Degradation of natural uranium containing micro-mineral phases: Implications to contaminats transport at abandoned mining sites

In this work a combination of multiple synchrotron and laboratory based micro-techniques is utilized to unveil the speciation, heterogeneities and degradation of uranium (U) micro-mineral phases accumulated on rock outcrop from natural U deposit area. The investigated system is sampled from the abandoned Krunkelbach U mine in Southern Germany with 2-3 km surrounding area which represents a natural analogue site with an unique 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 uraninite, mixed U oxy-hydroxides to uranyl silicates, representing a wide scale of U ore weathering events. 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 uranyl silicate and uranyl tungstate mineralization. Based on a multi-technique investigation 10-200 μm Cu(UO2)2(PO4)2-x(AsO4)x·8H2O particles with widely varying arsenic-phosphorus (As-P) content are analyzed. The evidences of a degradation on some zones of selected crystals are found which are associated with higher As 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 drastically 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.