Charging and ion adsorption behaviour of different iron (hydr)oxides

Metal (hydr)oxides are of importance for many soil systems. All metal (hydr)oxides have a surface charge that varies with the pH. The variation in this surface charge is caused by adsorption and desorption of protons. The adsorption of cat- and anions on the metal (hydr)oxide surface is strongly influenced by their variable surface charge. The description of the charging behaviour of metal (hydr)oxides should therefore always be the starting point for modeling.

For the modeling of the chemical adsorption behaviour of metal (hydr)oxide surfaces, many different models are available. Five of these models were compared in their ability to describe an extended data set of cadmium adsorption on goethite (iron oxyhydroxide). None of the models could describe all data simultaneously. The best results were obtained with a surface complexation model, based on the MUlti SIte Complexation MUSIC approach of Hiemstra and van Riemsdijk.

A combination of a new interface model and the MUSIC model, the charge distribution (CD) MUSIC model is used for the description of cadmium adsorption on goethite. The CD- MUSIC mode I could give a good simultaneous description of an extended data set for cadmium adsorption on goethite. The same model, with adapted parameters, could describe extended adsorption data sets for adsorption on goethite in a system with only cadmium, phosphate and a mixture of these two ions.

The MUSIC model predicts proton affinities for individual surface groups of metal- (hydr)oxides. A refinement of the MUSIC model shows that the prediction of the proton affinities of both dissolved and surface groups can be understood in one theoretical framework. The application of the refined MUSIC model to different iron (hydr)oxides shows that the model can predict the charging behaviour very well. For a good prediction of the charging behaviour, the crystal structure and morphology of the oxide must be well known.

Finaly, a comparison is made of the charging behaviour and the cadmium and phosphate adsorption for three different goethites, lepidocrocite and hematite. The differences between the chemical behaviour of the different goethites could be modelled well with parameters which were consistent with the refined MUSIC model. The modelling of the behaviour of lepidocrocite and hematite was more problematic because the crystal morphologies were less well known.