A versatile Zr(IV)-organophosphonate coordination polymer platform for the selective adsorption of lanthanides and actinides

Abstract

New coordination polymer materials based on the complexation of Zr(IV) by 1-hydroxy-2-(1H-imidazol-1-yl)ethane-1,1-diyl bisphosphonate (zoledronate) have been prepared by the hydrothermal reaction of ZrOCl2·8H2O with zoledronic acid. The materials were prepared with different compositions by varying the P/(P+Zr) mole fraction of the reactants. All compositions were X-ray amorphous and had virtually no open porosity which is consistent with the formation of a random gel-type network. The extraction of elements by these materials from solutions containing a range of lanthanide elements in addition to major and minor actinides and other typical fission product elements was investigated at different total cation concentrations. Through variation of the composition of the gels it was possible to tune the adsorption properties. Materials with high phosphorus mole fractions were found to have very high selectivity for lanthanides and Th4+ and low selectivity for typical fission product elements (Cs+, Sr2+ and Co2+) in 0.10M HNO3 solution. In contrast materials with lower phosphorus mole fractions did not sorb lanthanides strongly while they continued to extract Th4+ quantitatively from acid solutions. The materials with lower mole fractions of phosphorus were incorporated into polyacrylonitrile beads and the adsorption properties of the composites explored in greater detail. The kinetics, capacity and acid dependency of this composite were investigated. The present versatile coordination polymer adsorbent platform can be used for the selective extraction of Th4+ and possibly other major actinides from complex mixtures containing a variety of lanthanides in the HNO3 concentration range 0.5–0.01mol/L. The group extraction of the remaining lanthanides could then be accomplished with a change in composition of the adsorbent. The ability to convert the Th-saturated adsorbent materials directly into sodium zirconium phosphate (NZP) adds an additional dimension to the utility to the materials since sorbed Th, or other actinides, could be converted into one of the most versatile and stable phosphate ceramic materials for the immobilization of actinides.