Bioassociation of uranium onto extreme halophilic microorganisms relevant in nuclear waste repositories in rock salt

For the final storage of radioactive waste in a deep geological formation rock salt is a potential host rock. Indigenous microorganisms and its interactions with radionuclides have to be considered for the safety performance of the repository in terms of a worst case scenario, where radionuclides are potentially released from the storage site. Therefore, two extreme halophilic microorganisms, which originally occur in rock salt, were used to study its interactions with uranium. The kinetics of uranium bioassociation onto cells of the extreme halophilic archaeon Halobacterium noricense DSM 15987 and the moderate halophilic bacterium Brachybacterium sp. G1 were investigated in detail in batch experiments. For the understanding on a molecular level, in situ infrared spectroscopy was applied, monitoring the bioassociation processes online.
It turned out, that the mechanism of uranium association onto the two different microorganisms differs. The studies were performed at 1.7 M NaCl and 3 M NaCl for the bacterium and archaeon, respectively, to keep the essential osmotic pressure. Both experiments started with washed cells from the exponential growth phase at an initial U(VI) concentration of 40 µM U(VI) at pCH+ 6 (corrected pH due to the presence of high chloride concentration). The occurring process for Brachybacterium sp. G1 was a fast biosorption process, which was completed after 1 h. Infrared spectroscopy showed that only carboxylate functional groups were involved in uranium sorption. In contrast, the association onto H. noricense was a rather complex, multistage process [1]. Within the first hour, an association was observed, which was followed by a desorption phase for about 4 hours. Subsequently, uranium was bioassociated again over the timeframe of one week. Apart from carboxylate functional groups, contributions of phosphoryl groups to uranium binding were evidenced by infrared spectroscopy. The occurrence of the multistage uranium association was furthermore visualized with scanning electron microscopy.
[1] Bader, M et al. (2017) J. Hazard. Mater. 32, 225 – 232.