SURFACE STRUCTURAL MODELING OF CATION SORPTION AT THE RUTILE-WATER INTERFACE

Ion adsorption phenomena at mineral-solution interfaces are modeled most effectively when bulk macroscopic measurements are reconciled with crystallographic and molecular information. The Charge Distribution Multisite Complexation (CD-MUSIC) model is a surface complexation model (SCM) that successfully describes adsorption phenomena and the distribution of various surface sites within a single conceptual framework constrained by molecular scale data. For a complete description of interface phenomena, it is necessary in any electrostatic SCM to include a representation of the electrical double layer (EDL) structure.

Recently, it has been shown with X-ray standing wave spectroscopy (Zhang et al. 2004) that electrolyte ions may be present in the interface at positions where they directly coordinate to surface groups. This picture is differently from the common perception that simple electrolyte ions remain primarily hydrated, i.e. form outer-sphere complexes. In the present contribution, this issue will be discussed, by modeling an extensive set of potentiometric surface titration data available for rutile at 25°C with the CD-MUSIC model. The data sets in 0.03 and 0.3 m NaCl, RbCl and KCl media were modeled simultaneously. Such a simultaneous fitting of multiple electrolyte data assumes that each electrolyte ion has a unique position(s) within the EDL. In addition, multivalent ion titration data for the specific adsorption of Ca²⁺ and Sr²⁺ to rutile in NaCl and RbCl media were also fitted simultaneously within the same CD-MUSIC model. In all instances, X-ray standing wave measurements of Zhang et al. (2004) were used to define the predominant cation sorption species as an inner-sphere tetradentate complex. A spatial charge distribution between the rutile surface and the Stern layer was assumed for this inner-sphere complex. Outer-sphere electrolyte ion adsorption was also allowed. This modeling approach successfully described the pH-dependent proton binding in the presence of a variety of ions with a coherent set of adsorption parameters. It revealed that electrolyte ions may be present in the compact part of the EDL of rutile in two main forms, i.e. an outer- and inner-sphere complex. The relative contribution depends on solution conditions.

Research sponsored by: NSR – NIRT initiative EAR-0124001.