An electron paramagnetic resonance study of Cu(II) sorbed on kaolinite

Abstract Cu(II) sorbed on kaolinite (KGa-1b) was studied using electron paramagnetic resonance (EPR) spectroscopy over various pH, ionic strength, and Cu(II) concentration conditions. Two different absorption lines, namely, an isotropic ( g iso = 2.193) and an anisotropic signal ( g || = 2.365 and g ⊥ = 2.088) were observed at room temperature. The signals were assigned to an outer-sphere and an inner-sphere Cu(II) surface complex, respectively. The intensity of the two signals varied with the sorption conditions or post-sorptional treatments such as rinsing and dehydration/rehydration. The intensity of the isotropic signal was not directly proportional to the amount of sorbed Cu(II). Rinsing of the Cu-loaded surface had a different impact on the signal intensity depending on the sorption pH. The isotropic signal together with the anisotropic signal was simultaneously observed on the kaolinite surface air-dried at room temperature for as long as 13 months. On the other hand, only the anisotropic signal was observed on the surface dried at 105 °C overnight. The re-wetting of the once dried kaolinite with deionized water immediately restored the isotropic signal. These results show that an outer-sphere surface complex is the dominant form of the sorbed copper ions and that the drying and aging of the surface convert the outer-sphere complex into the inner-sphere complex or surface precipitate, at least a part of which was reversibly converted back into the outer-sphere complex by the rehydration of the surface. This work suggests that dehydration during aging as well as solution variables such as pH, concentrations of Cu(II) and competing cations can have an impact on the amount and long-term fate of the outer-sphere Cu(II) complex formed on the kaolinite surface.