Evolution of the chemical form of zirconium in aqueous solution during denitration and its influence on extraction by TRPO

Denitration by formic acid is a process that can reduce the acidity of High-Level Liquid Waste (HLLW) generated by the Plutonium Uranium Reduction EXtraction (PUREX) process, making it compatible with the feed requirements of the TRPO (trialkylphosphine oxide) process. Unfortunately, the denitration leads to the formation of interfacial crud in the TRPO extraction due to the presence of zirconium; however, the mechanism remains elusive. In this work, the evolution of the chemical form of Zr(IV) during the denitration process by formic acid was investigated. The aggregates formed in the Zr(IV) solutions after denitration were characterized by dynamic and static light scattering (DLS and SLS). Afterwards, the solutions were vacuum-dried and the solids were characterized by elemental analysis, scanning electron microscope (SEM), powder X-ray diffraction (PXRD), Xray photoelectron spectroscopy (XPS), and Raman spectroscopy. Zr(IV) is present in solutions with different forms such as the tetramers of Zr4(OH)8(NO3)8 and Zr4(OH)8(HCOO)8 and their aggregates, depending on the [HCOOH]/[HNO3] ratio. The evolution of the chemical form of Zr(IV) can impact the extraction of Zr(IV) by TRPO and is responsible for the formation of interfacial crud.