Molecular binding of Eu(III)/Cm(III) by Stenotrophomonas bentonitica and its impact on the safety of future geodisposal of radioactive waste

Microbial communities occurring in reference materials for artificial barriers (e.g. bentonites) in future deep geological repositories of radioactive waste can influence the migration behavior of radionuclides such as curium (CmIII). This study investigates the molecular interactions between CmIII and its inactive analogue europium (EuIII) with the indigenous bentonite bacterium Stenotrophomonas bentonitica at environmentally relevant concentrations. Potentiometric studies showed a remarkable high concentration of phosphates at the bacterial cell wall compared to other bacteria, revealing a great potential of S. bentonitica for metal binding. Infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) confirmed the role of phosphates and carboxylate groups from the cell envelope in the bioassociation of EuIII. The ATR-FTIR spectra also suggested a bidentate bridging EuIII complex with carboxylate groups. Additionally, time-resolved laser-induced fluorescence spectroscopy (TRLFS) identified phosphoryl and carboxyl groups from bacterial envelopes, among other released complexing agents, to be involved in the EuIII and CmIII coordination. Microscopic and kinetic Eu-binding studies indicated biosorption as the main interaction process, in addition to other mechanisms. The ability of this bacterium to form a biofilm at the surface of bentonites allow them to immobilize trivalent lanthanide and actinides in the environment.