‘Strongly bound’ uranium in marine waters: occurrence and analytical implications

Abstract

In the determination of dissolved 238U in aqueous solutions, a pre-concentration by chelating ion exchange chromatography with a Chelex-100 column at pH 4 (or Chelex- 238U) and co-precipitation with iron hydroxide (or Fe 238U) yielded indistinguishable results for pure solutions of inorganic uranium and samples of open-ocean waters. However, in coastal marine waters, the concentration of Fe 238U could be significantly higher than that of Chelex- 238U. The concentration ratio of Fe 238U. The concentration ratio of Fe 238U to Chelex- 238U seemed to be higher in waters with higher concentration of dissolved organic carbon (DOC). A concentration ratio of Fe 238U to Chelex- 238U as high as 1.27 was observed. The concentration of 238U (UV- 238U) in these coastal waters might be further elevated if dissolved organic matter in the sample was first mineralized, by subjecting it to UV irradiation in the presence of hydrogen peroxide under acidic conditions, before analysis. The ratio of the concentration of UV- 238U to Chelex- 238U increased even more dramatically with increasing concentration of DOC. The highest value observed was 1.83. The concentration of dissolved uranium thus increased in the order: Chelex- 238U ⩽ Fe 238U ⩽ UV 238U. These results are consistent with the notion that: (a) only inorganic and weakly bound species are removed by the Chelex 100 resin so that Chelex-U represents inorganic and weakly bound uranium: (b) iron hydroxide co-precipitation can remove a fraction of dissolved organic matter so that FeU includes inorganic as well as a variable fraction of ‘strongly bound’ uranium; (c) the UV irradiation of a sample of water converts ‘strongly bound’ uranium to the readily detectable inorganic uranium: (d) ‘strongly bound’ uranium is more likely to be found in coastal marine waters where the concentration of dissolved organic matter is higher. Previously published data are almost exclusively Chelex-U and FeU. In the open oceans, if the amount of ‘trongly bound’ uranium is small, they should be essentially identical. In coastal waters, however, Chelex-U and FeU may represent different fractions of dissolved uranium. Thus, some of the conclusions on the behavior of uranium in estuarine and coastal waters drawn from these data may have to be re-examined to take into consideration the speciation of uranium in these waters and the analytical methods used. By determining Chelex- 238U and UV- 238U simultaneously in a sample, ‘strongly bound’ uranium may be estimated as the difference in concentration in these two measurements.