Gamma-irradiation-induced molecular-weight distribution and complexation affinity of humic acid with Cs+, Sr2+, and Eu3+

Highlights: • Gamma irradiation degraded humic acid and changed the ratio of functional groups. • Phenolic –OH was predominant in smaller molecular-weight fraction of humic acid. • Cs⁺, Sr²⁺, and Eu³⁺ were mainly distributed in small molecular-weight fractions. • Molecular-weight distribution of TOC content was shifted toward larger fractions in the presence of Sr²⁺ and Eu³⁺. Solutions of humic acid (HA) were irradiated with 0, 1, 5, 10, 50, and 100 kGy of gamma irradiation using a ⁶⁰Co 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⁺, Sr²⁺, and Eu³⁺ 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 Sr²⁺ and Eu³⁺ ions redistributed organic carbon toward the larger molecular-weight fractions.