Selective Separation of Europium Using Polymer-Enhanced Ultrafiltration

The U.S. Department of Energy (DOE) is actively pursuing new and improved separation techniques for the cleanup of past nuclear defense production sites. Research and production activities at DOE's Hanford Site in Richland, Wash., have created large volumes of waste streams containing hazardous and toxic chemicals along with radioactive materials. Many of these wastes will require processing for segregation into high-level, transuranic, and/or low-level waste for permanent disposal. A process to selectively remove actinides, such as americium, from liquid radioactive waste was investigated for potential use at Hanford and other contaminated DOE sites. The objective of this research was to determine the effectiveness of polymer binding followed by ultrafiltration for removal of europium (Eu), a nonradioactive surrogate for trivalent actinides such as americium. A commercially available polyacrylic acid (PAA) and a Pacific Northwest Laboratory (PNL) synthesized copolymer were tested. Both polymers significantly increased Eu removal. A cation exchange mechanism was implied by examination of the${\rm Eu}\text{-}{\rm to}\text{-}{\rm RCO}_{2}{}^{-}$functional groups that comprise the acrylic acid monomer. The weight ratios of Eu-to-polymer needed to achieve 85% rejection of Eu were 1:6 for PAA and 1:10 for the PNL copolymer. Addition of sodium to the feed solution at a concentration three orders of magnitude greater than Eu did not adversely affect rejection of Eu; this showed the high selectivity of both polymers for Eu. Polymer binding of metals followed by ultrafiltration also has potential applications for selective separation of metals from various industrial process streams. The formation of metal hydroxide precipitates is also a possibility unless pH is controlled; these could be separated as well by ultrafiltration but defeat the intent of polymer addition. For the polymers tested, pH had to be above the${\rm pK}_{{\rm a}}$(4.25) of the ionizing functional groups but below a pH of 6 where precipitation may interfere.