Solubility and solid phase of trivalent lanthanide hydroxides and oxides

Abstract

An undersaturation approach was adopted to reveal the solubility and solubility-limiting solid phase of La, Eu and Tm in the sample solutions with the ionic strength (I) of 0.1 mol/L (M) and the hydrogen ion concentration range (pH c =-log [H + ]) of 6 to 12 after aging at 25 to 90°C for certain periods up to 12 weeks. Solubility and solubilitylimiting solid phases were discussed with the aid of X-ray diffraction (XRD) patterns of the solid phases and temperature dependence of solubility products (log K s°) values. The XRD patterns of the solid phases indicated that the initial solid phase of La 2 O 3 (cr) transformed to La(OH) 3 (cr) after aging at 25 °C and 90 °C. On the other hand, Eu 2 O 3 (cr) remained stable after aging at 25 °C, but after 90 °C, converted to Eu(OH) 3 (cr). It was observed that Tm 2 O 3 (cr) was not transformed after aging at both 25 °C or 90 °C. Thermodynamic properties obtained from the temperature dependence of the solubility products agreed well with the observations in the XRD patterns.

Thermodynamic Studies on the Hydrolysis of Trivalent Plutonium and Solubility of Pu(OH)3(am)

“…These thermodynamic parameters indicate that the hydrolysis of Pu­(III) is an endothermic reaction (Δ r H m ° = 34.10 ± 4.48 kJ mol –1 ). The exothermic characteristic of the dissolution of Pu­(OH) 3 (am) (Δ r H m ° = −294.29 ± 23.05 kJ mol –1 ) is validated by the decrease in solubility with increasing temperature, which is coincident with those for trivalent lanthanide elements [Ln­(III)]. − In the hydrolysis reaction, the net change of enthalpy and entropy is a combination effect of the dissociation of water molecules in the first hydration shell, the formation of Pu–OH, and the change in the hydration environment between Pu 3+ and Pu­(OH) n 3– n ions. The relatively large enthalpy change during the dissolution process can be ascribed to the energetically large contribution to the phase transition of the amorphous solid phase to the hydrated aqueous phase, Pu­(OH) 3 (aq).…”

“…To evaluate the compensation effect within the An­(III) series, further thermodynamic studies on other An­(III) ions, such as Am­(III) and Cm­(III), are encouraged. In the case of dissolution, a few studies on the solubility of Ln­(OH) 3 (cr) (Ln = La, Nd, and Gd) have been performed at temperatures ranging from 25 to 80 °C. − However, the amount of data is not sufficient to discuss the enthalpy and entropy relationship of the dissolution of Ln­(OH) 3 and Pu­(OH) 3 (am).…”

UV-Vis spectrophotometric determination of rare earth elements (REE) speciation at near-neutral to alkaline pH. Part I: m-cresol purple properties from 25–75 °C and Er hydrolysis

“…Several other studies 32,33 have investigated the solubility of La(OH) 3 (s), Eu(OH) 3 (s), and Am(OH) 3 (s) at 25 °C to discuss the formation of their hydroxyl species at alkaline pH up to 13. The lack of increased REE hydroxide solubility with pH, even at those alkaline pH values, indicates that a higher order REE hydroxyl complex such as Eu(OH) 4 − is unlikely to be stable.…”