Transport of exchanging Ca2+ and Na+ in clay systems

Abstract

Solute transport in clay systems is strongly governed by ion exchange phenomena. We carried out two kinds of experiments in order to investigate the effect of Ca+—Na2+ ion exchange on the transport of solutes. One was a leaching experiment of Na-bentonite with CaCl2 solutions. The other was an evaporation experi-ment of Ca-bentonite having groundwater consisting of NaCl solution. Since Ca2+—Na+ exchange is nonlinear, two types of ion exchange can take place: favorable exchange due to a high affinity for Ca2+ and non-favorable exchange due to a low affinity for Na+. Favorable exchange could be observed in the leaching experiment. Penetrating Ca2+ exchanged with almost all adsorbed Na+ at the moving front of the Ca2+ concentration profile. Then, the Ca2+ front became steep. On the other hand, non-favorable exchange could be observed in the evaporation experiment. The penetrating front of Na+ concentration profile became flat. Ca2+—Na+ ion exchange affects the hydraulic properties of soils. In the leaching experiment, the water flux was suddenly increased when the whole clay from the surface to the bottom was transformed into a Ca-clay system having higher hydraulic conductivity. Water retention also varies significantly depending on the clay systems. For a given water suction, the water content of Ca-bentonite leached with CaCl2 solution is less than that of Na-bentonite, whereas it is much greater than that of Ca-saturated-bentonite mixed with CaCl2 solution. Predicting the changes in the hydraulic properties caused by ion exchange is important to quantify the transport of exchanging solutes in soils.