Surface Complexation of Selenite on Goethite: MO/DFT Geometry and Charge Distribution*

The adsorption of selenite on goethite (a-FeOOH) has been analyzed with the charge distribution (CD) and the multi-site surface complexation (MUSIC) model being combined with an extended Stern (ES) layer model option. The geometry of a set of different types of hydrated iron-selenite complexes has been calculated using Molecular Orbital / Density Functional Theory (MO/DFT). The optimized geometries have been interpreted with the Brown bond valence approach resulting in a set of ionic charge distribution values. After correction for dipole orientation effects, it results in the interfacial charge distribution coefficients that can be applied to the analysis of adsorption data. The use of theoretical CD values has the practical advantage of a reduction of the number of adjustable parameters. From a theoretical perspective, the CD values can constrain the model, revealing a surface speciation that can be tested experimentally. Modeling of the adsorption of SeO3 in (pseudo-) monocomponent goethite systems, using the calculated CD values, has revealed the dominant presence of a bidentate surface species o(FeO)2SeO. The dominance of this surface species agrees with the interpretation of EXAFS measurements given in literature. The agreement supports the validity of the approach. To describe the adsorption at very low pH and a high loading, formation of an additional surface species is required in the modeling. The maximum contribution is about 20 % or less. In case of anion competition, as found in the PO4-SeO3 goethite system, the relative contribution increases. Analysis of the adsorption behavior in the PO4-SeO3 goethite systems revealed the probable nature of the additional surface complex, which is found to be a protonated monodentate surface complex oFeOSeOOH. With the affinity constants derived, the CD model is able to describe the SeO3 adsorption on goethite over a large range of pH, ionic strength, and loading conditions for a variety of goethite preparations. The CD model correctly predicts the proton co-adsorption of selenite and is able to describe the shift of the IEP upon addition of selenite.