Mechanism of Adsorption-desorption Hysteresis of Radionuclide in Marine Environment

The adsorption and desorption hysteresis of ¹¹⁰Ag^m on marine sediment interfaces under various particle concentrations, initial concentrations and adsorption kinetics were studied using traditional batch experiments. The results show that the adsorption and desorption process of ¹¹⁰Ag^m on marine sediments can be well described by the Freundlich isothermal model. The adsorption rate of ¹¹⁰Ag^m increases exponentially in 0.24 h, with a high adsorption rate immediately, and then gradually tends to be gentle. There is particle concentration effect phenomenon. Because with the increase of particle concentration, the isotherms tend to decrease. Desorption hysteresis is described by hysteresis angle (θ) and thermodynamic index (TII). With the particle concentrations increasing from 0.1 g/L to 3.0 g/L, the values of TII and θ first increase and then decrease. The adsorption expresses less reversible because as the adsorption of ¹¹⁰Ag^m to adsorption-sites of different combinations and particles could integrate after collision, this would decrease the specific adsorption surface area and sites. With the initial ¹¹⁰Ag^m concentration increasing, the value of TII first decreases and then increases, and the desorption hysteresis of ¹¹⁰Ag^m first decreases and then increases. There are two main reasons. Firstly, these phenomena are possibly caused by different microscopic structures of ¹¹⁰Ag^m on the marine sediment. Secondly, it is due to the diffusion restriction of narrow pore throat induced by the imbedded ¹¹⁰Ag^m molecules during high –concentration gradient force．Thus the adsorption system shows a certain degree of desorption hysteresis.