Chemistry And Geochemistry Of The Rare Earth Element Phosphates

The REE (rare earth element) phosphates are known to be very insoluble, and thus have potential importance in the immobilization of REE in nuclear waste forms and in ocean environments. The hydrous REE phosphates were synthesized with rhabdophane (light REE) and xenotime (heavy REE) structures from dilute nitric acid at or below 100{dollar}\sp\circ{dollar}C, using the acid hydrolysis of the respective REE-pyrophosphate (P{dollar}\sb 2{dollar}O{dollar}\sb 7\sp{lcub}4-{rcub}{dollar}), over a period of 4 to 6 weeks. These derivatives for several REE were characterized by scanning electron microscopy (SEM), powder x-ray diffraction (XRD), differential analysis (DTA) and thermogravimetric analysis (TGA). The DyPO{dollar}\sb 4\cdot{dollar}xH{dollar}\sb 2{dollar}O derivative was shown to be dimorphic. The behaviour of a specimen of natural rhabdophane (Idaho, U.S.A.), was consistent with its chemical composition, as determined by secondary ion mass spectrometry (SIMS).;Solubility products of several hydrous REE phosphates were measured using spectrophotometry and instrumental neutron activation analysis (INAA). The pK values (plus or minus 0.2), were found to be La = 24.5 (25{dollar}\sp\circ{dollar}C), Pr = 26.0 (100{dollar}\sp\circ{dollar}), Nd = 24.7 (25{dollar}\sp\circ{dollar}), Sm = 25.7 (25{dollar}\sp\circ{dollar}), Ho = 24.6 (25{dollar}\sp\circ{dollar}) and Er = 25.2 (25{dollar}\sp\circ{dollar}). The solubilities were predicted to decrease with increasing temperature. The hydrous REE phosphates dissolve in dilute acid with first order, reversible kinetics, with equilibrium achieved in 40 days. Seawater REE concentrations were predicted to be on the order of 1-200 pmol/L, the same range measured by other workers.;Anhydrous REE phosphate single crystals, (L. A. Boatner, ORNL) and natural monazite (Petaca district, New Mexico), were found to dissolve much more slowly, so that solubility products could not be determined.;Surface exchange reactions of hydrous and anhydrous phosphates were found to be slow but measurable, (reaction times were several months)