Trisodium Phosphate Phases and Solubility in Alkaline Solutions Relevant to Radioactive Waste Processing

The U.S. Department of Energy’s Hanford Site faces significant challenges in managing radioactive waste, where phosphate precipitation can obstruct operational processes. To investigate inconsistencies in the reported phosphate solubility and determine the factors influencing the precipitation of different trisodium phosphate hydrate phases, we examined the trisodium phosphate system (Na3PO4:NaOH:H2O). X-ray diffraction revealed that commercial starting materials undergo transformations to produce multiple hydrates, including phases comprising trisodium phosphate nonahydrate (Na3PO4·9H2O), an ordered polymorph of trisodium phosphate dodecahydrate containing sodium hydroxide (Na3PO4·12H2O·(1/6)NaOH), and a recently reported orthorhombic trisodium phosphate heptahydrate (Na3PO4·7H2O). Computational modeling indicated that the ordered dodecahydrate is more stable than its disordered counterpart, suggesting that kinetic barriers stabilize the disordered structure. Solubility measurements using either anhydrous Na3PO4 or Na3PO4·12H2O·(1/6)NaOH revealed that release of NaOH from the hydrate starting material elevated solution alkalinity, thereby reducing phosphate solubility relative to solutions prepared with anhydrous Na3PO4 over a temperature range of 20–44 °C. These results show that disparities in phosphate solubility data can arise from (i) incorporation of variable amounts of NaOH within the solid phase, which changes solution pH; (ii) diversity of hydrate phases that control equilibrium solubility; (iii) temperature dependence of phosphate phase stability; and (iv) rapid phosphate precipitation kinetics affecting the results of solubility studies due to thermal gradients that occur during sampling. Together, these factors explain historical inconsistencies in phosphate solubility under alkaline conditions, representative of radioactive waste stored at the Hanford Site.