NEW INSIGHT INTO THE ROLE OF ORGANIC MATTER IN THE CONCENTRATION OF U: SEQUESTRATION WITHOUT PRIOR REDUCTION

Samples taken from a uraniferous fen were studied to understand how reductive pathways observed under controlled laboratory settings translate into a complex, natural setting that is dominated by natural organic matter (NOM) and to test the hypothesis that U need not be reduced and precipitated in order for sequestration to take place in a reducing environment. In addition to Fe XANES, simultaneous U and S bulk XANES analyses were performed to determine the spatial relationship of these two elements. Representative areas of samples of interest were mapped using SXRFMA and fluorescence microtomography to determine if U is spatially correlated with Fe.

U is only partially reduced in a limited interval of a 1.4 m core and is not correlated with any other element. This zone of partial reduction is coincident with the completion of Fe reduction, substantial S reduction, and the interval of highest NOM concentrations. The SXFMA element distribution maps reveal no correlation between U and Fe at any depth. The microtomographic map of organic debris confirms this lack of correlation. The distribution of U and Fe at distinctly different sites on the organic matter indicates that NOM plays an important role and may prevent the incorporation of U as Fe minerals are precipitated or sorption of U onto Fe mineral surfaces. Furthermore, the complexing effect of humic acids may make the uranyl ion unavailable as an electron acceptor for Fe oxidation.

The recalcitrance of U(VI) under thermodynamically favorable conditions does not support a simple thermodynamic model for U reduction in this complex environment. Microbial reduction is likely active where Fe and S are reduced and (coupled with low Eh in the sediment) may contribute to partial U reduction. However, if present this process occurs substantially slower than in the laboratory. Humification of NOM provides ample carboxyl groups that may complex uranyl ion prior to reduction, providing the first step in a sequestration process. These complexes may persist until the organic matter is consumed thereby releasing uranyl ion and making it available either for reduction in a low Eh zone or migration in an oxidizing medium to the nearest organic matter where it is again sequestered without prior reduction.