Complexation of U (VI) with N,N,N', and N,Ndimethyl-3-oxa-glutaramic acid (DMOGA) was studied in comparison with their dicarboxylate analog, oxydiacetic acid (ODA). Thermodynamic parameters, including stability constants, enthalpy and entropy of complexation, were determined by spectrophotometry, potentiometry and calorimetry. Single-crystal X-ray diffractometry, EXAFS spectroscopy, FT-IR absorption and laser-induced luminescence spectroscopy were used to obtain structural information on the U(VI) complexes.
Glutardiamidoxime (H2B), a diamidoxime ligand that has implications in sequestering uranium from seawater, forms strong complexes with UO2(2+). Five U(VI) complexes were identified in 3% NaCl solution. The stability constants and the enthalpies of complexation were measured by potentiometry and microcalorimetry. The competition between glutardiamidoxime and carbonate for complexing U(VI) in 3% NaCl was also studied in comparison with the cyclic glutarimidedioxime ligand (H2A) previously studied.
In acidic aqueous solutions, gluconate protonation is coupled with lactonization of gluconic acid. With the decrease of pC H , two lactones (δ/γ) are sequentially formed. The δ-lactone forms more readily than the γ-lactone. In 0.1 M gluconate solutions, if pC H is above 2.5, only the δ-lactone is generated. When pC H is decreased below 2.0, the formation of the γ-lactone is observable although the δ-lactone predominates. At I = 0.1 M NaClO 4 and room temperature, the deprotonation constant of the carboxylic group, using the NMR technique, was determined to be log K a = 3.30 ± 0.02; the δ-lactonization constant, by the batch potentiometric titrations, was obtained to be log K L = -(0.54 ± 0.04). Using ESI-MS, the rate constants of the δ-lactonization and the hydrolysis at pC H ~ 5.0 were estimated to be k 1 = 3.2 x 10 -5 s -1 and k -1 = 1.1 x 10 -4 s -1 , respectively.
Uranium adsorbed on amidoxime-based polyethylene fiber in simulated seawater can be quantitatively eluted at room temperature using 1 M Na2CO3 containing 0.1 M H2O2. This efficient elution process is probably due to the formation of an extremely stable uranyl-peroxo-carbonato complex in the carbonate solution. After washing with water, the sorbent can be reused with minimal loss of uranium loading capacity. Possible existence of this stable uranyl species in ocean water is also discussed.
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