling proc- combined alkaline hydrolysis/peeling-off process. For ess. At room temperature, the degradation of cellu- the conditions existing in a repository for low and in- lose stops after about one year [6,16]. The extent of termediate level radioactive waste, only the fast peel- degradation is given by: ing off process is important [16] and will be discussed here. The peeling-off reaction is an endwise degrada- tion process by which a reducing end group is split off (celdeg) = ~ (Gr)0 -(i- (1) from the cellulose chain resulting in soluble degrada- kt tion products such as isosaccharinic acid. where: (celdeg) = fraction of cellulose degraded after degradation time t, k-| = first order rate constant of the peeling-off process, k = first order rate constant of the G-G-G-G-G-G-G-G-G-Gr t overall stopping process, (Gr)0 = mole fraction of re- peeling off OH ducing end-groups in cellulose. The maximum amount of cellulose degraded is:

(celdeg^ (G )c (2) G - G - G - G - G -G - Gr + xGe r

From equation (2) it can be deduced that (celdeg)max chemical stopping depends on the initial mole fraction of reducing end- groups in the cellulose, (Gr)0, and on the ratio of the rate constants of the propagation reaction and the -^ G-G-G-G-G-G- Gmsa stopping reactions. The initial mole fraction of reduc- ing end-groups is defined as: physical stopping _ number of reducing end-groups G-G-G-G-G-G-G r,c r ° number of glucose units in the chain

Fig. 3: Schematic presentation of the peeling-off re- One molecule of cellulose, containing n glucose units action. G is a glucose monomeric unit (gluco- (degree of polymerisation DP=n), has one reducing pyranose), Gr is a reducing end-group in the end-group. Consequently, the mole fraction of reduc- amorphous region of the cellulose fibre and ing end-groups can be written as: Grc is a reducing end-group in the crystalline region of the cellulose fibre. Gmsa is a stable 1_ (4) metasaccharinic acid end-group, the Ge elimi- DP nated glucose units and x is their number. where DP = the degree of polymerisation. Fig. 4 shows the extent of degradation as a function The peeling-off process is controlled by two compet- of time for cellulose with a degree of polymerisation ing reactions : a progressive shortening of the cellu- (DP) of 118 and of 1110. The figure clearly shows that lose molecule in which glucose monomeric units are the degradation practically comes to an end after ap- successively eliminated (peeled off) from the cellulose proximately one year of degradation time. molecule (starting at the reducing end-group) and a stopping reaction. The stopping reaction can be sub- The degree of polymerisation determines the extent of divided in a chemical and a physical stopping reac- degradation since, as shown in equation 4, the con- tion. The former is the transformation of a reducing centration of reducing endgroups, (Gr)0, is the inverse end-group into a stable metasaccharinic acid end- of the degree of polymerisation [16]. The larger the group. The latter implies that a reducing end-group degree of polymerisation, the lower is the mole frac- reaches the crystalline region of the cellulose and is tion of reducing endgroups and the smaller is the de- no longer accessible to alkali [14]. A schematic pres- gree of degradation. Fig. 5 clearly shows the depend- entation of the peeling off reaction is shown in Fig. 3. ence of the extent of degradation (celdeg) on the de- The endgroup split off (Ge) reacts further via a benzilic gree of polymerisation of cellulose (DP). 75

COOH COOH 40 1 i I HOCH2-C-OH HOCH2-C-O, 35 H-C-H H-C-H 3H 30 H —C —OH H-C-O I CH OH CH O 25 r - 2 2

a) 20 Fig. 6: Possible coordination of a trivalent metal ion 2 - d 15 - / —$_— DP=118 M(lll) with isosaccharinic acid according to a - -• - - DP = 1110 general coordination scheme for polyhydroxy 10 ligands [17]. 5 1 •- " Fig. 6 illustrates such a coordination for a trivalent 0 100 200 300 400 500 600 700 800 metal ion M(lll). The complexes formed with polyhy- Time [days] droxy ligands are in general very stable. An indirect indication of complexation is the effect of ligands on Fig. 4: Degradation of cellulose with different degree the sorption of metals on a solid phase. Fig. 7 shows of polymerisation as a function of time. Sym- the effect of pure ISA and cellulose degradation prod- bols represent experimental data [6]. The ucts on the sorption of Eu(lll) on feldspar at pH 13.3. lines are calculated using equation (1); DP = ISA and cellulose degradation products have an ad- 118: k, = 1.2-10-2 hrs-1, kt = 3.8-10-4 hrs-1; verse effect on the sorption of Eu(lll), the extent of DP= 1110: k, = 1.9-10-2 hrs-1 and kt = sorption depending strongly on the ISA concentration 4.5-10-4 hrs-1. in the liquid phase. From the similar effect on sorption observed for ISA and cellulose degradation products, it can be concluded that ISA is the main degradation 40 -|—i—r—i—v product. This was confirmed by additional chemical - theoretical analysis of the degradation products, showing that • cellulose (DP=118) about 80 % of the total organic carbon was identified 30 paper (DP=290) as ISA [6,18]. • tissues (DP=1110) • cotton (DP=1800) 1 20 O) 6 u 10 5 O3 4 500 2000 3

