2021 Volume 20 Issue 1 Pages 9-22
A new method of evaluating colloid diffusion and filtration in compacted bentonites using dendrimers was developed. Polyamidoamine (PAMAM) dendrimers with sizes of 5.7 and 7.2 nm were chosen as diffusion probes owing to their high monodispersity and well-defined molecular structure. The diffusion and filtration behaviors were investigated by a through-diffusion experiment in bentonite compacted to 0.8 Mg/m3 and saturated with 0.005–0.5 mol/L NaCl. The breakthrough curves were observed under all conditions, demonstrating that dendrimers could diffuse through the pore network in compacted bentonite. The effective diffusivity (De) and filtration ratio (Rf) of dendrimers were determined from the breakthrough curves and the depth profiles in compacted bentonite, respectively. The De values of the negatively charged dendrimers increased when porewater salinity increased and dendrimer size decreased owing to the anion exclusion effect on negatively charged clay surfaces. The Rf values increased when porewater salinity decreased and dendrimer size increased, demonstrating that significant fractions of dendrimers were filtered by the narrow pores in the complex pore networks. The results of this study confirm the validity of the evaluation method using dendrimers and the importance of the need for further investigation under various conditions to understand the factors controlling colloid diffusion and filtration and their relationships with the microstructure in compacted bentonite.
