SURFACE CHARACTERIZATION OF ZIRCONIUM AND THORIUM PHOSPHATE-COMPOUNDS

As part of a study involving sorption experiments with actinide ions s orbed onto phosphate compounds and to understand the sorption mechanis ms, we have first synthesized the following three solids: ZrP2O7 (zirc onium diphosphate), Zr2O(PO4)(2) (zirconium oxophosphate) and Th-4(PO4 )(4)P2O7 (thorium phosphate diphosphate). These compounds have been ch aracterized using X-ray powder diffraction, IR spectroscopy and EPMA. The specific surface areas, measured by the N-2-BET method, are 5.5 +/ - 0.1, 0.9 +/- 0.2 and 1.2 +/- 0.2 m(2) g(-1) respectively. The averag e grain size of the powdered samples is around 10 mu m. The pH(IEP) (p H corresponding to the isoelectric point) determined by electrokinetic measurements is 3.6 for ZrP2O7, 4.0 for Zr2O(PO4)(2) and 6.8 for Th-4 (PO4)(4)P2O7. By simulation of the titration curves obtained for the t hree solids, we have determined, using the FITEQL code (constant capac itance model), the surface acidity constants using a sorption site num ber close to 7 sites nm(-2) for the zirconium diphosphate, 7.5 sites n m-2 for the zirconium oxophosphate and 4 sites nm(-2) for the thorium phosphate diphosphate.