Fig. 5: Dependence of the extent of degradation of cel- • cellulose lulose on the degree of polymerisation (DP). The 1 pure ISA solid line is calculated using equation (2) with 0 k1 = 1.2-10-2 hrs-1, kt= 3.8-10-4 hrs-1. (Gr)0 is -7 -3 -2 -1 calculated using equation (4). Symbols repre- sent measured values [6]. Log(ISA) [M] Fig. 7: Effect of ISA and cellulose degradation prod- ucts on the sorption of Eu(lll) on feldspar at 4 EFFECT OF ISA ON THE SORPTION pH = 13.3 (artifical cement pore water). The OF RADIONUCLIDES shaded area represents the situation where no ligands are present in the liquid phase [6]. Isosaccharinic acid is a polyhydroxy carboxylic acid and - by analogy with gluconic acid - is assumed to form strong complexes with metals as follows: The final effect of ISA on the sorption of radionuclides depends strongly on the concentration of ISA in the

H4ISA"+M,n' + (5) cement pore water. Assessment of the effect of cellu- lose degradation on the sorption of radionuclides According to a general coordination scheme [17], tri- therefore requires a quantification of the concentration and tetravalent metals are exclusively bound to hy- of ISA in the cement pore water. This concentration droxylic groups when conditions are alkaline. depends on several factors such as: 76

• cellulose loading in the cement, TTTTTTTFI TTTTTTTTj TTTTTTIT1 r 1 I TITTIj 1 1 1 Mini • porosity of the cement, • extent of degradation, 10" • sorption of ISA on cement, • formation of weakly soluble salts with Ca,

• stability of ISA under alkaline conditions. 10"' The cellulose loading of the cement is fixed for a given type of waste. An average loading value is of the order of 2-5 %. The extent of degradation de- pends on the degree of polymerisation. A typical value 10-3 iii/il J\ i mill i i i mill i i i mill i for the cellulosic materials in a repository is about 10"6 10"5 10"4 10"3 10"2 10'1 1000. The corresponding extent of degradation by the (ISA) [moir1] peeling off reaction for these cellulosic materials will eq be about 3 %. The maximum concentration of ISA in the pore water of a cementitious repository could be Fig. 8: Adsorption isotherm of ISA on Portland ce- limited by solubility constraints. It is known from the ment at pH 13.3 [21]. literature that ISA forms weakly soluble salts with Ca2+[19]:

Ca(ISA)2 • Ca2+ +2 ISA" (6) The final concentration of ISA that can be expected in a cement pore water for a given cellulose loading can and hence the Ca2+ concentration in a cement pore be estimated by combining the different processes water could set an upper limit on ISA concentrations. discussed. Fig. 9 outlines the different processes in- The corresponding solubility product (Kso) of volved. Ca(ISA)2 is defined as

a SOLID CELLULOSE cellulose in cement - Ca (7) - celdeg with: -cellulose loading

= the activity of Ca2+ in solution, AMOUNT OF ISA IN ISA in pore water POREWATER a tne iSA = activity of ISA in solution. - porosity P

