Brown coal removes uranium from sea water where it is present mainly as [UO<sub>2</sub>(CO<sub>3</sub>)3]<sup>4−</sup>. The adsorption and binding of uranium is studied by infrared spectroscopy. The spectra of the coal-uranium adducts still exhibit the asymmetric stretching vibration of the uranyl ion, but no CO<sub>3</sub><sup>2−</sup> frequencies, suggesting that uranium is retained as UO<sub>2</sub><sup>2+</sup>. The coal humic acids are shown to be responsible for the decomposition of the carbonato complex. The subsequent uptake of uranium is not a pure cation exchange process since the shift of the asymmetric uranyl stretching frequency from 950 cm<sup>−1</sup> (hydrated UO<sub>2</sub><sup>2+</sup>) to 890 cm<sup>−1</sup> points to complexation of UO<sub>2</sub><sup>2+</sup> by carboxylate groups which act as bidentate ligands.
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