Effects of Microorganisms Growth on the Long-Term Stability of Cement and Bitumen
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EFFECTS OF MICROORGANISMS GROWTH ON THE LONG-TERM STABILITY OF CEMENT AND BITUMEN
MARIE F. LIBERT, R. SELLIER, G. JOUQUET, M. TRESCINSKI and H. SPOR, Department of waste management and disposal, Nuclear Research Center of Cadarache, 13108 St Paul Lez Durance Cedex, FRANCE. ABSTRACT Cement is used as a coating matrix for nuclear waste or as an engineered barrier of waste repositories situated in geological formations. The effect of mineral acids excreted by bacteria (Thiobacillus) or organic acids produced by fungi, on the biodegradation of cement is discussed. Organic acids are quantitatively and qualitatively determined during growth of fungi over a two-year period. Even with high pH conditions, pH of the cement = 11, growth of microorganisms occurs. Biodeterioration of cement is expressed in terms of bioleaching velocity of calcium and is observed by electron microscopy.
Bitumen is commonly used as a matrix for the long-term storage of radioactive wastes. Long-term biodegrability of bitumen is discussed as a function of its chemical composition and various studied microorganisms.
INTRODUCTION It is now well known that in geological formations, there are microorganisms well adapted to that particular environment.. These microbes show an important potential of mineral alteration, when provided with an adequate source of energy. For safety reasons,we must evaluate the effects of microbial activity on the long-term stability of materials such as cement or bitumen used for nuclear waste disposal. Biodegradation of Cement The biodegradation of cement is due to the products of microbial metabolism, such as organic or mineral acids, and thus is an indirect effect of microbial growth. Microorganisms can produce organic acids resulting from cellulose biodegradation; cellulose is representative of an organic solid waste. In the same way, microorganisms can produce mineral acid (H2S04) from sulfur, as a source of energy; sulfur can be present in the environment of nuclear waste, or in the waste itself. The aim of our study is to compare the effect of these two kinds of acids on the cement alteration. The experimental device used consists of two disk samples of ordinary portland cement containing approximatively 15 percents W/W of portlandite Ca (OH) 2, immersed in a mineral medium (Fig. 1).
Mat. Res. Soc. Symp. Proc. Vol. 294. c)1993 Materials Research Society
268
N
CYLINDER -
OF CEMENT DIAMETER: 70 mm
m
THICKNESS:8 mm
Figure 1. Biodegradation of Cement Experimental Device.
This medium is inoculated: - with a mixed culture of fungi, (Aspergillus niger, Trichoderma viride), heterotroptic microorganisms producing organic acids isolated from a soil sample. The carbon and energy source is glucose, which is an intermediate product of cellulose degradation. - or with a mixed culture of Thiobacillus, autotrophic microorganisms producing H2SO4 from sulfur as a energy source. During a twelve-month period, the evolution of produced acids and the amount of leached calcium are measured (Fig. 2a. organic acids; Fig. 2b. mine
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