AFM STUDY OF THE DISSOLUTION AND SURFACE REACTIVITY OF METATORBERNITE [CU(UO2)2(PO4)2(H20)8] - ETCH PIT FORMATION, SECONDARY PRECIPITATION AND EFFECTS OF CATIONS IN SOLUTION

Metatorbernite [Cu(UO₂)₂(PO₄)₂(H₂0)₈] is an environmentally important mineral occuring localities that have been significantly contaminated by uranium (e.g., Hanford, WA). Here, we investigate how the solubility and surface reactivity of metatorbernite may impact the mobility of uranium (and possibly other actinides) in contaminated groundwater. Crystals of metatorbernite were synthesized using a newly developed, slow diffusion method. Using this method, two morphologies of crystals may be grown: at pH ~ 2, square shaped crystals occur, while at pH ~ 4, irregular, “blade” shaped crystals occur. Micromorphology and microtopography of the (001) surface of metatorbernite was measured using atomic force microscopy (AFM) before and after being treated with one of three solutions. First, metatorbernite crystals were exposed only to HNO₃ solutions (pH < 2); these showed the formation of square etch pits on the basal surface. Second, metatorbernite crystals were exposed to a dilute 30% H₂O₂ solution. The occurrence of crystals with morphologies characteristic of studtite [[(UO₂)O₂(H₂O)₂]·2(H₂O)] was observed on the metatorbernite surface within hours. Third, metatorbernite crystals were exposed to a very dilute Th(NO₃)₄ solution at pH < 2, and the precipitation of a secondary (presumably Th-containing) phase was observed. The possibility of the secondary, insoluble Th-phase occurring on the crystal surface may give insight into the ability of metatorbernite to effect the transport of thorium and other tetravalent actinide elements at contaminated sites. For the latter set of experiments, we characterized the secondary precipitates by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Further, AFM images suggest that, with increased reaction time, secondary phases form on the mineral surface. Short batch dissolution experiments (< 3 hours), showed that the dissolution of uranyl minerals appears to occur only at low pH-values in contrast to weak acidic, neutral or basic conditions.