CH0400007 PAUL SC HERR ER I NSTITUT PSI Bericht Nr. 04-01 April 2004 ISSN 1019-0643 Nuclear Energy and Safety Research Department Laboratory for Waste Management Cellulose Degradation at Alkaline Conditions: Long-Term Experiments at Elevated Temperatures M. A. Glaus and L. R. Van Loon PAUL S HERR ER IN ST IT UT PSI Bericht Nr. 04-01 April 2004 ISSN 1019-0643 Nuclear Energy and Safety Research Department Laboratory for Waste Management Cellulose Degradation at Alkaline Conditions: Long-Term Experiments at Elevated Temperatures M. A. Glaus and L. R. Van Loon Paul Scherrer Institut 5232 Villigen PSI Switzerland Tel. 41 056 310 21 Fax 41 056 310 21 99 Internet: www.psi.ch PREFACE The Laboratory for Waste Management of the Nuclear Energy and Safety Research Department at the Paul Scherrer Institut is performing work to develop and test models as well as to acquire specific data relevant to performance assessments of planned Swiss nuclear waste repositories. These investigations are undertaken in close co- operation with, and with the partial financial support of, the National Cooperative for the Disposal of Radioactive Waste (Nagra). The present report is issued simultaneously as a PSI-Bericht and a Nagra Technical Report. The work presented in this report has further been co-funded by Nirex and SKB. This report has been prepared, verified and approved for publication by PSI and Nagra. The work was carried out in accordance with quality assurance arrangements established by PSI, Nagra, Nirex and SKB. The views expressed and conclusions reached are those of the authors and do not necessarily represent those of the co-funders. The report may be freely used for non-commercial purposes. However all commercial uses, including copying and republication require permission from PSI, Nagra, Nirex and SKB. All intellectual property rights reside with the co-funders of the work. PSI Bericht 04-01 TABLE OF CONTENTS TABLE OF CONTENTS .................................................................................................. I LIST OF FIGURES ........................................................................................................ 11 LIST OF TABLES ......................................................................................................... V LIST O F SYMBOLS AND ABBREVIATIONS ............................................................... VI ABSTRACT ................................................................................................................. VII ZUSAM MENFASSUNG ............................................................................................... Ix RESUM E .................................................................................................................... xi 1 INTRODUCTION ..................................................................................................... I 1.1 Impactofthedegreeofcellulosedegradation .................................................. 1 1.2 The alkaline degradation of cellulose ............................................................... 2 1.3 Conflicting long-term predictions for cellulose degradation ............................... 5 1.4 Purpose and strategy followed in the present study ......................................... 8 2 EXPERIMENTAL ................................................................................................... 3 3 RESULTS .............................................................................................................. 9 3.1 Degree of cellulose degraded ........................................................................ 19 3.2 Supporting information .................................................................................. 29 3.2.1 Dilution of reaction suspensions ......................................................... 29 3.2.2 Alkalinity titrations ............................................................................... 30 3.2.3 Elemental analyses ............................................................................ 34 3.2.4 Other degradation products than ISA and mass balance for carbon .... 36 4 DISCUSSION ........................................................................................................ 43 4.1 Qualitative model comparison ........................................................................ 43 4.2 Attempt to quantitatively model the alkaline hydrolysis ................................... 47 4.2.1 Evaluation based on Celdeg............................................................... 47 4.2.2 Evaluation based on DP ..................................................................... 50 4.3 Implications of the fit parameters obtained ..................................................... 50 4.4 Implications for the prediction of the long-term behaviour of cellulose ............. 52 Acknowledgements ..................................................................................................... 53 5 REFERENCES ...................................................................................................... 54 APPENDIX: HPAEC AND HPIEC CHROMATOGRAMS .............................................. 57 PSI Bericht 04-01 LIST OF FIGURES Figure 1: Prediction of pore water concentration of isosaccharinic acid (ISA) for typical conditions encountered in a cementitious repository for radioactive waste planned in Switzerland. The model calculations are based on parameters for cellulose degradation and sorption of ISA on cement according t VAN LOON GLAUS 1998) and VAN LON et al. 1997). In the presence of Ca 2, ions the solubility of Q-ISA cannot exceed the limit indicated by the gray bar . .............................................................................. ..................................................................... 1 Figure 2 Simplified representation of the processes involved in the alkaline degradation of cellulose. Note that the chain lengths depicted are not represented to scale and do not indicate changes in chain-length due to progression of the reactions. The uppermost rectangle refers to the peeling-off process, the middle rectangle to stopping processes and the low est rectangle to the alkaline hydrolysis . ..................................................................................... 3 Figure 3 Comparison of Celdeg calculated from TOC (plot A) and [ISAI (plot B) or degradation experiments carried out at pH 13.3 and at 1 00 gdM-3 of cellulose. No experimental uncertainty was assigned to the data of PAVASARS (1 999) . ............................................................ 6 Figure 4 Different ways to model the ISA data shown in Fig. 3 The solid and dotted lines are based on a model including alkaline hydrolysis, but using different reaction rate constants k. ............... 7 Figure 5: Cellulose degradation simulated using different values for the kinetic constant for alkaline h yd ro ly sis k . ...................................................................................................................................... Figure 6 Different scenarios by which the data in the literature may be explained. For details, the reader is referred to the text . ........................................................................................................... 9 Figure 7 Predictions for the degradation of Aldrich cellulose at elevated temperatures for the scena rio s listed in the text . ............................................................................................................ 1 1 Figure 8: Degradation of Aldrich cellulose in the presence of Ca(OH), at pH 13.3 and T = 90'C (open symbols: A90, closed symbols: C90). The initial ratio of weight of cellulose to volume of liquid is -1 0 gdM-3. The data of series A90 are average values from three independent experiments. The error bars represent the maximum of the sample-to-sample variations and the experimental uncertainties calculated for the single measurements . ............ 25 Figure 9 Degradation of Aldrich cellulose in the absence of Ca(OH)2 at pH 13.3 and T = 90'C (series B90). The initial ratio of weight of cellulose to volume of liquid is 0 g-dM-3. All data are from one single experiment. The error bars represent the experimental uncertainties calculated for the single measurements . ................................................................. 25 Figure 10: Degradation of cotton cellulose in the presence o Ca(OH)2 at pH 13.3 and T = 90'C (se- ries E90). The initial ratio of weight of cellulose to volume of liquid is -10 g-dM-3. Each time point represents the results of one single independent experiment. The error bars represent the experimental uncertainties calculated or the single measurements . .................... 26 Figure 1 1: Degradation of cotton cellulose in the absence of Ca(OH)2 at pH 13.3 and T = 90'C (series F90). The initial ratio of weight of cellulose to volume o liquid is 10 gdM-3. Each time point represents the results of one single independent experiment. The error bars represent the experimental uncertainties calculated for the single measurements . .................... 26 III PSI Bericht 04-01 Figure 12: Degradation of cotton cellulose in the presence of Ca(OH)2 at pH 13.3 and T = 90'C (se- ries G90). The initial ratio of weight of cellulose to volume of liquid is 67 g-dM-3 . Each time point represents the results of one single independent experiment. The error bars represent the experimental uncertainties calculated for the single measurements . .................... 27 Figure