Amorphization of a proposed sorbent of strontium, brushite, CaHPO4•2H2O, studied by X-ray diffraction and Raman spectroscopy

Abstract

We present a systematic study of the transformation of brushite (dicalcium phosphate dihydrate, CaHPO4•2H2O) under irradiation of electrons of well-defined energy (2.5 MeV) and flux as a function of the irradiation dose. Contrarily to model hydroxides such as portlandite and brucite, which are very resistant to electron radiation damage, the studied brushite decomposes quite easily, even for very low irradiation doses. Irradiated brushite samples were characterized using X-ray diffraction (XRD) and Raman spectroscopy to get complementary information about changes in atomic structure and chemical composition respectively under irradiation. XRD showed that irradiation causes a very limited dilatation of unit cell of crystalline brushite, which becomes progressively amorphous with increasing radiation dose. Raman spectroscopy complemented XRD results and confirmed that the transformation of brushite to the amorphous phase was not abrupt, but rather progressive. Raman spectroscopy allowed for the identification of the amorphous phase as a calcium pyrophosphate. Both techniques showed that the amorphization of brushite was not fully complete at the maximum dose used, 5.5 GGy (4 C). Interestingly, monetite phase, (CaHPO4 dicalcium phosphate), was not detected at any step of the transformation as it is the case when brushite is thermally decomposed. This study reveals the high sensitivity to electron radiation of both hydrogen bonds and protonated phosphate units in brushite, thus facilitating the transformation into pyrophosphate. The damage of brushite by energetic electrons is to be carefully considered for applications related to the use of brushite as ion-exchanger in the decontamination of effluents polluted with strontium-90, an efficient beta ray emitter.