Uranyl adsorption at the muscovite (mica)/water interface was studied by second harmonic generation (SHG). Using the nonresonant χ3 technique and the Gouy-Chapman model, the initial surface charge density of the mica surface was determined to be -0.022(1) C/m2 at pH 6 and in the presence of dissolved carbonate. Under these same conditions, uranyl adsorption isotherms collected using nonresonant χ3 experiments and resonantly enhanced SHG experiments that probe the ligand-to-metal charge transfer bands of the uranyl cation yielded a uranyl binding constant of 3(1) × 107 M-1, corresponding to a Gibbs free energy of adsorption of -52.6(8) kJ/mol, and a maximum surface charge density at monolayer uranyl coverage of 0.028(3) C/m2. These results suggest favorable adsorption of uranyl ions to the mica interface through strong ion-dipole or hydrogen interactions, with a 1:1 uranyl ion to surface site ratio that is indicative of monovalent cations ((UO2)3(OH) 5+, (UO2)4(OH)7 +, UO2OH+, UO2Cl+, UO2(CH3COO-)+) binding at the interface, in addition to neutral uranyl species (UO2(OH)2 and UO2CO3). This work provides benchmark measurements to be used in the improvement of contaminant transport modeling.
ASJC Scopus subject areas
- Environmental Chemistry