This thesis describes the design and selectivity of (new) 226Ra2+ extractants. The results presented provide insight in both the requirements needed to give Ra2+ selectivity and in the achievable Ra2+/Mn+ (Mn+ = alkali(ne earth) cation) separations.
Chapter 1 gives a general introduction into molecular recognition and the need for selective extraction of radioactive contaminants.
Chapter 2 describes the general industries and conditions in which the naturally occurring Ra2+ nuclides are encountered as well as their toxic nature. In addition, various known Ra2+ extraction techniques are discussed, with the emphasis on organic extractants.
In Chapter 3 the bridging of thiacalixarene derivatives with oligoethylene glycol ditosylates is described. The conformation of the resulting thiacalix(bis)crowns depends on the base and oligoethylene glycol ditosylates used. The conformational assignment relies on several X-ray crystal structures and extensive 2-D 1H NMR studies. This chapter provides the (synthetic) background necessary for the synthesis and characterization of the extractants described in Chapter 4.
Chapter 4 describes the synthesis of thiacalixcrown dicarboxylic acids, and their 226Ra2+ selectivity in the presence of a large excess of the most common alkali(ne earth) cations. Thiacalixcrowns show significantly improved 226Ra2+ selectivities over their parent calixcrowns and of the two crown-ether bridge sizes (5 and 6), the crown-6 gives the highest overall 226Ra2+ selectivity. This results in 226Ra2+ extraction from Mn+/226Ra2+ ratios up to 3.5 x 107 (Mn+ = Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, or Ba2+), using an excess of [M] to extractant. In addition, the effective pH range (6-13) of thiacalixcrown-6 dicarboxylic acid allows for full regeneration at acidic conditions.
Chapter 5 deals with the synthesis of (tri)substituted thiacalixarene derivatives. Based on their 226Ra2+ selectivities and those of known thiacalixarene derivatives, the optimal conditions are determined for the effective extraction of 226Ra2+ from solutions containing Ca2+, Sr2+, or Ba2+. The results clearly show the advantage of bringing together the 226Ra2+ extracting components of a synergistic system on a thiacalixarene platform. Consequently, the in Chapter 4 reported thiacalixcrown-6 dicarboxylic acid is the best thiacalixarene-based 226Ra2+ extractant under the conditions used.
In Chapter 6 the non-covalent assemblies formed by guanosine (hexadecamers) and isoguanosine (decamers) show 226Ra2+ extraction from Mn+/226Ra2+ ratios up to 106 (Mn+ = Na+, K+, Rb+, Cs+, Mg2+, Ca2+, Sr2+, or Ba2+), using a similar deficiency of extractant as described in Chapter 4. These systems are effective over a broad pH range (3-11). Where guanosine requires the presence of a lipophilic picrate anion to form 226Ra2+ selective assemblies, the more 226Ra2+ selective isoguanosine gives high selectivities without any additives. The results clearly demonstrate the power of molecular self-assembly for the construction of selective extractants.
The last experimental chapter, Chapter 7, shows that the thiacalixcrowns and (iso)guanosines described in Chapters 4 and 6, respectively, are also able to effectively extract 226Ra2+ from mixed metal ion-containing gas-field produced water and its model solution. Surprizingly, the extractants reported in Chapters 4-6 to have the highest overall 226Ra2+ selectivities in the alkali(ne earth) series, thiacalixcrown-6 dicarboxylic acid and isoguanosine, gave the poorest results with the produced water and its model solution. This is caused by their relatively low 226Ra2+/Na+ selectivities, since Na+ is the main competing cation in the gas-field produced water. The 226Ra2+ cations can be effectively stripped from the organic phases, underlining the potential for applications in industrial Ra2+ separation.
The outlook gives some suggestions for further improvements that can make the extractants described in Chapters 4, 6, and 7 better applicable in industrial Ra2+ containing (aqueous) waste streams.
The Ra2+ selectivities of conventional crown ether/acid systems have been improved and a new non-covalent method for the formation of Ra2+ extractants is introduced. Furthermore, the potential use of organic extractants in industrial Ra2+ containing TENORM waste streams is made clear. Although the work presented in this thesis is of a fundamental nature, it clearly illustrates the headway made towards the separation of Ra2+ from waste streams containing significant excesses of competing cations.