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

Abstract 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.