Separation of Uranium(VI) from Aqueous Solution by Textile Bound Calix[6]arenes

The remediation of former uranium mining and milling sites of Saxony and Thuringia requires, amongst others, the purification of uranium contaminated seepage and mine waters. For this, predominantly conventional water treatment methods (precipitation, flocculation) are applied.
We developed an alternative method for the separation of uranium(VI) from aqueous solution. For this, a uranophile calix[6]arene (p-tert-butyl-calix[6]arene hexacarboxylic acid) is permanently immobilized onto textile material (polyethylene terephthalate fabric) via spacer groups [1]. Calixarenes are macrocyclic molecules formed by 4, 6 or 8 para-substituted phenolic units linked by methylene bridges ortho-positioned to the OH functions. Thus, molecules of various ring sizes are formed. The substitution of calix[6]arenes on the hydroxyl group by carboxylic and hydroxamic groups, respectively, leads to ligands that are able to bind uranium(VI) selectively.
In the present work, the uranium(VI) binding onto the calixarene modified textile is studied as a function of pH value and initial uranium concentration in the absence and presence of competing ions by means of batch experiments. Furthermore, the kinetics of the uranium binding is studied as well as the possibility for remobilization of the bound uranium.
The results have shown, that the calixarene modified textile is suitable for the separation of uranium(VI) from aqueous solution at pH values higher than 4. Maximal 7.6x10-7 mol uranium per 1 g of the calixarene modified textile is bound at pH 5. The influence of competing ions (sodium, potassium, calcium, magnesium, carbonate and sulfate) on the uranium separation at pH 5 is very small. At pH 7 in carbonate rich waters calcium ions have a competing effect. Under environmentally relevant pH conditions (near-neutral pH range) the uranium is strongly bound to the calixarene modified textile and cannot be mobilized. Under acidic conditions an almost complete regeneration of the calixarene modified textile is feasible. The regenerated textile filter material can be utilized for further uranium separation cycles.
In dependence on the specific requirements on-site the textile filter material can be used in batch or column operation. Especially for smaller remediation objects and for objects with lower contamination levels ([UO22+] = 1×10-6 M) this method provides a lower cost alternative to conventional water treatment methods. The applicability of this separation principle to further actinides is expected.

[1]Schmeide, K., Heise, K.H., Bernhard, G., Keil, D., Jansen, K., Praschak, D.: Uranium(VI) separation from aqueous solution by calix[6]arene modified textiles. J. Radioanal. Nucl. Chem. 261 (1), 61-67 (2004).