Distribution of barium and fulvic acid at the mica-solution interface using in-situ X-ray reflectivity |
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| Authors: | Sang Soo Lee Kathryn L Nagy |
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| Institution: | a Department of Earth and Environmental Sciences, 845 West Taylor Street MC-186, University of Illinois at Chicago, Chicago, IL 60607, USA
b Chemistry Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA |
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| Abstract: | The interfacial structures of the basal surface of muscovite mica in solutions containing (1) 5 × 10−3 m BaCl2, (2) 500 ppm Elliott Soil Fulvic Acid I (ESFA I), (3) 100 ppm Elliott Soil Fulvic Acid II (ESFA II), (4) 100 ppm Pahokee Peat Fulvic Acid I (PPFA), and (5) 5 × 10−3 m BaCl2 and 100 ppm ESFA II were obtained with high resolution in-situ X-ray reflectivity. The derived electron-density profile in BaCl2 shows two sharp peaks near the mica surface at 1.98(2) and 3.02(4) Å corresponding to the heights of a mixture of Ba2+ ions and water molecules adsorbed in ditrigonal cavities and water molecules coordinated to the Ba2+ ions, respectively. This pattern indicates that most Ba2+ ions are adsorbed on the mica surface as inner-sphere complexes in a partially hydrated form. The amount of Ba2+ ions in the ditrigonal cavities compensates more than 90% of the layer charge of the mica surface. The electron-density profiles of the fulvic acids (FAs) adsorbed on the mica surface, in the absence of Ba2+, had overall thicknesses of 4.9-10.8 Å and consisted of one broad taller peak near the surface (likely hydrophobic and positively-charged groups) followed by a broad humped pattern (possibly containing negatively-charged functional groups). The total interfacial electron density and thickness of the FA layer increased as the solution FA concentration increased. The sorbed peat FA which has higher ash content showed a higher average electron density than the sorbed soil FA. When the muscovite reacted with a pre-mixed BaCl2-ESFA II solution, the positions of the two peaks nearest the surface matched those in the BaCl2 solution. However, the occupancy of the second peak decreased by about 30% implying that the hydration shell of surface-adsorbed Ba2+ was partially substituted by FA. The two surface peaks were followed by a broad less electron-dense layer suggesting a sorption mechanism in which Ba2+ acts dominantly as a bridging cation between the mica surface and FA. When the muscovite reacted first with FA and subsequently with BaCl2, more Ba2+ could be adsorbed on the FA-coated mica surface. The peak closest to the mica included Ba2+ ions adsorbed directly on the mica in an amount similar to that in the BaCl2 solution but more broadly distributed. A second peak observed within the FA layer suggests that the FA coating provides additional sites for Ba2+ sorption. The results indicate that enhanced uptake of heavy metals can occur when an organic coating already exists on a mineral surface. |
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