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Redox and Adsorption Processes of Hydrolyzed Uranyl Species on a Mercury Electrode from Sodium Chloride Solutions

Reduction and adsorption processes of hydrolyzed uranyl species on the mercury drop electrode, from 0.55 mol L-1 NaCl, were studied by electrochemical cathodic stripping techniques: cyclic voltammetry, alternating current voltammetry, chronoamperometry and double potential step chronocoulometry. The only uranyl species adsorbed on the Hg electrode (between pH 5 and 7) was found to be uncharged uranyl dihydroxide, as well as its reduction product. At bulk concentrations between 3 and 5 mmol L-1, the reactant forms an adsorbed compact film as a result of surface polymerization, nucleation or other condensation process. The adsorption isotherm at pH 6.2 and -0.2 V was measured. It was fitted with Frumkin type isotherm. At the plateau, a reduction current corresponding to 6.7x10-9 mol cm-2 of uranyl ions was measured. This represents on the average 7 adsorbed layers of UO2(OH)2o, presuming an orthorhombic crystal cell structure. Multilayer adsorption is certain even if a polymeric form of uranyl dihyroxide was considered.

uranyl(VI)-ion; hydrolysis; adsorption

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