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

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 (D(s)) of UO(2)(2+) 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 UO(2)(2+) 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 Sr(2+) 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 UO(2)(2+) in the fibrous membrane was found to be significantly lower than that of Sr(2+), which has a self-diffusion coefficient comparable to that of UO(2)(2+) in aqueous medium. This could be attributed to stronger binding of UO(2)(2+) with AO groups as compared to Sr(2+). 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) right harpoon over left harpoon ([UO(2)(CO(3))(3)](4-))(aq) and (UO(2)(2+))(S) right harpoon over left harpoon ([UO(2)(CO(3))(3)](4-))(aq) systems were experimentally measured. The exchange profiles thus obtained were found to be non-Fickian and much slower than (H(+))(S) right harpoon over left harpoon (UO(2)(2+))(aq) and (UO(2)(2+))(S) right harpoon over left harpoon (UO(2)(2+))(aq) exchanges. This seems to suggest that the reaction kinetics involved in decomplexation of [UO(2)(CO(3))(3)](4-) into UO(2)(2+), 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.

Poly(amidoxime)-reduced graphene oxide composites as adsorbents for the enrichment of uranium from seawater

“…The related thermodynamic parameters calculated from the temperature-dependent isotherms are listed in Table 4. The positive values of standard enthalpy changes (∆H 0 ) indicated an endothermic process for the adsorption of UO 2 2+ on 2.0PAO/rGO composite that was consistent with our observed results and the literature [15,16,36]. The negative values of Gibbs free energy change (∆G 0 ) indicated that UO 2 2+ adsorption on 2.0PAO/ rGO was a spontaneous process [37].…”

“…However, compared to the I D /I G value of rGO (~1.42), the I D /I G values of PAO/rGO composites were decreased to 1.04-1. 16, which suggests that the formed PAO on PAO/rGO composite surfaces mainly existed in the form of amorphous carbon materials. We have previously found that the modification of multiwalled carbon nanotubes (MWCNTs) with special functional groups, such as poly(aniline) [22], poly(methylmethacrylate) [23], and β-cyclodextrin [24], increased the disorder degree of MWCNTs.…”

“…Therefore, the introduction of proper functional groups onto GO surfaces is crucial for its real application in the high extraction of UO 2 2+ from seawater. Owing to its excellent affinity with UO 2 2+ , amidoxime (AO) was widely accepted as one of the most promising binding sites for UO 2 2+ due to its specific binding affinity with UO 2 2+ [15][16][17][18]. For this reason, modification of the GO surface with AO would thought to result in the high extraction of UO 2 2+ from seawater.…”

Extraction of uranium from seawater : design and testing of a symbiotic system

“…Owens et al (2011) -4 , considered to likely be the rate limiting step (Das et al, 2008(Das et al, , 2009. Kinetic analysis of the model displacement reaction confirmed that the rate-determining step in the extraction process, was both the the complexing of uranyl by amidoxime group coupled with the dissociation of the carbonate group from the uranyl tricarbonate complex…”

U(VI) adsorption onto cetyltrimethylammonium bromide modified bentonite in the presence of U(VI)-CO3 complexes

“…A variety of adsorbents have been evaluated, hydrous titanium oxide, as one of the most widely described inorganic material (Yamashita et al, 1980b;, was found that carbonate ion from the uranyl tricarbonate complex (UO 2 (CO 3 ) 3 4− ) was released into the solution during the adsorption process (Yamashita et al, 1980a). Recently, synthetic polymers with amidoxime functional groups had been mainly used because of their high affinity in chelating U(VI) in seawater (Zhang et al, 2003;Das et al, 2009;Kim et al, 2014;Lashley et al, 2016). The uranium adsorption proceeded by ligand exchange of three carbonate ions on the central uranyl ion with two amidoxime groups, accompanied by deprotonations (Hirotsu et al, 1986).…”