New sorbents for processing radioactive waste

Abstract

A great volume of higher activity waste (HAW) has been accumulated in the nuclear industry. The radionuclides contained in them are elements of nuclear fuel (233,235,238U, 239Pu, 241,243Am), uranium fission products (134Cs, 137Cs, 90Sr, 95Zr, 95Nb, 129I, 131I, 144Ce, 103,106Ru, 147Pm, 152,154Eu, 140Ba, etc.), and products of induced neutron activation of construction materials (58,60Co, 54Mn, 51Cr, 59Fe). The key principle of HAW processing is maximally efficient and feasibly selective recovery of both highly active and industrial and nuclear medicine radionuclides. At present only uranium and plutonium are recycled during the waste processing, while fission products and residues of uranium and plutonium are immobilized into solid matrices and sent for burial. The technology reduces the risk of radionuclides entering the environment. However it cannot be considered rational since the irreversibly lost elements are isotopes 90Sr and 147Pm used in fuel cells; 137Cs used as a source of ionizing radiation; 99Mo needed for nuclear pharmaceuticals; and nonferrous and precious metals Ag, Ru, Pd, Au, Rh, etc. which account for almost 30% of the whole mass of all radioactive wastes. The many years of development of routines of selective extraction of valuable components from radioactive solutions show that sorption technologies are highly promising. Therefore efficient sorption materials (sorbents) are required. Unfortunately, most selective sorbents used currently for the purification of radioactive wastes are quite costly and dependent on the chemical compositions of the solutions being purified. This is not always adequate for tackling the current technological challenges successfully, and this stimulates development of new high-level sorption materials. Among the known sorbents, the most interesting are synthetically feasible impregnated sorbents in which the stationary phase is complexation organic compounds, extractants non-covalently bound to the surface of a macroporous or nonorganic carrier. Metal ion sorption by impregnated sorbents is based on guest-host bonding between an organic extractant and a metal ion. Therefore the sorption properties of sorbents rely mainly on the extraction ability of the organic extractant. Its choice and structural optimization are determined. Quite common extractants are neutral organophosphorous compounds, i.e., phosphine oxides, carbamoyl phosphine oxides, as well as substituted calixarenes, which carry phosphor-containing moieties in the lower rim. Application of organophosphorous acids and zirconium salts of dialkyl phosphoric acids as extractants has been described. Macrocyclic compounds, i.e., crown ethers and crown-containing calixarenes, have also been studied quite well. High extraction properties are present in diamides of malonic, diglycolic, and heterocyclic acids, polynitrogen heterocyclic compounds, and synergistic mixtures of various extractants. This review represents the results of the development of physicochemical principles for the obtainment and practical application of new impregnated sorbents in which the extractants are synthetically feasible acidic phosphoryl podands, in particular, the results of the studies of influence of structure of phosphoryl podands on the efficiency of partitioning, extraction, and purification of U(VI), Th(IV), Np(IV), and I(cyrillic) Pu(IV) and rare earths La(III), Nd(III), 147Pm, Sm(III), and 99Mo.