Analysis of radioactive cesium and cobalt adsorption to rocks using the two-site kinetic model equations

Abstract

The present study shows that equations of the two-site kinetic model, which have been applied to trace-metal and pesticide adsorption to soil, could also be used to simulate diffusion-controlled adsorption behaviour of 60Co and 137Cs to rock powder (diameter less than 105 × 10 −5 m ), pieces (diameter 2 × 10 −3 m ∼ 3 × 10 −3 m ) and slabs ( 0.05 m × 0.05 m × 0.01 m ) of chert, shale and granodiorite in saline water. The adsorption rate coefficient was dependent on sizes of rocks and rock pore structures but was independent of distribution coefficients. The desorption rate coefficient was dependent on sizes of rocks, rock pore structures and distribution coefficients. The difference in adsorption and desorption rate coefficients between rock powder, pieces, and slabs was zero to two orders of magnitude for 137Cs and zero to three orders of magnitude for 60Co. Theoretical relationships between parameters of the two-site kinetic model and the Fickian diffusion model were also developed. Effective diffusion coefficients could be evaluated from the fitted rate parameter values of the two-site kinetic model. Observed characteristics of rate parameters agreed well with the theoretical equation. Dependence of adsorption and desorption rate coefficients and equilibrium distribution coefficient of 137Cs and 60Co on solid to liquid ratios were insignificant for rock powder. Applying rate parameter values obtained in this study to field conditions, it was concluded that kinetics of adsorption may often become important in predicting field-scale solute transport.