Solution to the particle concentration effect on determining K_d value of radionuclides

The particle concentration effect on K_d values of radionuclides has been observed but the underlying mechanism remains controversial. The hope is to use the relationship between particle concentration, adsorption-desorption isotherms and reversibility, in combination with surface component activity of model (SCA model), to solve this issue. ¹³⁷Cs, ⁶⁰Co, ⁹⁰Sr were used as tracers, batch experiments were conducted in freshwater-sediment and seawater-sediment. The experiment of each radionuclide was designed with five different particle concentrations C_p, and for each C_p there were seven different initial concentrations C₀. After adsorption experiments, four consecutive desorption experiments were carried out. At the fourth desorption experiment, radionuclide concentrations in the supernatant and sediment were measured. The results showed that adsorption and single desorption data of ¹³⁷Cs, ⁶⁰Co, ⁹⁰Sr might be described by linear isotherms. ¹³⁷Cs was reversible in the seawater-sediment, so hysteresis angles of the five-particle concentration were approximately 0°, all adsorption and desorption data could be classified into one line. In the remaining systems, besides the adsorption and single desorption isotherms moved upward with the decrease of particle concentration, hysteresis angles and irreversibility also increased, thus, the particle concentration effect was obvious. The reversible and resistant component concentrations calculated by adsorption, single desorption and consecutive desorption isotherm were linear functions of equilibrium concentration C_e1, respectively. Data from adsorption and desorption experiments with particle concentration effect could be classified into the same line using the Freundlich-SCA model. The results of this study indicate that the particle concentration effect is related to reversibility. When adsorption isotherm and single desorption isotherm are both linear, consecutive desorption isotherm, reversible and resistant component concentrations approach linearity too. After the Freundlich-SCA model eliminated the particle concentration effect on adsorption and desorption data, the data can be used to predict the adsorption, single desorption isotherm and K_d value at any particle concentration.