Exchanges of Uranium(VI) Species in Amidoxime-Functionalized Sorbents

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Abstract
Amidoxime (AO)-functionalized polymer sorbents used in this study were prepared by two different routes involving UV grafting and electron-beam grafting of acrylonitrile (AN) into poly(propylene) fibrous and microporous sheets, and subsequent conversion of AN to AO groups by reacting the precursor sorbent with hydroxylamine. The values of self-diffusion coefficient (Ds) of UO22+ in fibrous and sheet AO sorbents were found to be 1.1 × 10−6 and 2.3 × 10−10 cm2 s−1, respectively. The higher diffusion mobility of UO22+ in the fibrous AO sorbent was attributed to its higher free volume as observed in scanning electron microscopic studies. The water content was also found to be maximum in AO-fibrous sorbent (165−200 wt %) and minimum in AO-sheet sorbent (70 wt %). In fibrous AO sorbent, the values of Ds for Na+ and Sr2+ were found to be comparable to their self-diffusion coefficients in the aqueous medium. This indicated that the retardation in diffusion mobility of the ions was a minimum in the fibrous AO sorbent. However, Ds of UO22+ in the fibrous membrane was found to be significantly lower than that of Sr2+, which has a self-diffusion coefficient comparable to that of UO22+ in aqueous medium. This could be attributed to stronger binding of UO22+ with AO groups as compared to Sr2+. To understand the parameters affecting the U(VI) sorption from seawater, the U(VI) exchange rates between fibrous AO sorbent (S) and seawater (aq) involving (H+/Na+)S ⇌ ([UO2(CO3)3]4−)aq and (UO22+)S ⇌ ([UO2(CO3)3]4−)aq systems were experimentally measured. The exchange profiles thus obtained were found to be non-Fickian and much slower than (H+)S ⇌ (UO22+)aq and (UO22+)S ⇌ (UO22+)aq exchanges. This seems to suggest that the reaction kinetics involved in decomplexation of [UO2(CO3)3]4− into UO22+, which forms a complex with AO groups, is the rate-determining step in sorption of U(VI) from seawater. The kinetics of U(VI) sorption in AO-gel and AO-fibrous sorbents followed the pseudo-second-order rate equation. The density of AO groups in the sorbents and their conditioning were found to influence the U(VI) sorption from seawater.

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