Effect of gamma-irradiation on complexation of humic substances with divalent calcium ion.

The effect of gamma-irradiation doses of 0, 10, 100, and 500 kGy at the dose rates of 1 or 0.1 kGy/h on the molecular and chemical properties of humic substances (HS) were investigated using total organic carbon measurements, UV-Vis spectrometry, 13C nuclear magnetic resonance, and acid-base titration. A possible mechanism of the radiolysis on HS was also speculated. The complexation ability of irradiated HS with Ca2+ ions was studied using a Ca ion-selective electrode. The apparent formation constants of the Ca-HS complexes increased as the irradiation dose increased, and this was attributed to the relative increase in the ratio of phenolic -OH to carboxylic groups of HS. The contribution of the phenolic -OH groups to Ca-HS complexes was suppressed at pH 5 owing to its high acid dissociation constants. In addition, the radiation dose rates of 1 and 0.1 kGy/h did not significantly affect the properties of HS and the apparent formation constants of the Ca-HS complexes.