Complexation of Am with size-fractionated soil humic acids

As colloidal organic matter, humic substances are widely recognized as important complexing ligands for trace elements in aquatic environments. We studied the effects of bulk properties of humic acid (HA) on Am complexation using soil HAs and their size-separated fractions to elucidate complexation properties of trivalent actinides with HAs. The higher (HR) and lower molecular size fractions (FL) were fractionated from HAs prepared from soil of three types (entisol, andisol, and buried andisol) using Sephadex G-25 gel chromatography. In the presence of HAs, dominant size ranges of Am were 450 nm–100 kDa for the entisol and andisol HAs and their HR fractions, and 100–30 kDa for the HR fraction of buried andisol HA in a 0.01 M NaClO4 solution at pH 7. On the other hand, about 50% of Am in the presence of FL fraction was found in the molecular size range of 30–10 kDa. However, positive correlation was found between the percentages of Am and HAs in each size range except for the HAs less than 10 kDa. The complexation properties of Am with HAs depend on their macromolecular features such as the molecular size distribution and conformation of functional groups.