Speciation of the trivalent f-elements Eu(III) and Cm(III) in digestive media

In case radioactive materials are released into the environment, their incorporation into our digestive system would be a significant concern. Trivalent f-elements, i.e., trivalent actinides (An(III)) and lanthanides (Ln(III)), could potentially represent a serious health risk due to their chemo- and radiotoxicity, nevertheless the biochemical behavior of these elements are mostly unknown even to date. This study, therefore, focuses on the chemical speciation of trivalent f-elements in the human gastrointestinal tract. To simulate the digestive system artificial digestive juices (saliva, gastric juice, pancreatic juice and bile fluid) were prepared. The chemical speciation of Ln(III) (as Eu(III)) and An(III) (as Cm(III)) was determined experimentally by time-resolved laser-induced fluorescence spectroscopy (TRLFS) and the results were compared with thermodynamic modelling. The results indicate a dominant inorganic species with phosphate/carbonate in the mouth, while the aquo ion is predominantly formed with a minor contribution of the enzyme pepsin in the stomach. In the intestinal tract the most significant species are with the protein mucin. We demonstrated the first experimental results on the chemical speciation of trivalent f-elements in the digestive media by TRLFS. The results highlight a significant gap in chemical speciation between experiments and thermodynamic modelling due to the limited availability of thermodynamic stability constants particularly for organic species. Chemical speciation strongly influences the in vivo behavior of metal ions. Therefore, the results of this speciation study will help to enhance the assessment of health risks and to improve decorporation strategies after ingestion of these (radio-) toxic heavy metal ions.