Comparison of Adsorption Behaviors of Kaolin from Column and Batch Tests: Concept of Dual Porosity

Abstract

Batch equilibrium tests with various soil-liquid ratios (S/L=1∶2.5, 1∶5, 1∶10, and 1∶20) and column tests with a flow interruption and rest period were performed to evaluate the adsorption behavior of kaolin. Identical isotherms were obtained from the batch equilibrium tests, illustrating that the S/L ratio has minimal impacts on adsorption behavior if complete mixing is ensured. In the column tests, a long non-Fickian tailing was observed in the breakthrough curve, and a decrease in the concentration of the liquid phase was observed in the closed system during the flow interruption and rest period although the column achieved local equilibrium. These results demonstrated that sorption equilibrium in the soil columns was not reached even when a stable exit concentration was achieved. The dual porosity (DP) concept was used to interpret the test results. Insufficient adsorption in the immobile phase is the major cause of the relatively low adsorbed concentration in short-term column tests, and a higher ratio of mobile phase voids compared to that of mobile phase adsorption capacity leads to a lower apparent retardation factor. The long-term transfer between the two phases is another key point that should be accounted for in the data analysis and modeling of column tests. Differences in immobile phase adsorption levels between tests under laboratory conditions and those under in situ conditions lead to discrepancies, which may confound the apparent retardation factor.