SELECTIVITY IN THE COMPLEXATION OF ACTINIDES BY HUMIC SUBSTANCES

The interactions of a range of actinide elements (Th, U, Np, Pu, Am) w ith humic substances from the Needle's Eye natural analogue site were studied by gel permeation chromatography. Bulk humic substances were i solated by ammonia extraction, followed by dialysis against distilled water and freeze-drying. The gel permeation results suggest that Needl e's Eye humic substances can be fractionated into three incompletely r esolved fractions with average molecular weights determined by analyti cal ultracentrifugation around 49 000 for Fraction 1, around 14 700 fo r Fraction 2 and around 8000 for Fraction 3. Although there are signif icant differences between the organic matter elution patterns in indiv idual gel permeation experiments, presumably due to differences in col umn packing, these are much smaller than the differences between metal ions. The uranium that is naturally present in these humic substances is largely bound in the late-eluting fraction. Spikes of the early ac tinides, including Np and Pu in controlled valency states, have been a dded to the humic substances, and gel permeation of the spiked humic s ubstances shows that the three humic fractions vary greatly in their e ffectiveness and selectivity as ligands for early actinides. (C) 1997 Elsevier Science Ltd.