An Unsaturated Transient Flow Method for Determining Solute Adsorption by Variable‐Charge Soils

The amount of reactive ions adsorbed during transport in variable‐charge soils is often smaller than those predicted from batch adsorption experiments. This overestimation of adsorption for invading ions by the batch method is likely to be caused by excessive desorption of indigenous, strongly adsorbed ions. We propose an unsaturated transient flow method by which equilibrium adsorption of weakly reactive ions can be determined without causing appreciable desorption of indigenous ions. In our method, water is imbibed into horizontal columns packed with soil that has been mixed with a salt solution at different concentrations to attain adsorption equilibrium. We make use of the observation that during imbibition of water, the antecedent solution is pushed ahead by the invading solution and accumulates in the region beyond the plane of separation, with the solution concentration unchanged. From linear plots of the solute content vs. soil water content in this region, the amount of solutes adsorbed by soil and the equilibrium solution concentration prior to the water imbibition are obtained. The advantages of the method are that adsorption isotherms can be determined under conditions similar to those expected during transport processes in soil, and that it is exempt from uncertainty about attainment of adsorption equilibrium as in the miscible displacement methods. The proposed method is best suited for variable‐charge soils where adsorption of weakly reactive ions is largely due to an increase in the exchange capacity.