The Earth’s oceans are a source of uranium, but due to the low concentration of uranyl ions (UO₂²⁺) (3 ppb) and the abundance of other cations the extraction of uranium requires chelating agents with high affinity and selectivity.

Julius Rebek and colleagues, Scripps Research Institute, USA, report a tripodal ligand capable of extracting uranyl ions from aqueous solutions. The ligand consists of a rigid skeleton with three carboxylates that can bind uranyl in a bidentate manner.

Extraction experiments at 400 ppm of uranyl ion showed 59 % of the uranyl ions were extracted into the organic phase. In the presence of Cl⁻, Na⁺, Mg²⁺, Ca²⁺, K⁺, and SO₄^2–, at levels similar to those in seawater, the chelating ligand showed high selectivity and no lessening of function with 59 % of UO₂²⁺ being transferred to the organic phase.

• Selective Recognition and Extraction of the Uranyl Ion
  A. C. Sather, O. B. Berryman, J. Rebek,
  J. Am. Chem. Soc. 2010, 132.
  DOI: 10.1021/ja1035607