Experimental Investigation on the Active Range of Sulfate-Reducing Bacteria for Geological Disposal
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EXPERIMENTAL INVESTIGATION ON THE ACTIVE RANGE OF SULFATE-REDUCING BACTERIA FOR GEOLOGICAL DISPOSAL S. FUKUNAGA*, H. YOSHIKAWA**, K. FUJIKI* and H. ASANO* *: Research Institute, ***: Nuclear Power Division,
Ishikawajima-Harima Heavy Industries Co., Ltd. 1, Shin-nakahara-cho, Isogo-ku, Yokohama 235, Japan **: Power Reactor & Nuclear Fuel Development Corporation, Muramatsu, Tokai-mura, Naka-gun, lbaraki 319-11, Japan ABSTRACT
The active range of Desulfovibrio desulfuricans, a species of sulfate-reducing bacteria, was examined in terms of pH and Eh using a fermenter at controlled pH and Eh. Such research is important because sulfate-reducing bacteria (SRB) are thought to exist underground at depths equal to those of supposed repositories for high-level radioactive wastes and to be capable of inducing corrosion of the metals used in containment vessels. SRB activity was estimated at 35°C, with lactate as an electron donor, at a pH range from 7 to 11 and Eh range from 0 to -380 mV. Activity increased as pH approached neutral and Eh declined. The upper pH limit for activity was between 9.9 and 10.3, at Eh of -360 to -384 mV. The upper Eh limit for activity was between -68 and -3 mV, at pH 7.1. These results show that SRB can be made active at higher pH by decreasing Eh, and that the higher pH levels of 8 to 10 produced by use of the buffer material bentonite does not suppress SRB completely. A chart was obtained showing the active range of Desulfovibrio desulfuricans in terms of pH and Eh. Such charts can be used to estimate the viability of SRB and other microorganisms when the environmental conditions of a repository are specified. INTRODUCTION
Prior to the 1980s, studies on the disposal of high and intermediate-level radioactive waste in deep geological formations never considered the possible role of microorganisms, as repositories were expected to be sterile because of their extreme environmental conditions. Subsequent research has ascertained that this assumption can no longer be justified [I]. Much work has been carried out on the populations, activities and diversity of subterranean bacteria, using rock and groundwater samples collected from sites several hundreds to 4200 meters deep. Viable counts of 101 to 105/mL and total counts of 103 to 106/mL have been reported. The diversity extends to include heterotrophs, sulfate-reducing bacteria, methanogenic bacteria, fermenting bacteria, N2-fixating bacteria, nitrifiers, and sulfuroxidizers [2]. Iron-reducing bacteria have also been observed [3]. At present it is generally believed that microorganisms can exist several hundred meters underground, i.e., at depths equal to those of supposed repositories. Such microorganisms could disperse into a repository from the surrounding rock and groundwater. Contamination from barrier material or other sources is also possible [4]. If the incoming microorganisms are active, they can act on the materials used in storage containers and artificial barriers (such as steel, concrete, bentonite, and bitumen), decompose wastes, alte
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