Gamma-irradiation-induced molecular-weight distribution and complexation affinity of humic acid with Cs, Sr, and Eu.

Solutions of humic acid (HA) were irradiated with 0, 1, 5, 10, 50, and 100 kGy of gamma irradiation using a Co source. The non-irradiated and irradiated HA molecules were fractionated by ultrafiltration into four categories: > 100, 50-100, 10-50, and < 10 kDa. Total organic carbon measurements and potentiometric titration analysis suggested that (1) some gamma-irradiated HA molecules were degraded into smaller molecules and (2) radiolytic degradation caused phenolic -OH became the predominant functional group in the small molecular-weight fractions of HA. The effect of absorbed dose of gamma rays on the distributions of Cs, Sr, and Eu ions in the molecular-weight fractions of the metal-HA systems was examined to discuss the complexation affinity. The metal ions were distributed in the smaller molecular-weight fractions at different doses, which corresponded to the degradation of HA molecules. For a predetermined absorbed dose, Cs ions did not change the molecular-weight distribution of the total organic carbon content of the degraded HA molecules. Conversely, the Sr and Eu ions redistributed organic carbon toward the larger molecular-weight fractions.