Comparative Coprecipitation of Phosphate and Arsenate with Yttrium and the Rare Earths: The Influence of Solution Complexation

Abstract

Coprecipitation of yttrium (Y) and rare earth elements (REEs) with phosphate and arsenate removes these elements from solution in variable proportions. During both phosphate and arsenate Coprecipitation, middle REEs (Sm and Eu) are progressively depleted in solution relative to heavier and lighter elements. Solution complexation by oxalate (Ox2-) influences Y and REE removal patterns by strongly enhancing the retention of Y and the heaviest REEs in solution. The extent of this enhancement is well described by a quantitative account of the comparative solution complexation of Y and REEs as M(Ox)+ and M(Ox)\(_{\text{2}}^ - \). The comparative behavior of phosphate and arsenate coprecipitation exhibits both similarities and differences. During arsenate coprecipitation the light REEs are retained in solution, relative to the heavy REEs, to a greater extent than is the case for phosphate coprecipitation. Notable irregularities are observed in the comparative coprecipitation behavior of nearest-neighbor elements (e.g., Eu–Gd–Tb and Tm–Yb–Lu). Such irregularities are very similar for phosphate and arsenate coprecipitation in the absence and in the presence of solution complexation.