title: In situ nanoscale observations of metatorbernite surfaces interacted with aqueous solutions.
abstract: Metatorbernite (Cu(UO(2))(2)(PO(4))(2)·8H(2)O) has been identified in contaminated sediments as a phase controlling the fate of U. Here, we applied atomic force microscopy (AFM) to observe in situ the interaction between metatorbernite cleavage surfaces and flowing aqueous solutions (residence time = 1 min) with different pHs. In contact with deionized water the features of (001) surfaces barely modify. However, changes are remarkable both under acidic and basic conditions. In acidic solutions (pH = 2.5) metatorbernite surface develops a rough altered layer and large pits nucleate on it. The altered layer shows a low adhesion and is removed by the AFM tip during the scanning. The large pits spread rapidly, at few tens of nm/s, indicating a collapse of the structure. The combination of dissolution and the presence of defects in the metatorbernite structure can explain both the collapse process and the alteration of the surfaces under acidic conditions. Other mechanisms such as ion exchange reactions remain speculative. In NaOH solutions (pH = 11.5) metatorbernite dissolves by formation of etch pits bounded by steps parallel to [100], the direction of the most straight periodic bond chains (PBCs) in metatorbernite structure. These steps retreat at ∼0.15 nm/s. Under these conditions dissolution is promoted by the formation of stable uranyl carbonate complexes in solution.
authors: Departamento de Cristalografía y Mineralogía, Universidad Complutense de Madrid , C/José Antonio Novais 2, Madrid 28040, Spain. jmastill@ucm.es
authors: José Manuel
authors: JM
authors: Astilleros
authors: André Jorge
authors: AJ
authors: Pinto
authors: Mário A
authors: MA
authors: Gonçalves
authors: Nuria
authors: N
authors: Sánchez-Pastor
authors: Lurdes
authors: L
authors: Fernández-Díaz
mesh_terms: D019015:Geologic Sediments / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D010710:Phosphates / Q000737:chemistry*; D012996:Solutions; D013499:Surface Properties; D017974:Uranium Compounds / Q000737:chemistry*; D014867:Water / Q000737:chemistry*
chemical_list: D010710:Phosphates; D012996:Solutions; D017974:Uranium Compounds; C554754:metatorbernite; D014867:Water