Degradation of Cu-bearing uranyl As-P micromineral phases from Krunkelbach uranium deposit, Southern Germany

The abandoned Krunkelbach uranium (U) mine, Southern Germany, with 2-3 km surrounding area represents a unique natural analogue site with accumulation of U minerals suitable for investigations of potential mobilization-immobilization processes expected in a real spent nuclear fuel repository. A specific feature of the site is the occurrence of more than forty secondary U minerals, from mixed redox U oxy-hydroxides to alkaline metal uranyl silicates, thus representing a wide scale of U ore weathering events. In this work the combination of synchrotron and laboratory techniques is used to unveil U speciation and micro heterogeneities in U phases accumulated on granitic rock outcrop from U deposit area. Available data on the age of the secondary U mineralization indicates that oxidizing processes at the site started some 340,000 years ago and continues up to date. Several phases close to Cu(UO2)2(PO4)2-x(AsO4)x·8H2O are identified on 1×2 mm2 area with presumably older, more evenly distributed Cu(UO2)2(SiO3OH)2·6H2O and (Fe, Ba, Pb)(UO2)2(WO4) (OH)4·12H2O, mineralization. Based on a multi-technique investigation 10-200 µm Cu(UO2)2(PO4)2-x(AsO4)x·8H2O particles with widely varying As-P content are analyzed. The evidences of a degradation occurred on some zones on the selected crystals are found associated with higher As and decreased P content. This observation can be apparently attributed to different degradation properties of the mixed As-P phases depending on As-P ratio and originate from different solubility properties of Cu(UO2)2(PO4)2·8H2O and Cu(UO2)2(AsO4)2·8H2O species. The conditions for preferential formation of As rich Cu(UO2)2(XO4)2·8H2O [X=As, P] phases and its possible role on U transport in environment under oxidizing conditions are discussed.