Experimental and modelling studies of radiocesium sorption/desorption processes in the fixed-bed moss column

Highlights: • In the desorption experiment, only 25% of ¹³⁷Cs content was removed from moss. • Model solution showed that moss column has got only 43% of exchangeable sites. • The two site kinetic model is useful to merge the planar and specific sorption sites. • The model better agrees with the experimental data than one site kinetic model. • The analytic solution improves understanding of the dynamics of desorption process - Abstract: The experimental and modelling studies of the fixed-bed column filled with inanimate mosses Ptilium crista-castrensis containing background levels of ¹³⁷Cs, flushed with the distilled water are presented. Fitting of the experimental results by Langmuir and Freundlich sorption models gave significant deviations. A more sophisticated two kinetic site model created for the case of non-equilibrium transport of the radiocesium solute through porous media was more suitable to describe the experimental data. It has been shown that theoretically the two site sorption model can be simplified to the conventional one kinetic site model with time dependent parameters. The radiocesium transport equation included the terms taking into account advection, dispersion and decay. General analytical solution was derived for two types of the border conditions: 1) when a constant flux of the material on the surface layer of the absorbent is known (i.e. a Cauchy type border condition); 2) a border condition with constant initial material concentrations. The mathematical solution was derived using the Laplace transformation method. In such a way, the obtained simulated profile of the radionuclide concentration distribution within the fixed-bed column with inanimate mosses showed the best-fitting to the experimental results.