COMPARATIVE COPRECIPITATION OF PHOSPHATE AND ARSENATE WITH YTTRIUM AND THE RARE-EARTHS - THE INFLUENCE OF SOLUTION COMPLEXATION

Coprecipitation of yttrium (Y) and rare earth elements (REEs) with pho sphate and arsenate removes these elements from solution in variable p roportions. During both phosphate and arsenate coprecipitation, middle REEs (Sm and Eu) are progressively depleted in solution relative to h eavier and lighter elements. Solution complexation by oxalate (Ox(2-)) influences Y and REE removal patterns by strongly enhancing the reten tion of Y and the heaviest REEs in solution. The extent of this enhanc ement is well described by a quantitative account of the comparative s olution complexation of Y and REEs as M(Ox)(+) and M(Ox)(2)(-). The co mparative 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, Notabl e irregularities are observed in the comparative coprecipitation behav ior of nearest-neighbor elements (e.g., Eu-Gd-Tb and Tm-Yb-Lu). Such i rregularities are very similar for phosphate and arsenate coprecipitat ion in the absence and in the presence of solution complexation.