Uranium(VI) Complexation by Calix[6]arenes studied by Time-resolved Laser-induced Fluorescence Spectroscopy (TRLFS)

For the purification of uranium contaminated waters various water purification methods (e.g., application of ion exchangers, direct precipitation, microbiological methods) are usually applied. Also solvent extraction applying calix[6]arenes is suitable to separate uranium from aqueous solution [1]. To facilitate the uranium separation from uranium containing waters in practice, we are developing an alternative procedure. For this, calix[6]arenes are permanently fixed onto textile substrates via spacer groups [2]. This method is advantageous especially for smaller remediation objects and objects with lower contamination levels ([UO22+] ~ 1×10-6 M).
In the present work, the extraction efficiency of various calix[6]arenes including their selectivity towards uranium(VI) is studied in a two phases solvent extraction system in dependence on pH value, ligand concentration and the amount of competing ions. The complex formation between uranium(VI) and calix[6]arenes is further studied at pH 4 by means of TRLFS as well as by TRLFS with ultrashort laser pulses. By these methods it is possible to detect uranium and calixarene species at equilibrium directly in the aqueous and organic phase. Reaction mechanisms are discussed and extraction constants are determined. In the presence of competing ions (e.g., calcium, barium, magnesium) the uranium extraction is only slightly decreased. The suitability of the calix[6]arenes as efficient reagents for selective uranium separation from aqueous solution is shown.

[1] Shinkai, S. et al.: Molecular design of calixarene-based uranophiles which exhibit remarkably high stability and selectivity. J. Am. Chem. Soc. 109, 6371-6376 (1987).
[2] Schmeide, K. et al.: Separation of uranium from aqueous solution by textile bound calixarenes. In: Uranium in the aquatic environment. Springer Verlag, Berlin 2002, p. 417-424.