| The location of a rare earth element (europium) and phosphate in calcium sulfate was investigated using powder X-ray diffraction (PXRD), inductively coupled plasma mass spectrometry (ICP-MS), and scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) and photoluminescence spectroscopy (PLS). Thermogravimetric analysis with differential scanning calorimetry (TG-DSC) was performed to characterize the hydration state of samples synthesized at 80 °C. A leaching experiment was also carried to determine the amount of europium leached. The leachate was analyzed by Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). The combined results indicated that Eu forms separate phases rather than solid solutions in gypsum or bassanite. When there is no phosphate present, the europium forms an auxiliary phase with sulfate that is readily soluble in water. However, when phosphate is present, a small amount of europium sulfate may form but the majority of europium forms europium phosphate which is insoluble in water. Therefore, the europium in calcium sulfate is more leachable when there is less phosphate. |
