Incorporation versus adsorption: substitution of Ca2+ by Eu3+ and Cm3+ in aragonite and gypsum.

The aim of the present work is to understand the interaction behaviour of trivalent f-elements with Ca2+-bearing mineral phases on a molecular level. This is achieved by use of time-resolved laser fluorescence spectroscopy (TRLFS) with Eu3+ and Cm3+ as atomic sensors on a trace concentration level. These ions are of special interest as models for trivalent actinides (e.g., Cm(III), Am(III) or Pu(III)) which strongly contribute to radiotoxicity in high level nuclear waste. Results from TRLFS with these trivalent ions show a significantly different mode of interaction in the two cases: while in aragonite only structural incorporation is observed, on gypsum nothing but inner-sphere surface complexes can be found. This shows how the anions forming the coordination sphere of the foreign ion control the formation of solid solutions as opposed to adsorption complexes.