Influence of non-equilibrium and nonlinear sorption of 137Cs in soils. Study with stirred flow-through reactor experiments and quantification with a nonlinear equilibrium-kinetic model.

This paper addresses the modelling of cesium sorption in non-equilibrium and nonlinear conditions with a two-site model. Compared to the classical K d approach, the proposed model better reproduced the breakthrough curves observed during continuous-flow stirred tank reactor experiments conducted on two contrasted soils. Fitted parameters suggested contrasted conditions of cesium sorption between 1) equilibrium sites, with low affinity and high sorption capacity comparable to CEC and 2) non-equilibrium sites, with a fast sorption rate (half-time of 0.2–0.3 h), a slow desorption rate (half-time of 3–9 days) and a very low sorption capacity (0.02–0.04% of CEC). Comparison of EK sites densities with sorption capacities derived from the literature suggests that the EK equilibrium and kinetic sites might correspond to ion exchange and surface complexation of soil clay minerals respectively. This work stresses the limits of the K d model to predict 137Cs sorption in reactive transport conditions and supports an alternative non-equilibrium nonlinear approach. • Cs sorption was studied on two soils with stirred flow-through reactor experiments. • Observed Cs sorption was influenced by flowrate and concentration. • The proposed two-site model described non-equilibrium and non-linearity effects. • Equilibrium sites had low affinity and high sorption capacity comparable to CEC. • Non-equilibrium sites had a fast sorption rate and a very low sorption capacity. [ABSTRACT FROM AUTHOR]