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.
© 2008 American Chemical Society. Received June 28, 2007. Revised manuscript received October 22, 2007. Accepted October 29, 2007. Publication Date (Web): January 23, 2008. This work was carried out in the Materials Process Simulation Center of the Division of Chemistry and Chemical Engineering at the California Institute of Technology and in the Department of Civil Engineering at Howard University. The data collected in this study provided background information for two provisional patent applications on the development of dendrimer based chelating agents and separation systems for recovering actinides from aqueous solutions. Funding for this work was provided by the National Science Foundation (NIRT CBET Award no. 050695) (Caltech) and the Department of Energy (DOE Massie Chair Grant DE-FG02-94EW11423 (Howard University).
Supplemental Material - es0715905_si.pdf