Precipitates of Al(III), Sc(III), and La(III) at the Muscovite–Water Interface

The interaction of Al­(III), Sc­(III), and La­(III) with muscovite–water interfaces was studied at pH 4 and 10 mM NaCl using second harmonic generation (SHG) and X-ray photoelectron spectroscopy (XPS). SHG data for Sc­(III) and La­(III) suggest complete and/or partial irreversible adsorption that is attributed by XPS to the growth of Sc­(III) and La­(III) hydroxides/oxides on the muscovite surface. Al­(III) adsorption appears to coincide with the growth of gibbsite (Al­(OH)₃) deposits on the muscovite surface, as indicated by the magnitude of the interfacial potential computed from the SHG data. This interpretation of the data is consistent with previous studies reporting the epitaxial growth of gibbsite on the muscovite surface under similar conditions. The implication of our findings is that the surface charge density of mica may change (and in the case of Al­(III), even flip sign from negative (mica) to positive (gibbsite)) when Al­(III), Sc­(III), or La­(III) is present in aqueous phases in contact with heterogeneous geochemical media rich in mica-class minerals, even at subsaturation conditions

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