Uranyl adsorption at the muscovite (mica)/water interface studied by second harmonic generation

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.