Effects of synthesis conditions on ion exchange properties of α-zirconium phosphate for Eu and Am

Three zirconium phosphate products A, B and C, made through different synthesis routes, were investigated for their europium and americium ion exchange properties utilizing radiotracers Eu-152(3+) and Am-241(3+). Aim of this investigation was to see how material properties change based on different synthesis, and how does the changes effect on trivalent Eu and Am uptake and affinities on the materials. Ultimate goal of an ongoing research is to create inorganic exchanger suitable for separation of trivalent actinides and lanthanides. Powder X-ray diffraction showed that all three products had same a-zirconium phosphate crystal structure. The P:Zr ratio determined by microscope X-ray microanalysis was also the same for all products: 2.43 +/- 0.05. However, infrared absorbance, material acidity, particle morphology, and Eu and Am distribution coefficients differed significantly between products. The intensities of the strong IR absorption at approximately 960 cm(-1), attributed to vibrations of the orthophosphate group, were in descending order B > C > A. Material acidity showed the same descending order B > C > A. First acidity constants pK(al) were 2.3 for product B, 3.1 for C and 3.5 for A. Unit cell volumes increased in the reverse order: B < C < A. Distribution coefficients (K-D), studied for pH 0 to 3 nitric acid media, varied remarkably. For any given pH the K-D descended in the order A > C > B for both Eu and Am. Separation factors, defined as K-D (Eu): K-D (Am), were from 4 to 41 for product A, from 5 to 15 for B, and from 3 to 7 for C. Selectivity coefficients (k(M/H), M = Eu, Am) and sorption strength decreased along with increasing ZrP product acidity. Metal binding coefficients (k(M)) had high values, up to 109, especially in ZrP C and A, while the selectivity coefficients were low, 10-5 to 10(-1), because they relate to the third power of the low pK(al). It was observed that for ZrPs there are strong interdependencies between acidity of the product, unit cell volume, IR absorption, K-D, k(M/H) and k M. Finally, it can be concluded that the ion exchange properties of alpha-ZrP products can be modified considerably by varying their synthesis conditions, perhaps to tailor specific actinide/lanthanide separations.