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Determination of Uranium Coordination Environment on Ion Exchange Resins in Saline and Non-Saline Environments

Extended X-ray absorption fine structure (EXAFS) analysis has been successfully used to determine the coordination environment and therefore uptake mechanism towards the uranyl cation for a selection of commercially available ion exchange resins in non-saline and saline conditions ([Cl-] = 22.7 g L-1, 0.64 M) similar to those found in sea water. The resins tested were Purolite S985, S910 and S957, Dowex M4195, Ps-EDA, Ps-DETA and Ps-PEHA, which contain polyamine, amidoxime, mixed sulfonic/phosphonic acid, bispicolylamine, ethylenediamine, diethylenetriamine and pentaethylenehexamine functional groups, respectively. Purolite S910 and S957 were both found to extract the uranyl cation through a chelation mechanism. The uranium coordination environment on uranyl loaded Purolite S910 was found to be either tetraor hexa-coordinate in the equatorial plane, with two a 2:1 ratio of amidoxime:uranium in the fit suggesting either monodentate or η2 coordination via 2 amidoxime groups. The uranium environment for uranyl loaded Purolite S957 was found to be tetra-coordinate in the equatorial plane, with both sulfonic and phosphonic acid groups being involved in sorption. The presence of chloride in the loading solution had no effect on the uranyl coordination environment observed on any of the resins. A variation on the F-test was applied to the addition of a sulfur atom from a sulfate group to the fits for Purolite S910 and S957. The addition of this scattering path to the EXAFS fit was only found to be significant for the fit of uranyl bound to Purolite S957 under saline conditions. In contrast, Dowex M4195, Purolite S985, Ps-EDA, Ps-DETA and Ps-PEHA exhibited an anion exchange mechanism for uranyl uptake as the corresponding EXAFS data were fit to a [UO2(SO4)3] structure.