Use of attenuated total reflection - infrared spectroscopy for in situ study of inorganic ions adsorption on metal oxy-hydroxides

The long-term safety of radioactive waste depositories is based on the sorption of radionuclides on engineered and natural barriers. In order to predict the safety of such depositories, the extrapolation of sorption data measured on reference systems at the laboratory scale, to real systems and over a very long time is needed. This prediction must be performed through models based on physico-chemical processes as close as possible to the real ones. The structure of surface complexes is an important data when determining thermodynamic surface complexation constants. Differentiating between inner- (IS) and outer-sphere (OS) complexes has been often made by observing the effect of ionic strength on the quantity of sorbed ions. However, this method has been questioned, pointing out the need of spectroscopic methods to solve this problem. The fitting of experimental data, as the sorbed fraction of ions versus pH, has been used to determine both the surface inner-sphere complex structure (typically monodentate or bidentate) and the thermodynamic complexation constant. However, due to the number of adjustable parameters (site density, acidity constants, surface complexation constants, electrostatic parameters,..), such a method often led to several sets of surface complexes, what makes spectroscopic results of invaluable help for determining the nature of surface complexes. Due to its ability to probe chemical bonds, infrared spectroscopy has been applied for years to sorption studies. The first studies made use of invasive sampling techniques on dried samples. Since ions might be coordinated differently on dried and wet surfaces, in-situ FTIR measurements in the presence of water should be preferred. The problem of the strong IR absorption by water has been solved by using attenuated total reflection (ATR), able to probe a short depth of sample by the evanescent wave existing in a lower index refraction medium (water) in contact with a more dense medium in which an IR radiation is propagating (see figure below). This presentation will address the current state of knowledge in the use of ATR-IR to determine surface complexes between inorganic ions and oxides/hydroxides surfaces together with our recent results. (orig.)