Next Generation Extractants for Cesium Separation from High-Level Waste: From Fundamental Concepts to Site Implementation

Abstract

This project unites expertise at Oak Ridge National Laboratory (ORNL) and Texas Tech University (TTU, Prof. Richard A. Bartsch) to answer fundamental questions addressing the problem of cesium removal from high-level tank waste. Efforts focus on novel solvent-extraction systems containing calixcrown extractants designed for enhanced cesium binding and release. Exciting results are being obtained in three areas: (1) a new lipophilic cesium extractant with a high solubility in the solvent; (2) new proton-ionizable calixcrowns that both strongly extract cesium and "switch off" when protonated; and (3) an improved solvent system that may be stripped with more than 100-fold greater efficiency. Scientific questions primarily concern how to more effectively reverse extraction, focusing on the use of amino groups and proton-ionizable groups to enable pH-switching. Synthesis is being performed at ORNL (amino calixcrowns) and TTU (proton-ionizable calixcrowns). At ORNL, the extraction behavior is being surveyed to assess the effectiveness of candidate solvent systems, and systematic distribution measurements are under way to obtain a thermodynamic understanding of partitioning and complexation equilibria. Crystal structures obtained at ORNL are revealing the structural details of cesium binding. The overall objective is a significant advance in the predictability and efficiency of cesium extraction from high-level waste in support of potential implementation at U. S. Department of Energy (USDOE) sites. Research Objectives Purpose. The overall goal of this project, unchanged from the previous performance period (EMSP Project No. 73803), is to seek increased fundamental understanding and major improvement in cesium separation from high-level waste by the calix[4]arene-crown-6 family of cesium-selective extractants. Expected results are targeted toward needs in high-level-waste (HLW) remediation, primarily at the USDOE Savannah River Site (SRS) and Hanford Site. Results have in particular strongly benefited HLW cleanup at the SRS, where plans now call for implementation of the Caustic-Side Solvent Extraction (CSSX) process in the $1B SRS Salt Waste Processing Facility (SWPF) now being designed by private industry. The development of the CSSX process was made possible in part by results from this research, and its successful implementation will likely depend in part on basic knowledge gleaned from future efforts under this project. As discussed below, significant strides have been made on the specific objectives that had been set forth previously. Most notably, promising results have been obtained toward significantly improved extraction and stripping of cesium, and it is the primary purpose of the renewal proposal to bring such results to fruition. Specific objectives are proposed for synthesis of compounds of the types previously targeted (see below), characterization of their binding behavior toward cesium, and development of a thermodynamic understanding of cesium extraction behavior. The teaming to reach these objectives ...

High-Temperature Sorption of Cesium and Strontium on Dispersed Kaolinite Powders

“…Other approaches including catalytic oxidation, acid digestion (20), ion exchange (21,22), precipitation, and solvent extraction (23,24) are being developed and tested as methods to separate and concentrate radionuclides and reduce the waste volumes. These processes, however, result in a secondary radionuclide-laden waste that needs subsequent treatment and disposal.…”

Adsorption Property of Cesium onto Modified Macroporous Silica–Calix[4]arene-crown Based Supramolecular Recognition Materials

“…8,9 A caustic-side solvent extraction (CSSX) process for removal of cesium from alkaline solution was proposed utilizing calix [4]arene-bis(tert-octylbenzo-crown-6) (BOB CalixC6) as an extractant at Oak Ridge National Laboratory (ORNL). 10,11 The fission product solvent extraction (FPEX) process integrating two extractants, 4,4′(5′)di(tert-butylcyclohexano)-18-crown-6 (DtBuCH18C6) and BOB CalixC6, was reported to simultaneously separate cesium and strontium together from acidic media. 12,13 All the processes showed high selectivity and excellent extraction ability for cesium.…”

“…In the studies of Cs(I) separation, the supramolecular recognition agents calix[4]arenes, which are basket-shaped compounds of potential interest for host–guest complexation studies, have received considerable attention over the past 30 years. , Calix[4]arene-crown compounds, the popular derivatives of calix[4]arenes, show high affinity for the complexation of alkali and alkaline-earth metals. − Especially, the derivatives of the 1,3-alternate calix[4]arene-18-crown-6 exhibit high extraction selectivity for cesium over all of the other elements, which is exemplified by the following solvent extraction processes. A so-called cesium separation by calix-crown extraction (CCCEX) by 1,3-(dioctyloxy)-2,4-crown-6-calix[4]arene or 1,3-[(2,4-diethylheptylethoxy)oxy]-2,4-crown-6-calix[4]arene (Calix[4]arene-R14) was developed by the French Atomic Energy Commission (CEA). , A caustic-side solvent extraction (CSSX) process for removal of cesium from alkaline solution was proposed utilizing calix[4]arene-bis( tert -octylbenzo-crown-6) (BOB CalixC6) as an extractant at Oak Ridge National Laboratory (ORNL). , The fission product solvent extraction (FPEX) process integrating two extractants, 4,4′(5′)-di( tert -butylcyclohexano)-18-crown-6 (DtBuCH18C6) and BOB CalixC6, was reported to simultaneously separate cesium and strontium together from acidic media. , All the processes showed high selectivity and excellent extraction ability for cesium. However, some adverse diluents and phase modifiers such as tri- n -octylamine (TOA), 1-(2,2,3,3-tetrafluoropropoxy)-3-(4- sec -butylphenoxy)-2-propanol (Cs-7SB), and 1-(1,1,2,2-tetrafluoroethoxyl)-3-[4-( tert -octyl)phenoxyl]-2-propanol (Cs-3) were introduced to avoid the formation of the third phase, − which might induce a large quantity of secondary wastes.…”