Biomedical subjects

Rasoul Rahnemaie

Publications and source records attributed to Rasoul Rahnemaie.

3 recordsLinked to original sources

PubMed2006

Inner- and outer-sphere complexation of ions at the goethite-solution interface.

Formation of inner- and outer-sphere complexes of environmentally important divalent ions on the goethite surface was examined by applying the charge distribution CD model for inner- and outer-sphere complexation. The model assumes spatial charge distribution between the surface (0-plane) and the next electrostatic plane (1-plane) for innersphere complexation and between the 1-plane and the head end of the diffuse double layer (2-plane) for the outersphere complexation. The latter approach has been used because the distance of closest approach to a charged surface may differ for different ions. The surface structural approach implies the use of a Three-Plane model for the compact part (Stern layer) of solid-solution interface, which is divided into two layers. The thickness of each layer depends on the capacitance and the local dielectric constant. The new approach has been applied to describe the adsorption of magnesium, calcium, strontium, and sulfate ions. It is shown that the concept can successfully describe the development of surface charge in the presence of Ca(+2), Mg(+2), Sr(+2), and SO4(-2) as a function of loading, pH, and salt level, and also the shift in the isoelectric point (IEP) of goethite. The CD modeling revealed that, for the conditions studied, magnesium is mainly adsorbed as a bidentate innersphere complex, calcium can be a combination of bidentate innersphere and a monodentate inner- or outer-sphere complexes, and strontium is probably adsorbed as an outersphere complex. Sulfate is present as a mixture of inner- and outer-sphere monodentate complexes. Outersphere complexation is less pH dependent than innersphere complexation. The CD model predicts that the outersphere complexation of divalent cations and anions is relatively favorable at respectively low and high pH. Increase of ion loading favors the formation of innersphere complexes.

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PubMed2005

A new surface structural approach to ion adsorption: tracing the location of electrolyte ions.

Electrolyte ions differ in size leading to the possibility that the distance of closest approach to a charged surface differs for different ions. So far, ions bound as outersphere complexes have been treated as point charges present at one or two electrostatic plane(s). However, in a multicomponent system, each electrolyte ion may have its own distance of approach and corresponding electrostatic plane with an ion-specific capacitance. It is preferable to make the capacitance of the compact part of the double layer a general characteristic of the solid-solution interface. A new surface structural approach is presented that may account for variation in size of electrolyte ions. In this approach, the location of the charge of the outersphere surface complexes is described using the concept of charge distribution in which the ion charge is allowed to be distributed over two electrostatic planes. It was shown that the concept can successfully describe the pH dependent proton binding and the shift in the isoelectric point (IEP) in the presence of variety of monovalent electrolyte ions, including Li(+), Na(+), K(+), Cs(+), Cl(-), NO(-)(3), and ClO(-)(4) with a common set of parameters. The new concept also sheds more light on the degree of hydration of the ions when present as outersphere complexes. Interpretation of the charge distribution values obtained shows that Cl(-) ions are located relatively close to the surface. The large alkali ions K(+), Cs(+), and Rb(+) are at the largest distance. Li(+), Na(+), NO(-)(3), and ClO(-)(4) are present at intermediate positions.

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PubMed2004

Surface complexation of carbonate on goethite: IR spectroscopy, structure and charge distribution.

The adsorption of carbonate on goethite has been evaluated, focussing on the relation between the structure of the surface complex and corresponding adsorption characteristics, like pH dependency and proton co-adsorption. The surface structure of adsorbed CO3(-2) has been assessed with (1) a reinterpretation of IR spectroscopy data, (2) determination of the charge distribution within the carbonate complex using surface complexation modeling, and (3) evaluation of the proton co-adsorption of various oxyanions, including carbonate, in relation with structural differences. Carbonate adsorption leads to a degeneration of the nu3 IR vibration. Currently, the magnitude of the Deltanu3 band splitting is used as a criterion for metal coordination. However, the interpretation is not unambiguous, since the magnitude of Deltanu3 is influenced by polarization and additional field effects, due to, e.g., H bonding. Our evaluation shows that for goethite the magnitude of band splitting Deltanu3 falls within the range of values that is representative for bidentate complex formation, despite contrarily assignments made in literature. Determination of the charge distribution (CD), derived by modeling available carbonate adsorption data, shows that a very large part (2/3) of the carbonate charge resides in the surface. Interpretation of this result with a bond valence and a ligand charge analysis strongly favors the bidentate surface complexation option for adsorbed carbonate. This option is also supported by the proton co-adsorption of carbonate. The H co-adsorption is very high, which corresponds closely to an oxyanion surface complex in which 2/3 of the ligands are common with the surface. The high H co-adsorption is in conflict with the monodentate option for adsorbed CO3(-2). The study shows that the H co-adsorption of CO3(-2) is almost equal to the experimental H co-adsorption obtained for SeO3(-2) adsorption, which can be rationalized supposing for both XO3(-2) complexes the same ligand distribution in the interface, i.e., bidentate complex formation.

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