BIO-CEMENTATION OF CAPPING LAYERS IN NEAR-SURFACE RADIOACTIVE DISPOSAL SITES USING MICROBIAL INDUCED CALCITE PRECIPITATION

A near-surface disposal facility for low-level radioactive waste is designed to prevent ingress of water and retard radionuclide migration from the disposal facilities to the biosphere for hundreds of years. Distinct long-term properties are expected from the capping materials of the disposal facility, including mechanical and chemical stability, low penetration, and long durability. Thus, to ensure the safety of the disposal facility, soil improvement techniques are required in order to upgrade the physico-mechanical properties of its capping layers. In the present study, the feasibility of using an emerging soil improvement, bio-cementation technique involving indigenous urea-hydrolyzing bacteria was explored. Israel`s national near surface radioactive waste disposal site is situated at the north-eastern Negev desert and operated by the Nuclear Research Centre Negev. The site receives radioactive waste from hospitals, research institutions, higher education facilities and industry. Bio-stimulation experiments conducted on soils collected from the north-eastern Negev Desert demonstrated that indigenous urea-hydrolyzing bacteria are naturally present in this region. Incubating soil samples in urea and enrichment media demonstrated typical characteristics of urea hydrolysis, including pH evolution and urea depletion. Urea hydrolysis was observed in soil samples collected from different locations in the region albeit at different efficiencies. Increasing the yeast extract concentration in the enrichment media led to more rapid urea hydrolysis, supporting that bio-stimulation kinetics can be controlled by adjusting the energy source concentrations. Following stimulation, bacteria grown in the presence of calcium ions were shown to produce calcium carbonate (calcite) precipitates. Taken together, our research demonstrates that stimulating indigenous bacteria in the sandy soil of the north-eastern Negev Desert can lead to spontaneous production of calcite precipitates, which can serve as a soil stabilizing layer.