Amidoximes as ligand functionalities for braided polymeric materials for the recovery of uranium from seawater

Abstract

The formation constants of the UO 2 2+ cation with the amidoximate ligands bzam (benzamidoxime) and acetam (acetamidoxime) are reported. These are of interest in light of their proposed use as the functional groups of extractants for uranium in seawater. The formation constants of bzam with UO 2 2+ were measured by monitoring the absorbance of the π→π* transitions in the UV spectrum of the bzam ligand in the presence of 1:1 UO 2 2+ as a function of pH. This yielded log K 1 = 12.4 for UO 2 2+ with bzam, and log K = 6.9 for the equilibrium UO 2 (bzam) + + OH-= UO 2 (bzam)OH at 25 o C and ionic strength zero. The bzam complexes were also studied monitoring the fluorescence of the UO 2 2+ system. Analysis of the intense fluorescence that occurs in 5 x 10-6 M UO 2 2+ solutions between pH 5 and 9 suggested that this was due to the [(UO 2) 3 O(OH) 3 ] + trimer. Monomeric species such as UO 2 2+ and [UO 2 (OH) 4 ] 2-, and dimers such as [(UO 2)(OH) 2 ] 2+ , fluoresce only weakly. Titration of such solutions with bzam supported the above log K values measured by absorbance, and with higher bzam concentrations yielded log  2 = 22.3. The acetam ligand does not have any absorbance, so that complexformation was monitored by fluorescence only. Formation constants measured by fluorescence may differ from those measured by other techniques such as absorbance. The agreement obtained between log K values measured by absorbance and fluorescence for the bzam complex of UO 2 2+ supported the log K values measured for the acetam complexes by florescence alone were reliable: log K 1 = 13.6, log  2 = 23.7, and log K UO 2 (acetam) + + OH-= UO 2 (acetam)OH = 6.8. The high log K values found for the bzam and acetam complexes of UO 2 2+ were analyzed using DFT calculations. These log K values are related to the ability of polymer-based extractants bearing bzam or acetam type functional groups to extract UO 2 2+ at a concentration of 1.3 x 10-8 M and in the competing 0.0025 M CO 3 2present in the oceans.

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).…”

Development of electrospun polymer nanofiber mats for removal of uranium from aqueous systems

“…Previous studies indicated that amidoximation 21,77,[108][109][110][111][112][113][114][115][116] 33-43 of PAN led to uranyl uptake that was dependent on pH, and additional gains in U extraction efficiency were achieved using high surface area to volume nanomaterials 117,118 and electrospun fibers. 21,119,120 Here, high , and spectra are similar to previous literature reports.…”

“…Finally, intensities and Γ increase but vibrational frequencies remain constant when the solution pH used during uptake increases from 4 to 6.8. Because pH impacts both uranyl speciation 92 and AO-PAN functional group protonation 113,124 both would influence the resulting vibrational features.…”

“…Previously, several computational and well-controlled experimental studies were conducted to decipher the structure of amidoxime and uranyl complexes 110,114,116 and formation constants. 109,112,113,124,125 We build on this excellent foundation for understanding these spectral features and uranyl binding mechanisms. First, we consider the pK a values (assumed at infinite dilution) of acetamidoxime (5.78 and 13.50) 113,124 as a model compound to gain insights on the thermodynamics of uranyl uptake on the functionalized mat surface.…”

“…109,112,113,124,125 We build on this excellent foundation for understanding these spectral features and uranyl binding mechanisms. First, we consider the pK a values (assumed at infinite dilution) of acetamidoxime (5.78 and 13.50) 113,124 as a model compound to gain insights on the thermodynamics of uranyl uptake on the functionalized mat surface. 113,124 These details are summarized in Figure A-1A and Table B-1.…”