CONCENTRATION OF The value for log Kso was determined to be -6.22 ± ISA IN PORE WATER 0s A> in BEFORE SORPTION 0.03 at ionic strength I = 0 [20]. This solubility product -sorption q can be used to calculate the maximum concentration - solubility constraints Ks of ISA which may be present in cement pore waters. CONCENTRATION OF ISA IN PORE WATER 0SA)eq The equilibrium concentrations of ISA calculated ac- AFTER SORPTION sorption of radionuclides cording to the solubility limits of Ca(ISA)2 in a cement -as a function of ISA in pore water pore water (pH = 13.3) in equilibrium with Ca(OH)2 is EFFECTOF ISA ON THE SORPTION OF Sorption reduction 0.05 - 0.1 M [20]. METALS A more important process is the sorption of ISA on Fig. 9: Flow diagram for calculating the concentration cement. ISA was found to sorb strongly on the sur- of ISA in a cement pore water of a repository face of cement. The sorption behaviour can be de- with a given cellulose loading and the result- scribed by a Langmuir type of isotherm, assuming two ing effect on metal sorption. sorption sites [21]: q K 2 2 Fig. 10 shows the calculated ISA concentration in the porewater of a repository as a function of the cellulose loading. The parameters used are listed in Table 1. where q is the amount of ISA sorbed on the cement in The effect of sorption on the concentration is clearly 1 mol-kg- , qi is the sorption capacity of sorption site 1 illustrated in this plot. One can see that the sorption and q2 the one of sorption site 2. K<[ and K2 are the process reduces the concentration of ISA in the pore sorption coefficients for the two sorption sites. water by almost three orders of magnitude. For a re- Figure 8 shows the sorption isotherm of ISA on Port- pository with a typical cellulose loading of 3 - 5 %, the concentration of ISA is reduced from 0.1 M to about land cement. The sorption of ISA on cement is a very 4 important process since it can significantly reduce the 10-4 M. From Fig. 7 it is clear that the effect of 10- M concentration of ISA in the cement pore water and ISA on the sorption of Eu(lll) is negligible small. A consequently can reduce its adverse effect on the similarly small effect was also observed for Th(IV) and sorption of radionuclides. Ni(ll) [6]. 77

Parameter Value equilibrium on the cement phase. For a repository with a cellulose loading of 3-5 %, the concentration of cellulose loading 0.01-10% ISA expected is of the order of 10-4 M. At this porosity 10% concentration level, ISA has only a small effect on the DP 1000 sorption of radionuclides. 1 ki 3.7-10-2 hrs- In a second phase, the remaining cellulose will de- kt 6.9-10-4 hrs-1 grade slowly by a combined alkaline hydrolysis and Ki 1730 ImoH peeling-off process. However, this process results in qi 0.1 mol-kg-1 only negligible amounts of ISA in the pore water [16] and may be neglected in safety assessment studies. K2 12 ImoH O.17molkg-1 cement 580 kgm3 6 ACKNOWLEDGEMENTS

Table 1: Parameters used to estimate the concentra- The authors would like to thank the National Coopera- tion of ISA in the pore water of a cementi- tive for the Disposal of Radioactive Waste (Nagra) for tious repository with varying cellulose load- partly financing this work. ing. 7 REFERENCES U 1O "1 • • • • 1 i r I ITI.J- I )'1M [1] Nagra, Endlager fur schwach- und mittelaktive 10"1 Abfalle (Endlager SMA). Bericht zur Langzeit- sicherheit des Endlagers SMA am Standort Wel- 10* lenberg (Gemeinde Wolfenschiessen, NW), Nagra Technical Report NTB 94-06, Nagra, Wet- tingen, CH (1994). \ [2] L.R. Van Loon, Z. Kopajtic, Complexation of [ISA] = ~1C - 10* Cu2*, NP+ and UO#+ by Radiolytic Degradation Products of Bitumen, Radiochimica Acta 54 10* loading 3 -5 % (1991)193. 1 1 1 1 Hllf 1 1 1 11 ill i i 11 in .1 < J 1 mil , 10"3 10* 10"1 10° 101 [3] L.R. Van Loon, W. Hummel, The Radiolytic and Chemical Degradation of Organic Ion Exchange cellulose loading [%] Resins under Alkaline Conditions: Effect on Ra- Fig. 10: Estimated concentration of ISA in the pore dionuclide Speciation, PSI-Bericht 95-13, Paul water of a cementitious repository for varying Scherrer Institute, Villigen, CH (1995), Nagra cellulose loading. The solid line represents the Technical Report NTB 95-08, Nagra, Wettingen, situation when ignoring the sorption of ISA on CH(1995). the cement. The dashed line reflects the ef- [4] U. Berner, A Thermodynamic Description of the fect of sorption of ISA on the cement. Evolution of Pore Water Chemistry and Speciation during the Degradation of Cement, PSI-Bericht 62, Paul Scherrer Institute, Villigen, 5 CONCLUSIONS CH (1990), Nagra Technical Report NTB 90-12, Nagra, Wettingen, CH (1990). Under the alkaline conditions existing in a cementi- tious repository for low and intermediate radioactive [5] R.L. Whistler, J.N. BeMiller, Alkaline Degradation waste, cellulose initially degrades via a fast peeling-off of Polysaccharides, Advances in process to water soluble degradation products. The Chemistry and Biochemistry 13 (1958) 289. extent of degradation depends on the degree of po- lymerisation (DP) of the cellulose. For a cellulose with [6] LR. Van Loon, M.A. Glaus, S. Stallone, A. Lau- DP = 1000, at most only a few percent of the cellulose be, Degradation of Cellulosic Materials under Al- will degrade in this way. The main degradation pro- kaline Conditions: Characterisation and Quantifi- duct is isosaccharinic acid (ISA), a polyhydroxy type cation of the Degradation Products and their In- of ligand, which forms stable complexes with metals, teraction with Radionuclides, Internal Technical especially under the alkaline conditions of a cementi- Report TM-44-96-01, Paul Scherrer Institute, tious repository. As a consequence ISA has an ad- Villigen, CH (1996). verse effect on the sorption of metals on cement. The [7] A.D. Moreton, Thermodynamic Modelling of the effect depends strongly on the concentration of ISA in Effect of Hydroxycarboxylic on the Solubil- the cement pore water. The concentration of ISA in ity of at High pH, Mat. Res. Soc. the pore water is strongly controlled by its sorption Symp. Proc. 294 (1993) 753. 78

