Complexation of americium with humic acid

As a part of the interlaboratory comparison exercise for the complexation of humic acid and colloid generation (COCO-Club activities) in the CEC project MIRAGE-II, the complexation of americium with humic acid has been studied in our laboratory. Two humic acids were used for the study: Aldrich-HA(H+) which is a reference humic acid of the COCO-Club and Bradford-HA(H+) from Lake Bradford, Florida. A wide concentration range of humic acid and different ratios of Am to humic acid have been investigated between pH 5 and 6 with the ionic strength of 0.1 M and 1.0 M. The complexation has been studied by UV-spectroscopy, Laser-induced Photoacoustic Spectroscopy (LPAS) and ultrafiltration. LPAS is used for the submicromolar concentration range where the sensitivity of UV spectroscopy is not accessible. Ultrafiltration is used for low Am to humic acid ratios where both spectroscopic methods are not applicable. Varying the humic acid concentration over three orders of magnitude, only a 1:1 type of binding is observed. No significant variation of the stability constant is found in the investigated range of pH and ionic strength. However, the precipitation tendency and the loading capacity of humic acid are found to depend sensitively on pH and ionic strength. The complexation study provides a deep insight into the influence of humic acid on the migration behaviour of fission products and actinides in the geosphere. (orig.)