Study on the functional group affinity for aqueous uranium recovery from natural water

Abstract

Functional group $(NH_2, COOH, OH, and P=O(OH)_2)$ affinity toward major uranium species $(UO_2(CO_3)_3^{4-}, CaUO_2(CO_3)_3^{2-}$, and $Ca_2UO_2(CO_3)_3)$ present in natural water were measured and compared. Nano-silica surface was functionalized with monolayer thickness and was confirmed by various analytical techniques (ATR-FTIR, NMR, EA, and TGA). The surface density of functional groups were determined to be $1.38 ± 0.13 (NH_2)$, 1.45 ± 0.16 (COOH), 2.09 ± 0.02 (OH), and $1.11 ± 0.16 (P=O(OH)_2) ligand/nm^2$, respectively. The functional group affinity toward uranium was evaluated in terms of the sorption kinetic and capacity. Amine and carboxyl group showed a faster kinetics $(NH_2: 1.7 - 5.5 x 10^{-2}

COOH: 0.8 - 3.6 x 10^{-2} mmol/mg·min)$ and higher sorption capacity $(NH_2: 31.5 - 36.3

COOH: 32.3 - 37.4 mg/mmol)$ toward the negatively charged uranium species such as $UO_2(CO_3)_3^{4-}$, $CaUO_2(CO_3)_3^{2-}$. On the contrary, there was no noticeable difference in the sorption affinity toward the neutral charged uranium species $(Ca_2UO_2(CO_3)_3)$. The electrophoretic measurement of the functional groups revealed that amine was the most positively charged ligand (+7.5 mV) and carboxyl was the most negatively charged one (-25.0 mV), indicating that highly charged ligands provide the higher affinity toward uranium species. The charge of U-bound amine was significantly shifted to the negative region (-13.5 mV), however, that of U-bound carboxyl was not changed. The results imply that the complexation of U with positively and negatively charged ligands would occur via ionic and covalent bonding, respectively. It confirmed that the amine and carboxyl groups have a higher affinity for uranium carbonate species in nature-equivalent synthetic seawater conditions providing even better than that of amidoxime group. Therefore, this study proposes the amine and carboxyl group as the most reactive functional group for the recovery of uranium from natural water. In addition, it would be applicable to the remediation of uranium contaminated environments.