The pH value influence on the waylandite-structured BiAl3(PO4)2(OH)6 compound formation under hydrothermal conditions

Abstract

The present work has found that the rate of the waylandite-structured BiAl3(PO4)2(OH)6 compound formation under conditions of hydrothermal treatment at 200 °C of a suspension obtained at 25 °C as a result of the nanocrystalline hexagonal BiPO4 and the amorphous phase precipitation, increases in the 5 – 9 pH range specified for this water-salt suspension. The rate of this compound formation decreases under conditions of isothermal exposure of the initial suspension with pH 11. This is due to the increased solubility under these conditions of the amorphous phase, predominantly consisting of aluminum hydroxide. Thermodynamic calculation of phase equilibria at 200 °C in the BiAl3(PO4)2(OH)6-BiPO4-Al(OH)3-H2O(HNO3, NaOH) system showed that the BiAl3(PO4)2(OH)6 compound, resulting from the water-salt suspension treatment, is stable in the 4.2–11.7 pH range specified for this suspension. It was experimentally established that this compound is not formed under conditions of hydrothermal treatment of the suspension with the specified pH 3. This is consistent with the calculation results and is explained by the increased stability of nanocrystalline BiPO4 under these conditions.