Abstract
Leaching experiments of solid matrices (bitumen and cement pastes) have been first implemented to define the physicochemical conditions that microorganisms are likely to meet at the bitumen-concrete interface (see the paper of Bertron et al.). Of course, as might be suspected, the cement matrix imposes highly alkaline pH conditions (10 <pH <11). The screening of a range of anaerobic denitrifying bacterial strains led us to select Halomonas desiderata as a model bacterium capable of catalyzing the reaction of nitrate reduction in these extreme conditions of pH. The denitrifying activity of Halomonas desiderata was quantified in batch bioreactor in the presence of solid matrices and / or leachate from bitumen and cement matrices. Denitrification was relatively fast in the presence of cement matrix (<100 hours) and 2 to 3 times slower in the presence of bituminous matrix. Overall, the presence of solid cement promoted the kinetics of denitrification. The observation of solid surfaces at the end of the experiment revealed the presence of a biofilm of Halomonas desiderata on the cement paste surface. These attached bacteria showed a denitrifying activity comparable to planktonic bacterial culture. On the other side, no colonization of bitumen could be highlighted as either by SEM or epifluorescence microscopy. Now, we are currently developing a continuous experimental bioreactor which should allow us a more rational understanding of the bitumen-cement-microbe interactions.
Highlights
Before disposal, LILW-SL may be compacted to reduce its volume, or solidified by coating in a matrix of bitumen if it is a liquid [1,2]
Leaching experiments of solid matrices have been first implemented to define the physicochemical conditions that microorganisms are likely to meet at the bitumen-concrete interface
Considering physico-chemical conditions determined in the bitumen-cement interface: pH≈10.6 / 0.50 mM acetic acid / 0.33mM nitrates, we focused our attention to identify a model microorganism capable of (i) growing in alkaliphilic conditions, (ii) using nitrate as electron acceptor and catalyzing its reduction to the step of nitrogen gas, and (iii) oxidizing simple organic substrates such as carboxylic acids with short aliphatic chains
Summary
LILW-SL may be compacted to reduce its volume, or solidified by coating in a matrix of bitumen if it is a liquid [1,2]. It is placed in metal containers and embedded in concrete. During storage (Figure 1), driven by the combined effects of water resaturation and irradiation, bitumen and concrete are likely to release chemical species, especially soluble salts including, hydroxides, nitrates, organic matter (organic acids, phenols ...), gas and radionuclides [3,4,5,6].
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