Competitive adsorption of strontium and fulvic acid at the muscovite-solution interface observed with resonant anomalous X-ray reflectivity

Molecular-scale distributions of Sr²⁺ and fulvic acid (FA) adsorbed on the muscovite (0 0 1) surface were investigated using in situ specular X-ray reflectivity (XR) and resonant anomalous X-ray reflectivity (RAXR). The total amount of Sr²⁺ adsorbed from a 1 × 10⁻² mol/kg SrCl₂ and 100 mg/kg Elliott Soil Fulvic Acid II (ESFA II) solution at pH 5.5 compensated 81 ± 5% of the muscovite surface charge, less than previously measured (118 ± 5%) in an ESFA II-free solution with the same Sr concentration and pH. Inner-sphere (IS) and outer-sphere (OS) Sr²⁺ constituted 87% of the total adsorbed species in IS:OS proportions of 19:81 compared to 42:58 in the solution without FA, suggesting that adsorbed FA competes with the IS Sr²⁺ for surface sites. The coverage of both IS and OS Sr²⁺ decreased even more in a pH 3.5 solution containing the same concentration of FA and 0.5 × 10⁻² mol/kg Sr(NO₃)₂, whereas a significant amount of Sr²⁺ accumulated farther from the surface in the FA layer. The amount of Sr²⁺ incorporated in the ∼10 Å thick FA layer decreased by 79% with decreasing FA concentration (100 → 1 mg/kg) and increasing Sr²⁺ concentration (0.5 × 10⁻² → 1 × 10⁻² mol/kg) and pH (3.5 → 3.6). These results indicate not only that adsorbed FA molecules (and perhaps also H₃O⁺) displace Sr²⁺ near the muscovite surface, but also that the sorbed FA film provides binding sites for additional Sr²⁺ away from the surface. When a muscovite crystal pre-coated with FA after reaction in a 100 mg/kg ESFA II solution for 50 h was subsequently reacted with a 0.5 × 10⁻² mol/kg Sr(NO₃)₂ and 100 mg/kg ESFA II solution at pH 3.7, a significant fraction of Sr²⁺ was distributed in the outer part of the FA film similar to that observed on fresh muscovite reacted at pH 3.5 with a pre-mixed Sr-FA solution at the same concentrations. However, this Sr²⁺ sorbed in the pre-adsorbed organic film was more widely distributed and had a lower coverage, suggesting that pre-sorbed FA may undergo fractionation and/or conformational changes that diminish its capacity, and that of the muscovite (0 0 1) surface, for adsorbing the aqueous Sr cation.