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

Area of Science:

• Environmental Science
• Geochemistry
• Surface Chemistry

Background:

• Understanding uranyl adsorption at mineral-water interfaces is crucial for predicting contaminant transport.
• Muscovite (mica) is a common environmental mineral, making its interaction with uranyl ions significant.
• Previous studies have lacked precise measurements of uranyl adsorption parameters at the muscovite interface.

Purpose of the Study:

• To quantify uranyl adsorption at the muscovite/water interface.
• To determine uranyl binding constants and surface charge density.
• To provide benchmark data for improving contaminant transport models.

Main Methods:

• Second harmonic generation (SHG) spectroscopy was employed to study uranyl adsorption.
• Nonresonant χ(3) and resonantly enhanced SHG experiments were used.
• The Gouy-Chapman model was applied to determine surface charge density.

Main Results:

• The initial surface charge density of muscovite was determined to be -0.022(1) C/m(2) at pH 6 with dissolved carbonate.
• A uranyl binding constant of 3(1) × 10(7) M(-1) was measured, corresponding to a Gibbs free energy of -52.6(8) kJ/mol.
• Maximum surface charge density at monolayer coverage was 0.028(3) C/m(2), indicating strong interactions.

Conclusions:

• Uranyl ions exhibit favorable adsorption to the muscovite interface via ion-dipole or hydrogen bonding.
• A 1:1 uranyl ion to surface site ratio suggests binding of monovalent cations and neutral uranyl species.
• The findings provide essential data for refining environmental contaminant transport models.