Microbial interaction with bituminous materials used in radioactive waste disposal

Introduction: In Belgium, Boom Clay is a potential host rock for the geological disposal of radioactive waste. Part of this radioactive waste is immobilized in a bituminous matrix which typically contains also large amounts of soluble salts (e.g. sodium nitrate and sodium sulphate). Over time, water will infiltrate into the waste drums resulting in the solubilization and leaching of these salts and organic compounds from the waste. This could affect the redox conditions present in the Boom Clay and consequently decrease its reducing capacity. Sulphate is a known electron acceptor for microorganisms and most of the other leachates from bituminized waste are also biodegradable. In this study, we investigated whether the microbial population present in Boom Clay borehole water is able to reduce sulphate in the presence of relevant organic electron donors. Material and Methods: Batch experiments were performed in anoxic septum bottles with Boom Clay borehole water as water environment and as inoculum. Sulphate was added as electron acceptor and acetate or formate were added as electron donors. Acetate and formate concentrations were measured spectrophotometrically, while the microbial population was followed up by intracellular ATP measurements and flow cytometry, PCR and DGGE. Results: A significant decrease in formate was observed from day 2 onwards, while no decrease for acetate over the 84 days of testing was observed. Interestingly, without the addition of sulphate as electron acceptor, a significant decrease in formate was observed after 37 days. Microbial activity and cell count increased during the first week, but stayed constant onwards. There was variation in the bacteria population with time in all the samples, while the archaea population was present only at day 0, and not in all the samples of day 84. Discussion and conclusions: Based on our results, microorganisms present in Boom Clay borehole water can probably reduce sulphate. Furthermore, formate is more favorable as electron donor compared to acetate in this process. Finally, we observed a change in the microbial population with days. However, it should be investigated if this can occur in relevant in situ conditions.