Quantitative assessment of the dissociation behavior of Eu(III) with humic acid under varying pH, ionic strength, and pre-equilibrium period using a modified combined dissociation model

Abstract

Understanding the dissociation behavior of long half-life radionuclides with humic acid (HA) is critical for their migration behavior in aqueous environments. In this study, we developed a modified combined dissociation model comprising three kinetic terms and one constant, representing the concentrations of unbound, fast-dissociating, slow-dissociating, and non-dissociating metals, respectively. For the first time, the modified dissociation model incorporated the contributions of removal kinetics of unbound metals and metal state transformations to the dissociation processes. The modified dissociation model performed well in describing the metal dissociation curves from the view of standard errors of fit calculation and coefficients of determination. Furthermore, this modified dissociation model was employed to quantitatively evaluate the effect of solution pH, ionic strength (I), and pre-equilibrium period of Eu with HA on the dissociation behavior of Eu bound to HA. Results revealed that the fraction of fast-dissociating Eu increased as pH rose, while that of slow-dissociating Eu decreased. The fraction of fast-dissociating Eu was significantly inhibited with increasing I. As the pre-equilibrium period of Eu with HA was prolonged, the fraction of fast-dissociating Eu gradually transformed into the slow-dissociating and non-dissociating Eu.