[8] B.F. Greenfield, W.N. Harrison, G.P. Robertson, Derivatives: Chemistry, Biochemistry and Ap- P.J. Somers, M.W.Spindler, Mechanistic Studies plications (Eds. J.F. Kennedy, G.O. Phillips, on the Alkaline Degradation of Cellulose in Ce- D.J. Wedlock, P.A. Williams), Marcel Dekker ment, NSS/R272, AEA-D&R-0219, AEA Tech- Inc., New York, Chichester, Brisbane (1985) nology, Harwell, UK (1993). pp. 27-35. [9] B.F. Greenfield, M.H. Hurdus, N.J. Pilkington, [16] L.R. Van Loon, M. Glaus, A Review of the Kinet- M.W. Spindler, S.J. Williams, The Degradation of ics of Alkaline Degradation of Cellulose in View of Cellulose in the Near Field of a Radioactive its Relevance for Safety Assessment of Radioac- Waste Repository, Mat. Res. Soc. Symp. Proc. tive Waste Repositories, accepted for publication 333(1994)705. in Journal of Environmental Polymer Degradation (1997). [10] B.F. Greenfield, G.F. Holtom, M.H. Hurdus, N. O'Kelly, N.J. Pilkington, A. Rosevaer, [17] M. van Duin, J.A. Peters, A.P.G. Kieboom, M.W. Spindler, S.J. Williams, The Identification H. van Bekkum, A. General Coordination-loni- and Degradation of Isosaccharinic Acid, a Cellu- zation Scheme for Polyhydroxy Carboxylic lose Degradation Product, Mat. Res. Soc. Symp. Acids in Water, Reel. Trav. Chim. Pays-Bas Proc. 353(1995) 1151. 108(1989)57. [11] D.T. Sawyer, Metal-Gluconate Complexes, [18] K. Fischer, A. Chodura, J. Kotalik, L. Vollner, Chem. Rev. 64 (1964) 633. D. Bieniek, Aliphatische Carbonsauren und [12] K. Okamura, Structure of Cellulose, In: Wood Zuckersauren-Reaktionsprodukte der alkalischen and Cellulosic Chemistry (Eds. D.N.-S. Hon, Hydrolyse von Cellulose und Cellulosederivaten, N. Shiraishi), Marcel Dekker Inc., New York, Nagra unpublished internal report, Nagra, Wet- Basel (1991) pp. 89-112. tingen,CH(1996). [13] H. Krassig, Structure of Cellulose and its Relation [19] R.L. Whistler, J.N. BeMiller, a-D-lsosaccharino- to Properties of Cellulose Fibers, In: Cellulose 1,4-: Action of Lime Water on , In and its Derivatives: Chemistry, Biochemistry and Methods in Carbohydrate Chemistry (Eds. Applications (Eds. J.F. Kennedy, G.O. Phillips, M.L Wolfrom, J.N. BeMiller), Vol. II: Reactions of D.J. Wedlock, P.A. Williams), Marcel Dekker (1963) 477. Inc., New York, Chichester, Brisbane (1985) [20] L.R. Van Loon, M. Glaus, The Solubility of Ca- pp. 3-25. Gluconate and Ca-lsosaccharinate, Internal [14] D.W. Haas, B.F. Hrutfiord, K.V. Sarkanen, Kine- Technical Report TM-44-95-06, Paul Scherrer tic Study on the Alkaline Degradation of Cotton Institute, Villigen, CH (1995). and Hydrocellulose, J. Appl. Polym. Sci. 11 [21] LR. Van Loon, M. Glaus, S. Stallone, A. Laube, (1967)587. The Sorption of Isosaccharinic Acid - a Cellulose [15] M. Lewin, New Chemical Approaches to the Degradation Product - on Cement, accepted for Structure of Cellulose, In: Cellulose and its publication in Environ. Sci & Technol. (1997).