Role of cavitation on the dissolution of lanthanide oxides in nitric acid.

The dissolution of lanthanide oxides in nitric acid is a key step in hydrometallurgical processing, nuclear materials management, and the production of high purity lanthanides. However, dissolution kinetics are often limited by the refractory nature and high lattice stability of these oxides. The present study establishes a mechanistic framework for the ultrasound-assisted dissolution of selected lanthanide oxides (CeO2, Sm2O3, Gd2O3, Yb2O3 and Lu2O3) in nitric acid, with emphasis on the interplay between cavitation dynamics, material properties and reaction pathways. Under silent conditions, trivalent oxides exhibit dissolution rates following the order Sm2O3 > Gd2O3 > Yb2O3 > Lu2O3, governed by increasing lattice energy and bond strength across the lanthanide series. In contrast, CeO2 shows markedly sluggish dissolution due to its redox-limited pathway requiring reduction of Ce4+ to Ce3+. Application of low-frequency ultrasound (20-40 kHz) leads to significant intensification, with enhancement factors spanning up to two orders of magnitude. Parametric analysis reveals that dissolution increases with acid concentration, temperature and acoustic intensity, and decreases with increasing frequency, highlighting the dominant role of cavitation characteristics. A key outcome of this work is the identification of two distinct dissolution regimes: (i) surface reaction-controlled oxidative dissolution for trivalent oxides, and (ii) reductive, kinetically constrained dissolution for ceria. Dynamic light scattering confirms rapid cavitation-induced fragmentation of ceria, following first-order breakage kinetics, although this does not eliminate the intrinsic redox limitation. Sintering-induced microstructural evolution leads to diminished kinetics, which are partially recovered under ultrasound. The study provides new insight into structure-mechanism-process relationships governing sonochemical dissolution, with implications for intensified hydrometallurgical processing of lanthanide materials.