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

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 (D-s) of UO22+ in fibrous and sheet AO sorbents were found to be 1.1 x 10(-6) and 2.3 x 10(-10) cm(2) 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 D-s 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, D-s 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) reversible arrow ([UO2(CO3)(3)](4-))(aq) and (UO22+)(s) reversible arrow ([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) reversible arrow (UO22+)(aq) and (UO22+)(s) reversible arrow (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.