Dendritic Chelating Agents. 2. U(VI) Binding to Poly(amidoamine) and Poly(propyleneimine) Dendrimers in Aqueous Solutions

Chelating agents are widely employed in many separation processes used to recover uranyl [U(VI)] from contaminated aqueous solutions. This article describes an experimental investigation of the binding of U(VI) to poly(amidoamine) [PAMAM] and poly(propyleneimine) [PPI] dendrimers in aqueous solutions. We combine fluorescence spectroscopy with bench scale ultrafiltration experiments to measure the extent of binding and fractional binding of U(VI) in aqueous solutions of these dendrimers as a function of (i) metal−ion dendrimer loading, (ii) dendrimer generation, (iii) dendrimer core and terminal group chemistry, and (iv) solution pH and competing ligands (NO_3^−, PO_4^(3−), CO_3^(2−), and Cl^−). The overall results of this study suggest that uranyl binding to PAMAM and PPI dendrimers in aqueous solutions involves the coordination of the UO22+ ions with the dendrimer amine, amide, and carboxylic groups. We find significant binding of U(VI) to PAMAM dendrimers in (i) acidic solutions containing up to 1.0 M HNO_3 and H_3PO_4 and (ii) in basic solutions containing up to 0.5 M Na_2CO_3. However, no binding of U(VI) by PAMAM dendrimers is observed in aqueous solutions containing 1.0 M NaCl at pH 3.0. These results strongly suggest that PAMAM and PPI dendrimers can serve as high capacity and selective chelating ligands for U(VI) in aqueous solutions.