3 research outputs found
Precipitates of Al(III), Sc(III), and La(III) at the Muscovite–Water Interface
The
interaction of AlÂ(III), ScÂ(III), and LaÂ(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 ScÂ(III) and LaÂ(III) suggest complete and/or partial irreversible
adsorption that is attributed by XPS to the growth of ScÂ(III) and
LaÂ(III) hydroxides/oxides on the muscovite surface. AlÂ(III) adsorption
appears to coincide with the growth of gibbsite (AlÂ(OH)<sub>3</sub>) 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 AlÂ(III), even flip sign from
negative (mica) to positive (gibbsite)) when AlÂ(III), ScÂ(III), or
LaÂ(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 χ<sup>3</sup> technique and the Gouy–Chapman model, the initial
surface charge density of the mica surface was determined to be −0.022(1)
C/m<sup>2</sup> at pH 6 and in the presence of dissolved carbonate.
Under these same conditions, uranyl adsorption isotherms collected
using nonresonant χ<sup>3</sup> 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<sup>7</sup> M<sup>–1</sup>, 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<sup>2</sup>. 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<sub>2</sub>)<sub>3</sub>(OH)<sub>5</sub><sup>+</sup>, (UO<sub>2</sub>)<sub>4</sub>(OH)<sub>7</sub><sup>+</sup>, UO<sub>2</sub>OH<sup>+</sup>, UO<sub>2</sub>Cl<sup>+</sup>, UO<sub>2</sub>(CH<sub>3</sub>COO<sup>–</sup>)<sup>+</sup>) binding
at the interface, in addition to neutral uranyl species (UO<sub>2</sub>(OH)<sub>2</sub> and UO<sub>2</sub>CO<sub>3</sub>). This work provides
benchmark measurements to be used in the improvement of contaminant
transport modeling