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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.

Chapter 3 the bridging of thiacalix[4]arene derivatives with oligoethylene glycol ditosylates is described. The conformation of the resulting thiacalix[4](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 thiacalix[4]crown dicarboxylic acids, and their 226Ra2+ selectivity in the presence of a large excess of the most common alkali(ne earth) cations. Thiacalix[4]crowns show significantly improved 226Ra2+ selectivities over their parent calix[4]crowns 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 thiacalix[4]crown-6 dicarboxylic acid allows for full regeneration at acidic conditions.

Chapter 5 deals with the synthesis of (tri)substituted thiacalix[4]arene derivatives. Based on their 226Ra2+ selectivities and those of known thiacalix[4]arene 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 thiacalix[4]arene platform. Consequently, the in Chapter 4 reported thiacalix[4]crown-6 dicarboxylic acid is the best thiacalix[4]arene-based 226Ra2+ extractant under the conditions used.

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 thiacalix[4]crowns 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, thiacalix[4]crown-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.