Uranyl Adsorption at the Muscovite (Mica)/Water Interface Studied by Second Harmonic Generation

摘要

Uranyl adsorption at the muscovite (mica)/water interface was studied by second harmonic generation (SHG). Using the nonresonant X~3 technique and the Gouy-Chapman model, the initial surface charge density of the mica surface was determined to be -0.022(1) C/m~2 at pH 6 and in the presence of dissolved carbonate. Under these same conditions, uranyl adsorption isotherms collected using nonresonant X~3 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) × 10~7 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/m~2. 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 ((UO_2)_3(OH)_S~+, (UO_2)_4(OH)_7~+, UO_2OH~+, UO_2C l~+, UO_2(CH_3COO~-)~+) binding at the interface, in addition to neutral uranyl species (UO_2(OH)_2 and UO_2CO_3). This work provides benchmark measurements to be used in the improvement of contaminant transport modeling.