In Belgium, an important fraction of the current stored volume of long-lived intermediate level radioactive waste is immobilised in a bituminous matrix as Eurobitum, which contains besides bitumen and radionuclides, large amounts of soluble salts with sodium nitrate as the most dominant. Geological disposal of this waste in a water-saturated sedimentary formation such as Boom Clay will induce water uptake by the hygroscopic salts present in the bituminized waste. The dissolution of these salts will result in a further osmosis-induced water uptake through the semi-permeable bitumen membrane and thus in swelling of the Eurobitum. Additionally, sodium nitrate will slowly leach from the waste. The nitrate plume in the clay water could cause a geochemical perturbation of the surrounding clay, possibly affecting the redox conditions, causing ionic strength effects and cation exchange processes, which might result in an increase in the mobility of the radionuclides through the host rock. However, it is known that nitrate can also be removed inside the disposal gallery or in the near-field by various processes. Abiotically, the reduction of nitrate can occur with H2 (produced during radiolysis of bitumen or water and during corrosion of steel) and/or steel acting as electron donors and/or with the steel or even pyrite in the clay possibly serving as catalytic surface. These abiotic reactions would lead to the production of ammonium, which can sorb onto clay minerals and would therefore compete with some radionuclides for sorption. Biotically, nitrate can be consumed as electron acceptor by microorganisms, if proper growth conditions are provided. Respiratory microbial consumption of nitrate leads to the intermediate production of nitrite, and finally to nitrogenous gases. Most of the other leachates from bituminized waste (e.g. acetate, H2) are biodegradable and can be used as electron donor, to fuel this microbial nitrate reduction. Depending on the electron donor used in this denitrification process, the final overall result could be a gas pressure decrease or increase. During disposal conditions, the microbial population will be exposed to hyperalkaline conditions originating from the pore water from the concrete lining of the waste monolith and the backfill material, which could affect the viability and activity and thus denitrification potential of the microbial population. The efficiency of the Boom Clay borehole water microbial community to reduce nitrate leaching from regular and thermally aged non-radioactive Eurobitum in the presence or absence of known bitumen degradation products was investigated in different series of anoxic laboratory batch experiments. It was shown that acetate is the most preferred electron donor for microbial nitrate reduction and the highest nitrate reduction rates were observed in the presence of acetate. Formate seemed also to be an easy accessible electron donor, but related to the kinetics of the reaction, not as efficient as acetate to remove nitrate. On the other hand, oxalate was the least preferred electron donor for nitrate reduction and was only completely degraded in one out of three replicates. However, calcium oxalate crystals were formed, indicating that if oxalate is present, it will probably be less (bio)available compared to other organic compounds. Next to the added bitumen degradation products, the microbial community was able to use organics that leached from the Eurobitum blocks as electron donor to carry out nitrate reduction. Moreover, a clear biofilm formation on the solid bitumen block was observed in all conditions, suggesting that microorganisms could enhance the degradation of Eurobitum. However, in the high pH conditions expected to prevail in a repository, no biofilm on Eurobitum or nitrate reduction was observed when the community was exposed to pH 12.5 for 110 days. In general, pH 12.5 seems the upper limit for microbial nitrate reduction for both the microbial community of the Boom Clay borehole water and the pH adapted Harpur Hill sediment. However, stress evoked by this high pH was not enough to eliminate the complete microbial community as intact cells remained present and could be resuscitated when lowering the pH to a less alkaline pH. In addition, the Harpur Hill sediment revealed an initial increase in cell numbers a this high pH 12.5 similar as observed in the other conditions. The microbial community present in the Harpur Hill sediment is clearly adapted to high pH conditions as twice as much nitrate was reduced at pH 10.5 compared to pH 9. The Boom Clay microbial population was also still able to carry out nitrate reduction in at pH 10.5, although rates were lower compared to pH 9. It must be noted, however, that phosphate can quickly become a limiting nutrient in such experimental batch conditions, hence it is important to distinguish between pH and nutrient limitation in these laboratory test. Nevertheless, phosphate is not expected to be a limiting nutrient in situ as phosphate in Boom Clay is bioavailable the by the mineral fraction such as apatite. Beta diversity assessment of both communities based on flow cytometry profiles indicate that at pH can induce a shift in the microbial community which is different depending on the microbial community. Altogether, this study indicates that microbes can make biofilms on Eurobitum, and can reduce the nitrate leaching of the bitumen to nitrite and nitrous gasses. To carry out this nitrate reduction, the microbial community is able to use organics leaching from the bituminized waste. High alkaline pH (up to 12.5) alone is not sufficient to eliminate microbial presence in a geological repository, but it can induce a significant shift in the microbial community and inhibit microbial activity. Where a less alkaline pH will occur, e.g. at the boundary of the Boom Clay and the concrete liner of the disposal gallery, microbes can be present and are able to perform nitrate reduction.
Original languageEnglish
Number of pages57
StatePublished - 31 May 2018

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