Abstract:
The structure and mechanism of cation sorption at the (0 0 1) muscovite-water interface were investigated in 0.01 and 0.5 m KCl, CsCl, and CaCl2 and 0.01 m BaCl2 solutions at slightly acidic pH by high-resolution X-ray reflectivity. Structural relaxations of atom positions in the 2M1 muscovite were small (≤0.07 Å) and occurred over a distance of 30 to 40 Å perpendicular to the interface. Cations in all solutions were sorbed dominantly in the first and second solution layers adjacent to the mineral surface. The derived heights of the first solution layer in KCl and CsCl solutions, 1.67(6)-1.77(7) and 2.15(9)-2.16(2) Å, respectively, differ in magnitude by the approximate difference in crystallographic radii between K and Cs, and correspond closely to the interlayer cation positions in bulk K- and Cs-mica structures. The first solution layer heights in CaCl2 and BaCl2 solutions, 2.46(5)-2.56(11) and 2.02(5) Å, respectively, differ in a sense opposite to that expected based on crystallographic or hydrated radii of the divalent cations. The derived ion heights in all solutions imply that there is no intercalated water layer between the first solution layer and the muscovite surface. Molecular compositions were assigned to the first two solution layers in the electron density profiles using models that constrain the number density of sorbed cations, water molecules, and anions by considering the permanent negative charge of the muscovite and average solution density. The models result in partial charge balance (at least 50%) by cations sorbed in the first two layers in the 0.01 m solutions and approximately full charge balance in the 0.5 m solutions. Damped oscillations of model water density away from the first two solution layers agree with previous X-ray reflectivity results on the muscovite (0 0 1) surface in pure water. © 2006 Elsevier Inc. All rights reserved.
