Response of 90Sr precipitated into Sr5(PO4)3OH and SrHPO4 by Wet-Chemistry: New insights into the phase evolution during grain growth

Extraction of 90 Sr from groundwaters is of great significance in energy and ecological environment. Phosphate minerals have been proposed as a suitable matrix for the in-situ remediation of radionuclides from groundwaters by encouraging co-precipitation. However, the understanding of how Sr 2+ precipitate into phosphate minerals affected by grain growth has not been fully resolved. In this work, the precipitation reaction of Sr 5 (PO 4 ) 3 OH and SrHPO 4 were studied based on experiments and PHREEQC simulation. Some specific issues such as the precipitation reaction kinetics and the phase evolution during grain growth were discussed in detail, along with investigating the chemical stability of obtained precipitations before and after calcination . The results show that pH appears to be a prevailing factor with a recommended pH = 8 ∼ 11 to obtain the stability domain of Sr 5 (PO 4 ) 3 OH and SrHPO 4 to remove Sr 2+ with a removal rate over 98 %. Interestingly, SrHPO 4 is more likely to preferentially nucleate at pH = 8 solution compared to Sr 5 (PO 4 ) 3 OH, and the poorly crystalline SrHPO 4 tends to disappear over time. TEM and SAXS results show the plate-like nanoparticles of SrHPO 4 are wrapped into the rod-like particles of Sr 5 (PO 4 ) 3 OH during the grain growth, and the inner SrHPO 4 can react with Sr 5 (PO 4 ) 3 OH to form Sr 3 (PO 4 ) 2 at temperature over 600 ℃. This observation is inconsistent with the previous grain growth result of poorly crystalline SrHPO 4 , where it is believed to recrystallize into more stable Sr 5 (PO 4 ) 3 OH crystals over time. Moreover, the Sr-O binding energy plays an important role in controlling the degradation of Sr 5 (PO 4 ) 3 OH, Sr 3 (PO 4 ) 2 , SrHPO 4 and β -Sr 2 P 2 O 7 .